Patent Description:
In recent years, enhancement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environment protection. Therefore, there is a growing trend to increase the strength and reduce the thickness of steel sheets used as materials for automobile members to reduce the weight of automotive bodies. Further, the steel sheets used for automobile members are formed into complicated shapes, so that they are required to have good workability.

In response to such a request, for example, <CIT> (PTL <NUM>) describes "a high-strength cold-rolled steel sheet with excellent uniform deformability and local deformability, which contains, in mass%,.

with the balance being iron and inevitable impurities, wherein the steel sheet comprises a texture where the average value of the X-ray random intensity ratios of the {<NUM>}<<NUM>> to {<NUM>}<<NUM>> orientation group and the {<NUM>}<<NUM>> crystal orientation of at least the sheet surface at <NUM>/<NUM> to <NUM>/<NUM> sheet thickness from the surface of the steel sheet is <NUM> or less, the X-ray random intensity ratio of the {<NUM>}<<NUM>> crystal orientation is <NUM> or less, the r (rC) value in a direction orthogonal to the rolling direction is <NUM> or more, and the r value at <NUM>° (r30) from the rolling direction is <NUM> or less, and a microstructure where the total area ratio of ferrite and bainite is <NUM> % or more, and the area ratio of martensite is <NUM> % or more and <NUM> % or less".

Further, <CIT> (PTL <NUM>) describes "a high-strength steel sheet having excellent hardenability with very little aging deterioration, which contains, in mass%,.

with the balance being iron and inevitable impurities, wherein the steel sheet microstructure is mainly composed of ferrite and bainite, the BH after baking treatment is <NUM> MPa or more, and the maximum tensile strength is <NUM> MPa or more.

PTL <NUM> describes 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 sheet has a chemical composition comprising, in mass%, C: <NUM>% to <NUM>%, Si: <NUM>% to <NUM>%, Mn: <NUM>% to <NUM>%, P: <NUM>% to <NUM>%, S: <NUM>% to <NUM>%, Al: <NUM>% to <NUM>%, N: <NUM>% to <NUM>%, O: <NUM>% to <NUM>%, and a remainder of Fe and impurities. The base steel sheet includes in a range of <NUM>/<NUM> thickness to <NUM>/<NUM> thickness centered at a position of <NUM>/<NUM> thickness from the surface of the base steel sheet, by volume fraction, <NUM>% or more and <NUM>% or less of a ferrite phase, a total of <NUM>% or more of a hard structure comprising one or more of a bainite phase, a bainitic ferrite phase, a fresh martensite phase and a tempered martensite phase, a residual austenite phase is <NUM> to <NUM>% by volume fraction, and a total of a pearlite phase and a coarse cementite phase is <NUM> to <NUM>% by volume fraction. In a surface layer range of <NUM> depth in a steel sheet direction from an interface between the hot-dip galvanized layer and the base steel sheet, a volume fraction of a residual austenite is <NUM> to <NUM>%. The base steel sheet includes a microstructure in which V1/V2 which is a ratio of a volume fraction V1 of the hard structure in the surface layer range and a volume fraction V2 of the hard structure in the range of <NUM>/<NUM> thickness to <NUM>/<NUM> thickness centered at the position of <NUM>/<NUM> thickness from the surface of the base steel sheet is <NUM> or more and <NUM> or less. A Fe content is more than <NUM>% to <NUM>% or less and an Al content is more than <NUM>% to <NUM>% or less in the hot-dip galvanized layer, and columnar grains formed of a ζ phase are included in the hot-dip galvanized layer. A ratio ((A*/A)×<NUM>) of an interface (A*) between the ζ phase and the base steel sheet in an entire interface (A) between the hot-dip galvanized plated layer and the base steel sheet is <NUM>% or more. A refined layer is formed at the side of the interface in the base steel sheet, an average thickness of the refined layer is <NUM> to <NUM>, an average grain size of ferrite in the refined layer is <NUM> to <NUM>, one or two or more of oxides of Si and Mn are contained, and a maximum size of the oxide is <NUM> to <NUM>.

However, from the viewpoint of rust resistance of automobile bodies, steel sheets used as materials for automobile members are sometimes subjected to zinc or zinc alloy coating or plating, such as hot-dip galvanizing.

However, when the steel sheets described in PTLs <NUM> and <NUM> are subjected to hot-dip galvanizing, the coating or plating quality such as coating or plating appearance and coating or plating adhesion may be insufficient. Therefore, it is desired to make improvement in this regard.

It could thus be helpful to provide a hot-dip galvanized steel sheet that has both high strength and good workability, as well as excellent coating quality.

It is also helpful to provide a method of manufacturing the hot-dip galvanized steel sheet.

As a result of intensive studies, we discovered the following.

That is, it is effective to use Si and Mn in terms of increasing the strength of a steel sheet. However, elements such as Si and Mn are oxidizable elements, which combine with oxygen to form oxides on the steel sheet surface. The presence of such Si and Mn oxides on the surface of the base steel sheet during the coating or plating treatment reduces the wettability of the base steel sheet by a coating or plating bath (hot dip zinc), causing poor coating or plating appearance such as non-coating or non-plating and deterioration of coating or plating adhesion.

In this regard, if internal oxidation is caused in the surface layer of the base steel sheet to form oxides of Si and Mn before the coating or plating treatment, these oxides in the surface layer of the base steel sheet serve as a barrier, and the formation of oxides on the surface of the base steel sheet (hereinafter referred to as external oxidation) is suppressed. As a result, the coating or plating quality such as coating or plating appearance and coating or plating adhesion is improved.

Further, the coating or plating quality, especially coating or plating adhesion, is improved by containing an appropriate amount of Fe in a hot-dip galvanized layer.

