JP6777267B1 - High-strength galvanized steel sheet and its manufacturing method - Google Patents

Abstract

Provided are a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, which is excellent in punching property, stretch flangeability, bendability and plating property, and capable of manufacturing a part with high dimensional accuracy, and a method for manufacturing the same. The base steel sheet contains C, Si, Mn, P, S, Al, N, Ca and Cr, and [% Mn] / [% Si] satisfies the relationship of 2.9 or more and 11.7 or less. The balance has a composition of components consisting of Fe and unavoidable impurities, and has a steel structure containing one or two selected from the group consisting of bainite and ferrite, tempered martensite, hardened martensite and retained austenite. High strength in which the ratio of the amount of Si enrichment to the amount of Mn enrichment in the surface layer of the steel sheet is 0.7 or more and 1.3 or less, and the amount of diffusible hydrogen in the base steel sheet is 0.80 mass ppm or less. Hot-dip zinc-plated steel sheet.

Description

The present invention relates to a high-strength hot-dip galvanized steel sheet and a method for producing the same.

Attempts are being made to increase the strength and thinning of thin steel sheets for automobiles in order to achieve both reduction of CO ₂ emissions by reducing the weight of the vehicle and improvement of collision resistance performance by increasing the strength of the vehicle body. .. For example, for the purpose of increasing the strength of the vehicle body, there are an increasing number of cases where high-strength steel plates having a tensile strength (TS) of 780 MPa or higher are applied to the main structural parts forming the skeleton of the automobile cabin.

High-strength steel sheets used for reinforcing parts and skeletal structural parts of automobiles are required to have excellent formability. For example, since a part such as a crash box has a punched end face and a bent portion, a steel plate having high punching property, stretch flangeability, and bendability is preferable from the viewpoint of moldability.

Further, high-strength steel sheets used for reinforcing parts and skeletal structural parts of automobiles are required to be able to manufacture parts with high dimensional accuracy. In order to manufacture parts with high dimensional accuracy, it is important to control the yield ratio (YR = yield strength YS / tensile strength TS) of the steel sheet within a certain range. By controlling the yield ratio (YR) of the steel sheet within a certain range, it is possible to suppress springback after forming the steel sheet and improve the dimensional accuracy at the time of forming. Further, by increasing the yield ratio (YR) of the steel sheet, the impact absorption energy of the component at the time of collision can be increased.

Further, if the amount of the alloying element added is increased in order to increase the strength of the steel sheet, the wettability of the galvanizing to the base steel sheet is lowered, and the reactivity of the galvanized layer during the alloying treatment is hindered. As a result, the plating property is likely to be deteriorated. Therefore, there is a demand for a steel sheet having high strength and good plating properties.

In order to increase the application ratio of high-strength steel sheets to automobile parts, high-strength steel sheets that comprehensively satisfy the above-mentioned characteristics are required.

Conventionally, various high-strength steel sheets have been developed for the purpose of application to automobile parts. For example, in Patent Document 1, the steel sheet structure is mainly composed of ferrite and bainite, the number density of non-metal inclusions of more than 5 μm contained in the steel sheet is 15 pieces / mm ² or less, and the tensile strength is 540 MPa or more. Exhibits a high-strength steel sheet having excellent stretch flangeability and a method for producing the same.

In Patent Document 2, a ferrite phase and a martensite phase are contained, and the area ratio of the martensite phase to the entire structure is 30% or more, and (area occupied by the martensite phase) / (area occupied by the ferrite phase) is High-strength cold-rolled steel sheets, high-strength hot-dip galvanized steel sheets, which are more than 0.45 and less than 1.5 and have an average particle size of martensite phase of 2 μm or more and are excellent in hole expandability and bendability, and methods for producing them. Is disclosed.

In Patent Document 3, ferrite is contained in an amount of 20% or more as the main phase, one or more of martensite and bainite are contained in a total of 5 to 80% or less as the second phase, and the residual austenite volume ratio is 10%. High-strength steel sheet, hot-dip galvanized steel sheet, alloy with martensite and bainite shapes satisfying a predetermined relational expression, tensile strength of 780 MPa or more, and excellent 180 ° U bendability. Chemical hot-dip galvanized steel sheets and methods for producing them are disclosed.

Japanese Unexamined Patent Publication No. 2009-249732 Japanese Unexamined Patent Publication No. 2010-255094 JP-A-2015-78398

However, in Patent Document 1, bendability and plating property are not considered. Patent Document 2 does not consider YR, which is an index of dimensional accuracy at the time of molding, and plating property. Further, in Patent Document 3, YR and stretch flangeability are not considered. As described above, there is no steel sheet that comprehensively satisfies strength, punching property, stretch flangeability, bendability and plating property, and can manufacture a part with high dimensional accuracy.

The present invention has been made in view of such circumstances, and is excellent in punching property, stretch flange property, bendability and plating property, can manufacture a part with high dimensional accuracy, and has a high tensile strength of 780 MPa or more. An object of the present invention is to provide a strong hot-dip galvanized steel sheet and a method for producing the same.

In the present invention, it is possible to manufacture a part with high dimensional accuracy (high dimensional accuracy at the time of molding) means that YR is 45% or more and 75% or less in the TS: 780 MPa class, and TS: 980 MPa. In the class, it means that it is 50% or more and 80% or less, and in the TS: 1180 MPa class, it means that it is 60% or more and 90% or less. YR is calculated by the following equation (1).
YR = YS / TS ... (1)

Further, excellent punching property means that the total void number density after punching is 2000 / mm ² or less, and the quenching martensite and tempering after punching with respect to the total void number density after punching represented by the following formula. It means that the ratio of the number density of voids generated inside martensite is 0.85 or less.
[Ratio of void number density generated inside hardened martensite and tempered martensite after punching to total void number density after punching] = [Void number density generated inside hardened martensite and tempered martensite after punching] / [ Total void number density after punching]

Further, "excellent in stretch flangeability" means that the value of the hole expansion ratio (λ), which is an index of stretch flangeability, is 20% or more.

In addition, excellent bendability means that a bending test was performed by the V-block method with a bending angle of 90 °, and the ridgeline of the bending apex was observed with a 40x microscope, and cracks with a crack length of 200 μm or more were observed. The value (R / t) obtained by dividing the minimum bending radius (R) that disappears by the plate thickness (t) is 3.0 or less for the TS: 780 MPa class and 980 MPa class, and 4.0 or less for the TS: 1180 MPa class. Means that.

Further, excellent in plating property means that the surfaces of the manufactured hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet are visually observed and there are no non-plating defects on the surface of the steel sheet.

The present inventors have obtained the following findings as a result of repeated diligent studies in order to achieve the above-mentioned problems.
(1) A high-strength steel plate with excellent dimensional accuracy during molding can be obtained by using a structure mainly composed of ferrite and bainite, which are soft phases, and dispersing hardened martensite and tempered martensite, which are hard phases, in the structure. It can be realized.
(2) By setting the amount of diffusible hydrogen in the base steel sheet to 0.80 mass ppm or less, a high-strength steel sheet having excellent punching property and stretch flangeability can be realized.
(3) A high-strength steel sheet having excellent bendability can be realized by setting the Ca content of the steel sheet to 0.0200% or less and the amount of diffusible hydrogen in the steel to 0.80 mass ppm or less. Can be done.
(4) The ratio of the Si enrichment amount to the Mn enrichment amount of the surface layer of the base steel sheet (ISi _Surface / ISi _Bulk ) / (IMn _Surface / Imn _Bulk ) is 0.7 or more and 1.3 or less, and cold rolling. When the plate is annealed, the cold-rolled plate heated to the maximum temperature is maintained at the maximum temperature at the dew point of the atmosphere in the region where the cold-rolled plate reaches the maximum temperature in the annealing furnace at -40 ° C or less. As a result, a high-strength steel sheet having excellent plating properties can be realized.

The present invention has been made based on the above findings. That is, the gist structure of the present invention is as follows.

[1] A high-strength hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet and having a tensile strength of 780 MPa or more.
The base steel sheet is
By mass%
C: 0.050% or more and 0.200% or less,
Si: 0.10% or more and 0.90% or less,
Mn: 2.00% or more and 3.50% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0100% or less,
Ca: 0.0200% or less and Cr: 0.300% or less,
[% Mn] / [% Si] satisfies the relationship of 2.9 or more and 11.7 or less, and the balance has a component composition of Fe and unavoidable impurities.
One or two selected from the group consisting of bainite and ferrite has a total area ratio of 5% or more and 85% or less.
Area ratio of tempered martensite is 65% or less,
It has a steel structure in which the area ratio of hardened martensite is 5% or more and 40% or less and the volume ratio of retained austenite is 5.0% or less.
The ratio of the Si concentration to the Mn concentration on the surface layer of the base steel sheet is 0.7 or more and 1.3 or less, and the diffusible hydrogen amount in the base steel sheet is 0.80 mass ppm or less. , High-strength hot-dip galvanized steel sheet.
However, [% Mn] and [% Si] indicate the contents (mass%) of Mn and Si in steel, respectively.

[2] The high-strength hot-dip galvanized steel sheet according to the above [1], wherein the hot-dip galvanized layer has cracks.

[3] The component composition is further increased by mass%.
Ti: 0.001% or more and 0.100% or less,
Nb: 0.001% or more and 0.100% or less,
V: 0.001% or more and 0.100% or less,
B: 0.0001% or more and 0.0100% or less,
Mo: 0.005% or more and 2.000% or less,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 0.50% or less,
Sb: 0.001% or more and 0.200% or less,
Sn: 0.001% or more and 0.200% or less,
Ta: 0.001% or more and 0.100% or less,
Mg: 0.0001% or more and 0.0200% or less,
Zn: 0.001% or more and 0.020% or less,
Co: 0.001% or more and 0.020% or less,
The above-mentioned [1] or [2], which contains at least one selected from the group consisting of Zr: 0.001% or more and 0.020% or less, and REM: 0.0001% or more and 0.0200% or less. High-strength hot-dip galvanized steel sheet.

