WO2020196149A1 - 溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板およびその製造方法 - Google Patents

溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板およびその製造方法 Download PDF

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WO2020196149A1
WO2020196149A1 PCT/JP2020/011959 JP2020011959W WO2020196149A1 WO 2020196149 A1 WO2020196149 A1 WO 2020196149A1 JP 2020011959 W JP2020011959 W JP 2020011959W WO 2020196149 A1 WO2020196149 A1 WO 2020196149A1
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Prior art keywords
steel sheet
molten
less
hot
plated steel
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Ceased
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PCT/JP2020/011959
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English (en)
French (fr)
Inventor
真也 植杉
健太郎 平田
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to US17/438,440 priority Critical patent/US20220154320A1/en
Priority to MX2021010939A priority patent/MX2021010939A/es
Priority to KR1020217032242A priority patent/KR20210135577A/ko
Priority to CN202080020996.6A priority patent/CN113631736A/zh
Publication of WO2020196149A1 publication Critical patent/WO2020196149A1/ja
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • 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
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
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    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
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    • 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
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    • 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
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/10—Oxidising
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    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/003—Cementite
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Definitions

  • the present invention relates to a molten Zn-Al-Mg-based plated steel sheet and a method for producing the same.
  • Patent Document 1 by adding Si, Nb and Ti to a steel sheet, the hardness difference between a hard phase such as martensite and bainite structure and a ferrite phase is reduced, so that a high tensile strength of 780 MPa or more is obtained. And a technique for achieving both workability and workability are disclosed.
  • Patent Document 2 discloses a hot-dip Zn—Al—Mg-based galvanized steel sheet having a high tensile strength of 780 MPa or more and excellent bending workability.
  • Japanese Patent Publication Japanese Patent Laid-Open No. 2006-283156
  • Japanese Patent Publication Japanese Patent Laid-Open No. 2014-189812
  • Patent Document 1 the technique described in Patent Document 1 is not suitable for plated steel sheets. Further, according to the manufacturing method described in Patent Document 2, the amount of martensite after plating may decrease depending on the hot rolling conditions, so that the strength of 780 MPa or more may not be stably obtained. ..
  • One aspect of the present invention is to realize a molten Zn-Al-Mg-based galvanized steel sheet that stably achieves both a tensile strength of 780 MPa or more and high workability.
  • the molten Zn-Al-Mg-based plated steel sheet according to one aspect of the present invention has a fused Zn-Al-Mg based plated layer on the surface of the steel base material.
  • the steel base material is C: 0.050 to 0.180%, Si: 0.001 to 0.50%, Mn: 1.00 to 2.80%, Ti: in mass%. It contains 0.01 to 0.10% and B: 0.005 to 0.0100%, the balance contains Fe and unavoidable impurities, and the average grain size of cementite after winding in the hot rolling process is 2 ⁇ m.
  • the metal structure after the continuous hot-dip zinc plating step has a ferrite phase and a second phase having an area ratio of 15% or more and less than 45%, and the second phase is made of martensite or martensite and bainite. It is composed and has an average crystal grain size of 8 ⁇ m or less.
  • the method for producing a molten Zn—Al—Mg-based plated steel sheet includes a hot rolling step, a cold rolling step, annealing and molten Zn—Al—Mg.
  • the hot rolling step the average cooling rate after hot rolling is 20 ° C./sec or more and less than 80 ° C./sec, and the winding temperature is 400 ° C. or more and less than 600 ° C.
  • molten Zn-Al-Mg-based galvanized steel sheet that stably achieves both a tensile strength of 780 MPa or more and high workability.
  • (C) C is an element necessary for increasing the strength of steel.
  • a C content of 0.050% or more is required.
  • the C content is limited to 0.180% or less, and may be controlled to 0.160% or less.
  • Si Si
  • Si silicon
  • Mn Mn manganese Mn (manganese) is effective for increasing the strength.
  • a Mn content of 1.00% or more is secured in order to stably obtain a strength level of 780 MPa or more in tensile strength.
  • the Mn content is set to 2.80% or less.
  • Ti titanium
  • Ti titanium
  • S sulfur
  • N nitrogen
  • B which is necessary for suppressing the austenite-ferrite transformation, easily binds to N
  • the addition of Ti is effective in ensuring the content of solid-solved B.
  • the Ti content is 0.01% by mass or more, the content of solid-solved B necessary for suppressing the austenite-ferrite transformation is secured, and the effect of finely precipitating the precipitate appears remarkably. Further, when the Ti content is 0.1% by mass or less, the Ti content in the base steel sheet does not become excessive, and the manufacturing cost of the base steel sheet can be suppressed.
  • B B (boron) suppresses austenite-ferrite transformation of steel and contributes to strengthening the transformed structure. By suppressing the austenite-ferrite transformation, it has the effect of lowering the precipitation start temperature of Ti-based carbides and the like and refining those carbides. In order to obtain the above effect sufficiently, a B content of 0.0005% or more is secured. It is more effective to set it to 0.0010% or more. However, a large amount of B is a factor that causes a decrease in workability due to the formation of boride. When B is added, it is necessary to carry out in the range of 0.0100% or less, and it may be controlled to 0.0050% or less.
  • P phosphorus
  • P (phosphorus) is effective in strengthening the solid solution, it is preferable to secure a P content of 0.005% or more. It may be managed to 0.010% or more. However, if the P content is excessive, segregation is likely to occur and the workability is lowered. The P content is limited to 0.050% or less.
  • S sulfur
  • S sulfur
  • the S content is allowed up to 0.020%. However, since excessive reduction in S causes an increase in the steelmaking load, the S content may usually be 0.001% or more.
  • Al Al (aluminum) has a deoxidizing action. In order to fully exert its action, it is desirable to add Al so that the Al content in the steel is 0.005% or more. However, excessive Al content causes a decrease in workability. Therefore, the Al content is limited to 0.10% or less, and may be controlled to 0.050% or less.
  • Nb, V Similar to Ti, Nb (niobium) and V (vanadium) improve the uniformity of the structure by miniaturizing the structure, and improve the strength without deteriorating the workability such as bendability by strengthening the dispersion of carbide particles. Contribute to. Therefore, one or two types of Nb and V may be contained as required. In order to sufficiently obtain the above effects, it is more effective to secure a content of 0.01% or more for Nb and 0.03% or more for V. However, if a large amount of these elements is contained, the workability is deteriorated. Therefore, when one or two of these are added, the Nb content is in the range of 0.10% or less, and the V content is also in the range of 0.10% or less.
