JP7567739B2 - Manufacturing method for steel plate with excellent fatigue crack propagation resistance - Google Patents

Description

The present invention relates to a steel material with excellent fatigue crack propagation resistance and a manufacturing method thereof. More specifically, the present invention relates to a steel material that is used in various welded structures such as ships, marine structures, bridges, construction machinery, buildings, and tanks, and that has improved fatigue crack propagation resistance even when subjected to repeated loads, and a manufacturing method thereof.

In recent years, high-strength steel materials are increasingly being used in ships, marine structures, bridges, construction machinery, buildings, tanks and other structures in order to streamline design, reduce steel weight, thin walls and reduce the labor required for welding. In addition, these steel materials are required to have excellent fatigue resistance properties to ensure not only toughness and ductility but also weldability and structural safety.

In welded structures, fatigue failure often occurs when a fatigue crack initiates at the weld toe and propagates through the steel material, causing destruction. This is believed to be due to the fact that the weld toe is prone to becoming a stress concentration area due to its shape, and also due to the occurrence of tensile residual stress after welding.

For this reason, widely known methods for preventing cracks from occurring at the weld toe include techniques for improving the shape and reducing stress concentration, such as by applying additional welding, and techniques for introducing compressive residual stress, such as by peening.

However, it is nearly impossible to perform such treatment on an industrial scale on the numerous weld toes that exist, and it is also difficult to say that it is realistic in terms of cost. Therefore, recognizing that it is important to extend the fatigue life by reducing the rate at which fatigue cracks subsequently propagate through the steel, even if they do occur, there is a strong demand from the industry for improving the fatigue crack propagation characteristics of the steel itself.

Here, Patent Document 1 describes a steel having a chemical composition of 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, with the balance being Fe and unavoidable impurities, which is hot-rolled, reheated and quenched, reheated to a two-phase region temperature range of _Ac1 transformation point + 10°C to 790°C, quenched at an average cooling rate of 5 to 60°C/s, and tempered by holding at 400 to 650°C for 10 minutes or more, and has a metal structure that is 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 340 to 440 with an area fraction of 15 to 85%, and has an absolute value of surface residual stress of 150 N/mm A method for manufacturing a steel material having excellent fatigue crack propagation resistance of ² or less has been disclosed.

Patent Document 2 also describes a steel sheet containing C: 0.04-0.3%, Si: 0.01-2%, Mn: 0.1-3%, Al: 0.001-0.1%, N: 0.001-0.01%, P: 0.02% or less, S: 0.01% or less, with the balance being iron and unavoidable impurities, and having a two-phase structure consisting of a soft phase and a hard second phase surrounding the soft phase in a mesh-like structure, and the soft phase and the hard second phase are: (1) the soft phase is ferrite, tempered bainite, tempered martensite and having an average Vickers hardness of 150 or less; (2) the hard second phase is composed of one or more of bainite, martensite, tempered bainite, and tempered martensite, and has an average Vickers hardness of 250 or more; and (3) the grain boundary occupancy rate of the hard second phase, as expressed by a specific formula, is 0.5 or less. A thick steel material with excellent fatigue crack propagation resistance is disclosed.

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

Patent Document 4 also discloses a steel sheet containing, by mass%, C: 0.03-0.30%, Si: 0.01-0.5%, Mn: 0.3-2.0%, sol. Al: 0.001-0.1%, with the remainder being Fe and unavoidable impurities, and whose structure is made up of a hard portion and a soft portion, and in which the proportions of the structure of these two portions and the average hardness in Vickers hardness satisfy a specific formula.

Furthermore, Patent Document 5 discloses a thick steel plate with excellent fatigue crack propagation properties and toughness, which contains, by mass%, C: 0.02-0.16%, Si: 0.05-0.5%, Al: 0.005-0.060%, one or more selected from Mn: 0.1-2.5%, Cu: 0.1-2.0%, and Ni: 0.1-6.0% so that the value of 2Mn+Cu+Ni is 3.5-6.0%, with the remainder being Fe and unavoidable impurities, and which contains 5-20% by volume of retained austenite with an average diameter of 0.1-0.5 μm.

Patent No. 4924047 Patent No. 3785392 Patent No. 4998708 JP 2002-121640 A Patent No. 4497009

However, the manufacturing techniques described in Patent Documents 1 to 5 above have the following problems:

The manufacturing method of Patent Document 1 involves hot rolling, reheating and quenching, reheating to a two-phase temperature range of _Ac1 transformation point +10°C to 790°C, quenching at an average cooling rate of 5 to 60°C/s, and then tempering by holding at 400 to 650°C for 10 minutes or more. However, this manufacturing method is complicated.

