Classifications

• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/62—Quenching devices
  • C21D1/63—Quenching devices for bath quenching
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/573—Continuous furnaces for strip or wire with cooling
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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 invention relates to steel plates and components that have excellent tensile strength, yield ratio, ductility, fatigue resistance at sheared edges, and delayed fracture resistance in corrosive environments, as well as methods for manufacturing the same.
• the steel plates of the present invention can be suitably used as structural components for automobile parts and the like.
• Patent Document 1 discloses a high-strength steel plate of 1,180 MPa or more that has excellent yield ratio, flatness in the plate width direction, and work embrittlement resistance, as well as a manufacturing method for the same.
• the technology described in Patent Document 1 does not take into consideration high-strength steel plate that has excellent ductility, fatigue resistance at sheared edges, and delayed fracture resistance in corrosive environments.
• Patent Document 2 discloses a high-strength steel plate of 1,180 MPa or more that has excellent ductility and bending crack resistance at the sheared edge, and a high yield ratio, as well as a method for manufacturing the same.
• the technology described in Patent Document 2 does not take into consideration high-strength steel plate that has excellent fatigue resistance at the sheared edge and delayed fracture resistance in corrosive environments.
• Patent Document 3 discloses a high-strength steel plate of 980 MPa or more that has excellent surface properties, steel plate shape, and fatigue strength, and a method for manufacturing the same.
• the technology described in Patent Document 3 does not take into consideration high-strength steel plate that has excellent ductility, fatigue resistance at sheared end faces, and delayed fracture resistance in corrosive environments.
• Patent Document 4 discloses a high-strength steel plate of 1180 MPa or more that has excellent delayed fracture resistance in corrosive environments, corrosion resistance, and weldability, and a method for manufacturing the same.
• the technology described in Patent Document 4 does not take into consideration high-strength steel plate that has excellent yield ratio, ductility, and fatigue resistance at the sheared edge.
• the present invention was developed in light of these circumstances, and aims to provide steel plates, components, and methods for manufacturing them that have a tensile strength TS of 1180 MPa or more, a yield ratio YR of 65% or more, and excellent ductility, fatigue resistance at sheared edges, and delayed fracture resistance in corrosive environments.
• the tensile strength TS (hereinafter also simply referred to as TS) and the yield ratio YR (hereinafter also simply referred to as YR) can be measured in accordance with JIS Z 2241 (2022).
• Excellent ductility means that TS ⁇ El determined by the measurement method according to JIS Z 2241 (2022) is 14000 MPa ⁇ % or more.
• “Excellent delayed fracture resistance in a corrosive environment” means that the delayed fracture resistance is excellent in the corrosive environment specified in the present invention, that is, the number of days to cracking in the evaluation of the metallic material is 63 days or more.
• the present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows. [1] In mass%, C: 0.030% or more and 0.500% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; At the 1/4 position of the plate thickness, Area fraction of tempered martensite: 83% or more, Volume fraction of retained austenite: less than 3%; Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%; A steel sheet having a structure that satisfies the following formulas (1) and (2): P(C) ⁇ 70%...(1) ⁇ P(S)-P(C) ⁇ 20%...(2) During the ceremony, P(S):
• [4] The steel plate according to any one of [1] to [3], wherein the number of days to cracking is 63 days or more in an evaluation of the metal material.
• [5] A member made using the steel plate according to any one of [1] to [4].
• [6] A cold-rolled sheet produced by hot rolling, pickling and cold rolling a steel having the component composition according to [1] or [2], Annealing temperature T1: 750°C or higher and 850°C or lower, Heating is performed under the condition of a holding time t1 at an annealing temperature T1 of 10 seconds or more and 1000 seconds or less, Cooling at an average cooling rate of 700 to 600 ° C.
• a method for producing a steel sheet wherein, during cooling of the water quenching in the annealing step, pressure is applied from the front and back surfaces of the steel sheet with two rolls placed on either side of the steel sheet, and the pressure is applied under the conditions of a roll-to-roll distance of 20 mm or more and 250 mm or less in the steel sheet transport direction of the two rolls and a pressure of 196 N or more.
• a method for manufacturing a component comprising a step of subjecting the steel plate according to any one of [1] to [4] to at least one of forming processing and joining processing to form a component.
• the present invention it is possible to obtain steel sheets that have a TS of 1180 MPa or more, a YR of 65% or more, and excellent ductility, fatigue resistance at sheared edges, and delayed fracture resistance in corrosive environments. Furthermore, by applying the steel sheets of the present invention to, for example, automotive structural components, it is possible to reduce the weight of the vehicle body and thereby improve fuel efficiency. Therefore, the industrial value of this steel sheet is extremely great.
• FIG. 1 is a schematic diagram illustrating a packet having the maximum occupancy rate within a prior austenite grain.
• FIG. 2 is a schematic diagram illustrating the method for evaluating the delayed fracture properties of metallic materials (HeTsAce).
• FIG. 3 is a diagram showing an example of an image of the distribution of droplets on the evaluation surface of a metal material.
• FIG. 4 is a schematic diagram illustrating a case where a shielding material is placed between a spray nozzle and a metallic material in a method for evaluating delayed fracture properties of a metallic material.
• FIG. 5 is a diagram showing a schematic diagram of how the sprayed liquid in the atmosphere re-adheres to the evaluation surface when the distance between the shielding material and the evaluation surface of the metal material is changed.
• FIG. 6 is a diagram illustrating an embodiment of a corrosion test cycle according to a method for evaluating delayed fracture properties of a metallic material.
• FIG. 7 is a diagram illustrating another embodiment of a corrosion test cycle according to the method for evaluating delayed fracture properties of a metallic material.
• FIG. 8 is a diagram schematically showing a test piece for evaluating delayed fracture properties used in the examples.
