Hot-dip galvanized steel sheet, method of manufacturing the same, and parts

Abstract

This invention provides a hot-dip galvanized steel sheet with high YR (glucose content), high ductility, elongation flange and bending properties, and improved shear workability, with a TS (strength) of 1180 MPa or higher. The base steel sheet has a specified composition and microstructure, particularly with an aspect ratio of 2.0 or less: the proportion of residual γ (glucose content) exceeds 50%, the number of hardness frequency groups of 0.25 or higher is 1, the area ratio of quenched martensite in the surface layer is 80% or less, the surface softening thickness is 10 μm to 100 μm, and the diffusible hydrogen content in the low-temperature region is 0.015 ppm by mass or less.

Description

Technical Field

[0001] This invention relates to a hot-dip galvanized steel sheet, its manufacturing method, and components. Background Technology

[0002] To balance reducing CO2 emissions through vehicle lightweighting with improved crashworthiness, the use of high-strength steel sheets in automobiles is being promoted. Furthermore, new legal regulations are constantly being introduced. Therefore, to enhance vehicle body strength, the application of high-strength steel sheets, especially those with a tensile strength (TS) of 1180 MPa or higher, is gradually increasing as a primary structural component and reinforcing component (hereinafter also referred to as the vehicle's frame structural components, etc.) forming the chassis of the automobile.

[0003] Furthermore, high-strength steel sheets used in automotive frame structural components require high component strength when formed into these components. Increasing component strength, for example, by increasing the yield strength along the length of the component (hereinafter referred to as YS) or by increasing the yield ratio of the steel sheet (=YS / TS×100, hereinafter referred to as YR), is effective. This results in an increase in the impact energy absorbed during a car collision (hereinafter referred to as impact energy absorption).

[0004] Furthermore, components such as collision boxes in automotive skeleton structures have punched end faces and bent sections. Therefore, from a formability point of view, steel sheets with high elongation flanges and bending properties are preferred for such components, in addition to high ductility.

[0005] Furthermore, from the perspective of rust prevention performance of the car body, steel sheets used as materials for automotive frame structural components are sometimes hot-dip galvanized steel sheets obtained by hot-dip galvanizing.

[0006] As a technology involving such hot-dip galvanized steel sheets, for example, it is disclosed in Patent Document 1:

[0007] "A high-strength hot-dip galvanized steel sheet comprises a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, having a tensile strength of 780 MPa or higher. The base steel sheet has the following composition, by mass%, containing C: 0.050%–0.200%, Si: 0.10%–0.90%, Mn: 2.00%–3.50%, P: 0.001%–0.100%, S: less than 0.0200%, Al: less than 1.000%, N: less than 0.0100%, Ca: less than 0.0200%, and Cr: less than 0.300%, and the [%Mn] / [%Si] ratio satisfies a relationship of 2.9–11.7, with the remainder consisting of Fe and unavoidable impurities."

[0008] The base steel plate has the following steel structure:

[0009] The combined area percentage of one or two types of bainite and ferrite is 5% to 85%.

[0010] The surface area of ​​tempered martensite is below 65%.

[0011] The area fraction of quenched martensite is 5%–40%, and the area fraction of retained austenite is less than 5.0%.

[0012] The ratio of Si enrichment to Mn enrichment in the surface layer of the aforementioned base steel plate is 0.7 to 1.3, and the amount of diffusible hydrogen in the aforementioned base steel plate is less than 0.80 ppm by mass.

[0013] Wherein, [%Mn] and [%Si] represent the content (mass%) of Mn and Si in the steel, respectively.

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent No. 6777267 Summary of the Invention

[0017] However, for high-strength steel plates with a strength (TS) of 1180 MPa or higher, especially hot-dip galvanized steel plates, there is a concern about embrittlement and cracking during shearing. Therefore, good shearability is also required.

[0018] However, the hot-dip galvanized steel sheet described in Patent Document 1 does not take into account shearing processability. Therefore, from the viewpoint of increasing the application rate of high-strength steel sheets with a strength of 1180 MPa or higher, especially hot-dip galvanized steel sheets, in automotive frame structural components, there is a current demand for the development of hot-dip galvanized steel sheets with a strength of 1180 MPa or higher that have high strength (YR), high ductility, tensile flange and bending properties, and improved shearing processability.

[0019] The present invention was developed in view of the above-mentioned situation, and its object is to provide a hot-dip galvanized steel sheet with a TS of 1180 MPa or more that has high YR (thereby obtaining high component strength when applied to components), high ductility, tensile flange and bending properties, and improved shear workability.

[0020] In addition, the present invention aims to provide a method for manufacturing the above-mentioned hot-dip galvanized steel sheet.

[0021] Furthermore, the object of the present invention is to provide a component made using the above-mentioned hot-dip galvanized steel sheet.

[0022] Here, "high YR (high component strength)" means that the YR is 65% or higher.

[0023] It should be noted that YR is obtained by the following formula (2).

[0024] YR=YS / TS×100……(2)

[0025] In addition, TS and YS were measured according to JIS Z 2241.

[0026] "High ductility" means that the total elongation (hereinafter also referred to as E1) is 6% or more as measured according to JIS Z 2241.

[0027] "High tensile flange property" refers to a porosity (hereinafter referred to as λ) of 30% or more as measured by JIS Z 2256.

[0028] "High bending performance" means that the pass rate of the bending test conducted in accordance with JIS Z 2248 (for details, see the description of the examples below) is 100%.

[0029] "High shear workability" means that no cracks were observed on the shear end face of the test piece in the shear work test described in the examples described later.

[0030] Furthermore, the inventors conducted repeated and in-depth research to achieve the aforementioned objectives. The results yielded the following insights.

[0031] (1) For the base steel plate, based on the specified composition, the steel structure is set to be mainly martensite (quenched martensite, tempered martensite, and bainite). As a result, a high tensile flange strength of TS: above 1180MPa can be obtained.

[0032] (2) High ductility can be obtained by ensuring a certain amount of residual austenite in the steel structure of the base material steel plate.

[0033] (3) By setting the proportion of grains with an aspect ratio of less than 2.0 in the grains constituting the retained austenite to more than 50%, and setting the number of bins with a frequency of more than 0.25 in the histogram of the hardness distribution at 1 / 4 of the thickness of the base steel plate to 1, a high YR (high component strength) is obtained.

[0034] (4) By setting the area ratio of quenched martensite in the surface layer of the base steel plate to less than 80% and setting the surface softening thickness of the base steel plate to 10μm to 100μm, high flexibility can be obtained.

[0035] (5) High shear workability can be obtained by making the diffusible hydrogen content in the low-temperature region of the base steel plate below 0.015 ppm by mass.

[0036] This invention was completed based on further research conducted on the above-mentioned insights.

[0037] The main structure of this invention is as follows.

[0038] 1. A hot-dip galvanized steel sheet, comprising a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet, having a tensile strength of 1180 MPa or higher.

[0039] The aforementioned base steel plate has the following composition and steel structure:

[0040] The above composition, by mass%, is as follows: C: 0.090%–0.390%, Si: 0.01%–2.50%, Mn: 2.00%–4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 0.100%, and N: less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities.

[0041] In the aforementioned steel microstructure, at the position of 1 / 4 thickness of the base steel plate, the area fraction of martensite is 70% or more, the area fraction of ferrite is 10% or less, and the area fraction of retained austenite is 0.5% or more and less than 10.0%, and the proportion of grains with an aspect ratio of 2.0 or less among the grains constituting the retained austenite exceeds 50%.

[0042] In the histogram of the hardness distribution at 1 / 4 of the thickness of the aforementioned base steel plate, there is one group with a frequency of 0.25 or higher. The class range of the group intervals in the above histogram, measured in Vickers hardness (HV), exceeds (n-1)×20+450 but is less than n×20+450, where n is an integer from 1 to 10.

[0043] Furthermore, in the surface portion of the aforementioned base steel plate, the area ratio of quenched martensite is 80% or less, and the surface portion of the aforementioned base steel plate is the region from the surface of the aforementioned base steel plate to a depth of 10 μm.

[0044] The surface softening thickness of the aforementioned base steel plate is 10μm to 100μm.

[0045] The diffusible hydrogen content in the low-temperature region of the aforementioned base steel plate is below 0.015 ppm by mass.

[0046] Here, the diffusible hydrogen content in the low-temperature region of the base steel plate is the amount of hydrogen released from the base steel plate when it is heated from room temperature to 50°C.

[0047] 2. The hot-dip galvanized steel sheet according to claim 1 above, wherein the composition of the base steel sheet further comprises, by mass %, a component selected from O: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: At least one of the following: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

[0048] 3. The hot-dip galvanized steel sheet according to 1 or 2 above, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

[0049] 4. A method for manufacturing hot-dip galvanized steel sheet, comprising hot rolling a steel billet having the composition described in 1 or 2 above, under the conditions of a winding temperature of 350°C to 600°C and a residence time in a temperature range of 300°C or higher during cooling after winding of 5000 s or more, to obtain a hot-rolled steel sheet.

[0050] Next, the hot-rolled steel plates are pickled.

[0051] Next, the hot-rolled steel sheet is subjected to a first heat treatment under the conditions of a heat treatment temperature of 450℃~650℃ and a residence time of 10 minutes or more in a temperature range above 400℃ but below the heat treatment temperature.

[0052] Next, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.

[0053] Next, the cold-rolled steel sheet was annealed under the following conditions: an average heating rate of 10°C / s or more in the temperature range of 250°C to 700°C; an oxygen concentration of 0.5% to 5.0% by volume in the temperature range of 250°C to 700°C; an annealing temperature of 820°C to 950°C; and a dew point of -35°C or more in the annealing temperature range.

[0054] Next, the cold-rolled steel sheet is subjected to hot-dip galvanizing to produce coated steel sheet.

[0055] Next, the above-mentioned coated steel sheet is cooled at a cooling stop temperature below 150°C.

[0056] Next, the above-mentioned coated steel sheet is subjected to a second heat treatment under the condition that the following formula (1) is satisfied.

