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WO2019106895A1 - 高強度亜鉛めっき鋼板およびその製造方法 - Google Patents

高強度亜鉛めっき鋼板およびその製造方法 Download PDF

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Publication number
WO2019106895A1
WO2019106895A1 PCT/JP2018/030693 JP2018030693W WO2019106895A1 WO 2019106895 A1 WO2019106895 A1 WO 2019106895A1 JP 2018030693 W JP2018030693 W JP 2018030693W WO 2019106895 A1 WO2019106895 A1 WO 2019106895A1
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Prior art keywords
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steel sheet
plating
temperature
steel
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PCT/JP2018/030693
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English (en)
French (fr)
Inventor
裕美 吉冨
泰明 沖田
正貴 木庭
義彦 小野
松田　広志
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JFE Steel Corp
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JFE Steel Corp
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Publication date
Application filed by JFE Steel Corp filed Critical JFE Steel Corp
Priority to EP18884774.3A priority Critical patent/EP3719157B1/en
Priority to US16/766,398 priority patent/US11408059B2/en
Priority to MX2020005475A priority patent/MX2020005475A/es
Priority to JP2018560920A priority patent/JP6525114B1/ja
Priority to CN201880076648.3A priority patent/CN111433380B/zh
Priority to KR1020207015197A priority patent/KR102402864B1/ko
Publication of WO2019106895A1 publication Critical patent/WO2019106895A1/ja
Anticipated expiration legal-status Critical
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • B32—LAYERED PRODUCTS
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    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
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    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
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Definitions

  • the present invention relates to a high strength galvanized steel sheet which is excellent in resistance to hydrogen embrittlement which tends to be a problem as the strength increases and which is suitable for a construction material and a collision resistant part of a car, and a method of manufacturing the same.
  • Patent Documents 1 to 4 are documents relating to characteristics such as high strength.
  • Patent Document 1 discloses a hot-dip galvanized steel sheet excellent in workability and having a high yield ratio and high strength, and a method of manufacturing the same.
  • Patent Document 2 discloses a method of providing a steel plate having a tensile strength of 980 MPa or more, exhibiting a high yield ratio, and being excellent in workability (specifically, strength-ductility balance).
  • Patent Document 3 discloses high-strength hot-dip galvanized steel excellent in plating appearance, corrosion resistance, resistance to peeling during plating, resistance to peeling during high processing, and workability during high processing using a high strength steel plate containing Si and Mn as a base material. Disclosed is a plated steel plate and a method of manufacturing the same.
  • Patent Document 4 discloses a method of manufacturing a high strength plated steel sheet having excellent delayed fracture resistance.
  • Patent Documents 5 and 6 are documents relating to this hydrogen embrittlement.
  • Patent Document 5 discloses a plated steel sheet for hot press excellent in delayed fracture resistance and a method of manufacturing the same.
  • the precipitates in steel are utilized, and the penetration of diffusible hydrogen is suppressed as much as possible depending on the manufacturing process conditions before plating, and hydrogen in steel after plating is trapped as non-diffusible hydrogen.
  • Patent Document 6 discloses a high-strength steel plate excellent in welded portion hydrogen embrittlement of a steel plate having a base material strength (TS) of about 870 MPa and a method of manufacturing the same.
  • TS base material strength
  • Patent Document 6 the hydrogen embrittlement is improved by dispersing the oxide in the steel.
  • Patent Document 1 since the steel structure is a composite structure containing ferrite and martensite, the high yield ratio is only about 0.70 although the yield ratio is high. Moreover, in patent document 1, since Si and Mn are contained abundantly, plating quality is inadequate. Furthermore, the resistance to hydrogen embrittlement has not been positively improved.
  • Patent Document 3 in the annealing step before plating, the hydrogen concentration in the furnace atmosphere is limited to 20 vol% or more, and the annealing temperature is limited to 600 to 700 ° C. Due to the metallographic structure, it is difficult to apply to materials having an Ac3 point exceeding 800 ° C. Furthermore, when the hydrogen concentration in the atmosphere in the annealing furnace is high, the hydrogen concentration in the steel increases, and it can not be said that the hydrogen embrittlement resistance is sufficient.
  • Patent Document 4 although the delayed fracture resistance after processing is improved, the hydrogen concentration during annealing is also high, hydrogen remains in the base material itself, and the hydrogen embrittlement resistance is inferior.
  • Patent Document 5 when a large amount of precipitates on the order of several microns is present, the mechanical properties of the material itself are degraded.
  • Patent Document 6 since the upper limit of the in-furnace hydrogen concentration of the continuous plating line is 60%, a large amount of hydrogen is taken into the steel when annealing to a high temperature of A c3 or more. Therefore, the method of Patent Document 6 can not produce an ultra-high strength steel plate excellent in hydrogen embrittlement resistance of TS ⁇ 1100 MPa.
  • the present invention is an ultra-high-strength plated steel sheet that is concerned about hydrogen embrittlement, and is excellent in the appearance of plating and the resistance to hydrogen embrittlement of the material together with a material that achieves high yield strength with high demand.
  • An object of the present invention is to provide a high strength galvanized steel sheet having a suitable high yield strength and a method of manufacturing the same.
  • the inventors of the present invention have studied the relationship between tensile strength (TS) and yield strength (YS) for various steel plates, and resistance spot weld nugget as plating property and hydrogen embrittlement resistance. We examined the coexistence of overcoming of the crack of the crack. As a result, in addition to the component composition of the steel sheet, the above problems have been solved by appropriately adjusting the manufacturing conditions to form an optimal steel structure and controlling the amount of hydrogen in the steel. Specifically, the present invention provides the following.
  • the steel composition is mass%, C: 0.10% or more and 0.30% or less, Si: less than 1.2%, Mn: 2.0% or more and 3.5% or less, P: 0.010% 70% or less of S: 0.002% or less, Al: 1% or less, N: 0.006% or less, and the balance of the component composition consisting of Fe and unavoidable impurities, and the area ratio % Or more (including 100%), less than 20% of ferrite (including 0%), and steel structure containing less than 5% (including 0%) of retained austenite, and the amount of diffusible hydrogen in the steel 0.20 mass ppm or less of a steel plate, and a galvanized layer having an Fe content of 8 to 15% by mass on a surface of the steel plate and a plating adhesion amount per one surface of 20 to 120 g / m 2 , the provided, Mn oxide content contained in the galvanized layer is 0.050 g / m 2 or more , And the tensile strength of more than 1
  • the component composition may further contain, in mass%, one or more of Ti, Nb, V, and Zr in total of 0.005 to 0.1%, and one or more of Mo, Cr, Cu, and Ni.
  • the component composition further contains, in mass%, one or two selected from Sb: 0.001 to 0.1% and Sn: 0.001 to 0.1% [1] or The high strength galvanized steel sheet according to [2].
  • % And dew point Dp 50 ° C. or less, at a temperature T (° C.) of 200 ° C. or more and 350 ° C. or less, for a time t (hr) or more that satisfies the equation (1) at 0.005 (hr) or more A post heat treatment step, and a method of manufacturing a high strength galvanized steel sheet.
  • the width trim is performed before the post heat treatment step, and the residence time t (hr) of staying at a temperature T (° C.) of 200 ° C. or more and 350 ° C. or less in the post heat treatment step is 0.005 (hr)
  • T temperature
  • t residence time
  • the tensile strength is high at 1100 MPa or higher, preferably 1150 MPa or higher, and the yield ratio (yield strength ratio) is 0.85 or higher, and excellent in surface properties (platability and surface appearance)
  • yield strength ratio yield strength ratio
  • a high strength galvanized steel sheet excellent in hydrogen embrittlement can be obtained.
  • the high-strength galvanized steel sheet of the present invention comprises a steel sheet and a galvanized layer formed on the surface of the steel sheet. Below, it demonstrates in order of a steel plate and a zinc plating layer.
  • composition of the steel plate is as follows. In the following description, “%” which is a unit of content of a component means “mass%”.
  • C 0.10% to 0.30% (C: 0.10 to 0.30%)
  • C is an element effective for increasing the strength of the steel plate, and contributes to the increase in strength by forming martensite, which is one of the hard phases of the steel structure.
  • the C content needs to be 0.10% or more. Preferably it is 0.11% or more, More preferably, it is 0.12% or more.
  • the C content exceeds 0.30%, in the present invention, the spot weldability is significantly deteriorated, and at the same time, the steel sheet becomes hard due to the increase in the strength of martensite, and the formability such as ductility tends to decrease. Therefore, the C content is 0.30% or less. From the viewpoint of property improvement, the content is preferably 0.28% or less, more preferably 0.25% or less.
  • Si less than 1.2% Si is an element that contributes to high strength mainly by solid solution strengthening, and the decrease in ductility relative to the increase in strength is relatively small, and it contributes not only to strength but also to the balance improvement between strength and ductility Do.
  • Si easily forms a Si-based oxide on the surface of the steel sheet, which may cause non-plating, stabilizes austenite at the time of annealing, and facilitates formation of retained austenite in the final product. Therefore, it is sufficient to add only the amount necessary for securing the strength, and from that viewpoint, the Si content is desirably 0.01% or more. More preferably, it is 0.02% or more. More preferably, it is 0.05% or more.
  • the upper limit is made less than 1.2% from the viewpoint of plating property and retained austenite formation. Preferably it is 1.0% or less. More preferably, it is 0.9% or less.
  • Mn 2.0% or more and 3.5% or less Mn is effective as an element contributing to high strength by solid solution strengthening and martensite formation. In order to acquire this effect, it is necessary to make Mn content 2.0% or more. Preferably it is 2.1% or more, more preferably 2.2% or more. On the other hand, if the Mn content exceeds 3.5%, spot weld cracking occurs, and the steel structure is likely to be uneven due to segregation of Mn and the workability is reduced. If the Mn content exceeds 3.5%, Mn is likely to be concentrated as an oxide or a composite oxide on the surface of the steel sheet, which may cause non-plating. Therefore, the Mn content is set to 3.5% or less. Preferably it is 3.3% or less, more preferably 3.0% or less.
  • P 0.010% or less
  • P is an effective element that contributes to the strengthening of the steel sheet by solid solution strengthening.
  • the content exceeds 0.010%, workability such as weldability and stretch flangeability is reduced. Therefore, the P content is made 0.010% or less.
  • it is 0.008% or less, more preferably 0.007% or less.
  • the lower limit is not particularly specified, but if it is less than 0.001%, the production efficiency and the dephosphorization cost increase in the production process, so it is preferably 0.001% or more.
  • S 0.002% or less
  • S is a harmful element which causes hot embrittlement, causes a decrease in weldability, or is present as a sulfide-based inclusion in a steel to reduce the workability of a steel sheet. is there. For this reason, it is preferable to reduce the S content as much as possible. Therefore, the S content is made 0.002% or less.
  • the lower limit is not particularly specified, but if it is less than 0.0001%, production efficiency and cost increase in the current manufacturing process are caused, and therefore, it is preferable to be 0.0001% or more.
  • Al 1% or less Al is added as a deoxidizer.
  • the content is preferably 0.01% or more from the viewpoint of obtaining the effect. More preferably, it is 0.02% or more.
  • the Al content exceeds 1%, the raw material cost will rise, and it will also cause surface defects of the steel sheet, so 1% is made the upper limit.
  • it is 0.4% or less, more preferably 0.1% or less.
  • the N content is set to 0.006% or less, preferably 0.005% or less, and more preferably 0.004% or less.
  • the content is preferably as small as possible from the viewpoint of ductility improvement by cleaning of the ferrite, but in order to lower the production efficiency and increase the cost in the manufacturing process, the preferable lower limit is made 0.0001% or more. More preferably, it is 0.0010% or more, further preferably 0.0015% or more.
  • the component composition of the above-described steel sheet may contain, as optional components, one or more of Ti, Nb, V, and Zr in total of 0.005 to 0.1% and / or one or more of Mo, Cr, Cu, and Ni. A total of 0.005 to 0.5% and / or B: 0.0003 to 0.005% may be contained.