[4] The high-strength hot-dip galvanized steel sheet according to any one of [1] to [3] above, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

[5] A steel slab having the component composition according to the above [1] or [3] is hot-rolled to obtain a hot-rolled plate.
Next, the hot-rolled plate is pickled and washed.
Next, the hot-rolled plate was cold-rolled at a reduction rate of 30% or more to obtain a cold-rolled plate.
Next, the cold-rolled plate has a annealing furnace and a hot-dip galvanizing facility located downstream of the annealing furnace, and the hot-dip galvanizing facility is located on the hot-dip galvanizing bath and the steel strip side of the annealing furnace. Supplied to a continuous hot dip galvanizing apparatus, the tip of which is connected and has a snout immersed in the hot dip galvanizing bath.
First, in the annealing furnace, the cold rolled plate is heated to a maximum temperature of 750 ° C. or higher and 900 ° C. or lower with an average heating rate of 10 ° C./s or lower in a temperature range of 200 ° C. or higher and 650 ° C. or lower.
Next, the cold rolling is performed by setting the dew point of the atmosphere in the region where the cold rolled plate reaches the maximum temperature in the annealing furnace to −40 ° C. or lower and the primary hydrogen concentration in the atmosphere to 5% by volume or more and 10% by volume or less. Hold the plate at the maximum temperature reached for 5 s or more and 100 s or less.
Next, the cold rolled plate is cooled from the maximum temperature reached to 600 ° C. with an average cooling rate of 5 ° C./s or more.
Next, the cold-rolled plate is moved from the annealing furnace to the hot-dip galvanized bath via the snout, with a residence time of 15 s or more before entering the hot-dip galvanized bath. Further cooling is performed under the condition that the hydrogen concentration in the atmosphere of the section containing the roll passing immediately before the roll enters the snout is 50% or more and 95% or less of the first hydrogen concentration.
Next, a method for producing a high-strength hot-dip galvanized steel sheet, wherein the cold-rolled sheet is immersed in the hot-dip galvanized bath, subjected to a hot-dip galvanized treatment, and then cooled to 50 ° C. or lower.

[6] The method for producing a high-strength hot-dip galvanized steel sheet according to the above [5], wherein after the hot-dip galvanizing treatment, a galvanizing alloying treatment is further performed and then cooling is performed to 50 ° C. or lower.

[7] The high strength according to the above [5] or [6], wherein the high-strength hot-dip galvanized steel sheet is rolled at an elongation rate of 0.05% or more and 1.00% or less after cooling to 50 ° C. or lower. A method for manufacturing a hot-dip galvanized steel sheet.

According to the present invention, a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, which is excellent in punching property, stretch flangeability, bendability and plating property, and capable of manufacturing a part with high dimensional accuracy, and its manufacture. A method can be provided.

It is a figure which shows the outline of the cross section of a high-strength hot-dip galvanized steel sheet.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.
FIG. 1 shows an outline of a cross section of the high-strength galvanized steel sheet 1 according to the embodiment. As shown in FIG. 1, the high-strength hot-dip galvanized steel sheet 1 has a hot-dip galvanized layer 3 or an alloyed hot-dip galvanized layer on the surface of the base steel sheet 2. First, the appropriate range of the component composition of the base steel sheet and the reason for its limitation will be described. In the following description, "%" representing the content of the component elements of the base steel sheet means "mass%" unless otherwise specified.

C: 0.050% or more and 0.200% or less C is effective for generating a desired amount of hardened martensite or tempered martensite, setting the TS to 780 MPa or more, and obtaining excellent dimensional accuracy during molding. It is an element. If the C content is less than 0.050%, the area ratio of the hardened martensite decreases and the area ratio of ferrite and bainite increases, making it difficult to set the TS to 780 MPa or more. In addition, the dimensional accuracy at the time of molding is lowered. On the other hand, when the C content exceeds 0.200%, the carbon concentration in the hardened martensite and the tempered martensite increases, and the hardness of the hardened martensite and the tempered martensite increases. As a result, the hardness difference between the soft phase ferrite or bainite and the hard phase hardened martensite or tempered martensite becomes large, so that punching property, stretch flangeability, and bendability are lowered. Therefore, the content of C is set to 0.050% or more and 0.200% or less. The content of C is preferably 0.060% or more, more preferably 0.065% or more. The C content is preferably 0.150% or less, more preferably 0.110% or less.

Si: 0.10% or more and 0.90% or less By adding Si in combination with Mn, the base steel sheet is used for heating to the maximum temperature reached and holding at the maximum temperature, which will be described later. Si concentration and Mn concentration on the surface layer are suppressed, and good plating property can be realized. If the Si content is less than 0.10%, the amount of Mn concentrated on the surface of the steel sheet increases during the above heating and holding at the maximum temperature, and Mn oxide that causes non-plating defects is generated on the surface of the steel sheet. Therefore, it becomes difficult to realize good plating property. On the other hand, when the Si content exceeds 0.90%, the amount of Si concentrated on the surface of the steel sheet increases during the above heating and holding at the maximum temperature reached, and Si oxidation that causes non-plating defects on the surface of the steel sheet. Since a product is produced, it becomes difficult to achieve good plating property. Further, when the Si content exceeds 0.90%, the volume fraction of retained austenite increases. Since the produced retained austenite has a high hydrogen concentration, voids are generated inside the martensite when it undergoes processing during punching and during bending tests and undergoes martensitic transformation. Therefore, the total void number density after punching increases, and the punching property and the stretch flange property decrease. In addition, since the void inside the martensite during the bending test becomes the starting point of the crack, the bendability also decreases. Therefore, the Si content is 0.10% or more and 0.90% or less. The Si content is preferably 0.20% or more, more preferably 0.30% or more. The Si content is preferably 0.85% or less, more preferably 0.80% or less.

Mn: 2.00% or more and 3.50% or less Mn is effective for generating a desired amount of hardened martensite or tempered martensite to set the TS to 780 MPa or more and to obtain excellent dimensional accuracy during molding. It is an element. If the Mn content is less than 2.00%, the area ratio of the hardened martensite decreases and the area ratio of ferrite and bainite increases, making it difficult to set the TS to 780 MPa or more. In addition, the dimensional accuracy at the time of molding is also lowered. Further, it becomes difficult to realize good plating property. On the other hand, when the Mn content exceeds 3.50%, the area ratio of tempered martensite increases, the area ratio of ferrite and bainite decreases, and the dimensional accuracy at the time of molding decreases. In addition, the amount of diffusible hydrogen in the base steel sheet increases, and the punching property and bendability decrease. Further, it becomes difficult to realize good plating property. Therefore, the Mn content is set to 2.00% or more and 3.50% or less. The Mn content is preferably 2.20% or more, more preferably 2.30% or more. The Mn content is preferably 3.30% or less, more preferably 3.00% or less.

P: 0.001% or more and 0.100% or less P is an element for increasing the strength of the base steel sheet, which has a solid solution strengthening effect. In order to obtain such an effect, the content of P is set to 0.001% or more. On the other hand, when the P content exceeds 0.100%, P segregates at the old austenite grain boundaries and embrittles the grain boundaries, so that the punching property and the stretch flangeability are deteriorated. Therefore, the content of P is set to 0.001% or more and 0.100% or less. The P content is preferably 0.002% or more, more preferably 0.003% or more. The P content is preferably 0.050% or less, more preferably 0.030% or less.

S: 0.0200% or less S exists as a sulfide in steel, and when the content exceeds 0.0200%, the ultimate deformability of the base steel sheet is lowered, so that punching property and stretch flangeability, And bendability is reduced. Therefore, the S content should be 0.0200% or less. Although the lower limit of the S content is not particularly specified, the S content is preferably 0.0001% or more due to restrictions in production technology. The content of S is more preferably 0.0040% or less.

Al: 1.000% or less Al suppresses the formation of carbides during annealing and increases the volume fraction of retained austenite. Since the produced retained austenite has a high hydrogen concentration, voids are generated inside the martensite when it undergoes processing during punching and bending tests and undergoes martensitic transformation. Therefore, the punching property and the stretch flange property are lowered. Further, since the void becomes the starting point of the crack, the bendability is also lowered. Therefore, the Al content is set to 1.000% or less. Although the lower limit of the Al content is not particularly specified, the Al content is preferably 0.010% or more in order to obtain a sufficient effect as an antacid. The Al content is preferably 0.100% or less, more preferably 0.070% or less. The Al content is more preferably 0.020% or more.

N: 0.0100% or less N exists as a nitride in steel, and when the content exceeds 0.0100%, the ultimate deformability of the base steel sheet is lowered, so that punching property and stretch flangeability, And bendability is reduced. Therefore, the N content should be 0.0100% or less. Although the lower limit of the N content is not particularly specified, the N content is preferably 0.0005% or more due to restrictions in production technology. The content of N is preferably 0.0050% or less.

Cr: 0.300% or less Cr is an element that enhances hardenability, and a desired amount of hardened martensite or tempered martensite is generated to set TS to 780 MPa or more and obtain excellent dimensional accuracy during molding. It is an effective element for this. When the Cr content exceeds 0.300%, the area ratio of hardened martensite and tempered martensite increases, the area ratio of ferrite and bainite decreases, and the dimensional accuracy at the time of molding decreases. Therefore, the Cr content should be 0.300% or less. The lower limit of the Cr content may be 0.000%, but the Cr content is preferably 0.010% or more from the viewpoint of increasing hardenability. Therefore, the Cr content is set to 0.300% or less. The Cr content is preferably 0.010% or more. The Cr content is preferably 0.100% or less.

