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

Abstract

To provide a steel plate excellent in fatigue crack propagation resistance property which dispenses with complicated heat treatment and can be obtained by a simple production method.SOLUTION: A steel plate is provided, having a chemical composition comprised of, by mass%, 0.02 to 0.40% C, 0.010 to 0.500% Si, 0.05 to 2.00% Mn, 0.050% or less P, 0.050% or less S, 0.100% or less Al, 0.10% or less N, and the balance Fe with impurities, wherein metallic structure of the steel plate is any one structure of tempered bainite structure, tempered martensite structure or mixed structure of tempered bainite structure and tempered martensite structure, or structure in which ferrite structure having an area ratio of 50% or less exists in the metallic structure, and in a plate thickness center portion, 1 to 100 pieces / 3.5 μm2 of one or more of precipitates of carbide, nitride and carbonitride having sizes of 10 nm or more exist.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a steel material having excellent fatigue crack propagation characteristics and a method for manufacturing the same. More specifically, the present invention relates to a steel material used in various welded structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks, and having improved fatigue crack propagation characteristics even when subjected to repeated loads, and a method for manufacturing the same.

Background Art

[0002] In recent years, in structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks, there have been an increasing number of cases where high-strength steel materials are applied for the purpose of rationalizing design, reducing the weight of steel materials, thinning, and saving labor in welding. In addition, these steel materials are required to have excellent fatigue characteristics in order to ensure not only toughness and ductility but also weldability and structural safety.

[0003] In welded structures, fatigue failure often occurs when a fatigue crack occurs at the weld termination and propagates through the steel material to cause failure. This is considered to be due to the fact that the weld termination is likely to be a stress concentration part due to its shape factor, and in addition, tensile residual stress is generated after welding.

[0004] Therefore, as means for suppressing the generation of cracks from the weld termination, techniques for improving the shape and reducing stress concentration by performing additional welding, techniques for introducing compressive residual stress by peening, etc. are widely known.

[0005] However, it is almost impossible to perform such treatment on a large number of weld terminations on an industrial scale, and it is hardly realistic in terms of cost. Therefore, based on the recognition that it is important to extend the fatigue life by reducing the propagation speed of the crack in the steel material even if a fatigue crack occurs, there has been a strong demand from the industry to improve the fatigue crack propagation characteristics of the steel material 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] In addition, Patent Document 3 discloses a steel material with a steel composition by 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 inevitable impurities. In both the plate thickness direction and the plate length direction, 80% or more of the metal structure consists 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 a Vickers hardness of 340 or more and an aspect ratio of 2.5 or less and an area fraction of 15 to 85%. This steel material has low material anisotropy and excellent fatigue crack propagation characteristics.

[0009] In addition, Patent Document 4 discloses a steel plate 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 balance being Fe and inevitable impurities. The structure of the steel plate consists of a hard part and a soft part, and the steel plate in which the ratio occupied in the structures of these two parts and the average hardness in terms of Vickers hardness satisfy a specific formula is disclosed.

[0010] Furthermore, Patent Document 5 discloses a steel containing, by mass, C: 0.02 to 0.16%, Si: 0.05 to 0.5%, Al: 0.005 to 0.060%, and further containing one or more selected from Mn: 0.1 to 2.5%, Cu: 0.1 to 2.0%, Ni: 0.1 to 6.0% such that the value of 2Mn + Cu + Ni is 3.5 to 6.0%, with the balance being Fe and inevitable impurities. The steel is characterized by containing retained austenite with an average diameter of 0.1 to 0.5 μm in a volume ratio of 5 to 20%, and discloses a thick steel plate excellent in fatigue crack propagation characteristics and toughness.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

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

[0013] The manufacturing method of Patent Document 1 is to perform reheating after hot rolling, quenching treatment, reheating in the two-phase temperature range of Ac1 transformation point + 10°C to 790°C, quenching at an average cooling rate of 5 to 60°C / s, and then tempering by holding at 400 to 650°C for 10 minutes or more. However, the manufacturing method is complicated.