[0057] 6.5≤(T+273)×{log(t×3600)+20} / 1000≤13.0……(1)

[0058] Here, T is the heat treatment temperature (°C) of the second heat treatment, and t is the holding time (hr) of the second heat treatment.

[0059] 5. The method for manufacturing hot-dip galvanized steel sheet according to 4 above, wherein, after the hot-dip galvanizing treatment, the coated steel sheet is subjected to alloying treatment.

[0060] 6. A component made of hot-dip galvanized steel sheet as described in any one of 1 to 3 above.

[0061] 7. The component according to 6 above is used as a skeleton structural member of an automobile or as a reinforcing member of an automobile.

[0062] Invention Effects

[0063] According to the present invention, hot-dip galvanized steel sheets with a TS of 1180 MPa or higher, having high YR, high ductility, tensile flange and bending properties, and improved shear workability, can be obtained.

[0064] In particular, the hot-dip galvanized steel sheet of this invention possesses excellent properties, making it suitable for use in the frame structure components of automobiles of various sizes and shapes. This enables improved fuel efficiency through vehicle weight reduction, resulting in significant industrial value. Detailed Implementation

[0065] The present invention is described based on the following embodiments.

[0066] [1] Hot-dip galvanized steel sheet

[0067] [1-1] Base Material Steel Plate

[0068] First, the composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described. It should be noted that the unit of composition is "mass%", and hereafter, unless otherwise specified, it will only be expressed as "%".

[0069] C: 0.090%~0.390%

[0070] Carbon (C) is one of the important basic components. Specifically, C is an element that particularly affects the proportions of martensite, ferrite, and retained austenite, as well as the aspect ratio of the retained austenite. Here, when the C content is below 0.090%, the martensite content decreases, making it difficult to achieve a TS (Total Strength) of 1180 MPa or higher. On the other hand, if the C content exceeds 0.390%, the aspect ratio of the retained austenite increases, making it difficult to achieve the desired YR (Yet-Resistant Ratio). Therefore, the C content is set to 0.090% to 0.390%. The C content is preferably 0.100% or more, more preferably 0.110% or more. The C content is preferably 0.360% or less, more preferably 0.350% or less.

[0071] Si: 0.01%~2.50%

[0072] Si suppresses carbide formation during continuous annealing and promotes the formation of retained austenite. In other words, Si is an element that affects the fraction and aspect ratio of retained austenite. Furthermore, Si is an element that affects the hardness distribution of the base steel sheet, particularly the hardness distribution of martensite. If the Si content is less than 0.01%, uneven hardness of the martensite occurs during cooling after annealing or during the second heat treatment. Consequently, the number of groups with a frequency of 0.25 or higher in the histogram of the hardness distribution at the 1 / 4 thickness position of the base steel sheet increases, and YR and λ decrease. Additionally, bendability also decreases. On the other hand, if the Si content exceeds 2.50%, the aspect ratio of retained austenite increases, making it impossible to achieve the desired YR. Additionally, λ also decreases. Therefore, the Si content is set to Si: 0.01% to 2.50%. The Si content is preferably 0.10% or more, more preferably 0.15% or more. The Si content is preferably 2.00% or less, more preferably 1.50% or less.

[0073] Mn: 2.00%~4.00%

[0074] Mn is one of the important basic components. Specifically, Mn is a crucial element that significantly affects the martensite content. When the Mn content is below 2.00%, the martensite content decreases, making it difficult to achieve a hardness (TS) of 1180 MPa or higher. On the other hand, when the Mn content exceeds 4.00%, uneven martensite hardness occurs during cooling after annealing or during the second heat treatment. Consequently, the number of groups with a frequency of 0.25 or higher in the histogram of the hardness distribution at 1 / 4 of the plate thickness of the base steel increases, and YR and λ decrease. Furthermore, flexibility also decreases. Therefore, the Mn content is between 2.00% and 4.00%. The Mn content is preferably 2.20% or more, more preferably 2.50% or more. The Mn content is preferably 3.80% or less, more preferably 3.60% or less.

[0075] P: below 0.100%

[0076] Phosphorus (P) segregates at the original austenite grain boundaries, causing grain boundary embrittlement. Therefore, the ultimate deformation capacity of the steel sheet decreases, and thus the strength (λ) decreases. Furthermore, the flexibility also decreases. Therefore, the P content is 0.100% or less. The P content is preferably 0.070% or less. It should be noted that there is no particular lower limit for the P content; P is a solid solution strengthening element that can improve the strength of the steel sheet. Therefore, the P content is preferably 0.001% or more.

[0077] S: below 0.0200%

[0078] Sulfide (S) exists in the form of sulfides, reducing the steel's ultimate deformation capacity. Therefore, the bending strength (λ) decreases. Furthermore, the flexibility also decreases. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0050% or less. It should be noted that while there is no specific lower limit for the S content, due to limitations in production technology, the S content is preferably 0.0001% or more.

[0079] Al: below 0.100%

[0080] Al is an element that raises the A3 phase transformation point and generates ferrite phase in the steel microstructure. However, if a large amount of ferrite phase is generated in the steel microstructure, it is difficult to achieve the desired YR (yellow rust). Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.050% or less. It should be noted that there is no specific lower limit for the Al content. Al inhibits carbide formation during continuous annealing and promotes the formation of retained austenite. That is, Al affects the fraction and aspect ratio of retained austenite. Therefore, the Al content is preferably 0.001% or more.

[0081] N: below 0.0100%

[0082] Nitrogen (N) exists in the form of nitrides, which reduces the steel's ultimate deformation capacity. Therefore, the bending strength (λ) decreases. Furthermore, the flexibility also decreases. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0050% or less. It should be noted that while there is no specific lower limit for the N content, due to limitations in production technology, the N content is preferably 0.0005% or more.

[0083] In one embodiment of the present invention, the base steel sheet of the hot-dip galvanized steel sheet has a composition containing the aforementioned elements, with the remainder consisting of Fe and unavoidable impurities. Furthermore, in another embodiment of the present invention, the base steel sheet of the hot-dip galvanized steel sheet has a composition containing the aforementioned elements, with the remainder consisting of Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, and As. Their presence is permissible if the total content of these impurities is 0.100% or less.

[0084] The basic composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention has been described above, but it may also contain at least one of the following additive elements, either alone or in combination.

[0085] O: Below 0.0100%

[0086] Ti: below 0.200%

[0087] Nb: below 0.200%

[0088] V: Below 0.200%

[0089] Ta: below 0.10%

[0090] W: below 0.10%

[0091] B: Below 0.0100%

[0092] Cr: less than 1.00%

[0093] Mo: 1.00% or less,

[0094] Ni: below 1.00%

[0095] Co: less than 0.010%

[0096] Cu: less than 1.00%

[0097] Sn: below 0.200%

[0098] Sb: below 0.200%

[0099] Ca: below 0.0100%

[0100] Mg: less than 0.0100%

[0101] REM: below 0.0100%

[0102] Zr: below 0.100%

[0103] Te: less than 0.100%

[0104] Hf: below 0.10% and Bi: below 0.200%

[0105] The preferred content of each element when these arbitrarily added elements are included is explained below.

[0106] O: Below 0.0100%

[0107] O, existing in the form of oxides, reduces the steel's ultimate deformation capacity. Therefore, λ decreases. Furthermore, bendability also decreases. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0050% or less. It should be noted that while there is no specific lower limit for the O content, due to limitations in production technology, the O content is preferably 0.0001% or more.

[0108] Ti: below 0.200%, Nb: below 0.200%, V: below 0.200%

[0109] Ti, Nb, and V form precipitates and inclusions. When these precipitates and inclusions become coarse and form in large quantities, the ultimate deformation capacity of the steel sheet decreases. Therefore, λ decreases. Furthermore, bendability also decreases. Therefore, the contents of Ti, Nb, and V are each set to 0.200% or less. The contents of Ti, Nb, and V are preferably 0.100% or less. It should be noted that there is no particular lower limit for the contents of Ti, Nb, and V. By adding Ti, Nb, and V, the recrystallization temperature during the heating process in continuous annealing increases. This results in more uniform martensite hardness, which contributes to the increase of YR. Therefore, the contents of Ti, Nb, and V are preferably 0.001% or more.

[0110] Ta: 0.10% ~ W: 0.10%

[0111] Ta and W form precipitates and inclusions. When these precipitates and inclusions become coarse and form in large quantities, the ultimate deformation capacity of the steel sheet decreases. Therefore, λ decreases. In addition, the bendability also decreases. Therefore, the content of Ta and W is set to 0.10% or less each. The content of Ta and W is preferably 0.08% or less each. It should be noted that there is no particular specification for the lower limit of the content of Ta and W. Ta and W improve the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing. Therefore, the content of Ta and W is preferably 0.01% or more each.

[0112] B: Below 0.0100%

[0113] Botrytis cinerea (B) promotes the formation of internal cracks in steel sheets during casting or hot rolling, thus reducing the ultimate deformation capacity of the steel sheet. Therefore, λ (hardness index) decreases. Furthermore, bendability also decreases. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less. It should be noted that there is no particular lower limit for the B content. B is an element that segregates at austenite grain boundaries during annealing, improving hardenability. Therefore, the B content is preferably set to 0.0003% or more.

[0114] Cr: less than 1.00%, Mo: less than 1.00%, Ni: less than 1.00%

[0115] Excessive Cr, Mo, and Ni content increases coarse precipitates and inclusions, reducing the steel plate's ultimate deformation capacity. Consequently, λ decreases. Furthermore, bendability also decreases. Therefore, the contents of Cr, Mo, and Ni are each set to 1.00% or less. Preferably, the contents of Cr, Mo, and Ni are each 0.80% or less. It should be noted that there is no specific lower limit for the contents of Cr, Mo, and Ni. Cr, Mo, and Ni are all elements that improve hardenability. Therefore, the contents of Cr, Mo, and Ni are each preferably 0.01% or more.

[0116] Co: less than 0.010%

[0117] Excessive Co content increases coarse precipitates and inclusions, reducing the steel plate's ultimate deformation capacity. Consequently, λ decreases. Furthermore, bendability also decreases. Therefore, the Co content is set to 0.010% or less. Preferably, the Co content is 0.008% or less. It should be noted that there is no specific lower limit for the Co content. Co is an element that improves hardenability. Therefore, the Co content is preferably 0.001% or more.

