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 sheets, components, and methods for manufacturing them that have excellent tensile strength, yield ratio, stretch flangeability, fatigue resistance at sheared edges, and delayed fracture resistance in corrosive environments.
• the steel sheets of the present invention can be suitably used as structural components for automobile parts, etc.
• Patent Document 1 discloses a high-strength steel sheet of 1,180 MPa or more that has excellent yield ratio, flatness in the sheet width direction, and work embrittlement resistance, as well as a method for manufacturing the same.
• the technology described in Patent Document 1 does not take into consideration high-strength steel sheets that have excellent stretch flangeability, fatigue resistance at sheared edges, 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, stretch flangeability, and fatigue resistance at the sheared edge.
• [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: 800 ° C. or higher, Heating is performed under the condition that the holding time t1 at the annealing temperature T1 is 10 seconds or more, Cooling at an average cooling rate of 700 to 600 ° C.
• the present invention it is possible to obtain steel sheets that have a TS of 1,320 MPa or more, a YR of 75% or more, and that are excellent in stretch flangeability, 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.
• the steel sheet of the present invention has a component composition containing, 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 a quarter-thickness position of the sheet, the area fraction of tempered martensite is 95% or more, the volume fraction of retained austenite is less than 3%, and the total area fraction of ferrite and bainitic ferrite is less than 5%;
• the steel sheet has a structure that satisfies the following formulas (1) and (2).
• 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.
• the N content is preferably 0.0050% or less.
• 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.
• 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 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.
• Total area fraction of ferrite and bainitic ferrite less than 5% This is one of the important constituent elements of the present invention. If the total area fraction of ferrite and bainitic ferrite is 5% or more, it becomes difficult to achieve excellent stretch flangeability. Therefore, the total area fraction of ferrite and bainitic ferrite is less than 5%. The total area fraction of these is preferably 3% or less. The total area fraction of these is more preferably 2% or less. The lower limit of the total area fraction of ferrite and bainitic ferrite is not particularly limited, and may be 0%.
• 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 steel sheet of the present invention can be determined to have excellent delayed fracture resistance in a corrosive environment in accordance with the method for evaluating delayed fracture properties of metallic materials (HeTsAce): the number of days to crack is 63 days or more. Specifically, the steel sheet of the present invention has a cracking period of 63 days or more according to the following evaluation method for delayed fracture properties of metallic materials (HeTsAce).
• 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.
• ⁇ Chloride deposition amount 1,000 to 20,000 mg/m 2 >
• the amount of chloride attached to the metal material is 1,000 to 20,000 mg/ m2 .
• This amount corresponds to the amount of chloride attached that is expected in an atmospheric corrosive environment in which an actual automobile runs. In a corrosive environment where the amount is less than 1,000 mg/ m2 , corrosion hardly progresses, resulting in little hydrogen generation and hydrogen penetration into the metal material, making delayed fracture unlikely. On the other hand, if the amount exceeds 20,000 mg/ m2 , the corrosion rate will be significantly different from that in an actual environment, resulting in an excessive durability test that is not suitable for the purpose.
• the amount is set to 20,000 mg/ m2 or less.
• the amount of chloride attached is preferably more than 5,000 mg/ m2 .
• the amount of chloride attached is preferably 12,000 mg/ m2 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.
• the amount of chloride adhesion can be controlled by, for example, changing the chloride concentration of the chloride-containing aqueous solution or by changing the time for which the chloride-containing aqueous solution is applied (the process time of the chloride adhesion step (A)) to change the amount of chloride-containing aqueous solution applied to the metal material.
• chloride deposition step (A) chloride is deposited on a metal material to obtain a desired chloride deposition amount.
• the chloride preferably includes one or more selected from sodium salt (NaCl), potassium salt (KCl), calcium salt (CaCl 2 ), and magnesium salt (MgCl 2 ), which are present in the atmospheric environment in which typical metal materials are used.
• a chloride-based component containing chloride and a component other than chloride may be deposited.
• a chloride-based component refers to a component in which chloride accounts for more than 50 mass% of the total components in terms of solid content.
• 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
