Plated steel sheet and method for manufacturing same

Abstract

A plated steel sheet including a steel sheet and a plating layer disposed on a surface of the steel sheet, the steel sheet having a tensile strength of 1150 MPa or more, wherein the steel sheet included in the plated steel sheet has a predetermined composition and a metal microstructure in which a total of bainite and tempered martensite accounts for 95 area% or less, retained austenite accounts for 5 vol% or more, a total of polygonal ferrite and bainitic ferrite accounts for 5 area% or less, and a balance of the metal microstructure accounts for 5 area% or less.

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 high strength (for example, tensile strength of 1150 MPa or more) in order to achieve weight reduction of the vehicle body. Further, the plated steel sheet is required to have high workability (for example, elongation EL is high, hole expansion ratio λ is high) because it is necessary to form a component shape by working the plated steel sheet.

[0004] Patent Documents 1 and 2 disclose plated steel sheets that contain C, Si and Mn as essential elements and have a predetermined metal microstructure, thereby exhibiting high ductility and high strength.

[0005] Patent Document 3 discloses a plated steel sheet that contains C, Si, Mn and Al, as well as at least one selected from V, Mo, Ti and Nb, as essential elements, and has a predetermined metal microstructure, thereby exhibiting high ductility and high workability (high λ or the like).CONVENTIONAL ART DOCUMENTPATENT DOCUMENT

[0006] Patent Document 1: WO 2020 / 080402 A Patent Document 2: WO 2020 / 080401 A Patent Document 3: WO 2019 / 159771 A SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0007] In Patent Documents 1 to 3, in order to obtain desired performance, a manufacturing method is strictly controlled, especially the reheating and subsequent cooling after hot rolling (and after cold rolling).

[0008] For example, in Patent Document 1, it is necessary to control the cooling rate or hold the temperature within at least six temperature ranges (810 to 700°C, 700 to 490°C, 490 to 405°C, 405 to 310°C, 315 to 255°C, and 255°C to cooling stop temperature (254 to 220°C)).

[0009] In Patent Document 2, it is necessary to control the cooling rate or hold the temperature within at least five temperature ranges (810 to 650°C, 650 to 505°C, 505 to 405°C, 405 to 315°C, and 315 to cooling stop temperature (310 to 255°C)).

[0010] In Patent Document 3, it is necessary to control the cooling rate or hold the temperature within at least four temperature ranges (830 to 550°C or less, Ac1 + 60°C to 550°C, 550 to 400°C, and 375 to 150°C).

[0011] As described above, in the plated steel sheet as disclosed in Patent Documents 1 to 3, a manufacturing method for providing high strength and high workability is complicated, which may fail to perform stable production. In addition, there is also room for improvement in strength and workability.

[0012] The present disclosure has been made in view of such a circumstance, and an object thereof is to provide a plated steel sheet that can be more easily manufactured, has high strength, can contribute to weight reduction of a vehicle body, and exhibits high workability, and a method for manufacturing the plated steel sheet.MEANS FOR SOLVING THE PROBLEMS

[0013] 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 having a tensile strength of 1150 MPa or more, wherein the steel sheet included in the plated steel sheet has a composition including: C: 0.150 to 0.250 mass%; Si: 0.80 to 2.20 mass%; Mn: 1.50 to 2.80 mass%; Cr: 0.15 to 1.50 mass%; Ti: 0.012 to 0.100 mass%; B: 0.0015 to 0.0100 mass%; P: 0.100 mass% or less (including 0 mass%); S: 0.050 mass% or less (including 0 mass%); Al: 0.005 to 1.000 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 bainite and tempered martensite account for 95 area% or less; retained austenite accounts for 5 vol% or more; a total of polygonal ferrite and bainitic ferrite accounts for 5 area% or less; and a balance of the metal microstructure accounts for 5 area% or less.

[0014] A second aspect of the present invention is the plated steel sheet according to the first aspect, 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.

[0015] A third aspect of the present invention is the plated steel sheet according to the first or second aspect, 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.

[0016] A fourth aspect of the present invention is the plated steel sheet according to any one of the first to third aspects, wherein the retained austenite includes: 10 to 50 vol% of first retained austenite having a solid solution C content of 0.9 mass% or less; and 10 to 50 vol% of second retained austenite having a solid solution C content of more than 0.9 mass% and 1.1 mass% or less.

[0017] A fifth aspect of the present invention is the method for manufacturing a plated steel sheet according to any one of the first to fourth aspects, the method including: providing a rolled sheet by hot rolling a steel having the composition according to the first aspect; heating the rolled sheet to a first heating temperature of 850°C or more; holding at the first heating temperature for 50 seconds or more after the heating; cooling to a cooling stop temperature of 200 to 350°C after the holding, wherein an average cooling rate from the first heating temperature to 550°C is 5°C / s or more, and an average cooling rate from 550°C to the cooling stop temperature is 1°C / s or more; heating to a second heating temperature of 300°C to 450°C and holding for 50 to 1000 seconds after the cooling to provide a steel sheet; and forming a plating layer on the steel sheet.

[0018] A sixth aspect of the present invention is the manufacturing method according to the fifth 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 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.

[0019] A seventh aspect of the present invention is the manufacturing method according to the fifth or sixth aspects, wherein an average cooling rate from 400°C to the cooling stop temperature in the cooling is 40°C / sec or less.TECHNICAL EFFECTS OF THE INVENTION

[0020] Embodiments of the present invention can provide a plated steel sheet that can be more easily manufactured, has high strength, can contribute to weight reduction of a vehicle body, and exhibits high workability, and a method for manufacturing the plated steel sheet.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] [Fig. 1] Fig. 1 is a graph showing a relationship between a dew point at a center of a soaking zone and R / t during a second reduction treatment. [Fig. 2] Fig. 2 is a schematic view of an X-ray diffraction pattern. [Fig. 3] Fig. 3 is a view showing an example of a carbon concentration profile obtained in Examples. [Fig. 4] Fig. 4 is a graph showing a relationship between a position at which a carbon concentration (mass%) in a sheet thickness direction is 50% of a bulk carbon concentration, and R / t. [Fig. 5] Fig. 5 is a graph showing a relationship between a ratio of a maximum value of a carbon concentration (mass%) in a region from a surface of a steel sheet to 20 µm to a bulk carbon concentration, and R / t. MODE FOR CARRYING OUT THE INVENTION

