Plated steel sheet and method for manufacturing same

Abstract

A plated steel sheet includes a steel sheet and a plating layer disposed on a surface of the steel sheet, the steel sheet satisfying the following formulas (1) and (2), wherein the steel sheet included in the plated steel sheet has a predetermined composition, and has a metal microstructure in which a total of ferrite and upper bainite accounts for 30 to 70 area%, tempered martensite accounts for 10 to 45 area%, MA accounts for 5 to 35 area%, and retained austenite accounts for 5 to 20 vol%. TSmax−TSmin<20MPa TSmin≥980MPa

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a plated steel sheet and a method for manufacturing the same.BACKGROUND ART

[0002] Conventionally, improvement of safety of an occupant in a vehicle has been required, and strength of a material of a vehicle body has been improved for such a purpose. On the other hand, against the background of the growing global warming problem and the like, the movement for improving the fuel efficiency of automobiles is accelerating. It is known that weight reduction of a vehicle body is effective for improving fuel efficiency.

[0003] A plated steel sheet used for an automobile body is required to have further improved strength in order to ensure safety against collision and the like while reducing the weight of the vehicle body. Examples of the mechanical properties that greatly affect safety against collision and the like include tensile strength and yield ratio.

[0004] On the other hand, in order to apply a plated steel sheet to a member such as an automobile body having a complicated shape, excellent workability is also required. Examples of mechanical properties that greatly contribute to workability include ductility and hole expandability.

[0005] As a technique for achieving both collision safety (high strength, predetermined yield ratio) and high workability, utilization of soft ferrite (and upper bainite) and the like has been mainly proposed in addition to retained austenite (use of Transformation induced plasticity (TRIP) phenomenon).

[0006] Patent Document 1 discloses a plated steel sheet containing ferrite and having a tensile strength of 980 MPa or more and favorable workability by controlling the microstructure ratio of MA and retained austenite.CONVENTIONAL ART DOCUMENTPATENT DOCUMENT

[0007] Patent Document 1: JP 2016-194139 ASUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0008] A variation in tensile strength in the sheet width direction of the plated steel sheet causes preferential deformation and the like of a portion having low strength, which may deteriorate collision safety when applied to, for example, an automobile body. However, in the prior art, the variation in tensile strength in the sheet width direction is not considered, and there is room for further improvement.

[0009] The present disclosure has been made in view of such a circumstance, and an object thereof is to provide a plated steel sheet that has high strength, can contribute to weight reduction of a vehicle body, has a predetermined yield ratio and high workability, and suppresses variations in tensile strength in a sheet width direction, and a method for manufacturing the plated steel sheet.MEANS FOR SOLVING THE PROBLEMS

[0010] A first aspect of the present invention is a plated steel sheet including a steel sheet and a plating layer disposed on a surface of the steel sheet and satisfying the following formulas (1) and (2), wherein the steel sheet included in the plated steel sheet has a composition including: C: 0.150 to 0.300 mass%; Si: 0.80 to 2.20 mass%; Mn: 1.60 to 2.80 mass%; Al: 0.300 to 0.800 mass%; Ti: 0.010 to 0.050 mass%; P: 0.050 mass% or less (including 0 mass%); S: 0.0100 mass% or less (including 0 mass%); N: 0.0100 mass% or less (including 0 mass%); and a balance: iron and inevitable impurities, and wherein the steel sheet included in the plated steel sheet has a metal microstructure in which: a total of ferrite and upper bainite accounts for 30 to 70 area%, tempered martensite accounts for 10 to 45 area%, MA accounts for 5 to 35 area%, and retained austenite accounts for 5 to 20 vol%. TS max − TS min < 20 MPa TS min ≧ 980 MPa

[0011] In the formulas (1) and (2), TS max is the maximum tensile strength measured at five different positions in a sheet width direction of the plated steel sheet, and TS min is the minimum tensile strength measured at the five positions, and when a sheet width is W, the five positions include two positions of W / 10 to W / 8 from both ends toward a center in the sheet width direction, two positions of W / 4 from the both ends toward the center in the sheet width direction, and one position of the center in the sheet width direction.

[0012] A second aspect of the present invention is the plated steel sheet according to the first aspect, further including at least one selected from the group consisting of Nb: more than 0 mass% and 0.20 mass% or less, and V: more than 0 mass% and 0.50 mass% or less.

[0013] A third aspect of the present invention is the plated steel sheet according to the first or second aspect, further including at least one selected from the group consisting of Ni: more than 0 mass% and 2.0 mass% or less, Cr: more than 0 mass% and 2.0 mass% or less, and Mo: more than 0 mass% and 0.50 mass% or less.

[0014] A fourth aspect of the present invention is the plated steel sheet according to any one of the first to third aspects, further including B: more than 0 mass% and 0.0050 mass% or less.

[0015] A fifth aspect of the present invention is the plated steel sheet according to any one of the first to fourth aspects, further including at least one selected from the group consisting of Mg: more than 0 mass% and 0.040 mass% or less, REM: more than 0 mass% and 0.040 mass% or less, and Ca: more than 0 mass% and 0.040 mass% or less.

[0016] A sixth aspect of the present invention is the plated steel sheet according to any one of the first to fifth aspects, wherein in a sheet thickness direction, a position where a carbon concentration (mass%) is 50% of a bulk carbon concentration is in a region of 0.2% or more of the sheet thickness from the surface of the steel sheet.

[0017] A seventh aspect of the present invention is the plated steel sheet according to any one of the first to sixth aspects, wherein a maximum value of a carbon concentration (mass%) in a region from the surface of the steel sheet to 20 µm in a sheet thickness direction is less than 70% of a bulk carbon concentration.

[0018] Eighth aspect of the present invention is a method for manufacturing the plated steel sheet according to any one of the first to seventh aspects, the method including: providing a rolled sheet by hot rolling a steel having the composition according to any one of the first to fifth aspects; first heating the rolled sheet to a first heating temperature of 850°C or more and 910°C or less; holding at the first heating temperature for 5 to 1800 seconds after the first heating; first cooling to a first cooling stop temperature of 350°C to 750°C after the holding; dwelling within the temperature range from the first cooling stop temperature to a second cooling start temperature ranging from 350°C to the first cooling stop temperature for 20 to 300 seconds after the first cooling, at an average cooling rate of 10°C / s or less, which is slower than that of the first cooling; second cooling from the second cooling start temperature to a second cooling stop temperature of 100 to 300°C after the dwelling at an average cooling rate higher than that in the dwelling; second heating to a second heating temperature of 300 to 500°C after the second cooling; providing a steel sheet by holding at the second heating temperature for 300 to 1800 seconds after the second heating; and forming a plating layer on the steel sheet.

[0019] A ninth aspect of the present invention is the manufacturing method according to the eighth aspect, the method including: performing an oxidation treatment under conditions of an oxygen concentration of 0.1 to 2% and a reached temperature of 650 to 750°C after the providing the rolled sheet, wherein the holding includes performing a reduction treatment that comprises a first reduction treatment performed under conditions with a dew point of -35 to -15°C, followed by a second reduction treatment performed under conditions with a dew point of -25 to 0°C, which is higher than that of the first reduction treatment. TECHNICAL EFFECTS OF THE INVENTION

[0020] Embodiments of the present invention can provide a plated steel sheet that has high strength, can contribute to weight reduction of a vehicle body, has a predetermined yield ratio and high workability, and suppresses variations in tensile strength in a sheet width direction, and a method for manufacturing the plated steel sheet.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] [Fig. 1] Fig. 1 is a view showing an example of a carbon concentration profile obtained in Examples.MODE FOR CARRYING OUT THE INVENTION

[0022] The present inventors have conducted studies from various angles in order to achieve a plated steel sheet that has high strength (tensile strength TS of 980 MPa or more), can contribute to weight reduction of a vehicle body, has a yield ratio YR of predetermined (0.55 to 0.75) and high workability (elongation EL: 19% or more, hole expansion ratio: λ: 20% or more), and suppresses variations in tensile strength in a sheet width direction.

[0023] Typically, in order to sufficiently introduce soft ferrite and the like, it has been effective to perform annealing (hereinafter, also referred to as "first heating") after the hot rolling step at a relatively low temperature and the like.

[0024] The present inventors have found that introduction of soft ferrite or the like by a normal method as described above may cause the metal microstructure or the like in the steel sheet to become uneven, and variations in the tensile strength in the sheet width direction.

[0025] The present inventors have found that a predetermined metal microstructure including a sufficient amount of ferrite and the like can be achieved by adjusting the composition to a predetermined composition (particularly, adding a predetermined amount of Al capable of contributing to the formation of ferrite and upper bainite), performing annealing at a high temperature of 850°C or more after the rolling step to eliminate distortion and the like after the rolling, and then dwelling (or holding) at a predetermined temperature range for a predetermined time during subsequent cooling, and as a result, a plated steel sheet having high strength (high tensile strength), a predetermined yield ratio, and high workability, and suppressing variations in tensile strength in the sheet width direction can be obtained.