• FIG. 9 is a schematic diagram illustrating a method of applying pressure during water cooling in the method of manufacturing a steel sheet according to the present invention.
• C 0.030% to 0.500% C is one of the important basic components of steel, and in the present invention, it is an important element that affects the area fraction (hereinafter also referred to as fraction) of tempered martensite and the fatigue resistance of the sheared edge. If the C content is less than 0.030%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, if the C content exceeds 0.500%, the tempered martensite becomes embrittled, making it difficult to obtain excellent fatigue resistance at the sheared end faces. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably set to 0.050% or more. The C content is more preferably set to 0.100% or more. Furthermore, the C content is preferably set to 0.400% or less. The C content is more preferably set to 0.350% or less.
• Si 0.010% or more and 2.500% or less Si is one of the important basic components of steel, and in the present invention, in particular, it is an important element that affects the amount of retained austenite because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. If the Si content is less than 0.010%, it becomes difficult to achieve a TS of 1,180 MPa or more. On the other hand, if the Si content exceeds 2.500%, the amount of retained austenite increases excessively, making it difficult to achieve YR ⁇ 65%. Therefore, the Si content is set to 0.010% or more and 2.500% or less.
• the Si content is preferably set to 0.050% or more.
• the Si content is more preferably set to 0.100% or more.
• the Si content is also preferably set to 2.000% or less.
• the Si content is more preferably set to 1.200% or less.
• the Si content is more preferably set to 0.500% or less, and further preferably set to 0.300% or
• Mn 0.10% or more and 5.00% or less
• Mn is one of the important basic components of steel, and in the present invention, it is an important element that affects the fraction of tempered martensite and the delayed fracture resistance in a corrosive environment. If the Mn content is less than 0.10%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, if the Mn content exceeds 5.00%, corrosion of the steel sheet is accelerated, and hydrogen generation associated with corrosion is promoted, making it difficult to achieve excellent delayed fracture resistance in a corrosive environment. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably set to 0.50% or more. The Mn content is more preferably set to 0.80% or more. The Mn content is preferably set to 4.50% or less. The Mn content is more preferably set to 4.00% or less.
• P 0.100% or less P segregates at prior austenite grain boundaries and embrittles the grain boundaries. Therefore, if the P content exceeds 0.100%, the ultimate deformability of the steel sheet is reduced, making it difficult to achieve excellent fatigue resistance at the sheared edge. Therefore, the P content must be 0.100% or less.
• the P content is preferably 0.070% or less.
• the P content is more preferably 0.050% or less, and even more preferably 0.020% or less.
• the P content is preferably 0.001% or more, and more preferably 0.002% or more.
• N 0.0100% or less N exists as a nitride, and if it is contained in an amount exceeding 0.0100%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance at the sheared edge. Therefore, the N content must be 0.0100% or less. Therefore, the N content is set to 0.0100% or less. The N content is preferably set to 0.0050% or less. Although the lower limit of the N content is not particularly specified, due to constraints on production technology, the N content is preferably 0.0001% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.
• O 0.0100% or less
• O exists as an oxide, and if it is contained in an amount exceeding 0.0100%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance at the sheared edge. Therefore, the O content must be 0.0100% or less. Therefore, the O content is set to 0.0100% or less.
• the O content is preferably set to 0.0050% or less. Although the lower limit of the O content is not particularly specified, due to constraints on production technology, the O content is preferably 0.0001% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.
• a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities.
• unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. A total content of 0.100% or less of these impurities is acceptable. It is preferable that a steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities.
• the steel sheet of the present invention further contains, by mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, and Cu: 1.00% or less.
• the contents of Ti, Nb, and V are each preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more, because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing.
• Ta and W 0.10% or less
• the Ta and W contents are each 0.10% or less.
• these contents are each 0.08% or less.
• the Ta and W contents are preferably 0.01% or more, respectively, because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing.
• the content of each of these elements is more preferably 0.02% or more, and further preferably 0.03% or more.
• B 0.0100% or less If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not be reduced, so the fatigue resistance of the sheared edge portion will not be reduced. Therefore, if B is contained, the B content is set to 0.0100% or less. The B content is preferably set to 0.0080% or less. Although the lower limit of the B content is not particularly specified, since B is an element that segregates at austenite grain boundaries during annealing and improves hardenability, the B content is more preferably 0.0003% or more.
• Cr 1.00% or less
• Mo 1.00% or less
• Ni 1.00% or less. If Cr, Mo, and Ni are each 1.00% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. Therefore, when at least one of Cr, Mo, and Ni is contained, the Cr, Mo, and Ni contents are each 1.00% or less. Preferably, each of these contents is 0.80% or less. Each of these contents is more preferably 0.75% or less, and even more preferably 0.70% or less. Although there are no particular restrictions on the lower limits of the Cr, Mo, and Ni contents, since these elements improve hardenability, the Cr, Mo, and Ni contents are preferably 0.01% or more, and more preferably 0.02% or more.
• Sn 0.200% or less If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not be reduced, so the fatigue resistance of the sheared edge portion will not be reduced. Therefore, if Sn is contained, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.150% or less. Although the lower limit of the Sn content is not particularly specified, since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content is preferably 0.001% or more, and more preferably 0.002% or more.
• Sb 0.200% or less If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.150% or less. The Sb content is more preferably set to 0.100% or less. Although the lower limit of the Sb content is not particularly specified, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is preferably 0.001% or more, and more preferably 0.002% or more.
• the Ca, Mg, and REM contents are each 0.0100% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge surface does not deteriorate. Therefore, when at least one of Ca, Mg, and REM is contained, the Ca, Mg, and REM contents are preferably each 0.0100% or less. Preferably, each of these contents is 0.0050% or less.
• the Zr and Te contents are preferably 0.001% or more, and more preferably 0.002% or more.