  • Ti, Nb, V, and Zr form carbides and nitrides (which may be carbonitrides) with C and N, and contribute to high strengthening of the steel sheet, particularly to high YR by forming fine precipitates.
  • these elements are also effective for trap sites (detoxification) of hydrogen in steel.
  • an excessive content exceeding 0.1% in total increases the deformation resistance at the time of cold rolling to inhibit the productivity, and the presence of excessive or coarse precipitates reduces the ductility of the ferrite, and Decreases the formability such as ductility, bendability and stretch flangeability. Then, it is preferable to make the said total into 0.1% or less. More preferably, it is 0.08% or less, more preferably 0.06% or less.
  • Mo, Cr, Cu, Ni, and B are elements that contribute to high strength in order to enhance the hardenability and facilitate the formation of martensite. Then, it is preferable to make one or more types in Mo, Cr, Cu, and Ni into 0.005% or more in total. More preferably, it is 0.01% or more, further preferably 0.05% or more. Moreover, in the case of B, 0.0003% or more is preferable, More preferably, it is 0.0005% or more, More preferably, it is 0.0010% or more. In addition, with Mo, Cr, Cu, and Ni, excessive addition exceeding 0.5% in total leads to saturation of effects and cost increase.
  • the upper limit is made into 0.5%. Since Ni has the effect of suppressing the generation of surface defects due to the Cu content, it is desirable to include Ni when it is contained. In particular, it is preferable to contain Ni at a half or more of the Cu content. Also for B, the above lower limit is provided to obtain the effect of suppressing the formation of ferrite that occurs in the annealing and cooling process. In addition, when the B content exceeds 0.005%, the effect is saturated. Excess hardenability also has disadvantages such as weld cracking during welding.
  • the component composition of the above steel sheet may contain, as optional components, Sb: 0.001 to 0.1% and / or Sn: 0.001 to 0.1%.
  • Sb and Sn are elements effective for suppressing the reduction in strength of the steel sheet by suppressing decarburization, denitrification, deasphalting and the like. Moreover, since it is effective also to spot weld cracking suppression, the Sn content and the Sb content are each preferably 0.001% or more. More preferably, it is 0.003% or more, still more preferably 0.005% or more. However, in both Sn and Sb, the excess content exceeding 0.1% reduces the workability such as the stretch flangeability of the steel sheet. Therefore, the Sn content and the Sb content are both preferably 0.1% or less. More preferably, it is 0.030% or less, still more preferably 0.010% or less.
  • the component composition of the above-described steel sheet may contain Ca: 0.0010% or less as an optional component.
  • the Ca content is preferably 0.0010% or less. More preferably, it is 0.0005% or less, more preferably 0.0003% or less. Further, the lower limit is not particularly limited, but it may be difficult in some cases not to contain Ca at all in production, so in consideration of it, the Ca content is preferably 0.00001% or more. More preferably, it is 0.00005% or more.
  • the balance other than the above is Fe and unavoidable impurities.
  • the optional component is an unavoidable impurity because the effect of the present invention is not impaired.
  • Steel structure is area ratio, 70% or more (including 100%) of bainite and martensite (including tempered martensite), less than 20% (including 0%) of ferrite, and less than 5% of retained austenite Including 0%).
  • the area ratio of less than 20% is acceptable for obtaining the yield strength of the present invention. Preferably it is 15% or less, more preferably 10% or less.
  • the lower limit is not particularly limited, but is preferably 1% or more, more preferably 2% or more, and still more preferably 3% or more.
  • generated at comparatively high temperature here is regarded as a ferrite, without distinguishing with a ferrite by observation by the scanning electron microscope as described in the below-mentioned Example.
  • tempered martensite and bainite are hard phases, and the area ratio of these hard phases is important in the present invention because high tensile strength and yield strength are obtained. Martensite is particularly hard and is effective for enhancing the steel plate strength. Tempered martensite and bainite (as mentioned above, bainite without carbides is regarded as ferrite, so bainite as used herein means bainite containing carbides) is a structure that is particularly important for obtaining high yield strength is there. From the above, the total of these hard phases is made 70% or more in area ratio. Preferably it is 80% or more, More preferably, it is 85% or more, More preferably, it is 90% or more. The upper limit is preferably 100% or less, preferably 99% or less, and more preferably 98% or less.
  • the bainite in these hard phases is preferably 20% or more and 50% or less. Moreover, 49% or less is preferable with respect to the whole structure
  • Retained austenite less than 5% (including 0%) Austenite (fcc phase) has higher hydrogen storage capacity than ferrite (bcc phase). Therefore, when this retained austenite is transformed to martensite by stress induction such as processing, there is a concern that diffusible hydrogen in the steel may be increased. Therefore, make it less than 5%. More preferably, it is 4% or less, more preferably 3% or less. Although the lower limit is not particularly limited, it is preferably 0% or more, more preferably 1% or more.
  • the steel structure may contain precipitates such as pearlite and carbide in the remaining portion as a structure other than the above-described structure.
  • the area ratio of these other structures is preferably 10% or less, more preferably 5% or less.
  • the area ratio in the above steel structure adopts the result obtained by the method described in the examples.
  • a more specific method of measuring the area ratio is described in the examples, but briefly as follows.
  • the above area ratio is represented by the structure in the area of 1 ⁇ 4 thickness position (1/8 to 3 ⁇ 8) of the plate thickness from the surface, and after polishing the L cross section (plate thickness cross section parallel to the rolling direction) of the steel plate It corrodes with a liquid, and three or more fields of view are observed by SEM with a magnification of 1500 times, and an image photographed and analyzed is determined.
  • the said steel plate is 0.20 mass ppm (mass.ppm) or less of the amount of diffusible hydrogen in steel obtained by measuring by the method as described in an Example. Diffusive hydrogen in steel degrades hydrogen embrittlement resistance. If the amount of diffusible hydrogen in the steel exceeds 0.20 mass ppm and becomes excessive, for example, cracking of the weld nugget tends to occur during welding. In the present invention, it was revealed that the improvement effect is obtained by setting the amount of diffusible hydrogen in the steel which is the base material to 0.20 mass ppm or less before welding. Preferably it is 0.15 mass ppm or less, More preferably, it is 0.10 mass ppm or less, More preferably, it is 0.08 mass ppm or less.
  • the lower limit is not particularly limited, but is preferably as small as possible, so the lower limit is 0 mass ppm. It is necessary to reduce the amount of diffusible hydrogen of the base material to 0.20 mass ppm or less before welding, and in the product after welding, if the amount of diffusible hydrogen of the base material portion is 0.20 mass ppm or less It can be considered that it was 0.20 mass ppm or less before welding.
  • the zinc plating layer contains 20 to 120 g / m 2 of plating per one side and 8 to 15% by mass of Fe.
  • the adhesion amount is less than 20 g / m 2 , it will be difficult to ensure corrosion resistance. Therefore, the adhesion amount is 20 g / m 2 or more, preferably 25 g / m 2 or more, and more preferably 30 g / m 2 or more. On the other hand, if it exceeds 120 g / m 2 , the plating peeling resistance deteriorates. Therefore, the adhesion amount is 120 g / m 2 or less, preferably 100 g / m 2 or less, more preferably 80 g / m 2 or less.
  • the Mn oxide formed in the heat treatment step before plating reacts with the plating bath and the steel plate to form an FeAl or FeZn alloy phase, which is taken into the plating, and has a plating property, Plating resistance is improved.
  • the amount of Mn oxide contained in the galvanized layer is more than 0.050 g / m 2 , the formation reaction of the FeAl or FeZn alloy phase becomes insufficient, resulting in the occurrence of non-plating and a decrease in the peel resistance to plating.
  • the amount of Mn oxide in the plating layer is set to 0.050 g / m 2 or less.
  • the amount of Mn oxide in the plating layer is preferably 0.005 g / m 2 or more and 0.050 g / m 2 or less.
  • the measurement of the amount of Mn oxides in a zinc plating layer is performed by the method as described in an Example.
  • the galvanized layer contains 8 to 15% by mass of Fe.
  • the Fe content in the zinc plating layer is 8% or more by mass%, it can be said that the Fe—Zn alloy layer is sufficiently obtained.
  • it is 9% or more, more preferably 10% or more.
  • the Fe content is 15% or less.
  • it is 14% or less, More preferably, it is 13% or less.
  • components other than Fe, Zn of remainder, and unavoidable impurities contained in a zinc plating layer are not specifically limited.
  • the zinc plating layer contains, for example, 0.001% to 1.0% of Al by mass%, and Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, One or two or more selected from Ti, Be, Bi and REM may be contained in a total of 0 to 30%.
  • the zinc plating layer which has Fe content in the said range is an alloying galvanization layer.
  • the method for producing a high strength galvanized steel sheet according to the present invention includes an annealing step, a plating step, and a post heat treatment step.
  • a cold rolled material having the above-mentioned component composition is contained in an annealing furnace atmosphere having a hydrogen concentration H of 1 vol% or more and 13 vol% or less, and the temperature T in the annealing furnace: (A c 3 point ⁇ 20 ° C.) After heating (soaking) to a temperature of 5 s (seconds) or more, it is cooled and retained in a temperature range of 400 to 550 ° C. for 10 s or more.
  • the cold rolled material used in the manufacturing method of the present invention is manufactured from a steel material.
  • the steel material is manufactured by a continuous casting method generally called a slab (slab).
  • the purpose of adopting the continuous casting method is to prevent macro segregation of alloy components.
  • the steel material may be manufactured by an ingot method, a thin slab casting method, or the like.
  • the steel slab is manufactured, it is cooled to room temperature and then reheated, and then it is hot-rolled by charging it into a heating furnace as it is, without cooling to around room temperature.
  • Either a method of hot rolling immediately after performing a slight supplementary heat, or a method of hot rolling while maintaining a high temperature state after casting may be used.
  • the conditions of the hot rolling are not particularly limited, but the steel material having the above-mentioned component composition is heated at a temperature of 1100 ° C. or more and 1350 ° C. or less, subjected to hot rolling with a finish rolling temperature of 800 ° C. or more and 950 ° C. or less.
  • the condition of winding at a temperature of not less than ° C and not more than 700 ° C is preferable.
  • these preferable conditions are demonstrated.
  • the heating temperature of the steel slab is preferably in the range of 1100 ° C. or more and 1350 ° C. or less. If the temperature is out of the upper limit temperature range, the precipitates present in the steel slab are easily coarsened, and for example, it may be disadvantageous when securing strength by precipitation strengthening. In addition, coarse precipitates may act as nuclei in the subsequent heat treatment to adversely affect the structure formation. On the other hand, it is useful to reduce cracks and irregularities on the surface of the steel sheet by scaling off bubbles and defects on the surface of the slab by appropriate heating to achieve a smooth steel surface. In order to acquire such an effect, it is preferable to set it as 1100 degreeC or more.
  • the heated steel slab is subjected to hot rolling including rough rolling and finish rolling.
  • a steel slab becomes a sheet bar by rough rolling, and becomes a hot-rolled coil by finish rolling.
  • hot rolling conditions the following are preferable.
  • Finish rolling temperature 800 ° C. or more and 950 ° C. or less is preferable.
  • the finish rolling temperature 800 ° C. or more
  • the structure obtained by the hot-rolled coil tends to be uniform. Being able to make the tissue uniform at this stage contributes to the uniformity of the final product's tissue. If the structure is non-uniform, workability such as ductility, bendability, stretch flangeability and the like is reduced.
  • the temperature exceeds 950 ° C. the amount of oxide (scale) formation increases, the interface between the base iron and the oxide becomes rough, and the surface quality after pickling and cold rolling may deteriorate.
  • the formability of the steel plate similar to the steel slab such as strength, bendability and stretch flangeability, may be reduced.
  • cooling is started within 3 seconds after finishing rolling to finish the refining and homogenization of the structure, and the temperature range from [finish rolling temperature] to [finish rolling temperature -100] ° C. It is preferable to cool at an average cooling rate of 10 to 250 ° C./s.
