Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present disclosure relates to steel material.
• JP-A Japanese Patent Application Laid-Open
• JP-A No. 2010-064110 JP-A No. 2012-057236
• JP-A No. 2012-255184 disclose steel materials in which corrosion resistance in an environment containing chloride ions (Cl - ions) is improved by the inclusion of Sn in amounts of from 0.005 to 0.3 mass%, from 0.02 to 0.40 mass%, and from 0.01 to 0.50 mass%, respectively.
• JP-A No. 2012-144799 discloses a steel material for an offshore structure containing Sn in amount of from 0.03 to 0.5 mass%, and composed of ferrite and a hard second phase.
• JP 5958102 relates to a corrosion-resistant steel material for a ship ballast tank which is obtained by heating a steel stock having a composition comprising, by mass, 0.03 to 0.20% C, 0.05 to 0.50% Si, 0.7 to 2.0% Mn, ⁇ _0.035% P, ⁇ _0.01% S, ⁇ 0.10% Al, 0.02 to 0.2% Sn, 0.003 to 0.03% Nb, 0.005 to 0.030% Ti and 0.0010 to 0.010% N, and in which the contents of Cu, Ni and Cr are controlled to less than 0.20%, less than 0.20% and less than 0.20%, respectively, and the balance Fe with inevitable impurities at 1,000 to 1,350°C, thereafter finishing rolling in the temperature range of 600 to less than 800°C, and performing cooling.
• JP 2014-001450 A concerns a steel for ship has a composition that contains, by mass%, C: 0.03 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.7 to 2.00%, Al: 0.100% or less, Ti: 0.005 to 0.030%, N: 0.0010 to 0.010% and Sn: 0.02 to 0.2% and further contains one element or two or more elements selected from Sb: 0.02 to 0.2%, W: 0.005 to 0.50%, Mo: 0.01 to 0.50% and Nb: 0.003 to 0.03%, wherein the remainder of the composition consists of Fe and inevitable impurities, and an area ratio of a ferrite phase is set to 3% or more, and a solid solution amount of Sn after cooling is set to 0.01% or more.
• WO 2011/102244 A1 relates to a steel material which has a chemical composition including, by mass%, 0.01-0.14% C, 0.04-0.6% Si, 0.5-2.0% Mn, 0.01% or less P, 0.003% or less S, less than 0.2% Cu, more than 0.0007% but not more than 0.005% B, less than 0.05% Al, less than 0.007% N, 0.003% O, and 0.03-0.50% Sn, with the remainder being Fe and impurities, and the Cu/Sn ratio being 1 or less, and which is characterised in that the Bq value is 0.003 or less, the Ceq value is 0.15-0.35, and the oxide number in the region which is 2 mm or less from the surface layer is 5 ⁇ 10 4 per 1 square mm.
• JP-A Nos. 2010-064110 , 2012-057236 , and 2012-255184 which disclose improvement of corrosion resistance by the addition of Sn, there remains room for further improvement in terms of toughness and fatigue characteristics, and a technique that satisfies all of the demands regarding corrosion resistance, toughness, and fatigue characteristics is still required.
• the corrosion resistance of steel is improved by regulating the Sn concentration ratio between the soft structure and the hard structure by dividing water cooling into two stages; however, there remains room for further improvement with respect to low-temperature toughness and fatigue characteristics.
• An object of the present invention is to provide a steel material which has excellent corrosion resistance as well as low-temperature toughness and fatigue characteristics.
• the present invention was made to achieve the above object with a steel material which is as defined in the claims.
• a numerical range expressed by "from x to y" or “between x and y” includes herein the values of x and y in the range as the minimum and maximum values, respectively.
