Steel material with excellent fatigue crack propagation resistance and manufacturing method thereof

Abstract

To provide a steel sheet having excellent fatigue crack propagation resistance properties obtained by a simple manufacturing method without complicated heat treatment.SOLUTION: There is provided a method for manufacturing a steel sheet in which a steel stock 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 0.0001 to 0.0100% of B is heated to the Ac3 point or more and subjected to reheating and quenching treatment and then cooling is stopped at 355 to 450°C, wherein the metallic structure of the steel sheet is one of a bainitic structure, a martensitic structure or a mixed structure of a bainitic structure and a martensitic structure and further when a steel sheet has one or more precipitates of carbides, nitrides and carbonitrides having a size of 10 nm or more and 1 to 100 pieces per 3.8 μm2, a steel sheet having excellent fatigue crack propagation resistance properties and mechanical properties can be easily produced.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steel material having excellent fatigue crack propagation resistance and a method for manufacturing the same, and more particularly 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 method for manufacturing the same. [Background technology]

[0002] In recent years, high-strength steel has been increasingly used in ships, marine structures, bridges, construction machinery, buildings, tanks, and other structures to streamline design, reduce steel weight, thin walls, and reduce welding labor. In addition, these steels are required to have not only toughness and ductility, but also excellent fatigue resistance to ensure weldability and structural safety.

[0003] In welded structures, fatigue failure often occurs when a fatigue crack initiates at the weld toe and propagates through the steel material, resulting in failure. This is thought to be due to the fact that the weld toe is prone to become a stress concentration area due to its shape, and also due to the generation of tensile residual stress after welding.

[0004] For this reason, widely known means for suppressing crack initiation from the weld toe include techniques for improving the shape by applying additional welding to reduce stress concentration, and techniques for introducing compressive residual stress by peening or the like.

[0005] However, it is nearly impossible to apply such treatment to the numerous weld toes on an industrial scale, and it is also not 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 in the steel, even if they do occur, there is a strong demand from the industry for improving the fatigue crack propagation properties of the steel itself.

[0006] Here, Patent Document 1 describes a steel having a chemical composition containing, by mass %, 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, then reheated to a two-phase 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 which has a metal structure consisting of a mixed structure of a ferrite phase having a Vickers hardness of 85 to 130 and a tempered martensite phase having a Vickers hardness of 340 to 440 with an area fraction of 15 to 85%, and an absolute value of the surface residual stress is 150 N / mm 2 The following method for manufacturing a steel material having excellent fatigue crack propagation resistance has been disclosed.

[0007] Patent Document 2 also describes a steel sheet containing 0.04 to 0.3% C, 0.01 to 2% Si, 0.1 to 3% Mn, 0.001 to 0.1% Al, 0.001 to 0.01% N, 0.001 to 0.01% P or less, 0.02% or less S, and the balance being iron and unavoidable impurities, which has 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 classified as follows: (1) the soft phase is ferrite, tempered bainite, or tempered martensite; The present invention discloses a thick steel material with excellent fatigue crack propagation resistance that satisfies all of the following conditions: (1) 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 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.

[0008] Patent Document 3 also discloses a steel material having a steel composition, in mass %, 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, in which 80% or more of the metal structure in both the thickness direction and the length direction of the plate is composed of 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, and which has small anisotropy of the material and excellent fatigue crack propagation resistance.

[0009] Patent Document 4 also discloses a steel sheet containing, by mass%, C: 0.03 to 0.30%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, sol. Al: 0.001 to 0.1%, with the remainder being Fe and unavoidable impurities, and whose structure is composed of a hard portion and a soft portion, and whose proportions of these two portions in the structure and the average hardness in Vickers hardness satisfy a specific formula.

