High-strength galvanized steel sheet and parts and method for manufacturing the same

Abstract

This invention provides a high-strength galvanized steel sheet with a tensile strength of over 1320 MPa, excellent formability, and fatigue strength at the spot weld. The steel sheet's composition, by mass%, is as follows: C: 0.150–0.450%, Si: 0.80–3.00%, Mn: 2.00–4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 0.100%, O: less than 0.0100%, and N: less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities. The diffusible hydrogen content in the steel sheet is less than 0.60 ppm by mass. The total amount of tempered martensite and bainite is 60–95%, retained austenite is 5–30%, the proportion of retained austenite with an aspect ratio of 5.5 or greater relative to the total retained austenite is less than 50%, and fresh martensite with a diameter of less than 2.0 μm is less than 20%.

Description

Technical Field

[0001] This invention relates to high-strength galvanized steel sheets with a tensile strength (TS) of 1320 MPa or higher and a method for manufacturing the same.

[0002] Furthermore, the present invention relates to components made of high-strength galvanized steel sheets and methods for manufacturing the same. Background Technology

[0003] In recent years, for example in the automotive industry, there has been a desire to improve the fuel efficiency of automobiles in order to reduce carbon dioxide (CO2) emissions, from the perspective of protecting the Earth's environment.

[0004] For improving the fuel efficiency of automobiles, reducing the weight of the vehicle body is effective. However, in this case, it is necessary to maintain the strength of the vehicle body while achieving weight reduction.

[0005] For example, Patent Document 1 discloses a high-strength steel plate with good formability.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2020 / 017609 Summary of the Invention

[0009] High-strength steel plates with tensile strengths of 1320 MPa or higher typically contain a large amount of alloying elements required for achieving high strength. In the spot weld obtained by spot welding such high-strength steel plates together, the heat-affected zone around the weld nugget (molten solidified part) lacks toughness, and its strength (fatigue strength) may sometimes be insufficient under repeated load stress.

[0010] If the fatigue strength reduction of the spot weld can be suppressed, the overall collision strength of the vehicle can be fully maintained.

[0011] The present invention was made in view of the above problems, and its purpose is to provide a high-strength galvanized steel sheet with a tensile strength of more than 1320 MPa, excellent formability, and fatigue strength of the spot weld.

[0012] The inventors have conducted in-depth research and discovered that the above-mentioned objectives can be achieved by adopting the following configuration, thereby completing the present invention.

[0013] That is, the present invention provides the following [1] to

[15] .

[0014] [1] A high-strength galvanized steel sheet, comprising a steel sheet and a galvanized layer, having a tensile strength of 1320 MPa or higher, wherein the steel sheet has the following composition and microstructure: the composition, by mass %, contains C: 0.150–0.450%, Si: 0.80–3.00%, Mn: 2.00–4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 0.100%, O: less than 0.0100%, and N: 0. The content of the metal is less than 0.100%, with the remainder consisting of Fe and unavoidable impurities; and the amount of diffusible hydrogen in the steel is less than 0.60 ppm by mass. In the above microstructure, 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 above retained austenite with an aspect ratio of 5.5 or more relative to the total area of ​​the above retained austenite is less than 50%, and the area ratio of fresh martensite with a diameter of 2.0 μm or less is less than 20%.

[0015] [2] According to the high-strength galvanized steel sheet described in [1] above, the above-mentioned composition further contains, by mass %, the following components: 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% The following elements are present in amounts of at least one of the following: Cu: less than 1.000%, Ni: less than 1.000%, Ca: less than 0.0050%, Mg: less than 0.0050%, REM: less than 0.0050%, Co: less than 0.30%, Ta: less than 0.10%, As: less than 0.100%, Pb: less than 0.100%, Zn: less than 0.100%, Bi: less than 0.100%, and Hf: less than 0.10%.

[0016] [3] The high-strength galvanized steel sheet according to [1] or [2] above, wherein the steel sheet has a decarburized layer.

[0017] [4] A high-strength galvanized steel sheet according to any one of [1] to [3], wherein a metal coating is provided on at least one side of the steel sheet and between the steel sheet and the galvanized layer.

[0018] [5] According to the high-strength galvanized steel sheet described in [4] above, the composition of the metal coating consists of Fe and unavoidable impurities.

[0019] [6] The high-strength galvanized steel sheet according to [5] above, wherein the composition of the metal coating further contains at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, totaling less than 10% by mass.

[0020] [7] The high-strength galvanized steel sheet according to any one of [1] to [6] above is a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet.

[0021] [8] A component made of high-strength galvanized steel sheet as described in any one of [1] to [7] above.

[0022] [9] A method for manufacturing a high-strength galvanized steel sheet, comprising the method described in [1] or [2] above, wherein a steel slab having the composition described in [1] or [2] above is hot-rolled, the obtained hot-rolled steel sheet is wound at a winding temperature of 350 to 700°C, the wound hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet is subjected to a first heat treatment, a galvanizing treatment, and a second heat treatment in sequence. 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 up 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 of 300 to 500°C, which is higher than the cooling stop temperature T5. Then, the average cooling rate v2 that satisfies the following formula (1) is used to cool from (Ms point - 200)°C to 50°C. In the first heat treatment, the galvanizing treatment and the second heat treatment, the holding time t1 in the temperature range T1 of 300°C or higher and less than 450°C and the holding time t2 in the temperature range T2 of 450°C to 600°C satisfy the following formula (2).

[0023] v2≤3.8[C]+2.4[Mn]+1.2[Si]…(1)

[0024] 1.1≤3t1 / t2≤6.5…(2)

[0025] Wherein, [C], [Mn] and [Si] in the above formula (1) are the contents of C, Mn and Si in the above composition, respectively, and the unit of the contents is mass%.

[0026]

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

[0027]

[11] In the method for manufacturing high-strength galvanized steel sheet according to [9] or

[10] above, the cold-rolled steel sheet is subjected to metal plating treatment before the first heat treatment, and a metal coating is formed on at least one side of the cold-rolled steel sheet.

[0028]

[12] In the manufacturing method of the high-strength galvanized steel sheet described in

[11] above, the composition of the metal coating consists of Fe and unavoidable impurities.

[0029]

[13] According to the manufacturing method of the high-strength galvanized steel sheet described in

[12] above, the composition of the metal coating further contains at least 10% by mass of an element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co.

[0030]

[14] The method for manufacturing high-strength galvanized steel sheet according to any one of [9] to

[13] above, wherein the galvanizing treatment is a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment.

[0031]

[15] A method for manufacturing a component, wherein the component is obtained by performing at least one of forming and joining processes on the high-strength galvanized steel sheet described in any one of [1] to [7] above.

[0032] According to the present invention, a high-strength galvanized steel sheet with a tensile strength of 1320 MPa or more, excellent formability, and fatigue strength of the spot welded portion can be provided. Attached Figure Description

[0033] Figure 1 This is a line diagram showing an example of the first heat treatment, galvanizing treatment, and second heat treatment.

[0034] Figure 2 This is a cross-sectional view of the plate assembly used for resistance welding.

[0035] Figure 3 This is a top view showing the plate assembly after resistance welding.

[0036] Figure 4 yes Figure 3 A-A line cross-section. Detailed Implementation

[0037] High-strength galvanized steel sheet

[0038] The high-strength galvanized steel sheet of the present invention has a steel sheet (base steel sheet) and a galvanized layer, the steel sheet having the composition and microstructure described later, and satisfying the diffusible hydrogen content in the steel described later.

[0039] High strength refers to a tensile strength (TS) of 1320 MPa or higher.

[0040] Hereinafter, "high-strength galvanized steel sheet" will also be referred to as "galvanized steel sheet".

[0041] The high-strength galvanized steel sheet of this invention has a tensile strength of over 1320 MPa, and also exhibits excellent formability and fatigue strength at the spot welds. Therefore, it can adequately maintain impact strength, making it preferably used in transportation vehicles such as automobiles.

[0042] It should be noted that, as a method for forming and processing the high-strength galvanized steel sheet of the present invention, conventional processing methods such as pressing can be used without limitation. As a method for welding the high-strength galvanized steel sheet of the present invention, conventional welding methods such as spot welding and arc welding can be used without limitation.

[0043] <Steel Plate>

[0044] First, the steel plate (base steel plate) that makes up the galvanized steel sheet will be explained.

[0045] The steel plate is, for example, a cold-rolled steel plate that has undergone the second heat treatment described later.

[0046] There is no particular limitation on the thickness of the steel plate, for example, it can be 0.5mm to 3.0mm.

[0047] Composition

[0048] The composition of the steel plate (hereinafter, for convenience, also referred to as "the composition of the present invention") will be described.

[0049] Unless otherwise stated, "%" in the composition of this invention refers to "mass %".

[0050] (C: 0.150~0.450%)

[0051] Carbon (C) induces martensite formation and increases the strength of the steel sheet. If the amount of C is too low, the hardness of the martensite decreases, and the combined area fraction of tempered martensite and bainite decreases, resulting in a tensile strength of 1320 MPa or higher. Therefore, the amount of C is 0.150% or more, preferably 0.180% or more, and more preferably 0.190% or more.

[0052] On the other hand, if the amount of carbon is too high, a large amount of cementite will be generated in the heat-affected zone, which will reduce the toughness and fatigue strength of the spot weld. Therefore, the amount of carbon is 0.450% or less, preferably 0.400% or less, and more preferably 0.370% or less.

