JP7787438B2 - Hot-dip galvanized steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to hot-dip galvanized steel sheets and their manufacturing methods, and more particularly to high-strength hot-dip galvanized steel sheets used primarily as automotive steel sheets and their manufacturing methods.

In recent years, there has been a demand for improved automobile fuel efficiency in light of greenhouse gas emission regulations as part of measures to combat global warming, and the use of high-strength steel sheets is becoming increasingly widespread in order to reduce the weight of vehicle bodies and ensure collision safety. In particular, there has been a growing need for high-strength steel sheets with a tensile strength of 980 MPa or more. Furthermore, high-strength hot-dip galvanized steel sheets with a hot-dip galvanized surface are required for areas of the vehicle body that require corrosion resistance.

Steel sheets used for automotive parts require not only strength but also various workability properties necessary for part formation, such as press formability and weldability. Specifically, from the perspective of press formability, steel sheets are required to have excellent elongation (total elongation in tensile tests: El) and stretch flangeability (hole expansion ratio: λ).

Generally, as the strength of steel plate increases, its press formability deteriorates. TRIP (Transformation Induced Plasticity) steel plate, which utilizes the transformation-induced plasticity of retained austenite, is known as a means of achieving both high strength and press formability in steel.

Patent Documents 1 to 3 disclose high-strength TRIP steel sheets with improved elongation and hole expansion ratios by controlling the structural fraction within a predetermined range. Patent Document 4 also describes a high-strength steel sheet with a predetermined chemical composition, including a volume fraction of 15% or less of ferrite with an average grain size of 2 μm or less, a volume fraction of 2 to 15% of retained austenite with an average grain size of 2 μm or less, a volume fraction of 10% or less of martensite with an average grain size of 3 μm or less, and the remainder being bainite and tempered martensite with an average grain size of 6 μm or less, and containing an average of 10 or more cementite particles with a grain size of 0.04 μm or more within the bainite and tempered martensite grains. It also describes that the high-strength steel sheet has a tensile strength of 1180 MPa or more, high elongation and hole expansion properties, and therefore excellent bending workability.

Patent document 5 discloses a TRIP steel sheet that improves stretch flange formability by limiting the area fraction of massive (low aspect ratio) retained austenite.

Patent document 6 discloses a high-strength TRIP steel sheet that has a large amount of work hardening in the early stages of forming, excellent shape fixability, and workability, achieved by controlling the amount of solute Si and solute Mn contained in retained austenite to be equal to or greater than a predetermined value.

In addition to press formability, automotive steel sheets also require excellent weldability. In particular, when welding hot-dip galvanized steel sheets together, or hot-dip galvanized steel sheets to ungalvanized steel sheets, it is necessary to suppress liquid metal embrittlement (LME) cracking. This phenomenon occurs when zinc, which has been liquefied by welding heat input, infiltrates and embrittles the steel sheet along the grain boundaries, causing cracks when tensile stress generated by welding acts on these areas.

Patent Document 7 discloses that such LME cracking is more likely to occur in steels with a higher Si content. Therefore, this document discloses TRIP steel sheets in which Al, which has a similar effect, is added in place of some of the Si added to TRIP steel to obtain retained austenite. Furthermore, Patent Documents 8 and 9 also disclose TRIP steel sheets in which Al is added in place of some of the Si.

In addition, Patent Document 10 discloses a method for manufacturing hot-dip galvanized steel sheets with excellent LME cracking resistance, which is characterized by controlling the atmosphere during heating and annealing in a hot-dip galvanizing line.

International Publication No. 2013/051238 Japanese Patent Application Laid-Open No. 2006-104532 JP 2011-184757 A International Publication No. 2017/179372 International Publication No. 2018/190416 International Publication No. 2013/018741 International Publication No. 2018/202916 JP 2011-17046 A International Publication No. 2013/144377 International Publication No. 2018/234938

In this technical field, there is an ongoing need for hot-dip galvanized steel sheets that combine high strength and press formability while also having excellent LME cracking resistance, and there is still room for improvement in these respects even with conventional steel sheets or hot-dip galvanized steel sheets.

Therefore, the object of the present invention is to provide a hot-dip galvanized steel sheet having excellent press formability and LME cracking resistance in spot welds, and a method for manufacturing the same.

As a result of extensive research conducted by the inventors to achieve the above objective, they discovered that the LME cracking susceptibility of spot welds in hot-dip galvanized steel sheets can be significantly improved in the following cases:

- When the interface between the base steel sheet and the hot-dip galvanized layer is smooth: Depressions present at the interface between the base steel sheet and the hot-dip galvanized layer (hereinafter also referred to simply as the "steel sheet/coating interface") are likely to accumulate molten Zn due to heat input during spot welding and become stress concentration areas, and are therefore likely to become the initiation points of LME cracking. Note that in the present invention, it is not the unevenness on the surface of the hot-dip galvanized steel sheet that is important, but rather the depressions at the steel sheet/coating interface, and this characteristic is essentially different from the steel sheet roughness that is generally measured. Specifically, it was found that a significant improvement effect can be obtained when the number density of depressions with a depth exceeding 2 μm at the steel sheet/coating interface is 2.0/100 μm or less per interface length.
When the Al concentration of the Al-enriched layer at the steel sheet/coating interface is high: Although the detailed mechanism is unknown, it is possible that the Al-enriched layer at the steel sheet/coating interface inhibits the penetration of molten Zn into the base steel sheet. Furthermore, it was found that the effect is greater as the Al concentration in the Al-enriched layer increases. Specifically, when the Al concentration was measured in the depth direction from the surface of the hot-dip galvanized steel sheet using a high-frequency glow discharge optical emission spectrometer (GDS), it was found that a significant improvement effect was obtained when the maximum Al concentration in the Al-enriched layer at the steel sheet/coating interface was 2.0 mass% or more.
- When a low-Si layer (Si-dilute layer) is present in the base steel sheet directly below the steel sheet/coating interface: Because Si is an element that deteriorates LME cracking susceptibility, it is thought that LME cracking susceptibility is improved when the Si concentration is low, particularly in the region of the base steel sheet directly below the coating interface in contact with molten Zn. Specifically, when the Si emission intensity was measured in the depth direction from the surface of the hot-dip galvanized steel sheet using a high-frequency glow discharge optical emission spectrometer (GDS), it was found that a significant improvement effect could be obtained when the Si _s /Si _b ratio in the base steel sheet directly below the steel sheet/coating interface was 0.90 or less.