[0014] Patent Document 2 is characterized in that after performing diffusion heat treatment on a steel slab before hot rolling at a heating temperature of 1200 to 1350°C and a holding time in this temperature range of 2 to 100 h, hot rolling is performed at a heating temperature of Ac3 transformation point to 1250°C and accelerated cooling at 5 to 100°C / s from above the Ar3 transformation point to 400°C or less after rolling, and further two-phase region heat treatment is performed at a heating temperature of (Ac1 transformation point + 30°C) to (Ac3 transformation point - 10°C) and accelerated cooling at 5 to 100°C / s to 400°C or less. However, manufacturing methods such as diffusion heat treatment and two-phase region heat treatment are complicated.

[0015] Patent Document 3 is characterized in that solution heat treatment is performed by holding steel at 1200 to 1300°C for 25 to 60 hours and then air cooling, reheating to 1000 to 1200°C, performing hot rolling at a rolling finish temperature above the Ar3 transformation point and air cooling, reheating and holding above the Ac3 transformation point and then air cooling, further reheating to the two-phase region temperature of Ac1 transformation point + 10°C to Ac3 transformation point - 10°C, then quenching at an average cooling rate of 5°C / s or more, and tempering at 400°C 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 technology 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] The test steel A is a steel plate (plate thickness: 25 mm) having a steel composition of 0.12% C - 0.28% Si - 1.33% Mn - 0.013% P - 0.004% S - 0.01% Ni - 0.02% Cr - 0.011% Ti - 0.047% Al - 0.0038% N. Using this, the steel plate was reheated to 900°C, quenched, and then tempered at 200°C. Thereafter, the relationship between the stress intensity factor range [ΔK] and the fatigue crack propagation rate [da / dN] was examined using a CT test piece with both sides thinned to 12 mm. The test steel B is a soft phase / hard phase dispersion steel (a steel material obtained by heating a steel ingot having 0.15% C - 0.25% Si - 1.51% Mn - 0.009% P - 0.005% S - 0.07% Cr - 0.038% Al - 0.0025% N at 950°C or higher and 1250°C or lower, performing rolling with a cumulative reduction ratio of 50% or more at a temperature above the Ar3 point, starting accelerated cooling at a cooling rate of 10°C / s or more and less than 30°C / s from a temperature range of Ar3 point - 60°C to 730°C or higher, and then cooling to 650°C or lower and 500°C or higher). The test steel C is a conventional steel (a steel material obtained by heating a steel ingot having 0.13% C - 0.33% Si - 1.45% Mn - 0.009% P - 0.008% S - 0.05% Cr - 0.012 Ti - 0.048% Al - 0.0039% N at 950°C or higher and 1250°C or lower, performing rolling with a cumulative reduction ratio of 50% or more at 900°C or higher, starting accelerated cooling from a temperature range of the Ar3 point to Ar3 point - 100°C, and then cooling to 700°C or lower and 500°C or higher).

[0023] The results are shown in Fig. 1. The □ in Fig. 1 (shown as "white frame filled with black" in the figure) represents the data of the test steel A, △ represents the data of the test steel B, and ○ represents the test steel C. Also, the solid line in the figure represents the level of the fatigue crack propagation rate [da / dN] with respect to the stress intensity factor range [ΔK] in a general conventional steel such as the test steel C, and it is a straight line connecting 3.50×10 1 / 2 m / cycle when ΔK = 15 MPa·m -8 and 1.70×10<s 1 / 2 m / cycle when ΔK = 25 MPa·m -7 The dotted line below it represents a level that is 1 / 2 of the fatigue crack propagation rate level of the test steel C, and it is 1.75×10 1 / 2 m / cycle when ΔK = 15 MPa·m -8m / cycle and ΔK = 25 MPa·m 1 / 2 When it is, it is a straight line connecting 8.50×10 -8 m / cycle. The one-dot chain line below it is at 1 / 5 level of the fatigue crack propagation speed level of the test steel C, and when ΔK = 15 MPa·m 1 / 2 When it is, it is 7.00×10 -9 m / cycle and a straight line connecting 3.40×10 1 / 2 m / cycle when ΔK = 25 MPa·m -8 From this experimental result, it was confirmed that the test steel B is about 1 / 2 of the fatigue crack propagation speed level of the test steel C, while the fatigue crack propagation speed level of the test steel A is about 1 / 5 of that of the test steel C, and it has the most excellent fatigue crack propagation resistance characteristics.

