steel

Abstract

To provide a steel material excellent in fatigue resistant crack propagation properties even in one of a thickness direction and a direction orthogonal thereto and having high strength.SOLUTION: The chemical composition of a steel material includes 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 a remainder:Fe and impurities. Ceq is 0.25-0.55; a metallographic structure at a position of 1 / 4t from the surface of the steel material includes 15% of one or more kinds selected from bainite, martensite and high distortion ferrite by total area%; and in a tensile test using a test piece sampled so that the rolling direction of the steel material is coincident with the longitudinal direction, the ratio σSL / σSU of a lower yield point σSL to an upper yield point σSU is 0.97 or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to steel materials.

Background Art

[0002] In recent years, when constructing welded structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks, high-strength steel materials have been increasingly applied in order to rationalize the design, reduce the weight of the steel materials used, and save labor in welding work by making the thickness thinner. For this reason, the high-strength steel materials to be applied are required to have excellent fatigue resistance characteristics in addition to excellent ductility in order to ensure structural safety.

[0003] In welded structures, there are many cases where fatigue cracks occur at the welded termination ends and propagate through the steel materials of the welded structures and cause fracture (fatigue fracture). This is considered to be due to the fact that the welded termination ends are likely to become stress concentration parts due to their shapes, and in addition, tensile residual stress occurs after welding.

[0004] Even if a fatigue crack occurs, if the crack propagation speed in the subsequent steel material can be reduced, the fatigue life of the welded structure can be extended. For this reason, there is a strong demand for improving 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. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2008-214646 [Patent Document 2] Japanese Patent Publication No. 2010-242211 [Overview of the project] [Problems that the invention aims to solve]

[0008] The techniques described in Patent Documents 1 and 2 involve strong processing in the α-γ two-phase region or the ferrite single-phase region to develop a texture in a specific orientation.

[0009] Furthermore, while the technologies described in Patent Documents 1 and 2 can reduce the fatigue crack propagation rate in the thickness direction of the plate, they do not consider reducing the fatigue crack propagation rate in directions other than the thickness direction. In actual welded structures, fatigue cracks propagate not only in the thickness direction of the steel material but also in the width and length directions, and in many cases this leads to failure. The technologies described in Patent Documents 1 and 2, while drastically reducing the fatigue crack propagation rate in one direction, inherently contain the possibility that, for example, fatigue cracks in the length direction of the steel plate may be accelerated and propagate, leading to ultimate failure.

[0010] The present invention aims to solve the problems of the prior art and provide a steel material that has excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, and has high strength. [Means for solving the problem]

[0011] This invention was made to solve the above-mentioned problems and is summarized in the following steel materials. In this invention, "steel materials" include thick steel plates, steel pipes, structural steel, thin steel plates, etc.

[0012] (1) The chemical composition of the steel material is, in 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%, The remainder consists of Fe and impurities. The carbon equivalent Ceq value, as defined by equation (i) below, is between 0.25 and 0.55. In a cross-section of the steel material parallel to the rolling direction and thickness direction, 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 15% or more of one or more materials selected from the group consisting of bainite, martensite, and high-strain ferrite having a KAM value of 0.5° or higher, in total area percentage. In a tensile test using a test specimen taken such that the rolling direction of the steel material coincides with the longitudinal direction, the upper yield point σ SU and the lowering point σ SL The ratio σ SL / σ SU The value is 0.97 or higher. Steel material. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 ···(i) However, the element symbols in the above formula represent the content (mass %) of each element, and zero is used if an element is not present.

[0013] (2) The chemical composition satisfies the following formula (ii): The steel materials described in (1) above. 0.5 ≤ Ti / N ≤ 5.0 ···(ii) However, the element symbols in the above formula represent the content (mass %) of each element.

[0014] (3) The chemical composition is such that a portion of the Fe is replaced by, in mass%, Cu: 2.00% or less, Ni: 3.00% or less, Cr: 1.00% or less, Mo: 1.00% or less Nb: 0.060% or less, V: 1.00% or less, and B: 0.0030% or less, It contains one or more selected from the group consisting of the following: The steel materials described in (1) or (2) above.

[0015] (4) The chemical composition is such that a portion of the Fe is replaced by, in mass%, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.010% or less, It contains one or more selected from the group consisting of, The following equation (iii) is satisfied: The steel material described in any of the above (1) to (3). 0.0005≦Ca+Mg+REM≦0.0080 (iii) However, the element symbols in the above formula represent the content (mass %) of each element, and zero is used if an element is not present.

[0016] (5) The average dislocation density at a position 1 / 2t from the surface of the steel material is 3.0 × 10 14 / m 2 That's all. The steel material described in any of the above (1) to (4). [Effects of the Invention]

[0017] According to the present invention, it is possible to obtain a steel material that has excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, and has high strength. [Brief explanation of the drawing]

[0018] [Figure 1] This figure shows the dimensions of the CT test specimen used for fatigue crack extension testing in the in-plate direction. [Figure 2] 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]

[0019] The inventors of this invention have conducted various studies on the relationship between the microstructure of steel and its fatigue crack propagation characteristics. As a result, they found that when the microstructure near the fatigue crack tip softens repeatedly, the crack closing load increases, and the crack propagation speed can be reduced. Furthermore, they found that by ensuring a certain percentage of microstructure with high dislocation density and suppressing the occurrence of upper yield in tensile tests, the microstructure softens with repeated loading, and the crack propagation speed can be significantly reduced regardless of the crack propagation direction.