  • the winding temperature is preferably 450 to 700.degree. If the temperature immediately before coil winding after hot rolling, that is, the coiling temperature is 450 ° C. or higher, it is preferable from the viewpoint of fine precipitation of carbides when Nb or the like is added, if the coiling temperature is 700 ° C. or lower It is preferable because cementite precipitates do not become too coarse. Also, if the temperature range is less than 450 ° C. or more than 700 ° C., the structure is likely to change during holding after being wound into a coil, and rolling due to non-uniformity of the metal structure of the material in cold rolling in the post process It is easy for troubles to occur. A more preferable winding temperature is 500 ° C. or more and 680 ° C. or less from the viewpoint of granulation of the hot-rolled sheet structure.
  • a cold rolling process is performed. Usually, after removing the scale by pickling, cold rolling is applied to form a cold rolled coil. This pickling is performed as needed.
  • the cold rolling is preferably performed at a rolling reduction of 20% or more. This is to obtain a uniform and fine microstructure in the subsequent heating. If the amount is less than 20%, coarse particles may be easily formed during heating, or an uneven structure may be easily formed, and as described above, there is a concern that the strength and processability of the final product plate after heat treatment may be reduced. Deteriorate the surface properties.
  • the upper limit of the rolling reduction is not particularly defined, but because of the high strength steel plate, the high rolling reduction may result in poor shape in addition to the reduction in productivity due to the rolling load.
  • the rolling reduction is preferably 90% or less.
  • the above is the method for producing a cold rolled material.
  • the annealing step the cold rolled material, the hydrogen concentration H: at 1 vol% or more 13 vol% or less of the annealing furnace atmosphere, the annealing furnace temperature T: (A c3 point -20 °C) ⁇ 900 °C 5 seconds or more at a temperature After heating, it is cooled and retained in a temperature range of 400 to 550 ° C. for 10 seconds or more.
  • Annealing furnace temperature T (A c3 point ⁇ 20 ° C.) to the temperature range of 900 ° C. or less
  • the average heating rate is not particularly limited, but the average heating rate is less than 10 ° C./s because of uniform structure. Is preferred. Further, from the viewpoint of suppressing a decrease in production efficiency, the average heating rate is preferably 1 ° C./s or more.
  • the heating temperature (temperature in the annealing furnace) T is set to (Ac 3 point ⁇ 20 ° C.) to 900 ° C. in order to secure both the material and the plating property.
  • the heating temperature is less than (A c3 point ⁇ 20 ° C.)
  • the ferrite fraction becomes high, so that the strength can not be obtained or the formation of bainite becomes difficult.
  • the heating temperature exceeds 900 ° C., the crystal grains are coarsened to deteriorate workability such as bendability and stretch flangeability, which is not preferable.
  • Mn and Si are easily concentrated on the surface to inhibit the plating property.
  • the heating temperature exceeds the Ac3 point and exceeds 900 ° C., the load on the facility is high, and there is a possibility that the production can not be stably performed.
  • heating is performed for 5 seconds or more at a temperature T in an annealing furnace: (A c 3 point ⁇ 20 ° C.) to 900 ° C. 180 seconds or less is preferable in order to prevent excessive austenite grain size coarsening. Further, the heating time is set to 5 seconds or more from the viewpoint of homogenization of the tissue.
  • the hydrogen concentration H in the temperature range from (A c3 point ⁇ 20 ° C.) to 900 ° C. is 1 to 13 vol%.
  • the plating property is secured, and at the same time, excessive hydrogen penetration into the steel is prevented. If the hydrogen concentration is less than 1 vol%, non-plating occurs frequently.
  • the hydrogen concentration exceeds 13 vol%, the effect on the plateability is saturated, and at the same time, the penetration of hydrogen into the steel is remarkably increased, and the hydrogen embrittlement resistance of the final product is deteriorated.
  • the hydrogen concentration does not have to be in the range of 1 vol% or more except for the temperature range of (A c3 point ⁇ 20 ° C.) to 900 ° C.
  • bainite is an important structure to obtain high YS. In order to produce this and to make the bainite area ratio 20% or more of the hard phase, it is necessary to stay for 10 s or more in this temperature range. Residence below 400 ° C. tends to fall below the temperature of the subsequent plating bath, which is not preferable because it degrades the quality of the plating bath. In that case, the plate temperature may be heated to the plating bath, so the lower limit of the above temperature range is set to 400.degree.
  • the cooling stop temperature may be 400 to 550 ° C. as described above, but it is also possible to once cool to a temperature lower than this and to make it stay in a temperature range of 400 to 550 ° C. by reheating. In this case, when the temperature is cooled to the Ms point or lower, tempering may be performed after martensite is generated.
  • the steel sheet after the annealing step is subjected to plating treatment, alloying treatment, and cooling to 100 ° C. or less at an average cooling rate of 3 ° C./s or more.
  • the plating adhesion amount per one surface is made to be 20 to 120 g / m 2 .
  • the Fe content is 8 to 15% by mass.
  • the galvanized layer in which the Fe content is in the above range is an alloyed galvanized layer.
  • Al 0.001% to 1.0% is contained.
  • the zinc plating layer contains a predetermined amount of Mn oxide, it contains Mn. Containing 0 to 30% in total of one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM It is also good. The remainder is Zn and unavoidable impurities.
  • the method of plating treatment is preferably hot-dip galvanizing treatment.
  • the conditions may be set as appropriate.
  • the alloying process heated after hot dip galvanization is performed. For example, a process of holding in a temperature range of 480 to 600 ° C. for about 1 to 60 seconds can be exemplified. By this treatment, an alloyed zinc plated layer having an Fe content of 8 to 15% is obtained.
  • the alloy After the above alloying treatment, the alloy is cooled to 100 ° C. or less at an average cooling rate of 3 ° C./s or more. This is to obtain martensite which is essential for high strength. If it is less than 3 ° C./s, it is difficult to obtain martensite necessary for strength, and if cooling is stopped at a temperature higher than 100 ° C., martensite is excessively tempered (self-tempered) at this point, austenite It is because it does not become martensite and transforms to ferrite, making it difficult to obtain the required strength.
  • a post heat treatment step is performed after the plating step.
  • the plated steel sheet after the plating step was treated with a hydrogen concentration of H: 10 vol. % And dew point Dp: 50 ° C. or less, at a temperature T (° C.) of 200 ° C. or more and 350 ° C. or less, for a time t (hr) or more that satisfies the equation (1) at 0.005 (hr) or more Process.
  • a post heat treatment step is performed to obtain high yield strength and to reduce the amount of diffusible hydrogen in the steel.
  • Hydrogen concentration H 10 vol.
  • the hydrogen concentration H is preferably as low as 5 vol. % Or less is preferable, more preferably 2 vol. % Or less.
  • the lower limit of the hydrogen concentration H is not particularly limited and is preferably as small as described above, but it is difficult to excessively reduce the hydrogen concentration, so the preferable lower limit is 1 vol% or more.
  • preferable dew point Dp is 45 degrees C or less, More preferably, it is 40 degrees C or less.
  • the lower limit of the dew point Dp is not particularly limited, but is preferably ⁇ 80 ° C. or more from the viewpoint of production cost.
  • the upper limit of the temperature is set to 350 ° C.
  • it is 340 degrees C or less, More preferably, it is 330 degrees C or less.
  • the lower limit of the temperature to be retained is less than 200 ° C.
  • YR does not increase.
  • the lower limit of the temperature is set to 200.degree.
  • it is 210 degreeC or more, More preferably, it is 220 degreeC or more.
  • the above is the method for producing a high strength galvanized steel sheet according to the present invention, but after the cold rolling and before annealing, the cold rolled sheet obtained by cold rolling is subjected to a temperature from Ac1 point to Ac3 point + 50 ° C. It is also possible to carry out a pretreatment step of heating the area and pickling.
  • a c1 point-heated to a temperature range of A c3 point + 50 °C "heating to a temperature range of c1 point A-A c3 point + 50 °C", in order to guarantee a high yield strength and plating properties due to the formation of steel structure in the final product It is a condition of It is preferable in terms of material to obtain a structure including ferrite and martensite before the subsequent annealing step. Furthermore, it is preferable to concentrate oxides such as Mn in the surface layer portion of the steel sheet by this heating also from the viewpoint of the plating property. From that point of view, heating is performed to a temperature range of A c1 point to A c3 point + 50 ° C.
  • the acid pickling after the heating is subsequent heat treatment to ensure the plating property by heating at a temperature range of 3 or more of A c , so oxides such as Si and Mn concentrated in the surface layer of the steel sheet are removed by acid pickling Do.
  • temper rolling is performed at an elongation rate of 0.1% or more after cooling of the plating process. Temper rolling may not be performed. In addition to the purpose of shape correction and surface roughness adjustment, temper rolling is performed at an elongation rate of 0.1% or more for the purpose of stably obtaining YS. For shape correction and surface roughness adjustment, leveler processing may be performed instead of temper rolling. Excessive temper rolling introduces excessive strain on the surface of the steel sheet and lowers the evaluation value of ductility and stretch flangeability. In addition, excessive temper rolling reduces ductility, and because of the high strength steel plate, equipment load also increases. Then, it is preferable to make the rolling reduction of temper rolling into 3% or less.
  • width trimming before or after the temper rolling.
  • coil width adjustment can be performed.
  • width trimming prior to the post heat treatment step hydrogen in the steel can be efficiently released by the subsequent post heat treatment.
  • a residence time t (hr) of staying at a temperature T (° C.) of 200 ° C. or more and 350 ° C. or less in the post heat treatment step is 0.005 (hr) or more and (2) It suffices to satisfy the condition. 115-18.3 ⁇ ln (t) ⁇ T (2)
  • the temperature can be shortened if the temperature conditions are the same, and the temperature can be lowered if the conditions of the residence time are the same.
  • Molten steel of the composition shown in Table 1 was melted by a converter and made into a slab by a continuous casting machine.
  • the slab was heated to 1200 ° C. to form a hot rolled coil at a finish rolling temperature of 840 ° C. and a coil winding temperature of 610 ° C.
  • This hot rolled coil was used as a cold rolled material having a thickness of 1.4 mm and a cold rolling reduction of 50%.
  • This cold rolled material is heated to 810 ° C. ((Ac3 point ⁇ 20 ° C.) to 900 ° C.) and retained for 60 seconds by annealing treatment in an annealing furnace atmosphere with a hydrogen concentration of 9 vol% and a dew point of ⁇ 30 ° C.
  • Amount of hydrogen in steel was measured by the following method. First, a test piece of about 5 ⁇ 30 mm was cut out from the alloyed galvanized steel sheet subjected to the post heat treatment. Then, using a router, the plating on the surface of the test specimen was removed and placed in a quartz tube. Then, after replacing the inside of the quartz tube with Ar, the temperature was raised at 200 ° C./hr and hydrogen generated up to 400 ° C. was measured by gas chromatography. Thus, the amount of released hydrogen was measured by a temperature rising analysis method. The cumulative value of the amount of hydrogen detected in the temperature range from room temperature (25 ° C.) to less than 210 ° C. was taken as the diffusible hydrogen amount.
  • FIG. 1 shows the relationship between the amount of diffusible hydrogen and the minimum nugget diameter.
  • the steel structure and the like are within the scope of the present invention.
  • the molten steel of the composition shown in Table 2 is melted by a converter and made into a slab by a continuous casting machine, then hot rolling, cold rolling, heating (annealing), pickling under various conditions shown in Table 2 (Table 2 In the case of “ ⁇ ” in the above, the concentration of HCl in the pickling solution was adjusted to 5 mass% and the solution temperature was adjusted to 60 ° C.), annealing and plating treatment, temper rolling, width trim, post heat treatment A high strength galvanized steel sheet (product sheet) of .4 mm thickness was manufactured. In addition, in cooling (cooling after plating treatment), it was cooled to 100 ° C. or less by passing it through a water bath having a water temperature of 40 ° C.
  • the evaluation method is as follows.
  • Texture observation Specimens for texture observation were taken from the galvanized steel sheet, and after grinding the L cross section (plate thickness cross section parallel to the rolling direction), it was corroded with nital solution and it was near 1/4 t (t is the total thickness) from the surface by SEM.