[0024] Next, a thin film sample was made from 1 / 2 thickness of the steel plate of the above test steel A, and the metal structure was observed by TEM (transmission electron microscope). Also, carbides, nitrides, and carbonitrides were identified by the attached energy dispersive X-ray spectrometer (EDX) analysis. An example of the result is shown in Fig. 2. In Fig. 2, precipitates composed of 3 carbides were observed in one field of view (3.8 μm 2 , 10,000 times magnification). As a result of performing the same observation on 10 fields at a field interval of 50 μm, precipitates composed of one or more carbides, nitrides, or carbonitrides were confirmed in 6 fields (equivalent to 60% of 10 fields).

[0025] In addition, for the observation of the metal structure, a test piece taken from the center of the plate thickness (position of 1 / 2 of the plate thickness) of the steel material (steel plate) was polished and corroded (nital etching), and the structure was observed with a scanning electron microscope (SEM) (magnification: 500 times), imaged, and the metal structure was determined using an image analysis device. As a result, in the above test steel A, the observed metal structure was a tempered bainite structure. Also, the above test steel B was a structure in which massive pearlite and massive bainite were formed in a ferrite matrix. Furthermore, the above test steel C was a structure in which banded pearlite was formed in a ferrite matrix.

[0026] From the experimental results as described above, it has been found that the fatigue crack propagation characteristics are significantly improved by using a steel material having a specific composition, in which the metallographic structure is a tempered bainite structure and in which at least one of carbide, nitride, and carbonitride precipitates is present.

[0027] Similarly, in the case of a tempered martensite structure, or a mixed structure of a tempered bainite structure and a tempered martensite structure, and even when a ferrite structure is present in those structures, if the ferrite structure has an area ratio of 50% or less, and in a steel material in which at least one of carbide, nitride, and carbonitride precipitates is present, a decrease in the fatigue crack propagation rate was similarly observed.

[0028] The present invention has been completed through further study based on such findings, 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 inevitable impurities, wherein the metallographic structure of the steel material is any one of a tempered bainite structure, a tempered martensite structure, or a mixed structure of a tempered bainite structure and a tempered martensite structure, and further, at least one of carbide, nitride, and carbonitride precipitates is present, and which is characterized by excellent fatigue crack propagation resistance. 〔2〕The steel material according to 〔1〕, characterized in that in addition to the above chemical composition, it further contains, by mass%, one or more selected from Cu: 0.01 to 2.00%, Ni: 0.01 to 5.00%, Cr: 0.01 to 3.00%, Mo: 0.01 to 1.00%, Nb: 0.001 to 0.100%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, B: 0.0001 to 0.0100%, REM: 0.001 to 0.100%, and which is characterized by excellent fatigue crack propagation resistance. 〔3〕In [1] or [2], a steel material excellent in fatigue crack propagation resistance, characterized in that ferrite structures with an area ratio of 50% or less exist in each of the metal structures. 〔4〕In any one of [1] to [3], a steel material excellent in fatigue crack propagation resistance, characterized in that the plate thickness of the steel material is 5 to 100 mm. 〔5〕In any one of [1] to [4], the number of the precipitates is 1 to 100 pieces / 3.8 μm 2 at the center of the plate thickness of the steel material, and the steel material is excellent in fatigue crack propagation resistance. 〔6〕In any one of [1] to [5], when the stress intensity factor range [ΔK] in the fatigue crack propagation test of the steel material is 20 MPa·m 1 / 2 and the fatigue crack propagation rate [da / dN] is 4.26×10 -8 m / cycle or less, the steel material is excellent in fatigue crack propagation resistance. 〔7〕A method for manufacturing the steel material according to any one of [1] to [6], including heating 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, and the balance being Fe and inevitable impurities, to a temperature equal to or higher than the Ac3 point, performing a reheat quenching treatment, and then performing a tempering treatment, and the method for manufacturing a steel material excellent in fatigue crack propagation resistance is characterized in this. 〔8〕In [7], in addition to the chemical composition of the steel material, further containing, 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%, REM: 0.001 to 0.100%, and the method for manufacturing a steel material excellent in fatigue crack propagation resistance is characterized in this. 〔9〕In [7] or [8], the reheat quenching treatment is performed at a cooling stop temperature of 25°C or less and a cooling rate of 5 to 250°C / s, and the method for manufacturing a steel material excellent in fatigue crack propagation resistance is characterized in this. In any one of

