KR20240007934A - High-strength galvanized steel sheets and members and their manufacturing methods - Google Patents

Abstract

Provided is a high-strength galvanized steel sheet with a tensile strength of 1320 MPa or more and excellent formability and fatigue strength of spot welds. The component composition of the steel sheet, in mass%, is C: 0.150 to 0.450%, Si: 0.80 to 3.00%, Mn: 2.00 to 4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, O : 0.0100% or less and N: 0.0100% or less, and the balance consists of Fe and inevitable impurities. The amount of diffusible hydrogen in the steel sheet is 0.60 ppm by mass or less. Fresh marten where the total of tempered martensite and bainite is 60 to 95%, retained austenite is 5 to 30%, the ratio of retained austenite with an aspect ratio of 5.5 or more to the total retained austenite is 50% or less, and the diameter is 2.0 ㎛ or less. The site is less than 20%.

Description

High-strength galvanized steel sheets and members and their manufacturing methods

The present invention relates to a high-strength galvanized steel sheet having a tensile strength (TS) of 1320 MPa or more and a method of manufacturing the same.

Additionally, the present invention relates to a member made using a high-strength galvanized steel sheet and a method of manufacturing the same.

Recently, for example, in the automobile industry, there is a demand for improving the fuel efficiency of automobiles in order to reduce carbon dioxide (CO ₂ ) emissions from the viewpoint of preserving the global environment.

In order to improve the fuel efficiency of automobiles, it is effective to reduce the weight of the automotive body, but in this case, it is necessary to reduce the weight of the automobile body while maintaining the strength of the automobile body.

For example, Patent Document 1 discloses a high-strength steel plate with good formability.

International Publication No. 2020/017609

High-strength steel sheets with a tensile strength of 1320 MPa or more usually contain a lot of alloying elements necessary for high strength. In spot-welded sections obtained by spot welding these high-strength steel plates, the toughness of the heat-affected zone around the nugget (melted and solidified zone) is insufficient, and when stress is repeatedly applied, the heat-affected zone is insufficient. The strength (fatigue strength) may become insufficient.

If the decline in fatigue strength of the spot weld zone can be suppressed, the collision strength of the entire automobile can be sufficiently maintained.

The present invention has been made in consideration of the above points, and its purpose is to provide a high-strength galvanized steel sheet that has a tensile strength of 1320 MPa or more and is also excellent in formability and fatigue strength of spot welds.

As a result of intensive study, the present inventors have found that the above object can be achieved by adopting the following configuration, and have completed the present invention.

That is, the present invention provides the following [1] to [15].

[1] A high-strength galvanized steel sheet having a steel sheet and a galvanized layer and having a tensile strength of 1320 MPa or more, wherein the steel sheet has C: 0.150 to 0.450%, Si: 0.80 to 3.00%, and Mn: 2.00 to 4.00 in mass%. %, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, O: 0.0100% or less, and N: 0.0100% or less, with the remainder being Fe and inevitable impurities, and micro It has a structure, the amount of diffusible hydrogen in the steel is 0.60 mass ppm or less, and in the microstructure, the total area ratio of tempered martensite and bainite is 60 to 95%, and the area ratio of retained austenite is 60 to 95%. , the area ratio of the retained austenite, which is 5 to 30% and has an aspect ratio of 5.5 or more to the entire retained austenite, and the area ratio of fresh martensite, which is 50% or less and has a diameter of 2.0 μm or less, 20% or less, high-strength galvanized steel sheet.

[2] The above component composition, in mass%, is further: B: 0.0050% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less, Mo: 1.000% or less. , Cr: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Zr: 0.1000% or less, Cu: 1.000% or less, Ni: 1.000% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM : 0.0050% or less, Co: 0.30% or less, Ta: 0.10% or less, As: 0.100% or less, Pb: 0.100% or less, Zn: 0.100% or less, Bi: 0.100% or less, and Hf: 0.10% or less. The high-strength galvanized steel sheet according to [1] above, containing at least one element selected from the following.

[3] The high-strength galvanized steel sheet according to [1] or [2] above, wherein the steel sheet has a decarburization layer.

[4] The high-strength galvanized steel sheet according to any of the above [1] to [3], which has a metal plating layer on at least one side of the steel sheet and between the steel sheet and the zinc plating layer.

[5] The high-strength galvanized steel sheet according to [4] above, wherein the chemical composition of the metal plating layer consists of Fe and inevitable impurities.

[6] The component composition of the metal plating layer is further selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co. The high-strength galvanized steel sheet according to [5] above, which contains a total of 10% by mass or less of one type of element.

[7] The high-strength galvanized steel sheet according to any of [1] to [6] above, which is a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet.

[8] A member formed using the high-strength galvanized steel sheet according to any of the above [1] to [7].

[9] A method of manufacturing a high-strength galvanized steel sheet according to [1] or [2] above, wherein a steel slab having the component composition described in [1] or [2] is hot-rolled, and the obtained hot-rolled steel sheet is , coiling at a coiling temperature of 350 to 700°C, cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet, and performing the first heat treatment, galvanization treatment, and second heat treatment on the cold-rolled steel sheet in this order. In the first heat treatment, the cold-rolled steel sheet is heated at a heating temperature (T3) of 750 to 950 ° C., and cooled from the heating temperature (T3) to a cooling stop temperature (T4) of 350 ° C. to 550 ° C. , the average cooling rate (v1) from the heating temperature (T3) to 550°C is 10°C/s or more, and in the second heat treatment, the cold rolled steel sheet is cooled to a cooling stop temperature (T5) of 50 to 350°C. After that, the cooling stop temperature (T5) is exceeded and reheated to a reheating temperature (T6) of 300 to 500°C, and thereafter, the average cooling satisfies the following equation (1) from (Ms point - 200)°C to 50°C. cooling at a rate v2, and in the first heat treatment, the zinc plating treatment, and the second heat treatment, a holding time (t ₁ ) in a temperature range (T1) of 300°C or more and less than 450°C, and 450°C or more. A method for manufacturing a high-strength galvanized steel sheet, wherein the holding time (t ₂ ) in the temperature range (T2) of 600°C or lower satisfies the following equation (2).

v2≤3.8[C]+2.4[Mn]+1.2[Si]… (One)

1.1≤3t ₁ /t ₂ ≤6.5 … (2)

However, [C], [Mn], and [Si] in the above formula (1) are the contents of C, Mn, and Si, respectively, in the above component composition, and the unit of the above contents is mass%.

[10] In the first heat treatment, heating at the heating temperature T3 is performed in an atmosphere with a dew point exceeding -30°C to form a decarburization layer on the outermost layer of the cold rolled steel sheet, [9] The manufacturing method of high-strength galvanized steel sheet described in.

[11] Manufacturing of the high-strength galvanized steel sheet according to [9] or [10] above, wherein before the first heat treatment, the cold rolled steel sheet is subjected to a metal plating treatment to form a metal plating layer on at least one side of the cold rolled steel sheet. method.

[12] The method for producing a high-strength galvanized steel sheet according to [11] above, wherein the chemical composition of the metal plating layer consists of Fe and inevitable impurities.

[13] The component composition of the metal plating layer is further selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co. The method for producing a high-strength galvanized steel sheet according to [12] above, which contains a total of 10% by mass or less of one type of element.

[14] The method for producing a high-strength galvanized steel sheet according to any one of [9] to [13], wherein the zinc plating treatment is a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment.

[15] A method of manufacturing a member, wherein the high-strength galvanized steel sheet according to any of [1] to [7] above is subjected to at least one of forming processing and joining processing to obtain a member.

According to the present invention, it is possible to provide a high-strength galvanized steel sheet that has a tensile strength of 1320 MPa or more and is also excellent in formability and fatigue strength of spot welds.

1 is a chart showing an example of first heat treatment, zinc plating treatment, and second heat treatment.
Figure 2 is a cross-sectional view showing a sheet set used for resistance welding.
Figure 3 is a plan view showing the plate after resistance welding.
Figure 4 is a cross-sectional view taken along line AA of Figure 3.

(Form for carrying out the invention)

[High-strength galvanized steel sheet]

The high-strength galvanized steel sheet of the present invention has a steel sheet (base steel sheet) and a galvanized layer. This steel sheet has a component composition and microstructure described later, and satisfies the amount of diffusible hydrogen in steel described later.

High strength means that the tensile strength (TS) is 1320 MPa or more.

Hereinafter, “high-strength galvanized steel sheet” is also simply referred to as “galvanized steel sheet.”

The high-strength galvanized steel sheet of the present invention has a tensile strength of 1320 MPa or more, and is also excellent in formability and fatigue strength of spot welds. For this reason, since the collision strength can be sufficiently maintained, it is suitably used in transportation vehicles such as automobiles.

Additionally, as a method for forming and processing the high-strength galvanized steel sheet of the present invention, general processing methods such as press processing can be used without limitation. As a method for welding the high-strength galvanized steel sheet of the present invention, general welding methods such as spot welding and arc welding can be used without limitation.

〈Steel plate〉

First, the steel sheet (underlying steel sheet) constituting the galvanized steel sheet will be explained.

The steel sheet is, for example, a cold-rolled steel sheet that has undergone the second heat treatment described later.

The plate thickness of the steel plate is not particularly limited and is, for example, 0.5 mm or more and 3.0 mm or less.

《Ingredients Composition》

The component composition of the steel plate (hereinafter, also referred to as the “component composition of the present invention”) will be explained.