  • the volume fraction of retained austenite was quantified by X-ray diffraction intensity.
  • F means ferrite
  • M means hardened martensite
  • B means bainite
  • residual ⁇ means retained austenite.
  • JIS No. 5 tensile test specimen (JIS Z 2201) was taken from a galvanized steel sheet in a direction perpendicular to the rolling direction, and a tensile test was conducted at a constant tensile speed (cross head speed) of 10 mm / min.
  • the yield strength (YS) is the value obtained by reading 0.2% proof stress from the slope of the stress range of 150 to 350 MPa
  • the tensile strength is the value obtained by dividing the maximum load in the tensile test by the initial cross section of the test piece parallel portion
  • the plate thickness in calculation of the cross-sectional area of the parallel portion was the plate thickness value including plating thickness.
  • the non-plating defect means an area on the order of several ⁇ m to several mm where plating does not exist and the steel plate is exposed.
  • the amount of diffusible hydrogen in the steel was measured in the same manner as in Example 1.
  • the amount of Mn oxide in the zinc plating layer is carried out by dissolving the plating layer with dilute hydrochloric acid to which an inhibitor is added and using ICP emission spectrometry for the amount of Mn oxide in the zinc plating layer. It measured using.
  • Hydrogen Embrittlement As an evaluation of hydrogen embrittlement resistance, hydrogen embrittlement of resistance spot welds of steel plates was evaluated. The evaluation method is the same as in Example 1.
  • the welding current value was set as the condition for forming the nugget diameter according to each steel plate strength.
  • the nugget diameter was 3.8 mm at 1100 to 1250 MPa, 4.8 mm at 1250 to 1400 MPa, and 6 mm at 1400 MPa or more.
  • the spacer spacing at both ends was 40 mm, and the steel plate and the spacer were previously fixed by welding. After leaving it for 24 hours after welding, the spacer portion was cut off, and the cross section of the weld nugget was observed to evaluate the presence or absence of a crack (crack). In the table, no crack is indicated by "o", and crack is indicated by "x”.
  • the obtained results are shown in Table 4.
  • the invention examples had good TS, YR, surface properties and weldability. Either of the comparative examples was inferior. Further, from the comparison between the inventive example and the comparative example, the relationship between the diffusible hydrogen amount and the weldability is the same as that in FIG. 1 within the range of the component composition and steel structure of the present invention. When the content is 20 mass ppm or less, it can be understood that the resistance spot weld nugget cracking evaluation is good as resistance to hydrogen embrittlement.
  • the high strength galvanized steel sheet of the present invention not only has high tensile strength but also has a high yield strength ratio, good surface properties and resistance to hydrogen embrittlement. For this reason, when the present invention is applied mainly to a framework part of a car body, especially the cabin periphery which affects the collision safety, it contributes to the weight reduction of the vehicle body by the high strength and thin wall effect together with the improvement of its safety performance. As a result, the present invention can also contribute to environmental aspects such as CO 2 emission. In addition, since the present invention has good surface properties and plating quality, it is possible to positively apply it to a part where there is a concern about corrosion due to rain and snow, such as a foot. For this reason, according to the present invention, the performance improvement can also be expected with regard to the rust and corrosion resistance of the vehicle body. Such characteristics are effective not only to automobile parts but also to civil engineering, construction and home appliance fields.

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Abstract

水素脆化が懸念される超高強度めっき鋼板において、需要が高い高降伏強さを達成した材質と共に、めっき外観や素材の耐水素脆性に優れ、建材や自動車の耐衝突部品に好適な高い降伏強さを持つ高強度溶融亜鉛めっき鋼板およびその製造方法を提供する。　特定の成分組成と、マルテンサイトおよびベイナイトを７０％以上（１００％を含む）、フェライトを２０％未満（０％を含む）、残留オーステナイトを５％未満（０％を含む）含む鋼組織と、を有し、鋼中の拡散性水素量が０．２０質量ｐｐｍ以下の鋼板と、該鋼板の表面に、Ｆｅ含有量が質量％で８～１５％であり、片面あたりのめっき付着量が２０～１２０g／ｍ２である亜鉛めっき層と、を備え、亜鉛めっき層に含まれるＭｎ酸化物量が０．０５０ｇ／ｍ２以下であり、引張強さ１１００ＭＰａ以上、降伏比０．８５以上である高強度亜鉛めっき鋼板とする。

Description

高強度亜鉛めっき鋼板およびその製造方法
　本発明は、強度が高くなるほど問題となりやすい耐水素脆性に優れ、建材や自動車の耐衝突部品に好適な高強度亜鉛めっき鋼板およびその製造方法に関する。
　自動車の衝突安全性および燃費改善が強く求められている昨今、部品素材である鋼板の高強度化が進んでいる。中でも、自動車が衝突した際に乗員の安全を確保する観点から、キャビン周りに使われる部品素材には、高い引張強さだけでなく、高い降伏強さも求められる。さらに、世界規模で自動車の普及が広がっており、多種多様な地域・気候のなか種々の用途で自動車が使われることに対し、部品素材である鋼板には高い防錆性が求められる。高強度等の特性に関する文献として下記特許文献１～４がある。
　特許文献１には、加工性に優れた高降伏比高強度の溶融亜鉛めっき鋼板とその製造方法が開示されている。
　また、特許文献２には、引張強さが９８０ＭＰａ以上であり、高降伏比を示し、且つ加工性（詳細には、強度－延性バランス）に優れた鋼板を提供する方法が開示されている。
　また、特許文献３には、ＳｉおよびＭｎを含有する高強度鋼板を母材とする、めっき外観、耐食性、高加工時の耐めっき剥離性および高加工時の加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法が開示されている。
　また、特許文献４には、耐遅れ破壊特性が良好な高強度めっき鋼板の製造方法が開示されている。