[10] , [7] or [9], the tempering treatment is carried out at a heating temperature of 100 to 450 °C for a heating time of 10 seconds or more, and a method for producing a steel material excellent in fatigue crack propagation characteristics is characterized in that.

Effects of the Invention

[0029] According to the present invention, a steel material excellent in fatigue crack propagation characteristics can be produced by a simple manufacturing method, and it has an extremely high industrial effect.

Brief Description of the Drawings

[0030]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

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

[0032] [Basic Chemical Composition of Steel Material] Next, the basic chemical composition of the steel material according to the present invention will be described. In the following, “%” in the chemical composition means “mass %”.

[0033] [C: 0.02 to 0.40%] C needs to be added in an amount of 0.02% or more to ensure strength. However, an addition of 0.40% or more inhibits weldability. Therefore, it 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%. Even more preferably, it is 0.02 to 0.30%.

[0034] [Si: 0.010 to 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 widely used in the deoxidation process of molten steel for high-tensile steel. It also fixes N in the steel as AlN, contributing to the improvement of the toughness of the base material. On the other hand, an addition exceeding 0.100% will reduce the toughness of the base material and mix into the weld metal during welding, deteriorating the toughness. Therefore, Al is limited to 0.100% or less. Preferably, it is 0.080% or less. More preferably, it is 0.070% or less. Even more preferably, it is 0.060% or less.

[0039] [N: 0.1000% or less] N is not preferable because it has an adverse effect on ductility and toughness in the solid solution state. However, it combines with V, Al, or Ti and effectively works for austenite grain refinement and precipitation strengthening. Therefore, even a small amount is effective for improving mechanical properties. However, if it is contained in excess, the amount of solid solution N may increase, possibly having an adverse effect on ductility and toughness. Therefore, N is limited to the 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] [Optional chemical composition of steel] The above-described chemical composition is the basic chemical composition of the steel of the present invention. In the present invention, in addition to this basic chemical composition, as an optional chemical composition, if necessary, for the purpose of adjusting strength, toughness, weldability, etc., imparting weather resistance, etc., Cu: 0.01 - 2.00%, Ni: 0.01 - 5.00%, Cr: 0.01 - 3.00%, Mo: 0.01 - 1.00%, Nb: 0.001 - 0.100%, V: 0.001 - 0.100%, Ti: 0.001 - 0.100%, B: 0.0001 - 0.0100%, REM: 0.001 - 0.100%, one or more selected from these can be selected and contained.

[0041] [Cu: 0.01 - 2.00%] Cu brings about the effect of increasing strength by solid solution and improves weather resistance. Therefore, it is preferable to set the lower limit at 0.01%. However, if its content exceeds 2.00%, weldability will be impaired and defects are likely to occur during steel manufacturing. Therefore, when it is contained, it is preferably set at 0.01 - 2.00%. More preferably, it is 0.01 - 1.50%. Even more preferably, it is 0.01 - 1.00%.

[0042] [Ni: 0.01 - 5.00%] Ni improves low-temperature toughness and is effective in improving weather resistance and hot brittleness that occurs when Cu is added. Therefore, it is preferable to set the lower limit at 0.01%. However, if its content exceeds 5.00%, it will inhibit weldability and lead to an increase in cost. Therefore, when it is contained, it is preferably set at 0.01 - 5.00%. More preferably, it is 0.01 - 4.00%. Even more preferably, it is 0.01 - 3.00%.