“%” in the component composition of the present invention means “% by mass” unless otherwise specified.

(C: 0.150 to 0.450%)

C generates martensite and increases the strength of the steel sheet. If the amount of C is too small, the hardness of martensite decreases and the total area ratio of tempered martensite and bainite decreases, so a tensile strength of 1320 MPa or more cannot be obtained. For this reason, the amount of C is 0.150% or more, preferably 0.180% or more, and more preferably 0.190% or more.

On the other hand, if the amount of C is too large, a large amount of cementite is generated in the heat-affected zone, the toughness of the spot weld zone decreases, and the fatigue strength of the spot weld zone decreases. For this reason, the amount of C is 0.450% or less, preferably 0.400% or less, and more preferably 0.370% or less.

(Si: 0.80∼3.00%)

Si increases the strength of the steel sheet through solid solution strengthening. From the viewpoint of obtaining a tensile strength of 1320 MPa or more, the Si amount is 0.80% or more, preferably 1.00% or more, and more preferably 1.10% or more.

On the other hand, if the amount of Si is too large, the toughness of the spot welded portion decreases and the fatigue strength of the spot welded portion decreases. Additionally, if the amount of Si is too large, there is concern that the resistance weld cracking resistance (described later) in the spot weld zone will decrease. For this reason, the amount of Si is 3.00% or less, preferably 2.60% or less, and more preferably 2.40% or less.

(Mn: 2.00∼4.00%)

Mn increases the strength of the steel sheet through solid solution strengthening. From the viewpoint of obtaining a tensile strength of 1320 MPa or more, the amount of Mn is 2.00% or more, preferably 2.20% or more, and more preferably 2.40% or more.

On the other hand, if the amount of Mn is too large, a large amount of cementite is generated during tempering, the toughness of the spot weld zone decreases, and the fatigue strength of the spot weld zone decreases. For this reason, the amount of Mn is 4.00% or less, preferably 3.60% or less, and more preferably 3.50% or less.

(P: 0.100% or less)

P segregates at grain boundaries, lowering the toughness of the spot weld zone and reducing the fatigue strength of the spot weld zone. For this reason, the amount of P is 0.100% or less, preferably 0.030% or less, and more preferably 0.010% or less.

(S: 0.0200% or less)

S combines with Mn to form coarse MnS, which reduces the toughness of the spot weld zone and the fatigue strength of the spot weld zone. For this reason, the amount of S is 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0020% or less.

(Al: 0.100% or less)

Al acts as a deoxidizing agent. If the amount of Al is too large, oxides and nitrides aggregate and coarsen, which lowers the toughness of the spot weld zone and reduces the fatigue strength of the spot weld zone. For this reason, the amount of Al is 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less.

The lower limit of the amount of Al is not particularly limited, but from the viewpoint of obtaining the effect of adding Al, for example, it is 0.010%, and 0.020% is preferable.

(O: 0.0100% or less)

O forms an oxide, which reduces the toughness of the spot weld zone and reduces the fatigue strength of the spot weld zone. For this reason, the O amount is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less.

(N: 0.0100% or less)

N combines with Ti to form TiN. If the amount of N is too large, the toughness of the spot welded portion decreases due to the increased amount of TiN formed, and the fatigue strength of the spot welded portion decreases. For this reason, the amount of N is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less.

The component composition of the present invention contains the above components, and the remainder consists of Fe and inevitable impurities.

(Other elements)

The component composition of the present invention replaces a part of the remainder (Fe and inevitable impurities) and additionally contains at least one element (also referred to as “arbitrary element”) selected from the group consisting of the elements described below in mass percent. It may contain).

((B: 0.0050% or less))

B is an element that can improve the hardenability of a steel sheet by segregating at austenite grain boundaries, and is preferably added because it increases the tensile strength of the steel sheet.

However, if the amount of B is too large, Fe ₂₃ (CB) ₆ is formed, which reduces the toughness of the spot weld zone and reduces the fatigue strength of the spot weld zone. For this reason, the amount of B is preferably 0.0050% or less, more preferably 0.0040% or less, and still more preferably 0.0030% or less.

The lower limit of the amount of B is not particularly limited, but from the viewpoint of obtaining the effect of adding B, for example, it is 0.0005%, and 0.0010% is preferable.

((Ti: 0.200% or less))

Ti is preferably added because it increases the tensile strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment.

However, if the amount of Ti is too large, it combines with N to form a coarse nitride, which reduces the toughness of the spot weld zone and reduces the fatigue strength of the spot weld zone. For this reason, the amount of Ti is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less.

The lower limit of the amount of Ti is not particularly limited, but from the viewpoint of obtaining the effect of adding Ti, for example, it is 0.005%, and 0.010% is preferable.

((Nb: 0.200% or less, V: 0.500% or less, W: 0.500% or less))

Nb, V, and W are preferably added because they increase the tensile strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment.

However, if the amount of these elements is excessively large, they do not dissolve when the steel slab is heated and remain as coarse carbides. Coarse carbides reduce the toughness of the spot weld zone and reduce the fatigue strength of the spot weld zone.

For this reason, the amount of Nb is preferably 0.200% or less, more preferably 0.100% or less, and still more preferably 0.050% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Nb, it is, for example, 0.005%, and 0.010% is preferable.

The amount of V is preferably 0.500% or less, more preferably 0.300% or less, and still more preferably 0.100% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding V, for example, it is 0.005%, and 0.010% is preferable.

The amount of W is preferably 0.500% or less, more preferably 0.200% or less, and still more preferably 0.050% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding W, for example, it is 0.001%, and 0.002% is preferable.

((Mo: 1.000% or less, Cr: 1.000% or less))

Mo and Cr are preferably added because they increase the tensile strength of the steel sheet by increasing the quenching properties of the steel sheet. However, when the amount of these elements is excessively large, hard martensite is excessively generated, the toughness of the spot weld zone decreases, and the fatigue strength of the spot weld zone decreases.

For this reason, the Mo amount is preferably 1.000% or less, more preferably 0.700% or less, and still more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Mo, for example, it is 0.005%, and 0.020% is preferable.

The Cr amount is preferably 1.000% or less, more preferably 0.700% or less, and still more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Cr, it is, for example, 0.005%, and 0.020% is preferable.

((Sb: 0.200% or less, Sn: 0.200% or less))

Sb and Sn are preferably added because they increase the tensile strength of the steel sheet by suppressing decarburization of the surface of the steel sheet. However, when the amount of these elements is excessively high, the steel becomes embrittled, cracks occur in the heat-affected zone, and the fatigue strength of the spot weld zone is reduced.

For this reason, the amount of Sb is preferably 0.200% or less, more preferably 0.080% or less, and still more preferably 0.040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Sb, it is, for example, 0.001%, and 0.002% is preferable.

The amount of Sn is preferably 0.200% or less, more preferably 0.080% or less, and still more preferably 0.040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Sn, for example, it is 0.001%, and 0.002% is preferable.

((Zr: 0.1000% or less))

Zr is preferably added because it spheroidizes the shape of the precipitate and increases the toughness of the spot weld zone. However, when the amount of Zr is excessively large, coarse precipitates remaining undissolved increase when the hot-rolled steel slab is heated, the toughness of the spot weld zone decreases, and the fatigue strength of the spot weld zone decreases.

For this reason, the Zr amount is preferably 0.1000% or less, more preferably 0.0700% or less, and even more preferably 0.0400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Zr, for example, it is 0.0005%, and 0.0010% is preferable.

((Cu: 1.000% or less))

Cu is preferably added because it increases the tensile strength of the steel sheet by increasing the quenching properties of the steel sheet. However, when the amount of Cu is excessively large, the toughness of the spot welded portion decreases due to an increase in Cu inclusions, and the fatigue strength of the spot welded portion decreases.

For this reason, the Cu amount is preferably 1.000% or less, more preferably 0.700% or less, and still more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Cu, for example, it is 0.005%, and 0.010% is preferable.

((Ni: 1.000% or less))

Ni is preferably added because it increases the tensile strength of the steel sheet by increasing the quenching properties of the steel sheet. However, when the amount of Ni is excessively large, hard martensite increases, thereby lowering the toughness of the spot weld zone and reducing the fatigue strength of the spot weld zone.

For this reason, the Ni amount is preferably 1.000% or less, more preferably 0.700% or less, and still more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Ni, for example, it is 0.003%, and 0.005% is preferable.

((Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less))

Ca, Mg, and REM (Rare Earth Metal) are preferably added because they increase the toughness of the spot weld zone by spheroidizing the shape of precipitates such as sulfides and oxides. However, when the amount of these elements is excessively large, the toughness of the spot weld zone decreases due to the coarsening of the sulfide, and the fatigue strength of the spot weld zone decreases.

For this reason, the amount of Ca is preferably 0.0050% or less, more preferably 0.0045% or less, and still more preferably 0.0040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Ca, for example, it is 0.0005%, and 0.0010% is preferable.

The amount of Mg is preferably 0.0050% or less, more preferably 0.0048% or less, and still more preferably 0.0045% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Mg, it is, for example, 0.0005%, and 0.0010% is preferable.

The amount of REM is preferably 0.0050% or less, more preferably 0.0040% or less, and still more preferably 0.0030% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding REM, for example, it is 0.0005%, and 0.0010% is preferable.

((Co: 0.30% or less))

Co is preferably added because it spheroidizes the shape of the precipitate and increases the toughness of the spot weld zone. However, when the amount of Co is excessively large, hard martensite increases, thereby reducing the toughness of the spot weld zone and reducing the fatigue strength of the spot weld zone.

