High-strength alloyed hot-dip galvanized steel sheet and method for producing same

Abstract

Provided is an alloyed hot-dip galvanized steel sheet which has a tensile strength of 980 MPa or more and excellent ductility and microcrack resistance, and which is provided with a steel sheet and an alloyed hot-dip galvanized layer. The component composition of the steel sheet has a carbon equivalent Ceq of 0.520 or more but less than 0.700, and the total content of Nb and Ti is 0.010-0.080 mass%. The martensite area ratio at the 1 / 4 plate thickness position of the steel plate is more than 30% and 70% or less. In a range from the surface of the steel sheet to a depth of 20 [mu] m, the area ratio of martensite is more than 30% and 70% or less, the grain size of prior austenite having a martensite area ratio of 50% or more is 3.0 [mu] m or less, the total content of Nb and Ti in precipitates having a particle size of 100 nm or less is 50 ppm by mass or more, and the total content of Nb and Ti in precipitates having a particle size of more than 100 nm is 350 ppm by mass or less. In a range from the surface of the steel sheet to a depth of 1 [mu] m, the long sides of the Si and Mn oxides present on the grain boundaries are 200 nm or less.

Description

Technical Field

[0001] This invention relates to high-strength alloyed hot-dip galvanized steel sheets and their manufacturing methods. Background Technology

[0002] From the perspective of protecting the Earth's environment, in order to improve the fuel efficiency of automobiles, the steel plates used for automobile parts are sometimes made stronger and thinner, thereby making the car body lighter.

[0003] In addition, from the perspective of rust prevention performance of the car body, galvanizing is sometimes applied to the steel sheets used as automotive parts.

[0004] For example, alloyed hot-dip galvanized steel sheets with a tensile strength (TS) of 980 MPa or higher are being developed as steel sheets for the frame of automobile cockpits (Patent Documents 1-2).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2012-219342

[0008] Patent Document 2: Japanese Patent Application Publication No. 2013-117042 Summary of the Invention

[0009] High-strength steel plates with a tensile strength of 980 MPa or higher may develop cracks (crazing) when subjected to bending deformation due to reduced local ductility and sometimes deteriorated bending properties.

[0010] The inventors applied a bending deformation to an alloyed hot-dip galvanized steel sheet (hereinafter, also simply "coated steel sheet") with a bending radius R greater than the bending radius R that causes cracking according to the JIS bending test, thus forming a bent portion. The bent portion of the coated steel sheet was then observed using a scanning electron microscope (SEM).

[0011] Figure 1 This is a SEM image representing a cross-section of an alloyed hot-dip galvanized steel sheet.

[0012] like Figure 1 As shown, it can be seen that in the bent portion of the alloyed hot-dip galvanized steel sheet, fine microcracks 3, which originate from cracks in the alloyed hot-dip galvanized layer 2 and cannot be observed without SEM, are generated in the steel sheet 1.

[0013] If microcracks occur in the steel plate (base steel plate), the corrosion resistance of the bent part of the coated steel plate will deteriorate, and consequently, the impact absorption characteristics and fatigue characteristics may also deteriorate.

[0014] To suppress the formation of microcracks, it is necessary to stop the cracks generated in the alloyed hot-dip galvanized layer (hereinafter also referred to as the "coating") from forming on the steel plate. This is very different from the formation mechanism of previous cracks (large-sized cracks that can be observed with a magnifying glass) that formed on the side of the steel plate.

[0015] In the aforementioned patent documents 1 to 3, techniques for improving flexibility (suppressing crack formation) are disclosed, but the targets are conventional cracks, not microcracks.

[0016] Hereinafter, the bending property of microcracks will also be referred to as "microcrack resistance".

[0017] The criteria for evaluating excellent microcrack resistance are explained later (see the [Example] column).

[0018] It should be noted that, for convenience, "resistance to microcracks" is sometimes referred to as "flexibility".

[0019] The present invention was made in view of the above points, with the aim of providing an alloyed hot-dip galvanized steel sheet with a tensile strength of 980 MPa or more and excellent ductility and resistance to microcracks.

[0020] Furthermore, the present invention aims to provide a method for manufacturing the above-mentioned alloyed hot-dip galvanized steel sheet.

[0021] The inventors conducted in-depth research and found that the above-mentioned objectives could be achieved by adopting the following configuration, thus completing the present invention.

[0022] That is, the present invention provides the following [1] to [6].

[0023] [1] A high-strength alloyed hot-dip galvanized steel sheet, comprising a steel sheet and an alloyed hot-dip galvanized layer disposed on the surface of the steel sheet, wherein the composition of the steel sheet comprises the following: a carbon equivalent Ceq of 0.520 or more and less than 0.700 as shown in the following formula (1), a total content of Nb and Ti of 0.010 to 0.080% by mass, the remainder being Fe and unavoidable impurities, and a martensite area ratio of more than 30% and less than 70% at 1 / 4 of the thickness of the steel sheet, wherein the composition of the steel sheet comprises the following: Within a range from the surface to a depth of 20 μm, the martensite area fraction exceeds 30% but is less than 70%, the original austenite grain size is less than 3.0 μm, the combined Nb and Ti content in precipitates less than 100 nm is more than 50 ppm by mass, and the combined Nb and Ti content in precipitates greater than 100 nm is less than 350 ppm by mass. Within a range from the surface of the aforementioned steel plate to a depth of 1 μm, the long side of Si and Mn oxides present at grain boundaries is less than 200 nm.

[0024] Ceq=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) (1)

[0025] In the above formula (1), [M%] is the content of element M in the above composition in terms of mass%, which is 0 when element M is not present.

[0026] [2] The high-strength alloyed hot-dip galvanized steel sheet according to [1] above, wherein the above composition further contains, by mass%, C: 0.070-0.170%, Si: less than 0.30%, Mn: 1.70-3.50%, P: less than 0.100%, S: less than 0.0200%, Al: less than 0.100%, N: less than 0.0100%, and O: less than 0.0100%.