　ところで、一般的にＺｎやＮｉなどのめっきが施されると水素は素材から放出・侵入がしにくいため拡散性水素と呼ばれる鋼中水素が残存しやすくなり、素材の水素脆化が起こりやすくなる。この水素脆化に関する文献として、下記特許文献５、６がある。
　特許文献５では、耐遅れ破壊特性に優れたホットプレス用めっき鋼板とその製造方法が開示されている。特許文献５においては、鋼中の析出物を活用し、めっき前には製造プロセス条件により拡散性水素の侵入を極力抑制し、鍍金後の鋼中水素を非拡散性水素としてトラップさせている。
　特許文献６では、母材強度（ＴＳ）＜８７０ＭＰａ程度の鋼板の溶接部水素脆性に優れる高強度鋼板とその製造方法が開示されている。この特許文献６においては、鋼中に酸化物を分散させることで水素脆性を改善している。
特許第５４３８３０２号公報 特開２０１３－２１３２３２号公報 特開２０１５－１５１６０７号公報 特開２０１１－１１１６７１号公報 特開２０１２－４１５９７号公報 特開２００７－２３１３７３号公報
　特許文献１では、鋼組織がフェライトとマルテンサイトを含有する複合組織であるため、高降伏比とはいえ降伏比が０．７０程度までしか高降伏比になっていない。また、特許文献１では、ＳｉやＭｎを多量に含有しているためめっき品質が不十分である。さらに、耐水素脆性を積極的に改善していない。
　特許文献２では、めっき密着性を低下させるＳｉの添加を抑えているが、２．０％を超えるＭｎ添加量がある場合、鋼板表面にはＭｎ系酸化物ができやすく一般的にめっき性を損なう。特許文献２ではめっき層を形成するときの条件は特に限定しておらず、通常用いられる条件を採用しており、めっき性が劣る。さらに、耐水素脆性を積極的に改善していない。
　特許文献３では、めっき前の焼鈍工程において、炉内雰囲気の水素濃度が２０ｖｏｌ％以上且つ焼鈍温度が６００～７００℃に制限されている。金属組織構成上、Ａｃ３点が８００℃を超える素材には適用するのが困難である。さらに焼鈍炉内雰囲気中の水素濃度が高いと鋼中水素濃度が増大し、耐水素脆性が十分とはいえない。
　特許文献４では、加工後の耐遅れ破壊特性は改善されているものの、焼鈍中の水素濃度も高く、母材そのものに水素が残留し耐水素脆性が劣る。
　特許文献５では、数ミクロンオーダーの析出物が多量に存在すると、素材そのものの機械的特性が劣化する。
　特許文献６では、連続めっきラインの炉内水素濃度の上限が６０％であることから、Ａｃ３点以上の高温に焼鈍した場合に大量の水素が鋼中に取り込まれる。したがって、特許文献６の方法でＴＳ≧１１００ＭＰａの耐水素脆性に優れる超高強度鋼板を製造することはできない。
　本発明は、水素脆化が懸念される超高強度めっき鋼板において、需要が高い高降伏強さを達成した材質と共に、めっき外観や素材の耐水素脆性に優れ、建材や自動車の耐衝突部品に好適な高い降伏強さを持つ高強度亜鉛めっき鋼板およびその製造方法を提供することを目的とする。
　本発明者らは、上記課題を解決するために、種々の鋼板に対して、引張強さ（ＴＳ）と降伏強さ（ＹＳ）の関係と、めっき性および耐水素脆性として抵抗スポット溶接部ナゲットの亀裂割れ克服の両立を検討した。その結果、鋼板の成分組成に加え、製造条件の適切な調整によって、最適な鋼組織を作り込むと共に鋼中水素量を制御することで、上記課題を解決するに至った。具体的には、本発明は以下のものを提供する。
　［１］鋼組成が質量％で、Ｃ：０．１０％以上０．３０％以下、Ｓｉ：１．２％未満、Ｍｎ：２．０％以上３．５％以下、Ｐ：０．０１０％以下、Ｓ：０．００２％以下、Ａｌ：１％以下、Ｎ：０．００６％以下を含有し、残部がＦｅおよび不可避的不純物からなる成分組成と、面積率で、マルテンサイトおよびベイナイトを７０％以上（１００％を含む）、フェライトを２０％未満（０％を含む）、残留オーステナイトを５％未満（０％を含む）含む鋼組織と、を有し、鋼中の拡散性水素量が０．２０質量ｐｐｍ以下の鋼板と、該鋼板の表面に、Ｆｅ含有量が質量％で８～１５％であり、片面あたりのめっき付着量が２０～１２０g／ｍ２である亜鉛めっき層と、を備え、前記亜鉛めっき層に含まれるＭｎ酸化物量が０．０５０ｇ／ｍ２以下であり、引張強さが１１００ＭＰａ以上、降伏比が０．８５以上である高強度亜鉛めっき鋼板。
　［２］前記成分組成は、さらに、質量％で、Ｔｉ、Ｎｂ、Ｖ、Ｚｒのうち１種以上を合計で０．００５～０．１％、Ｍｏ、Ｃｒ、Ｃｕ、Ｎｉのうち1種以上を合計で０．００５～０．５％およびＢ：０．０００３～０．００５％から選ばれる少なくとも１つを含有する［１］に記載の高強度亜鉛めっき鋼板。
　［３］前記成分組成は、さらに、質量％で、Ｓｂ：０．００１～０．１％およびＳｎ：０．００１～０．１％から選ばれる１種又は２種を含有する［１］または［２］に記載の高強度亜鉛めっき鋼板。
　［４］前記成分組成は、さらに、質量％で、Ｃａ：０．００１０％以下を含有する［１］～［３］のいずれかに記載の高強度亜鉛めっき鋼板。
　［５］［１］～［４］のいずれかに記載の成分組成を有する冷延素材を、水素濃度Ｈ：１ｖｏｌ．％以上１３ｖｏｌ．％以下の焼鈍炉内雰囲気で、焼鈍炉内温度Ｔ：Ａｃ３点－２０℃～９００℃の温度域に５秒滞留させた後、冷却し、４００～５５０℃の温度域に１０秒以上滞留させる焼鈍工程と、前記焼鈍工程後の鋼板を、めっき処理し、合金化処理し、平均冷却速度３℃／ｓ以上で、１００℃以下まで冷却するめっき工程と、前記めっき工程後のめっき鋼板を、水素濃度Ｈ：１０ｖｏｌ．％以下かつ露点Ｄｐ：５０℃以下の炉内雰囲気で、２００℃以上３５０℃以下の温度Ｔ（℃）に、０．００５（ｈｒ）以上で（１）式を満たす時間ｔ（ｈｒ）以上滞留させる後熱処理工程と、を備える高強度亜鉛めっき鋼板の製造方法。
　１３０－１８．３×ｌｎ（ｔ）≦Ｔ　　　（１）
　［６］さらに、前記焼鈍工程の前に冷延素材を、Ａｃ１点～Ａｃ３点＋５０℃の温度域まで加熱し、酸洗する前処理工程を備える［５］に記載の高強度亜鉛めっき鋼板の製造方法。
　［７］前記めっき工程後、０．１％以上の伸長率で調質圧延を施す［５］または［６］に記載の高強度亜鉛めっき鋼板の製造方法。
　［８］前記後熱処理工程後に、幅トリムをする［７］に記載の高強度亜鉛めっき鋼板の製造方法。
　［９］前記後熱処理工程前に、幅トリムを行い、前記後熱処理工程における、２００℃以上３５０℃以下の温度Ｔ（℃）で滞留する滞留時間ｔ（ｈｒ）が、０．００５（ｈｒ）以上かつ（２）式を満たす［７］に記載の高強度亜鉛めっき鋼板の製造方法。
　１１５－１８．３×ｌｎ（ｔ）≦　Ｔ　　　（２）
　本発明によれば，引張強さが１１００ＭＰａ以上、好ましくは１１５０ＭＰａ以上の高強度で、かつ降伏比（降伏強度比）が０．８５以上で、表面性状（めっき性・表面外観）に優れ、耐水素脆性にも優れた高強度亜鉛めっき鋼板が得られる。
拡散性水素量と最小ナゲット径との関係の一例を示す図である。
　以下、本発明の実施形態について説明する。なお、本発明は以下の実施形態に限定されない。
　＜高強度亜鉛めっき鋼板＞
　本発明の高強度亜鉛めっき鋼板は、鋼板と、該鋼板の表面に形成された亜鉛めっき層とを備える。以下では、鋼板、亜鉛めっき層の順で説明する。
　鋼板の成分組成は以下の通りである。以下の説明において、成分の含有量の単位である「％」は「質量％」を意味する。
　Ｃ：０．１０％以上０．３０％以下（Ｃ：０．１０～０．３０％）
　Ｃは鋼板の高強度化に有効な元素であり、鋼組織の硬質相の一つであるマルテンサイトを形成することで高強度化に寄与する。これらの効果を得るためには、Ｃ含有量は０．１０％以上とすることが必要である。好ましくは０．１１％以上、より好ましくは０．１２％以上である。一方、Ｃ含有量が０．３０％を超えると、本発明ではスポット溶接性が顕著に劣化すると同時に、マルテンサイトの強度増加により鋼板が硬質化し、延性などの成形性が低下する傾向にある。したがってＣ含有量は０．３０％以下とする。特性改善の観点から、好ましくは０．２８％以下、より好ましくは０．２５％以下とする。
　Ｓｉ：１．２％未満
　Ｓｉは主に固溶強化により高強度化に寄与する元素であり、強度上昇に対して延性の低下が比較的少なく強度のみならず強度と延性のバランス向上にも寄与する。一方でＳｉは鋼板表面にＳｉ系酸化物を形成しやすく、不めっきの原因となる場合があると共に、焼鈍時にオーステナイトを安定化させ、最終製品に残留オーステナイトを形成させやすくする。したがって、強度確保に必要な分だけ添加すればよく、その観点からはＳｉ含有量は０．０１％以上が望ましい。より好ましくは０．０２％以上である。さらに好ましくは０．０５％以上である。めっき性や残留オーステナイト生成の観点からその上限を１．２％未満とする。好ましくは１．０％以下である。より好ましくは０．９％以下である。
　Ｍｎ：２．０％以上３．５％以下
　Ｍｎは固溶強化およびマルテンサイト形成により高強度化に寄与する元素として有効である。この効果を得るためにＭｎ含有量は２．０％以上にする必要がある。好ましくは２．１％以上、より好ましくは２．２％以上である。一方、Ｍｎ含有量が３．５％を超えるとスポット溶接部割れを招くと共に、Ｍｎの偏析などに起因して鋼組織にムラを生じやすくなり、加工性の低下を招く。また、Ｍｎ含有量が３．５％を超えると、Ｍｎは鋼板表面に酸化物あるいは複合酸化物として濃化しやすく、不めっきの原因となる場合がある。そこで、Ｍｎ含有量を３．５％以下とする。好ましくは３．３％以下、より好ましくは３．０％以下である。
　Ｐ：０．０１０％以下
　Ｐは、固溶強化により鋼板の高強度化に寄与する有効な元素である。その含有量が０．０１０％を超えると溶接性や、伸びフランジ性などの加工性が低下する。そこで、Ｐ含有量は０．０１０％以下とする。好ましくは０．００８％以下、より好ましくは０．００７％以下である。下限は特に規定しないが０．００１％未満では製造過程において生産能率低下と脱燐コスト増を招くため、好ましくは０．００１％以上とする。
　Ｓ：０．００２％以下
　Ｓは熱間脆性を起こす原因となったり、溶接性の低下をもたらしたり、鋼中に硫化物系介在物として存在して鋼板の加工性を低下させる有害な元素である。このため、Ｓ含有量は極力低減することが好ましい。そこで、Ｓ含有量は０．００２％以下とする。下限は特に規定しないが０．０００１％未満では現状の製造過程において生産能率低下とコスト増を招くため、０．０００１％以上とすることが好ましい。
　Ａｌ：１％以下
　Ａｌは脱酸材として添加される。その効果を得る観点から好ましい含有量は０．０１％以上である。より好ましくは０．０２％以上である。一方Ａｌ含有量が１％を超えると原料コストの上昇を招くほか、鋼板の表面欠陥を誘発する原因にもなるため１％を上限とする。好ましくは０．４％以下、より好ましくは０．１％以下である。
　Ｎ：０．００６％以下
　Ｎ含有量が０．００６％を超えると鋼中に過剰な窒化物が生成して延性や靭性を低下させるほか、鋼板の表面性状の悪化を招くことがある。このためＮ含有量は０．００６％以下、好ましくは０．００５％以下、より好ましくは０．００４％以下とする。フェライトの清浄化による延性向上の観点からは含有量は極力少ない方が好ましいが、製造過程における生産能率低下とコスト増を招くため好ましい下限は０．０００１％以上とする。より好ましくは０．００１０％以上、さらに好ましくは０．００１５％以上である。
　上記鋼板の成分組成は、任意成分として、Ｔｉ、Ｎｂ、Ｖ、Ｚｒのうち１種以上を合計で０．００５～０．１％および／またはＭｏ、Ｃｒ、Ｃｕ、Ｎｉのうち1種以上を合計で０．００５～０．５％および／またはＢ：０．０００３～０．００５％を含有してもよい。