(e) In addition, it is important to properly control the annealing conditions prior to the coating or plating treatment and the coating or plating treatment conditions to create a complex structure as described above, to form oxides of Si and Mn in the surface layer of the base steel sheet by causing internal oxidation in the surface layer of the base steel sheet, and also to contain an appropriate amount of Fe in the hot-dip galvanized layer. It is particularly important to control the atmosphere during the holding of annealing and to control the temperature of the cold-rolled steel sheet when it enters the coating or plating bath in the coating or plating treatment.

Specifically, when the dew point is set in a range of -<NUM> or higher and <NUM> or lower and a certain amount of oxygen is ensured in the holding atmosphere of annealing, the internal oxidation in the surface layer of the base steel sheet is promoted. On the other hand, when the hydrogen concentration is set to <NUM> mass% or more and <NUM> mass% or less, oxides that have been formed on the surface of the base steel sheet (and oxides that have been formed during the holding of annealing) are reduced. Therefore, it is important to suppress the external oxidation while introducing sufficient oxygen from the atmosphere into the interior (surface layer) of the base steel sheet. It is also important to promote the diffusion of Fe from the base steel sheet to the coated or plated layer by setting the temperature of the cold-rolled steel sheet when it enters the coating or plating bath to at least <NUM> higher than the coating or plating bath temperature.

The present disclosure is based on these discoveries and further studies.

The present invention is directed to a hot-dip galvanized steel sheet and a method of manufacturing a hot-dip galvanized steel sheet as defined in the claims.

According to the present disclosure, it is possible to obtain a hot-dip galvanized steel sheet having both high strength and good workability, as well as excellent coating quality.

By applying the hot-dip galvanized steel sheet of the present disclosure to automobile members, the performance of automobile bodies can be significantly improved.

The present disclosure will be described based on the following embodiments.

First, the chemical composition of a base steel sheet of a hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described. The "%" representations below indicating the chemical composition are in "mass%" unless stated otherwise.

C is an element that improves the hardenability. C also plays a role in increasing the strength of ferrite. Therefore, it is required to contain C to ensure a desired tensile strength (TS) of <NUM> MPa or more. When the C content is less than <NUM> %, the desired tensile strength cannot be obtained. Therefore, the C content is set to <NUM> % or more. The C content is preferably <NUM> % or more and more preferably <NUM> % or more. On the other hand, if the C content exceeds <NUM> %, the stability of austenite increases, and it is difficult to form bainite. In addition, the strength of martensite increases excessively, and the yield ratio decreases. Therefore, the C content is set to <NUM> % or less. The C content is preferably <NUM> % or less and more preferably <NUM> % or less.

Si is a solid-solution-strengthening element. Si also plays a role in increasing the yield ratio by increasing the strength of ferrite. To obtain this effect, the Si content is set to <NUM> % or more. The Si content is preferably <NUM> % or more and more preferably <NUM> % or more. On the other hand, if the Si content is too high, Si concentrates on the surface of the base steel sheet, causing external oxidation and deteriorating the coating quality such as coating appearance. Therefore, the Si content is set to <NUM> % or less. The Si content is preferably <NUM> % or less and more preferably <NUM> % or less.

Mn is an element that improves the hardenability of steel. Therefore, it is required to contain Mn to ensure the desired tensile strength. When the Mn content is less than <NUM> %, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to <NUM> % or more. The Mn content is preferably <NUM> % or more and more preferably <NUM> % or more. On the other hand, if the Mn content is too high, Mn concentrates on the surface of the base steel sheet, causing external oxidation and deteriorating the coating quality such as coating appearance. In addition, Mn tends to concentrate into austenite during, for example, the holding of annealing, and the strength of martensite that transforms from austenite excessively increases. Therefore, the Mn content is set to <NUM> % or less. The Mn content is preferably <NUM> % or less and more preferably <NUM> % or less.

P is an element that strengthens steel. However, if the P content is too high, P segregates to grain boundaries and deteriorates the hole expansion formability. Therefore, the P content is set to <NUM> % or less. The P content is preferably <NUM> % or less and more preferably <NUM>% or less. Although the lower limit of the P content is not particularly limited, it is preferably <NUM> % or more from the viewpoint of cost, for example. The P content is more preferably <NUM> % or more and even more preferably <NUM> % or more.

S is an element that deteriorates the elongation through the formation of MnS and the like. If Ti is contained together with S, the hole expansion formability may be deteriorated due to the formation of, for example, TiS and Ti(C,S). Therefore, the S content is set to <NUM> % or less. The S content is preferably <NUM> % or less, more preferably <NUM> % or less, and even more preferably <NUM> % or less. Although the lower limit of the S content is not particularly limited, it is preferably <NUM> % or more from the viewpoint of cost, for example. The S content is more preferably <NUM> % or more.

Al is an element added as a deoxidizing material. Al also plays a role in reducing coarse inclusions in the steel and improving the hole expansion formability. When the Al content is less than <NUM> %, the above effect is insufficient. Therefore, the Al content is set to <NUM> % or more. The Al content is preferably <NUM> % or more. On the other hand, if the Al content exceeds <NUM> %, nitride-based precipitates such as AlN are coarsened, and the hole expansion formability is deteriorated. Therefore, the Al content is set to <NUM> % or less. The Al content is preferably <NUM> % or less and more preferably <NUM> % or less.

N is an element that contributes to the improvement of hole expansion formability by forming nitride-based precipitates such as AlN that pin crystal grain boundaries. However, if the N content exceeds <NUM> %, nitride-based precipitates such as AlN are coarsened, and the hole expansion formability is deteriorated. Therefore, the N content is set to <NUM> % or less. The N content is preferably <NUM> % or less and more preferably <NUM> % or less. Although the lower limit of the N content is not particularly limited, it is preferably <NUM> % or more from the viewpoint of cost, for example. The N content is more preferably <NUM> % or more.

The base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a chemical composition containing the above elements and the balance of Fe (iron) and inevitable impurities. It is particularly preferable that the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a chemical composition containing the above elements, with the balance consisting of Fe and inevitable impurities.