Patent Document 2 is characterized in that a steel slab before hot rolling is subjected to a diffusion heat treatment at a heating temperature of 1200 to 1350°C and a holding time in this temperature range of 2 to 100 hours, and then hot rolling is performed at a heating temperature of the _Ac3 transformation point to 1250°C, and after rolling, accelerated cooling is performed at 5 to 100°C/s from the _Ar3 transformation point or higher to 400°C or lower, and further a two-phase region heat treatment is performed at a heating temperature of ( _Ac1 transformation point + 30°C) to ( _Ac3 transformation point - 10°C) and accelerated cooling is performed at 5 to 100°C/s to 400°C or lower, but the manufacturing methods such as the diffusion heat treatment and the two-phase region heat treatment are complicated.

Patent Document 3 describes a method in which steel is subjected to solution heat treatment at 1200 to 1300°C for 25 to 60 hours, followed by air cooling, reheating to 1000 to 1200°C, hot rolling to a rolling end temperature of the _Ar3 transformation point or higher, air cooling, reheating and holding at the _Ac3 transformation point or higher, air cooling, and further reheating to a two-phase region temperature of _Ac1 transformation point + 10°C to _Ac3 transformation point - 10°C, followed by quenching at an average cooling rate of 5°C/s or more, and tempering at 400°C to 650°C. However, the manufacturing methods, such as solution heat treatment and two-phase region heat treatment, are complicated.

Patent Document 4 proposes the relational equation fA.HA - fB.HB ≧ -3500. Here, fA and fB are the percentages of the structure that are occupied by the hard and soft parts, respectively, and HA and HB are the average Vickers hardness values of the hard and soft parts, respectively. However, there is no linear relationship between fA.HA - fB.HB and the fatigue crack propagation rate, and it is unclear as an indicator.

Furthermore, Patent Document 5 is characterized in that after hot rolling, the steel is directly quenched or reheated and quenched, and then is subjected to a two-phase region heat treatment in which the steel is heated to a temperature of 650° C. or higher and lower than the Ac _point and then cooled. However, the manufacturing method such as the two-phase region heat treatment is complicated.

The present invention aims to solve the problems of the conventional technology described above and provide a steel material with excellent fatigue crack propagation resistance that can be manufactured by a simple manufacturing method without complicated heat treatment. Here, "steel material" includes steel plate, steel bar, steel strip, etc., but the following explanation will focus mainly on "steel plate."

The inventors conducted the following experiments and carried out extensive research to solve the above problems.

[Fatigue crack propagation rate]
Three types of test steels (A, B, and C) were used in the experiment. Their chemical compositions are shown in Table 1.

Test steel A was a steel plate (thickness 25 mm) with a steel composition of 0.12%C-0.28%Si-1.33%Mn-0.013%P-0.004%S-0.01%Ni-0.02%Cr-0.011%Ti-0.047%Al-0.0038%N, which was reheated to 900°C, quenched, and then tempered at 200°C. Thereafter, the relationship between the stress intensity factor range [ΔK] and the fatigue crack propagation rate [da/dN] was investigated using a CT test piece whose thickness was reduced to 12 mm on both sides. Test steel B is a soft phase/hard phase dispersion steel (a steel ingot having 0.15%C-0.25%Si-1.51%Mn-0.009%P-0.005%S-0.07%Cr-0.038%Al-0.0025%N) heated to 950°C or higher and 1250°C or lower, and then rolled at ₃ Ar or higher with a cumulative rolling reduction of 50% or more. The steel used was a steel material obtained by starting accelerated cooling from the temperature range of _{Ar 3} point -60°C to 730°C or higher at a cooling rate of 10°C/s or more but less than 30°C/s, and then cooling to 650°C or lower and 500°C or higher, and the test steel C was a conventional steel (a steel material obtained by heating a steel ingot containing 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 to 950°C or higher and rolling it at 900°C or higher with a cumulative reduction rate of 50% or more, and then starting accelerated cooling from the temperature range of Ar ₃ point to Ar ₃ point -100°C, and then cooling to 700°C or lower and 500°C or higher).

The results are shown in Figure 1. In Figure 1, squares (shown as "white frame with black fill" in the figure) represent data for test steel A, triangles represent data for test steel B, and circles represent data for test steel C. The solid line in the figure represents the level of fatigue crack propagation rate [da/dN] versus stress intensity factor range [ΔK] for a typical conventional steel such as test steel C, and is a straight line connecting 3.50 x 10 ^-8 m/cycle when ΔK = 15 MPa·m ^1/2 and 1.70 x 10 ^-7 m/cycle when ΔK = 25 MPa·m ^1/2 . The dotted line below is half the fatigue crack propagation rate level of sample steel C, and is a straight line connecting 1.75×10 ^-8 m/cycle when ΔK = 15 MPa·m ^1/2 and 8.50×10 ^-8 m/cycle when ΔK = 25 MPa·m ^1/2 . The dashed dotted line below is one-fifth the fatigue crack propagation rate level of sample steel C, and is a straight line connecting 7.00×10 ^-9 m/cycle when ΔK = 15 MPa·m ^1/2 and 3.40×10 ^-8 m/cycle when ΔK = 25 MPa·m ^1/2 . From the results of this experiment, it was confirmed that sample steel B had a fatigue crack propagation rate level that was approximately half that of sample steel C, while sample steel A had a fatigue crack propagation rate level that was approximately 1/5 that of sample steel C, indicating that sample steel A has the best fatigue crack propagation resistance.