[0027] [3] The high-strength alloyed hot-dip galvanized steel sheet according to [2] above, wherein the above composition further contains, by mass%, at least one element selected from B: less than 0.0050%, Ta: less than 0.10%, W: less than 0.10%, Cr: less than 1.00%, Ni: less than 1.00%, Mo: less than 1.00%, V: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

[0028] [4] A method for manufacturing a high-strength alloyed hot-dip galvanized steel sheet is a method for manufacturing a high-strength alloyed hot-dip galvanized steel sheet as described in any one of [1] to [3] above. The method involves hot rolling a slab having the composition described in any one of [1] to [3] above to obtain a hot-rolled steel sheet under the following conditions: a slab heating temperature of 1200°C or higher, a final reduction rate of 5% or higher, a rolling end temperature of 850–970°C, and a cooling time from the final reduction to below 700°C of 6.0 s or less. Then, the hot-rolled steel sheet is pickled under the condition of a reduction rate of 30% or higher. The hot-rolled steel sheet after pickling is cold-rolled to obtain a cold-rolled steel sheet. Then, the cold-rolled steel sheet is pickled for more than 2.0 seconds. The pickled cold-rolled steel sheet is then annealed under the following conditions: a heating rate of 2.0 to 7.0 °C / s from 500 °C to 700 °C, a dew point of -40 °C or less in an atmosphere above 700 °C, a maximum temperature of 740 to 860 °C, and a cooling rate v1 of 2.0 °C / s or less from 530 °C to 480 °C. Finally, the annealed cold-rolled steel sheet is subjected to an alloying hot-dip galvanizing treatment, which includes alloying at a temperature above 480 °C.

[0029] [5] In the method for manufacturing high-strength alloyed hot-dip galvanized steel sheet according to [4] above, the cooling rate v2 from 700°C to 600°C in the annealing process is 5.0°C / s or less.

[0030] [6] In the method for manufacturing high-strength alloyed hot-dip galvanized steel sheet according to [4] or [5] above, the CO concentration in the atmosphere above 700°C during the annealing is less than 200 ppm by volume.

[0031] According to the present invention, it is possible to provide alloyed hot-dip galvanized steel sheets with a tensile strength of 980 MPa or more and excellent ductility and resistance to microcracks. Attached Figure Description

[0032] Figure 1 This is a SEM image representing a cross-section of an alloyed hot-dip galvanized steel sheet. Detailed Implementation

[0033] [Alloyed hot-dip galvanized steel sheet]

[0034] In short, the high-strength alloyed hot-dip galvanized steel sheet of this embodiment includes a steel sheet and an alloyed hot-dip galvanized layer disposed on the surface of the steel sheet.

[0035] High strength refers to a tensile strength (TS) of 980 MPa or higher.

[0036] In this embodiment, the high-strength alloyed hot-dip galvanized steel sheet exhibits excellent ductility, resistance to microcracks, and corrosion resistance due to its composition and microstructure described later. Therefore, it is suitable for automotive components (e.g., the frame of a car's cockpit). In this case, it improves safety performance while also contributing to vehicle weight reduction, resulting in improved fuel efficiency and reduced CO2 emissions, thus contributing to environmental benefits. Furthermore, it can be actively applied to areas of concern such as automotive suspension components, where corrosion from rain and snow is a concern.

[0037] It should be noted that the high-strength alloyed hot-dip galvanized steel sheet of this embodiment is not limited to automobiles, but can also be applied to civil engineering, construction, home appliances and other fields.

[0038] <Steel Plate>

[0039] First, the steel plate (base steel plate) of the high-strength alloyed hot-dip galvanized steel sheet of this embodiment will be described.

[0040] There is no particular limitation on the thickness of the steel plate, for example, it can be 0.5mm to 3.0mm.

[0041] Composition

[0042] First, the composition of the steel plate (base steel plate) will be explained.

[0043] Unless otherwise specified, the unit "%" in the composition of ingredients refers to "mass %".

[0044] (Carbon equivalent Ceq: ≥0.520 and <0.700)

[0045] Based on the premise of obtaining a tensile strength of 980 MPa or higher, the carbon equivalent Ceq is 0.520 or higher, preferably 0.540 or higher, and more preferably 0.560 or higher.

[0046] On the other hand, if the carbon equivalent Ceq is too high, the ductility decreases. For the sake of obtaining good ductility, the carbon equivalent Ceq is less than 0.700, preferably less than 0.698, more preferably less than 0.690, and even more preferably less than 0.670.

[0047] The carbon equivalent Ceq is expressed by the following formula (1).

[0048] Ceq=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) (1)

[0049] In the above formula (1), [M%] is the content of element M in the composition of the steel plate (unit: mass%), which is 0 (zero) when element M is not present.

[0050] (Nb+Ti: 0.010~0.080%)

[0051] Nb and Ti exist as carbides and / or nitrides dispersed on the surface of the steel plate, thereby suppressing stress concentration and the formation of microcracks. To achieve this effect, the combined content of Nb and Ti is 0.010% or more, preferably 0.012% or more, more preferably 0.016% or more, and even more preferably 0.020% or more.

[0052] On the other hand, if excessive amounts of Nb and Ti are added, the precipitates on the surface of the steel plate become coarse, promoting the formation of microcracks. Therefore, the combined content of Nb and Ti is 0.080% or less, preferably 0.060% or less, and more preferably 0.040% or less.

[0053] (Other elements, of which 1)

[0054] The composition of the steel plate may further include the elements described below.

[0055] ((C: 0.070~0.170%))

[0056] C is an effective element for increasing the strength of steel, particularly by forming martensite, one of the hard phases, in the steel microstructure.

[0057] From the viewpoint of obtaining the desired high strength, specifically a tensile strength of 980 MPa or more, the C content is preferably 0.070% or more, more preferably 0.075% or more, and even more preferably 0.080% or more.

[0058] On the other hand, if the carbon content is too high, the ductility will decrease. Therefore, the carbon content is preferably 0.170% or less, more preferably 0.160% or less, and even more preferably 0.150% or less.

[0059] (Si: below 0.30%)

[0060] If excessive Si is added, Si oxide will form on the surface of the steel plate, reducing its resistance to microcracks. Therefore, the Si content is preferably 0.30% or less, more preferably 0.25% or less, and even more preferably 0.20% or less.

[0061] There is no specific lower limit for the Si content, for example, it can be 0.01%, but it can also be 0 (zero).

[0062] (Mn: 1.70-3.50%)

[0063] Mn is an element that contributes to the high strength of steel through solid solution strengthening and the formation of martensite. To achieve this effect, the Mn content is preferably 1.70% or more, more preferably 1.80% or more, and even more preferably 2.00% or more.