[0053] (Si: 0.80~3.00%)

[0054] Si enhances the strength of steel sheets through solid solution strengthening. From the viewpoint of obtaining a tensile strength of 1320 MPa or more, the Si content is 0.80% or more, preferably 1.00% or more, and more preferably 1.10% or more.

[0055] On the other hand, if the Si content is too high, the toughness and fatigue strength of the spot weld will decrease. Furthermore, if the Si content is too high, the resistance to welding cracks (described later) of the spot weld may decrease. Therefore, the Si content is 3.00% or less, preferably 2.60% or less, and more preferably 2.40% or less.

[0056] (Mn: 2.00~4.00%)

[0057] Mn enhances the strength of steel plates through solid solution strengthening. From the viewpoint of obtaining a tensile strength of 1320 MPa or more, the Mn content is 2.00% or more, preferably 2.20% or more, and more preferably 2.40% or more.

[0058] On the other hand, if the Mn content is too high, a large amount of cementite will be generated during tempering, and the toughness and fatigue strength of the spot weld will decrease. Therefore, the Mn content is 4.00% or less, preferably 3.60% or less, and more preferably 3.50% or less.

[0059] (P: below 0.100%)

[0060] Segregation of phosphorus (P) at grain boundaries reduces the toughness and fatigue strength of the spot weld. Therefore, the amount of P is 0.100% or less, preferably 0.030% or less, and more preferably 0.010% or less.

[0061] (S: below 0.0200%)

[0062] S combines with Mn to form coarse MnS, which reduces the toughness and fatigue strength of the spot weld. Therefore, the amount of S is 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0020% or less.

[0063] (Al: below 0.100%)

[0064] Al acts as a deoxidizer. If the amount of Al is too high, the toughness of the spot weld will decrease due to the coarsening of oxides and nitrides, and the fatigue strength of the spot weld will also decrease. Therefore, the amount of Al is 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less.

[0065] There is no particular limitation on the lower limit of the Al content, but from the viewpoint of obtaining the effect of adding Al, it is, for example, 0.010%, preferably 0.020%.

[0066] (O: below 0.0100%)

[0067] O forms oxides, which reduces the toughness of the spot weld and decreases its fatigue strength. Therefore, the amount of O is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less.

[0068] (N: below 0.0100%)

[0069] Ni combines with Ti to form TiN. If the amount of Ni is too high, the excessive amount of TiN formed will reduce the toughness and fatigue strength of the spot weld. Therefore, the amount of Ni is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less.

[0070] The composition of the present invention contains the above-mentioned components and the remainder consists of Fe and unavoidable impurities.

[0071] (Other elements)

[0072] In the composition of the present invention, a portion of the remaining portion (Fe and unavoidable impurities) may be replaced by an element selected from at least one of the elements described below (also referred to as "any element") in a mass percentage.

[0073] ((B: below 0.0050%))

[0074] B is an element that can improve the hardenability of steel sheets by segregating at austenite grain boundaries, and is preferred to be added because it increases the tensile strength of the steel sheets.

[0075] However, if there is too much B, Fe will form. 23 (CB)6 reduces the toughness of the spot weld and decreases its fatigue strength. Therefore, the amount of B is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0076] There is no particular limit to the lower limit of the amount of B. From the viewpoint of obtaining the effect of adding B, it is, for example, 0.0005%, preferably 0.0010%.

[0077] ((Ti: below 0.200%))

[0078] Ti increases the tensile strength of steel sheets by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment, and is therefore preferred to be added.

[0079] However, if the Ti content is too high, it will combine with N to form coarse nitrides, which will reduce the toughness and fatigue strength of the spot weld. Therefore, the Ti content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less.

[0080] There is no particular limitation on the lower limit of the Ti amount, but from the viewpoint of obtaining the effect of Ti addition, it is, for example, 0.005%, preferably 0.010%.

[0081] ((Nb: less than 0.200%, V: less than 0.500%, W: less than 0.500%))

[0082] Nb, V, and W increase the tensile strength of steel sheets by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment, and are therefore preferred additions.

[0083] However, when these elements are present in excessive amounts, they do not dissolve during heating and remain as coarse carbides. These coarse carbides reduce the toughness and fatigue strength of the spot welds.

[0084] Therefore, the amount of Nb is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The lower limit is not particularly limited, but from the viewpoint of achieving the desired Nb addition effect, it is, for example, 0.005%, preferably 0.010%.

[0085] The amount of V is preferably 0.500% or less, more preferably 0.300% or less, and even more preferably 0.100% or less. The lower limit is not particularly limited, but from the viewpoint of achieving the desired V-addition effect, it is, for example, 0.005%, preferably 0.010%.

[0086] The amount of W is preferably 0.500% or less, more preferably 0.200% or less, and even more preferably 0.050% or less. There is no particular limitation on the lower limit, but from the viewpoint of achieving the desired effect of W addition, it is, for example, 0.001%, preferably 0.002%.

[0087] ((Mo: less than 1.000%, Cr: less than 1.000%))

[0088] Mo and Cr improve the hardenability of steel plates, thus increasing their tensile strength, and are therefore preferred additions. However, excessive amounts of these elements can lead to the over-formation of hard martensite, resulting in decreased toughness and fatigue strength of the spot welds.

[0089] Therefore, the amount of Mo is preferably 1.000% or less, more preferably 0.700% or less, and even more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the desired effect of Mo addition, it is, for example, 0.005%, preferably 0.020%.

[0090] The Cr content is preferably 1.000% or less, more preferably 0.700% or less, and even more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the desired effect of Cr addition, it is, for example, 0.005%, preferably 0.020%.

[0091] ((Sb: less than 0.200%, Sn: less than 0.200%))

[0092] Sb and Sn increase the tensile strength of steel plates by inhibiting decarburization on the surface, and are therefore preferred additions. However, excessive amounts of these elements can cause embrittlement of the steel, leading to cracks in the heat-affected zone and a decrease in the fatigue strength of spot welds.

[0093] Therefore, the amount of Sb is preferably 0.200% or less, more preferably 0.080% or less, and even more preferably 0.040% or less. The lower limit is not particularly limited, but from the viewpoint of achieving the desired effect of Sb addition, it is, for example, 0.001%, preferably 0.002%.

[0094] The amount of Sn is preferably 0.200% or less, more preferably 0.080% or less, and even more preferably 0.040% or less. There is no particular limitation on the lower limit, but from the viewpoint of achieving the desired Sn addition effect, it is, for example, 0.001%, preferably 0.002%.

[0095] ((Zr: below 0.1000%))

[0096] Zr spherizes the precipitates, improving the toughness of the spot weld, and is therefore preferred. However, if the amount of Zr is excessive, the amount of coarse precipitates that remain undissolved during the heating of the hot-rolled steel slab increases, resulting in a decrease in the toughness and fatigue strength of the spot weld.

[0097] Therefore, the Zr content 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 desired Zr addition effect, it is, for example, 0.0005%, preferably 0.0010%.

[0098] (Cu: less than 1.000%)

[0099] Cu is preferred because it improves the hardenability of steel plates, thereby increasing their tensile strength. However, excessive Cu content can lead to decreased toughness and fatigue strength of the spot welds due to increased Cu inclusions.

[0100] Therefore, the amount of Cu is preferably 1.000% or less, more preferably 0.700% or less, and even more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the desired Cu addition effect, it is, for example, 0.005%, preferably 0.010%.

[0101] (Ni: less than 1.000%)

[0102] Ni is preferred because it improves the hardenability of steel plates, thereby increasing their tensile strength. However, excessive Ni can lead to a decrease in the toughness and fatigue strength of spot welds due to the increase in hard martensite.

[0103] Therefore, the amount of Ni is preferably 1.000% or less, more preferably 0.700% or less, and even more preferably 0.400% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the desired effect of Ni addition, it is, for example, 0.003%, preferably 0.005%.

[0104] (Ca: less than 0.0050%, Mg: less than 0.0050%, REM: less than 0.0050%)

[0105] Ca, Mg, and REM (Rare Earth Metal) spheroidize the precipitates of sulfides, oxides, etc., increasing the toughness of the spot weld, and are therefore preferred additions. However, in excessive amounts of these elements, the toughness of the spot weld decreases due to the coarsening of sulfides, and the fatigue strength of the spot weld decreases.

[0106] Therefore, the amount of Ca is preferably 0.0050% or less, more preferably 0.0045% or less, and even more preferably 0.0040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the desired effect of Ca addition, it is, for example, 0.0005%, preferably 0.0010%.

[0107] The amount of Mg is preferably 0.0050% or less, more preferably 0.0048% or less, and even more preferably 0.0045% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the desired effect of Mg addition, it is, for example, 0.0005%, preferably 0.0010%.

[0108] The amount of REM is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. The lower limit is not particularly limited, but from the viewpoint of achieving the desired effect of adding REM, it is, for example, 0.0005%, preferably 0.0010%.

[0109] (Co: less than 0.30%)

[0110] Co spherizes the precipitates, increasing the toughness of the spot weld, and is therefore preferred. However, excessive Co content leads to a decrease in the toughness and fatigue strength of the spot weld due to the increase in hard martensite.

[0111] Therefore, the amount of Co is preferably 0.30% or less, more preferably 0.20% or less, and even more preferably 0.10% or less. There is no particular limitation on the lower limit, but from the viewpoint of obtaining the desired effect of Co addition, it is, for example, 0.01%, preferably 0.02%.

[0112] ((Ta: below 0.10%))

[0113] Ta spherizes the precipitates, increasing the toughness of the spot weld, and is therefore preferred. However, excessive Ta leads to a decrease in the toughness and fatigue strength of the spot weld due to the increase in coarse carbides.

