JP7747244B2 - High-strength galvannealed steel sheet and method for manufacturing the same - Google Patents

Description

The present invention relates to a high-strength galvannealed steel sheet and a method for manufacturing the same.

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 having a tensile strength (TS) of 980 MPa or more have been developed as steel sheets to be used for the framework of automobile cabins (Patent Documents 1 and 2).

JP 2012-219342 A JP 2013-117042 A

High-strength steel plates with a tensile strength of 980 MPa or more may experience poor bendability due to a decrease in local ductility, and cracks may occur when subjected to bending deformation.

The inventors formed a bent portion by applying bending deformation to a galvannealed steel sheet (hereinafter simply referred to as "galvanized steel sheet") at a bending radius R greater than the bending radius R at which cracks occur in a bending test conforming to JIS. The bent portion of the galvanized steel sheet was then observed using a scanning electron microscope (SEM).

FIG. 1 is an SEM image showing a cross section of a galvannealed steel sheet.
As shown in FIG. 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 occur in the steel sheet 1.

When microcracks occur in the 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") at 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 the targets of all of these are conventional cracks, not microcracks.

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."

The present invention has been made in view of the above points, and an object of the present invention is to provide a galvannealed steel sheet having a tensile strength of 980 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.

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 component composition in which a carbon equivalent Ceq represented by the following 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 mass%, with the balance being Fe and unavoidable impurities; the martensite area ratio is more than 30% and 70% or less at a quarter-thickness position of the steel plate; and the martensite area ratio is more than 30% and 70% or less 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 more than 30% and not more than 70%, a prior austenite grain size of 3.0 μm or less and a martensite area ratio of 50% or more, a total content of Nb and Ti in precipitates of 100 nm or less of 50 ppm by mass or more, and a total content of Nb and Ti 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 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 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: 0.070 to 0.170%, 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 the component composition according to any one of [1] to [3] above 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 A method for producing a high-strength galvannealed steel sheet, comprising: pickling a steel sheet; cold-rolling the hot-rolled steel sheet after the pickling at a rolling reduction of 30% or more to obtain a cold-rolled steel sheet; pickling the cold-rolled steel sheet for 2.0 s or more; annealing the cold-rolled steel sheet after the pickling under conditions of a heating rate of 2.0 to 7.0°C/s from 500°C to 700°C, a dew point of an atmosphere at 700°C or higher being −40°C or lower, a maximum temperature reached being 740 to 860°C, and a cooling rate _v1 of 2.0°C/s from 530°C to 480°C; and then subjecting the annealed cold-rolled steel sheet 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], 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 the above [4] or [5], wherein, in the annealing, the CO concentration in an atmosphere at 700°C or higher is 200 volume ppm or less.

The present invention provides a galvannealed steel sheet having a tensile strength of 980 MPa or more and excellent ductility and microcrack resistance.

1 is an SEM image showing a cross section of a galvannealed steel sheet.

[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 980 MPa or more.

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 component composition and microstructure described below. Therefore, it is suitable for use in automobile components (e.g., 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 an automobile where corrosion due to rain and snow is a concern, such as automobile suspension parts.

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.

<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.

《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.

(Carbon equivalent Ceq: 0.520 or more and less than 0.700)
The carbon equivalent Ceq is 0.520 or more, preferably 0.540 or more, and more preferably 0.560 or more, because a tensile strength of 980 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 0.700, preferably 0.698 or less, more preferably 0.690 or less, and even more preferably 0.670 or less.

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.

(Nb+Ti: 0.010-0.080%)
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 coarsening of precipitates in the surface layer of the steel sheet, 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.

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

((C: 0.070-0.170%))
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 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.
On the other hand, if the C content is too high, the ductility decreases, so the C content is preferably 0.170% or less, more preferably 0.160% or less, and even more preferably 0.150% or less.

