JP7111279B1 - Hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Abstract

To provide a hot-dip galvanized steel sheet having both high strength and good workability and excellent plating quality. The chemical composition of the base steel plate is set within a predetermined range, the steel structure of the base steel plate is a composite structure of ferrite, martensite and bainite, and the amount of oxygen present as oxides in the surface layer of the base steel plate is 0.05 g/m2 or more per side. The Fe content in the hot-dip galvanized layer is set to 0.40% by mass or more.

Description

TECHNICAL FIELD The present invention relates to a hot-dip galvanized steel sheet suitable for automobile members and the like, and a method for producing the same.

2. Description of the Related Art In recent years, from the viewpoint of protecting the global environment, improving the fuel efficiency of automobiles has become an important issue. For this reason, there has been a growing movement to reduce the weight of automobile bodies by increasing the strength and reducing the thickness of steel sheets that are used as materials for automobile members. In addition, since steel sheets used for automobile members are formed into complicated shapes, good workability is also required.

In response to such a request, for example, in Patent Document 1,
"In % by mass,
C: 0.01% or more and 0.4% or less Si: 0.001% or more and 2.5% or less,
Mn: 0.001% or more and 4.0% or less,
P: 0.001% or more and 0.15% or less,
S: 0.0005% or more and 0.03% or less,
Al: 0.001% or more and 2.0% or less,
N: 0.0005% or more and 0.01% or less,
O: Contains 0.0005% or more and 0.01% or less, the balance is iron and inevitable impurities, and the texture is at least { 112} <110> to {113} <110> orientation groups and the average value of the X-ray random intensity ratio of the {112} <131> crystal orientation group is 5.0 or less, and the {001} <110> crystal orientation The X-ray random intensity ratio is 4.0 or less, the r (rC) value in the direction perpendicular to the rolling direction is 0.70 or more, and the r value at 30° (r30) to the rolling direction is 1.10 or less, Furthermore, as the steel sheet structure, the combined area ratio of ferrite and bainite is 50% or more, and martensite is 1% or more and 50% or less. . ”
is disclosed.

Moreover, in Patent Document 2,
"In % by mass,
C: 0.05% to 0.20%,
Si: 0.3 to 1.50%,
Mn: 1.3-2.6%,
P: 0.001 to 0.03%,
S: 0.0001 to 0.01%,
Al: 0.0005 to 0.1%,
N: 0.0005 to 0.0040%,
O: 0.0015 to 0.007%,
and the balance being iron and unavoidable impurities, the steel sheet structure is mainly composed of ferrite and bainite structures, the BH after baking is 60 MPa or more, and the maximum tensile strength is 540 MPa or more. A high-strength steel sheet with excellent bake hardenability with very little aging deterioration. ”
is disclosed.

JP 2012-172159 A Japanese Patent Application No. 2009-249733

From the viewpoint of anti-corrosion performance of automobile bodies, steel sheets used as materials for automobile members are sometimes subjected to zinc-based plating, for example, hot-dip galvanization.

However, when the steel sheets disclosed in Patent Documents 1 and 2 are subjected to hot-dip galvanizing, there are cases where sufficient plating qualities such as plating appearance and plating adhesion cannot be obtained. Therefore, it is the present condition that the improvement of this point is calculated|required.

It is an object of the present invention to provide a hot-dip galvanized steel sheet that has both high strength and good workability and is excellent in plating quality.
Another object of the present invention is to provide a method for manufacturing the hot-dip galvanized steel sheet.

In order to achieve the above object, the inventors made extensive studies and obtained the following findings.
(a) In order to obtain good workability, it is necessary to increase the hole expansibility and elongation of the steel sheet. From the viewpoint of preventing cracks during forming, it is effective to increase the yield ratio YR (=yield strength (YS)/tensile strength (TS)) of the steel sheet.
(b) Utilization of martensite is effective for obtaining high strength. On the other hand, utilization of ferrite is effective for obtaining excellent elongation. Moreover, in order to obtain excellent hole expansibility, it is necessary to reduce the difference in hardness between ferrite, which is a soft phase, and martensite, which is a hard phase. For this purpose, it is effective to utilize bainite, which is an intermediate phase. Also, the use of bainite increases the yield ratio.
(c) That is, the steel structure is a composite structure of ferrite, martensite and bainite controlled to a predetermined area ratio (hereinafter simply referred to as a composite structure) to achieve both high strength and good workability. becomes possible.

(d) Also, in order to obtain good plating quality,
・Internal oxidation is caused in the surface layer of the base steel sheet before the plating treatment, and oxides of Si and Mn are formed on the surface layer of the base steel sheet, and
・Including an appropriate amount of Fe in the hot-dip galvanized layer,
is effective.
That is, from the viewpoint of increasing the strength of steel sheets, it is effective to use Si and Mn. However, elements such as Si and Mn are easily oxidizable elements, and combine with oxygen to form oxides on the surface of the steel sheet. If such oxides of Si and Mn are present on the surface of the base steel sheet during plating, the wettability of the base steel sheet by the plating bath (molten zinc) is reduced, resulting in poor coating appearance such as non-coating and coating adhesion. cause a decline in sex.
In this regard, if internal oxidation is caused in the surface layer of the base steel plate to form oxides of Si or Mn before the plating treatment, the oxides present in the surface layer of the base steel plate act as a barrier, The formation of oxides on the surface (hereinafter also referred to as external oxidation) is suppressed. As a result, plating qualities such as plating appearance and plating adhesion are improved.
In addition, by including an appropriate amount of Fe in the hot-dip galvanized layer, the plating quality, particularly the plating adhesion, is improved.

(e) In addition, the above composite structure is created, and internal oxidation is caused in the surface layer of the base steel sheet to form an oxide of Si or Mn on the surface layer of the base steel sheet, and further in the hot-dip galvanized layer In order to contain an appropriate amount of Fe in the steel, it is important to appropriately control the annealing conditions before plating and the plating conditions. In particular, it is important to control the atmosphere during annealing and control the plate temperature entering the plating bath during plating.
Specifically, the dew point is in the range of −20° C. or higher and 5° C. or lower, and a certain amount of oxygen is secured in the atmosphere for holding the annealing to promote internal oxidation in the surface layer of the base steel sheet, while the hydrogen concentration is set to 3 mass. % or more and 20% by mass or less, the oxide formed on the surface of the base steel sheet (and formed during the holding of the annealing) is reduced. Therefore, it is important to suppress external oxidation while sufficiently introducing oxygen in the atmosphere into the interior (surface layer) of the base steel sheet. Further, it is important to promote the diffusion of Fe from the substrate steel sheet into the coating layer by setting the plate temperature entering the coating bath to +10°C or higher.
The present invention has been completed based on the above findings and further studies.