[0022] The present inventors have studied from various angles in order to achieve a plated steel sheet that can be more easily manufactured and exhibits high strength (the tensile strength TS is 1150 MPa or more, and the yield stress YS is 850 MPa or more) and high workability (the extension EL is 14% or more, and the hole expansion ratio λ is 30% or more). As a result, it has been possible to achieve a plated steel sheet exhibiting high strength and high workability by containing a predetermined composition, particularly Cr, Ti and B in a predetermined amount as essential elements, and further using a steel sheet having a predetermined metal microstructure. In addition, the present inventors have found that the plated steel sheet can be manufactured by an easier manufacturing method in which in cooling after hot rolling (cold rolling) and reheating, the cooling rate may be controlled in temperatures of two ranges (first heating temperature to 550°C, 550°C to cooling stop temperature) smaller than those in the conventional art.

[0023] Hereinafter, details of each requirement defined by the embodiments of the present invention will be described.<1. Plated steel sheet>

[0024] 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.

[0025] 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>

[0026] 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 0.250 mass%, Si: 0.80 to 2.20 mass%, Mn: 1.50 to 2.80 mass%, Cr: 0.15 to 1.50 mass%, Ti: 0.012 to 0.100 mass%, B: 0.0015 to 0.0100 mass%, P: 0.100 mass% or less (including 0 mass%), S: 0.050 mass% or less (including 0 mass%), Al: 0.005 to 1.000 mass%, and N: 0.0100 mass% or less (including 0 mass%), and the balance is iron and inevitable impurities.

[0027] Hereinafter, each element will be described in detail.(C: 0.150 to 0.250 mass%)

[0028] C is an element important for improving the strength of steel. In addition, C is an important element for stabilizing austenite and securing retained γ-phase. Further, C also has an effect of suppressing generation of polygonal ferrite during cooling from a high temperature. In order to exhibit these effects, the C content is 0.150 mass% or more, preferably 0.170 mass% or more, and more preferably 0.190 mass% or more. In contrast, when C is excessively contained, weldability is deteriorated, and thus, the C content is set to 0.250 mass% or less, preferably 0.240 mass% or less, and more preferably 0.230 mass% or less.(Si: 0.80 to 2.20 mass%)

[0029] Si is an element that contributes to high strength of steel as a solid-solution strengthening element. Si also has an effect of improving the tempering softening resistance and suppressing softening of martensite during tempering. In addition, Si is an important element for exerting an effect of suppressing the formation of carbide, condensing and stabilizing C in austenite, and securing the retained γ-phase. In order to exhibit these effects, the Si content is 0.80 mass% or more, preferably 1.00 mass% or more, and more preferably 1.20 mass% or more. In contrast, when Si is excessively contained, a large amount of scale is formed at the time of hot rolling, and the surface state is deteriorated such that scale marks are formed on the surface of the steel sheet. Therefore, the Si content is 2.20 mass% or less, preferably 2.00 mass% or less, and more preferably 1.80 mass% or less.(Mn: 1.50 to 2.80 mass%)

[0030] Mn is an important element that not only increases the strength of the steel but also directly contributes to the stabilization of austenite. In addition, Mn is an element that is also an element for improving hardenability and exhibits an effect of suppressing the generation of polygonal ferrite. In order to exhibit these effects, the Mn content is 1.50 mass% or more, preferably 1.70 mass% or more, and more preferably 1.90 mass% or more. In contrast, when Mn is excessively contained, an adverse effect such as generation of slab cracking is caused, and therefore the Mn content is set to 2.80 mass% or less, preferably 2.70 mass% or less, more preferably 2.60 mass% or less, and still more preferably 2.20 mass% or less.(Cr: 0.15 to 1.50 mass%)

[0031] Cr is an element that suppresses the generation of polygonal ferrite and bainitic ferrite during cooling from a high temperature. Further, Cr is one of the important elements because of improving the tempering softening resistance and also has the effect of suppressing the softening of martensite during tempering and the retained γ-phase decomposition during the alloying treatment. In order to exhibit the above-described effects, the Cr content is 0.15 mass% or more, preferably 0.17 mass% or more, and more preferably 0.20 mass% or more. In contrast, when Cr is excessively contained, not only the effects are saturated, but also formability is deteriorated, and thus the Cr content is set to 1.50 mass% or less, preferably 1.20 mass% or less, and more preferably 1.00 mass% or less.(Ti: 0.012 to 0.100 mass%)

[0032] Ti contributes to the improvement of the strength and toughness of the steel sheet by refining the metal microstructure. Further, Ti is one of the important elements that has an effect of preventing the formation of BN by being associated with the solid solution N. In order to exhibit the above-described effects, the Ti content is 0.012 mass% or more, preferably 0.014 mass% or more, and more preferably 0.016 mass% or more. In contrast, when Ti is excessively contained, not only the above effects are saturated, but also the yield ratio is increased and the shape fixability is deteriorated, and thus the Ti content is set to 0.100 mass% or less, preferably 0.080 mass% or less, and more preferably 0.060 mass% or less.(B: 0.0015 to 0.0100 mass%)

[0033] B is one of the important elements that suppress the formation of polygonal ferrite and bainitic ferrite during cooling from high temperatures. In order to exhibit the above-described effects, the B content is set to 0.0015 mass% or more, preferably 0.0017 mass% or more, and more preferably 0.0020 mass% or more. In contrast, when B is excessively contained, not only the effects are saturated, but also formability is deteriorated, and thus the B content is set to 0.0100 mass% or less, preferably 0.0080 mass% or less, and more preferably 0.0060 mass% or less.(P: 0.100 mass% or less (including 0 mass%))

[0034] P is an element inevitably present as an impurity element. P is an element that promotes grain boundary embrittlement due to grain boundary segregation and deteriorates formability. Therefore, it is better to reduce P, and the P content is preferably 0.100 mass% or less, preferably 0.080 mass% or less, and more preferably 0.050 mass% or less.