[0020] The inventors hypothesize that mobile dislocation density is involved in this mechanism. They believe that repeated softening characteristics depend on the density of mobile dislocations among the dislocations in the microstructure, and that by making the mobile dislocation density sufficiently high, the microstructure at the fatigue crack tip repeatedly softens, making it possible to improve fatigue crack propagation resistance regardless of the crack propagation direction. Microstructures with high dislocation density can be identified by EBSD, which will be described later, but it is difficult to distinguish between mobile and immobile dislocations. Therefore, direct measurement of mobile dislocation density is difficult, but they have come to hypothesize that the proportion of microstructures with high dislocation density and the presence or absence of upper yield in tensile tests can serve as indicators of mobile dislocation density.

[0021] Furthermore, the inventors have discovered that by transforming austenite at a low temperature and adjusting the temperature history after the transformation to an appropriate range, steel sheets can be obtained without compromising productivity, in which the proportion of structures with high dislocation density is above a certain level and the occurrence of upper yield in tensile tests is suppressed. Moreover, they have found that by adjusting the chemical composition and manufacturing conditions to an appropriate range, desired strength, ductility, and toughness can be stably obtained regardless of the thickness of the steel material.

[0022] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0023] (A) Chemical composition The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0024] 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 weldability and joint toughness, and to suppress the decrease in the amount of movable dislocations that contribute to repeated softening, the C content should be 0.30% or less. Preferably, the C content is 0.04% or more, and preferably 0.20% or less.

[0025] Si: 0.03~0.60% Si is an inexpensive deoxidizing element that contributes to increased strength through solid solution strengthening. To obtain this effect, the Si content should be 0.03% or higher. On the other hand, to suppress the decrease in weldability and joint toughness, the Si content should be 0.60% or lower. For steel materials with strict requirements for weldability or base material and joint toughness, the Si content is preferably 0.03% or higher, and preferably 0.50% or lower.

[0026] Mn: 0.50~2.50% Mn is an effective element for improving 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 weldability and joint toughness, the Mn content should be 2.50% or less. Preferably, the Mn content is 0.80% or more, and more preferably 0.90% or more. Furthermore, preferably, the Mn content is 2.00% or less, and more preferably 1.80% or less.

[0027] P:0.030% or less P is inevitably present in steel. Since P promotes embrittlement, 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 is preferable.

[0028] S: 0.010% or less S is an unavoidable impurity and exists in steel as a sulfide inclusion. S significantly degrades mechanical properties, particularly ductility and toughness. Therefore, the S content should be 0.010% or less. To ensure ductility and toughness, a lower S content is desirable, and a S content of 0.005% or less is preferable. However, since reducing S leads to increased costs, the S content may be 0.001% or more.

[0029] Al: 0.002~0.050% Al is a deoxidizing element and, through AlN, is an effective element for refining the austenite grain size. To achieve these effects, the Al content should be 0.002% or more. On the other hand, to suppress the formation of inclusions that are detrimental to the surface quality and toughness of the steel billet, the Al content should be 0.050% or less. Preferably, the Al content is 0.020% or more, and preferably 0.040% or less.

[0030] N: 0.0010~0.0080% N, together with Al, forms nitrides and is an element effective in refining austenite grain size. To achieve this effect, the N content should be 0.0010% or more. On the other hand, to suppress embrittlement and a decrease in elongation properties due to solid-solution N, the N content should be 0.0080% or less. Preferably, the N content is 0.0015% or more. Furthermore, preferably, the N content is 0.0060% or less, and more preferably, 0.0050% or less.

[0031] Ti: 0.003~0.030% Ti contributes to improved ductility even in trace amounts. To achieve this effect, the Ti content should be between 0.003% and 0.030%. Preferably, the Ti content is 0.006% or more, and preferably 0.020% or less.

[0032] Furthermore, by setting the ratio of Ti content to N content to 0.5 or higher, it is possible to reduce dissolved N, improve elongation properties, and prevent the occurrence of surface defects in the slab. In addition, by setting the ratio of Ti content to N content to 5.0 or lower, the formation of TiC can be suppressed, improving elongation properties. Therefore, in order to obtain excellent ductility, it is preferable that the Ti content satisfies the following equation (ii) in relation to the N content. 0.5 ≤ Ti / N ≤ 5.0 ···(ii) However, the element symbols in the above formula represent the content (mass %) of each element.

[0033] In the chemical composition of the steel material of the present invention, in addition to the elements mentioned above, at least one element selected from the group consisting of Cu, Ni, Cr, Mo, Nb, V, and B may be included within the range shown below for the purpose of improving strength. The reasons for limiting each element will be explained below.

[0034] Cu:2.00% or less Cu may be included as needed, as it contributes to increased strength through solid solution and is also effective in improving overall corrosion resistance and localized corrosion resistance. However, if the Cu content is excessive, adverse effects such as increased surface cracking of the steel billet and deterioration of joint toughness will become apparent. Therefore, the Cu content should be 2.00% or less. Preferably, the Cu content is 1.50% or less, and more preferably less than 1.00%. If the above effects are to be obtained more reliably, preferably the Cu content is 0.01% or more, and more preferably 0.03% or more.

[0035] Ni: 3.00% or less Ni is effective in ensuring strength and improving toughness, and also effectively contributes to improving the hot brittleness that occurs when Cu is added, so it may be included as needed. However, if the Ni content is excessive, weldability will decrease and costs will increase. For this reason, the Ni content should be 3.00% or less. Preferably, the Ni content should be 2.00% or less. If the above effects are to be obtained more reliably, the Ni content should be 0.01% or more, and more preferably 0.03% or more.