That is, the gist and configuration of the present invention are as follows.
1. A hot-dip galvanized steel sheet having a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet,
The base steel plate is
in % by mass,
C: 0.09% or more and 0.17% or less,
Si: 0.3% or more and 1.1% or less,
Mn: 1.9% or more and 2.7% or less,
P: 0.10% or less,
S: 0.050% or less,
Al: 0.01% or more and 0.20% or less and N: 0.10% or less, with the balance being Fe and unavoidable impurities,
In addition, the area ratio to the entire steel structure is
Ferrite is 30% or more and 85% or less,
Martensite is 5% or more and 30% or less,
Having a steel structure in which bainite is 10% or more and 60% or less and other metal phases are 15% or less,
The amount of oxygen present as an oxide in the surface layer of the base steel plate is 0.05 g/m ² or more and 0.50 g/m ² or less per side, and the surface layer has a depth from the surface of the base steel plate It is a region up to a position of 100 μm,
A hot-dip galvanized steel sheet, wherein the Fe content in the hot-dip galvanized layer is 0.40% by mass or more.

2. 2. The hot-dip galvanized steel sheet according to 1 above, wherein the other metal phase has an area ratio of 5% or less.

3. 3. The hot-dip galvanized steel sheet according to 1 or 2 above, wherein the Fe content in the hot-dip galvanized layer is 8.0% by mass or less.

4. 4. The hot-dip galvanized steel sheet according to any one of 1 to 3 above, wherein the hot-dip galvanized layer has a coating weight per side of 20 g/m ² or more.

5. The chemical composition of the base steel plate is further, in mass%,
Nb: 0.040% or less,
Ti: 0.030% or less,
B: 0.0030% or less,
Cr: 0.3% or less,
5. The hot-dip galvanized steel sheet according to any one of 1 to 4 above, containing one or more of Mo: 0.2% or less and V: 0.065% or less.

6. The chemical composition of the base steel plate is further, in mass%,
6. Any one of 1 to 5 above, containing 0.1% or less in total: one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr. Hot-dip galvanized steel sheet.

7. A hot rolling step of hot rolling a steel slab having the chemical composition according to 1, 5 or 6 to form a hot rolled steel sheet;
A cold-rolling step of cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet;
An annealing step of heating the cold-rolled steel sheet to an annealing temperature, holding it at the annealing temperature, and then cooling it;
Next, a plating treatment step of subjecting the cold-rolled steel sheet to a hot-dip galvanizing treatment;
has
In the annealing step,
The average heating rate in the temperature range from 500° C. to the annealing temperature is 1° C./second or more and 7° C./second or less,
The annealing temperature is (A _{C 1} point + 50 ° C.) or more (A _{C 3} point + 20 ° C.) or less,
The holding time in the holding is 1 second or more and 40 seconds or less,
The dew point of the atmosphere during the holding is −20° C. or more and 5° C. or less, and the hydrogen concentration is 3% by mass or more and 20% by mass or less,
The average cooling rate in the temperature range from the annealing temperature to the primary cooling stop temperature is 10 ° C./sec or more,
The primary cooling stop temperature is 450° C. or higher and 600° C. or lower,
The secondary cooling time is 20 seconds or more and 100 seconds or less,
The secondary cooling stop temperature is 400° C. or higher and 500° C. or lower,
In the plating process,
A method for producing a hot-dip galvanized steel sheet, wherein the sheet temperature entering the plating bath is the plating bath temperature +10°C or higher.

According to the present invention, a hot-dip galvanized steel sheet having both high strength and good workability and excellent plating quality can be obtained.
By applying the hot-dip galvanized steel sheet of the present invention to automobile members, it is possible to greatly contribute to improving the performance of automobile bodies.

The present invention will be described based on the following embodiments.
First, the chemical composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described. Incidentally, although the units in all component compositions are "% by mass", they are indicated simply as "%" unless otherwise specified.

C: 0.09% or more and 0.17% or less C is an element that improves hardenability. C also plays a role in increasing the strength of ferrite. Therefore, C is required to ensure a desired tensile strength (TS) of 750 MPa or more. Here, if the C content is less than 0.09%, the desired tensile strength cannot be obtained. Therefore, the C content should be 0.09% or more. The C content is preferably 0.10% or more, more preferably 0.11% or more. On the other hand, when the C content exceeds 0.17%, the stability of austenite increases and bainite is less likely to form. Also, the strength of martensite increases excessively and the yield ratio decreases. Therefore, the C content should be 0.17% or less. The C content is preferably 0.16% or less, more preferably 0.15% or less.

Si: 0.3% to 1.1% Si is a strengthening element by solid solution strengthening. Si also plays a role in increasing the yield ratio by increasing the strength of ferrite. From the viewpoint of obtaining such effects, the Si content is set to 0.3% or more. The Si content is preferably 0.4% or more, more preferably 0.5% or more. On the other hand, when Si is contained excessively, Si concentrates on the surface of the base steel sheet, causing external oxidation and degrading the coating quality such as coating appearance. Therefore, the Si content should be 1.1% or less. The Si content is preferably 1.0% or less, more preferably 0.9% or less.

Mn: 1.9% or more and 2.7% or less Mn is an element that improves the hardenability of steel. Therefore, Mn is required in order to secure the desired tensile strength. Here, if the Mn content is less than 1.9%, the desired tensile strength cannot be obtained. Therefore, the Mn content should be 1.9% or more. The Mn content is preferably 2.0% or more, more preferably 2.1% or more. On the other hand, when Mn is excessively contained, Mn concentrates on the surface of the base steel sheet, causing external oxidation and degrading the plating quality such as plating appearance. In addition, Mn tends to concentrate into austenite when annealing is maintained, and the strength of martensite transformed from austenite increases excessively. As a result, the yield ratio is lowered. Therefore, the Mn content should be 2.7% or less. The Mn content is preferably 2.6% or less, more preferably 2.5% or less.

P: 0.10% or less P is an element that strengthens steel. However, when P is excessively contained, P segregates at the grain boundary and deteriorates the hole expansibility. Therefore, the P content should be 0.10% or less. The P content is preferably 0.05% or less, more preferably 0.03% or less. Although the lower limit of the P content is not particularly limited, it is preferably 0.001% or more from the viewpoint of cost and the like. The P content is more preferably 0.003% or more, still more preferably 0.005% or more.

S: 0.050% or less S is an element that deteriorates elongation through the formation of MnS and the like. Moreover, when Ti is contained together with S, there is a possibility that the hole expansibility may be deteriorated through the formation of TiS, Ti(C, S), and the like. Therefore, the S content should be 0.050% or less. The S content is preferably 0.030% or less, more preferably 0.020% or less, still more preferably 0.015% or less. Although the lower limit of the S content is not particularly limited, it is preferably 0.0002% or more from the viewpoint of cost and the like. The S content is more preferably 0.0005% or more.

Al: 0.01% to 0.20% Al is an element added as a deoxidizer. Al also serves to reduce coarse inclusions in steel and improve hole expandability. Here, if the Al content is less than 0.01%, the above effects cannot be sufficiently obtained. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.02% or more. On the other hand, when the Al content exceeds 0.20%, nitride-based precipitates such as AlN become coarse, and the hole expansibility deteriorates. Therefore, the Al content is set to 0.20% or less. The Al content is preferably 0.17% or less, more preferably 0.15% or less.