The present invention has been realized based on the above findings, and is specifically as follows.
(1) A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on at least one surface of the base steel sheet,
The base steel plate is, in mass%,
C: 0.15-0.30%,
Si: 0.30-2.50%,
Mn: 1.40-3.49%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001-1.50%,
N: 0.0100% or less,
O: 0.0100% or less,
Cr: 0-1.00%,
Mo: 0-1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Co: 0-1.00%,
W: 0-1.00%,
Sn: 0-1.00%,
Sb: 0 to 0.50%,
Nb: 0 to 0.200%,
Ti: 0-0.200%,
V: 0-1.00%,
B: 0 to 0.0050%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Ce: 0 to 0.0150%,
Zr: 0 to 0.0100%,
La: 0 to 0.0150%,
Hf: 0-0.0100%,
Bi: 0 to 0.0100%,
REM other than Ce and La: 0 to 0.0100%, and the balance: Fe and impurities,
The steel structure in the range of 1/8 thickness to 3/8 thickness centered on 1/4 thickness from the surface of the base steel plate is, in volume %,
Ferrite: 0 to 50%,
Tempered martensite: 1% or more,
Retained austenite: 5% or more,
Fresh martensite: 0 to 15%
Sum of pearlite and cementite: 0 to 5%, and the balance: bainite,
when the hot-dip galvanized steel sheet is measured with a high-frequency glow discharge optical emission spectrometer (GDS), the maximum value of the Al concentration in an Al-enriched layer present at the interface between the base steel sheet and the hot-dip galvanized layer is 2.0 mass% or more, and Si _s /Si _b in the base steel sheet immediately below the interface between the base steel sheet and the hot-dip galvanized layer is 0.90 or less,
The number density of recesses having a depth exceeding 2 μm at the interface between the base steel sheet and the hot-dip galvanized layer is 2.0/100 μm or less per interface length, and
A hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more.
Si _s : minimum value of Si emission intensity in the base steel sheet directly below the interface between the base steel sheet and the hot-dip galvanized layer Si _b : average value of Si emission intensity in the base steel sheet (2) The chemical composition is, in mass%,
Si: 0.30 to 1.20%, and Al: 0.30 to 1.50%
The hot-dip galvanized steel sheet according to (1) above, characterized in that it comprises
(3) (A) a hot rolling step comprising hot rolling a slab having the chemical composition described in (1) or (2) above, and then coiling and cooling the resulting hot-rolled steel sheet, wherein the cooling satisfies the following formula (1):
where:
T(t): Steel plate temperature when t seconds have passed after winding [K]
tf: time [seconds] for the steel plate temperature to reach 673K
Nx: total atomic fraction [-] of Si, Mn and Al in steel; (B) a pickling step comprising: bending and unbending the hot-rolled steel sheet at least once, and then passing the hot-rolled steel sheet through an aqueous solution containing 1.0 to 5.0 mol/L of HCl, less than 3.0 mol/L of Fe ²⁺ and less than 0.10 mol/L of Fe ³⁺ at a temperature of 70 to 90°C at an average speed of 10 m/min or more for 30 seconds or more;
(C) A cold rolling process in which the hot-rolled steel sheet after pickling treatment is cold-rolled at a reduction ratio of 30 to 75%;
(D) A heat treatment and plating step, which includes subjecting the obtained cold-rolled steel sheet to heat treatment and plating, and which satisfies the following conditions (D1) to (D4): (D1) the cold-rolled steel sheet is heated from 600°C to a maximum heating temperature of Ac1+30°C to 950°C at an average heating rate of 0.2 to 20°C/sec, and the atmosphere around the cold-rolled steel sheet satisfies the following formula (4);
pH ₂ O: water vapor partial pressure pH ₂ : hydrogen partial pressure (D2) holding the cold-rolled steel sheet at the maximum heating temperature for 1 to 1000 seconds;
(D3) The time from immersion in the coating bath until gas wiping is performed is 0.1 to 5 seconds, and the steel sheet temperature after gas wiping is 440°C or less;
(D4) A method for producing a hot-dip galvanized steel sheet according to (1) or (2) above, comprising cooling the steel sheet to a range of Ms to Ms-200°C, then reheating it to a temperature range of 300 to 420°C, and holding it in the temperature range for 100 to 600 seconds.

The present invention makes it possible to obtain hot-dip galvanized steel sheets that have excellent press formability and LME cracking resistance in spot welds.

FIG. 1 is a diagram for explaining a method for measuring the number density of recesses having a depth exceeding 2 μm at the interface between a base steel sheet and a hot-dip galvanized layer. FIG. 2 is a diagram for explaining a method for measuring the maximum Al concentration of an Al-enriched layer present at the interface between a base steel sheet and a hot-dip galvanized layer. FIG. 2 is a diagram for explaining a method for measuring the maximum Al concentration of an Al-enriched layer present at the interface between a base steel sheet and a hot-dip galvanized layer. FIG. 2 is a diagram for explaining a method for measuring Si _s /Si _b in a base steel sheet immediately below the interface between the base steel sheet and the hot-dip galvanized layer.

"Chemical composition of base steel plate"
First, the reasons for limiting the chemical composition of the base steel sheet (hereinafter also simply referred to as steel sheet) according to an embodiment of the present invention as described above will be explained. Note that in this specification, all "%" defining the chemical composition is "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values before and after it are included as the lower and upper limits, unless otherwise specified.

[C: 0.15-0.30%]
C (carbon) is an essential element for ensuring the strength of steel sheets. To fully obtain this effect, the C content is set to 0.15% or more. The C content may be 0.16% or more, 0.18% or more, or 0.20% or more. On the other hand, excessive C content may deteriorate workability, such as press formability, and weldability. For this reason, the C content is set to 0.30% or less. The C content may be 0.28% or less, 0.27% or less, or 0.25% or less.

[Si:0.30-2.50%]
Si (silicon) is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. To fully obtain these effects, the Si content is set to 0.30% or more. The Si content may be 0.40% or more, 0.50% or more, 0.51% or more, 0.52% or more, 0.55% or more, 0.60% or more, or 0.70% or more. On the other hand, excessive addition may promote LME cracking during welding. Therefore, the Si content is set to 2.50% or less. From the viewpoint of suppressing LME cracking, a lower Si content is desirable; specifically, 2.00% or less is desirable, and 1.50% or less is more desirable. In particular, when the Si content is limited to 1.20% or less, particularly excellent resistance to LME cracking susceptibility can be obtained.

[Mn: 1.40-3.49%]
Mn (manganese) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel sheets. To fully obtain this effect, the Mn content is set to 1.40% or more. The Mn content may be 1.50% or more, 1.70% or more, or 2.00% or more. On the other hand, excessive addition may deteriorate workability such as press formability, weldability, and low-temperature toughness. Therefore, the Mn content is set to 3.49% or less. The Mn content may also be 3.20% or less, 3.00% or less, or 2.90% or less.

[P: 0.050% or less]
P (phosphorus) is a solid solution strengthening element and is effective in increasing the strength of steel sheets, but excessive addition of P deteriorates weldability and toughness. Therefore, the P content is limited to 0.050% or less. The P content is preferably 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but excessively reducing the P content increases the cost of dephosphorization, so from an economical standpoint, the lower limit is preferably set to 0.001%.

[S: 0.0100% or less]
S (sulfur) is an element contained as an impurity, and forms MnS in steel, which deteriorates toughness and hole expandability. Therefore, the S content is limited to 0.0100% or less, as a range in which deterioration of toughness and hole expandability is not significant. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. The S content may be 0%, but if the S content is reduced too much, the desulfurization cost will be high, so from an economical viewpoint, the lower limit is preferably set to 0.0001%.

[Al: 0.001-1.50%]
At least 0.001% of aluminum (Al) is added to deoxidize the steel. The Al content may be 0.005% or more, 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive addition of Al not only saturates the effect and leads to unnecessary cost increases, but also raises the transformation temperature of the steel, increasing the load during hot rolling and resulting in reduced mechanical properties of the steel sheet. Therefore, the upper limit of the Al content is 1.50%. The Al content may be 1.40% or less, 1.20% or less, or 1.00% or less. Furthermore, Al also has the effect of suppressing the formation of iron carbides and thereby increasing retained austenite. To achieve this effect, 0.30% or more of Al must be added. The Al content may be 0.50% or more or 0.70% or more.

[N: 0.0100% or less]
N (nitrogen) is an element contained as an impurity, and if its content is high, it may form coarse nitrides in the steel, deteriorating bendability and hole expandability. Therefore, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but if the N content is reduced too much, the cost of denitrification will be high, so from an economical standpoint, it is preferable to set the lower limit to 0.0001%.

