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

• High-strength steel sheets used in automobiles are required to have an excellent yield ratio, excellent ductility, and excellent stress corrosion cracking resistance.
• steel sheets with excellent ductility are preferable from the perspective of formability.
• excellent yield ratios and excellent stress corrosion cracking resistance are required.
• one issue with using high-strength steel sheets for components is that the increased strength of steel sheets results in increased springback, significantly reducing shape fixability during press forming. Therefore, in the field of press technology, to ensure shape fixability, it is common to predict the amount of shape deformation after demolding during press forming and design the press die shape accordingly.
• 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, stress corrosion cracking resistance, and material stability.
• Patent Document 2 discloses a high-strength steel plate of 1,180 MPa or more that has excellent ductility and bending crack resistance at sheared edges, a high yield ratio, and 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 stress corrosion cracking resistance and material stability.
• Patent Document 3 discloses a method for manufacturing high-strength steel sheet of 980 MPa or more, which has excellent ductility, stretch flangeability, and stable mechanical properties. However, the technology described in Patent Document 3 does not take into consideration high-strength steel sheet with excellent yield ratio and stress corrosion cracking resistance.
• the present invention was developed in light of these circumstances, and aims to provide steel plates and components with a tensile strength TS of 1180 MPa or more, a yield ratio YR of 65% or more, and excellent ductility, stress corrosion cracking resistance, and material stability, as well as methods for manufacturing the same.
• TS tensile strength
• YR yield ratio
• Excellent stress corrosion cracking resistance means that a test specimen obtained from a steel plate is subjected to four-point bending in accordance with ASTM (G39-99), stresses equivalent to YS and TS are applied to the bent vertices of the test specimen, and the test specimen in the stressed state is immersed in 1 mass % sulfuric acid at 25°C for 100 hours, and the test specimen to which a stress equivalent to YS has been applied shows no cracks.
• the present inventors have conducted extensive research to achieve the above-mentioned object and have found the following. (1) By setting the amount of tempered martensite to 83% or more, a TS of 1180 MPa or more can be achieved. (2) By setting the total amount of ferrite and bainitic ferrite to 5% or more, excellent ductility can be achieved. (3) By keeping the amount of retained austenite to less than 3% and the total amount of ferrite and bainitic ferrite to less than 15%, a YR of 65% or more can be achieved.
• the component composition further comprises, in 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, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less,
• the steel sheet according to [1] above containing at least one element selected from the following: [3] The steel sheet according to [1] or [2], having a plating layer on the surface of the steel sheet.
• [4] A member made using the steel plate according to any one of [1] to [3] above.
• [5] 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 CR1 of 700 to 500 ° C.: less than 5 ° C./s; and cooling by water quenching at an average cooling rate CR2 of 300 ° C./s or more from a quenching start temperature T2 of (Ms - 80 ° C.) or more to 80 ° C., which is equal to or higher than Ms.
• Tempering temperature T3 100 ° C or more and less than 250 ° C
• An annealing process is performed at a tempering temperature T3 for a holding time t3 of 10 seconds or more and 10,000 seconds or less.
• the time t2 during which the steel sheet is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or higher and 10.0 seconds or lower
• 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.
• [6] The method for producing a steel sheet according to [5], wherein a plating treatment is performed.
• [7] A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of [1] to [3] to at least one of forming and joining 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, stress corrosion cracking resistance, and material stability. Furthermore, by applying the steel sheets of the present invention to, for example, automotive structural components, it is possible to improve fuel efficiency by reducing the vehicle body weight. Therefore, the industrial value of this steel sheet is extremely great.
• the steel sheet of the present invention has a component composition containing, in mass%, C: 0.030% or more and 0.450% 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;
• the steel sheet has a structure that satisfies the following conditions at a 1/4 position in the sheet thickness: an area fraction of tempered martensite: 83% or more, a volume fraction of retained austenite: less than 3%, a total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%, and an area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 ⁇ m or more and 1.0
• S 0.0200% or less S exists as sulfide and becomes the initiation point of stress corrosion cracking. Therefore, if the content exceeds 0.0200%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the S content must be 0.0200% or less.
• the S content is preferably 0.0050% or less.