For this reason, the Co amount is preferably 0.30% or less, more preferably 0.20% or less, and still more preferably 0.10% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Co, it is, for example, 0.01%, and 0.02% is preferable.

((Ta: 0.10% or less))

Ta is preferably added because it spheroidizes the shape of the precipitate and increases the toughness of the spot weld zone. However, when the amount of Ta is excessively large, the toughness of the spot weld zone decreases due to the increase in coarse carbides, and the fatigue strength of the spot weld zone decreases.

For this reason, the Ta amount is preferably 0.10% or less, more preferably 0.08% or less, and still more preferably 0.06% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Ta, it is, for example, 0.01%, and 0.02% is preferable.

((As: 0.100% or less, Pb: 0.100% or less, Zn: 0.100% or less, and Bi: 0.100% or less))

As, Pb, Zn, and Bi are preferably added because they make the shape of the precipitate spherical and increase the toughness of the spot weld zone. However, when the amount of these elements is excessively large, a large amount of coarse precipitates or inclusions are generated, which reduces the toughness of the spot weld zone and reduces the fatigue strength of the spot weld zone.

For this reason, the As amount is preferably 0.100% or less, more preferably 0.050% or less, and still more preferably 0.010% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding As, for example, it is 0.001%, and 0.002% is preferable.

The amount of Pb is preferably 0.100% or less, more preferably 0.050% or less, and still more preferably 0.010% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Pb, it is, for example, 0.001%, and 0.002% is preferable.

The amount of Zn is preferably 0.100% or less, more preferably 0.050% or less, and still more preferably 0.010% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Zn, for example, it is 0.001%, and 0.002% is preferable.

The amount of Bi is preferably 0.100% or less, more preferably 0.050% or less, and still more preferably 0.010% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Bi, for example, it is 0.001%, and 0.002% is preferable.

((Hf: 0.10% or less))

Hf is preferably added because it spheroidizes the shape of the precipitate and increases the toughness of the spot weld zone. However, when the amount of Hf is excessively large, the toughness of the spot welded portion decreases due to the increase in coarse carbides, and the fatigue strength of the spot welded portion decreases.

For this reason, the amount of Hf is preferably 0.10% or less, more preferably 0.08% or less, and still more preferably 0.06% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Hf, it is, for example, 0.01%, and 0.02% is preferable.

When the component composition of the present invention contains the above-described optional element in an amount less than the above-mentioned lower limit, the optional element is contained as an unavoidable impurity.

《Micro Organization》

Next, the microstructure of the steel plate (hereinafter also referred to as the “microstructure of the present invention”) is explained.

In order to obtain the effect of the present invention, it is not enough to satisfy the component composition of the present invention described above, and it is necessary to satisfy the microstructure of the present invention described below.

Hereinafter, the area ratio is the area ratio with respect to the entire microstructure. The area ratio of each tissue is obtained by the method described in the Examples described later.

(Total area ratio of tempered martensite and bainite: 60 to 95%)

From the viewpoint of stably securing a tensile strength of 1320 MPa or more, the total area ratio of tempered martensite and bainite is 60% or more, preferably 65% or more, and more preferably 70% or more.

On the other hand, if this total area ratio is too high, the area ratio of retained austenite decreases, the ductility of the steel sheet decreases, and the formability decreases. For this reason, this total area ratio is 95% or less, preferably 92% or less, and more preferably 88% or less.

(Area ratio of retained austenite: 5 to 30%)

Retained austenite improves the ductility of steel sheets. For this reason, the area ratio of retained austenite is 5% or more, preferably 6% or more, and more preferably 8% or more.

On the other hand, if the area ratio of retained austenite is too high, the retained austenite that transforms to martensite when stressed increases, cracks occur in the heat-affected zone, and the fatigue strength of the spot weld zone decreases. For this reason, the area ratio of retained austenite is 30% or less, preferably 25% or less, and more preferably 20% or less.

(Area ratio of retained austenite with an aspect ratio of 5.5 or more: 50% or less)

When stress is repeatedly applied, the retained austenite is transformed into hard martensite by work hardening. If there is too much retained austenite with an aspect ratio of 5.5 or more, stress is concentrated at the tip of martensite after transformation, and voids are likely to occur. Additionally, since cracks easily occur around the nugget as voids are connected, the fatigue strength of the spot weld zone decreases.

Additionally, retained austenite with an aspect ratio of 5.5 or more is unstable, so if there is too much retained austenite, the TRIP (Transformation Induced plasticity) effect due to retained austenite cannot be obtained, and ductility decreases.

For this reason, the area ratio of retained austenite with an aspect ratio of 5.5 or more to the entire retained austenite is 50% or less, preferably 45% or less, and more preferably 40% or less.

On the other hand, the lower limit of the area ratio of retained austenite with an aspect ratio of 5.5 or more to all retained austenite is not particularly limited, and is, for example, 2%, and 4% is preferable.

(Area ratio of fresh martensite with a diameter of 2.0㎛ or less: 20% or less)

Fine fresh martensite improves the strength of steel sheets. To obtain this effect, the diameter of fresh martensite is set to 2.0 μm or less.

However, if there is too much fresh martensite with a diameter of 2.0 ㎛ or less, voids are generated due to differences in hardness of the structure, and formability is reduced. Additionally, since cracks easily occur around the nugget when voids are connected, the fatigue strength of the spot weld zone decreases. For this reason, the area ratio of fresh martensite with a diameter of 2.0 μm or less is 20% or less, preferably 17% or less, and more preferably 15% or less.

On the other hand, the lower limit of the area ratio of fresh martensite with a diameter of 2.0 μm or less is not particularly limited, and is, for example, 1%, and 3% is preferable.

The microstructure of the present invention includes structures (residual structures) other than tempered martensite, bainite, retained austenite, and fresh martensite, for example, pearlite; ferrite; iron-based carbonitride; alloy carbonitride; Inclusions such as MnS, Al ₂ O ₃ ; It may also include known organizations such as:

The area ratio of the remaining tissue is preferably 10% or less, more preferably 8% or less, and still more preferably 5% or less. If the area ratio of the remaining tissue is within this range, the effect of the present invention is not impaired.

《Amount of diffusible hydrogen in steel: 0.60 ppm by mass or less》

If the amount of diffusible hydrogen in the steel is too high, the spot weld zone is likely to break during welding, and the fatigue strength of the spot weld zone decreases. For this reason, the amount of diffusible hydrogen in steel is 0.60 ppm by mass or less, preferably 0.50 ppm by mass or less, and more preferably 0.40 ppm by mass or less.

The amount of diffusible hydrogen in steel is determined by the method described in the Examples described later.

《Decarburization layer》

When galvanized steel sheets are spot welded (resistance welded), the following points are of concern.

That is, during resistance welding, residual stress is generated near the spot weld zone, and in that state, the zinc in the galvanized layer melts and diffuses into the grain boundaries of the steel sheet, causing liquid metal embrittlement (LME). , grain boundary cracking (LME cracking) may occur in the steel sheet. If the steel sheet contains Si (especially if the amount of Si is large), LME cracking is likely to occur.

For this reason, galvanized steel sheets have resistance to such cracking (hereinafter referred to as “resistance weld cracking characteristics in spot welds”, “resistance weld cracking characteristics in welds”, or simply “resistance weld cracking characteristics”) There are cases where excellent performance is required.

Therefore, it is preferable that the steel sheet (underlying steel sheet) constituting the galvanized steel sheet has a decarburization layer, which is a layer with a low C concentration, in its outermost layer. Accordingly, the galvanized steel sheet has excellent resistance weld cracking resistance in the spot weld zone.

The reason is not clear, but it is thought that the decarburization layer is sticky due to the low C concentration, and therefore the above-mentioned cracks are unlikely to occur.

When the steel sheet has a decarburization layer, the thickness (depth in the sheet thickness direction) of the decarburization layer is, for example, 10 μm or more.

For the reason that the resistance weld cracking resistance in the spot weld zone is superior, the thickness of the decarburization layer (depth in the sheet thickness direction) is preferably 20 μm or more, more preferably 30 μm or more, and even more preferably 40 μm or more. do.

On the other hand, the upper limit of the thickness of the decarburization layer is not particularly limited. However, from the viewpoint of keeping the tensile strength of the steel plate within a good range, the thickness of the decarburization layer is preferably 130 μm or less, more preferably 100 μm or less, and more preferably 70 μm or less.

The decarburization layer (and its thickness) is calculated as follows.

First, the C concentration is measured in the direction of the sheet thickness from the interface between the zinc-plated layer and the steel sheet (if the galvanized steel sheet has a metal-plated layer described later, the interface between the zinc-plated layer and the metal-plated layer. The same applies hereinafter).

To measure C concentration, an Electron Probe Micro Analyzer (EPMA) is used.

Specifically, first, the resin-filled galvanized steel sheet is polished, and a cross section perpendicular to the rolling direction is finished for observation. After that, a galvanized steel sheet is taken out from the resin and used as a sample.

The acceleration voltage is set to 7 kV, the irradiation current is set to 50 mA, and the cross-section of the sample is analyzed in 1 μm steps in a range of 300 × 300 μm including the outermost layer of the steel plate, and the C intensity is measured by surface or line analysis. .

At this time, in order to suppress contamination, a plasma cleaner is used to remove hydrocarbon from the surface and surroundings of the sample in two locations, the measurement room and the sample preparation room, before starting the measurement.