• a steel material according to the invention consists of, in terms of percentage by mass, from 0.01 to 0.20% of C, from 0.01 to 1.00% of Si, from 0.05 to 3.00% of Mn, from 0 to 0.050% of P, from 0 to 0.0100% of S, from 0.05 to 0.25% of Sn, from 0 to 0.100% of Al, from 0.0005 to 0.0100% of N, from 0.0001 to 0.0100% of O, from 0 to 0.050% of Ti, from 0 to 0.050% of Nb, from 0 to 0.050% of V, from 0 to 0.050% of W, from 0 to 0.050% of Mo, from 0 to 0.10% of Cu, from 0 to 0.05% of Ni, from 0 to 0.10% of Cr, from 0 to 0.05% of Sb, from 0 to 0.0010% of B, from 0 to 0.0100% of Ca, from 0 to 0.0100% of Mg, from 0 to 0.0100% of REM, and
• a steel material of the invention When a steel material of the invention has the above composition, it can be a steel material excellent in corrosion resistance and also in low-temperature toughness and fatigue characteristics. Although the reason therefor is not very clear, it is presumed as follows.
• the present inventors prepared various steel plates with different Sn contents, and investigated the relationship between corrosion resistance and toughness. As a result, it has been found that as the Sn content increases, the corrosion resistance improves, but the absorbed energy (low temperature toughness) at 0°C in the Charpy impact test may deteriorate in some cases. For example, when the threshold of corrosion resistance in a SAE J 2334 test is set at 0.6 mm or less, and the threshold of absorbed energy at 0°C is set at 150 J or more, it has been found that not both can be easily satisfied stably.
• the inventors further studied diligently about a steel material excellent in all of corrosion resistance, low-temperature toughness, and fatigue characteristics, and as a result, the following findings were obtained.
• the steel material of the present is obtainable by a process including the steps set out in claim 1.
• a steel after the finish rolling is slowly cooled first, then held at a predetermined temperature for a certain period of time allowing recuperation. Thereafter by conducting strong-cooling to a temperature of 550°C or less, segregation of Sn at a grain boundary can be suppressed, and the Sn ratio can be regulated within the above range.
• the C content is an element for improving the strength of a steel material.
• the C content is excessive, the weldability is remarkably deteriorated.
• the formation amount of cementite that acts as a cathode to promote corrosion in a low pH environment increases, and the corrosion resistance of a steel material decreases. Therefore, the C content is defined in a range of from 0.01 to 0.20%.
• the C content is preferably 0.02% or more, and more preferably 0.03% or more.
• the lower limit of the C content may be 0.05%, 0.07%, or 0.09%.
• the C content is preferably 0.18% or less, and more preferably 0.16% or less.
• the upper limit of the C content may be 0.15%, or 0.13%.
• the Si is an element necessary for deoxidation. In order to obtain a sufficient deoxidation effect, it is necessary to contain it 0.01% or more. On the other hand, when the Si content is excessive, the toughness of a steel material, especially when welding is performed, the toughness of a base metal and a weld heat affected zone is impaired. Therefore, the Si content is defined in a range of from 0.01 to 1 .00%.
• the Si content is preferably 0.03% or more, and more preferably 0.05% or more.
• the lower limit of the Si content may be 0.10%, 0.15%, or 0.20%.
• the Si content is preferably 0.80% or less, and more preferably 0.60% or less.
• the upper limit of the Si content may be 0.50%, 0.40%, or 0.30%.
• Mn is an element having an action of increasing the strength of a steel material at low cost.
• the Mn content is defined in a range of from 0.05 to 3.00%.
• the Mn content is preferably 0.50% or more, and more preferably 0.80% or more. Further, the Mn content is preferably 2.50% or less, and more preferably 2.00% or less.
• P is an element existing as an impurity in a steel material.
• P is an element that lowers acid resistance of a steel material, and lowers the corrosion resistance of a steel material in a chloride corrosion environment where the pH at a corrosion interface decreases. P also deteriorates the weldability and toughness of a steel material. Therefore, the P content is limited to 0.050% or less.
• the P content is preferably 0.040% or less, and more preferably 0.030% or less.
