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
  • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C18/00—Alloys based on zinc
• • 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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon

Definitions

• the present invention relates to high-strength galvanized steel sheets with excellent workability, members made from the high-strength galvanized steel sheets, and methods for manufacturing them.
• the galvanized steel sheet of the present invention can be suitably used mainly as a steel sheet for automobiles.
• Hydrogen embrittlement of high-strength steel sheets may be caused not only by hydrogen that enters from the outside due to corrosion during use, but also by hydrogen that enters during the manufacturing process.
• the general method is to continuously perform a series of steps from annealing to hot-dip galvanizing in a hydrogen-containing atmosphere for the purpose of reducing and suppressing oxidation of the steel sheet. Therefore, a galvanized layer is formed in a state where hydrogen that has penetrated into the steel from the atmosphere remains.
• Hydrogen diffusion in galvanizing is very slow and acts as a barrier to the release of diffusible hydrogen in steel to the outside, so diffusible hydrogen may remain and hydrogen embrittlement may occur during processing, which may promote cracking. , there is a risk of causing local ductility deterioration, especially in bendability. Therefore, it is important to establish a hot-dip galvanized steel sheet with excellent hydrogen embrittlement resistance in which hydrogen in the steel is sufficiently reduced, and a method for manufacturing the same.
• Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet with excellent bendability and a method for manufacturing the same.
• Patent Document 2 discloses a high strength steel plate with a TS of 900 MPa or more that has excellent ductility and delayed fracture resistance, a method for manufacturing a high strength cold rolled steel sheet, and a method for manufacturing a high strength galvanized steel sheet.
• Patent Document 1 describes improvement in bendability by making martensite grains finer, it does not disclose a method for improving hydrogen embrittlement resistance.
• Patent Document 2 describes the coexistence of ductility and hydrogen embrittlement resistance, what it deals with is hydrogen embrittlement resistance after processing, and the hydrogen embrittlement resistance caused by hydrogen penetrating into the steel during manufacturing. There is no mention of improvements. It is thought that a large amount of hydrogen remains in the steel due to the manufacturing methods disclosed in these patent documents, and hydrogen embrittlement may occur during the processing process, such as deterioration of bendability or cracking in the nugget of spot welds. There is a risk.
• the present invention has been made in view of the above circumstances, and has a tensile strength (TS) of 980 MPa, which is suitable for automobile use, has a high yield ratio (YR), excellent bendability, and excellent hydrogen embrittlement resistance.
• TS tensile strength
• Yield ratio yield ratio
• the present invention provides galvanized steel sheets and members having a pressure of less than 1470 MPa, and methods for manufacturing them.
• a high yield ratio is defined by JIS Z2241 (2011), which requires that a JIS No. 5 tensile test piece (JIS Z2201) is taken in the direction perpendicular to the rolling direction, and the strain rate is 10 -3 /s. It means that YR is 0.60 or more when a tensile test is conducted according to .
• Tensile strength is determined by a tensile test in accordance with the regulations of JIS Z2241 (2011), in which a JIS No. 5 tensile test piece (JIS Z2201) is taken in the direction perpendicular to the rolling direction, and the strain rate is 10 -3 /s. Refers to the tensile strength obtained by
• Excellent bendability means that a strip-shaped test piece of 35 mm x 100 mm is taken from a galvanized steel sheet so that the bending test axis is parallel to the rolling direction, the stroke speed is 50 mm/s, the indentation load is 10 tons, and the test piece is pressed.
• a 90 degree V-bending test was performed at various bending radii, and the ridgeline at the bending apex of the test piece was observed with a 10x magnifying glass, and no cracks with a length of 0.5 mm or more were observed.
• R/t which is the value obtained by dividing the minimum bending radius R (mm) by the plate thickness (mm), is calculated and refers to satisfying any of the following (A), (B), and (C).
• A TS: 980 MPa or more and less than 1100 MPa and R/t: 5.0 or less
• B TS: 1100 MPa or more and less than 1300 MPa and R/t: 6.0 or less
• C TS: 1300 MPa or more and less than 1470 MPa and R/t: 7.0 or less
• Excellent hydrogen embrittlement resistance means that spot welding is performed by the following method, and the resulting nugget has a crack of 100 ⁇ m or less.
• Spacers with a thickness of 2 mm are sandwiched between both ends of a 30 mm x 100 mm test piece taken from a galvanized steel plate, and the center between the spacers is joined by spot welding to produce a welded test piece.
• an inverter DC resistance spot welding machine is used, and a dome-shaped electrode made of chrome copper with a tip diameter of 6 mm is used.
• the pressurizing force is 380 kgf
• the current application time is 16 cycles/50 Hz
• the holding time is 5 cycles/50 Hz.
• the welding current value is adjusted to form a nugget diameter according to the TS.
• the nugget diameter is 3.8 mm when the TS is 980 MPa or more and less than 1250 MPa, and 4.8 mm when the TS is 1250 MPa or more and less than 1470 MPa.
• the present inventors have made extensive studies to solve the above problems. As a result, in addition to optimizing the steel sheet structure, it is possible to reduce diffusible hydrogen in the steel by appropriately controlling the precipitation form of carbides in the steel and using them as hydrogen trap sites, thereby achieving a high yield ratio. We have discovered that it is possible to obtain a high-strength galvanized steel sheet that has excellent bendability and a low risk of cracking in the nuggets of spot welds.
• a galvanized steel sheet comprising a base steel sheet and a galvanized layer formed on the surface of the base steel sheet,
• the base steel plate is In area ratio, Ferrite: less than 50%, Total of martensite and bainite: 50% or more, Retained austenite: has a steel structure of 10% or less,
• area ratio 75% or more of the martensite is tempered martensite having carbides with an average grain size of 50 nm or more and 200 nm or less,
• the ratio TM1/TM2 is 0.70 or more, The tensile strength is 980 MPa or more and less than 1470 MPa, The yield ratio is 0.60 or more, A galvanized steel sheet, wherein the integrated value of the amount of hydrogen released when the base steel sheet from which the galvanized layer has been peeled is heated from room temperature to 200° C. is 0.45 mass ppm or less.
• the steel composition of the base steel sheet is in mass%, C: 0.080% or more and 0.300% or less, Si: 0.01% or more and 2.00% or less, Mn: 1.00% or more and 4.00% or less, P: 0.10% or less, S: 0.0200% or less, Al: 0.003% or more and 0.100% or less, N: Contains 0.0100% or less,
• the steel component further comprises, in mass%, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, Sb: 0.200% or less, Sn: 0.200% or less, V: 0.100% or less, Cu: 2.00% or less, Cr: 2.00% or less, Ni: 2.00% or less, Mo: 1.00% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, The galvanized steel sheet according to [2] above, containing
• a slab having the steel composition described in [2] or [3] above is heated to a temperature range of 1100 to 1350°C, hot rolled at a finish rolling finish temperature of 800 to 950°C, and then rolled at a coiling temperature of 800 to 950°C.