[0064] On the other hand, if the Mn content is too high, Mn oxides will form on the surface of the steel plate, reducing its resistance to microcracks. Therefore, the Mn content is preferably 3.50% or less, more preferably 3.20% or less, and even more preferably 3.00% or less.

[0065] (P: below 0.100%)

[0066] Phosphorus (P) segregates at the original austenite grain boundaries, causing grain boundary embrittlement. Therefore, from the perspective of reducing the ultimate deformation energy of steel plates, if the P content is too high, the bending performance will decrease. Therefore, the P content is preferably 0.100% or less, more preferably 0.070% or less, and even more preferably 0.040% or less.

[0067] On the other hand, there is no particular lower limit for the phosphorus (P) content. However, P is a solid solution strengthening element that can increase the strength of steel plates. From this point of view, the P content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0068] (S: below 0.0200%)

[0069] Sulfide (S) exists as a sulfide. From the perspective of reducing the ultimate deformation energy of steel plates, excessive S content reduces flexibility. Therefore, the S content is preferably 0.0200% or less, more preferably 0.0120% or less, and even more preferably 0.0050% or less.

[0070] On the other hand, there is no particular lower limit for the sulfur content. However, from the perspective of production technology constraints, the sulfur content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0071] (Al: below 0.100%)

[0072] Al is added as a deoxidizing material. From the viewpoint of achieving this effect, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.

[0073] However, if the Al content is too high, it may cause surface defects in the coating. Therefore, the Al content is preferably 0.100% or less, more preferably 0.090% or less, and even more preferably 0.080% or less.

[0074] (N: below 0.0100%)

[0075] Nitrogen (N) exists as a nitride. From the perspective of reducing the ultimate deformation energy of steel plates, excessive N content reduces flexibility. Therefore, the N content is preferably 0.0100% or less, more preferably 0.0070% or less, and even more preferably 0.0050% or less.

[0076] On the other hand, there is no particular lower limit for the nitrogen content. However, from the perspective of production technology constraints, the nitrogen content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0077] (O: below 0.0100%)

[0078] O, as an oxide, reduces the ultimate deformation energy of steel plates. Excessive O content decreases flexibility. Therefore, the O content is preferably 0.0100% or less, more preferably 0.0070% or less, and even more preferably 0.0050% or less.

[0079] On the other hand, there is no particular lower limit for the O content. However, from the perspective of production technology constraints, the O content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0080] (Other elements, number 2)

[0081] The steel plate may further contain at least one element selected from the elements listed below.

[0082] (B)

[0083] B is included in Si and Mn oxides that can be formed on the surface of the steel plate. By improving the wettability of the aforementioned oxides with molten zinc, it achieves the effect of giving the coating a good appearance. From the viewpoint of obtaining this effect, the B content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0084] On the other hand, if the B content is too high, coarse B compounds are formed, which sometimes reduces flexibility. Therefore, the B content is preferably 0.0050% or less, more preferably 0.0045% or less, and even more preferably 0.0040% or less.

[0085] (Ta and W)

[0086] Regarding Ta and W, if their contents are appropriate, large amounts of coarse precipitates and inclusions will not be generated, thus preventing a decrease in the ultimate deformation energy of the steel plate and maintaining its flexibility. Therefore, the contents of Ta and W are preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less, respectively.

[0087] There is no particular limitation on the lower limit of the content of Ta and W. However, Ta and W form fine carbides, nitrides or carbonitrides during hot rolling or annealing, thereby increasing the strength of the steel sheet. Therefore, the content of Ta and W is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0088] ((Cr, Ni and Mo))

[0089] Regarding Cr, Ni, and Mo, if their contents are appropriate, coarse precipitates and inclusions will not increase, the ultimate deformation energy of the steel plate will not decrease, and therefore the bending properties will not decrease. Therefore, the contents of Cr, Ni, and Mo are preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less.

[0090] There is no particular lower limit for the content of Cr, Ni, and Mo. However, Cr, Ni, and Mo are elements that improve hardenability. Therefore, the content of Cr, Ni, and Mo is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0091] ((V))

[0092] V is useful for precipitation strengthening of steel. However, if the V content is too high, the workability may be insufficient. Therefore, the V content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less.

[0093] On the other hand, in the case of containing V, in order to obtain the effect of adding V, the content of V is preferably 0.01% or more, more preferably 0.04% or more, and even more preferably 0.07% or more.

[0094] (Co)

[0095] Regarding Co, if the Co content is appropriate, coarse precipitates and inclusions will not increase, the ultimate deformation energy of the steel plate will not decrease, and therefore the bending performance will not decrease. Therefore, the Co content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.006% or less.

[0096] There is no particular lower limit for the Co content. However, Co is an element that improves hardenability. Therefore, the Co content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

[0097] (Cu)

[0098] Regarding Cu, if the Cu content is appropriate, coarse precipitates and inclusions will not increase, the ultimate deformation energy of the steel plate will not decrease, and therefore the bending performance will not decrease. Therefore, the Cu content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less.

[0099] There is no particular lower limit for the Cu content. However, Cu is an element that improves hardenability. Therefore, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0100] ((Sn))

[0101] Regarding Sn, if the Sn content is appropriate, no cracks will form inside the steel sheet during casting or hot rolling, and the ultimate deformation energy of the steel sheet will not decrease, thus the bending properties will not be reduced. Therefore, the Sn content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.

[0102] There is no particular lower limit for the Sn content. However, Sn is an element that improves hardenability. Therefore, the Sn content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0103] ((Sb))

[0104] Regarding Sb, if the Sb content is appropriate, coarse precipitates and inclusions will not increase, thus not reducing the ultimate deformation energy of the steel plate, and therefore the bending performance will not decrease. Therefore, the Sb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.

[0105] There is no particular limitation on the lower limit of Sb content. However, Sb is an element that controls the thickness of the softening layer present in the steel plate and can adjust the strength. Therefore, the Sb content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0106] (Ca, Mg, and REM)

[0107] Regarding Ca, Mg, and REM (rare earth metals), if their contents are appropriate, coarse precipitates and inclusions will not increase, the ultimate deformation energy of the steel plate will not decrease, and therefore the bending properties will not decrease. Therefore, the contents of Ca, Mg, and REM are preferably 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less, respectively.