The above describes the basic chemical composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure. The base steel sheet may contain, as optional elements, at least one selected from the group consisting of.

Further, it may contain, as optional elements, at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr, where the selected elements are contained in a total amount of <NUM> % or less.

If any of the above optional elements is contained in an amount less than the suitable lower limit described below, this element is regarded as an inevitable impurity.

Nb contributes to increasing the strength through the refinement of prior γ grains and the formation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to increasing the yield ratio. To obtain this effect, the Nb content is preferably <NUM> % or more. The Nb content is more preferably <NUM> % or more and even more preferably <NUM> % or more. On the other hand, an excessively high Nb content results in an excessive amount of carbonitride-based precipitate, which deteriorates the hole expansion formability. Therefore, when Nb is contained, the Nb content is preferably <NUM> % or less. The Nb content is more preferably <NUM> % or less and even more preferably <NUM> % or less.

Ti, like Nb, contributes to increasing the strength through the refinement of prior γ grains and the formation of fine precipitates. In addition, the fine precipitates increase the strength of ferrite and contribute to increasing the yield ratio. To obtain this effect, the Ti content is preferably <NUM> % or more. The Ti content is more preferably <NUM> % or more and even more preferably <NUM> % or more. On the other hand, an excessively high Ti content results in an excessive amount of carbonitride-based precipitate, which deteriorates the hole expansion formability. Therefore, when Ti is contained, the Ti content is preferably <NUM> % or less. The Ti content is more preferably <NUM> % or less and even more preferably <NUM> % or less.

B is an element that improves the hardenability of steel. The inclusion of B renders it possible to achieve the desired tensile strength even when the Mn content is low. To obtain this effect, the B content is preferably <NUM> % or more. The B content is more preferably <NUM> % or more. On the other hand, a B content of <NUM> % or more results in an excessive amount of nitride-based precipitate such as BN, which deteriorates the hole expansion formability. Therefore, when B is contained, the B content is preferably <NUM> % or less. The B content is more preferably <NUM> % or less and even more preferably <NUM> % or less.

Cr is an element that improves the hardenability of steel. To obtain this effect, the Cr content is preferably <NUM> % or more. However, an excessively high Cr content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when Cr is contained, the Cr content is preferably <NUM> % or less. The Cr content is more preferably <NUM> % or less and even more preferably <NUM> % or less.

Mo, like Cr, is an element that improves the hardenability of steel. To obtain this effect, the Mo content is preferably <NUM> % or more. However, an excessively high Mo content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when Mo is contained, the Mo content is preferably <NUM> % or less. The Mo content is more preferably <NUM> % or less and even more preferably <NUM> % or less.

V, like Cr, is an element that improves the hardenability of steel. To obtain this effect, the V content is preferably <NUM> % or more. However, an excessively high V content may cause oxide formation reaction accompanied by the formation of hydrogen ions, which may deteriorate the coating quality. Further, precipitates such as carbides are excessively precipitated, and the hole expansion formability is deteriorated. Therefore, when V is contained, the V content is preferably <NUM> % or less. The V content is more preferably <NUM> % or less and even more preferably <NUM> % or less.

At least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of <NUM> % or less.

Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr are elements that increase the strength without deteriorating the coating quality. To obtain this effect, the content of these elements is preferably <NUM> % or more, either singly or in total. However, when the total content of these elements exceeds <NUM> %, the above effect is saturated. Therefore, when at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr are contained, the total content of these elements is preferably <NUM> % or less.

The balance other than the aforementioned elements is Fe and inevitable impurities.

Next, the steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

The steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure is a complex structure where.

with respect to the entire steel microstructure. Note that the area ratio refers to a ratio of the area of each metallic phase to the area of the entire steel microstructure.

Ferrite is a necessary phase from the viewpoint of obtaining desired elongation. Therefore, the area ratio of ferrite is set to <NUM> % or more. The area ratio of ferrite is preferably <NUM> % or more and more preferably <NUM> % or more. On the other hand, an excess of ferrite reduces the area ratio of martensite required to ensure the strength, rendering it difficult to ensure the strength. It also suppresses the formation of bainite and reduces the hole expansion formability and the yield ratio. Therefore, the area ratio of ferrite is set to <NUM> % or less. The area ratio of ferrite is preferably <NUM> % or less.

As used herein, the ferrite is a microstructure containing crystal grains of BCC lattice, which is formed by transformation from austenite at relatively high temperatures.

Martensite contributes to the improvement of strength and is a phase necessary for ensuring the desired tensile strength. Therefore, the area ratio of martensite is set to <NUM> % or more. The area ratio of martensite is preferably <NUM> % or more and more preferably <NUM> % or more. On the other hand, an excess of martensite deteriorates the elongation. Therefore, the area ratio of martensite is set to <NUM> % or less. The area ratio of martensite is preferably <NUM> % or less and more preferably <NUM> % or less.

As used herein, the martensite refers to a hard microstructure formed from austenite at or below the martensite transformation temperature (also referred to simply as "Ms point"), which includes both so-called fresh martensite as quenched and so-called tempered martensite where fresh martensite is reheated and tempered.

Bainite is a phase necessary for improving the hole expansion formability and increasing the yield ratio. Therefore, the area ratio of bainite is set to <NUM> % or more. The area ratio of bainite is preferably <NUM> % or more and more preferably <NUM> % or more. On the other hand, an excess of bainite deteriorates the elongation. Therefore, the area ratio of bainite is set to <NUM> % or less. The area ratio of bainite is preferably <NUM> % or less and more preferably <NUM> % or less.

As used herein, the bainite is a hard microstructure in which fine carbides are dispersed in needle-like or plate-like ferrite, and it is formed from austenite at relatively low temperatures (at or above the martensitic transformation temperature).