Next, a thin film sample was prepared from 1/2 the thickness of the steel plate of the above-mentioned test steel A, and metal structure observation was performed using a TEM (transmission electron microscope). In addition, carbides, nitrides, and carbonitrides were identified using an attached energy dispersive X-ray spectrometer (EDX) analysis. An example of the results is shown in Figure 2. In Figure 2, three precipitates consisting of carbides were observed in one field of view (3.8 μm ² , 10,000 times magnification). As a result of similar observations being performed in 10 fields of view with a field interval of 50 μm, precipitates consisting of one or more carbides, nitrides, or carbonitrides were confirmed in six fields of view (corresponding to 60% of the 10 fields of view).

For the observation of the metal structure, a test piece taken from the center of the thickness of the steel material (steel plate) (at 1/2 the thickness) was polished and corroded (nital etched), and the structure was observed with a scanning electron microscope (SEM) (magnification: 500x). An image was taken and the metal structure was determined using an image analyzer. As a result, the metal structure observed in the above sample steel A was a tempered bainite structure. The above sample steel B had a structure in which clumped pearlite and clumped bainite had formed in a ferrite matrix. Furthermore, the above sample steel C had a structure in which band-shaped pearlite had formed in a ferrite matrix.

From the above experimental results, it was discovered that fatigue crack propagation characteristics are significantly improved by using a steel material with a specific composition, whose metal structure is a tempered bainite structure, and which further contains one or more types of precipitates of carbides, nitrides, and carbonitrides.

Similarly, a decrease in fatigue crack propagation rate was observed in steel materials that had tempered martensite structures or mixed structures of tempered bainite and tempered martensite structures, and even if ferrite structures were present within these structures, the area ratio of the ferrite structures was 50% or less, and that contained one or more of the following precipitates: carbide, nitride, and carbonitride.

The present invention was completed based on these findings and through further investigation, and the gist of the present invention is as follows.
[1] A steel material having a chemical composition containing, by mass%, C: 0.02-0.40%, Si: 0.010-0.500%, Mn: 0.05-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.100% or less, N: 0.1000% or less, with the balance being Fe and unavoidable impurities, wherein the metal structure of the steel material is any one of a tempered bainite structure, a tempered martensite structure, or a mixed structure of a tempered bainite structure and a tempered martensite structure, and further characterized in that one or more types of precipitates of carbide, nitride, and carbonitride are present.
[2] In [1], in addition to the chemical composition, the steel material has excellent fatigue crack propagation resistance, characterized in that it further contains, by mass%, one or more selected from 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%, and REM: 0.001 to 0.100%.
[3] A steel material having excellent fatigue crack propagation resistance according to [1] or [2], characterized in that each of the metal structures 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 plate thickness of the steel material is 5 to 100 mm.
[5] In any one of [1] to [4], the number of the precipitates is 1 to 100 precipitates/3.8 ^μm2 at the center of the plate thickness of the steel material.
[6] In any one of [1] to [5], a steel material having excellent fatigue crack propagation resistance, characterized in that when the stress intensity factor range [ΔK] in a fatigue crack propagation test of the steel material is 20 MPa·m ^1/2 , the fatigue crack propagation rate [da/dN] is 4.26×10 ^-8 m/cycle or less.
[7] A method for producing a steel material according to any one of [1] to [6], comprising heating a steel material having a chemical composition, by mass%, containing 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, with the balance being Fe and unavoidable impurities, to the Ac _point or higher, subjecting the steel material to a reheating and quenching treatment, and then subjecting the steel material to a tempering treatment, thereby producing a steel material having excellent fatigue crack propagation resistance.
[8] In [7], a method for producing a steel material having excellent fatigue crack propagation resistance, characterized in that in addition to the chemical composition of the steel material, the steel material further contains, by mass%, one or more selected from 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%, and REM: 0.001 to 0.100%.
[9] In the method of [7] or [8], the reheating and quenching treatment is carried out at a cooling stop temperature of 25°C or less and a cooling rate of 5 to 250°C/s.
[10] A method for producing a steel material having excellent fatigue crack propagation resistance, as set forth in any one of [7] to [9], characterized in that the tempering treatment is carried out at a heating temperature of 100 to 450°C for a heating time of 10 seconds or more.

The present invention makes it possible to produce steel materials with excellent fatigue crack propagation resistance using a simple manufacturing method, which has a significant industrial effect.

FIG. 1 is a correlation diagram showing the results of fatigue crack propagation resistance tests of the steel material of the present invention and conventional steel materials. 1 is a photograph taken in observation of the metal structure of a steel material according to the present invention.