[0043] [Cr: 0.01 - 3.00%] Cr improves weather resistance and strength. Therefore, it is preferable to set 0.01% as the lower limit of addition. However, if its content exceeds 3.00%, weldability and toughness will be impaired. Therefore, when it is contained, it is preferably set at 0.01 - 3.00%. More preferably, it is 0.01 - 2.50%. Even more preferably, it is 0.01 - 2.00%.

[0044] [Mo: 0.01 - 1.00%] Mo may contain 0.01% or more to increase strength. However, if its content exceeds 1.00%, deterioration of weldability and toughness will occur. Therefore, when it is contained, it is preferably set at 0.01 - 1.00%. More preferably, it is 0.01 - 0.90%. Even more preferably, it is 0.01 - 0.80%.

[0045] [Nb: 0.001 - 0.100%] Nb may be contained in an amount of 0.001% or more because it has the function of suppressing austenite recrystallization during rolling to achieve grain refinement and at the same time bringing about high strength by precipitation. However, when the content exceeds 0.100%, the toughness deteriorates. Therefore, when it 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%.

[0046] [V: 0.001 - 0.100%] Similar to Nb, V may be contained in an amount of 0.001% or more because it has the function of bringing about high strength by precipitation. However, when the content exceeds 0.100%, it causes a decrease in weldability and toughness. Therefore, when it 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%.

[0047] [Ti: 0.001 - 0.100%] Ti may be contained in an amount of 0.001% or more to improve strength increase and toughness of the welded part. However, when the content exceeds 0.100%, it tends to cause an increase in cost. Therefore, when it 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%.

[0048] [B: 0.0001 - 0.0100%] B may be contained in an amount of 0.0001% or more to enhance hardenability and contribute to strength increase. However, when the content exceeds 0.0100%, it harms weldability. Therefore, when it is contained, it is preferably 0.0001 to 0.0100%. More preferably, it is 0.0001 to 0.0090%. Even more preferably, it is 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 portion of the plate thickness of the obtained steel material has the following metallographic structure due to differences in chemical composition, changes in the cooling rate depending on the plate thickness of the steel material, and heat treatment conditions such as tempering. The 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, the metallographic structure 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 higher) tends to result in a bainite structure. Even slower cooling rates (5°C / s or lower) result in a bainite or martensite structure, or a ferrite structure mixed into the matrix of bainite and martensite. Then, by tempering these reheated and quenched steels, the structures (a) to (f) described above can be obtained.

[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 material, and further carbides, nitrides or carbonitrides of Cr, Mo, Nb, V, Ti, etc. which are optional chemical compositions. The precipitates in these metal structures are scattered in the form of fine particles of various sizes as shown in the example of Fig. 2, and their size is about 10 to 500 nm.

[0056] In addition, these precipitates can be confirmed by cutting out a 1-mm-thick slice from the center of the plate thickness of the steel material (steel plate), grinding the cut-out slice from both sides to reduce the thickness to 50 μm, punching it out to a diameter of 3 mm with a disk punch, thinning the disk by electrolytic polishing, washing it with methanol, drying it on filter paper to make an observation sample for TEM, and performing TEM observation (10,000 times). Furthermore, identification of carbides, nitrides, and carbonitrides was carried out by energy-dispersive X-ray spectroscopy (EDX) analysis attached. Here, as a method for identifying carbides, nitrides, and carbonitrides, the above-mentioned EDX analysis method, etc. can be mentioned. According to this method, in the case of carbides, peaks of Fe and C appear, in the case of nitrides, peaks of Fe and N appear, and in the case of carbonitrides, peaks of Fe, C, and N appear.

