High-strength cold-rolled steel sheet, high-strength coated steel sheet, method for manufacturing high-strength cold-rolled steel sheet, and method for manufacturing high-strength coated steel sheet

Abstract

The present invention provides a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1080 MPa or more, and excellent ductility, stretch flangeability, and bendability, and a method for manufacturing the same. A high-strength cold-rolled steel sheet having the following composition and steel structure, the composition containing C, Si, Mn, P, S, N, Al, Ti, Nb, and B, with the balance being composed of Fe and inevitable impurities, satisfying [mol% N] / [mol% Ti] < 1. In the steel structure, ferrite: 12% or more and less than 30% in area fraction, tempered martensite and bainite: 55% - 85% in total area fraction, quenched martensite: 15% or less in area fraction, retained austenite: 1% - 10% in area fraction, the area fraction of low-Mn ferrite having an Mn concentration of 0.8 × [% Mn] or less is 5% - 20%, (the area fraction of the above ferrite) - (the area fraction of the above low-Mn ferrite): 10% or more, and the remaining structure: less than 3% in area fraction, and the average crystal grain size of the above low-Mn ferrite is 10 μm or less.

Description

Technical Field

[0001] The present invention relates to a high-strength cold-rolled steel sheet, a high-strength coated steel sheet, a method for manufacturing a high-strength cold-rolled steel sheet, and a method for manufacturing a high-strength coated steel sheet. Background Art

[0002] In order to achieve both the collision safety of automobiles and the low fuel consumption brought about by weight reduction, high-strength steel sheets are required. Moreover, in order to obtain excellent formability through stamping, automotive steel sheets having excellent ductility, stretch flangeability, and bendability are required.

[0003] Patent Document 1 describes a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility and bendability. Patent Document 2 discloses a high-strength steel sheet having an excellent balance between ductility and stretch flangeability and a method for manufacturing the same.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2015-193897

[0007] Patent Document 2: Japanese Patent No. 5464302 Summary of the Invention

[0008] However, stretch flangeability is not considered in Patent Document 1. Bendability is not considered in Patent Document 2. Thus, there is no steel sheet that comprehensively satisfies strength, ductility, stretch flangeability, and bendability.

[0009] The present invention has been completed in view of the above circumstances, and an object thereof is to provide a high-strength cold-rolled steel sheet having a tensile strength (TS) of 1080 MPa or more and excellent ductility, stretch flangeability, and bendability, and a method for manufacturing the same.

[0010] It should be noted that in the present invention, high strength means that the tensile strength TS measured based on JIS Z2201 is 1080 MPa or more.

[0011] In addition, excellent stretchability means that the tensile El measured based on JIS Z2201 is 10% or more.

[0012] In addition, excellent stretch flangeability means that the value of the hole expansion ratio (λ) measured based on JIS Z2256, which is an index of stretch flangeability, is 50% or more.

[0013] In addition, excellent bendability means that the VDA bend angle measured based on the German Automotive Industry Association standard VDA328-100 is 85° or more.

[0014] In order to solve the above problems, the present inventors conducted intensive research repeatedly, and as a result, the following was obtained.

[0015] (1) When a steel sheet containing Mn is annealed in a two-phase region of ferrite and austenite, element partitioning (Mn partitioning) occurs in which the Mn concentration in the ferrite phase decreases while the Mn concentration in the austenite phase increases.

[0016] (2) When the steel sheet having undergone the above Mn partitioning is cooled at an appropriate cooling rate, new ferrite phase transformation occurs in the austenite with the ferrite having a low Mn concentration as the nucleus. Since the Mn concentration of the newly formed ferrite during cooling retains the Mn concentration of the austenite before the phase transformation, ferrite with a high Mn concentration is formed.

[0017] (3) Ferrite with a high Mn concentration is harder than ferrite with a low Mn concentration. Since this hard ferrite is arranged to be sandwiched between ferrite with a low Mn concentration and soft ferrite and hard bainite or tempered martensite, it has the effect of reducing the hardness difference between the soft phase (ferrite with a high Mn concentration) and the hard phase (bainite or tempered martensite). As a result, the stretch flange formability of the high-strength cold-rolled steel sheet is improved.

[0018] (4) By finely dispersing ferrite with a low Mn concentration, it is effective in improving bendability.

[0019] The present invention was completed based on the above circumstances. That is, the main configuration of the present invention is as follows.

[0020] [1] A high-strength cold-rolled steel sheet having the following composition and steel structure in mass%, the composition comprising: C: 0.06% to 0.15%, Si: 0.10% to 1.8%, Mn: 2.00% to 3.50%, P: 0.050% or less, S: 0.0050% or less, N: 0.0060% or less, Al: 0.010% to 1.0%, Ti: 0.005% to 0.075%, Nb: 0.005% to 0.075%, and B: 0.0002% to 0.0040%, with the balance being composed of Fe and inevitable impurities, satisfying [mol% N] / [mol% Ti] < 1,

[0021] In the steel structure,

[0022] Ferrite: 12% or more and less than 30% in area fraction,

[0023] Tempered martensite and bainite: 55% to 85% in total area fraction,

[0024] Quenched martensite: 15% or less in area fraction,

[0025] Retained austenite: 1% to 10% by area fraction,

[0026] The area fraction of low-Mn ferrite having an Mn concentration of 0.8×[%Mn] or less is 5% to 20%,

[0027] (The area fraction of the above ferrite) - (the area fraction of the above low-Mn ferrite): 10% or more,

[0028] The remaining structure: less than 3% by area fraction,

[0029] And the average crystal grain size of the above low-Mn ferrite is 10 μm or less.

[0030] Among them, [mol%N] and [mol%Ti] respectively represent the contents (mol%) of N and Ti in the steel, and [%Mn] represents the content (mass%) of Mn in the steel.

[0031] [2] The high-strength cold-rolled steel sheet according to the above [1], wherein the above composition further contains, by mass%, V: 0.200% or less, Cr: 0.20% or less, Mo: 0.20% or less, Cu: 0.30% or less, Ni: 0.30% or less, Sb: 0.100% or less, Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.0200% or less, and Co: 0.100% or less of at least one element.

[0032] [3] A high-strength plated steel sheet having a plating layer on at least one side of the high-strength cold-rolled steel sheet described in the above [1] or [2].

[0033] [4] A method for manufacturing a high-strength cold-rolled steel sheet, which performs hot rolling on a steel slab having the composition described in the above [1] or [2] to obtain a hot-rolled sheet,

[0034] Performs pickling on the above hot-rolled sheet,

[0035] Performs cold rolling on the pickled above hot-rolled sheet to obtain a cold-rolled sheet,

[0036] Then, a first heating step is carried out: heating the above cold-rolled sheet to a first heating temperature of the Ac 1 point to (Ac 3 point - 50 °C), and maintaining it for 10 s or more in the first heating temperature range of the Ac 1 point to (Ac 3 point - 50 °C),

[0037] Next, perform the second heating process: Heat the above cold-rolled sheet to a second heating temperature of Ac 3 point to (Ac 3 point + 40°C) at a heating rate of 10°C / s or more, and hold it in the second heating temperature range of Ac 3 point to (Ac 3 point + 40°C) for 5 s to 60 s.