• the upper limit of the P content may be 0.020%, 0.015%, or 0.010%.
• the lower limit of the P content is 0%. Since the desulfurization cost for dephosphorization to ultra-low concentration is high, the lower limit of the P content may be 0.0005%, 0.001%, or 0.003%.
• S is an element existing as an impurity in a steel material.
• S forms MnS as a starting point of corrosion in a steel material.
• the S content is preferably 0.0080% or less, more preferably 0.0060% or less, and even more preferably 0.0040% or less.
• the lower limit of the S content is 0%. Since the refining cost for desulphurization to ultra-low concentration is high, the lower limit of the S content may be 0.0005%, or 0.0010%.
• the Sn content is defined in a range of from 0.05 to 0.25%.
• the Sn content is preferably 0.07% or more, more preferably 0.09% or more, and further preferably 0.10% or more. Further, the Sn content is preferably 0.20% or less, more preferably 0.18% or less, and further preferably 0.016% or less.
• Al is an element effective for deoxidizing a steel material. Since Si is contained, deoxidation is performed by Si. Therefore, a deoxidation treatment with Al is not absolutely necessary, and the lower limit of the Al content is 0%. However, deoxidation with Al may be performed in addition to the same with Si. Meanwhile, when the Al content exceeds 0.100%, the corrosion resistance of a steel material in a low pH environment is lowered, so that the corrosion resistance of the steel material in a chloride corrosion environment is lowered. Further, when the Al content exceeds 0.100%, a nitride becomes coarse and the toughness of a steel material decreases. Therefore, the Al content is defined in a range of from 0 to 0.100%.
• the Al content is preferably 0.005% or more, more preferably 0.010% or more, further preferably 0.015% or more, still further preferably 0.020% or more, and particularly preferably 0.025% or more. Further, the Al content is preferably 0.060% or less, and more preferably 0.045% or less.
• N dissolves in the form of ammonia and has an effect of improving the corrosion resistance of a steel plate in a saline environment by suppressing the pH decrease due to the hydrolysis of Fe 3+ in an environment where the amount of air borne salt particle is high.
• the N content is regulated in a range of from 0.0005 to 0.0100%. Since it is not easy to lower the lower limit of N below 0.0005%, and it is also costly, the lower limit is set at 0.0005%.
• the lower limit of the N content may be set at 0.0010% or 0.0020%, according to need.
• the upper limit of the N content may be set at 0.0080% or 0.0060%.
• the toughness of the same is improved, and especially when welding is applied, the toughness of a welded joint is improved.
• O forms an oxide, such as SnO and SnO 2 . Therefore, when the O content becomes excessive, when the O content becomes excessive, the Sn concentration in the steel cannot be sufficiently secured.
• the above oxide acts as a starting point of corrosion, the corrosion resistance of a steel material decreases. Therefore, the O content is regulated in a range of from 0.0001 to 0.0100%.
• the content of O is preferably 0.0002% or more, and more preferably 0.0003% or more.
• the lower limit of the O content may be 0.0005%, 0.0010%, 0.0015%, or 0.0019%. Also, the O content is preferably 0.0090% or less, and more preferably 0.0080% or less. The upper limit of the O content may be 0.0060%, 0.0040%, or 0.0030%.
• All of Ti, Nb, and V are elements which form precipitates to enhance the strength of a steel material, and may be contained according to need. It is not prerequisite to contain the elements, and the lower limits of their contents are all 0%. On the other hand, when Ti, Nb, or V are excessively contained, the toughness is apt to decrease. Therefore, each content should be 0.050% or less. Each content is preferably 0.0030% or less, and more preferably 0.020% or less. In order to obtain the above effect, one or more kinds selected from Ti, Nb, and V may be contained at 0.001% or more.
• the contents of W and Mo should be respectively 0.050% or less. It is preferable that both the contents are 0.040% or less.
• the upper limit of each of the W content and the Mo content may be 0.030%, 0.020%, 0.010%, or 0.005%.