[0010] Furthermore, Patent Document 5 discloses a thick steel plate with excellent fatigue crack propagation properties and toughness, which contains, by mass%, C: 0.02 to 0.16%, Si: 0.05 to 0.5%, Al: 0.005 to 0.060%, and one or more elements selected from Mn: 0.1 to 2.5%, Cu: 0.1 to 2.0%, and Ni: 0.1 to 6.0% so that the value of 2Mn + Cu + Ni is 3.5 to 6.0%, with the remainder being Fe and unavoidable impurities, and which contains 5 to 20% by volume of retained austenite having an average diameter of 0.1 to 0.5 μm. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 4924047 [Patent Document 2] Patent No. 3785392 [Patent Document 3] Patent No. 4998708 [Patent Document 4] Japanese Patent Application Laid-Open No. 2002-121640 [Patent Document 5] Patent No. 4497009 Summary of the Invention [Problem to be solved by the invention]

[0012] However, the manufacturing techniques according to the above Patent Documents 1 to 5 have the following problems.

[0013] The manufacturing method described in 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, but this manufacturing method is complicated.

[0014] 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 for a holding time within that temperature range of 2 to 100 hours, followed by hot rolling at a heating temperature of the Ac3 transformation point to 1250°C, in which the slab is cooled after rolling at an accelerated rate of 5 to 100°C / s from the Ar3 transformation point or higher to 400°C or lower, and further subjected to a two-phase region heat treatment at a heating temperature of (Ac1 transformation point +30°C) to (Ac3 transformation point -10°C), in which the slab is cooled at an accelerated rate of 5 to 100°C / s to 400°C or lower; however, the manufacturing methods such as the diffusion heat treatment and the two-phase region heat treatment are complicated.

[0015] 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 at least the Ar3 transformation point, air cooling, reheating to at least the Ac3 transformation point, air cooling, and further reheating to a two-phase region temperature of between the Ac1 transformation point +10°C and the Ac3 transformation point -10°C, followed by quenching at an average cooling rate of 5°C / s or more, and tempering at 400 to 650°C. However, manufacturing methods such as solution heat treatment and two-phase region heat treatment are complicated.

[0016] Patent Document 4 proposes the relationship fA·HA-fB·HB≧-3500, where fA and fB are the percentages of the hard and soft parts in the structure, 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, making it unclear as an indicator.

[0017] Furthermore, Patent Document 5 is characterized in that after hot rolling, the steel is subjected to direct quenching or reheating and quenching, and then 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 Ac3 point and then cooled. However, the manufacturing method such as the two-phase region heat treatment is complicated.

[0018] Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional art and to 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 mainly focus on "steel plate". [Means for solving the problem]

[0019] The present inventors conducted the following experiments and conducted extensive research to solve the above problems.

[0020] [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.

[0021] [Table 1]

[0022] Test steel A was a 25mm thick steel plate with a 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. The steel plate was reheated to 900°C, quenched, and cooled to 200°C. The thickness was then reduced to 12mm on both sides using a CT test specimen, and the relationship between the stress intensity factor range [ΔK] and the fatigue crack propagation rate [da / dN] was investigated. Test steel B is a soft / hard phase dispersion steel (0.15%C-0.25%Si-1.51%Mn-0.009%P-0.005%S-0.07%Cr-0.038%Al-0.0025%N) made by heating an ingot of steel between 950°C and 1250°C, rolling it at a cumulative reduction rate of 50% or more at Ar3 or higher, and then starting accelerated cooling at a rate of 10°C / s to 30°C / s from the temperature range of Ar3 - 60°C to 730°C or higher, and then cooling it to 650°C or lower and 500°C or higher. The steel used was a conventional steel (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, heated to 950°C or higher and 1250°C or lower, rolled to 900°C or higher with a cumulative reduction of 50% or higher, and then accelerated cooling was initiated from the temperature range of Ar3 to Ar3 - 100°C, followed by cooling to 700°C or lower and 500°C or higher).

[0023] The results are shown in Figure 1. In Figure 1, squares (shown as "white frames with black fills") 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, where ΔK = 15 MPa m 1 / 2 3.50×10 -8 m / cycle and ΔK=25MPa·m 1 / 2 When -7 The dotted line below it represents half the fatigue crack propagation rate level of test steel C, ΔK = 15 MPa m 1 / 2 When -8m / cycle and ΔK=25MPa·m 1 / 2 When -8 The dashed line below it represents a level that is 1 / 5 of the fatigue crack propagation rate level of test steel C, ΔK = 15 MPa m 1 / 2 When -9 m / cycle and ΔK=25MPa·m 1 / 2 3.40×10 -8 The results of this experiment show that test steel B has a fatigue crack propagation rate level that is about half that of test steel C, while test steel A has a fatigue crack propagation rate level that is about one-fifth that of test steel C, demonstrating that test steel A has the best fatigue crack propagation resistance.