• the Hf content is preferably 0.001% or more, and more preferably 0.010% or more, because Hf is an element that spheroidizes the shapes of nitrides and sulfides and improves the ultimate deformability of the steel sheet.
• Bi 0.200% or less If Bi is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. Therefore, if Bi is contained, the Bi content is set to 0.200% or less. The Bi content is preferably set to 0.190% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
• Tempered martensite area fraction of 83% or more This is one of the important constituent elements of the present invention.
• the area fraction of tempered martensite must be 83% or more. Therefore, the area fraction of tempered martensite is set to 83% or more.
• the area fraction of tempered martensite is preferably 84% or more.
• the area fraction of tempered martensite is preferably 95% or less, and more preferably 94% or less.
• the area fraction of tempered martensite is measured as follows: After polishing the L-section of a steel sheet, it is corroded with 1 vol. % nital, and a portion of the sheet that is 1/4 of the sheet thickness (a position corresponding to 1/4 of the sheet thickness in the depth direction from the surface of the steel sheet) is observed using an SEM at a magnification of 2000 times and a field of view of 30 ⁇ m ⁇ 30 ⁇ m in 10 fields of view.
• the tempered martensite has fine irregularities inside and contains carbides.
• the area fraction of the tempered martensite can be calculated from the average value of these values.
• Retained austenite volume fraction less than 3% This is one of the important constituent elements of the present invention. If the volume fraction of retained austenite is 3% or more, it becomes difficult to achieve YR ⁇ 65%. The reason why it becomes difficult to achieve YR ⁇ 65% is that the YS decreases due to the transformation of retained austenite to martensite during the tensile test. Therefore, the volume fraction of retained austenite is less than 3%. Preferably, the volume fraction of retained austenite is 1% or less. The lower limit of the volume fraction of retained austenite is not particularly limited and may be 0%.
• the volume fraction of retained austenite is measured as follows: After polishing the steel plate to 1/4 of the plate thickness, the surface is further polished by 0.1 mm using chemical polishing.
• the retained austenite volume fraction is determined by measuring the integrated intensity ratios of the diffraction peaks of the ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of fcc iron and the ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ planes of bcc iron using CoK ⁇ radiation in an X-ray diffractometer, and then averaging the nine integrated intensity ratios obtained.
• the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. This is one of the important constituent elements of the present invention. If the total area fraction of ferrite and bainitic ferrite is 15% or more, it becomes difficult to achieve YR ⁇ 65%. On the other hand, if the total area fraction of ferrite and bainitic ferrite is less than 5%, it becomes difficult to achieve excellent ductility. Therefore, the total area fraction of ferrite and bainitic ferrite is set to 5% or more and less than 15%. The total area fraction of these is preferably 6% or more. The total area fraction of these is preferably 13% or less.
• the method for measuring the total area fraction of ferrite and bainitic ferrite is as follows: After polishing the L-section of the steel plate, it is etched with 1 vol. % nital, and a portion of 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed using an SEM at 2000x magnification with a field of view of 30 ⁇ m x 30 ⁇ m, using 10 fields of view. Note that in the above structural image, the ferrite and bainitic ferrite are recessed and the interior of the structure is flat, and does not contain carbides. The total area fraction of ferrite and bainitic ferrite can be calculated from the average of these values.
• Remaining structures other than the above-mentioned total structure may include pearlite, fresh martensite, acicular ferrite, etc. These remaining structures do not affect the properties as long as their area fraction is 5% or less, so they may be included.
• P(C) The average value of the occupancy rate of the packet with the largest occupancy rate within the prior austenite grain at the center of the thickness of the steel plate. This is one of the important constituent elements of the present invention.
• the occupancy rate of the packet with the largest occupancy rate within the prior austenite grain at the center of the thickness of the steel plate affects the fatigue resistance of the sheared edge surface and the delayed fracture resistance in a corrosive environment.
• the occupancy rate of one packet within a prior austenite grain is calculated by dividing the area of the specified packet by the total area within the prior austenite grain.
• the inventors have found that by reducing the occupancy rate of packets that have the maximum occupancy rate within prior austenite grains at the center of the thickness of the steel plate, specifically by setting the average value of the occupancy rate of packets that have the maximum occupancy rate within prior austenite grains at the center of the thickness of the steel plate to 70% or less, the structure becomes finer and crack propagation can be suppressed, thereby improving the fatigue resistance of the shear end surface and the resistance to delayed fracture in a corrosive environment. Therefore, the average value of the occupancy rate of packets having the maximum occupancy rate in the prior austenite grains at the center position of the thickness of the steel plate: P(C) is set to 70% or less.
• the average value of this occupancy rate: P(C) is set to 60% or less.
• the occupancy rate of packets having the maximum occupancy rate in prior austenite grains is 25%. Therefore, the average value of the occupancy rate of packets having the maximum occupancy rate in prior austenite grains at the center position of the thickness of the steel plate: P(C) is preferably 25% or more, but is not necessarily limited to this.
• P(S) The average value of the packet occupancy rate with the highest occupancy rate within prior austenite grains at a depth of 100 ⁇ m from the steel sheet surface.
• P(C) The average value of the packet occupancy rate with the highest occupancy rate within prior austenite grains at the center of the steel sheet thickness. This is one of the important constituent elements of the present invention. The difference between P(S) and P(C) affects the fatigue resistance of the sheared edge and the delayed fracture resistance in a corrosive environment.
• ⁇ P(S) - P(C) ⁇ Failure to satisfy ⁇ P(S) - P(C) ⁇ ⁇ 20% means that the packet occupancy rate in the surface layer of the steel sheet is excessively large compared to the packet occupancy rate in the center of the steel sheet, accelerating the initiation of fatigue-induced cracks and delayed fracture-induced cracks in the surface layer of the steel sheet. Therefore, ⁇ P(S) - P(C) ⁇ is set to 20% or less. ⁇ P(S) - P(C) ⁇ is preferably set to 15% or less.
