Steel materials and steel structures containing them

Abstract

This invention provides a steel material that exhibits excellent fatigue crack propagation characteristics, high strength, a good yield ratio, and excellent ductility. [Solution] A steel material is provided having a predetermined chemical composition, a carbon equivalent Ceq of 0.25 to 0.50, a metal structure at a position 1 / 4 t from the surface containing 80% or more of a low-strain structure with a KAM value of less than 0.5° in area percent, and an average residual stress at a position 0.1 to 0.5 mm from the surface in the depth direction of the rolling direction and in the direction perpendicular to the rolling direction of less than -50 MPa.

Description

[Technical Field]

[0001] This invention relates to steel materials and steel structures containing them. [Background technology]

[0002] In recent years, when constructing welded structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks, high-strength steel is increasingly being used to streamline design, reduce the weight of steel used, and save labor in welding work through thinner walls. Therefore, the high-strength steel used must possess not only excellent ductility but also superior fatigue resistance to ensure structural safety.

[0003] In welded structures, there is concern that fatigue cracks may originate at the weld toe, propagate through the steel of the welded structure, and eventually lead to failure (fatigue fracture). This is attributed to factors such as the weld toe being prone to stress concentration due to its shape, as well as the generation of tensile residual stress after welding.

[0004] Even if fatigue cracks occur, reducing the subsequent crack propagation speed within the steel material can extend the fatigue life of welded structures. For this reason, there is a strong demand to improve the fatigue crack propagation resistance characteristics of steel materials.

[0005] In response to such demands, for example, Patent Document 1 describes a thick steel sheet for welded structures in which, by mass%, C: 0.06-0.20%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.006% or less, Al: 0.10% or less, and a ferrite phase having an average hardness of less than 150 HV, comprising 60% or more by volume, and the second phase having an average hardness of less than 240 HV, and the X-ray diffraction intensity ratio of the (200) plane at the center of the plate thickness and at the 1 / 4 position of the plate thickness is 2.0 or more or the X-ray diffraction intensity ratio of the (110) plane is 2.5 or more, and the thickness in the plate thickness direction of ferrite grain colonies aligned within 5° of the rolling surface on any of the {100}, {110}, {111}, or {211} planes is 5 μm or less on average at the center of the plate thickness and at the 1 / 4 position of the plate thickness.

[0006] Furthermore, Patent Document 2 describes a thick steel plate having a composition in mass%, comprising C: 0.03~0.15%, Si: 0.60% or less, Mn: 0.80~1.80%, and one or two selected from Ti: 0.005~0.050% and Nb: 0.001~0.1%, wherein the X-ray intensity ratio of the (110) plane parallel to the plate surface in the range from 2 mm in the thickness direction from the front and back surfaces to 3 / 10 in the thickness direction is 2.0 or more.

[0007] Furthermore, Patent Document 3 describes a material containing, by mass%, C: 0.01~0.30%, Si: 0.03~0.60%, Mn: 0.50~2.50%, P: 0.030% or less, S: 0.010% or less, Al: 0.002~0.050%, N: 0.0010~0.0080%, Ti: 0.003~0.030%, and containing bainite, martensite, and a high-strain ferrite phase having a KAM value of 0.5° or higher, with an upper yield point σ SU and the lowering point σ SL The ratio σ SL / σ SU Steel materials with a ratio of 0.97 or higher are listed. [Prior art documents] [Patent Documents]

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] In the technologies described in Patent Documents 1 and 2, severe plastic deformation is applied in the α-γ two-phase region or the ferrite single-phase region to develop a texture with a specific orientation.

[0010] Furthermore, according to the technologies described in Patent Documents 1 and 2, it is taught that the fatigue crack propagation rate in the plate thickness direction can be reduced. However, no consideration is given to reducing the fatigue crack propagation rate in directions other than the plate thickness direction.

[0011] In the technology of Patent Document 3, severe water cooling is performed from a temperature range of Ar3 - 50°C or higher to a predetermined temperature, and the tempering parameter is also restricted to develop a high-strain structure and suppress the appearance of the upper yield point.

[0012] According to the technology described in Patent Document 3, it is taught that the fatigue crack propagation resistance can be reduced in both the thickness direction and the direction orthogonal thereto.

[0013] The present invention solves the problems of the prior art, and with a novel configuration, provides a steel material that is excellent in fatigue crack propagation resistance in both the thickness direction and the direction orthogonal thereto, has high strength, a good yield ratio, and excellent ductility.

Means for Solving the Problems

[0014] This invention was made to solve the above problems and is based on the following steel materials. In this invention, "steel materials" include thick steel plates, steel pipes, structural steel, thin steel plates, etc. The direction perpendicular to the thickness direction is, for example, the rolling direction and the width direction, etc., if the steel material is a steel plate.

[0015] (1) The chemical composition is expressed in mass%, C: 0.01~0.30%, Si: 0.01~0.60%, Mn: 0.50~2.50%, P: 0.030% or less, S: 0.010% or less, N: 0.0010~0.0080%, O: 0.0100% or less, One or more elements selected from the group consisting of Al: 0.002-0.050%, Nb: 0.002-0.060%, and Ti: 0.002-0.030%. Cu: 0~2.00%, Ni: 0~3.00%, Cr: 0~1.00%, Mo: 0~1.00%, W: 0~1.00%, V: 0~1.00%, B: 0~0.0030%, Ca: 0~0.0100%, Mg: 0~0.0050%, REM: 0~0.0200%, and The remainder consists of Fe and impurities. The carbon equivalent Ceq value, as defined by equation (i) below, is between 0.25 and 0.50. In a cross-section parallel to the rolling direction and thickness direction of the steel material, when the thickness of the steel material is t, the metallographic structure at a position 1 / 4t from the surface of the steel material contains 80% or more of a low-strain structure with a KAM value of less than 0.5°, in terms of area percentage. A steel material in which the average residual stress in the rolling direction and the direction perpendicular to rolling, at a position 0.1 to 0.5 mm in depth from the surface of the steel material, is less than -50 MPa. Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15 ···(i) However, in the above formula, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent the mass percentage of each element, and if an element is not present, it is set to zero. (2) The above chemical composition is in mass %, Cu: 0.01~2.00%, Ni: 0.01~3.00%, Cr: 0.01~1.00%, Mo: 0.01~1.00%, W: 0.01~1.00%, V: 0.01~1.00%, B: 0.0003~0.0030%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0050%, and REM: 0.0001~0.0200% The steel material described in (1) above, which contains one or more selected from the group consisting of the above. (3) In the metallographic structure at a position 1 / 4 t from the surface of the steel material, the number density of precipitates is 15 particles / μm 2 The steel material described in (1) or (2) above, wherein the equivalent circular diameter of the precipitate is 3 to 500 nm. (4) In the metallographic structure at a position 1 / 4t from the surface of the steel material, the number density of precipitates with one or more dislocations within a 50nm radius is 10 particles / μm 2 The steel materials described in any one of the above items (1) to (3). (5) A steel structure that includes the steel materials described in any one of the above items (1) to (4). [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain a steel material that exhibits excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, as well as high strength, a good yield ratio, and excellent ductility. [Brief explanation of the drawing]