N: 0.10% or less N is an element that forms nitride-based precipitates such as AlN that pin grain boundaries and contributes to improvement in hole expandability. However, if the N content exceeds 0.10%, nitride-based precipitates such as AlN become coarse, and the hole expansibility deteriorates. Therefore, the N content should be 0.10% or less. The N content is preferably 0.05% or less, more preferably 0.010% or less. Although the lower limit of the N content is not particularly limited, the N content is preferably 0.0006% or more from the viewpoint of cost and the like. The N content is more preferably 0.0010% or more.

The base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention has a chemical composition containing the above elements and the balance of Fe (iron) and unavoidable impurities. In particular, the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention preferably has a chemical composition containing the above elements with the balance being Fe and unavoidable impurities.

As described above, the basic chemical composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention has been described.
Nb: 0.040% or less,
Ti: 0.030% or less,
B: 0.0030% or less,
Cr: 0.3% or less,
One or more of Mo: 0.2% or less and V: 0.065% or less can be contained.
Furthermore, one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr can be contained in a total amount of 0.1% or less as an optional additive element.
In addition, when the above optional additive element is contained below the preferable lower limit value described later, the element is assumed to be contained as an unavoidable impurity.

Nb: 0.040% or less Nb contributes to high strength through refinement of prior γ grains and generation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to an increase in the yield ratio. In order to obtain such effects, the Nb content is preferably 0.0010% or more. The Nb content is more preferably 0.0015% or more, still more preferably 0.0020% or more. On the other hand, if Nb is contained excessively, the amount of carbonitride-based precipitates becomes excessive, and the hole expansibility deteriorates. Therefore, when Nb is contained, the content is preferably 0.040% or less. The Nb content is more preferably 0.035% or less, still more preferably 0.030% or less.

Ti: 0.030% or less Ti, like Nb, contributes to high strength through refinement of prior γ grains and generation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to an increase in the yield ratio. In order to obtain such effects, the Ti content is preferably 0.0010% or more. The Ti content is more preferably 0.0015% or more, still more preferably 0.0020% or more. On the other hand, if Ti is contained excessively, the amount of carbonitride-based precipitates becomes excessive and the hole expansibility deteriorates. Therefore, when Ti is contained, the content is preferably 0.030% or less. The Ti content is more preferably 0.025% or less, still more preferably 0.020% or less.

B: 0.0030% or less B is an element that improves the hardenability of steel. By containing B, it becomes possible to secure the desired tensile strength even when the Mn content is small. In order to obtain such effects, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, when the B content is 0.0030% or more, nitride-based precipitates such as BN become excessive, and the hole expansibility deteriorates. Therefore, when B is contained, the content is preferably 0.0030% or less. The B content is more preferably 0.0025% or less, still more preferably 0.0020% or less.

Cr: 0.3% or less Cr is an element that improves the hardenability of steel. In order to obtain such effects, the Cr content is preferably 0.005% or more. However, if Cr is contained excessively, an oxide formation reaction accompanied by the generation of hydrogen ions may occur, which may deteriorate the plating quality. In addition, the amount of precipitates such as carbides becomes excessive, and the hole expansibility deteriorates. Therefore, when Cr is contained, the content is preferably 0.3% or less. The Cr content is more preferably 0.2% or less, still more preferably 0.1% or less.

Mo: 0.2% or less Mo, like Cr, is an element that improves the hardenability of steel. In order to obtain such effects, the Mo content is preferably 0.005% or more. However, if Mo is contained excessively, an oxide formation reaction accompanied by the generation of hydrogen ions may occur, which may deteriorate the plating quality. In addition, the amount of precipitates such as carbides becomes excessive, and the hole expansibility deteriorates. Therefore, when Mo is contained, the content is preferably 0.2% or less. The Mo content is more preferably 0.1% or less, still more preferably 0.04% or less.

V: 0.065% or less V, like Cr, is an element that improves the hardenability of steel. In order to obtain such effects, the V content is preferably 0.005% or more. However, if V is contained excessively, an oxide formation reaction accompanied by the generation of hydrogen ions may occur, which may deteriorate the plating quality. In addition, the amount of precipitates such as carbides becomes excessive, and the hole expansibility deteriorates. Therefore, when V is contained, the content is preferably 0.065% or less. The V content is more preferably 0.050% or less, still more preferably 0.035% or less.

One or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr: 0.1% or less in total Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr are elements that increase strength without deteriorating plating quality. In order to obtain such effects, the content of these elements is preferably 0.0010% or more, either singly or in total. However, when the total content of these elements exceeds 0.1%, the above effects are saturated. Therefore, when one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr are contained, the total content of these elements is 0.1% or less. preferably.

The balance other than the above elements is Fe and unavoidable impurities.

Next, the steel structure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.
The steel structure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention has an area ratio of
Ferrite is 30% or more and 85% or less,
It is a composite structure containing 5% or more and 30% or less of martensite and 10% or more and 60% or less of bainite. The area ratio refers to the ratio of the area of each metal phase to the area of the entire steel structure.

Area ratio of ferrite: 30% or more and 85% or less Ferrite is a necessary phase from the viewpoint of obtaining desired elongation. Therefore, the area ratio of ferrite is set to 30% or more. The area ratio of ferrite is preferably 35% or more, more preferably 40% or more. On the other hand, if ferrite is excessive, the area ratio of martensite required to ensure strength decreases, making it difficult to ensure strength. In addition, the formation of bainite is suppressed, and the hole expansibility and yield ratio are lowered. Therefore, the area ratio of ferrite is set to 85% or less. The area ratio of ferrite is preferably 80% or less.
The ferrite referred to here is a structure composed of crystal grains of a BCC lattice, and is generated by transformation from austenite at a relatively high temperature.

Area ratio of martensite: 5% or more and 30% or less Martensite contributes to the improvement of strength and is a necessary phase for ensuring desired tensile strength. Therefore, the area ratio of martensite is set to 5% or more. The area ratio of martensite is preferably 8% or more, more preferably 10% or more. On the other hand, excess martensite reduces elongation. Therefore, the area ratio of martensite is set to 30% or less. The area ratio of martensite is preferably 28% or less, more preferably 25% or less.
Note that the martensite here refers to a hard structure generated from austenite below the martensite transformation point (also simply referred to as the Ms point). It includes both tempered so-called tempered martensite.

Area ratio of bainite: 10% or more and 60% or less Bainite is a phase necessary for improving hole expansibility and increasing the yield ratio. Therefore, the area ratio of bainite is set to 10% or more. The area ratio of bainite is preferably 15% or more, more preferably 20% or more. On the other hand, excess bainite reduces elongation. Therefore, the area ratio of bainite is set to 60% or less. The area ratio of bainite is preferably 55% or less, more preferably 50% or less.
The bainite referred to here is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite, and is generated from austenite at a relatively low temperature (above the martensitic transformation point).