The steel microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure may contain metallic phases other than martensite, ferrite, and bainite. It is acceptable if the total area ratio of other metallic phases is <NUM> % or less. Therefore, the area ratio of other metallic phases is set to <NUM> % or less. The area ratio of other metallic phases is preferably <NUM> % or less and more preferably <NUM> % or less. The area ratio of other metallic phases may be <NUM> %.

Examples of the other metallic phases include pearlite, retained austenite, and non-recrystallized ferrite. Among these phases, pearlite and non-recrystallized ferrite deteriorate the workability (El and λ), so that the total area ratio of pearlite and non-recrystallized ferrite is set to <NUM> % or less. The area ratios of pearlite and non-recrystallized ferrite may each be <NUM> %. Because retained austenite does not deteriorate the workability (El and λ), there is no problem if the area ratio of retained austenite is <NUM> % or less. The area ratio of retained austenite is preferably <NUM> % or less and more preferably <NUM> % or less. The area ratio of retained austenite may be <NUM> % or less.

As used herein, the pearlite is a microstructure containing ferrite and needle-like cementite. The retained austenite is austenite remaining without being transformed into martensite. The non-recrystallized ferrite is ferrite that is not recrystallized, in which crystal grains include sub-boundaries.

As used herein, the area ratio of each phase is measured as follows.

A test piece is collected from the base steel sheet of the hot-dip galvanized steel sheet so that an L-section parallel to the rolling direction serves as a test surface. Next, the test surface of the test piece is subjected to mirror polishing, and the microstructure is revealed with a nital solution. The test surface of the test piece with the revealed microstructure is observed with a SEM at a magnification of 1500x, and the area ratio of martensite, the area ratio of ferrite, and the area ratio of bainite at the <NUM>/<NUM> thickness position of the base steel sheet are measured with a point counting method.

In the SEM image, martensite is a white microstructure. Further, fine carbides are precipitated inside tempered martensite among the martensite. Ferrite is a black microstructure. Bainite has white carbides precipitated in a black microstructure. Each phase in the SEM image is identified based on the above description. However, depending on the plane orientation of block grains and the degree of etching, it may be difficult to reveal the internal carbides. In that case, etching is thoroughly performed for confirmation.

The total area ratio of the other metallic phases is calculated by subtracting the area ratio of martensite, the area ratio of ferrite, and the area ratio of bainite from <NUM> %.

Among the other metallic phases, pearlite is a microstructure containing ferrite and needle-like cementite as described above. Based on this, pearlite is identified in the SEM image, and the area ratio of pearlite is measured. Non-recrystallized ferrite has sub-boundaries inside crystal grains as described above. Based on this, non-recrystallized ferrite is identified in the SEM image, and the area ratio of non-recrystallized ferrite is measured.

The area ratio of retained austenite is measured as follows.

The base steel sheet of the hot-dip galvanized steel sheet is polished in the thickness direction (depth direction) to the <NUM>/<NUM> thickness position and then chemically polished by <NUM> to obtain an observation plane. Next, the observation plane is observed with the X-ray diffraction method. Using a Mo Kα source as an incident X-ray, ratios of the diffraction intensity of each of (<NUM>), (<NUM>) and (<NUM>) planes of fcc iron (austenite) to the diffraction intensity of each of (<NUM>), (<NUM>), and (<NUM>) planes of bcc iron are determined, and the volume fraction of retained austenite is calculated based on the ratio of diffraction intensity of each plane. Next, assuming that the retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is taken as the area ratio of retained austenite.

Amount of oxygen present as oxide in the surface layer of the base steel sheet (hereinafter also referred to as "amount of oxygen in oxide form in the surface layer of the base steel sheet"): <NUM>/m<NUM> or more and <NUM>/m<NUM> or less per surface.

As described above, it is effective to use Si and Mn in terms of increasing the strength of a steel sheet. However, elements such as Si and Mn are oxidizable elements, which combine with oxygen to form oxides on the steel sheet surface. The presence of such Si and Mn oxides on the surface of the base steel sheet during coating treatment reduces the wettability of the base steel sheet by a coating bath (hot-dip zinc), causing poor coating appearance such as non-coating and deterioration of coating adhesion.

In this regard, if internal oxidation is caused in the surface layer of the base steel sheet to form oxides of Si and Mn before the coating treatment, these oxides present in the surface layer of the base steel sheet serve as a barrier, and the formation of oxides on the surface of the base steel sheet (hereinafter referred to as "external oxidation") is suppressed. As a result, the coating quality such as coating appearance and coating adhesion is improved. Therefore, the amount of oxygen in oxide form in the surface layer of the base steel sheet is set to <NUM>/m<NUM> or more per surface (note that all the amount of oxygen described below is the amount for one surface). The amount of oxygen in oxide form in the surface layer of the base steel sheet is preferably <NUM>/m<NUM> or more. On the other hand, if the amount of oxygen in oxide form in the surface layer of the base steel sheet exceeds <NUM>/m<NUM>, the oxides promote fracture and deteriorate the elongation and the hole expansion formability. Therefore, the amount of oxygen in oxide form in the surface layer of the base steel sheet is set to <NUM>/m<NUM> or less. The amount of oxygen in oxide form in the surface layer of the base steel sheet is preferably <NUM>/m<NUM> or less.

As used herein, the surface layer is an area from the surface of the base steel sheet to a position at a depth of <NUM>.

Oxides are compounds of oxygen and elements such as Si, Mn, Fe, P, Al, Nb, Ti, B, Cr, Mo, and V contained in the base steel sheet, and the oxides are mainly Si oxides and Mn oxides.

The amount of internal oxidation is inversely related to the amount of external oxidation. Therefore, if external oxidation occurs in the base steel sheet, the amount of oxygen in oxide form in the surface layer of the base steel sheet is less than <NUM>/m<NUM>.