In order to suppress the accumulation of hydrocarbon during measurement, the sample is heated on a stage in the measurement chamber, and measurement is performed while maintaining the temperature of the sample at a maximum of 100°C.

Due to the effect of suppressing contamination, it is confirmed that the lower detection limit of C is sufficiently lower than 0.10 mass%.

Details of the device used and the method for suppressing contamination are described in Reference 1 below.

Reference 1: Yamashita et al., “Carbon distribution in the early stage of pro-eutectoid ferrite transformation of low carbon steel by high-precision FE-EPMA”, Iron and Steel, Japan Iron and Steel Association, 2017, Volume 103, No. 11, p.14-20

However, since suppression of contamination depends on the type of device used, conditions, etc., it is not necessarily essential, and it is sufficient if the C intensity can be measured with sufficient precision.

Next, the measured C intensity is converted to C concentration (unit: mass%) to obtain a concentration map. For conversion, a calibration curve prepared in advance using standard samples is used.

From the obtained concentration map, a line profile in the plate thickness direction is extracted. By averaging the line profile at 300 points along the direction parallel to the surface of the steel sheet (direction perpendicular to the sheet thickness direction), a profile of the C concentration in the sheet thickness direction is obtained.

Smoothing processing using the simple moving average method is performed on the obtained C concentration profile in the plate thickness direction. The smoothing score is set to about 21 points.

In the profile after the smoothing process, the area where the C concentration is 80% by mass or less of the maximum value is set as the decarburization layer, and the distance in the sheet thickness direction of that area is set as the thickness of the decarburization layer.

For each sample, the average value of the measurement results of two fields of view is used.

<Zinc plating layer>

The zinc plating layer is formed by the zinc plating process described later.

The adhesion amount of the zinc plating layer is preferably 20 to 80 g/m2 per side.

<Metal plating layer>

The high-strength galvanized steel sheet of the present invention may further have a metal plating layer (a plating layer different from the zinc plating layer described above) on at least one side of the steel sheet and between the steel sheet and the zinc plating layer.

Accordingly, the resistance weld cracking resistance in the spot welded portion is excellent.

The reason is not clear, but it is thought that the metal plating layer suppresses the zinc (Zn) in the zinc plating layer from diffusing and invading the steel sheet during resistance welding (zinc intrusion suppression effect).

Examples of metals used in the metal plating layer include metals (Fe, Ni, etc.) with a higher melting point than Zn, but Fe is preferred because the following effects can be expected in addition to the zinc penetration suppression effect described above. do.

That is, the metal plating layer is preferably a metal plating layer (hereinafter also referred to as “Fe-based plating layer”) having a component composition consisting of Fe and inevitable impurities.

The component composition of the Fe-based plating layer further includes at least one selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co. You may contain 10 mass % or less of elements in total.

When there is a large amount of Si on the surface of a steel sheet, it is thought that the toughness decreases in the spot weld zone and the resistance weld cracking resistance tends to deteriorate.

In this case, by forming the Fe-based plating layer on the surface of the steel sheet, the Fe-based plating layer acts as a solid-solution Si-deficient layer, and as a result, the amount of Si dissolved in the spot weld zone is reduced, thereby suppressing the decline in toughness of the spot weld zone. It is thought to have superior resistance to weld cracking properties (toughness reduction suppression effect).

In addition, the Fe-based plating layer functions as a soft layer, relieves the stress applied to the surface of the steel sheet during spot welding, and reduces the residual stress in the spot weld zone, so it is thought to have superior resistance weld cracking resistance (stress palliative effect).

From the viewpoint of improving the resistance weld cracking resistance in the spot weld zone, it is preferable to form a metal plating layer such as an Fe-based plating layer on the surface of the steel sheet having the decarburization layer described above.

The adhesion amount of the metal plating layer per side is, for example, more than 0 g/m 2 , preferably 2.0 g/m 2 or more, more preferably 4.0 g/m 2 or more, and even more preferably 6.0 g/m 2 or more.

On the other hand, the upper limit is not particularly limited, but from the viewpoint of cost, the adhesion amount of the metal plating layer is preferably 60 g/m 2 or less, more preferably 50 g/m 2 or less, more preferably 40 g/m 2 or less, and 30 g per side from the viewpoint of cost. /㎡ or less is particularly preferable.

The adhesion amount of the metal plating layer is calculated as follows.

First, a test piece measuring 10 mm x 15 mm is taken from a galvanized steel sheet with a metal plating layer and embedded in resin to obtain an embedded sample with the cross section of the galvanized steel sheet exposed. Any three locations of this cross section are observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and at a magnification of 2,000 to 10,000 times depending on the thickness of the metal plating layer. The average value of the thickness of the metal plating layer in three views is multiplied by the specific gravity of the metal to convert it into the adhesion amount per side of the metal plating layer.

[Evaluation test of resistance weld cracking characteristics in spot weld zone]

A test method for evaluating the resistance weld cracking characteristics of a spot welded portion will be explained based on FIGS. 2 to 4.

Fig. 2 is a cross-sectional view showing the plate 5 used for resistance welding. Fig. 3 is a plan view showing the plate 5 after resistance welding. FIG. 4 is a cross-sectional view taken along line A-A of FIG. 3.

First, a test piece 4 is cut out from the galvanized steel sheet to be evaluated. The test piece 4 has the rolling direction (TD) in the longitudinal direction and the rolling direction in the short direction, and its size is the length in the long direction (L): 150 mm, and the width (W) in the short direction. : 50 mm, plate thickness (t): 1.6 mm.

Similarly, a counterpart test piece 3 of the same size is cut from another galvanized steel sheet.

The evaluation target surface (surface of the zinc plating layer) of the test piece 4 is brought into contact with the zinc plating layer of the other test piece 3 to obtain the plate 5.

The plate 5 is attached to the fixture 7 through a pair of steel plate spacers 6 (length in the long direction: 50 mm, length in the short direction: 45 mm, thickness (t _S ): 2.0 mm). Fix it. The spacer 6 is arranged so that its longitudinal end surface matches both short end surfaces of the plate 5. For this reason, the distance D between the pair of spacers 6 is 60 mm. The fixing stand 7 is a single plate with a hole 7a drilled in the center.

Next, using a single-phase alternating current (50 Hz) resistance welder using servomotor pressure, the plate 5 was bent while pressing it with a pair of electrodes 8 (tip diameter: 6 mm). In this state, resistance welding is performed.

More specifically, resistance welding is performed under predetermined conditions (pressure force, hold time, and welding time) with a welding current corresponding to a predetermined nugget diameter d to form a welded portion including the nugget 9.

The hold time refers to the time from when the welding current flows until the electrode 8 begins to open.

The nugget diameter d is the distance between the ends of the nugget 9 in the longitudinal direction of the plate 5.

During resistance welding, the pair of electrodes 8 presses the plate 5 from above and below in the vertical direction.

The lower electrode 8a presses the test piece 4 through the hole 7a of the fixture 7. During pressurization, the lower electrode 8a and the fixture 7 are in contact with the virtual plane S extending the surface where the spacer 6 and the fixture 7 are in contact. Fix it. The upper electrode 8b is movable at a position where it can contact the central part of the other test piece 3.

Resistance welding is performed with the plate 5 tilted 5° toward the longitudinal side of the plate 5 with respect to the horizontal direction (that is, with the angle θ with respect to the horizontal direction set to 5°).

The plate 5 after resistance welding is cut along line A-A in FIG. 3 so as to include the center of the welded portion including the nugget 9. The cross section of the welded portion is observed using an optical microscope (x200), and the resistance resistance weld cracking characteristics of the welded portion are evaluated.

In addition, FIG. 4 schematically shows a crack 10 that occurred in the test piece 4.

When cracks occur in the counterpart test piece 3, the stress in the test piece 4 is distributed, and appropriate evaluation cannot be obtained. For this reason, data showing no cracks occurring in the counterpart test piece 3 is adopted.

[Manufacturing method of high-strength galvanized steel sheet]

Next, the manufacturing method of the high-strength galvanized steel sheet of the present invention (hereinafter also referred to as the “manufacturing method of the present invention” for convenience) will be described. The manufacturing method of the present invention is also a method of manufacturing the high-strength galvanized steel sheet of the present invention described above.

Unless otherwise specified, the temperatures shown below when heating or cooling steel slabs, steel sheets (hot-rolled steel sheets, cold-rolled steel sheets), etc. mean their surface temperatures.

The method of melting the steel slab (steel material) is not particularly limited, and known melting methods such as a converter or electric furnace can be adopted. After melting, it is desirable to obtain the steel slab by continuous casting. However, the steel slab may be obtained using other known casting methods such as the ingot casting-slabbing rolling method and the thin slab continuous casting method.

In the manufacturing method of the present invention, first, a steel slab having the above-described component composition is hot rolled. Accordingly, a hot rolled steel sheet is obtained.

In hot rolling, the steel slab may be reheated in a heating furnace and then rolled. When the steel slab maintains a temperature above a predetermined temperature, direct rolling may be performed without heating the steel slab.

〈Hot Rolling〉

In hot rolling, rough rolling and finish rolling are performed on the steel slab.

Before rough rolling, it is preferable to heat the steel slab to dissolve carbides in the steel slab.

From the viewpoint of dissolving carbides and preventing an increase in rolling load, the temperature at which the steel slab is heated (steel slab heating temperature) is preferably 1100°C or higher, and more preferably 1150°C or higher.