[0108] There is no particular lower limit for the content of Ca, Mg, and REM. However, Ca, Mg, and REM are elements that shape nitrides and sulfides into spherical shapes, thereby increasing the ultimate deformation energy of steel plates. Therefore, the content of Ca, Mg, and REM is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0109] ((Zr and Te))

[0110] Regarding Zr and Te, if their contents are appropriate, coarse precipitates and inclusions will not increase, the ultimate deformation energy of the steel plate will not decrease, and therefore the bending properties will not decrease. Therefore, the contents of Zr and Te are preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less, respectively.

[0111] There is no particular lower limit for the content of Zr and Te. However, Zr and Te are elements that shape nitrides and sulfides into spherical shapes, thereby improving the ultimate deformation energy of steel plates. Therefore, the content of Zr and Te is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more, respectively.

[0112] (Hf)

[0113] Regarding Hf, if the Hf content is appropriate, coarse precipitates and inclusions will not increase, thus not reducing the ultimate deformation energy of the steel plate, and therefore the bending performance will not decrease. Therefore, the Hf content is preferably below 0.10%, more preferably below 0.08%, and even more preferably below 0.06%.

[0114] There is no particular lower limit for the Hf content. However, Hf is an element that shapes nitrides and sulfides into spherical forms, thereby increasing the ultimate deformation energy of steel plates. Therefore, the Hf content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0115] (Bi)

[0116] Regarding Bi, if the Bi content is appropriate, coarse precipitates and inclusions will not increase, the ultimate deformation energy of the steel plate will not decrease, and therefore the bending performance will not decrease. Therefore, the Bi content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.

[0117] There is no particular limitation on the lower limit of Bi content. However, Bi is an element that reduces segregation. Therefore, the Bi content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0118] If the content of any 2 of the other elements mentioned above is less than the preferred lower limit value, it is considered an unavoidable impurity since it does not impair the effect of the present invention.

[0119] (The rest of the text)

[0120] The remaining components consist of Fe and unavoidable impurities.

[0121] Examples of unavoidable impurities include Zn, Pb, and As. The total content of these unavoidable impurities is preferably less than 0.100%.

[0122] In this embodiment, the steel plate preferably contains only the aforementioned elements and the remaining portion as its composition, and the remaining portion is Fe (iron) and unavoidable impurities.

[0123] Micro-organization

[0124] Next, the microstructure (steel structure) of the steel plate (base steel plate) will be explained.

[0125] (1 / 4 of the plate thickness)

[0126] First, the microstructure at the 1 / 4 position of the steel plate thickness will be described.

[0127] (Martensite area ratio m1: exceeding 30% but below 70%)

[0128] The martensite area ratio is also referred to as the "M area ratio".

[0129] Martensite contributes to the high strength of steel sheets. From the viewpoint of obtaining a tensile strength of 980 MPa or more, the martensite area fraction (M area fraction m1) at the 1 / 4 position of the steel sheet thickness is more than 30%, preferably more than 32%, and more preferably more than 35%.

[0130] On the other hand, if there is too much martensite, the desired ductility cannot be obtained. Therefore, the area fraction m1 is 70% or less, preferably 65% ​​or less, and more preferably 60% or less.

[0131] (The remaining part of the organization)

[0132] In the 1 / 4 position of the steel plate thickness, the microstructure other than martensite (the remaining microstructure) can include, for example, ferrite, retained austenite, bainite, pearlite, and cementite.

[0133] In the section of the steel plate at 1 / 4 of its thickness, the area ratio of the remaining part is, for example, more than 30% and less than 70%, which can be 35-68% or 40-65%.

[0134] (From surface to a depth of 20 μm)

[0135] Next, the microstructure from the surface of the steel plate to a depth of 20 μm will be described.

[0136] (Martensite area ratio m2: exceeding 30% but below 70%)

[0137] From the viewpoint of obtaining a tensile strength of 980 MPa or more, the martensite area ratio (M area ratio m2) from the surface of the steel plate to a depth of 20 μm is more than 30%, preferably more than 32%, and more preferably more than 35%.

[0138] On the other hand, if there is too much martensite, the desired ductility cannot be obtained. Therefore, the area fraction m2 is 70% or less, preferably 65% ​​or less, and more preferably 60% or less.

[0139] (The remaining part of the organization)

[0140] Within a range from the surface of the steel plate to a depth of 20 μm, the microstructure other than martensite (remaining microstructure) can be exemplified by ferrite, retained austenite, bainite, pearlite, and cementite.

[0141] In the range from the surface of the steel plate to a depth of 20 μm, the area fraction of the remaining microstructure is, for example, more than 30% and less than 70%, which can be 35-68% or 40-65%.

[0142] (Original austenite grain size: below 3.0 μm)

[0143] Austenite is denoted as "γ".

[0144] Microcracks in the steel plate (base plate) propagate from the original γ grain boundaries. By finely dispersing the martensite present on the surface of the steel plate, the stress attached to the original γ grain boundaries is dispersed, thereby suppressing the formation of microcracks.

[0145] Therefore, based on the reason of excellent resistance to microcracks, the original γ grain size (hereinafter referred to as "original γ grain size") with a martensite area ratio of 50% or more is 3.0 μm or less, preferably 2.7 μm or less, and more preferably 2.0 μm or less.

[0146] There is no specific lower limit, for example, it is 0.5μm.

[0147] The martensite area ratio is calculated as follows.

[0148] First, test pieces are taken from alloyed hot-dip galvanized steel sheets, with the observation surface being a section parallel to the rolling direction and the thickness direction of the steel sheet (base steel sheet). The coating layer of the test piece is dissolved and removed using hydrochloric acid with added inhibitors. Next, the observation surface of the test piece is mirror-polished and then etched with a 3% (v / v) nitric acid ethanol solution to reveal the microstructure on the observation surface.

[0149] Then, scanning electron microscopy (SEM) was used to observe the desired area of ​​the test specimen at 3000x magnification. More specifically, five fields of view were observed at the 1 / 4 thickness position of the steel plate (including the 1 / 4 thickness position) and five fields of view from the surface of the steel plate to a depth of 20 μm, and SEM images were obtained for each.

[0150] The obtained SEM images were coated separately for each tissue, and the martensite area ratio (average of 5 fields of view) was calculated based on the number of pixels.

[0151] In SEM images, martensite appears as a white or light gray structure. It should be noted, for example, that ferrite is a gray or dark gray structure with smooth grain boundaries, distinguishing it from martensite. Martensite includes auto-tempered martensite containing carbides.