[O: 0.0100% or less]
O (oxygen) is an element contained as an impurity, and if its content is high, it may form coarse oxides in the steel, deteriorating bendability and hole expandability. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but from the viewpoint of manufacturing costs, the lower limit is preferably set to 0.0001%.

The basic chemical composition of the base steel plate and the slab used in its manufacture according to an embodiment of the present invention is as described above. Furthermore, the base steel plate and slab may contain the following optional elements as needed. Note that when these optional elements are not contained, the lower limit of their content is 0%.

[Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Sn : 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, Ti: 0-0.200%, V: 0-1.00% and B: 0-0.0050%]
Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Sn (tin), Sb (antimony), Nb (niobium), Ti (titanium), V (vanadium), and B (boron) are all elements effective in increasing the strength of steel sheets. Therefore, one or more of these elements may be added as needed. However, adding these elements in excess saturates the effect and unnecessarily increases costs. Therefore, the contents of these elements are set to Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, Ti: 0-0.200%, V: 0-1.00%, and B: 0-0.0050%. Each element may be 0.001% or more, 0.005% or more, or 0.010% or more. In particular, the B content may be 0.0001% or more or 0.0002% or more. Similarly, the B content may be 0.0030% or less, 0.0010% or less, less than 0.0005%, 0.0004% or less, or 0.0003% or less.

[Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Ce: 0 to 0.0150%, Zr: 0 to 0.0100%, La: 0 to 0.0150%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, and REM other than Ce and La: 0 to 0.0100%]
Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), and REMs (rare earth elements) other than Ce and La are elements that contribute to the fine dispersion of inclusions in steel, while Bi (bismuth) is an element that reduces the microsegregation of substitutional alloying elements such as Mn and Si in steel. Since each of these elements contributes to improving the workability of steel sheets, one or more of these elements may be added as needed. However, excessive addition can cause deterioration of ductility. Therefore, the upper limit of their content is 0.0150% or 0.0100%. Furthermore, each element may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

In the base steel plate according to an embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the base steel plate.

"Steel structure inside the base steel plate"
Next, the reasons for limiting the internal structure of the base steel sheet according to the embodiment of the present invention will be described.

[Ferrite: 0 to 50%]
Ferrite has excellent ductility but is a soft structure, and is desirably contained as needed. In this case, the content may be 1% or more, 5% or more, or 10% or more by volume. On the other hand, if ferrite is contained in an excessive amount, it becomes difficult to ensure the desired steel sheet strength. Therefore, the content may be 50% or less by volume, 45% or less, 40% or less, or 35% or less.

[Tempered martensite: 1% or more]
Tempered martensite is a high-strength and tough structure, and is an essential metal structure in embodiments of the present invention. In order to achieve a high level of balance between strength and elongation, the tempered martensite content is set to 1% or more by volume. The tempered martensite content is preferably 5% or more, and may be 10% or more or 20% or more. There is no particular upper limit, but the tempered martensite content may be, for example, 90% or less, 80% or less, 70% or less, or 50% or less by volume.

[Residual austenite: 5% or more]
Retained austenite improves the ductility of steel sheets through the TRIP effect, which transforms into martensite through stress-induced transformation during deformation of the steel sheet. Therefore, the retained austenite content is set to 5% or more by volume, and may be 8% or more, 9% or more, 10% or more, or 11% or more. Since the greater the amount of retained austenite, the greater the elongation, so there is no need to specify an upper limit. However, to obtain a large amount of retained austenite, it becomes necessary to include a large amount of alloying elements such as C. Since the present invention sets an upper limit on the C content, it is practically difficult to obtain 30% or more of retained austenite. Therefore, the retained austenite content may be 30% or less, 25% or less, or 20% or less by volume.

[Fresh martensite: 0 to 15%]
In an embodiment of the present invention, fresh martensite refers to martensite that has not been tempered, i.e., martensite that does not contain carbides. Because this fresh martensite has a brittle structure, it becomes the origin of fracture during plastic deformation and deteriorates the local ductility of the steel sheet. Therefore, its content is set to 0 to 15% by volume. The fresh martensite content is preferably 0 to 10% or 0 to 5% by volume. The fresh martensite content may be 1% or more, or 2% or more, by volume.

[Total of pearlite and cementite: 0 to 5%]
Pearlite contains hard and coarse cementite, which acts as a fracture origin during plastic deformation and deteriorates the local ductility of the steel sheet. Therefore, its content, including cementite, is set to 0 to 5% by volume, and may be 0 to 3% or 0 to 2%.

[Bainite: Remainder]
The remainder of the metal structure of the base steel plate according to the embodiment of the present invention is composed of bainite. The bainite in the remainder structure may be any of upper bainite having carbides between laths, lower bainite having carbides within the laths, bainitic ferrite having no carbides, and granular bainitic ferrite in which the lath boundaries of bainite have been restored and become unclear, or a mixture thereof. The bainite content in the remainder may be 0%. For example, the bainite content in the remainder may be 1% or more, 5% or more, or 10% or more by volume. There is no particular upper limit, but the bainite content in the remainder may be, for example, 70% or less, 60% or less, 57% or less, 55% or less, 50% or less, or 40% or less by volume.

The fraction of the steel structure is evaluated using secondary electron images taken using an FE-SEM and X-ray diffraction. First, a sample is taken from a thickness cross-section parallel to the rolling direction of the steel sheet at the center position in the width direction. The observation surface is mechanically polished to a mirror finish and then etched using a nital solution. Next, secondary electron images are taken over a total area of 2.0 × 10 ^{−9 m2} or more in one or more observation fields ranging from 1/8 to 3/8 thickness, centered at 1/4 thickness from the surface of the base steel sheet, on the observation surface. From the obtained secondary electron images, the area fractions of ferrite, retained austenite, bainite, tempered martensite, fresh martensite, and pearlite are measured, and these are considered to be volume fractions. A region having a substructure within grains and where cementite precipitates with multiple variants is determined to be tempered martensite. A region where cementite precipitates in a lamellar form is determined to be pearlite (or the sum of pearlite and cementite). Areas with low brightness and no visible substructure are judged to be ferrite. Areas with high brightness and no visible substructure due to etching are judged to be fresh martensite and retained austenite. Areas that do not fall into any of the above categories are judged to be bainite. The volume fraction of each structure is determined by calculating the volume fraction of each structure using the point counting method. The volume fraction of fresh martensite can be determined by subtracting the volume fraction of retained austenite determined by X-ray diffraction.

The volume fraction of retained austenite is measured using X-ray diffraction. Specifically, the base steel plate is mechanically and chemically polished to remove the material from the plate surface to a depth of 1/4 of the way through the plate thickness. The polished sample is then scanned using MoKα1 radiation as a characteristic X-ray. The structural fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), and (311) of the fcc phase. This is the volume fraction of retained austenite.

"Hot-dip galvanized layer"
The base steel sheet according to the embodiment of the present invention has a hot-dip galvanized layer on at least one surface, preferably both surfaces. The galvanized layer may be a hot-dip galvanized (GI) layer having any composition known to those skilled in the art, and may contain additional elements such as Al, Mg, Si, and Fe in addition to Zn. The coating weight of the galvanized layer is not particularly limited and may be a general coating weight. A general coating weight for automotive applications is, for example, 20 to 100 g/ ^m2 per side.