[0036] Cr:1.00% or less Cr is an element that improves hardenability, contributes to increased strength, and also contributes to improved weather resistance, so it may be included as needed. However, if the Cr content is excessive, weldability and toughness will decrease. For this reason, the Cr content should be 1.00% or less. Preferably, the Cr content is 0.50% or less. If the above effects are to be obtained more reliably, the Cr content is preferably 0.01% or more, and more preferably 0.03% or more.

[0037] Mo: 1.00% or less Mo is an element that contributes to increased strength, so it may be included as needed. However, excessive Mo content leads to a decrease in weldability and toughness, as well as an increase in cost. Therefore, the Mo content should be 1.00% or less. Preferably, the Mo content is 0.50% or less. To more reliably obtain the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.

[0038] Nb: 0.060% or less Nb, when added in small amounts, contributes to microstructure refinement and is an effective element for ensuring the strength of the base material; therefore, it may be included as needed. However, if the Nb content is excessive, it hardens the weld and significantly deteriorates its toughness. For this reason, the Nb content should be 0.060% or less. Preferably, the Nb content is 0.030% 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.

[0039] V:1.00% or less V contributes to increased strength through precipitation strengthening and may be included as needed. However, excessive V content may impair joint toughness. Therefore, the V content should be 1.00% or less. Preferably, the V content is 0.50% or less. To more reliably obtain the above effects, the V content is preferably 0.01% or more, and more preferably 0.03% or more.

[0040] B: 0.0030% or less B may be included as needed, as its small addition enhances hardenability and contributes to improving the strength of the base material. However, excessive B content degrades elongation and joint toughness. Therefore, the B content should be 0.0030% or less. To more reliably obtain the above effects, a B content of 0.0003% or more is preferable.

[0041] In the chemical composition of the steel material of the present invention, in addition to the elements mentioned above, at least one element selected from the group consisting of Ca, Mg, and REM may be included within the range shown below, for the purpose of improving toughness and ductility. The reasons for limiting each element will be explained below.

[0042] Ca: 0.010% or less Mg: 0.010% or less REM: 0.010% or less Ca, Mg, and REM may be included as needed, as they all suppress the formation of coarse inclusions (such as stretched MnS) by forming sulfides. On the other hand, if the content of any of these is excessive, the effect will saturate, and coarse oxides or sulfides will form, degrading toughness and elongation. Therefore, the content of Ca, Mg, and REM should each be 0.010% or less. Preferably, the content of Ca, Mg, and REM should each be 0.008% or less.

[0043] To obtain excellent ductility, it is preferable that the total content of these elements be 0.0005% or more. Furthermore, from the viewpoint of preventing deterioration of toughness and elongation properties due to coarse oxides or sulfides, it is preferable that the total content of these elements be 0.0080% or less.

[0044] In other words, it is preferable that the following formula (iii) is satisfied. The total content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. Furthermore, the total content is more preferably 0.0060% or less, and even more preferably 0.0040% or less. 0.0005≦Ca+Mg+REM≦0.0080 (iii) However, the element symbols in the above formula represent the content (mass %) of each element, and zero is used if an element is not present.

[0045] In this invention, REM refers to a total of 17 elements including Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. Industrially, lanthanides are added in the form of mischmetal.

[0046] Ceq: 0.25~0.55 The steel material of the present invention has the above-described composition, and furthermore, as an indicator of hardenability and weldability, the value of the carbon equivalent Ceq, defined by the following formula (i), is set to 0.25 to 0.55. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 ···(i) However, the element symbols in the above formula represent the content (mass %) of each element, and zero is used if an element is not present.

[0047] From the viewpoint of obtaining the desired microstructure in the accelerated cooling process, the carbon equivalent Ceq defined by equation (i) above should be set to 0.25 or higher. Furthermore, from the viewpoint of suppressing a decrease in ductility, toughness, and weldability, the Ceq should be set to 0.55 or lower. Preferably, the Ceq is 0.27 or higher, and preferably 0.45 or lower.

[0048] In the chemical composition of the steel material of the present invention, the remainder is Fe and impurities. Here, "impurities" refers 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, and which are acceptable as long as they do not adversely affect the present invention. For example, an impurity of O: 0.01% or less is acceptable.

[0049] (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 in the rolling direction, 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.

[0050] Bainite, martensite, and high-strain ferrite: more than 15% in total In the present invention, the mobile dislocations necessary to obtain the desired cyclic softening characteristics are introduced by transforming austenite at low temperatures. If the total area ratio of one or more microstructures selected from the group consisting of bainite, martensite, and high-strain ferrites having a KAM (Kernel Average Misorientation) value of 0.5° or higher (hereinafter, these microstructures are collectively referred to as "high dislocation density microstructures") is less than 15%, the mobile dislocation density will be insufficient, the desired cyclic softening characteristics will not be exhibited, and excellent fatigue crack propagation resistance cannot be obtained. Therefore, the total area ratio of the high dislocation density microstructures should be 15% or higher. Preferably, the area ratio of the high dislocation density microstructures is 30% or higher, and may be 100%. High-strain ferrites having a KAM value of 0.5° or higher include bainite ferrite, Widmannstetten ferrite, and acicular ferrite.

[0051] The remaining tissue consists of low-strain ferrite and pearlite with a KAM value of less than 0.5°, and may also contain retained austenite (retained gamma), etc.