[0038] Next, perform the first cooling process: Cool the above cold-rolled sheet at a first cooling rate of 10°C / s or more to a first cooling stop temperature below 500°C and exceeding the Ms point, and then hold it at this first cooling stop temperature for 10 s to 60 s, or cool it from this first cooling stop temperature to a temperature exceeding the Ms point at a third cooling rate of less than 10°C / s for 10 s to 60 s.

[0039] Next, perform the second cooling process: Cool the above cold-rolled sheet at a second cooling rate of 10°C / s or more to a second cooling stop temperature of (Ms point - 100°C) to 100°C.

[0040] Next, perform the reheating process: Reheat the above cold-rolled sheet to a reheating temperature range of the above second cooling stop temperature to 450°C, and hold it in the reheating temperature range of the above second cooling stop temperature to 450°C for 10 s to 1800 s to obtain a high-strength cold-rolled steel sheet.

[0041] [5] A method for manufacturing a high-strength plated steel sheet, which performs a plating process on the above high-strength cold-rolled steel sheet after the reheating process described in the above [4] to obtain a high-strength plated steel sheet.

[0042] [6] An automotive component, which is made of at least a part of the high-strength cold-rolled steel sheet described in the above [1] or [2].

[0043] [7] An automotive component, which is made of at least a part of the high-strength plated steel sheet described in the above [3].

[0044] According to the present invention, a high-strength cold-rolled steel sheet having a tensile strength of 1080 MPa or more and excellent ductility, stretch flangeability, and bendability, and a manufacturing method thereof can be provided. Detailed Embodiments

[0045] Hereinafter, embodiments of the present invention will be described. It should be noted that the present invention is not limited to the following embodiments.

[0046] First, the appropriate range of the composition of the high-strength cold-rolled steel sheet and the reasons for its limitation will be described. It should be noted that in the following description, "%" indicating the content of the component elements of the steel sheet refers to "mass%" unless otherwise specified. Also, in this specification, the numerical range represented by "~" means the range of the numerical values described before and after "~" including the lower limit value and the upper limit value.

[0047] [Essential components]

[0048] C: 0.06% - 0.15%

[0049] C is contained in bainite or tempered martensite and contributes to the increase in strength. In addition, C has the effect of stabilizing retained austenite that contributes to ductility by thickening austenite. In order to obtain such an effect, the C content is 0.06% or more. On the other hand, if the C content exceeds 0.15%, the quenched martensite increases and the stretch flangeability decreases. Therefore, the C content is 0.06% - 0.15%. The C content is preferably 0.07% or more, more preferably 0.10% or more. In addition, the C content is preferably 0.14% or less, more preferably 0.12% or less.

[0050] Si: 0.10% - 1.8%

[0051] Si contributes to the increase in strength through solid solution strengthening. In addition, since it inhibits the formation of cementite and contributes to the stabilization of retained austenite, Si needs to contain 0.10% or more. On the other hand, since Si thickens in ferrite in the ferrite-austenite two-phase region, it thickens in the region of low-Mn ferrite. If the Si thickening in ferrite becomes excessive, the dislocation movement system changes, resulting in a decrease in bendability. Therefore, the Si content is 1.8% or less. The Si content is preferably 0.3% or more, more preferably 0.5% or more. In addition, the Si content is preferably 1.6% or less, more preferably 1.4% or less.

[0052] Mn: 2.00% - 3.50%

[0053] Mn is an important element for solid solution strengthening of ferrite by using element partitioning. If the Mn content is less than 2.00%, the effect of sufficient solid solution strengthening cannot be obtained. On the other hand, if the Mn content exceeds 3.50%, the ferrite phase transformation is excessively inhibited during cooling after the reheating process, and ferrite with a high Mn concentration cannot be sufficiently formed. As a result, the stretch flangeability deteriorates. Therefore, the Mn content is 2.00% - 3.50%. The Mn content is preferably 2.1% or more, more preferably 2.3% or more. In addition, the Mn content is preferably 3.2% or less, more preferably 3.0% or less.

[0054] P: 0.050% or less

[0055] When P exceeds 0.050%, the weldability decreases. Therefore, the P content is 0.050% or less. The lower limit of the P content is not particularly limited and can be 0.000%. From the viewpoint of manufacturing cost, the P content is preferably 0.0001% or more. The P content is preferably 0.020% or less.

[0056] S: 0.0050% or less

[0057] When S exceeds 0.0050%, the stretch flange property decreases. Therefore, the S content is 0.0050% or less. The lower limit of the S content is not particularly limited and can be 0.0000%. From the viewpoint of manufacturing cost, the S content is preferably 0.0001% or more. The S content is more preferably 0.0020% or less.

[0058] N: 0.0060% or less

[0059] When N is excessively contained, nitrides are formed, reducing the ductility and bendability. In addition, if combined with B to form BN, the effect of strength increase caused by B cannot be obtained. Therefore, the N content is 0.0060% or less. The lower limit of the N content is not particularly limited and can be 0.0000%. From the viewpoint of manufacturing cost, the N content is preferably 0.0001% or more. The N content is more preferably 0.0045% or less.

[0060] Al: 0.010% - 1.0%

[0061] When Al is 0.010% or more, it acts as a deoxidizing material. On the other hand, if the Al content exceeds 1.0%, not only does the effect saturate, but the weldability also decreases. Therefore, the Al content is 0.010% - 1.0%. The Al content is preferably 0.02% or more. In addition, the Al content is preferably 0.9% or less.

[0062] Ti: 0.005% - 0.075%

[0063] Ti has the effect of fixing N in steel as nitride TiN. To obtain this effect, the Ti content is 0.005% or more. On the other hand, if the Ti content exceeds 0.075%, excessive carbides are generated and the ductility decreases. The Ti content is preferably 0.008% or more. In addition, the Ti content is preferably 0.05% or less.

[0064] Nb: 0.005% - 0.075%

[0065] Nb segregates at grain boundaries in a solid solution state or precipitates as fine carbides with a pinning effect, and has the effect of finely dispersing the ferrite phase with a low Mn concentration in the first heating process in the ferrite-austenite two-phase region. To obtain this effect, Nb is contained in an amount of 0.005% or more. On the other hand, if the Nb content exceeds 0.075%, not only does the effect saturate but carbides are excessively generated, resulting in a reduction in ductility. Therefore, the Nb content is 0.005% to 0.075%. The Nb content is preferably 0.008% or more. Additionally, the Nb content is preferably 0.05% or less.