• a hot rolling process of winding at 650°C or less An oxidation step in which the steel sheet after the hot rolling step is heated to 600° C. or higher in an oxidizing atmosphere containing an oxygen concentration of 1000 volume ppm or more and 30000 volume ppm or less;
• a reduction step in which the steel plate after the oxidation step is held for 20 seconds or more in a reducing atmosphere at 700° C. or higher, dew point: -20° C.
• a first cooling step of cooling the steel plate after the soaking step A galvanizing step of forming a galvanized layer on the surface of the steel sheet after the first cooling step;
• the average cooling rate of the steel sheet after the galvanizing process from the temperature range of 350 to 450 °C until reaching (Ms point -100 °C): 20 °C / s or less, and 5 seconds or more in the temperature range of 100 to 200 °C a second cooling step of retaining and cooling to a cooling stop temperature of 100° C.
• a method for producing galvanized steel sheets including: [8] The method for manufacturing a galvanized steel sheet according to [7] above, wherein after the galvanizing step, the galvanized layer is formed on the surface of the steel sheet, and then alloying treatment is further performed. [9] The method for producing a galvanized steel sheet according to [7] or [8], which includes a cold rolling step of cold rolling at a rolling reduction of 20% or more after the hot rolling step and before the oxidation step. .
• the steel plate after the soaking step is average cooled from 600 to 900°C in an atmosphere with a hydrogen concentration of 0.5% by volume or more and 30% by volume or less and a dew point of 0°C or less.
• the average cooling rate from the temperature range of 350 to 450 °C to (Ms point -100 °C) is 10 °C/s or less, and the temperature range of 100 to 200 °C is 10 seconds.
• a method for producing a member comprising the step of subjecting the galvanized steel sheet according to any one of [1] to [5] to at least one of forming and bonding to produce a member.
• the galvanized steel sheet provided by the present invention has a tensile strength of 980 MPa or more and less than 1470 MPa, a yield ratio of 0.60 or more, and excellent bendability and hydrogen embrittlement resistance. Further, the member provided by the present invention has a tensile strength of 780 MPa or more and less than 1180 MPa, a yield ratio of 0.60 or more, and excellent bendability, stretch flangeability, and hydrogen embrittlement resistance.
• the galvanized steel sheet of the present invention is a galvanized steel sheet having a base steel sheet and a galvanized layer formed on the surface of the base steel sheet, wherein the base steel sheet has an area ratio of less than 50% ferrite. and has a steel structure in which the total of martensite and bainite is 50% or more and retained austenite is 10% or less, and in terms of area ratio, 75% or more of martensite is carbide with an average grain size of 50 nm or more and 200 nm or less.
• the base steel plate which has a TM2 ratio of TM1/TM2 of 0.70 or more, a tensile strength of 980 MPa or more and less than 1470 MPa, and a yield ratio of 0.60 or more, and from which the galvanized layer has been peeled, is heated from room temperature to 200°C. It is characterized in that the integrated value of the amount of hydrogen released when the temperature is increased is 0.45 mass ppm or less.
• the area ratio of each structure refers to the area ratio of each constituent phase to the total area observed.
• the area ratio of each structure can be determined from the image data obtained by polishing a steel plate cross section parallel to the rolling direction, corroding it with nital solution, and photographing it with a SEM (scanning electron microscope) at a magnification of 1500x. I can do it.
• the measurement area is a region of 1/8 of the plate thickness from the surface and a region of 1/8 to 3/8 of the plate thickness.
• ferrite is black
• bainite is black containing island-shaped retained austenite or gray containing aligned carbides
• tempered martensite is light gray containing fine carbides with random orientation
• retained austenite is distinguished as white. can.
• the area ratio of retained austenite is determined individually by measuring the X-ray diffraction intensity, and the area ratio of as-quenched martensite is determined by subtracting it from the area ratio of the white portion described above.
• the area ratio of retained austenite is the (200), (220), (311) of FCC iron relative to the integrated X-ray diffraction intensity of (200), (211), and (220) planes of BCC iron in the 1/4 plate thickness plane. It is determined from the ratio of the surface X-ray diffraction integrated intensity.
• the retained austenite is calculated as a volume fraction by the above measurement, but assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.
• the martensite defined in the present invention includes both as-quenched martensite and tempered martensite, and may be composed of as-quenched martensite and tempered martensite.
• Ferrite area ratio less than 50% While the presence of ferrite is advantageous in improving ductility, it is disadvantageous in obtaining high TS and high yield ratio. If the area ratio of ferrite is 50% or more, a TS of 980 MPa or more may not be obtained. Therefore, the area percentage of ferrite is less than 50%, preferably less than 45%.
• the lower limit is not particularly limited, and the area ratio of ferrite may be 0%, but is preferably 1% or more, more preferably 2% or more.
• Martensite is a hard structure that is generated from austenite at a low temperature below the martensite transformation start point (Ms point).
• Martensite in the present invention includes not only as-quenched martensite (as-quenched martensite) but also tempered martensite obtained by tempering generated martensite at a predetermined temperature.
• Bainite is generated from austenite at a relatively low temperature above the Ms point, and is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite.
• Martensite and bainite are structures responsible for the strength of the steel sheet of the present invention, and if the total area ratio is less than 50%, a TS of 980 MPa or more may not be obtained. Therefore, the total area ratio of martensite and bainite is 50% or more, preferably 55% or more. Note that the total area ratio of martensite and bainite may be 100%, or only one of them may be within the above range.
• Area ratio of retained austenite 10% or less Retained austenite may transform into hard martensite during bending and promote cracking, and this effect becomes significant when the retained austenite exceeds 10%. Therefore, the area ratio of retained austenite is 10% or less, preferably 5% or less.
• the retained austenite may be 0%.
• the remaining structure may contain other structures at an area ratio of 5% or less.
• Other organizations include perlite and the like.
• tempered martensite having carbides with an average grain size of 50 nm or more and 200 nm or less
• the area ratio of tempered martensite to all martensite is 75% or more, preferably 80% or more, and may be 100%. This effect can be obtained by setting the carbide size to an average particle size of 50 nm or more and 200 nm or less.
• the average particle size of the carbide is 200 nm or less.
• the average particle size is the average value of the major axis length and minor axis length when approximated to an ellipse. The average particle size can be measured by the method shown in Examples, for example.
• the galvanized steel sheet of the present invention has an area ratio of tempered martensite to the entire structure in the region from the steel sheet surface to 1/8 of the sheet thickness (hereinafter also referred to as the steel sheet surface layer).
• TM1/TM2 which is the ratio of TM1 to the area ratio TM2 (vol%) of tempered martensite in the entire structure in the region from 1/8 to 3/8 of the plate thickness, must be 0.70 or more. .