((Si: 0.30% or less))
If Si is added in excess, it forms Si oxides in the surface layer of the steel sheet, which reduces the microcrack resistance. Therefore, 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).

((Mn: 1.70-3.50%))
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 in 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.

((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, the lower limit of the P content is not particularly limited. 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.

((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.

((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, and therefore the Al content is preferably 0.100% or less, more preferably 0.090% or less, and even more preferably 0.080% or less.

((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.

((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 will decrease. 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.

(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.

((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.

((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.
There are no particular lower limits for the contents of Ta and W. 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.

((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 a decrease in 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.

((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.

((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.
The lower limit of the Co content is not particularly limited. 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.

((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.

((Sn))
If the Sn content is appropriate, it does not cause cracks to form inside the steel sheet during casting or hot rolling, and does not reduce the ultimate deformability of the steel sheet, so that the bendability does not decrease. 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.

((Sb))
If the Sb 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 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 control the thickness of the softened layer present in the steel sheet and 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.

((Ca, Mg and REM))
When the contents of Ca, Mg, and REM (rare earth metals) are appropriate, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so that the bendability is not reduced. Therefore, the contents of Ca, Mg, and REM 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 shapes 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.

((Zr and Te))
Appropriate amounts of Zr and Te do not increase coarse precipitates or inclusions, do not reduce the ultimate deformability of the steel sheet, and do not reduce 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 contents of Zr and Te are not particularly limited. However, Zr and Te are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Zr and Te are each preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

((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.

((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.

With regard to the second other element mentioned above, if its content is less than the preferred lower limit value mentioned above, it is considered an unavoidable impurity since it does not impair the effects of the present invention.

(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.

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

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

((Martensite area ratio m ₁ : more than 30% and less than 70%))
The martensite area ratio is also referred to as "M area ratio."
Martensite contributes to increasing the strength of the steel sheet. From the viewpoint of obtaining a tensile strength of 980 MPa or more, the martensite area fraction (M area fraction m ₁ ) at the 1/4 position in the sheet thickness direction of the steel sheet is more than 30%, preferably 32% or more, and more preferably 35% or more.
On the other hand, if the martensite content is too high, the desired ductility cannot be obtained. Therefore, the M area fraction _m1 is 70% or less, preferably 65% or less, and more preferably 60% or less.

((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, 30% or more and less than 70%, and may be 35 to 68%, or may be 40 to 65%.

(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.

((Martensite area ratio m ₂ : more than 30% and not more than 70%))
From the viewpoint of obtaining a tensile strength of 980 MPa or more, the martensite area fraction (M area fraction m ₂ ) in the range from the surface of the steel plate to a depth of 20 μm is more than 30%, preferably 32% or more, and more preferably 35% or more.
On the other hand, if the martensite content is too high, the desired ductility cannot be obtained. Therefore, the M area fraction _m2 is 70% or less, preferably 65% or less, and more preferably 60% or less.

((residual tissue))
In the range from the surface of the steel plate to a depth of 20 μm, examples of structures other than martensite (residual 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, 30% or more and less than 70%, and may be 35 to 68%, or 40 to 65%.

((Prior austenite grain size: 3.0 μm or less))
Austenite is represented as "γ".
Microcracks that occur in steel sheets (base steel sheets) originate from 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 obtain excellent microcrack resistance, the prior γ grain size having 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.

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 1/4 position 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 image is 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.

When determining the prior γ grain size when the martensite area fraction is 50% or more, in addition to the method using the SEM described above, the electron backscatter diffraction (EBSD) method is also used. That is, the prior austenite grain boundaries are identified using 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.

((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 ppm by mass or more, preferably 100 ppm by mass or more, and more preferably 150 ppm by mass 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.

((Nb + Ti in precipitates larger than 100 nm: 350 mass ppm or less))
On the other hand, if the surface layer of the steel sheet contains a large number of coarse precipitates (Nb and Ti precipitates), 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 (Nb, Ti precipitates) exceeding 100 nm is 350 ppm by mass or less, preferably 310 ppm by mass or less, more preferably 250 ppm by mass or less, still more preferably 200 ppm by mass or less, and particularly preferably 150 ppm by mass 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.