The amount of oxygen in oxide form in the surface layer of the base steel sheet is measured with an "impulse furnace-infrared absorption method".

First, the hot-dip galvanized layer is removed from the hot-dip galvanized steel sheet. The method of removing the hot-dip galvanized layer is not limited if the hot-dip galvanized layer can be totally removed. Examples thereof include pickling, alkali dissolution, and mechanical polishing.

Next, the amount of oxygen in the steel of the base steel sheet is measured. The measured value is taken as the total amount of oxygen OI (g) contained in the base steel sheet.

Next, at least the surface layers (an area from the surface of the base steel sheet to a position at a depth of <NUM>) on both sides of the base steel sheet is removed by polishing, and the amount of oxygen in the steel of the base steel sheet is measured after the surface layers have been removed. The measured value is taken as OH (g).

The amount of oxygen in oxide form in the surface layer of the base steel sheet is calculated based on the following formula.

In the above formula, the amount of oxygen in oxide form in the surface layer of the base steel sheet is calculated by.

The thickness of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure is preferably <NUM> or more. The thickness is preferably <NUM> or less.

Next, the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

It is preferable to contain a large amount of Fe in the hot-dip galvanized layer to improve the coating adhesion. Therefore, the Fe content in the hot-dip galvanized layer is set to <NUM> mass% or more. The Fe content in the hot-dip galvanized layer is preferably <NUM> mass% or more. On the other hand, an excess of Fe in the hot-dip galvanized layer results in the formation of a hard Fe-Zn alloy phase in the hot-dip galvanized layer. As a result, the coating itself is likely to be broken, resulting in deterioration of coating adhesion. Therefore, the Fe content in the hot-dip galvanized layer is <NUM> mass% or less. The Fe content in the hot-dip galvanized layer is preferably <NUM> mass% or less and more preferably <NUM> mass% or less.

A large coating weight is desirable to improve the corrosion resistance. Therefore, the coating weight is preferably <NUM>/m<NUM> or more per surface (note that all the coating weight described below is the amount for one surface). The coating weight is more preferably <NUM>/m<NUM> or more and even more preferably <NUM>/m<NUM> or more. The upper limit of the coating weight is not particularly limited. However, if the coating weight exceeds <NUM>/m<NUM>, the above effect is saturated. Therefore, the coating weight is preferably <NUM>/m<NUM> or less.

The Fe content and the coating weight in the hot-dip galvanized layer are measured as follows.

After degreasing the surface of the hot-dip galvanized steel sheet as a test piece, the mass of the test piece is weighed for the first time. Next, two or three drops of inhibitor, which is a corrosion inhibitor for Fe, are added to <NUM> cc of <NUM>:<NUM> HCl solution (HCl solution with a concentration of <NUM> vol. %), and then the test piece is immersed in the solution to dissolve the hot-dip galvanized layer of the test piece. After dissolving the hot-dip galvanized layer (when there is no more H<NUM> gas formed on the surface of the test piece), the solution is collected. After the test piece is collected and dried, the mass of the test piece is weighed for the second time.

The coating weight is calculated by the following formula.

The masses of Fe, Zn, and Al dissolved in the collected solution (hereinafter referred to as dissolved amount of Fe, dissolved amount of Zn, and dissolved amount of Al) are measured with the inductively coupled plasma (ICP) method, and the Fe content in the hot-dip galvanized layer is determined by the following formula.

The hot-dip galvanized layer is mainly composed of Zn and is basically composed of Zn and the aforementioned Fe. Depending on the composition of the coating bath, the hot-dip galvanized layer may contain <NUM> mass% or less, specifically <NUM> mass% to <NUM> mass%, of Al. The balance other than Zn, Fe and Al is inevitable impurities. The hot-dip galvanized layer may be provided on only one side or on both sides of the base steel sheet.

Next, the mechanical properties of the hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

The hot-dip galvanized steel sheet according to one embodiment of the present disclosure has a tensile strength (TS) of <NUM> MPa or more. The tensile strength (TS) is preferably <NUM> MPa or more. Although the upper limit of the tensile strength is not particularly limited, a tensile strength of less than <NUM> MPa is preferred considering the balance with other properties.

Further, from the viewpoint of workability,.

TS×El is preferably <NUM> MPa·% or more and more preferably <NUM> MPa·% or more.

TS×λ is preferably <NUM> MPa·% or more and more preferably <NUM> MPa·% or more.

YR is preferably <NUM> or more and more preferably <NUM> or more.

As used herein, the tensile strength (TS), the yield stress (YS), and the elongation (El) are measured as follows.

A JIS No. <NUM> test piece with a gauge length of <NUM> and a gauge width of <NUM> is collected from the center of the width of the hot-dip galvanized steel sheet, with the rolling direction being the longitudinal direction. Next, the collected JIS No. <NUM> test piece is subjected to a tensile test in accordance with the provisions of JIS Z <NUM> (<NUM>) to measure the tensile strength (TS), the yield stress (YS), and the elongation (El). The tensile speed is <NUM>/min.

Further, λ is the maximum hole expansion ratio (%), which is measured as follows.

A <NUM> square test piece is collected from the center of the width of the hot-dip galvanized steel sheet. Next, the collected test piece is subjected to a hole expanding test according to the Japan Iron and Steel Federation standard JFST1001 to measure λ. Specifically, after punching a hole with a diameter of <NUM> in the test piece, a <NUM>-degree conical punch is pressed into the hole while the surrounding area is being restrained, and the diameter of the hole at the crack initiation limit is measured. The maximum hole expansion ratio λ (%) is determined by the following formula. <MAT> where Df is the diameter of the hole at the crack initiation limit (mm), and D<NUM> is the initial (before the punch is pressed in) diameter of the hole (mm).