[0035] 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: 0.050 mass% or less (including 0 mass%))

[0036] S is an element inevitably present as an impurity element. S is an element that forms sulfide-based inclusions such as MnS, and this serves as a starting point of cracking and deteriorates formability. Therefore, it is better to reduce S, and the S content is 0.050 mass% or less, preferably 0.030 mass% or less, and more preferably 0.020 mass% or less.(Al: 0.005 to 1.000 mass%)

[0037] Al is an element that acts as a deoxidizing material, and in order to exert such an effect, the Al content is set to 0.005 mass% or more, preferably 0.010 mass% or more, more preferably 0.020 mass% or more, and still more preferably 0.030 mass% or more. In contrast, when Al is excessively contained, inclusions such as alumina are generated in a large amount in the steel sheet, and formability is deteriorated, and thus, the Al content is set to 1.000 mass% or less, preferably 0.800 mass% or less, and more preferably 0.500 mass% or less.(N: 0.0100 mass% or less (including 0 mass%))

[0038] N is an element inevitably present as an impurity element. N is an element that forms a nitride, and this nitride causes a starting point of cracking to deteriorate formability. Therefore, it is better to reduce N, and the N content is 0.0100 mass% or less, preferably 0.0080 mass% or less, and more preferably 0.0060 mass% or less.

[0039] 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 balance indicate a concept excluding elements with composition range separately defined.<3. Metal microstructure of steel sheet included in plated steel sheet>

[0040] In the metal microstructure of the steel sheet included in the plated steel sheet according to embodiments of the present invention, the total of bainite and tempered martensite accounts for 95 area% or less, the retained austenite accounts for 5 vol% or more, the total of polygonal ferrite and bainitic ferrite accounts for 5 area% or less, and the balance of metal microstructure accounts for 5 area% or less. Each microstructure will be described below.(Total of bainite and tempered martensite: 95 area% or less)

[0041] Bainite and tempered martensite are metal microstructures necessary for ensuring strength. However, when these metal microstructures are excessively included, workability such as elongation deteriorates, and therefore the total of bainite and tempered martensite is set to 95 area% or less, preferably 92 area% or less, and more preferably 90 area% or less. The lower limit of the total of bainite and tempered martensite preferably accounts for 50 area% or more, more preferably 60 area% or more, and still more preferably 70 area% or more.(Retained austenite: 5 vol% or more)

[0042] The retained austenite (hereinafter, also referred to as "retained γ-phase") is mainly present between laths of the metal microstructure, but may be present in a lump on an aggregate of lath-like microstructures (for example, a block or packet) and grain boundaries of the prior γ-phase. This retained γ-phase is transformed into martensite when the steel sheet is deformed by being subjected to strain, thereby exhibiting high workability such as high elongation. That is, including the retained γ-phase promotes hardening of a portion deformed by receiving distortion, allowing concentration of distortion to be prevented. These effects are generally called TRIP effects and contribute to the development of a balance between high strength and high workability.

[0043] In order to exert the TRIP effect, the retained γ-phase accounts for 5 vol% or more, preferably 6 vol% or more, and more preferably 7 vol% or more based on the entire metal microstructure. The upper limit of the retained γ-phase is not particularly limited, but may account for 40 area% or less, 30 area% or less, or 20 area% or less.

[0044] The amount of the solid solution C in the retained γ-phase can be an index that affects the stability of martensitic transformation of the retained γ-phase at the time of deformation, and including a predetermined amount of both the retained γ-phase having a relatively low amount of the solid solution C and the retained γ-phase having a relatively large amount of the solid solution C is likely to achieve desired elongation. According to an embodiment of the present invention, the retained γ-phase preferably includes 10 to 50 vol% of the retained γ-phase (hereinafter also referred to as "first retained γ-phase") having a solid solution C amount of 0.9 mass% or less, and 10 to 50 vol% of the retained γ-phase (hereinafter also referred to as "second retained γ-phase") having a solid solution C amount of more than 0.9 mass% and 1.1 mass% or less. More preferably, the retained γ-phase includes 15 to 50 vol% of both the first retained γ-phase and the second retained γ-phase.(Total of polygonal ferrite and bainitic ferrite: 5 area% or less)

[0045] Polygonal ferrite and bainitic ferrite are soft microstructures that lead to a reduction in strength and, in some cases, can also reduce the hole expansion ratio λ. Therefore, the total of polygonal ferrite and bainitic ferrite accounts for 5 area% or less, preferably 3 area% or less, and more preferably 1 area% or less based on the entire metal microstructure. The lower limit value of the total of polygonal ferrite and bainitic ferrite is not particularly limited, and may account for 0 area%.(Balance of metal microstructure: 5 area% or less)

[0046] In order to ensure high strength and high workability, the balance of metal microstructure of fresh martensite, pearlite, and the like needs to account for 5 area% or less in total. The lower limit value of the balance of metal microstructure is not particularly limited, and may account for 0 area%.

[0047] The method for measuring the area ratio (or volume ratio) of the metal microstructure is a method described in Examples described later.<4. Mechanical properties of plated steel sheet>

[0048] The plated steel sheet according to the present embodiments can exhibit high strength (high tensile strength, high yield stress) and high workability (high elongation and high hole expansion ratio). In the present specification, a plated steel sheet having a tensile strength TS of 1150 MPa or more and a yield stress YS of 850 MPa or more is defined as a high-strength plated steel sheet. In addition, in the present specification, a plated steel sheet having an elongation EL of 14% or more and a hole expansion ratio λ of 30% or more is defined as a plated steel sheet having high workability.

[0049] The tensile strength is preferably 1180 MPa or more. The yield stress is preferably 900 MPa or more. The elongation EL is preferably 15% or more. The hole expansion ratio λ is preferably 35% or more.

[0050] 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.<5. Decarburized layer of surface layer of steel sheet included in plated steel sheet>

[0051] 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.