Area ratio of other metal phases: 15% or less Further, the steel structure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention may contain other metal phases than martensite, ferrite and bainite. good. Here, if the total area ratio of the other metal phases is 15% or less, it is allowed. Therefore, the area ratio of other metal phases is set to 15% or less. The area ratio of other metal phases is preferably 10% or less, more preferably 5% or less. The area ratio of other metal phases may be 0%.

Other metal phases include, for example, pearlite, retained austenite, and non-recrystallized ferrite. Of these, pearlite and unrecrystallized ferrite deteriorate workability (El and λ), so the total area ratio of pearlite and unrecrystallized ferrite is set to 5% or less. The area ratios of pearlite and non-recrystallized ferrite may each be 0%. Since retained austenite does not deteriorate the workability (El and λ), there is no problem if the area ratio of retained austenite is 15% or less. The area ratio of retained austenite is preferably 10% or less, more preferably 5% or less. The area ratio of retained austenite may be 0% or less.

The pearlite referred to here is a structure composed of ferrite and acicular cementite. Retained austenite is austenite remaining without being transformed into martensite. Non-recrystallized ferrite is ferrite that has not been recrystallized, and sub-grain boundaries exist in crystal grains.

Here, the area ratio of each phase is measured as follows.
That is, a test piece is taken from a base steel sheet of a hot-dip galvanized steel sheet so that the L cross section parallel to the rolling direction serves as the test surface. Next, the test surface of the test piece is mirror-polished, and the structure is revealed with a nital solution. The test surface of the test piece with the revealed structure was observed with an SEM at a magnification of 1500, and the area ratio of martensite, ferrite, and bainite at the 1/4 position of the plate thickness of the base steel plate was determined by the point counting method. to measure.
In addition, in the SEM image, martensite presents a white structure. Among martensites, tempered martensite has fine carbides precipitated therein. Ferrite presents a black texture. Bainite has white carbide precipitated in a black structure. These points identify each phase in the SEM image. However, depending on the plane orientation of the block grains and the degree of etching, it may be difficult for the internal carbides to appear.
The total area ratio of other metal phases is calculated by subtracting the area ratio of martensite, the area ratio of ferrite and the area ratio of bainite from 100%.

Among the other metal phases, pearlite has a structure composed of ferrite and acicular cementite as described above. From this point, pearlite is identified in the above SEM image, and the area ratio of pearlite is measured. As described above, unrecrystallized ferrite has subgrain boundaries in the grains, and from this point, unrecrystallized ferrite is identified in the above SEM image, and the area ratio of unrecrystallized ferrite is measured. .

The area ratio of retained austenite is measured as follows.
That is, after polishing the base steel plate of the hot-dip galvanized steel plate in the plate thickness direction (depth direction) to the position of 1/4 of the plate thickness, the surface which is further polished by 0.1 mm by chemical polishing is used as the observation surface. Then, the observation surface is observed by the X-ray diffraction method. Using the Kα line of Mo as the incident X-ray, the diffraction intensity of the (200), (211) and (220) planes of the bcc iron is compared with the (200), (220) and (311) of the fcc iron (austenite). The diffraction intensity ratio of each surface is obtained, and the volume fraction of retained austenite is calculated from the diffraction intensity ratio of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume ratio of the retained austenite is defined as the area ratio of the retained austenite.

Amount of oxygen present as an oxide in the surface layer of the base steel sheet (hereinafter also referred to as the amount of oxygen in the form of oxide in the surface layer of the base steel sheet): 0.05 g/m ² or more and 0.50 g/m ² or less per side As described above As described above, it is effective to use Si or Mn from the viewpoint of increasing the strength of the steel sheet. However, elements such as Si and Mn are easily oxidizable elements, and combine with oxygen to form oxides on the surface of the steel sheet. If such oxides of Si and Mn are present on the surface of the base steel sheet during plating, the wettability of the base steel sheet by the plating bath (molten zinc) is reduced, resulting in poor coating appearance such as non-coating and coating adhesion. cause a decline in sex.
In this regard, if internal oxidation is caused in the surface layer of the base steel plate to form oxides of Si or Mn before the plating treatment, the oxides present in the surface layer of the base steel plate act as a barrier, The formation of oxides on the surface (hereinafter also referred to as external oxidation) is suppressed. As a result, plating qualities such as plating appearance and plating adhesion are improved. Therefore, the amount of oxygen in the form of oxides in the surface layer of the base steel sheet should be 0.05 g/m ² or more per side (every oxygen amount described below is for one side). The amount of oxide-form oxygen in the surface layer of the base steel sheet is preferably 0.06 g/m ² or more. On the other hand, if the amount of oxygen in the form of oxides in the surface layer of the base steel sheet exceeds 0.50 g/m ² , the oxides promote fracture, resulting in reduced elongation and hole expansibility. Therefore, the amount of oxygen in the form of oxides in the surface layer of the base steel sheet is set to 0.50 g/m ² or less. The amount of oxide-form oxygen in the surface layer of the base steel sheet is preferably 0.45 g/m ² or less.

Here, the surface layer portion is a region from the surface of the base steel plate to a position at a depth of 100 μm.
Further, oxides are compounds of O and elements such as Si, Mn, Fe, P, Al, Nb, Ti, B, Cr, Mo and V contained in the base steel plate, and are mainly Si oxides. and Mn oxide.
Since the amount of internal oxidation and the amount of external oxidation are inversely correlated, when external oxidation occurs in the base steel sheet, the amount of oxygen in the form of oxides in the surface layer of the base steel sheet is less than 0.05 g/m ² . Become.

The amount of oxygen in the form of oxides in the surface layer of the base steel sheet is measured by an "impulse furnace-infrared absorption method".
That is, first, the hot dip galvanized layer is removed from the hot dip galvanized steel sheet. The method for removing the hot-dip galvanized layer is not particularly limited as long as the hot-dip galvanized layer can be sufficiently removed, and examples thereof include pickling, alkali peeling, and mechanical polishing.
Next, the amount of oxygen in the steel of the base steel plate is measured. Then, the measured value is taken as the total amount of oxygen OI (g) contained in the base steel sheet.
Next, on both sides of the base steel plate, at least the surface layer portion (region from the surface of the base steel plate to a position of 100 μm in depth) is removed by polishing, and the oxygen content in the steel of the base steel plate after removing the surface layer portion is measured. . And let the measured value be OH (g).
Then, the amount of oxygen in the form of oxides in the surface layer of the base steel sheet is calculated by the following equation.
[Oxygen content in the surface layer of the base steel sheet]
= {OI (g) - OH (g) × ([thickness of base steel plate before polishing (mm)] / [thickness of base steel plate after polishing (mm)])} ÷ ([surface of base steel plate ( area (m ² )]/2
In addition, in the above formula,
By dividing OH (g) by ([thickness of base steel plate before polishing (mm)]/[thickness of base steel plate after polishing (mm)]), the amount of dissolved oxygen contained in the base steel plate to calculate
Next, the amount of dissolved oxygen contained in the base steel sheet is subtracted from the total oxygen amount OI (g) contained in the base steel sheet,
Furthermore, the value is divided by [the area (m ² ) of the surface (per side) of the base steel plate] and 2,
Thus, the amount of oxygen in the form of oxides in the surface layer of the base steel sheet is calculated.