• W content and the Mo content are as small as possible, and the lower limits of their contents are 0%.
• W and Mo may be contained, and the lower limits of their contents may be 0.010%, or 0.020%.
• Cu is an element that improves the corrosion resistance of a steel material.
• the Cu content is preferably as low as possible, and the lower limit of the Cu content should be 0%. Meanwhile, considering possibility of contamination as an impurity, the Cu content is set at 0.10% or less.
• the Cu content is preferably 0.07% or less, more preferably 0.05% or less, further preferably 0.03% or less, and still further preferably 0.02% or less.
• the Cu content is particularly preferably 0.01% or less.
• Ni improves, similarly to Cu, the corrosion resistance of a steel material.
• the Ni content is preferably as low as possible, and the lower limit of the Ni content should be 0%. Meanwhile, even when it is mixed in as an impurity, so long as the Ni content is 0.05% or less, the corrosion resistance decrease is only slight. Therefore, the Ni content is set at 0.05% or less.
• the Ni content is preferably 0.03% or less, more preferably 0.02% or less, and further preferably 0.01% or less.
• Cr is an element that improves corrosion resistance of steel material.
• the Cr content is preferably as low as possible, and the lower limit of the Cr content should be 0%. Meanwhile, considering possibility of contamination as an impurity, the Cr content is set at 0.10% or less.
• the Cr content is preferably 0.07% or less, more preferably less than 0.05%, further preferably 0.03% or less, and still further preferably 0.02% or less.
• the Cr content is particularly preferably 0.01% or less.
• Sb is an element that improves the acid resistance
• Sb may be contained as necessary. It is not indispensable to contain Sb, and the lower limit of its content is 0%. Incidentally, even when Sb is contained in an amount exceeding 0.05%, not only the effect is saturated, but also the toughness and the like of the steel material are deteriorated. Therefore, the Sb content is set at 0.05% or less.
• the upper limit of the Sb content may be 0.04% or less, or 0.03% or less.
• the Sb content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.015% or more. When it is not necessary to obtain the above effect, the upper limit of the Sb content may be 0.015%, 0.010%, or 0.005% according to need.
• B is an element for increasing the strength of a steel material by addition of a trace amount thereof, so it may be added optionally. It is not indispensable to contain B, and the lower limit of its content is 0%. When B is added in an amount exceeding 0.0010%, the toughness may be deteriorated, so the B content is set at 0.0010% or less. In order to obtain the above effect, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. When it is not necessary to obtain the above effect, the upper limit of the B content may be 0.0005%, or 0.0003%, according to need.
• Ca is present in the form of an oxide in a steel material, and has an action of suppressing decrease in pH at the interface in a corrosion reaction zone to prevent corrosion, and therefore Ca may be included if necessary. It is not indispensable to contain Ca, and the lower limit of its content is 0%. When the Ca content exceeds 0.0100%, the above effect is saturated. Accordingly, the Ca content is set at 0.0100% or less.
• the Ca content is preferably 0.0050% or less, and more preferably 0.0040% or less.
• the Ca content is preferably 0.0002% or more, and more preferably 0.0005% or more.
• the upper limit of the Ca content may be 0.0030%, 0.0005%, or 0.0002% or less, according to need.
• Mg has an action of suppressing decrease in pH at the interface in a corrosion reaction zone to prevent corrosion of a steel material, and therefore Mg may be included according to need. It is not indispensable to contain Mg, and the lower limit of its content is 0%. When the Mg content exceeds 0.0100%, the above effect is saturated. Therefore, the Mg content is set at 0.0100% or less.
• the Mg content is preferably 0.0050% or less, and more preferably 0.0040% or less. In order to obtain the above effect, the Mg content is preferably 0.0002% or more, and more preferably 0.0005% or more. When it is not necessary to obtain the above effect, the upper limit of the Mg content may be set at 0.0030%, 0.0005%, or 0.0002%, according to need.