On the other hand, from the viewpoint of preventing increase in scale loss, the steel slab heating temperature is preferably 1300°C or lower, and more preferably 1280°C or lower.

As described above, when the steel slab before rough rolling maintains a temperature above a predetermined temperature and the carbides in the steel slab are dissolved, heating of the steel slab before rough rolling can be omitted.

The conditions for rough rolling and finish rolling are not particularly limited, but for example, the finish rolling temperature is preferably 700 to 1,100°C, and more preferably 800 to 1,000°C.

《Winding temperature: 350∼700℃》

Next, the hot rolled steel sheet obtained by hot rolling the steel slab is wound.

If the temperature at which the hot-rolled steel sheet is wound (coiling temperature) is too low, hard martensite with a high carbon concentration is generated, and thus coarse fresh martensite increases after heat treatment. Additionally, when cold rolling is performed on a hard hot-rolled steel sheet mainly composed of martensite, the fatigue strength of the spot weld zone decreases.

For this reason, the coiling temperature is 350°C or higher, preferably 400°C or higher, and more preferably 450°C or higher.

On the other hand, if the coiling temperature is too high, ferrite and pearlite are excessively generated in the microstructure of the hot rolled steel sheet, and the nucleation sites of austenite during heat treatment are reduced (i.e., the total area ratio of tempered martensite and bainite is reduced). decreases), it becomes difficult to secure the desired strength after heat treatment.

For this reason, the coiling temperature is 700°C or lower, preferably 650°C or lower, and more preferably 600°C or lower.

〈Cold Rolling〉

Next, cold rolling is performed on the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet.

The rolling reduction rate of cold rolling is preferably 30% or more, and more preferably 35% or more. The upper limit is not particularly limited and is, for example, 70% or less, preferably 65% or less.

〈Metal plating treatment〉

A metal plating treatment may be performed on a cold rolled steel sheet obtained by cold rolling before performing the first heat treatment described later. Accordingly, the above-mentioned metal plating layer is formed on at least one side of the cold rolled steel sheet.

As the metal plating layer to be formed, the Fe-based plating layer described above is preferable.

The metal plating treatment is not particularly limited, but electroplating is preferable from the viewpoint of manufacturability. Examples of metal plating baths used in electroplating include sulfuric acid baths, hydrochloric acid baths, and baths containing a mixture of both. When performing electroplating treatment, the adhesion amount of the metal plating layer formed can be adjusted by the current application time, etc.

When forming an Fe-based plating layer by metal plating treatment, an Fe-based plating bath is used.

The Fe-based plating bath is, for example, Fe and at least one selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co. Contains one element. The content of these elements is appropriately adjusted depending on the component composition of the Fe-based plating layer to be formed.

In the Fe-based plating bath, metal elements may be contained as metal ions, and non-metal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acid, etc.

When using an iron sulfate plating bath as an Fe-based plating bath, conductive additives such as sodium sulfate and potassium sulfate; Chelating agent; pH buffering agent; It may additionally contain the like.

The cold rolled steel sheet before metal plating treatment may be subjected to degreasing and water washing in order to clean the surface of the cold rolled steel sheet. Additionally, in order to activate the surface of the cold rolled steel sheet, acid washing and water washing may be performed.

The method of degreasing and water washing is not particularly limited, and conventionally known methods can be adopted.

For acid washing, various acid solutions such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used, and among these, sulfuric acid, hydrochloric acid, or mixtures thereof are preferable. The concentration of the acid solution is not particularly limited, but is preferably 1 to 20% by mass, considering the ability to remove the oxide film and prevention of surface roughening (surface defects) due to peracid cleaning, etc. A defoaming agent, an acid cleaning accelerator, an acid cleaning inhibitor, etc. may be added to the acid solution.

Next, the first heat treatment, zinc plating treatment, and second heat treatment described below are performed in this order on the cold rolled steel sheet obtained by cold rolling (or the cold rolled steel sheet on which metal plating treatment has been performed).

〈First heat treatment〉

In the first heat treatment, the cold-rolled steel sheet is heated to a heating temperature (T3) described later, and then cooled to a cooling stop temperature (T4) described later. This cooling includes cooling at an average cooling rate (v1) described later.

《Heating temperature (T3): 750∼950℃》

If the heating temperature (T3) is too low, heating occurs in the two-phase region of ferrite and austenite, the final microstructure contains ferrite, and the total area ratio of tempered martensite and bainite decreases, so that the desired It becomes difficult to secure the desired tensile strength. For this reason, the heating temperature (T3) is 750°C or higher, preferably 800°C or higher, and more preferably 830°C or higher.

On the other hand, if the heating temperature T3 is too high, the amount of hydrogen penetrating into the steel increases due to an increase in the hydrogen partial pressure, and thus the amount of diffusible hydrogen in the steel increases. For this reason, the heating temperature is 950°C or lower, preferably 930°C or lower, and more preferably 900°C or lower.

The time (heating time) for holding the cold-rolled steel sheet at the heating temperature (T3) is not particularly limited and is, for example, 10 to 500 seconds, preferably 50 to 300 seconds, and more preferably 80 to 200 seconds.

《Dew point: exceeding -30℃》

The dew point of the atmosphere when heating the cold rolled steel sheet at the heating temperature (T3) may be greater than -30°C.

Accordingly, the decarburization reaction of the cold rolled steel sheet is promoted, and the C concentration in the outermost layer of the cold rolled steel sheet can be reduced. That is, the decarburization layer described above can be formed. In this case, the galvanized steel sheet finally obtained has excellent resistance weld cracking resistance in the spot weld zone.

Since the thickness of the formed decarburization layer increases and the resistance weld cracking resistance in the spot weld zone is improved, the dew point is preferably -20°C or higher, and more preferably -5°C or higher.

The upper limit of the dew point is not particularly limited. However, from the viewpoint of suppressing oxidation of the surface of the cold rolled steel sheet and improving the adhesion of the zinc plating layer formed by the zinc plating treatment described later, the dew point is preferably 40°C or lower, and more preferably 30°C or lower. 20°C or lower is more preferable.

《Average cooling rate (v1): 10℃/s or more》

Next, the cold rolled steel sheet heated at the heating temperature T3 is cooled to the cooling stop temperature T4 described later. Here, the average cooling rate from the heating temperature (T3) to 550°C is taken as v1.

If the average cooling rate (v1) is too low, ferrite transformation occurs during cooling and the total area ratio of tempered martensite and bainite decreases, making it difficult to secure the desired tensile strength. For this reason, the average cooling rate (v1) is 10°C/s or more, preferably 11°C/s or more, and more preferably 13°C/s or more.

The upper limit of the average cooling rate (v1) is not particularly limited, and is, for example, 45°C/s, and 30°C/s is preferable.

《Cooling stop temperature (T4): 350∼550℃》

If the cooling stop temperature (T4) is too low, the total area ratio of tempered martensite and bainite becomes too high, the area ratio of retained austenite decreases, the ductility of the steel sheet (cold rolled steel sheet) decreases, and the formability decreases. do. For this reason, the cooling stop temperature (T4) is 350°C or higher, preferably 370°C or higher, and more preferably 390°C or higher.

On the other hand, if the cooling stop temperature (T4) is too high, pearlite is generated, the area ratio of retained austenite decreases, the ductility of the steel sheet (cold rolled steel sheet) decreases, and the formability decreases. For this reason, the cooling stop temperature (T4) is 550°C or lower, preferably 530°C or lower, and more preferably 510°C or lower.

〈Zinc plating treatment〉

Next, a galvanizing layer is formed on the surface of the cold-rolled steel sheet (hereinafter, simply referred to as “cold-rolled steel sheet”) to which the first heat treatment has been applied.

As the zinc plating treatment, hot-dip galvanizing treatment or alloyed hot-dip galvanizing treatment is preferable.

When performing zinc plating treatment, an apparatus configured to continuously perform heat treatment (first heat treatment and second heat treatment) and zinc plating treatment may be used.

Hereinafter, suitable conditions for galvanizing treatment will be explained.

However, in the manufacturing method of the present invention, it is necessary that the holding time (t ₁ ) and holding time (t ₂ ) described later satisfy the conditions described later.

When performing hot-dip galvanizing treatment, for example, a cold-rolled steel sheet is immersed in a zinc bath with a bath temperature of 440 to 500°C. After that, it is desirable to adjust the adhesion amount of the zinc plating layer (hot-dip galvanizing layer) by gas wiping or the like.

As a zinc bath, a zinc bath having an Al content of 0.10 to 0.23% by mass and the balance consisting of Zn and inevitable impurities is preferable.

When performing alloying hot-dip galvanizing treatment, if the alloying temperature is too low, the Zn-Fe alloying speed may be excessively slow and alloying may become significantly difficult. On the other hand, if the alloying temperature is too high, untransformed austenite may transform into pearlite, and tensile strength and ductility may decrease. For this reason, the alloying temperature is preferably 450 to 600°C, more preferably 470 to 550°C, and even more preferably 470 to 530°C.

The adhesion amount of the zinc plating layer on hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) is preferably 20 to 80 g/m 2 per side.

〈Second heat treatment〉

In the second heat treatment, the cold-rolled steel sheet (hereinafter, simply referred to as “cold-rolled steel sheet”) to which galvanizing treatment has been applied is cooled to a cooling stop temperature (T5) described later, and then reheated to a reheating temperature (T6) described later, Thereafter, cool to at least 50°C. This cooling (hereinafter also referred to as “re-cooling”) includes cooling at an average cooling rate (v2) described later.

