Manufacturing method for steel plate with excellent fatigue crack propagation resistance

Abstract

To provide a manufacturing method for obtaining a steel sheet having excellent fatigue crack propagation resistance properties 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 and the balance Fe with inevitable impurities is heated to 1000 to 1300°C, rolled at a cumulative reduction ratio of 50% or more at 900°C or more, quenched and then tempered. The metallic structure of the resulting steel sheet is one of a tempered bainitic structure, a tempered martensitic structure or a mixed structure thereof or a structure in which a ferrite structure having an area ratio of 50% or less is present in these metallic structures and one or more precipitates of carbides, nitrides and carbinides having a size of 10 nm or more and 1 to 100 pieces / 3.8 μm2 are present in the center of the sheet thickness.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a steel plate having excellent fatigue crack propagation resistance, and more particularly to a method for manufacturing a steel plate 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. [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 manufacturing method for easily obtaining a steel material having excellent fatigue crack propagation resistance without requiring complicated heat treatment. Here, "steel material" includes steel plate, steel bar, steel strip, etc., but the following description 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] Steel A was prepared from an ingot with a composition of 0.13%C, 0.26%Si, 1.34%Mn, 0.007%P, 0.005%S, 0.01%Ni, 0.07%Cr, 0.014%Ti, 0.043%Al, and 0.0038%N. The ingot was heated to 1200°C and rolled to a final thickness of 25 mm with a cumulative reduction of 80% above 900°C. The plate was then rapidly cooled from 900°C to 20°C at a rate of 70°C / s to a finishing temperature of 940°C, and tempered at 200°C for 3600 seconds. The relationship between the stress intensity factor range [ΔK] and the fatigue crack propagation rate [da / dN] was then investigated using CT specimens with a thickness reduced on both sides to 12 mm. 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 m1 / 2 When -8 m / 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 transmission electron microscope (TEM). In addition, carbides, nitrides, and carbonitrides were identified using an attached energy dispersive X-ray spectrometer (EDX). An example of the results is shown in Figure 2. In Figure 2, a thin film sample was prepared from one field of view (3.8 μm 2 , 10,000x), 13 precipitates consisting of carbides were observed. Similar observations were made in 10 fields of view at intervals of 50 μm, and precipitates consisting of one or more carbides, nitrides, or carbonitrides were confirmed in 7 of the fields of view (equivalent to 70% 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 images were taken and analyzed using an image analyzer. As a result, the observed metallographic structure of the steel A was a tempered bainite structure. The steel B had a structure in which massive pearlite and massive bainite were formed in a ferrite matrix. The steel C had a structure in which band-shaped pearlite was formed in a ferrite matrix.

[0026] From the above experimental results, it was found that the fatigue crack propagation properties can be significantly improved by using a steel material having a specific composition, whose metal structure is a tempered bainite structure, and which further contains one or more types of precipitates selected from the group consisting of carbide, nitride, and carbonitride.

[0027] Similarly, a decrease in fatigue crack propagation rate was observed in steel materials with tempered martensite structures or mixed structures of tempered bainite and tempered 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, B: 0.0001 to 0.0100%, with the balance being Fe and unavoidable impurities, is heated to a temperature of 1000 to 1300°C at the center of the plate thickness, rolled to 900°C or more with a cumulative reduction rate of 50% or more, then quenched and tempered to produce a steel plate, The quenching treatment is a treatment of cooling from the Ar3 point or higher to 40°C or lower at a cooling rate of 5°C / s or higher, and the tempering treatment is a treatment of heating at a temperature of 100 to 450°C for a heating time of 10 minutes or longer, The steel sheet has a metal structure selected from the group consisting of a tempered bainite structure, a tempered martensite structure, and a mixed structure of a tempered bainite structure and a tempered martensite structure, and further has at least one of carbide, nitride, and carbonitride precipitates with a size of 10 nm or more and a distribution of 1 to 100 precipitates / 3.8 μm at the center of the sheet thickness. 2 Assume that the steel plate exists A method for manufacturing a steel plate having excellent fatigue crack propagation resistance, characterized by: [2] A method for producing a steel plate having excellent fatigue crack propagation resistance according to [1], characterized in that in addition to the chemical composition of the steel material, the steel plate further contains, by mass%, one or more elements 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%, and REM: 0.001 to 0.100%. [3] A method for producing a steel plate having excellent fatigue crack propagation resistance according to [1] or [2], characterized in that the above-mentioned metal structures and a ferrite structure with an area ratio of 50% or less are present. [4] A method for producing a steel plate having excellent fatigue crack propagation resistance in any one of [1] to [3] above, characterized in that the thickness of the steel plate is 5 to 100 mm. [5] In any one of [1] to [4], the steel plate is subjected to a fatigue crack propagation test in which the stress intensity factor range [ΔK] is 20 MPa m 1 / 2 When the fatigue crack propagation rate [da / dN] is 4.26×10 -8 A method for producing a steel plate with excellent fatigue crack propagation resistance, characterized in that the steel plate has a fatigue crack propagation resistance of 0.1 m / cycle or less. [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, with regard to an embodiment of the present invention, first, a steel material obtained by the manufacturing method 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] [B:0.0001~0.0100%] B is contained in an amount of 0.0001% or more to improve hardenability and contribute to increasing 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%.