• the obtained local orientation data is analyzed using OIM Analysis 7 (OIM), and a diagram (CP map) is created in which each close-packed plane group (CP group) is color-coded using the method described in Non-Patent Document 1 (Smart Processing Society Journal, 2013, Vol. 2, No. 3, pp. 110-118).
• a packet is defined as an area belonging to the same CP group.
• the area of the packet with the largest occupancy rate in the obtained CP map is determined, and this is divided by the total area within the prior austenite grain to determine the occupancy rate of the packet with the largest occupancy rate within the prior austenite grain.
• This analysis is performed on 10 or more adjacent prior austenite grains, and the average value is used as the average occupancy rate of the packet with the largest occupancy rate within the prior austenite grain.
• the thickness of the steel plate of the present invention is preferably 0.5 mm or more. It is also preferable that the thickness be 3.0 mm or less.
• the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride deposition step (A) is a mean contact area of the droplets on the evaluation surface of the metallic material of 0.1 mm2 or more and less than 3.0 mm2 ; an area ratio of the total contact area of the droplets to the area of the evaluation surface of the metallic material of 40% or more and 80% or less; and a standard deviation of the contact area of the droplets on the evaluation surface of the metallic material of 3.0 mm2 or less.
• Drying step (b1) A step of drying a metal material by maintaining it in an atmosphere having a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less;
• Wetting step (b2) a step of wetting the metal material by holding it in an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less;
• transition step (b3) a step of transitioning from the atmosphere having the relative humidity Hb1 to the atmosphere having the relative humidity Hb2 at a rate of change of the relative humidity of 30%/h or less;
• the evaluation target is a metallic material extracted from a steel plate.
• the method for extracting the metallic material from the steel plate is not particularly limited.
• the metallic material can be extracted by shearing the steel plate to a predetermined size.
• Methods for applying stress to the metallic material include processing the metallic material (processing method). Examples of processing methods include bending, bulging, stretching, and twisting. Other examples include fixing the material in a stressed shape using bolts or the like, and using residual stress remaining after processing.
• a process comprising a chloride adhesion process (A) and a corrosion process (B) can be performed at least once (once or twice or more) on the metallic material to which stress has been applied as described above. Furthermore, in the evaluation method of the present invention, the state of the metallic material is confirmed after performing the process comprising the chloride adhesion process (A) and the corrosion process (B) once or more times, and the delayed fracture properties of the metallic material can be evaluated based on the confirmed state of the metallic material. This confirmation can be performed, for example, by visually observing the presence or absence of cracks in the metal material and the extent of those cracks.
• a chloride adhesion process (A) and a corrosion process (B) are carried out while stress is applied to the metallic material. After each of these processes is carried out one or more times, the presence or absence and extent of cracks in the metallic material are confirmed, thereby evaluating the delayed fracture properties.
• the chloride deposition step (A) is a step of depositing droplets of a chloride-containing aqueous solution on the evaluation surface of the metal material so that the amount of chloride deposition is 1,000 to 20,000 mg/m 2.
• the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material (droplet deposition distribution) in at least the first (first) chloride deposition step (A) is set to have an average contact area of the droplets on the evaluation surface of the metal material of 0.1 mm 2 or more and less than 3.0 mm 2 , an area ratio of the total contact area of the droplets to the area of the evaluation surface of the metal material of 40% or more and 80% or less, and a standard deviation of the contact area of the droplets on the evaluation surface of the metal material of 3.0 mm 2 or less.
• the amount of chloride adhesion can be calculated by multiplying the difference in mass of the test piece (metal material) before and after application of the chloride-containing aqueous solution in the chloride adhesion step (A) by the chloride concentration of the chloride-containing aqueous solution and dividing the result by the area of the test piece's evaluation surface. If, when measuring the mass difference, the chloride-containing aqueous solution adheres to areas other than the evaluation surface of the test piece, appropriate measures can be taken, such as masking the areas other than the evaluation surface or wiping off the chloride-containing aqueous solution that has adhered to the areas other than the evaluation surface.
• components other than chloride include, but are not limited to, sulfides and nitrate compounds.
• a NaCl-based component a component in which NaCl accounts for more than 50 mass% of the total components
• a component containing a combination of multiple metal salts may be used as the chloride to be adhered to the metal material.
• a component containing a combination of multiple metal salts include the Society of Automotive Engineers standard (SAE J2334) (0.5% by mass NaCl-0.1% by mass CaCl 2 -0.075% by mass NaHCO 3 ), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl 2 -0.12% by mass CaCl 2 -0.07% by mass KCl, and others (for example, an aqueous solution of Aquamarine (registered trademark) manufactured by Yashima Pharmaceutical Co., Ltd.)).
• SAE J2334 Society of Automotive Engineers standard
• artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl 2 -0.12% by mass CaCl 2 -0.07% by mass KCl
• others for example, an aqueous solution of Aquamarine (registered trademark) manufactured by Yashima Pharmaceutical Co., Ltd.
• the method for depositing chloride on a metal material is not particularly limited as long as it is a method that can achieve the desired distribution of chloride-containing aqueous solution droplets for evaluating the metal material.
• the chloride-containing aqueous solution include a chloride-containing aqueous solution containing a component primarily composed of chloride (usually an aqueous solution such as salt water, hereinafter also referred to as salt water).
• salt water is used as the chloride-containing aqueous solution.
• Spray nozzles come in a variety of types, including single-fluid spray nozzles (nozzles that atomize liquid sent under pressure) and two-fluid spray nozzles (nozzles that use high-speed fluids such as compressed air to atomize the liquid).
• Two-fluid spray nozzles also differ in the liquid supply method, and are classified as liquid pressure types (liquid is pressurized and supplied to the two-fluid nozzle) and suction types (liquid is sucked up and sprayed using the force of compressed air). It is preferable to select a spray nozzle that ensures an even distribution of droplets.