  • Mo, Cr Since both Mo (molybdenum) and Cr (chromium) have an effect of improving the strength by strengthening the solid solution, one or two types of Mo and Cr may be contained as needed. In order to fully exert the above action, it is more effective to secure a content of 0.01% or more for Mo and 0.01% or more for Cr. However, if a large amount of these elements is contained, the ductility will be lowered. Therefore, when one or two of these are added, the Mo content is set to the range of 1.00% or less, and the Cr content is also set to the range of 1.00% or less.
  • the steel sheet according to the present embodiment contains C, Si, Mn, Ti, and B, and may further contain each of the above-mentioned components as other components.
  • a preferred embodiment further comprises one or more of P, S, and Al. In a more preferred embodiment, all of these are included.
  • one or more, preferably all, of P, S, and Al are included, and Nb, V, Cr, and Examples include those further containing one or more types of Mo.
  • the balance contains Fe and unavoidable impurities.
  • the steel base material is applied to a DP (dual phase) steel sheet having martensite or a composite structure in which martensite and bainite are dispersed as the second phase in the main phase ferrite.
  • the total area ratio of martensite dispersed in the main phase ferrite or the second phase composed of martensite and bainite is 15% or more and less than 45%. If the area ratio of the second phase is less than 15%, it becomes difficult to stably obtain a tensile strength of 780 MPa or more. On the contrary, when it becomes 45% or more, it becomes too hard and the workability deteriorates.
  • the second phase is most preferably martensite only, but bainite may be partially dispersed.
  • the ratio of the volume of bainite to the total volume of martensite and bainite is more preferably in the range of 0 to 5%. All of the examples of the present invention in the examples described later satisfy this condition.
  • the average crystal grain size of the second phase is refined to 8 ⁇ m or less. It was found that sufficient workability was ensured and it was useful for expanding the degree of freedom in design. It is preferable that ferrite, which is the main phase, is also miniaturized, but the average crystal grain size of the second phase is particularly important for processability.
  • the ferrite phase is also sufficiently refined.
  • the average crystal grain size of the ferrite phase is 10 ⁇ m or less.
  • the average crystal grain size of the ferrite phase is 10 ⁇ m or less.
  • the above-mentioned hot-dip Zn-Al-Mg-based plated steel sheet is a general hot-dip galvanized steel sheet in which each step of hot rolling, pickling, cold rolling, annealing, and hot-dip galvanizing is performed on a steel slab in this order. It can be manufactured using the line.
  • the average cooling rate is 20 ° C./sec or more and less than 80 ° C./sec
  • the winding temperature is 400 ° C. or more and less than 600 ° C.
  • hot rolling was performed at a finish rolling temperature of 830 to 940 ° C.
  • the cold rolling ratio was set to 40 to 70%
  • annealing was performed at 740 to 880 ° C.
  • the average cooling rate up to at least 450 ° C. is 5 ° C./sec or more in the cooling process until the film is immersed in the plating bath.
  • the finish rolling temperature in hot rolling is 830 to 940 ° C.
  • the finish rolling temperature is 830 ° C. or higher, the deformation resistance of the steel sheet does not increase, and it is possible to prevent a decrease in the manufacturability of the steel sheet due to hot rolling.
  • the finish rolling temperature is 940 ° C. or lower, it is possible to prevent the occurrence of scale defects on the coil surface and suppress the deterioration of the surface quality.
  • the finished rolled steel sheet (hot-rolled steel sheet) is cooled to a winding temperature of 400 ° C. or higher and lower than 600 ° C. at an average cooling rate of 20 ° C./sec or more and less than 80 ° C./sec.
  • the average cooling rate is 20 ° C./sec or less, or when the winding temperature is 600 ° C. or more
  • the cementite of the hot-dip steel sheet structure becomes coarse, and some of the coarse cementite is not present in the reduction heating in the hot-dip galvanizing process. It remains as a dissolved carbide.
  • the amount of martensite after hot-dip galvanizing is reduced, and a tensile strength of 780 MPa or more cannot be obtained.
  • the average cooling rate is 80 ° C./sec or more, or when the winding temperature is less than 400 ° C.
  • the hardness of the hot-rolled steel sheet increases due to the high dislocation density, and the load in the cold rolling process increases. Not only is it increased, but it also causes a decrease in workability after the hot dip galvanizing process.
  • the cementite particle size after winding by hot rolling is 2 ⁇ m or less.
  • the residual undissolved carbide can be suppressed in the reduction heating in the hot-dip galvanizing step, so that the amount of martensite after hot-dip galvanizing increases. Therefore, it is possible to stably produce a plated steel sheet having both strength and workability at a high level of 780 MPa or more.
  • the cold rolling ratio is preferably 40 to 70%. If the cold rolling ratio is less than 40%, the structure after annealing becomes coarse and the bendability deteriorates. On the other hand, when the cold rolling rate exceeds 70%, the structure miniaturization effect of cold rolling is saturated. Further, it is not preferable to give an excessively high cold rolling rate because it increases the load in the cold rolling process.
  • the plate thickness after hot rolling is adjusted according to the final target plate thickness so that the cold rolling ratio in this cold rolling step is within the above range. In some cases, after hot rolling, an intermediate cold rolling + intermediate annealing step may be inserted before the cold rolling step.
  • Continuous hot dip galvanizing process In the continuous hot-dip galvanizing step, annealing and hot-dip Zn-Al-Mg plating are sequentially performed.
  • the material temperature (maximum temperature reached) may be heated to 740 to 880 ° C. in a reducing atmosphere. If the material temperature does not reach 740 ° C., recrystallization is insufficient and an unrecrystallized structure tends to remain, so that it is difficult to stably obtain good processability. If the temperature exceeds 880 ° C., the crystal grains of the austenite matrix become coarse, and the miniaturization of the second phase necessary for imparting good processability becomes insufficient.
  • the time for holding the material temperature in the range of 740 to 880 ° C. may be set in the range of, for example, 60 seconds or less.
  • the average cooling rate up to at least 450 ° C is 5 ° C / sec or more. If the cooling rate in this temperature range is slower than this, pearlite is likely to be partially generated, and it becomes difficult to stably obtain a high intensity of 780 MPa or more. Further, from the viewpoint of miniaturization of the ferrite particle size and the second phase particle size, it is effective to set the cooling rate to 5 ° C./sec or more. Since the steel targeted in the present invention contains a predetermined Ti and, if necessary, Nb as described above, the average crystal grain size of ferrite is 10 ⁇ m or less by selecting the cooling rate after heating in this way. In addition, a fine structure having an average crystal grain size of 8 ⁇ m or less in the second phase can be obtained.