The following is a detailed description of an embodiment of the present invention.

[Basic chemical composition of steel]
Next, the basic chemical composition of the steel material according to the present invention will be described. Note that, hereinafter, "%" in the chemical composition means "mass %".

[C:0.02-0.40%]
C must be added at 0.02% or more to ensure strength. However, adding 0.40% or more impairs weldability. Therefore, the range of C is 0.02 to 0.40%. The content is preferably 0.02 to 0.35%. More preferably, it is 0.02 to 0.33%. Further preferably, it is 0.02 to 0.30%.

[Si:0.010-0.500%]
Silicon is effective as a deoxidizer and at least 0.010% is necessary to increase strength. However, if it is added in excess of 0.500%, it deteriorates weldability and toughness. The range is limited to 0.050 to 0.500%. The range is preferably 0.050 to 0.450%. The range is more preferably 0.050 to 0.430%. The range is further preferably 0.050 to 0. It is 400%.

[Mn: 0.05-2.00%]
Mn is inexpensive and increases strength by increasing hardenability, and from the viewpoint of improving toughness, 0.05% or more is necessary. However, if it exceeds 2.00%, it leads to deterioration of weldability. The range of the content of Cr is limited to 0.05 to 2.00%. The range is preferably 0.05 to 1.90%. The range is more preferably 0.05 to 1.85%. The range is further preferably 0.05 to 1. . 80%.

[P: 0.050% or less]
P is an impurity that deteriorates toughness, so the lower the content, the better. From the viewpoint of production costs, the content is limited to a range of 0.050% or less. Preferably, it is 0.040% or less. More preferably, it is 0.050% or less. It is preferably 0.030% or less, and more preferably 0.020% or less.

[S: 0.050% or less]
S is an impurity that deteriorates toughness, so the lower the content, the better. From the viewpoint of production costs, the content is limited to a range of 0.050% or less. Preferably, it is 0.040% or less. More preferably, it is 0.050% or less. It is preferably 0.030% or less, and more preferably 0.020% or less.

[Al: 0.100% or less]
Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process for high tensile steel. It also fixes N in the steel as AlN, contributing to improving the toughness of the base material. Addition of more than 100% of Al reduces the toughness of the base metal and also degrades the toughness of the weld metal when it is mixed into the weld metal during welding. For this reason, Al is limited to 0.100% or less. Preferably, it is 0.080% or less. % or less. More preferably, it is 0.070% or less. Further preferably, it is 0.060% or less.

[N: 0.1000% or less]
N is undesirable because it adversely affects ductility and toughness in a solid solution state, but it is effective in refining austenite grains and strengthening precipitation when combined with V, Al, and Ti, so even a small amount can improve mechanical properties. However, excessive N content increases the amount of dissolved N, which may adversely affect ductility and toughness, so the N content is limited to a range of 0.1000% or less. Preferably, it is 0.0900%. The content is preferably 0.0800% or less, more preferably 0.0700% or less.

[Optional chemical composition of steel]
The above-mentioned chemical composition is the basic chemical composition of the steel material of the present invention. In addition to this basic chemical composition, the present invention further includes, as an optional chemical composition, one or more selected from 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%, and REM: 0.001 to 0.100% can be selected and contained for the purpose of adjusting strength, toughness, weldability, and the like, and imparting weather resistance, as necessary.

[Cu: 0.01-2.00%]
Cu has the effect of increasing strength through solid solution and improving weather resistance. For this reason, the lower limit is preferably set to 0.01%. However, if the Cu content exceeds 2.00%, weldability is impaired. At the same time, defects are likely to occur during the production of steel materials. Therefore, when it is contained, it is preferable that the content is 0.01 to 2.00%. More preferably, it is 0.01 to 1.50%. Even more preferably,
It is 0.01 to 1.00%.

[Ni:0.01-5.00%]
Ni is effective in improving low-temperature toughness, weather resistance, and hot brittleness that occurs when Cu is added. Therefore, the lower limit is preferably set at 0.01%. However, if the Ni content is 5 If it exceeds 0.00%, it impairs weldability and leads to increased costs. Therefore, if it is contained, it is preferable to limit it to 0.01 to 5.00%. More preferably, it is preferable to limit it to 0.01 to 4.00%. %, and more preferably, it is 0.01 to 3.00%.

[Cr:0.01-3.00%]
Cr improves weather resistance and strength. For this reason, the lower limit of its addition is preferably set at 0.01%. However, if its content exceeds 3.00%, it impairs weldability and toughness. Therefore, its content is If added, the content is preferably 0.01 to 3.00%, more preferably 0.01 to 2.50%, and even more preferably 0.01 to 2.00%.