[0057] [Manufacturing method of steel material] The manufacturing method of the steel material according to the present invention is characterized in that a steel material (steel plate) adjusted to have the above-mentioned composition is heated to a temperature equal to or higher than the Ac3 point, subjected to reheat quenching treatment, and then subjected to tempering treatment. Here, the temperature is the temperature at the center of the plate thickness of the steel material (steel plate). Thereby, the structure at the center of the plate thickness of the steel material adjusts the cooling rate according to the plate thickness, and the above-mentioned metal structure and precipitates in the metal structure are obtained. And in the case of a steel material having such a metal structure, it has been confirmed that the fatigue crack propagation resistance characteristics are improved as described later. Such an effect has been confirmed in the range where the plate thickness of the steel material is 5 to 100 mm. Furthermore, each process in the manufacturing method of the steel material of the present invention will be described in detail.

[0058] [Heating to a temperature equal to or higher than the Ac3 point] First, in order to completely austenitize the metal structure of the steel material, it is heated above the Ac3 point (austenite transformation temperature). This heating is also called re-heating, and this temperature is also called the re-heating temperature. Heating below the Ac3 point will cause the pre-heating structure to remain and result in a strength reduction. This Ac3 point can be calculated by the following formula (1) based on the composition percentage of the steel material. 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) In addition, when not contained, it is regarded as 0.

[0059] [Quenching treatment] To make the metal structure at the center of the plate thickness of the steel material be a bainite structure, a martensite structure, or a mixed structure of a bainite structure and a martensite structure as described above, a quenching treatment is performed. These structures can be appropriately obtained by changing the cooling rate during water cooling according to the plate thickness and adjusting the chemical composition. Even if the ferrite structure does not precipitate, or if the precipitation is 50% or less, the desired mechanical properties described later can be obtained. However, when the ferrite structure exceeds 50% in area ratio, those properties cannot be obtained.

[0060] Specifically, in laboratory rolling, the heated steel material is cooled by spraying flowing water, and it is preferably cooled at a cooling rate of 297°C / s or less, preferably 5 - 250°C / s, and the cooling stop temperature is 25°C or less. In actual equipment, there are quenching devices such as spraying flowing water cooling and laminar cooling. Here, the cooling rate is set to 250°C / s or less because a steel material having a tempered martensite structure obtained by re-heating quenching - tempering of a 6-mm-thick steel material having the components of the present invention satisfies the desired properties. Also, the cooling rate is set to 5°C / s or more in the case of re-heating quenching a 100-mm-thick steel material, and the cooling stop temperature is set to 25°C or less because it is likely to become a martensite structure when sufficiently quenched.

[0061] As described above, each of the above-mentioned structures can be obtained by adjusting heat treatment conditions such as chemical composition, cooling rate, tempering treatment, etc.

[0062] Specifically, when the cooling rate is high (50°C / s or more), it is likely to form a martensite structure, and when the cooling rate is low (10°C / s or more), it is likely to form a bainite structure. When the cooling rate is even lower (5°C / s or less), a bainite structure or a martensite structure, or a ferrite structure is mixed into the matrix of the bainite structure and the martensite structure. By tempering these reheated and quenched steel materials, the structures (a) to (f) described below can be obtained. (a) Tempered bainite structure (b) Tempered martensite structure (c) Mixed structure of tempered bainite structure and tempered martensite structure (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) Mixed structure of ferrite structure with an area ratio of 50% or less, tempered bainite structure and tempered martensite structure

[0063] [Tempering treatment] By subjecting the metal structure generated in the above quenching treatment to a tempering treatment at a heating temperature of 100 to 450°C for a heating time of 10 seconds or more, the bainite structure becomes a tempered bainite structure, the martensite structure becomes a tempered martensite structure, and precipitates that are the carbides, nitrides or carbonitrides described above are formed.

[0064] As a specific heating temperature, if it is not 100°C or higher, none of the carbides, nitrides, and carbonitrides will precipitate, and if it exceeds 450°C, a strength reduction will occur. Therefore, 100 to 450°C is preferable. More preferably, it is 150 to 450°C. The heating time is preferably 10 seconds or more, but if it exceeds 100 minutes, the manufacturing cost will be too high, so 10 seconds to 100 minutes is preferable. More preferably, it is 10 seconds to 60 minutes. By this treatment, the martensite structure becomes a tempered martensite structure, and the bainite structure becomes a tempered bainite structure, and carbides and the like precipitate. The tempering at 100 to 450°C can be simply performed in an air furnace or a vacuum furnace. Also, in an actual machine, a large air furnace can be used.