[0118] Cu: below 1.00%

[0119] Excessive Cu content increases coarse precipitates and inclusions, reducing the steel plate's ultimate deformation capacity. Consequently, λ decreases. Furthermore, bendability also decreases. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less. It should be noted that there is no specific lower limit for the Cu content. Cu is an element that improves hardenability. Therefore, the Cu content is preferably 0.01% or more.

[0120] Sn: below 0.200%

[0121] Sn promotes the formation of internal cracks in steel sheets during casting or hot rolling, reducing the ultimate deformation capacity of the steel sheet. Therefore, λ decreases. Furthermore, bendability also decreases. Therefore, the Sn content is 0.200% or less. The Sn content is preferably 0.100% or less. It should be noted that there is no particular lower limit for the Sn content. Sn is an element that improves hardenability. Therefore, the Sn content is preferably set to 0.001% or more.

[0122] Sb: below 0.200%

[0123] Excessive Sb content increases coarse precipitates and inclusions, reducing the steel plate's ultimate deformation capacity. Consequently, λ decreases. Furthermore, bendability also decreases. Therefore, the Sb content is set to 0.200% or less. The Sb content is preferably 0.100% or less. It should be noted that there is no specific lower limit for the Sb content. Therefore, Sb is an element that controls the surface softening thickness and allows for strength adjustment. Therefore, the Sb content is preferably 0.001% or more.

[0124] Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%

[0125] Excessive content of Ca, Mg, and REM increases coarse precipitates and inclusions, reducing the ultimate deformation capacity of the steel plate. Therefore, λ decreases. Furthermore, bendability also decreases. Therefore, the contents of Ca, Mg, and REM are each 0.0100% or less. The contents of Ca, Mg, and REM are preferably 0.0050% or less. It should be noted that there is no specific lower limit for the contents of Ca, Mg, and REM. Ca, Mg, and REM are all elements that spheroidize nitrides and sulfides, improving the ultimate deformation capacity of the steel plate. Therefore, the contents of Ca, Mg, and REM are preferably 0.0005% or more.

[0126] Zr: below 0.100%, Te: below 0.100%

[0127] Excessive Zr and Te content increases coarse precipitates and inclusions, reducing the ultimate deformation capacity of the steel plate. Consequently, λ decreases. Furthermore, bendability also decreases. Therefore, the Zr and Te content is 0.100% or less. Preferably, the Zr and Te content is 0.080% or less. It should be noted that there is no specific lower limit for the Zr and Te content. Both Zr and Te are elements that spheroidize nitrides and sulfides, improving the ultimate deformation capacity of the steel plate. Therefore, the Zr and Te content is preferably 0.001% or more.

[0128] Hf: below 0.10%

[0129] If the Hf content is excessive, it increases the amount of coarse precipitates and inclusions, reducing the ultimate deformation capacity of the steel plate. Therefore, λ decreases. Furthermore, bendability also decreases. Therefore, the Hf content is set to 0.10% or less. The Hf content is preferably 0.08% or less. It should be noted that there is no specific lower limit for the Hf content. Hf is an element that spheroidizes nitrides and sulfides, improving the ultimate deformation capacity of the steel plate. Therefore, the Hf content is preferably 0.01% or more.

[0130] Bi: below 0.200%

[0131] Excessive Bi content increases coarse precipitates and inclusions, reducing the ultimate deformation capacity of the steel plate. Therefore, λ decreases. Furthermore, bendability also decreases. Therefore, the Bi content is 0.200% or less. The Bi content is preferably 0.100% or less. It should be noted that there is no specific lower limit for the Bi content. Bi is an element that reduces segregation. Therefore, the Bi content is preferably 0.001% or more.

[0132] It should be noted that the contents of O, Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi mentioned above will not impair the effectiveness of the present invention when they are below the preferred lower limit values, and are therefore included as unavoidable impurities.

[0133] Next, the steel structure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0134] In one embodiment of the present invention, the microstructure of the base steel sheet of the hot-dip galvanized steel sheet, at a position of 1 / 4 of the sheet thickness, has an area ratio of martensite of 70% or more, an area ratio of ferrite of 10% or less, and an area ratio of retained austenite of 0.5% or more and less than 10.0%, and in the grains constituting the retained austenite, the proportion of grains with an aspect ratio of 2.0 or less exceeds 50%.

[0135] In the histogram of the hardness distribution at 1 / 4 of the thickness of the aforementioned base steel plate, there is one group with a frequency of 0.25 or higher. The class range of the group intervals in the above histogram, measured in Vickers hardness (HV), exceeds (n-1)×20+450 but is less than n×20+450, where n is an integer from 1 to 10.

[0136] Furthermore, in the surface portion of the aforementioned base steel plate, the area ratio of quenched martensite is 80% or less, and the surface portion of the aforementioned base steel plate is the region from the surface of the aforementioned base steel plate to a depth of 10 μm.

[0137] The surface softening thickness of the aforementioned base steel plate is 10μm to 100μm.

[0138] The martensite area ratio at 1 / 4 of the thickness of the base steel plate is over 70%.

[0139] In the microstructure of the base steel plate, martensite is the main phase. Specifically, by setting the area fraction of martensite at the 1 / 4 thickness position of the base steel plate to 70% or more, high tensile flange strength (TS) of 1180 MPa or more can be achieved. Therefore, the area fraction of martensite at the 1 / 4 thickness position of the base steel plate is 70% or more. The area fraction of martensite at the 1 / 4 thickness position of the base steel plate is preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more. It should be noted that there is no particular upper limit to the area fraction of martensite at the 1 / 4 thickness position of the base steel plate. From the viewpoint of obtaining high ductility, the area fraction of martensite at the 1 / 4 thickness position of the base steel plate is preferably 99% or less, more preferably 98% or less, and even more preferably 97% or less. It should be noted that the martensite here includes not only quenched martensite (fresh martensite), but also tempered martensite and bainite.

[0140] The ferrite area fraction at 1 / 4 of the plate thickness of the base steel plate is less than 10%.

[0141] By ensuring that the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate is 10% or less, YR increases. Furthermore, λ increases, and the bendability also improves. Therefore, the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate is 10% or less. Preferably, the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate is 8% or less. Furthermore, the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate can be 0%. From the viewpoint of improving ductility, the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate is preferably 1% or more, more preferably 2% or more. It should be noted that ferrite here is sometimes also defined as bainitic ferrite.

[0142] Here, the area ratios of martensite and ferrite at the 1 / 4 position of the base steel plate thickness are determined as follows.

[0143] Specifically, the specimen was cut with the thickness section (L-section) of the base steel plate parallel to the rolling direction as the observation surface. Next, the observation surface of the specimen was mirror-polished using diamond polishing paste, followed by fine polishing using colloidal silica. Then, the observation surface of the specimen was etched with 3 vol.% nitric acid ethanol to reveal the microstructure. Next, using a SEM (Scanning Electron Microscope) equipped with an InLens detector, the observation position was set at 1 / 4 of the thickness of the base steel plate (i.e., the 1 / 4 thickness of the steel plate became the center of the field of view in the thickness direction). Three fields of view were observed under the following conditions: accelerating voltage: 1 kV, magnification: 5000x, and field of view: 17 μm × 23 μm. The areas of martensite and ferrite were calculated from the obtained microstructure images using Adobe Photoshop (Adobe Systems). Next, the areas of martensite and ferrite calculated for each field of view are divided by the area of ​​the field of view (17μm×23μm), and the arithmetic mean of these values ​​is taken as the area ratio of martensite and ferrite.

[0144] It should be noted that in the above microstructure images, the phases can be distinguished from each other in the following ways: Quenched martensite is a convex structure with fine internal irregularities. Tempered martensite and bainite are concave structures containing fine carbides. Ferrite is a concave structure, a flat structure without carbides.

[0145] The area fraction of retained austenite at 1 / 4 of the plate thickness of the base steel plate: greater than 0.5% and less than 10.0%.

[0146] By including a certain amount of retained austenite in the microstructure of the base steel plate, the desired El can be achieved. To obtain this effect, the area fraction of retained austenite at the 1 / 4 position of the base steel plate thickness is set to 0.5% or more. On the other hand, if the area fraction of retained austenite at the 1 / 4 position of the base steel plate thickness is 10.0% or more, it is difficult to achieve the desired YR. Therefore, the area fraction of retained austenite at the 1 / 4 position of the base steel plate thickness is 0.5% or more and less than 10.0%. The area fraction of retained austenite at the 1 / 4 position of the base steel plate thickness is preferably 1.0% or more, more preferably 1.5% or more. The area fraction of retained austenite at the 1 / 4 position of the base steel plate thickness is preferably 8.0% or less, more preferably 6.0% or less.

[0147] Here, the area ratio of retained austenite at the 1 / 4 position of the base steel plate thickness is determined as follows.

[0148] The steel plate was ground with the observation surface at a distance of 1 / 4 of the plate thickness from the surface (equivalent to 1 / 4 of the plate thickness in the depth direction). Further grinding of 0.1 mm was performed using chemical grinding. Next, the observation surface was examined using an X-ray diffraction apparatus with a Co Kα source to measure the integrated reflection intensity of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron. The volume fraction of austenite was calculated by the intensity ratio of the integrated reflection intensity from each plane of fcc iron (austenite) to that from each plane of bcc iron. However, this volume fraction of austenite was considered three-dimensionally uniform, and the area fraction of residual austenite at the 1 / 4 plate thickness position of the base steel plate was taken as the base material.

[0149] Furthermore, at the 1 / 4 position of the base steel plate thickness, the area fraction of the remaining microstructure other than martensite, ferrite, and retained austenite is preferably 5% or less. Examples of this remaining microstructure include microstructures known as other steel plate microstructures, such as pearlite, cementite, and metastable carbides (ε-carbides, η-carbides, χ-carbides, etc.). The remaining microstructure can be identified, for example, by observation using a SEM (Scanning Electron Microscope).