• salt water since salt water is used, it is preferable to use a corrosion-resistant metal such as stainless steel for the spray nozzle material.
• the chloride concentration in the saltwater is not particularly limited. However, when controlling the saltwater droplet distribution using a spray nozzle, if saltwater with a chloride concentration of less than 2.0% by mass is used to deposit saltwater on a metal material, the spray time is long to achieve an appropriate chloride deposition amount, making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. Therefore, the chloride concentration in the saltwater is preferably 2.0% by mass or more, and more preferably 5.0% by mass or more.
• the chloride concentration in the salt water is preferably 20% by mass or less, and more preferably 15% by mass or less.
• the chloride concentration in the salt water depends on the target amount of chloride adhesion; it is preferable to use low-concentration salt water when the amount of chloride adhesion is relatively low, and high-concentration salt water when the amount of chloride adhesion is relatively high.
• the total amount of salt water sprayed must be reduced, which will likely result in the average contact area of salt water droplets adhering to the evaluation surface of the metal material and the ratio of the total contact area of salt water droplets to the total area of the evaluation surface being too small.
• the total amount of salt water sprayed must be increased, which will likely result in the average contact area of salt water droplets adhering to the evaluation surface of the metal material and the ratio of the total contact area of salt water droplets to the area of the evaluation surface being too large.
• the average contact area of the droplets on the evaluation surface of the metallic material (average contact area per droplet) is set to 0.1 mm2 or more and less than 3.0 mm2 . If the average contact area is less than 0.1 mm2 , the droplet volume is small, and the target chloride adhesion amount is not achieved. Therefore, the average contact area is set to 0.1 mm2 or more.
• the average contact area is preferably set to 0.5 mm2 or more, and more preferably set to 1.0 mm2 or more.
• the average contact area is set to less than 3.0 mm2 .
• the average contact area is preferably set to 2.8 mm2 or less, and more preferably set to 2.5 mm2 or less.
• the average contact area of the droplets on the evaluation surface of the metallic material can be measured by the measurement method described below.
• the area ratio of the total contact area of the droplets to the area of the evaluation surface of the metal material is set to 40% or more and 80% or less. If the total contact area ratio of droplets is less than 40%, the droplet adhesion distribution will be non-uniform, resulting in large variations in delayed fracture evaluation. Therefore, the total contact area ratio of droplets is set to 40% or more.
• the total contact area ratio of droplets is preferably set to 50% or more, more preferably 55% or more.
• the total contact area ratio of droplets is set to 80% or less.
• the total contact area ratio of droplets is preferably set to 75% or less, more preferably 70% or less.
• the total contact area ratio of droplets can be measured using the measurement method described below.
• the standard deviation in the distribution of droplet contact areas on the evaluation surface of the metallic material is set to 3.0 mm2 or less. If the standard deviation of the droplet contact areas on the evaluation surface of the metallic material is greater than 3.0 mm2 , the droplet adhesion distribution will be non-uniform, resulting in greater variability in delayed fracture evaluation. Therefore, the standard deviation of the droplet contact areas is set to 3.0 mm2 or less.
• the standard deviation of the droplet contact areas is preferably set to 2.8 mm2 or less, and more preferably set to 2.5 mm2 or less. The standard deviation of the droplet contact areas can be measured by the measurement method described below.
• the distribution of saltwater droplets attached to the evaluation surface of the metal material can be determined by attaching saltwater droplets to the evaluation surface of the metal material in the chloride attachment step (A), capturing an image of the droplet distribution across the entire evaluation surface of the metal material, and performing image analysis.
• the image can be acquired using a digital camera, microscope, optical microscope, or the like.
• the image can also be acquired by photographing the evaluation surface from above (from the direction of the spray nozzle shown in Figure 2) the evaluation surface of the metal material.
• the evaluation surface can be photographed either inside a test tank equipped with a spray nozzle, or after the metal material is removed from the test tank.
• the metal material is removed from the test tank equipped with a spray nozzle and the evaluation surface of the metal material is photographed.
• the image is also acquired immediately (within 30 seconds) after the saltwater droplets are attached to the evaluation surface of the metal material.
• Figure 3 is a schematic diagram showing an image of the droplet distribution on the evaluation surface of the metal material obtained as described above.
• the area indicated by a circle in Figure 3 is the droplet contact area. From this image, the average droplet contact area (average contact area per droplet), the total droplet contact area ratio, and the standard deviation of the droplet contact area are determined by image analysis.
• the evaluation surface of a metallic material refers to the surface of the metallic material for evaluating its delayed fracture properties.
• the evaluation surface can be determined appropriately depending on the metallic material being evaluated.
• the surface of the plate facing the spray nozzle can be the evaluation surface (see Figure 2).
• the surface facing the spray nozzle in the stressed portion can be the evaluation surface (the surface corresponding to the plan view (top view) of the metallic material when the direction in which the spray nozzle is installed relative to the metallic material is upward).
• the evaluation target is a bent metallic material
• the surface facing the spray nozzle in the bent portion can be the evaluation surface (see Figure 8).
• one method is to use a spray nozzle to apply saltwater to the evaluation surface of the metal material, as mentioned above.
• a two-fluid nozzle is preferable as the spray nozzle.
• two-fluid nozzles include the KSMMS (product name) manufactured by Kyoritsu Alloy Manufacturing Co., Ltd., the Two-Fluid Air Atomizing Nozzle (product name) manufactured by Spraying Systems Japan LLC, and the Fine Mist Generating Nozzle (product name) manufactured by Ikeuchi Co., Ltd.
• the distance from the tip of the spray nozzle to the evaluation surface of the metal material is preferably 10 to 30 cm.