[0052] 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 in the rolling direction of the sample (the so-called L-direction cross section) is observed.

[0053] 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 2On the above area, electron backscattering diffraction (EBSD) analysis is performed using a field emission scanning electron microscope (FE-SEM), and the local orientation difference around each measurement point is mapped by KAM. Next, after nitriding the area measured by EBSD, carbide precipitation was observed with an FE-SEM (observation magnification: 1000 times).

[0054] The KAM method is a method in which the orientation differences between six adjacent pixels (first approximation), twelve outer pixels (second approximation), and eighteen outer pixels (third approximation) of a certain regular hexagonal pixel in the measurement data are averaged, and the value is taken as the local orientation difference (KAM value) of the pixel at the center, and this calculation is performed for each pixel.

[0055] In the present invention, the measurement step was set to 0.2 μm, and a region where the KAM value of the third approximation is 1° or more and no carbide is precipitated is defined as martensite, a region where the KAM value is 1° or more and carbide is precipitated is defined as bainite, and a region where the KAM value is 1° or less and 0.5° or more is defined as high-strain ferrite.

[0056] σ SL / σ SU : 0.97 or more As described above, in order for the structure near the fatigue crack tip to repeatedly soften, it is particularly necessary to increase the mobile dislocation density. And in order to increase the mobile dislocation density, in addition to setting the area ratio of the high dislocation density structure within the above range, in the tensile characteristics in the rolling direction, the upper yield point σ SU and the lower yield point σ SL The ratio σ SL / σ SU must be 0.97 or more. When σ SL / σ SU is less than 0.97, the dislocations are immobilized, and it becomes difficult to obtain the repeated softening characteristics necessary for excellent fatigue crack growth resistance characteristics.

[0057] The upper yield point σ SU , and the lower yield point σ SLThe measurements shall be taken in accordance with JIS Z 2241:2011. Specifically, for steel plates, a No. 1B tensile test specimen shall be used, taken parallel to the rolling direction from a position 1 / 2t from the surface and 1 / 4W from the end face of the steel plate, where the plate thickness is t and the width is W. For steel pipes, a No. 14B test specimen shall be used, taken perpendicular to the rolling direction from a position 1 / 2t from the surface of the steel material. Furthermore, for structural steel, the plate thickness of the flange shall be t. f When the width is F, 1 / 2t from the surface of the flange f A specimen No. 1B, taken from the position and 1 / 2F from the outside of the flange in a direction perpendicular to the rolling direction, will be used. If the upper yield point and lower yield point are not observed, σ SL / σ SU We consider this to be = 1.0.

[0058] Average dislocation density: 3.0 × 10⁻⁶ 14 / m 2 That's all. As mentioned above, in order to increase the mobile dislocation density, a certain amount or more of high dislocation density structure is secured. A high proportion of high dislocation density structure means an increase in dislocation density in the metal structure. In other words, in order to more reliably increase the mobile dislocation density, the average dislocation density should be 3.0 × 10⁻⁶. 14 / m 2 It is preferable that the above is true. There is no need to set an upper limit on the average dislocation density, but if it is excessively high, the ductility may deteriorate significantly. Therefore, the average dislocation density should be 14.0 × 10⁻⁶. 14 / m 2 The following is preferable:

[0059] The average dislocation density can be determined by the following method. First, a test specimen for dislocation density measurement is taken from the steel material so that the thickness direction of the test specimen coincides with the thickness direction of the steel material. Depending on the shape of the steel material, the size of the test specimen is, for example, 20 mm wide x 20 mm long x 2 mm thick. In this case, the measurement surface of the test specimen is a surface of 20 mm wide x 20 mm long. In the case of steel plates, the test specimen is taken from a position 1 / 2t from the surface and 1 / 4W from the end face of the steel plate. In the case of steel pipes, the test specimen is taken from a position 1 / 2t from the surface. In the case of structural steel, it is taken from 1 / 2t from the surface of the flange.f A test specimen is taken from the position and from a position 1 / 2F from the outside of the flange.

[0060] The observation surface of the test specimen is mirror-polished, and then electropolished using 10% by volume perchloric acid (acetic acid solvent) to remove processing strain from the surface. The full width at half maximum (FWHM) β of the peaks of the (110), (211), and (220) planes of the body-centered cubic structure (iron) is determined from the treated measurement surface by X-ray diffraction (XRD).

[0061] In XRD, the full width at half maximum (FWHM) β is measured using a CoKα radiation source, a tube voltage of 30kV, and a tube current of 10mA. Furthermore, LaB (lanthanum hexaboride) powder is used to measure the FWHM originating from the X-ray diffractometer.

[0062] The heterogeneous strain ε of the specimen is determined from the full width at half maximum β obtained by the method described above and the Williamson-Hall equation (equation (I) below). β×cosθ / λ=0.9 / D+2ε×sinθ / λ (I) Here, θ: diffraction angle (rad), λ: X-ray wavelength (nm), D: crystallite size (nm)

[0063] Furthermore, using the obtained heterogeneous strain ε and equation (II) below, the average dislocation density ρ(m -2 ) can be calculated. ρ = 14.4 × ε 2 / b 2 ...(II) Here, b: Burgers vector of body-centered cubic structure (iron) (b = 0.248 (nm))

[0064] The steel material in this invention exhibits excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, and possesses high strength.