[0066] B: 0.0002% to 0.0040%

[0067] B contributes to an increase in strength and, in the first heating process in the ferrite-austenite two-phase region, has the effect of making the ferrite phase with a low Mn concentration fine and improving bendability, and needs to be contained in an amount of 0.0002% or more. On the other hand, if the B content exceeds 0.0040%, the ductility decreases. Therefore, the B content is 0.0002% to 0.0040%. The B content is preferably 0.0007% or more. Additionally, the B content is preferably 0.0030% or less.

[0068] [mol% N] / [mol% Ti] < 1

[0069] Ti has the effect of fixing N as TiN, but when the molar amount of the Ti content is the same as or less than the molar amount of the N content, the N not fixed by Ti combines with B, reducing or disappearing the effect containing B.

[0070] [Optional component]

[0071] In addition to the above-described composition, the high-strength cold-rolled steel sheet of the present embodiment may further contain, by mass%, at least one element selected from V: 0.200% or less, Cr: 0.20% or less, Mo: 0.20% or less, Cu: 0.30% or less, Ni: 0.30% or less, Sb: 0.100% or less, Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.0200% or less, and Co: 0.100% or less.

[0072] V: 0.200% or less

[0073] V is contained in an amount of 0.005% or more to form fine carbides, which helps to increase the strength. Therefore, when contained, the V content is preferably 0.005% or more. On the other hand, in order to prevent the coarsening of carbides and further increase the strength and obtain more excellent ductility, the V content is preferably set to 0.200% or less. Therefore, when contained, the V content is preferably 0.200% or less. The V content is more preferably 0.008% or more. In addition, the V content is more preferably 0.1% or less.

[0074] Cr: 0.20% or less

[0075] Cr is contained in an amount of 0.05% or more, which helps to increase the strength due to solid solution strengthening. Therefore, when contained, the Cr content is preferably 0.05% or more. On the other hand, from the viewpoints of preventing the formation of cementite and further improving the ductility and stretch flange formability, when contained, the Cr content is preferably 0.20% or less. The Cr content is more preferably 0.06% or more. In addition, the Cr content is more preferably 0.15% or less.

[0076] Mo: 0.20% or less

[0077] Mo is contained in an amount of 0.01% or more, which helps to increase the strength due to solid solution strengthening. Therefore, when contained, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 0.20%, the effect saturates. Therefore, in order to further reduce the manufacturing cost, when contained, the Mo content is preferably 0.20% or less. The Mo content is more preferably 0.02% or more. In addition, the Mo content is more preferably 0.15% or less.

[0078] Cu: 0.30% or less

[0079] Cu is contained in an amount of 0.01% or more, which helps to increase the strength due to solid solution strengthening. Therefore, when contained, the Cu content is preferably 0.01% or more. On the other hand, in order to obtain better ductility and stretch flange formability, when contained, the Cu content is preferably 0.30% or less. The Cu content is more preferably 0.02% or more. In addition, the Cu content is more preferably 0.20% or less.

[0080] Ni: 0.30% or less

[0081] Ni is contained in an amount of 0.01% or more, which helps to increase the strength due to solid solution strengthening. Therefore, when contained, the Ni amount is preferably 0.01% or more. On the other hand, if the Ni content exceeds 0.30%, the effect saturates. Therefore, in order to further reduce the manufacturing cost, the Ni content is preferably 0.30% or less. The Ni content is more preferably 0.02% or more. In addition, the Ni content is more preferably 0.20% or less.

[0082] Sb: 0.100% or less

[0083] Sn: 0.100% or less

[0084] Sb and Sn are each contained at 0.002% or more to have the effect of suppressing decarburization of the steel sheet surface. Therefore, when contained, Sb and Sn are each preferably 0.002% or more. On the other hand, when the contents of Sb and Sn each exceed 0.100%, the effect saturates. Therefore, from the viewpoint of further reducing the manufacturing cost, when contained, the contents of Sb and Sn are each preferably 0.100% or less. Sb and Sn are more preferably each 0.004% or more. In addition, the contents of Sb and Sn are more preferably each 0.05% or less.

[0085] Ca: 0.0050% or less

[0086] Mg: 0.0050% or less

[0087] REM: 0.0050% or less

[0088] Ca, Mg, and REM each act as a deoxidizing material when contained at 0.0001% or more. Therefore, when contained, Ca, Mg, and REM are each preferably 0.0001% or more. On the other hand, from the viewpoint of further improving the stretch flangeability, when contained, the contents of Ca, Mg, and REM are each preferably 0.0050% or less. The contents of Ca, Mg, and REM are more preferably each 0.0002% or more. In addition, the contents of Ca, Mg, and REM are more preferably each 0.0040% or less.

[0089] Ta: 0.100% or less

[0090] Ta has the effect of increasing the strength of the steel sheet by forming fine carbides. When Ta is contained, in order to obtain such an effect, it is preferable to set the Ta content to 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, excessive precipitation of Ta carbides occurs and the ductility decreases. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.050% or less.

[0091] W: 0.500% or less

[0092] W has the effect of enhancing the strength of the steel sheet by solid solution strengthening. When W is contained, in order to obtain such an effect, it is preferable to set the W content to 0.005% or more. On the other hand, if the W content exceeds 0.500%, excessive precipitation of W carbide occurs and the ductility decreases. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.300% or less.

[0093] Zr: 0.0200% or less

[0094] Zr can be used as a deoxidizing material. When Zr is contained, in order to obtain such an effect, it is preferable to set the Zr content to 0.0001% or more. On the other hand, if the Zr content exceeds 0.0200%, excessive precipitation of Zr carbide occurs and the ductility decreases. Therefore, when Zr is contained, the Zr content is preferably 0.0200% or less. The Zr content is more preferably 0.0150% or less.

[0095] Co: 0.100% or less

[0096] Co has the effect of enhancing the strength of the steel sheet by solid solution strengthening. When Co is contained, in order to obtain such an effect, it is preferable to set the Co content to 0.005% or less. On the other hand, if the Co content exceeds 0.100%, the effect saturates. Therefore, when Co is contained, the Co content is preferably 0.100% or less. The Co content is more preferably 0.080% or less.

[0097] The remainder other than the above components is Fe and inevitable impurities. It should be noted that for any of the above components, when the content is less than the lower limit value, since the effects of the present invention are not impaired, these arbitrary elements can be treated as inevitable impurities when contained in an amount less than the lower limit value.

[0098] [Steel structure]

[0099] Next, the steel structure of the high-strength cold-rolled steel sheet will be described.