  • this annealing be performed on a continuous plating line where annealing and molten Zn-Al-Mg-based plating can be performed with a single line through plate.
  • the steel sheet is directly immersed in the hot-dip galvanizing bath after being cooled to the appropriate material temperature for immersion in the hot-dip galvanizing bath.
  • the annealing atmosphere is a reducing atmosphere, and the steel sheet is controlled so as not to come into contact with the atmosphere until it is immersed in the plating bath.
  • the plating bath composition is, for example, by mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, B: 0 to 0.05%. , Si: 0 to 2.0%, Fe: 0 to 2.0%, and the balance is Zn and unavoidable impurities.
  • the plating layer composition of the obtained plated steel sheet substantially reflects the plating bath composition.
  • the obtained plated steel sheet is brought into contact with water vapor in a closed container to blacken the plated layer.
  • the brightness (L * value) of the surface of the plating layer can be reduced to 60 or less (preferably 40 or less, more preferably 35 or less).
  • a steel sheet in which a black oxide of Zn is present on the surface layer of the molten Zn—Al—Mg-based plating layer and the surface brightness L * is 60 or less can be obtained. If the surface layer of the plated steel sheet has such brightness, a black plated steel sheet having excellent design can be obtained.
  • the contact time with water vapor and the like are appropriately set according to the required brightness L * .
  • the brightness (L * value) of the surface of the plating layer is measured using a spectroscopic color difference meter.
  • a slab having the chemical composition shown in FIG. 1 is hot-rolled at a heating temperature of 1250 ° C., a finish rolling temperature of 880 ° C., an average cooling rate from finish rolling to winding at 15 to 70 ° C./sec, and a winding temperature of 420 to 630 ° C. Then, a hot-rolled steel sheet having a plate thickness of 1.8 to 2.8 mm was obtained.
  • the hot-rolled steel sheet After pickling the hot-rolled steel sheet, it is cold-rolled at a rolling rate of 45 to 65% to obtain a plating base plate (steel base material) with a plate thickness of 1.0 mm, which is passed through a continuous hot-dip galvanizing line to hydrogen- It was annealed at various temperatures of 750 to 850 ° C. in a nitrogen mixed gas atmosphere, and cooled to about 420 ° C. at a cooling rate of 8 to 12 ° C./sec.
  • a plating base plate steel base material
  • a plate thickness of 1.0 mm which is passed through a continuous hot-dip galvanizing line to hydrogen- It was annealed at various temperatures of 750 to 850 ° C. in a nitrogen mixed gas atmosphere, and cooled to about 420 ° C. at a cooling rate of 8 to 12 ° C./sec.
  • molten Zn-Al-Mg-based plating bath having the following bath composition, then pulled up, and the amount of plating adhered to each side is about 90 g / m 2 by the gas wiping method.
  • a molten Zn-Al-Mg-based plated steel sheet was produced by adjusting to, and this was used as a test material.
  • the plating bath temperature was about 410 ° C.
  • the plating bath composition is as follows; By mass%, Al: 6%, Mg: 3%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, the balance: Zn.
  • each steel (example of the present invention: steels A to G, comparative example: steel a)
  • CT indicates the winding temperature
  • cooling rate indicates the average cooling rate from finish rolling to winding in hot rolling
  • annealing temperature indicates reduction heating in a continuous hot-dip galvanizing line. Indicates the temperature.
  • Tensile strength TS and total elongation T.El were determined according to JIS Z2241 using JIS No. 5 test pieces collected so that the longitudinal direction of the test pieces was perpendicular to the rolling direction of the plating base plate (steel base material).
  • Metal structure The metal structures of the hot-rolled material and the plating material were observed with a scanning electron microscope in a cross section (L cross section) parallel to the rolling direction.
  • the hot-rolled material was etched with Piclar's reagent and then image-analyzed in 10 fields to determine the average grain size of cementite.
  • the plating materials all had a metal structure in which ferrite was the main phase and martensite or martensite and bainite were present as the second phase.
  • Image analysis of 10 fields of view was performed to determine the area ratio of the second phase and the average crystal grain size (diameter equivalent to a circle).
  • the cementite particle size of the hot-rolled material is 2 ⁇ m or less, and the area ratio of the second phase composed of martensite or martensite and bainite of the plating material is 15% or more and less than 45%.
  • the average crystal grain size of the second phase is 8 ⁇ m or less, the tensile strength TS is 780 MPa or more, and the tensile strength TS ⁇ total elongation T.I. El was 14000 MPa ⁇ % or more, and the bendability index R / t was 1.5 or less. That is, in the example of the present invention, a plated steel sheet having both strength and workability at a high level was stably obtained.
  • CT take-up temperature
  • the area ratio of the second phase was less than 15%, and the tensile strength TS was 780 MPa or less. That is, a plated steel sheet satisfying the strength required in the present invention could not be obtained.
  • the molten Zn-Al-Mg-based plated steel sheet according to one aspect of the present invention is a molten Zn-Al-Mg-based plated steel sheet having a molten Zn-Al-Mg-based plating layer on the surface of the steel substrate.
  • B containing 0.0005 to 0.0100%, the balance containing Fe and unavoidable impurities, the average particle size of cementite after winding in the hot rolling step is 2 ⁇ m or less, and the continuous hot-dip zinc plating step.
  • the later metallographic structure has a ferrite phase and a second phase having an area ratio of 15% or more and less than 45%.
  • the second phase is composed of martensite or martensite and baynite, and has an average crystal grain size of 8 ⁇ m. It is as follows.
  • the molten Zn—Al—Mg-based galvanized steel sheet according to one aspect of the present invention has P: 0.005 to 0.050%, S: 0.001 to 0.020%, and Al: 0.005 in mass%. It may further contain one or more of ⁇ 0.10%.
  • the molten Zn—Al—Mg-based galvanized steel sheet according to one aspect of the present invention has Nb: 0 to 0.10%, V: 0 to 0.10%, Cr: 0 to 1.00%, and Cr: 0 to 1.00% in mass%. Mo: One or more of 0 to 1.00% may be further contained.
  • the surface layer of the molten Zn-Al-Mg-based plating layer contains a black oxide of Zn, and the surface brightness L * is 60 or less. There may be.
  • the method for producing a molten Zn-Al-Mg-based plated steel sheet according to one aspect of the present invention is continuous melting in which a hot rolling step, a cold rolling step, and annealing and molten Zn-Al-Mg-based plating are performed in this order.