Ca: 0.0200% or less Ca exists as an inclusion in the base steel sheet. When the Ca content exceeds 0.0200%, when diffusible hydrogen is contained in the base steel sheet, the inclusions become the starting points of cracks during the bending test, so that the bendability is lowered. Therefore, the Ca content should be 0.0200% or less. The lower limit of the Ca content may be 0.0000%, but the Ca content is preferably 0.0001% or more due to restrictions in production technology. The Ca content is preferably 0.0020% or less.

[% Mn] / [% Si] is 2.9 or more and 11.7 or less This is an extremely important constituent requirement of the invention in the present invention. By setting [% Mn] / [% Si] to 2.9 or more and 11.7 or less, both Si concentration and Mn concentration on the surface of the base steel sheet are suppressed, and the surface layer of the base steel sheet described later is suppressed. The ratio of the Si concentration to the Mn concentration can be within a desired range, and good plating properties can be realized. If [% Mn] / [% Si] is less than 2.9, the amount of Si concentrated on the surface of the base steel sheet increases during the above heating and holding at the maximum temperature, which causes non-plating defects on the surface of the steel sheet. Since Si oxide is produced, it becomes difficult to realize good plating property. On the other hand, when [% Mn] / [% Si] exceeds 11.7, the amount of Mn enrichment on the surface of the base steel sheet increases during the heating and holding at the maximum temperature reached, and the surface of the base steel sheet is not suitable. Since Mn oxide that causes plating defects is generated, it becomes difficult to achieve good plating properties. [% Mn] / [% Si] is preferably 3.1 or more, more preferably 3.3 or more. Further, [% Mn] / [% Si] is preferably 11.0 or less, more preferably 10.0 or less.

[Arbitrary component]
In addition to the above-mentioned composition, the high-strength steel plate of the present invention further has Ti: 0.001% or more and 0.100% or less, Nb: 0.001% or more and 0.100% or less, V: in mass%. 0.001% or more and 0.100% or less, B: 0.0001% or more and 0.0100% or less, Mo: 0.005% or more and 2.000% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 0.50% or less, Sb: 0.001% or more and 0.200% or less, Sn: 0.001% or more and 0.200% or less, Ta: 0.001% or more and 0.100% Hereinafter, Mg: 0.0001% or more and 0.0200% or less, Zn: 0.001% or more and 0.020% or less, Co: 0.001% or more and 0.020% or less, Zr: 0.001% or more and 0. It is preferable to contain at least one selected from the group consisting of 020% or less and REM: 0.0001% or more and 0.0200% or less, alone or in combination.

Ti, Nb and V increase TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In order to obtain such an effect, the content of at least one of Ti, Nb and V is set to 0.001% or more, respectively. On the other hand, when the content of at least one of Ti, Nb and V exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, and when the steel sheet contains diffusible hydrogen, the said. Since the precipitates and inclusions become the starting points of cracks during the bending test, the bendability is lowered. Therefore, when at least one of Ti, Nb and V is added, their contents are 0.001% or more and 0.100% or less, respectively. When at least one of Ti, Nb and V is added, their content is preferably 0.005% or more, respectively. When at least one of Ti, Nb and V is added, the content thereof is preferably 0.060% or less, respectively.

B is an element that can improve hardenability by segregating at the austenite grain boundaries. By adding B to the steel, it is possible to suppress the formation of ferrite and grain growth during annealing cooling. In order to obtain such an effect, the content of B is set to 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, cracks occur inside the steel sheet during hot rolling and the ultimate deformability of the steel sheet is lowered, so that punching property, stretch flangeability and bendability are lowered. To do. Therefore, when B is added, its content is 0.0001% or more and 0.0100% or less. When B is added, its content is preferably 0.0002% or more. When B is added, its content is preferably 0.0050% or less.

Mo is an element that enhances hardenability, and is an element that is effective for further improving TS and further improving dimensional accuracy during molding by keeping the area ratio of hardened martensite and tempered martensite within a more preferable range. Is. In order to obtain such an effect, the Mo content is set to 0.005% or more. On the other hand, when the Mo content exceeds 2.000%, the area ratio of the hardened martensite and the tempered martensite increases, it becomes difficult to set the TS to 780 MPa, and the dimensional accuracy at the time of molding decreases. Further, when coarse precipitates and inclusions increase and diffusible hydrogen is contained in the steel sheet, the precipitates and inclusions become the starting points of cracks during the bending test, so that the bendability is lowered. Therefore, when Mo is added, its content is 0.005% or more and 2.000% or less. When Mo is added, its content is preferably 0.020% or more. When Mo is added, its content is preferably 0.500% or less.

Cu is an element that enhances hardenability, and is an element that is effective for further improving TS and further improving dimensional accuracy during molding by keeping the area ratio of hardened martensite and tempered martensite within a more preferable range. Is. In order to obtain such an effect, the Cu content is set to 0.01% or more. On the other hand, when the Cu content exceeds 1.00%, the area ratio of the hardened martensite and the tempered martensite increases, the TS is 780 MPa or more, and it becomes difficult to obtain excellent dimensional accuracy at the time of molding. Further, when coarse precipitates and inclusions increase and diffusible hydrogen is contained in the steel sheet, the precipitates and inclusions become the starting points of cracks during the bending test, so that the bendability is lowered. Therefore, when Cu is added, its content is 0.01% or more and 1.00% or less. When Cu is added, its content is preferably 0.02% or more. When Cu is added, its content is preferably 0.20% or less.

Ni is an element that enhances hardenability, and is an element that is effective for further improving TS and further improving dimensional accuracy during molding by keeping the area ratio of hardened martensite and tempered martensite within a more preferable range. Is. In order to obtain such an effect, the Ni content is set to 0.01% or more. On the other hand, when the Ni content exceeds 0.50%, the area ratio of hardened martensite and tempered martensite increases, and the dimensional accuracy at the time of TS and molding decreases. Further, when coarse precipitates and inclusions increase and diffusible hydrogen is contained in the steel sheet, the precipitates and inclusions become the starting points of cracks during the bending test, so that the bendability is lowered. Therefore, when Ni is added, its content is 0.01% or more and 0.50% or less. When Ni is added, its content is preferably 0.02% or more. When Ni is added, it is preferably 0.20% or less.

Sb and Sn are elements effective for suppressing oxidation of the surface of the base steel sheet during annealing and obtaining better plating properties. In order to obtain such an effect, the content of one or two Sb and Sn should be 0.001% or more, respectively. On the other hand, when the contents of one or two of Sb and Sn each exceed 0.200%, coarse precipitates and inclusions increase, and when diffusible hydrogen is contained in the base steel sheet, the precipitates. Bendability is reduced because objects and inclusions serve as the starting point of cracks during the bending test. Therefore, when one or two kinds of Sb and Sn are added, the content thereof is 0.001% or more and 0.200% or less, respectively. When one or two kinds of Sb and Sn are added, their contents are preferably 0.005% or more, respectively. When one or two kinds of Sb and Sn are added, their contents are preferably 0.050% or less, respectively.

Ta, like Ti, Nb and V, raises TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In addition, Ta is partially dissolved in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta) (C, N), which significantly suppresses the coarsening of the precipitates. Therefore, it is considered that there is an effect of improving the strength of the base steel plate by stabilizing the precipitation strengthening. In order to obtain such an effect, the Ta content should be 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, and when the steel sheet contains diffusible hydrogen, the precipitates and inclusions crack during the bending test. Since it is the starting point of, the bendability is reduced. Therefore, when Ta is added, its content is 0.001% or more and 0.100% or less. When Ta is added, their content is preferably 0.005% or more, respectively. When Ta is added, the content thereof is preferably 0.020% or less.

Mg is an element effective for spheroidizing the shape of inclusions such as sulfides and oxides, improving the ultimate deformability of the steel sheet, and improving the stretch flangeability. In order to obtain such an effect, the Mg content is set to 0.0001% or more. On the other hand, when the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, and when the steel sheet contains diffusible hydrogen, the precipitates and inclusions crack during the bending test. Since it is the starting point of, the bendability is reduced. Therefore, when Mg is added, its content should be 0.0001% or more and 0.0200% or less. When Mg is added, their contents are preferably 0.0005% or more, respectively. When Mg is added, the content thereof is preferably 0.0050% or less.

Zn, Co, and Zr are all effective elements for spheroidizing the shape of inclusions, improving the ultimate deformability of the steel sheet, and improving the stretch flangeability. In order to obtain such an effect, the content of one or more of Zn, Co and Zr is set to 0.001% or more, respectively. On the other hand, when the content of one or more of Zn, Co and Zr exceeds 0.020%, a large amount of coarse precipitates and inclusions are generated, and the steel sheet contains diffusible hydrogen. Since the precipitates and inclusions serve as the starting points of cracks during the bending test, the bendability is lowered. Therefore, when one or more of Zn, Co and Zr are added, the content of one or more of Zn, Co and Zr is 0.0001% or more and 0.020% or less, respectively. When at least one of Zn, Co and Zr is added, the content thereof is preferably 0.002% or more, respectively. When at least one of Zn, Co and Zr is added, the content thereof is preferably 0.010% or less, respectively.