[0152] When determining the original γ grain size with a martensite area ratio of 50% or higher, in addition to the SEM method mentioned above, electron backscatter diffraction (EBSD) is also used. Specifically, EBSD is used to determine the original austenite grain boundaries. Among the original γ grains, those with a martensite area ratio of 50% or higher are selected, and for each selected original γ grain, the equivalent circle diameter is calculated based on its area. The average value of the calculated equivalent circle diameters (the average value of 5 fields of view) is taken as the original γ grain size.

[0153] (Nb+Ti in precipitates below 100nm: ≥50 ppm by mass)

[0154] By dispersing fine precipitates (Nb, Ti precipitates) on the surface of the steel plate, stress concentration at grain boundaries can be prevented and the formation of microcracks can be suppressed.

[0155] Therefore, based on the reason of excellent resistance to microcracks, the total content of Nb and Ti in the fine precipitates (Nb and Ti precipitates) below 100 nm is 50 ppm by mass or more, preferably 100 ppm by mass or more, and more preferably 150 ppm by mass or more.

[0156] There is no particular upper limit, such as 500 ppm by mass, preferably 400 ppm by mass, and more preferably 300 ppm by mass.

[0157] (Nb+Ti in precipitates exceeding 100nm: less than 350 ppm by mass)

[0158] On the other hand, if there are more coarse precipitates (Nb, Ti precipitates) on the surface of the steel plate, stress concentration will occur at the precipitate, which can easily become the starting point of microcracks.

[0159] Therefore, based on the reason of excellent resistance to microcracks, the content of Nb and Ti in coarse precipitates (Nb and Ti precipitates) exceeding 100 nm is preferably 350 ppm by mass or less, more preferably 310 ppm by mass or less, more preferably 250 ppm by mass or less, even more preferably 200 ppm by mass or less, and particularly preferably 150 ppm by mass or less.

[0160] There is no particular limitation on the lower limit, for example, it is 10 ppm by mass, preferably 30 ppm by mass, and more preferably 70 ppm by mass.

[0161] The contents of Nb and Ti in the precipitate were calculated as follows.

[0162] First, test pieces measuring 20mm × 50mm were taken from alloyed hot-dip galvanized steel sheets. The coating layer on the test pieces was dissolved and removed using hydrochloric acid with added inhibitors. Next, the test pieces were electrolyzed using a solution of 10% acetylacetone – 1% tetramethylammonium chloride – methanol as the electrolyte for extracting the precipitates. The electrolysis rate was determined based on the mass reduction of the test pieces. The electrolysis time was adjusted so that the electrolysis rate was 20 μm deep from the surface of the steel sheet. Using a 100 nm pore size filter, the residues (precipitates) in the electrolyte after electrolysis were separated into residues smaller than 100 nm and residues larger than 100 nm. After acid decomposition of each precipitate, the Nb and Ti contents (in ppm by mass) were determined using ICP (inductively coupled plasma) luminescence spectrophotometry. Five test pieces were taken from an alloyed hot-dip galvanized steel sheet, and electrolysis was performed on each piece; the average value was used.

[0163] (From surface to a depth of 1 μm)

[0164] Next, the microstructure from the surface of the steel plate to a depth of 1 μm will be described.

[0165] (The long side of Si and Mn oxides is below 200 nm.)

[0166] Si and Mn oxides mainly form near grain boundaries, increasing stress concentration at the grain boundaries. Therefore, if their size is too large, their resistance to microcracks deteriorates.

[0167] Therefore, based on the reason of excellent resistance to microcracks, the long side of the Si and Mn oxides (hereinafter referred to as "Si and Mn oxides") existing on the grain boundaries is 200 nm or less, preferably 180 nm or less, and more preferably 160 nm or less.

[0168] There is no specific limit to the lower limit, for example, it is 10nm, but 30nm is preferred.

[0169] The long sides of Si and Mn oxides are calculated as follows.

[0170] First, samples were taken from alloyed hot-dip galvanized steel sheets using a FIB (Focused Ion Beam). The cross-section of the samples (specifically, from the surface of the steel sheet to a depth of 1 μm) was observed using a transmission electron microscope (TEM) at magnification of 10,000–30,000x, and the dimensions of the long sides of Si and Mn oxides present at grain boundaries were measured. Si and Mn oxides present within 10 nm of the grain boundaries were also considered to be present at the grain boundaries.

[0171] Here, the grain boundary is the original austenite grain boundary, not any of the lath block boundary, lath bundle boundary, or lath boundary.

[0172] When measuring the long side of Si and Mn oxides, rectangles inscribed with Si and Mn oxides are drawn on the TEM image. Multiple rectangles are drawn for each Si and Mn oxide, and the rectangle with the largest aspect ratio is selected. The long side of this rectangle is then used as the long side of the Si or Mn oxide. The average value of the measurement results from five fields of view is used.

[0173] On TEM images, whether an object is a Si or Mn oxide is determined using an EDX (energy-dispersive X-ray diffraction) device attached to the TEM. Objects with a Si or Mn concentration more than twice that of the parent phase are treated as Si or Mn oxides.

[0174] It should be noted that the area ratio of each microstructure (martensite, etc.) in the range "from the surface of the steel plate to a depth of 1 μm" is the same as the area ratio of each microstructure in the range "from the surface of the steel plate to a depth of 20 μm" mentioned above.

[0175] <Alloyed hot-dip galvanized layer>

[0176] The high-strength alloyed hot-dip galvanized steel sheet of this embodiment has an alloyed hot-dip galvanized layer (coating) on ​​the surface of the steel sheet (base steel sheet). The coating may be disposed on only one side of the steel sheet, but preferably on both sides of the steel sheet.

[0177] The coating is formed by alloying hot-dip galvanizing, which will be described later.

[0178] For example, the coating adhesion amount is 20-80 g / m² per single side. 2 .

[0179] [Manufacturing method of alloyed hot-dip galvanized steel sheet]

[0180] Next, the method for manufacturing the high-strength alloyed hot-dip galvanized steel sheet of this embodiment will be described.

[0181] Unless otherwise specified, all temperatures mentioned below refer to the surface temperature of the slab or steel plate (hot-rolled steel plate, cold-rolled steel plate, etc.).

[0182] In this embodiment, briefly speaking, a slab having the above-described composition is used to perform hot rolling, cold rolling, annealing, and alloying hot-dip galvanizing treatments.