[0150] In addition, the area ratio of the remaining tissue is calculated using the following formula.

[0151] [Area percentage of remaining microstructure (%)] = 100 - [Area percentage of martensite (%)] - [Area percentage of ferrite (%)] - [Area percentage of retained austenite (%)]

[0152] The proportion of grains with an aspect ratio of 2.0 or less constituting retained austenite at 1 / 4 of the thickness of the base steel plate (also referred to as the proportion of retained γ grains with an aspect ratio of 2.0 or less): exceeding 50%.

[0153] The proportion of residual γ grains with an aspect ratio of 2.0 or less is an extremely important factor. By reducing the aspect ratio (major axis length / minor axis length) of the grains constituting the retained austenite, i.e., increasing the proportion of near-equiaxed grains, the phase transformation from retained austenite to martensite in the early stages of deformation is suppressed, thereby improving the YR (yellow refractive index). To achieve this effect, the proportion of residual γ grains with an aspect ratio of 2.0 or less needs to exceed 50%. It should be noted that a higher proportion of residual γ grains with an aspect ratio of 2.0 or less is preferred, preferably 60% or more, and more preferably 70% or more.

[0154] Here, the proportion of residual γ with an aspect ratio of 2.0 or less is determined as follows.

[0155] Specifically, a 300 μm thick sample was manufactured by machining with the observation surface at a position 1 / 4 of the plate thickness from the surface (equivalent to 1 / 4 of the plate thickness in the depth direction). Then, the sample, which had undergone thin-film formation using electrolytic polishing and a dual-jet electrolytic polishing method, was measured by EBSD. EBSD measurements were performed under conditions sufficient for evaluating retained austenite (measurement field: 15 μm × 15 μm, measurement point interval: 20 nm, accelerating voltage: 20 kV) across three fields of view. Based on the obtained crystal orientation data, the phase was reduced to Iron-Gamma using AMETEK EDAX's OIMAnalysis, first removing information about the BCC phase. Next, a CI (Confidence Index) > 0.1 was set as the threshold for the obtained crystal orientation data. Then, the aspect ratio of the retained austenite grains and the number fraction of retained γ grains with an aspect ratio of 2.0 or less were calculated using the Grain Shape Aspect Ratio.

[0156] In the histogram of hardness distribution at 1 / 4 of the plate thickness of the base steel plate, the number of group intervals with a frequency of 0.25 or higher (hereinafter also referred to as the number of group intervals with hardness frequencies of 0.25 or higher): 1.

[0157] The number of hardness frequency groups above 0.25 is extremely important. Setting the number of hardness frequency groups above 0.25 to one, in other words, by making the hardness of the base steel plate uniform and reducing the hardness deviation of the base steel plate, the YR can be increased. In addition, λ and bendability are also improved. Therefore, the number of hardness frequency groups above 0.25 is set to one. It should be noted that the class range of the hardness distribution histogram at 1 / 4 of the plate thickness of the base steel plate is set in Vickers hardness HV to be greater than (n-1)×20+450 and less than n×20+450, and n is set to an integer from 1 to 10.

[0158] In addition, the number of groups with hardness frequencies of 0.25 or higher was determined as follows.

[0159] The test specimen was cut using a section (L-section) of the base steel plate parallel to the rolling direction as the test surface. Next, the test surface of the specimen was mirror-polished using diamond polishing paste. At a position 1 / 4 of the thickness of the base steel plate on the test surface of the specimen, a micro Vickers hardness tester was used to measure the Vickers hardness (HV) at 50 points under a load of 5 gf. Then, the class intervals were set to a range exceeding (n-1)×20+450 and below n×20+450 in Vickers hardness HV, with n set to an integer from 1 to 10. A histogram of hardness distribution was created, and the number of class intervals with a frequency of 0.25 or higher was counted in the created histogram. It should be noted that the frequency of each class interval is the value obtained by dividing the number of Vickers hardness HV measurements classified into each class interval by the total number of Vickers hardness HV measurements (50). For example, if there are 10 test results in the range of Vickers hardness HV exceeding 450 and below 470, the frequency of the class interval for the grade range exceeding 450 and below 470 is 0.20 (=10÷50).

[0160] The area fraction of quenched martensite (fresh martensite) in the surface layer of the base steel plate is below 80%.

[0161] The area ratio of quenched martensite in the surface layer of the base steel plate is an extremely important factor. By reducing the area ratio of quenched martensite in the surface layer of the base steel plate, in other words, by increasing the area ratio of phases with low hydrogen solubility such as ferrite, bainitic ferrite, bainite, and tempered martensite in the surface layer of the base steel plate, the desired flexibility can be achieved. Furthermore, increasing the area ratio of phases with low hydrogen solubility in the surface layer of the base steel plate also contributes to reducing the amount of diffusible hydrogen in the low-temperature region of the base steel plate, as described later. Additionally, EL can be increased, and λ can also be improved. To achieve this effect, the area ratio of quenched martensite in the surface layer of the base steel plate is set to 80% or less. The area ratio of quenched martensite in the surface layer of the base steel plate is preferably 75% or less, more preferably 70% or less. It should be noted that there is no particular limitation on the lower limit of the area ratio of quenched martensite in the surface layer of the base steel plate. From the viewpoint of achieving the desired TS (transfer rate), the area ratio of quenched martensite in the surface layer of the base steel plate is preferably 20% or more. More preferably, the area ratio of quenched martensite in the surface layer of the base steel plate is 30% or more. It should be noted that the surface layer of the base steel plate is the region from the surface of the base steel plate to a depth of 10 μm.

[0162] Here, the area ratio of quenched martensite in the surface layer of the base steel plate is determined as follows.

[0163] Specifically, the specimen was cut with the thickness section (L-section) of the base steel plate parallel to the rolling direction as the observation surface. Next, the observation surface of the specimen was ground. Then, the observation surface of the specimen was etched with 3 vol.% nitric acid ethanol to reveal the microstructure. Next, the surface of the base steel plate was used as the observation position, and three fields of view were observed using a SEM (Scanning Electron Microscope) at 3000x magnification. From the obtained microstructure images, the area of ​​quenched martensite in the three fields of view was calculated using Adobe Photoshop from Adobe Systems. Then, the area of ​​quenched martensite calculated for each field of view was divided by the area of ​​its respective field of view, and the arithmetic mean of these values ​​was taken as the area ratio of quenched martensite in the surface of the base steel plate. It should be noted that in the above microstructure images, the quenched martensite appears as a white microstructure.

[0164] The area fraction of the remaining microstructure in the surface layer of the base steel plate, excluding quenched martensite, is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more. Furthermore, the area fraction of the remaining microstructure in the surface layer of the base steel plate, excluding quenched martensite, is preferably 80% or less, more preferably 70% or less. The remaining microstructure in the surface layer of the base steel plate is essentially composed of phases such as ferrite, bainitic ferrite, bainite, and tempered martensite; however, as long as the area fraction is 5% or less, it may also include microstructures known as other steel plate microstructures, such as pearlite, cementite, metastable carbides (ε carbides, η carbides, χ carbides, etc.). The remaining microstructure can be identified, for example, by observation using a SEM (Scanning Electron Microscope).

[0165] In addition, the area ratio of the remaining tissue is calculated using the following formula.

[0166] [Area percentage of remaining microstructure (%)] = 100 - [Area percentage of quenched martensite (%)]

[0167] Surface softening thickness of the base steel plate: 10μm~100μm

[0168] Compared to the portion of the base steel sheet at 1 / 4 of its thickness, softening the surface layer of the base steel sheet achieves the desired flexibility. Therefore, the surface softening thickness of the base steel sheet is preferably set to 10 μm or more. On the other hand, when the surface softening thickness of the base steel sheet exceeds 100 μm, it sometimes leads to a decrease in TS (flexibility strength). Therefore, the surface softening thickness of the base steel sheet is preferably set to 10 μm to 100 μm. More preferably, the surface softening thickness of the base steel sheet is 12 μm or more, and even more preferably 15 μm or more. Furthermore, more preferably, the surface softening thickness of the base steel sheet is 80 μm or less, and even more preferably 60 μm or less.

[0169] Here, the surface softening thickness is measured as follows.

[0170] The surface of the base steel plate, specifically the thickness section (L section) parallel to the rolling direction, is smoothed by wet grinding. Next, using a Vickers hardness tester under a load of 5 gf, hardness measurements are taken at 5 μm intervals along the thickness (depth) direction, from a position 10 μm deep from the surface of the base steel plate to the center of the base steel plate. The hardness obtained at 1 / 4 of the base steel plate thickness is then used as the reference hardness. The distance (depth) from the surface of the base steel plate to the deepest point where the hardness is less than or equal to 0.85 times the reference hardness is measured, and this measured value is taken as the surface softening thickness.

[0171] It should be noted that the microstructure of the base steel plate is generally roughly symmetrical vertically along the thickness direction. Therefore, in the identification of the microstructure at the 1 / 4 thickness position and the surface layer of the base steel plate, as well as the proportion of residual γ particles with an aspect ratio of 2.0 or less, the number of hardness frequency groups of 0.25 or higher, and the determination of the surface softening thickness of the base steel plate, it is sufficient to use any one of the surfaces (front and back) of the base steel plate as the starting point for the thickness position (0 thickness position), such as the 1 / 4 thickness position. The same applies below.

[0172] In addition, in one embodiment of the hot-dip galvanized steel sheet, it is important to properly control the amount of diffusible hydrogen in the low-temperature region of the base steel sheet.

[0173] Low-temperature diffusible hydrogen content of the base steel plate: below 0.015 ppm by mass.