• the chlorides to be attached to the metal material have a composition similar to that of the snow-melting agents sprayed in that area.
• components having a composition similar to that of snow-melting agents include a component mainly composed of CaCl2 (a component in which CaCl2 is more than 50 mass% of the total components), a component mainly composed of MgCl2 (a component in which MgCl2 is more than 50 mass% of the total components), and a component mainly composed of NaCl (a component in which NaCl is 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 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 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 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.
• 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.
• 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 chloride adhesion process (A) is preferably carried out in an atmosphere with a relative humidity Ha1 of 30% or more. If the relative humidity Ha1 in the chloride adhesion process (A) is less than 30%, the droplets sprayed from the spray nozzle, particularly when spraying droplets of a chloride-containing aqueous solution using a spray nozzle, will tend to dry out before reaching the evaluation surface of the metal material. As a result, it may be difficult to control the droplet distribution to obtain the desired distribution on the evaluation surface of the metal material. Furthermore, the chloride adhesion process (A) is preferably carried out in an atmosphere with a relative humidity of 80% or less. If the relative humidity in the chloride adhesion process (A) is greater than 80%, the droplets that adhere to the metal material will tend to become coarse.
• a characteristic of atmospheric corrosion environments is the repeated alternation of wet (humid) and dry (dry) states, and simulating this environmental change is important in approximating the corrosion patterns found in the actual environment in which a vehicle travels.
• the corrosion products that form on the steel material change depending on whether it is wet or dry, and it is known that hydrogen is generated during the process of changing from a wet state to a dry state, or from a dry state to a wet state. Therefore, the conditions during the cycle of changing relative humidity (corrosion process (B)) are also important in evaluating delayed fracture properties.
• the corrosion step (B) is a step of performing a cycle at least once (once or twice or more) in an atmosphere at a temperature Tb1 that is 60°C or less and within a certain range, the cycle comprising the following drying step (b1), the following wetting step (b2), the following transition step (b3), and the following transition step (b4).
• the corrosion step (B) is performed in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 of the corrosion step (B) exceeds 60°C, not only will the evaluation be performed in an environment far removed from the corrosive environment in which the metal material is actually used, but the corrosion mechanism may also change. Therefore, the temperature Tb1 of the corrosion step (B) is set to 60°C or less, preferably 50°C or less. On the other hand, the lower limit of the temperature Tb1 of the corrosion step (B) is not particularly limited.
• 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 water rinsing method in the water rinsing step (C) is not particularly limited, but examples include a method in which water is sprayed onto the evaluation surface of the metal material from a spray nozzle to rinse the evaluation surface, or a method in which the evaluation surface is immersed in water to rinse the evaluation surface.
• a cycle of the chloride adhesion process (A) and corrosion process (B) as described above is carried out, and then a corrosion test cycle in which a water rinsing process (C) is carried out before the chloride adhesion process (A) is carried out again is carried out at least once.
• the condition of the metal material the presence or absence of cracks in the metal material, the extent of cracks, etc.
• the delayed fracture properties of the metal material are evaluated based on the confirmed condition of the metal material.
• the steel sheet of the present invention has a cracking period of 63 days or more under the following conditions in the evaluation method for delayed fracture properties of the above-mentioned metallic materials (HeTsAce).
• Corrosion test cycle After the first step (A) was performed, the distribution of droplets on the evaluation surface was confirmed. Subsequent steps (A) were performed under the same conditions as the first step (A).
• One cycle of step (B) was defined as the drying step (b1) ⁇ transition step (b3) ⁇ wetting step (b2) ⁇ transition step (b4) performed in this order.
• a water washing step (C) was inserted before performing step (A), and then step (A) was performed.
• Metallic material used for evaluation The obtained steel plate is sheared into a size of 16 mm x 75 mm, with the longitudinal direction perpendicular to the rolling direction, to prepare test specimens. The clearance during shearing is 15% in all cases.
• a four-point bending test is performed according to ASTM (G39-99), and stresses equivalent to YS and TS are applied to the bend vertex of the test specimen.