[0017] [Figure 1] This is an example of a bright-field image obtained by TEM observation. [Figure 2] This figure shows the dimensions of the CT test specimen used for fatigue crack extension testing in the in-plate direction. [Figure 3] This figure shows the dimensions of a three-point bending test specimen used for fatigue crack extension testing in the thickness direction of the plate. [Modes for carrying out the invention]

[0018] The inventors have conducted various studies on the relationship between metal structure and fatigue crack propagation characteristics. Specifically, the inventors first found that by optimizing the chemical composition of steel and constructing the metal structure of the steel with a low-strain structure, more specifically, a low-strain structure with a KAM value of less than 0.5°, it is possible to increase the strength of the steel, for example, to a tensile strength of 490 MPa or more, while also improving the yield ratio and ductility of the steel. Next, the inventors investigated how to introduce compressive residual stress into the surface layer of the steel while maintaining the low-strain structure by devising a heat treatment process for steel cooled after hot rolling. More specifically, as will be explained in detail later in relation to the manufacturing method, the inventors found that by heating steel cooled after hot rolling and accelerating cooling from a heat treatment temperature below the Ac1 point, it is possible to control the average residual stress in the rolling direction and the direction perpendicular to rolling to less than -50 MPa while maintaining the low-strain structure. As a result, we found that fatigue crack propagation characteristics can be improved in both the thickness direction and the direction perpendicular to it, while maintaining a good yield ratio.

[0019] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0020] (A) Chemical composition The reasons for the limitations on each element are as follows. In the following explanation, "%" for content refers to "mass%". Also, in this specification, unless otherwise specified, "~" indicating a numerical range means that the values ​​before and after it are included as the lower and upper limits.

[0021] C: 0.01~0.30% Carbon (C) is an element that contributes to increased strength through solid solution strengthening and improved hardenability. To ensure the desired high strength through these effects, the C content should be 0.01% or more. On the other hand, to suppress the decrease in ductility and toughness, and to suppress the decrease in mobile dislocations that contribute to repeated softening, the C content should be 0.30% or less. Preferably, the C content is 0.04% or more, 0.06% or more, 0.08% or more, or 0.10% or more, and preferably 0.26% or less, 0.24% or less, 0.22% or less, 0.20% or less, 0.18% or less, or 0.16% or less.

[0022] Si: 0.01~0.60% Si is an inexpensive deoxidizing element that contributes to increased strength through solid solution strengthening. To obtain this effect, the Si content should be 0.01% or more. On the other hand, to suppress the decrease in ductility and toughness, the Si content should be 0.60% or less. The Si content is preferably 0.03% or more, 0.06% or more, 0.09% or more, 0.12% or more, or 0.15% or more, and preferably 0.50% or less, 0.40% or less, 0.30% or less, or 0.20% or less.

[0023] Mn: 0.50~2.50% Mn is an element that improves the strength and toughness of the base material. To obtain such effects, the Mn content should be 0.50% or more. On the other hand, to suppress the decrease in ductility and toughness, the Mn content should be 2.50% or less. A Mn content of 0.60% or more is preferable, and 0.90% or more is more preferable. Furthermore, a Mn content of 2.00% or less, 0.18% or less, 0.16% or less, or 0.14% or less is preferable.

[0024] P:0.030% or less Since phosphorus (P) is an impurity and reduces ductility and toughness, the P content should be 0.030% or less. It is desirable to have as little P content as possible. However, reducing P would lead to a significant increase in melting costs and impair practicality, so the P content may be 0.001% or more. A P content of 0.025% or less, 0.020% or less, or 0.015% or less is preferable.

[0025] S: 0.010% or less S is an impurity and exists in steel as a sulfide inclusion. Since S degrades ductility and toughness, the S content should be kept below 0.010%. To ensure ductility and toughness, a lower S content is preferable, and it is preferable that the S content be 0.007% or less, 0.005% or less, or 0.003% or less. However, since reducing S leads to increased costs, the S content may be 0.001% or more.

[0026] N: 0.0010~0.0080% N, along with Al, Nb, and / or Ti, forms nitrides and is an element effective in refining the metal structure. To achieve this effect, the N content should be 0.0010% or more. On the other hand, to suppress the decrease in ductility and toughness, the N content should be 0.0080% or less. Preferably, the N content is 0.0015% or more, 0.0020% or more, 0.0025% or more, or 0.0030% or more. Furthermore, preferably, the N content is 0.0060% or less, and more preferably 0.0050% or less.

[0027] O: 0.0100% or less O is an impurity and exists in steel as an oxide inclusion. Excessive O content can reduce the ductility and toughness of the steel due to the formation of inclusions. Therefore, the O content should be 0.0100% or less. There is no particular lower limit to the O content, but reducing it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. Furthermore, it is preferable that the O content be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0028] One or more elements selected from the group consisting of Al: 0.002-0.050%, Nb: 0.002-0.060%, and Ti: 0.002-0.030%. The steel material of the present invention contains one or more elements selected from the group consisting of Al: 0.002-0.050%, Nb: 0.002-0.060%, and Ti: 0.002-0.030%. These elements are important for improving fatigue crack propagation resistance in both the thickness direction and the direction perpendicular thereto. By including these elements, when precipitates containing Al, Nb, and / or Ti are formed during the hot rolling process, dislocations are trapped in these precipitates when the average residual stress in the rolling direction and the direction perpendicular to rolling is controlled to less than -50 MPa, thereby further improving fatigue crack propagation resistance regardless of the crack propagation direction. Al, Nb, and Ti may be included individually or in any specific combination of two or more of the above elements. Al, Nb, and Ti will be described in detail below.