[0065] In this invention, "high strength" refers to a tensile strength TS: 490 MPa or higher. Furthermore, "excellent fatigue crack propagation characteristics" means that, regardless of the crack propagation direction, the fatigue crack propagation rate da / dN is at least ΔK: 15 MPa·m0.5 1.60 × 10 -8 (m / cycle) or less, ΔK: 25MPa m 0.5 8.0 x 10 -8 This refers to cases where the value is less than or equal to (m / cycle).

[0066] 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.

[0067] Repeated softening rate: 0.96 or less The steel material according to the present invention is subjected to a cyclic stress-strain curve measurement test with a maximum tensile and compressive strain of ±0.010, a cyclic strain rate of 0.8% / s, and a wavenumber of 10 up to the maximum strain, followed by a wavenumber of 10 from the maximum strain to zero strain, repeated 20 times. The results are the stress σ1 at the first maximum strain and the average stress σ at the maximum strain from the 15th to the 20th cycle. 15-20 The ratio σ 15-20 It is preferable that the repeated softening characteristics are such that the repeated softening rate, indicated by / σ1, is 0.96 or less.

[0068] In the case of steel plates, the cyclic softening rate is measured by a cyclic stress-strain curve measurement test in the rolling direction and width direction at a position 1 / 4W from the end face of the steel plate and 1 / 2t from the surface of the steel plate, where W is the width and t is the thickness of the steel plate, respectively. In the case of steel pipes, the cyclic softening rate is measured by a cyclic stress-strain curve measurement test in the rolling direction at a position 1 / 2t from the surface of the steel pipe. In the case of structural steel, the measurement is taken at a position 1 / 2t from the surface of the flange. f The cyclic softening rate is measured by a cyclic stress-strain curve measurement test in the rolling direction and flange height direction at the position and at a position 1 / 6F from the outside of the flange.

[0069] In the aforementioned cyclic stress-strain curve measurement test, the strain applied to the steel material was a bidirectional waveform with alternating tension and compression, and the strain range after the gradual increase in strain was set to 0.024 using a gradually increasing / decreasing waveform. The cyclic strain rate was set to 0.8% / s through strain control. In the gradual increase process, the maximum strain was reached after 12 waves, and in the gradual decrease process, the strain amount was set to 0 after 12 waves. This gradual increase / decreasing process was considered one block, and the stress corresponding to the maximum strain in the first block was defined as stress σ1, and the average value of the stress corresponding to the maximum strain in each block from the 15th to the 20th block was defined as σ 15-20 Let's assume that.

[0070] The repeatable softening rate σ obtained in this way 15-20 / σ1 is a stress intensity factor with a stress intensity factor range of ΔK of 15 MPa·m 0.5 From 25 MPa·m 0.5 The microstructure exhibits a good correlation with the repeated softening characteristics of fatigue cracks propagated under these conditions, particularly near the crack tip. Here, "near the fatigue crack tip" refers to the repeated plastic region of the crack tip where tensile plastic deformation occurs under maximum load and compressive plastic deformation occurs under minimum load.

[0071] Repeated softening near the fatigue crack tip increases the stress intensity factor Kcl at which the fatigue crack closes, thus slowing down the fatigue crack propagation rate. To obtain excellent fatigue crack propagation resistance regardless of the crack propagation direction, the repeated softening rate σ in the rolling direction and width direction is important. 15-20 It is preferable that / σ1 is 0.96 or less.

[0072] The reduction in fatigue crack propagation due to repeated softening almost saturates when the repeated softening rate is 0.94 or less. Therefore, the desirable range in which repeated softening can be utilized to the fullest extent is the repeated softening rate σ 15-20 The / σ1 is 0.94 or less. However, if the repeated softening rate is excessively low, the steel may become brittle, so it is preferable that it be 0.80 or higher.

[0073] (D) Manufacturing method 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 process and accelerated cooling process described later on the resulting steel material. Each process will be explained below.

[0074] 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 a cumulative reduction ratio of 50 to 75% within a temperature range of 900°C or less above Ar3. The temperature refers to the surface temperature of the steel material.

[0075] Heating temperature: 900~1300℃ If the steel material is heated to a temperature 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 reheating temperature of the steel material should be in the range of 900 to 1300°C.

[0076] Cumulative reduction ratio in the temperature range above Ar3 and below 900°C: 50-75% The reheated steel material is hot-rolled to achieve the desired plate thickness and shape. To improve ductility, after rough rolling, finish rolling is performed under conditions where the surface temperature of the steel billet exceeds Ar3 and is within a temperature range of 900°C or less, and the cumulative reduction ratio is 50-75%.

[0077] Ar3 is the temperature at which ferrite transformation begins when steel is cooled, and can be calculated using equation (v) below. Here, the higher the value of Ar3 in the steel composition, the higher the temperature at which ferrite transformation occurs, which reduces the dislocation density within the ferrite grains and improves the elongation properties, but deteriorates the fatigue crack propagation properties. Therefore, it is preferable that Ar3 be 820°C or lower. Ar3=910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo...(v) However, the element symbols in the above formula represent the mass percentage of each element, and elements that are not present are calculated as 0.

[0078] If the finishing rolling temperature is below Ar3, two-phase rolling occurs, forming stretched ferrite and degrading the fatigue crack propagation characteristics in the direction perpendicular to the thickness direction. On the other hand, if the finishing rolling temperature is above 900°C, recrystallization rolling occurs, causing the ferrite to coarseen and degrading the strength-ductility balance.