[0024] Next, a thin film sample was prepared from half the thickness of the steel plate of the above-mentioned test steel A, and the metal structure was observed using a TEM (transmission electron microscope) (magnification: 10,000 times). An example of the results is shown in Figure 2. In Figure 2, the metal structure was observed in one field of view (3.8 μm 2 ) 28 precipitates consisting of carbides were observed. Similar observations were made in 10 fields of view at intervals of 50 μm, and one or more precipitates consisting of carbides, nitrides, or carbonitrides were confirmed in 6 of the fields of view (equivalent to 60% of the 10 fields of view).

[0025] The metallographic structure was observed by polishing and etching (nital etching) test specimens taken from the center of the steel plate thickness (at half the plate thickness). The specimens were then subjected to microstructural observation using a scanning electron microscope (SEM) at 500x magnification, and the metallographic structure was evaluated using an image analyzer. As a result, the metallographic structure observed in the above-mentioned sample steel A was a bainite structure. The above-mentioned sample steel B had a structure in which blocky pearlite and blocky bainite were formed in a ferrite matrix. Furthermore, the above-mentioned sample steel C had a structure in which band-shaped pearlite was formed in a ferrite matrix.

[0026] From the above experimental results, it was discovered that the fatigue crack propagation properties can be significantly improved by using a steel material having a specific composition, a bainite metal structure, and the presence of one or more types of precipitates of carbide, nitride, and carbonitride.

[0027] Similarly, a decrease in fatigue crack propagation rate was observed in steel materials with martensite structures or mixed structures of bainite and martensite structures, and even in those structures where ferrite structures are present, the ferrite structures account for 50% or less in area ratio, and where one or more of carbide, nitride, and carbonitride precipitates are present.

[0028] The present invention was completed based on these findings and further investigations, and the gist of the present invention is as follows. [1] A steel material having a chemical composition containing, 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, with the balance being Fe and unavoidable impurities, wherein the metal structure of the steel material is any one of a bainite structure, a martensite structure, or a mixed structure of a bainite structure and a martensite structure, and further characterized in that one or more types of precipitates of carbide, nitride, and carbonitride are present. This steel material has excellent fatigue crack propagation resistance. [2] In [1], in addition to the chemical composition, the steel material having excellent fatigue crack propagation resistance according to [1] is characterized in that it further contains, in 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] In [1] or [2], a steel material having excellent fatigue crack propagation resistance, characterized in that the above-mentioned metal structures and a ferrite structure with an area ratio of 50% or less are present. [4] In any one of [1] to [3], the steel material has excellent fatigue crack propagation resistance, 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 the precipitates is 1 to 100 / 3.8 μm at the center of the plate thickness of the steel material. 2 A steel material having excellent fatigue crack propagation resistance, characterized by: [6] In any one of [1] to [5], the stress intensity factor range [ΔK] in a fatigue crack propagation test of the steel material is 20 MPa m 1 / 2 When the fatigue crack propagation rate [da / dN] is 4.26×10 -8 A steel material with excellent fatigue crack propagation resistance characterized by a fatigue crack propagation strength of 0.1 m / cycle or less. [7] A method for producing a steel material according to any one of [1] to [6], characterized in that a steel material having excellent fatigue crack propagation resistance is heated to the Ac3 point or higher, subjected to a reheating and quenching treatment, and cooled at 100 to 450°C, the steel material having a chemical composition containing, 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, with the balance being Fe and unavoidable impurities. [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] A method for producing a steel material having excellent fatigue crack propagation resistance according to [7] or [8], characterized in that the reheating and quenching treatment is carried out at a cooling rate of 5 to 275°C / s. [Effects of the Invention]

[0029] According to the present invention, a steel material having excellent fatigue crack propagation resistance can be produced by a simple production method, and this has a significant industrial effect. [Brief explanation of the drawings]

[0030] [Figure 1] 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. [Figure 2] 1 is a photograph taken in observation of the metal structure of a steel material according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, embodiments of the present invention will be specifically described.