[0114] Therefore, the amount of Ta is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less. There is no particular limitation on the lower limit, but from the viewpoint of achieving the desired effect of Ta addition, it is, for example, 0.01%, preferably 0.02%.

[0115] ((As: less than 0.100%, Pb: less than 0.100%, Zn: less than 0.100%, and Bi: less than 0.100%))

[0116] As, Pb, Zn, and Bi sphericalize the precipitates, increasing the toughness of the spot weld, and are therefore preferred additions. However, excessive amounts of these elements generate a large number of coarse precipitates and inclusions, thereby reducing the toughness and fatigue strength of the spot weld.

[0117] Therefore, the amount of As is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less. There is no particular limitation on the lower limit, but from the viewpoint of achieving the desired effect of As addition, it is, for example, 0.001%, preferably 0.002%.

[0118] The amount of Pb is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less. There is no particular limitation on the lower limit, but from the viewpoint of achieving the desired Pb addition effect, it is, for example, 0.001%, preferably 0.002%.

[0119] The Zn content is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less. There is no particular limitation on the lower limit, but from the viewpoint of obtaining the desired Zn addition effect, it is, for example, 0.001%, preferably 0.002%.

[0120] The amount of Bi is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less. There is no particular limitation on the lower limit, but from the viewpoint of achieving the desired effect of Bi addition, it is, for example, 0.001%, preferably 0.002%.

[0121] ((Hf: below 0.10%))

[0122] Hf spherizes the precipitates, increasing the toughness of the spot weld, and is therefore preferred. However, excessive Hf leads to a decrease in the toughness and fatigue strength of the spot weld due to the increase in coarse carbides.

[0123] Therefore, the amount of Hf is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less. There is no particular limitation on the lower limit, but from the viewpoint of achieving the desired Hf addition effect, it is, for example, 0.01%, preferably 0.02%.

[0124] When the composition of the present invention contains any of the above-mentioned elements in an amount less than the lower limit value, the arbitrary element is contained as an unavoidable impurity.

[0125] Micro-organization

[0126] Next, the microstructure of the steel plate (hereinafter, for convenience, also referred to as "the microstructure of the present invention") will be described.

[0127] In order to achieve the effects of the present invention, it is not sufficient to simply satisfy the composition of the present invention as described above; it is also necessary to satisfy the microstructure of the present invention as described below.

[0128] Hereinafter, the area ratio is relative to the area ratio of the entire microstructure. The area ratio of each structure is determined using the methods described in the examples below.

[0129] (Total area ratio of tempered martensite and bainite: 60-95%)

[0130] From the viewpoint of stably ensuring 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.

[0131] On the other hand, if the total area ratio is too high, the area ratio of retained austenite becomes low, resulting in decreased ductility and formability of the steel sheet. Therefore, the total area ratio is 95% or less, preferably 92% or less, and more preferably 88% or less.

[0132] (Area ratio of retained austenite: 5-30%)

[0133] Retained austenite improves the ductility of the steel sheet. Therefore, the area fraction of retained austenite is 5% or more, preferably 6% or more, and more preferably 8% or more.

[0134] On the other hand, if the area ratio of retained austenite is too high, the amount of retained austenite undergoing martensitic transformation under stress increases, leading to cracks in the heat-affected zone and a decrease in the fatigue strength of the spot weld. Therefore, the area ratio of retained austenite is 30% or less, preferably 25% or less, and more preferably 20% or less.

[0135] (Area ratio of retained austenite with an aspect ratio of 5.5 or higher: less than 50%)

[0136] Under repeated stress loading, the retained austenite transforms into hard martensite through work hardening. If there is too much retained austenite with an aspect ratio greater than 5.5, stress concentrates at the leading edge of the transformed martensite, easily leading to porosity. Moreover, the interconnectedness of these pores makes it easy for cracks to form around the weld nugget, thus reducing the fatigue strength of the spot weld.

[0137] In addition, since the retained austenite with an aspect ratio of 5.5 or higher is unstable, if there is too much of it, the TRIP (Transformation Induced Plasticity) effect brought by the retained austenite will not be obtained, and the ductility will decrease.

[0138] Therefore, the area ratio of the retained austenite with an aspect ratio of 5.5 or more relative to the total retained austenite is 50% or less, preferably 45% or less, and more preferably 40% or less.

[0139] On the other hand, there is no particular limitation on the lower limit of the area ratio of retained austenite with an aspect ratio of 5.5 or more relative to the total retained austenite, for example, it is 2%, preferably 4%.

[0140] (Area fraction of fresh martensite with a diameter of less than 2.0 μm: less than 20%)

[0141] Fine, fresh martensite increases the strength of steel sheets. To achieve this effect, the diameter of the fresh martensite should be less than 2.0 μm.

[0142] However, if there is too much fresh martensite with a diameter of 2.0 μm or less, porosity will be generated due to the difference in hardness of the microstructure, resulting in decreased formability. In addition, the interconnectedness of pores makes it easy for cracks to form around the weld nugget, thereby reducing the fatigue strength of the spot weld. Therefore, the area fraction 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.

[0143] On the other hand, there is no particular limit to the lower limit of the area ratio of fresh martensite with a diameter of 2.0 μm or less, for example, it is 1%, preferably 3%.

[0144] In the microstructure of the present invention, in addition to tempered martensite, bainite, retained austenite and fresh martensite (the remaining part of the microstructure), it may also contain, for example, pearlite; ferrite; iron-based carbonitriding compounds; alloy carbonitriding compounds; inclusions such as MnS and Al2O3; and other known microstructures.

[0145] The area fraction of the remaining tissue is preferably 10% or less, more preferably 8% or less, and even more preferably 5% or less. If the area fraction of the remaining tissue is within this range, the effect of the present invention will not be impaired.

[0146] Diffusible hydrogen content in steel: below 0.60 ppm by mass

[0147] If the diffusible hydrogen content in the steel is too high, the spot weld is prone to cracking during welding, and the fatigue strength of the spot weld decreases. Therefore, the diffusible hydrogen content in the 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.

[0148] The amount of diffusible hydrogen in the steel was determined by the method described in the examples described later.

[0149] Decarburized layer

[0150] When spot welding (resistance welding) galvanized steel sheets, there are several concerns.

[0151] That is, during resistance welding, residual stress is generated near the spot weld. Under this condition, the zinc in the galvanized layer melts and diffuses into the grain boundaries of the steel plate, resulting in liquid metal embrittlement (LME) and the formation of grain boundary cracks (LME cracks) in the steel plate. LME cracks are more likely to occur when the steel plate contains Si (especially when the Si content is high).

[0152] Therefore, galvanized steel sheets are sometimes required to have excellent resistance to such cracks (hereinafter also referred to as "resistance to welding cracks of spot welds", "resistance to welding cracks of welded parts", or simply "resistance to welding cracks").

[0153] Therefore, the steel sheet (base steel sheet) constituting the galvanized steel sheet preferably has a decarburized layer as a low-C concentration layer on its outermost surface. As a result, the spot welds of the galvanized steel sheet exhibit excellent resistance to resistance welding cracks.

[0154] The reason is not yet clear, but it can be considered that the low C concentration and viscosity of the decarburized layer make it less prone to the aforementioned cracks.

[0155] When the steel plate has a decarburized layer, the thickness of the decarburized layer (depth in the thickness direction) is, for example, 10 μm or more.

[0156] Based on the reason that the spot weld has better resistance to welding cracks, the thickness (depth in the thickness direction of the plate) of the decarburized layer is preferably 20 μm or more, more preferably 30 μm or more, and even more preferably 40 μm or more.

[0157] On the other hand, there is no particular upper limit to the thickness of the decarburized layer. However, from the viewpoint of achieving a good range of tensile strength for the steel sheet, the thickness of the decarburized layer is preferably 130 μm or less, more preferably 100 μm or less, and even more preferably 70 μm or less.

[0158] The decarburized layer (and its thickness) is calculated as follows.

[0159] First, the C concentration is measured in the thickness direction at the interface between the galvanized layer and the steel sheet (in the case of a galvanized steel sheet with a metallic coating described later; the same applies below).

[0160] To determine the C concentration, an electron beam microanalyzer (EPMA) was used.

[0161] Specifically, first, the galvanized steel sheet embedded in resin is ground, and the cross-section perpendicular to the rolling direction is precision machined into an observation surface. Then, the galvanized steel sheet is removed from the resin as a sample.

[0162] With the accelerating voltage set to 7kV and the irradiation current set to 50nA, the cross-section of the sample, including the outermost layer of the steel plate, was analyzed by surface or line analysis in 1μm increments to determine the C strength.

[0163] To suppress contamination, before the measurement begins, a plasma cleaning machine is used to remove hydrocarbons from the surface and surrounding area of ​​the sample at two locations: the measurement chamber and the sample preparation chamber.

[0164] To suppress the accumulation of hydrocarbons during the measurement, the sample is heated on the workbench in the measurement chamber, and the measurement is performed while maintaining the sample temperature at a maximum of 100°C.

[0165] It was confirmed that the detection limit of C was much lower than 0.10% of the mass due to the inhibitory effect of contamination.

[0166] Details of the apparatus used and the method for suppressing pollution are described in the following reference 1.

