High-strength galvannealed steel sheet and method for manufacturing the same

Abstract

Provided is an alloyed hot-dip galvanized steel sheet comprising a steel sheet and an alloyed hot-dip galvanized layer and having a tensile strength of 590 MPa or more and excellent ductility and microcrack resistance. In the component composition of the steel sheet, the carbon equivalent Ceq is 0.370 to less than 0.520, and the total Nb and Ti content is 0.010-0.080 mass%. At a position at 1 / 4 of the thickness of the steel sheet, the martensite area ratio is 5-30%. In a region from the surface of the steel sheet to a depth of 20 μm, the martensite area ratio is 5-30%, the prior austenite grain size when the martensite area ratio is 50% or more is 3.0 μm or less, the total Nb and Ti content in precipitate having a size of 100 nm or less is 50 mass ppm or more, and the total Nb and Ti content in precipitate having a size greater than 100 nm is 350 mass ppm or less. In a region from the surface of the steel sheet to a depth of 1 μm, the sizes of the long sides of Si and Mn oxides present on the crystal grain boundary are 200 nm or less.

Description

[Technical Field]

[0001] The present invention relates to a high-strength galvannealed steel sheet and a method for producing the same. [Background technology]

[0002] From the viewpoint of protecting the global environment, in order to improve the fuel efficiency of automobiles, the weight of automobile bodies may be reduced by increasing the strength and thinning of steel sheets used as automobile components. Furthermore, from the viewpoint of the rust prevention performance of the vehicle body, the steel sheets used as automobile members are sometimes subjected to a zinc-based plating treatment. For example, galvannealed steel sheets with a tensile strength (TS) of 590 MPa or more have been developed as steel sheets to be used for the framework of automobile cabins (Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-219342 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-117042 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-37574 Summary of the Invention [Problem to be solved by the invention]

[0004] High-strength steel plates with a tensile strength of 590 MPa or more may have poor bendability due to a decrease in local ductility, which can lead to cracks occurring when subjected to bending deformation.

[0005] The present inventors formed a bent portion by applying bending deformation to a galvannealed steel sheet (hereinafter also simply referred to as "galvanized steel sheet") at a bending radius R larger than the bending radius R at which cracks occur in a bending test according to JIS. Thereafter, the bent portion of the galvannealed steel sheet was observed using a scanning electron microscope (SEM).

[0006] FIG. 1 is an SEM image showing a cross section of a galvannealed steel sheet. As shown in Figure 1, it was found that in the bent portion of the galvannealed steel sheet, fine microcracks 3 that cannot be observed without using an SEM originate from cracks in the galvannealed layer 2 and develop in the steel sheet 1.

[0007] When microcracks occur in a steel sheet (base steel sheet), the corrosion resistance of the bent portion of the plated steel sheet deteriorates, and furthermore, the impact absorption characteristics and fatigue characteristics may also deteriorate. To prevent the occurrence of microcracks, it is necessary to stop cracks that occur in the galvannealed layer (hereinafter simply referred to as the "coated layer") with the steel sheet. The mechanism by which these cracks occur is significantly different from that of conventional cracks that occur on the steel sheet side (cracks large enough to be observed with a magnifying glass). The above-mentioned Patent Documents 1 to 3 disclose techniques for improving bendability (suppressing the occurrence of cracks), but all of them are aimed at conventional cracks, not microcracks.

[0008] Hereinafter, the bending property related to microcracks will also be referred to as "microcrack resistance." The criteria for determining whether a material has excellent microcrack resistance will be described later (see the "Examples" section). For convenience, "microcrack resistance" may be referred to as "bending property."

[0009] The present invention has been made in view of the above points, and has an object to provide a galvannealed steel sheet having a tensile strength of 590 MPa or more and excellent ductility and microcrack resistance. A further object of the present invention is to provide a method for producing the above-mentioned galvannealed steel sheet. [Means for solving the problem]