[0183] For hot rolling, cold rolling, annealing, and alloyed hot-dip galvanizing treatments, there are no special restrictions other than those listed below, and conditions that follow conventional methods may be appropriately adopted.

[0184] Hot Rolled

[0185] First, the slab with the above-mentioned composition is hot-rolled to obtain a hot-rolled steel sheet. The hot-rolling conditions are described below.

[0186] Slab heating temperature: above 1200℃

[0187] If the slab heating temperature is too low, the precipitates (Nb and Ti precipitates) generated during slab casting will not dissolve sufficiently, resulting in an increase in coarse precipitates. In this case, the Nb and Ti content in the fine precipitates decreases. Therefore, the slab heating temperature is 1200°C or higher, preferably 1210°C or higher, and more preferably 1220°C or higher.

[0188] There is no particular upper limit to the slab heating temperature, for example, 1350°C, preferably 1330°C, and more preferably 1310°C.

[0189] Final reduction rate: 5% or higher

[0190] By introducing a large amount of strain into the surface layer through rolling, the grains (original γ grains) can be refined. Therefore, the final reduction rate is 5% or more, preferably 6% or more, and more preferably 7% or more.

[0191] It should be noted that if the final reduction rate is too high, the rolling load will increase, so it is preferable to be below 30%, more preferably below 25%, and even more preferably below 20%.

[0192] The final reduction rate is calculated based on the roll gap of the rolling mill stand used for hot rolling. Specifically, the final reduction rate is the value calculated using the following formula based on the roll gap (R1) of the final rolling stand and the roll gap (R2) of the penultimate rolling stand.

[0193] (R2-R1) / R2×100

[0194] Rolling finish temperature: 850~970℃

[0195] If the rolling end temperature is too low, dynamic recrystallization will not occur, and the grains (original γ grains) cannot be refined. Therefore, the rolling end temperature is 850°C or higher, preferably 860°C or higher, and more preferably 870°C or higher.

[0196] On the other hand, if the rolling end temperature is too high, the grains (original γ grains) cannot be refined due to grain growth after rolling. Therefore, the rolling end temperature is 970°C or below, preferably 950°C or below, and more preferably 930°C or below.

[0197] Cooldown time: less than 6.0 seconds

[0198] When cooling from the final reduction (based on the final rolling stand) to below 700°C, fine precipitates (Nb, Ti precipitates) are formed on the surface. If the cooling time from the final reduction to below 700°C (also referred to as the "cooling time") is too long, the precipitates grow and become coarse. In addition, the amount of fine precipitates decreases, and the original γ-particle size becomes coarser.

[0199] Therefore, the cooling time is 6.0 s or less, preferably 5.5 s or less, and more preferably 5.0 s or less. There is no particular limitation on the lower limit of the cooling time; for example, it is 2.0 s, preferably 2.5 s.

[0200] Acid washing

[0201] Pickling is performed on hot-rolled steel sheets to remove oxide scale and Si and Mn oxides generated during hot rolling. There are no particular limitations on the pickling conditions; conventional methods can be used.

[0202] Cold rolling

[0203] Next, the pickled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The conditions for cold rolling are described below.

[0204] Compression reduction rate: 30%

[0205] By utilizing the strain introduced during rolling, the original γ-grains containing martensite are refined. To achieve this effect, the reduction rate of cold rolling is 30% or more, preferably 35% or more, and more preferably 40% or more.

[0206] There is no particular upper limit to the reduction rate of cold rolling, for example, it is 70%, preferably 65%, and more preferably 60%.

[0207] Pickling time: 2.0s or more

[0208] To remove Si and Mn oxides (reduce the long sides of Si and Mn oxides), the cold-rolled steel sheet obtained by cold rolling is pickled again. To achieve this effect, the pickling time is 2.0 s or more, preferably 2.4 s or more, and more preferably 2.8 s or more.

[0209] There is no particular upper limit to the pickling time, such as 6.0s, but 5.0s is preferred.

[0210] There are no special restrictions on conditions other than pickling time; pickling can be carried out according to conventional methods.

[0211] <annealing>

[0212] Next, the pickled cold-rolled steel sheet is annealed (heat treated).

[0213] The conditions for annealing are explained below.

[0214] Heating rate: 2.0~7.0℃ / s

[0215] By accelerating the heating rate from 500°C to 700°C (hereinafter referred to as "heating rate"), the original γ-grains containing martensite are refined. To achieve this effect, the heating rate is 2.0°C / s or more, preferably 2.5°C / s or more, and more preferably 3.0°C / s or more.

[0216] On the other hand, if the heating rate is too fast, the recrystallization of ferrite will be insufficient, resulting in excessive martensite and reduced ductility. Therefore, the heating rate is preferably 7.0°C / s or less, more preferably 6.5°C / s or less, and even more preferably 6.0°C / s or less.

[0217] The heating rate is the average heating rate.

[0218] Dew point: below -40℃

[0219] In order to suppress the formation of Si and Mn oxides and reduce their size (long side), the dew point (referred to as "dew point") of the atmosphere above 700°C is below -40°C, preferably below -41°C, and more preferably below -42°C.

[0220] On the other hand, if the dew point is lowered too much, the cost will increase. Therefore, a dew point of -60°C or higher is preferred.

[0221] There are no particular limitations on the method for adjusting the dew point inside the furnace. For example, one method is to remove moisture from the gas introduced into the furnace and the gas circulating inside the furnace using a filter.

[0222] Maximum temperature reached: 740–860℃

[0223] The martensite area ratio is controlled by the highest temperature reached.

[0224] In order to control the martensite area ratio within the above range, the maximum reaching temperature (soaking temperature) is 740°C or higher, preferably 760°C or higher, and more preferably 780°C or higher.

[0225] For the same reason, the maximum temperature reached is below 860°C, preferably below 840°C, and more preferably below 820°C.

[0226] Cooling rate v1 from 530°C to 480°C: less than 2.0°C / s

[0227] At temperatures ranging from 530°C to 480°C, a bainitic phase transformation occurs, and martensite decreases. In order to control the martensite area ratio within the aforementioned range, the cooling rate v1 (hereinafter referred to as "cooling rate v1") from 530°C to 480°C is 2.0°C / s or less, preferably 1.8°C / s or less, and more preferably 1.5°C / s or less.

[0228] The lower limit of the cooling rate v1 is not particularly limited, for example, it is 0.5℃ / s, preferably 0.8℃ / s.