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

[Nb: 0.001 to 0.100%]
Nb suppresses austenite recrystallization during rolling to refine the grains, and at the same time, has the effect of increasing strength through precipitation, so it may be contained in an amount of 0.001% or more. However, it should not be more than 0.100%. If it is contained in excess, toughness will deteriorate. Therefore, if it is contained, it is preferable that the content be 0.001 to 0.100%, more preferably 0.001 to 0.090%, and even more preferably 0.001 ~0.080%.

[V:0.001-0.100%]
Like Nb, V also has the effect of increasing strength through precipitation, so it may be contained in an amount of 0.001% or more. However, if it is contained in an amount exceeding 0.100%, it will cause a decrease in weldability and toughness. Therefore, When it is contained, its content is preferably 0.001 to 0.100%, more preferably 0.001 to 0.090%, and further preferably 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 welded portion. However, if the content exceeds 0.100%, the cost tends to increase. Therefore, the content In this case, the content is preferably 0.001 to 0.100%, more preferably 0.001 to 0.090%, and further preferably 0.001 to 0.080%.

[B:0.0001-0.0100%]
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 it is contained in an amount exceeding 0.0100%, it impairs weldability. Therefore, if it is contained, the content should be less than 0. The content is preferably 0.0001 to 0.0100%, more preferably 0.0001 to 0.0090%, and further preferably 0.0001 to 0.0080%.

[REM: 0.001-0.100%]
REM refers to rare earth elements such as Sc, Y, La, and Ce. When added in small amounts, they may be contained in an amount of 0.001% or more because they contribute to improving HAZ toughness. However, if they exceed 0.100%, If REM is contained in the steel, it will impair weldability. Therefore, if REM is contained, it is preferable that the content be 0.001 to 0.100%. More preferably, it is 0.001 to 0.090%. Even more preferably, , 0.001 to 0.080%.

[Remainder Chemical Composition]
The remaining chemical composition other than the above-mentioned chemical composition is composed of Fe and inevitable impurities. Examples of the inevitable impurity elements include O (oxygen), Sn, Sb, As, Pb, Bi, Ca, Mg, etc., and a total of 0.10% or less is acceptable. In addition, as long as the above-mentioned basic chemical composition and optional chemical composition are satisfied, the inclusion of inevitable impurity elements other than these is not prevented, and such embodiments are also included in the technical scope of the present invention.

[Metal structure of steel material]
In the present invention, a steel material having the above-mentioned chemical composition can be obtained by a manufacturing method described below. It has been found that the metal structure of the central part of the plate thickness of the obtained steel material has the following metal structure due to the difference in chemical composition, the change in cooling rate depending on the plate thickness of the steel material, and the heat treatment conditions such as tempering treatment. The structure is any one of tempered bainite structure, tempered martensite structure, and a mixed structure of tempered bainite structure and tempered martensite structure, and a ferrite structure with an area ratio of 50% or less may be present in the structure of these metal structures. In other words, it has the following metal structure.
(a) Tempered bainite structure (b) Tempered martensite structure (c) A mixed structure of tempered bainite structure and tempered martensite structure (d) A ferrite structure and a tempered bainite structure with an area ratio of 50% or less (e) A ferrite structure and a tempered martensite structure with an area ratio of 50% or less (f) A mixed structure of a ferrite structure, a tempered bainite structure, and a tempered martensite structure with an area ratio of 50% or less. As mentioned above, these metal structures can be determined by observing the structure using a scanning electron microscope (SEM) (magnification: 500 times).

As mentioned above, each of the above structures can be obtained by adjusting the chemical composition, cooling rate, and heat treatment conditions such as tempering.

Specifically, a high cooling rate (50°C/s or more) tends to result in a martensite structure, while a low cooling rate (10°C/s or more) tends to result in a bainite structure. If the cooling rate is even slower (5°C/s or less), the bainite structure or martensite structure, or ferrite structure will be mixed into the matrix of the bainite and martensite structures, will result. Then, by tempering these reheated and quenched steel materials, the structures (a) to (f) mentioned above can be obtained.

[Precipitates in metal structure]
In the above-mentioned 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, in the metallographic observation, the number of precipitates is 1 to 100/3.8 μm ² in the center of the plate thickness of the steel material, so that a steel material having excellent fatigue crack propagation resistance can be obtained. That is, in the metallographic observation at the center of the plate thickness of the steel material, it is preferable that one measurement field of view is 3.8 μm ² , 10 fields of view are observed at intervals of 50 μm, and the number of the above precipitates is 1 to 100/3.8 μm ² somewhere in each of the fields of view (at least one field of view).

The above-mentioned precipitates are carbides, nitrides, or carbonitrides of elements such as Fe, Si, Mn, and Al in the basic chemical composition of the steel, as well as carbides, nitrides, or carbonitrides of elements such as Cr, Mo, Nb, V, and Ti in the optional chemical composition. These precipitates in the metal structure are scattered as small and large particles, as in the example shown in Figure 2, and their size is approximately 10 to 500 nm.