[0010] As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention. That is, the present invention provides the following [1] to [6]. [1] A steel sheet comprising: a steel plate; and a galvannealed layer disposed on a surface of the steel plate, wherein the steel plate has a chemical composition in which a carbon equivalent Ceq represented by the following formula (1) is 0.370 or more and less than 0.520; the total content of Nb and Ti is 0.010 to 0.080 mass%, with the balance being Fe and unavoidable impurities; the steel plate has a martensite area ratio of 5 to 30% at a quarter-thickness position of the steel plate; and the martensite area ratio is 5 to 30% within a range from the surface of the steel plate to a depth of 20 μm. A high-strength galvannealed steel sheet having a martensite area ratio of 5 to 30%, a prior austenite grain size of 3.0 μm or less with a martensite area ratio of 50% or more, a total Nb and Ti content in precipitates of 100 nm or less of 50 ppm by mass or more, and a total Nb and Ti content in precipitates of more than 100 nm of 350 ppm by mass or less, and wherein the long sides of Si and Mn oxides present on grain boundaries within a range of 1 μm deep from the surface of the steel sheet are 200 nm or less. Ceq=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14)...(1) In the above formula (1), [M %] is the content of element M in the above composition in unit mass %, and is 0 when element M is not contained. [2] The high-strength galvannealed steel sheet according to [1] above, wherein the chemical composition further contains, in mass%, C: 0.050 to 0.150%, Si: 0.30% or less, Mn: 1.70 to 3.50%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less. [3] The high-strength galvannealed steel sheet according to [2] above, wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of B: 0.0050% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, V: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less. [4] A method for producing a high-strength galvannealed steel sheet according to any one of [1] to [3] above, comprising: hot rolling a slab having a chemical composition according to any one of [1] to [3] above under conditions of a slab heating temperature of 1200°C or higher, a final reduction of 5% or higher, a rolling completion temperature of 850 to 970°C, and a cooling time from the final reduction to 700°C or lower of 6.0 seconds or shorter to obtain a hot-rolled steel sheet; thereafter, pickling the hot-rolled steel sheet; and cold rolling the hot-rolled steel sheet after pickling under conditions of a reduction of 30% or higher. a cold-rolled steel sheet obtained by subjecting the cold-rolled steel sheet to pickling for 2.0 seconds or more; annealing the cold-rolled steel sheet after the pickling under conditions in which a heating rate from 500°C to 700°C is 2.0 to 7.0°C / s, a dew point of an atmosphere at 700°C or higher is −40°C or lower, a maximum temperature is 740 to 860°C, and a cooling rate v1 from 530°C to 480°C is 2.0°C / s or lower; and then subjecting the cold-rolled steel sheet after the annealing to a galvannealed coating treatment including alloying at a temperature of 480°C or higher. [5] The method for producing a high-strength galvannealed steel sheet according to [4] above, wherein in the annealing, the cooling rate v2 from 700°C to 600°C is 5.0°C / s or less. [6] The method for producing a high-strength galvannealed steel sheet according to [4] or [5] above, wherein, in the annealing, the CO concentration in the atmosphere at 700°C or higher is 200 ppm by volume or less. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a galvannealed steel sheet having a tensile strength of 590 MPa or more and excellent ductility and microcrack resistance. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an SEM image showing a cross section of a galvannealed steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0013] [Galvannealed steel sheet] The high-strength galvannealed steel sheet of this embodiment generally includes a steel sheet and a galvannealed layer disposed on the surface of the steel sheet. High strength means that the tensile strength (TS) is 590 MPa or more.

[0014] The high-strength galvannealed steel sheet of this embodiment has excellent ductility and microcrack resistance, and also excellent corrosion resistance, due to the steel sheet having the chemical composition and microstructure described below. Therefore, it is suitable for use in automobile components (for example, the framework of an automobile cabin). In this case, it can contribute to improving safety performance and reducing the weight of the automobile body, thereby improving automobile fuel efficiency and reducing CO2 emissions, thereby contributing to the environment. It can also be actively applied to parts of automobiles where corrosion due to rain and snow is a concern, such as suspension parts.

[0015] The high-strength galvannealed steel sheet of this embodiment can be applied not only to automobiles but also to fields such as civil engineering, construction, and home appliances.

[0016] <Steel plate> First, the steel sheet (base steel sheet) included in the high-strength galvannealed steel sheet of this embodiment will be described. The thickness of the steel plate is not particularly limited, and is, for example, 0.5 mm or more and 3.0 mm or less.

[0017] 《Component composition》 First, the chemical composition of the steel sheet (base steel sheet) will be described. The unit "%" in the composition of a component means "% by mass" unless otherwise specified.

[0018] (Carbon equivalent Ceq: 0.370 or more but less than 0.520) The carbon equivalent Ceq is 0.370 or more, preferably 0.390 or more, and more preferably 0.410 or more, for the reason that a tensile strength of 590 MPa or more can be obtained. On the other hand, if the carbon equivalent Ceq is too high, the ductility decreases. In order to obtain good ductility, the carbon equivalent Ceq is less than 0.520, preferably 0.518 or less, more preferably 0.510 or less, and even more preferably 0.490 or less.

[0019] The carbon equivalent Ceq is expressed by the following formula (1). Ceq=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14)...(1) In the above formula (1), [M %] is the content (unit: mass %) of element M in the chemical composition of the steel sheet, and is 0 (zero) when element M is not contained.

[0020] (Nb+Ti: 0.010 to 0.080%) Nb and Ti are dispersed in the surface layer of the steel sheet as carbides and / or nitrides, thereby suppressing stress concentration and the occurrence of microcracks. To achieve this effect, the total 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. On the other hand, excessive addition of Nb and Ti causes the precipitates in the surface layer of the steel sheet to become coarse, promoting the occurrence of microcracks. Therefore, the total content of Nb and Ti is 0.080% or less, preferably 0.060% or less, and more preferably 0.040% or less.

[0021] (Other elements 1) The composition of the steel sheet may further contain the elements described below.

[0022] ((C: 0.050~0.150%)) C is an element that is effective in increasing the strength of steel, and in particular contributes to increasing the strength by forming martensite, which is one of the hard phases in the steel structure. From the viewpoint of obtaining a desired high strength, specifically a tensile strength of 590 MPa or more, the C content is preferably 0.050% or more, more preferably 0.055% or more, and even more preferably 0.060% or more. On the other hand, if the C content is too high, the ductility decreases, so the C content is preferably 0.150% or less, more preferably 0.140% or less, and even more preferably 0.130% or less.

[0023] ((Si:0.30% or less)) If Si is added in excess, Si oxides are formed in the surface layer of the steel sheet, which reduces the microcrack resistance, so the Si content is preferably 0.30% or less, more preferably 0.25% or less, and even more preferably 0.20% or less. The lower limit of the Si content is not particularly limited and is, for example, 0.01%, but may be 0 (zero).

[0024] ((Mn: 1.70 to 3.50%)) Mn is an element that contributes to increasing the strength of steel by solid solution strengthening and martensite formation. To obtain these effects, the Mn content is preferably 1.70% or more, more preferably 1.80% or more, and even more preferably 2.00% or more. On the other hand, if the Mn content is too high, Mn oxides are formed on the surface layer of the steel sheet, which reduces the microcrack resistance. Therefore, the Mn content is preferably 3.50% or less, more preferably 3.20% or less, and even more preferably 3.00% or less.