　Ｔｉ、Ｎｂ、Ｖ、Ｚｒは、ＣやＮと炭化物や窒化物（炭窒化物の場合もある）を形成する、微細析出物とすることで鋼板の高強度化、特に高ＹＲ化に寄与する。この効果を得る観点から、Ｔｉ、Ｎｂ、Ｖ、Ｚｒのうち１種以上を合計で０．００５％以上含有することが好ましい。より好ましくは０．０１５％以上、さらに好ましくは０．０３０％以上である。また、これらの元素は、鋼中水素のトラップサイト（無害化）のためにも有効である。しかしながら合計が０．１％を超える過剰な含有は、冷間圧延時の変形抵抗を高めて生産性を阻害するほか、過剰な或いは粗大な析出物の存在はフェライトの延性を低下させ、鋼板の延性や曲げ性、伸びフランジ性などの加工性を低下させる。そこで、上記合計を０．１％以下とすることが好ましい。より好ましくは０．０８％以下、さらに好ましくは０．０６％以下である。
　Ｍｏ、Ｃｒ、Ｃｕ、Ｎｉ、Ｂは、焼入れ性を高めてマルテンサイトを生成させやすくするため、高強度化に寄与する元素である。そこで、Ｍｏ、Ｃｒ、Ｃｕ、Ｎｉのうち1種以上を合計で０．００５％以上とすることが好ましい。より好ましくは０．０１％以上、さらに好ましくは０．０５％以上である。また、Ｂの場合は０．０００３％以上が好ましく、より好ましくは０．０００５％以上、さらに好ましくは０．００１０％以上である。また、Ｍｏ、Ｃｒ、Ｃｕ、Ｎｉについては、合計が０．５％を超える過剰な添加は効果の飽和やコスト増につながる。また、Ｃｕについては熱間圧延時の割れを誘発し表面疵の発生原因となるためその上限を０．５％とする。ＮｉについてはＣｕ含有による表面疵の発生を抑止する効果があるためＣｕ含有時に含有することが望ましい。特にＣｕ含有量の１／２以上のＮｉ含有することが好ましい。Ｂについても焼鈍冷却過程で起こるフェライト生成の抑制効果を得るための上記下限を設ける。また、Ｂ含有量が０．００５％を超える過剰な含有は効果が飽和する。過剰な焼入れ性は溶接時の溶接部割れなどの不利益もある。
　上記鋼板の成分組成は、任意成分として、Ｓｂ：０．００１～０．１％および／またはＳｎ：０．００１～０．１％を含有してもよい。
　ＳｂやＳｎは脱炭や脱窒、脱硼などを抑制して、鋼板の強度低下抑制に有効な元素である。またスポット溶接割れ抑制にも有効であるためＳｎ含有量、Ｓｂ含有量はぞれぞれ０．００１％以上が好ましい。より好ましくは０．００３％以上、さらに好ましくは０．００５％以上である。しかしながら、Ｓｎ、Ｓｂいずれも、０．１％を超える過剰な含有は鋼板の伸びフランジ性などの加工性を低下させる。そこで、Ｓｎ含有量、Ｓｂ含有量いずれも、０．１％以下とすることが好ましい。より好ましくは０．０３０％以下、さらに好ましくは０．０１０％以下である。
　上記鋼板の成分組成は、任意成分として、Ｃａ：０．００１０％以下を含有してもよい。
　Ｃａは鋼中で硫化物や酸化物を形成し、鋼板の加工性を低下させる。このため、Ｃａ含有量は０．００１０％以下が好ましい。より好ましくは０．０００５％以下、さらに好ましくは０．０００３％以下である。また、下限は特に限定されないが、製造上、Ｃａを全く含まないようにすることが困難な場合もあることから、それを考慮すると、Ｃａ含有量は０．００００１％以上が好ましい。より好ましくは０．００００５％以上である。
　上記鋼板の成分組成において、上記以外の残部はＦｅおよび不可避的不純物である。上記任意成分において、含有量の下限が存在する成分を上記下限値未満で含む場合、本発明の効果が害されないため、その任意成分は不可避的不純物とする。
　続いて、上記鋼板の鋼組織について説明する。
　鋼組織は、面積率で、ベイナイトおよびマルテンサイト（焼き戻しマルテンサイトを含む）を７０％以上（１００％を含む）、フェライトを２０％未満（０％を含む）、残留オーステナイトを５％未満（０％を含む）含む。
　フェライトが２０％未満（０％を含む）
　フェライトの存在は、高い引張強さおよび降伏強さを得る観点から好ましくないが、本発明の降伏強さを得るためには面積率で２０％未満まで許容される。好ましくは１５％以下、より好ましくは１０％以下である。下限は特に限定されないが、１％以上が好ましく、より好ましくは２％以上、さらに好ましくは３％以上である。なお、ここで比較的高温で生成した炭化物を含まないベイナイトは後述の実施例に記載の走査電子顕微鏡での観察ではフェライトとの区別はせず、フェライトとみなす。
　ベイナイトおよびマルテンサイト（焼き戻しマルテンサイトを含む）の合計が７０％以上（１００％を含む）
　焼入れままのマルテンサイト、焼戻しマルテンサイトおよびベイナイトは硬質相であり、高い引張強さと降伏強さを得るため、これら硬質相の面積率が本発明において重要である。マルテンサイトは特に硬質であり、鋼板強度を高めるために有効である。焼戻しマルテンサイトとベイナイト（上記の通り、炭化物を含まないベイナイトはフェライトとみなされるため、ここで言うベイナイトは炭化物を含むベイナイトを意味する）は、特に高い降伏強さを得るために重要な組織である。以上より、これら硬質相の合計を面積率で７０％以上とする。好ましくは８０％以上、より好ましくは８５％以上、さらに好ましくは９０％以上である。上限について好ましくは、１００％以下、好ましくは９９％以下、さらに好ましくは９８％以下である。
　これら硬質相における、ベイナイトは、２０％以上５０％以下であることが好ましい。また、組織全体に対して、ベイナイトは４９％以下が好ましく、より好ましくは４５％以下、さらに好ましくは４０％以下である。
　残留オーステナイトが５％未満（０％を含む）
　オーステナイト（ｆｃｃ相）はフェライト（ｂｃｃ相）に比べ、水素吸蔵能が高い。そのため、この残留オーステナイトが、加工など応力誘起によりマルテンサイトに変態した場合、鋼中の拡散性水素を増加させる懸念がある。そのため５％未満にする。より好ましくは４％以下、さらに好ましくは３％以下である。下限は特に限定しないが好ましくは０％以上、より好ましくは１％以上である。
　なお、鋼組織は上記した組織以外の組織として、残部にパーライトおよび炭化物などの析出物を含む場合がある。これらのその他の組織（硬質相、フェライト、残留オーステナイト以外の残部）は、面積率で１０％以下であることが好ましく、より好ましくは５％以下である。
　上記の鋼組織における面積率は、実施例に記載の方法で得られる結果を採用する。より具体的な面積率の測定方法は実施例に記載するが、簡潔には以下の通りである。上記面積率は表面から板厚の１／４厚み位置（１／８～３／８）の領域における組織で代表し、鋼板のＬ断面（圧延方向に平行な板厚断面）を研磨後、ナイタール液で腐食しＳＥＭで１５００倍の倍率で３視野以上を観察して撮影した画像を解析して求める。
　上記鋼板は、実施例に記載の方法で測定して得られる鋼中の拡散性水素量が０．２０質量ｐｐｍ（ｍａｓｓ．ｐｐｍ）以下である。鋼中の拡散性水素は、耐水素脆性を劣化させる。鋼中の拡散性水素量が０．２０質量ｐｐｍを超えて過剰になれば、たとえば溶接時に溶接部ナゲットの亀裂割れが生じやすくなる。本発明では、溶接前に母材である鋼中の拡散性水素量を０．２０質量ｐｐｍ以下とすることで改善効果があることを明らかにした。好ましくは０．１５質量ｐｐｍ以下、より好ましくは０．１０質量ｐｐｍ以下、さらに好ましくは０．０８質量ｐｐｍ以下である。下限は特に限定しないが、少ないほど好ましいため、下限は０質量ｐｐｍである。溶接前に母材の上記拡散性水素量を０．２０質量ｐｐｍ以下にすることが必要であり、溶接後の製品において、母材部分の拡散性水素量が０．２０質量ｐｐｍ以下であれば、溶接前に０．２０質量ｐｐｍ以下であったとみなせる。
　続いて、亜鉛めっき層について説明する。
　亜鉛めっき層は、片面あたりのめっき付着量が２０～１２０ｇ／ｍ２でＦｅを質量％で８～１５％含む。
　付着量が２０ｇ／ｍ２未満では耐食性の確保が困難になる。したがって、付着量は２０ｇ／ｍ２以上とし、好ましくは２５ｇ／ｍ２以上、より好ましくは３０ｇ／ｍ２以上である。一方、１２０ｇ／ｍ２を超えると耐めっき剥離性が劣化する。したがって、付着量は１２０ｇ／ｍ２以下、好ましくは１００ｇ／ｍ２以下、より好ましくは８０ｇ／ｍ２以下である。
　また、亜鉛めっき層中では、めっき前の熱処理工程で形成されたＭｎ酸化物は、めっき浴と鋼板が反応し、ＦｅＡｌあるいはＦｅＺｎ合金相が形成されることでめっき中に取り込まれ、めっき性、耐めっき剥離性が改善する。
　亜鉛めっき層中に含まれるＭｎ酸化物量は低いほど好ましい。また、めっき層中のＭｎ酸化物量が０．０５０ｇ／ｍ２超ではＦｅＡｌあるいはＦｅＺｎ合金相の形成反応が不十分となり、不めっきの発生や耐めっき剥離性の低下を招く。しかしながら、Ｍｎ酸化物量を０．００５ｇ／ｍ２未満に抑制するためには、通常操業条件よりも露点を低く制御する必要があり困難である。そこで、めっき層中のＭｎ酸化物量は０．０５０ｇ／ｍ２以下とする。また、以上の通り、めっき層中のＭｎ酸化物量は０．００５ｇ／ｍ２以上０．０５０ｇ／ｍ２以下が好ましい。なお、亜鉛めっき層中のＭｎ酸化物量の測定は実施例に記載の方法で行う。
　また、亜鉛めっき層は、Ｆｅを質量％で８～１５％含む。亜鉛めっき層中のＦｅ含有量が、質量％で８％以上であると、Ｆｅ－Ｚｎの合金層が十分に得られているといえる。好ましくは９％以上、より好ましくは１０％以上である。また、Ｆｅ含有量が１５％を超えるとめっき密着性が悪くなり、プレス時にパウダリングと呼ばれる不具合を引き起こす。このため、上記Ｆｅ含有量は１５％以下とする。好ましくは１４％以下、より好ましくは１３％以下である。
　また、亜鉛めっき層に含まれる、Ｆｅと残部のＺｎおよび不可避的不純物以外の成分は特に限定されない。亜鉛めっき層は、例えば、質量％で、Ａｌ：０．００１％～１．０％を含有し、Ｐｂ、Ｓｂ、Ｓｉ、Ｓｎ、Ｍｇ、Ｍｎ、Ｎｉ、Ｃｒ、Ｃｏ、Ｃａ、Ｃｕ、Ｌｉ、Ｔｉ、Ｂｅ、ＢｉおよびＲＥＭから選択する１種または２種以上を合計で０～３０％を含有してもよい。なお、Ｆｅ含有量が上記範囲にある亜鉛めっき層は、合金化溶融亜鉛めっき層である。
　＜高強度亜鉛めっき鋼板の製造方法＞
　本発明の高強度亜鉛めっき鋼板の製造方法は、焼鈍工程と、めっき工程と、後熱処理工程とを有する。
　焼鈍工程とは、上記成分組成を有する冷延素材を、水素濃度Ｈ：１ｖｏｌ％以上１３ｖｏｌ％以下の焼鈍炉内雰囲気で、焼鈍炉内温度Ｔ：（Ａｃ３点－２０℃）～９００℃以下の温度に５ｓ（秒）以上加熱（均熱処理）した後、冷却し、４００～５５０℃の温度域に１０ｓ以上滞留させる工程である。
　先ず、冷延素材の製造方法について説明する。
　本発明の製造方法で使用する冷延素材は、鋼素材から製造される。鋼素材は、一般的にスラブ（鋳片）とよばれる連続鋳造方法で製造されたものである。連続鋳造法を採用するのは、合金成分のマクロ偏析を防止する目的である。鋼素材は、造塊法や薄スラブ鋳造法などで製造してもよい。
　また、鋼スラブを製造したあと、一旦室温まで冷却してその後再加熱する従来法に加え、室温付近まで冷却せずに温片のままで加熱炉に装入して熱間圧延する方法や、わずかの補熱を行った後に直ちに熱間圧延する方法、或いは鋳造後高温状態を保ったまま熱間圧延する方法のいずれでもよい。
　熱間圧延の条件は特に限定されないが、上記成分組成を有する鋼素材を、１１００℃以上１３５０℃以下の温度で加熱し、仕上げ圧延温度が８００℃以上９５０℃以下の熱間圧延を施し、４５０℃以上７００℃以下の温度で巻き取る条件が好ましい。以下、これらの好ましい条件について説明する。
　鋼スラブの加熱温度は、１１００℃以上１３５０℃以下の範囲とすることが好ましい。上記上限温度範囲外であると、鋼スラブ中に存在する析出物は粗大化しやすく、例えば析出強化による強度確保をする場合には不利となる場合がある。また、粗大な析出物を核として後の熱処理において組織形成に悪影響を及ぼす可能性がある。一方、適切な加熱によりスラブ表面の気泡や欠陥などをスケールオフさせることで鋼板表面の亀裂や凹凸を低減し、平滑な鋼板表面を達成することは有益である。このような効果を得るために１１００℃以上とすることが好ましい。一方で、１３５０℃を超えるとオーステナイト粒の粗大化が起こり、最終製品の金属組織も粗大化して、鋼板の強度や曲げ性や伸びフランジ性などの加工性が低下する原因となる場合がある。