[0026] Hereinafter, details of each requirement defined by the embodiments of the present invention will be described.<1. Plated steel sheet>

[0027] A plated steel sheet according to embodiments of the present invention includes a steel sheet and a plating layer disposed on a surface of the steel sheet. The plating layer may be a hot-dip galvanizing layer, a zinc-based plating layer such as an alloyed hot-dip galvanizing layer or an electrogalvanizing layer, an aluminum plating layer such as a hot-dip aluminum plating layer, or the like. The zinc-based plating layer may be a zinc-based alloyed plating layer such as zinc-Ni, zinc-Fe, or zinc-Al. The plating layer may be formed on at least one surface (for example, a rolled surface) of the steel sheet, may be formed on both opposing surfaces, or may be formed on the entire surface of the steel sheet.

[0028] Specific examples of the plated steel sheet including the zinc-based plating layer include hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), electrogalvanized steel sheets (EG) and the like.<2. Composition of steel sheet included in plated steel sheet>

[0029] It is preferable that the steel sheet (for example, a portion that may be obtained by removing the plating layer from the plated steel sheet) included in the plated steel sheet according to embodiments of the present invention includes C: 0.150 to 0.300 mass%, Si: 0.80 to 2.20 mass%, Mn: 1.60 to 2.80 mass%, Al: 0.300 to 0.800 mass%, Ti: 0.010 to 0.050 mass%, P: 0.050 mass% or less (including 0 mass%), S: 0.0100 mass% or less (including 0 mass%), N: 0.0100 mass% or less (including 0 mass%), and the balance is iron and inevitable impurities.

[0030] Hereinafter, each component will be described in detail.(C: 0.150 to 0.300 mass%)

[0031] C is an element effective for obtaining a desired microstructure such as retained austenite (hereinafter also referred to as "retained γ-phase") and improving characteristics such as tensile strength TS and elongation EL. In order to exhibit this effect, the C content is 0.150 mass% or more, preferably 0.170 mass% or more, more preferably 0.180 mass% or more, and still more preferably 0.200 mass% or more. In contrast, when the C content is excessive, MA (Martensite-Austenite constituent) and the retained γ-phase become coarse, the hole expandability λ decreases, and the weldability is also adversely affected. Therefore, the C content is 0.300 mass% or less, preferably 0.280 mass% or less, more preferably 0.260 mass% or less, and still more preferably 0.250 mass% or less.(Si: 0.80 to 2.20 mass%)

[0032] Si suppresses precipitation of cementite and promotes formation of retained γ-phase. In order to exhibit this effect, the Si content is 0.80 mass% or more, preferably 0.90 mass% or more, more preferably 1.00 mass% or more, and still more preferably 1.20 mass% or more. In contrast, when the Si content is excessive, the MA and the retained γ-phase become coarse, the hole expansion ratio λ decreases, and the weldability also decreases. Therefore, the Si content is 2.20 mass% or less, preferably 2.10 mass% or less, and more preferably 2.00 mass% or less.(Mn: 1.60 to 2.80 mass%)

[0033] Mn is an element that contributes to improvement of the strength of the steel sheet by solid solution strengthening, and is an element necessary for ensuring high strength. In order to exhibit this effect, the Mn content is 1.60 mass% or more, preferably 1.80 mass% or more, and more preferably 2.00 mass% or more. In contrast, when the Mn content is excessive, the formation of ferrite and upper bainite is suppressed in the manufacturing process, and thus a soft microstructure contributing to improvement of ductility cannot be sufficiently introduced, and as a result, high elongation EL cannot be achieved. Therefore, the Mn content is 2.80 mass% or less, preferably 2.70 mass% or less, more preferably 2.60 mass% or less, and still more preferably 2.50 mass% or less.(Al: 0.300 to 0.800 mass%)

[0034] Al has an effect of promoting the formation of ferrite and upper bainite, and is an important element for securing a desired area ratio of ferrite and upper bainite by short-time retention in a predetermined temperature range described later. In order to exhibit this effect, the Al content is 0.300 mass% or more, preferably 0.350 mass% or more, and more preferably 0.400 mass% or more. In contrast, when the Al content is excessive, it may be difficult to eliminate the inhomogeneity of the microstructure derived from the manufacturing process, which may thus cause the strength variation in the steel sheet. Therefore, the Al content is 0.800 mass% or less, preferably 0.750 mass% or less, and more preferably 0.700 mass% or less.(Ti: 0.010 to 0.050 mass%)

[0035] Ti is an element that exhibits effects of precipitation strengthening and metal microstructure refinement, and contributes to an increase in strength of the steel sheet. In order to exhibit these effects, the Ti content is 0.010 mass% or more, preferably 0.015 mass% or more, and more preferably 0.020 mass% or more. In contrast, when the Ti content is excessive, carbonitrides of Ti are excessively precipitated, and the workability of the steel sheet is deteriorated. Therefore, the Ti content is 0.050 mass% or less, preferably 0.045 mass% or less, and more preferably 0.040 mass% or less.(P: 0.050 mass% or less (including 0 mass%))

[0036] P is an element inevitably present as an impurity element. When the P content is excessive, the elongation EL and the hole expansion ratio λ decrease. Therefore, the P content is 0.050 mass% or less, preferably 0.030 mass% or less.

[0037] In the present specification, "including 0 mass%" means including embodiments in which the impurities are not intentionally added, that is, a case of the content being equal to or less than the inevitable impurity level (the case in which the impurities are intentionally added is not excluded).(S: 0.0100 mass% or less (including 0 mass%))

[0038] S is an element inevitably present as an impurity element. When the S content is excessive, a sulfide-based inclusion such as MnS is formed, and this causes a starting point of cracking, and the hole expansion ratio λ decreases. Therefore, the S content is 0.0100 mass% or less, preferably 0.0050 mass% or less.(N: 0.0100 mass% or less (including 0 mass%))

[0039] N is an element inevitably present as an impurity element. When the N content is excessive, a coarse nitride is formed, which causes deterioration of bendability and hole expandability and generation of a blowhole during welding. Therefore, the N content is 0.0100 mass% or less. The reduction of the N content is costly, and if the N content is to be reduced to less than 0.0005 mass%, the cost is significantly increased. Therefore, the lower limit of the N content is preferably 0.0005 mass% or more.

[0040] The steel sheet included in the plated steel sheet according to embodiments of the present invention includes the above-described composition, and in one embodiment of the present invention, the balance is preferably iron and inevitable impurities. As the inevitable impurities, contamination of elements brought depending on the situation of raw materials, materials, manufacturing facilities, and the like is allowed. For example, there are elements such as P, S, and N, which are typically preferable as the content is smaller, and thus are inevitable impurities, but are separately defined as the composition range as described above. Therefore, in the present specification, the "inevitable impurities" constituting the residual portion indicates a concept excluding elements with composition range separately defined.

[0041] As a preferable embodiment of the present invention, if necessary, it is also effective to further contain, in addition to the above elements, (a) at least one selected from the group consisting of Nb: more than 0 mass% and 0.20 mass% or less and V: more than 0 mass% and 0.50 mass% or less, (b) at least one selected from the group consisting of Ni: more than 0 mass% and 2.0 mass% or less, Cr: more than 0 mass% and 2.0 mass% or less, and Mo: more than 0 mass% and 0.50 mass% or less, (c) B: more than 0 mass% and 0.0050 mass% or less, (d) at least one selected from the group consisting of Mg: more than 0 mass% and 0.040 mass% or less, REM: more than 0 mass% and 0.040 mass% or less, and Ca: more than 0 mass% and 0.040 mass% or less, and the characteristics of the plated steel sheet are further improved according to the types of elements to be contained.((a) At least one selected from the group consisting of Nb: more than 0 mass% and 0.20 mass% or less, and V: more than 0 mass% and 0.50 mass% or less)

[0042] Nb is an element that exerts effects of precipitation strengthening and metal microstructure refinement, and contributes to improvement of the strength of the steel sheet. Therefore, the Nb content is preferably more than 0 mass%. These effects increase as the Nb content increases, and the Nb content is more preferably 0.005 mass% or more, still more preferably 0.010 mass% or more. In contrast, when the Nb content is excessive, carbonitrides of Nb may be excessively precipitated to deteriorate the workability of the steel sheet. Therefore, the Nb content is preferably 0.20 mass% or less, more preferably 0.10 mass% or less, and still more preferably 0.060 mass% or less.

[0043] Similarly to Nb, V is an element that exerts effects of precipitation strengthening and metal microstructure refinement, and contributes to improvement of the strength of the steel sheet. Therefore, the V content is preferably more than 0 mass%. These effects increase as the V content increases, and the V content is more preferably 0.010 mass% or more, still more preferably 0.020 mass% or more. In contrast, when the V content is excessive, carbonitrides of V may be excessively precipitated to deteriorate the workability of the steel sheet. Therefore, the V content is preferably 0.50 mass% or less, more preferably 0.40 mass% or less, still more preferably 0.30 mass% or less, and still more preferably 0.10 mass% or less.((b) At least one selected from the group consisting of Ni: more than 0 mass% and 2.0 mass% or less, Cr: more than 0 mass% and 2.0 mass% or less, and Mo: more than 0 mass% and 0.50 mass% or less)

[0044] Ni is an element that contributes to improvement of the strength of the steel sheet, stabilizes the retained γ-phase, and is effective for securing a desired amount of the retained γ-phase. Therefore, the Ni content is preferably more than 0 mass%. These effects increase as the Ni content increases, and the Ni content is more preferably 0.001 mass% or more, still more preferably 0.01 mass% or more. In contrast, when the Ni content is excessive, manufacturability during hot rolling may be deteriorated. Therefore, the Ni content is preferably 2.0 mass% or less, more preferably 1.0 mass% or less.