[0100] Ferrite: 12% or more and less than 30% in area fraction

[0101] Ferrite is effective in improving ductility. Also, through the occurrence of ferrite phase transformation, C can be concentrated in the retained austenite, further improving ductility. Therefore, it is necessary to set the area fraction of ferrite to 12% or more. If the area fraction of ferrite is 30% or more, the stretch flangeability decreases. The area fraction of ferrite is preferably 15% or more, more preferably 18% or more. In addition, the area fraction of ferrite is preferably 28% or less, more preferably 25% or less. It should be noted that the above area fraction is the total area fraction of ferrite with a high Mn concentration and low-Mn ferrite described later.

[0102] Tempered martensite and bainite: 55% to 85% in total by area fraction

[0103] Tempered martensite and bainite have a higher dislocation density than ferrite and are structures containing cementite. Tempered martensite and bainite are effective for increasing strength. In order to achieve high strength, the total of tempered martensite and bainite by area fraction needs to be 55% or more. On the other hand, when the total area fraction of tempered martensite and bainite exceeds 85%, the ductility decreases. The total area fraction of tempered martensite and bainite is preferably 60% or more, more preferably 65% or more. In addition, the total area fraction of tempered martensite and bainite is preferably 75% or less.

[0104] Quenched martensite: 15% or less by area fraction

[0105] Quenched martensite has a higher dislocation density than ferrite and is a very hard structure that does not contain cementite and has C solid-solved. If the area fraction of quenched martensite exceeds 15%, the ductility, stretch flange formability, and bendability decrease. The area fraction of quenched martensite is preferably 13% or less, more preferably 10% or less. The lower limit of the area fraction of quenched martensite is not particularly limited and can be 0%, but since it is difficult to suppress the formation of quenched martensite by bending, it is preferably 3% or more.

[0106] Retained austenite: 1% to 10% by area fraction

[0107] When the area fraction of retained austenite is 1% or more, due to the effect of plasticity caused by phase transformation, it contributes to the improvement of ductility. On the other hand, when the area fraction of retained austenite exceeds 10%, the stretch flange formability decreases. The area fraction of retained austenite is preferably 3% or more. In addition, the area fraction of retained austenite is preferably 8% or less.

[0108] Area fraction of the remaining structure: less than 3%

[0109] In addition to the above-mentioned ferrite, tempered martensite, bainite, quenched martensite, and retained austenite, the steel structure may also contain pearlite, carbides such as cementite, and other structures (remaining structures) within the range that does not impair the effects of this embodiment. If the area fraction of the remaining structure is less than 3%, it reduces the ductility, stretch flange formability, and bendability. Therefore, the area fraction of the remaining structure is less than 3%. It should be noted that the type and area fraction of the remaining structure can be confirmed and determined by SEM observation, for example.

[0110] Low-Mn ferrite with an Mn concentration of 0.8×[%Mn] or less: 5% to 20% by area fraction

[0111] Ferrite with a low Mn concentration of 0.8×[%Mn] or less is referred to as low-Mn ferrite. Low-Mn ferrite is generated during the heating and holding in the ferrite-austenite two-phase region in the first heating process and during the cooling in the first cooling process. Subsequently, according to the heating and holding in the single-phase region of the second heating process, the ferrite turns into austenite. However, depending on the control of the second heating temperature and the holding time of the second heating temperature, the diffusion of Mn from the low-Mn ferrite into the austenite is suppressed. The structure is a single-phase austenite, but regions with a Mn concentration of 0.8×[%Mn] or less remain locally. The structure of this region with a low Mn concentration then preferentially undergoes a ferrite phase transformation while maintaining the Mn concentration during the first cooling process to form low-Mn ferrite. Taking this low-Mn ferrite as the nucleus, a ferrite phase transformation occurs in the austenite with a Mn concentration exceeding 0.8×[%Mn], generating ferrite with a Mn concentration exceeding 0.8×[%Mn] (hereinafter, also referred to as high-Mn ferrite). High-Mn ferrite is hard and improves the stretch flangeability. Therefore, in order to generate high-Mn ferrite and improve the stretch flangeability, it is indispensable to generate low-Mn ferrite at a specified area fraction. If the area fraction of the low-Mn ferrite is less than 5%, the number of ferrite nuclei is small, and insufficient amounts of ferrite cannot be generated during the first cooling process, resulting in a decrease in ductility and stretch flangeability. On the other hand, if the area fraction of the low-Mn ferrite exceeds 20%, the amount of ferrite increases, leading to a decrease in strength and stretch flangeability. Therefore, the area fraction of the low-Mn ferrite is 5% to 20%. The area fraction of the low-Mn ferrite is preferably 7% or more, more preferably 10% or more. In addition, the area fraction of the low-Mn ferrite is preferably 18% or less, more preferably 15% or less.

[0112] (Area fraction of ferrite) - (Area fraction of low-Mn ferrite): 10% or more

[0113] By subtracting the area fraction of the low-Mn ferrite from the area fraction of the total ferrite, the area fraction of the high-hardness ferrite with a high Mn concentration (hereinafter, also referred to as high-Mn ferrite) generated during the first cooling process can be obtained. It should be noted that high-Mn ferrite refers to ferrite with a higher Mn concentration compared to low-Mn ferrite, that is, ferrite with a Mn concentration exceeding 0.8×[%Mn]. When (Area fraction of ferrite) - (Area fraction of low-Mn ferrite) is less than 10%, the high-hardness Mn ferrite is insufficient, resulting in a decrease in stretch flangeability. (Area fraction of ferrite) - (Area fraction of low-Mn ferrite) is preferably 12% or more, more preferably 15% or more. It should be noted that the upper limit of (Area fraction of ferrite) - (Area fraction of low-Mn ferrite) is not particularly limited and is preferably 30% or less.

[0114] Average crystal grain size of low-Mn ferrite: 10 μm or less

[0115] By finely dispersing a soft low-Mn ferrite in a steel sheet, the bendability can be improved. To improve the bendability, it is necessary for the low-Mn ferrite to be finely dispersed without being interconnected. When the average crystal grain size (equivalent circle diameter) of the low-Mn ferrite exceeds 10 μm, the effect of improving the bendability cannot be obtained. The average crystal grain size of the low-Mn ferrite is preferably 8 μm or less, more preferably 6 μm or less.

[0116] Here, the area fraction of each structure is measured as follows. First, a test piece for microstructure observation is taken from a high-strength cold-rolled steel sheet. A cross-section (L-section) parallel to the rolling direction of the test piece is obtained, and polishing is carried out in such a way that the position equivalent to 1 / 4 of the sheet thickness in the plate thickness depth direction from the steel sheet surface is called the observation surface, and corrosion is carried out using 3% nitric acid ethanol. Using a Scanning Electron Microscope (SEM), the observation surface is observed at a magnification of 2000 times to obtain a microstructure image.