• the spray pressure of the spray nozzle is preferably 0.05 to 0.7 MPa.
• the spray angle ( ⁇ in Figure 2) is preferably 30 to 120°.
• the salt water spray time is preferably 10 seconds or less.
• a particularly preferred method for achieving the above-described saltwater droplet distribution is to place a shielding material with an opening between the spray nozzle and the metal material, and allow droplets of the chloride-containing aqueous solution sprayed from the spray nozzle to adhere to the evaluation surface of the metal material through the opening in the shielding material.
• the opening in the shielding material preferably has a shape and size substantially equivalent to the shape and size of the evaluation surface of the metal material. “Substantially equivalent” means that the opening in the shielding material is equivalent to the outer shape and size of the evaluation surface of the metal material when viewed from above the surface, or that the area of the opening in the shielding material is within ⁇ 10% of the area of the evaluation surface of the metal material.
• the shielding material be capable of shielding areas other than the evaluation surface of the metal material.
• the evaluation surface of the metal material can be seen through the opening in the shielding material, while other areas are not visible (are shielded).
• Figure 4 is a schematic diagram illustrating the case where a shielding material is placed between the spray nozzle and the metal material in the evaluation method of the present invention.
• a shielding material is placed between the spray nozzle and the metal material in the evaluation method of the present invention.
• the distance Y is the shortest distance from the shielding material to the evaluation surface of the metal material.
• the material of the shielding material is not limited as long as it prevents the sprayed liquid from penetrating. Examples of materials include resin, ceramic, metal, and wood. These materials can be processed and used as shielding materials.
• the evaluation method of the present invention it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material in the initial (first) chloride deposition step (A).
• the droplet distribution on the evaluation surface as described above is controlled at least in the initial chloride deposition step (A).
• the average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area can be measured at the end of the initial chloride deposition step (A) (within 30 seconds after the end of saltwater spraying).
• a test specimen other than the one to be tested may be used, and the conditions set in advance to achieve a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area.
• chloride deposition may be carried out under the same conditions as the initial chloride deposition step (A), or chloride deposition may be carried out under different conditions from the initial chloride deposition step (A) as long as the desired chloride deposition amount is achieved.
• conditions that result in a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of droplet contact area are set in advance, and the first chloride deposition step (A) is carried out under these set conditions. If the chloride deposition step (A) is carried out two or more times, it is preferable to carry out the second and subsequent chloride deposition steps (A) under these set conditions.
• the temperature Tb1 of the corrosion step (B) is preferably set to 5°C or more, and more preferably 10°C or more.
• the temperature Tb1 of the corrosion step (B) is preferably within ⁇ 5°C, and more preferably within ⁇ 2°C.
• the drying step (b1) is a step of drying the metal material by maintaining an atmosphere with a relative humidity Hb1 of 45% or less for 1.0 to 5.0 hours.
• the relative humidity Hb1 in the drying step (b1) is set to 45% or less. This is to simulate the dry state, which is one of the characteristics of an atmospheric corrosive environment. Furthermore, if the relative humidity Hb1 in the drying step (b1) exceeds 45%, a long period of time is required to sufficiently dry the metal material surface, resulting in a longer evaluation time.
• the relative humidity Hb1 in the drying step (b1) is preferably 40% or less.
• the lower limit of the relative humidity Hb1 in the drying step (b1) is not particularly limited.
• the relative humidity Hb1 in the drying step (b1) is preferably 20% or more. Furthermore, if the components to be attached to the metal material surface contain substances that exhibit deliquescent properties at lower relative humidities, such as magnesium chloride or calcium chloride, it is preferable to set the relative humidity Hb1 in the drying step (b1) low.
• the processing time for the drying process (b1) (the time spent in an atmosphere with relative humidity Hb1) should be between 1.0 and 5.0 hours. If the processing time for the drying process (b1) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the processing time for the drying process (b1) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it will take a long time to evaluate the delayed fracture properties.
• the wetting step (b2) is a step of wetting the metal material by maintaining it in an atmosphere with a relative humidity Hb2 of 80% or higher for 1.0 to 5.0 hours.
• the relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. This is to simulate the wet state, which is one of the characteristics of an atmospheric corrosive environment. If the relative humidity Hb2 in the wetting step (b2) is less than 80%, the effect of wetting will be insufficient, making it impossible to simulate an actual corrosive environment.
• sodium chloride has the highest saturated critical vapor pressure, which is approximately 75 to 78% in relative humidity terms.
• the relative humidity Hb2 in the wetting step (b2) is set to 80% or higher.
• the relative humidity Hb2 in the wetting step (b2) be less than 98%. This is because when the relative humidity Hb2 is 98% or higher, the water film formed by condensation becomes too thick, making it easier for the attached chlorides to be washed away. This phenomenon is particularly likely to occur when evaluating processed test specimens. Therefore, when evaluating processed test specimens, it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%.
• the process time for the wetting step (b2) (the time for which the specimen is maintained in an atmosphere with a relative humidity Hb2 of 80% or higher) should be 1.0 hour or more and 5.0 hours or less. If the process time for the wetting step (b2) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the process time for the wetting step (b2) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it will take a long time to evaluate the delayed fracture properties.
• transition step (b3) is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2
• transition step (b4) is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1.
• transition step (b3) which is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2
• moisture absorption begins due to deliquescence of chlorides present on the surface of the metal material, and corrosion of the metal material begins. It is known that corrosion products present on the surface of the metal material change at this time, and hydrogen is thought to be generated along with this change in corrosion products.
• the transition step (b4) which is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1
• the moisture becomes a concentrated solution containing a large amount of chlorides and metal ions eluted by corrosion, and in the case of steel materials, iron ions, and the pH of the solution is thought to decrease.