"Excellent coating quality" means that there is no peeling of the hot-dip galvanized layer in a ball impact test under the following conditions, and that there is no non-coating defect in the hot-dip galvanized layer (preferably, there is no uneven coating appearance) found by appearance observation. The non-coating defect refers to an area of several micrometers to several millimeters in size where the base steel sheet is exposed without the hot-dip galvanized layer.

(After dropping the ball under the above conditions and causing the ball to impact the hot-dip galvanized steel sheet, the area that has been impacted by the ball is peeled by tape (tape with an adhesive strength of <NUM> N per <NUM> width in accordance with JIS Z <NUM> (<NUM>)), and the peeling of the hot-dip galvanized layer is determined visually.

Next, a method of manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present disclosure will be described.

The method of manufacturing a hot-dip galvanized steel sheet according to one embodiment of the present disclosure comprises.

In the following description, "temperature" is the surface temperature of the steel sheet or slab unless otherwise specified. The surface temperature of the steel sheet or slab is measured, for example, using a radiation thermometer.

In this process, a steel material (steel slab) having the chemical composition described above is subjected to hot rolling to obtain a hot-rolled steel sheet.

The steel material used is preferably obtained by continuous casting to prevent macro-segregation of components. The steel material can also be obtained by ingot casting or thin slab casting.

The following describes the optimum manufacturing conditions of the hot rolling process.

If the heating temperature of the slab is lower than <NUM>, precipitates such as AlN are not sufficiently dissolved. As a result, precipitates such as AlN may be coarsened during the hot rolling, which deteriorates the hole expansion formability. Therefore, the heating temperature of the slab is preferably <NUM> or higher. The heating temperature of the slab is more preferably <NUM> or higher and even more preferably <NUM> or higher. The upper limit of the heating temperature of the slab is not particularly limited, but <NUM> or lower is preferred. The heating temperature of the slab is more preferably <NUM> or lower.

If the rolling finish temperature is lower than <NUM>, inclusions and coarse carbides may be formed, which deteriorates the hole expansion formability. The quality of the interior of the base steel sheet may also be deteriorated. Therefore, the rolling finish temperature is <NUM> or higher. The rolling finish temperature is preferably <NUM> or higher. On the other hand, if the holding time at high temperatures is increased, coarse inclusions may be formed, which deteriorates the hole expansion formability. Therefore, the rolling finish temperature is <NUM> or lower. The rolling finish temperature is preferably <NUM> or lower.

The steel material is subjected to hot rolling as described above to obtain a hot-rolled steel sheet, and then the hot-rolled steel sheet is coiled. When the coiling temperature is higher than <NUM>, the surface of the steel substrate may be decarburized. This may cause a difference in microstructure between the interior and the surface of the base steel sheet, resulting in uneven alloy concentration. Further, coarse carbides and nitrides may be formed, which deteriorates the hole expansion formability. Therefore, the coiling temperature is <NUM> or lower. The coiling temperature is preferably <NUM> or lower. On the other hand, the coiling temperature is preferably <NUM> or higher to prevent deterioration of cold rolling manufacturability. The coiling temperature is more preferably <NUM> or higher.

The hot-rolled steel sheet may be subjected to pickling after coiling. The conditions of the pickling are not particularly limited, and conventional methods may be followed. Further, the hot-rolled steel sheet may be subjected to heat treatment after coiling to soften the microstructure.

In this process, the hot-rolled steel sheet obtained in the hot rolling process is subjected to cold rolling to obtain a cold-rolled steel sheet. There is no limit on the cold rolling ratio if the sheet thickness is controlled within a desired range. However, if the cold rolling ratio is too small, it is difficult to cause recrystallization in the subsequent annealing process. That is, non-recrystallized ferrite may be formed, which deteriorates the elongation. Therefore, the cold rolling ratio is <NUM> % or more. The cold rolling ratio is preferably <NUM> % or more. On the other hand, if the cold rolling ratio is too high, it is also difficult to cause recrystallization in the subsequent annealing process due to excessive strain. That is, non-recrystallized ferrite may be formed, which deteriorates the elongation. Therefore, the cold rolling ratio is <NUM> % or less. The cold rolling ratio is preferably <NUM> % or less.

In this process, the cold-rolled steel sheet obtained in the cold rolling process is heated to an annealing temperature, held at the annealing temperature, and then cooled.

Further, from the viewpoint of creating a complex structure as described above, forming oxides of Si and Mn in the surface layer of the base steel sheet by causing internal oxidation in the surface layer of the base steel sheet, and containing an appropriate amount of Fe in the hot-dip galvanized layer, it is important in this process to set.

The average heating rate is preferably a low rate so that ferrite is recrystallized and the desired area ratio of ferrite is ensured. Therefore, the average heating rate is set to <NUM>/s or lower. The average heating rate is preferably <NUM>/s or lower and more preferably <NUM>/s or lower. On the other hand, as the average heating rate decreases, Mn, which diffuses at a low rate, also concentrates into austenite and stabilizes the austenite. As a result, it is difficult to cause bainite transformation, and the desired complex structure cannot be obtained. Therefore, the average heating rate is set to <NUM>/s or higher. The average heating rate is preferably <NUM>/s or higher and more preferably <NUM>/s or higher.

If the annealing temperature is lower than (AC1 point + <NUM>), coarse Fe-based precipitates are formed, which deteriorates the strength and the hole expansion formability. Therefore, the annealing temperature is set to (AC1 point + <NUM>) or higher. The annealing temperature is preferably (AC1 point + <NUM>) or higher. On the other hand, if the annealing temperature exceeds (AC3 point + <NUM>), the area ratio of ferrite decreases, and the elongation deteriorates. Therefore, the annealing temperature is set to (AC3 point + <NUM>) or lower. The annealing temperature is preferably (AC3 point + <NUM>) or lower.

As used herein, the AC1 point and the AC3 point are calculated by the following formulas, respectively. Note that in the following formulas, (% element symbol) refers to the content (mass %) of each element in the chemical composition of the base steel sheet. If the element is not contained (including cases where it is inevitably contained), it is calculated as <NUM>. <MAT> <MAT>.