[Number density of recesses exceeding 2 μm in depth at the interface between the base steel sheet and the hot-dip galvanized layer: 2.0 recesses/100 μm or less per interface length]
In the hot-dip galvanized steel sheet according to an embodiment of the present invention, the number density of recesses having a depth exceeding 2 μm at the interface between the base steel sheet and the hot-dip galvanized layer is 2.0 recesses/100 μm or less per interface length. Recesses having a depth exceeding 2 μm are prone to accumulate molten Zn due to heat input during spot welding and act as stress concentration areas, thereby becoming the starting point for LME cracking. Therefore, when the number density of such recesses increases, specifically when the number density exceeds 2.0 recesses/100 μm, the LME cracking susceptibility significantly deteriorates. Therefore, from the viewpoint of suppressing or reducing LME cracking, it is desirable to reduce the number density of such recesses to achieve a flatter interface shape; specifically, the number density is more desirably 1.0 recesses/100 μm or less. From the viewpoint of suppressing or reducing LME cracking, the smaller the number density of recesses having a depth exceeding 2 μm, the better. Therefore, the lower limit of the number density is preferably 0.0 recesses/100 μm, and may be 0.1 recesses/100 μm.

The number density of these recesses is measured as follows. Referring to Figure 1, a sample is first taken from a thickness cross-section of the hot-dip galvanized steel sheet parallel to the rolling direction, with the thickness cross-section at the center of the width as the observation surface. The observation surface is then mechanically polished to a mirror finish, and a backscattered electron image of the coating/steel sheet interface is taken at a magnification of 500x using an FE-SEM (Figure 1(a)). The obtained backscattered electron image is binarized to clarify the interface between the hot-dip galvanized layer and the base steel sheet (Figure 1(b)). The converted binarized image is then converted into numerical data to obtain a height profile of the interface (Figure 1(c)). Image analysis software capable of this operation includes, for example, Image J. The center line of the height profile is determined using the least-squares method, and regions where the surface height deviates from the center line by more than 2 μm on the negative side are defined as "recesses with a depth of more than 2 μm at the interface between the base steel sheet and the hot-dip galvanized layer." A similar analysis is performed over a total measurement range of more than 1 mm in the X direction (rolling direction). For example, if the size in the X direction (rolling direction) in one field of view is 200 μm, the above analysis is performed at least five times, changing the field of view. The number of "depressions with a depth exceeding 2 μm at the interface between the base steel sheet and the hot-dip galvanized layer" obtained in each field of view is summed, and this is converted to a number density per 100 μm of interface length. This is determined as the "number density of depressions with a depth exceeding 2 μm at the interface between the base steel sheet and the hot-dip galvanized layer." Here, the interface length refers to the length along the height profile of the interface as shown in Figure 1(c) and can be measured using image analysis software. While contact-type or laser-type roughness meters are commonly used to measure the height profile of steel sheets, when measuring the interface between the coating and the base steel sheet (steel substrate), as in the present invention, the coating must first be dissolved and stripped with acid. However, these methods are not recommended because there is a concern that the acid dissolution will simultaneously corrode not only the coating but also the steel substrate interface, resulting in changes to the original unevenness.

[Maximum Al concentration of Al-enriched layer present at the interface between base steel sheet and hot-dip galvanized layer: 2.0 mass% or more]
A hot-dip galvanized steel sheet according to an embodiment of the present invention has an Al-enriched layer at the interface between the base steel sheet and the hot-dip galvanized layer. Here, the Al-enriched layer refers to a region where the Al concentration is 10% or more higher than the Al concentration in the hot-dip galvanized layer and 10% or more higher than the Al concentration in the base steel sheet. The Al concentration in the hot-dip galvanized layer refers to the Al concentration measured by a high-frequency glow discharge optical emission spectrometer (GDS) at a position halfway through the thickness of the hot-dip galvanized layer. The halfway through the thickness of the hot-dip galvanized layer corresponds to the intermediate position between the interface between the base steel sheet and the hot-dip galvanized layer, as determined by GDS measurement (described later), and the surface of the hot-dip galvanized steel sheet. Furthermore, the Al concentration in the base steel sheet refers to the Al concentration corresponding to the average value of the Al emission intensity measured by GDS at a depth of 100 to 150 μm from the surface of the hot-dip galvanized steel sheet. The presence of an Al-enriched layer at the interface between the base steel sheet and the hot-dip galvanized layer is thought to suppress the penetration of molten Zn into the base steel sheet, which in turn makes it possible to improve LME cracking susceptibility. From the viewpoint of reliably improving LME cracking susceptibility, the maximum Al concentration of the Al-enriched layer is 2.0 mass% or more, preferably 2.5 mass% or more. The effect of improving LME cracking susceptibility is greater as the Al concentration in the Al-enriched layer is higher, and therefore the upper limit is not particularly limited. For example, the maximum Al concentration of the Al-enriched layer may be 8.0 mass% or less, 6.0 mass% or less, 5.0 mass% or less, 4.5 mass% or less, or 4.1 mass% or less.

The maximum Al concentration in the Al-enriched layer is measured using a high-frequency glow discharge optical emission spectrometer (GDS). Specifically, the surface of the hot-dip galvanized steel sheet is placed in an Ar atmosphere, a voltage is applied to generate glow plasma, and the steel sheet surface is sputtered while analysis is performed in the depth direction. The elements contained in the material (hot-dip galvanized steel sheet) are identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. Depth data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth can be calculated in advance using a standard sample, allowing the sputtering time to be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. Furthermore, commercially available analytical equipment can be used for the high-frequency GDS analysis of the galvanized steel sheet in this embodiment. In this embodiment, a high-frequency glow discharge optical emission spectrometer, GD-Profiler 2, manufactured by Horiba, Ltd., is used. The obtained emission intensity is converted to mass% by creating a calibration curve as follows. The average value of the emission intensity is calculated within a certain depth range where the emission intensity is sufficiently stable. For example, it is the average value of the emission intensity at a depth of 100 to 150 μm from the surface of the hot-dip galvanized steel sheet. This average value corresponds to the Al content [mass%] of the base steel sheet. Furthermore, when the emission intensity is 0, the mass% is also set to 0. A calibration curve is created using these two points. The position of the interface between the base steel sheet and the hot-dip galvanized layer can be determined from the emission intensity of Zn.

An example of Zn emission intensity measured by GDS is shown in Figure 2. The position where the Zn emission intensity drops sharply corresponds to the interface between the base steel sheet and the hot-dip galvanized layer. Similarly, an example of Al concentration (emission intensity) measured by GDS is shown in Figure 3. It can be seen that an Al concentration peak appears at the position where the Zn emission intensity drops sharply in Figure 2. The Al concentration (Almax in Figure 3) calculated from the emission intensity of such a peak that appears at or near the position where the Zn emission intensity drops sharply when measured by GDS is determined to be the "maximum Al concentration of the Al-enriched layer present at the interface between the base steel sheet and the hot-dip galvanized layer."