• TM1/TM2 is set to 0.70 or more.
• TM1/TM2 is preferably 0.72 or more, more preferably 0.75 or more.
• the upper limit is not particularly limited and may be 1.00.
• the surface (steel plate surface) here refers to the surface of the base steel plate (base iron, base steel plate) under the galvanized layer.
• TM1 and TM2 After polishing the cross section of the plate parallel to the rolling direction, it was corroded with nital liquid, and the area from the surface to 1/8 of the plate thickness and the area from 1/8 to 3/8 of the plate thickness were examined using SEM. , each tissue in three or more fields of view is photographed at a magnification of 1500 times. Then, TM1/TM2 is calculated from the area ratio of the obtained tempered martensite.
• the cumulative value of the amount of hydrogen released when the base steel sheet from which the galvanized layer has been peeled is heated from room temperature to 200°C: 0.45 mass ppm or less
• the galvanized steel sheet of the present invention has the carbides in martensite within the above-mentioned range. By controlling the steel so as to effectively trap hydrogen that has entered the steel during the manufacturing process, diffusible hydrogen is reduced, and the steel has excellent bendability and hydrogen embrittlement resistance.
• the cumulative amount of hydrogen released when the base steel sheet from which the plating layer has been peeled is heated from room temperature (15 to 35°C) to 200°C at 200°C/h is calculated as the amount of diffusible hydrogen in the steel. do.
• the amount of diffusible hydrogen is 0.45 mass ppm or less, preferably 0.40 mass ppm or less.
• the lower limit is not particularly limited, but since it is preferable that the amount of diffusible hydrogen is as small as possible, it may be set to 0 mass ppm.
• the galvanized steel sheet of the present invention aims to reduce diffusible hydrogen by effectively trapping hydrogen that has entered the steel during the manufacturing process by controlling the carbides in martensite within the above-mentioned range. . Hydrogen trapped in carbides is less likely to be desorbed from the steel than diffusible hydrogen, and is not released when heated up to 200°C, but is released at 350°C or higher.
• the integrated value (trapped hydrogen amount) is 0.05 mass ppm or more. More preferably, it is 0.07 mass ppm or more. Although the upper limit is not particularly limited, the integrated value of the amount of hydrogen released at 350 to 600° C. may be 1.00 mass ppm or less.
• the above-mentioned method for quantifying hydrogen in steel is to take a test piece for hydrogen analysis of approximately 5 x 30 mm from a galvanized steel sheet, remove the surface galvanized layer using a precision grinder, and immediately place it in an atmosphere of Ar gas.
• the sample is placed in a substituted quartz tube, heated to 600°C at a heating rate of 200°C/h, and the amount of hydrogen released during heating is measured using a gas chromatograph.
• the integrated value of the amount of hydrogen released in the temperature range from room temperature to 200°C is defined as the "diffusible hydrogen amount", and the integrated value of the amount of hydrogen released in the temperature range of 350°C to 600°C as the "trapped hydrogen amount". Ask for each.
• the amount of hydrogen after the production of the steel sheet is completed it is preferable to measure the amount of hydrogen after the production of the steel sheet is completed. That is, it is preferable to measure the amount of diffusible hydrogen after the production of the steel sheet is completed. Further, it is preferable that the amount of trapped hydrogen is measured after the production of the steel plate is completed. Further, it is further preferred that the measurement of the amount of diffusible hydrogen and the measurement of the amount of trapped hydrogen be carried out within 72 hours after the steel plate first reaches room temperature (40°C or less) after the reheating process specified in the present invention. preferable.
• the galvanized steel sheet of the present invention has a galvanized layer on the surface of the base steel sheet, and the galvanized layer may be an alloyed galvanized layer, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, etc. It may be a layer. This galvanized layer may be provided only on one surface of the base steel sheet, or may be provided on both surfaces.
• the galvanized layer here refers to a plating layer whose main component is Zn (Zn content is 50.0% or more), and includes, for example, a galvanized layer such as a hot-dip galvanized layer or an alloyed hot-dip galvanized layer. Examples include alloyed galvanized layers such as.
• the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al. .
• the hot-dip galvanized layer may optionally include one selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM.
• the total content of a species or two or more elements may be 0.0% by mass or more and 3.5% by mass or less.
• the Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. Note that the remainder other than the above elements are unavoidable impurities.
• the alloyed hot-dip galvanized layer (alloyed galvanized layer) is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. Additionally, the alloyed hot-dip galvanized layer may optionally be selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM. One or more types of elements may be contained in a total amount of 0.0% by mass or more and 3.5% by mass or less.
• the Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0% by mass or more, and still more preferably 8.0% by mass or more. Further, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0% by mass or less, still more preferably 13.0% by mass or less. Note that the remainder other than the above elements are unavoidable impurities.
• the amount of plating deposited on one side of the galvanized layer is not particularly limited, but is preferably 20 g/m 2 or more. Further, the amount of plating deposited on one side of the galvanized layer is preferably 80 g/m 2 or less.
• the plating adhesion amount of the galvanized layer is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("IBIT 700BK” (registered trademark) manufactured by Asahi Chemical Co., Ltd.) to 1 L of a 10% by mass hydrochloric acid aqueous solution. Next, a steel plate serving as a test material is immersed in the treatment liquid to dissolve the galvanized layer. Then, by measuring the amount of mass loss of the test material before and after melting, and dividing that value by the surface area of the base steel sheet (the surface area of the part covered with plating), the amount of plating coating (g/m 2 ) is calculated.
• a corrosion inhibitor for Fe (“IBIT 700BK” (registered trademark) manufactured by Asahi Chemical Co., Ltd.)
• C 0.080% or more and 0.300% or less C improves hardenability and makes it easier to obtain martensite and bainite. Further, by precipitating in the structure as fine carbides, hydrogen in the steel can be trapped, the amount of diffusible hydrogen is reduced, and bendability is improved. If the C content is less than 0.080%, it is impossible to satisfy all of the requirements of TS ⁇ 980 MPa, yield ratio: 0.60 or more, and excellent bendability. Therefore, the C content is preferably 0.080% or more, more preferably 0.100% or more. On the other hand, when the C content exceeds 0.300%, the hydrogen trapping effect by fine carbides is saturated, and the strength of martensite becomes excessively high, which may impair bendability. Therefore, the C content is preferably 0.300% or less, more preferably 0.250% or less.
• Si 0.01% or more and 2.00% or less
• Si is an element effective in strengthening ferrite, which has excellent ductility, and contributes to improving the balance between strength and ductility. This effect cannot be obtained when the Si content is less than 0.01%. Therefore, the Si content is preferably 0.01% or more, more preferably 0.1% or more.
• Si is also an element that suppresses the formation of carbides. If the Si content exceeds 2.00%, the formation of carbides necessary for the present invention is suppressed, and excellent bendability may not be obtained. Therefore, the Si content is preferably 2.00% or less, more preferably 1.50% or less.