[0029] Al is a deoxidizing element and also forms AlN, which improves fatigue crack propagation characteristics. However, if the Al content is excessive, it may form inclusions that are detrimental to ductility and toughness. For this reason, the Al content should be 0.050% or less. Preferably, the Al content is 0.040% or less, 0.035% or less, or 0.030% or less. To more reliably obtain the above effects, the Al content is preferably 0.002% or more, and more preferably 0.003% or more, 0.010% or more, 0.015% or more, or 0.020% or more.

[0030] Nb is an element that forms precipitates such as carbides and nitrides, improving fatigue crack propagation characteristics. However, excessive Nb content can degrade ductility and toughness. Therefore, the Nb content should be 0.060% or less. Preferably, the Nb content is 0.030% or less, 0.025% or less, 0.020% or less, or 0.015% or less. To more reliably obtain the above effects, the Nb content is preferably 0.002% or more, and more preferably 0.003% or more, 0.005% or more, or 0.010% or more.

[0031] Ti is an element that forms precipitates such as carbides and nitrides, improving fatigue crack propagation characteristics. To achieve this effect, the Ti content is preferably 0.002% or more, and more preferably 0.006% or more, 0.008% or more, or 0.010% or more. On the other hand, excessive Ti content may form inclusions that are detrimental to ductility and toughness, so the Ti content should be 0.030% or less. The Ti content is preferably 0.020% or less, 0.018% or less, 0.016% or less, or 0.014% or less.

[0032] In the chemical composition of the steel material of the present invention, in addition to the elements mentioned above, one or more elements selected from the group consisting of Cu, Ni, Cr, Mo, W, V, B, Ca, Mg, and REM may be included within the ranges shown below, for the purpose of improving mechanical properties such as strength, ductility, and toughness. The reasons for limiting each element are explained below.

[0033] Cu: 0~2.00% Cu may be included as needed to improve hardenability and contribute to increased strength. However, excessive Cu content can lead to adverse effects such as increased surface cracking of the steel billet. Therefore, the Cu content should be 2.00% or less. Preferably, the Cu content is 1.50% or less, 1.25% or less, 1.00% or less, 0.75% or less, or 0.50% or less. To obtain the above effects, the Cu content is preferably 0.01% or more, and more preferably 0.03% or more.

[0034] Ni: 0~3.00% Ni is effective in improving strength and toughness, and also effectively contributes to improving surface cracking of steel billets that occurs when Cu is included, so it may be included as needed. However, if the Ni content is excessive, the cost will increase. For this reason, the Ni content should be 3.00% or less. Preferably, the Ni content is 2.00% or less, 1.50% or less, 1.25% or less, 1.00% or less, 0.75% or less, or 0.50% or less. To obtain the above effects, the Ni content is preferably 0.01% or more, and more preferably 0.03% or more.

[0035] Cr: 0~1.00% Cr is an element that improves hardenability and contributes to increased strength, so it may be included as needed. However, if the Cr content is excessive, ductility and toughness may decrease. Therefore, the Cr content should be 1.00% or less. Preferably, the Cr content is 0.70% or less, 0.50% or less, or 0.30% or less. To obtain the above effects, the Cr content is preferably 0.01% or more, and more preferably 0.03% or more.

[0036] Mo: 0~1.00% Mo is an element that contributes to improving strength and toughness, and may be included as needed. However, excessive Mo content increases costs. Therefore, the Mo content should be 1.00% or less. Preferably, the Mo content is 0.70% or less, 0.50% or less, or 0.30% or less. To obtain the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.

[0037] W: 0~1.00% Since W is an element that contributes to improving strength and toughness, it may be included as needed. However, if the W content is excessive, the cost will increase. Therefore, the W content should be 1.00% or less. Preferably, the W content is 0.70% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.05% or less. To obtain the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more.

[0038] V: 0~1.00% V may be included as needed because it contributes to increased strength through improved hardenability and precipitation strengthening. However, excessive V content may impair ductility and toughness. Therefore, the V content should be 1.00% or less. Preferably, the V content is 0.50% or less, 0.30% or less, or 0.20% or less. To obtain the above effects, the V content is preferably 0.01% or more, and more preferably 0.03% or more.

[0039] B: 0~0.0030% B may be included as needed because it enhances hardenability and contributes to improved strength. However, excessive B content may degrade ductility and toughness. Therefore, the B content should be 0.0030% or less. Preferably, the B content is 0.0020% or less or 0.0015% or less. To obtain the above effects, it is preferable that the B content be 0.0003% or more.

[0040] Ca: 0~0.0100% Ca is an element that controls the morphology of nonmetallic inclusions and contributes to improving ductility and toughness, and may be included as needed. On the other hand, excessive Ca content will saturate the effect and lead to increased manufacturing costs. Therefore, the Ca content should be 0.0100% or less. Preferably, the Ca content is 0.0050% or less, 0.0030% or less, or 0.0020% or less. To obtain the above effects, the Ca content is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0041] Mg: 0~0.0050% Mg is an element that controls the morphology of nonmetallic inclusions and contributes to improving ductility and toughness, and may be included as needed. On the other hand, excessive Mg content will saturate the effect and lead to increased manufacturing costs. Therefore, the Mg content should be 0.0050% or less. Preferably, the Mg content is 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less. To obtain the above effects, the Mg content is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0042] REM: 0~0.0200% REM is an element that controls the morphology of nonmetallic inclusions and contributes to improving ductility and toughness, and may be included as needed. On the other hand, excessive REM content will saturate the effect and lead to increased manufacturing costs. Therefore, the REM content should be 0.0200% or less. Preferably, the REM content is 0.0100% or less, 0.080% or less, or 0.0060% or less. To obtain the above effects, the REM content is preferably 0.0001% or more, and more preferably 0.0003% or more. In this embodiment, REM is a collective term for 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0043] Ceq: 0.25~0.50 The steel material of the present invention has the above-described composition, and furthermore, as an indicator of hardenability, the value of the carbon equivalent Ceq, defined by the following formula (i), is set to 0.25 to 0.50. Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15 ···(i) However, in the above formula, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent the mass percentage of each element, and if an element is not present, it is set to zero.

[0044] From the viewpoint of ensuring strength, the carbon equivalent Ceq defined by equation (i) above should be 0.25 or higher. Furthermore, from the viewpoint of suppressing a decrease in ductility and toughness, the Ceq should be 0.50 or lower. Preferably, the Ceq is 0.27 or higher or 0.30 or higher, and preferably 0.45 or lower or 0.40 or lower.