[0032] [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 %."

[0033] [C: 0.02~0.40%] C must be added in an amount of 0.02% or more to ensure strength. However, adding 0.40% or more impairs weldability. Therefore, the C content is limited to the range of 0.02 to 0.40%. Preferably, it is 0.02 to 0.35%. More preferably, it is 0.02 to 0.33%. Still more preferably, it is 0.02 to 0.30%.

[0034] [Si: 0.010~0.500%] Si is effective as a deoxidizer and 0.010% or more is necessary to increase strength, but adding more than 0.500% will deteriorate weldability and toughness. Therefore, the content is limited to the range of 0.010 to 0.500%. Preferably, it is 0.050 to 0.450%. More preferably, it is 0.050 to 0.430%. Still more preferably, it is 0.050 to 0.400%.

[0035] [Mn: 0.05~2.00%] Mn not only increases strength by increasing hardenability at low cost, but also improves toughness. A content of 0.05% or more is necessary from the viewpoint of improving toughness. However, if it exceeds 2.00%, it leads to deterioration of weldability. Therefore, the content is limited to the range of 0.05 to 2.00%. It is preferably 0.05 to 1.90%, more preferably 0.05 to 1.85%, and even more preferably 0.05 to 1.80%.

[0036] [P:0.050% or less] P is an impurity that deteriorates toughness, so the lower the content, the better. In terms of production costs, the P content is limited to a range of 0.050% or less, preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less.

[0037] [S:0.050% or less] S is an impurity that deteriorates toughness, so the lower the content, the better. In terms of production costs, the S content is limited to a range of 0.050% or less, preferably 0.040% or less, more preferably 0.030% or less, and even more preferably 0.020% or less.

[0038] [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. On the other hand, adding more than 0.100% Al reduces the toughness of the base material and also degrades the toughness of the weld metal by being 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. More preferably, it is 0.070% or less. Even more preferably, it is 0.060% or less.

[0039] [N:0.1000% or less] N is undesirable in a solid solution state because it adversely affects ductility and toughness, but it effectively combines with V, Al, and Ti to refine austenite grains and strengthen precipitation, so a small amount is effective in improving mechanical properties. However, excessive N content increases the amount of solute 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% or less. More preferably, it is 0.0800% or less. Even more preferably, it is 0.0700% or less.

[0040] [Optionally selected 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 may further contain, 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%, for the purpose of adjusting strength, toughness, weldability, etc., or imparting weather resistance, as necessary.

[0041] [Cu: 0.01 to 2.00%] Cu has the effect of increasing strength through solid solution and also improves weather resistance. For this reason, the lower limit is preferably set to 0.01%. However, if its content exceeds 2.00%, it impairs weldability and makes it more likely for defects to occur during steel production. Therefore, when Cu is contained, it is preferably set to 0.01 to 2.00%. More preferably, it is 0.01 to 1.50%. Even more preferably, It is 0.01 to 1.00%.

[0042] [Ni: 0.01~5.00%] Ni improves low-temperature toughness and is effective in improving weather resistance and hot embrittlement that occurs when Cu is added. For this reason, the lower limit is preferably set to 0.01%. However, if the content exceeds 5.00%, it impairs weldability and leads to increased costs. Therefore, when Ni is contained, it is preferably set to 0.01 to 5.00%. More preferably, it is 0.01 to 4.00%. Even more preferably, it is 0.01 to 3.00%.

[0043] [Cr: 0.01~3.00%] Cr improves weather resistance and strength. For this reason, the lower limit of its addition is preferably 0.01%. However, if its content exceeds 3.00%, it impairs weldability and toughness. Therefore, when Cr is contained, its content is preferably 0.01 to 3.00%, more preferably 0.01 to 2.50%, and even more preferably 0.01 to 2.00%.

[0044] [Mo: 0.01-1.00%] Mo may be contained in an amount of 0.01% or more to increase strength. However, if its content exceeds 1.00%, it causes deterioration of weldability and toughness. Therefore, if Mo is contained, it is preferably 0.01 to 1.00%, more preferably 0.01 to 0.90%, and even more preferably 0.01 to 0.80%.