REM is an element effective for spheroidizing the shape of inclusions, improving the ultimate deformability of the steel sheet, and improving the stretch flangeability. In order to obtain such an effect, the content of REM should be 0.0001% or more. On the other hand, when the REM content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, and when the steel sheet contains diffusible hydrogen, the precipitates and inclusions crack during the bending test. Since it is the starting point of, the bendability is reduced. Therefore, when REM is added, its content is 0.0001% or more and 0.0200% or less. When REM is added, their contents are preferably 0.0010% or more, respectively. When REM is added, their contents are preferably 0.0100% or less.

The rest other than the above-mentioned components are Fe and unavoidable impurities. When the content of the optional component is less than the lower limit, the effect of the present invention is not impaired. Therefore, when the content is less than the lower limit, it is treated as an unavoidable impurity.

Next, the steel structure of the base steel sheet will be described.
Ferrite and bainite, which are soft phases in which one or two species selected from the group consisting of bainite and ferrite have a total area ratio of 5% or more and 85% or less, are mainly composed of these phases because the solid solubility of hydrogen is low. By forming the structure, the amount of diffusible hydrogen in the base steel plate can be reduced. Further, by including ferrite and bainite, YR can be controlled in a desired range. As a result, excellent punching property, stretch flangeability, bendability, and dimensional accuracy can be obtained. In order to obtain such an effect, the total area ratio of one or two types of ferrite and bainite is set to 5% or more. On the other hand, if the total area ratio of one or two types of ferrite and bainite exceeds 85%, it becomes difficult to set the TS to 780 MPa or more. Therefore, the total area ratio of one or two types of ferrite and bainite shall be 5% or more and 85% or less. The total area ratio of one or two types of ferrite and bainite is preferably 80% or less, more preferably 70% or less. In addition, bainite is a structure composed of bainite ferrite and a very small amount of carbides, and the area ratio of bainite can also be defined as the area ratio of bainite ferrite.

Area ratio of tempered martensite: 65% or less By including tempered martensite having a hardness intermediate between ferrite and bainite and hardened martensite, dimensional accuracy during molding is further improved and excellent elongation flangeability is achieved. Can be obtained. When the area ratio of tempered martensite exceeds 65%, YS increases, YR cannot be controlled within a desired range, and dimensional accuracy at the time of molding decreases. The lower limit of the area ratio of tempered martensite is not particularly specified, but in order to further improve the dimensional accuracy at the time of molding and obtain better stretch flangeability, the area ratio of tempered martensite should be 3% or more. It is preferably present, and more preferably 5% or more. The area ratio of tempered martensite is preferably 60% or less, more preferably 55% or less.

Area ratio of hardened martensite: 5% or more and 40% or less By dispersing the hardened martensite generated during cooling to 50 ° C or less after hot dip galvanizing treatment in the soft phases ferrite and bainite, it is excellent at the time of molding. Dimensional accuracy can be obtained, and the TS can be set to 780 MPa or more. In order to obtain such an effect, the area ratio of hardened martensite is set to 5% or more. Further, hardened martensite has a high hydrogen concentration because it is produced from austenite having a high solid solubility of hydrogen. Therefore, if the area ratio of the hardened martensite exceeds 40%, voids are generated inside the hardened martensite at the time of punching, so that the punching property and the stretch flange property are deteriorated. Further, during the bending test, voids are generated inside the hardened martensite and become the starting point of cracks, so that the bendability is also lowered. Therefore, the area ratio of hardened martensite is 5% or more and 40% or less. The area ratio of the hardened martensite is preferably 6% or more, more preferably 7% or more. The area ratio of the hardened martensite is preferably 35% or less, more preferably 30% or less.

Here, the methods for measuring the area ratios of ferrite and bainite, tempered martensite, and hardened martensite are as follows.
After cutting out the sample so that the cross section (L cross section) parallel to the rolling direction of the steel sheet becomes the observation surface, the observation surface is polished with diamond paste. Next, the observation surface is finish-polished using alumina. Then, 1 vol. % Corrodes the observation surface with nital. Three-field observation at a magnification of 3000 times using a scanning electron microscope (SEM) at a position of 1/4 of the thickness of the steel plate (a position corresponding to 1/4 of the thickness in the depth direction from the surface of the steel plate). To do. The obtained tissue image is analyzed using Adobe Photoshop of Adobe Systems, Inc. The area ratio of each constituent phase is obtained by averaging the analysis results for the three fields of view. In the above structure image, ferrite and bainite have a gray structure (base structure), tempered martensite has a structure in which fine white carbides are deposited on a gray base, and hardened martensite has a white structure. Therefore, it is possible to identify each tissue and measure the area ratio.

Volume fraction of retained austenite: 5.0% or less When the volume fraction of retained austenite exceeds 5.0%, the retained austenite has a high hydrogen concentration, so it is processed during punching or bending test to martensite. Voids occur inside martensite when the site is transformed. Therefore, the total void number density after punching increases, and the punching property and the stretch flange property decrease. Further, since the void becomes the starting point of the crack, the bendability is also lowered. Therefore, the volume fraction of retained austenite should be 5.0% or less. Although the lower limit of the volume fraction of retained austenite is not particularly specified, it is preferably 0.2% or more, more preferably 0.4% or more in order to further improve the dimensional accuracy at the time of molding. The volume fraction of retained austenite is preferably 4.0% or less, more preferably 3.5% or less.

Here, the method for measuring the volume fraction of retained austenite is as follows. The steel sheet was mechanically ground to 1/4 of the sheet thickness in the plate thickness direction (depth direction), and then chemically polished with oxalic acid to prepare an observation surface. The observation surface was observed by an X-ray diffraction method. As the incident X-ray, a Kα source of Co is used, and the surface of each of (200), (220), and (311) of retained austenite with respect to the diffraction intensity of each of the (200), (211), and (220) surfaces of ferrite is used. The volume ratio of retained austenite was calculated from the ratio of diffraction intensities.

As for the steel structure, in addition to the above-mentioned ferrite, bainite (bainitic ferrite), tempered martensite, hardened martensite, and retained austenite, carbides such as pearlite and cementite, and other structures can be used to obtain the effects of the present invention. It may be included as long as it does not impair. The area ratio of other tissues is preferably 3% or less. The type and area ratio of other tissues (remaining tissues) may be confirmed and determined by, for example, SEM observation.

Next, requirements other than the above that the base steel sheet satisfies will be described.
Ratio of Si concentration to Mn concentration on the surface layer of the base steel sheet: 0.7 or more and 1.3 or less This is an extremely important constituent requirement of the invention in the present invention. The ratio of the Si concentration to the Mn concentration of the surface layer of the base steel sheet is expressed by the following formula.
(Ratio of Si enrichment amount to Mn concentration amount of base steel sheet surface layer) = (ISi _Surface / ISi _Bulk ) / (IMn _Surface / IMn _Bulk )
Here, ISi _Surface , ISi _Bulk , IMn _Surface, and IMn _Bulk are Si intensities in a region having a depth of 0.6 μm from the surface of the steel sheet, as measured by glow discharge spectroscopy (GDS), respectively. The Si strength in the region where the depth from the steel plate surface is 6.0 μm, the Mn strength in the region where the depth from the steel plate surface is 0.6 μm, and the Mn strength in the region where the depth from the steel plate surface is 6.0 μm. is there. In the GDS analysis, the hot-dip galvanized layer or the alloyed hot-dip galvanized layer on the surface of the steel sheet is removed as described later, and then the analysis is performed. The amount of Si enrichment and the amount of Mn enrichment on the outermost layer of the base steel sheet affect the platenability, but the outermost layer of the base steel sheet is incorporated into the hot-dip galvanized layer or the alloyed hot-dip galvanized layer. Therefore, it is not possible to directly evaluate the amount of Si enrichment and the amount of Mn enrichment in the outermost layer of the base steel sheet by GDS analysis. Here, as Si or Mn is concentrated on the outermost surface layer of the base steel sheet, the amount of Si or Mn decreases in the region where the depth from the steel sheet surface is 6.0 μm. When the amount of Si or Mn concentrated on the outermost layer of the base steel sheet is large, a Si-deficient layer or Mn-deficient layer in which the amount of Si or Mn is deficient is formed in the region where the depth from the steel sheet surface is 6.0 μm. Will be done. Therefore, in the present invention, it is decided to evaluate the concentration of Si and Mn on the outermost layer of the base steel sheet based on the Si strength and the Mn strength in the region where the depth from the surface of the steel sheet is 6.0 μm. ..

By compoundly adding Si and Mn to the steel sheet at the above ratios and controlling the dew point and hydrogen concentration in the atmosphere where the cold-rolled sheet reaches the maximum temperature as described below, the amount of Mn enrichment in the surface layer of the base steel sheet. The ratio of the amount of Si enrichment to the amount of Si can be controlled to 0.7 or more and 1.3 or less, and good plating property can be realized. When the ratio of the Si concentration to the Mn concentration of the surface layer of the base steel sheet is less than 0.7, the Si strength is low in the region where the depth from the steel sheet surface is 0.6 μm, and a Si deficient layer is formed. In other words, it is estimated that the amount of Si concentrated on the outermost layer of the base steel sheet was large, and Si oxide, which causes non-plating defects, is generated on the surface of the base steel sheet, so that good plating properties can be achieved. Becomes difficult. On the other hand, when the ratio of the Si concentration to the Mn concentration on the surface layer of the base steel sheet exceeds 1.3, the Mn strength is low in the region where the depth from the steel sheet surface is 0.6 μm, and a Mn-deficient layer is formed. In other words, it is estimated that the amount of Mn concentrated on the outermost layer of the base steel sheet was large, and Mn oxide, which causes non-plating defects, is generated on the surface of the base steel sheet, thus achieving good plating properties. It becomes difficult to do. Therefore, the ratio of the Si concentration to the Mn concentration of the surface layer of the base steel sheet is 0.7 or more and 1.3 or less. The ratio of the Si concentration amount to the Mn concentration amount of the surface layer of the base steel sheet is preferably 0.8 or more, more preferably 0.9 or more. The ratio of the Si concentration to the Mn concentration on the surface layer of the base steel sheet is preferably 1.2 or less, more preferably 1.1 or less.