[0052] 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.

[0053] 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).

[0054] 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.

[0055] In addition to the high tensile strength (1150 MPa or more), the high yield stress (850 MPa or more), the high elongation EL (14% or more), and the high hole expansion ratio λ (30% or more), the plated steel sheet according to the present embodiments can exhibit excellent bendability in which R / t is less than 2.5 when a bending test shown in Examples described later is further performed.<6. Method for manufacturing plated steel sheet>

[0056] The method for manufacturing a 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) heating the rolled sheet to a first heating temperature of 850°C or more; (C) holding at the first heating temperature for 50 seconds or more; (D) cooling to a cooling stop temperature of 200 to 350°C after the holding, wherein an average cooling rate from the first heating temperature to 550°C is 5°C / s or more, and an average cooling rate from 550°C to the cooling stop temperature is 1°C / s or more; (E) heating to a second heating temperature of 300°C to 450°C and holding for 50 to 1000 seconds after the cooling to provide a steel sheet; and (F) forming a plating layer on the steel sheet. Hereinafter, each step will be described in detail. (A) Providing rolled sheet

[0057] 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 may be cast by a continuous casting method, an ingot method, a thin slab casting method, or the like, reheated to about 1150 to 1300°C, hot-rolled with a finish rolling temperature of about 850 to 950°C, and coiled at about 500 to 700°C to provide a rolled sheet.

[0058] 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) Heating rolled sheet to first heating temperature

[0059] The rolled sheet is heated to a first heating temperature of 850°C or more. Setting the first heating temperature to 850°C or more allows recrystallization to be promoted and the metal microstructure to be made uniform. The upper limit of the first heating temperature is not particularly limited, but may be, for example, 1000°C or less.(C) Holding at first heating temperature

[0060] The rolled sheet is held at the first heating temperature for 50 seconds or more. Holding at the first heating temperature for 50 seconds or more allows recrystallization to be promoted and the metal microstructure to be made uniform. The upper limit of the holding time is not particularly limited, and may be, for example, 30 minutes or less from the viewpoint of productivity.

[0061] 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).

[0062] 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 carbon concentration of the surface layer of the steel sheet capable of achieving excellent bendability in addition to high tensile strength, high yield stress, 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

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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

[0068] 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 embodiments, 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

[0069] 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.

[0070] 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.

[0071] 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 800°C in the atmosphere, and then heating is performed in a temperature range of 850 to 920°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

[0072] Then, the dew point of the second reduction treatment will be described. Fig. 1 is a graph created using a reference example to be described later, and is a graph showing the relationship between the dew point at the center of a soaking zone at the time of the second reduction treatment and R / t. From Fig. 1, it is found that the dew point in the second reduction treatment needs to be -25°C or more in order to achieve excellent bendability with R / t of less than 2.5. 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.

[0073] 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 920°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.

[0074] 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.

[0075] Between the first reduction treatment and the second reduction treatment, for example, a holding step or the like 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.

[0076] 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) Cooling to cooling stop temperature after step (C)

[0077] After the step (C), cooling is performed up to a cooling stop temperature of 200 to 350°C at a predetermined cooling rate.

[0078] First, an average cooling rate (hereinafter, also referred to as a "first average cooling rate") from the first heating temperature to 550°C is set to 5°C / sec or more. When the first average cooling rate is less than 5°C / sec, polygonal ferrite may be excessively generated. The first average cooling rate is preferably 7°C / sec or more. The upper limit of the first average cooling rate is not particularly limited, but may be, for example, 100°C / sec or less from the viewpoint of ease of control.

[0079] Then, the average cooling rate (hereinafter, also referred to as a "second average cooling rate") from 550°C to the cooling stop temperature (200 to 350°C) is set to 1°C / sec or more. When the second average cooling rate is less than 1°C / sec, bainitic ferrite may be excessively generated. The second average cooling rate is preferably 2°C / sec or more. The upper limit of the second average cooling rate is not particularly limited, but may be, for example, 100°C / sec or less from the viewpoint of ease of control. In addition, when the cooling stop temperature is less than 200°C, the amount of martensite may be excessive, and the amount of retained austenite may not be sufficiently secured, and when the cooling stop temperature is more than 350°C, the amount of martensite may not be sufficiently secured, and the balance of metal microstructure of fresh martensite, pearlite, and the like may be excessive.

[0080] Further, an average cooling rate (hereinafter, also referred to as a "third average cooling rate") from 400°C to the cooling stop temperature is preferably 40°C / sec or less. As a result, the enrichment of C into untransformed austenite due to the bainite transformation is promoted, the retained γ-phase is stabilized, and a predetermined amount of the first retained γ-phase, which is relatively unstable and has a small amount of solid solution C content, is easily secured.(E) Heating to the second heating temperature after step (D) to provide steel sheet

[0081] After the step (D), heating (reheating) is performed to the second heating temperature of 300°C to 450°C and held for 50 to 1000 seconds to provide a steel sheet used for the plated steel sheet according to the embodiments of the present invention. Promoting the bainite transformation and promoting the enrichment of C into untransformed austenite by this step allows the amount of retained austenite to be sufficiently secured, and further, a predetermined amount of the second retained γ-phase, which is relatively stable and has a high amount of solid solution C content, to be easily secured. The second heating temperature is out of the above temperature range or the holding time is less than 50 seconds, leading to a risk that the amount of retained austenite cannot be sufficiently secured. In contrast, the holding time is preferably 1000 seconds or less because the above effect becomes saturated if the holding time is too long.

[0082] The metal microstructure defined in the embodiments of the present invention can be obtained mainly through the step (D) and step (E).(F) Forming a plating layer on steel sheet.

[0083] 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.

[0084] 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.

[0085] 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]

[0086] 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.

[0087] A steel (steel slab) having the composition shown in Table 1 was heated at 1250°C for 30 minutes, then hot rolling was performed such that the rolling reduction ratio was about 90% and the finish rolling temperature was 920°C, and the steel was cooled to 660°C and coiled, and then cooled to room temperature to provide a rolled sheet having a sheet thickness of 1.8 to 2.6 mm. The obtained rolled sheet was subjected to pickling to remove the surface scale, and then cold-rolled to have a sheet thickness of 1.0 to 1.6 mm.