Further, the plate thickness of the base steel plate of the hot-dip galvanized steel plate according to one embodiment of the present invention is preferably 0.2 mm or more and 3.2 mm or less.

Next, the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

Fe content in the hot-dip galvanized layer: 0.40% by mass or more In order to improve coating adhesion, it is preferable that the Fe content in the hot-dip galvanized layer is large. Therefore, the Fe content in the hot-dip galvanized layer is set to 0.40% by mass or more. The Fe content in the hot-dip galvanized layer is preferably 0.50% by mass or more. On the other hand, when Fe in the hot-dip galvanized layer becomes excessive, a hard Fe--Zn alloy phase is formed in the hot-dip galvanized layer. As a result, the plating itself is likely to be destroyed, which may lead to deterioration in plating adhesion. Therefore, the Fe content in the hot-dip galvanized layer is preferably 8.0% by mass or less. The Fe content in the hot-dip galvanized layer is more preferably 7.5% by mass or less, still more preferably 7.0% by mass or less.

Coating weight on hot-dip galvanized layer: 20 g/m ² or more per side For improving corrosion resistance, the coating weight is preferably as large as possible. Therefore, the plating weight is preferably 20 g/m ² or more per side (all plating weights described below are for one side). The coating weight is more preferably 25 g/m ² or more, and still more preferably 30 g/m ² or more. The upper limit of the coating weight is not particularly limited, but when the coating weight exceeds 120 g/m ² , the above effects are saturated. Therefore, it is preferable that the plating amount is 120 g/m ² or less.

Here, the Fe content and coating weight in the hot-dip galvanized layer are measured in the following manner.
That is, after degreasing the surface of a hot-dip galvanized steel sheet to be a test piece, the mass of the test piece is primarily weighed. Next, 2 to 3 drops of an inhibitor, which is a corrosion inhibitor for Fe, was added to 30 cc of a 1:3 HCl aqueous solution (HCl aqueous solution with a concentration of 25% by volume), and then the test material was immersed in the solution. Dissolve the hot-dip galvanized layer of the specimen. After dissolving the hot-dip galvanized layer (after H ₂ gas evolution on the surface of the test piece has ended), the solution is collected. Also, after collecting and drying the test piece, the mass of the test piece is secondarily weighed.
Then, the plating deposition amount is calculated by the following equation.
[Plating adhesion amount (g / m ² )] = ([mass of test piece at primary basis weight (g)] - [mass of test piece at secondary basis weight (g)]) ÷ [test piece Area of the plated portion (area of the portion covered with the hot-dip galvanized layer in the test piece before the hot-dip galvanized layer is dissolved) (m ² )]

In addition, the mass of Fe, Zn and Al dissolved in the collected solution (hereinafter also referred to as Fe dissolved amount, Zn dissolved amount and Al dissolved amount) is measured by the ICP (Inductively Coupled Plasma) method, and the following formula is obtained. Determine the Fe content in the hot-dip galvanized layer.
[Fe content in hot-dip galvanized layer (% by mass)]
= [Amount of Fe dissolved (g)]/([Amount of Fe dissolved (g)] + [Amount of Zn dissolved (g)] + [Amount of Al dissolved (g)]) × 100

The hot-dip galvanized layer contains Zn as a main component, and is basically composed of Zn and Fe described above. Further, depending on the plating bath composition, the hot-dip galvanized layer may contain 0.30% by mass or less, particularly 0.15 to 0.30% by mass of Al. The remainder other than Zn, Fe and Al is unavoidable impurities. Further, the hot-dip galvanized layer may be provided only on one surface of the base steel sheet, or may be provided on both surfaces.

Next, the mechanical properties of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.
A hot-dip galvanized steel sheet according to one embodiment of the present invention has a tensile strength (TS) of 750 MPa or more. Tensile strength (TS) is preferably 780 MPa or more. Although the upper limit of the tensile strength is not particularly limited, the tensile strength is preferably less than 980 MPa from the viewpoint of easy balance with other properties.

In addition, from the viewpoint of workability,
TS×El is 18000 MPa·% or more,
TS×λ is 40000 MPa·% or more, and
Yield ratio YR (=YS/TS) is 0.55 or more.
TS×El is preferably 19000 MPa·% or more, more preferably 20000 MPa·% or more.
TS×λ, preferably 45000 MPa·% or more, more preferably 50000 MPa·% or more.
YR is preferably 0.60 or more, more preferably 0.65 or more.

Here, tensile strength (TS), yield strength (YS) and elongation (El) are measured as follows.
That is, a JIS No. 5 test piece having a gage length of 50 mm and a gage length width of 25 mm is taken from the width center of the hot-dip galvanized steel sheet so that the rolling direction is the longitudinal direction. Then, using the JIS No. 5 test piece taken, a tensile test is performed in accordance with JIS Z 2241 (2011) to measure tensile strength (TS), yield strength (YS) and elongation (El). In addition, the tensile speed shall be 10 mm/min.

λ is the limit hole expansion rate (%), which is measured as follows.
That is, a 100 mm square test piece is taken from the width center of the hot-dip galvanized steel sheet. Then, using the sampled test piece, a hole expansion test is performed according to the Japan Iron and Steel Federation standard JFST1001, and λ is measured. Specifically, after punching a hole with a diameter of 10 mm in the test piece, a 60° conical punch is pushed into the hole while the circumference is restrained, and the diameter of the hole at the crack initiation limit is measured. Then, the limit hole expansion rate λ (%) is obtained from the following formula.
Limit hole expansion rate λ (%) = {(D _f −D ₀ )/D ₀ }×100
Here, _Df is the hole diameter (mm) at the crack generation limit, and _D0 is the initial hole diameter (mm) (before pushing in the punch).

In addition, "excellent plating quality" means that there is no peeling of the hot-dip galvanized layer in the ball impact test under the following conditions, and that there is no unplated defect in the hot-dip galvanized layer by visual observation (preferably plating appearance It means that there is no unevenness). The unplated defect means a region having a size of several μm to several mm, where the base steel sheet is exposed without the hot-dip galvanized layer.
・Ball impact test conditions Ball mass: 2.8 kg Drop height: 1 m
(After the ball is dropped under the above conditions and collided with the hot-dip galvanized steel sheet, the ball collision part is peeled off with tape (JIS Z 1522 (2009) compliant tape with an adhesive strength of 8 N per 25 mm width). , Visually determine the presence or absence of peeling of the hot-dip galvanized layer.)