• the rate of change of the relative humidity when changing the relative humidity is set to 30%/h or less. If the rate of change of the relative humidity in the transition steps (b3) and (b4) is 30%/h or less, hydrogen generated by corrosion can be sufficiently penetrated into the metal material, enabling appropriate evaluation of delayed fracture properties. On the other hand, the lower limit of the rate of change of the relative humidity is not particularly specified. However, if the transition steps (b3) and (b4) are too long, it will take a long time to evaluate the delayed fracture properties. Therefore, the rate of change of the relative humidity is preferably 1.5%/h or more, and more preferably 10%/h or more.
• the purpose of the method for evaluating the delayed fracture properties of metallic materials of the present invention is to simulate the changes in relative humidity between day and night in an actual environment. Therefore, if the process time (time for one cycle) of the corrosion process (B), which simulates the changes in relative humidity between day and night in an actual environment, exceeds 24 hours, this means that corrosion will be slower than in an actual environment, and a long time will be required to evaluate the delayed fracture properties. In other words, it is preferable to set the process time of the corrosion process (B) to 24 hours or less. To expedite the evaluation, it is more preferable to set the process time of the corrosion process (B) to 12 hours or less.
• the process time of the corrosion process (B) is shortened, the relative humidity will change rapidly, which will reduce the correlation with corrosion in an actual environment and may result in a discrepancy with the delayed fracture properties in an actual environment. Therefore, it is preferable to set the process time of the corrosion process (B) to 5 hours or more.
• the chloride adhesion process (A) and the corrosion process (B) are each performed at least once.
• the chloride adhesion process (A) may be performed after a random number of cycles of the corrosion process (B), or after a predetermined number of cycles of the corrosion process (B).
• the process comprising the chloride adhesion process (A) and the corrosion process (B) can be performed until cracks occur in the metal material.
• the number of test days may be determined in advance, and the process may be performed for a number of days corresponding to that number of test days.
• the number of times the process is performed can be set appropriately, taking into account, for example, simulating corrosion patterns in an actual environment.
• the process can be performed 200 times or less, or may be performed 100 times or less.
• FIG. 6 is a diagram illustrating one embodiment of a corrosion test cycle according to the evaluation method of the present invention.
• the corrosion test cycle shown in Figure 6 shows an example of a corrosion test cycle in which the chloride adhesion process (A) and the corrosion process (B) are each performed once.
• the corrosion process (B) has one cycle consisting of a drying process (b1), a transition process (b3), a wetting process (b2), and a transition process (b4).
• the corrosion process (B) cycle following the chloride adhesion process (A) begin with the drying process (b1).
• the initiation points of condensation are uniformly dispersed when humidity increases, reducing variability in the evaluation of delayed fracture properties.
• the corrosion process (B) cycle begins with the transition process (b3), wetting process (b2), or transition process (b4), the saltwater applied in the chloride adhesion process (A) may not be sufficiently dried, or the saltwater applied in a high-humidity environment may absorb moisture and become coarse, resulting in uneven dispersion of the initiation points of condensation. For this reason, it is best to avoid starting the corrosion process (B) cycle from any process other than the drying process (b1).
• FIG. 7 shows an example of a corrosion test cycle in which a cycle of the chloride deposition process (A) followed by the corrosion process (B) is performed, and then a water rinsing process (C) is performed before performing the chloride deposition process (A) again.
• the average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 is less than 5°C/s, the total area fraction of ferrite and bainitic ferrite will be 15% or more, making it difficult to achieve a YR of 65% or more.
• the average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 exceeds 30°C/s, the average occupancy rate of packets with the largest occupancy rate within the prior austenite grains at the center of the steel plate thickness exceeds 70%, and the fatigue resistance of the sheared edge and the delayed fracture resistance in a corrosive environment deteriorate.
• the average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 is set to 5°C/s or more and 30°C/s or less.
• the average cooling rate CR2 is preferably 10°C/s or more.
• the average cooling rate CR2 is preferably 20°C/s or less.
• the average cooling rate CR2 is calculated by (cooling start temperature (Ms+100°C) - cooling stop temperature (quenching start temperature T2)) / cooling time (s) from the cooling start temperature (Ms+100°C) to the cooling stop temperature (quenching start temperature T2).
• Specific examples of cooling at the average cooling rate CR2 include mist cooling and gas cooling.
• Rapid cooling start temperature T2 (Ms - 50°C) or more but less than (Ms + 50°C) This is one of the important constituent elements of the present invention.
• the rapid cooling start temperature T2 By setting the rapid cooling start temperature T2 to (Ms - 50°C) or more but less than (Ms + 50°C), it is possible to obtain a structure in which the average occupancy rate of packets having the largest occupancy rate within prior austenite grains at the center of the thickness of the steel plate is 70% or less and the retained austenite volume fraction is less than 3%. If the rapid cooling start temperature T2 is less than (Ms - 50°C), the retained austenite volume fraction will be 3% or more, making it difficult to achieve a YR of 65% or more.
• the quenching start temperature T2 is set to (Ms - 50°C) or higher but lower than (Ms + 50°C).
• the quenching start temperature T2 is preferably (Ms - 40°C) or higher.
• the quenching start temperature T2 is preferably (Ms + 40°C) or lower.
• Ms is the martensitic transformation start temperature (°C)
• [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the content (mass%) of C, Mn, Cr, Mo, and Ni in the steel (steel plate), respectively, and if no element is contained, it is set to 0.
• Average cooling rate CR3 from quenching start temperature T2 to 80°C 300°C/s or more If the average cooling rate CR3 from quenching start temperature T2 to 80°C is less than 300°C/s, the volume fraction of retained austenite will be 3% or more, making it difficult to achieve a YR of 65% or more. Therefore, the average cooling rate CR3 from quenching start temperature T2 to 80°C is set to 300°C/s or more.
• the average cooling rate CR3 is preferably 800°C/s or more. There is no particular need to limit the upper limit, but the average cooling rate CR3 is preferably 3000°C/s or less.