The contents of Nb and Ti in the precipitates are determined as follows.
First, a test piece measuring 20 mm x 50 mm was taken from the galvannealed steel sheet. The plating 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 in the depth direction from the surface of the steel sheet. The residue (precipitates) contained in the electrolytic solution after electrolysis was separated into those with a size of 100 nm or less and those with a size 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.

(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.

((Long side of Si, Mn oxide: 200 nm or less))
Since Si and Mn oxides are mainly formed in the vicinity of grain boundaries and increase stress concentration at the grain boundaries, if their size is too large, microcrack resistance will be deteriorated.
For this reason, in order to obtain excellent microcrack resistance, the long side of the Si, Mn oxides (simply referred to as "Si, Mn oxides") present on the grain boundaries 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.

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 size of the long sides of the Si and Mn oxides present on the grain boundaries is measured. Si and Mn oxides present within 10 nm of the grain boundaries are also considered to be Si and 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 the Si-Mn oxides, a rectangle inscribed with the Si-Mn oxides 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 determined 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 oxide of Si or Mn is determined using an EDX (energy dispersive X-ray) analyzer attached to the TEM. Observed objects with a Si or Mn concentration at least twice that of the matrix are treated as Si or Mn oxides.

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.

<Galvannealed layer>
The high-strength galvannealed steel sheet of this embodiment has a galvannealed layer (plating layer) on the surface of a steel sheet (base steel sheet). The plating 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 ² per side.

[Method of manufacturing 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.

<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.

<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.

《Final rolling reduction rate: 5% or more》
The 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 from the roll gap (R1) of the final rolling stand and the roll gap (R2) of the rolling stand before the final one by the following formula:
(R2-R1)/R2×100

《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 less, preferably 950°C or less, and more preferably 930°C or less.

《Cooling time: 6.0s or less》
During cooling from the final reduction (reduction by the final rolling stand) to 700° C. or less, fine precipitates (Nb and Ti precipitates) are precipitated in the surface layer. If the cooling time from the final reduction to 700° C. or less (simply referred to as the "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.

《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.

<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.

《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%.

《Pickling time: 2.0s or more》
The cold-rolled steel sheet obtained by cold rolling is also subjected to pickling again for the purpose of removing Si and Mn oxides (reducing 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.

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

<<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 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.

《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.

《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-mentioned 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.

<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. In order to control the martensite area ratio within the above-mentioned range, the cooling rate v ₁ from 530° C. to 480° C. (simply referred to as "cooling rate v ₁ ") 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 preferably 0.8°C/s.

<Cooling rate _v2 from 700°C to 600°C: 5.0°C/s or less>
The cooling rate v ₂ from 700° C. to 600° C. (simply referred to as "cooling rate v ₂ ") 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 obtain 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.
In this case, by reducing the CO concentration, coarsening of precipitates (Nb and Ti precipitates) in the surface layer can be further suppressed. 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 reduced 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 CO ₂ (carbon dioxide) from the gas introduced into the furnace and circulating within the furnace using a filter or the like.

<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 coating 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 balance being Zn and unavoidable impurities.
The temperature of the zinc bath is, for example, 440 to 500°C.

《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 less, more preferably 550°C or less, and even more preferably 530°C or less.

The present invention will be specifically explained below using examples. However, the present invention is not limited to the examples described below.

<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 plating treatment, a zinc bath (bath temperature: 470°C) containing 0.14 mass% Al, with the remainder being Zn and unavoidable impurities, was used, and the coating weight of the plating layer was adjusted to 45 g/ ^m2 per side.