[Si _s /Si _b in base steel sheet directly below the interface between base steel sheet and hot-dip galvanized layer: 0.90 or less]
The hot-dip galvanized steel sheet according to an embodiment of the present invention has a Si-depleted region in the base steel sheet immediately below the interface between the base steel sheet and the hot-dip galvanized layer. Here, "immediately below the interface" refers to a region extending from the interface between the base steel sheet and the hot-dip galvanized layer to a depth of 10 μm, more specifically, a region extending from the interface between the base steel sheet and the hot-dip galvanized layer to a depth of 10 μm, as determined by GDS measurement. Since Si is an element that deteriorates LME cracking susceptibility, the presence of such a Si-depleted region in the base steel sheet immediately below the coating interface in contact with molten Zn makes it possible to improve LME cracking susceptibility. From the viewpoint of reliably improving LME cracking susceptibility, the ratio Si _s /Si _b (where Si _s is the minimum value of Si emission intensity in the base steel sheet immediately below the interface between the base steel sheet and the hot-dip galvanized layer, and Si _b is the average value of Si emission intensity in the base steel sheet) is set to 0.90 or less, preferably 0.85 or less. The effect of improving LME cracking susceptibility is greater as the Si _s /Si _b ratio decreases. Therefore, the lower limit is not particularly limited. For example, Si _s /Si _b may be 0.10 or more, 0.30 or more, 0.50 or more, 0.60 or more, or 0.65 or more. _{Si s} and Si _b are measured using a high-frequency glow discharge optical emission spectrometer (GDS) in the same manner as in the case of the maximum Al concentration in the Al-enriched layer. The detailed measurement conditions are as described in relation to the maximum Al concentration in the Al-enriched layer. Si _b can be calculated as an average value within a certain depth range where the emission intensity is sufficiently stable. For example, it can be the average emission intensity within a depth range of 100 to 150 μm from the surface of the hot-dip galvanized steel sheet. A measurement example is shown in Figure 4. Referring to Figure 4, it can be seen that a minimum value of Si (Si _s ) appears in the base steel sheet directly below the interface between the base steel sheet and the hot-dip galvanized layer.

"Mechanical properties"
The hot-dip galvanized steel sheet according to an embodiment of the present invention can achieve excellent mechanical properties, such as high strength, specifically a tensile strength (TS) of 980 MPa or more. The tensile strength is preferably 1080 MPa or more, more preferably 1180 MPa or more. The upper limit is not particularly limited, and the tensile strength may be, for example, 2000 MPa or less, 1800 MPa or less, 1600 MPa or less, or 1500 MPa or less. The hot-dip galvanized steel sheet according to an embodiment of the present invention can also achieve high ductility, more specifically, a total elongation (El) of 8.0% or more, preferably 10.0% or more, more preferably 12.0% or more, or 15.0% or more. The upper limit is not particularly limited, and the total elongation may be, for example, 40.0% or less or 35.0% or less. The tensile strength and total elongation are measured by taking a JIS No. 5 tensile test specimen from a direction perpendicular to the rolling direction of the steel sheet and conducting a tensile test in accordance with JIS Z2241:2011. Furthermore, the hot-dip galvanized steel sheet according to the embodiment of the present invention can achieve high hole expandability, more specifically, a hole expansion ratio (λ) of 18% or more, preferably 20% or more, and more preferably 25% or more. The upper limit is not particularly limited, but the hole expansion ratio may be, for example, 80% or less or 70% or less. The hole expansion ratio is measured by performing the "JFS T 1001 Hole Expansion Test Method" of the Japan Iron and Steel Federation standard. The hot-dip galvanized steel sheet according to the embodiment of the present invention can achieve a high level of balance between tensile strength (TS), total elongation (El), and hole expansion ratio (λ), thereby achieving press formability suitable for use as an automotive component.

[Thickness]
The hot-dip galvanized steel sheet according to the embodiment of the present invention has a thickness of, for example, 0.6 to 4.0 mm. Although not particularly limited, the thickness may be 0.8 mm or more, 1.0 mm or more, or 1.2 mm or more. Similarly, the thickness may be 3.0 mm or less, 2.5 mm or less, or 2.0 mm or less.

"Manufacturing method"
Next, a method for manufacturing a hot-dip galvanized steel sheet will be described. The following description is intended to exemplify a characteristic method for manufacturing a hot-dip galvanized steel sheet according to an embodiment of the present invention, but is not intended to limit the hot-dip galvanized steel sheet to one manufactured by the manufacturing method described below.

"(A) Hot rolling process"
First, in the hot rolling process, a slab having the same chemical composition as that described above for the base steel plate is heated before hot rolling, and then subjected to rough rolling and finish rolling. The heating temperature of the slab is not particularly limited, but is generally preferably 1150°C or higher in order to sufficiently dissolve borides, carbides, etc. Note that the steel slab used is preferably cast by a continuous casting method from the viewpoint of manufacturability, but may also be produced by an ingot casting method or a thin slab casting method.

[Rough rolling]
The heated slab may be subjected to rough rolling before finish rolling. The rough rolling conditions are not particularly limited, but it is preferable to perform the rough rolling so that the completion temperature is 1050°C or higher and the total reduction is 60% or more. If the total reduction is less than 60%, recrystallization during hot rolling will be insufficient, which may lead to heterogeneity in the hot-rolled steel sheet structure. The total reduction may be, for example, 90% or less.

[Finishing rolling]
After optional rough rolling, finish rolling is performed. The conditions are not particularly limited, but it is desirable to perform finish rolling within a range that satisfies the following conditions: a finish rolling entry temperature of 950 to 1100 ° C, a finish rolling exit temperature of 850 to 1000 ° C, and a total rolling reduction of 80 to 95%. If the finish rolling entry temperature is below 950 ° C, the finish rolling exit temperature is below 850 ° C, or the total rolling reduction exceeds 95%, the texture of the hot-rolled steel sheet develops, which may result in anisotropy in the final product sheet. On the other hand, if the finish rolling entry temperature exceeds 1100 ° C, the finish rolling exit temperature exceeds 1000 ° C, or the total rolling reduction is below 80%, the crystal grain size of the hot-rolled steel sheet may become coarse, causing coarsening of the final product sheet structure.

[Winding temperature]
The hot-rolled steel sheet after finish rolling is cooled to, for example, 700°C or less and then wound into a coil. The coiling temperature is preferably 450 to 680°C. If the coiling temperature is below 450°C, the strength of the hot-rolled sheet becomes excessive, which may impair cold rolling properties. On the other hand, if the coiling temperature exceeds 680°C, alloy elements such as Mn are concentrated in the cementite, which may delay the dissolution of cementite in the final annealing process and cause a decrease in strength. The coiling temperature may be 500°C or higher and/or 650°C or lower, or 600°C or lower.

[Cooling of hot rolled coil after coiling]
The hot-rolled coil (hot-rolled steel sheet) after coiling is cooled so as to satisfy the following formula (1).
where:
T(t): Steel plate temperature when t seconds have passed after winding [K]
tf: time [seconds] for the steel plate temperature to reach 673K
Nx: Sum of the atomic fractions [-] of Si, Mn, and Al in the steel. Equation (1) indicates that the larger the Σ value, the more the internal oxidation reaction progresses on the surface of the hot-rolled steel sheet. Σ in equation (1) is calculated using the quadrature method by division. Δt is a finite value corresponding to the measurement interval of temperature T(t). For example, it is 100 seconds. Do is the diffusion coefficient of oxygen atoms at temperature T(t), No is the amount of oxygen atoms dissolved in the steel at temperature T(t), and Nx is the total amount of the main elements to be internally oxidized in the steel. Nx can be calculated by converting the mass fraction of each element (Si, Mn, Al) to atomic fraction and summing them. This can be expressed mathematically as equation (5).
Here, [X] is the mass fraction of element X, and M _X is the atomic weight of element X. The denominator of formula (5) is the sum of all elements added to the steel of interest.
That is, Equation (1) indicates that the internal oxidation reaction proceeds more easily with a larger oxygen atom diffusion coefficient and a larger amount of oxygen dissolved in solid solution, and more slowly with a larger amount of the internally oxidized element. As the internal oxidation reaction proceeds, the Si dissolved in the steel is consumed in the formation of the internal oxide, resulting in the formation of a Si-dilute layer directly below the internal oxide layer. Here, Si, particularly Si dissolved in the steel, is an element that reduces LME cracking susceptibility. However, as described below, this Si-dilute layer can be retained in the final product by limiting the pickling conditions, thereby reducing the amount of solute Si in the base steel sheet directly below the interface with the molten Zn. This significantly improves LME resistance. On the other hand, excessive internal oxidation leads to increased unevenness in the steel sheet after pickling. Although these unevennesses are smoothed to a certain extent by cold rolling, their effects persist in the final product. As a result, the number density of recesses exceeding 2 μm in depth at the steel sheet/coating interface in the final product increases, deteriorating LME resistance. For these reasons, the value of formula (1) is limited to more than 0.05 and less than 1.50, preferably 0.10 to 1.00, and more preferably 0.20 to 0.70.