[0041] [Optionally selected chemical composition of steel] The above-mentioned chemical composition is the basic chemical composition of the steel material of the present invention, and in the present invention, in addition to this basic chemical composition, 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%, and REM: 0.001 to 0.100% can be optionally contained as a chemical composition, if necessary, for the purpose of adjusting strength, toughness, weldability, etc., or imparting weather resistance.

[0042] [Cu: 0.01 to 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 content exceeds 2.00%, weldability is impaired and defects are more likely 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%.

[0043] [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%.

[0044] [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%.

[0045] [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%.

[0046] [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%.

[0047] [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%.

[0048] [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%.

[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 is manufactured by the manufacturing method described below to obtain a steel material with excellent fatigue crack propagation resistance. It has been found that the metallographic structure of the central part of the plate thickness of this obtained steel material has the following metallographic structure depending on the heat treatment conditions, such as tempering, due to differences in chemical composition and changes in the cooling rate depending on the plate thickness of the steel material. This structure is either a tempered bainite structure, a tempered martensite structure, or a mixed structure of tempered bainite and tempered martensite, and these metallographic structures may also contain a ferrite structure with an area ratio of 50% or less. In other words, it has the following metallographic structure. (a) Tempered bainite structure (b) Tempered martensite structure (c) Mixed structure of tempered bainite and tempered martensite (d) Ferrite structure with an area ratio of 50% or less and tempered bainite structure (e) Ferrite structure with an area ratio of 50% or less and tempered martensite structure (f) A mixed structure of ferrite, tempered bainite, and tempered 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 tempering treatment, and the like.

[0053] Specifically, a high cooling rate (50°C / s or higher) tends to result in a martensite structure, while a low cooling rate (10°C / s or lower) tends to result in a bainite structure. Even slower cooling rates (5°C / s or lower) result in a bainite structure, a martensite structure, or a bainite / martensite matrix with ferrite intermixed. In other words, the above-mentioned heat treatments can produce the structures (a) to (f) described above.

[0054] [Precipitates in 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 shown in the example in Figure 2, and their size is 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. 2 The presence of carbides, nitrides, and carbonitrides can be confirmed by analyzing them with an attached energy dispersive X-ray spectrometer (EDX). The carbides, nitrides, and carbonitrides were then identified. The EDX analysis method described above can be used to identify carbides, nitrides, and carbonitrides. With this method, peaks for Fe and C appear in carbides, peaks for Fe and N appear in nitrides, and peaks for Fe, C, and N appear in carbonitrides.

[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 1000 to 1300°C, rolled at 900°C or higher with a cumulative reduction of 50% or more, quenched, and then tempered. Here, the temperature is the temperature at the center of the steel material's thickness. As a result, the structure at the center of the steel material's thickness 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 materials having such metal structure exhibit 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. Furthermore, each step in the method for producing a steel material of the present invention will be described in detail.

[0058] [Heating temperature: 1000~1300℃] If the heating temperature is less than 1000°C, the subsequent rolling temperature cannot be ensured. Furthermore, if the temperature exceeds 1300°C, the steel grains become coarse, making it difficult to ensure toughness. Therefore, the heating temperature is specified to be 1000 to 1300°C. It is preferably 1000 to 1250°C, and more preferably 1000 to 1200°C. The heating time is adjusted appropriately depending on the thickness of the steel material, but is preferably in the range of 1 to 24 hours. It is more preferably 1 to 12 hours.