[0045] In the chemical composition of the steel material of the present invention, the remainder is Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of steel material due to various factors in the raw materials such as ore and scrap, and the manufacturing process. In addition, one or more of Co, Zr, Hf, Sr, Ta, Sn, Sb, Zn, Bi, Se, Pb, As, and Te may be included to the extent that they do not adversely affect the present invention. For Co, Zr, Hf, Sr, and Ta, one or more of the following may be included in amounts of Co: 0-0.100%, Zr: 0-0.050%, Hf: 0-0.020%, Sr: 0-0.020%, and Ta: 0-0.020% in order to improve the properties of the weld heat-affected zone. For Sn, Sb, and Zn, one or more of the following may be included to improve corrosion resistance: Sn: 0-0.300%, Sb: 0-0.050%, and Zn: 0-0.020%. For Bi, Se, Pb, As, and Te, one or more of the following may be included to improve machinability: Bi: 0-0.100%, Se: 0-0.020%, Pb: 0-0.090%, As: 0-0.050%, and Te: 0-0.050%.

[0046] (B) Metallographic structure and mechanical properties The metallographic structure of the steel material of the present invention will now be described. In the following description, "%" means "area %". Furthermore, in the present invention, the "metallographic structure" of the steel material is defined as the structure at a position 1 / 4t from the surface of the steel material in a cross section parallel to the rolling direction and thickness direction of the steel material, where the thickness of the steel material is t. The thickness of the steel material referred to here is the plate thickness in the case of a steel plate, the wall thickness in the case of a steel pipe, and the plate thickness of the flange in the case of a structural steel.

[0047] Low-strain tissue: 80% or more Regarding the metal structure of the steel material of the present invention, if the area ratio of low-strain structures with a KAM (Kernel Average Misorientation) value of less than 0.5° is less than 80%, it may lead to a decrease in the yield ratio. Therefore, the area ratio of low-strain structures should be 80% or more. Preferably, the area ratio of low-strain structures is 85% or more, and may be 90% or more, 95% or more, or 97% or more. The upper limit of the area ratio of low-strain structures is not particularly limited, but may be 100% or less, 99% or less, or 98% or less. Low-strain structures with a KAM value of less than 0.5° include, for example, ferrite, pearlite, and tempered bainite.

[0048] The remaining tissue is a high-strain structure with a KAM value of 0.5° or higher, and may include, for example, martensite or retained austenite (retained gamma). The area percentage of the remaining tissue is not particularly limited, but may be, for example, 0-20%, 0-10%, 0-5%, or 0-3%.

[0049] Identification of metallographic structures and calculation of area ratios In this invention, the area ratio of the metal structure is determined as follows. As described above, first, a sample is taken from a position 1 / 4t from the surface of the steel material, in a cross section parallel to the rolling direction and thickness direction of the steel material, where the thickness of the steel material is t. Then, the cross section of the sample in the rolling direction (the so-called L-direction cross section) is observed. Although it is preferable that the observation surface is parallel to the rolling direction, if the rolling direction of the steel material cannot be determined, it is not necessarily required to be parallel to the rolling direction as long as it is parallel to the thickness direction.

[0050] Specifically, the observation surface of the sample is polished to a mirror finish, the strain-influenced layer is removed by electropolishing, and then a total of 2.0 × 10⁻⁶ layers are removed in one or more fields of view. -8 m 2 Using a field emission scanning electron microscope (FE-SEM), electron back scattering diffraction (EBSD) analysis is performed on the above-mentioned area. The local azimuthal difference around each measurement point is then mapped using the KAM method to determine the area ratio.

[0051] The KAM method is a calculation performed for each pixel in a given hexagonal pixel in the measurement data. It involves averaging the orientation differences between six adjacent pixels (first approximation) and the twelve pixels outside of those (second approximation), and then using that average value as the local orientation difference (KAM value) of the central pixel.

[0052] In this invention, the measurement step is set to 0.1 μm, and the region where the second approximate KAM value is less than 0.5° is defined as a low-strain structure. On the other hand, the region where the second approximate KAM value is 0.5° or more is defined as a high-strain structure.

[0053] Average residual stress at a depth of 0.1-0.5 mm: less than -50 MPa The steel material of the present invention has an average residual stress of less than -50 MPa in the rolling direction and the direction perpendicular to rolling (width direction) at a position 0.1 to 0.5 mm in depth from the surface of the steel material. As will be explained in detail later, by increasing the cooling rate in the heat treatment process, compressive residual stress is introduced into the surface layer of the steel material due to the internal temperature difference, making it possible to suppress fatigue crack initiation and propagation from the surface of the steel material. Here, compressive residual stress means that the residual stress is less than 0 MPa, and in order to ensure excellent fatigue crack propagation characteristics, the average residual stress at a depth of 0.1 to 0.5 mm from the surface of the steel material must be less than -50 MPa. Preferably, it is -100 MPa or less, -120 MPa or less, or -140 MPa or less.

[0054] The measurement of residual stress is carried out by the hole-drilling method based on ASTM E837-13a. For the measurement of residual stress, a test piece with a length of 200 mm in the rolling direction, a length of 200 mm in the width direction, and the full thickness, taken from the steel material, is used to measure the residual stress in the rolling direction and the direction perpendicular to the rolling direction at the center of the test piece steel material. The measurement interval in the depth direction is set to 0.01 mm. The residual stress in the rolling direction and the direction perpendicular to the rolling direction at positions with a depth of 0.1 to 0.5 mm is measured at 5 points each, and the average value of the 10 residual stresses is calculated, and this value is taken as the average residual stress.

[0055] In addition, if the compressive residual stress becomes excessive, the steel material is likely to undergo out-of-plane deformation, and the flatness of the steel material may decrease. Therefore, the residual stress on the surface layer of the steel material may be, for example, -1000 MPa or more, -300 MPa or more, or -250 MPa or more.