[0167] Reference 1: Yamashita et al., “Analysis of Carbon Distribution in the Early Stage of Proeutectoid Ferrite Phase Transformation in Low-Carbon Steel Using High-Precision FE-EPMA,” Iron and Steel, Japan Iron and Steel Association, 2017, Vol. 103, No. 11, pp. 14-20

[0168] Since pollution suppression depends on the type and conditions of the equipment used, it is not necessarily required, as long as the intensity of C can be measured with sufficient accuracy.

[0169] Next, the measured C intensity was converted into C concentration (unit: mass%) to obtain a concentration graph. In the conversion, a calibration curve prepared in advance using standard samples was used.

[0170] Based on the obtained concentration map, the line profile in the thickness direction is extracted. The thickness direction profile of C concentration is obtained by averaging the line profiles at 300 locations along a direction parallel to the surface of the steel plate (orthogonal to the thickness direction).

[0171] The thickness profile of the obtained C concentration plate was smoothed using a simple moving average method. The number of smoothing points was approximately 21.

[0172] In the smoothed contour, the region with a C concentration below 80% by mass of the maximum value is taken as the decarburization layer, and the distance of this region in the thickness direction is taken as the thickness of the decarburization layer.

[0173] For each sample, the average of the measurement results from two fields of view is used.

[0174] <Zinc coating>

[0175] The zinc coating is formed through the zinc plating process described later.

[0176] The preferred coating thickness for zinc plating is 20–80 g / m² on one side. 2 .

[0177] <Metallic coating>

[0178] In the high-strength galvanized steel sheet of the present invention, a metal coating (a coating different from the galvanized layer) may be further provided on at least one side of the steel sheet and between the steel sheet and the galvanized layer.

[0179] Therefore, the spot weld exhibits excellent resistance to welding cracks.

[0180] The reason is not yet clear, but it can be considered that the metal coating inhibits the diffusion of zinc (Zn) from the zinc coating into the steel plate during resistance welding (zinc intrusion inhibition effect).

[0181] Metals used as metal coatings can include those with melting points higher than Zn (Fe, Ni, etc.), but Fe is preferred because in addition to the zinc intrusion inhibition effect mentioned above, the following effects are also expected.

[0182] That is, the metal coating preferably has a composition consisting of Fe and unavoidable impurities (hereinafter also referred to as "Fe-based coating").

[0183] The composition of the Fe-based coating may further contain at least 10% by mass of an element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co.

[0184] It can be assumed that when there is a high Si content on the surface of the steel plate, the toughness decreases in the spot weld area, and the resistance to resistance welding cracks is easily deteriorated.

[0185] At this point, by setting an Fe-based coating on the surface of the steel plate, the Fe-based coating acts as a layer lacking the Si solution layer. As a result, the amount of Si dissolved in the spot weld is reduced, thereby suppressing the decrease in the toughness of the spot weld. Therefore, it can be considered that the resistance welding crack characteristics are better (toughness reduction suppression effect).

[0186] In addition, the Fe-based coating acts as a soft layer, mitigating the stress applied to the steel plate surface during spot welding and reducing residual stress in the spot weld area. Therefore, it can be considered to have superior resistance to welding cracks (stress mitigation effect).

[0187] From the viewpoint of improving the resistance to resistance welding cracks of spot welds, it is preferable to apply a metal coating such as an Fe-based coating to the surface of the steel plate having the aforementioned decarburized layer.

[0188] The adhesion amount of the metal coating on one side is, for example, greater than 0 g / m. 2 The preferred value is 2.0 g / m 2 The above, more preferably 4.0 g / m 2 The above is further preferred to be 6.0 g / m 2 above.

[0189] On the other hand, there is no specific upper limit, but from a cost perspective, the preferred adhesion amount of the metal coating on one side is 60 g / m². 2 The following is more preferably 50g / m 2 The following is a further preferred value: 40g / m 2 The following is particularly preferred: 30g / m 2 the following.

[0190] The amount of metal coating adhesion is calculated as follows.

[0191] First, 10mm × 15mm specimens were collected from galvanized steel sheets with metallic coatings and embedded in resin to obtain embedded samples with exposed cross-sections of the galvanized steel sheets. Using a scanning electron microscope (SEM) at an accelerating voltage of 15kV, three arbitrary locations on this cross-section were observed at magnifications ranging from 2000 to 10000x, depending on the thickness of the metallic coating. The average thickness of the metallic coating in the three fields of view was multiplied by the specific gravity of the metal to calculate the amount of metallic coating adhering to one side.

[0192] [Evaluation Test of Resistance Welding Crack Characteristics of Spot Welds]

[0193] based on Figures 2-4 This describes the test method used to evaluate the resistance to welding cracks in spot welds.

[0194] Figure 2 This is a cross-sectional view of plate group 5 used for resistance welding. Figure 3 This is a top view of plate group 5 after resistance welding. Figure 4 for Figure 3 A-A line cross-section.

[0195] First, test piece 4 is cut from the galvanized steel sheet to be evaluated. Test piece 4 has the rolling right angle direction (TD) as the long side direction and the rolling direction as the short side direction. Its dimensions are: length L: 150 mm in the long side direction, width W: 50 mm in the short side direction, and plate thickness t: 1.6 mm.

[0196] Similarly, cut out test pieces of the same size from other galvanized steel sheets.

[0197] The surface of the evaluation object (the surface of the galvanized layer) of test piece 4 is brought into contact with the galvanized layer of the other test piece 3 to obtain plate group 5.

[0198] Plate 5 is connected to a pair of spacers 6 (length in the long side: 50mm, length in the short side: 45mm, thickness t) which are steel plates. SThe shims 6 (2.0mm) are fixed to the mounting base 7. The long side end face of the shims 6 is aligned with the short side end faces of the plate assembly 5. Therefore, the distance D between a pair of shims 6 is 60mm. The mounting base 7 is a plate with a hole 7a in the center.

[0199] Next, resistance welding is performed using a servo motor pressurized single-phase AC (50Hz) resistance welding machine while the plate assembly 5 is being flexed by applying pressure to it with a pair of electrodes 8 (front diameter: 6mm).

[0200] More specifically, resistance welding is performed under specified conditions (applied pressure, holding time, and welding time) and at a welding current that becomes a specified weld nugget diameter d, to form a welded part containing a weld nugget 9.

[0201] The holding time refers to the time from when the welding current stops flowing until electrode 8 begins to open.

[0202] The diameter d of the melt core is the distance between the ends of the melt cores 9 along the long side of the plate group 5.

[0203] During resistance welding, a pair of electrodes 8 apply pressure to the plate assembly 5 from above and below in the vertical direction.

[0204] The lower electrode 8a applies pressure to the test piece 4 through the hole 7a in the fixing stage 7. During pressure application, the lower electrode 8a is fixed to the fixing stage 7 at a position where it contacts an imaginary plane S obtained by extending the contact surface between the gasket 6 and the fixing stage 7. This allows the upper electrode 8b to move to a position where it can contact the center of the test piece 3.

[0205] Resistance welding is performed on the plate group 5 with the long side of the plate group 5 tilted at 5° relative to the horizontal direction (i.e., the angle θ relative to the horizontal direction is 5°).

[0206] After resistance welding, the board assembly 5 is along... Figure 3 The weld section was cut along line A-A to include the center of the weld containing the weld nugget 9. The cross-section of the weld was observed using an optical microscope (200x) to evaluate its resistance to welding cracks.

[0207] It should be explained that Figure 4 The diagram schematically illustrates the crack 10 that occurred in specimen 4.

[0208] When a crack appeared in the other party's specimen 3, the stress in specimen 4 was dispersed, and a proper evaluation could not be obtained. Therefore, the data from the other party's specimen 3, which did not develop cracks, were used.

[0209] [Manufacturing method of high-strength galvanized steel sheet]

[0210] Next, a method for manufacturing the high-strength galvanized steel sheet of the present invention (hereinafter, for convenience, also referred to as "the manufacturing method of the present invention") will be described. The manufacturing method of the present invention is also a method for manufacturing the high-strength galvanized steel sheet of the present invention described above.

[0211] Unless otherwise stated, the temperatures used to heat or cool the steel slabs, plates (hot-rolled steel plates, cold-rolled steel plates), etc. shown below refer to their surface temperatures.

[0212] There are no particular limitations on the smelting method for steel slabs (steel billets), and well-known smelting methods such as converters and electric furnaces can be used. After smelting, continuous casting is the preferred method to obtain steel slabs. However, other well-known casting methods such as ingot-rolling and thin slab continuous casting can also be used to obtain steel slabs.

[0213] In the manufacturing method of the present invention, firstly, a steel slab having the above-described composition is hot-rolled. This yields a hot-rolled steel sheet.

[0214] During hot rolling, the steel slab can also be reheated in a heating furnace before rolling. Alternatively, if the steel slab is kept at a temperature above the specified temperature, it can be directly fed for rolling without heating.

[0215] Hot Rolled

[0216] In hot rolling, steel slabs undergo rough rolling and finish rolling.

[0217] It is preferable to heat the steel slab before rough rolling to dissolve the carbides in the steel slab.

[0218] 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.

[0219] On the other hand, from the viewpoint of preventing increased oxide scale loss, the heating temperature of the steel slab is preferably below 1300°C, and more preferably below 1280°C.

[0220] As mentioned above, if the steel slab before rough rolling is kept at a temperature above the specified temperature and the carbides in the steel slab are dissolved, the heating of the steel slab before rough rolling can be omitted.

[0221] There are no particular limitations on the conditions for roughing and finishing rolling, but the finishing temperature is preferably 700 to 1100°C, and more preferably 800 to 1000°C.

[0222] Winding temperature: 350~700℃

[0223] Next, the hot-rolled steel sheet obtained by hot rolling the steel slab is wound.