[0045] Similarly to Ni, Cr is an element that contributes to improvement of the strength of the steel sheet, stabilizes the retained γ-phase, and secures a desired amount of the retained γ-phase. Therefore, the Cr content is preferably more than 0 mass%. These effects increase as the Cr content increases, and the Cr content is more preferably 0.001 mass% or more, still more preferably 0.02 mass% or more. In contrast, when the Cr content is excessive, manufacturability during hot rolling is deteriorated. Therefore, the Cr content is preferably 2.0 mass% or less, more preferably 1.0 mass% or less.

[0046] Similarly to Ni and Cr, Mo is an element that contributes to improvement of the strength of the steel sheet, stabilizes the retained γ-phase, and secures a desired amount of the retained γ-phase. Therefore, the Mo content is preferably more than 0 mass%. These effects increase as the Mo content increases, and the Mo content is more preferably 0.001 mass% or more, still more preferably 0.01 mass% or more. In contrast, when the Mo content is excessive, manufacturability during hot rolling may be deteriorated. Therefore, the Mo content is preferably 0.50 mass% or less, more preferably 0.20 mass% or less.

[0047] Ni, Cr, and Mo may be contained singly, or may be contained in combination of two or three.((c) B: more than 0 mass% and 0.0050 mass% or less)

[0048] B is an element effective for suppressing excessive ferrite transformation by enhancing hardenability. Therefore, the B content is preferably more than 0 mass%. This effect increases as the B content increases, and the B content is more preferably 0.0001 mass% or more, still more preferably 0.0003 mass% or more, and still more preferably 0.0005 mass% or more. However, when the B content is excessive, hardenability may be excessively increased, making it difficult to introduce ferrite and upper bainite during cooling. Therefore, the B content is preferably 0.0050 mass% or less, more preferably 0.0030 mass% or less, and still more preferably 0.0020 mass% or less.((d) At least one selected from the group consisting of Mg: more than 0 mass% and 0.040 mass% or less, REM: more than 0 mass% and 0.040 mass% or less, and Ca: more than 0 mass% and 0.040 mass% or less)

[0049] Mg, REM (rare earth element), and Ca form fine oxides and / or sulfides, and suppress a decrease in hole expandability due to coarse oxides and / or sulfides. Therefore, the content of each of Mg, REM and Ca is preferably more than 0 mass%. This effect increases as the content of each of Mg, REM and Ca increases, and the content of each of Mg, REM and Ca is more preferably 0.0005 mass% or more, still more preferably 0.0010 mass% or more. However, when the content of each of Mg, REM and Ca is excessive, the oxide and / or sulfide may become coarse, leading to deterioration of hole expandability. Therefore, the content of each of Mg, REM and Ca is preferably 0.040 mass% or less, and more preferably 0.010 mass% or less. The REM contains a total of 17 types of lanthanoid series rare earth elements from La (atomic number 15) to Lu (atomic number 71), including Sc and Y, and the REM content means the total amount of these 17 types of elements.<3. Metal microstructure of steel sheet included in plated steel sheet>

[0050] In the metal microstructure of the steel sheet included in the plated steel sheet according to embodiments of the present invention, the total of ferrite and upper bainite accounts for 30 to 70 area%, tempered martensite accounts for 10 to 45 area%, MA accounts for 5 to 35 area%, and retained austenite accounts for 5 to 20 vol%. Each microstructure will be described below.(Total of ferrite and upper bainite: 30 to 70 area%)

[0051] Ferrite and upper bainite are important microstructures for improving workability (particularly elongation EL). Therefore, the total of ferrite and upper bainite accounts for 30 area% or more, preferably 35 area% or more, and more preferably 40 area% or more. In contrast, both ferrite and upper bainite have low strength, and thus when the total area ratio of ferrite and upper bainite is excessive, the tensile strength TS and the yield stress YS decrease. Therefore, the total of ferrite and upper bainite accounts for 70 area% or less, preferably 65 area% or less, and more preferably 60 area% or less.

[0052] The area ratio of ferrite and upper bainite can be determined by observing a Nital-etched steel sheet with a scanning electron microscope (SEM) and measuring a black portion not containing carbide by a point calculation method or the like.(Tempered martensite: 10 to 45 area%)

[0053] Tempered martensite is an important metal microstructure for ensuring the desired tensile strength TS and yield ratio YR. Therefore, the tempered martensite accounts for 10 area% or more, preferably 15 area% or more, more preferably 18 area% or more, and still more preferably 20 area% or more. In contrast, when the tempered martensite is excessive, the elongation EL decreases. Therefore, the tempered martensite accounts for 45 area% or less, preferably 40 area% or less, and more preferably 35 area% or less.

[0054] The area ratio of the tempered martensite can be determined as a portion excluding ferrite, upper bainite, MA, and the balance of metal microstructure including pearlite and cementite.(MA: 5 to 35 area%)

[0055] MA is a composite microstructure of fresh martensite and retained γ-phase. MA contains the retained γ-phase, and thus increasing the amount of MA is effective for improving workability such as elongation EL. Therefore, the MA accounts for 5 area% or more, preferably 7 area% or more, and more preferably 10 area% or more. However, significantly hard fresh martensite is also contained in MA, and thus the interface between the fresh martensite and the other microstructure increases due to an increase in the amount of MA, and the hole expansion ratio λ decreases. Therefore, the MA accounts for 35 area% or less, preferably 30 area% or less, and more preferably 25 area% or less.

[0056] The area ratio of MA can be determined by observing a steel sheet subjected to Nital-etched steel sheet with SEM, and measuring a gray area not containing carbide by a point calculation method.(Retained austenite: 5 to 20 vol%)

[0057] The retained γ-phase causes a TRIP (transformation induced plasticity) phenomenon that transforms into martensite in press working or the like, and contributes to improvement of elongation EL. Martensite formed by the TRIP phenomenon has high hardness, and thus the retained γ-phase is an effective microstructure for improving the tensile strength TS and the elongation EL. In order to achieve desired strength and workability, the retained γ-phase accounts for 5 vol% or more, preferably 6 vol% or more, more preferably 8 vol% or more, and still more preferably 10 vol% or more. In contrast, the retained γ-phase is a microstructure having a low yield ratio, and thus the volume ratio of the retained γ-phase for securing a desired yield ratio is set to 20 vol% or less, preferably 19 vol% or less, more preferably 18 vol% or less, and still more preferably 16 vol% or less.

[0058] The volume ratio of the retained γ-phase can be measured by, for example, X-ray diffraction. In this method, for example, a portion up to 1 / 4 of the thickness of the steel sheet from the surface of the steel sheet is removed by mechanical polishing, chemical polishing, or the like, and an X-ray diffraction pattern on the surface exposed thereby is acquired using a Co-Kα line as a characteristic X-ray. Then, the volume ratio of the retained γ-phase can be measured from the integrated intensity ratio of the diffraction peaks of the body centered cubic lattice (bcc) phase, the body centered tetragonal lattice (bet) phase, and the face centered cubic lattice (fcc) phase.

[0059] Examples of the balance of metal microstructure include pearlite and cementite. The balance of metal microstructure may contain 10 area% or less in total.<4. Mechanical properties of plated steel sheet>

[0060] The plated steel sheet according to the present embodiments can exhibit high strength (high tensile strength), a predetermined yield ratio, and high workability (high elongation and high hole expansion ratio). In the present specification, a steel sheet having a tensile strength TS of 980 MPa or more, specifically, a steel sheet satisfying the following formula (2) is defined as a high-strength plated steel sheet. TS min ≥ 980 MPa

[0061] In the formula (2), TS min is the minimum tensile strength measured at five different points in a sheet width direction of the plated steel sheet, and when the sheet width is W, the five points include two points at positions of W / 10 to W / 8 from both ends toward the center, two points at positions of W / 4 from both ends toward the center, and one point at the center in the sheet width direction.

[0062] The predetermined yield ratio YR is set to 0.55 to 0.75. Setting the tensile strength and the yield ratio as described above is considered to allow sufficient collision safety to be secured when the present invention is applied to, for example, an automobile body. In addition, in the present specification, a plated steel sheet having an elongation EL of 19% or more and a hole expansion ratio λ of 20% or more is defined as a plated steel sheet having high workability.

[0063] The left side of the above formula (2) is preferably 990 MPa or more, and more preferably 1000 MPa or more. The yield ratio is preferably 0.57 to 0.72. The elongation EL is preferably 20% or more, more preferably 21% or more. The hole expansion ratio λ is preferably 23% or more, more preferably 25% or more, and still more preferably 30% or more. In addition, the yield stress YS is preferably 550 MPa or more, more preferably 570 MPa or more, still more preferably 600 MPa or more for securing collision safety.