The annealing temperature may be constant during the holding. The annealing temperature may not be constant during the holding, if it is within the above temperature range and the temperature fluctuation range is within ±<NUM> of the set temperature.

The annealing time is an important condition to transform austenite to bainite. From the viewpoint of avoiding concentration of Mn in austenite, i.e., avoiding excessive stabilization of austenite and obtaining an appropriate amount of bainite, the annealing time is preferably short. Therefore, the annealing time is set to <NUM> seconds or shorter. The annealing time is preferably <NUM> seconds or shorter and more preferably <NUM> seconds or shorter. On the other hand, if the annealing time is shorter than <NUM> second, recrystallization of ferrite is not promoted, resulting in deteriorated hole expansion formability. Therefore, the annealing time is set to <NUM> second or longer. The annealing time is preferably <NUM> seconds or longer. The annealing time is the holding time at the annealing temperature.

As described above, it is necessary to ensure a certain amount of oxygen in the holding atmosphere to cause internal oxidation in the surface layer of the base steel sheet and to form appropriate amounts of Si and Mn oxides in the surface layer of the base steel sheet. Further, it is necessary to raise the dew point to some extent from the viewpoint of ensuring an appropriate amount of Fe in the hot-dip galvanized layer. Therefore, the dew point of the holding atmosphere is set to -<NUM> or higher. The dew point of the holding atmosphere is preferably -<NUM> or higher and more preferably -<NUM> or higher. On the other hand, if the dew point is too high, excessive internal oxidation is caused in the surface layer of the base steel sheet, which deteriorates the elongation and the hole expansion formability. If the dew point is too high, iron diffusion is excessively promoted during the coating treatment, resulting in excessive diffusion of iron in the coated layer. Therefore, the dew point of the holding atmosphere is set to <NUM> or lower. The dew point of the holding atmosphere is preferably <NUM> or lower.

To promote internal oxidation in the surface layer of the base steel sheet and to ensure the coating weight of the hot-dip galvanized layer, the oxides formed on the surface of the base steel sheet (and formed during the holding of the annealing process) need to be reduced. Therefore, the hydrogen concentration in the holding atmosphere is set to <NUM> mass% or more. The hydrogen concentration in the holding atmosphere is preferably <NUM> mass% or more. On the other hand, if the hydrogen concentration in the holding atmosphere is too high, hydrogen penetrates into the steel, and the elongation and the hole expansion formability are deteriorated. Therefore, the hydrogen concentration in the holding atmosphere is set to <NUM> mass% or less. The hydrogen concentration in the holding atmosphere is preferably <NUM> mass% or less.

During the cooling process in a temperature range from the annealing temperature to the primary cooling stop temperature, it is necessary to properly control the cooling rate to form bainite. That is, if the primary cooling rate is low, pearlite is formed in addition to ferrite, and an appropriate amount of bainite cannot be obtained. Therefore, the primary cooling rate is set to <NUM>/s or higher. The primary cooling rate is preferably <NUM>/s or higher and more preferably <NUM>/s or higher. The upper limit of the primary cooling rate is not limited, because a high primary cooling rate is preferred to suppress pearlite transformation. For example, there is no problem if the primary cooling rate reaches <NUM>/s or higher by water cooling or like.

The primary cooling stop temperature is set to <NUM> or higher and <NUM> or lower to suppress pearlite transformation during the primary cooling and to ensure the specified amount of bainite during the secondary cooling. That is, if the primary cooling stop temperature exceeds <NUM>, pearlite transformation is accelerated during the secondary cooling. Therefore, the primary cooling stop temperature is set to <NUM> or lower. The primary cooling stop temperature is preferably <NUM> or lower and more preferably <NUM> or lower. On the other hand, if the primary cooling stop temperature is lower than <NUM>, bainite transformation is suppressed during the secondary cooling, rendering it difficult to ensure the specified fraction of bainite. Therefore, the primary cooling stop temperature is set to <NUM> or higher. The primary cooling stop temperature is preferably <NUM> or higher and more preferably <NUM> or higher.

In the secondary cooling process from the primary cooling stop temperature to the secondary cooling stop temperature following the primary cooling process, it is necessary properly control the secondary cooling time to form bainite. That is, a long secondary cooling time promotes bainite transformation. Therefore, the secondary cooling time is set to <NUM> seconds or longer. The secondary cooling time is preferably <NUM> seconds or longer and more preferably <NUM> seconds or longer. On the other hand, if the secondary cooling time is too long, bainite is excessively formed, and the area ratio of martensite necessary for ensuring strength cannot be obtained. Therefore, the secondary cooling time is set to <NUM> seconds or shorter. The secondary cooling time is preferably <NUM> seconds or shorter and more preferably <NUM> seconds or shorter.

The secondary cooling stop temperature is set to <NUM> or higher and <NUM> or lower from the viewpoint of ensuring the specified fraction of bainite and controlling the temperature of the cold-rolled steel sheet when it enters the coating bath in the coating treatment process, which will be described later, within the specified range. That is, if the secondary cooling stop temperature exceeds <NUM>, bainite transformation is accelerated during the secondary cooling, and the fraction of bainite becomes too high. Therefore, the secondary cooling stop temperature is set to <NUM> or lower. The secondary cooling stop temperature is preferably <NUM> or lower and more preferably <NUM> or lower. On the other hand, if the secondary cooling stop temperature is lower than <NUM>, it is difficult to control the temperature of the cold-rolled steel sheet when it enters the coating bath to a temperature at least <NUM> higher than the coating bath temperature even if heat treatment is applied immediately before the coating treatment, especially in a case of using a continuous annealing hot-dip galvanizing line (CGL). Therefore, the secondary cooling stop temperature is set to <NUM> or higher. The secondary cooling stop temperature is preferably <NUM> or higher and more preferably <NUM> or higher.