• the above-mentioned corrosive environment cycle (HeTsAce) is performed for 63 days. After the test, the presence or absence of cracks is visually confirmed for each test specimen, and samples that are loaded with a stress equivalent to YS and do not exhibit cracks are judged to have excellent delayed fracture resistance in a corrosive environment.
• the method for producing a steel sheet of the present invention comprises the steps of: heating a cold-rolled sheet produced by hot-rolling, pickling, and cold-rolling a steel having the aforementioned chemical composition under conditions of an annealing temperature T1 of 800°C or higher and a holding time t1 at the annealing temperature T1 of 10 seconds or longer; cooling from 700 to 600°C at an average cooling rate CR1 of 5°C/s or higher; cooling from (Ms+100°C) to a quenching start temperature T2 of (Ms-50°C) or higher but lower than (Ms+50°C) at an average cooling rate CR2 of 5°C/s or higher but 30°C/s or lower; and cooling from the quenching start temperature T2 at an average cooling rate CR2 of 5°C/s or higher but 30°C/s or lower.
• the method includes an annealing step in which the steel sheet is cooled by water quenching from 0°C to 80°C at an average cooling rate CR3 of 300°C/s or more, and heated under the conditions of a tempering temperature T3 of 100°C to 400°C and a holding time t3 at the tempering temperature T3 of 10 seconds to 10,000 seconds, and during the water quenching cooling in the annealing step, the steel sheet is pressed from the front and back sides by two rolls placed on either side of the steel sheet, and the pressing is performed under the conditions of a distance between the two rolls in the steel sheet conveying direction of 20 mm to 250 mm and a pressing force of 196 N or more.
• steel slabs there are no particular limitations on the method for producing steel material (steel slabs), and any known production method, such as a converter or electric furnace, is suitable. It is preferable to produce steel slabs (slabs) using a continuous casting method to prevent macrosegregation.
• the slab heating temperature, slab soaking time, and coiling temperature in hot rolling are not particularly limited.
• Methods for hot rolling a steel slab include a method of heating the slab and then rolling it, a method of directly rolling the slab after continuous casting without heating it, and a method of subjecting the slab after continuous casting to a short-term heat treatment and then rolling it.
• the slab heating temperature, slab soaking time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, but the slab heating temperature is preferably 1100°C or higher and 1300°C or lower.
• the slab soaking time is preferably 30 minutes or more, and 250 minutes or less.
• the finish rolling temperature is preferably the Ar3 transformation point or higher.
• the coiling temperature is preferably 350° C. or higher, and 650° C. or lower.
• the hot-rolled steel sheet produced in this manner is then pickled.
• Pickling is capable of removing oxides from the steel sheet surface, and is therefore important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet product.
• Pickling may be performed once or in multiple steps. After hot rolling, the pickled sheet may be cold-rolled as is, or it may be heat-treated and then cold-rolled.
• the cold-rolled sheet obtained as described above is then annealed.
• the annealing conditions are as follows:
• Annealing temperature T1 800°C or higher If the annealing temperature T1 is lower than 800°C, the total area fraction of ferrite and bainitic ferrite will be 5% or higher, making it difficult to achieve a TS of 1320 MPa or higher and also difficult to achieve excellent stretch flangeability. Therefore, the annealing temperature T1 is set to 800°C or higher.
• the annealing temperature T1 is preferably 820°C or higher. There is no particular need to limit the upper limit, but the annealing temperature T1 is preferably 1000°C or lower.
• the annealing temperature here refers to the holding temperature in the annealing step.
• the annealing temperature may be constant during holding. Also, the annealing temperature does not have to be constant during holding as long as it is in a temperature range of 800°C or higher and the temperature fluctuation is within ⁇ 10°C of the set temperature.
• Holding time t1 at annealing temperature T1 10 seconds or more If the holding time t1 at annealing temperature T1 is less than 10 seconds, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve a TS of 1320 MPa or more and also difficult to achieve excellent stretch flangeability. Therefore, the holding time t1 at the annealing temperature T1 is set to 10 seconds or more.
• the holding time t1 at the annealing temperature T1 is preferably 30 seconds or more. Although there is no particular need to limit the upper limit, the holding time t1 at the annealing temperature T1 is preferably 1000 seconds or less.
• Average cooling rate CR1 from 700 to 600 ° C 5 ° C/s or more If the average cooling rate CR1 from 700 to 600 ° C is less than 5 ° C/s, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve a TS of 1320 MPa or more and achieving excellent stretch flangeability. Therefore, the average cooling rate CR1 from 700 to 600 ° C is set to 5 ° C/s or more.
• the average cooling rate CR1 is preferably 10 ° C/s or more. There is no particular need to limit the upper limit, but the average cooling rate CR1 is preferably 50 ° C/s or less.