[0088] In Table 1, "-" indicates that the element was not intentionally added. In addition, in the following tables, numerical values marked with * indicate that they are out of the scope of the embodiments of the present invention. [Table 1]Test No.Composition of steel sheet [mass%] * balance: iron and inevitable impuritiesCSiMnCrTiBPSAlN10.2211.491.990.200.0170.00300.0070.00090.0400.001520.2231.491.970.200.0170.00280.0040.00090.3430.003130.2111.482.000.390.0160.00260.0050.00100.0380.003240.2251.482.000.580.0160.00260.0040.00090.0380.003150.2161.502.030.200.0610.00280.0050.00100.0380.002260.2251.482.000.580.0160.00260.0040.00090.0380.003170.2251.482.000.580.0160.00260.0040.00090.0380.003180.2180.971.970.990.0160.00280.0030.00080.0410.002090.2170.982.370.600.0160.00270.0030.00090.0420.0011100.2221.352.020.510.0320.00300.009<0.00100.0400.0056110.2221.692.010.500.0320.00280.0070.00060.0430.0039120.2231.702.030.490.0310.00280.0060.00050.0480.0030130.2231.702.030.490.0310.00280.0060.00050.0480.0030140.2231.702.000.500.0290.00280.0090.00060.0420.0031150.222*0.682.060.200.0160.00250.0050.00090.0390.0016160.2111.602.19*0.030.0330.00310.0070.00050.4420.0023170.2170.952.370.600.016*-0.0030.00090.0420.0014

[0089] The rolled sheet was subjected to the above-described step (B1) and steps (B) to (F) to provide a plated steel sheet of Test Nos. 1 to 17. The conditions for each step are shown in Table 2. Although not shown in Table 2, in Test Nos. 11 to 14, the step (B1) was performed during the temperature rise to the first heating temperature in the step (B), the oxygen concentration was 0.1 to 2%, the reached temperature was 650 to 750°C, and the temperature rise time in the oxidation treatment was 10 seconds or more and 120 seconds or less. The step (C) included a step (C1) of performing reduction treatment that includes a first reduction treatment (C1a) performed under conditions with a dew point of -35 to -15°C, followed by a second reduction treatment (C1b) performed under conditions with a dew point of -25 to 0°C, which is higher than that of the first reduction treatment, and heating times (holding times) of the first reduction treatment and the second reduction treatment were 60 seconds or more, respectively. In addition, although not shown in Table 2, in Test Nos. 1 to 17, an alloyed hot-dip galvanizing layer was formed in the step (F) by a publicly-known method. Specifically, in the step (F), the steel sheet was immersed in a hot-dip galvanizing bath at 300°C or more and 550°C or less to perform a hot-dip galvanizing treatment (plating time: 1 to 10 seconds), and then subjected to an alloying treatment, and the alloying treatment temperature was 480 to 500°C, and the alloying treatment time was 20 to 25 seconds. [Table 2]Test No.Step (B)Step (C)Step (D)Step (E)First heating temperature (°C)Step (C1a)Holding time (see)First average cooling rate (°C / s)Second average cooling rate (°C / s)Third average cooling rate (°C / s)Cooling stop temperature (°C)Second heating temperature (°C)Holding time (see)Dew point (°C)1930-1264031.23004004452930-1264020.63504004453930-1264030.93254004454930-1264030.93254004455930-1264030.93254004456900-654040403104001327930-1264035.03253254458930-1264035.03254004459930-1264035.032540044510900-135869.130040028011904-61621436.532939933612896-213514413.525240328013907-1013514412.825939128014911-30162738.930241133615930-1264031.230040044516939-2136310.928042644217930-1264035.0325400445

[0090] The steel sheets of Test Nos. 1 to 17 described above were evaluated as follows.<Evaluation of metal microstructure>

[0091] A test piece was taken from a central portion in the rolling direction and a direction (sheet width direction) perpendicular to the thickness direction, and from the test piece, a section parallel to the rolling direction and the sheet width direction and at a position of 1 / 4 of the sheet thickness from the surface of the steel plate was exposed by electrolytic polishing or the like. The section was observed by SEM, and the area ratios of polygonal ferrite, bainitic ferrite, MA, and pearlite were measured. The sum of the area ratio of bainite and the area ratio of tempered martensite was determined by subtracting the sum of the area ratios of polygonal ferrite, bainitic ferrite, MA, and pearlite from 100 area%.

[0092] The volume ratio of retained γ-phase in the metal microstructure constituting the steel sheet was measured by an X-ray diffraction method. Specifically, an X-ray diffraction pattern was acquired from the section using an X-ray source: Co-Kα, and the volume ratio of the retained γ-phase with respect to the entire metal microstructure was measured on the basis of a publicly-known method (ISIJ Int. Vol. 33. (1993), No. 7, P. 776).

[0093] The area ratio of the fresh martensite in the balance of metal microstructure was determined by subtracting the volume ratio (≈ area ratio) of the retained γ-phase from the area ratio of the MA, considering the volume ratio of the retained γ-phase as the area ratio. In addition, the ratios of the first retained γ-phase and the second retained γ-phase to the total retained γ-phase were determined as follows.

[0094] First, the distribution of the carbon concentration of the retained γ-phase was determined using three diffraction peaks of (200) y, (220) y, and (311) γ measured by the X-ray diffractometer. Fig. 2 is a schematic view of an X-ray diffraction pattern. For the three diffraction peaks (200) y, (220) γ, and (311) γ, as shown in Fig. 2, 2θ (2θ avg (hkl)) at each of which the diffraction intensity became maximum and the half width thereof Δ2θ (hkl) were determined. Herein, (hkl) is intended to mean (200), (220), or (311) (the same applies hereinafter).