　加熱された鋼スラブに対し、粗圧延および仕上げ圧延を含む熱間圧延を施す。一般的に鋼スラブは粗圧延でシートバーとなり、仕上げ圧延によって熱延コイルとなる。また、ミル能力等によってはそのような区分けにこだわらず、所定のサイズになれば問題ない。熱間圧延条件としては、以下が好ましい。
　仕上げ圧延温度：８００℃以上９５０℃以下が好ましい。仕上げ圧延温度を８００℃以上とすることで、熱延コイルで得られる組織を均一にできる傾向にある。この段階で組織を均一にできることは、最終製品の組織が均一になることに寄与する。組織が不均一だと、延性や曲げ性、伸びフランジ性などの加工性が低下する。一方９５０℃を超えると酸化物（スケール）生成量が多くなり地鉄と酸化物の界面が荒れて、酸洗および冷間圧延後の表面品質が劣化する場合がある。
　また、組織において結晶粒径が粗大になることで、鋼スラブ同様鋼板の強度や曲げ性や伸びフランジ性などの加工性が低下する原因となる場合がある。上記熱間圧延を終了した後、組織の微細化や均一化のため、仕上げ圧延終了後３秒以内に冷却を開始し、［仕上げ圧延温度］～［仕上げ圧延温度－１００］℃の温度域を１０～２５０℃／ｓの平均冷却速度で冷却することが好ましい。
　巻取り温度は４５０～７００℃とすることが好ましい。熱延後のコイル巻取り直前の温度、すなわち巻取り温度が４５０℃以上であれば、Ｎｂなどを添加した際には炭化物の微細析出の観点から好ましく、巻取り温度が７００℃以下であればセメンタイト析出物が粗大になりすぎないため好ましい。また、４５０℃未満や７００℃超の温度域になると、コイルに巻き取った後の保持中に組織が変化しやすく、後工程の冷間圧延において素材の金属組織の不均一性に起因した圧延トラブルなどが起こりやすい。熱延板組織の整粒化などの観点からより好ましい巻取温度は５００℃以上６８０℃以下とする。
　次いで、冷間圧延工程を行う。通常、酸洗によりスケールを落とした後、冷間圧延が施され冷延コイルとなる。この酸洗は必要に応じて行われる。
　冷間圧延は圧下率２０％以上とすることが好ましい。これは引続き行う加熱において均一微細なミクロ組織を得るためである。２０％未満では加熱時に粗粒になりやすい場合や、不均一な組織になりやすい場合があり、前述したように、その後の熱処理後最終製品板での強度や加工性低下が懸念されるほか、表面性状を劣化させる。圧下率の上限は特に規定しないが、高強度の鋼板ゆえ、高い圧下率は圧延負荷による生産性低下のほか、形状不良となる場合がある。圧下率は９０％以下が好ましい。
　上記が冷延素材の製造方法である。
　焼鈍工程では、冷延素材を、水素濃度Ｈ：１ｖｏｌ％以上１３ｖｏｌ％以下の焼鈍炉内雰囲気で、焼鈍炉内温度Ｔ：（Ａｃ３点－２０℃）～９００℃以下の温度で５秒以上加熱した後、冷却し、４００～５５０℃の温度域に１０ｓ以上滞留させる。
　焼鈍炉内温度Ｔ：（Ａｃ３点－２０℃）～９００℃以下の温度域にするための平均加熱速度は特に限定されないが、平均加熱速度は組織の均一化という理由で１０℃／ｓ未満が好ましい。また、製造効率低下を抑える観点から平均加熱速度は１℃／ｓ以上が好ましい。
　加熱温度（焼鈍炉内温度）Ｔは、材質とめっき性いずれも担保するために、（Ａｃ３点－２０℃）～９００℃に設定する。加熱温度が（Ａｃ３点－２０℃）未満では、最終的に得られる金属組織で、フェライトの分率が高くなるため強度が得られなかったり、ベイナイトの生成が難しくなる。また、加熱温度が９００℃を超えると結晶粒が粗大化して曲げ性や伸びフランジ性などの加工性が低下するため好ましくない。また、加熱温度が９００℃を超えると、表面にＭｎやＳｉが濃化しやすくなってめっき性を阻害する。また、加熱温度がＡｃ３点を超えて且つ９００℃を超えると設備への負荷も高く安定して製造できなくなる可能性がある。
　また、本発明の製造方法では、焼鈍炉内温度Ｔ：（Ａｃ３点－２０℃）～９００℃の温度で５秒以上加熱する。過剰なオーステナイト粒径の粗大化を防ぐという理由で１８０秒以下が好ましい。また、組織の均一化の観点から、加熱時間は５秒以上とする。
　（Ａｃ３点－２０℃）～９００℃の温度域における水素濃度Ｈは１～１３ｖｏｌ％とする。本発明においては、上述の加熱温度に対し炉内雰囲気も同時に制御することでめっき性が担保されると同時に、鋼中への過剰な水素侵入を防ぐ。水素濃度が１ｖｏｌ％未満では不めっきが多発する。１３ｖｏｌ％を超える水素濃度ではめっき性に対する効果が飽和すると同時に、鋼中への水素侵入が著しく増大し、最終製品の耐水素脆性を劣化させる。なお、上記（Ａｃ３点－２０℃）～９００℃の温度域以外については、水素濃度は１ｖｏｌ％以上の範囲になくてもよい。
　上記水素濃度雰囲気での滞留の後、冷却するに際し、４００～５５０℃の温度域で１０ｓ以上滞留させる。これはベイナイトの生成と鋼中水素低減の促進のためである。金属組織の規定として、高ＹＳを得るためにベイナイトは重要な組織である。これを生成し、ベイナイト面積率を硬質相の２０％以上にするためにはこの温度域で１０ｓ以上滞留させる必要がある。４００℃未満での滞留は、後に続くめっき浴温を下回ることになりやすく、めっき浴の品質を落とすため好ましくない。その場合はめっき浴までに板温を加熱すればよく、そのため上記温度域の下限を４００℃とする。一方、５５０℃を超える温度域ではベイナイトではなくフェライトやパーライトが出やすくなる。加熱温度からこの温度域までの冷却については、３℃／s以上の冷却速度（平均冷却速度）とすることが好ましい。冷却速度が３℃／ｓ未満ではフェライト変態を起こしやすく、所望の金属組織が得られなくなる場合があるためである。好ましい冷却速度の上限は特に規定はない。また、冷却停止温度としては、上述の４００～５５０℃とすればよいが、これ以下の温度に一旦冷却し、再加熱により４００～５５０℃の温度域での滞留をさせることも可能である。この場合、Ｍｓ点以下まで冷却した場合にはマルテンサイトが生成された後、焼戻されることもある。
　めっき工程では、焼鈍工程後の鋼板を、めっき処理し、合金化処理し、平均冷却速度３℃／s以上で１００℃以下まで冷却する。
　めっき処理および合金化処理では、片面あたりのめっき付着量が２０～１２０ｇ／ｍ２になるようにする。また、Ｆｅ含有量が質量％で８～１５％である。上記の通り、Ｆｅ含有量が上記範囲の亜鉛めっき層は、合金化溶融亜鉛めっき層である。Ｆｅ以外にＡｌ：０．００１％～１．０％を含有する。また、上記の通り、亜鉛めっき層は所定量のＭｎ酸化物を含むため、Ｍｎを含有する。Ｐｂ、Ｓｂ、Ｓｉ、Ｓｎ、Ｍｇ、Ｍｎ、Ｎｉ、Ｃｒ、Ｃｏ、Ｃａ、Ｃｕ、Ｌｉ、Ｔｉ、Ｂｅ、ＢｉおよびＲＥＭから選択する１種または２種以上を合計０～３０％を含有してもよい。また、残部はＺｎ及び不可避的不純物である。
　めっき処理の方法は、溶融亜鉛めっき処理が好ましい。条件は適宜設定すればよい。また、溶融亜鉛めっき後に加熱する合金化処理を行う。例えば、４８０～６００℃の温度域に１～６０秒程度保持する処理が例示できる。この処理により、Ｆｅ含有量が８～１５％の合金化亜鉛めっき層が得られる。
　上記合金化処理後、平均冷却速度３℃／ｓ以上で１００℃以下まで冷却する。これは高強度化に必須なマルテンサイトを得るためである。３℃／ｓ未満では強度に必要なマルテンサイトを得ることが難しく、また１００℃より高い温度で冷却を止めてしまうと、マルテンサイトがこの時点で過度に焼戻され（自己焼戻し）たり、オーステナイトがマルテンサイトにならずフェライトに変態してしまい必要な強度を得にくくなるためである。
　上記めっき工程後に後熱処理工程を行う。後熱処理工程は、めっき工程後のめっき鋼板を、水素濃度Ｈ：１０ｖｏｌ．％以下かつ露点Ｄｐ：５０℃以下の炉内雰囲気で、２００℃以上３５０℃以下の温度Ｔ（℃）に、０．００５（ｈｒ）以上で（１）式を満たす時間ｔ（ｈｒ）以上滞留させる工程である。
　１３０－１８．３×ｌｎ（ｔ）≦Ｔ　　（１）
　高い降伏強さを得るため、また、鋼中の拡散性水素量を低減させるため、後熱処理工程を行う。水素濃度Ｈ：１０ｖｏｌ．％以下かつ露点Ｄｐ：５０℃以下の炉内雰囲気にすることで、鋼中の拡散性水素量の増加を抑えることができる。水素濃度Ｈは少ない方が好ましく５ｖｏｌ．％以下が好ましく、より好ましくは２ｖｏｌ．％以下である。水素濃度Ｈの下限は特に限定されず、上記の通り少ない方が好ましいが、水素濃度を過度に低下させるのは困難であることから、好ましい下限は１ｖｏｌ％以上である。また、上記効果を得るために、好ましい露点Ｄｐは４５℃以下、より好ましくは４０℃以下である。露点Ｄｐの下限は特に限定されないが、製造コストの観点からは－８０℃以上が好ましい。
　滞留させる温度について、３５０℃を超える温度では引張強さの低下や、めっき層の劣化や外観の劣化が起きるため温度の上限は３５０℃とした。好ましくは３４０℃以下、より好ましくは３３０℃以下である。また、滞留させる温度の下限が２００℃未満になると、ＹＲが高まらない。また、鋼中の拡散性水素量を十分に低下させることが難しくなり、溶接亀裂割れが生じる。そこで、上記温度の下限を２００℃とした。好ましくは２１０℃以上、より好ましくは２２０℃以上である。
　また、鋼中の水素を低減させるためには、温度だけでなく時間を適正化することが重要である。滞留させる時間を０．００５ｈｒ以上かつ（１）式を満たすように調整することで、鋼中の拡散性水素量を低減できるとともに、高い降伏強さにすることができる。
　以上が本発明の高強度亜鉛めっき鋼板の製造方法であるが、上記冷間圧延後、焼鈍の前に、冷間圧延で得られた冷延板をＡｃ１点～Ａｃ３点＋５０℃の温度域に加熱し、酸洗する前処理工程を行うことも可能である。
　Ａｃ１点～Ａｃ３点＋５０℃の温度域に加熱
　「Ａｃ１点～Ａｃ３点＋５０℃の温度域に加熱」は、鋼組織の形成による高い降伏強さとめっき性を最終製品で担保するための条件である。引続く焼鈍工程の前に、フェライトとマルテンサイトを含む組織を得ておくことが材質上好ましい。さらに、めっき性の観点からもこの加熱により鋼板表層部にＭｎなどの酸化物を濃化させることが望ましい。その観点で、Ａｃ１点～Ａｃ３点＋５０℃の温度域に加熱する。ここで、上述のＡｃ１やＡｃ３については以下の式で得られる値を用いた。
Ａｃ１＝７５１－２７Ｃ＋１８Ｓｉ－１２Ｍｎ－２３Ｃｕ－２３Ｎｉ＋２４Ｃｒ＋２３Ｍｏ－４０Ｖ－６Ｔｉ＋３２Ｚｒ＋２３３Ｎｂ－１６９Ａｌ－８９５Ｂ
Ａｃ３＝９１０－２０３（Ｃ）１／２＋４４．７Ｓｉ－３０Ｍｎ－１１Ｐ＋７００Ｓ＋４００Ａｌ＋４００Ｔｉとする。
なお、上記式における元素記号は各元素の含有量を意味し、含有しない成分は０とする。
　上記加熱後の酸洗は、引続く熱処理において、Ａｃ３点以上の温度域での加熱によりめっき性を担保するため、鋼板表層部に濃化したＳｉやＭｎなどの酸化物を酸洗により除去する。
　また、めっき工程後に調質圧延を行ってもよい。
　調質圧延は、めっき工程の冷却の後に、０．１％以上の伸長率で行われる。調質圧延は行わなくてもよい。形状矯正や表面粗度調整の目的に加え、ＹＳを安定的に得る目的で、０．１％以上の伸長率で調質圧延をする。形状矯正や表面粗度調整については調質圧延に代えてレベラー加工を施してもよい。過度な調質圧延は、鋼板表面に過剰な歪が導入されて延性や伸びフランジ性の評価値を下げる。また、過度な調質圧延は延性も低下させるほか、高強度鋼板ゆえ設備負荷も高くなる。そこで、調質圧延の圧下率は３％以下とすることが好ましい。
　上記調質圧延の前または後に幅トリムを行うことが好ましい。この幅トリムにより、コイル幅調整を行うことができる。また、下記の通り、幅トリムを後熱処理工程より前に行うことで、引続く後熱処理で効率的に鋼中水素を放出させることができる。