[0056] Number density of precipitates: 15 particles / μm 2 Above Equivalent circle diameter of precipitates: 3 - 500 nm In the steel material according to the preferred embodiment of the present invention, in the metallographic structure at a position of 1 / 4t from the surface of the steel material, the number density of precipitates is 15 particles / μm 2 or more, and the equivalent circle diameter of the precipitates is 3 - 500 nm. That is, in the steel material according to the preferred embodiment of the present invention, the number density of precipitates with an equivalent circle diameter of 3 - 500 nm is 15 particles / μm 2 or more. From the viewpoint of improving ductility and / or fatigue crack propagation characteristics, the number density of the precipitates is 18 particles / μm 2 or more, 20 particles / μm 2 or more, or 25 particles / μm [[ID=…]] 2 or more is preferred. The upper limit of the number density of the precipitates is not particularly limited, but may be, for example, 100 particles / μm 2 or less, or 50 particles / μm 2 or less. An example of the bright-field image obtained by TEM observation described later is shown in Fig. 1.

[0057] Method for measuring the number density of precipitates with an equivalent circle diameter of 3 - 500 nm The number density of precipitates with an equivalent circular diameter of 3 to 500 nm is measured by a transmission electron microscope (TEM). First, a sample is taken from a position 1 / 4 t from the surface of the steel material in a cross section parallel to the rolling direction and thickness direction of the steel material. Although it is preferable that the observation surface be parallel to the rolling direction, if the rolling direction of the steel material cannot be determined, it is not necessary for it to be parallel to the rolling direction as long as it is parallel to the thickness direction. After wet polishing the sample to a thickness of 60 μm, double-sided jet electrolytic polishing is performed using a mixture of perchloric acid (10 vol.%) and ethanol (90 vol.%) to obtain a thin film sample. TEM observation is performed on any 5 or more fields of view in the low-strain structure with a thickness of 100 ± 50 nm of the thin film sample. The acceleration voltage during TEM observation is set to 200 kV. <001> Crystal grains observable by the incident electron beam were used as the observation target. Bright-field images with a field size of 1 μm × 1 μm were generated in each field of view. To measure the number density of precipitates, the bright-field images were binarized using image processing techniques. In the binarization process, an appropriate threshold was set for the extraction of precipitates. Furthermore, objects where connected pixels had an equivalent circle diameter of less than 3 nm and greater than 500 nm were removed, and the number of precipitates with an equivalent circle diameter of 3 to 500 nm was determined. The obtained number of precipitates and the field of view area (1 μm) were used. 2 Based on this, the number density of precipitates (particles / μm 2 The number density of precipitates (particles / μm) was measured. The arithmetic mean of the number densities of five or more observation fields was then used as the number density of precipitates (particles / μm). 2 ) was defined as follows.

[0058] Number density of precipitates with one or more dislocations within a 50 nm radius: 10 particles / μm 2 That's all. In a steel material according to a more preferred embodiment of the present invention, the number density of precipitates having one or more dislocations within a 50 nm radius is 10 particles / μm 2This concludes the explanation. In this embodiment, "precipitates in which one or more dislocations are present within a 50 nm radius" may be referred to as "precipitates with trapped dislocations." As described above, from the viewpoint of improving fatigue crack propagation characteristics, the number density of precipitates in which one or more dislocations are present within a 50 nm radius is set to 10 particles / μm 2 It is preferable to have more than 15 particles / μm, and more preferably 15 particles / μm 2 That concludes the explanation. There is no particular upper limit to the number density of precipitates in which one or more dislocations exist within a 50 nm radius, but for example, 80 particles / μm 2 Below, 60 pieces / μm 2 Less than or 40 pieces / μm 2 The following is also acceptable.

[0059] Method for measuring the number density of precipitates containing one or more dislocations within a 50 nm radius. The number density of precipitates containing one or more dislocations within a 50 nm radius can be determined by the following method: Take a sample from a position 1 / 4 t from the surface of the steel material, in a cross section parallel to the rolling direction and thickness direction of the steel material, where t is the thickness of the steel material. While it is preferable that the observation surface be parallel to the rolling direction, if the rolling direction of the steel material cannot be determined, it is not necessarily required to be parallel to the rolling direction as long as it is parallel to the thickness direction.

[0060] Specifically, precipitates containing one or more dislocations within a 50 nm radius (precipitates with trapped dislocations) are identified by the following method. After wet polishing the sample to a thickness of 60 μm, double-sided jet electrolytic polishing is performed using a mixture of perchloric acid (10 vol.%) and ethanol (90 vol.%) to obtain a thin film sample. TEM observation is performed on any five or more fields of view within the low-strain structure of the thin film sample with a thickness of 100 ± 50 nm. The acceleration voltage during TEM observation is set to 200 kV. <001> The target of observation was crystal grains observable by the incident electron beam. Bright-field images with a field size of 2 μm × 2 μm were generated in each field of view. Using the bright-field images of each field of view, image processing techniques were used to binarize the images in order to identify precipitates that had captured dislocations. In the binarization process, an appropriate threshold was set so that precipitates that had captured dislocations could be extracted. Then, the contour length was calculated for each connected location, and targets with a contour length of 0.15 μm or less were extracted. Furthermore, targets where the connected pixels had a circular equivalent diameter of less than 3 nm and greater than 500 nm were removed as noise. The number of precipitates that had captured dislocations was then determined from the obtained binarized images. 2 Based on this, the number density of dislocation-trapped precipitates (particles / μm 2 The number density of the dislocation-captured precipitate (particles / μm) was measured. The arithmetic mean of the five number densities was then used as the number density (particles / μm). 2 ) was defined as follows.

[0061] Next, the mechanical properties of the steel material of the present invention will be described. The steel material of the present invention has excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, as well as high strength, a good yield ratio, and excellent ductility.

[0062] Fatigue crack propagation characteristics The steel material of the present invention exhibits excellent fatigue crack propagation characteristics, and more specifically, regardless of the crack propagation direction, the fatigue crack propagation rate da / dN is ΔK: 15 MPa·m 0.5 1.60 × 10 -8 (m / cycle) or less, and ΔK: 25MPa·m 0.5 8.0 x 10 -8A value of (m / cycle) or less can be achieved. The fatigue crack propagation characteristics in the plate direction are measured in accordance with ASTM E647. The fatigue crack propagation velocity in the plate direction is the greater of the fatigue crack propagation velocities measured using two types of CT test specimens in which the fatigue crack propagates in the direction of rolling or perpendicular thereto. The fatigue crack propagation characteristics in the plate thickness direction are measured using a small three-point bending test specimen in which the fatigue crack propagates in the direction of plate thickness.