[0117] (Ferrite)

[0118] The area fraction of ferrite is obtained as follows. The high-Mn ferrite and the low-Mn ferrite are observed with the same contrast in the secondary electron image observation using SEM and can be identified as other microstructures. By performing image analysis on the microstructure image obtained as described above, the area fraction of ferrite and the area fraction of other microstructures are thus obtained.

[0119] (Quenched martensite)

[0120] The area fraction of quenched martensite is obtained as follows. The same field of view as the above microstructure image is observed using Electron backscatter diffraction (EBSD) of SEM, and analysis is carried out using an ImageQuality map (IQ map). The area where the IQ value is lower than the surrounding area is regarded as quenched martensite, and the area fraction is obtained.

[0121] (Retained austenite)

[0122] The area fraction of retained austenite is obtained as follows. A test piece is taken from a high-strength cold-rolled steel sheet. The test piece is ground and polished in the plate thickness direction (depth direction) in such a way that the position equivalent to 1 / 4 of the sheet thickness in the plate thickness depth direction from the steel sheet surface becomes the measurement surface. The measurement surface is analyzed by X-ray diffraction to obtain the amount of retained austenite. The ratio of the peak intensities of the {111}, {200}, {220}, and {311} planes of austenite to the peak intensities of the {110}, {200}, and {211} planes of ferrite is obtained, and the amount of austenite is calculated from each average value. Although the volume fraction of austenite is obtained in this method, this value is set as the area fraction of austenite.

[0123] (Tempered martensite and bainite)

[0124] The total area fraction of tempered martensite and bainite is obtained by subtracting the area fraction of the above-mentioned quenched martensite and the area fraction of the above-mentioned retained austenite from the tissue fraction other than the above-mentioned ferrite.

[0125] (Low-Mn ferrite)

[0126] The area fraction and average crystal grain size of low-Mn ferrite are obtained as follows. A test piece is taken from the high-strength cold-rolled steel sheet, and the test piece is ground in the thickness direction (depth direction) so that the position equivalent to 1 / 4 of the sheet thickness from the surface of the steel sheet becomes the analysis surface. Using an Electron Probe MicroAnalyzer (EPMA), the Mn concentration in the area of 100x100μm 2 of the analysis surface is measured. The area fraction of low-Mn ferrite is obtained as the area fraction of the region of 0.8×[%Mn] or less by image analysis based on the measurement result of the Mn concentration obtained by EPMA. The average crystal grain size (equivalent circle diameter) of low-Mn ferrite is obtained by image analysis based on the region of 0.8×[%Mn] or less.

[0127] It should be noted that the thickness of the high-strength cold-rolled steel sheet is not particularly limited, and is usually 0.3 mm to 2.8 mm.

[0128] In order to improve the corrosion resistance, the above-mentioned high-strength cold-rolled steel sheet may have a plating layer on at least one side. As the plating layer, any one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electro-galvanized layer is preferred. The composition of the galvanized layer is not particularly limited and may be a known composition.

[0129] The composition of the hot-dip galvanized layer is not particularly limited as long as it is general. In one example, the plating layer has the following composition: containing Fe: 20% by mass or less, Al: 0.001% by mass to 1.0% by mass, and 1 or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in total containing 0% by mass to 3.5% by mass, and the balance is composed of Zn and inevitable impurities. In the case where the plating layer is a hot-dip galvanized layer, in one example, the Fe content in the plating layer is less than 7% by mass, and in the case of an alloyed hot-dip galvanized layer, in one example, the Fe content in the plating layer is 7% by mass to 15% by mass, more preferably 8% by mass to 13% by mass.

[0130] The coating amount is not particularly limited, and the coating amount on one side of the high-strength cold-rolled steel sheet is preferably set to 20 to 80 g / m 2 . In one example, the coating layer is formed on the front and back surfaces of the high-strength cold-rolled steel sheet.

[0131] Next, a method for manufacturing the high-strength cold-rolled steel sheet will be described. The manufacturing method of the high-strength cold-rolled steel sheet in this embodiment may be a method for manufacturing a high-strength cold-rolled steel sheet, in which a hot-rolled sheet is obtained by hot-rolling a steel billet having the above-described composition,

[0132] the above hot-rolled sheet is pickled,

[0133] the pickled hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet,

[0134] Then, a first heating step is carried out: the above cold-rolled sheet is heated to a first heating temperature of Ac 1 point to (Ac 3 point - 50 °C), and held for 10 s or more in the first heating temperature range of Ac 1 point to (Ac 3 point - 50 °C),

[0135] Then, a second heating step is carried out: the above cold-rolled sheet is heated at a heating rate of 10 °C / s or more to a second heating temperature of Ac 3 point to (Ac 3 point + 40 °C), and held for 5 s to 60 s in the second heating temperature range of Ac 3 point to (Ac 3 point + 40 °C),

[0136] Then, a first cooling step is carried out: the above cold-rolled sheet is cooled at a first cooling rate of 10 °C / s or more to a first cooling stop temperature below 500 °C and exceeding the Ms point, and then held at this first cooling stop temperature for 10 s to 60 s, or cooled at a third cooling rate of less than 10 °C / s from this first cooling stop temperature to a temperature exceeding the Ms point for 10 s to 60 s,

[0137] Then, a second cooling step is carried out: the above cold-rolled sheet is cooled at a second cooling rate of 10 °C / s or more to a second cooling stop temperature of (Ms point - 100 °C) to 100 °C,

[0138] Then, a reheating step is carried out: the above cold-rolled sheet is reheated to a reheating temperature of the above second cooling stop temperature to 450 °C, and held for 10 s to 1800 s in the reheating temperature range of the above second cooling stop temperature to 450 °C to obtain a high-strength cold-rolled steel sheet.

[0139] First, a hot-rolled sheet is obtained by hot-rolling a steel billet having the above-described composition.

[0140] First, a steel billet having the above-described composition is manufactured. First, the steel billet is melted to obtain molten steel having the above-described composition. The melting method is not particularly limited and may be any of well-known melting methods such as converter melting and electric furnace melting. The obtained molten steel is solidified to manufacture a steel billet (slab). The method for manufacturing a steel billet from molten steel is not particularly limited, and a continuous casting method, an ingot-making method, or a thin slab casting method may be used. The steel billet may be temporarily cooled and then reheated before hot rolling, or the cast steel billet may be continuously hot rolled without being cooled to room temperature.

[0141] Next, the manufactured steel billet is subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot rolled sheet.

[0142] In one example, the manufactured steel billet is temporarily cooled to room temperature as described above, and then the slab is heated and rolled. In addition, hot rolling can be performed using an energy-saving process. As the energy-saving process, there can be mentioned direct rolling in which the manufactured steel billet is not cooled to room temperature and is charged into a heating furnace in a warm sheet state and hot rolled directly, or direct rolling in which the manufactured steel billet is slightly heat-retained and then immediately rolled.