《Cooling stop temperature (T5): 50∼350℃》

If the cooling stop temperature (T5) is too low, the area ratio of retained austenite decreases, the ductility of the steel sheet (cold rolled steel sheet) decreases, and the formability decreases. For this reason, the cooling stop temperature (T5) is 50°C or higher, preferably 100°C or higher, and more preferably 120°C or higher.

On the other hand, if the cooling stop temperature (T5) is too high, the area ratio of fresh martensite with a diameter of 2.0 μm or less becomes too high. For this reason, the cooling stop temperature (T5) is preferably 350°C or lower, and more preferably 300°C or lower.

In addition, if the cold-rolled steel sheet has been cooled to the cooling stop temperature (T5), the temperature may be immediately increased to the reheating temperature (T6) described later, without maintaining the cold rolled steel sheet at the cooling stop temperature (T5). Alternatively, after maintaining the cooling stop temperature (T5) for a certain period of time, the temperature may be raised to the reheating temperature (T6) described later.

《Reheating temperature (T6): exceeds cooling stop temperature (T5) and is also 300-500℃》

The reheating temperature (T6) exceeds the cooling stop temperature (T5) and is a temperature within the range of 300 to 500°C. Accordingly, distribution of carbon from martensite formed when cooled to the cooling stop temperature (T5) to untransformed austenite is promoted, and a desired area ratio is obtained with respect to retained austenite.

《Average cooling rate (v2): v2≤3.8[C]+2.4[Mn]+1.2[Si]》

In cooling (recooling) from the reheating temperature (T6) to at least 50°C, the average cooling rate that satisfies the following equation (1) from (Ms point - 200)°C to 50°C is taken as v2.

In the manufacturing method of the present invention, the average cooling rate (v2) satisfies the following equation (1).

v2≤3.8[C]+2.4[Mn]+1.2[Si]… (One)

[C], [Mn], and [Si] in the above formula (1) are the contents (unit: mass%) of C, Mn, and Si in the above-mentioned component composition (component composition of the present invention), respectively.

When the average cooling rate (v2) satisfies the above equation (1), the martensite generated during re-cooling is self-tempered, and fine carbides are generated in the martensite. As a result, in the ultimately obtained microstructure, the area ratio of fresh martensite with a diameter of 2.0 μm or less can be reduced.

The lower limit of the average cooling rate (v2) is not particularly limited, and is, for example, 1°C/s, preferably 2°C/s.

Additionally, the Ms point (unit: °C) is obtained from the following equation (3).

Ms＝550－350×[C]－40×[Mn]－35×[V]－20×[Cr]－17×[Ni]－10×[Cu]－10×[Mo]－5×[W ]+15×[Co]+30×[Al]… (3)

In the above formula (3), [X] is the content (unit: mass%) of element X in the above-mentioned component composition (component composition of the present invention).

〈Relationship between holding time (t ₁ ) and holding time (t ₂ ): 1.1≤3t ₁ /t ₂ ≤6.5>

1 is a chart showing an example of the above-described first heat treatment, galvanizing treatment, and second heat treatment. In Figure 1, the above-described heating temperature (T3), cooling-stop temperature (T4), cooling-stop temperature (T5), and reheating temperature (T6) are shown.

In addition, in Figure 1, a temperature range (T1) of 300°C or more and less than 450°C and a temperature range (T2) of 450°C or more and 600°C or less are shown.

Here, the holding time (residence time) of the cold rolled steel sheet in the temperature range T1 is set to _t1 . The holding time (t ₁ ) is indicated by diagonal hatching going from the upper right to the lower left in FIG. 1 .

Additionally, the holding time (residence time) of the cold rolled steel sheet in the temperature range T2 is set to _t2 . The holding time (t ₂ ) is indicated by diagonal hatching going from the upper left to the lower right in FIG. 1 .

In the manufacturing method of the present invention, the holding time (t ₁ ) and the holding time (t ₂ ) satisfy the following equation (2).

1.1≤3t ₁ /t ₂ ≤6.5 … (2)

By setting the value of 3t ₁ /t ₂ to a certain value or more, bainite transformation progresses, carbon concentration in the retained austenite progresses, and the amount of retained austenite with a small aspect ratio and good stability increases. In other words, the area ratio of retained austenite with an aspect ratio of 5.5 or more can be reduced.

Specifically, the value of 3t ₁ /t ₂ is 1.1 or more, preferably 1.3 or more, and more preferably 1.5 or more.

On the other hand, if the value of 3t ₁ /t ₂ is too high, the bainite transformation progresses too much, and the total area ratio of tempered martensite and bainite becomes too high.

For this reason, the value of 3t ₁ /t ₂ is 6.5 or less, preferably 6.3 or less, and more preferably 6.1 or less.

The holding time (t ₁ ) in the temperature range (T1) is not particularly limited as long as the value of 3t ₁ /t ₂ satisfies the above equation (2), but is, for example, 10 to 110 seconds, and 20 to 110 seconds. 100 seconds is preferable, and 30 to 90 seconds is more preferable.

The holding time (t ₂ ) in the temperature range (T2) is also not particularly limited as long as the value of 3t ₁ /t ₂ satisfies the above equation (2), but is, for example, 10 to 100 seconds, and 20 to 20 seconds. 90 seconds is preferable, and 30 to 80 seconds is more preferable.

In the above-described first heat treatment, galvanizing treatment, and second heat treatment, the time for maintaining the cold rolled steel sheet at each temperature (cooling stop temperature (T4), reheating temperature (T6), etc.) is the value of 3t ₁ /t ₂ There is no particular limitation as long as the above formula (2) is satisfied.

In the manufacturing method of the present invention described above, for example, the holding temperature, such as heating temperature or reheating temperature, does not need to be constant as long as it is within the temperature range described above. The cooling rate may vary during cooling as long as it is within the above-mentioned rate range. Heat treatment may be performed using any equipment as long as the conditions such as the above-mentioned temperature range are met.

[Member and its manufacturing method]

Next, a member (hereinafter also referred to as “member of the present invention”) made using the high-strength galvanized steel sheet of the present invention described above will be described.

The member of the present invention is obtained by subjecting the high-strength galvanized steel sheet of the present invention to at least one of forming processing and joining processing.

The molding process is not particularly limited, and examples include general molding processing such as press processing.

The joining process is not particularly limited, and examples include welding such as spot welding, laser welding, and arc welding; riveted joints; caulking joints; etc. can be mentioned.

The conditions for molding processing and joining processing are not particularly limited, and commercial methods may be used.

(Example)

Below, the present invention will be described in detail through examples. However, the present invention is not limited to the embodiments described below.

[Test Example 1]

〈Manufacture of galvanized steel sheets〉

Molten steel with the component composition shown in Tables 1 and 2 below, the balance being Fe and inevitable impurities, was melted in a converter, and a steel slab was obtained by a continuous casting method. In addition, Table 2 below is a continuation of Table 1 below, and underlines in Tables 1 to 2 below mean outside the scope of the present invention (the same applies to other tables).

The obtained steel slab was hot rolled under the conditions shown in Table 3 below to obtain a hot rolled steel sheet. Specifically, the steel slab was heated to 1250°C and rough rolled. Next, finish rolling was performed at a finish rolling temperature of 900°C, and coiling was performed at a coiling temperature shown in Table 3 below.

A cold rolled steel sheet was obtained by cold rolling the coiled hot rolled steel sheet at a rolling rate shown in Table 3 below.

The obtained cold-rolled steel sheet was subjected to first heat treatment, galvanization treatment, and second heat treatment under the conditions shown in Table 3 below.

In addition, in any example, at the heating temperature T3, the cold rolled steel sheet was held for 100 seconds in an atmosphere with a dew point of -30°C or higher.

In addition, in any example, after the cold rolled steel sheet was cooled to the cooling stop temperature (T5), the temperature was immediately raised to the reheating temperature (T6) without maintaining the cooling stop temperature (T5).

In the galvanizing treatment, hot-dip galvanizing or alloyed hot-dip galvanizing is performed on both sides of the cold-rolled steel sheet after the first heat treatment to obtain a hot-dip galvanized steel sheet (GI) or alloyed hot-dip galvanized steel sheet (GA).

As a hot dip galvanizing bath, when producing GI, a zinc bath containing Al: 0.20% by mass, with the balance being Zn and inevitable impurities, is used, and when producing GA, a zinc bath containing Al: 0.14% by mass is used. A zinc bath containing Zn and the remainder consisting of Zn and inevitable impurities was used.

The bath temperature was set at 470°C even when producing either GI or GA.

The adhesion amount of the zinc plating layer was set to 45 to 72 g/m 2 per side when producing GI, and 45 g/m 2 per side when producing GA.

When producing GA, the alloying temperature was set to 530°C.

The composition of the GI zinc plating layer contained 0.1 to 1.0 mass% of Fe and 0.2 to 1.0 mass% of Al, with the remainder being Fe and inevitable impurities. The composition of the galvanized layer of GA contained 7 to 15% by mass of Fe and 0.1 to 1.0% by mass of Al, with the remainder being Fe and inevitable impurities.

Hereinafter, the galvanized steel sheet (hot-dip galvanized steel sheet (GI) or alloyed hot-dip galvanized steel sheet (GA)) after the second heat treatment is also simply referred to as “galvanized steel sheet.”

〈Observation of micro-organization〉

Regarding the obtained galvanized steel sheet, the microstructure of the steel sheet was observed as follows. The results are shown in Table 4 below.