[0063] The upper limits of the crack propagation velocity in each of the stress intensity factor ranges described above were determined using the upper limit of the data band for the relationship between the stress intensity factor range and fatigue crack propagation velocity for NK-class KA steel, as described in "Data Collection on Fatigue Crack Propagation Resistance of Metallic Materials," Vol. 1, p. 55, edited by the Materials Society of Japan, as a reference value, and using the case where the fatigue crack propagation velocity in the same stress intensity factor range is 1 / 2 or less of the reference value as a guideline.

[0064] Yield strength (YS) Tensile strength (TS) Furthermore, the steel material of the present invention has high strength, and more specifically, it can achieve a yield strength YS of 325 MPa or more and a tensile strength TS of 490 MPa or more. YS may be 350 MPa or more, 375 MPa or more, or 400 MPa or more. The upper limit of YS is not particularly limited, but for example it may be 800 MPa or less, 750 MPa or less, or 700 MPa or less. TS may be 550 MPa or more, 575 MPa or more, or 600 MPa or more. The upper limit of TS is not particularly limited, but for example it may be 900 MPa or less, 800 MPa or less, or 750 MPa or less.

[0065] Yield ratio (YR) The steel material of the present invention has a good yield ratio, and more specifically, it has an upper yield point in a tensile test and can achieve a yield ratio (YR), which is the ratio of yield stress (YS) to tensile strength (TS), of 80% or more. YR may be 82% or more, 84% or more, or 86% or more. The upper limit of YR is not particularly limited, but may be, for example, 95% or less or 93% or less.

[0066] Ductility (Total elongation (t-EL)) The steel material of the present invention has high ductility, and more specifically, can achieve a total elongation (t-EL) of 20% or more. The t-EL may be 22.0% or more, 24.0% or more, or 26.0% or more. The upper limit of t-EL is not particularly limited, but may be, for example, 50.0% or less, 45.0% or less, or 40.0% or less.

[0067] Tensile strength (TS), yield stress (YS), yield ratio (YR), and total elongation (t-EL) are measured using a No. 14 tensile test specimen, taken from the center in the thickness direction (at a position 1 / 2t from the surface of the steel material, where the thickness of the steel material is t), with the longitudinal direction being the direction perpendicular to the rolling direction and the thickness direction (width direction), in accordance with JIS Z 2241:2011. In detail, for steel materials in which upper yield occurs, the yield stress (YS) is the upper yield point. On the other hand, for steel materials in which an upper yield point does not occur, the yield stress (YS) is the proof strength using the permanent elongation method at a permanent elongation of 0.2%. Also, total elongation (t-EL) is the total elongation at fracture.

[0068] [plate thickness] The steel material of the present invention is not particularly limited, but generally has a thickness of 5 to 150 mm. For example, the plate thickness may be 12 mm or more, 15 mm or more, or 20 mm or more, and / or 100 mm or less, 90 mm or less, or 80 mm or less. The thickness of the steel material may be 2.5 mm or less.

[0069] As described above, the steel material of the present invention exhibits excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, as well as high strength and excellent yield ratio and ductility. Therefore, the steel material of the present invention is particularly useful for use in steel structures in technical fields where these properties are required at a high level, and especially useful for use in steel structures in fields such as ships, offshore structures, bridges, construction machinery, buildings, and tanks. In a preferred embodiment, steel structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks containing the steel material of the present invention are provided. Such steel structures only need to contain the steel material of the present invention in at least a portion thereof, and therefore at least a portion of these steel structures will satisfy the characteristics of the steel material described above.

[0070] (C) Manufacturing method Next, preferred manufacturing methods for the steel materials of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing the steel sheets of the present invention and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below.

[0071] There are no particular restrictions on the manufacturing conditions for the steel material according to the present invention, but it can be manufactured by melting molten steel having the above chemical composition using known methods such as a converter, electric furnace, or vacuum melting furnace, and then sequentially performing the hot rolling, cooling, and heat treatment processes described later on the resulting steel material. Each process will be explained below.

[0072] Hot rolling process In the hot rolling process, the steel material having the above chemical composition is heated to a temperature range of 900 to 1300°C, and then hot-rolled to satisfy the desired thickness and shape. To improve ductility, rough rolling is performed before finish rolling during the hot rolling process. Finish rolling is hot rolling at temperatures below 900°C until the end of rolling. Hot rolling is completed within the temperature range of Ar3-50 to 900°C, with a cumulative reduction ratio of 30 to 75% at the end of rolling at temperatures below 900°C. Note that the temperature refers to the surface temperature of the steel material. In the following, the temperature at which hot rolling is completed may be referred to as the rolling completion temperature. Also, in the following, the cumulative reduction ratio at temperatures below 900°C until the end of rolling may be referred to as the finish rolling cumulative reduction ratio.

[0073] Heating temperature: 900~1300℃ If the heating temperature of the steel material is below 900°C, the deformation resistance increases, increasing the load on the rolling mill and reducing productivity. On the other hand, if the reheating temperature exceeds 1300°C, surface defects are more likely to occur due to scaling during heating, increasing the burden of post-rolling maintenance, and the crystal grains become coarser, making it difficult to secure the desired toughness. For this reason, the heating temperature of the steel material should be in the range of 900 to 1300°C.

[0074] Hot rolling completion temperature: Ar3-50~900℃ Rolling at temperatures below Ar3-50°C degrades toughness and ductility; therefore, the end temperature of hot rolling should be Ar3-50°C or higher. On the other hand, if the end temperature of hot rolling exceeds 900°C, it becomes recrystallized rolling, causing the metal structure to coarseen and degrading the strength-ductility balance. For this reason, the end temperature of hot rolling should be between Ar3-50°C and 900°C. Preferably, the end temperature of hot rolling is 860°C or lower.

[0075] Ar3 is the temperature at which ferrite transformation begins when steel is cooled, and can be calculated using equation (ii) below. Ar3(℃)=910-310×[C]+65×[Si]-80×[Mn]-20×[Cu]-55×[Ni]-15×[Cr]-80×[Mo]...(ii) However, in the above formula, [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] represent the mass percentage of each element, and elements that are not present are calculated as 0.