[0025] ((P:0.100% or less)) Since P segregates at prior austenite grain boundaries and embrittles the grain boundaries, reducing the ultimate deformability of the steel sheet, too much P content reduces bendability, so the P content is preferably 0.100% or less, more preferably 0.070% or less, and even more preferably 0.040% or less. On the other hand, there is no particular lower limit for the P content. However, P is a solid solution strengthening element and can increase the strength of the steel sheet. From this viewpoint, the P content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0026] ((S:0.0200% or less)) S exists as sulfide and reduces the ultimate deformability of the steel sheet, so if the S content is too high, the bendability will decrease. Therefore, the S content is preferably 0.0200% or less, more preferably 0.0120% or less, and even more preferably 0.0050% or less. On the other hand, there is no particular lower limit for the S content, although, due to constraints on production technology, the S content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0027] ((Al: 0.100% or less)) Al is added as a deoxidizer, and from the viewpoint of obtaining 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. However, if the Al content is too high, it may cause surface defects in the coating layer, so the Al content is preferably 0.100% or less, more preferably 0.090% or less, and even more preferably 0.080% or less.

[0028] ((N:0.0100% or less)) Since N exists as nitride and reduces the ultimate deformability of the steel sheet, if the N content is too high, the bendability will decrease. Therefore, the N content is preferably 0.0100% or less, more preferably 0.0070% or less, and even more preferably 0.0050% or less. On the other hand, there is no particular lower limit for the N content, although, due to constraints on production technology, the N content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0029] ((O:0.0100% or less)) O exists as an oxide and reduces the ultimate deformability of the steel sheet, so if the O content is too high, the bendability decreases. Therefore, the O content is preferably 0.0100% or less, more preferably 0.0070% or less, and even more preferably 0.0050% or less. On the other hand, there is no particular lower limit for the O content, although, due to constraints on production technology, the O content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0030] (Other elements 2) The composition of the steel sheet may further contain at least one element selected from the group consisting of the elements described below.

[0031] ((B)) B is contained in Si and Mn oxides formed on the surface of the steel sheet and improves the wettability of these oxides with molten zinc, thereby improving the appearance of the coating layer. 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. On the other hand, if the B content is too high, coarse B compounds are formed, which may result in a decrease in bendability. Therefore, the B content is preferably 0.0050% or less, more preferably 0.0045% or less, and even more preferably 0.0040% or less.

[0032] ((Ta and W)) Appropriate amounts of Ta and W do not produce large amounts of coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, thereby preventing a decrease in bendability. Therefore, the Ta and W contents are preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less, respectively. The lower limits of the contents of Ta and W are not particularly limited. However, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the contents of Ta and W are each preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0033] ((Cr, Ni and Mo)) Appropriate amounts of Cr, Ni, and Mo do not increase coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, thereby preventing deterioration of bendability. Therefore, the contents of Cr, Ni, and Mo are each preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less. There are no particular lower limits for the contents of Cr, Ni, and Mo. However, Cr, Ni, and Mo are elements that improve hardenability. Therefore, the contents of Cr, Ni, and Mo are each preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.

[0034] ((V)) V is useful for precipitation strengthening of steel. However, if the V content is too high, workability may become 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. On the other hand, when V is contained, in order to obtain the effect of adding V, the V content is preferably 0.01% or more, more preferably 0.04% or more, and even more preferably 0.07% or more.

[0035] ((Co)) If the Co content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that the bendability will not be reduced. Therefore, the Co content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.006% or less. 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.

[0036] ((Cu)) If the Cu content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that the bendability will not be reduced. Therefore, the Cu content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less. The lower limit of the Cu content is not particularly limited. 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.

[0037] ((Sn)) An appropriate Sn content prevents cracks from forming inside the steel sheet during casting or hot rolling, and does not reduce the ultimate deformability of the steel sheet, thereby preventing a decrease in bendability. Therefore, the Sn content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. The lower limit of the Sn content is not particularly limited. 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.

[0038] ((Sb)) An appropriate Sb content does not increase coarse precipitates or inclusions, and does not reduce the ultimate deformability of the steel sheet, thereby preventing a decrease in bendability. Therefore, the Sb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. The lower limit of the Sb content is not particularly limited. However, Sb is an element that can adjust the strength by controlling the thickness of the softened layer present in the steel sheet. Therefore, the Sb content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0039] ((Ca, Mg and REM)) Appropriate amounts of Ca, Mg, and REM (rare earth metals) do not increase coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, thereby preventing deterioration of bendability. Therefore, the Ca, Mg, and REM contents are each preferably 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less. The lower limits of the contents of Ca, Mg, and REM are not particularly limited. However, Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Ca, Mg, and REM are each preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0040] ((Zr and Te)) Appropriate amounts of Zr and Te do not increase coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, thereby preventing deterioration of bendability. Therefore, the Zr and Te contents are preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less, respectively. The lower limits of the Zr and Te contents are not particularly limited. However, Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the Zr and Te contents are each preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

[0041] ((Hf)) If the Hf content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that the bendability will not be reduced. Therefore, the Hf content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less. The lower limit of the Hf content is not particularly limited. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0042] ((Bi)) If the Bi content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so the bendability will not be reduced. Therefore, the Bi content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. The lower limit of the Bi content is not particularly limited. However, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0043] Regarding the second other element described above, if the content is less than the preferred lower limit value described above, it does not impair the effects of the present invention and is therefore considered to be an unavoidable impurity.