• REM rare earth element
• the lower limit of its content is 0%.
• the REM content is preferably 0.0050% or less, and more preferably 0.0040% or less.
• the REM content is preferably 0.0002% or more, and more preferably 0.0005% or more.
• the upper limit of the Mg content may be set at 0.0030%, 0.0005%, or 0.0002%, according to need.
• REM is a collective term of 17 elements including 15 elements of lanthanoid, as well as Y and Sc.
• 17 elements may be included in a steel material, and the REM content means the sum of the contents of such elements.
• the balance of the chemical composition is Fe and impurities.
• impurity means a component which is mixed in when a steel material is produced industrially due to various factors related to the raw material, such as ore, and scrap, or a production process, and which is tolerable so long as the invention is not adversely affected.
• the Sn ratio between the crystal grain boundary and the crystal grain inside affects the low-temperature toughness, the fatigue characteristics and the corrosion resistance of a steel.
• the Sn ratio between the crystal grain boundary and the crystal grain inside is set at 1.2 or less.
• the Sn ratio is preferably 1.1 or less, and more preferably 1.05 or less.
• the lower limit of the Sn ratio need not be particularly determined, the lower limit may be set at 0.7, 0.8, 0.9, or 1.0.
• An Sn ratio between the crystal grain boundary and the crystal grain inside of the invention may be determined by the following method. Firstly, a cylindrical specimen having a diameter of 3 mm and a length of 10 mm is prepared from a steel material at a position of 1/4 t (t represents plate thickness or wall thickness). Then, the specimen was subjected to an ultra-high vacuum impact fracture mechanism attached to an Auger spectroscopic analyzer (Model 670i, manufactured by ULVAC-PHI, Inc.), and the fracture surface, which is formed by fracture in vacuum (1.0E -9 Torr or less) in an atmosphere at the liquid nitrogen temperature (-150°C), is observed.
• Auger spectroscopic analyzer Model 670i, manufactured by ULVAC-PHI, Inc.
• the fracture surface is mostly occupied by cleavage fracture surfaces having a river pattern, and dimple fracture surfaces, and sparse intergranular fractured surfaces are also recognized.
• the crystal grain boundary and the crystal grain inside of the fracture surface are discriminated by a macro-fractographic method, and Auger spectra are measured at 10 positions in each crystal grain boundary and crystal grain inside.
• the fracture surface examined by the macro-fractographic method is analyzed by Auger spectroscopy with respect to C, which is apt to segregate at a crystal grain boundary, to confirm discrimination between the crystal grain boundary and the crystal grain inside.
• the Sn ratio is calculated by measuring the ratio of the concentrations (atom%) of Sn between the crystal grain boundary and the crystal grain inside.
• the relative sensitivity coefficient is calibrated with Au.
• the thickness (plate thickness) of a steel plate is preferably from 10 to 40 mm.
• a steel material may be a steel pipe or a section steel, and its thickness or wall thickness may be from about 3 to 50 mm.
• a steel material according to the invention can be produced using, for example, the production method described below.
• a method of producing a steel material including:
• the heating temperature in the heating step is from 1000 to 1150°C.
• the austenite grain size at the time of heating can be kept small, so that grain refining of the rolled structure can be achieved.
• the heating temperature is 1150°C or lower, coarsening of austenite grains is suppressed and coarsening of the structure after cooling transformation is also suppressed, so that excellent low-temperature toughness can be achieved.
• the heating temperature is 1000°C or higher, the alloy elements are sufficiently solutionized, so that the deterioration of the internal quality of the steel is suppressed, and the finishing temperature in rolling is not excessively lowered, so that the enhancement of low-temperature toughness can be expected.
• the finishing temperature at the surface in the rolling step is 900°C or lower, the growth of recrystallized austenite grains is suppressed, and grain refining thereof is promoted. Further, when the finishing temperature is 750°C or higher, the ferrite structure becomes less susceptible to processing, so that the low-temperature toughness is improved. Consequently, the finishing temperature is set from 900 to 750°C.