[0143] The hot rolling start temperature is preferably 1100 °C or higher. By setting the hot rolling start temperature to 1100 °C or higher, the rolling load can be further reduced. In addition, from the viewpoint of further reducing the heating cost, the hot rolling start temperature is preferably 1300 °C or lower.

[0144] The finish rolling temperature is preferably above the Ar 3 point. By setting the finish rolling temperature above the Ar 3 point, the hot rolled structure can be made more uniform, and the ductility of the high-strength cold rolled steel sheet can be further improved. In addition, the finish rolling temperature is preferably 1000 °C or lower. By setting the finish rolling temperature to 1000 °C or lower, coarsening of the hot rolled structure can be prevented, and the bendability of the high-strength cold rolled steel sheet can be further improved.

[0145] The coiling temperature of the hot rolled sheet after hot rolling is preferably 500 °C or lower. By setting the coiling temperature to 500 °C or lower, formation of a ferrite-pearlite lamellar structure can be prevented, ferrite connection can be prevented in the first heating process, and the bendability can be further improved.

[0146] Next, the above hot rolled sheet is pickled. By pickling, the scale on the surface of the hot rolled sheet can be removed. The pickling conditions can be carried out according to common methods.

[0147] Next, the pickled hot rolled sheet is cold rolled to obtain a cold rolled sheet. The cold rolling conditions can be carried out according to common methods. The cold rolling reduction ratio is not particularly limited, and for example, it can be 30% or more and can be 80% or less.

[0148] Next, annealing including a first heating step, a second heating step, a first cooling step, a second cooling step, and a re-heating step is performed on the cold-rolled sheet. In one example, the cold-rolled sheet obtained as described above is supplied to a continuous annealing furnace and annealed. Additionally, when a hot-dip galvanized layer or an alloyed hot-dip galvanized layer is formed on the surface of the high-strength cold-rolled steel sheet, the cold-rolled sheet can also be supplied to a continuous hot-dip galvanizing apparatus and annealed and coated continuously.

[0149] First, in the first heating step, the cold-rolled sheet is heated to a first heating temperature of the Ac 1 point to (Ac 3 point - 50°C), and held for 10 s or more in the first heating temperature range of the Ac 1 point to (Ac 3 point - 50°C).

[0150] The first heating temperature and the first heating temperature range: Ac 1 point to (Ac 3 point - 50°C)

[0151] In the first heating step, the cold-rolled sheet is heated and held in the two-phase region of ferrite and austenite, resulting in an Mn partitioning where the Mn concentration in the ferrite phase decreases and the Mn concentration in the austenite phase increases. As a result, low-Mn ferrite is formed. When the first heating temperature and the first heating temperature range are lower than the Ac 1 point, no Mn partitioning occurs. On the other hand, if the first heating temperature and the first heating temperature range exceed (Ac 3 point - 50°C), coarse ferrite is formed. This coarse ferrite cannot be refined even by the second heating step, the average crystal grain size of the low-Mn ferrite exceeds 10 μm, and the bendability decreases. Therefore, the first heating temperature and the first heating temperature range are the Ac 1 point to (Ac 3 point - 50°C). The first heating temperature and the first heating temperature range are preferably (Ac 1 point + 10°C) or higher, more preferably (Ac 1 point + 30°C) or higher. Additionally, the first heating temperature and the first heating temperature range are preferably (Ac 3 point - 60°C) or lower. It should be noted that in this application, the temperature holding in the specified temperature range means that the temperature can change while being held in this temperature range, and it does not need to be isothermal holding at a specified temperature. Additionally, Ac 1 and Ac 3 are calculated according to the following formulas (1) and (2) respectively.

[0152] Ac 1Ac1(℃) = 751 - 16[%C] + 35[%Si] - 28[%Mn] - 5.5[%Cu] - 16[%Ni] + 13[%Cr] + 3.4[%Mo]…(1)

[0153] Ac1 3 Ac3(℃) = 881 - 206[%C] + 53[%Si] - 15[%Mn] - 27[%Cu] - 20[%Ni] - 1[%Cr] + 41[%Mo]…(2)

[0154] Herein, [%M] represents the content of M in the steel (mass%).

[0155] First holding time: 10 s or more

[0156] The holding time in the first heating temperature range (the first holding time) is 10 s or more. When the first holding time is less than 10 s, Mn is not sufficiently distributed, and high-Mn ferrite cannot be sufficiently formed, resulting in a decrease in the drawing flange property. The upper limit of the first holding time is not particularly limited. From the viewpoint of productivity, the first holding time is preferably 1800 s or less. The first holding time is preferably 20 s or more, more preferably 100 s or more. Additionally, the first holding time is preferably 1500 s or less.

[0157] Next, a second heating process is performed, heating the cold-rolled sheet at a heating rate of 10℃ / s or more to the second heating temperature of Ac1 point~(Ac1 point + 40℃), and holding it for 5 s to 60 s in the second heating temperature range of Ac1 point~(Ac1 point + 40℃). 3 point~(Ac1 3 point + 40℃) of the second heating temperature, and holding it for 5 s to 60 s in the second heating temperature range of Ac1 3 point~(Ac1 3 point + 40℃).

[0158] In the second heating process, while maintaining the Mn concentration of the low-Mn ferrite generated in the above first heating process, it is formed into an austenite single-phase structure.

[0159] Heating rate: 10℃ / s or more

[0160] When the heating rate is less than 10℃ / s, Mn diffuses uniformly when heated above the Ac1 point, and low-Mn ferrite cannot be obtained, resulting in a decrease in ductility and drawing flange property. The heating rate is preferably 15℃ / s or more. The upper limit of the heating rate is not particularly limited. From the viewpoint of production technology, it is preferably 50℃ / s or less. 3 point~(Ac1

[0161] Second heating temperature and second heating temperature range: Ac1 3 point~(Ac1 3 point + 40℃)

[0162] The second heating temperature and the second heating temperature range are less than Ac1 3At this point, ferrite remains. Taking this ferrite as the nucleus, a large amount of ferrite is then generated in the first cooling process, reducing the stretch flangeability. In addition, the strength also decreases. On the other hand, when the second heating temperature and the second heating temperature range exceed (Ac 3 point + 40°C), Mn diffusion from the low-Mn ferrite to the region with a lower Mn concentration occurs, resulting in a decrease in the area fraction of the region with a lower Mn concentration, and the austenite coarsens, reducing the grain boundary area that becomes the nucleation site of ferrite in the subsequent first cooling process. Therefore, during the cooling in the first cooling process, it is difficult to generate a ferrite phase transformation on both sides of the Mn concentration and the nucleation site, reducing the ductility. In addition, the bendability decreases. The second heating temperature and the second heating temperature range are preferably Ac 3 + 5°C or higher, more preferably (the first heating temperature + 10°C) or higher. In addition, the second heating temperature and the second heating temperature range are preferably (Ac 3 point + 35°C) or lower, more preferably (Ac 3 point + 30°C) or lower.