[0044] (Remainder) The balance of the composition is composed of Fe and unavoidable impurities. Examples of the unavoidable impurities include Zn, Pb, and As. The total content of the unavoidable impurities is preferably 0.100% or less. In this embodiment, the steel sheet preferably contains, as a chemical composition, only the above-mentioned elements and the balance, with the balance being Fe (iron) and unavoidable impurities.

[0045] Microstructure Next, the microstructure (steel structure) of the steel sheet (base steel sheet) will be described.

[0046] (plate thickness 1 / 4 position) First, the microstructure at the 1 / 4 position of the steel plate thickness will be described.

[0047] ((Martensite area fraction m1: 5-30%)) The martensite area fraction is also referred to as the "M area fraction." Martensite contributes to increasing the strength of steel sheets. From the viewpoint of obtaining a tensile strength of 590 MPa or more, the martensite area fraction (M area fraction m1) at the 1 / 4 position in the sheet thickness direction of the steel sheet is 5% or more, preferably 7% or more, and more preferably 11% or more. On the other hand, if the martensite content is too high, the desired ductility cannot be obtained, and therefore the M area fraction m1 is 30% or less, preferably 28% or less, and more preferably 26% or less.

[0048] ((residual tissue)) At the 1 / 4 position in the plate thickness of the steel plate, examples of the structure (remaining structure) other than martensite include ferrite, retained austenite, bainite, pearlite, and cementite. At the 1 / 4 position in the plate thickness of the steel plate, the area ratio of the remaining structure is, for example, 70 to 95%, may be 72 to 93%, or may be 74 to 89%.

[0049] (Range from the surface to a depth of 20 μm) Next, the microstructure in the range from the surface of the steel sheet to a depth of 20 μm will be described.

[0050] ((Martensite area ratio m2: 5-30%)) In order to obtain a tensile strength of 590 MPa or more, the martensite area fraction (M area fraction m2) in the range from the surface of the steel plate to a depth of 20 μm is 5% or more, preferably 7% or more, and more preferably 11% or more. On the other hand, if the martensite content is too high, the desired ductility cannot be obtained, and therefore the M area fraction m2 is 30% or less, preferably 28% or less, and more preferably 26% or less.

[0051] ((residual tissue)) In the range from the surface of the steel plate to a depth of 20 μm, examples of structures other than martensite (remaining structures) include ferrite, retained austenite, bainite, pearlite, and cementite. In the range from the surface of the steel plate to a depth of 20 μm, the area ratio of the remaining structure is, for example, 70 to 95%, or alternatively 72 to 93%, or 74 to 89%.

[0052] ((Prior austenite grain size: 3.0 μm or less)) Austenite is represented as "γ". Microcracks that occur in steel sheets (base steel sheets) start at prior-γ grain boundaries and propagate. Microcrack occurrence can be suppressed by finely dispersing martensite in the surface layer of the steel sheet and dispersing the stress applied to the prior-γ grain boundaries. For this reason, in order to achieve excellent microcrack resistance, the prior γ grain size with a martensite area ratio of 50% or more (simply referred to as "prior γ grain size") is 3.0 μm or less, preferably 2.7 μm or less, and more preferably 2.0 μm or less. The lower limit is not particularly limited, and is, for example, 0.5 μm.

[0053] The martensite area ratio is determined as follows. First, a test piece is taken from a galvannealed steel sheet so that the cross section parallel to the rolling direction of the steel sheet (base steel sheet) and parallel to the sheet thickness direction serves as the observation surface. The coating layer of the test piece is dissolved and removed using hydrochloric acid containing an inhibitor. Next, the observation surface of the test piece is mirror-polished and then etched with 3% by volume of nital to reveal the structure on the observation surface. Thereafter, a desired area on the observation surface of the test piece is observed using a scanning electron microscope (SEM) at a magnification of 3,000. More specifically, five fields of view are observed at a position at 1 / 4 of the plate thickness of the steel plate (a range including the position at 1 / 4 of the plate thickness) and a range at a depth of 20 μm from the surface of the steel plate, and SEM images are obtained for each. The obtained SEM images are colored according to the structure, and the martensite area ratio (average value of five fields of view) is calculated from the number of pixels. In SEM images, martensite appears as a white or light gray structure. Ferrite, for example, appears as a gray or dark gray structure with smooth grain boundaries, and is distinguishable from martensite. Martensite includes autotempered martensite, which contains carbides.

[0054] When determining the prior γ grain size with a martensite area fraction of 50% or more, in addition to the above-mentioned method using SEM, the electron backscatter diffraction (EBSD) method is also used. That is, the prior austenite grain boundaries are identified by the EBSD method. Among the prior γ grains, those with a martensite area fraction of 50% or more are selected, and the circle-equivalent diameter of each selected prior γ grain is determined from its area. The average of the determined circle-equivalent diameters (average of five fields of view) is taken as the prior γ grain size.

[0055] ((Nb+Ti in precipitates of 100 nm or less: 50 mass ppm or more)) By dispersing fine precipitates (Nb and Ti precipitates) in the surface layer of the steel sheet, stress concentration at the grain boundaries can be prevented, and the occurrence of microcracks can be suppressed. For this reason, in order to obtain excellent microcrack resistance, the total content of Nb and Ti in fine precipitates (Nb, Ti precipitates) of 100 nm or less is 50 mass ppm or more, preferably 100 mass ppm or more, and more preferably 150 mass ppm or more. The upper limit is not particularly limited, and is, for example, 500 ppm by mass, preferably 400 ppm by mass, and more preferably 300 ppm by mass.