[0065] [Fatigue crack propagation characteristics] The fatigue crack propagation characteristics of the steel material are evaluated by obtaining the stress intensity factor range [ΔK] and the fatigue crack propagation rate [da / dN] through a fatigue crack propagation test conforming to the standard of ASTM E647. This stress intensity factor range [ΔK] is ΔK = Kmax - Kmin, which represents the difference between the maximum value and the minimum value of the stress intensity factor. Also, the fatigue crack propagation rate [da / dN] is the amount of fatigue crack propagation [m / cycle] in one cycle with the stress waveform. In particular, in the intermediate region of ΔK (approximately ΔK = about 10 MPa·m 1 / 2 to ΔK = about 70 MPa·m 1 / 2 ), a linear relational expression called Paris' law, da / dN = CΔK m (C and m are constants), holds.

[0066] Here, in the present invention, as a steel material having excellent fatigue crack propagation characteristics, when the stress intensity factor range [ΔK] is 20 MPa·m 1 / 2 , those having a fatigue crack propagation rate of 4.26×10 -8 m / cycle or less are referred to.

[0067] [Mechanical properties of steel material] The target values of the mechanical properties of the steel material of the present invention are: yield stress [YS] ≥ 400 MPa, tensile strength [TS] ≥ 500 MPa in a tensile test, and absorbed energy [at a test temperature of 0°C in a Charpy impact test]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 (reheating to the Ac3 point or higher, quenching, and then tempering at a low temperature of 100 to 450°C for 10 minutes or more).

[0069] [Table 2]

[0070] [Table 3]

[0071] A 1 mm thin section was cut from the center of the steel (steel plate) that had been treated as described above, polished on both sides to reduce the thickness to 50 μm, and then punched out to 3 mm diameter using a disc punch. The discs were thinned by electrolytic polishing, washed with methanol, and dried on filter paper to prepare TEM observation samples. TEM observation (10,000x magnification) was performed with 10 fields of view at 50 μm intervals. Furthermore, carbides, nitrides, and carbonitrides were identified using an attached energy dispersive X-ray spectrometer (EDX).

[0072] The results of observing precipitates such as carbides are shown in Table 4. All of the test specimens of the present invention had at least one precipitate in their metal structures, but no precipitates were observed at all in test specimens / plates Nos. 24, 27, 29, 30, and 32 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 in the direction perpendicular to the rolling direction and used for the test. For the Charpy impact test, three full-size test pieces were taken from the center of the plate thickness in the rolling direction and tested at a test temperature of 0°C. For the fatigue crack propagation rate, CT test pieces with both sides thinned to a thickness of 12 mm (if the thickness of the steel plate is less than 12 mm, no thinning is performed and the original thickness is used) were subjected to a fatigue crack propagation test at an R ratio of 0.1, a sine wave, and 10 Hz. At this time, YS [MPa], TS [MPa], V E0 [J], ΔK = 20 MPa·m 1 / 2 The fatigue crack propagation rate [da / dN] [m / cycle] at this time is shown in Table 5. Microscopic observation was performed by nitriding etching at the 1 / 2 position of the plate thickness and observing with SEM. TEM observation was performed by taking a thin film from the 1 / 2 position of the plate thickness.

[0075] [Mechanical properties of steel] From the obtained test pieces, tensile test pieces (parallel part diameter 6 mmφ) and Charpy impact test pieces (V-notch) were taken in accordance with the regulations of JIS Z 2241 and 2242, and tensile tests and impact tests were carried out.

[0076] In the tensile test, when the yield point appeared, the upper yield point was used, and when the yield point did not appear, the value obtained as the 0.2% proof stress was used as the YS of the test piece, and the value obtained by taking the maximum value in the stress-strain curve as the tensile stress was used as the TS.