Here, the calculation method of the Si concentration amount and the Mn concentration amount of the surface layer of the base steel sheet is as follows. First, the hot-dip galvanized layer or the alloyed hot-dip galvanized layer of the high-strength hot-dip galvanized steel sheet is removed with a lutor to obtain a test piece. The measurement sample is analyzed in the depth direction from the surface layer of the steel sheet by GDS. The GDS device uses System3580 manufactured by Rigaku Denki Kogyo Co., Ltd., and the measurement conditions are as follows.
・ Measurement mode: DC mode ・ Electrode size: φ4 [mm]
-Ar gas flow rate: 250 [cc / min]
・ Current: 20 [mA]
A sample is cut out from a high-strength hot-dip galvanized steel sheet separately from the test piece, analyzed by changing the sputtering time, and the sputtering rate is calculated from the sputtering trace depth of the sample. On the basis of the sputtering rate, determined for the measurement sample, a Si intensity ISi _Surface in sputtering time as a sputtering distance 0.6 .mu.m, and ISi _Bulk the Si intensity at sputtering time as a sputtering distance 6.0 .mu.m, ISi _Surface / Calculate ISi _Bulk . Further, the Mn intensity Imn _Surface at a sputtering time of 0.6 μm and the Mn intensity Imn _Bulk at a sputtering time of 6.0 μm are obtained, and the Imn _Surface / Imn _Bulk is calculated.

Amount of diffusible hydrogen in base steel sheet: 0.80 mass ppm or less This is an extremely important constituent requirement of the invention in the present invention. As a result of diligent studies to realize a high-strength steel sheet having excellent punching property and stretch flangeability, the present inventors have determined that the amount of diffusible hydrogen in the base steel sheet is punching property, stretch flange property, and bending. I found that it was related to sex. As a result of further studies, it has been found that excellent punching property, stretch flangeability, and bendability can be obtained by reducing the amount of diffusible hydrogen in the base steel sheet to 0.80 mass ppm or less. It came to be completed. Although the lower limit of the amount of diffusible hydrogen in the base steel sheet is not particularly specified, the amount of diffusible hydrogen in the base steel sheet is preferably 0.01 mass ppm or more due to restrictions on production technology. The amount of diffusible hydrogen in the base steel sheet is more preferably 0.05 mass ppm or more. The amount of diffusible hydrogen in the base steel sheet is preferably 0.50 mass ppm or less, more preferably 0.40 mass ppm or less, and further preferably 0.35 mass ppm or less. The base steel sheet for measuring the amount of diffusible hydrogen is not limited to the base steel sheet of the high-strength hot-dip galvanized steel sheet after the plating process and before the processing, and after the plating process, punching, stretch flange forming, bending, etc. It may be a base steel plate of a processed steel plate, or may be a base material portion of a product manufactured by welding a processed steel plate.

Here, the method for measuring the amount of diffusible hydrogen in the base steel sheet is as follows. A test piece having a length of 30 mm and a width of 5 mm is collected from a high-strength hot-dip galvanized steel sheet, and the hot-dip galvanized layer or the alloyed hot-dip galvanized layer is alkali-removed. Then, the amount of hydrogen released from the test piece is measured by a thermal desorption analysis method. Specifically, after continuously heating from room temperature to 300 ° C. at a heating rate of 200 ° C./h, the mixture is cooled to room temperature, and the cumulative amount of hydrogen released from the test piece is measured from room temperature to 210 ° C. The amount of diffusible hydrogen in the steel sheet.

Next, the hot-dip galvanized layer and the alloyed hot-dip galvanized layer formed on the surface of the base steel sheet described above will be described.
The composition of the hot-dip galvanized layer is not particularly limited, and may be a general one. In one example, the plating layer contains Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr. , Co, Ca, Cu, Li, Ti, Be, Bi, and REM containing one or more selected from the group consisting of 0% by mass or more and 3.5% by mass or less in total, and the balance is Zn and unavoidable. It has a composition consisting of target impurities. When the plating layer is a hot-dip galvanized layer, the Fe content in the plating layer is less than 7% by mass in one example, and in the case of an alloyed hot-dip galvanized layer, the Fe content in the plating layer is contained in one example. The amount is 7% by mass or more and 15% by mass or less, more preferably 8% by mass or more and 13% by mass or less.

The amount of plating adhered is not particularly limited, but it is preferable that the amount of plating adhered per one side of the base steel sheet is 20 to 80 g / m ² .

The hot-dip galvanized layer preferably has cracks. Since the hot-dip galvanized layer has cracks, the amount of diffusible hydrogen in the base steel sheet can be reduced to a more suitable range. As a result, punching property, stretch flange property, and bendability can be improved.

Here, it was determined as follows whether the hot-dip galvanized layer had cracks. With respect to the hot-dip galvanized layer formed on the surface of the base steel sheet, a total of 4 fields of view were observed for each of the front surface and the back surface of the base steel sheet at a magnification of 1500 times using SEM, and 10 μm or more. When one or more cracks having a length of 3 are present in any of the above 4 visual fields, it is determined that the cracks are present.

The high-strength hot-dip galvanized steel sheet provided with the above-mentioned base steel sheet and the hot-dip galvanized layer has a tensile strength (TS) of 780 MPa or more.
The TS is measured as follows in accordance with JIS Z 2241. From the high-strength hot-dip galvanized steel sheet, take JIS No. 5 test pieces so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using the test piece, a tensile test is performed under the condition that the crosshead displacement velocity Vc is 1.67 × 10 ^-1 mm / s, and TS is measured.

The thickness of the high-strength galvanized steel sheet according to the present invention is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.

Next, a method for producing a high-strength hot-dip galvanized steel sheet of the present invention will be described.
First, a steel slab having the above-mentioned composition is produced. First, the steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and any known melting method such as converter melting or electric furnace melting is suitable. The obtained molten steel is solidified to produce a steel slab (slab). The method for producing a steel slab from molten steel is not particularly limited, and a continuous casting method, an ingot forming method, a thin slab casting method, or the like can be used. In order to prevent macrosegregation, the steel slab is preferably manufactured by a continuous casting method.

Next, the manufactured steel slab is subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot-rolled plate.
In one example, the steel slab produced as described above is once cooled to room temperature, then slab-heated and then rolled. The slab heating temperature is preferably 1100 ° C. or higher from the viewpoint of melting carbides and reducing the rolling load. Further, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300 ° C. or lower. The slab heating temperature is based on the temperature of the slab surface during heating.

In addition, hot rolling may be performed by applying an energy saving process. As an energy-saving process, the manufactured steel slab is not cooled to room temperature, but is charged into a heating furnace as a hot piece and hot-rolled by direct rolling, or after the manufactured steel slab is slightly heat-retained. Direct rolling that rolls immediately can be mentioned.

Next, the steel slab is roughly rolled under normal conditions to obtain a seat bar. The sheet bar is subjected to finish rolling to obtain a hot-rolled plate. When the heating temperature of the slab is lowered, it is preferable to heat the seat bar using a bar heater or the like before the finish rolling from the viewpoint of preventing troubles during the finish rolling. The finish rolling temperature is for reducing the rolling load, and when the rolling reduction rate of austenite in the unrecrystallized state is high, an abnormal structure extending in the rolling direction develops and the workability of the annealed plate is lowered. It is preferable that the temperature is equal to or higher than the Ar ₃ transformation point. After finish rolling, the hot-rolled plate is wound up and recovered. The winding temperature is preferably 300 ° C. or higher and 700 ° C. or lower from the viewpoint of moldability of the annealed plate.

It should be noted that the rough-rolled plates may be joined to each other during hot rolling to continuously perform finish rolling. Further, the rough-rolled plate (sheet bar) may be wound once before the finish rolling. Further, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Lubrication rolling is also effective from the viewpoint of uniform steel sheet shape and uniform material. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 or more and 0.25 or less.

Next, the hot-rolled plate is pickled. By pickling, oxides on the surface of the steel sheet can be removed, and good plating quality can be ensured in the high-strength hot-dip galvanized steel sheet of the final product. The pickling may be performed only once or may be divided into a plurality of times.

Next, after hot rolling, the pickled hot-rolled plate is cold-rolled at a reduction rate of 30% or more to obtain a cold-rolled plate. After hot rolling, cold rolling may be performed without heat treatment, or cold rolling may be performed after heat treatment. When cold rolling is performed after the heat treatment, it is preferable to hold the product at a holding temperature of 450 ° C. or higher and 800 ° C. or lower for 900 s or more and 36000 s or less.

Cold rolling reduction rate: 30% or more By setting the cold rolling reduction rate to 30% or more, the total area ratio of one or two selected from the group consisting of ferrite and bainite is 85% or less. The TS can be 780 MPa or more. Although the upper limit of the rolling reduction in cold rolling is not particularly specified, it is preferably 80% or less, more preferably 70% or less in order to further improve the dimensional accuracy at the time of molding. The rolling reduction of cold rolling is preferably 35% or more, more preferably 40% or more. The number of rolling passes and the reduction rate of each pass are not particularly limited, and the cumulative reduction rate may be 30% or more.