[0056] ((Nb+Ti in precipitates larger than 100 nm: 350 mass ppm or less)) On the other hand, if there are many coarse precipitates (Nb and Ti precipitates) in the surface layer of the steel sheet, stress will be concentrated on these precipitates, which will easily become the starting points for microcracks. For this reason, in order to obtain excellent microcrack resistance, the total content of Nb and Ti in coarse precipitates exceeding 100 nm (Nb, Ti precipitates) is 350 mass ppm or less, preferably 310 mass ppm or less, more preferably 250 mass ppm or less, even more preferably 200 mass ppm or less, and particularly preferably 150 mass ppm or less. The lower limit is not particularly limited, and is, for example, 10 ppm by mass, preferably 30 ppm by mass, and more preferably 70 ppm by mass.

[0057] The contents of Nb and Ti in the precipitates are determined as follows. First, a 20 mm x 50 mm test piece was taken from the galvannealed steel sheet. The coating layer of the test piece was dissolved and removed using hydrochloric acid with an inhibitor added. Next, the test piece was electrolyzed using 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol as an electrolyte for deposit extraction. The amount of electrolysis was determined from the mass loss of the test piece. The electrolysis time was adjusted so that the amount of electrolysis was 20 μm from the surface of the steel sheet in the depth direction. The residue (precipitates) contained in the electrolytic solution after electrolysis was separated into those with a diameter of 100 nm or less and those with a diameter of more than 100 nm using a filter with a pore size of 100 nm. Each precipitate was subjected to acid decomposition and then to ICP (inductively coupled plasma) atomic emission spectroscopy to determine the Nb and Ti contents (unit: ppm by mass). Five test pieces were taken from one galvannealed steel sheet, and electrolysis was performed on each, and the average value was used.

[0058] (Range from the surface to a depth of 1 μm) Next, the microstructure in the range from the surface of the steel sheet to a depth of 1 μm will be described.

[0059] ((Long side of Si, Mn oxide: 200 nm or less)) Since Si and Mn oxides are mainly formed near grain boundaries and increase stress concentration at the grain boundaries, if their size is too large, microcrack resistance will deteriorate. For this reason, in order to obtain excellent microcrack resistance, the long side of the Si, Mn oxides present on the grain boundaries (simply referred to as "Si, Mn oxides") is 200 nm or less, preferably 180 nm or less, and more preferably 160 nm or less. The lower limit is not particularly limited, and is, for example, 10 nm, preferably 30 nm.

[0060] The long side of the Si and Mn oxides is determined as follows. First, a sample is taken from a galvannealed steel sheet using a focused ion beam (FIB). The cross section of the sample (specifically, the area from the surface of the steel sheet to a depth of 1 μm) is observed at 10,000 to 30,000 magnifications using a transmission electron microscope (TEM), and the length of the long side of the Si, Mn oxides present on the grain boundaries is measured. Si, Mn oxides present within 10 nm of the grain boundaries are also considered to be Si, Mn oxides present on the grain boundaries. Here, the grain boundary is a prior austenite grain boundary, and is not a block boundary, packet boundary, or lath boundary. When measuring the long side of Si, Mn oxides, a rectangle inscribed in the Si, Mn oxide is drawn on the TEM image. Multiple rectangles are drawn for each Si, Mn oxide, and the rectangle with the largest aspect ratio is selected. The long side of that rectangle is then taken as the long side of the Si, Mn oxide. The average value of the measurement results for five fields of view is used. Whether or not an observed object in a TEM image is an Si, Mn oxide is determined using an EDX (energy dispersive X-ray) analyzer attached to the TEM. Observed objects with Si or Mn concentrations twice or more higher than that of the parent phase are treated as Si, Mn oxides.

[0061] The area ratio of each structure (such as martensite) 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 structure in the "range from the surface of the steel plate to a depth of 20 μm" described above.

[0062] <Galvannealed layer> The high-strength galvannealed steel sheet of this embodiment has a galvannealed layer (plated layer) on the surface of a steel sheet (base steel sheet). The plated layer may be disposed on only one side of the steel sheet, but is preferably disposed on both sides of the steel sheet. The plating layer is formed by a galvannealing treatment described later. The coating weight of the plating layer is, for example, 20 to 80 g / m per side. 2 is.

[0063] [Manufacturing method of galvannealed steel sheet] Next, a method for producing the high-strength galvannealed steel sheet of this embodiment will be described. Hereinafter, unless otherwise specified, each temperature refers to the surface temperature of a slab or steel plate (hot-rolled steel plate, cold-rolled steel plate, etc.). In this embodiment, generally, a slab having the above-described component composition is subjected to hot rolling, cold rolling, annealing, and galvannealing treatment. The conditions for the hot rolling, cold rolling, annealing and galvannealing treatment are not particularly limited except for those described below, and conventional conditions can be appropriately adopted.

[0064] <Hot rolling> First, a slab having the above-described composition is hot-rolled to obtain a hot-rolled steel sheet. The hot-rolling conditions are described below.

[0065] Slab heating temperature: 1200°C or higher If the slab heating temperature is too low, the precipitates (Nb and Ti precipitates) formed during the casting of the slab will not dissolve sufficiently, resulting in an increase in coarse precipitates. In this case, the Nb and Ti contents in the fine precipitates will decrease. Therefore, the slab heating temperature should be 1200°C or higher, preferably 1210°C or higher, and more preferably 1220°C or higher. The upper limit of the slab heating temperature is not particularly limited, and is, for example, 1350°C, preferably 1330°C, and more preferably 1310°C.