• the S content is more preferably 0.0030% or less, and even more preferably 0.0020% or less. Although there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more, and more preferably 0.0002% or more.
• N 0.0100% or less N exists as nitrides and can be the starting point for stress corrosion cracking. Therefore, if the N content exceeds 0.0100%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the N content must be 0.0100% or less.
• the N content is preferably 0.0050% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more, more preferably 0.0002% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.
• the Cr, Mo, and Ni contents are each preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% 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.
• 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.
• 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.
• Remaining structures other than the above-mentioned total structure may include pearlite and fresh martensite. These remaining structures do not affect the properties as long as their area fraction is 5% or less, so they may be included.
• the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m within grains surrounded by grain boundaries at an angle of 15° or more affects stress corrosion cracking resistance and material stability. If the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m within grains surrounded by grain boundaries at an angle of 15° or more exceeds 50%, stress corrosion cracking will begin.
• the inventors have discovered that by increasing the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m, an excessive increase in TS due to an increase in the area fraction of tempered martensite can be suppressed, thereby suppressing changes in TS when the tempered martensite fraction changes in the sheet width direction, thereby improving material stability. Therefore, within the tempered martensite grains surrounded by grain boundaries at an angle of 15° or more, the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m is set to 5% to 50%. This area fraction is preferably 7% or more. Furthermore, this area fraction is preferably 30% or less.
• the method for measuring the fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m is as follows. Using a TEM (transmission electron microscope), the steel sheet structure is observed at a position 1/4 of the sheet thickness from the steel sheet surface at 20,000x magnification with a field of view of 4 ⁇ m x 4 ⁇ m, and the grain size and number of carbides present within all tempered martensite grains in the field of view are calculated.
• the grain size of the carbides is determined by importing data in which carbides have been identified in advance into Image-Pro from MediaCybernetics and calculating the circle equivalent diameter.
• the total area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m is calculated.
• the total area fraction of all tempered martensite is also calculated.
• the total area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 ⁇ m or more and 1.0 ⁇ m or less within the grains is divided by the total area fraction of all tempered martensite to calculate the area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 ⁇ m or more and 1.0 ⁇ m or less.
• the grains surrounded by grain boundaries at an angle of 15° or more in the tempered martensite are determined, for example, as follows. Transmission electron backscatter diffraction (transmission EBSD) measurement is performed using a test piece that has undergone TEM observation to obtain local crystal orientation data. Then, the obtained local crystal orientation data is analyzed using analysis software: OIM Analysis 7. Furthermore, prior to the analysis of the local crystal orientation data, a cleanup process is performed once using the grain dilation function of the analysis software (grain tolerance angle: 5, minimum grain size: 2, single iteration: ON). Next, by displaying the grain boundaries in the tempered martensite at an angle of 15° or more, the grains surrounded by grain boundaries at an angle of 15° or more in the tempered martensite are determined. Furthermore, for one carbide, a region in the microscope image that is surrounded by something other than the carbide and is formed seamlessly and integrally is counted as one carbide.
• transmission EBSD transmission electron backscatter diffraction
• the thickness of the steel plate of the present invention is preferably 0.5 mm or more, and more preferably 3.0 mm or less.
• the plate width is preferably 600 mm or more, and more preferably 1500 mm or less.
• the method for producing a steel sheet of the present invention comprises: hot rolling, pickling, and cold rolling a steel having a chemical composition to produce a cold-rolled sheet; heating the cold-rolled sheet under the conditions of an annealing temperature T1 of 750°C or higher and 850°C or lower, and a holding time t1 at the annealing temperature T1 of 10 seconds or higher and 1000 seconds or lower; A cooling treatment is performed, which includes cooling at an average cooling rate CR1 of less than 5°C/s from 700 to 500°C, and cooling by water quenching at an average cooling rate CR2 of 300°C/s or more from a quenching start temperature T2 of (Ms-80°C) or more and Ms or less to 80°C, Tempering temperature T3: 100 ° C.
• the time t2 during which the steel sheet is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or higher and 10.0 seconds or lower.
• pressure is applied to the front and back surfaces of the steel sheet using two rolls placed on either side of the steel sheet, and the pressure is applied under the conditions of a distance between the two rolls in the steel sheet conveying direction of 20 mm or more and 250 mm or less, and a pressure of 196 N or more.