"(B) Pickling process"
The hot-rolled steel sheet obtained in the hot rolling process is subjected to pickling treatment by passing it through an aqueous solution containing 1.0 to 5.0 mol/L of HCl, less than 3.0 mol/L of Fe ²⁺ , and less than 0.10 mol/L of Fe ³⁺ at an average speed of 10 m/min or more at a temperature of 70 to 90°C for 30 seconds or more. To efficiently remove the internal oxide layer containing silicon oxides, the hot-rolled steel sheet is subjected to at least one bending and unbending deformation using a tension leveler or the like before pickling. If the HCl concentration in the pickling solution is below 1.0 mol/L, the Fe ²⁺ concentration is 3.0 mol/L or more, the temperature of the aqueous solution is below 70°C, the average speed of the hot-rolled steel sheet is below 10 m/min, or the pickling time is less than 30 seconds, the pickling does not proceed sufficiently, resulting in uneven removal of the internal oxide layer. As a result, the number density of recesses exceeding 2 μm in depth at the steel sheet/coating interface in the final product increases. On the other hand, if the HCl concentration exceeds 5.0 mol/L or the temperature exceeds 90°C, the pickling process proceeds excessively, even removing the Si-poor layer formed on the hot-rolled steel sheet, resulting in insufficient formation of a Si-poor region immediately below the interface between the base steel sheet and the hot-dip galvanized layer in the final product. Furthermore, Fe ³⁺ in the pickling solution is known to promote the dissolution of the base steel sheet by suppressing the inhibitor effect, and this effect becomes significant when the Fe ³⁺ content is 0.10 mol/L or higher. As a result, the formation of a Si-poor region immediately below the interface between the base steel sheet and the hot-dip galvanized layer in the final product is also insufficient. For these reasons, the pickling conditions are limited as described above.

"(C) Cold rolling process"
The hot-rolled steel sheet after pickling is then subjected to cold rolling. The reduction ratio in cold rolling is set to 30% or more to promote recrystallization and/or smooth out irregularities on the steel sheet after pickling. If the reduction ratio is less than 30%, the irregularities on the steel sheet surface cannot be sufficiently smoothed, and the number density of depressions exceeding 2 μm in depth at the steel sheet/coating interface in the final product increases. The reduction ratio may be 40% or more. However, excessive reduction increases the rolling load, leading to an increased load on the cold rolling mill, so the upper limit is set to 75% or 70%.

(D) Heat Treatment and Plating Process
[(D1) Average heating rate from 600°C to the maximum heating temperature of Ac1+30°C to 950°C is 0.2 to 20°C/sec]
Next, the obtained cold-rolled steel sheet is subjected to a predetermined heat treatment and plating in a heat treatment and plating process. Specifically, the cold-rolled steel sheet is first heated from 600°C to a maximum heating temperature of Ac1+30°C to 950°C at an average heating rate of 0.2 to 20°C/s, and the atmosphere around the cold-rolled steel sheet satisfies the following formula (4):
pH ₂ O: Water vapor partial pressure pH ₂ : Hydrogen partial pressure Promoting internal oxidation reactions not only during the hot rolling process but also during the heat treatment process after cold rolling further promotes the formation of Si-poor regions. The log(pH ₂ O/pH ₂ ) in equation (4) is also called the oxygen potential. The higher this value, the more readily oxidized elements present in the surface layer of the steel are internally oxidized, such as Si, Mn, and Al, and the more the Si-poor regions grow. To achieve this effect, this value must exceed at least -1.0. On the other hand, if this value exceeds -0.1, oxidation of not only Si, Mn, Al, etc. but also Fe progresses, resulting in defects such as bare spots. A more preferred range is -0.9 to -0.2, and even more preferred is -0.8 to -0.3. The average heating rate from 600°C to the maximum heating temperature of Ac1+30°C to 950°C is limited to 0.2 to 20°C/s. If the rate exceeds 20°C/s, the internal oxidation reaction does not proceed sufficiently. On the other hand, if the heating rate is below 0.2°C/sec, the strength decreases due to the coarsening of the structure and the excessive progress of the decarburization reaction. A preferred average heating rate is 0.5 to 10°C/sec, and more preferably 1 to 7°C/sec. Ac1 (°C) is calculated using the following formula. In the formula below, the element symbol is substituted with the mass% of the element in the base steel plate. For elements that are not contained, 0 mass% is substituted.
Ac1 (℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr

[(D2) Ac1 + 30°C to 950°C maximum heating temperature held for 1 second to 1000 seconds]
In order to sufficiently advance austenitization and obtain the desired structure by the subsequent cooling treatment, the cold-rolled steel sheet is heated to at least Ac1 + 30°C or higher and soaked at that temperature (maximum heating temperature). Insufficient austenitization may result in the formation of a large amount of ferrite in the final structure. However, excessively increasing the heating temperature not only leads to deterioration of toughness due to coarsening of austenite grains but also to damage to the annealing equipment. Therefore, the upper limit is set to 950°C, preferably 900°C. A short soaking time does not sufficiently advance austenitization, so the soaking time is set to at least 1 second. The soaking time is preferably 30 seconds or more or 60 seconds or more. On the other hand, a too long soaking time impairs productivity, so the upper limit is set to 1000 seconds, preferably 600 seconds. During soaking, the cold-rolled steel sheet does not necessarily need to be maintained at a constant temperature; it may vary within a range that satisfies the above conditions.

[(D3) The time from immersion in the coating bath until gas wiping is performed is 0.1 to 5 seconds, and the steel sheet temperature after gas wiping is 440°C or less]
After the heating and holding, the cold-rolled steel sheet is cooled and immersed in a hot-dip galvanizing bath. The average cooling rate in the temperature range of 550 to 700°C is preferably 10 to 100°C/sec. Regarding the temperature of the steel sheet during immersion in the hot-dip galvanizing bath, if the difference between the steel sheet temperature and the galvanizing bath temperature is too large, the galvanizing bath temperature may change, causing operational problems. Therefore, the steel sheet temperature is preferably set to between -20°C and +20°C above the galvanizing bath temperature. Hot-dip galvanizing may be performed according to conventional methods. For example, the galvanizing bath temperature may be 440 to 480°C, and the immersion time may be 5 seconds or less. To form an Al-enriched layer at the steel sheet/galvanizing interface, the galvanizing bath preferably contains 0.1 to 0.5 mass% Al. Other impurities may include Fe, Si, Mg, Mn, Cr, Ti, Pb, etc. The coating weight is controlled by gas wiping. The coating weight may be varied as appropriate depending on the required corrosion resistance, but a preferred range is, for example, 20 to 100 g/ ^m² per side. The time between immersion in the coating bath and gas wiping is limited to 0.1 to 5 seconds. If the time exceeds 5 seconds or the steel sheet temperature after gas wiping exceeds 440°C, the Al-enriched layer begins to disintegrate, causing the maximum Al concentration in the Al-enriched layer present at the steel sheet/coating interface to fall below a predetermined value. The lower limit is not particularly limited, but for example, the steel sheet temperature after gas wiping may be 300°C or higher. Meanwhile, the lower limit of the time before gas wiping is determined by the equipment configuration, but it is difficult to achieve a time below 0.1 seconds in a typical hot-dip galvanizing line.