These precipitates can be confirmed by cutting a 1 mm slice from the center of the thickness of the steel material (steel plate), polishing both sides of the slice to reduce the thickness to 50 μm, punching out a 3 mm diameter slice with a disk punch, thinning the disk by electrolytic polishing, washing with methanol, and drying on filter paper to prepare a TEM observation sample, and observing with a TEM (10,000x magnification). Furthermore, the carbides, nitrides, and carbonitrides were identified by an attached energy dispersive X-ray spectrometer (EDX) analysis. Here, the above-mentioned EDX analysis method can be mentioned as a method for identifying carbides, nitrides, and carbonitrides. According to this method, peaks of Fe and C appear in carbides, peaks of Fe and N appear in nitrides, and peaks of Fe, C, and N appear in carbonitrides.

[Method of manufacturing steel material]
The method for producing a steel material according to the present invention is characterized in that a steel material (steel material) adjusted to have the above-mentioned composition is heated to the _Ac3 point or higher, reheated and quenched, and then tempered. Here, the temperature is the temperature at the center of the plate thickness of the steel material (steel material). As a result, the structure at the center of the plate thickness of the steel material is cooled at a rate adjusted according to the plate thickness, and the above-mentioned metal structure and precipitates within the metal structure are obtained. In the case of a steel material having such a metal structure, it has been confirmed that the fatigue crack propagation resistance is improved as described below. Such an effect has been confirmed when the plate thickness of the steel material is in the range of 5 to 100 mm.
Furthermore, each step in the method for producing a steel material according to the present invention will be described in detail.

[Ac heating for ₃ points or more]
First, in order to completely austenitize the metal structure of the steel, it is heated to the _Ac3 point (austenite transformation temperature) or higher. This heating is also called reheating, and this temperature is also called the reheating temperature. Heating below the _Ac3 point leaves the pre-heating structure remaining, resulting in a decrease in strength. This _Ac3 point can be calculated using the following formula (1) based on the composition percentage of the steel.
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×Nb+198.4×Al+3315×B... (1)
If not contained, the value is set to 0.

[Quenching treatment]
In order to make the metal structure in the center of the plate thickness of the steel material into the above-mentioned bainite structure, martensite structure, or mixed structure of bainite and martensite structures, a quenching treatment is performed. These structures are appropriately obtained by changing the cooling rate during water cooling according to the plate thickness and adjusting the chemical composition. If no ferrite structure is precipitated or if its precipitation is 50% or less, the desired mechanical properties described below can be obtained, but if the area ratio of the ferrite structure exceeds 50%, the properties cannot be obtained.

Specifically, in laboratory rolling, the heated steel is cooled with a water jet at a cooling rate of 297°C/s or less, preferably 5 to 250°C/s, with the cooling stop temperature being 25°C or less. In actual equipment, there are water jet cooling and laminar cooling quenching devices. The cooling rate is set to 250°C/s or less because a steel material with a plate thickness of 6 mm containing the composition of the present invention and having a tempered martensite structure that has been reheated, quenched, and tempered satisfies the desired characteristics. The cooling rate is set to 5°C/s or more when a steel material with a plate thickness of 100 mm is reheated and quenched, and the cooling stop temperature is set to 25°C or less because it is easy to become a martensite structure if it is quenched sufficiently.

As mentioned above, each of the above structures can be obtained by adjusting the chemical composition, cooling rate, and heat treatment conditions such as tempering.

Specifically, a high cooling rate (50°C/s or higher) tends to result in a martensite structure, whereas a low cooling rate (10°C/s or higher) tends to result in a bainite structure. If the cooling rate is even slower (5°C/s or lower), a bainite structure or a martensite structure, or a ferrite structure will be mixed into the matrix of the bainite structure and the martensite structure. Then, by tempering these reheated and quenched steel materials, the structures (a) to (f) described below can be obtained.
(a) Tempered bainite structure; (b) Tempered martensite structure; (c) A mixed structure of tempered bainite structure and tempered martensite structure; (d) A ferrite structure with an area ratio of 50% or less and a tempered bainite structure; (e) A ferrite structure with an area ratio of 50% or less and a tempered martensite structure; (f) A mixed structure of a ferrite structure, a tempered bainite structure, and a tempered martensite structure with an area ratio of 50% or less and a tempered martensite structure.

[Tempering treatment]
By subjecting the metal structure produced by the above-mentioned quenching treatment to a tempering treatment at a heating temperature of 100 to 450°C for a heating time of 10 seconds or more, the bainite structure becomes a tempered bainite structure, the martensite structure becomes a tempered martensite structure, and the above-mentioned precipitates which are carbides, nitrides or carbonitrides appear.