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 ratio m ₁ : martensite area ratio at 1/4 position of the steel plate thickness M area ratio m ₂ : 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 in the range from the surface of the steel plate to a depth of 20 μm Nb + Ti in precipitates (100 nm or less): total content of Nb and Ti in precipitates of 100 nm or less in the range from the surface of the steel plate to a depth of 20 μm Nb + Ti in precipitates (more than 100 nm): total content of Nb and Ti in precipitates of more than 100 nm in the range from the surface of the steel plate to a depth of 20 μm Long side of Si, Mn oxides: long side of Si, Mn oxides present on grain boundaries in the range from the surface of the steel plate to a depth of 1 μm

<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.

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, a tensile test in accordance with JIS Z 2241 was carried out five times, and the tensile strength (TS) and total elongation at break (El) were determined from the average values of the five tests.
If TS is 980 MPa or more, it can be evaluated as having high strength.
If El is 10.0% or more, it can be evaluated as having excellent ductility.

<Bending test>
A test piece measuring 30 mm × 100 mm, the end faces of which were ground, was taken from the widthwise center of the plated steel sheet. 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 test pieces were subjected to a 90-degree V-bend test, and a bent portion was formed in each test piece. 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 a magnification of 3,000 times 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 determined as the amount of microcracks (unit: number/mm). Five test pieces were taken from each plated steel sheet, and bending tests were performed on each, and the average value was used.
In the "Microcrack Resistance" column of Table 3 below, the following was recorded: "A" when the amount of microcracks was 25 cracks/mm or less; "B" when it was more than 25 cracks/mm and 50 cracks/mm or less; "C" when it was more than 50 cracks/mm and 75 cracks/mm or less; and "D" when it was more than 75 cracks/mm.
If the rating is "A" or "B", it can be evaluated as having excellent microcrack resistance.

<Post-bending corrosion resistance test>
First, a 90-degree V-bend 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)).
Using the test specimens after the bending test, a salt spray test was carried out for two days in accordance with JIS Z 2371 (2000). Thereafter, the specimens were washed for one minute using chromic acid (concentration: 200 g/L, temperature: 80°C) to remove corrosion products formed by corrosion.
The amount of loss of the plating layer per day (unit: g/( ^m2 ·day)) was determined using a gravimetric method. In the "Corrosion Resistance" column of Table 3 below, the following was recorded: "A ^" when the amount of loss of the plating layer was less than 15 g/( ^m2 ·day), "B" when it was 15 g/( ^m2 ·day) or more but less than 25 g/( ^m2 ·day), and "C" when it was 25 g/(m2·day) or more.
If it is rated as "A" or "B", it can be evaluated as having excellent corrosion resistance.

<Summary of evaluation results>
As shown in Table 3 above, all of the plated steel sheets (invention examples) of 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 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 to 7, 11 to 12, 14 to 17, 22 to 23, 25, 28 to 29, 32, 37, 41 to 42 and 44 (comparative examples) were insufficient in at least one of TS, ductility and microcrack resistance.

1: Steel plate 2: Galvannealed layer 3: Microcrack d: Depth of microcrack

Claims (6)

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 0.520 or more and less than 0.700, a total content of Nb and Ti is 0.010 to 0.080 mass%, and further contains, in mass%, C: 0.070 to 0.170%, 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 more than 30% and 70% or less,
In a range from the surface of the steel plate to a depth of 20 μm, a martensite area ratio is more than 30% and not more than 70%, 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. 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. 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;
Annealing is performed on the cold-rolled steel sheet after the pickling under the conditions that the heating rate from 500 ° C. to 700 ° C. is 2.0 to 7.0 ° C./s, the dew point of the atmosphere at 700 ° C. or higher is −40 ° C. or less, the maximum temperature reached is 740 to 860 ° C., and the cooling rate _v1 from 530 ° C. to 480 ° C. 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. The method for producing a high-strength galvannealed steel sheet according to claim 3 , wherein in the annealing, a cooling rate _v2 from 700°C to 600°C is 5.0°C/s or less. 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. 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.