[0059] [Cumulative reduction rate above 900℃: 50% or more] In order to refine the austenite grains by rolling and thereby improve toughness, rolling is performed at a cumulative reduction rate of 50% or more at 900°C or higher. Preferably, it is 52% or higher. More preferably, it is 55% or higher. As long as the temperature is 900°C or higher, it does not matter whether the rolling is performed in the austenite recrystallization region or the austenite non-recrystallization region. However, since excessive reduction in the austenite non-recrystallization region causes anisotropy in the mechanical properties, it is desirable that the cumulative reduction rate in the austenite non-recrystallization region be 25% or less.

[0060] [Finishing temperature] The finishing temperature refers to the final rolling temperature. This temperature is preferably 1000 to 850°C. If the temperature is outside this range, excessive ferrite structure will be generated in the subsequent quenching process, which is undesirable. A temperature of 1000 to 900°C is more preferable.

[0061] [Quenching treatment] Quenching is a manufacturing condition for converting steel material into a bainite structure, a martensite structure, or a mixed structure thereof. Specifically, the process involves cooling from the Ar3 point or higher to 100°C or lower at a cooling rate of 5°C / s or higher. If the cooling start temperature is below the Ar3 point, excessive ferrite will form, making it impossible to obtain the desired fatigue crack propagation rate. Therefore, the cooling start temperature is set to the Ar3 point or higher. If the cooling rate is below 5°C / s, excessive ferrite will form, making it impossible to obtain the desired fatigue crack propagation rate. Therefore, the cooling rate is set to 5°C / s or higher, preferably 6°C / s or higher. If the cooling stop temperature exceeds 100°C, the desired strength will not be obtained. Therefore, the cooling stop temperature is set to 100°C or lower, preferably 80°C or lower, and more preferably 40°C or lower.

[0062] The Ar3 point can be calculated using the following formula (1) based on the composition percentage of the steel material. Ar3=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo... (1) If not contained, the value is set to 0.

[0063] In laboratory experiments, the quenching method was 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.

[0064] [Tempering treatment] By subjecting the metal structure produced by the above-mentioned quenching treatment to a tempering treatment at 100 to 450°C for 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.

[0065] Specifically, tempering involves heating the quenched steel material at a low temperature of 100 to 450°C for 10 seconds or longer. The heating time is preferably 10 seconds or longer, but if it exceeds 100 minutes, the manufacturing cost becomes too high, so it is preferably 10 seconds to 100 minutes. More preferably, it is 10 to 60 minutes. The heating temperature must be 100°C or higher in order to precipitate carbides, nitrides, and carbonitrides, and if it exceeds 450°C, a decrease in strength occurs. Therefore, the heating temperature is preferably 100 to 450°C. More preferably, it is 150 to 450°C.

[0066] [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 ) is called the Paris law, da / dN=CΔK m A linear relationship holds: (C and m are constants).

[0067] 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.

[0068] [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]

[0069] Twenty-six types of steel having steel compositions A to Z shown in Table 2 were prepared, and test pieces (plates No. 1 to 34) shown in Table 3 were processed under the manufacturing conditions shown in Table 3 (heating and rolling treatment, quenching treatment, and then tempering treatment at a low temperature of 100 to 450°C for 10 seconds or more).

[0070] [Table 2]

[0071] [Table 3]

[0072] 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 section was polished from both sides to reduce the thickness to 50 μm. Then, it was punched out to 3 mm diameter with a disc punch, and the disc was thinned by electrolytic polishing. After washing with methanol, it was dried on filter paper and used as a TEM observation sample. TEM observation (field of view: 3.8 μm) was performed. 2 A 10-field microscope (magnification: 10,000x) was performed at 50 μm intervals in 10 fields of view. Furthermore, carbides, nitrides, and carbonitrides were identified by analysis using an attached energy dispersive X-ray spectrometer (EDX). The results of observing precipitates such as carbides are shown in Table 4. All of the test specimens of the present 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, 31, 32, and 34 of the comparative examples.

[0073] [Table 4]

[0074] 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 5. 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 observed and photographed (field of view: 3.8 μm 2 , magnification: 10,000x).

[0075] [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.

[0076] 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; and the maximum tensile stress value on the stress-strain curve was taken as TS.

[0077] 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.

[0078] The results obtained are shown in Table 5.

[0079] [Table 5]

[0080] 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, observation of the metallographic structure revealed the presence of one or more types of precipitates, as shown in Table 4. Furthermore, as shown in Table 3, the metallographic structure was either a tempered bainite structure, a tempered martensite structure, or a mixed structure of tempered bainite and tempered martensite, or a structure in which ferrite structure was present in an area ratio of 50% or less within these metallographic structures.