[(D4) Cooling the steel plate to a temperature range of Ms to Ms-200°C, then reheating to a temperature range of 300 to 420°C, and holding at that temperature range for 100 to 600 seconds]
To transform a portion of the untransformed austenite into martensite, the steel sheet is cooled to a temperature ranging from the martensitic transformation start temperature (Ms) to Ms - 200°C. The martensite formed here is subsequently tempered by reheating and holding to form tempered martensite. If the cooling stop temperature exceeds Ms, tempered martensite is not formed, and the desired metal structure is not obtained. On the other hand, if the cooling stop temperature is below Ms - 200°C, the untransformed austenite is excessively reduced, and the desired retained austenite content is not obtained. The preferred range of the cooling stop temperature is Ms - 20 to Ms - 150°C, and more preferably Ms - 40 to Ms - 100°C. Note that martensitic transformation occurs after ferrite transformation and/or bainite transformation. Carbon partitions into austenite during this transformation. Therefore, Ms does not match the Ms obtained when the steel sheet is heated to a single austenite phase and then rapidly cooled. In embodiments of the present invention, Ms is determined by measuring the thermal expansion temperature. For example, Ms can be determined by using a device capable of measuring the amount of thermal expansion during continuous heat treatment, such as a Formaster testing machine, to reproduce the heat cycle from the start of heat treatment (equivalent to room temperature) until cooling to the temperature below Ms, and measuring the amount of thermal expansion during that period. In the temperature-thermal expansion curve obtained by simulating the heat cycle using a thermal expansion measuring device, the steel sheet thermally shrinks linearly during cooling, but deviates from the linear relationship at a certain temperature. The temperature at this point is Ms in this embodiment of the present invention.

After cooling to a temperature range of Ms to Ms - 200°C, the steel is reheated to a temperature range of 300°C to 420°C and held there. This process concentrates carbon in the austenite, stabilizing the austenite (austempering), and simultaneously tempers the martensite formed during cooling to obtain the desired retained austenite content. If the reheating temperature is below 300°C or the holding time is less than 100 seconds, carbon concentration in the austenite is insufficient. During the subsequent cooling process to room temperature, the proportion of untransformed austenite that remains at room temperature and becomes retained austenite decreases, while the proportion that becomes fresh martensite increases. As a result, the retained austenite content falls below the lower limit of 5% by volume and/or the fresh martensite content exceeds the upper limit of 15% by volume. On the other hand, if the reheating temperature exceeds 420°C or the holding time exceeds 600 seconds, austenite decomposes into cementite, and the desired retained austenite content cannot be obtained. The order of steps (D3) and (D4) does not matter. For example, after immersion in the plating bath, the steel sheet may be cooled to a temperature range of Ms to Ms-200°C, or the steel sheet may be immersed in the plating bath after step (D4) is completed.

Finally, the steel plate is cooled to room temperature to produce the final product. Temper rolling may be performed to flatten the steel plate and adjust the surface roughness. In this case, it is preferable to keep the elongation to 2% or less to avoid deterioration of ductility.

Next, an example of the present invention will be described. The conditions in the example are an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Steel having the chemical composition shown in Table 1 was cast to produce slabs. The balance other than the components shown in Table 1 consisted of Fe and impurities. These slabs were hot-rolled, including rough rolling and finish rolling, under the conditions shown in Table 2 to produce hot-rolled steel sheets. They were then coiled and cooled under the conditions shown in Table 2. Next, after bending and unbending deformation at least once using a tension leveler, the hot-rolled steel sheets were pickled under the conditions shown in Table 2 to remove the internal oxide layer, and then cold-rolled. The thickness of each sheet after cold rolling was 1.6 mm. The obtained cold-rolled steel sheets were further heat-treated and hot-dip galvanized (GI) under the conditions shown in Table 2.

JIS No. 5 tensile test specimens were taken from the hot-dip galvanized steel sheets obtained in this manner in a direction perpendicular to the rolling direction, and tensile tests were conducted in accordance with JIS Z2241:2011 to measure the tensile strength (TS) and total elongation (El). Furthermore, the hole expansion ratio (λ) was measured according to the "JFS T 1001 Hole Expansion Test Method" of the Japan Iron and Steel Federation standard. Steel sheets with a TS of 980 MPa or more and a TS × El × λ ^0.5 /1000 of 90 or more were judged to have good mechanical properties and press formability suitable for use as automotive components.

In addition, to evaluate the liquid metal embrittlement (LME) cracking resistance of spot welds, test pieces measuring 150 mm wide x 50 mm long were taken and spot welding tests were conducted on pairs of sheets. The sheets were paired using the steel sheets shown in Table 3, and were welded with a 3° impact angle. A servomotor-driven stationary spot welding test machine was used. The power source was single-phase AC 50 Hz, pressure 400 kgf, current flow time 20 cycles, and hold time 5 cycles. The welding current values were set to produce molten nugget diameters 4.0, 5.0, and 5.5 times √t (t: sheet thickness/mm). A chromium-copper electrode with a tip diameter of 6 mm and a tip curvature radius of 40 mm was used. The cross section of the nugget portion of the welded sample was observed, and those in which a crack of 0.2 mm or more was observed at any current value were judged as × (fail), those in which a crack of 0.1 mm or more but less than 0.2 mm was observed at any current value were judged as ○ (pass), and those in which the crack was less than 0.1 mm at any current value were judged as ⊚ (pass). The results are shown in Table 3.

In Comparative Examples 4, 10, and 38, the pickling conditions were not controlled within the specified range, which is believed to have caused excessive pickling and insufficient formation of a Si-depleted region immediately below the interface between the base steel sheet and the hot-dip galvanized layer. As a result, the Si _s /Si _b value was high, resulting in cracks occurring in the spot welds. In Comparative Examples 5, 8, and 39, the pickling conditions were not controlled within the specified range, which is believed to have caused insufficient pickling and uneven removal of the internal oxide layer. As a result, the number density of recesses exceeding 2 μm in depth at the interface between the base steel sheet and the hot-dip galvanized layer increased, resulting in cracks occurring in the spot welds. In Comparative Example 6, the value of formula (1) was high, which is believed to have caused excessive internal oxidation, resulting in large irregularities on the steel sheet after pickling. As a result, the number density of recesses exceeding 2 μm in depth at the interface between the base steel sheet and the hot-dip galvanized layer increased, resulting in cracks occurring in the spot welds. In Comparative Example 7, the value of formula (1) was low, which is believed to have caused insufficient internal oxidation, resulting in insufficient formation of a Si-depleted layer immediately below the internal oxide layer. As a result, the Si _s /Si _b value increased, and cracks occurred in the spot welds. In Comparative Example 16, the maximum heating temperature in the heat treatment and plating processes was low, resulting in insufficient austenitization and the formation of excessive ferrite, resulting in poor press formability. In Comparative Example 16, the maximum heating temperature was lower than Ac1 (717°C), and there was no austenite capable of martensitic transformation. Therefore, "Ms - cooling temperature" and "Ms" in Table 2 are indicated as "-." In Comparative Example 17, the time between immersion in the plating bath and gas wiping was long, which is thought to have initiated the collapse of the Al-enriched layer present at the interface between the base steel sheet and the hot-dip galvanized layer. As a result, the maximum Al concentration of the Al-enriched layer decreased, and cracks occurred in the spot welds. In Comparative Example 18, the steel sheet temperature after gas wiping was high, which is thought to have initiated the collapse of the Al-enriched layer. As a result, the maximum Al concentration of the Al-enriched layer decreased, and cracks occurred in the spot welds. In Comparative Example 19, the value of formula (4) was low, and therefore internal oxidation did not progress sufficiently, which is thought to have prevented the formation of a sufficient Si-poor layer. As a result, the value of Si _s /Si _b increased, and cracks occurred in the spot welds.