[0064] Further, the plated steel sheet according to the present embodiments suppresses variations in tensile strength in the sheet width direction (that is, the rolling direction and the direction perpendicular to the sheet thickness direction), and specifically satisfies the following formula (1). TS max − TS min < 20 MPa

[0065] In the formula (1), TS max is a maximum tensile strength measured at five different positions in the sheet width direction (that is, the rolling direction and the direction perpendicular to the sheet thickness direction) of the plated steel sheet, and TS min is a minimum tensile strength measured at the five positions, and when the sheet width is W, the five positions include two positions of W / 10 to W / 8 from both ends toward the center, two positions of W / 4 from both ends toward the center, and one position of the center in the sheet width direction.

[0066] The left side of the above formula (1) is preferably 18 MPa or less.

[0067] The thickness of the steel sheet included in the plated steel sheet according to the present embodiments is not particularly limited. The sheet thickness of the steel sheet included in the plated steel sheet according to the present embodiments may be, for example, 0.8 mm or more and 2.3 mm or less. The plating deposition amount of the plated steel sheet according to the embodiments is not particularly limited, and may be, for example, about 10 to 100 g / m 2< per one surface. The sheet width W (that is, the lengths in the rolling direction and the direction perpendicular to the sheet thickness direction) of the plated steel sheet according to the present embodiments is not particularly limited, but may be, for example, 800 mm or more and 1200 mm or less.<5. Decarburized layer of surface layer of steel sheet included in plated steel sheet>

[0068] For the steel sheet included in the plated steel sheet according to the present embodiments, (I) the position where the carbon concentration (mass%) is 50% of the bulk carbon concentration in the sheet thickness direction is preferably in a region of 0.2% or more of the sheet thickness from the surface of the steel sheet. The position relates to the thickness of the decarburized layer that contributes to the bendability. The position is in a region of 0.2% or more of the sheet thickness from the surface of the steel sheet, and the decarburized layer contributing to bendability is ensured to have a certain thickness or more, thereby allowing excellent bendability to be reliably exhibited in bending. The position where the carbon concentration (mass%) is 50% of the bulk carbon concentration may be in a region of 0.5% or more, or 1.0% or more of the sheet thickness from the surface of the steel sheet.

[0069] For the steel sheet included in the plated steel sheet according to the present embodiments, (II) the maximum value of the carbon concentration (mass%) in the region from the surface of the steel sheet to 20 µm in the sheet thickness direction is preferably less than 70% of the bulk carbon concentration. The present inventors have confirmed a steel sheet included in a conventional plated steel sheet, and have first found that a region having a significantly high carbon concentration exists on a surface layer of the steel sheet, and this is a cause of deterioration in bendability. As described later, the present inventors have studied manufacturing conditions of a steel sheet, and found that decarburization of the steel sheet surface layer is promoted to suppress the carbon concentration of the steel sheet surface layer, specifically, when the maximum value of the carbon concentration (mass%) in a region from the steel sheet surface to 20 µm in the sheet thickness direction is suppressed to less than 70% of the bulk carbon concentration, thereby providing excellent bendability. The maximum value of the carbon concentration is preferably 65% or less of the bulk carbon concentration, and more preferably 60% or less of the bulk carbon concentration.

[0070] From the viewpoint of exhibiting excellent bendability, the plated steel sheet according to the present embodiments preferably satisfies at least one of the above (I) and (II), and more preferably satisfies both of the above (I) and (II).

[0071] The "surface of the steel sheet" refers to a position of an interface between the plating layer and the steel sheet. For example, in the case of zinc-based plating, the position of the interface between the plating layer and the steel sheet refers to a point at which Zn constituting the plating layer is not detected (the analysis value of Zn becomes 0) when Zn is analyzed in the thickness direction of the plating layer from the surface of the plating layer by GD-OES as measured in Examples described later, and this point is defined as the start point of the distance (depth) from the surface of the steel sheet.

[0072] The plated steel sheet according to the present embodiments exhibits high strength (high tensile strength), a predetermined yield ratio, and high workability (high elongation and high hole expansion ratio), satisfies the above formula (1), and can exhibit excellent bendability with a limit R / t of 2.0 or less when a bending test shown in Examples described later is performed.<6. Method for manufacturing plated steel sheet>

[0073] The method for manufacturing the plated steel sheet according to embodiments of the present invention includes: (A) providing a rolled sheet by hot rolling a steel having the above-described composition; (B) first heating the rolled sheet to a first heating temperature of 850°C or more and 910°C or less; (C) holding at the first heating temperature for 5 to 1800 seconds after the first heating; (D) first cooling to a first cooling stop temperature of 350°C to 750°C after the holding; (E) dwelling within the temperature range from the first cooling stop temperature to a second cooling start temperature ranging from 350°C to the first cooling stop temperature for 20 to 300 seconds after the first cooling, at an average cooling rate of 10°C / s or less, which is slower than that of the first cooling; (F) second cooling from the second cooling start temperature to a second cooling stop temperature of 100 to 300°C after the dwelling at an average cooling rate higher than that in the dwelling; (G) second heating to a second heating temperature of 300 to 500°C after the second cooling; (H) providing a steel sheet by holding at the second heating temperature for 300 to 1800 seconds after the second heating; and (I) forming a plating layer on the steel sheet. Hereinafter, each step will be described in detail. (A) Providing rolled sheet

[0074] The steel having the composition described above is smelted and cast by a method typically performed, and the steel (steel slab) is subjected to a hot rolling by a method typically performed, thereby allowing a rolled sheet to be provided. As an example, a steel (steel slab) having the above-described composition cast by a continuous casting method, an ingot method, a thin slab casting method, or the like is reheated to about 1150 to 1300°C, hot-rolled to a finish rolling temperature of about 850 to 950°C, and wound at about 500 to 700°C, which may provide a rolled sheet.

[0075] If necessary, the rolled sheet may be pickled by a method typically performed to remove the surface scale. In addition, if necessary, the rolled sheet may be further subjected to cold rolling by a method typically performed.(B) First heating

[0076] The rolled sheet is heated to a first heating temperature of 850°C or more and 910°C or less. When the first heating temperature is less than 850°C, the reverse transformation to austenite becomes insufficient, a worked microstructure including distortion due to rolling remains, and strength variation occurs in the sheet width direction of the plated steel sheet. Therefore, the first heating temperature is 850°C or more, preferably 860°C or more, and more preferably 870°C or more. In contrast, when the first heating temperature is more than 910°C, the crystal grain size is coarsened, which may cause a decrease in the amount of transformation of ferrite and upper bainite in the cooling step (accordingly, the area ratio of tempered martensite may be excessive) and a decrease in workability such as hole expansion ratio. Therefore, therefore, the first heating temperature is set to 910°C or less, preferably 900°C or less. The heating rate is not particularly limited, and the temperature may be raised at any heating rate. For example, the heating is performed from room temperature to the first heating temperature at an average heating rate of 1°C / s or more and 100°C / s or less.(C) Holding at first heating temperature

[0077] The rolled sheet is held at the first heating temperature for 5 to 1800 seconds. When the holding time is less than 5 seconds, the reverse transformation to austenite is insufficient, the worked microstructure cannot be sufficiently eliminated, and strength variation occurs in the sheet width direction of the plated steel sheet. Therefore, the holding time is set to 5 seconds or more, preferably 10 seconds or more, and more preferably 20 seconds or more. In contrast, when the holding time is more than 1800 seconds, in addition to the decrease in productivity, there is a risk that the grain of the steel sheet microstructure is coarsened, resulting in a decrease in workability such as the hole expansion ratio λ. Therefore, the holding time is 1800 seconds or less, preferably 1500 seconds or less, and more preferably 1000 seconds or less.

[0078] It is preferable that after the step (A), performing an oxidation treatment under the condition of an oxygen concentration of 0.1 to 2% and a reached temperature of 650 to 750°C (hereinafter, also referred to as "step (B1)") is included, and the step (C) includes performing a reduction treatment (hereinafter, also referred to as "step (C1)") that includes a first reduction treatment performed under conditions with a dew point of -35 to -15°C, followed by a second reduction treatment performed under conditions with a dew point of -25 to 0°C, which is higher than that of the first reduction treatment. For example, the oxidation treatment in the step (B1) may be performed after the step (A) and before the step (B), or may be performed during the temperature rise to the first heating temperature in the step (B) (that is, the step (B) may include the oxidation treatment of the step (B1)). The reduction treatment in the step (C1) can be performed during the holding at the first heating temperature in the step (C).