[0066] 《Final rolling reduction rate: 5% or more》 Rolling introduces a large amount of strain into the surface layer, which can refine the crystal grains (prior γ grains). Therefore, the final rolling reduction is 5% or more, preferably 6% or more, and more preferably 7% or more. If the final rolling reduction is too high, the rolling load increases, so it is preferably 30% or less, more preferably 25% or less, and even more preferably 20% or less. The final reduction is calculated from the roll gap of the rolling stand used in hot rolling. Specifically, the final reduction is a value calculated using the roll gap (R1) of the final rolling stand and the roll gap (R2) of the rolling stand before the final one, using the following formula: (R2-R1) / R2×100

[0067] 《Rolling completion temperature: 850~970℃》 If the rolling completion temperature is too low, dynamic recrystallization does not occur and the crystal grains (prior γ grains) cannot be refined. Therefore, the rolling completion temperature is 850°C or higher, preferably 860°C or higher, and more preferably 870°C or higher. On the other hand, if the rolling completion temperature is too high, the crystal grains (prior γ grains) cannot be refined by grain growth after rolling. Therefore, the rolling completion temperature is 970°C or lower, preferably 950°C or lower, and more preferably 930°C or lower.

[0068] 《Cooling time: 6.0s or less》 During cooling from the final reduction (reduction by the final rolling stand) to below 700°C, fine precipitates (Nb and Ti precipitates) are precipitated in the surface layer. If the cooling time from the final reduction to below 700°C (simply referred to as "cooling time") is too long, the precipitates grow and become coarse, and the amount of fine precipitates decreases, causing the prior γ grain size to become coarse. Therefore, the cooling time is 6.0 seconds or less, preferably 5.5 seconds or less, and more preferably 5.0 seconds or less. The lower limit of the cooling time is not particularly limited, and is, for example, 2.0 seconds, and preferably 2.5 seconds.

[0069] 《Pickling》 The hot-rolled steel sheet obtained by hot rolling is subjected to pickling in order to remove scale and Si and Mn oxides formed by the hot rolling. The conditions for pickling are not particularly limited, and the pickling may be carried out according to a conventional method.

[0070] <Cold rolling> Next, the pickled hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The cold rolling conditions are described below.

[0071] 《Reduction rate: 30%》 The strain introduced by rolling refines prior γ grains containing martensite. To obtain this effect, the reduction ratio of cold rolling is 30% or more, preferably 35% or more, and more preferably 40% or more. The upper limit of the reduction rate in cold rolling is not particularly limited, and is, for example, 70%, preferably 65%, and more preferably 60%.

[0072] 《Pickling time: 2.0s or more》 The cold-rolled steel sheet obtained by cold rolling is also subjected to pickling again in order to remove Si and Mn oxides (to reduce the long sides of the Si and Mn oxides). To obtain this effect, the pickling time is 2.0 seconds or more, preferably 2.4 seconds or more, and more preferably 2.8 seconds or more. The upper limit of the pickling time is not particularly limited, and is, for example, 6.0 seconds, preferably 5.0 seconds. The conditions other than the pickling time are not particularly limited, and pickling may be carried out according to a conventional method.

[0073] Annealing Next, the pickled cold-rolled steel sheet is subjected to annealing (heat treatment). The annealing conditions will be explained below.

[0074] Heating rate: 2.0 to 7.0°C / s By increasing the heating rate from 500°C to 700°C (simply referred to as "heating rate"), prior 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. On the other hand, if the heating rate is too fast, the recrystallization of ferrite becomes insufficient, resulting in an excessive amount of martensite, which reduces ductility. Therefore, the heating rate is set to 7.0°C / s or less, preferably 6.5°C / s or less, and more preferably 6.0°C / s or less. The temperature increase rate is an average temperature increase rate.

[0075] 《Dew point: -40℃ or less》 In order to suppress the generation of Si and Mn oxides and reduce their size (long side), the dew point of an atmosphere at 700°C or higher (simply referred to as "dew point") is -40°C or lower, preferably -41°C or lower, and more preferably -42°C or lower. On the other hand, if the dew point is lowered too much, the cost increases, so the dew point is preferably -60°C or higher. The method for adjusting the dew point inside the furnace is not particularly limited, and examples thereof include a method in which moisture is removed from the gas introduced into the furnace and the gas circulating inside the furnace using a filter or the like.

[0076] 《Maximum temperature: 740~860℃》 The area ratio of martensite is controlled by the maximum temperature reached. In order to control the martensite area ratio within the above range, the maximum temperature (soaking temperature) is 740°C or higher, preferably 760°C or higher, and more preferably 780°C or higher. For the same reason, the maximum temperature to be reached is 860°C or less, preferably 840°C or less, and more preferably 820°C or less.

[0077] <Cooling rate v1 from 530°C to 480°C: 2.0°C / s or less> Bainite transformation occurs and martensite decreases at temperatures between 530°C and 480°C. To control the martensite area ratio within the above-mentioned range, the cooling rate v1 from 530°C to 480°C (simply referred to as "cooling rate v1") is 2.0°C / s or less, preferably 1.8°C / s or less, and more preferably 1.5°C / s or less. The lower limit of the cooling rate v1 is not particularly limited, and is, for example, 0.5°C / s, and 0.8°C / s is preferable.