[0174] The diffusible hydrogen content in the low-temperature region of the base steel sheet is an extremely important factor. Specifically, the inventors conducted repeated and in-depth research to obtain hot-dip galvanized steel sheets with high YR (Yeast Ratio), high ductility, tensile flange strength, and bendability, and improved shear workability (TS = 1180 MPa or higher). Their results showed that the diffusible hydrogen content in the low-temperature region of the base steel sheet, i.e., the amount of hydrogen released from the base steel sheet when heated from room temperature to 50°C, has a significant impact on the aforementioned properties, especially shear workability. Specifically, the inventors realized that shear workability, when the base steel sheet is heated, depends more on the amount of hydrogen released from the base steel sheet in the low-temperature region, specifically the temperature range from room temperature to 50°C, than on the amount of hydrogen released in the high-temperature region. Furthermore, they realized that in order to significantly improve shear workability while achieving high YR (Yellow Reduction), high ductility, tensile flangeability, and bendability, reducing the diffusive hydrogen content in the low-temperature region of the base steel sheet, particularly setting it to 0.015 ppm by mass or less, is essential; thus, the present invention was developed. Therefore, the diffusive hydrogen content in the low-temperature region of the base steel sheet is set to 0.015 ppm by mass or less. Lower diffusive hydrogen content in the low-temperature region of the base steel sheet is preferred, preferably 0.010 ppm by mass or less, and more preferably 0.006 ppm by mass or less. It should be noted that the lower limit of the diffusive hydrogen content in the low-temperature region of the base steel sheet is not particularly limited, and it can also be 0 ppm by mass. However, due to limitations in production technology, the diffusive hydrogen content in the low-temperature region of the base steel sheet is preferably 0.001 ppm by mass or more.

[0175] Here, the diffusible hydrogen content in the low-temperature region of the base steel plate is determined by the following procedure.

[0176] A 30mm long and 5mm wide test piece is cut from the center of a hot-dip galvanized steel sheet sample by shearing. Immediately after cutting, the test piece is immersed in liquid nitrogen. Then, while maintaining the temperature of the treatment solution below room temperature, the hot-dip galvanized layer is removed using alkali. Next, the amount of hydrogen released from the test piece during heating is measured using a programmed temperature desorption method. Specifically, the test piece is heated from room temperature to a temperature of 300°C at a heating rate of 200°C / hr, and then cooled to room temperature. The cumulative amount of hydrogen released from the test piece within the temperature range from room temperature to 50°C during heating (hereinafter also referred to as cumulative hydrogen release) is measured. Then, the diffusible hydrogen content in the low-temperature region of the base steel sheet is calculated using the following formula.

[0177] [Low-temperature diffusible hydrogen content of the base steel plate (mass ppm)] = [Cumulative hydrogen release (g)] ÷ [Mass of the test piece (g)] × 10 6

[0178] Furthermore, as long as the room temperature is within the range of 10–25°C, it will not have a significant impact on the determination of the diffusible hydrogen content in the low-temperature region of the base steel plate. Specifically, when the room temperature is outside the range of 10–25°C, 25°C is used as the representative temperature for room temperature, and the cumulative hydrogen release from the test piece in the temperature range of 25°C to 50°C is measured.

[0179] It should be noted that for steel sheets that have undergone punching, stretching and flange forming, and bending processes, and for products (parts) manufactured by welding the aforementioned processed steel sheets, it is sufficient to measure the low-temperature diffusible hydrogen content of the base steel sheet portion according to the same procedure as described above.

[0180] In addition, in one embodiment of the present invention, the thickness of the base steel sheet of the hot-dip galvanized steel sheet is not particularly limited, and is usually 0.3 mm to 2.8 mm.

[0181] [1-2] Hot-dip galvanized layer

[0182] Next, the hot-dip galvanized layer of a hot-dip galvanized steel sheet according to one embodiment of the present invention will be described. It should be noted that the hot-dip galvanized layer referred to herein also includes an alloyed hot-dip galvanized layer (a coating obtained by alloying hot-dip galvanizing). Furthermore, the hot-dip galvanized layer is provided on both sides of the surface of the base steel sheet.

[0183] The composition of the hot-dip galvanized layer is not particularly limited, as long as it is a common composition. In one example, it has the following composition: containing Fe: less than 20% by mass, Al: 0.001% to 1.0% by mass, and also containing a total of 0% to 3.5% by mass of one or more of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the remainder consisting of Zn and unavoidable impurities. In the case of the (unalloyed) hot-dip galvanized layer, in one example, the Fe content in the coating is less than 7% by mass. In the case of the alloyed hot-dip galvanized layer, in one example, the Fe content in the coating is 7% to 15% by mass, more preferably 8% to 13% by mass.

[0184] In addition, there is no particular limitation on the amount of plating applied to each single side, but it is preferably 20 to 80 g / m². 2 .

[0185] Furthermore, the hot-dip galvanized layer preferably has cracks. By intentionally incorporating cracks into the hot-dip galvanized layer, the diffusible hydrogen content in the low-temperature region of the base steel sheet can be further reduced. It should be noted that the presence or absence of cracks in the hot-dip galvanized layer is determined as follows: For the surface (front and back) of the hot-dip galvanized layer of the hot-dip galvanized steel sheet, two fields of view are observed on each surface using SEM at a magnification of 1500x, for a total of four fields of view. If one or more cracks with a length of 10 μm or more are present in any of the four fields of view, it is determined that a crack exists. Conversely, if no cracks with a length of 10 μm or more are present in any of the four fields of view, it is determined that no crack exists.

[0186] [1-3] Others

[0187] Tensile strength (TS): ≥1180MPa

[0188] In one embodiment of the present invention, the TS (steel strength) of the hot-dip galvanized steel sheet is 1180 MPa or higher. Furthermore, the TS is measured based on JIS Z2241, according to the methods described in the embodiments described later.

[0189] In addition, the thickness of the hot-dip galvanized steel sheet in one embodiment of the present invention is not particularly limited, but is generally 0.3 mm or more and 2.8 mm or less.

[0190] [2] Manufacturing method of hot-dip galvanized steel sheet

[0191] Next, a method for manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0192] One embodiment of the present invention includes a method for manufacturing hot-dip galvanized steel sheet, comprising:

[0193] Hot-rolled steel sheets are produced by hot rolling a steel billet with the above-mentioned composition under the conditions of a winding temperature of 350℃~600℃ and a residence time of more than 5000s in a temperature range of 300℃ or higher during cooling after winding.

[0194] Next, the hot-rolled steel plates are pickled.

[0195] Next, the hot-rolled steel sheet is subjected to a first heat treatment at a temperature of 450℃ to 650℃ and a residence time of 10 minutes or more in a temperature range above 400℃ but below the heat treatment temperature.

[0196] Next, the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet.

[0197] Next, the cold-rolled steel sheet was annealed under the following conditions: an average heating rate of 10°C / s or more in the temperature range of 250°C to 700°C; an oxygen concentration of 0.5% to 5.0% by volume in the temperature range of 250°C to 700°C; an annealing temperature of 820°C to 950°C; and a dew point of -35°C or more in the annealing temperature range.

[0198] Next, the cold-rolled steel sheet is subjected to hot-dip galvanizing to produce coated steel sheet.

[0199] Next, the above-mentioned coated steel sheet was cooled at a cooling stop temperature below 150°C.

[0200] Next, the above-mentioned coated steel sheet is subjected to a second heat treatment under the condition that the following formula (1) is satisfied.

[0201] 6.5≤(T+273)×{log(t×3600)+20} / 1000≤13.0……(1)

[0202] Here, T is the heat treatment temperature (°C) of the second heat treatment, and t is the holding time (hr) of the second heat treatment.

[0203] In addition, a method for manufacturing hot-dip galvanized steel sheet according to one embodiment of the present invention is a method for manufacturing the hot-dip galvanized steel sheet according to one embodiment of the present invention described above.

[0204] It should be noted that, unless otherwise specified, the temperatures mentioned above are based on the surface temperature of the billet or plate.

[0205] [Hot rolling process]

[0206] First, hot-rolled steel sheets are produced by hot rolling of steel billets. There are no particular limitations on the smelting method for the steel billets (billet material); known smelting methods such as converters and electric furnaces are suitable. Furthermore, to prevent macroscopic segregation, continuous casting is preferred for manufacturing steel billets. Alternatively, steel billets can also be manufactured using ingot casting, slab casting, etc. It should be noted that, in addition to the existing method of temporarily cooling the steel billet to room temperature after manufacturing and then reheating it, energy-saving processes such as direct rolling and feed rolling can be applied without problems. Direct rolling is a process in which the steel billet is loaded into the heating furnace as a hot sheet without cooling to room temperature. Feed rolling is a process in which rolling is performed immediately after a slight holding time.

[0207] When heating the steel billet, from the viewpoint of dissolving carbides and reducing rolling load, it is preferable to set the slab heating temperature to 1100°C or higher. Furthermore, to prevent increased oxide scale loss, the slab heating temperature is preferably 1300°C or lower. It should be noted that the slab heating temperature refers to the temperature of the slab surface.

[0208] Next, the steel billet is produced into sheet steel by rough rolling under normal conditions. It should be noted that, from the viewpoint of preventing rolling defects, it is preferable to heat the sheet steel using a barheater or similar device before finish rolling, while lowering the slab heating temperature. Furthermore, the finish rolling temperature is preferably above the Ar3 phase transformation point. Excessively lowering the finish rolling temperature leads to increased rolling load and increased reduction in the non-recrystallized state of austenite. This results in the development of an abnormal microstructure elongated along the rolling direction, which sometimes reduces the workability of the annealed steel sheet. It should be noted that the Ar3 phase transformation point is calculated using the following formula.

[0209] Ar3(℃)=868-396×[%C]+24.6×[%Si]-68.1×[%Mn]-36.1×[%Ni]-20.7×[%Cu]-24.8×[%Cr]

[0210] It should be noted that the [% element symbol] in the above formula represents the content (mass %) of that element in the above composition.

[0211] It should be noted that thin steel sheets can also be joined together and continuously finished rolled. Alternatively, thin steel sheets can be temporarily wound. Furthermore, to reduce the rolling load during rolling, part or all of the finishing rolling can be lubricated rolling. Lubricated rolling is also effective from the viewpoint of homogenizing the shape and material of the steel sheet. It should be noted that the coefficient of friction during lubricated rolling is preferably set in the range of 0.10 to 0.25.

[0212] In addition, in the hot rolling process, the control of residence time in the temperature range above 300°C during winding and cooling after winding becomes particularly important.