[0229] Cooling rate v2 from 700°C to 600°C: below 5.0°C / s

[0230] The cooling rate v2 (referred to as "cooling rate v2") from 700°C to 600°C is, for example, less than 10.0°C / s.

[0231] At this point, the temperature is slowly cooled from 700°C to 600°C, resulting in ferrite grain growth and further refinement of the original γ grains containing martensite. To achieve this effect, the cooling rate v2 is preferably 5.0°C / s or less, more preferably 4.5°C / s or less, and even more preferably 4.0°C / s or less.

[0232] CO concentration: below 200 ppm (by volume)

[0233] The CO concentration (referred to as "CO concentration") in an atmosphere above 700°C is, for example, below 400 ppm by volume.

[0234] At this point, by reducing the CO concentration, the coarsening of precipitates (Nb, Ti precipitates) on the surface can be further suppressed. To achieve this effect, the CO concentration is 200 ppm by volume or less, preferably 180 ppm by volume or less, and more preferably 150 ppm by volume or less.

[0235] On the other hand, if the CO concentration is reduced too much, the cost will increase. Therefore, the CO concentration is preferably above 10 ppm by volume.

[0236] There are no particular limitations on the methods for adjusting the CO concentration inside the furnace. For example, methods such as removing the gas introduced into the furnace and the CO (carbon monoxide) and CO2 (carbon dioxide) circulating inside the furnace can be used.

[0237] <Alloying hot-dip galvanizing treatment>

[0238] Annealed cold-rolled steel sheets are subjected to alloyed hot-dip galvanizing treatment. This yields alloyed hot-dip galvanized steel sheets.

[0239] In alloyed hot-dip galvanizing, the first step is to perform hot-dip galvanizing.

[0240] In hot-dip galvanizing, for example, annealed cold-rolled steel sheets are immersed in a zinc bath (Zn bath). Then, the adhesion of the coating can be adjusted appropriately by methods such as gas wiping.

[0241] Examples of zinc baths include those with an Al content of 0.10 to 0.23% by mass, with the remainder consisting of Zn and unavoidable impurities.

[0242] The temperature of the zinc bath is, for example, 440–500℃.

[0243] Alloying temperature: above 480℃

[0244] After hot-dip galvanizing, alloying (specifically, for example, Zn-Fe alloying) is performed. Alloying can be carried out using conventional methods. At this time, the alloying temperature is 480°C or higher, preferably 485°C or higher, and more preferably 490°C or higher.

[0245] On the other hand, the alloying temperature is preferably below 600°C, more preferably below 550°C, and even more preferably below 530°C.

[0246] Example

[0247] The present invention will now be specifically described with reference to specific embodiments. However, the present invention is not limited to the embodiments described below.

[0248] Manufacturing of Alloyed Hot-Dip Galvanized Steel Sheets

[0249] Using slabs with the composition shown in Table 1 below (the remainder consisting of Fe and unavoidable impurities), hot rolling and cold rolling are performed under the conditions shown in Table 2 below to manufacture cold-rolled steel sheets with the sheet thickness and width shown in Table 3 below. Pickling is performed after both hot rolling and cold rolling.

[0250] Then, using a continuous hot-dip galvanizing equipment, the cold-rolled steel sheet is subjected to annealing and alloying hot-dip galvanizing treatment under the conditions shown in Table 2 below to manufacture alloyed hot-dip galvanized steel sheet (coated steel sheet).

[0251] In alloyed hot-dip galvanizing, a zinc bath (bath temperature: 470℃) is used, consisting of 0.14% Al by mass, with the remainder being Zn and unavoidable impurities, to achieve a coating adhesion of 45 g / m² per single side. 2 Adjustments will be made in this manner.

[0252] <Observations on Microstructures>

[0253] The following items were measured on the obtained coated steel sheet using the method described above. The results are shown in Table 3 below.

[0254] M area ratio m1: Martensite area ratio at 1 / 4 of the plate thickness.

[0255] M-area ratio m2: Martensite area ratio from the surface of the steel plate to a depth of 20 μm.

[0256] Original γ grain size: The original γ grain size with a martensite area ratio of 50% or more, ranging from the surface of the steel plate to a depth of 20 μm.

[0257] Nb+Ti in precipitates (below 100 nm): The total content of Nb and Ti in precipitates below 100 nm in the range from the surface of the steel plate to a depth of 20 μm.

[0258] Nb+Ti in precipitates (over 100 nm): The total content of Nb and Ti in precipitates exceeding 100 nm in the range from the surface of the steel plate to a depth of 20 μm.

[0259] Long sides of Si and Mn oxides: Long sides of Si and Mn oxides present at grain boundaries, extending from the surface of the steel plate to a depth of 1 μm.

[0260] <evaluate>

[0261] The obtained coated steel sheet was subjected to the following tests to evaluate various properties. The results are shown in Table 3 below.

[0262] Tensile Testing

[0263] From the obtained coated steel sheet, take test piece No. 5 as described in JIS Z 2241, with the long side direction (tension direction) perpendicular to the rolling direction (90° to the rolling direction). Using the taken test piece, perform 5 tensile tests according to JIS Z 2241, and determine the tensile strength (TS) and total elongation at fracture (E1) based on the average of the 5 tests.

[0264] If the strength (TS) is above 980 MPa, it can be evaluated as high strength.

[0265] If El is above 10.0%, it can be evaluated as having excellent extensibility.

[0266] Bending Test

[0267] A 30mm × 100mm test piece is taken from the center of the width of the galvanized steel sheet, with the end face being the ground surface. The 30mm edge of the test piece is parallel to the rolling direction (L direction) of the steel sheet, and the 100mm edge of the test piece is parallel to the width direction (C direction) of the steel sheet.

[0268] A 90-degree V-shaped bend test was performed on the test piece to form a bend. As shown in Table 3 below, the bend radius R to plate thickness t ratio (R / t) was in the range of 4.9 to 5.1.

[0269] Then, with the so-called C-section exposed, the test piece was embedded in resin and ground. Using SEM, the curved portion of the C-section (including the V-shaped bend apex) was observed at 3000x magnification, and the number of microcracks was determined. At this point, cracks connected to the coating and extending into the steel plate to a depth d (refer to...) were considered. Figure 1 Cracks larger than 1 μm are considered microcracks, and their number is determined.

[0270] The number of microcracks per unit length along the surface of the steel plate is used as the microcrack quantity (unit: cracks / mm). Five test pieces are taken for each coated steel plate, and bending tests are performed on each piece. The average value is used.