[0078] <Cooling rate v2 from 700°C to 600°C: 5.0°C / s or less> The cooling rate v2 from 700°C to 600°C (simply referred to as "cooling rate v2") is, for example, 10.0°C / s or less. At this time, slow cooling from 700°C to 600°C causes ferrite grain growth, further refining prior 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. <CO concentration: 200 ppm by volume or less> The CO concentration in the atmosphere at 700° C. or higher (simply referred to as "CO concentration") is, for example, 400 ppm by volume or less. At this time, coarsening of precipitates (Nb and Ti precipitates) in the surface layer can be further suppressed by reducing the CO concentration. To achieve this effect, the CO concentration is 200 volume ppm or less, preferably 180 volume ppm or less, and more preferably 150 volume ppm or less. On the other hand, if the CO concentration is lowered too much, the cost increases, so the CO concentration is preferably 10 ppm by volume or more. The method for adjusting the CO concentration in the furnace is not particularly limited, and examples thereof include a method of removing CO (carbon monoxide) and CO2 (carbon dioxide) circulating in the gas fed into the furnace and in the furnace using a filter or the like.

[0079] <Zinc alloyed hot-dip plating treatment> The annealed cold-rolled steel sheet is subjected to a galvannealing treatment, thereby obtaining a galvannealed steel sheet. In the galvannealing treatment, first, a hot-dip galvanizing treatment is carried out. In the hot dip galvanizing treatment, for example, the annealed cold rolled steel sheet is immersed in a zinc bath (Zn bath), and then the coating weight of the plating layer may be adjusted as appropriate by gas wiping or the like. The zinc bath may be, for example, a zinc bath having a component composition in which the Al content is 0.10 to 0.23 mass %, with the remainder being Zn and unavoidable impurities. The temperature of the zinc bath is, for example, 440 to 500°C.

[0080] 《Alloying temperature: 480℃ or higher》 After the hot dip galvanizing treatment, alloying (specifically, for example, Zn-Fe alloying) is carried out. The alloying may be carried out according to a conventional method. In this case, the alloying temperature is 480°C or higher, preferably 485°C or higher, and more preferably 490°C or higher. On the other hand, the alloying temperature is preferably 600°C or lower, more preferably 550°C or lower, and even more preferably 530°C or lower. [Example]

[0081] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0082] <Manufacturing of galvannealed steel sheets> Using a slab having the chemical composition shown in Table 1 below (the balance consisting of Fe and unavoidable impurities), hot rolling and cold rolling were carried out under the conditions shown in Table 2 below to produce cold-rolled steel sheets having the thickness and width shown in Table 3 below. After the hot rolling and cold rolling, pickling was carried out. Thereafter, the cold-rolled steel sheets were subjected to annealing and galvannealing treatment using continuous hot-dip galvanizing equipment under the conditions shown in Table 2 below, to produce galvannealed steel sheets (plated steel sheets). In the galvannealed hot-dip coating process, a zinc bath (bath temperature: 470°C) containing 0.14 mass% Al, with the remainder consisting of Zn and unavoidable impurities, was used, and the coating weight of the coating layer was 45 g / m per side.2 It was adjusted so that

[0083] Observation of microstructure The obtained plated steel sheets were measured for the following items by the above-mentioned methods, and the results are shown in Table 3 below. M area fraction m1: Martensite area fraction at 1 / 4 of the steel plate thickness M area ratio m2: Martensite area ratio in the range from the surface of the steel plate to a depth of 20 μm Prior γ grain size: Prior γ grain size with a martensite area ratio of 50% or more within a range of 20 μm from the surface of the steel plate. Nb+Ti in precipitates (100 nm or less): The total content of Nb and Ti in precipitates of 100 nm or less in the area from the surface of the steel sheet to a depth of 20 μm Nb+Ti in precipitates (over 100 nm): The total content of Nb and Ti in precipitates over 100 nm in size within a range from the surface of the steel sheet to a depth of 20 μm Long side of Si, Mn oxides: Long side of Si, Mn oxides present on grain boundaries within a depth of 1 μm from the surface of the steel sheet

[0084] <evaluation> The obtained plated steel sheets were subjected to the tests described below to evaluate various properties, and the results are shown in Table 3 below.

[0085] Tensile test No. 5 test pieces according to JIS Z 2241 were taken from the obtained plated steel sheets, with the longitudinal direction (tensile direction) at an angle of 90° to the rolling direction (perpendicular to the rolling direction). Using the taken test pieces, tensile tests in accordance with JIS Z 2241 were carried out five times, and the tensile strength (TS) and total elongation at break (El) were calculated from the average values ​​of the five tests. If the TS is 590 MPa or more, it can be evaluated as having high strength. If El is 14.0% or more, it can be evaluated as having excellent ductility.

[0086] <Bending test> A 30 mm × 100 mm test piece was taken from the width center of the plated steel sheet, with the edge being a ground surface. The 30 mm side of the test piece was parallel to the rolling direction (L direction) of the steel sheet, and the 100 mm side of the test piece was parallel to the width direction (C direction) of the steel sheet. The specimens were subjected to a 90-degree V-bend test to form a bent portion in each specimen. As shown in Table 3 below, the bending test conditions were such that the ratio of the bending radius R to the plate thickness t (R / t) was in the range of 4.9 to 5.1. The test specimen was then embedded in resin with the so-called C-section exposed and polished, and the bent portion of the C-section of the test specimen (including the apex of the V-bend) was observed using an SEM at 3,000x magnification to count the number of microcracks. At this time, cracks that were connected to cracks in the plating layer and penetrated into the steel sheet and had a depth d (see Figure 1) of more than 1 μm were defined as microcracks, and the number of such cracks was counted. The number of microcracks per unit length along the surface of the steel sheet was calculated as the amount of microcracks (unit: pieces / mm). Five test pieces were taken from each plated steel sheet, and bending tests were carried out on each, and the average value was used. In the "Microcrack Resistance" column of Table 3 below, the following was recorded: "A" if the amount of microcracks was 25 cracks / mm or less; "B" if it was more than 25 cracks / mm and 50 cracks / mm or less; "C" if it was more than 50 cracks / mm and 75 cracks / mm or less; and "D" if it was more than 75 cracks / mm. If it is rated as "A" or "B", it can be evaluated as having excellent microcrack resistance.