[0045] [Nb: 0.001 to 0.100%] Nb suppresses austenite recrystallization during rolling, leading to grain refinement, and at the same time, it has the function 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%, toughness deteriorates. Therefore, if it is contained, it is preferably 0.001 to 0.100%, more preferably 0.001 to 0.090%, and even more preferably 0.001 to 0.080%.

[0046] [V:0.001~0.100%] Like Nb, V also has the function 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 result in a decrease in weldability and toughness. Therefore, if V is contained, it is preferably 0.001 to 0.100%, more preferably 0.001 to 0.090%, and even more preferably 0.001 to 0.080%.

[0047] [Ti: 0.001~0.100%] Ti may be contained in an amount of 0.001% or more to increase strength and improve weld toughness. However, if the content exceeds 0.100%, costs tend to increase. Therefore, if Ti is contained, it is preferably 0.001 to 0.100%, more preferably 0.001 to 0.090%, and even more preferably 0.001 to 0.080%.

[0048] [B:0.0001~0.0100%] B may be contained in an amount of 0.0001% or more to improve hardenability and contribute to increased strength. However, if it is contained in an amount exceeding 0.0100%, it impairs weldability. Therefore, if B is contained, it is preferably 0.0001 to 0.0100%, more preferably 0.0001 to 0.0090%, and even more preferably 0.0001 to 0.0080%.

[0049] [REM: 0.001~0.100%] REM refers to rare earth elements such as Sc, Y, La, and Ce. When added in trace amounts, 0.001% or more may be contained because it contributes to improving HAZ toughness. However, if the content exceeds 0.100%, it will impair weldability. Therefore, when REM is contained, it is preferably 0.001 to 0.100%. More preferably, it is 0.001 to 0.090%. Even more preferably, it is 0.001 to 0.080%.

[0050] [Remaining chemical composition] The remainder of the chemical composition other than the above-mentioned chemical composition consists of Fe and unavoidable impurities. Examples of these unavoidable impurity elements include O (oxygen), Sn, Sb, As, Pb, Bi, Ca, and Mg, and a total of 0.10% or less is acceptable. Furthermore, as long as the above-mentioned basic chemical composition and optional chemical composition are satisfied, the inclusion of other unavoidable impurity elements is not prohibited, and such embodiments are also within the technical scope of the present invention.

[0051] [Metal structure of steel] In the present invention, a steel material having the aforementioned chemical composition can be obtained by the manufacturing method described below. It has been found that the metallographic structure of the central part of the plate thickness of the obtained steel material has the following metallographic structure depending on the chemical composition and the heat treatment conditions such as quenching by changing the cooling rate according to the plate thickness of the steel material. This structure is either a bainite structure, a martensite structure, or a mixed structure of bainite and martensite, and there are also cases where a ferrite structure with an area ratio of 50% or less is present within these metallographic structures. In other words, it has the following metallographic structure. (a) Bainite structure (b) Martensite structure (c) Mixed structure of bainite and martensite (d) Ferrite and bainite structures with an area ratio of 50% or less (e) Ferrite structure and martensite structure with an area ratio of 50% or less (f) A mixed structure of ferrite, bainite, and martensite 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).

[0052] As mentioned above, each of the above structures can be obtained by adjusting the chemical composition, cooling rate, heat treatment conditions such as quenching, and the like.

[0053] Specifically, a high cooling rate (50°C / s or higher) and a low cooling stop temperature (150°C or lower) tend to result in a martensite structure, while a low cooling rate (30°C / s or lower) and a high cooling stop temperature (100 to 450°C) tend to result in a bainite structure. An even slower cooling rate (5°C / s or lower) results in a bainite or martensite structure, or a ferrite structure mixed into the matrix of bainite and martensite. In other words, the above-mentioned heat treatment can produce the structures (a) to (f) described above.