• 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:
• Quenching start temperature T2 (Ms - 80°C) or higher and Ms or lower This is one of the important constituent elements of the present invention.
• the quenching start temperature T2 By setting the quenching start temperature T2 to (Ms - 80°C) or higher and Ms or lower, it is possible to obtain a structure in which the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m in grains surrounded by grain boundaries at an angle of 15° or more is 5% to 50%.
• the quenching start temperature T2 is lower than (Ms - 80°C)
• the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m in grain size exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance.
• the quenching start temperature T2 exceeds Ms
• the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m becomes less than 5%, making it difficult to achieve excellent material stability. Therefore, the quenching start temperature T2 is set to (Ms - 80°C) or more and Ms or less.
• the quenching start temperature T2 is preferably (Ms - 40°C) or more.
• the quenching start temperature T2 is preferably (Ms - 10°C) or less.
• 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 CR2 from quenching start temperature T2 to 80°C 300°C/s or more If the average cooling rate CR2 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 CR2 from quenching start temperature T2 to 80°C is set to 300°C/s or more.
• the average cooling rate CR2 is preferably 800°C/s or more. There is no particular need to limit the upper limit, but the average cooling rate CR2 is preferably 3000°C/s or less.
• the cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for less than 1.0 second, the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m within grains surrounded by grain boundaries at an angle of 15° or more will be less than 5%, making it difficult to achieve excellent material stability. Therefore, the cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for 1.0 second to 10.0 seconds. The cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for preferably 1.5 seconds or more. The cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for preferably 5.0 seconds or less.
• the tempering temperature T3 is 250°C or higher, the tempering of martensite proceeds excessively, and the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m or more and 1.0 ⁇ m or less in the grains surrounded by grain boundaries at an angle of 15° or more exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the tempering temperature T3 is set to 100°C or higher and lower than 250°C.
• the tempering temperature T3 is preferably set to 150°C or higher.
• the tempering temperature T3 is preferably set to 220°C or lower.
• 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 at the tempering temperature T3 is preferably set to 50 seconds or more.
• the holding time t3 at the tempering temperature 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.
• the above-mentioned steel plate (high-strength steel plate) may be processed under conditions that result in an equivalent plastic strain of 0.05% or more and 5.00% or less. Furthermore, after processing, it may be reheated again under conditions that result in a temperature of 100°C or more and 400°C or less.
• 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 1. If pressure is not applied during the water-quenching process, the proportion of tempered martensite containing five or more carbides with a grain size of 0.1 ⁇ m to 1.0 ⁇ m within grains surrounded by grain boundaries at an angle of 15° or greater will be less than 5%, making it difficult to achieve excellent material stability. After extensive research, the inventors discovered that applying pressure during the cooling process (water-quenching) of water-quenching affects the progression of martensitic transformation.
• Pressure is applied by sandwiching the material between rolls spaced apart from each other, and the pressure required to achieve the above effect is 196 N or more.
• This pressure corresponds to the applied load of one roll.
• a pressure of 196 N or more means that each of the two rolls applies pressure with a force of 196 N or more.
• the distance between the rolls is preferably 30 mm or more, and the distance between the rolls is preferably 220 mm or less.
• the pressure is preferably 294 N or more, and more preferably 4900 N 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.
• the obtained steel plate was sheared to a size of 20 mm x 75 mm with the direction perpendicular to the rolling direction as the longitudinal direction, and 2 mm was removed from both sides of the longitudinal end face by mechanical grinding to process into 16 mm x 75 mm test specimens.
• the obtained test specimens were subjected to four-point bending in accordance with ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bend vertices of the test specimens.
• the test specimens in the stressed state were immersed in 1 mass% sulfuric acid at 25 °C for 100 hours. After the test, the presence or absence of cracks was confirmed for each test specimen by visual inspection.
• 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, stress corrosion cracking resistance, and material stability, while the comparative examples were inferior in at least one of these characteristics.
• the components obtained by forming and joining the steel plates of the present invention have a tensile strength TS of 1180 MPa or more, a yield ratio YR of 65% or more, and excellent ductility, stress corrosion cracking resistance, and material stability, similar to the steel plates of the present invention.

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