[0077] Also, for the Charpy impact test, three tests were carried out each, and the absorbed energy V E0 at a test temperature of 0°C was obtained, and its average value was used as the value of the test piece.

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

[0079]

Table 5

[0080] The test pieces of plate Nos. 1 to 22 having the steel material chemical composition defined in the present invention and produced by the steel material manufacturing method defined in the present invention have a YS of 400 MPa or more, a TS of 500 MPa or more, V an E0 of 27 J or more, and a fatigue crack propagation rate [da / dN] at ΔK = 20 MPa·m 1 / 2 is 4.26×10 -8 [m / cycle] or less, and a steel plate excellent in fatigue crack propagation resistance was obtained. At this time, as shown in Table 4, at least one precipitate was present in the metallographic observation. Also, as shown in Table 3, the metallographic structure was any one of a tempered bainite structure, a tempered martensite structure, or a mixed structure of a tempered bainite structure and a tempered martensite structure, or a structure in which a ferrite structure with an area ratio of 50% or less was present in these metallographic structures.

[0081] However, for the steel plate of plate No. 23, since C exceeded the specified value of the present invention, the Charpy impact test value was less than 27 J. For the steel plate of plate No. 24, since C, Si, and Mn were below the specified values of the present invention, no precipitate such as carbide was observed in any of the 10 fields in the metallographic observation, YS and TS were below the target values of the present invention, the Charpy impact test value was below the target value of the present invention, and the fatigue crack propagation rate at ΔK = 20 MPa·m 1 / 2 is 4.26×10 -8[m / cycle], and thus 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, resulting in a Charpy impact test value 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, resulting in a Charpy impact test value 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, resulting in a metallographic structure with an area ratio of 80% ferrite and 20% pearlite, 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 tempering temperature exceeding the value specified in the present invention, resulting in a YS and TS below the target values of the present invention. For steel sheet No. 29, the reheating temperature was below the Ac3 point specified in the present invention, resulting in a metallographic structure of 85% ferrite + 15% pearlite, 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. For steel sheet No. 30, the reheating temperature was below the Ac3 point and the tempering temperature exceeded the specified value of the present invention, resulting in a metallographic structure of 20% pearlite + 80% ferrite, 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. For steel sheet No. 31, the tempering temperature exceeded the specified value of the present invention, resulting in YS and TS being below the target values of the present invention. For steel plate No. 32, the reheating temperature was below the Ac3 point and the tempering temperature was below the specified value of the present invention, so the metallographic structure was 18% pearlite and 82% ferrite, 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, 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.

Claims

1. A steel sheet 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 inevitable impurities, wherein the metallographic structure of the steel sheet is any one of a tempered bainite structure, a tempered martensite structure, or a mixed structure of a tempered bainite structure and a tempered martensite structure, and further, at the center of the sheet thickness, one or more of precipitates of carbides, nitrides, and carbonitrides are present with a size of 10 nm or more and 1 to 100 pieces / 3.8 μm 2 exist, the yield stress in the tensile test: 400 MPa or more, the tensile strength: 500 MPa or more, the absorbed energy at a test temperature of 0 °C in the Charpy impact test is 27 J or more, and the stress intensity factor range ΔK in the fatigue crack propagation test is 20 MPa·m 1 / 2 when the fatigue crack propagation rate da / dN is 4.26×10 -8 m / cycle or less, characterized by a steel sheet having excellent fatigue crack propagation resistance characteristics.

2. In addition to the above chemical composition, the steel sheet excellent in fatigue crack propagation resistance according to Claim 1, 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%, REM: 0.001 to 0.100%.

3. The steel sheet excellent in fatigue crack propagation resistance according to Claim 1 or 2, characterized in that a ferrite structure with an area ratio of 50% or less exists together with the above metal structures.

4. The steel sheet excellent in fatigue crack propagation resistance according to any one of Claims 1 to 3, characterized in that the thickness of the steel sheet is 5 to 100 mm.

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