The cold-rolled plate obtained as described above is supplied to a continuous hot-dip galvanizing apparatus and annealed.
The continuous hot-dip galvanizing apparatus includes an annealing furnace and a hot-dip galvanizing facility located downstream of the annealing furnace, and the hot-dip galvanizing facility is connected to the hot-dip galvanizing bath and the steel strip side of the annealing furnace. , A device equipped with a snout whose tip is immersed in a hot-dip galvanizing bath is used. Such a continuous hot-dip galvanizing apparatus is a general continuous hot-dip galvanizing line configured to continuously perform a series of treatments including heating, cooling, hot-dip galvanizing, and alloying of hot-dip galvanizing. (CGL: Hot-dip galvanizing line) can be applied. The cold-rolled plate supplied to the continuous hot-dip galvanizing apparatus is first annealed while being conveyed in an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are juxtaposed in this order. The specific annealing conditions are as follows. The number of times of annealing is not particularly limited, but is preferably once (single annealing method).

First, the cold rolled sheet is supplied to the annealing furnace, and the average heating rate in the temperature range of 200 ° C. or higher and 650 ° C. or lower is set to 10 ° C./s or lower, and the cold rolled sheet is heated to the maximum temperature reached at 750 ° C. or higher and 900 ° C. or lower.

Average heating rate in the temperature range of 200 ° C. or higher and 650 ° C. or lower: 10 ° C./s or lower By reducing the average heating rate in the temperature range of 200 ° C. or higher and 650 ° C. or lower, a structure mainly composed of ferrite and bainite, which are soft phases. It is possible to have a structure in which hardened martensite and tempered martensite, which are hard phases, are dispersed, and excellent dimensional accuracy can be obtained at the time of molding. When the average heating rate in the temperature range of 200 ° C. or higher and 650 ° C. or lower exceeds 10 ° C./s, the area ratio of the hardened martensite increases, and the punching property and the stretch flangeability decrease. In addition, the bendability is also reduced. Therefore, the average heating rate in the temperature range of 200 ° C. or higher and 650 ° C. or lower is 10 ° C./s or lower. The lower limit of the average heating rate in the temperature range of 200 ° C. or higher and 650 ° C. or lower is not particularly specified, but it may be 2 ° C./s or higher in order to further increase the TS and further improve the dimensional accuracy at the time of molding. It is preferably, more preferably 4 ° C./s or higher. The average heating rate in the temperature range of 200 ° C. or higher and 650 ° C. or lower is preferably 9 ° C./s or lower, more preferably 8 ° C./s or lower.

Maximum ultimate temperature: 750 ° C or more and 900 ° C or less If the maximum ultimate temperature is less than 750 ° C, the production ratio of austenite during heating in the two-phase region of ferrite and austenite becomes insufficient, so the area ratio of ferrite and bainite increases. As a result, it becomes difficult to set the TS to 780 MPa, and the dimensional accuracy at the time of molding decreases. On the other hand, when the maximum temperature reaches 900 ° C., heating is performed in the austenite single-phase region, so that the area ratio of ferrite and bainite, which are the soft phases after annealing, decreases, and the area ratio of tempered martensite increases. , It becomes difficult to obtain excellent dimensional accuracy at the time of molding. Therefore, the maximum temperature reached is 750 ° C or higher and 900 ° C or lower. The maximum temperature reached is preferably 770 ° C. or higher, more preferably 780 ° C. or higher. The maximum temperature reached is preferably 880 ° C. or lower, more preferably 860 ° C. or lower. The maximum temperature reached is based on the surface temperature of the base steel sheet.

Next, the cold rolled plate heated to the maximum temperature reached as described above is held at the maximum temperature for 5 s or more and 100 s or less. At this time, the holding is performed in a state where the dew point of the atmosphere in the region where the cold rolled plate reaches the maximum temperature is −40 ° C. or less and the first hydrogen concentration in the atmosphere is 5% by volume or more and 10% by volume or less. As described above, the annealing furnace is composed of a heating zone, a soaking zone and a cooling zone in order from the entry side. Since the temperature of the steel sheet is raised from the heating zone to the tropics and then cooled in the cooling zone, it is estimated that the region where the cold rolled sheet reaches the maximum temperature is the tropics exit side before the start of cooling. Can be done.

Retention time at maximum temperature of cold rolled plate: 5 s or more and 100 s or less If the retention time at maximum temperature is less than 5 s, the production ratio of austenite during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite and bainite increases, it becomes difficult to set the TS to 780 MPa, and the dimensional accuracy at the time of molding decreases. On the other hand, if the holding time at the maximum temperature exceeds 100 s, the austenite formation ratio becomes excessive, so that the area ratio of ferrite and bainite, which are soft phases, decreases, and the area ratio of tempered martensite increases. , The dimensional accuracy at the time of molding is lowered. Further, when the holding time at the maximum temperature reaches 100 s, the amount of Si enrichment and the amount of Mn enrichment on the surface layer of the steel sheet increase, and Si oxide and Mn oxide causing non-plating defects are generated on the surface of the steel sheet. Therefore, it becomes difficult to realize good plating property. Therefore, the holding time at the maximum temperature reached is 5 s or more and 100 s or less. The holding time at the maximum temperature reached is preferably 10 s or more, more preferably 15 s or more. The holding time at the maximum temperature reached is preferably 80 s or less, more preferably 60 s or less.

Dew point of atmosphere in the region where the cold rolled plate reaches the maximum temperature: -40 ° C or less This is an extremely important constituent requirement of the invention in the present invention. By setting the dew point of the atmosphere in the region where the maximum temperature is reached to -40 ° C or less in the annealing furnace, the amount of Si concentration and the amount of Mn concentration on the surface layer of the base steel sheet can be reduced, and good plating performance can be achieved. Can be realized. Although the lower limit of the dew point in the region where the maximum temperature is reached is not particularly specified, it is preferable that the dew point in the atmosphere in the region where the cold rolled plate reaches the maximum temperature is -60 ° C or higher due to restrictions on production technology. More preferably, it is −55 ° C. or higher. The dew point of the atmosphere in the region where the cold rolled plate reaches the maximum temperature is preferably −42 ° C. or lower, more preferably −45 ° C. or lower.

1st hydrogen concentration in the atmosphere in the region where the cold rolled plate reaches the maximum temperature: 5% by volume or more and 10% by volume or less This is an extremely important constituent requirement of the invention in the present invention. When the hydrogen concentration (first hydrogen concentration) in the atmosphere in the region where the cold-rolled sheet reaches the maximum temperature is less than 5% by volume, the amount of Si concentration and Mn concentration on the surface layer of the steel sheet increases, and the surface of the steel sheet is not plated. Since Si oxide and Mn oxide that cause defects are generated, it becomes difficult to realize good plating property. On the other hand, when the first hydrogen concentration exceeds 10% by volume, the amount of diffusible hydrogen in the steel sheet increases, so that the total void number density after punching increases, the punching property and the stretch flangeability decrease, and bending. The sex is reduced. Therefore, the first hydrogen concentration is set to 5% by volume or more and 10% by volume or less. The first hydrogen concentration is preferably 6% by volume or more. The first hydrogen concentration is preferably 9% by volume or less. In addition, nitrogen is preferable for the balance other than hydrogen in the atmosphere in the region where the cold rolled plate reaches the maximum temperature.

Next, the cold rolled plate held at the maximum temperature reached under the above-mentioned conditions is cooled from the maximum temperature to 600 ° C in the annealing furnace at an average cooling rate of 5 ° C./s or more.

Average cooling rate from maximum temperature reached to 600 ° C: 5 ° C / s or more By increasing the average cooling rate from maximum temperature to 600 ° C, the area ratio of ferrite and bainite can be controlled within the desired range. It is possible to set the TS to 780 MPa or more and obtain excellent dimensional accuracy at the time of molding. In order to obtain such an effect, the average cooling rate from the maximum temperature reached to 600 ° C. is set to 5 ° C./s or more. The upper limit of the average cooling rate from the maximum temperature reached to 600 ° C. is not particularly specified, but it is the highest in order to further improve the dimensional accuracy at the time of molding and to improve the stretch flangeability and bendability. The upper limit of the average cooling rate from the reached temperature to 600 ° C. is preferably 40 ° C./s or less, more preferably 30 ° C./s or less. The average cooling rate from the maximum temperature reached to 600 ° C. is preferably 7 ° C./s or higher, more preferably 10 ° C./s or higher.

Next, the cold-rolled plate cooled to 600 ° C. as described above is passed through the steel strip out side of the cooling zone, and the tip portion is moved to the hot-dip galvanizing bath via a snout immersed in the hot-dip galvanizing bath. Let it cool further. At this time, the residence time until the cold-rolled plate cooled to 600 ° C. enters the hot-dip galvanizing bath is set to 15 s or more. Immediately before the connecting portion between the cooling zone and the snout, a roll is provided for changing the traveling direction of the cold-rolled plate to allow it to enter the snout, and the cold-rolled plate passes through the roll. Invades the snout from. It is an extremely important constitutional requirement of the present invention that the hydrogen concentration in the atmosphere of the section containing the roll in the cooling zone is 50% or more and 95% or less of the above-mentioned first hydrogen concentration. In addition, nitrogen is preferable for the balance other than hydrogen in the atmosphere of the section accommodating the roll in the cooling zone.