Specific heating temperatures must be 100°C or higher to precipitate any of the carbides, nitrides, or carbonitrides, and if the temperature exceeds 450°C, strength will decrease. Therefore, 100 to 450°C is preferable. More preferably, it is 150 to 450°C. The heating time is preferably 10 seconds or more, but if it exceeds 100 minutes, the manufacturing costs will be too high, so 10 seconds to 100 minutes is preferable. More preferably, it is 10 seconds to 60 minutes. This treatment causes the martensite structure to become a tempered martensite structure, and the bainite structure to become a tempered bainite structure, and causes carbides and the like to precipitate. Tempering at 100 to 450°C can be easily performed in an atmospheric furnace or vacuum furnace. Also, a large atmospheric furnace can be used in actual equipment.

[Fatigue crack propagation resistance]
The fatigue crack propagation resistance of steel materials is evaluated by determining the stress intensity factor range [ΔK] and fatigue crack propagation rate [da/dN] through fatigue crack propagation tests conforming to the ASTM E647 standard. 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. The fatigue crack propagation rate [da/dN] is the fatigue crack extension amount [m/cycle] in one cycle of the stress waveform, and in particular, in the intermediate range of ΔK (approximately ΔK = about 10 MPa·m ^1/2 to ΔK = about 70 MPa·m ^1/2 ), a linear relationship called the Paris law, da/dN = CΔK ^m (C and m are constants), holds.

In the present invention, a steel material having excellent fatigue crack propagation resistance is one having a fatigue crack propagation rate of 4.26×10 ^-8 m/cycle or less when the stress intensity factor range [ΔK] is 20 MPa·m ^1/2 .

[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 in a tensile test, tensile strength [TS] ≧ 500 MPa, absorbed energy [ _VE0 ] ≧ 27 J at a test temperature of ₀ °C in a Charpy impact test, and fatigue crack propagation rate [da/dN] ≦ 4.26 × ^10-8 [m/cycle] when the stress intensity factor range [ΔK] is 20 MPa·m1 ^/2 in a fatigue crack propagation resistance test.

Twenty-six types of steel having steel compositions A to Z shown in Table 2 were prepared, and test pieces (plates No. 1 to 32) shown in Table 3 were treated under the manufacturing conditions shown in Table 3 (reheating to _Ac3 point or higher, quenching treatment, and then tempering treatment at a low temperature of 100 to 450°C for 10 minutes or more).

A 1 mm slice was cut from the center of the thickness of the steel material (steel plate) that had been treated as described above, and the slice was polished from both sides to reduce the thickness to 50 μm, then punched out to 3 mm diameter with a disk punch. The disk was thinned by electrolytic polishing, washed with methanol, and dried on filter paper to prepare a TEM observation sample. TEM observation (10,000x magnification) was performed with 10 fields of view at a 50 μm pitch. Furthermore, carbides, nitrides, and carbonitrides were identified using an attached energy dispersive X-ray spectrometer (EDX) analysis.

The results of observing precipitates such as carbides are shown in Table 4. All of the test pieces according to the present invention had at least one precipitate in the metal structure, but no precipitates were observed at all in the comparative test pieces and plates No. 24, 27, 29, 30, and 32.

Next, for the tensile test, ASTM-F (6φ×24GL) round bar test pieces were taken from the center of the plate thickness and in the direction perpendicular to the rolling direction and used for the test. For the Charpy impact test, three full-size test pieces were taken from the center of the plate thickness and in the rolling direction and used for the test at a test temperature of 0°C. For the fatigue crack propagation rate, a fatigue crack propagation test was performed on a CT test piece with a plate thickness of 12 mm on both sides (when the plate thickness of the steel plate was less than 12 mm, the plate thickness was not reduced and the original thickness was used) at an R ratio of 0.1, a sine wave, and 10 Hz. Table 5 shows the YS [MPa], TS [MPa], _V E ₀ [J], and the fatigue crack propagation rate [da/dN] [m/cycle] at ΔK = 20 MPa·m ^1/2 at this time. Microscopic observation was performed by nital etching at the plate thickness 1/2 position and SEM observation. The TEM observation was carried out by taking a thin film from the position of 1/2 the plate thickness.

[Mechanical properties of steel]
Tensile test specimens (parallel portion diameter 6 mmφ) and Charpy impact test specimens (V notch) were taken from the obtained test specimens in accordance with the provisions of JIS Z 2241 and 2242, and tensile tests and impact tests were carried out.

In the tensile test, if a yield point was found, the upper yield point was taken as the YS of the test piece. If no yield point was found, the 0.2% yield strength was taken as the YS. The maximum value on the stress-strain curve was taken as the tensile stress, and the TS was taken as the TS.

The Charpy impact test was carried out on three specimens each, and the absorbed energy _V E ₀ at a test temperature of 0° C. was calculated, and the average value was regarded as the value of the test specimen.

The results are shown in Table 5.