In this process, the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment after the annealing treatment.

Further, in this process, it is important that the temperature of the cold-rolled steel sheet when it enters the coating bath be at least <NUM> higher than the coating bath temperature.

Temperature of the cold-rolled steel sheet when it enters the coating bath: coating bath temperature + <NUM> or higher.

To ensure an appropriate amount of Fe in the hot-dip galvanized layer, it is necessary to control the temperature of the cold-rolled steel sheet when it enters the coating bath higher than the coating bath temperature, especially to a temperature at least <NUM> higher than the coating bath temperature. The temperature of the cold-rolled steel sheet when it enters the coating bath is preferably at least <NUM> higher than the coating bath temperature and more preferably at least <NUM> higher than the coating bath temperature. The upper limit of the temperature of the cold-rolled steel sheet when it enters the coating bath is not particularly limited, but it is preferably <NUM> or lower.

The coating bath is basically composed of Zn, and it may contain <NUM> mass% to <NUM> mass% of Al. The balance other than Zn and Al is inevitable impurities.

The coating bath temperature is preferably <NUM> to <NUM>.

In addition, the annealing process and the coating treatment process may be performed on a continuous annealing line (CAL) or on a continuous annealing hot-dip galvanizing line (CGL). Each process may be performed by batch processing.

The conditions of each process other than the above are not limited, and conventional methods may be followed. After the annealing process, temper rolling may be performed for shape adjustment.

According to the above manufacturing method, it is possible to obtain a hot-dip galvanized steel sheet that has both high strength and good workability as well as excellent coating quality, and this hot-dip galvanized steel sheet can be suitably used for automotive members.

Steel materials having the chemical compositions listed in Table <NUM> (with the balance being Fe and inevitable impurities) were melted in a vacuum melting furnace and then subjected to blooming to obtain bloomed materials with a thickness of <NUM>. The obtained bloomed materials were subjected to hot rolling under the conditions listed in Table <NUM> to obtain hot-rolled steel sheets with a thickness of <NUM>. Next, the obtained hot-rolled steel sheets were ground to a thickness of <NUM>, and then they were subjected to cold rolling under the conditions listed in Table <NUM> to obtain cold-rolled steel sheets with a thickness <NUM> to <NUM>. Next, the obtained cold-rolled steel sheets were subjected to annealing and coating treatment under the conditions listed in Table <NUM> to obtain hot-dip galvanized steel sheets with hot-dip galvanized layers on both sides. Blank cells in Table <NUM> indicate that the element is not intentionally added (it is not necessarily <NUM> mass%, and it may be inevitably contained).

Next, each obtained hot-dip galvanized steel sheet was used to identify the microstructure in the base steel sheet, measure the amount of oxygen in oxide form in the surface layer of the base steel sheet, and measure the coating weight and the Fe content per surface in the hot-dip galvanized layer, according to the procedure described above.

For the identification of the microstructure in the base steel sheet (point counting method), <NUM> × <NUM> grids were evenly spaced over an area to be observed by a SEM (an area of <NUM> × <NUM>). The number of grid points in each phase was counted, and a ratio of the number of grid points occupied by each phase to the total number of grid points was taken as the area ratio of each phase. Note that the area ratio of each phase was the average value of the area ratios of each phase obtained from three separate SEM images.

Further, the mechanical properties of each of the obtained hot-dip galvanized steel sheets were measured according to the procedure described above. The results are listed in Table <NUM>.

The desired tensile strength (TS) is <NUM> MPa or more.

From the viewpoint of workability, the desired TS×El is <NUM> MPa·% or more, TS×λ is <NUM> MPa·% or more, and yield ratio YR (= YS/TS) is <NUM> or more.

Furthermore, the coating quality (coating adhesion and coating appearance) of each of the obtained hot-dip galvanized steel sheets was examined according to the procedure described above and evaluated according to the following criteria. The evaluation results are listed in Table <NUM>.

As listed in Table <NUM>, all Examples had both high strength and good workability, as well as excellent coating quality.

Claim 1:
A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, wherein
the base steel sheet comprises
a chemical composition containing, in mass%,
C: <NUM> % or more and <NUM> % or less,
Si: <NUM> % or more and <NUM> % or less,
Mn: <NUM> % or more and <NUM> % or less,
P: <NUM> % or less,
S: <NUM> % or less,
Al: <NUM> % or more and <NUM> % or less, and
N: <NUM> % or less, and,
optionally,
at least one selected from the group consisting of
Nb: <NUM> % or less,
Ti: <NUM> % or less,
B: <NUM> % or less,
Cr: <NUM> % or less,
Mo: <NUM> % or less, and
V: <NUM> % or less, and
at least one selected from the group consisting of Ta, W, Ni, Cu, Sn, Sb, Ca, Mg and Zr in a total amount of <NUM> % or less,
with the balance being Fe and inevitable impurities, and
a steel microstructure where
ferrite has an area ratio of <NUM> % or more and <NUM> % or less, martensite has an area ratio of <NUM> % or more and <NUM> % or less, bainite has an area ratio of <NUM> % or more and <NUM> % or less, other metallic phases have an area ratio of <NUM> % or less, and among the other metallic phases, pearlite and non-recrystallized ferrite have a total area ratio of <NUM> % or less
with respect to the entire steel microstructure being measured utilizing the method described in the description,
oxygen is present as oxides in a surface layer of the base steel sheet in an amount of <NUM>/m<NUM> or more and <NUM>/m<NUM> or less per surface being measured utilizing the method described in the description, where the surface layer is an area from a surface of the base steel sheet to a position at a depth of <NUM>, and the oxides are compounds of oxygen and elements contained in the base steel sheet,
the hot-dip galvanized layer contains Fe in an amount of <NUM> mass% or more and <NUM> mass% or less being measured utilizing the method described in the description.