• Mn 1.00% or more and 4.00% or less Mn improves the hardenability of steel, making it easier to obtain martensite and bainite.
• the Mn content is preferably 1.00% or more, more preferably 1.50% or more.
• the Mn content is preferably 4.00% or less, more preferably 3.50% or less.
• the P content is preferably 0.10% or less, more preferably 0.05% or less.
• the P content is preferably 0.03% or less, more preferably 0.02% or less.
• the lower limit is not particularly limited, since P is an element effective in increasing the strength of steel sheets through solid solution strengthening, in order to obtain such an effect, the P content is preferably 0.001% or more. .
• the P content may be 0.002% or more, or 0.005% or more.
• the S content is preferably 0.0200% or less, more preferably 0.0100% or less.
• the S content may be set to 0.0001% or more due to production technology constraints.
• the S content may be 0.0002% or more, or 0.0004% or more.
• Al 0.003% or more and 0.100% or less
• Al also contributes to solid solution strengthening of steel. These effects may not be obtained if the Al content is less than 0.003%. Therefore, the Al content is preferably 0.003% or more.
• the Al content is preferably 0.005% or more, and preferably 0.007% or more.
• the Al content is preferably 0.100% or less, more preferably 0.050% or less.
• N 0.0100% or less N forms coarse nitrides and becomes a starting point for void generation, which may reduce bendability.
• the N content is preferably 0.0100% or less, more preferably 0.0060% or less.
• the N content may be set to 0.0005% or more due to production technology constraints.
• the remainder other than the above consists of Fe and unavoidable impurities. It is preferable that the steel sheet of the present invention has a composition containing the above-mentioned components, with the remainder consisting of Fe and inevitable impurities.
• the component composition may further optionally contain a predetermined amount of at least one selected from the following element groups. When the following arbitrary elements are included in amounts less than the preferable lower limit, the arbitrary elements can be included as unavoidable impurities.
• B 0.0100% or less
• B is an effective element for improving the hardenability of steel.
• the B content is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more. More preferably, the content is 0.0005% or more. It is more preferable that the B content is 0.0007 or more.
• the B content is preferably 0.0100% or less, more preferably 0.0050% or less, and 0.0030% or less. It is even more preferable.
• Ti 0.200% or less Ti precipitates fine carbides and contributes to increased strength and hydrogen trapping effect.
• the lower limit of Ti is not particularly limited, but in order to obtain these effects, it is preferably 0.001% or more, more preferably 0.005% or more, and preferably 0.010% or more. More preferred.
• the Ti content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.060% or less.
• Nb 0.200% or less Nb precipitates fine carbides and contributes to increased strength and hydrogen trapping effect.
• the lower limit of Nb is not particularly limited, but in order to obtain these effects, it is preferably 0.001% or more, more preferably 0.005% or more, and preferably 0.010% or more. More preferred.
• the Nb content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.060% or less.
• Sb 0.200% or less
• Sb is an element effective in suppressing excessive decarburization of the steel plate surface and preventing a decrease in the amount of martensite produced.
• the Sb content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.
• the Sb content is preferably 0.200% or less.
• the Sb content is more preferably 0.060% or less, and even more preferably 0.020% or less.
• Sn 0.200% or less
• Sn is an effective element for suppressing decarburization, denitrification, etc., and suppressing a decrease in strength of steel.
• the Sn content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.
• the Sn content is preferably 0.200% or less.
• the Sn content is more preferably 0.060% or less, and even more preferably 0.020% or less.
• V 0.100% or less V precipitates fine carbides and contributes to increased strength and hydrogen trapping effect.
• the lower limit of V is not particularly limited, but in order to obtain these effects, it is preferably 0.001% or more, more preferably 0.002% or more, and preferably 0.005% or more. More preferred.
• the V content is preferably 0.007% or more, and preferably 0.009% or more.
• carbides may become coarse and cause deterioration in bendability. Therefore, when containing V, the V content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less.
• Cu 2.00% or less
• Cu is an element that increases hardenability and is an effective element for bringing the area ratio of the hard phase within a more suitable range.
• the Cu content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.
• the Cu content is preferably 0.040% or more, and preferably 0.060% or more.
• the Cu content is preferably 2.00% or less, more preferably 1.00% or less, and 0.50% or less. is even more preferable.
• the Cr content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.
• the Cr content is preferably 0.007% or more, and preferably 0.009% or more.
• the Cr content is preferably 2.00% or less, more preferably 1.00% or less, and 0.80% or less. is even more preferable.
• Ni 2.00% or less
• the Ni content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.
• the Ni content is preferably 0.030% or more, and preferably 0.040% or more.
• the Ni content is preferably 2.00% or less, more preferably 1.00% or less, and 0.80% or less. is even more preferable.
• the Ni content is preferably 0.60% or less, more preferably 0.40% or less.
• the Mo content is preferably 0.005% or more, more preferably 0.01% or more, and even more preferably 0.02% or more.
• the Mo content is preferably 0.03% or more, and preferably 0.04% or more.
• the Mo content is preferably 1.00% or less, more preferably 0.80% or less, and 0.60% or less. is even more preferable.
• the Mo content is preferably 0.50% or less, more preferably 0.40% or less.
• the Ta content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. Further, when Ta is contained, the Ta content is preferably 0.100% or less from the viewpoint of preventing cost increases. The Ta content is more preferably 0.050% or less, and even more preferably 0.020% or less. The Ta content is preferably 0.010% or less, more preferably 0.008% or less.
• the W content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
• the W content is preferably 0.005% or more, and preferably 0.007% or more.
• the W content is preferably 0.500% or less, more preferably 0.450% or less, and 0.400% or less. is even more preferable.
• the W content is preferably 0.350% or less, more preferably 0.300% or less.
• the Zr content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more. Further, when containing Zr, the Zr content is preferably 0.020% or less from the viewpoint of preventing cost increases. The Zr content is more preferably 0.010% or less, and even more preferably 0.0050% or less.
• the Ca content is preferably 0.0200% or less.
• the Ca content is more preferably 0.0100% or less, and even more preferably 0.0050% or less.
• the Ca content is preferably 0.0040% or less, more preferably 0.0030% or less.
• the lower limit of the Ca content is not particularly limited and may be 0.0000%, but due to production technology constraints, the Ca content is preferably 0.0001% or more.
• the Ca content is more preferably 0.0005% or more.
• Mg 0.0200% or less
• Mg content is preferably 0.0200% or less.
• the Mg content is more preferably 0.0100% or less, and even more preferably 0.0050% or less.
• the Zn content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. Further, when containing Zn, the Zn content is preferably 0.020% or less from the viewpoint of preventing cost increases. The Zn content is more preferably 0.010% or less, and even more preferably 0.008% or less.