　後熱処理工程前に幅トリムを行うことが好ましい。後熱処理工程前に幅トリムを行うと、後熱処理工程における、２００℃以上３５０℃以下の温度Ｔ（℃）で滞留する滞留時間ｔ（ｈｒ）を、０．００５（ｈｒ）以上かつ（２）式を満たす条件にすればよい。
１１５－１８．３×ｌｎ（ｔ）≦Ｔ　　　（２）
　（２）式から明らかなように、（１）式の場合と比較して、温度条件が同じであれば短時間化でき、滞留時間の条件が同じであれば低温化することができる。
　表1に示す組成の溶鋼を転炉で溶製し、連続鋳造機でスラブとした。このスラブを１２００℃に加熱し、仕上圧延温度８４０℃、コイル巻取温度６１０℃で熱延コイルとした。この熱延コイルを冷間圧下率５０％で板厚１．４ｍｍの冷延素材とした。この冷延素材を、水素濃度９ｖｏｌ％および露点－３０℃の焼鈍炉内雰囲気の焼鈍処理で、８１０℃（（Ａｃ３点－２０℃）～９００℃の範囲内）まで加熱し、６０秒滞留させた後、５００℃まで冷却し、５０秒滞留させた。その後亜鉛めっきを施して合金化処理をおこない、めっき後は水温４０℃の水槽を通すことで１００℃以下まで冷却し、平均冷却速度を３℃／sとして高強度合金化亜鉛めっき鋼板（製品板）を製造した。ここで、めっき層のＦｅ含有量および付着量は本願発明範囲になるように調整した。その後、水素濃度０ｖｏｌ．％および露点－１０℃の炉内雰囲気で、種々の温度および時間で後熱処理を行った。調質圧延はめっき後に実施し伸長率は０．２％とした。幅トリムは実施しなかった。
　それぞれからサンプルを切出し、鋼中の水素分析、耐水素脆性の評価として溶接部のナゲット割れを評価した。結果を図１に示す。
　鋼中の水素量
　鋼中の水素量を以下の方法で測定した。先ず、後熱処理まで施した合金化亜鉛めっき鋼板から、５×３０ｍｍ程度の試験片を切り出した。次いで、ルータを使って試験片表面のめっきを除去して石英管中に入れた。次いで、石英管中をＡｒで置換した後、２００℃／ｈｒで昇温し、４００℃までに発生した水素をガスクロマトグラフにより測定した。このように、昇温分析法にて放出水素量を測定した。室温（２５℃）から２１０℃未満の温度域で検出された水素量の累積値を拡散性水素量とした。
　耐水素脆性
　耐水素脆性の評価として、鋼板の抵抗スポット溶接部のナゲット割れを評価した。評価方法は、３０×１００ｍｍの板の両端に板厚２ｍｍの板をスペーサとして挟み、スペーサ間の中央をスポット溶接にて接合して試験片を作製した。この際、スポット溶接は、インバータ直流抵抗スポット溶接機を用い、電極はクロム銅製の先端径６ｍｍのドーム型を用いた。加圧力は３８０ｋｇｆ、通電時間は１６サイクル／５０Ｈｚ、保持時間は５サイクル／５０Ｈｚとした。溶接電流値を変化させて種々のナゲット径のサンプルを作製した。
　両端のスペーサ間隔は４０ｍｍとし、鋼板とスペーサは、予め溶接により固縛した。溶接後２４時間放置したのち、スペーサ部を切り落として、溶接ナゲットの断面観察を行い、水素脆化による割れ（亀裂）の有無の評価を行い、亀裂がなかった最小のナゲット径を求めた。図１に拡散性水素量と最小ナゲット径との関係を示した。
　図１に示す通り、鋼中の拡散性水素量が０．２０質量ｐｐｍを超えると最少ナゲット径が急激に大きくなり、最少ナゲット径が４を超え劣化している。
　なお、拡散性水素量が本発明範囲の場合、鋼組織等も本発明範囲である。
Figure JPOXMLDOC01-appb-T000001
  
　表２に示す成分組成の溶鋼を転炉で溶製し、連続鋳造機でスラブとしたあと、表２に示す種々の条件で熱延、冷延、加熱（焼鈍）、酸洗、（表２において「○」の場合は、酸洗液のＨＣｌ濃度を５ｍａｓｓ％、液温を６０℃に調整したものを使用した）、焼鈍およびめっき処理、調質圧延、幅トリム、後熱処理を施し、１．４ｍｍ厚の高強度亜鉛めっき鋼板（製品板）を製造した。なお、冷却（めっき処理後の冷却）では水温４０℃の水槽を通すことで１００℃以下まで冷却した。
　以上により得られた亜鉛めっき鋼板のサンプルを採取し、下記の方法で鋼組織観察および引張試験を行って組織の分率（面積率）、降伏強さ（ＹＳ）、引張強さ（ＴＳ）、降伏強度比（ＹＲ＝ＹＳ／ＴＳ）を測定・算出した。また、外観を目視観察してめっき性（表面性状）を評価した。評価方法は以下の通りである。
　組織観察
　亜鉛めっき鋼板から組織観察用試験片を採取し、Ｌ断面（圧延方向に平行な板厚断面）を研磨後、ナイタール液で腐食しＳＥＭで表面から１／４ｔ（ｔは全厚）近傍の位置を１５００倍の倍率で３視野以上を観察して撮影した画像を解析した（観察視野ごとに面積率を測定し、平均値を算出した）。ただし、残留オーステナイトの体積率（体積率を面積率とみなす）についてはＸ線回折強度により定量した。表４のＦはフェライト、Ｍは焼入れままマルテンサイト、Ｍ’は焼戻しマルテンサイト、Ｂはベイナイト、残留γは残留オーステナイトを意味する。
　なお、上記組織観察において、一部の例においては、その他の相として、パーライト、析出物や介在物の凝集が観察された。
　引張試験
　亜鉛めっき鋼板から圧延方向に対して直角方向にＪＩＳ５号引張試験片（ＪＩＳＺ２２０１）を採取し、引張速度（クロスヘッドスピード）１０ｍｍ／ｍｉｎ一定で引張試験を行った。降伏強さ（ＹＳ）は、応力１５０－３５０ＭＰａ弾性域の傾きから０．２％耐力を読み取った値とし、引張強さは引張試験における最大荷重を初期の試験片平行部断面積で除した値とした。平行部の断面積算出における板厚はめっき厚込みの板厚値を用いた。
　表面性状（外観）
　めっき後，後熱処理したのちの外観を目視観察し、不めっき欠陥が全くないものを「○」、不めっき欠陥が発生したものを「×」、不めっき欠陥はないがめっき外観ムラなどが生じたものは「△」とした。なお、不めっき欠陥とは数μｍ～数ｍｍ程度のオーダーで、めっきが存在せず鋼板が露出している領域を意味する。
　鋼中の拡散性水素量
　鋼中の拡散性水素量の測定は、実施例１と同様の方法で行った。
　亜鉛めっき層中のＭｎ酸化物量
　亜鉛めっき層中のＭｎ酸化物量の測定は、亜鉛めっき層中のＭｎ酸化物量については、めっき層を、インヒビターを添加した希塩酸で溶解し、ＩＣＰ発光分光分析法を使用して測定した。
　耐水素脆性
　耐水素脆性の評価として、鋼板の抵抗スポット溶接部の水素脆性を評価した。評価方法は、実施例１と同様である。溶接電流値は、それぞれの鋼板強度に応じたナゲット径を形成する条件とした。１１００～１２５０ＭＰａでは、３．８ｍｍ、１２５０～１４００ＭＰａでは４．８ｍｍ、１４００ＭＰａ以上では６ｍｍのナゲット径とした。実施例１同様、両端のスペーサ間隔は４０ｍｍとし、鋼板とスペーサは、予め溶接により固縛した。溶接後２４時間放置したのち、スペーサ部を切り落として、溶接ナゲットの断面観察をおこない、割れ（亀裂）の有無の評価をおこなった。表中、亀裂なしを「○」、亀裂ありを「×」であらわした。
　得られた結果を表４に示す。発明例はＴＳ、ＹＲ、表面性状、溶接性がいずれも良好であった。比較例はいずれかが劣っていた。また、発明例と比較例との対比から、本発明の成分組成や鋼組織の範囲内において、拡散性水素量と溶接性との関係は図１と同様であり、拡散性水素量が０．２０質量ｐｐｍ以下のときに、耐水素脆性として、抵抗スポット溶接部ナゲット割れの評価が良好になることが分かる。
Figure JPOXMLDOC01-appb-T000002
  
Figure JPOXMLDOC01-appb-T000003
  
Figure JPOXMLDOC01-appb-T000004
  
　本発明の高強度亜鉛めっき鋼板は、高い引張強さを有するだけでなく、高い降伏強度比と良好な表面性状および耐水素脆性を兼ね備える。このため、自動車車体の骨格部品、特に衝突安全性に影響するキャビン周辺を中心に、本発明を適用した場合、その安全性能の向上と共に、高強度薄肉化効果による車体軽量化に寄与する。その結果、本発明は、ＣＯ２排出など環境面にも貢献することができる。また、本発明は、良好な表面性状・めっき品質を兼ね備えているため、足回りなど雨雪による腐食が懸念される箇所にも積極的に適用することが可能である。このため、本発明によれば、車体の防錆・耐腐食性についても性能向上が期待できる。このような特性は自動車部品に限らず、土木・建築、家電分野にも有効である。

Claims (9)

  1. 　鋼組成が質量％で、
    Ｃ：０．１０％以上０．３０％以下、
    Ｓｉ：１．２％未満、
    Ｍｎ：２．０％以上３．５％以下、
    Ｐ：０．０１０％以下、
    Ｓ：０．００２％以下、
    Ａｌ：１％以下、
    Ｎ：０．００６％以下を含有し、残部がＦｅおよび不可避的不純物からなる成分組成と、
    　面積率で、マルテンサイトおよびベイナイトを７０％以上（１００％を含む）、フェライトを２０％未満（０％を含む）、残留オーステナイトを５％未満（０％を含む）含む鋼組織と、を有し、
    　鋼中の拡散性水素量が０．２０質量ｐｐｍ以下の鋼板と、
    　該鋼板の表面に、Ｆｅ含有量が質量％で８～１５％であり、片面あたりのめっき付着量が２０～１２０g／ｍ２である亜鉛めっき層と、を備え、
    　前記亜鉛めっき層に含まれるＭｎ酸化物量が０．０５０ｇ／ｍ２以下であり、
    　引張強さが１１００ＭＰａ以上、降伏比が０．８５以上である高強度亜鉛めっき鋼板。
  2. 　前記成分組成は、さらに、質量％で、
    Ｔｉ、Ｎｂ、Ｖ、Ｚｒのうち１種以上を合計で０．００５～０．１％、
    Ｍｏ、Ｃｒ、Ｃｕ、Ｎｉのうち1種以上を合計で０．００５～０．５％および
    Ｂ：０．０００３～０．００５％から選ばれる少なくとも１つを含有する請求項1に記載の高強度亜鉛めっき鋼板。
  3. 　前記成分組成は、さらに、質量％で、
    Ｓｂ：０．００１～０．１％およびＳｎ：０．００１～０．１％から選ばれる１種又は２種を含有する請求項１または２に記載の高強度亜鉛めっき鋼板。
  4. 　前記成分組成は、さらに、質量％で、
    Ｃａ：０．００１０％以下を含有する請求項１～３のいずれかに記載の高強度亜鉛めっき鋼板。
  5. 　請求項１～４のいずれかに記載の成分組成を有する冷延素材を、水素濃度Ｈ：１ｖｏｌ．％以上１３ｖｏｌ．％以下の焼鈍炉内雰囲気で、焼鈍炉内温度Ｔ：Ａｃ３点－２０℃～９００℃の温度域に５秒以上滞留させた後、冷却し、４００～５５０℃の温度域に１０秒以上滞留させる焼鈍工程と、
    　前記焼鈍工程後の鋼板を、めっき処理し、合金化処理し、平均冷却速度３℃／ｓ以上で、１００℃以下まで冷却するめっき工程と、
    　前記めっき工程後のめっき鋼板を、水素濃度Ｈ：１０ｖｏｌ．％以下かつ露点Ｄｐ：５０℃以下の炉内雰囲気で、２００℃以上３５０℃以下の温度Ｔ（℃）に、０．００５（ｈｒ）以上で（１）式を満たす時間ｔ（ｈｒ）以上滞留させる後熱処理工程と、を備える高強度亜鉛めっき鋼板の製造方法。
    　１３０－１８．３×ｌｎ（ｔ）≦Ｔ　　　（１）
  6. 　さらに、前記焼鈍工程の前に冷延素材を、Ａｃ１点～Ａｃ３点＋５０℃の温度域まで加熱し、酸洗する前処理工程を備える請求項５に記載の高強度亜鉛めっき鋼板の製造方法。
  7. 　前記めっき工程後、０．１％以上の伸長率で調質圧延を施す請求項５または６に記載の高強度亜鉛めっき鋼板の製造方法。
  8. 　前記後熱処理工程後に、幅トリムをする請求項７に記載の高強度亜鉛めっき鋼板の製造方法。
  9. 　前記後熱処理工程前に、幅トリムを行い、
    　前記後熱処理工程における、２００℃以上３５０℃以下の温度Ｔ（℃）で滞留する滞留時間ｔ（ｈｒ）が、０．００５（ｈｒ）以上かつ（２）式を満たす請求項７に記載の高強度亜鉛めっき鋼板の製造方法。
    　１１５－１８．３×ｌｎ（ｔ）≦　Ｔ　　　（２）
     
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