[0079] Furthermore, when the cumulative reduction ratio exceeds 50%, the number of ferrite nucleation sites in austenite increases, and by refining the ferrite, elongation and strength are improved in a balanced manner. On the other hand, productivity deteriorates when the cumulative reduction ratio exceeds 75%. Therefore, the cumulative reduction ratio should be kept between 50% and 75%. A cumulative reduction ratio of 55% to 65% is preferable.

[0080] Next, in the accelerated cooling process, from the temperature range of Ar3-50℃ or higher, T is calculated using the following equation (iv). f The material is cooled to a temperature range below ℃, and the cooling is terminated when the cumulative tempering parameter is 16500 or less. The temperature at this point is also the surface temperature of the steel material. T f =511+254×C+18.2×Mn+10.6×Ni+7.26×Cr+4.5×Mo...(iv) In the above formula, the element symbols represent the mass percentage of each element, and elements that are not present are calculated as 0.

[0081] Also, from the starting temperature of accelerated cooling, T f The average cooling rate to °C shall be, for example, greater than 25 °C / s and less than or equal to 70 °C / s. The average cooling rate shall be the average value in the thickness direction of the steel material. If the cooling rate inside the steel material is slow, the steel material will not harden sufficiently, leading to a decrease in dislocation density, as well as σ SL / σ SUThis may decrease. In that case, the fatigue crack propagation rate of the steel may decrease. Therefore, considering not only the cooling rate of the surface but also the cooling rate inside the steel, the average cooling rate in the thickness direction of the steel should be set to be greater than 25°C / s and less than or equal to 70°C / s, as described above. The cooling rate inside the plate thickness is determined from the surface temperature of the steel by heat transfer analysis.

[0082] Accelerated cooling start temperature: Ar3-50℃ or higher If the accelerated cooling start temperature is below Ar3-50°C, the microstructure becomes predominantly ferrite and pearlite, making it impossible to secure the desired high strength. Furthermore, the dislocation density decreases, and the fatigue crack propagation characteristics deteriorate.

[0083] Accelerated cooling stop temperature: T f Below ℃ The stop temperature for accelerated cooling is T f In temperatures above °C, dislocation recovery and pinning of dislocations by solid-solution carbon occur, preventing the desired repeated softening characteristics from being achieved and degrading the fatigue crack propagation characteristics. Therefore, the accelerated cooling stop temperature is set to T f The temperature should be below ℃. More preferably, T f The temperature should be -30°C or lower. There is no particular lower limit to the stop temperature, but temperatures below 200°C will lead to a deterioration of ductility and a decrease in productivity, so temperatures of 200°C or higher are preferable if you want to improve ductility or productivity.

[0084] Average cooling rate: greater than 25°C / s and less than or equal to 70°C / s From the starting temperature of accelerated cooling, T f If the average cooling rate to °C is below 25 °C / s, the microstructure becomes predominantly ferrite and pearlite, making it impossible to achieve the desired high strength. Furthermore, the dislocation density decreases, and the fatigue crack propagation characteristics deteriorate. On the other hand, exceeding 70 °C / s degrades ductility and toughness. Therefore, the average cooling rate should be between 25 °C / s and 70 °C / s.

[0085] Cumulative tempering parameter: 16500 or less Dislocations introduced by the transformation during the accelerated cooling process undergo recovery and pinning by solid-solution carbon during the accelerated cooling process and the subsequent thawing process. In accelerated cooling, T f If the cumulative tempering parameter LMP exceeds 16500 during the cooling process below °C and the radiating process after accelerated cooling, significant dislocation recovery and dislocation pinning by solid solution carbon will occur, preventing the desired repeated softening characteristics from being secured and degrading the fatigue crack propagation characteristics. Therefore, in accelerated cooling, T f The integrated tempering parameter LMP during the cooling process after the temperature drops below °C and during the dissipation cooling process after accelerated cooling shall be 16500 or less. More preferably, it shall be 15000 or less.

[0086] The cumulative tempering parameter LMP can be determined by the following method: In accelerated cooling, T f The temperature history of the subsequent cooling process and the dissipation process after accelerated cooling is measured, starting from the point when the temperature falls below °C. Then, the integrated equivalent tempering time τ(h) is calculated from equations (III) and (IV) below. Δτ i =10 (Ti+273) / (Tf+273)(20logti)-20 ...(III) τ = ΣΔτ i ...(IV) However, the meaning of each symbol in the formula is as follows: t i : Sampling time (h) T i :Average temperature (°C) for each sampling time Δτ i : Equivalent tempering time (h) for each sampling time

[0087] Furthermore, the integrated tempering parameter LMP can be determined using the calculated integrated equivalent tempering time τ and equation (V) below. LMP=T f (20 + logτ) ···(V)

[0088] In addition, in this invention, a tempering step may be performed after the accelerated cooling step or quenching step described above is completed. The tempering step may be performed online immediately after the accelerated cooling step is completed without cooling to room temperature, or it may be performed by cooling to room temperature and then reheating on a separate line.

[0089] The tempering process involves performing a tempering treatment so that the cumulative tempering parameter LMP is between 11,000 and 16,000. This allows for adjustment of strength, ductility, and toughness. If the LMP is less than 11,000, tempering treatment will have little effect on ductility and toughness. On the other hand, if the LMP is 16,000 or higher, dislocation recovery and pinning of dislocations by solid solution carbon will occur, making it impossible to ensure the desired repeated softening characteristics and degrading fatigue crack propagation characteristics. For this reason, the tempering parameter LMP in the tempering treatment is limited to between 11,000 and 16,000.

[0090] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0091] Table 1 shows the chemical composition of the test steels used in the examples. Each test steel was formed into a billet by ingot formation followed by bloc rolling or continuous casting.