[0076] Below 900℃, cumulative reduction ratio until completion of hot rolling: 30-75% When the cumulative reduction ratio of finish rolling exceeds 30%, the fineness of the metal structure due to phase transformation is promoted, and elongation and strength are improved in a balanced manner. On the other hand, productivity deteriorates when the cumulative reduction ratio of finish rolling exceeds 75%. Therefore, the cumulative reduction ratio of finish rolling should be set between 30% and 75%. Preferably, the cumulative reduction ratio of finish rolling should be between 35% and 65%. Here, the cumulative reduction ratio below 900°C until the end of hot rolling refers to the reduction ratio determined by the thickness after hot rolling relative to the thickness at 900°C, and is calculated using the following formula. For temperatures below 900℃, the cumulative reduction ratio until the end of hot rolling = (Thickness at 900℃ (mm) - Thickness after hot rolling (mm)) / Thickness at 900℃ (mm)

[0077] cooling process The finished-rolled steel is then cooled. There are no restrictions on the cooling process; accelerated cooling by water cooling or other methods may be used, or it may be air-cooled without accelerated cooling.

[0078] Heat treatment process In this invention, after the cooling process is completed, the steel material is subjected to a reheating heat treatment at a temperature of 400°C to Ac1, and then the steel material is cooled to a temperature range of 250°C or lower at an average cooling rate of 20 to 100°C / second. The temperature at this stage is also the surface temperature of the steel material.

[0079] Ac1 (°C) is the austenite transformation onset temperature when heating steel, and can be calculated using equation (iii) below. Ac1(℃)=750.8-26.6×[C]+17.6×[Si]-11.6×[Mn]-22.9×[Cu]-23.0×[Ni]+24.1×[Cr]+22.5×[Mo]-39.7×[V]+232.4×[Nb]-169.4×[Al]-894.7×[B] ···(iii) However, in the above formula, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Nb], [Al], and [B] represent the content (mass%) of each element, and elements that are not present are calculated as 0.

[0080] Reheating temperature: 400℃~Ac1 If the reheating temperature is less than 400°C, the average residual stress in the rolling direction and the direction perpendicular to rolling at a depth of 0.1 to 0.5 mm from the surface of the steel material may exceed -50 MPa, which may result in a deterioration of fatigue crack propagation characteristics. Therefore, the reheating temperature should be 400°C or higher, preferably 450°C or higher, and more preferably 480°C or higher. Furthermore, if the temperature exceeds Ac1, the area ratio of the high-strain structure increases, which may decrease the yield ratio and / or ductility, so the reheating temperature should be kept below Ac1.

[0081] The holding time at the reheating temperature is not particularly limited, but it is preferable to set it to 0.1 hours or more to ensure a uniform temperature distribution inside the steel. The upper limit of the holding time at the reheating temperature is not particularly limited, but it may be, for example, 1.0 hour or less.

[0082] Average cooling rate: 20~100℃ / sec If the average cooling rate from the reheating temperature to a temperature range of 250°C or less is less than 20°C / second, the residual stress on the surface of the steel material will not be compressed, which may degrade the fatigue crack propagation characteristics. On the other hand, if it exceeds 100°C / second, the toughness and flatness may deteriorate. The average cooling rate should be 20°C / second or higher or 25°C / second or higher, and 100°C / second or lower or 60°C / second or lower.

[0083] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0084] Table 1 shows the chemical composition of the test steels used in the examples. Each test steel was produced as a billet by continuous casting.

[0085] The obtained steel billets were heated under the conditions shown in Table 2, and then hot-rolled. Hot rolling was carried out by rough rolling and finish rolling. Rough rolling was performed at a temperature above 900°C and was carried out under the same conditions as all examples and comparative examples. Finish rolling was carried out under the conditions shown in Table 2. Next, the finish-rolled steel material was cooled under the conditions shown in Table 2, and then heat-treated under the conditions shown in Table 2 to produce steel material with the plate thickness shown in Table 2.

[0086] [Table 1]

[0087] [Table 2]

[0088] The obtained steel material was subjected to metallographic observation using the following method, and the area ratio of low-strain structures was measured. First, in the rolling direction cross-section of the steel material, with the width and thickness being W and t respectively, a specimen for metallographic observation was cut from a position 1 / 4W from the end face and 1 / 4t from the surface of the steel material. The rolling direction cross-section of the specimen was then polished to a mirror finish, and the strain-influenced layer was removed by electropolishing. Then, EBSD was performed in a 200 μm × 150 μm field of view with a measurement step of 0.1 μm to measure and map the KAM. From the obtained KAM map, low-strain structures were identified and their area ratio was calculated. Residual stress was measured by the drilling method based on ASTM E837-13a under the conditions described above. The number density and equivalent diameter of precipitates, and the number density of precipitates with one or more dislocations within a 50 nm radius were measured by TEM under the conditions described above.

[0089] Furthermore, test specimens were taken from the aforementioned steel material, and tensile tests and fatigue crack propagation tests were conducted. The test methods were as follows.

[0090] (1) Tensile test Tensile tests were conducted in accordance with JIS Z 2241:2011. A No. 14 tensile test specimen was taken from a position 1 / 4 t from the surface of the steel material, with the tensile direction perpendicular to the rolling direction and thickness direction (width direction). Tensile tests were performed, and the yield stress (YS), tensile strength (TS), yield ratio (YR), and total elongation (t-EL) were determined. A specimen was judged to pass (G) if YS was 325 MPa or higher, TS was 490 MPa or higher, t-EL was 20% or higher, and YR was 80% or higher. Any other result was a failure (B).

[0091] (2) Fatigue crack propagation test The fatigue crack propagation characteristics in the plate direction were assessed in accordance with ASTM E647. Two types of CT test specimens were taken: one in the direction perpendicular to the rolling direction (plate width direction) and another in the direction of fatigue crack propagation (plate length direction). For steel plates with a thickness of 12.5 mm or less, the entire thickness was used. For steel plates with a thickness between 12.5 mm and 25 mm, one side was machined from the surface layer of the steel plate, and for steel plates with a thickness exceeding 25 mm, both sides were thinned around a point 1 / 4th of the way from the surface to a thickness of 12.5 mm. The specimen dimensions are shown in Figure 2, and the conditions for the fatigue crack propagation test using CT test specimens were as follows. Stress ratio: 0.1 • Test frequency: 10Hz • Environment: Room temperature, ambient air • Crack length measurement: Unloading elastic compliance method using back strain gauges • Back gauge length: 2mm