[0054] [Precipitates in the metal structure] In the metallographic observation using TEM mentioned above, it was found that it is important that one or more of the precipitates of carbide, nitride, and carbonitride exist in the metallographic structure. Specifically, in the metallographic observation, the number of precipitates is 1 to 100 per 3.8 μm at the center of the steel plate thickness. 2 This results in a steel material with excellent fatigue crack propagation resistance. 2 The visual field is observed in 10 fields at 50 μm intervals, and the number of the above precipitates is 1 to 100 / 3.8 μm somewhere in each of the visual fields (at least in one visual field). 2 It is preferable that:

[0055] The above-mentioned precipitates are carbides, nitrides, or carbonitrides of Fe, Si, Mn, Al, etc. in the basic chemical composition of the steel, as well as carbides, nitrides, or carbonitrides of Cr, Mo, Nb, V, Ti, etc. in the optional chemical composition. These precipitates in the metal structure are scattered in the form of small and large particles, as in the example shown in Figure 2, and their size is about 10 to 500 nm.

[0056] These precipitates were obtained by cutting a 1 mm thin section from the center of the steel plate, polishing the cut section from both sides to reduce the thickness to 50 μm, punching it out to 3 mm diameter with a disc punch, thinning the disc by electrolytic polishing, washing it with methanol, and drying it on filter paper to prepare a TEM observation sample. The TEM observation (observation field: 3.8 μm) was then carried out. 2The precipitates were observed using an attached energy dispersive X-ray spectrometer (EDX) for magnification of 10,000x. Carbide, nitride, and carbonitride were then identified using the attached energy dispersive X-ray spectrometer (EDX). This allows for the determination of whether the precipitates are carbide, nitride, or carbonitride precipitates. Here, methods for identifying carbides, nitrides, and carbonitrides include energy dispersive X-ray spectrometer (EDX) analysis. With this method, carbides exhibit peaks for Fe and C, nitrides exhibit peaks for Fe and N, and carbonitrides exhibit peaks for Fe, C, and N.

[0057] [Steel manufacturing method] The method for producing a steel material according to the present invention is characterized in that a steel material prepared to have the aforementioned composition is heated to or above the Ac3 point, subjected to a reheating and quenching treatment, and cooled to a temperature between 100 and 450°C. Here, the temperature is the temperature at the center of the thickness of the steel material (steel plate). As a result, the structure at the center of the thickness of the steel material is obtained by adjusting the cooling rate according to the thickness, and the aforementioned metal structure and precipitates within the metal structure are obtained. It has been confirmed that steel material having such a metal structure exhibits improved fatigue crack propagation resistance, as described below. This effect has been confirmed when the steel material (steel plate) has a thickness in the range of 5 to 100 mm.

[0058] Furthermore, each step in the method for producing a steel material of the present invention will be described in detail.

[0059] [Heating above Ac3 point] In order to fully austenitize the metal structure of steel, it is heated to above the Ac3 point (austenite transformation temperature). 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 and reduces strength. This Ac3 point can be calculated using the following formula (1) based on the composition percentage of the steel. Ac3=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.

[0060] [Quenching treatment] Quenching is performed to convert the metal structure at the center of the steel plate thickness into the aforementioned bainite structure, martensite structure, or a mixed structure of bainite and martensite structure. These structures can be obtained by adjusting the chemical composition and changing the cooling rate during water cooling depending on the plate thickness. The desired mechanical properties described below can be obtained even if no ferrite structure is precipitated, or if its precipitation is 50% or less in area fraction. However, if the ferrite structure is more than 50% in area fraction, these properties cannot be obtained.

[0061] Specifically, the quenching treatment involves controlled cooling of the heated steel material with a water jet at a cooling rate of 5 to 275°C / s or less, preferably 5 to 260°C / s. The cooling rate of 275°C / s or less is set in order to obtain a martensite structure or a bainite structure.

[0062] In addition, as a quenching method, in laboratory experiments, controlled cooling was performed using water jet cooling from above the Ar3 point. In the case of actual equipment, the desired metal (micro)structure can be obtained by cooling using water jet cooling or laminar cooling.

[0063] [Cooling stop temperature] By performing the above quenching treatment and stopping the cooling at 100 to 450°C during the cooling process, a bainite structure, a martensite structure, or a mixed structure of bainite and martensite structures is formed, and the above-mentioned precipitates, which are carbides, nitrides, or carbonitrides, appear.