[0095] Then, from the above 2θ avg (hkl), the Bragg condition: λ = 2dsinθ (d: lattice plane interval, λ: wavelength of Co-Kα ray) was used to determine d(hkl) from the following formula (1). d hkl = λ / 2 sin 2 θ avg hkl / 2

[0096] Each lattice constant a0 (hkl) was determined by the following formula (2), and the arithmetic average thereof was defined as the lattice constant a0. a 0 hkl = d hkl √ h 2 + k 2 + l 2

[0097] The carbon concentration % C avg (unit: mass%) was determined using the following formula (3). % C avg = 1 / 0.033 × a 0 − 3.572

[0098] Then, the half width Δ%C of the carbon concentration distribution of the retained γ-phase was determined by the following procedure.

[0099] First, the diffraction angles at the upper and lower limits of the half width Δ2θ (hkl) of the diffraction angle 20 (hkl) of each peak were obtained by the following formulas (4) and (5) (refer to Fig. 2). 2 θL hkl = 2 θ avg hkl − Δ 2 θ hkl / 2 2 θH hkl = 2 θ avg hkl + Δ 2 θ hkl / 2

[0100] The upper and lower limit values %CL and %CH of the half width of the carbon concentration distribution were obtained by using 2θL (hkl) and 2θH (hkl), respectively, and using the Bragg condition and the above formulas (1) to (3) in the same procedure as described above. Then, the half width Δ%C of the carbon concentration distribution was determined by the following formula (6). Δ % C = % CH − % CL

[0101] Assuming that the carbon concentration distribution was a normal distribution, the standard deviation σ%C was calculated from the half width Δ%C as follows. That is, the probability density function f(x) of the normal distribution is represented by the following formula (7) from the average value u and the standard deviation σ. f x = 1 / √ 2 πσ 2 × exp − x − u 2 / 2 σ 2

[0102] The probability f(u) at the average value was obtained by the following formula (8) by substituting x = u into the above formula (7). f u = 1 / √ 2 πσ 2

[0103] The probability density f(%C avg ± Δ%C / 2) at a value obtained by moving up and down by 1 / 2 of half width Δ%C from the average value u = %C avg (%C avg ± Δ%C / 2) is 1 / 2 of the probability f(u) = f(%C avg ) at the average value u = %C avg , and thus the relationship of the following formula (9) is obtained from the above formula (7) and the above formula (8). 1 / √ 2 πσ % C 2 × exp − Δ % C / 2 2 / 2 σ % C 2 = 1 / 2 √ 2 πσ % C 2

[0104] The following formula (10) is derived by modifying the above formula (9) as a formula for determining the standard deviation σ%C from the half width Δ%C, and thus the standard deviation σ%C was calculated by substituting the half width Δ%C into the formula (10). σ % C = √ Δ % C / 2 2 / 2 ln 2

[0105] Using the average value %C avg and the standard deviation σ%C of the carbon concentration distribution in the retained γ-phase obtained as described above, the following formula (12) was derived as a formula for obtaining the volume ratio VγR (C ≤ 1.0%) of the retained γ-phase having a carbon concentration of 1.0 mass% or less with respect to the entire metal microstructure by the cumulative distribution function g(x) shown in the following formula (11), and VγR (C ≤ 1.0%) was calculated using this formula (12). g x = 1 / 2 × 1 + erf x − u / √ 2 σ 2 VγR C ≤ 1.0 % = VγR × g 1.0 = VγR × 1 / 2 × 1 + erf 1.0 − % C avg / √ 2 σ % C 2

[0106] Using the average value% C avg and the standard deviation σ%C of the carbon concentration distribution in the retained γ-phase determined as described above, the following formulas (13) and (14) were derived as formulas for determining volume ratios VγR (C ≤ 0.9%) and VγR (0.9% < C ≤ 1.1%) of the retained γ-phase having a carbon concentration of 0.9 mass% or less and more than 0.9 mass% and 1.1 mass% or less with respect to the entire metal microstructure, respectively, by the cumulative distribution function g(x) shown in the above formula (11), and VγR (C ≤ 0.9%) was calculated using the formulas (13) and (14), and then the ratio to the total volume ratio of the retained γ-phase was determined.

[0107] Regarding the balance of metal microstructure, microstructures other than fresh martensite and pearlite were not observed.<Means for measuring decarburization behavior: measurement of carbon profile by GD-OES>

[0108] As described below, the carbon profile was measured by glow discharge optical emission spectrometry (GD-OES), and the decarburization behavior was examined.(Preparing of sample)

[0109] 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)

[0110] 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)

[0111] 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)

[0112] 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).

[0113] Details of the calibration curve method for converting the measured emission intensity of each element into a concentration will be described below.

[0114] The relationship between the sputtering weight W i (g / sec) per unit time of the element i and the emission intensity I i is represented by the following formula (15) using the slope a and the intercept b of the calibration curve. W i = aI i + b

[0115] The sputtering weight W i per unit time of the element i is determined by the following formula (16) 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 / sec) are known. W i = C i × ρ × Δd × S

[0116] 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 (15) 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 samples 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)------

[0117] Then, a carbon profile was obtained using the analysis result on carbon. An example thereof is illustrated in Fig. 3. Fig. 3 shows the carbon profiles of Test No. 12 and Test No. 14.

[0118] In Fig. 3, 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. 14 is higher than the bulk carbon concentration, whereas the carbon profile of Test No. 12 is sufficiently lower than the bulk carbon concentration.