• the average cooling rate CR1 is calculated by (cooling start temperature (700° C.) ⁇ cooling stop temperature (600° C.))/cooling time (s) from the cooling start temperature (700° C.) to the cooling stop temperature.
• Specific examples of cooling at the average cooling rate CR1 include water cooling and mist cooling.
• Average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 5°C/s or more and 30°C/s or less. This is one of the important constituent elements of the present invention.
• the average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 affects the total area fraction of ferrite and bainitic ferrite and the average occupancy rate of the packet with the largest occupancy rate within the prior austenite grains at the center of the steel plate thickness.
• 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 5% or more, making it difficult to achieve excellent stretch flangeability.
• 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 75% 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 during water quenching in the annealing process, the volume fraction of retained austenite will be 3% or more, making it difficult to achieve a YR of 75% 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 set an upper limit, but the average cooling rate CR3 is preferably 3000°C/s or less.
• 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 1320 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.
• the holding time t3 at the tempering temperature T3 is set to 10 seconds or more and 10,000 seconds or less.
• the holding time t3 is preferably set to 50 seconds or more.
• the holding time t3 is preferably set to 5,000 seconds or less.
• 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.
• steel plates high-strength steel plates
• they are usually cooled to room temperature before being traded.
• the "roll distance between two rolls” refers to the distance between the contact points of one roll and the steel sheet and the contact point of the other roll and the steel sheet, as shown in Figure 9. If pressure is not applied during the water-quenching process, P(S) becomes excessively large relative to P(C), making it difficult to achieve ⁇ P(S) - P(C) ⁇ ⁇ 20%, and the fatigue resistance of the sheared edge and the delayed fracture resistance in a corrosive environment are reduced. After extensive research, the inventors discovered that applying pressure during the water-quenching process affects the difference between P(S) and P(C).
• 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.
• the steel sheet of the present invention has a tensile strength of 1320 MPa or more, a YR of 75% or more, and is excellent in stretch flangeability, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment. Therefore, members obtained using the steel sheet of the present invention also have a tensile strength of 1320 MPa or more, a YR of 75% or more, and are excellent in stretch flangeability, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment.
• the use of members of the present invention enables weight reduction. Therefore, members of the present invention can be suitably used, for example, in automotive structural members.
• the hole expansion test was performed in accordance with JIS Z 2256 (2020). After shearing the obtained steel plate to 100 mm x 100 mm, a hole having a diameter of 10 mm was punched with a clearance of 12.5%, and then a conical punch having an apex angle of 60 ° was pressed into the hole while holding it down with a blank holding force of 9 ton (88.26 kN) using a die having an inner diameter of 75 mm. The hole diameter at the crack initiation limit was measured, and the limit hole expansion ratio: ⁇ (%) was calculated from the following formula, and the hole expandability was evaluated from the value of this limit hole expansion ratio.
• the stretch flangeability was determined to be good when the hole expanding ratio ( ⁇ ), which is an index of stretch flangeability, was 30% or more, regardless of the strength of the steel sheet.
• 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 amount of chloride deposited was calculated from the chloride concentration of the aqueous solution by calculating the difference in mass of the test piece before and after salt water spraying and dividing it by the area of the evaluation surface of the test piece. In addition, the following conditions were adopted: Chloride-containing aqueous solution: 15 mass% NaCl aqueous solution sprayed by spraying.
• the water-washing process (C) is a process in which the evaluation surface of the metal material is washed with water.
• the water-washing method in the water-washing process (C) is not particularly limited, but water was sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface with water.
• 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.
• the components obtained by forming and joining the steel sheets of the present invention had a tensile strength TS of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment.
• these components had a tensile strength of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=97218383

Family Applications (1)

Country Status (2)

Citations (2)

• 2025
  • 2025-01-28 WO PCT/JP2025/002535 patent/WO2025204079A1/ja active Pending
  • 2025-01-28 JP JP2025536025A patent/JP7845581B2/ja active Active

Patent Citations (2)

Also Published As

Similar Documents

Legal Events