[0079] The surface of the steel sheet is subjected to the oxidation treatment in the step (B1), thereby allowing an Fe oxide layer to be formed on the surface of the steel sheet. Then, the reduction treatment in the step (C1) under a reducing atmosphere makes it possible to form a reduced Fe layer capable of favorably forming, for example, a plating layer while forming a decarburized layer on the surface layer of the steel sheet. The present inventors have conducted studies, and found that the oxidation treatment is performed as described above, the range of the dew point of each of the first reduction treatment and the second reduction treatment is determined in the reduction treatment, and the dew point of the second reduction treatment is set to be higher than the dew point of the first reduction treatment, thereby allowing easily securing the decarburized state of the surface layer of the steel sheet capable of achieving excellent bendability in addition to high tensile strength, a predetermined yield ratio, high elongation, and high hole expansion ratio. Hereinafter, each treatment of the oxidation treatment and the reduction treatment will be described.(B1) Performing oxidation treatment

[0080] The oxidation treatment is preferably performed under the condition of an oxygen concentration of 0.1 to 2%. The oxygen concentration is preferably 0.1% or more, more preferably 0.15% or more, and still more preferably 0.2% or more from the viewpoint of obtaining an excellent plating appearance. In contrast, when the oxygen concentration is too high, the oxide scale adheres to the roll in the furnace due to excessive oxidation, and a defect called pick-up, in which a press flaw occurs in the steel sheet, easily occurs. From the viewpoint of suppressing the occurrence of the defect, the oxygen concentration is preferably 2% or less, more preferably 1.7% or less, and still more preferably 1.5% or less. The concentration of elements other than oxygen is not particularly limited, and examples thereof include a gas atmosphere including CO 2 , N 2 , H 2 O, and other inevitable impurities together with oxygen having the above concentration. For example, the oxidation treatment can be performed in a combustion gas such as cokes oven gas (COG) or liquefied petroleum gas (LPG) in a direct fired furnace (DFF) type annealing furnace or the like under a gas atmosphere in which the concentration of unburned O 2 is controlled.

[0081] The oxidation treatment is preferably performed under the condition of a reached temperature of 650 to 750°C. Examples thereof include heating to a temperature within a range of a reached temperature of 650 to 750°C in an oxidation heating zone in a DFF type annealing furnace. Setting the reached temperature to 750°C or less makes it possible to suppress the reaction between SiO 2 and FeO generated by the oxidation treatment particularly on the surface in the vicinity of the edge in the sheet width direction of the steel sheet, and makes it possible to improve the adhesion between the steel sheet and the plating layer.

[0082] In the present specification, the "reached temperature" at the time of heating in the oxidation treatment means the maximum temperature reached by the rolled sheet under heating control in the oxidation heating zone.

[0083] The reached temperature in the oxidation treatment is more preferably 730°C or less, still more preferably 720°C or less, and still more preferably 700°C or less. In contrast, the temperature of the steel sheet in the oxidation treatment is preferably 650°C or more from the viewpoint of forming the Fe oxide layer in the gas atmosphere described above. The temperature of the steel sheet in the oxidation treatment is more preferably 670°C or more.

[0084] The temperature rise time in the oxidation treatment is not particularly limited, and may be adjusted so as not to form, for example, a fire light layer that adversely affects the plating property by the oxidation treatment due to being excessively long. Specifically, the temperature rise time in the oxidation treatment may be appropriately adjusted in consideration of the conditions of hot rolling (particularly the coiling temperature), the annealing conditions before pickling, the pickling conditions, and the temperature of the steel sheet during heating in the oxidation treatment. For example, the temperature rise time in the oxidation treatment is preferably 10 seconds or more, and more preferably 15 seconds or more. In addition, for example, the temperature rise time in the oxidation treatment is preferably 120 seconds or less, and more preferably 90 seconds or less. Although the case where the oxidation treatment is performed while raising the temperature has been described above, an aspect of the oxidation treatment is not limited thereto, and the oxidation treatment may be performed by raising to the reached temperature and holding at the reached temperature.(C1) Performing reduction treatment

[0085] In the reduction treatment, for example, a reduced Fe layer capable of favorably forming a plating layer is formed on the surface layer of the steel sheet while a decarburized layer is formed. In the manufacturing method according to the present embodiment, the reduction treatment is preferably performed through a first reduction treatment performed under conditions with a dew point of -35 to -15°C, followed by a second reduction treatment performed under conditions with a dew point of -25 to 0°C, which is higher than that of the first reduction treatment. Hereinafter, each of the first reduction treatment and the second reduction treatment will be described.(C1a) First reduction treatment

[0086] In the first reduction treatment, the dew point is preferably in the range of - 35 to -15°C. When the dew point is higher than -15°C, decarburization easily proceeds more than necessary, which may lead to a decrease in strength. In addition, the reduction treatment is less likely to proceed, and a defect called a pickup in which iron oxide generated by the oxidation treatment adheres to a roll and causes a press flaw on a steel plate is likely to occur. Therefore, the dew point is preferably -15°C or less. The dew point is more preferably -20°C or less. In contrast, from the viewpoint of suppressing additional equipment and cost, the dew point in the first reduction treatment is preferably -35°C or more. The dew point is more preferably -30°C or more.

[0087] As the dew point, the dew point in the atmosphere at the central portion in the front stage of the reduction zone in which the first reduction treatment is performed is within the above range. The control of the dew point in the first reduction treatment and the second reduction treatment described later can be performed by, for example, a method in which a water vapor gas is charged and mixed with an atmospheric gas in a furnace, a method in which an atmospheric gas is bubbled and water vapor is mixed, or the like.

[0088] The atmosphere of the first reduction treatment includes an atmosphere that satisfies the dew point described above and contains N 2 , H 2 , CO, H 2 O, O 2 , and other inevitable impurities. In the first reduction treatment, the reached temperature of the steel sheet reaches 850°C in the atmosphere, and then heating is performed in a temperature range of 850 to 910°C, for example, for 60 to 240 seconds. The "holding" includes not only a case where the temperature is constant but also a case where the temperature varies in the temperature range.(C1b) Second reduction treatment

[0089] Then, the dew point of the second reduction treatment will be described. In order to achieve excellent bendability with an R / t of less than 2.0, the dew point in the second reduction treatment is preferably -25°C or more. In contrast, the upper limit of the dew point in the second reduction treatment is preferably 0°C or less from the viewpoint of exhibiting higher strength. The dew point in the second reduction treatment is more preferably -15°C or less.

[0090] The atmosphere of the second reduction treatment includes an atmosphere that satisfies the dew point described above and contains N 2 , H 2 , CO, H 2 O, O 2 , and other inevitable impurities. In the second reduction treatment, in the atmosphere and a temperature range in which the reached temperature of the steel sheet is 850 to 910°C, heating is performed, for example, for 60 to 240 seconds. The "holding" includes not only a case where the temperature is constant but also a case where the temperature varies in the temperature range.

[0091] The first reduction treatment and the second reduction treatment may be classified into atmospheres having different dew points as described above, and the specific aspect thereof is not limited. For example, in addition to providing a reducing furnace for performing each of the first reduction treatment and the second reduction treatment, when the steel sheet is a plated steel sheet, a front region for performing the first reduction treatment and a rear region for performing the second reduction treatment may be separated by installing a partition wall having an opening area ratio of 20% or less, for example, in the middle of the reduction zone of the continuous hot-dip plating line.

[0092] Between the first reduction treatment and the second reduction treatment, for example, a step such as the holding step for holding at least any condition (dew point or the like) of the first reduction treatment may be performed as long as there is no adverse effect on ensuring the decarburized state of the steel sheet according to the present embodiments. Preferably, the second reduction treatment is performed immediately after the first reduction treatment.

[0093] The oxidation treatment and the reduction treatment may be performed using any publicly-known single or plurality of facilities. Preferably, equipment of a continuous galvanizing line (CGL) is used from the viewpoint of manufacturing efficiency, cost, and quality retention. Using a continuous galvanizing line, an oxidation treatment and a reduction treatment by an oxidation-reduction method, and a hot-dip galvanizing treatment and an alloying treatment when a galvanized steel sheet for example is manufactured as a steel sheet can be continuously performed in a series of manufacturing lines. More specifically, the oxidation treatment and the reduction treatment by the oxidation-reduction method may be performed using, for example, an annealing furnace in a DFF type continuous galvanizing line. For example, as described above, the oxidation treatment is performed in a heating zone in a DFF type annealing furnace. In addition, the reduction treatment may be performed, for example, in a soaking zone in a DFF type annealing furnace.(D) First cooling step

[0094] After the step (C), cooling is performed to a first cooling stop temperature of 350°C to 750°C.

[0095] When the first cooling stop temperature is less than 350°C, ferrite and upper bainite cannot be sufficiently transformed (accordingly, the area ratio of tempered martensite may be excessive), and desired workability (high elongation EL or the like) cannot be achieved. Therefore, the first cooling stop temperature is 350°C or more, preferably 360°C or more, and more preferably 370°C or more. In contrast, when the first cooling stop temperature is more than 750°C, transformation of ferrite and upper bainite does not occur, and thus desired workability cannot be achieved. Therefore, the first cooling stop temperature is 750°C or less, preferably 720°C or less, and more preferably 700°C or less.

[0096] The cooling rate from the first heating temperature to the first cooling stop temperature is not particularly limited, and the average cooling rate may be, for example, 1°C / s or more and 50°C / s or less.(E) Dwelling

[0097] After the step (D), dwelling is performed within the temperature range from the first cooling stop temperature to a second cooling start temperature ranging from 350°C to the first cooling stop temperature for 20 to 300 seconds, at an average cooling rate of 10°C / s or less, which is slower than that of the first cooling. The transformation of ferrite and upper bainite can be promoted by the steps (D) and (E).