[0213] Winding temperature: 350℃~600℃

[0214] In the hot rolling process, after finishing rolling, the hot-rolled sheet is wound and collected, and then cooled. At this time, by setting the winding temperature to 350°C or higher, carbon (C) diffuses into the oxide scale generated during rolling. That is, decarburization of the surface layer of the hot-rolled steel sheet is promoted, and the surface softening thickness of the base steel sheet and the martensite area ratio in the surface layer of the base steel sheet can be controlled within a desired range. This results in high flexibility. Furthermore, by reducing the martensite area ratio in the surface layer of the base steel sheet, the diffusible hydrogen content in the low-temperature region of the base steel sheet is also reduced. This results in excellent El, λ, and shear workability. Moreover, the steel microstructure of the hot-rolled steel sheet becomes a microstructure dominated by low-temperature phase transformation, and the number of hardness frequencies above 0.25 can be controlled to one. As a result, YR also increases. However, when the winding temperature exceeds 600°C, the surface softening thickness of the base steel sheet increases, making it difficult to achieve a TS of 1180 MPa or higher. Furthermore, the microstructure of hot-rolled steel sheets does not become the main body of the low-temperature metamorphic phase, making it difficult to control the number of hardness frequency groups of 0.25 or higher to one. As a result, it is difficult to achieve the desired YR, λ, and flexibility. Therefore, the winding temperature is set to 350°C to 600°C. The winding temperature is preferably 380°C or higher, more preferably 410°C or higher. In addition, the winding temperature is preferably 570°C or lower, more preferably 550°C or lower.

[0215] Residence time in the temperature zone above 300°C during cooling after winding (hereinafter also referred to as residence time in the temperature zone above 300°C): 5000s or more

[0216] After hot-rolled steel sheet is wound into a coil, it is held in a temperature region of 300°C or higher for 5000 s or more. This allows carbon (C) to diffuse into the oxide scale generated during rolling. In other words, decarburization of the surface layer of the hot-rolled steel sheet is promoted, and the surface softening thickness of the base steel sheet and the martensite area ratio in the surface layer of the base steel sheet can be controlled within a desired range. This results in high flexibility. Furthermore, by reducing the martensite area ratio in the surface layer of the base steel sheet, the diffusible hydrogen content in the low-temperature region of the base steel sheet is also reduced. As a result, excellent E1, λ, and shear workability are obtained. Therefore, the holding time in the temperature region of 300°C or higher is 5000 s or more. The holding time in the temperature region of 300°C or higher is preferably 7000 s or more, more preferably 9000 s or more. There is no particular upper limit to the holding time in the temperature region of 300°C or higher, but from the viewpoint of keeping TS (temperature scale) within a specified range, the holding time in the temperature region of 300°C or higher is preferably 100,000 s or less. The residence time in the temperature range of 300°C or above is more preferably 80,000 s or less, and even more preferably 40,000 s or less. It should be noted that the starting point for the residence time in the temperature range of 300°C or above is set at the moment when the hot-rolled steel sheet is wound into a coil (the moment of completion of winding). Furthermore, there is no particular limitation on the residence temperature as long as it is in the temperature range of 300°C or above, but it is preferably 350°C or above, and more preferably 600°C or below.

[0217] It should be noted that there are no particular limitations on the cooling conditions after the temperature range of 300°C or above; conventional methods can be used. For example, the preferred cooling rate is 0.001°C / s to 1°C / s, and the preferred cooling stop temperature is 20°C to 200°C.

[0218] [Pickling process]

[0219] After hot rolling, the hot-rolled steel sheet undergoes pickling. Pickling removes oxides from the steel sheet surface, ensuring good chemical treatment properties and coating quality. It should be noted that pickling can be performed once or in multiple stages. There are no particular limitations on pickling conditions; conventional methods are sufficient.

[0220] [First heat treatment process]

[0221] Next, the hot-rolled steel sheet undergoes its first heat treatment. At this point, it is important that the following conditions are met.

[0222] Heat treatment temperature: 450℃~650℃

[0223] By heat-treating hot-rolled steel sheets, fine carbides are uniformly generated within the steel microstructure, allowing the number of hardness frequency groups of 0.25 or higher to be controlled to one. To achieve this effect, the heat treatment temperature needs to be set to 450°C or higher. However, if the heat treatment temperature exceeds 650°C, the carbides become coarse and spherical, or pearlite is formed. Therefore, it is difficult to control the number of hardness frequency groups of 0.25 or higher to one. Thus, the heat treatment temperature is typically between 450°C and 650°C. A heat treatment temperature of 460°C or higher is preferred, and 470°C or higher is more preferred. A heat treatment temperature of 600°C or lower is preferred, and 550°C or lower is more preferred.

[0224] Residence time in the temperature range above 400°C and below the heat treatment temperature (hereinafter also referred to as residence time in the heat treatment temperature range): 10 minutes or more

[0225] By setting the residence time in the heat treatment temperature zone to 10 minutes or more, fine carbides are uniformly generated in the steel structure of the hot-rolled steel sheet, and the number of hardness frequency groups of 0.25 or higher can be controlled to be 1. Therefore, the residence time in the heat treatment temperature zone is set to 10 minutes or more. The residence time in the heat treatment temperature zone is preferably 100 minutes or more, more preferably 500 minutes or more. In addition, there is no particular upper limit to the residence time in the heat treatment temperature zone, but from the viewpoint of uniformly generating fine carbides in the steel structure of the hot-rolled steel sheet, it is preferably 3000 minutes or less. The residence time in the heat treatment temperature zone is more preferably 2000 minutes or less.

[0226] [Cold rolling process]

[0227] Next, the hot-rolled steel sheet is cold-rolled to produce cold-rolled steel sheet. There are no special restrictions on the cold rolling conditions; conventional methods can be used.

[0228] For example, cold rolling based on two or more rolling passes can be carried out through tandem multi-stand rolling or reversible rolling. Furthermore, the cumulative reduction rate of cold rolling is preferably set to, for example, 20% to 75%. It should be noted that there are no particular limitations on the number of rolling passes or the reduction rate of each pass; conventional methods can be followed.

[0229] [Annealing process]

[0230] Annealing is then performed on the cold-rolled steel sheet obtained as described above. At this point, it is important to meet the following conditions.

[0231] Average heating rate in the temperature range of 250℃ to 700℃ (hereinafter also referred to as the heating temperature range): 10℃ / s or more

[0232] By increasing the average heating rate of the heating temperature zone, the austenite formed during heating can be refined, and the number of groups with a hardness frequency of 0.25 or higher can be controlled to one. Therefore, the average heating rate of the heating temperature zone is set to 10°C / s or higher. The average heating rate of the heating temperature zone is preferably 12°C / s or higher, more preferably 14°C / s or higher. Furthermore, there is no specific upper limit for the average heating rate of the heating temperature zone; due to limitations in production technology, it is preferably 50°C / s or lower, more preferably 40°C / s or lower.

[0233] Oxygen concentration in the heating temperature zone: 0.5% to 5.0% by volume

[0234] By increasing the oxygen concentration in the heating temperature zone, decarburization occurs through oxygen in the atmosphere, forming a softened layer on the surface of the steel plate. This results in the desired flexibility. Furthermore, the martensite area ratio in the surface of the base steel plate decreases, and the diffusible hydrogen content in the low-temperature region of the base steel plate also decreases. To achieve this effect, the oxygen concentration in the heating temperature zone is set to 0.5 vol% or more. On the other hand, if the oxygen concentration in the heating temperature zone exceeds 5.0 vol%, the softened thickness of the base steel plate increases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the oxygen concentration in the heating temperature zone is 0.5 vol% to 5.0 vol%. Preferably, the oxygen concentration in the heating temperature zone is 1.0 vol% or more, more preferably 1.5 vol% or more. Preferably, the oxygen concentration in the heating temperature zone is 4.5 vol% or less, more preferably 4.0 vol% or less. It should be noted that the temperature of the heating temperature zone is based on the surface temperature of the steel plate. That is, when the surface temperature of the steel plate is within the heating temperature zone, the oxygen concentration can be adjusted to the above range.

[0235] Annealing temperature: 820℃~950℃

[0236] When the annealing temperature is below 820°C, the annealing process results in a two-phase region consisting of ferrite and austenite. In this case, the annealed steel sheet contains a large amount of ferrite, making it difficult to achieve the desired YR, λ, and bendability. On the other hand, if the annealing temperature exceeds 950°C, the austenite grains coarsen unevenly during annealing, making it difficult to control the number of grains with a hardness frequency of 0.25 or higher to one. Consequently, it is difficult to achieve the desired YR, λ, and bendability. Therefore, the annealing temperature is set between 820°C and 950°C. The annealing temperature is preferably 850°C or higher, more preferably 870°C or higher. The annealing temperature is preferably 930°C or lower, more preferably 920°C or lower. It should be noted that the annealing temperature is the highest temperature reached during the annealing process.

[0237] Dew point in the annealing temperature range: above -35°C

[0238] By increasing the dew point in the annealing temperature range (820°C to 950°C), decarburization occurs through oxygen in the atmosphere, forming a softened layer on the surface of the steel sheet. This results in achieving the desired flexibility. Furthermore, the martensite area ratio in the surface of the base steel sheet decreases, and the diffusible hydrogen content in the low-temperature region of the base steel sheet also decreases. To achieve this effect, the dew point in the annealing temperature range is -35°C or higher. Preferably, the dew point in the annealing temperature range is -30°C or higher, more preferably -25°C or higher. There is no particular upper limit to the dew point in the annealing temperature range; from the viewpoint of controlling the surface softening thickness of the base steel sheet within a specified range and ensuring a specified total saturation (TS), the dew point in the annealing temperature range is preferably 15°C or lower, more preferably 5°C or lower. It should be noted that the temperature in the annealing temperature range is based on the surface temperature of the steel sheet. That is, when the surface temperature of the steel sheet is within the annealing temperature range, the dew point is adjusted to the above range. The same applies to the oxygen concentration described later.

[0239] It should be noted that the holding time (hereinafter also referred to as annealing time) and oxygen concentration within the annealing temperature range are not particularly limited. From the viewpoint of controlling the softening thickness of the base steel sheet and the area ratio of martensite in the surface portion of the base steel sheet within a specified range, it is preferable to set them to 10s to 600s and 2 ppm to 30 ppm, respectively. In addition, the holding temperature does not always need to be constant.