[0163] Second holding time: 5 s to 60 s

[0164] When the holding time (second holding time) in the second heating temperature range is less than 5 s, the austenite phase transformation does not end, and ferrite remains. Then, an excessive ferrite phase transformation occurs in the first cooling process, so the strength and stretch flangeability decrease. On the other hand, when the second holding time exceeds 60 s, Mn diffusion from the low-Mn ferrite to the region with a lower Mn concentration occurs, resulting in a decrease in the area fraction of the region with a lower Mn concentration. As a result, the area fraction of the finally obtained low-Mn ferrite is less than 5%, and the ductility decreases. The second holding time is preferably 10 s or longer, more preferably 20 s or longer. In addition, the second holding time is preferably 40 s or shorter, more preferably 30 s or shorter.

[0165] Next, the first cooling process is carried out. The cold-rolled sheet is cooled at a first cooling rate of 10°C / s or higher to a first cooling stop temperature that is below 500°C and exceeds the Ms point, and then held at this first cooling stop temperature for 10 s to 60 s, or cooled from this first cooling stop temperature to a temperature exceeding the Ms point at a third cooling rate of less than 10°C / s for 10 s to 60 s.

[0166] First cooling stop temperature: below 500°C and exceeding the Ms point

[0167] By holding the cold-rolled sheet cooled to the first cooling stop temperature at the first cooling stop temperature, or slowly cooling it from the first cooling stop temperature to a temperature exceeding the Ms point at a third cooling rate of less than 10 °C / s, high-hardness Mn ferrite is produced according to the ferrite phase transformation. When the first cooling stop temperature exceeds 500 °C, instead of hard ferrite, pearlite is formed, and the ductility, stretch flangeability, and bendability are reduced. On the other hand, when the first cooling stop temperature is below the Ms point, instead of the ferrite phase transformation, high-hardness Mn ferrite is formed, and a martensite phase transformation occurs, reducing the ductility and stretch flangeability. Therefore, the first cooling stop temperature is below 500 °C and exceeds the Ms point. The first cooling stop temperature is preferably 470 °C or lower, more preferably 450 °C or lower. In addition, the first cooling stop temperature is preferably (Ms point + 10 °C) or higher, more preferably (Ms point + 20 °C) or higher. It should be noted that Ms is calculated according to the following formula (3).

[0168] Ms (°C) = 561 - 474 [%C] - 7.5 [%Si] - 33 [Mn] - 17 [%Ni] - 17 [%Cr] - 21 [%Mo]...(3)

[0169] Among them, [%M] represents the content of M in the steel (mass %).

[0170] First cooling rate: 10 °C / s or more

[0171] When the first cooling rate to the first cooling stop temperature is less than 10 °C / s, pearlite is formed in an area fraction of 3% or more, and the ductility, stretch flangeability, and bendability are reduced. The first cooling rate is preferably 15 °C / s or more. The upper limit of the first cooling rate is not particularly limited, and from the perspective of production equipment, the first cooling rate is preferably 100 °C / s or less.

[0172] Holding or slow cooling time: 10 s to 60 s

[0173] Maintain at the first cooling stop temperature, or cool slowly from the first cooling stop temperature to a temperature exceeding the Ms point at a third cooling rate of less than 10 °C / s to produce hard high-Mn ferrite. When the holding time at the first cooling stop temperature (the third holding time) or the slow cooling time from the first cooling stop temperature to a temperature exceeding the Ms point is less than 10 s, (area fraction of ferrite) - (area fraction of low-Mn ferrite) is less than 10%, and the stretch flangeability decreases. On the other hand, when the holding time at the first cooling stop temperature or the slow cooling time from the first cooling stop temperature to a temperature exceeding the Ms point exceeds 60 s, the area fractions of tempered martensite and bainite are less than 55%, and the strength decreases. Therefore, the holding time at the first cooling stop temperature or the slow cooling time from the first cooling stop temperature to a temperature exceeding the Ms point is 10 s to 60 s. The holding time at the first cooling stop temperature or the slow cooling time from the first cooling stop temperature to a temperature exceeding the Ms point is preferably 20 s or more, more preferably 30 s or more. In addition, the holding time at the first cooling stop temperature or the slow cooling time from the first cooling stop temperature to a temperature exceeding the Ms point is preferably 50 s or less, more preferably 40 s or less.

[0174] Third cooling rate: less than 10 °C / s

[0175] In the case of slow cooling from the first cooling stop temperature, the slow cooling rate (the third cooling rate) is less than 10 °C / s. When the third cooling rate is 10 °C / s or more, (area fraction of ferrite) - (area fraction of low-Mn ferrite) is less than 10%, and the stretch flangeability decreases. It is preferably 5 °C / s or less. When the temperature is below the Ms point during cooling, the area fraction of ferrite is less than 12%, and the ductility decreases.

[0176] Next, perform the second cooling process, and cool the cold-rolled sheet to the second cooling stop temperature of (Ms point - 100 °C) to 100 °C at a second cooling rate of 10 °C / s or more.

[0177] Second cooling stop temperature: (Ms point - 100 °C) to 100 °C

[0178] By cooling to a second cooling stop temperature of (Ms point - 100 °C) to 100 °C, the austenite that has not undergone a phase transformation is transformed into martensite transformation or bainite. When the second cooling stop temperature exceeds (Ms point - 100 °C), the quenched martensite increases and the ductility decreases. On the other hand, since C has not yet thickened in the untransformed austenite during the cooling process to the second cooling stop temperature position, when the second cooling stop temperature is set to less than 100 °C, the area fraction of the retained austenite is less than 1%, and the ductility decreases. Therefore, the second cooling stop temperature is (Ms point - 100 °C) to 100 °C. The second cooling stop temperature is preferably below (Ms point - 120 °C), more preferably below (Ms point - 150 °C). In addition, the second cooling stop temperature is preferably 120 °C or higher, more preferably 150 °C or higher.

[0179] Second cooling rate: 10 °C / s or more

[0180] The second cooling rate is 10 °C / s or more. If the second cooling rate is less than 10 °C / s, the untransformed austenite is stabilized, and the martensite transformation or bainite transformation is suppressed. This untransformed austenite transforms into quenched martensite during the final cooling after the reheating process, reducing the ductility and stretch flangeability. The second cooling rate is preferably 15 °C / s or more, more preferably 20 °C / s or more. There is no particular limitation on the upper limit of the second cooling rate. From the perspective of production equipment, the second cooling rate is preferably 100 °C / s or less.