[0271] If the number of microcracks is less than 25 / mm, use "A"; if it is more than 25 / mm but less than 50 / mm, use "B"; if it is more than 50 / mm but less than 75 / mm, use "C"; if it is more than 75 / mm, use "D". These values ​​are recorded in the "Microcrack Resistance" column of Table 3 below.

[0272] If it is "A" or "B", it can be evaluated as having excellent resistance to microcracks.

[0273] Corrosion Resistance Test After Bending

[0274] First, as described above, a 90-degree V-shaped bending test was performed (where the size of the test piece was 70 mm (L direction) × 100 mm (C direction)).

[0275] Using the test pieces after the bending test, a 2-day salt spray test was conducted according to JIS Z 2371 (2000). Then, a 1-minute cleaning was performed using chromic acid (concentration: 200 g / L, temperature: 80 °C) to remove corrosion products generated due to corrosion.

[0276] The reduction in coating thickness per day was determined using a gravimetric method (unit: g / (m²)). 2 The reduction in coating thickness is less than 15 g / (m²). 2 In the case of (days), "A", 15g / (m 2 More than 25g / (m²) 2 In the case of (day), "B", 25g / (m 2 In cases where the number of days exceeds 100 days, "C" will be recorded in the "Corrosion Resistance" column of Table 3 below.

[0277] If it is "A" or "B", it can be evaluated as having excellent corrosion resistance.

[0278]

[0279]

[0280]

[0281]

[0282]

[0283]

[0284]

[0285]

[0286] <Summary of Evaluation Results>

[0287] As shown in Table 3 above, the TS of the galvanized steel sheets (invention examples) No.1~3, 5, 8~10, 13, 18~21, 24, 26~27, 30~31, 33~36, 38~40, 43 and 45~62 are all above 590MPa, and have excellent ductility and resistance to microcracks, as well as excellent corrosion resistance.

[0288] In contrast, the galvanized steel sheets (comparative examples) of No. 4, 6-7, 11-12, 14-17, 22-23, 25, 28-29, 32, 37, 41-42 and 44 are inadequate in at least one of TS, ductility and microcrack resistance.

[0289] Symbol Explanation

[0290] 1: Steel plate

[0291] 2: Alloyed hot-dip galvanized layer

[0292] 3: Microcracks

[0293] d: Depth of microcracks

Claims

1. A high-strength alloyed hot-dip galvanized steel sheet, comprising a steel sheet and an alloyed hot-dip galvanized layer disposed on the surface of said steel sheet. The steel plate is composed of the following components: the carbon equivalent Ceq shown in formula (1) is 0.520 or more and less than 0.700, the total content of Nb and Ti is 0.010 to 0.080% by mass, and the remainder is Fe and unavoidable impurities. In the steel plate at a position of 1 / 4 of its thickness, the martensite area ratio exceeds 30% but is less than 70%. Within a range from the surface of the steel plate to a depth of 20 μm, the martensite area fraction exceeds 30% but is less than 70%, the original austenite grain size is less than 3.0 μm for precipitates with a martensite area fraction exceeding 50%, and the combined Nb and Ti content in precipitates less than 100 nm is more than 50 ppm by mass, while the combined Nb and Ti content in precipitates greater than 100 nm is less than 350 ppm by mass. Within a range from the surface of the steel plate to a depth of 1 μm, the long sides of the Si and Mn oxides present on the grain boundaries are less than 200 nm. Ceq = [C%] + ([Si%] / 24) + ([Mn%] / 6) + ([Ni%] / 40) + ([Cr%] / 5) + ([Mo%] / 4) + ([V%] / 14) (1) In the formula (1), [M%] is the content of element M in the composition of the component, expressed as mass%, and is 0 when element M is not present.

2. The high-strength alloyed hot-dip galvanized steel sheet according to claim 1, wherein, The composition of the ingredients, expressed in % by mass, further contains... C：0.070～0.170%、 Si: below 0.30% Mn: 1.70~3.50%, P: Below 0.100% S: Below 0.0200% Al: Below 0.100% N: below 0.0100%, and O: Below 0.0100%.

3. The high-strength alloyed hot-dip galvanized steel sheet according to claim 2, wherein, The composition of the ingredients, expressed in % by mass, further contains ingredients selected from... B: Below 0.0050% Ta: Below 0.10% W: Below 0.10% Cr: less than 1.00% Ni: below 1.00% Mo: 1.00% or less, V: Below 1.00% Co: less than 0.010% Cu: below 1.00% Sn: below 0.200% Sb: below 0.200% Ca: below 0.0100% Mg: below 0.0100% REM: below 0.0100% Zr: below 0.100% Te: below 0.100% Hf: below 0.10%, and Bi: At least one element in the range of 0.200% or less.

4. A method for manufacturing a high-strength alloyed hot-dip galvanized steel sheet, comprising the method for manufacturing the high-strength alloyed hot-dip galvanized steel sheet as described in any one of claims 1 to 3. A slab having the composition described in any one of claims 1 to 3 is hot-rolled under the following conditions: a slab heating temperature of 1200°C or higher, a final reduction rate of 5% or higher, a rolling end temperature of 850–970°C, and a cooling time of 6.0 s or less from the final reduction to below 700°C. A hot-rolled steel sheet is then subjected to pickling. The hot-rolled steel sheet after pickling is cold-rolled under a reduction rate of 30% or more to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is pickled for more than 2.0 seconds. The pickled cold-rolled steel sheet is annealed under the following conditions: a heating rate of 2.0–7.0 °C / s from 500 °C to 700 °C, a dew point of -40 °C or less in the atmosphere above 700 °C, a maximum reaching temperature of 740–860 °C, and a cooling rate v1 of 2.0 °C / s or less from 530 °C to 480 °C. The annealed cold-rolled steel sheet is subjected to an alloying hot-dip galvanizing treatment, which includes alloying at a temperature above 480°C.

5. The method for manufacturing high-strength alloyed hot-dip galvanized steel sheet according to claim 4, wherein, In the annealing process, the cooling rate v2 from 700°C to 600°C is less than 5.0°C / s.

6. The method for manufacturing high-strength alloyed hot-dip galvanized steel sheet according to claim 4 or 5, wherein, In the annealing process, the CO concentration in the atmosphere above 700°C is less than 200 ppm by volume.

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