• the average cooling rate CR3 is calculated by (cooling start temperature (quenching start temperature T2) - cooling stop temperature (80°C) / cooling time (s) from the cooling start temperature (quenching start temperature T2) to the cooling stop temperature (80°C).
• Tempering temperature T3 100°C or higher and 400°C or lower
• tempered martensite refers to a structure in which martensite at 80°C or lower has been heat-treated at a tempering temperature of 100°C or higher for a holding time of 10 seconds or longer. If the tempering temperature T3 in the tempering step during the annealing process is lower than 100°C, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, which deteriorates the fatigue resistance of the sheared end surface. On the other hand, if the tempering temperature T3 exceeds 400°C, the tempering of martensite progresses excessively, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the tempering temperature T3 is set to 100°C or more and 400°C or less. The tempering temperature T3 is preferably set to 150°C or more. The tempering temperature T3 is preferably set to 350°C or less.
• tempered martensite refers to a structure in which martensite at 80°C or less is subjected to heat treatment at a tempering temperature of 100°C or more and for a holding time of 10 seconds or more.
• the holding time t3 at the tempering temperature T3 is less than 10 seconds, the martensite will not be sufficiently tempered, resulting in a structure mainly consisting of as-quenched martensite, which will degrade the fatigue resistance properties of the sheared end surface.
• cooling after tempering there is no particular requirement for cooling after tempering, and any method may be used to cool to the desired temperature. It is desirable for the desired temperature to be around room temperature.
• hot-dip galvanizing and galvannealed hot-dip galvanizing are preferably performed during cooling from the annealing temperature T1 to the quenching start temperature T2.
• Electrogalvanizing and Zn-Ni electroalloy plating are preferably performed after the tempering process.
• hot-dip galvanizing and galvannealed hot-dip galvanizing processes from the standpoint of productivity, it is preferable to carry out the series of processes, including the heating, annealing, and plating processes, on a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight.
• a steel sheet can be obtained that has a TS of 1180 MPa or more, a YR of 65% or more, and excellent ductility, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment.
• the resulting steel sheet can be suitably used, for example, as a material for automobile parts.
• the material may be processed again under conditions that result in an equivalent plastic strain of 0.05% or more and 5.00% or less. Furthermore, after processing, the material may be reheated again under conditions that result in a temperature of 100°C or more and 400°C or less.
• the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Furthermore, the method for manufacturing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form the member.
• Forming can be performed using common processing methods such as press working without any restrictions.
• joining can be performed using common welding methods such as spot welding and arc welding, as well as rivet joining and crimping without any restrictions.
• the steel plate (high-strength cold-rolled steel plate) obtained in this manner was used as the test steel, and the tensile properties, fatigue resistance of the sheared edge, and delayed fracture resistance in a corrosive environment were evaluated according to the following test methods.
• the fatigue resistance properties of the sheared end surface were evaluated by a complete reverse bending test in accordance with JIS Z 2275 (1978).
• the test geometry was as described in JIS Z 2275 (1978), with a punch hole drilled in the center of the fatigue test specimen with a punch diameter of 10 mm and a clearance of 12%.
• the above test pieces were subjected to the following corrosion test cycle (corrosion test) consisting of a chloride adhesion step (A) and a corrosion step (B), and a water washing step (C).
• corrosion test consisting of a chloride adhesion step (A) and a corrosion step (B), and a water washing step (C).
• the chloride deposition step (A) droplets of a chloride-containing aqueous solution were deposited so that the amount of chloride deposited (in terms of solid content) was 10,000 mg/ m2 .
• the distance between the spray nozzle and the evaluation surface of the test piece (X in Figure 2) was 30 cm.
• the spray pressure of the spray nozzle was 0.2 MPa.
• the salt water spray time was 5 seconds.
• the corrosion step (B) is a process in which the drying step (b1) ⁇ the transition step (b3) ⁇ the wetting step (b2) ⁇ the transition step (b4) are carried out in this order, and one cycle is defined as the execution of the four steps described above.
• the temperature fluctuation range of the corrosion step (B) was set within 30 ⁇ 5°C.
• Drying step (b1) A step of drying the metal material by holding it in an atmosphere of 40% relative humidity Hb1 for 2.0 hours.
• Wetting step (b2) A step of wetting the metal material by holding it in an atmosphere of 90% relative humidity Hb2 for 2.0 hours.
• Transition step (b3) A step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 at a relative humidity change rate of 25%/h.
• Transition step (b4) A step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a relative humidity change rate of 25%/h.
• the corrosion test cycle was such that the chloride adhesion step (A) was followed by the corrosion step (B), and the water-rinsing step (C) was performed before the chloride adhesion step (A) was performed again. That is, in this example, the delayed fracture properties were evaluated using the following corrosion test cycle.
• the cycle was a repeated cycle of chloride adhesion step (A) ⁇ corrosion step (B) ⁇ water washing step (C).
• the period for evaluating delayed fracture resistance using the corrosive environment cycle was 63 days.
• each test piece was visually inspected for the presence or absence of cracks. Then, the delayed fracture resistance in a corrosive environment was evaluated according to the following criteria.
• a sample loaded with a stress equivalent to YS and showing no cracks was judged to have excellent delayed fracture resistance in a corrosive environment.
• ⁇ (Pass, particularly excellent) No cracks in the sample loaded with stress equivalent to YS and TS.
• ⁇ (Pass, excellent) No cracks in the sample loaded with stress equivalent to YS.
• ⁇ (Fail) Cracks in both the sample loaded with stress equivalent to YS and TS.
• Table 3 (Table 3-1, Table 3-2) had a tensile strength TS of 1180 MPa or more, a yield ratio YR of 65% or more, and were excellent in ductility, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment, whereas the comparative examples were inferior in at least one of these characteristics.

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