• the rolling reduction rate from 950°C is 50% or more, partial recrystallization of austenite hardly occurs, so that a duplex grain structure is suppressed to enhance the low-temperature toughness. Consequently, the rolling reduction rate from 950°C is set at 50% or more.
• water cooling is carried out under the following conditions.
• accelerated cooling is immediately performed at a cooling rate of from 5 to 10°C/s until the surface temperature of a steel material becomes 630°C or lower.
• the cooling rate within the above range, grain boundary segregation of Sn can be suppressed.
• the cooling rate is 5°C/s or more, Sn diffusion is suppressed.
• it is 10°C/s or less, the Sn ratio between the crystal grain boundary and the crystal grain inside is reduced, although the reason therefor is not very clear. As a result, in both cases, the low-temperature toughness and the fatigue characteristics are improved.
• the accelerated cooling is suspended allowing cooling in the air (holding) for recuperation until the surface temperature of the cooled steel material rises again due to the internal temperature of the steel material and the surface temperature is equalized in a temperature range of from 650 to 700°C.
• the holding time (this time is the accelerated cooling suspension time corresponding to the recuperation time) is 30 to 120 sec. Owing to the recuperation step, it is possible to segregate elements that are apt to segregate, such as S, P, and C, into the crystal grain boundary, and to suppress diffusion of Sn.
• the holding time is 30 sec or more, uniform recuperation deep into the inside of a steel material becomes possible.
• the holding time is 120 seconds or less, elevation of the surface temperature of a steel material up to a temperature range exceeding 700°C may be suppressed more easily so that diffusion of Sn may be reduced to suppress segregation.
• cooling is resumed at a cooling rate of from 10 to 60°C/s until the surface temperature reaches 550°C or less.
• the tensile strength should preferably be in a range of from 400 to 650 MPa.
• the tensile strength may be also from 480 to 580 MPa.
• a steel having the chemical composition shown in Table 1 was melted in a furnace and then cast in to a slab with a thickness of 300 mm.
• the slab was heated, subjected to rough rolling and finish rolling, and then quickly cooled to a steel plate having a plate thickness of 20 mm.
• the production conditions are shown in Table 2.
• a cylindrical specimen having a diameter of 3 mm and a length of 10 mm was cut out from each steel plate, and subjected to an ultra-high vacuum impact fracture mechanism attached to an Auger spectroscopic analyzer (Model 670i, manufactured by Ulvac Inc.), and the fracture surface, which was formed by fracture in vacuum (1.0E-9 Torr or less) in an atmosphere at the liquid nitrogen temperature (-150°C), was observed.
• the fracture surface was mostly occupied by cleavage fracture surfaces having a river pattern, and dimple fracture surfaces, and sparse intergranular fractured surfaces were also recognized.
• the crystal grain boundary and the crystal grain inside of the fracture surface were discriminated by a macro-fractographic method, and Auger spectra were measured at 10 positions in each crystal grain boundary and crystal grain inside.
• the fracture surface examined by the macro-fractographic method was analyzed by Auger spectroscopy with respect to C, which was apt to segregate at a crystal grain boundary, to confirm discrimination between the crystal grain boundary and the crystal grain inside.
• the Sn ratio was calculated by measuring the ratio of the concentrations (atom%) of Sn between the crystal grain boundary and the crystal grain inside. In this regard, the relative sensitivity coefficient was calibrated with Au.
• a specimen having a length of 60 mm, a width of 100 mm, and a thickness of 3 mm was cut out from each steel plate, and subjected to the SAE J 2334 test. In doing so, two specimens were taken from each steel plate, and on one of them, an anticorrosion coating was formed in advance.
• the SAE J 2334 test will be described below.