[0092] Steel plates were manufactured from the obtained steel billets under the conditions shown in Table 2. Although steel plates are used in this embodiment, the invention is not limited to steel plates, and the same applies to steel pipes or structural steel.

[0093] [Table 1]

[0094] [Table 2]

[0095] The obtained steel sheets were subjected to metallographic observation using the following method, and the area percentage of each microstructure was measured. First, in the rolling direction cross-section of the steel sheet, with the width and thickness of the steel sheet being W and t respectively, test specimens for metallographic observation were cut from a position 1 / 4W from the end face and 1 / 4t from the surface of the steel sheet. The rolling direction cross-section of the test specimens 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 0.2 μm to measure and map the KAM. From the obtained KAM map, the microstructure was identified, and the area percentage of the high dislocation density microstructure was calculated using commercially available image analysis software.

[0096] Furthermore, test specimens were taken from the steel plate and subjected to average dislocation density evaluation tests, tensile tests, repeated softening rate evaluation tests, and fatigue crack propagation tests. The test methods were as follows.

[0097] (1) Mean dislocation density evaluation test A test specimen measuring 20 mm wide × 20 mm long × 2 mm thick was taken from a position 1 / 2 t from the surface of the steel plate for measuring dislocation density. The 20 mm wide × 20 mm long surface was mirror-polished, and then electropolished using 10 vol% perchloric acid (acetic acid solvent) to remove processing strain from the surface. Then, the full width at half maximum (FWHM) β of the peaks of the (110), (211), and (220) planes of the body-centered cubic structure (iron) was determined by X-ray diffraction on the treated measurement surface. From the determined FWHM β, the heterogeneous strain ε of the test specimen was determined from equation (I) below, and the average dislocation density ρ(m) was calculated using equation (II). -2 ) was calculated. β×cosθ / λ=0.9 / D+2ε×sinθ / λ (I) ρ = 14.4 × ε 2 / b 2 ...(II) However, the meaning of each symbol in the above formula is as follows: θ: Diffraction angle (rad) λ: Wavelength of X-ray (nm) D: Crystallite diameter (nm) b: Burgers vector of body-centered cubic structure (iron) (b=0.248(nm))

[0098] (2) Tensile test Tensile testing was conducted in accordance with JIS Z 2241:2011. A No. 1B tensile test specimen was taken from a position 1 / 2t from the surface of the steel plate, with the tensile direction parallel to the rolling direction, and the tensile test was performed to determine the upper yield point σ. SU and the lowering point σ SL ratio σ SL / σ SU The 0.2% yield strength (YS), tensile strength (TS), and total elongation (t-EL) were calculated. A "○" was given if the tensile strength (TS) was 490 MPa or higher, and the total elongation (t-EL) was 15% or higher for steel plate thicknesses of 5 mm to 10 mm, 16% or higher for steel plate thicknesses of 10 mm to 15 mm, 17% or higher for steel plate thicknesses of 15 mm to 20 mm, 18% or higher for steel plate thicknesses of 20 mm to 25 mm, 19% or higher for steel plate thicknesses of 25 mm to 30 mm, 20% or higher for steel plate thicknesses of 30 mm to 40 mm, and 21% or higher for steel plate thicknesses of 40 mm to 50 mm. All other cases were marked with a "×".

[0099] (3) Repeated softening rate evaluation test Two types of round bar test specimens were taken from a position 1 / 2t from the surface of the steel plate, with a diameter of 10 mm and a parallel length of 30 mm, and the load direction being perpendicular to the rolling direction (plate width direction), and the load direction being in the rolling direction. A high-precision extensometer was attached to the parallel section of the test specimens, and axial force loading with gradually increasing and decreasing strain waveforms was applied using an electro-hydraulic servo fatigue testing machine with strain control. The strain range after the gradually increasing strain was set to 2.0%, with the maximum strain reached after 10 waves in the gradually increasing process and the strain becoming zero after 10 waves in the gradually decreasing process. This gradually increasing and gradually decreasing process is considered as one set, and this set will be referred to as a "block" below. The stress corresponding to the maximum strain in the first block is stress σ1, and the average value of the stress corresponding to the maximum strain in each block from the 15th block to the 20th block is σ 15-20 The repeated softening rate σ 15-20 The / σ1 value was calculated. Other load conditions were as follows: Stress ratio: -1.0 • Environmental meter: Room temperature air • Gauge length: 25mm • Strain rate: 0.8% / s

[0100] (4) 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 in the rolling direction (plate length direction). For steel plates with a thickness of 25 mm or less, the entire thickness was used for sampling. For steel plates with a thickness exceeding 25 mm, both sides were reduced in thickness to 25 mm, centered at a point 1 / 2t from the surface of the steel plate. The specimen dimensions are shown in Figure 1, and the conditions for the fatigue crack propagation test using CT test specimens were as follows. Stress ratio: 0.1 • Test frequency: 15Hz • Environment: Room temperature, ambient air • Crack length measurement: Unloading elastic compliance method using back strain gauges Back gauge length: 2mm

[0101] To assess the fatigue crack propagation characteristics in the thickness direction, three-point bending test specimens were taken as shown in Figure 2, ensuring that the direction of fatigue crack propagation from the steel plate was in the thickness direction. For steel plates with a thickness of 10 mm or less, the entire thickness was used for sampling. For steel plates with a thickness exceeding 10 mm, the thickness was reduced on both sides, centering on a point 1 / 2t from the surface of the steel plate, to a thickness of 10 mm. 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

[0102] 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, ΔK: 25MPa m 0.5 The fatigue crack propagation rate in is 8.0 × 10 -8 We evaluated it as ○ if it was less than or equal to (m / cycle). All other cases were marked as ×.