Dwelling time until the cold-rolled plate cooled to 600 ° C. or less penetrates into the hot-dip galvanizing bath: 15 s or more This is an extremely important constituent requirement of the invention in the present invention. The amount of diffusible hydrogen in the base steel sheet is reduced by diffusing to the outside until the cold-rolled sheet cooled to 600 ° C. or lower penetrates into the hot-dip galvanizing bath. Therefore, the amount of diffusible hydrogen in the steel sheet can be reduced by increasing the residence time until the cold-rolled sheet cooled to 600 ° C. or lower penetrates into the hot-dip galvanizing bath, resulting in excellent punching property and elongation. Flangeability and bendability can be obtained. In order to obtain such an effect, the residence time in the temperature range from 600 ° C. or lower to infiltration into the hot-dip galvanizing bath is set to 15 s or more. The upper limit of the residence time until the cold-rolled plate cooled to 600 ° C. or lower infiltrates into the hot-dip galvanizing bath is not particularly specified, but from the viewpoint of securing the area ratio of hardened martensite and tempered martensite, which are hard phases. , 700 s or less, more preferably 500 s or less. The residence time until the cold-rolled plate cooled to 600 ° C. or lower penetrates into the hot-dip galvanizing bath is preferably 20 s or more, more preferably 25 s or more.

Hydrogen concentration in the atmosphere of the compartment accommodating the roll passing immediately before the cold-rolled plate enters the snout: 50% or more and 95% or less of the above-mentioned first hydrogen concentration This is an extremely important constitutional requirement of the invention in the present invention. When the cold-rolled sheet passes through the roll, the hydrogen concentration in the atmosphere of the section accommodating the roll that passes immediately before the cold-rolled sheet enters the snout is set to 95% or less of the above-mentioned first hydrogen concentration. Since the amount of diffusible hydrogen in the steel sheet is reduced, excellent punching property, stretch flangeability, and bendability can be obtained. On the other hand, when the hydrogen concentration of the section accommodating the roll passing immediately before the cold-rolled plate enters the snout is less than 50% of the above-mentioned first hydrogen concentration, the steel plate surface layer is formed when the cold-rolled plate passes through the roll. Since it is oxidized, the ratio of the Si concentration amount to the Mn concentration amount of the surface layer of the base steel sheet increases, and it becomes difficult to realize good plating property. Therefore, the hydrogen concentration of the section accommodating the roll passing immediately before the cold rolled plate invades the snout is set to 50% or more and 95% or less of the above-mentioned first hydrogen concentration. The hydrogen concentration of the section accommodating the roll passing immediately before the cold-rolled plate invades the snout is preferably 60% or more, more preferably 70% or more of the above-mentioned first hydrogen concentration. Further, the hydrogen concentration of the section accommodating the roll passing immediately before the cold-rolled sheet invades the snout is preferably 93% or less, more preferably 90% or less of the above-mentioned first hydrogen concentration.

Next, the cold-rolled plate led to the hot-dip galvanizing bath via the snout is immersed in the hot-dip galvanizing bath to perform the hot-dip galvanizing treatment. At this time, the bath temperature of the hot-dip galvanizing bath is, in one example, 440 ° C. or higher and 500 ° C. or lower. As the hot-dip galvanizing bath, it is preferable to use one having an Al content of 0.10% by mass or more and 0.23% by mass or less and a composition in which the balance is Zn and unavoidable impurities.

After the hot-dip galvanizing treatment described above, a zinc-plating alloying treatment may be further performed to form an alloyed hot-dip galvanizing layer. The alloying treatment is preferably performed in a temperature range of 460 ° C. or higher and 600 ° C. or lower. If the alloying temperature is less than 460 ° C., the Zn—Fe alloying rate becomes excessively slow, and alloying becomes extremely difficult. On the other hand, when the alloying temperature exceeds 600 ° C., untransformed austenite may be transformed into pearlite, and TS and El may decrease. The alloying treatment is more preferably carried out in a temperature range of 470 ° C. or higher and 560 ° C. or lower, more preferably 470 ° C. or higher and 530 ° C. or lower.

The amount of adhesion of the hot-dip galvanized steel sheet (GI) and the alloyed hot-dip galvanized steel sheet (GA) is preferably ^{20 to} 80 g / m ² (double-sided plating) per side. The amount of plating adhered can be adjusted by performing gas wiping or the like after hot-dip galvanizing.

As described above, after galvanizing or further galvanizing alloying treatment, the mixture is cooled to room temperature. The cooling rate at this time is not particularly specified, but in order to further increase the TS, the average cooling rate up to 50 ° C. after galvanizing treatment or further galvanizing alloying treatment is 5 ° C./s or more. It is preferable to do so. On the other hand, due to restrictions on production technology, it is preferable that the average cooling rate up to 50 ° C. after galvanizing treatment or further galvanizing alloying treatment is 40 ° C./s or less. Further, after the galvanizing treatment or the zinc plating alloying treatment, the average cooling rate up to 50 ° C. is more preferably 7 ° C./s or more. Further, after galvanizing treatment or further galvanizing alloying treatment, the average cooling rate up to 50 ° C. is more preferably 30 ° C./s or less.

The cooling rate of less than 50 ° C. is not particularly limited, and cooling to a predetermined temperature can be performed by any method. As the cooling method, gas jet cooling, mist cooling, water cooling, air cooling and the like can be applied. Normally, high-strength hot-dip galvanized steel sheets are subject to trading after being cooled to room temperature.

As described above, a high-strength hot-dip galvanized steel sheet that has been subjected to hot-dip galvanizing treatment or further alloying hot-dip galvanizing treatment is cooled to 50 ° C. or lower, and then has an elongation rate of 0.05% or more and 1.00% or less. It may be rolled with. By setting the elongation rate of rolling performed after cooling to 50 ° C. or lower to 0.05% or more, cracks can be introduced into the galvanized layer. By introducing cracks into the galvanized layer, the amount of diffusible hydrogen in the steel sheet can be reduced, and as a result, punching property and stretch flangeability can be further improved. On the other hand, if the elongation rate of rolling after cooling to 50 ° C. or lower exceeds 1.00%, YS increases and the dimensional accuracy at the time of molding decreases. Therefore, the elongation rate of rolling after cooling to 50 ° C. or lower is preferably 1.00% or less, more preferably 0.70% or less. Further, the elongation rate of rolling after cooling to 50 ° C. or lower is more preferably 0.10% or more.

Rolling after cooling to 50 ° C. or lower may be performed (online) on an apparatus continuous with the above-mentioned continuous hot-dip galvanizing apparatus, or by an apparatus discontinuous with the above-mentioned continuous hot-dip galvanizing apparatus (on the apparatus). You may go (offline). Further, the desired elongation rate may be achieved by one rolling, or a plurality of rolling times may be performed to achieve a total elongation rate of 0.05% or more and 1.00% or less. The rolling described here generally refers to temper rolling, but as long as an elongation rate equivalent to that of temper rolling can be imparted, rolling by a method such as processing by a leveler may be used.

After rolling after cooling to 50 ° C. or lower as described above, heat may be retained in a temperature range of room temperature or higher and 300 ° C. or lower. By retaining heat in a temperature range of room temperature or higher and 300 ° C or lower, the amount of diffusible hydrogen in the steel sheet can be further reduced, and as a result, the total void number density can be reduced and λ can be set to 20% or higher. it can. Further, R / t can also be set to a desired value or less. The heat retention time is usually about 3 to 7 days, but the heat retention may be up to about 6 months.

The manufacturing conditions other than the above conditions can be applied by a conventional method.

A steel material having the composition shown in Table 1 and having the balance of Fe and unavoidable impurities was melted in a converter and made into a steel slab by a continuous casting method. The obtained steel slab was heated to 1250 ° C. and roughly rolled. Next, finish rolling was performed at a finish rolling temperature of 900 ° C., and winding was performed at a winding temperature of 450 ° C. to obtain a hot-rolled plate. The hot-rolled plate was pickled and then cold-rolled. The rolling reduction of cold rolling was as shown in Table 2. The plate thickness after cold rolling was 1.2 mm.

Next, the rolled cold-rolled sheet was annealed under the conditions shown in Table 2. Next, the cold-rolled sheet after annealing was subjected to hot-dip galvanizing treatment, cooled to 50 ° C. or lower, and then rolled under the conditions shown in Table 2 to obtain a high-strength hot-dip galvanized steel sheet (GI). Some high-strength hot-dip galvanized steel sheets are further alloyed after hot-dip galvanizing, cooled to 50 ° C or lower, rolled under the conditions shown in Table 2, and high-strength hot-dip galvanized. A steel plate (GA) was obtained.

As the hot-dip galvanizing bath, when GI was produced, a hot-dip galvanizing bath containing 0.20% by mass of Al and the balance being Zn and unavoidable impurities was used. When GA was produced, a hot-dip galvanizing bath containing 0.14% by mass of Al and the balance being Zn and unavoidable impurities was used. The bath temperature was 470 ° C. The amount of plating adhered was about 45 to 72 g / m ² (double-sided plating) per side when GI was manufactured, and about 45 g / m ² (double-sided plating) per side when GA was manufactured. The alloying treatment for producing GA was carried out at about 550 ° C.
The composition of the GI plating layer contained Fe: 0.1 to 1.0% by mass and Al: 0.2 to 1.0% by mass, and the balance consisted of Fe and unavoidable impurities. The composition of the plating layer of GA contained Fe: 7 to 15% by mass and Al: 0.1 to 1.0% by mass, and the balance consisted of Fe and unavoidable impurities.

Using the high-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel sheet obtained as described above as test steels, tensile properties, punching properties, stretch flangeability, bendability and plating properties are followed according to the following test methods. Was evaluated.

The tensile test was performed in accordance with JIS Z 2241. From the obtained steel sheet, JIS No. 5 test pieces were collected so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet. Using the test piece, a tensile test was performed under the condition that the crosshead displacement velocity Vc was 1.67 × 10 ^-1 mm / s, and YS, TS and El were measured. In the present invention, TS: 780 MPa or more was judged to be acceptable. Further, YR, which is an index of dimensional accuracy at the time of molding, is 45% or more and 75% or less in the TS: 780 MPa class, 50% or more and 80% or less in the TS: 980 MPa class, and 60% or more and 90% in the TS: 1180 MPa class. In the following cases, it was judged that the dimensional accuracy at the time of molding was good. YR was calculated based on the above formula (1).