[0119] 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, and elongation>

[0120] A JIS No. 5 test piece (sheet-shaped test piece) was taken such that a direction perpendicular to a rolling direction on a plane parallel to a rolled surface of a steel sheet during cold rolling was a longitudinal direction of the test piece. A tensile test was performed using the test piece, and the tensile strength TS, the yield stress YS, and the elongation EL were measured. An example in which the tensile strength TS was 1150 MPa or more was evaluated as sufficient, and an example in which the tensile strength TS was less than 1150 MPa was evaluated as insufficient. An example in which the yield stress YS was 850 MPa or more was evaluated as sufficient, and an example in which the yield stress YS was less than 850 MPa was evaluated as insufficient. An example in which the elongation EL was 14% or more was evaluated as sufficient, and an example in which the elongation EL was less than 14% was evaluated as insufficient.<Evaluation of hole expansion ratio>

[0121] A test piece having each sheet thickness × 90 mm × 90 mm was taken from the cold-rolled steel sheet. Using the test piece, a hole expansion test was performed according to JIS Z 2256: 2010, and a hole expansion ratio λ was measured. Then, an example in which the hole expansion ratio λ was 30% or more was evaluated as sufficient, and an example in which the hole expansion ratio λ was less than 30% was evaluated as insufficient.<Evaluation of bendability (R / t)>

[0122] The bendability of the steel sheet was evaluated by the following procedure. A long axis was taken in a direction perpendicular to the rolling 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 bending radius R (mm) to calculate R (mm) / sheet thickness t (mm). Then, an example in which the bending radius R (mm) / sheet thickness t (mm) was less than 2.5 was evaluated as excellent in bendability (Good), and an example in which the bending radius R (mm) / sheet thickness t (mm) was 2.5 or more was evaluated as poor in bendability (Poor).

[0123] The results are shown in Table 4. In Table 4, "PF + BF" is the sum of the area ratio of polygonal ferrite and the area ratio of bainitic ferrite, "B + M" is the sum of the area ratio of bainite and the area ratio of tempered martensite, "retained γ-phase" is the volume ratio of total retained austenite, "first retained γ-phase / retained γ-phase" is the ratio (vol%) of first retained γ-phase to total retained γ-phase, "second retained γ-phase / retained γ-phase" is the ratio (vol%) of second retained γ-phase to total retained γ-phase, "Balance of metal microstructure" is the area ratio of the balance of metal microstructure, and "maximum value of surface layer carbon concentration" is the maximum value of carbon concentration (mass%) in the region from the surface of the steel sheet to 20 µm in the sheet thickness direction. In addition, the composition shown in Table 4 is obtained by extracting a part of the composition shown in Table 1 such that the difference of Test Nos. 1 to 17 becomes clear. In addition, the balance of metal microstructures were fresh martensite and pearlite. [Table 4]Test No.Composition of steel sheet (partially extracted from Table 1)Metallographic structure of steel sheetCarbon concentration of steel sheetStrength of plated steel sheetWorkability of plated steel sheetSiCrBB + M (area%)Retained γ-phase (vol%)First retained γ-phase / retained γ-phase (vol%)Second retained γ-phase / retained γ-phase (vol%)PF + BF (area%)Balance of metal microstruct ure (area%)Bulk carbon concentration (mass%)Position at 50% of bulk carbon concentration from surface of steel sheet (µm)Position at 50% of bulk carbon concentration from sheet surface / sheet thickness (%)Maximum value of surface layer carbon concentration (mass%)Maximum value of surface layer carbon concentration / bulk carbon concentration (%)Tensile strength TS (MPa)Yield Stress YS (MPa)Elongation EL (%)Hole expansion ratio λ (%)Bendability11.490.200.003087-92833280≤5-----118610101452-21.490.200.002885~901017230≤5-----116210041460-31.480.390.002685~901030260≤5-----12099991449-41.480.580.002683-881235250≤5-----125110101437-51.500.200.002886~91933290≤5-----12029861446-61.480.580.002685~901043220≤5-----125610421439-71.480.580.002688~93748280≤5-----124410241452-80.970.990.002882-871346260≤5-----12329741434-90.980.600.002780-851547270≤5-----12138671430-101.350.510.003085~901032260≤5-----119610191533-111.690.500.002883-881236300≤50.22227.52.00.0944211979731537Good121.700.490.002883-881235280≤50.22329.42.10.0914111869551545Good131.700.490.002884~891138260≤50.22322.41.60.1104912179801446Good141.700.500.002885~901044270≤50.2231.70.10.23610612298741532Poor15*0.680.200.002593-98*251210≤5-----*1088978*995-161.60*0.030.0031----*>5≤5-----*10909051852-170.950.60*--125325*>5≤5-----*1130*69417*28-

[0124] From the results of Table 4, it can be considered as follows. Test Nos. 1 to 14 in Table 4 satisfied the requirements specified in the embodiments of the present invention, could be more easily manufactured, and had high strength and high workability. In addition, when comparing the test Nos. 11 to 14, the test Nos. 11 to 13 satisfied the preferable requirements defined in the embodiments of the present invention, and were excellent in bendability.

[0125] In contrast, all of Test Nos. 15 to 17 in Table 4 did not satisfy the requirements defined in the embodiments of the present invention, and the strength and / or workability were insufficient.

[0126] Test No. 15, which had a Si content of less than 0.80 mass% and a retained γ-phase of less than 5 vol%, had a tensile strength of less than 1150 MPa and an elongation of less than 14%.

[0127] Test No. 16 had a Cr content of less than 0.15 mass%, and the total of polygonal ferrite and bainitic ferrite was more than 5 area%, and thus the tensile strength was less than 1150 MPa.

[0128] Test No. 17 did not contain B, and the total of polygonal ferrite and bainitic ferrite accounted for more than 5 area%, and thus the tensile strength was less than 1150 MPa, the yield stress was less than 850 MPa, and the hole expansion ratio λ was less than 30%.[Reference Example]

[0129] Hereinafter, a reference example in which the relationship between the requirements (I) and (II) described above and the bendability is investigated in more detail will be described.

[0130] As a sample for annealing, an base sheet was prepared, having a bulk carbon concentration, that is, a C content of a steel sheet (base sheet) of 0.22 mass% (0.22 mass% C), a Si content of 1.7 mass%, a Mn content of 2.0 mass%, a Cr content of 0.5 mass%, an Al content of 0.04 mass%, and the balance being Fe and inevitable impurities (herein, the "inevitable impurities" may include P: more than 0 mass% and 0.1 mass% or less, S: more than 0 mass% and 0.05 mass% or less, and N: more than 0 mass% and 0.01 mass% or less), and having a structure of ferrite + pearlite and a strength of 700 to 800 MPa.