• Co 0.020% or less
• the Co content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
• the Co content is preferably 0.020% or less from the viewpoint of preventing cost increases.
• the Co content is more preferably 0.010% or less, and even more preferably 0.008% or less.
• each content of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, REM (excluding Ce): 0.0200% or less By adding these elements, the ultimate deformation of the steel sheet It is possible to obtain the effect of improving performance and stretch flangeability. In order to obtain this effect, it is preferable that at least one of these elements be contained in an amount of 0.0001% or more. On the other hand, from the viewpoint of preventing cost increases, when at least one of these elements is contained, the content of each is preferably 0.0200% or less. Ce is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Ce is 0.0150% or less, and even more preferably that it is 0.0100% or less.
• Ce is preferably 0.0080% or less, more preferably 0.0060% or less.
• Se is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Se is preferably 0.0007% or more, more preferably 0.0009% or more.
• Se is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Se is preferably 0.0080% or less, more preferably 0.0060% or less.
• Te is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Te is preferably 0.0007% or more, more preferably 0.0009% or more.
• Te is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Ge is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Ge is preferably 0.0007% or more, more preferably 0.0009% or more.
• Ge is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• it is more preferable that it is 0.0002% or more, and it is still more preferable that it is 0.0005% or more.
• Sr is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Sr is preferably 0.0007% or more, more preferably 0.0009% or more.
• Sr is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Cs is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Cs is preferably 0.0007% or more, more preferably 0.0009% or more.
• Cs is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Hf is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Hf is preferably 0.0007% or more, more preferably 0.0009% or more.
• Hf is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Pb is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Pb is preferably 0.0007% or more, more preferably 0.0009% or more.
• Pb is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Pb is preferably at most 0.0080%, more preferably at most 0.0060%.
• Bi is more preferably 0.0002% or more, and even more preferably 0.0005% or more.
• Bi is preferably 0.0007% or more, more preferably 0.0009% or more. Bi is more preferably 0.0150% or less, and even more preferably 0.0100% or less. Bi is preferably at most 0.0080%, more preferably at most 0.0060%. REM is more preferably 0.0002% or more, and even more preferably 0.0005% or more. REM is preferably 0.0007% or more, more preferably 0.0009% or more. REM is more preferably 0.0150% or less, and even more preferably 0.0100% or less. Note that the REM defined in the present invention excludes the above-mentioned Ce.
• REM as used in the present invention refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• the REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REMs.
• REM is not particularly limited, Sc, Y, and La are preferable.
• the method for producing galvanized steel sheets of the present invention involves heating a slab having the above-mentioned composition to a temperature range of 1100 to 1350°C, hot rolling at a finish rolling end temperature of 800 to 950°C, and coiling at a temperature of 650°C.
• a hot rolling process in which the steel sheet is coiled at a temperature below °C; an oxidation process in which the steel sheet after the hot rolling process is heated to 600°C or above in an oxidizing atmosphere containing an oxygen concentration of 1000 volume ppm or more and 30000 volume ppm or less;
• a reduction process in which the steel plate is held at 700°C or higher in a reducing atmosphere with a dew point of -20°C or lower and a hydrogen concentration of 8% to 30% by volume for 20 seconds or more, and the steel plate after the reduction process is heated to 750°C or higher.
• the steel plate after the soaking step is cooled by holding it for 20 seconds or more and 300 seconds or less in a soaking atmosphere with a dew point of -20 ° C. or less and a hydrogen concentration of 0.2 volume % or more and 8 volume % or less.
• the first cooling step forming a galvanized layer on the surface of the steel sheet after the first cooling step, the galvanizing step, and the steel sheet after the plating step from a temperature range of 350°C to 450°C (Ms point -
• a second cooling step in which the average cooling rate until the temperature reaches 100°C is 20°C/s or less, and the temperature is maintained in the temperature range of 100 to 200°C for 5 seconds or more to cool down to a cooling stop temperature of 100°C or less.
• a reheating step in which the steel plate after the cooling step is held in a temperature range of 100 to 450°C for 5 to 600 seconds.
• the slab heating temperature is set to 1100° C. or higher.
• the slab heating temperature is preferably 1150°C or higher.
• the slab heating temperature is 1350°C or lower, preferably 1300°C or lower.
• Finish rolling finish temperature 800-950°C If the finish rolling end temperature is less than 800° C., ferrite transformation occurs during rolling, and expanded ferrite is generated on the surface layer of the hot rolled sheet and remains even after the next step, which may deteriorate the final bendability. Therefore, the finish rolling end temperature is 800°C or higher, preferably 850°C or higher. On the other hand, if the finish rolling end temperature exceeds 950° C., crystal grains become coarse, which may cause insufficient strength or deterioration of bendability. Therefore, the finishing rolling temperature is 950°C or lower, preferably 930°C or lower.
• the winding temperature is 650°C or lower, preferably 600°C or lower.
• the lower limit of the winding temperature is not particularly limited, it is preferably 400° C. or higher from the viewpoint of suppressing the occurrence of shape defects in the steel sheet and preventing the steel sheet from becoming excessively hard.
• Cold rolling process Before the steel plate after the hot rolling process (hot rolling process) is subjected to the oxidation process, cold rolling may be performed as necessary.
• the hot rolled steel sheet obtained after the hot rolling process (hot rolling process) may be subjected to pretreatment such as pickling and degreasing by a known method, and then the process may be carried out under the following conditions. preferable.
• the rolling reduction rate (cumulative reduction rate) is not particularly limited, but if the rolling reduction rate (cumulative reduction rate) of cold rolling is less than 20%, recrystallization of ferrite will be promoted. First, unrecrystallized ferrite remains, and the steel structure of the present invention may not be obtained. Therefore, the rolling reduction (cumulative rolling reduction) in the cold rolling process is preferably 20% or more, more preferably 30% or more. Although the upper limit of the rolling reduction rate is not particularly limited, it is preferable that the rolling reduction rate is 80% or less.
• the oxidation step is a necessary step in order to improve the wettability of the steel sheet during hot-dip galvanizing immersion in the galvanizing step, etc., and to obtain a good appearance.
• an iron oxide layer is formed on the surface of the steel sheet, and a reduced iron layer is generated during subsequent annealing in a reducing atmosphere, thereby improving the wettability of the steel sheet.
• Oxygen concentration 1000 volume ppm or more and 30000 volume ppm or less
• the oxygen concentration in the oxidizing atmosphere needs to be within a predetermined range. If the oxygen concentration is less than 1000 ppm by volume, the formation of the iron oxide layer may be insufficient. Therefore, the oxygen concentration is 1000 volume ppm or more, preferably 1500 volume ppm or more. On the other hand, if the oxygen concentration exceeds 30,000 volume ppm, excessive iron oxide is formed, and the reduction is not completed before the galvanizing process such as hot-dip galvanizing immersion, and the plating wettability may deteriorate due to the remaining iron oxide. There is. Therefore, the oxygen concentration is 30,000 volume ppm or less, preferably 25,000 volume ppm or less. The oxygen concentration is preferably 22,000 volume ppm or less, more preferably 20,000 volume ppm or less.