[0092] To assess the fatigue crack propagation characteristics in the thickness direction, three-point bending test specimens were taken as shown in Figure 3, ensuring that the direction of fatigue crack propagation from the steel material was in the thickness direction. For steel plates with a thickness of 10 mm or less, the entire thickness was used. For steel plates with a thickness between 10 mm and 20 mm, one side was machined from the surface layer of the steel plate. For steel plates with a thickness exceeding 20 mm, both sides were thinned to a thickness of 10 mm, centering on a point 1 / 4th of the thickness from the surface of the steel plate. The conditions for the fatigue crack propagation test using the three-point bending test specimens were as follows. • Load application method: 3-point bending Stress ratio: 0.1 • Environment: Room temperature, ambient air • Crack length measurement: DC potentiometer method

[0093] Furthermore, the stress intensity factor range ΔK during fatigue crack propagation is 15 MPa·m. 0.5 The fatigue crack propagation rate in is 1.60 × 10⁻⁶ -8 (m / cycle) or less, and ΔK: 25MPa·m 0.5 The fatigue crack propagation rate in is 8.0 × 10 -8 A score of (G) was given if the value was less than or equal to (m / cycle). All other scores were given as a failure (B).

[0094] These measurement results are shown in Table 3. Note that "crack propagation direction: in-plate direction" refers to the direction in which the fatigue crack propagates, where the crack propagation velocity measured in the plate width direction and rolling direction is greater.

[0095] [Table 3]

[0096] Tests No. 1 to 19 all involved steel materials manufactured according to the preferred requirements of the present invention, using steel billets with the chemical composition of the present invention. These materials possessed high strength, a good yield ratio, and excellent ductility. Furthermore, the fatigue crack propagation rate was independent of the crack propagation direction, and the stress intensity factor range ΔK was 15 MPa·m. 0.5 At that time, 1.60 × 10 -8 (m / cycle) or less, ΔK is 25 MPa·m 0.5 At that time, 8.00 × 10 -8 The steel material satisfies the requirement of (m / cycle) or less and has excellent fatigue crack propagation characteristics. In particular, in addition to the requirements of the steel material according to the embodiment of the present invention, the number density of precipitates with an equivalent circular diameter of 3 to 500 nm is 15 particles / μm 2 Tests No. 1 to 17 that met the above criteria had a t-EL of 24.0% or higher, and in addition to the above, exhibited superior ductility. Furthermore, in addition to the requirements for the steel material according to the embodiment of the present invention, the number density of dislocation-captured precipitates was 10 particles / μm 2Tests No. 1-16 and 18 that satisfy the above conditions show that the fatigue crack propagation rate is independent of the crack propagation direction, and the stress intensity factor range ΔK is 25 MPa·m. 0.5 At that time, 7.00 × 10 -8 The value was less than (m / cycle), and in addition to the above, it had superior fatigue crack propagation characteristics.

[0097] On the other hand, tests No. 20-23 exhibited excellent fatigue crack propagation resistance but poor mechanical properties because the preferred manufacturing requirements or chemical composition of the present invention fell outside the specified range. Test No. 20 lacked sufficient tensile strength due to its low carbon equivalent (Ceq). Test No. 21 had poor ductility due to its high carbon equivalent (Ceq). Test No. 22 was not heat-treated, and Test No. 23 underwent a reverse transformation to austenite due to its high heat treatment temperature. It is believed that a high-strain structure was generated during cooling, resulting in an increased area ratio of the high-strain structure. Consequently, the yield ratio was poor.

[0098] Tests No. 24 and 25 satisfied the chemical composition of the present invention, but deviated from the preferred manufacturing requirements, resulting in poor fatigue crack propagation resistance. Specifically, it is believed that the residual stress on the surface layer did not satisfy the provisions of the present invention because the heat treatment temperature of No. 24 was low, and the cooling rate after heat treatment of No. 25 was slow. As a result, the fatigue crack propagation resistance was poor. Test No. 26 did not contain one or more elements selected from the group consisting of Al, Nb, and Ti, and therefore exhibited poor fatigue crack propagation resistance. [Industrial applicability]

[0099] According to the present invention, it is possible to obtain a steel material that exhibits excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, has high strength and a good yield ratio, and also has excellent ductility. The steel material according to the present invention is suitable for various welded structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks.

Claims

1. The chemical composition is expressed in mass percent. C: 0.01-0.30%, Si: 0.01-0.60%, Mn: 0.50 to 2.50%, P: 0.030% or less, S: 0.010% or less, N: 0.0010 to 0.0080%, O: 0.0100% or less, One or more elements selected from the group consisting of Al: 0.002–0.050%, Nb: 0.002–0.060%, and Ti: 0.002–0.030%. Cu: 0-2.00%, Ni: 0-3.00%, Cr: 0-1.00%, Mo: 0-1.00%, W: 0-1.00%, V: 0 to 1.00%, B: 0 to 0.0030%, Ca: 0-0.0100%, Mg: 0 to 0.0050%, REM: 0-0.0200%, and The remainder consists of Fe and impurities. The carbon equivalent Ceq value, as defined by equation (i) below, is between 0.25 and 0.

50. In a cross-section parallel to the rolling direction and thickness direction of the steel material, when the thickness of the steel material is t, the metallographic structure at a position 1 / 4t from the surface of the steel material contains 80% or more of a low-strain structure with a KAM value of less than 0.5°, in terms of area percentage. A steel material in which the average residual stress in the rolling direction and the direction perpendicular to rolling at a position 0.1 to 0.5 mm in depth from the surface of the steel material is less than -50 MPa. Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15...(i) However, in the above formula, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent the content (mass%) of each element, and if an element is not present, it is set to zero.

2. The aforementioned chemical composition is, in mass%, Cu: 0.01-2.00%, Ni: 0.01 to 3.00%, Cr: 0.01-1.00%, Mo: 0.01-1.00%, W: 0.01-1.00%, V: 0.01 to 1.00%, B: 0.0003 to 0.0030%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0050%, and REM: 0.0001-0.0200% The steel material according to claim 1, which contains one or more selected from the group consisting of the following.

3. In the metallographic structure at a position 1 / 4 t from the surface of the steel material, the number density of precipitates is 15 particles / μm ２ The steel material according to claim 1, wherein the equivalent circular diameter of the precipitate is 3 to 500 nm.

4. In the metallographic structure at a position 1 / 4 t from the surface of the steel material, the number density of precipitates with one or more dislocations within a 50 nm radius is 10 particles / μm ２ The steel material described in claim 1 is as described above.

5. A steel structure comprising the steel material described in any one of claims 1 to 4.

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