[0103] These measurement results are shown in Tables 3 and 4. In Table 3, "cyclic softening rate" refers to the average value of the cyclic softening rate measured in the plate width direction and the rolling direction when the load direction is the plate width direction, and in Table 4, "crack propagation direction: plate inward direction" refers to the crack propagation velocity measured in the plate width direction and the rolling direction when the fatigue crack is propagating, and refers to the value in the direction with the largest value.

[0104] [Table 3]

[0105] [Table 4]

[0106] Tests No. 1-26 all involved steel materials manufactured according to the requirements of the present invention, using steel billets with the chemical composition of the present invention. These materials possessed both high strength and excellent ductility, and the fatigue crack propagation rate was independent of the crack propagation direction, with a stress intensity factor range ΔK of 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, 8.0 × 10 -8 The steel sheet satisfies the requirement of (m / cycle) or less and possesses excellent fatigue crack propagation resistance. In Test No. 35, the ductility deteriorated due to the high finishing temperature during the final rolling process, but the steel sheet still possesses excellent fatigue crack propagation resistance.

[0107] On the other hand, while test samples No. 27 and 28 satisfy the manufacturing requirements of the present invention, their chemical compositions fall outside the specified range. Sample No. 27 has a high carbon content and a repeated softening rate that does not satisfy the requirements of the present invention, resulting in poor crack propagation velocity in the in-plate direction. It also has a high carbon equivalent (Ceq), leading to poor ductility. Similarly, sample No. 28 has a low carbon equivalent (Ceq), resulting in a low area ratio of the high dislocation density structure, a low average dislocation density, and a repeated softening rate that does not satisfy the requirements of the present invention, thus resulting in poor crack propagation velocity.

[0108] Furthermore, in tests No. 29-34, although the chemical composition of the present invention was within the specified range, the crack propagation velocity was inferior because the manufacturing requirements were not met. In test No. 29, the water cooling stop temperature and LMP were high in the accelerated cooling process, in test No. 31, the average cooling rate in the accelerated cooling process was low, and in test No. 34, the LMP was high in the accelerated cooling process. Although the area ratio of the high dislocation density structure satisfied the requirements of the present invention, σ SL / σ SU However, the requirements of the present invention were not met. Furthermore, tests No. 29 and No. 31 also lacked sufficient tensile strength (TS).

[0109] Test No. 30 did not involve an accelerated cooling process, and the area ratio and σ of the high dislocation density structure were measured. SL / σ SU However, the requirements of the present invention were not satisfied. In tests No. 32 and 33, the finishing temperature in the finish rolling was low, so the area ratio of the high dislocation density structure did not satisfy the requirements of the present invention. As a result, in these examples, the repeated softening rate did not satisfy the provisions of the present invention, resulting in poor fatigue crack propagation characteristics in the in-plate direction. [Industrial applicability]

[0110] According to the present invention, it is possible to obtain a steel material that has excellent fatigue crack propagation characteristics in both the thickness direction and the direction perpendicular thereto, and has high strength. 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 of the steel material, in mass percent, C: 0.01-0.30%, Si: 0.03 to 0.60%, Mn: 0.50 to 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 to 0.030%, The remainder consists of Fe and impurities. The carbon equivalent Ceq value, as defined by equation (i) below, is between 0.25 and 0.

55. In a cross-section of the steel material parallel to the rolling direction and thickness direction, when the thickness of the steel material is t, the metallographic structure at a position 1 / 4 t from the surface of the steel material contains 30% or more of one or more materials selected from the group consisting of bainite, martensite, and high-strain ferrite having a KAM value of 0.5° or higher, in total area percentage. In a tensile test using a test specimen taken such that the rolling direction of the steel material coincides with the longitudinal direction, the upper yield point σ ＳＵ and the lowering point σ ＳＬ The ratio σ ＳＬ / σ ＳＵ The value is 0.97 or higher. Steel material. Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15...(i) However, the element symbols in the above formula represent the content (mass %) of each element, and zero is used if an element is not present.

2. The aforementioned chemical composition satisfies equation (ii) below, The steel material according to claim 1. 0.5≦Ti / N≦5.0...(ii) However, the element symbols in the above formula represent the content (mass %) of each element.

3. The aforementioned chemical composition is C: 0.08-0.30%, It contains, and In place of a portion of the aforementioned Fe, in mass%, Cu: 2.00% or less, Ni: 3.00% or less, Cr: 1.00% or less, Mo: 1.00% or less Nb: 0.060% or less, V: 1.00% or less, and B: 0.0030% or less, It contains one or more selected from the group consisting of the following: The steel material according to claim 1 or claim 2.

4. The aforementioned chemical composition, in place of a portion of the Fe, is expressed in mass % as follows: Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.010% or less, It contains one or more selected from the group consisting of, The following equation (iii) is satisfied: The steel material according to any one of claims 1 to 3. 0.0005≦Ca+Mg+REM≦0.0080...(iii) However, the element symbols in the above formula represent the content (mass %) of each element, and zero is used if an element is not present.

5. The average dislocation density at a position 1 / 2t from the surface of the steel material is 3.0 × 10⁻¹⁴ １４ / m ２ That's all. The steel material according to any one of claims 1 to 4.

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