Next, a method for manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.
A method for manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present invention comprises:
A hot-rolling step of hot-rolling a steel slab having the above chemical composition to form a hot-rolled steel sheet;
A cold-rolling step of cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet;
An annealing step of heating the cold-rolled steel sheet to an annealing temperature, holding it at the annealing temperature, and then cooling it;
Next, a plating treatment step of subjecting the cold-rolled steel sheet to a hot-dip galvanizing treatment;
have
In the following description, the temperature refers to the surface temperature of the steel plate or slab unless otherwise specified. The surface temperature of the steel plate or slab is measured using, for example, a radiation thermometer.

- Hot rolling process This process is a process of hot-rolling the steel material (steel slab) having the chemical composition described above to form a hot-rolled steel sheet.
It should be noted that the steel material to be used is preferably manufactured by a continuous casting method in order to prevent macrosegregation of components. The steel material can also be produced by an ingot casting method or a thin slab casting method.
Preferred manufacturing conditions in the hot rolling step will be described below.

Slab heating temperature: 1200° C. or more If the slab heating temperature is less than 1200° C., precipitates such as AlN are not sufficiently solid-dissolved. Therefore, precipitates such as AlN may become coarse during hot rolling, degrading the hole expandability. Therefore, the heating temperature of the slab is preferably 1200° C. or higher. The heating temperature of the slab is more preferably 1230° C. or higher, still more preferably 1250° C. or higher. Although the upper limit of the slab heating temperature is not particularly limited, it is preferably 1400° C. or less. The heating temperature of the slab is more preferably 1350° C. or less.

Finish rolling temperature: 840° C. or more and 900° C. or less If the finish rolling temperature is less than 840° C., inclusions and coarse carbides may be formed to deteriorate the hole expansibility. Moreover, there is a possibility that the quality of the inside of the base steel plate may be deteriorated. Therefore, the finish rolling temperature is preferably 840°C or higher. The finish rolling temperature is more preferably 860° C. or higher. On the other hand, if the holding time at high temperature is long, coarse inclusions may be formed, deteriorating the expansibility. Therefore, the finish rolling temperature is preferably 900° C. or lower. The finish rolling temperature is more preferably 880° C. or lower.

Coiling temperature: 450° C. or higher and 650° C. or lower After the steel material is hot-rolled as described above, the obtained hot-rolled steel sheet is coiled. Here, when the coiling temperature exceeds 650°C, the surface of the base iron may be decarburized. In this case, a structural difference occurs between the inside and the surface of the base steel sheet, which may cause uneven alloy concentration. In addition, coarse carbides and nitrides may be formed, degrading the expansibility. Therefore, the winding temperature is preferably 650°C or less. The winding temperature is more preferably 630° C. or lower. On the other hand, the coiling temperature is preferably 450° C. or higher in order to prevent deterioration of cold rolling properties. The winding temperature is more preferably 470°C or higher.

Also, the hot-rolled steel sheet after winding may be pickled. Pickling conditions are not particularly limited, and conventional methods may be followed. In addition, the hot-rolled steel sheet after coiling may be subjected to heat treatment for structure softening.

- Cold-rolling process This process is a process of cold-rolling the hot-rolled steel sheet obtained in the hot-rolling process to obtain a cold-rolled steel sheet. Here, if the target thickness can be controlled, there is no restriction on the cold rolling reduction, but if the cold rolling reduction is excessively small, recrystallization is less likely to occur in the subsequent annealing process. That is, non-recrystallized ferrite may be generated and the elongation may be lowered. Therefore, the cold rolling rate is preferably 20% or more. The cold rolling rate is more preferably 30% or more. On the other hand, when the cold rolling rate is high, recrystallization is less likely to occur during the subsequent annealing step due to excessive strain. That is, non-recrystallized ferrite may be generated and the elongation may be lowered. Therefore, the cold rolling rate is preferably 90% or less. The cold rolling rate is more preferably 80% or less.

- Annealing step This step is a step of heating the cold-rolled steel sheet obtained in the cold rolling step to an annealing temperature, holding the steel sheet at the annealing temperature, and then cooling it.
Then, in this step, the above composite structure is created, and internal oxidation is caused in the surface layer of the base steel sheet to form an oxide of Si or Mn on the surface layer of the base steel sheet. From the viewpoint of containing an appropriate amount of Fe in
The average heating rate (hereinafter also referred to as the average heating rate) in the temperature range from 500 ° C. to the annealing temperature in heating is 1 ° C./sec or more and 7 ° C./sec or less,
The annealing temperature is (A _C1 point +50 ° C) or more (A _C3 point +20 ° C) or less,
The holding time in holding (hereinafter also referred to as annealing time) is 1 second or more and 40 seconds or less,
Atmosphere in holding, dew point: -20 ° C. or higher and 5 ° C. or lower, hydrogen concentration: 3 mass% or higher and 20 mass% or lower,
An average cooling rate (hereinafter also referred to as primary cooling rate) in the temperature range from the annealing temperature to the primary cooling stop temperature in cooling is 10 ° C./sec or more,
The primary cooling stop temperature is 450° C. or higher and 600° C. or lower,
Secondary cooling time (time from reaching the primary cooling stop temperature to reaching the secondary cooling stop temperature (if primary cooling stop temperature = secondary cooling stop temperature, the time after reaching the primary cooling stop temperature residence time at temperature)) of 20 seconds or more and 100 seconds or less,
a secondary cooling stop temperature of 400° C. or higher and 500° C. or lower;
It is important to

Average heating rate: 1° C./second or more and 7° C./second or less The average heating rate is preferably slow in order to recrystallize ferrite and secure a desired ferrite area ratio. Therefore, the average heating rate should be 7° C./sec or less. The average heating rate is preferably 6° C./second or less, more preferably 5° C./second or less. On the other hand, when the average heating rate is slow, Mn, which has a low diffusion rate, also concentrates in austenite, and austenite is stabilized. As a result, bainite transformation becomes difficult to occur, and a desired composite structure cannot be obtained. Therefore, the average heating rate should be 1° C./second or more. The average heating rate is preferably 2° C./second or more, more preferably 3° C./second or more.