  • the average cooling rate after hot rolling is 20 ° C./sec or more and less than 80 ° C./sec, and the winding temperature is 400 ° C. or more and less than 600 ° C.
  • the steel base material has a mass% of C: 0.050 to 0.180% and Si: 0.001 to 0. 50%, Mn: 1.00 to 2.80%, Ti: 0.01 to 0.10%, and B: 0.005 to 0.0100%, the balance containing Fe and unavoidable impurities, as described above.
  • the average particle size of cementite after winding in the hot rolling step is 2 ⁇ m or less
  • the metal structure after the continuous hot-dip galvanizing step has a ferrite phase and a second phase having an area ratio of 15% or more and less than 45%.
  • the second phase may be composed of martensite or martensite and bainite, and the average crystal grain size may be 8 ⁇ m or less.
  • the steel base material is mass%, P: 0.005 to 0.050%, S: 0.001 to 0. It may further contain one or more of 020% and Al: 0.005 to 0.10%.
  • the steel base material is mass%, Nb: 0 to 0.10%, V: 0 to 0.10%, Cr. It may further contain one or more of: 0 to 1.00% and Mo: 0 to 1.00%.
  • the surface layer of the molten Zn—Al—Mg-based plating layer contains a black oxide of Zn, and the surface brightness L * is high. It may be 60 or less.

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Abstract

強度と加工性とを両立しためっき鋼板を実現する。溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板において、鋼基材は、質量％で、Ｃ：０.０５０～０.１８０％、Ｓｉ：０.００１～０．５０％、Ｍｎ：１.００～２.８０％、Ｔｉ：０.０１～０.１０％、およびＢ：０.０００５～０.０１００％を含み、熱間圧延工程での巻取り後のセメンタイト平均粒径が２μｍ以下であり、連続溶融亜鉛めっき工程後の金属組織は、フェライト相と、面積率１５％以上４５％未満の第二相とを有し、第二相は、マルテンサイトまたは、マルテンサイトおよびベイナイトにより構成され平均結晶粒径が８μｍ以下である。

Description

溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板およびその製造方法
　本発明は、溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板およびその製造方法に関する。
　近年、自動車や建材の分野では軽量化および省資源化を目的とした高強度高防錆鋼板のニーズが高まっている。その高強度高防錆鋼板は、プレス加工や曲げ加工をはじめ様々な加工が施されるため、高強度および高耐食性であることに加え、加工性に優れることも重要である。しかしながら、材料の加工性は強度が上昇するのに伴って劣化するため、例えば、自動車の構造用部材および補強用部材において要求される最大引張強度７８０ＭＰａ以上のような高強度と、加工性とを両立させることができる技術の確立が望まれる。
　例えば、特許文献１には、鋼板にＳｉ、ＮｂおよびＴｉを添加することにより、マルテンサイトおよびベイナイト組織等の硬質相と、フェライト相との硬度差を小さくすることで、７８０ＭＰａ以上の高い引張強度と、加工性とを両立させる技術が開示されている。
　また、耐食性の観点からは、防錆効果の高い表面処理鋼板として溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板が知られている。近年、意匠性などの観点から黒色の外観を有する鋼板のニーズが高まってきていることから、めっき層自体が黒色化されている溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の需要はますます増大している。特許文献２には、引張強度７８０ＭＰa以上の高強度を有し、曲げ加工性に優れる溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板が開示されている。
日本国公開特許公報「特開２００６－２８３１５６号公報」 日本国公開特許公報「特開２０１４－１８９８１２号公報」
　しかしながら、鋼板へ多量にＴｉを添加すると再結晶温度の上昇を招くため、めっき工程での還元加熱温度を高くする必要がある。還元加熱温度の高温化はめっき不良の原因となるため、特許文献１に記載されている技術はめっき鋼板に好適とはいえない。また、特許文献２に記載の製造方法によれば、熱間圧延条件によってはめっき後のマルテンサイト量が減少する恐れがあることから、７８０ＭＰａ以上の強度が安定して得られない場合があった。
　本発明の一態様は、７８０ＭＰａ以上の引張強度と、高い加工性とを安定して両立した溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板を実現することを目的とする。
　上記の課題を解決するために、本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板は、鋼基材の表面に溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層を有する溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板において、前記鋼基材は、質量％で、Ｃ：０.０５０～０.１８０％、Ｓｉ：０.００１～０．５０％、Ｍｎ：１.００～２.８０％、Ｔｉ：０.０１～０.１０％、およびＢ：０.０００５～０.０１００％を含み、残部がＦｅおよび不可避的不純物を含み、熱間圧延工程での巻取り後におけるセメンタイトの平均粒径が２μｍ以下であり、連続溶融亜鉛めっき工程後の金属組織は、フェライト相と、面積率１５％以上４５％未満の第二相を有し、前記第二相は、マルテンサイトまたは、マルテンサイトおよびベイナイトにより構成され、平均結晶粒径が８μｍ以下である。