《Area ratio of tempered martensite, bainite and fresh martensite》

The obtained galvanized steel sheet was polished so that the cross section parallel to the rolling direction (L cross section) became the observation surface. The observation surface was corroded using 1% by volume Nital, and then observed at 3000 times magnification using a scanning electron microscope (SEM). The L cross section at a position corresponding to 1/4 of the plate thickness from the surface of the steel plate was observed at 10 views and an SEM image was acquired.

For the acquired SEM images, the area ratio of each tissue was determined, and the average area ratio of 10 views was taken as the area ratio of each tissue. For analysis of SEM images, Image-Pro, manufactured by Media Cybernetics, was used as analysis software.

In the SEM image of each field of view, the dark gray portion was determined to be tempered martensite and bainite, and the total area ratio (unit: %) of both was determined.

Similarly, in the SEM images of each field of view, white or light gray portions were determined to be fresh martensite and retained austenite.

From there, retained austenite with a face-centered cubic structure (fcc) was excluded. Specifically, EBSD (electron beam backscattering diffraction) data from the same field of view was acquired, and based on this data, excluding the fcc structure, the remaining structures were determined to be fresh martensite.

Afterwards, in the SEM image, the area of each structure determined to be fresh martensite was determined. From the calculated area, the equivalent circle diameter of each fresh martensite was determined, and this was taken as the diameter of each fresh martensite. In this way, the area ratio (unit: %) of fresh martensite with a diameter of 2.0 μm or less was determined.

《Area ratio of retained austenite》

The observation surface (L cross section at a position corresponding to 1/4 of the plate thickness) was observed using an X-ray diffraction method. As the incident X-ray, a Kα ray source of Co was used. The ratio of the diffraction intensity of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of the (200), (211), and (220) planes of bcc iron was determined. The value obtained by averaging the nine ratios determined was taken as the volume fraction of retained austenite.

The obtained volume ratio was regarded as the area ratio of retained austenite (unit: %).

《Area ratio of retained austenite with an aspect ratio of 5.5 or more》

For the observation surface (L cross section at a position corresponding to 1/4 of the plate thickness), EBSD data (phase map data) was acquired, the fcc structure was specified, and this was used as retained austenite particles. An area of 30 μm × 30 μm on the observation surface was considered one field of view, and data was acquired for 10 fields of view that were 50 μm or more apart from each other.

For each grain of retained austenite, the longest grain length was called the major axis length a, the grain length when crossing the grain the longest in the direction perpendicular to this was called the minor axis length b, and a/b was taken as the aspect ratio. When multiple particles were in contact with each other, they were divided approximately equally and regarded as individual particles.

The area of retained austenite with an aspect ratio (a/b) of 5.5 or more was determined, and the ratio to the entire area of retained austenite in the same field of view was determined. The average value of the 10 views of the obtained ratio was taken as the area ratio (unit: %) of retained austenite with an aspect ratio of 5.5 or more to the entire retained austenite in each steel sheet.

<evaluation>

The obtained galvanized steel sheet was evaluated by the following method. The results are shown in Table 4 below.

《Tensile test》

From the obtained galvanized steel sheet, a No. 5 test piece described in JIS Z 2201 with the longitudinal direction (tensile direction) at 90° from the rolling direction was taken. Using the collected test pieces, a tensile test based on JIS Z 2241 was performed five times, and the tensile strength (TS) and butt elongation (EL) were determined from the average value of the five tests. If TS is 1320 MPa or more, it can be evaluated as high strength. If EL is 10.0% or more, moldability can be evaluated as excellent.

《Hole expansion test》

A hole expansion test was performed on the obtained galvanized steel sheet in accordance with JIS Z 2256.

Specifically, first, the obtained galvanized steel sheet was sheared, and a test piece with a size of 100 mm x 100 mm was taken. A hole with a diameter of 10 mm was punched into the collected test piece with a clearance of 12.5%. Afterwards, using a die with an inner diameter of 75 mm, a cone punch with an apex angle of 60° is pushed into the hole while the wrinkle pressing force is suppressed to 9 tons (88.26 kN), and the hole diameter (D _f ) at the limit of crack occurrence is determined. (Unit: mm) was measured. The initial hole diameter was set to D ₀ (unit: mm), and the hole expansion rate (λ) (unit: %) was obtained from the following equation. If λ is 20% or more, the moldability can be evaluated as excellent.

λ={(D _f -D ₀ )/D ₀ }×100

《Measurement of the amount of diffusible hydrogen in steel plate》

From the obtained galvanized steel sheet, the galvanized layer was removed using a router (precision grinder), and then a test piece with a length of 30 mm and a width of 5 mm was taken. For the collected test pieces, the amount of diffusible hydrogen in the steel sheet was measured using a temperature-elevated desorption analysis method. The temperature increase rate was 200°C/hr. The cumulative value of the amount of hydrogen detected in the temperature range from room temperature (25°C) to less than 210°C was taken as the amount of diffusible hydrogen in steel (unit: mass ppm). The amount of diffusible hydrogen in the steel sheet is preferably 0.60 ppm by mass or less.

《Fatigue test of spot welds》

Using the obtained galvanized steel sheet, a cross tensile test piece with a spot weld was produced based on JIS Z 3138, and a fatigue test was performed.

First, spot welding was performed under the conditions of electrode: DR6mm-40R, pressing force: 4802N (490kgf), and energization time: 17 cycles, and the current value was adjusted so that the nugget diameter was 6.5mm to produce a cross tensile test piece.

A load was applied under the conditions of a minimum and maximum load ratio of 0.05, a frequency of 20 Hz, and a number of repetitions of 10 ⁷ , and then a cross tensile test was performed at a tensile speed of 5 mm/min. The fatigue strength of the spot welded portion was evaluated from the maximum cross tensile strength at which peeling of the test piece did not occur.

Specifically, when the cross tensile strength was 250N or more, “A” was listed, when it was 180N or more but less than 250N, “B” was listed, and when it was less than 180N, “C” was listed in Table 4 below. If it is “A” or “B”, it can be evaluated that the fatigue strength of the spot welded portion is excellent.

〈Summary of evaluation results〉

As shown in Tables 1 to 4 above, the galvanized steel sheets Nos. 1 to 3, 5 to 6, 9, 14 to 16, 19, 27 and 32 to 41 all have a tensile strength of 1320 MPa or more, In addition, the formability and fatigue strength of the spot weld zone were excellent.

In contrast, the galvanized steel sheets No. 4, 7 to 8, 10 to 13, 17 to 18, 20 to 26, and 28 to 31 were insufficient in at least tensile strength, formability, and fatigue strength of the spot weld zone.

Furthermore, forming or joining processing was performed on the galvanized steel sheets No. 1 to 3, 5 to 6, 9, 14 to 16, 19, 27, and 32 to 41 to obtain members.

All of the obtained members had a tensile strength of 1320 MPa or more, and were also excellent in formability and fatigue strength of the spot weld zone.

[Test Example 2]

〈Manufacture of galvanized steel sheets〉

Steel slabs with steel symbols shown in Table 5 below (see Tables 1 to 2 above) were hot rolled under the conditions shown in Table 5 below to obtain hot rolled steel sheets. Specifically, the steel slab was heated to 1250°C and rough rolled. Next, finish rolling was performed at a finish rolling temperature of 900°C, and coiling was performed at a coiling temperature shown in Table 5 below.

A cold rolled steel sheet was obtained by cold rolling the coiled hot rolled steel sheet at a rolling rate shown in Table 5 below.

In some examples, before performing the first heat treatment described later on the obtained cold rolled steel sheet, electroplating was performed using an Fe-based plating bath to form an Fe-based plating layer.

When electroplating treatment was performed, “Yes” was indicated, and when electroplating treatment was not performed, “-” was indicated in Table 5 below. The adhesion amount of the formed Fe-based plating layer is shown in Table 6 below.

Additionally, before performing the electroplating treatment, the surface of the cold rolled steel sheet was degreased using alkali.

As the Fe-based plating bath, a sulfuric acid bath (bath temperature: 50°C, pH: 2.0) containing 1.5 mol/L of Fe ²⁺ ions was used.

In an Fe-based plating bath, electrolytic treatment was performed using a cold-rolled steel sheet as a cathode and an iridium oxide electrode as an anode at a current density of 45 A/dm2. The adhesion amount of the Fe-based plating layer was controlled by adjusting the current application time.

The obtained cold-rolled steel sheet (or electroplated cold-rolled steel sheet) was subjected to the first heat treatment, galvanization treatment, and second heat treatment under the conditions shown in Table 5 below.

In addition, in both examples, at the heating temperature (T3), the cold rolled steel sheet was maintained for 100 seconds in an atmosphere with a dew point shown in Table 5 below.

In addition, in any example, after the cold rolled steel sheet was cooled to the cooling stop temperature (T5), the temperature was immediately raised to the reheating temperature (T6) without maintaining the cooling stop temperature (T5).

In the galvanizing treatment, alloyed hot-dip galvanized steel sheet (GA) was obtained by performing alloyed hot-dip galvanized steel sheet on both sides of the cold-rolled steel sheet after the first heat treatment.

As a hot-dip galvanizing bath, a zinc bath (bath temperature: 470°C) containing Al: 0.14% by mass and the balance consisting of Zn and inevitable impurities was used.

The adhesion amount of the zinc plating layer was 45 g/m2 per side. The alloying temperature was 530°C.

The composition of the galvanized layer of GA contained 7 to 15% by mass of Fe and 0.1 to 1.0% by mass of Al, with the remainder being Fe and inevitable impurities.