Annealing temperature: (AC1 point +50° _C ) or higher ( _AC3 point +20°C) or lower If the annealing temperature is less than (AC1 point +50° _C ), coarse Fe-based precipitates are formed, resulting in reduced strength and hole expandability. do. Therefore, the annealing temperature should be (AC1 point +50° _C ) or higher. The annealing temperature is preferably (A _C1 point + 60°C) or higher. On the other hand, when the annealing temperature exceeds (AC3 point +20° _C ), the area ratio of ferrite decreases and the elongation decreases. Therefore, the annealing temperature should be (AC3 point +20° _C ) or less. The annealing temperature is preferably (AC3 point +10° _C ) or less.
Note that the A _C1 point and the A _C3 point referred to here are calculated by the following formulas. In the following formulas, (% element symbol) means the content (% by mass) of each element in the chemical composition of the base steel sheet. However, when the element is not contained (including when it is unavoidably contained), it is calculated as 0.
A _C1 =723+22(%Si)−18(%Mn)+17(%Cr)+4.5(%Mo)+16(%V)
A _C3 =910-203√(%C)+45(%Si)-30(%Mn)-20(%Cu)-15(%Ni)+11(%Cr)+32(%Mo)+104(%V)+400 (% Ti) + 460 (% Al)
Also, the annealing temperature may be constant during the holding. Also, the annealing temperature does not have to be constant during the holding as long as it is within the above temperature range and the temperature fluctuation range is within ±10° C. of the set temperature.

Annealing time: 1 second or more and 40 seconds or less Annealing time is an important condition for transforming austenite into bainite. Here, from the viewpoint of preventing austenite from concentrating Mn, that is, from the viewpoint of avoiding excessive stabilization of austenite and obtaining an appropriate amount of bainite, the shorter the annealing time, the better. Therefore, the annealing time should be 40 seconds or less. The annealing time is preferably 30 seconds or less, more preferably 25 seconds or less. On the other hand, when the annealing time is less than 1 second, the recrystallization of ferrite is not promoted, so the hole expansibility is lowered. Therefore, the annealing time should be 1 second or longer. Annealing time is preferably 5 seconds or longer. The annealing time is the holding time at the annealing temperature.

Dew point of the holding atmosphere: −20° C. or higher and 5° C. or lower As described above, in order to cause internal oxidation in the surface layer of the base steel plate and form an appropriate amount of Si or Mn oxide on the surface layer of the base steel plate, It is necessary to ensure a certain amount of oxygen in the holding atmosphere. In addition, the dew point must be raised to some extent from the viewpoint of ensuring an appropriate amount of Fe content in the hot-dip galvanized layer. Therefore, the dew point of the holding atmosphere is set to −20° C. or higher. The dew point of the holding atmosphere is preferably -18°C or higher, more preferably -15°C or higher. On the other hand, if the dew point is too high, internal oxidation will be excessive in the surface layer of the base steel sheet, resulting in reduced elongation and hole expandability. On the other hand, if the dew point becomes too high, the diffusion of iron is excessively promoted during the plating process, resulting in an excessive amount of iron diffusion in the plating layer. Therefore, the dew point of the holding atmosphere should be 5° C. or less. The dew point of the holding atmosphere is preferably 0° C. or less.

Hydrogen concentration in the holding atmosphere: 3% by mass or more and 20% by mass or less Oxides (and formed during holding of the anneal) need to be reduced. Therefore, the hydrogen concentration in the holding atmosphere is set to 3% by mass or more. The hydrogen concentration in the holding atmosphere is preferably 5% by mass or more. On the other hand, if the hydrogen concentration in the holding atmosphere becomes excessively high, hydrogen penetrates into the steel, reducing elongation and hole expansibility. Therefore, the hydrogen concentration in the holding atmosphere is set to 20% by mass or less. The hydrogen concentration in the holding atmosphere is preferably 17% by mass or less.

Primary cooling rate: 10°C/sec or more In the cooling process in the temperature range from the annealing temperature to the primary cooling stop temperature, bainite is generated, so the cooling rate must be controlled appropriately. That is, when the primary cooling rate is slow, pearlite is produced in addition to ferrite, and an appropriate amount of bainite cannot be obtained. Therefore, the primary cooling rate is set at 10° C./second or more. The primary cooling rate is preferably 12° C./second or higher, more preferably 15° C./second or higher. In order to suppress the pearlite transformation, the primary cooling rate should be fast, so the upper limit of the primary cooling rate is not particularly limited. For example, there is no problem even if the primary cooling rate is set to 2000° C./second or more by water cooling or the like.

Primary cooling stop temperature: 450°C or higher and 600°C or lower The primary cooling stop temperature is 450°C or higher and 600°C or lower in order to suppress pearlite transformation during primary cooling and to secure a predetermined amount of bainite during secondary cooling. . That is, when the primary cooling stop temperature exceeds 600°C, pearlite transformation is promoted during secondary cooling. Therefore, the primary cooling stop temperature shall be 600° C. or lower. The primary cooling stop temperature is preferably 580°C or lower, more preferably 560°C or lower. On the other hand, if the primary cooling stop temperature is less than 450°C, bainite transformation during secondary cooling is suppressed, making it difficult to secure a predetermined bainite fraction. Therefore, the primary cooling stop temperature shall be 450° C. or higher. The primary cooling stop temperature is preferably 460° C. or higher, more preferably 470° C. or higher.

Secondary cooling time: 20 seconds or more and 100 seconds or less In the secondary cooling process from the primary cooling stop temperature to the secondary cooling stop temperature following the primary cooling process, bainite is generated, so the secondary cooling time is appropriately controlled. There is a need. That is, the longer the secondary cooling time is, the more the bainite transformation is promoted. Therefore, the secondary cooling time should be 20 seconds or longer. The secondary cooling time is preferably 25 seconds or longer, more preferably 30 seconds or longer. On the other hand, if the secondary cooling time is too long, the amount of bainite becomes excessive and the area ratio of martensite necessary for ensuring strength cannot be obtained. Therefore, the secondary cooling time is set to 100 seconds or less. The secondary cooling time is preferably 90 seconds or less, more preferably 80 seconds or less.

Secondary cooling stop temperature: 400° C. or higher and 500° C. or lower The secondary cooling stop temperature secures a predetermined bainite fraction and controls the plate temperature entering the plating bath in the plating process described later within a predetermined range. 400° C. or more and 500° C. or less from the viewpoint of That is, when the secondary cooling stop temperature exceeds 500°C, bainite transformation is accelerated during secondary cooling and the bainite fraction becomes excessive. Therefore, the secondary cooling stop temperature is set to 500° C. or less. The secondary cooling stop temperature is preferably 495°C or lower, more preferably 490°C or lower. On the other hand, when the secondary cooling stop temperature is less than 400°C, especially when using a CGL (continuous annealing hot-dip galvanizing line), even if heat treatment is performed immediately before the plating treatment, the plate temperature entering the plating bath is reduced. It becomes difficult to set the plating bath temperature to +10°C or more. Therefore, the secondary cooling stop temperature is set to 400° C. or higher. The secondary cooling stop temperature is preferably 420° C. or higher, more preferably 440° C. or higher.

- Plating treatment step This step is a step of subjecting the cold-rolled steel sheet to a hot-dip galvanizing treatment after the above-described annealing treatment.
In this step, it is important to set the plate temperature entering the plating bath to the plating bath temperature +10°C or higher.

Plate temperature entering the plating bath: Plating bath temperature + 10°C or more It is necessary to control the bath temperature to +10°C or higher. The plate temperature entering the plating bath is preferably plating bath temperature +15°C or higher, more preferably plating bath temperature +20°C or higher. Although the upper limit of the plate temperature entering the plating bath is not particularly limited, it is preferably 500°C or less.