　上記の課題を解決するために、本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法は、熱間圧延工程と、冷間圧延工程と、焼鈍および溶融Ｚｎ－Ａｌ－Ｍｇ系めっきをこの順で行う連続溶融亜鉛めっき工程と、をこの順で含む、鋼基材の表面に溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層を有する溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法であって、前記熱間圧延工程は、熱間圧延後の平均冷却速度が２０℃／秒以上８０℃／秒未満であり、巻取り温度が４００℃以上６００℃未満である。
　本発明の一態様によれば、７８０ＭＰａ以上の引張強度と、高い加工性とを安定して両立した溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板を実現できる。
本発明の一実施例における各鋼の成分を示す図である。 本発明の一実施例における各鋼の製造条件および特性を示す図である。
　以下、本発明の一実施形態について詳細に説明する。なお、以下の記載は発明の趣旨をより良く理解させるためのものであり、特に指定のない限り、本発明を限定するものではない。また、本明細書において特記しない限り、数値範囲を表す「Ａ～Ｂ」は、「Ａ以上Ｂ以下」を意味する。
　〔基材鋼板の化学組成〕
　めっき原板に相当する基材鋼板の成分元素について説明する。本明細書において、基材鋼板の化学組成に関する「％」は特に断らない限り「質量％」を意味する。
　（Ｃ）
　Ｃ（炭素）は、鋼の高強度化に必要な元素である。引張強さ７８０ＭＰａ以上の強度レベルを得るためには０.０５０％以上のＣ含有量を必要とする。ただし、Ｃ含有量が過剰になると組織の不均一性が顕著となり、加工性が低下する。そのため、Ｃ含有量は０.１８０％以下に制限され、０.１６０％以下に管理してもよい。
　（Ｓｉ）
　Ｓｉ（ケイ素）は、高強度化に有効である他、セメンタイトの析出を抑制する作用を有し、パーライト等の生成を抑制するうえで有効である。これらの作用を十分に発揮させるために０.００１％以上のＳｉ含有量を確保する。ただし、多量にＳｉを含有すると、鋼板表面にＳｉ濃化層が生じ、めっき性の低下を招く要因となる。そのため、Ｓｉ含有量は０.５０％以下に制限され、０.２５％以下とすることがより好ましい。
　（Ｍｎ）
　Ｍｎ（マンガン）は、高強度化に有効である。引張強さ７８０ＭＰａ以上の強度レベルを安定して得るために１.００％以上のＭｎ含有量を確保する。ただし、Ｍｎ含有量が過大になると偏析が生じやすくなり加工性が低下する。そのため、Ｍｎ含有量は２.８０％以下とする。
　（Ｔｉ）
　鋼板は、０．０１質量％以上０．１質量％以下のＴｉ（チタン）を含む。ＴｉはＣと反応することにより、Ｔｉを含む炭化物が微細な粒子として析出し、鋼板の高強度化に有効な元素である。また、Ｔｉは鋼中のＳ（硫黄）およびＮ（窒素）との親和性も高いため、Ｃと反応し析出物を生成するだけでなく、ＳおよびＮとも反応し析出物を生成する。オーステナイト－フェライト変態を抑制するのに必要なＢはＮと結合しやすいことから、Ｔｉの添加は、固溶したＢの含有量の確保に有効である。Ｔｉの含有量が０．０１質量％以上であることにより、オーステナイト－フェライト変態を抑制するのに必要な固溶したＢの含有量が確保され、析出物を微細析出させる効果が顕著に表れる。また、Ｔｉの含有量が０．１質量％以下であることにより、基材鋼板におけるＴｉの含有量が過剰とならず、基材鋼板の製造コストを抑制できる。
　（Ｂ）
　Ｂ（ホウ素）は、鋼のオーステナイト－フェライト変態を抑制し、変態組織強化に寄与する。オーステナイト－フェライト変態の抑制によりＴｉ系炭化物等の析出開始温度を低下させ、それらの炭化物を微細化させる効果を有する。上記効果を十分に得るために、０.０００５％以上のＢ含有量を確保する。０.００１０％以上とすることがより効果的である。ただし、多量のＢ含有は硼化物の生成による加工性低下を招く要因となる。Ｂを添加する場合は０.０１００％以下の範囲で行う必要があり、０.００５０％以下に管理してもよい。
　（Ｐ）
　Ｐ（リン）は、固溶強化に有効であるため、０.００５％以上のＰ含有量を確保することが好ましい。０.０１０％以上に管理してもよい。ただし、Ｐ含有量が過大になると偏析が生じやすくなり加工性が低下する。Ｐ含有量は０.０５０％以下に制限される。
　（Ｓ）
　Ｓ（硫黄）は加工性を低下させる要因となる。Ｓの含有量は０.０２０％まで許容される。ただし、過剰な低Ｓ化は製鋼負荷の増大を招くので、通常、Ｓ含有量は０.００１％以上であってよい。
　（Ａｌ）
　Ａｌ（アルミニウム）は、脱酸作用を有する。その作用を十分に発揮させるために、鋼中のＡｌ含有量が０.００５％以上となるようにＡｌを添加することが望ましい。ただし、過剰のＡｌ含有は加工性の低下を招く。そのため、Ａｌ含有量は０.１００％以下に制限され、０.０５０％以下に管理してもよい。
　（Ｎｂ、Ｖ）
　Ｎｂ（ニオブ）およびＶ（バナジウム）は、Ｔｉと同様に、組織の微細化によって組織の均一性を向上させるとともに、炭化物の粒子分散強化により、曲げ性等の加工性を劣化させることなく強度向上に寄与する。したがって、必要に応じてＮｂ、Ｖの１種または２種を含有させてもよい。上記効果を十分に得るためには、Ｎｂについては０.０１％以上、Ｖについては０．０３％以上の含有量を確保することがより効果的である。ただし、これらの元素を多量に含有すると加工性の低下を招く。そのため、これらの１種または２種を添加する場合、Ｎｂ含有量は０.１０％以下、Ｖ含有量も０.１０％以下の範囲とする。
　（Ｍｏ、Ｃｒ）
　Ｍｏ（モリブデン）およびＣｒ（クロム）は、いずれも固溶強化によって強度を向上させる作用を有するので、必要に応じてＭｏ、Ｃｒの１種または２種を含有させてもよい。上記作用を十分に発揮させるためには、Ｍｏについては０.０１％以上、Ｃｒについても０.０１％以上の含有量を確保することがより効果的である。ただし、これらの元素を多量に含有すると延性の低下を招く。そのため、これらの１種または２種を添加する場合、Ｍｏ含有量は１.００％以下、Ｃｒ含有量も１.００％以下の範囲とする。
　本実施形態に係る鋼板は、Ｃ、Ｓｉ、Ｍｎ、Ｔｉ、およびＢを含み、さらにその他の成分として上述した各成分を含み得る。好適な態様としては、Ｐ、Ｓ、およびＡｌの１種以上をさらに含むものである。より好適な態様では、これらをすべて含んでいる。さらに別の態様では、Ｃ、Ｓｉ、Ｍｎ、Ｔｉ、およびＢに加えて、Ｐ、Ｓ、およびＡｌの１種以上を含み、好ましくは全てを含んでいる態様において、Ｎｂ、Ｖ、Ｃｒ、およびＭｏの１種以上をさらに含むものが挙げられる。なお、残部はＦｅおよび不可避的不純物を含む。