• the steel plate temperature (maximum temperature reached during oxidation treatment) 600°C or higher If the steel plate temperature (maximum temperature reached during oxidation treatment) is less than 600°C, a small amount of iron oxide is formed on the surface, which may cause plating wetting during hot-dip plating, etc. The properties may be insufficient and a good appearance may not be obtained. Therefore, the steel plate temperature (the highest temperature reached during oxidation treatment) is 600°C or higher, preferably 620°C or higher.
• the upper limit of the steel plate temperature is not particularly limited, but is preferably 900°C in order to better prevent unreduced iron oxide from remaining in the subsequent reduction process due to excessive oxidation of the steel plate and to more stably obtain excellent plating properties.
• the temperature is preferably 880°C or lower, more preferably 880°C or lower.
• the steel plate temperature in the oxidation step (the highest temperature reached during oxidation treatment) is preferably 860°C or lower, more preferably 840°C or lower.
• a reduced iron layer is formed by heating the surface iron oxide generated in the oxidation step in an iron-reducing atmosphere.
• the atmosphere has a relatively low hydrogen concentration, so the iron reduction reaction rate becomes slow. Therefore, it is necessary to complete the reduction of iron oxide in the reduction step.
• the steel plate temperature in the reduction step is 700°C or higher, preferably 750°C or higher.
• the upper limit of the steel plate temperature is not particularly limited, it is preferably 950° C. or lower from the viewpoint of reducing the load on the furnace body.
• the steel plate temperature in the reduction step is preferably 920°C or lower, more preferably 900°C or lower.
• Retention time (retention time during reduction treatment): 20 seconds or more If the retention time in the reduction step (retention time during reduction treatment) is less than 20 seconds, the reduction of iron oxide may not be completed. Therefore, the holding time of the reduction step is 20 seconds or more, preferably 25 seconds or more.
• the upper limit of the holding time is not particularly limited, but from the viewpoint of productivity it is preferably 150 seconds or less.
• the holding time is preferably 120 seconds or less, more preferably 100 seconds or less.
• Hydrogen concentration (hydrogen concentration during reduction treatment): 8% by volume or more and 30% by volume or less If the hydrogen concentration in the atmosphere during the reduction process (hydrogen concentration during reduction treatment) is less than 8% by volume, unreduced iron oxide remains. There is. Therefore, the hydrogen concentration in the reduction step is 8% by volume or more, preferably 10% by volume or more. On the other hand, if it exceeds 30% by volume, the reduction rate becomes saturated and hydrogen penetration into the steel increases excessively, making it difficult to sufficiently reduce the amount of hydrogen in the steel in the next soaking step. Therefore, the hydrogen concentration in the reduction step is 30% by volume or less, preferably 28% by volume or less. The hydrogen concentration is preferably 26% by volume or less, more preferably 24% by volume or less.
• the lower limit of the dew point is not particularly limited, but is preferably ⁇ 50° C. or higher for ease of industrial control.
• the upper limit of the steel plate temperature in the soaking step is not particularly limited, but is preferably 950° C. or lower from the viewpoint of reducing the load on the furnace body and preventing excessive coarsening of austenite grains.
• the annealing temperature is preferably 930°C or lower, more preferably 910°C or lower.
• Holding time (holding time during soaking treatment): 20 s or more and 300 s or less If the holding time in the soaking step is less than 20 s, hydrogen in the steel may not be sufficiently reduced. Further, the formation of austenite may be insufficient, and the steel structure of the present invention may not be obtained. Therefore, the holding time is 20 seconds or more, preferably 50 seconds or more. On the other hand, if the holding time exceeds 300 seconds, coarsening of austenite grains and decarburization of the surface layer progress, and the steel structure of the present invention may not be obtained. Therefore, the holding time is 300 seconds or less, preferably 200 seconds or less. The holding time is preferably 180 seconds or less, more preferably 160 seconds or less.
• Hydrogen concentration (hydrogen concentration during soaking treatment): 0.2% by volume or more and 8% by volume or less Since the reduction of iron oxide has been completed in the reduction process, the atmosphere in the soaking process should be kept as low as possible so that the reduced iron does not re-oxidize. The aim is to reduce the amount of hydrogen that has entered the steel during the reduction process. When the hydrogen concentration exceeds 8% by volume, hydrogen in the steel cannot be sufficiently reduced and the steel sheet of the present invention cannot be obtained. Therefore, the hydrogen concentration is 8% by volume or less, preferably 5% by volume or less. On the other hand, if the hydrogen concentration is less than 0.2% by volume, there is a risk that reduced iron will be reoxidized. Therefore, the hydrogen concentration in the soaking atmosphere is 0.2% by volume or more, preferably 0.5% by volume or more. The hydrogen concentration is preferably 0.8% by volume or more, more preferably 1.0% by volume or more.
• the dew point of the atmosphere in the reduction step is -20°C or lower, preferably -25°C or lower.
• the lower limit of the dew point is not particularly limited, but is preferably ⁇ 50° C. or higher for ease of industrial control.
• Hydrogen concentration 0.5% by volume or more and 30% by volume or less
• dew point dew point during first cooling treatment: 0°C or less
• the hydrogen concentration of the cooling atmosphere in the first cooling step 0.5% by volume or more and the dew point to 0° C. or less
• the hydrogen concentration is 1.0% by volume or more and the dew point is -20°C or less.
• the hydrogen concentration is preferably 1.5% by volume or more, more preferably 2.0% by volume or more.
• the dew point is preferably -25°C or lower, more preferably -30°C or lower.
• the lower limit of the dew point is also not particularly limited, but the dew point is preferably -55°C or higher, more preferably -50°C or higher.
• the hydrogen concentration is more preferably 20% by volume or less, and even more preferably 10% by volume or less.
• the cooling stop temperature is 150°C or higher.
• the average cooling rate (° C./s) is obtained from (steel plate temperature (600 to 900° C.) ⁇ cooling stop temperature (150 to 500° C.))/cooling time (s) from the start of cooling to the stop of cooling.
• a galvanized layer is formed on the surface of the steel sheet after the first cooling step.
• a typical method is to immerse a steel plate in a hot-dip galvanizing bath. Below, immersion in a hot-dip galvanizing bath will be explained as an example.
• the conditions for immersion in the hot-dip galvanizing bath are not particularly limited, and a general method may be used.