[0064] If the cooling stop temperature, which is the temperature at which the cooling is stopped, is not 100°C or higher, any of carbides, nitrides, and carbonitrides will not precipitate, and if it exceeds 450°C, 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 techniques such as controlled cooling are used. In the case of an actual machine, controlled cooling is performed by performing jet cooling or laminar cooling, and stopping water cooling at 100 to 450°C. In the case of laboratory cooling, controlled cooling is performed 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 to 450°C. A martensite structure is likely to be obtained when the cooling stop temperature is less than 150°C.

[0065] [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 in accordance with 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 amount of fatigue crack growth [m / cycle] in one cycle of the stress waveform, and is particularly high in the intermediate range of ΔK (approximately ΔK = 10 MPa m 1 / 2 Therefore, ΔK = approximately 70 MPa m 1 / 2 ) in the equation, da / dN=CΔK, which is called the Paris law m A linear relationship holds: (C and m are constants).

[0066] In the present invention, a steel material with excellent fatigue crack propagation resistance has a stress intensity factor range [ΔK] of 20 MPa m 1 / 2 When the fatigue crack propagation rate is 4.26×10 -8 m / cycle or less.

[0067] [Mechanical properties of steel] The target values ​​of the mechanical properties of the steel material of the present invention are a yield stress [YS] ≥ 400 MPa, a tensile strength [TS] ≥ 500 MPa in a tensile test, and an absorbed energy [ V E0) ≥ 27J, and the stress intensity factor range in the fatigue crack propagation resistance test [ΔK] is 20 MPa m 1 / 2 Fatigue crack propagation rate at [da / dN] ≦ 4.26 × 10 -8 [m / cycle]. [Example]

[0068] 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 processed under the manufacturing conditions shown in Table 3 (heating to the Ac3 point or higher, quenching, and stopping cooling during the cooling process).

[0069] [Table 2]

[0070] [Table 3]

[0071] A 1 mm thin section was cut from the center of the thickness of the steel material (steel plate) that had been treated as described above, and the cut thin section was polished from both sides to reduce the thickness to 50 μm. Then, it was punched out to 3 mm diameter with a disk punch, and the disk was thinned by electrolytic polishing, washed with methanol, dried on filter paper, and used as a TEM observation sample. 2 A 10000x magnification was performed on 10 fields of view at 50 μm pitches. Furthermore, carbides, nitrides, and carbonitrides were identified using an attached energy dispersive X-ray spectrometer (EDX). The observation results are shown in Table 4. All of the test specimens of the invention had one or more types of precipitates in their metal structures, but no precipitates were observed at all in test specimens / plates Nos. 24, 27 to 29, and 32 of the comparative examples.

[0072] [Table 4]

[0073] Next, for the tensile test, ASTM-F (6φ×24GL) round bar test pieces were taken from the center of the plate thickness, 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, 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 thickness reduced on both sides to 12 mm, with an R ratio of 0.1, a sine wave, and 10 Hz. At this time, YS [MPa], TS [MPa], V E0[J], ΔK=20MPa·m 1 / 2 The fatigue crack propagation rates [da / dN] [m / cycle] at these times are shown in Table 4. Microscopic observations were performed using an SEM (magnification 500x) after nital etching at the half-thickness position. For TEM observations, a thin film was taken from the half-thickness position as described above, and then observed and photographed (field of view: 3.8 μm 2 , magnification: 10,000x).

[0074] [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.

[0075] 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% proof stress was taken as the YS. The maximum value on the stress-strain curve was taken as the tensile stress, and the value obtained was taken as TS.

[0076] In addition, Charpy impact tests were carried out on three specimens each, and the absorbed energy at test temperature: 0°C V E0 was determined, and the average value was used as the value for the test piece.

[0077] The results obtained are shown in Table 5.

[0078] [Table 5]

[0079] The test pieces Nos. 1 to 22 have the steel chemical composition specified in the present invention and are produced by the steel manufacturing method specified in the present invention, and have a YS of 400 MPa or more and a TS of 500 MPa or more. V E0 is 27 J or more, ΔK = 20 MPa m 1 / 2 The fatigue crack propagation rate [da / dN] at this time is 4.26×10 -8 The fatigue crack propagation rate was 100 [m / cycle] or less, and a steel plate with excellent fatigue crack propagation rate resistance was obtained. Furthermore, at this time, one or more types of precipitates were present in the metallographic observation, as shown in Table 4. Furthermore, as shown in Table 3, the metallographic structure was either a bainite structure, a martensite structure, or a mixed structure of bainite and martensite, or a structure in which ferrite structure was present in an area ratio of 50% or less within these metallographic structures.