[0224] If the temperature during the winding of hot-rolled steel sheet is too low (winding temperature), hard martensite with a high carbon concentration will be generated, resulting in an increase in coarse fresh martensite after heat treatment. Furthermore, if a hard, martensite-dominated hot-rolled steel sheet is cold-rolled, the fatigue strength of the spot welds will decrease.

[0225] Therefore, the winding temperature is 350°C or higher, preferably 400°C or higher, and more preferably 450°C or higher.

[0226] On the other hand, if the winding temperature is too high, excessive ferrite and pearlite will be generated in the microstructure of the hot-rolled steel sheet, and the nucleation sites of austenite during heat treatment will be reduced (i.e., the total area ratio of tempered martensite and bainite will be reduced), so it is difficult to ensure the desired strength after heat treatment.

[0227] Therefore, the winding temperature is below 700°C, preferably below 650°C, and more preferably below 600°C.

[0228] Cold rolling

[0229] Next, the hot-rolled steel sheet that has been wound is cold-rolled to obtain a cold-rolled steel sheet.

[0230] The cold rolling rate is preferably 30% or more, more preferably 35% or more. There is no particular upper limit, for example, it is 70% or less, preferably 65% ​​or less.

[0231] <Metal plating treatment>

[0232] Before performing the first heat treatment described later, the cold-rolled steel sheet obtained by cold rolling may also be subjected to a metal plating treatment. As a result, the aforementioned metal plating layer is formed on at least one side of the cold-rolled steel sheet.

[0233] The Fe-based coating described above is preferred as the metal coating to be formed.

[0234] There are no particular limitations on the metal plating process, but from a manufacturing point of view, electroplating is preferred. Examples of metal plating baths used for electroplating include sulfuric acid baths, hydrochloric acid baths, and baths obtained by mixing the two. When performing electroplating, the amount of metal coating formed can be adjusted according to factors such as the energizing time.

[0235] When forming an Fe-based coating through metal plating, an Fe-based plating bath is used.

[0236] Fe-based plating baths contain, for example, Fe, and at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The content of these elements can be appropriately adjusted according to the composition of the resulting Fe-based plating layer.

[0237] In Fe-based plating baths, as long as metal ions are present, non-metallic elements can be included as part of boric acid, phosphoric acid, nitric acid, organic acids, etc.

[0238] As an Fe-based plating bath, when using a ferric sulfate plating bath, it may further contain conductive additives such as sodium sulfate and potassium sulfate; chelating agents; pH buffers, etc.

[0239] To clean the surface of cold-rolled steel sheets, degreasing and washing can be performed on the cold-rolled steel sheets before metal plating. Furthermore, to improve the surface activity of cold-rolled steel sheets, pickling and washing can be performed.

[0240] There are no particular restrictions on the methods of degreasing and washing; previously known methods can be used.

[0241] Various acid solutions, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof, can be used in pickling, with sulfuric acid, hydrochloric acid, or mixtures thereof being preferred. The concentration of the acid solution is not particularly limited, but considering the ability to remove oxide scale and prevent surface roughness (surface defects) caused by over-pickling, 1-20% by mass is preferred. Defoamers, pickling accelerators, pickling inhibitors, etc., may also be added to the acid solution.

[0242] Next, the cold-rolled steel sheet obtained by cold rolling (or the cold-rolled steel sheet that has undergone metal plating treatment) is subjected to the first heat treatment, galvanizing treatment and second heat treatment as described below in sequence.

[0243] <First Heat Treatment>

[0244] In the first heat treatment, the cold-rolled steel sheet is heated at 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).

[0245] Heating temperature T3: 750~950℃

[0246] When the heating temperature T3 is too low, the final microstructure contains ferrite due to heating in the two-phase region of ferrite and austenite, and the total area ratio of tempered martensite and bainite decreases, making it difficult to ensure the desired tensile strength. Therefore, the heating temperature T3 is 750°C or higher, preferably 800°C or higher, and more preferably 830°C or higher.

[0247] On the other hand, if the heating temperature T3 is too high, the amount of hydrogen penetrating into the steel increases due to the increase in hydrogen partial pressure, thus increasing the amount of diffusing hydrogen in the steel. Therefore, the heating temperature is preferably 950°C or below, preferably 930°C or below, and more preferably 900°C or below.

[0248] The holding time (heating time) of cold-rolled steel sheet at heating temperature T3 is not particularly limited, for example, it is 10 to 500 seconds, preferably 50 to 300 seconds, and more preferably 80 to 200 seconds.

[0249] Dew point: greater than -30℃

[0250] It can make the dew point of the atmosphere when heating cold-rolled steel sheets at heating temperature T3 greater than -30℃.

[0251] This promotes the decarburization reaction of cold-rolled steel sheets, reducing the carbon concentration in the outermost layer. In other words, the aforementioned decarburized layer can be formed. Consequently, the resulting galvanized steel sheet exhibits excellent resistance to resistance welding cracks at the spot welds.

[0252] Based on the reason that the increased thickness of the decarburized layer results in better resistance to welding cracks in the spot weld, the dew point is preferably -20°C or higher, and more preferably -5°C or higher.

[0253] There is no particular upper limit to the dew point. However, from the viewpoint of suppressing oxidation of the surface of cold-rolled steel sheet and ensuring good adhesion of the galvanized layer formed by the galvanizing process described later, the dew point is preferably 40°C or lower, more preferably 30°C or lower, and even more preferably 20°C or lower.

[0254] Average cooling rate v1: 10℃ / s or higher

[0255] Next, the cold-rolled steel sheet heated at heating temperature T3 is cooled to the cooling stop temperature T4, which will be described later. Here, the average cooling rate from heating temperature T3 to 550°C is set as v1.

[0256] If the average cooling rate v1 is too low, ferrite phase transformation occurs during cooling, and the combined area fraction of tempered martensite and bainite decreases, making it difficult to ensure the desired tensile strength. Therefore, 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.

[0257] There is no particular upper limit to the average cooling rate v1, for example, it is 45°C / s, preferably 30°C / s.

[0258] Cooling stop temperature T4: 350~550℃

[0259] If the cooling stop temperature T4 is too low, the combined area ratio of tempered martensite and bainite will be too high, the area ratio of retained austenite will decrease, and the ductility and formability of the steel sheet (cold-rolled steel sheet) will decrease. Therefore, the cooling stop temperature T4 is 350°C or higher, preferably 370°C or higher, and more preferably 390°C or higher.

[0260] On the other hand, if the cooling stop temperature T4 is too high, pearlite will form, the area ratio of retained austenite will decrease, and the ductility and formability of the steel sheet (cold-rolled steel sheet) will decrease. Therefore, the cooling stop temperature T4 is 550°C or below, preferably 530°C or below, and more preferably 510°C or below.

[0261] <Zinc plating>

[0262] Next, a galvanized layer is formed on the surface of the cold-rolled steel sheet (hereinafter also referred to as "cold-rolled steel sheet") that has undergone the first heat treatment.

[0263] For galvanizing treatment, hot-dip galvanizing or alloyed hot-dip galvanizing is preferred.

[0264] When performing galvanizing, an apparatus configured to continuously perform heat treatment (first heat treatment and second heat treatment) and galvanizing can be used.

[0265] The preferred conditions for galvanizing are described below.

[0266] In the manufacturing method of the present invention, the holding time t1 and holding time t2, which will be described later, need to satisfy the conditions described later.

[0267] When performing molten galvanizing, for example, cold-rolled steel sheet is immersed in a zinc bath at a temperature of 440–500°C. Then, the amount of galvanized layer (molten galvanized layer) is preferably adjusted by means of gas wiping or the like.

[0268] As a zinc bath, a zinc bath with an Al content of 0.10 to 0.23% by mass and the remainder consisting of Zn and unavoidable impurities is preferred.

[0269] When performing alloyed melt-plating, if the alloying temperature is too low, the Zn-Fe alloying rate is too slow, and alloying can sometimes become significantly difficult. On the other hand, if the alloying temperature is too high, the untransformed austenite phase transforms into pearlite, and the tensile strength and ductility can sometimes decrease. Therefore, the alloying temperature is preferably 450–600°C, more preferably 470–550°C, and even more preferably 470–530°C.

[0270] The preferred zinc coating thickness for galvanized steel sheet (GI) and alloyed galvanized steel sheet (GA) is 20–80 g / m² on one side. 2 .

[0271] <Second Heat Treatment>

[0272] In the second heat treatment, the galvanized cold-rolled steel sheet (hereinafter also referred to as "cold-rolled steel sheet") is cooled to the cooling stop temperature T5 described later, then reheated to the reheating temperature T6 described later, and then cooled to at least 50°C. This cooling (hereinafter also referred to as "re-cooling") includes cooling at the average cooling rate v2 described later.

[0273] Cooling stop temperature T5: 50~350℃

[0274] If the cooling stop temperature T5 is too low, the area ratio of retained austenite will decrease, and the ductility and formability of the steel sheet (cold-rolled steel sheet) will decrease. Therefore, the cooling stop temperature T5 is 50°C or higher, preferably 100°C or higher, and more preferably 120°C or higher.

[0275] 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 will be too high. Therefore, the cooling stop temperature T5 is preferably 350°C or less, and more preferably 300°C or less.

[0276] It should be noted that if the cold-rolled steel sheet has been cooled to the cooling stop temperature T5, it may not be held at the cooling stop temperature T5 and the temperature may be immediately raised to the reheating temperature T6 (described later). Alternatively, it may be held at the cooling stop temperature T5 for a certain period of time and then raised to the reheating temperature T6 (described later).