In Comparative Example 20, the reheating temperature in the heat treatment and plating process was low, making it impossible to obtain the desired retained austenite content, and the press formability was poor. In Comparative Example 21, the cooling temperature in the heat treatment and plating process was high, making it impossible to form tempered martensite, and the press formability was poor. In Comparative Example 22, the holding time at the reheating temperature in the heat treatment and plating process was short, resulting in a high content of fresh martensite and poor press formability. In Comparative Example 33, the reheating temperature in the heat treatment and plating process was high, making it impossible to obtain the desired retained austenite content and poor press formability. In Comparative Example 34, the holding time at the reheating temperature in the heat treatment and plating process was long, making it impossible to obtain the desired retained austenite content and poor press formability. In Comparative Example 35, the cooling temperature in the heat treatment and plating process was too low, making it impossible to obtain the desired retained austenite content and poor press formability. In Comparative Example 36, the cold rolling reduction was low, which prevented the steel sheet surface from being sufficiently smoothed. As a result, the number density of recesses exceeding 2 μm in depth at the interface between the base steel sheet and the hot-dip galvanized layer increased in the final hot-dip galvanized steel sheet, resulting in cracks at the spot welds. In Comparative Example 37, the value of formula (4) was high, which resulted in internal oxidation of not only Si and other elements but also Fe, resulting in bare spots. For this reason, Comparative Example 37 was excluded from evaluation as a hot-dip galvanized steel sheet. In Comparative Examples 46 to 48 and 50 to 52, the chemical compositions were not controlled within the specified range, resulting in poor press formability. In Comparative Example 49, cracks occurred at the spot welds due to the high Si content. In Comparative Examples 53 and 54, the pickling time was short, which is thought to have resulted in insufficient pickling and uneven removal of the internal oxide layer. As a result, the number density of recesses exceeding 2 μm in depth at the interface between the base steel sheet and the hot-dip galvanized layer increased, resulting in cracks at the spot welds.

In contrast to this, the steel sheets of the examples have a TS of 980 MPa or more and a TS×El× ^λ0.5 /1000 of 90 or more, and furthermore, the test results for LME cracking resistance of the spot welds were good, which shows that they have excellent press formability and LME cracking resistance of the spot welds.

Claims (3)

A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on at least one surface of the base steel sheet,
The base steel plate is, in mass%,
C: 0.15-0.30%,
Si: 0.30-2.50%,
Mn: 1.40-3.49%,
P: 0.050% or less,
S: 0.0100% or less,
Al: 0.001-1.50%,
N: 0.0100% or less,
O: 0.0100% or less,
Cr: 0-1.00%,
Mo: 0-1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Co: 0-1.00%,
W: 0-1.00%,
Sn: 0-1.00%,
Sb: 0 to 0.50%,
Nb: 0 to 0.200%,
Ti: 0-0.200%,
V: 0-1.00%,
B: 0 to 0.0050%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Ce: 0 to 0.0150%,
Zr: 0 to 0.0100%,
La: 0 to 0.0150%,
Hf: 0-0.0100%,
Bi: 0 to 0.0100%,
REM other than Ce and La: 0 to 0.0100%, and the balance: Fe and impurities,
The steel structure in the range of 1/8 thickness to 3/8 thickness centered on 1/4 thickness from the surface of the base steel plate is, in volume %,
Ferrite: 0 to 50%,
Tempered martensite: 1% or more,
Retained austenite: 5 to 30%
Fresh martensite: 0 to 15%
Sum of pearlite and cementite: 0 to 5%, and the balance: bainite,
when the hot-dip galvanized steel sheet is measured with a high-frequency glow discharge optical emission spectrometer (GDS), the maximum value of the Al concentration in an Al-enriched layer present at the interface between the base steel sheet and the hot-dip galvanized layer is 2.0 mass% or more, and Si _s /Si _b in the base steel sheet immediately below the interface between the base steel sheet and the hot-dip galvanized layer is 0.90 or less,
The number density of recesses having a depth exceeding 2 μm at the interface between the base steel sheet and the hot-dip galvanized layer is 2.0/100 μm or less per interface length, and
A hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more.
Si _s : minimum value of Si emission intensity in the base steel sheet directly below the interface between the base steel sheet and the hot-dip galvanized layer Si _b : average value of Si emission intensity in the base steel sheet The chemical composition is, in mass %,
Si: 0.30 to 1.20%, and Al: 0.30 to 1.50%
The hot-dip galvanized steel sheet according to claim 1, comprising: (A) a hot rolling step comprising hot rolling a slab having the chemical composition according to claim 1 or 2, and then coiling and cooling the obtained hot-rolled steel sheet, wherein the cooling satisfies the following formula (1):
where:
T(t): Steel plate temperature when t seconds have passed after winding [K]
tf: time [seconds] for the steel plate temperature to reach 673K
Nx: total atomic fraction [-] of Si, Mn and Al in steel; (B) a pickling step comprising: bending and unbending the hot-rolled steel sheet at least once, and then passing the hot-rolled steel sheet through an aqueous solution containing 1.0 to 5.0 mol/L of HCl, less than 3.0 mol/L of Fe ²⁺ and less than 0.10 mol/L of Fe ³⁺ at a temperature of 70 to 90°C at an average speed of 10 m/min or more for 30 seconds or more;
(C) A cold rolling process in which the hot-rolled steel sheet after pickling treatment is cold-rolled at a reduction ratio of 30 to 75%;
(D) A heat treatment and plating step, which includes subjecting the obtained cold-rolled steel sheet to heat treatment and plating, and which satisfies the following conditions (D1) to (D4): (D1) the cold-rolled steel sheet is heated from 600°C to a maximum heating temperature of Ac1+30°C to 950°C at an average heating rate of 0.2 to 20°C/sec, and the atmosphere around the cold-rolled steel sheet satisfies the following formula (4);
pH ₂ O: water vapor partial pressure pH ₂ : hydrogen partial pressure (D2) holding the cold-rolled steel sheet at the maximum heating temperature for 1 to 1000 seconds;
(D3) The time from immersion in the coating bath until gas wiping is performed is 0.1 to 5 seconds, and the steel sheet temperature after gas wiping is 440°C or less;
(D4) A method for producing a hot-dip galvanized steel sheet according to claim 1 or 2, comprising cooling the steel sheet to a range of Ms to Ms-200°C, then reheating it to a temperature range of 300 to 420°C, and holding it in the temperature range for 100 to 600 seconds.