　〔鋼基材の金属組織〕
　本発明では、主相フェライトに第二相としてマルテンサイトまたは、マルテンサイトとベイナイトとが分散した複合組織を持つＤＰ（デュアルフェイズ）鋼板を鋼基材の適用対象としている。溶融亜鉛めっき後の金属組織において、主相フェライトに分散するマルテンサイトまたは、マルテンサイトとベイナイトとにより構成された第二相は、面積率で合計１５％以上４５％未満とする。第二相の面積率が１５％に満たないと７８０ＭＰａ以上の引張強さを安定して得ることが困難となる。逆に４５％以上になると硬くなりすぎて加工性が低下する。
　第二相はマルテンサイトのみであることが最も好ましいが、部分的にベイナイトが分散していてもよい。例えば、マルテンサイトとベイナイトとの合計体積に占めるベイナイトの体積の割合は、０～５％の範囲であることがより好ましい。後述する実施例における本発明例はいずれもこの条件を満たしている。
　本発明では、組織を微細化することにより加工性を向上させている。板厚０．８～２．０ｍｍ程度のめっき鋼板を使用して自動車の構造用部材および補強用部材を製造する場合を考慮すると、第二相の平均結晶粒径が８μｍ以下に微細化されているとき、十分な加工性が確保され、設計自由度の拡大に有用となることがわかった。主相であるフェライトも微細化されていることが好ましいが、加工性に関しては特に第二相の平均結晶粒径が重要である。
　第二相の平均結晶粒径が８μｍ以下となる後述の製造条件を採用すれば、フェライト相も十分に微細化される。例えば、フェライト相の平均結晶粒径は１０μｍ以下となる。後述する実施例において第二相の平均結晶粒径が８μｍ以下であるものは、いずれもフェライト相の平均結晶粒径は１０μｍ以下である。
　〔製造方法〕
　上述の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板は、鋼スラブに熱間圧延、酸洗、冷間圧延、焼鈍、溶融亜鉛めっきの各工程をこの順に施す、一般的な溶融亜鉛系めっき鋼板の製造ラインを利用して製造することができる。鋼材の強度および加工性を両立させるためには、鋼基材の化学組成をコントロールすることに加え、結晶粒径が十分に微細化するよう、製造条件を工夫する必要がある。具体的には、熱間圧延工程において、平均冷却速度を２０℃／秒以上８０℃／秒未満とし、巻取り温度を４００℃以上６００℃未満とする。
　なお、熱間圧延工程において８３０～９４０℃の仕上圧延温度により熱間圧延を施し、冷間圧延工程において冷間圧延率を４０～７０％とし、焼鈍工程において７４０～８８０℃により焼鈍を施した後、めっき浴に浸漬するまでの冷却過程で少なくとも４５０℃までの平均冷却速度を５℃／秒以上とすることが、より好ましい。
　（熱間圧延工程）
　上記の熱間圧延工程では、熱間圧延における仕上圧延温度を８３０～９４０℃とすることが好ましい。仕上圧延温度が８３０℃以上であることにより、鋼板の変形抵抗が高くならず、熱間圧延による鋼板の製造性の低下を防止することができる。また、仕上圧延温度が９４０℃以下であることにより、コイル表面におけるスケール疵の発生を防ぎ、表面品質の低下を抑制することができる。
　仕上圧延後の鋼板（熱延鋼板）は、２０℃／秒以上８０℃／秒未満の平均冷却速度で、４００℃以上６００℃未満の巻取り温度まで冷却される。平均冷却速度が２０℃／秒以下の場合、または巻取り温度が６００℃以上の場合、熱延鋼板組織のセメンタイトが粗大化し、溶融亜鉛めっき工程での還元加熱において粗大なセメンタイトの一部が未溶解炭化物として残存する。その結果、溶融亜鉛めっき後のマルテンサイト量が減少し、７８０ＭＰａ以上の引張強度が得られない。また、平均冷却速度が８０℃／秒以上の場合、または巻取り温度が４００℃未満の場合、転位密度が高くなることで熱延鋼板の硬さが増大し、冷間圧延工程での負荷が増大するだけでなく、溶融亜鉛めっき工程後の加工性の低下を招く。
　２０℃／秒以上８０℃／秒未満の平均冷却速度および４００℃以上６００℃未満の巻取り温度であれば、熱間圧延での巻取り後におけるセメンタイト粒径は２μｍ以下となる。これにより、溶融亜鉛めっき工程での還元加熱において、未溶解炭化物の残存を抑制することができるため、溶融亜鉛めっき後のマルテンサイト量が増加する。したがって、７８０ＭＰａ以上の、強度および加工性を高いレベルで両立しためっき鋼板を安定して製造することができる。
　（冷間圧延工程）
　上記の冷間圧延工程では、冷間圧延率を４０～７０％とすることが好ましい。４０％未満の冷間圧延率では焼鈍後の組織が粗大となり曲げ性が低下する。一方、７０％を超える冷間圧延率では、冷間圧延による組織微細化効果が飽和する。また、過度に高い冷間圧延率を付与することは冷間圧延工程の負荷を増大させ好ましくない。この冷間圧延工程での冷間圧延率が上記の範囲となるように、最終的な目標板厚に応じて熱間圧延後の板厚を調整しておく。場合によっては、熱間圧延後、この冷間圧延工程の前に、中間冷間圧延＋中間焼鈍の工程を挿入してもよい。
　（連続溶融亜鉛めっき工程）
　連続溶融亜鉛めっき工程では、焼鈍および溶融Ｚｎ－Ａｌ－Ｍｇ系めっきを順次行う。
　溶融亜鉛めっき浴に浸漬する直前に行う焼鈍では、還元性雰囲気下において、材料温度（最高到達温度）が７４０～８８０℃となるように加熱してもよい。材料温度が７４０℃に達しないと再結晶化が不十分となって未再結晶組織が残存しやすいため、良好な加工性を安定して得ることが難しい。８８０℃を超えるとオーステナイト母相の結晶粒が粗大化し、良好な加工性を付与するために必要な第二相の微細化が不十分となる。材料温度が７４０～８８０℃の範囲に保持する時間は、例えば６０秒以下の範囲で設定すればよい。
　焼鈍後の冷却過程では、少なくとも４５０℃までの平均冷却速度が５℃／秒以上となるようにすることが好ましい。この温度域での冷却速度がこれより遅いと、部分的にパーライトが生成しやすくなり、７８０ＭＰａ以上の高強度を安定して得ることが困難となる。また、フェライト粒径および第二相粒径の微細化の点からも、冷却速度は５℃／秒以上とすることが有効である。本発明で対象とする鋼は上記のように所定のＴｉおよび必要に応じてＮｂを含有しているので、加熱後の冷却速度をこのように選定することでフェライトの平均結晶粒径が１０μｍ以下、かつ第二相の平均結晶粒径が８μｍ以下である微細な組織を得ることができる。
　この焼鈍は、焼鈍および溶融Ｚｎ－Ａｌ－Ｍｇ系めっきを１回のライン通板で行うことが可能な連続めっきラインで行うことが望ましい。焼鈍後の上記冷却において、溶融亜鉛めっき浴に浸漬する際の適正材温まで冷却した後、鋼板を直接溶融亜鉛めっき浴に浸漬する。焼鈍雰囲気は還元性雰囲気とし、めっき浴中に浸漬されるまで鋼板が大気に触れないように管理される。
　溶融Ｚｎ－Ａｌ－Ｍｇ系めっきは、従来から実施されている方法を採用すればよい。めっき浴組成は、例えば、質量％で、Ａｌ：３.０～２２.０％、Ｍｇ：０.０５～１０.０％、Ｔｉ：０～０.１０％、Ｂ：０～０.０５％、Ｓｉ：０～２.０％、Ｆｅ：０～２.０％、残部がＺｎおよび不可避的不純物である組成が好適である。得られるめっき鋼板のめっき層組成は、めっき浴組成をほぼ反映したものとなる。