The plating bath composition is basically composed of Zn and may contain 0.15 to 0.30% by mass of Al. The remainder other than Zn and Al is unavoidable impurities.
Also, the plating bath temperature is preferably 440 to 500°C.
In addition, the above annealing process and plating process may be performed in a CAL (continuous annealing line) or in a CGL (continuous annealing hot-dip galvanizing line). Also, each of them may be performed by batch processing.

The conditions of each step other than the above are not particularly limited, and conventional methods may be followed. Further, after the annealing step, temper rolling may be performed for shape adjustment.
Then, according to the above-described manufacturing method, a hot-dip galvanized steel sheet having both high strength and good workability and excellent plating quality can be obtained, and the hot-dip galvanized steel sheet can be suitably used for automobile members. .

A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a vacuum melting furnace and then bloomed to obtain a bloomed rolled material having a thickness of 27 mm. The obtained blooming-rolled material was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 4.0 mm. Next, the obtained hot-rolled steel sheet is ground to a thickness of 3.0 mm, and then cold-rolled under the conditions shown in Table 2 to produce a cold-rolled steel sheet with a thickness of 0.9 to 1.8 mm. did. Then, the obtained cold-rolled steel sheets were subjected to annealing and plating treatment under the conditions shown in Table 2 to produce hot-dip galvanized steel sheets having hot-dip galvanized layers on both sides. A blank in Table 1 indicates that the element is not intentionally added (not 0% by mass, but may be unavoidably contained).

Next, using the obtained hot-dip galvanized steel sheet, the structure of the base steel sheet was identified, the amount of oxygen in the form of oxide in the surface layer of the base steel sheet was measured, and the amount of oxygen per side of the hot-dip galvanized layer was measured according to the procedure described above. The coating weight and Fe content were measured.
Table 3 shows the results.
In identifying the structure of the substrate steel sheet (point counting method), 16×15 grids were placed at equal intervals on the SEM observation area (82 μm×57 μm area). Then, the number of points of each phase in the lattice points was counted, and the ratio of the number of lattice points occupied by each phase to the total number of lattice points was defined as the area ratio of each phase. The area ratio of each phase was the average value of the area ratios of each phase obtained from three separate SEM images.

Also, using the obtained hot-dip galvanized steel sheet, the mechanical properties were measured according to the procedure described above. Table 4 shows the results.
The target tensile strength (TS) is 750 MPa or more.
Also, from the viewpoint of workability, the target TS×El is 18000 MPa·% or more, TS×λ is 40000 MPa·% or more, and the yield ratio YR (=YS/TS) is 0.55 or more.

Furthermore, using the obtained hot-dip galvanized steel sheets, the coating quality (coating adhesion and coating appearance) was investigated according to the procedure described above, and evaluated according to the following criteria. Table 4 shows the evaluation results.
・Plating adhesion 〇 (passed, excellent): No peeling of the hot-dip galvanized layer in the ball impact test according to the above-mentioned procedure × (Fail): Peeling of the hot-dip galvanized layer in the ball impact test according to the above-mentioned procedure Plating appearance ◎ (pass, particularly excellent): No non-plating defects and uneven plating appearance of the hot-dip galvanized layer 〇 (pass, excellent): There is uneven plating appearance of the hot-dip galvanizing layer, but no non-plating defects × (failed ): There is a non-plating defect in the hot-dip galvanized layer

As shown in Table 4, all the invention examples had both high strength and good workability, and were excellent in plating quality.
On the other hand, in Comparative Examples, at least one of strength, workability and plating quality was not sufficient.

Claims (7)

A hot-dip galvanized steel sheet having a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet,
The base steel plate is
in % by mass,
C: 0.09% or more and 0.17% or less,
Si: 0.3% or more and 1.1% or less,
Mn: 1.9% or more and 2.7% or less,
P: 0.10% or less,
S: 0.050% or less,
Al: 0.01% or more and 0.20% or less and N: 0.10% or less, with the balance being Fe and unavoidable impurities,
In addition, the area ratio to the entire steel structure is
Ferrite is 30% or more and 85% or less,
Martensite is 5% or more and 30% or less,
Having a steel structure in which bainite is 10% or more and 60% or less and other metal phases are 15% or less,
The amount of oxygen present as an oxide in the surface layer of the base steel plate is 0.05 g/m ² or more and 0.50 g/m ² or less per side, and the surface layer has a depth from the surface of the base steel plate It is a region up to a position of 100 μm,
A hot-dip galvanized steel sheet, wherein the Fe content in the hot-dip galvanized layer is 0.40% by mass or more. The hot-dip galvanized steel sheet according to claim 1, wherein the other metal phase has an area ratio of 5% or less. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the Fe content in the hot-dip galvanized layer is 8.0% by mass or less. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the coating weight per side of the hot-dip galvanized layer is 20 g/m ² or more. The chemical composition of the base steel plate is further, in mass%,
Nb: 0.040% or less,
Ti: 0.030% or less,
B: 0.0030% or less,
Cr: 0.3% or less,
The hot-dip galvanized steel sheet according to any one of claims 1 to 4, containing one or more of Mo: 0.2% or less and V: 0.065% or less. The chemical composition of the base steel plate is further, in mass%,
One or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr: 0.1% or less in total, according to any one of claims 1 to 5 hot-dip galvanized steel sheet. A method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 6,
A hot rolling step of hot rolling a steel slab having the chemical composition according to claim 1, 5 or 6 to form a hot rolled steel sheet;
A cold-rolling step of cold-rolling the hot-rolled steel sheet to form a cold-rolled steel sheet;
An annealing step of heating the cold-rolled steel sheet to an annealing temperature, holding it at the annealing temperature, and then cooling it;
Next, a plating treatment step of subjecting the cold-rolled steel sheet to a hot-dip galvanizing treatment;
has
In the annealing step,
The average heating rate in the temperature range from 500 ° C. to the annealing temperature is 1 ° C./sec or more and 7 ° C./sec or less,
The annealing temperature is (A _{C 1} point + 50 ° C.) or higher (A _{C 3} point + 20 ° C.) or lower,
The holding time in the holding is 1 second or more and 40 seconds or less,
The dew point of the atmosphere during the holding is −20° C. or more and 5° C. or less, and the hydrogen concentration is 3% by mass or more and 20% by mass or less,
The average cooling rate in the temperature range from the annealing temperature to the primary cooling stop temperature is 10 ° C./sec or more,
The primary cooling stop temperature is 450° C. or higher and 600° C. or lower,
The secondary cooling time is 20 seconds or more and 100 seconds or less,
The secondary cooling stop temperature is 400° C. or higher and 500° C. or lower,
In the plating process,
A method for producing a hot-dip galvanized steel sheet, wherein the sheet temperature entering the plating bath is the plating bath temperature +10°C or higher.