[0240] After the above annealing, the cold-rolled steel sheet is cooled. There are no particular limitations on the conditions at this time, and conventional methods can be used. For example, there is no particular limitation on the average cooling rate in the temperature range below the annealing temperature and above 500°C. From the viewpoint of controlling the number of hardness frequencies of 0.25 or higher, it is preferable to set it to 5°C / s to 50°C / s.

[0241] [Plating Process]

[0242] Next, the cold-rolled steel sheet is subjected to a coating process to produce coated steel sheet. Hot-dip galvanizing is an example of such a coating process. Alternatively, alloying can be performed after hot-dip galvanizing. Furthermore, annealing, cooling, and coating processes can be performed continuously on a single production line (CGL). For example, after annealing, the cold-rolled steel sheet is cooled to a temperature range of approximately 500°C. Then, the cold-rolled steel sheet is passed through the strip output side of the cooling zone, moving into the hot-dip galvanizing bath via a snout at its front end, while being further cooled. The time from the end of cooling of the cold-rolled steel sheet to its immersion in the hot-dip galvanizing bath is not particularly limited, but from the viewpoint of controlling the number of groups with a hardness frequency of 0.25 or higher, it is preferable to set it to 1 to 300 seconds. It should be noted that a roller is provided immediately in front of the connection between the cooling belt and the nozzle to change the direction of travel of the cold-rolled steel sheet and allow it to enter the nozzle. After passing through this roller, the cold-rolled steel sheet enters the nozzle. Then, the cold-rolled steel sheet, which has been introduced into the hot-dip galvanizing bath through the nozzle, is immersed in the hot-dip galvanizing bath to perform hot-dip galvanizing treatment and produce a coated steel sheet.

[0243] In hot-dip galvanizing, for example, cold-rolled steel sheets are immersed in a hot-dip galvanizing bath at 440°C to 500°C. Furthermore, it is preferable to use a hot-dip galvanizing bath composed of 0.10% to 0.23% by mass of Al, with the remainder being Zn and unavoidable impurities.

[0244] Furthermore, after the aforementioned hot-dip galvanizing treatment, alloying treatment can be performed within a temperature range of 460°C to 600°C. When the alloying temperature is below 460°C, the Zn-Fe alloying rate becomes too slow, reducing productivity. On the other hand, when the alloying temperature exceeds 600°C, the untransformed austenite transforms into pearlite, sometimes decreasing TS and E1. Therefore, the alloying temperature is preferably between 460°C and 600°C. The alloying temperature is preferably 470°C or higher. Additionally, the alloying temperature is preferably 560°C or lower.

[0245] In addition, the preferred coating adhesion amount is 20-80 g / m² per single side. 2 (Double-sided plating). It should be noted that the coating adhesion can be adjusted by gas wiping or other methods after hot-dip galvanizing.

[0246] [Cooling Process]

[0247] After plating, i.e. hot-dip galvanizing or alloying, the plated steel sheet is cooled at a cooling stop temperature of 150°C or below.

[0248] Cooling stop temperature: below 150℃

[0249] If the cooling stop temperature exceeds 150°C, it becomes difficult to control the number of groups with a hardness frequency of 0.25 or higher to only one. As a result, it is difficult to achieve the desired YR, λ, and flexibility. Therefore, the cooling stop temperature is below 150°C. Preferably, the cooling stop temperature is below 100°C. It should be noted that there is no specific specification for the lower limit of the cooling stop temperature; from a productivity point of view, it is preferable to set it above room temperature.

[0250] For cooling conditions other than those mentioned above, there are no particular limitations, and conventional methods are acceptable. For example, the average cooling rate from the end of hot-dip galvanizing or alloying treatment to the aforementioned cooling stop temperature is not particularly limited, but from the viewpoint of further improving TS (cooling temperature range), it is preferably 2°C / s or more, more preferably 5°C / s or more. Furthermore, due to limitations in production technology, this average cooling rate is preferably 50°C / s or less, more preferably 40°C / s or less. Additionally, the cooling method is not particularly limited, and gas jet cooling, spray cooling, water cooling, and air cooling can be used.

[0251] After the aforementioned cooling, it is preferable to perform additional rolling on the galvanized steel sheet with an elongation of 0.05% to 1.00%. By making the elongation in the additional rolling 0.05% or more, cracks can be introduced into the hot-dip galvanized layer. As a result, a further reduction in the diffusible hydrogen content in the low-temperature region of the base steel sheet can be expected. In addition, the proportion of residual γ with an aspect ratio of 2.0 or less increases, which can further improve YS. On the other hand, when the elongation in the additional rolling exceeds 1.00%, the area fraction of residual austenite decreases, and E1 sometimes decreases. Therefore, the elongation in the additional rolling is set to 0.05% to 1.00%. The elongation in the additional rolling is more preferably 0.10% or more. The elongation in the additional rolling is more preferably 0.50% or less.

[0252] It should be noted that the aforementioned additional rolling can be performed on a unit continuous with the continuous hot-dip galvanizing unit (online) or on a unit discontinuous with the continuous hot-dip galvanizing unit (offline). Furthermore, the target elongation can be achieved in a single rolling pass, or multiple rolling passes can be performed to achieve a total elongation of 0.05% to 1.00%. It should be noted that the rolling described here typically refers to temper rolling, but any method using a tension straightener or similar equipment can be employed as long as it imparts the same elongation as temper rolling.

[0253] [Second heat treatment]

[0254] Next, it is important to perform a second heat treatment on the galvanized steel sheet under the condition that the following formula (1) is satisfied.

[0255] 6.5≤(T+273)×{log(t×3600)+20} / 1000≤13.0……(1)

[0256] Here, T is the heat treatment temperature (°C) of the second heat treatment, and t is the holding time (hr) of the second heat treatment.

[0257] That is, by performing a second heat treatment under the condition of satisfying the above formula (1), the diffusible hydrogen content in the low-temperature region of the base steel plate can be reduced. In addition, the proportion of residual γ with an aspect ratio of 2.0 or less increases. To obtain this effect, (T+273)×{log(t×3600)+20} / 1000 needs to be 6.5 or more. On the other hand, when (T+273)×{log(t×3600)+20} / 1000 exceeds 13.0, the area ratio of residual austenite decreases, making it difficult to achieve the desired El. Therefore, it is important to perform the second heat treatment under the condition of satisfying the above formula (1). (T+273)×{log(t×3600)+20} / 1000 is preferably 6.8 or more, more preferably 7.0 or more. In addition, (T+273)×{log(t×3600)+20} / 1000 is preferably 12.5 or less, more preferably 12.0 or less. In addition, log in the above formula (1) is the common logarithm with a base of 10.

[0258] It should be noted that the heat treatment temperature in the second heat treatment is the highest temperature reached in the second heat treatment. Furthermore, the holding time in the second heat treatment is the residence time within the temperature range from the heat treatment temperature in the second heat treatment to -15°C and then to the heat treatment temperature in the second heat treatment.

[0259] It should be noted that there are no particular restrictions on the cooling conditions after the heat treatment; conventional methods are acceptable. For example, cooling methods can include gas jet cooling, spray cooling, water cooling, and air cooling.

[0260] Furthermore, after the second heat treatment described above, additional rolling can be performed. The elongation of this additional rolling is preferably set to 0.05% to 1.00% or less. It should be noted that the target elongation can be achieved by a single rolling operation, or by multiple rolling operations to achieve a total elongation of 0.05% to 1.00%. Additionally, the rolling described here generally refers to heat rolling, but any processing method using a tension straightener or the like can be used as long as it imparts the same elongation as heat rolling.

[0261] It should be noted that when hot-dip galvanized steel sheets are to be processed, they are usually processed after being cooled to room temperature.

[0262] There are no special restrictions on manufacturing conditions other than those mentioned above; conventional methods can be followed.

[0263] [3] Components

[0264] Next, the components of one embodiment of the present invention will be described.

[0265] One embodiment of the present invention comprises a component made using the hot-dip galvanized steel sheet described in one embodiment of the present invention. The component of one embodiment of the present invention is formed into the desired shape, for example, by stamping or the like. The component of one embodiment of the present invention is preferably a component for a car frame structure or a car reinforcement component.

[0266] Here, the hot-dip galvanized steel sheet described in one embodiment of the present invention is a hot-dip galvanized steel sheet with a TS of 1180 MPa or higher, exhibiting high YR, high ductility, tensile flange strength, and bendability, and improved shear workability. Therefore, the components of one embodiment of the present invention have excellent dimensional accuracy and can also contribute to the lightweighting of the vehicle body; thus, they are particularly suitable for use in all components for automotive frame structures or automotive reinforcement components.

[0267] Example

[0268] A steel billet (steel billet material) with the composition shown in Table 1, the remainder consisting of Fe and unavoidable impurities, is smelted in a converter and obtained by continuous casting. The obtained steel billet is heated to 1250°C and rough rolled to obtain sheet steel. Next, the sheet steel is finish rolled at a finishing temperature of 900°C, wound and cooled under the conditions shown in Table 2 to obtain hot-rolled steel sheet. After pickling, the hot-rolled steel sheet undergoes a first heat treatment and cold rolling under the conditions shown in Table 2 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm.

[0269] Next, the obtained cold-rolled steel sheet was annealed under the conditions shown in Table 2. Then, the cold-rolled steel sheet was subjected to the coating treatments shown in Table 2 to obtain coated steel sheets (with hot-dip galvanized layers on both sides). In Table 2, GI indicates only hot-dip galvanizing (resulting in hot-dip galvanized steel sheet without alloying), and GA indicates hot-dip galvanizing + alloying (resulting in alloyed hot-dip galvanized steel sheet).