[0080] However, in the case of steel plate No. 23, the C content exceeded the value specified in the present invention, and therefore the Charpy impact test value was below 27 J. In the case of steel plate No. 24, the C, Si, and Mn contents were below the values ​​specified in the present invention, and therefore no precipitates such as carbides were observed in any of the 10 visual fields in the metallographic observation, and the YS and TS were below the target values ​​of the present invention, and the Charpy impact test value was below the target values ​​of the present invention, resulting in a ΔK of 20 MPa m. 1 / 2 The fatigue crack propagation rate at this time is 4.26×10 -8 [m / cycle], and the fatigue crack propagation resistance was poor. Steel plate No. 25 had P, S, N, and Al contents exceeding the values ​​specified in the present invention, and therefore the Charpy impact test value was below the target value of the present invention. Steel plate No. 26 had Si and Mn contents exceeding the values ​​specified in the present invention, and therefore the Charpy impact test value was below the target value of the present invention. Steel plate No. 27 had a reheating temperature below the Ac3 point specified in the present invention, and therefore the metallographic structure was an 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 metallographic observation. As a result, the YS and TS were below the target values ​​of the present invention. Steel plate No. 28 had a cooling stop temperature below the specified value of the present invention, and therefore no precipitates such as carbides were observed in any of the 10 visual fields in the metallographic observation, and therefore the Charpy impact test value was below the target value of the present invention, and ΔK = 20 MPa m1 / 2 The fatigue crack propagation rate at this time is 4.26×10 -8 [m / cycle], and the fatigue crack propagation resistance was poor. For steel sheet No. 29, the heating temperature was below the Ac3 point defined in the present invention, so the metallographic structure was 85% ferrite + 15% pearlite, and no precipitates such as carbides were observed in any of the 10 visual fields during metallographic observation. As a result, the YS and TS were below the target values ​​defined in the present invention. For steel sheet No. 30, the cooling stop temperature exceeded the specified value defined in the present invention, so the metallographic structure was 82% ferrite + 18% pearlite, and the YS and TS were below the target values ​​defined in the present invention. Similarly, for steel sheet No. 31, the cooling stop temperature exceeded the specified value defined in the present invention, so the metallographic structure was 81% ferrite + 19% pearlite, and the YS and TS were below the target values ​​defined in the present invention. For steel plate No. 32, the cooling stop temperature was below the specified value of the present invention, so no precipitates such as carbides were observed in any of the 10 visual fields in the metallographic observation, and the Charpy impact test value was below the target value of the present invention, with ΔK = 20 MPa m 1 / 2 The fatigue crack propagation rate at this time is 4.26×10 -8 [m / cycle], and the fatigue crack propagation resistance was poor.

Claims

1. In mass%, it contains 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, B: 0.0001 to 0.0100%, and further contains 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%, REM: 0.001 to 0.100%, with the balance being Fe and unavoidable impurities. 3 Heat to above this point, reheat and harden, and stop cooling at 355-450°C. The metal structure of the steel plate is either a bainite structure, a martensite structure, or a mixed structure of a bainite structure and a martensite structure, and further, at the center of the thickness of the steel plate, one or more of precipitates of carbide, nitride, and carbonitride are 10 nm or more in size and 1 to 100 precipitates / 3.8 μm 2 A method for manufacturing a steel plate having excellent fatigue crack propagation resistance, characterized in that the steel plate is made of a material having a high fatigue crack propagation resistance.

2. 2. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, wherein the thickness of the steel plate is 5 to 100 mm.

3. The stress intensity factor range [ΔK] in the fatigue crack propagation test of the steel plate is 20 MPa m 1 / 2 When the fatigue crack propagation rate [da / dN] is 4.26 × 10 -8 3. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, wherein the fatigue crack propagation resistance is m / cycle or less.

4. 4. 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 rate of 5 to 275° C. / s.

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