Hereinafter, the galvanized steel sheet (alloyed hot-dip galvanized steel sheet (GA)) after the second heat treatment is also simply referred to as “galvanized steel sheet.”

〈Observation of micro-organization〉

Regarding the obtained galvanized steel sheet, the microstructure of the steel sheet was observed in the same manner as in [Test Example 1] above. The results are shown in Table 6 below.

<Measurement of the thickness of the decarburization layer>

For the obtained galvanized steel sheet, the thickness of the decarburization layer was measured according to the method described above. The results are shown in Table 6 below.

<evaluation>

For the obtained galvanized steel sheet, a tensile test and a hole expansion test were performed in the same manner as in [Test Example 1] above, and TS, EL, and λ were determined.

In addition, in the same manner as in [Test Example 1] above, the amount of hydrogen diffused in the steel plate was determined. At this time, when the zinc plating layer had an Fe-based plating layer, not only the zinc plating layer but also the Fe-based plating layer was removed, and then the amount of diffused hydrogen in the steel sheet was measured.

Additionally, in the same manner as in [Test Example 1] above, a fatigue test was conducted to evaluate the fatigue strength of the spot welded portion.

All results are shown in Table 6 below.

《Resistance weld cracking characteristics in spot welds》

A test piece was cut out from the obtained galvanized steel sheet, and the resistance weld cracking resistance in the spot welded portion was evaluated according to the test method explained based on FIGS. 2 to 4.

The other test piece was cut from an alloyed hot-dip galvanized steel sheet for testing (sheet thickness (t): 1.6 mm) with a tensile strength of 980 MPa and an adhesion amount of the zinc plating layer of 50 g/m2.

With the angle (θ) set to 5°, the test piece and the counterpart test piece are welded with a pressing force of 3.5 kN, a hold time of 0.12 seconds, 0.18 seconds or 0.24 seconds, and a nugget diameter (d) of 5.9 mm. Resistance welding was performed under the conditions of current and welding time to form a welded portion.

The cross section of the welded portion was observed, and the resistance resistance weld cracking characteristics of the welded portion were evaluated based on the following criteria.

If it was A+, A or B, it was judged that the resistance weld cracking resistance in the weld zone was excellent. The results are shown in Table 6 below.

A+: No cracks with a length of 0.1 mm or more were confirmed at a hold time of 0.12 seconds.

A: Cracks longer than 0.1 mm were confirmed at a hold time of 0.12 seconds, but no cracks longer than 0.1 mm were confirmed at a hold time of 0.18 seconds.

B: Cracks with a length of 0.1 mm or more were confirmed at a hold time of 0.18 seconds, but no cracks with a length of 0.1 mm or more were confirmed with a hold time of 0.24 seconds.

C: A crack with a length of 0.1 mm or more was confirmed at a hold time of 0.24 seconds.

〈Summary of evaluation results〉

As shown in Tables 5 to 6 above, the galvanized steel sheets Nos. 1, 15, and 42 to 46 all had tensile strengths of 1320 MPa or more, and were also excellent in formability and fatigue strength of spot welds.

Furthermore, forming or joining processing was performed on the galvanized steel sheets No. 1, 15, and 42 to 46 to obtain members.

All of the obtained members had a tensile strength of 1320 MPa or more, and were also excellent in formability and fatigue strength of the spot weld zone.

Next, compare No. 1 and 42 to 45 using the strong symbol A.

Nos. 42 to 43, which had a decarburization layer (but did not have an Fe-based plating layer), had superior resistance weld cracking resistance than No. 1, which did not have a decarburization layer and an Fe-based plating layer.

In addition, No. 45, which has both a decarburization layer and an Fe-based plating layer, was even more excellent in resistance weld cracking resistance than Nos. 42 to 43.

In addition, No. 44 has both a decarburization layer and an Fe-based plating layer, but since the thickness of the decarburization layer is small, it is presumed that the resistance weld cracking resistance was equivalent to that of Nos. 42 to 43.

Next, compare No.15 and 46 using the strong symbol E.

No. 46, which has both a decarburization layer and an Fe-based plating layer, had superior resistance weld cracking resistance than No. 15, which did not have a decarburization layer and an Fe-based plating layer.

The members obtained by subjecting galvanized steel sheets No. 1 and No. 42 to No. 46 to forming or joining processing had excellent resistance weld cracking resistance.

Claims (15)

A high-strength galvanized steel sheet having a steel sheet and a galvanized layer and having a tensile strength of 1320 MPa or more,
The steel plate,
In mass%,
C: 0.150 to 0.450%,
Si: 0.80 to 3.00%,
Mn: 2.00 to 4.00%,
P: 0.100% or less,
S: 0.0200% or less,
Al: 0.100% or less,
O: 0.0100% or less, and
N: Contains 0.0100% or less,
It has a component composition, the balance of which is Fe and inevitable impurities, and a microstructure,
The amount of diffusible hydrogen in the steel is 0.60 ppm by mass or less,
In the micro-organization,
The total area ratio of tempered martensite and bainite is 60 to 95%,
The area ratio of retained austenite is 5 to 30%,
The area ratio of the retained austenite having an aspect ratio of 5.5 or more to the entire retained austenite is 50% or less,
A high-strength galvanized steel sheet with an area ratio of fresh martensite with a diameter of 2.0 μm or less of 20% or less. According to paragraph 1,
The above component composition is further expressed in mass%,
B: 0.0050% or less,
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.500% or less,
W: 0.500% or less,
Mo: 1.000% or less,
Cr: 1.000% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Zr: 0.1000% or less,
Cu: 1.000% or less,
Ni: 1.000% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
REM: 0.0050% or less,
Co: 0.30% or less,
Ta: 0.10% or less,
As: 0.100% or less,
Pb: 0.100% or less,
Zn: 0.100% or less,
Bi: 0.100% or less, and
Hf: A high-strength galvanized steel sheet containing at least one element selected from the group consisting of 0.10% or less. According to claim 1 or 2,
The steel sheet is a high-strength galvanized steel sheet having a decarburization layer. According to any one of claims 1 to 3,
A high-strength galvanized steel sheet having a metal plating layer on at least one side of the steel sheet and between the steel sheet and the zinc plating layer. According to paragraph 4,
A high-strength galvanized steel sheet in which the chemical composition of the metal plating layer consists of Fe and inevitable impurities. According to clause 5,
The component composition of the metal plating layer further includes at least one selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co. A high-strength galvanized steel sheet containing a total of 10% by mass or less of the element. According to any one of claims 1 to 6,
High-strength galvanized steel, which is either hot-dip galvanized steel or alloyed hot-dip galvanized steel. A member formed using the high-strength galvanized steel sheet according to any one of claims 1 to 7. A method for manufacturing the high-strength galvanized steel sheet according to claim 1 or 2, comprising:
Hot rolling a steel slab having the component composition according to claim 1 or 2, and winding the obtained hot rolled steel sheet at a coiling temperature of 350 to 700°C,
Cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet,
A first heat treatment, a galvanizing treatment, and a second heat treatment are performed on the cold rolled steel sheet in this order,
In the first heat treatment, the cold rolled steel sheet is heated at a heating temperature (T3) of 750 to 950°C, and cooled from the heating temperature (T3) to a cooling stop temperature (T4) of 350°C to 550°C, provided that, The average cooling rate (v1) from the heating temperature (T3) to 550°C is 10°C/s or more,
In the second heat treatment, the cold rolled steel sheet is cooled to a cooling stop temperature (T5) of 50 to 350°C, and then reheated to a reheating temperature (T6) that exceeds the cooling stop temperature (T5) and is 300 to 500°C, Thereafter, cooling is performed from (Ms point - 200)°C to 50°C at an average cooling rate (v2) that satisfies the following equation (1),
In the first heat treatment, the zinc plating treatment, and the second heat treatment, a holding time (t ₁ ) in a temperature range (T1) of 300°C to 450°C, and a temperature range (T2) of 450°C to 600°C ) A method of manufacturing a high-strength galvanized steel sheet in which the holding time (t ₂ ) satisfies the following equation (2).
v2≤3.8[C]+2.4[Mn]+1.2[Si]… (One)
1.1≤3t ₁ /t ₂ ≤6.5 … (2)
However, [C], [Mn], and [Si] in the above formula (1) are the contents of C, Mn, and Si, respectively, in the above component composition, and the unit of the above contents is mass%. According to clause 9,
In the first heat treatment, heating at the heating temperature T3 is performed in an atmosphere with a dew point exceeding -30°C to form a decarburization layer on the outermost layer of the cold rolled steel sheet. . According to claim 9 or 10,
A method of producing a high-strength galvanized steel sheet, wherein before the first heat treatment, the cold-rolled steel sheet is subjected to a metal plating treatment to form a metal plating layer on at least one side of the cold-rolled steel sheet. According to clause 11,
A method for producing a high-strength galvanized steel sheet, wherein the component composition of the metal plating layer consists of Fe and inevitable impurities. According to clause 12,
The component composition of the metal plating layer further includes at least one selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co. A method for producing a high-strength galvanized steel sheet containing a total of 10% by mass or less of the element. According to any one of claims 9 to 13,
The zinc plating treatment is a method of manufacturing a high-strength galvanized steel sheet, which is a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment. A method of manufacturing a member, comprising subjecting the high-strength galvanized steel sheet according to any one of claims 1 to 7 to at least one of forming processing and joining processing to obtain a member.