[0098] When the average cooling rate from the first cooling stop temperature to the second cooling start temperature that is from 350°C to the first cooling stop temperature exceeds 10°C / s or is an average cooling rate equal to or more than that in the step (E), ferrite and upper bainite cannot be sufficiently transformed, and desired workability cannot be achieved. Therefore, the average cooling rate is 10°C / s or less, preferably 8°C / s or less, more preferably 5°C / s or less, and still more preferably 3°C / s or less. When the second cooling start temperature is the first cooling stop temperature, the average cooling rate is 0°C / s.

[0099] When the time in the temperature range from the first cooling stop temperature to the second cooling start temperature is less than 20 seconds, ferrite and upper bainite cannot be sufficiently transformed, and desired workability cannot be achieved. Therefore, the time is set to 20 seconds or more, preferably 25 seconds or more, and more preferably 30 seconds or more. In contrast, when the time exceeds 300 seconds, ferrite and upper bainite are excessively formed, and it is difficult to secure a tensile strength of 980 MPa or more. Therefore, the time is set to 300 seconds or less, preferably 200 seconds or less, more preferably 150 seconds or less, and still more preferably 100 seconds or less.(F) Second cooling

[0100] After the step (E), cooling is performed at an average cooling rate higher than that in the step (E) from the second cooling start temperature to a second cooling stop temperature of 100 to 300°C. As a result, martensite transformation is allowed to proceed, and tempered martensite is formed by performing a second heating step (tempering) described later.

[0101] When the second cooling stop temperature is less than 100°C, martensitic transformation excessively proceeds, a desired amount of retained γ-phase cannot be secured, and workability is deteriorated. Therefore, the second cooling stop temperature is 100°C or more, preferably 120°C or more, and more preferably 140°C or more. In contrast, when the second cooling stop temperature exceeds 300°C, a desired amount of martensite cannot be secured, and the yield stress YS (and the yield ratio YR) decreases. Therefore, the second cooling stop temperature is 300°C or less, preferably 250°C or less, and more preferably 200°C or less.

[0102] The cooling rate from the second cooling start temperature to the second cooling stop temperature is set to an average cooling rate faster than that in the step (E) from the viewpoint of productivity. The average cooling rate may be, for example, 1°C / s or more and 50°C / s or less.

[0103] In this step, the cooling is stopped and then holding may be performed for a predetermined time, but the second heating step to be described later may be performed without holding. When the holding time at the second cooling stop temperature is long, the characteristics are hardly affected, but from the viewpoint of productivity, for example, the holding time is preferably 600 seconds or less.(G) Second heating

[0104] After the step (F), heating is performed to a second heating temperature of 300 to 500°C. This step and the holding step (H) described later cause, in addition to the martensite being tempered, a part of the untransformed austenite to be transformed into bainite. As a result, C is concentrated in untransformed austenite to lead to a stabilization and retention as retained γ-phase.

[0105] When the second heating temperature is less than 300°C, the bainite formed with tempered martensite becomes excessively hard, and the workability is deteriorated. Therefore, the second heating temperature is 300°C or more, preferably 320°C or more, and more preferably 340°C or more. In contrast, when the second heating temperature exceeds 500°C, the bainite formed with tempered martensite becomes excessively soft, and the tensile strength TS decreases. Therefore, the second heating temperature is 500°C or less, preferably 480°C or less, and more preferably 450°C or less.

[0106] The heating rate from the second cooling stop temperature of 300°C or less to the second heating temperature is not particularly limited, and the average heating rate may be, for example, 1°C / s or more and 100°C / s or less.(H) Providing steel sheet by holding at second heating temperature for 300 to 1800 seconds

[0107] After the step (G), the steel sheet included in the plated steel sheet according to the present embodiments is obtained by holding at the second heating temperature for 300 to 1800 seconds. When the holding time is less than 300 seconds, the bainite transformation does not sufficiently proceed, the concentration of C to untransformed γ-phase becomes insufficient, and the amount of retained γ-phase decreases. Therefore, the holding time is set to 300 seconds or more, preferably 350 seconds or more, and more preferably 400 seconds or more. In contrast, when the holding time is more than 1800 seconds, the mechanical properties are hardly affected, but productivity decreases. Therefore, the holding time is set to 1800 seconds or less, preferably 1200 seconds or less, and more preferably 600 seconds or less.(I) Forming plating layer on the steel sheet

[0108] The above-described plating layer is formed on the surface of the steel sheet. The conditions for forming the plating layer are not particularly limited, and a conventional plating treatment can be employed. For example, in the case of forming a hot-dip galvanizing layer, for hot-dip galvanizing, for example, the above steel sheet is immersed in a hot-dip galvanizing bath at 300°C or more and 550°C or less to perform hot-dip galvanizing treatment. The plating time may be appropriately adjusted such that a desired plating deposition amount can be secured, and is preferably, for example, 1 to 10 seconds.

[0109] For alloying hot-dip galvanizing, the alloying treatment may be performed after the above hot-dip galvanizing. The alloying treatment temperature is not particularly limited, but is preferably 450°C or more, more preferably 460°C or more, and still more preferably 480°C or more because alloying does not sufficiently proceed when the alloying treatment temperature is too low. However, when the alloying treatment temperature is too high, alloying excessively proceeds, the concentration of Fe in the plating layer becomes excessive, and plating adhesion is deteriorated. From such a viewpoint, the alloying treatment temperature is preferably 550°C or less, more preferably 540°C or less, and still more preferably 530°C or less. The alloying treatment time is not particularly limited, and may be adjusted such that hot-dip galvanizing is alloyed. The alloying treatment time is, for example, 10 to 60 seconds.

[0110] The method for manufacturing a plated steel sheet according to the embodiments of the present invention may include other steps without departing from the object of the present disclosure.EXAMPLE

[0111] Hereinafter, embodiments of the present invention will be described more specifically with reference to Examples. The embodiments of the present invention are not limited by the following examples, and can be implemented with appropriate modifications within the scope that can be consistent with the above-described and later-described gist, and all of them are included in the scope of the embodiments of the present invention.

[0112] A steel having a composition shown in Table 1 was cast, cooled to room temperature after casting, and then heated to 1200 to 1350°C. Thereafter, hot rolling including rough rolling at a temperature of 1100°C or more and finish rolling at a temperature of 920°C or more was performed, cooling after finish rolling was performed, and coiling was performed at a temperature of 600°C or more to provide a rolled sheet. This rolled sheet was subjected to pickling to remove scale on the surface, and then subjected to cold rolling with a rolling ratio of 40 to 50% to provide a sheet thickness of 1.0 to 1.6 mm. [Table 1]Test No.Composition of steel sheet [mass%] * balance: iron and inevitable impurities (other than P, S and N)CSiMnAlTiPSN10.2131.822.210.4190.0340.0050.00100.002320.2071.792.190.4470.0280.0060.00050.002430.2071.792.190.4470.0280.0060.00050.002440.2111.852.210.4760.0350.0060.00050.002150.2111.852.210.4760.0350.0060.00050.002160.2131.822.210.4190.0340.0050.00100.002370.2111.852.210.4760.0350.0060.00050.002180.3032.002.000.0400.0250.0100.00150.004090.2030.792.000.5950.0000.0070.00080.0037100.2111.592.070.3800.0520.0100.00100.0039

[0113] The rolled sheet was subjected to the above-described step (B1) and steps (B) to (I) under the conditions shown in Table 2 to provide a plated steel sheets of Test Nos. 1 to 10 (for Test Nos. 8 to 10, as will be described later, the steel sheet is a steel sheet on which plating is not formed, and only the temperature history of the step (I) of forming a plating layer is provided). In Table 2, "-" indicates unmeasured. Although not shown in Table 2, in Test Nos. 1 to 10, the step (B 1) was performed during the temperature rise to the first heating temperature in the step (B), the oxygen concentration at each reached temperature was 0.1 to 2%, and the temperature rise time in the oxidation treatment was 10 seconds or more and 120 seconds or less. In addition, the heating time (holding time) of each of the first reduction treatment (step (C1a)) and the second reduction treatment (step (C1b)) was 60 seconds or more. In addition, in the step (E), holding was performed at the first cooling stop temperature (that is, the first cooling stop temperature and the second cooling start temperature were equivalent), the "time in the temperature range from the first cooling stop temperature to the second cooling start temperature" corresponds to the holding time at the first cooling stop temperature, and the average cooling rate from the first cooling stop temperature to the second cooling start temperature was 0°C / s. In the step (F), cooling to the second cooling stop temperature was performed at an average cooling rate faster than that in the step (E). In addition, in the step (I), the steel sheet was immersed in a hot-dip galvanizing bath at 300°C or more and 550°C or less to be subjected to a hot-dip galvanizing treatment (plating time: 1 to 10 seconds), and then to an alloying treatment, and the alloying treatment time was 10 to 60 seconds. In addition, in Test Nos. 8 to 10, only the temperature history was provided to the step (I). Specifically, the steel sheet was heated to 300°C or more and 550°C or less, held for 1 to 10 seconds, and then held at the "alloying treatment temperature" shown in Table 2 for 10 to 60 seconds. [Table 2]Test No.Step (B1)Step (B)Step (C)Step (D)Step (E)Step (F)Step (G)Step (H)Step (I)Reached temperature (°C)First heating temperature (°C)Step (C1a)Step (C1b)Holding time (sec)First cooling stop temperature (°C)Average cooling rate from first heating temperature to first cooling stop temperature (°C / s)Time in temperature range from first cooling stop temperature to second cooling start temperature (sec)Second cooling stop temperature (°C)Second heating temperature (°C)Holding time (sec)Alloying treatment temperature (°C)Dew point (°C)Dew point (°C)1701875-25.4-8.120844817381773954314922697871-20.2-9.214739927271623933874903713881-24.1-8.714742226271794023054904692850-21.0-11.416245221301564043364875701847-22.7-12.016245720301563973364846645913-22.7-13.122544718421903904674937668841-23.8-55.016250118301574063364838750930--16143050232004003855009900900--5920050--4204050010930930--1257001150350400300480