　得られためっき鋼板は、密閉容器中で水蒸気に接触させて、めっき層を黒色化する。この工程により、めっき層表面の明度（Ｌ＊値）を６０以下（好ましくは４０以下、さらに好ましくは３５以下）にまで低下させることができる。これにより、溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層の表層に、Ｚｎの黒色酸化物が存在し、表面の明度Ｌ＊が６０以下である鋼板が得られる。めっき鋼板の表層がこのような明度であれば、黒色の、意匠性に優れためっき鋼板が得られる。なお、必要とする明度Ｌ＊に応じて、水蒸気への接触時間等が適宜設定される。めっき層表面の明度（Ｌ＊値）は、分光型色差計を用いて測定される。
　本発明は上述した各実施形態に限定されるものではなく、請求項に示した範囲で種々の変更が可能であり、異なる実施形態にそれぞれ開示された技術的手段を適宜組み合わせて得られる実施形態についても本発明の技術的範囲に含まれる。
　本発明の一実施例について以下に説明する。
　〔試験方法〕
　図１に示す化学組成を有するスラブを加熱温度１２５０℃、仕上げ圧延温度８８０℃、仕上圧延から巻取りまでの平均冷却速度を１５～７０℃／秒、巻取り温度４２０～６３０℃で熱間圧延して、板厚１．８～２．８ｍｍの熱延鋼板を得た。熱延鋼板を酸洗後、４５～６５％の圧延率で冷間圧延して板厚１．０ｍｍのめっき原板（鋼基材）とし、これを連続溶融めっきラインに通板して、水素－窒素混合ガス雰囲気中７５０～８５０℃の種々の温度で焼鈍し、８～１２℃／秒の冷却速度により約４２０℃まで冷却した。
　その後、鋼板表面が大気に触れない状態のまま下記の浴組成の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき浴中に浸漬した後引き上げ、ガスワイピング法にて、めっき付着量を片面あたり約９０ｇ／ｍ２に調整することにより溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板を製造し、これを供試材とした。めっき浴温は約４１０℃であった。
　めっき浴組成は以下の通りである；
　質量％で、Ａｌ：６％、Ｍｇ：３％、Ｔｉ：０.００２％、Ｂ：０.０００５％、Ｓｉ：０.０１％、Ｆｅ：０.１％、残部：Ｚｎ。
　各鋼（本発明例：鋼Ａ～Ｇ、比較例：鋼ａ）の製造条件を図２に示した。このうち、「ＣＴ」は巻取り温度を示し、「冷却速度」は、熱間圧延における仕上圧延から巻取りまでの平均冷却速度を示し、「焼鈍温度」は連続溶融亜鉛めっきラインでの還元加熱温度を示す。
　〔試験項目〕
　得られた供試材のめっき鋼板について、以下の試験を行った。
　（引張特性）
　試験片の長手方向がめっき原板（鋼基材）の圧延方向に対し直角となるように採取したＪＩＳ５号試験片を用い、ＪＩＳ　Ｚ２２４１に従い引張強さＴＳ、全伸びＴ.Ｅｌを求めた。
　（曲げ試験）
　試験片の長手方向がめっき原板（鋼基材）の圧延方向に対し直角となるように採取した曲げ試験片を用いて、ＪＩＳ　Ｚ２２４８に従い曲げ角度４５度のＶブロック曲げ試験を実施した。試験後に、曲げ部を曲げの外側から目視にて観察し、割れが認められない最小の曲げ先端内側半径を限界曲げ半径Ｒとして算出し、限界曲げ半径Ｒを板厚ｔにより除した値を、曲げ性の指標Ｒ／ｔとして求めた。
　（金属組織）
　熱延材およびめっき材の金属組織は、圧延方向と平行な断面（Ｌ断面）を走査型電子顕微鏡にて観察した。熱延材については、ピクラール試薬によってエッチングした後、１０視野の画像解析を行ってセメンタイトの平均粒径を求めた。
　また、めっき材については、いずれもフェライトを主相とし、第二相としてマルテンサイトまたはマルテンサイトとベイナイトが存在する金属組織を呈していた。１０視野の画像解析を行い、第二相の面積率および平均結晶粒径（円相当径）を求めた。
　〔試験結果〕
　以上の試験項目についての試験結果を図２にまとめて示す。なお、図２中の下線を付した項目は、本発明規定範囲外または特性不十分であることを示す。
　本発明例のものは、いずれも熱延材のセメンタイト粒径が２μｍ以下であり、めっき材のマルテンサイトまたはマルテンサイトおよびベイナイトにより構成される第二相の面積率が１５％以上４５％未満、当該第二相の平均結晶粒径が８μｍ以下、引張強さＴＳが７８０ＭＰａ以上、引張強さＴＳ×全伸びＴ．Ｅｌが１４０００ＭＰａ・％以上、かつ曲げ性の指標Ｒ／ｔが１．５以下であった。すなわち、本発明例では、強度と加工性とを高いレベルで両立しためっき鋼板が安定して得られた。
　一方、鋼板の化学組成、巻取り温度（ＣＴ）、または仕上圧延から巻取りまでの平均冷却速度のいずれかの一つ以上の条件において、本発明規定範囲外の条件により製造された供試材（比較例）はいずれも、上記第二相の面積率が１５％未満となり、引張強さＴＳが７８０ＭＰａ以下となった。すなわち、本発明において求められる強度を満たすめっき鋼板は得られなかった。
　〔まとめ〕
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板は、鋼基材の表面に溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層を有する溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板において、前記鋼基材は、質量％で、Ｃ：０.０５０～０.１８０％、Ｓｉ：０.００１～０．５０％、Ｍｎ：１.００～２.８０％、Ｔｉ：０.０１～０.１０％、およびＢ：０.０００５～０.０１００％を含み、残部がＦｅおよび不可避的不純物を含み、熱間圧延工程での巻取り後におけるセメンタイトの平均粒径が２μｍ以下であり、連続溶融亜鉛めっき工程後の金属組織は、フェライト相と、面積率１５％以上４５％未満の第二相を有し、前記第二相は、マルテンサイトまたは、マルテンサイトおよびベイナイトにより構成され、平均結晶粒径が８μｍ以下である。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板は、質量％で、Ｐ：０.００５～０.０５０％、Ｓ：０.００１～０.０２０％、およびＡｌ：０.００５～０.１００％の１種以上をさらに含んでいてもよい。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板は、質量％で、Ｎｂ：０～０.１０％、Ｖ：０～０.１０％、Ｃｒ：０～１.００％、およびＭｏ：０～１.００％の１種以上をさらに含んでいてもよい。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板は、前記溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層の表層は、Ｚｎの黒色酸化物が存在し、表面の明度Ｌ＊が６０以下であってもよい。