The test pieces No. 1 to 22, which have the steel chemical composition specified in the present invention and are produced by the steel manufacturing method specified in the present invention, have YS of 400 MPa or more, TS of 500 MPa or more, _V E ₀ of 27 J or more, and fatigue crack propagation rate [da/dN] at ΔK=20 MPa·m ^1/2 of 4.26×10 ⁻⁸ [m/cycle] or less, and thus have excellent fatigue crack propagation rate resistance. In addition, at this time, at least one precipitate was present in the metal structure observation as shown in Table 4. In addition, as shown in Table 3, the metal structure was either a tempered bainite structure, a tempered martensite structure, or a mixed structure of a tempered bainite structure and a tempered martensite structure, or a structure in which a ferrite structure is present in an area ratio of 50% or less in these metal structures.

However, the steel plate No. 23 had a Charpy impact test value below 27J because C exceeded the specified value of the present invention. The steel plate No. 24 had C, Si, and Mn below the specified values of the present invention, so no precipitates such as carbides were observed in any of the 10 visual fields in the metallographic observation, the YS and TS were below the target values of the present invention, the Charpy impact test value was below the target values of the present invention, and the fatigue crack propagation rate at ΔK = 20 MPa·m ^1/2 exceeded 4.26 × 10 ^-8 [m/cycle], and the fatigue crack propagation resistance was poor. The steel plate No. 25 had P, S, Al, and N above the specified values of the present invention, so the Charpy impact test value was below the target value of the present invention. The steel plate No. 26 had Si and Mn above the specified values of the present invention, so the Charpy impact test value was below the target value of the present invention. In the case of steel plate No. 27, the reheating temperature was below the Ac ₃ point of the present invention, so the metal structure was 80% ferrite structure + 20% pearlite structure in terms of area ratio, and no precipitates such as carbides were observed in any of the 10 visual fields in the metal structure observation. As a result, the YS and TS were below the target values of the present invention. In the case of steel plate No. 28, the tempering treatment temperature exceeded the specified value of the present invention, so the YS and TS were below the target values of the present invention. In the case of steel plate No. 29, the reheating temperature was below the Ac ₃ point of the present invention, so the metal structure was 85% ferrite structure + 15% pearlite structure, and no precipitates such as carbides were observed in any of the 10 visual fields in the metal structure observation. As a result, the YS and TS were below the target values of the present invention. In the case of steel plate No. 30, the reheating temperature was below the Ac ₃ point and the tempering temperature was above the specified value of the present invention, so the metal structure was a 20% pearlite structure + 80% ferrite structure, and no precipitates such as carbides were observed in any of the 10 visual fields in the metal structure observation. As a result, the YS and TS were below the target values of the present invention. In the case of steel plate No. 31, the tempering temperature was above the specified value of the present invention, so the YS and TS were below the target values of the present invention. In the case of steel plate No. 32, the reheating temperature was below the Ac ₃ point and the tempering temperature was below the specified value of the present invention, so the metal structure was a 18% pearlite structure + 82% ferrite structure, and no precipitates such as carbides were observed in any of the 10 visual fields in the metal structure observation. As a result, the YS and TS were below the target values of the present invention, the Charpy impact test values were below the target values of the present invention, and the fatigue crack propagation rate at ΔK = 20 MPa·m ^1/2 exceeded 4.26 × 10 ^-8 [m/cycle], indicating poor fatigue crack propagation resistance.

Claims (7)

A method for producing a steel plate, the metal structure of which is any one of a tempered bainite structure, a tempered martensite structure, or a mixed structure of a tempered bainite structure and a tempered martensite structure, and further, one or more precipitates of carbide, nitride, and carbonitride are present,
A method for producing a steel plate with excellent fatigue crack propagation resistance, comprising heating a steel material having a chemical composition containing, by mass%, C: 0.02-0.40%, Si: 0.010-0.500%, Mn: 0.05-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.100% or less, N: 0.1000% or less, with the balance being Fe and unavoidable impurities, to the Ac _{point or} higher, subjecting the steel material to a reheating and quenching treatment, and then subjecting the steel material to a tempering treatment at a heating temperature of 100-450°C for a heating time of 10 seconds or more . 2. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, further comprising, in addition to the chemical composition of the steel material, one or more selected from, by mass%, 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%, and REM: 0.001 to 0.100%. 3. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1 , wherein the reheating and quenching treatment is carried out at a cooling stop temperature of 25° C. or less and a cooling rate of 5 to 250° C./s. 4. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, wherein the metal structures and a ferrite structure are present at an area ratio of 50% or less. 5. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, wherein the steel plate has a thickness of 5 to 100 mm. The number of the precipitates is 10 nm or more in size at the center of the thickness of the steel plate, and is 1 to 100 precipitates/3.8 μm. ² 6. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, The stress intensity factor range [ΔK] in the fatigue crack propagation test of the steel plate is 20 MPa m ^1/2 In this case, the fatigue crack propagation rate [da/dN] is 4.26×10 ^-8 7. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, wherein the fatigue crack propagation resistance is less than m/cycle.

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