[0131] The annealing sample was subjected to an oxidation treatment and a reduction treatment using an actual machine. Specifically, the oxidation treatment was performed at the oxygen concentration and the reached temperature shown in Table 5, and then the first reduction treatment (heating zone) and the second reduction treatment (soaking zone) shown in Table 5 were performed in this order under each condition shown in Table 5 (reached temperature, dew point).

[0132] The steel sheet after the second reduction treatment was cooled to 460°C, and then immersed in a plating bath of Al-Zn containing 0.08 to 0.13 mass% of Al (Al is effective Al%, and the plating bath temperature was 460 to 480°C) to perform zinc plating. The deposition amount of plating was controlled to 40 g / m 2< or more and 90 g / m 2< or less by gas wiping, and then alloying treatment was performed at 480 to 490°C for 20 to 30 seconds to provide an alloyed hot-dip galvanized steel sheet. [Table 5]No.Oxidation treatmentReduction treatmentOxygen concentration (%)Reached temperature (°C)First reduction zone reached temperature (°C)First reduction zone dew point (°C)Second reduction zone reached temperature (°C)Second reduction zone dew point (°C)1010.2673901-36.0892-29.51020.1681918-32.9893-33.81030.1676918-33.1893-33.71040.0674902-39.8916-27.11050.0665900-24.1910-2.81060.0675888-28.6897-10.81070.0695893-28.8896-10.31080.0695901-26.7913-9.91090.3697894-21.9903-8.51100.1711899-25.0914-9.91110.1697907-26.5904-11.0

[0133] The resulting plated steel sheet was subjected to the carbon profile measurement by GD-OES described above.

[0134] 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.

[0135] Further, the obtained plated steel sheet was evaluated for the following limit bendability (R / t).[Evaluation of limit bendability (R / t)]

[0136] The bendability of the steel sheet was evaluated by the following procedure. A test piece having a width of 40 mm × a length of 100 mm with a long axis in a direction perpendicular to the rolling direction was prepared, and a bending test was performed by a V block method in accordance with JIS Z 2248: 2014. In the bending test, the bending radius 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 limit bending radius R (mm) to calculate the limit bending radius R (mm) / sheet thickness t (mm). Then, a case where the limit bending radius R (mm) / sheet thickness t (mm) was less than 2.5 was evaluated as excellent in bendability, and a case where the limit bending radius R (mm) / sheet thickness t (mm) was 2.5 or more was evaluated as poor in bendability. [Table 6]No.Analysis resultPosition at 50% of bulk carbon (% of sheet thickness)Ratio of maximum value of carbon concentration (mass%) in region from surface of steel sheet to 20 µm (%)Bending R / t1010.1884.83.21020.0298.23.21030.1888.32.51040.0089.02.51052.0439.01.11060.2461.01.41070.2850.41.81080.2360.01.81092.3142.41.11100.2564.21.81112.0538.02.2

[0137] On the basis of the results in Table 6, a graph is shown in Fig. 4, showing a relationship between a position where the carbon concentration (mass%) in the sheet thickness direction is 50% of the bulk carbon concentration and R / t. As is found from Fig. 4, in order to achieve excellent bendability with R / t of less than 2.5, the position where the carbon concentration (mass%) in the sheet thickness direction is 50% of the bulk carbon concentration may be in a region of 0.20% or more of the sheet thickness from the surface of the steel sheet.

[0138] On the basis of the results in Table 6 above, a graph is shown in Fig. 5, showing the relationship between the ratio of the maximum value of the carbon concentration (mass%) in the region from the surface of the steel sheet to 20 µm from the surface of the steel sheet and R / t with respect to the bulk carbon concentration. From Fig. 5, it is found that in order to achieve excellent bendability with an R / t of less than 2.5, the ratio of the maximum value of the carbon concentration (mass%) in the region 20 µm from the surface of the steel sheet to the bulk carbon concentration is preferably less than 70%.

[0139] This application claims priority based on JP-2023-056175, filed on March 30, 2023, and JP-2024-022773, filed on February 19, 2024. JP-2023-056175 and JP-2024-022773 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 having a tensile strength of 1150 MPa or more, wherein the steel sheet included in the plated steel sheet has a composition comprising: C: 0.150 to 0.250 mass%; Si: 0.80 to 2.20 mass%; Mn: 1.50 to 2.80 mass%; Cr: 0.15 to 1.50 mass%; Ti: 0.012 to 0.100 mass%; B: 0.0015 to 0.0100 mass%; P: 0.100 mass% or less (including 0 mass%); S: 0.050 mass% or less (including 0 mass%); Al: 0.005 to 1.000 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 bainite and tempered martensite account for 95 area% or less; retained austenite accounts for 5 vol% or more; a total of polygonal ferrite and bainitic ferrite accounts for 5 area% or less; and a balance of the metal microstructure accounts for 5 area% or less.

2. 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.

3. 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.

4. The plated steel sheet according to claim 1, wherein the retained austenite comprises: 10 to 50 vol% of first retained austenite having a solid solution C content of 0.9 mass% or less; and 10 to 50 vol% of second retained austenite having a solid solution C content of more than 0.9 mass% and 1.1 mass% or less.

5. A method for manufacturing the plated steel sheet according to any one of claims 1 to 4, the method comprising: providing a rolled sheet by hot rolling a steel having the composition according to claim 1; heating the rolled sheet to a first heating temperature of 850°C or more; holding at the first heating temperature for 50 seconds or more after the heating; cooling to a cooling stop temperature of 200 to 350°C after the holding, wherein an average cooling rate from the first heating temperature to 550°C is 5°C / s or more, and an average cooling rate from 550°C to the cooling stop temperature is 1°C / s or more; heating to a second heating temperature of 300°C to 450°C and holding for 50 to 1000 seconds after the cooling seconds to provide a steel sheet; and forming a plating layer on the steel sheet.

6. The manufacturing method according to claim 5, 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 comprises 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.

7. The manufacturing method according to claim 5, wherein an average cooling rate from 400°C to the cooling stop temperature in the cooling is 40°C / sec or less.

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