[0270] It should be noted that in GI, a hot-dip galvanizing bath containing 0.20% by mass Al, with the remainder consisting of Zn and unavoidable impurities, is used as the plating bath. In GA, a hot-dip galvanizing bath containing 0.14% by mass Al, with the remainder consisting of Zn and unavoidable impurities, is used. The plating bath temperature is set to 470°C. The coating weight in GI is 45–72 g / m² per side. 2 (Double-sided plating) approximately, and in GA, it is 45g / m² per single side. 2(Double-sided plating) around. Additionally, in GA, the alloying temperature is set to around 550℃.

[0271] Furthermore, the composition of the hot-dip galvanized layer of GI is Fe: 0.1–1.0 wt%, Al: 0.2–1.0 wt%, with the remainder being Zn and unavoidable impurities. The composition of the (alloyed) hot-dip galvanized layer of GA is Fe: 7–15 wt%, Al: 0.1–1.0 wt%, with the remainder being Zn and unavoidable impurities.

[0272] Next, the obtained coated steel sheet was cooled under the conditions shown in Table 2. After cooling, the coated steel sheet was subjected to a second heat treatment under the conditions shown in Table 2. It should be noted that for conditions not explicitly stated, conventional methods were followed.

[0273] Thus, the microstructure of the obtained steel sheet at the 1 / 4 thickness position and the surface layer of the base steel sheet was identified using the method described above. The proportion of residual γ atoms with an aspect ratio of 2.0 or less, the number of hardness frequencies with a group interval of 0.25 or more, the surface softening thickness of the base steel sheet, and the diffusible hydrogen content in the low-temperature region of the base steel sheet were measured. The results are shown in Table 3. It should be noted that the composition of the base steel sheet of the obtained steel sheet is substantially the same as that of the billet stage. Suitable steels are all within the composition range of the above-described embodiments, while comparative steels are outside the composition range of the above-described embodiments.

[0274] In addition, the tensile properties, tensile flange properties, bending properties, and shear workability of the obtained steel plates were evaluated according to the following test methods. The results are shown in Table 3.

[0275] [Tension Test]

[0276] Tensile tests were conducted according to JIS Z 2241. Specifically, JIS No. 5 test pieces were taken from the obtained steel sheet with the rolling right angle (C direction) as the long side direction. Then, using the taken test pieces, tensile tests were performed at a crosshead speed of 1.67 × 10⁻⁶. －1 Tensile tests were conducted under conditions of mm / s to determine YS, TS, and El. Then, for TS, a value of 1180 MPa or higher was considered acceptable. Furthermore, YR was calculated from the measured YS and TS using the above formula (2). Then, a YR value of 65% or higher was considered acceptable. Additionally, an El value of 6% or higher was considered acceptable.

[0277] [Drilling Test]

[0278] The hole enlargement test was conducted according to JIS Z 2256. The obtained steel sheet was cut into 100mm × 100mm pieces. Then, a hole with a diameter of 10mm was punched into the cut steel sheet with a clearance of 12.5%. Next, the steel sheet was pressed using a die with an inner diameter of 75mm and a wrinkle holding force of 9ton (88.26kN). Under this condition, a conical punch with a apex angle of 60° was pressed into the hole, and the hole diameter at the point where cracking occurs was measured. Then, the (limit) hole enlargement rate λ (%) was calculated using the following formula.

[0279] (Limited) Pore Expansion Ratio: λ(%)={(D) f -D0) / D0}×100

[0280] Here, D f D0 is the initial pore diameter (mm) at which cracks occur. Then, the tensile flange performance is judged to be qualified when the (limiting) expansion ratio: λ is 30% or more.

[0281] [Bending Test]

[0282] The bending test was conducted according to JIS Z 2248. A rectangular test piece with a width of 30 mm and a length of 100 mm was taken from the obtained steel sheet, with the bending test axis parallel to the rolling direction of the base steel sheet. Then, a 90°V bending test was performed under an indentation load of 100 kN and a holding time of 5 seconds. Here, the value R / t, obtained by dividing the bending radius (R) by the sheet thickness (t), was set to 5 (for example, in the case of a sheet thickness of 1.4 mm, the bending radius is 7.0 mm), and bending tests were performed on 5 samples. Next, the presence or absence of cracks at the bend apex was checked. If no cracks were observed in all 5 samples, i.e., the pass rate was 100%, the bending performance was deemed acceptable. Here, the presence or absence of cracks was determined by examining the bend apex at 40x magnification using a digital microscope (RH-2000: manufactured by HIROX Corporation).

[0283] [Shearing Processing Test]

[0284] From the obtained steel sheet, a rectangular test piece with a width (rolling angle direction) of 114 mm and a length (rolling direction) of 32 mm was cut using a crank press, with the shear separation surface at the right angle to the rolling of the base steel sheet. It should be noted that the shearing conditions of the crank press were set to a shear angle of 0.5 degrees and a clearance of 20%. Next, the test piece was cut parallel to the rolling direction, with the center of its width as the observation position. The cut section of the test piece was then ground and etched with a solution of picric acid containing LION F. The shear separation surface of the test piece was then observed using an optical microscope at 25x magnification. If no cracks were observed in the obtained image, the shear workability was deemed acceptable.

[0285]

[0286]

[0287]

[0288]

[0289]

[0290] As shown in Table 3, in the invention examples, TS is above 1180MPa, resulting in high YR, high ductility, high tensile flange properties, high bending properties, and high shear processability.

[0291] On the other hand, in the comparative example, at least one of TS, YR, ductility, tensile flange properties, bending properties, and shear processability is insufficient.

[0292] The embodiments of the present invention have been described above, but the present invention is not limited to the descriptions that constitute a part of the disclosure of the present invention based on the above embodiments. That is, all other embodiments, examples, and techniques applied by those skilled in the art based on the above embodiments are included within the scope of the present invention. For example, in a series of heat treatments in the above manufacturing method, there are no particular limitations on the equipment used to perform heat treatment on the steel plate, as long as the heat process conditions are met.

[0293] Industrial availability

[0294] According to the present invention, a hot-dip galvanized steel sheet with a TS of 1180 MPa or higher can be obtained, which has high YR, high ductility, tensile flange and bending properties, and improved shear workability.

[0295] In particular, the hot-dip galvanized steel sheet of this invention possesses excellent properties, making it suitable for use in the frame structure components of automobiles of various sizes and shapes. This enables improved fuel efficiency through vehicle weight reduction, resulting in significant industrial value.

Claims

1. A hot-dip galvanized steel sheet, comprising a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet, having a tensile strength of 1180 MPa or higher. The base steel plate has the following composition and steel structure: The composition, by mass%, is as follows: C 0.090%–0.390%, Si 0.01%–2.50%, Mn 2.00%–4.00%, P less than 0.100%, S less than 0.0200%, Al less than 0.100%, and N less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities. In the steel microstructure, at a position of 1 / 4 thickness of the base steel plate, the area fraction of martensite is 70% or more, the area fraction of ferrite is 10% or less, and the area fraction of retained austenite is 0.5% or more and less than 10.0%, and the proportion of grains with an aspect ratio of 2.0 or less among the grains constituting the retained austenite exceeds 50%. In the histogram of hardness distribution at 1 / 4 of the thickness of the base steel plate, the number of group intervals with a frequency of 0.25 or higher is 1. The class range of the group intervals in the histogram, measured in Vickers hardness (HV), exceeds (n-1)×20+450 but is less than n×20+450, where n is an integer from 1 to 10. Furthermore, in the surface layer of the base steel plate, the area ratio of quenched martensite is 80% or less, and the surface layer of the base steel plate is the region from the surface of the base steel plate to a depth of 10 μm. The surface softening thickness of the base steel plate is 10μm to 100μm. Here, the surface softening thickness of the base steel plate is the distance from the surface of the base steel plate to the deepest point where the hardness is less than or equal to the base hardness multiplied by 0.85, using the hardness obtained at 1 / 4 of the plate thickness as the reference hardness. The diffusible hydrogen content in the low-temperature region of the base steel plate is below 0.015 ppm by mass. Here, the diffusible hydrogen content in the low-temperature region of the base steel plate is the amount of hydrogen released from the base steel plate when it is heated from room temperature to 50°C.

2. The hot-dip galvanized steel sheet according to claim 1, wherein, The composition of the base steel plate, by mass%, further contains ingredients selected from O: At least one of the following: less than 0.0100%, Ti: less than 0.200%, Nb: less than 0.200%, V: less than 0.200%, Ta: less than 0.10%, W: less than 0.10%, B: less than 0.0100%, Cr: less than 1.00%, Mo: less than 1.00%, Ni: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

3. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

4. A method for manufacturing hot-dip galvanized steel sheet, comprising hot rolling a steel billet having the composition described in claim 1 or 2 under conditions of a winding temperature of 350°C to 600°C and a residence time of more than 5000 seconds in a temperature range of 300°C or higher during cooling after winding to obtain a hot-rolled steel sheet. Next, the hot-rolled steel sheet is pickled. Next, the hot-rolled steel plate undergoes a first heat treatment at a heat treatment temperature of 450℃ to 650℃ and a temperature range of 400℃ or higher but below the heat treatment temperature for a residence time of 10 minutes or more. Next, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Next, the cold-rolled steel sheet is annealed under the following conditions: an average heating rate of 10°C / s or more in a temperature range of 250°C to 700°C; an oxygen concentration of 0.5% to 5.0% by volume in the temperature range of 250°C to 700°C; an annealing temperature of 820°C to 950°C; and a dew point of -35°C or more in the annealing temperature range. Next, the cold-rolled steel sheet is subjected to hot-dip galvanizing to produce a coated steel sheet. Next, the coated steel sheet is cooled at a temperature below 150°C (the cooling stop temperature). Next, the coated steel sheet is subjected to a second heat treatment under the condition that the following formula (1) is satisfied. 6.5≤(T+273)×{log(t×3600)+20} / 1000≤13.0……(1), Here, T is the heat treatment temperature of the second heat treatment, in °C, and t is the holding time of the second heat treatment, in hours.

5. The method for manufacturing hot-dip galvanized steel sheet according to claim 4, wherein, After the hot-dip galvanizing process, the coated steel sheet is subjected to alloying treatment.

6. A component made of hot-dip galvanized steel sheet according to any one of claims 1 to 3.

7. The component according to claim 6, used as a frame structure component of an automobile.

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