[0114] The plated steel sheets of Test Nos. 1 to 10 described above were evaluated as follows.<Evaluation of metal microstructure>

[0115] For the plated steel sheets of Test Nos. 1 to 10, a test piece was taken so as to include a position of W (plate width) / 4 from the end toward the center in the sheet width direction (the direction perpendicular to the rolling direction and the sheet thickness direction), and from the test piece, a section at a position that is parallel to the rolling direction and the sheet width direction and is 1 / 4 of the sheet thickness from the surface of the steel sheet included in the plated steel sheet was exposed by Nital-etching. The section was observed by SEM, and the sum of the area ratio of ferrite and the area ratio of upper bainite, the area ratio of tempered martensite, and the area ratio of MA were determined by the above-described method. In addition, an X-ray diffraction pattern was acquired from the section using a Co-Kα ray as a characteristic X-ray, and the volume ratio of the retained γ-phase was determined by the above-described method. Regarding the balance of metal microstructure, microstructures other than pearlite and cementite were not observed, and the balance of metal microstructure accounted for 10 area% or less in total.<Means for measuring decarburization behavior: measurement of carbon profile by GD-OES>

[0116] As described below, the carbon profile was measured by glow discharge optical emission spectrometry (GD-OES), and the decarburization behavior was examined.(Preparing sample)

[0117] A material having a size of 50 mm × 40 mm × sheet thickness or 30 mm × 30 mm × sheet thickness was taken. Thereafter, degreasing was performed according to a conventional method to prepare a sample. Then, using the sample, the concentration of mass% of each element was measured by GD-OES under the following conditions.(Measurement conditions)

[0118] Device used: Markus high-frequency glow discharge emission surface analyzer (rf-GD-OES) GD-Profiler2 manufactured by HORIBA, Ltd. Sputtering method: normal sputtering Measurement range: φ4 mm Gas type: Ar Element to be analyzed: B, C, O, Al, Si, Ti, Cr, Mn, Fe, Zn, P, S, and N (in this example, these elements were evaluated, but when elements other than the above elements are contained in, for example, a plating layer and / or a steel sheet, elements other than the above elements are also to be analyzed) (Measurement method)

[0119] The surface of the sample on which the plating was formed was subjected to GD-OES measurement until the depth reached 150 µm in the sheet thickness direction.(Analysis method)

[0120] The sputtering rate of the above device was substantially constant, and thus the sputter crater depth of the sample after the analysis was measured, and the horizontal axis was taken as the value (sputtering depth).

[0121] Details of the calibration curve method for converting the measured emission intensity of each element into a concentration will be described below.

[0122] The relationship between the sputtering weight W i (g / s) per unit time of the element i and the emission intensity I i is represented by the following formula (3) using the slope a and the intercept b of the calibration curve. W i = aI i + b

[0123] The sputtering weight W i per unit time of the element i is determined by the following formula (4) using the sputtering area S (cm 2< ) in the reference sample in which the concentration C i (wt%), the density ρ (g / cm 3< ), and the sputtering rate Δd (cm / s) are known. W i = C i × ρ × Δd × S

[0124] The emission intensity I i was measured using two or more types of reference samples in which W i was known, and the slope a and the intercept b of the above formula (3) were obtained to prepare a calibration curve in which the horizontal axis was the emission intensity and the vertical axis was the sputtering weight. The reference materials used are shown in Table 3 below. Using the prepared calibration curve, the sputtering weight was determined from the emission intensity of each target element, and the weight ratio was converted into the concentration. The calibration curve used for the conversion of the O concentration was corrected using SiO 2 such that the concentration ratio between Si and O was 1:2. [Table 3]Reference sampleMain elementBCOAlSiTiCrMnFeZnBAS 113Fe0.00660.837-0.01510.9310.0391.2481.20794.99-BAS 114Fe0.00080.403-0.0780.2950.00960.1870.41696.47-MBH 13X NSD1Fe-0.046-0.0130.411-24.5123.5349.09-MBH 13X 8110LFe(1.09)0.792-0.0090.960.05512.330.77276.08-MBH 31X BIB3Cu---0.02980.061--0.2430.09932.46SPEX 185-CO2Cu--------0.0960.15JAPAN FINE CERAMICS CO., LTD. Al2O3O--47.07(52.93)------*Numerical value indicated in parentheses in the table was excluded from reference value used for concentration conversion.

[0125] Then, a carbon profile was obtained using the analysis result on carbon. An example thereof is illustrated in Fig. 1. Fig. 1 shows the carbon profiles of Test No. 5 and Test No. 7.

[0126] In Fig. 1, it is found that the carbon profile of any material has a peak of the carbon concentration within 20 µm from the surface layer (specifically, in the internal oxide layer), but the carbon profile of Test No. 7 is higher than the bulk carbon concentration, whereas the carbon profile of Test No. 5 is sufficiently lower than the bulk carbon concentration.

[0127] From the obtained carbon concentration profile in the sheet thickness direction, the position where the carbon concentration (mass%) was 50% of the bulk carbon concentration, and the maximum value of the carbon concentration (mass%) in the region from the surface of the steel sheet to 20 µm were determined. The bulk carbon concentration was used for analysis by correcting the carbon concentration at a sufficiently deep position (120 to 150 µm) measured by GD-OES to a value obtained by ordinary steel analysis. For example, when the iron / steel analysis value was 0.22% and the analysis value by GD-OES was 0.25%, the analysis value by GD-OES was used for analysis as a 0.22 / 0.25-fold value.<Evaluation of tensile strength, yield stress, yield ratio, elongation, and variation of tensile strength in sheet width direction>

[0128] A JIS No. 5 test piece (sheet-shaped test piece) was taken such that a sheet width direction on a plane parallel to a rolled surface of a steel sheet during cold rolling was a longitudinal direction of the test piece. When the sheet width is W with respect to the sampling position, test pieces were sampled from the five positions, that is, two positions that were positions of W / 8 from both ends toward the center, two positions that are positions of W / 4 from both ends toward the center, and one position at the center in the sheet width direction. A tensile test was performed in JIS Z 2241: 2011 using the test piece, and the tensile strength TS, the yield stress YS, and the elongation EL were measured. The yield ratio YR was determined as YS / TS. The example in which the minimum tensile strength TS min at the five points was 980 MPa or more was evaluated as sufficient, and the example in which the minimum tensile strength TS min was less than 980 MPa was evaluated as insufficient. An example in which the yield ratio YS was 0.55 to 0.75 was evaluated as sufficient, and an example in which the yield ratio YS was out of the range of 0.55 to 0.75 was evaluated as insufficient. An example in which the elongation EL was 19% or more was evaluated as sufficient, and an example in which the elongation EL was less than 19% was evaluated as insufficient. For each physical property value other than the tensile strength, a value was adopted, measured using a test piece taken from one position that was a position of W / 4 from both ends toward the center in the sheet width direction.<Evaluation of hole expansion ratio>

[0129] For Test Nos. 1 to 10, a hole expansion test defined in JIS Z 2256: 2010 was performed to measure the hole expansion ratio λ. Then, an example in which the hole expansion ratio λ was 20% or more was evaluated as sufficient, and an example in which the hole expansion ratio λ was less than 20% was evaluated as insufficient.<Evaluation of limiting bendability (R / t)>

[0130] The bendability of Test Nos. 1 to 10 was evaluated by the following procedure. A long axis was taken in a sheet width direction to prepare a test piece having a width of 40 mm × a length of 100 mm, a bending test was performed by a V block method in accordance with JIS Z 2248: 2014, the bending radius in this case was variously changed to 0 to 7 mm, the minimum bending radius at which the material could be bent without being broken was determined, and this was used as the limiting bending radius R (mm) to calculate the limiting bending radius R (mm) / sheet thickness t (mm). Then, an example in which the limiting bending radius R (mm) / sheet thickness t (mm) was 2.0 or less was evaluated as excellent in bendability, and an example in which the limiting bending radius R (mm) / sheet thickness t (mm) was more than 2.0 was evaluated as poor in bendability.