The punching property was evaluated by the number of voids after punching. A 100 mm × 100 mm sample was taken from the obtained steel sheet by shearing. A hole having a diameter of 10 mm was punched in the sample with a clearance of 12.5%. Next, a 15 mm L × 10 mm W observation sample was cut out from the sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel sheet became the observation surface and included the punched end surface. Next, the observation surface was polished with diamond paste, and then 1 vol. Corroded with% nital. After punching, the total number of voids generated in the region 150 μm away from the punched end face was counted from the obtained tissue image by continuously photographing the region 300 μm away from the punched end face using SEM at a magnification of 3000 times. The total void number density of was evaluated. In addition, all voids are separated into voids generated inside hardened martensite and tempered martensite and voids generated at the interface between the soft phase and the hard phase, and the total void number density after punching is compared with the voids after punching. The ratio of the number of voids generated inside the hardened martensite and the tempered martensite was calculated. When the total void number density after punching is 2000 / mm ² or less, and the ratio of the void number density generated inside the hardened martensite and tempered martensite after punching to the total void number density after punching is 0.85 or less. Was judged to have good punching performance.

The stretch flangeability was evaluated by a hole expansion test. The drilling test was performed in accordance with JIS Z 2256. From the obtained steel sheet, a sample of 100 mm × 100 mm was collected by shearing. A hole having a diameter of 10 mm was punched in the sample with a clearance of 12.5%. Using a die with an inner diameter of 75 mm, a conical punch with an apex angle of 60 ° was pushed into the hole with a wrinkle pressing force of 9 ton (88.26 kN) around the hole, and the hole diameter at the crack generation limit was measured. The limit hole expansion rate: λ (%) was obtained from the following formula, and the hole expansion property was evaluated from the value of this limit hole expansion rate.
Limit hole expansion rate: λ (%) = {(D _f −D ₀ ) / D ₀ } × 100
However, in the above equation, D _f is the hole diameter (mm) at the time of crack occurrence, and D ₀ is the initial hole diameter (mm). Regardless of the strength of the steel sheet, when the value of λ was 20% or more, it was judged that the stretch flangeability was good.

The bending test was performed in accordance with JIS Z 2248. From the obtained steel sheet, strip-shaped test pieces having a width of 30 mm and a length of 100 mm were collected so that the direction parallel to the rolling direction of the steel sheet was the axial direction of the bending test. Then, a bending test was performed by the V-block method with a bending angle of 90 ° under the condition that the pressing load was 100 kN and the pressing holding time was 5 seconds. In the present invention, a 90 ° V bending test was performed, and the ridgeline of the bending apex was observed with a 40x microscope (RH-2000: manufactured by Hirox Co., Ltd.), and a crack with a crack length of 200 μm or more was observed. The minimum bending radius (R) was defined as the bending radius when the bending radius became impossible. Bending test when the value (R / t) obtained by dividing R by the plate thickness (t) is 3.0 or less for TS: 780 MPa class and TS: 980 MPa class, and 4.0 or less for TS: 1180 MPa class. Was judged to be good.

The platenability was evaluated by visually observing the presence or absence of non-plating defects on the front surface and the back surface of the obtained hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, and no non-plating defects were present. The case was judged to be excellent in plating property.

Further, according to the method described above, the area ratio of ferrite, bainite, tempered martensite and hardened martensite, the volume ratio of retained austenite, the ratio of the Si enrichment amount to the Mn concentration amount of the steel plate surface layer, and the diffusible hydrogen in the steel plate. The amount was measured. In addition, the presence or absence of cracks in the hot-dip galvanized layer or the alloyed hot-dip galvanized layer was determined. The remaining tissue was also confirmed by tissue observation.
The results are shown in Table 3.

As shown in Table 3, in the example of the present invention, the TS is 780 MPa or more, and the dimensional accuracy (YR), punching property, stretch flangeability (λ), bendability (R / t) and plating property at the time of molding are exhibited. Both were excellent. On the other hand, in the comparative example, any one or more of TS, dimensional accuracy at the time of molding, punching property, stretch flange property, bendability and plating property was inferior.

By applying the high-strength hot-dip galvanized steel sheet according to the present invention to structural members such as automobile parts, it is possible to improve fuel efficiency by reducing the weight of the vehicle body.

1 High-strength hot-dip galvanized steel sheet 2 Base steel sheet 3 Hot-dip galvanized layer (or alloyed hot-dip galvanized layer)

Claims (7)

A high-strength hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet and having a tensile strength of 780 MPa or more.
The base steel sheet is
By mass%
C: 0.050% or more and 0.200% or less,
Si: 0.10% or more and 0.90% or less,
Mn: 2.00% or more and 3.50% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0100% or less,
Ca: 0.0200% or less and Cr: 0.300% or less,
[% Mn] / [% Si] satisfies the relationship of 2.9 or more and 11.7 or less, and the balance has a component composition of Fe and unavoidable impurities.
One or two selected from the group consisting of bainite and ferrite has a total area ratio of 5% or more and 85% or less.
Area ratio of tempered martensite is 65% or less,
It has a steel structure in which the area ratio of hardened martensite is 5% or more and 40% or less and the volume ratio of retained austenite is 5.0% or less.
The ratio of the Si concentration to the Mn concentration on the surface layer of the base steel sheet is 0.7 or more and 1.3 or less, and the diffusible hydrogen amount in the base steel sheet is 0.80 mass ppm or less. , High-strength hot-dip galvanized steel sheet.
However, [% Mn] and [% Si] indicate the contents (mass%) of Mn and Si in steel, respectively. The high-strength hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanized layer has cracks. The component composition is further increased by mass%.
Ti: 0.001% or more and 0.100% or less,
Nb: 0.001% or more and 0.100% or less,
V: 0.001% or more and 0.100% or less,
B: 0.0001% or more and 0.0100% or less,
Mo: 0.005% or more and 2.000% or less,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 0.50% or less,
Sb: 0.001% or more and 0.200% or less,
Sn: 0.001% or more and 0.200% or less,
Ta: 0.001% or more and 0.100% or less,
Mg: 0.0001% or more and 0.0200% or less,
Zn: 0.001% or more and 0.020% or less,
Co: 0.001% or more and 0.020% or less,
The high-strength melt according to claim 1 or 2, which contains at least one selected from the group consisting of Zr: 0.001% or more and 0.020% or less, and REM: 0.0001% or more and 0.0200% or less. Galvanized steel sheet. The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer. A steel slab having the component composition according to claim 1 or 3 is hot-rolled to obtain a hot-rolled plate.
Next, the hot-rolled plate is pickled and washed.
Next, the hot-rolled plate was cold-rolled at a reduction rate of 30% or more to obtain a cold-rolled plate.
Next, the cold-rolled plate has a annealing furnace and a hot-dip galvanizing facility located downstream of the annealing furnace, and the hot-dip galvanizing facility is located on the hot-dip galvanizing bath and the steel strip side of the annealing furnace. Supplied to a continuous hot dip galvanizing apparatus, the tip of which is connected and has a snout immersed in the hot dip galvanizing bath.
First, in the annealing furnace, the cold rolled plate is heated to a maximum temperature of 750 ° C. or higher and 900 ° C. or lower with an average heating rate of 10 ° C./s or lower in a temperature range of 200 ° C. or higher and 650 ° C. or lower.
Next, the cold rolling is performed by setting the dew point of the atmosphere in the region where the cold rolled plate reaches the maximum temperature in the annealing furnace to −40 ° C. or lower and the primary hydrogen concentration in the atmosphere to 5% by volume or more and 10% by volume or less. Hold the plate at the maximum temperature reached for 5 s or more and 100 s or less.
Next, the cold rolled plate is cooled from the maximum temperature reached to 600 ° C. with an average cooling rate of 5 ° C./s or more.
Next, the cold-rolled plate is moved from the annealing furnace to the hot-dip galvanized bath via the snout, with a residence time of 15 s or more before entering the hot-dip galvanized bath. Further cooling is performed under the condition that the hydrogen concentration in the atmosphere of the section containing the roll passing immediately before the roll enters the snout is 50% or more and 95% or less of the first hydrogen concentration.
Next, a method for producing a high-strength hot-dip galvanized steel sheet, wherein the cold-rolled plate is immersed in the hot-dip galvanized bath, subjected to hot-dip galvanizing treatment, and then cooled to 50 ° C. or lower .
The high-strength hot-dip galvanized steel sheet includes a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, and has a tensile strength of 780 MPa or more.
One or two selected from the group consisting of bainite and ferrite has a total area ratio of 5% or more and 85% or less.
Area ratio of tempered martensite is 65% or less,
Area ratio of hardened martensite is 5% or more and 40% or less and
It has a steel structure with a volume fraction of retained austenite of 5.0% or less,
The ratio of the Si concentration to the Mn concentration of the surface layer of the base steel sheet is 0.7 or more and 1.3 or less, and the diffusible hydrogen amount in the base steel sheet is 0.80 mass ppm or less. , A method for manufacturing high-strength hot-dip galvanized steel sheets . The method for producing a high-strength hot-dip galvanized steel sheet according to claim 5, wherein after the hot-dip galvanizing treatment, a galvanizing alloying treatment is further performed and then cooling is performed to 50 ° C. or lower. The production of the high-strength hot-dip galvanized steel sheet according to claim 5 or 6, wherein the high-strength hot-dip galvanized steel sheet is rolled at an elongation rate of 0.05% or more and 1.00% or less after cooling to 50 ° C. or lower. Method.

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