• the SAE J 2334 test is an accelerated deterioration test in which a cycle of humid stage and dry stage (humid ⁇ salt application ⁇ dry; 24 hours in total) is repeated to simulate a severely corrosive environment where the amount of air borne salt particle exceeds 1 mdd.
• the SAE J 2334 test was conducted repeating a cycle under the following conditions.
• the corrosion form under the following conditions is similar to the corrosion form of an atmospheric exposure test.
• a shotblasting treatment was applied to the surface of each specimen.
• an anticorrosion primary coating, an under coating, an intermediate coating, and an over coating were applied one on another to form an anticorrosion coating having a total thickness of 250 ⁇ m.
• an inorganic zinc rich paint (“SHINTO-ZINC #2000" produced by SHINTO PAINT CO., LTD.) was coated to a thickness of 75 ⁇ m, and as a mist coating an epoxy resin paint ("NEO-GOSE #2300 MC" produced by SHINTO PAINT CO., LTD.) was applied.
• an epoxy resin coating (“NEO-GOSE #2300 PS” produced by Shinto Paint Co., Ltd.) was spray-coated to a film thickness of 120 ⁇ m.
• an intermediate paint for a fluorine resin paint (“SHINTO-FLON #100 intermediate paint” produced by SHINTO PAINT CO., LTD.) was coated to a thickness of 30 ⁇ m.
• a fluorine resin coating (“SHINTO-FLON #100" produced by SHINTO PAINT CO., LTD.) was spray-coated to a film thickness of 25 ⁇ m.
• a cross scratch was formed on the anticorrosion coating to expose part of the steel material.
• rust was formed uniformly over the entire specimen surface after the test, and therefore its corrosion amount was determined.
• the "corrosion amount” was determined as the average plate thickness decrement of the specimen when a surface rust layer on the surface was removed. Specifically, the plate thickness decrement was calculated using the weight change of the specimen before and after the test, and the surface area of the specimen, and used as the corrosion amount.
• the criteria for pass or fail at a corrosion resistance test were as follows. A SAE J 2334 test was conducted for 120 cycles using a specimen on which an anticorrosion coating was not formed, and one in which the corrosion amount was 0.60 mm or less was judged for passed. Further, a SAE J2334 test was conducted for 200 cycles using a specimen on which an anticorrosion coating was formed, and one in which the detached area at a scratched zone was 20% or less, and the maximum corrosion depth was 0.40 mm or less was judged as pass.
• the low-temperature toughness was evaluated on an impact test specimen taken from a central part in the plate thickness direction of the plate and in the direction perpendicular to the rolling direction, by determining the absorbed energy (vE 0 ) at 0°C using a V-notch specimen according to JIS Z 2242. A specimen with an absorbed energy of 150 J or more was judged as pass.
• the stress amplitude was changed as a test parameter, and the relationship between the stress amplitude and the fatigue fracture life was represented by a S-N diagram, and a fatigue limit was derived therefrom.
• a No. 2 specimen specified in JIS Z 2275 was used, and the load ratio (the value obtained by dividing the minimum load by the maximum load) was set at 0.1.
• the fatigue fracture life was defined as the time point at which the displacement (the displacement of the cylinder of an actuator that applied the load to a specimen) at the maximum load increased by 1 mm as compared with the start of the test. When the fatigue fracture life was 5.5 ⁇ 10 5 cycles or more, the fatigue resistance was judged as pass.
• Test Nos. 1 to 10 are Examples of the present disclosure which satisfy all the requirements of the present disclosure.
• the corrosion amount of the specimen without a coating was 0.60 mm or less, and in the scratched zone of the specimen with a coating the detached area was 20% or less, and the maximum corrosion depth was 0.40 mm or less.
• the Charpy absorbed energy at 0°C was 150 J or more.
• the fatigue fracture life was 5.5 ⁇ 10 5 cycles or more.
• a steel material according to the present disclosure is suitable for use as a material for a large structure, such as an offshore structure used in a cold district, and a bridge.

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