• the hot-dip galvanizing bath consists of Al, Zn, and inevitable impurities, and its composition is not particularly specified, but in one example, the Al concentration in the bath may be 0.05% by mass or more and 0.190% by mass or less. could be. If the Al concentration in the bath is 0.05% by mass or more, the generation of bottom dross can be more suitably prevented. Moreover, if the Al concentration in the bath is 0.190% by mass or less, the generation of top dross can be more suitably prevented. Also from the viewpoint of cost, it is preferable that the Al concentration in the bath is 0.190% by mass or less.
• the plating bath temperature is also not particularly specified, but may be 440°C or higher and 500°C or lower.
• the galvanizing applied in the present invention is not limited to the above-described hot-dip galvanizing, but may be other methods such as electrolytic galvanizing or electrolytic zinc-based plating, as long as the desired steel sheet temperature is satisfied in the preceding and succeeding steps.
• the amount of plating deposited per side is not particularly limited, but in one example, it is 25 g/m 2 or more and 120 g/m 2 or less. When the amount of plating deposited on one side is 25 g/m 2 or more, corrosion resistance is particularly good, and control of the amount of plating deposited is particularly easy. Further, if the amount of plating deposited per side is 120 g/m 2 or less, the plating adhesion is particularly good.
• the method for adjusting the coating amount is not particularly limited, but as an example in the case of hot-dip galvanizing, gas wiping can be used and adjustment can be made by adjusting the gas pressure and the distance between the wiping nozzle and the steel plate.
• alloying treatment may be performed to produce an alloyed galvanized steel sheet.
• an alloyed galvanized layer as the galvanized layer, desorption of hydrogen in the steel in the subsequent reheating step can be further promoted.
• the conditions for the alloying treatment are not particularly limited, as long as a desired degree of alloying can be obtained.
• the steel plate temperature can be 440°C or higher.
• the steel plate temperature can be 600°C or less.
• the time for holding can be 5 seconds or more.
• the holding time can be 60 seconds or less.
• the Fe content (alloying degree) in the plating layer is 7% by mass or more. Further, the Fe content (degree of alloying) is preferably 15% by mass or less.
• the degree of alloying is preferably 15% by mass or less.
• the degree of alloying exceeds 15% by mass, the formation of ⁇ phase at the interface between the alloyed galvanized layer and the base steel sheet may be promoted and the plating adhesion may decrease, so the degree of alloying should be 15% by mass or less. is preferred.
• the steel plate after the galvanizing process is cooled to a cooling stop temperature of 100°C or less.
• the cooling method is not particularly limited as long as it satisfies the following cooling conditions, and for example, N 2 gas cooling, water cooling, or a combination thereof may be used.
• Cooling stop temperature 100°C or less
• martensite is generated by cooling untransformed austenite to a lower temperature range than the Ms point. Since the hydrogen diffusion rate is faster in martensite, which is BCT, than in austenite, which has a crystal structure of FCC, hydrogen reduction in the subsequent reheating step can be promoted.
• BCT martensite
• austenite which has a crystal structure of FCC
• the steel sheet of the present invention may have 0% residual austenite, all untransformed austenite may be transformed to martensite at this point. Therefore, the lower limit of the cooling stop temperature is not particularly limited, but from the viewpoint of industrial ease of implementation, the cooling stop temperature is, for example, 0° C. or higher.
• Average cooling rate from the temperature range of 350 to 450°C to (Ms point -100°C) (average cooling rate during second cooling process): 20°C/s or less
• Ms cooling stop temperature
• the average cooling rate is more than 20° C./s, self-tempering becomes insufficient and the carbide cannot be sufficiently obtained.
• the average cooling rate is 20°C/s or less, preferably 10°C/s or less, more preferably 5°C/s or less.
• the lower limit of the cooling rate is not particularly limited, but from the viewpoint of more stably preventing the carbide size (average particle size of carbides contained in martensite) from exceeding 200 nm due to excessive slow cooling, it is 1°C. /s or more is preferable.
• the average cooling rate (°C/s) is "(cooling start temperature (350 to 450°C)) - (Ms - 100°C)) / cooling time from the start of cooling to (Ms - 100°C) (s)" ” can be obtained from
• the residence time is 5 seconds or more, preferably 10 seconds or more, more preferably 20 seconds or more.
• the residence time is preferably 100 seconds or less, more preferably 80 seconds or less, in order to more stably prevent the carbide size from exceeding 200 nm.
• Ms point is the martensitic transformation start temperature, and can be determined by the following formula.
• Ms (°C) 539-423 ⁇ ⁇ [C mass%] ⁇ 100/(100-[ ⁇ area %]) ⁇ -30 ⁇ [Mn mass%] -12 ⁇ [Cr mass%] -18 ⁇ [Ni mass %]-8 ⁇ [Mo mass %]
• [M mass %] (M: element) is the amount of each element contained in the steel sheet.
• [ ⁇ area %] is the ferrite area ratio (%) in the steel sheet structure after annealing.
• the hydrogen concentration in the reheating step is 0.2% by volume or less.
• Reheating temperature 100° C. or more and 450° C. or less If the reheating temperature is less than 100° C., martensite is insufficiently tempered, and the carbide and tempered martensite required for the present invention cannot be obtained. Further, if the temperature is lower than 100°C, the effect of reducing diffusible hydrogen in steel will be insufficient. Therefore, the reheating temperature is 100°C or higher, preferably 120°C. On the other hand, if the reheating temperature exceeds 450°C, excessive tempering causes a decrease in tensile strength and coarsening of carbides, making it impossible to obtain the steel plate of the present invention. Moreover, if the temperature exceeds 450°C, the appearance of the zinc plating may be impaired. Therefore, the reheating temperature is 450°C or lower, preferably 430°C or lower.
• Holding time (holding time during reheating treatment): 5-600s If the holding time is less than 5 seconds, martensite will not be sufficiently tempered, the carbides and tempered martensite required for the present invention will not be obtained, and diffusible hydrogen in steel will not be sufficiently reduced. Therefore, the holding time is 5 seconds or more, preferably 10 seconds or more. The holding time is more preferably 15 seconds or more, more preferably 20 seconds or more. On the other hand, if the holding time exceeds 600 seconds, not only will the production efficiency be reduced, but also excessive tempering will cause a decrease in tensile strength and coarsening of carbides, and the steel sheet of the present invention may not be obtained. Therefore, the retention time is 600 seconds or less, preferably 500 seconds or less. The holding time is more preferably 400 seconds or less, more preferably 300 seconds or less.
• the plate thickness is not particularly limited, it is preferably 0.4 mm or more, and more preferably 0.6 mm or more. Further, the plate thickness is preferably 3.2 mm or less, more preferably 3.0 mm or less.
• a member according to an embodiment of the present invention is a member made of (made of) the above-mentioned galvanized steel plate.
• the material is a galvanized steel plate that is subjected to at least one of forming and joining processes to produce a member.
• the above-mentioned galvanized steel sheet has a high yield ratio (YR), excellent bendability, excellent hydrogen embrittlement resistance, and a tensile strength (TS) of 980 MPa or more and less than 1470 MPa.