[0277] Reheating temperature T6: greater than cooling stop temperature T5 and 300-500℃

[0278] The reheating temperature T6 is a temperature greater than the cooling stop temperature T5 and within the range of 300–500 °C. This promotes the carbon distribution from the martensite formed during cooling to the cooling stop temperature T5 to the untransformed austenite, resulting in a desired area ratio for the retained austenite.

[0279] Average cooling rate v2: v2≤3.8[C]+2.4[Mn]+1.2[Si]

[0280] In the cooling process from reheating temperature T6 to at least 50°C (recooling), the average cooling rate from (Ms point - 200)°C to 50°C that satisfies the following equation (1) is set as v2.

[0281] In the manufacturing method of the present invention, the average cooling rate v2 satisfies the following formula (1).

[0282] v2≤3.8[C]+2.4[Mn]+1.2[Si]···(1)

[0283] In formula (1) above, [C], [Mn] and [Si] are the contents of C, Mn and Si in the above-mentioned composition (the composition of the present invention) (unit: mass%).

[0284] By ensuring that the average cooling rate v2 satisfies equation (1) above, the martensite generated during recooling undergoes self-tempering, resulting in the formation of fine carbides within the martensite. As a result, the area fraction of fresh martensite with a diameter of less than 2.0 μm can be reduced in the final microstructure.

[0285] The lower limit of the average cooling rate v2 is not particularly limited, for example, it is 1℃ / s, preferably 2℃ / s.

[0286] It should be noted that the Ms point (unit: °C) is obtained by the following formula (3).

[0287] Ms=550-350×[C]-40×[Mn]-35×[V]-20×[Cr]-17×[Ni]-10×[Cu]-10×[Mo]-5×[W]+15×[Co]+30×[Al]…(3)

[0288] In formula (3) above, [X] is the content of element X in the above-mentioned composition (the composition of the present invention) (unit: mass%).

[0289] <Relationship between hold time t1 and hold time t2: 1.1 ≤ 3t1 / t2 ≤ 6.5>

[0290] Figure 1 This is a line diagram illustrating an example of the first heat treatment, galvanizing treatment, and second heat treatment described above. Figure 1 The above heating temperature T3, cooling stop temperature T4, cooling stop temperature T5, and reheating temperature T6 are shown in the figure.

[0291] Furthermore, in Figure 1 The diagram shows the temperature range T1, which is above 300℃ and below 450℃, and the temperature range T2, which is between 450℃ and 600℃.

[0292] Here, the holding time (dwelling time) of the cold-rolled steel sheet in the temperature range T1 is set as t1. Figure 1 In the diagram, the holding time t1 is represented by a shading line running from the upper right to the lower left.

[0293] In addition, the holding time (dwelling time) of the cold-rolled steel sheet in the temperature range T2 is set as t2. Figure 1 In the diagram, the holding time t2 is represented by a shading line running from the top left to the bottom right.

[0294] In the manufacturing method of the present invention, the holding time t1 and the holding time t2 satisfy the following equation (2).

[0295] 1.1≤3t1 / t2≤6.5…(2)

[0296] By maintaining a value of 3t1 / t2 above a fixed value, a bainitic phase transformation is performed, resulting in the concentration of carbon in the retained austenite, thereby increasing the amount of retained austenite with good stability and a small aspect ratio. In other words, the area ratio of retained austenite with an aspect ratio of 5.5 or higher can be reduced.

[0297] Specifically, the value of 3t1 / t2 is 1.1 or more, preferably 1.3 or more, and more preferably 1.5 or more.

[0298] On the other hand, if the value of 3t1 / t2 is too high, the bainitic phase transformation will proceed excessively, and the total area ratio of tempered martensite and bainite will be too high.

[0299] Therefore, the value of 3t1 / t2 is 6.5 or less, preferably 6.3 or less, and more preferably 6.1 or less.

[0300] The holding time t1 within the temperature range T1 is not particularly limited as long as the value of 3t1 / t2 satisfies the above formula (2). For example, it is 10 to 110 seconds, preferably 20 to 100 seconds, and more preferably 30 to 90 seconds.

[0301] The holding time t2 within the temperature range T2 is not particularly limited as long as the value of 3t1 / t2 satisfies the above formula (2). For example, it is 10 to 100 seconds, preferably 20 to 90 seconds, and more preferably 30 to 80 seconds.

[0302] In the first heat treatment, galvanizing treatment and the second heat treatment mentioned above, there is no special limitation on the time that the cold-rolled steel sheet is held at each temperature (cooling stop temperature T4, reheating temperature T6, etc.) as long as the value of 3t1 / t2 satisfies the above formula (2).

[0303] In the manufacturing method of the present invention described above, for example, the holding temperature such as the heating temperature and the reheating temperature only needs to be within the aforementioned temperature range, and may not be fixed. The cooling rate only needs to be within the aforementioned rate range, and may vary during cooling. As long as the aforementioned temperature range and other conditions are met, heat treatment can be performed using any equipment.

[0304] [Components and their manufacturing methods]

[0305] Next, a component made using the high-strength galvanized steel sheet of the present invention described above (hereinafter also referred to as "the component of the present invention") will be described.

[0306] The components of the present invention are obtained by performing at least one of forming and joining processes on the high-strength galvanized steel sheet of the present invention.

[0307] As a forming process, there are no particular limitations; examples include common forming processes such as pressing.

[0308] As a joining process, there are no particular limitations; examples include spot welding, laser welding, arc welding, riveting, and rivet joining.

[0309] There are no special restrictions on the conditions for forming and joining processes; conventional methods can be followed.

[0310] Example

[0311] The present invention will be specifically described below with examples. However, the present invention is not limited to the examples described below.

[0312] [Experimental Example 1]

[0313] Manufacturing of galvanized steel sheets

[0314] Molten steel having the composition shown in Tables 1 and 2 below, with the remainder consisting of Fe and unavoidable impurities, is smelted in a converter and then continuously cast to obtain steel slabs. It should be noted that Table 2 is a continuation of Table 1, and the underlines in Tables 1 and 2 indicate items outside the scope of this invention (the same applies to the other tables).

[0315] The obtained steel slab is hot-rolled under the conditions shown in Table 3 below to obtain hot-rolled steel sheet. Specifically, the steel slab is heated to 1250°C and rough-rolled. Next, finish rolling is performed at a finish rolling end temperature of 900°C, and winding is carried out at the winding temperatures shown in Table 3 below.

[0316] The hot-rolled steel sheet that has been wound is cold-rolled according to the rolling rate shown in Table 3 to obtain a cold-rolled steel sheet.

[0317] The obtained cold-rolled steel sheets were subjected to a first heat treatment, a galvanizing treatment, and a second heat treatment under the conditions shown in Table 3 below.

[0318] It should be noted that in either case, the cold-rolled steel sheet was held for 100 seconds at a heating temperature T3 in an atmosphere with a dew point above -30°C.

[0319] In addition, in any case, after the cold-rolled steel sheet is cooled to the cooling stop temperature T5, it is not held at the cooling stop temperature T5, but is immediately heated to the reheating temperature T6.

[0320] In the galvanizing process, both sides of the cold-rolled steel sheet after the first heat treatment are subjected to hot-dip galvanizing or alloy hot-dip galvanizing to obtain hot-dip galvanized steel sheet (GI) or alloy hot-dip galvanized steel sheet (GA).

[0321] As a molten zinc plating bath, in the case of manufacturing GI, a zinc bath containing Al: 0.20% by mass and the remainder consisting of Zn and unavoidable impurities is used; in the case of manufacturing GA, a zinc bath containing Al: 0.14% by mass and the remainder consisting of Zn and unavoidable impurities is used.

[0322] In the manufacture of either GI or GA, the bath temperature is 470°C.

[0323] For GI manufacturing, the zinc coating adhesion is set at 45–72 g / m² per side. 2 In the case of manufacturing GA, it is set to 45g / m on one side. 2 .

[0324] In the case of manufacturing GA, the alloying temperature is set to 530°C.

[0325] The zinc plating of GI consists of 0.1–1.0% by mass Fe, 0.2–1.0% by mass Al, with the remainder consisting of Fe and unavoidable impurities. The zinc plating of GA consists of 7–15% by mass Fe, 0.1–1.0% by mass Al, with the remainder consisting of Fe and unavoidable impurities.

[0326] Hereinafter, the galvanized steel sheet after the second heat treatment (molten galvanized steel sheet (GI) or alloyed molten galvanized steel sheet (GA)) will be referred to as "galvanized steel sheet".

[0327] <Observations on Microstructures>

[0328] The microstructure of the obtained galvanized steel sheet was observed as follows. The results are shown in Table 4 below.

[0329] Area ratio of tempered martensite, bainite, and fresh martensite

[0330] The obtained galvanized steel sheet was ground with a section parallel to the rolling direction (L-section) as the observation surface. The observation surface was etched with 1% nitric acid ethanol and then observed using a scanning electron microscope (SEM) at magnification up to 3000x. Ten fields of view were observed on the L-section at a position equivalent to 1 / 4 of the sheet thickness from the surface of the steel sheet, and SEM images were acquired.

[0331] For the acquired SEM images, the area ratio of each tissue was calculated, and the average area ratio of 10 fields of view was taken as the area ratio of each tissue. Image-Pro, manufactured by Media Cybernetics, was used as the analysis software for the SEM images.