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法は、熱間圧延工程と、冷間圧延工程と、焼鈍および溶融Ｚｎ－Ａｌ－Ｍｇ系めっきをこの順で行う連続溶融亜鉛めっき工程と、をこの順で含む、鋼基材の表面に溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層を有する溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法であって、前記熱間圧延工程は、熱間圧延後の平均冷却速度が２０℃／秒以上８０℃／秒未満であり、巻取り温度が４００℃以上６００℃未満である。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法は、前記鋼基材は、質量％で、Ｃ：０.０５０～０.１８０％、Ｓｉ：０.００１～０．５０％、Ｍｎ：１.００～２.８０％、Ｔｉ：０.０１～０.１０％、およびＢ：０.０００５～０.０１００％を含み、残部がＦｅおよび不可避的不純物を含み、前記熱間圧延工程での巻取り後におけるセメンタイトの平均粒径が２μｍ以下であり、前記連続溶融亜鉛めっき工程後の金属組織は、フェライト相と、面積率１５％以上４５％未満の第二相を有し、前記第二相は、マルテンサイトまたは、マルテンサイトおよびベイナイトにより構成され、平均結晶粒径が８μｍ以下であってもよい。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法は、前記鋼基材は、質量％で、Ｐ：０.００５～０.０５０％、Ｓ：０.００１～０.０２０％、およびＡｌ：０.００５～０.１００％の１種以上をさらに含んでいてもよい。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法は、前記鋼基材は、質量％で、Ｎｂ：０～０.１０％、Ｖ：０～０.１０％、Ｃｒ：０～１.００％、およびＭｏ：０～１.００％の１種以上をさらに含んでいてもよい。
　本発明の一態様に係る溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法は、前記溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層の表層は、Ｚｎの黒色酸化物が存在し、表面の明度Ｌ＊が６０以下であってもよい。

Claims (9)

  1. 　鋼基材の表面に溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層を有する溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板において、
    　前記鋼基材は、質量％で、Ｃ：０.０５０～０.１８０％、Ｓｉ：０.００１～０．５０％、Ｍｎ：１.００～２.８０％、Ｔｉ：０.０１～０.１０％、およびＢ：０.０００５～０.０１００％を含み、残部がＦｅおよび不可避的不純物を含み、
    　熱間圧延工程での巻取り後におけるセメンタイトの平均粒径が２μｍ以下であり、
    　連続溶融亜鉛めっき工程後の金属組織は、フェライト相と、面積率１５％以上４５％未満の第二相とを有し、
    　前記第二相は、マルテンサイトまたは、マルテンサイトおよびベイナイトにより構成され、平均結晶粒径が８μｍ以下である、溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板。
  2. 　質量％で、Ｐ：０.００５～０.０５０％、Ｓ：０.００１～０.０２０％、およびＡｌ：０.００５～０.１００％の１種以上をさらに含む、請求項１に記載の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板。
  3. 　質量％で、Ｎｂ：０～０.１０％、Ｖ：０～０.１０％、Ｃｒ：０～１.００％、およびＭｏ：０～１.００％の１種以上をさらに含む、請求項１または２に記載の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板。
  4. 　前記溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層の表層は、
    　Ｚｎの黒色酸化物が存在し、表面の明度Ｌ＊が６０以下である、請求項１から３の何れか１項に記載の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板。
  5. 　熱間圧延工程と、冷間圧延工程と、焼鈍および溶融Ｚｎ－Ａｌ－Ｍｇ系めっきを順次行う連続溶融亜鉛めっき工程と、をこの順で含む、鋼基材の表面に溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層を有する溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法であって、
    　前記熱間圧延工程は、
    　　熱間圧延後の平均冷却速度が２０℃／秒以上８０℃／秒未満であり、
    　　巻取り温度が４００℃以上６００℃未満である、溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法。
  6. 　前記鋼基材は、質量％で、Ｃ：０.０５０～０.１８０％、Ｓｉ：０.００１～０．５０％、Ｍｎ：１.００～２.８０％、Ｔｉ：０.０１～０.１０％、およびＢ：０.０００５～０.０１００％を含み、残部がＦｅおよび不可避的不純物を含み、
    　前記熱間圧延工程での巻取り後におけるセメンタイトの平均粒径が２μｍ以下であり、
    　前記連続溶融亜鉛めっき工程後の金属組織は、フェライト相と、面積率１５％以上４５％未満の第二相を有し、
    　前記第二相は、マルテンサイトまたは、マルテンサイトおよびベイナイトにより構成され、平均結晶粒径が８μｍ以下である、請求項５に記載の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法。
  7. 　前記鋼基材は、質量％で、Ｐ：０.００５～０.０５０％、Ｓ：０.００１～０.０２０％、およびＡｌ：０.００５～０.１００％の１種以上をさらに含む、請求項６に記載の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法。
  8. 　前記鋼基材は、質量％で、Ｎｂ：０～０.１０％、Ｖ：０～０.１０％、Ｃｒ：０～１.００％、およびＭｏ：０～１.００％の１種以上をさらに含む、請求項６または７に記載の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法。
  9. 　前記溶融Ｚｎ－Ａｌ－Ｍｇ系めっき層の表層は、
    　Ｚｎの黒色酸化物が存在し、表面の明度Ｌ＊が６０以下である、請求項５から８の何れか１項に記載の溶融Ｚｎ－Ａｌ－Ｍｇ系めっき鋼板の製造方法。
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