• the member according to one embodiment of the present invention has a high yield ratio (YR), excellent bendability, excellent hydrogen embrittlement resistance, and a tensile strength (TS) of 980 MPa or more and less than 1470 MPa. Therefore, a member according to an embodiment of the present invention is particularly suitable as a member for use in the automotive field.
• YiR yield ratio
• TS tensile strength
• a method for manufacturing a member according to an embodiment of the present invention includes performing at least one of forming processing and joining processing on the above-described galvanized steel sheet (for example, a galvanized steel sheet manufactured by the above-described method for manufacturing a galvanized steel sheet). It has a process of making it into a member.
• the molding method is not particularly limited, and for example, a general processing method such as press working can be used.
• the joining method is not particularly limited, and for example, common welding such as spot welding, laser welding, arc welding, rivet joining, caulking joining, etc. can be used.
• the molding conditions and bonding conditions are not particularly limited, and conventional methods may be followed.
• hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets were manufactured under the conditions shown in Table 2, and the following evaluations were performed.
• the slab heating temperature was 1250°C
• the finish rolling end temperature was 900°C
• the winding temperature was 520°C
• the rolling ratio was 50%.
• the reheating treatment in the reheating step was performed in the atmosphere.
• the evaluation results are shown in Table 3. Note that the galvanizing treatment (hot-dip galvanizing treatment) and alloying treatment in the galvanizing process were performed under the following conditions.
• ⁇ Tensile test> A JIS No. 5 tensile test piece (JIS Z2201) was taken from the obtained galvanized steel sheet in a direction perpendicular to the rolling direction, and subjected to tensile testing in accordance with the provisions of JIS Z2241 (2011) at a strain rate of 10 ⁇ 3 /s.
• TS yield strength
• TS tensile strength
• EL elongation
• those with TS of 980 MPa or more and less than 1470 MPa and YR of 0.60 or more were defined as examples of the present invention.
• a test piece for microstructural observation was taken from the obtained galvanized steel sheet, and after polishing the thickness cross section parallel to the rolling direction, it was corroded with nital liquid, and an area of 1/8 of the sheet thickness from the surface and the sheet thickness were examined using SEM. Tissues were photographed in three fields of view at a magnification of 1500 times for a region of 1/8 to 3/8.
• the area ratio of retained austenite was determined individually by measuring the X-ray diffraction intensity, and the area ratio of as-quenched martensite was determined by subtracting it from the area ratio of the white part (as-quenched martensite and retained austenite).
• the area ratio of retained austenite is the (200), (220), (311) of FCC iron relative to the integrated X-ray diffraction intensity of (200), (211), and (220) planes of BCC iron in the 1/4 plate thickness plane. It was determined from the ratio of the surface X-ray diffraction integrated intensity.
• the retained austenite was calculated as a volume fraction by the above measurement, but assuming that the retained austenite was three-dimensionally homogeneous, the volume fraction of the retained austenite was taken as the area fraction of the retained austenite.
• the area ratio of each constituent phase was determined from the obtained image by the method described above. For the constituent phase area ratio of each region, the average value of all the images taken was used.
• the structure of one field of view was photographed using SEM at a magnification of 5,000 times in a region between 1/8 and 3/8 of the plate thickness of the steel plate (base steel plate), and the images revealed the carbides present inside the prior austenite grains containing martensite.
• the number and total area of the carbides were determined, and the area per carbide was calculated to obtain the average particle size of the carbides.
• the average particle size is the average value of the major axis length and minor axis length when approximated to an ellipse.
• a region that is integrally formed without interruption is measured as one.
• those having a diffusible hydrogen amount of 0.45 mass ppm or less were defined as examples of the present invention.
• the measurement of the amount of diffusible hydrogen and the measurement of the amount of trapped hydrogen were performed after the production of the steel plate was completed.
• ⁇ Bending test> A strip-shaped test piece of 35 mm x 100 mm was taken from the obtained galvanized steel sheet so that the direction parallel to the rolling direction was the bending test axis, and the stroke speed was 50 mm/s, the pushing load was 10 tons, and the pushing holding time was 5 seconds.
• TS 980 MPa or more and less than 1100 MPa and R/t: 5.0 or less
• B TS: 1100 MPa or more and less than 1300 MPa and R/t: 6.0 or less
• C TS: 1300 MPa or more and less than 1470 MPa and R/t: 7.0 or less
• ⁇ Hydrogen embrittlement resistance evaluation> A 30 mm x 100 mm test piece was taken from the obtained galvanized steel plate, spacers with a thickness of 2 mm were sandwiched between both ends, and the center between the spacers was joined by spot welding to prepare a welded test piece.
• spot welding an inverter DC resistance spot welding machine was used, and a dome-shaped electrode made of chromium copper and having a tip diameter of 6 mm was used.
• the pressurizing force was 380 kgf
• the current application time was 16 cycles/50 Hz
• the holding time was 5 cycles/50 Hz.
• the welding current value was adjusted to form a nugget diameter according to the TS.
• the nugget diameter was 3.8 mm when the TS was less than 1250 MPa, and 4.8 mm when the TS was 1250 MPa or more. After being left for 24 hours after spot welding, the spacer portion was cut off, the cross section of the nugget was observed, and the nugget was evaluated based on the following criteria, and those with a rank of 1 or 2 were considered to be within the preferred range of the present invention. Crack observation results Rank: No cracks: 1 (particularly excellent in hydrogen embrittlement resistance) Only microcracks of 100 ⁇ m or less occur: 2 (excellent hydrogen embrittlement resistance) Cracks exceeding 100 ⁇ m: 3 (poor hydrogen embrittlement resistance)
• the galvanized steel sheet of the invention example has a TS of 980 MPa or more and less than 1470 MPa, a YR of 0.60 or more, and a diffusible hydrogen in the steel of 0.45 mass ppm or less, and has excellent bendability and hydrogen embrittlement resistance.
• Ta The steel sheet of the comparative example was inferior in at least one of TS, YR, bendability, and hydrogen embrittlement resistance.
• the members obtained by forming or joining the galvanized steel sheets of the invention examples have a TS of 980 MPa or more and less than 1470 MPa, a YR of 0.60 or more, and The medium diffusible hydrogen was 0.45 mass ppm or less, and the bendability and hydrogen embrittlement resistance were excellent.
• a galvanized steel sheet having a TS of 980 MPa or more and less than 1470 MPa, a yield ratio of 0.60 or more, and excellent bendability and hydrogen embrittlement resistance, which is suitable mainly for use in automobile parts. Can be done.

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• 2023
  • 2023-03-31 JP JP2024512883A patent/JP7613636B2/ja active Active
  • 2023-03-31 WO PCT/JP2023/013450 patent/WO2023191021A1/ja not_active Ceased

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