[0332] In the SEM images of each field of view, the dark gray areas are identified as tempered martensite and bainite, and the combined area ratio of the two is calculated (unit: %).

[0333] Similarly, in the SEM images of each field of view, the white or light gray parts are identified as fresh martensite and retained austenite.

[0334] Residual austenite with a face-centered cubic (fcc) structure is removed. Specifically, EBSD (electron beam backscattering diffraction) data of the same field of view are acquired, and the fcc structure is removed based on this data, with the remaining structure identified as fresh martensite.

[0335] Then, in the SEM image, the area of ​​each tissue identified as fresh martensite is calculated. Based on the calculated area, the equivalent circle diameter of each fresh martensite is determined and used as the diameter of each fresh martensite. Thus, the area percentage (in %) of fresh martensite with a diameter of 2.0 μm or less is calculated.

[0336] Area ratio of retained austenite

[0337] The observation plane (L-section at 1 / 4 of the plate thickness) was observed using X-ray diffraction. A Co Kα-ray source was used as the incident X-ray. The ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron were calculated. The average of the nine ratios was taken as the volume fraction of retained austenite.

[0338] The calculated volume fraction is taken as the area fraction of the retained austenite (unit: %).

[0339] Area ratio of retained austenite with an aspect ratio of 5.5 or higher

[0340] For the observation plane (L-section at 1 / 4 of the plate thickness), EBSD data (phase diagram data) were acquired to determine the fcc microstructure, which was then used as the particles of retained austenite. Data was obtained for 10 fields of view, each 30 μm × 30 μm region within the observation plane, spaced more than 50 μm apart.

[0341] Among the particles in the retained austenite, the longest particle is designated as the major axis length *a*, and the longest particle length that crosses the particle in the direction perpendicular to it is designated as the minor axis length *b*. *a* / *b* is then defined as the aspect ratio. In cases where multiple particles are interconnected, they are divided approximately equally and treated as individual particles.

[0342] Calculate the area of ​​retained austenite with an aspect ratio (a / b) of 5.5 or greater, and determine its proportion relative to the total area of ​​retained austenite in the same field of view. Take the average of the calculated proportions from 10 fields of view as the area ratio (in %) of retained austenite with an aspect ratio of 5.5 or greater relative to the total area of ​​retained austenite in each steel plate.

[0343] <evaluate>

[0344] The obtained galvanized steel sheets were evaluated using the following methods. The results are shown in Table 4 below.

[0345] Tensile Testing

[0346] From the obtained galvanized steel sheet, take specimen No. 5 as described in JIS Z 2201, with the long side direction (tensile direction) at 90° to the rolling direction. Using the specimen, perform 5 tensile tests according to JIS Z 2241, and determine the tensile strength (TS) and elongation after fracture (EL) from the average of the 5 tests. If TS is 1320 MPa or higher, it can be evaluated as high strength. If EL is 10.0% or higher, it can be evaluated as excellent formability.

[0347] Hole Enlargement Test

[0348] The obtained galvanized steel sheet was subjected to a hole expansion test in accordance with JIS Z 2256.

[0349] Specifically, firstly, galvanized steel sheets were sheared, and 100mm × 100mm test pieces were taken. Holes with a diameter of 10mm were punched into the test pieces with a 12.5% ​​gap. Then, using a die with an inner diameter of 75mm, a conical punch with a 60° apex angle was pressed into the hole under controlled pressure of 9 tons (88.26kN), and the hole diameter D at which crack initiation was measured. f (Unit: mm). The initial aperture is D0 (unit: mm). The aperture expansion ratio λ (unit: %) is calculated according to the following formula. If λ is greater than 20%, the formability is considered excellent.

[0350] λ={(D f -D0) / D0}×100

[0351] Determination of the diffusible hydrogen content in steel plates

[0352] The zinc coating is removed from the obtained galvanized steel sheet using a grooving machine (precision grinding machine), and then test pieces with a length of 30 mm and a width of 5 mm are taken. For the taken test pieces, the diffusible hydrogen content in the steel sheet is determined by a temperature-induced desorption analysis method. The heating rate is 200 °C / hr. The cumulative value of hydrogen detected from room temperature (25 °C) to below 210 °C is taken as the diffusible hydrogen content in the steel (unit: ppm by mass). The diffusible hydrogen content in the steel sheet is preferably below 0.60 ppm by mass.

[0353] Fatigue test of spot welded parts

[0354] Using the obtained galvanized steel sheet, cross tensile test specimens with spot welds were prepared according to JIS Z 3138, and fatigue tests were carried out.

[0355] First, spot welding was performed under the conditions of electrode: DR6mm-40R, applied pressure: 4802N (490kgf), and energizing time: 17 cycles. The current value was adjusted to make the weld nugget diameter 6.5mm, and cross-tension test pieces were made.

[0356] At a minimum to maximum load ratio of 0.05, a frequency of 20 Hz, and a repetition count of 10... 7 Under the same conditions, a load was applied, and then a cross-tensile test was performed at a tensile speed of 5 mm / min. The fatigue strength of the spot weld was evaluated based on the maximum cross-tensile strength without any peeling of the specimen.

[0357] Specifically, in Table 4 below, cases with a cross tensile strength of 250 N or more are denoted as "A", cases with a cross tensile strength of 180 N or more but less than 250 N are denoted as "B", and cases with a cross tensile strength of less than 180 N are denoted as "C". If it is "A" or "B", the fatigue strength of the spot weld can be evaluated as excellent.

[0358]

[0359]

[0360] Table 3

[0361]

[0362] Table 4

[0363]

[0364] <Summary of Evaluation Results>

[0365] As shown in Tables 1 to 4 above, the tensile strength of galvanized steel sheets No. 1 to 3, 5 to 6, 9, 14 to 16, 19, 27 and 32 to 41 are all above 1320 MPa, and their formability and fatigue strength of the spot welds are also excellent.

[0366] In contrast, at least one of the tensile strength, formability and fatigue strength of the spot welds of the galvanized steel sheets No.4, 7-8, 10-13, 17-18, 20-26 and 28-31 is insufficient.

[0367] Then, the galvanized steel sheets No.1~3, 5~6, 9, 14~16, 19, 27 and 32~41 are subjected to forming or joining processes to obtain components.

[0368] The tensile strength of the obtained parts is all above 1320MPa, and the formability and fatigue strength of the spot welds are also excellent.

[0369] [Experimental Example 2]

[0370] Manufacturing of galvanized steel sheets

[0371] The steel slabs with the steel symbols shown in Table 5 (refer to Tables 1 and 2 above) are hot-rolled under the conditions shown in Table 5 to obtain hot-rolled steel sheets. Specifically, the steel slabs are heated to 1250°C and rough-rolled. Next, finish rolling is performed at a finish rolling temperature of 900°C, and winding is carried out at the winding temperatures shown in Table 5.

[0372] The hot-rolled steel sheet that has been wound is cold-rolled according to the rolling rate shown in Table 5 to obtain a cold-rolled steel sheet.

[0373] In some embodiments, before performing the first heat treatment described later on the obtained cold-rolled steel sheet, an electroplating treatment is performed using an Fe-based plating bath to form an Fe-based coating.

[0374] In Table 5 below, cases where electroplating was performed are marked "Yes", and cases where no electroplating was performed are marked "-". The amount of Fe-based coatings formed is shown in Table 6 below.

[0375] It should be noted that the surface of the cold-rolled steel sheet was degreased using alkali before electroplating.

[0376] As an Fe-based plating bath, a solution containing 1.5 mol / L Fe was used. 2+ Sulfuric acid bath for ions (bath temperature: 50℃, pH: 2.0).

[0377] In an Fe-based plating bath, using cold-rolled steel sheet as the cathode and iridium oxide electrode as the anode, at 45 A / dm... 2 Electrolytic treatment is performed at a specific current density. The amount of Fe-based coating can be controlled by adjusting the energizing time.

[0378] The obtained cold-rolled steel sheets (or, cold-rolled steel sheets that have undergone electroplating) are subjected to a first heat treatment, a galvanizing treatment, and a second heat treatment under the conditions shown in Table 5 below.

[0379] It should be noted that in any case, the cold-rolled steel sheet was held for 100 seconds at a heating temperature T3 in an atmosphere with the dew points shown in Table 5 below.

[0380] In addition, in any case, after the cold-rolled steel sheet is cooled to the cooling stop temperature T5, it is not held at the cooling stop temperature T5, but is immediately heated to the reheating temperature T6.

[0381] In the galvanizing process, alloyed molten galvanizing is performed on both sides of the cold-rolled steel sheet after the first heat treatment to obtain alloyed molten galvanized steel sheet (GA).

[0382] As a molten zinc plating bath, a zinc bath containing 0.14% by mass of Al and the remainder consisting of Zn and unavoidable impurities is used (bath temperature: 470°C).

[0383] The zinc coating adhesion amount is set at 45g / m² on one side. 2 The alloying temperature is set to 530℃.

[0384] The zinc plating of GA consists of Fe: 7-15% by mass, Al: 0.1-1.0% by mass, with the remainder consisting of Fe and unavoidable impurities.

[0385] Hereinafter, the galvanized steel sheet after the second heat treatment (alloyed molten galvanized steel sheet (GA)) will be referred to as "galvanized steel sheet".

[0386] <Observations on Microstructures>

[0387] The microstructure of the obtained galvanized steel sheet was observed in the same manner as in [Experimental Example 1] above. The results are shown in Table 6 below.