[0081] However, in the case of steel sheet No. 23, the C content exceeded the value specified in the present invention, and therefore the Charpy impact test value was below 27J. In the case of steel sheet 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. As a result, the YS and TS were below the target values ​​of the present invention, and the Charpy impact test value was below the target value of the present invention, and 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. Steel plate No. 25 had P, S, Al, and N values ​​exceeding the values ​​specified in the present invention, so the Charpy impact test value was below the target value of the present invention. Steel plate No. 26 had Si and Mn values ​​exceeding the values ​​specified in the present invention, so the Charpy impact test value was below the target value of the present invention. Steel plate No. 27 had a heating temperature exceeding the value specified in the present invention, so the Charpy impact test value was below the target value of the present invention. Steel plate No. 28 had a heating temperature below the value specified in the present invention, so sufficient reduction above 900°C was not achieved. As a result, the Charpy impact test value was below the target value of the present invention. Steel plate No. 29 had a cumulative reduction rate above 900°C below the value specified in the present invention, so the Charpy impact test value was below the target value of the present invention, and 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. In the case of steel plate No. 30, the cooling start temperature was below the Ar3 point, and the metal structure was a tempered bainite structure with an area ratio of ferrite exceeding 50%. As a result, the YS and TS were below the target values ​​of the present invention, and 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. Steel plate No. 31 had a cooling rate below the value specified in the present invention, resulting in a pearlite structure with a ferrite structure area ratio of more than 50%. YS and TS were below the target values ​​of the present invention, and no precipitates such as carbides were observed in any of the 10 visual fields observed in the metallographic structure. As a result, ΔK = 20 MPa m 1 / 2 The fatigue crack propagation rate at this time is 4.26×10 -8 [m / cycle], and thus had poor fatigue crack propagation resistance. Steel plate No. 32 had a cooling stop temperature that exceeded the value specified in the present invention, resulting in a pearlite structure with a ferrite structure area ratio of more than 50%. YS and TS were below the target values ​​of the present invention, and no precipitates such as carbides were observed in any of the 10 visual fields observed in the metallographic structure. As a result, 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. Steel plate No. 33 had a tempering temperature that exceeded the specified value of the present invention, so the YS and TS were below the target values ​​of the present invention. As a result, 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. The tempering temperature of steel plate No. 34 was below the specified value of the present invention, and no precipitates such as carbides were observed in any of the 10 visual fields in the metallographic observation. As a result, the Charpy impact test value was below the target value of the present invention, and 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. 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, B: 0.0001 to 0.0100%, with the balance being Fe and unavoidable impurities, is heated to a temperature of 1000 to 1300°C at the center of the plate thickness, rolled to 900°C or higher with a cumulative reduction rate of 50% or more, then quenched and further tempered to obtain a steel plate, The quenching treatment is performed using Ar ３ The treatment is performed by cooling from above the temperature limit to 40°C at a cooling rate of 5°C / s or more, and the tempering treatment is performed at a heating temperature of 100 to 450°C for a heating time of 10 minutes or more. The steel plate has a metal structure selected from the group consisting of a tempered bainite structure, a tempered martensite structure, and a mixed structure of a tempered bainite structure and a tempered martensite structure, and further has at least one of carbide, nitride, and carbonitride precipitates with a size of 10 nm or more and a distribution of 1 to 100 precipitates / 3.8 μm in the center of the plate thickness. ２ Assume that the steel plate exists A method for manufacturing a steel plate having excellent fatigue crack propagation resistance, characterized by:

2. 2. The method for producing a steel plate with excellent fatigue crack propagation resistance according to claim 1, characterized in that in addition to the chemical composition of the steel material, the steel material further contains, by mass%, one or more elements 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%, and REM: 0.001 to 0.100%.

3. 3. A method for manufacturing a steel plate having excellent fatigue crack propagation resistance according to claim 1, characterized in that the metal structures and a ferrite structure are present at an area ratio of 50% or less (excluding 0%).

4. 4. 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.

5. The steel plate is subjected to a fatigue crack propagation test in which the stress intensity factor range [ΔK] is 20 MPa m １／２ When the fatigue crack propagation rate [da / dN] is 4.26 × 10 －８ 5. The method for producing a steel plate having excellent fatigue crack propagation resistance according to claim 1, wherein the steel plate has a fatigue crack propagation rate of 100 m / cycle or less.

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