[0087] <Post-bending corrosion resistance test> First, a 90-degree V-bending test was carried out in the same manner as above (however, the size of the test piece was 70 mm (L direction) × 100 mm (C direction)). After the bending test, the test pieces were subjected to a salt spray test for two days in accordance with JIS Z 2371 (2000). After that, the test pieces were washed for one minute with chromic acid (concentration: 200 g / L, temperature: 80°C) to remove the corrosion products formed by corrosion. The amount of plating layer reduced per day (unit: g / (m2 The reduction in the plating layer was determined using a gravimetric method. 2 ·days), "A" is given, and 2 ·day) or more 25g / (m 2 ·days), "B" is given, and 2 If the test result was 10 days or more, the test result was recorded as "C" in the "Corrosion Resistance" column in Table 3 below. If it is rated as "A" or "B", it can be evaluated as having excellent corrosion resistance.

[0088] [Table 1] TIFF0007747245000002.tif20295

[0089] [Table 2] TIFF0007747245000004.tif151150TIFF0007747245000005.tif162150

[0090] [Table 3] TIFF0007747245000007.tif176150TIFF0007747245000008.tif164150

[0091] <Summary of evaluation results> As shown in Table 3 above, the plated steel sheets Nos. 1 to 3, 5, 8 to 10, 13, 18 to 21, 24, 26 to 27, 30 to 31, 33 to 36, 38 to 40, 43 and 45 to 62 (invention examples) all had a TS of 590 MPa or more, and were excellent in ductility and microcrack resistance, as well as in corrosion resistance. In contrast, the plated steel sheets Nos. 4, 6-7, 11-12, 14-17, 22-23, 25, 28-29, 32, 37, 41-42 and 44 (comparative examples) were insufficient in at least one of TS, ductility and microcrack resistance. [Explanation of symbols]

[0092] 1: Steel plate 2: Galvannealed layer 3: Microcracks d: Microcrack depth

Claims

1. A steel plate and a galvannealed layer disposed on a surface of the steel plate, The steel sheet has a chemical composition in which a carbon equivalent Ceq represented by the following formula (1) is 0.370 or more and less than 0.520, a total content of Nb and Ti is 0.010 to 0.080 mass%, and further contains, in mass%, C: 0.050 to 0.150%, Si: 0.30% or less, Mn: 1.70 to 3.50%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; At a quarter-thickness position of the steel plate, the martensite area ratio is 5 to 30%, In a range from the surface of the steel plate to a depth of 20 μm, a martensite area ratio is 5 to 30%, a prior austenite grain size having a martensite area ratio of 50% or more is 3.0 μm or less, the total content of Nb and Ti in precipitates of 100 nm or less is 50 ppm by mass or more, and the total content of Nb and Ti in precipitates of more than 100 nm is 350 ppm by mass or less, A high-strength galvannealed steel sheet, wherein the long sides of Si and Mn oxides present on grain boundaries within a range from the surface of the steel sheet to a depth of 1 μm are 200 nm or less. 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 in unit mass %, and is 0 when element M is not contained.

2. The component composition further includes, in mass %, B: 0.0050% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, V: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and The high-strength galvannealed steel sheet according to claim 1, further comprising at least one element selected from the group consisting of Bi: 0.200% or less.

3. A method for producing the high-strength galvannealed steel sheet according to claim 1 or 2, A slab having the component composition according to claim 1 or 2 is hot-rolled under the conditions of a slab heating temperature of 1200 ° C. or higher, a final reduction rate of 5% or higher, a rolling completion temperature of 850 to 970 ° C., and a cooling time from the final reduction to 700 ° C. or lower of 6.0 seconds or less to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is pickled; The hot-rolled steel sheet after the pickling is subjected to cold rolling under the condition of a rolling reduction rate of 30% or more to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is subjected to pickling for 2.0 s or more; The cold-rolled steel sheet after the pickling is heated at a rate of 2.0 to 7.0 ° C. / s from 500 ° C. to 700 ° C., the dew point of the atmosphere at 700 ° C. or higher is −40 ° C. or lower, the maximum temperature is 740 to 860 ° C., and the cooling rate v from 530 ° C. to 480 ° C. １ Annealing is performed under the condition that the rate of change is 2.0 ° C. / s or less, A method for producing a high-strength galvannealed steel sheet, comprising: subjecting the annealed cold-rolled steel sheet to a galvannealed hot-dip coating treatment including alloying at a temperature of 480°C or higher.

4. In the annealing, the cooling rate v from 700 ° C to 600 ° C ２ The method for producing a high-strength galvannealed steel sheet according to claim 3, wherein the melting point is 5.0°C / s or less.

5. The method for producing a high-strength galvannealed steel sheet according to claim 3, wherein, in the annealing, a CO concentration in an atmosphere at 700°C or higher is 200 ppm by volume or less.

6. The method for producing a high-strength galvannealed steel sheet according to claim 4, wherein, in the annealing, a CO concentration in an atmosphere at 700°C or higher is 200 ppm by volume or less.

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