[0131] The results are shown in Table 4. In Table 4, "-" indicates unmeasured. In addition, in Table 4, "F + UB" is the total area ratio of ferrite and upper bainite, "M" is the area ratio of tempered martensite, "MA" is the area ratio of MA, "retained γ-phase" is the volume ratio of retained austenite, and "maximum value of surface layer carbon concentration" is the maximum value of carbon concentration (mass%) in a region from the surface of the steel sheet to 20 µm in the sheet thickness direction. [Table 4]Test No.Metal microstructure of steel sheetCarbon concentration of steel sheetStrength of plated steel sheetYield ratio of plated steel sheetWorkability of plated steel sheetVariation in tensile strength of plated steel sheet in sheet width directionF + UB (area%)M (area%)MA (area%)Retained γ-phase (vol%)Bulk carbon concentration (mass%)Position at 50% of bulk carbon concentration from sheet surface / sheet thickness (%)Maximum value of surface layer carbon concentration / bulk carbon concentration (%)Tensile strength TS (MPa)Yield Stress YS (MPa)Yield ratio YR (-)Elongation EL (%)Hole expansion ratio λ (%)Limit bendability R / t (-)TS max -TS min (MPa)1523711110.2132.63210096750.67021.430.0≦2.0112454510110.2073.8399856100.61922.631.0≦2.073484111110.2073.5369886670.67521.027.0≦2.0184582814120.2113.43910206240.61221.030.7≦2.0185493615120.2113.53510086220.61721.631.7≦2.0346246610110.2132.13910047810.77920.838.0≦2.0107474112110.2110.00111910436210.59619.539.3>2.02087921-0.303--122510010.81715.050.0--96940-0.203--10239050.88510.374.0--1054739-0.211--11245430.48316.98.0--

[0132] From the results of Table 4, it can be considered as follows. Test Nos. 1 to 4 in Table 4 satisfied the requirements defined in the embodiments of the present invention, exhibited high strength, a predetermined yield ratio, and high workability, and satisfied the formula (1). In addition, Test Nos. 1 to 4 satisfied the requirement for exhibiting excellent bendability (satisfying at least one of (I) and (II) described above), and were excellent in bendability.

[0133] In contrast, Test Nos. 5 to 7 in Table 4 did not satisfy the requirements defined in the embodiments of the present invention, and were insufficient in the evaluation of the strength, the yield ratio, and the variation in the workability and / or tensile strength in sheet width direction.

[0134] In Test No. 5, the first heating temperature in the step (B) was less than 850°C, and thus the variation in tensile strength in the sheet width direction was large.

[0135] Test No. 5 satisfied the requirement for exhibiting excellent bendability (satisfying at least one of (I) and (II) described above), and was excellent in bendability.

[0136] In Test No. 6, the first heating temperature was more than 910°C, and thus the total of ferrite and upper bainite accounted for less than 30 area%, the tempered martensite accounted for more than 45 area%, and the yield ratio YR was more than 0.75.

[0137] Test No. 6 satisfied the requirement for exhibiting excellent bendability (satisfying at least one of (I) and (II) described above), and was excellent in bendability.

[0138] In Test No. 7, the first heating temperature in the step (B) was less than 850°C, and thus the variation in tensile strength in the sheet width direction was large. In addition, the dew point in the step (C1b) was less than -25°C and less than the dew point in the step (C1a), and the requirement for exhibiting excellent bendability (satisfying at least one of the above (I) and (II)) was not satisfied, and thus the bendability was poor.

[0139] In Test No. 8, the C content was more than 0.300 mass%, the Al content was less than 0.300 mass%, and the first heating temperature in the step (B) was more than 910°C, and thus the total of ferrite and upper bainite accounted for less than 30 area%, the tempered martensite accounted for more than 45 area%, and the MA accounted for less than 5 area%, the yield ratio YR was more than 0.75, and the elongation EL was less than 19%.

[0140] In Test No. 9, the Ti content was less than 0.010 mass%, the first cooling stop temperature in the step (D) was less than 350°C, and the holding time in the second heating in the step (H) was less than 300 seconds, and thus the total of ferrite and upper bainite accounted for less than 30 area%, the tempered martensite accounted for more than 45 area%, and the MA accounted for less than 5 area%, the yield ratio YR was more than 0.75, and the elongation EL was less than 19%.

[0141] In Test No. 10, the first heating temperature in the step (B) was more than 910°C and the second cooling stop temperature in the step (F) was more than 300°C, and thus the tempered martensite accounted for less than 10 area%, the MA accounted for more than 35 area%, the yield ratio YR was less than 0.55, the elongation EL was less than 19%, and the hole expansion ratio λ was less than 20%.

[0142] This application claims priority based on Japanese Patent Application No. 2023-056191, filed on March 30, 2023, and Japanese Patent Application No. 2024-022772, filed on February 19, 2024. Japanese Patent Application Nos. 2023-056191 and 2024-022772 are incorporated herein by reference.

Claims

1. A plated steel sheet comprising a steel sheet and a plating layer disposed on a surface of the steel sheet and satisfying following formulas (1) and (2), wherein the steel sheet included in the plated steel sheet has a composition comprising: C: 0.150 to 0.300 mass%; Si: 0.80 to 2.20 mass%; Mn: 1.60 to 2.80 mass%; Al: 0.300 to 0.800 mass%; Ti: 0.010 to 0.050 mass%; P: 0.050 mass% or less (including 0 mass%); S: 0.0100 mass% or less (including 0 mass%); N: 0.0100 mass% or less (including 0 mass%); and a balance: iron and inevitable impurities, and wherein the steel sheet included in the plated steel sheet has a metal microstructure in which: a total of ferrite and upper bainite accounts for 30 to 70 area%, tempered martensite accounts for 10 to 45 area%, MA accounts for 5 to 35 area% and retained austenite accounts for 5 to 20 vol%. TS max − TS min < 20 MPa TS min ≥ 980 MPa wherein TSmax is a maximum tensile strength measured at five different positions in a sheet width direction of the plated steel sheet, and TSmin is a minimum tensile strength measured at the five positions, and when a sheet width is W, the five positions include two positions of W / 10 to W / 8 from both ends toward a center in the sheet width direction, two positions of W / 4 from the both ends toward the center in the sheet width direction, and one position of the center in the sheet width direction.

2. The plated steel sheet according to claim 1, further comprising at least one selected from the group consisting of Nb: more than 0 mass% and 0.20 mass% or less, and V: more than 0 mass% and 0.50 mass% or less.

3. The plated steel sheet according to claim 1, further comprising at least one selected from the group consisting of Ni: more than 0 mass% and 2.0 mass% or less, Cr: more than 0 mass% and 2.0 mass% or less, and Mo: more than 0 mass% and 0.50 mass% or less.

4. The plated steel sheet according to claim 1, further comprising B: more than 0 mass% and 0.0050 mass% or less.

5. The plated steel sheet according to claim 1, further comprising at least one selected from the group consisting of Mg: more than 0 mass% and 0.040 mass% or less, REM: more than 0 mass% and 0.040 mass% or less, and Ca: more than 0 mass% and 0.040 mass% or less.

6. The plated steel sheet according to claim 1, wherein in a sheet thickness direction, a position where a carbon concentration (mass%) is 50% of a bulk carbon concentration is in a region of 0.2% or more of a sheet thickness from a surface of the steel sheet.

7. The plated steel sheet according to claim 1, wherein a maximum value of a carbon concentration (mass%) in a region from a surface of the steel sheet to 20 µm in a sheet thickness direction is less than 70% of a bulk carbon concentration.

8. A method for manufacturing the plated steel sheet according to any one of claims 1 to 7, the method comprising: providing a rolled sheet by hot rolling a steel having the composition according to any one of claims 1 to 5; first heating the rolled sheet to a first heating temperature of 850°C or more and 910°C or less; holding at the first heating temperature for 5 to 1800 seconds after the first heating; first cooling to a first cooling stop temperature of 350°C to 750°C after the holding; dwelling within the temperature range from the first cooling stop temperature to a second cooling start temperature ranging from 350°C to the first cooling stop temperature for 20 to 300 seconds after the first cooling, at an average cooling rate of 10°C / s or less, which is slower than that of the first cooling; second cooling from the second cooling start temperature to a second cooling stop temperature of 100 to 300°C after the dwelling at an average cooling rate higher than that in the dwelling; second heating to a second heating temperature of 300 to 500°C after the second cooling; providing a steel sheet by holding at the second heating temperature for 300 to 1800 seconds after the second heating; and forming a plating layer on the steel sheet.

9. The manufacturing method according to claim 8, the method comprising: performing an oxidation treatment under conditions of an oxygen concentration of 0.1 to 2% and a reached temperature of 650 to 750°C after the providing the rolled sheet, wherein the holding includes performing a reduction treatment that comprises a first reduction treatment performed under conditions with a dew point of -35 to -15°C, followed by a second reduction treatment performed under conditions with a dew point of -25 to 0°C, which is higher than that of the first reduction treatment.

Images