[0388] <Determination of the thickness of the decarburized layer>

[0389] The thickness of the decarburized layer was measured on the obtained galvanized steel sheet using the method described above. The results are shown in Table 6 below.

[0390] <evaluate>

[0391] For the obtained galvanized steel sheet, tensile tests and hole expansion tests were performed in the same manner as in [Experimental Example 1] above to determine TS, EL and λ.

[0392] Then, the amount of diffusible hydrogen in the steel plate was determined in the same manner as in [Experimental Example 1] above. In this case, if the zinc coating has an Fe-based coating, not only the zinc coating but also the Fe-based coating must be removed before the amount of diffusible hydrogen in the steel plate is measured.

[0393] Furthermore, fatigue tests were conducted in the same manner as in [Experimental Example 1] above to evaluate the fatigue strength of the spot weld.

[0394] The results are shown in Table 6 below.

[0395] Characteristics of resistance welding cracks in spot welded parts

[0396] Test pieces were cut from the obtained galvanized steel sheet, according to the... Figures 2-4 The test method described is used to evaluate the resistance to welding cracks in spot welds.

[0397] The test piece was taken from a sample with a tensile strength of 980 MPa and a zinc coating adhesion of 50 g / m. 2 The test was cut from an alloyed molten galvanized steel sheet (thickness t: 1.6mm).

[0398] With an angle θ of 5°, resistance welding is performed on the plate assembly of the test piece and the opposite test piece, with a pressure of 3.5 kN, a holding time of 0.12 seconds, 0.18 seconds or 0.24 seconds, and a welding current and welding time that makes the weld nugget diameter d reach 5.9 mm, to form a welded part.

[0399] Observe the cross-section of the welded section and evaluate its resistance to welding cracks according to the following criteria.

[0400] If the result is A+, A, or B, the resistance to welding cracks of the welded part is judged to be excellent. The results are shown in Table 6 below.

[0401] A+: No cracks longer than 0.1 mm were observed during a holding time of 0.12 seconds.

[0402] A: Cracks longer than 0.1 mm were observed when the holding time was 0.12 seconds, but no cracks longer than 0.1 mm were observed when the holding time was 0.18 seconds.

[0403] B: Cracks longer than 0.1 mm were observed when the holding time was 0.18 seconds, but no cracks longer than 0.1 mm were observed when the holding time was 0.24 seconds.

[0404] C: Cracks longer than 0.1 mm were observed when the holding time was 0.24 seconds.

[0405] Table 5

[0406]

[0407] Table 6

[0408]

[0409] <Summary of Evaluation Results>

[0410] As shown in Tables 5 and 6 above, the tensile strength of galvanized steel sheets No.1, 15, and 42-46 is all above 1320 MPa, and their formability and fatigue strength of the spot welds are also excellent.

[0411] Then, the galvanized steel sheets No.1, 15, and 42-46 are subjected to forming or joining processes to obtain components.

[0412] The tensile strength of the obtained parts is all above 1320MPa, and the formability and fatigue strength of the spot welds are also excellent.

[0413] Next, compare No.1 and 42-45 using the steel symbol A.

[0414] Compared to No.1, which has no decarburization layer and Fe-based coating, No.42-43, which has a decarburization layer (but no Fe-based coating), exhibits superior resistance to resistance welding cracks.

[0415] In addition, No.45, which has both a decarburized layer and an Fe-based coating, has superior resistance to resistance welding cracks compared to No.42-43.

[0416] It should be noted that No. 44 has both a decarburized layer and an Fe-based coating, but due to the small thickness of the decarburized layer, it is speculated that its resistance to welding cracks is equivalent to that of No. 42-43.

[0417] Next, compare No. 15 and 46, which use the steel symbol E.

[0418] Compared to No.15, which lacks both a decarburization layer and an Fe-based coating, No.46, which possesses both a decarburization layer and an Fe-based coating, exhibits superior resistance to resistance welding cracking.

[0419] Components obtained by forming or joining galvanized steel sheets of No.1 and 42-46 exhibit excellent resistance to welding cracks.

Claims

1. A high-strength galvanized steel sheet, comprising a steel sheet and a galvanized layer, having a tensile strength of 1320 MPa or higher. The steel plate has the following composition and microstructure, with a diffusible hydrogen content of less than 0.60 ppm by mass. The composition, by mass%, contains C: 0.150–0.450%, Si: 0.80–3.00%, Mn: 2.00–4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 0.100%, O: less than 0.0100%, and N: less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities. In the aforementioned microstructure, The combined area ratio of tempered martensite and bainite is 60–95%. The area fraction of retained austenite is 5-30%. The area ratio of the retained austenite with an aspect ratio of 5.5 or greater is less than 50% relative to the total retained austenite. The area fraction of fresh martensite with a diameter of less than 2.0 μm is 1% to 20%. The area fraction of the remaining microstructure, excluding tempered martensite, bainite, retained austenite, and fresh martensite, is less than 10%.

2. The high-strength galvanized steel sheet according to claim 1, wherein, The composition further contains, by mass%, at least one element selected from the following: 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.

3. The high-strength galvanized steel sheet according to claim 1, wherein, The steel plate has a decarburized layer.

4. The high-strength galvanized steel sheet according to claim 2, wherein, The steel plate has a decarburized layer.

5. The high-strength galvanized steel sheet according to any one of claims 1 to 4, wherein, A metallic coating is provided on at least one side of the steel plate and between the steel plate and the galvanized layer.

6. The high-strength galvanized steel sheet according to claim 5, wherein, The metal coating is composed of Fe and unavoidable impurities.

7. The high-strength galvanized steel sheet according to claim 6, wherein, The composition of the metal coating further includes at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, totaling less than 10% by mass.

8. The high-strength galvanized steel sheet according to any one of claims 1 to 4, wherein it is a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet.

9. A component made of high-strength galvanized steel sheet according to any one of claims 1 to 8.

10. A method for manufacturing a high-strength galvanized steel sheet, comprising the method for manufacturing the high-strength galvanized steel sheet as described in claim 1. A steel slab having the composition described in claim 1 is hot-rolled, and the resulting hot-rolled steel sheet is wound at a winding temperature of 350–700°C. The hot-rolled steel sheet that has been wound is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is subjected to a first heat treatment, a galvanizing treatment, and a second heat treatment in sequence. In the first heat treatment, the cold-rolled steel sheet is heated at a heating temperature T3 of 750–950°C, and then cooled from the heating temperature T3 to a cooling stop temperature T4 of 350–550°C. 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 50-350°C, and then reheated to a reheating temperature T6 of 300-500°C, which is higher than the cooling stop temperature T5. Then, it is cooled from (Ms point - 200)°C to 50°C at an average cooling rate v2 satisfying the following formula (1). In the first heat treatment, the galvanizing treatment and the second heat treatment, the holding time t1 in the temperature range T1 above 300°C and below 450°C, and the holding time t2 in the temperature range T2 from 450°C to 600°C satisfy the following formula (2). Wherein, [C], [Mn] and [Si] in formula (1) are the contents of C, Mn and Si in the composition, respectively, and the unit of the contents is mass.

11. A method for manufacturing a high-strength galvanized steel sheet, comprising the method for manufacturing the high-strength galvanized steel sheet as described in claim 2. A steel slab having the composition described in claim 2 is hot-rolled, and the resulting hot-rolled steel sheet is wound at a winding temperature of 350–700°C. The hot-rolled steel sheet that has been wound is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is subjected to a first heat treatment, a galvanizing treatment, and a second heat treatment in sequence. In the first heat treatment, the cold-rolled steel sheet is heated at a heating temperature T3 of 750–950°C, and then cooled from the heating temperature T3 to a cooling stop temperature T4 of 350–550°C. 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 50-350°C, and then reheated to a reheating temperature T6 of 300-500°C, which is higher than the cooling stop temperature T5. Then, it is cooled from (Ms point - 200)°C to 50°C at an average cooling rate v2 satisfying the following formula (1). In the first heat treatment, the galvanizing treatment and the second heat treatment, the holding time t1 in the temperature range T1 above 300°C and below 450°C, and the holding time t2 in the temperature range T2 from 450°C to 600°C satisfy the following formula (2). Wherein, [C], [Mn] and [Si] in formula (1) are the contents of C, Mn and Si in the composition, respectively, and the unit of the contents is mass.

12. The method for manufacturing high-strength galvanized steel sheet according to claim 10, wherein, In the first heat treatment, heating at the heating temperature T3 is carried out in an atmosphere with a dew point greater than -30°C, forming a decarburized layer on the outermost surface of the cold-rolled steel sheet.

13. The method for manufacturing high-strength galvanized steel sheet according to claim 11, wherein, In the first heat treatment, heating at the heating temperature T3 is carried out in an atmosphere with a dew point greater than -30°C, forming a decarburized layer on the outermost surface of the cold-rolled steel sheet.

14. The method for manufacturing high-strength galvanized steel sheet according to any one of claims 10 to 13, 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.

15. The method for manufacturing high-strength galvanized steel sheet according to claim 14, wherein, The metal coating is composed of Fe and unavoidable impurities.

16. The method for manufacturing high-strength galvanized steel sheet according to claim 15, wherein, The composition of the metal coating further includes at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, totaling less than 10% by mass.

17. The method for manufacturing high-strength galvanized steel sheet according to any one of claims 10 to 13, wherein, The galvanizing process is either melt galvanizing or alloy melt galvanizing.

18. A method for manufacturing a component, wherein the component is obtained by performing at least one of forming and joining processes on a high-strength galvanized steel sheet as described in any one of claims 1 to 8.

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