[0181] Next, a reheating process is carried out, reheating the cold-rolled sheet to a reheating temperature of the second cooling stop temperature to 450 °C, and holding it for 10 s to 1800 s in the reheating temperature range of the second cooling stop temperature to 450 °C.

[0182] Reheating temperature and reheating temperature range: second cooling stop temperature to 450 °C

[0183] By reheating, the martensite or bainite is tempered to improve the ductility, and by C partitioning to the untransformed austenite, the retained austenite is stabilized, further improving the ductility. When the reheating temperature and reheating temperature range exceed 450 °C, the C supersaturated in the martensite or bainite precipitates as cementite, and the C thickening to the retained austenite is suppressed, reducing the ductility. The reheating temperature and reheating temperature range are preferably 420 °C or lower, more preferably 400 °C or lower. It should be noted that there is no particular limitation on the heating rate up to the reheating temperature.

[0184] Fourth holding time: 10 s to 1800 s

[0185] When the holding time (the fourth holding time) in the reheating temperature range is less than 10 s, no C partitioning to retained austenite occurs, and quenched martensite is formed during the final cooling after the reheating process, resulting in a decrease in ductility and stretch flangeability. The fourth holding time is preferably 20 s or more, more preferably 100 s or more. When the fourth holding time exceeds 1800 s, the retained austenite decomposes into pearlite, and the area ratio thereof is 3% or more, leading to a decrease in ductility, stretch flangeability, and bendability. The fourth holding time is more preferably 1500 s or less.

[0186] It should be noted that the manufacturing conditions other than the above conditions can be carried out according to common methods.

[0187] Next, a method for manufacturing a high-strength plated steel sheet will be described.

[0188] In the manufacturing method of the high-strength plated steel sheet of the present embodiment, after the above reheating process, the high-strength cold-rolled steel sheet is subjected to a plating treatment to obtain a high-strength plated steel sheet.

[0189] The plating treatment can be carried out under known conditions. As the plating treatment, hot-dip galvanizing, alloyed hot-dip galvanizing, or electrogalvanizing is preferred.

[0190] [Automobile parts]

[0191] An automobile part made of at least a part of the above high-strength steel or high-strength plated steel sheet can be provided. In one example, the above high-strength steel or high-strength plated steel sheet can be formed into a target shape by stamping to manufacture an automobile part. It should be noted that the automobile part may include a steel sheet other than the high-strength steel sheet or high-strength plated steel sheet of the present embodiment as a material. According to the present embodiment, a high-strength steel sheet with a TS of 1080 MPa or more, having ductility, stretch flangeability, and bendability can be provided. Therefore, it is suitable as an automobile part contributing to the weight reduction of the vehicle body. The present high-strength steel sheet or high-strength plated steel sheet can be suitably used in all parts used as frame structure parts or reinforcement parts in automobile parts.

[0192] Examples

[0193] A steel billet is produced by melting a steel billet material having the composition shown in Table 1, with the balance being composed of Fe and inevitable impurities. After reheating the steel billet, hot rolling is performed to obtain a hot-rolled sheet, and the hot-rolled sheet is pickled and cold-rolled to obtain a cold-rolled sheet. Then, the cold-rolled sheet is subjected to a first heating process, a second heating process, a first cooling process, a second cooling process, and a reheating process to obtain a cold-rolled steel sheet (CR). The thickness of the hot-rolled sheet is 3.0 mm, the cold rolling ratio is 60%, and the thickness of the cold-rolled sheet is 1.2 mm. The slab reheating temperature (SRT), finish rolling temperature (FDT), coiling temperature (CT), first heating temperature, first holding time, heating rate, second heating temperature, second holding time, first cooling stop temperature, first cooling rate, third holding time, second cooling stop temperature, second cooling rate, reheating temperature, and fourth holding time are shown in Tables 2-1 and 2-2. It should be noted that in the first heating process, second heating process, first cooling process, second cooling process, and reheating process, except for Example No. 42, isothermal holding is performed at the first heating temperature, second heating temperature, first cooling stop temperature, second cooling stop temperature, and reheating temperature. For Example No. 42, in the first cooling process, slow cooling is performed from the first cooling stop temperature to 415°C over 35 s.

[0194] It should be noted that after the reheating process of a part of the cold-rolled steel sheet, hot-dip galvanizing treatment is performed to form a hot-dip galvanized layer on the surface, and a hot-dip galvanized steel sheet (GI) is obtained. The hot-dip galvanizing treatment uses a continuous hot-dip galvanizing production line. If necessary, the annealed cold-rolled annealed sheet is reheated to a temperature in the range of 430 to 480°C and immersed in a hot-dip galvanizing bath (bath temperature: 470°C) to adjust the galvanized layer adhesion amount to 45 g / m per side. 2 It should be noted that in the case of manufacturing a hot-dip galvanized steel sheet, the bath composition of the hot-dip galvanizing bath is a composition containing 0.18% by mass of Al, with the balance being composed of Fe and inevitable impurities. In addition, for a part of the hot-dip galvanized steel sheets, the bath composition of the hot-dip galvanizing bath is a composition containing 0.18% by mass of Al, with the balance being Fe and inevitable impurities. After the plating treatment, an alloying treatment is performed at 520°C to alloy the hot-dip galvanized layer, and an alloyed hot-dip galvanized steel sheet (GA) is obtained. It should be noted that the Fe concentration in the alloyed hot-dip galvanized layer is 9% to 12% by mass. In addition, for a part of the cold-rolled steel sheets, after the annealing process is completed, an electro-galvanizing production line is further used to perform electro-galvanizing treatment at a plating adhesion amount of 30 g / m per side 2 to obtain an electro-galvanized steel sheet (EG).

[0195] Take test pieces from the obtained high-strength cold-rolled steel sheets and conduct microstructure observation according to the above method. In addition, conduct tensile tests, hole expansion tests, and VDA bending tests according to the following methods. The results are shown in Tables 3-1 and 3-2.

[0196] (Tensile test)

[0197] Conduct a tensile test using No. 5 test pieces specified in JIS Z2201, and measure the tensile strength and elongation based on JIS Z2201. The test pieces are cut out in such a way that the direction perpendicular to the rolling direction becomes the long side direction.

[0198] (Hole expansion test)

[0199] Take test pieces of 100 mm W × 100 mm L from cold-rolled steel sheets or plated steel sheets and conduct a hole expansion test based on JIS Z2256 (2010). Punch a 10 mm φ hole in the test piece under the condition of a gap of 12 ± 1%, raise a conical punch with a vertex angle of 60° to expand the hole, and generate cracks in the thickness direction of the plate. As a result, the punch rises. According to the hole diameter after the crack is generated and the hole diameter before the test, calculate the hole expansion rate λ using the following formula.

[0200] Ultimate hole expansion rate: λ (%) = {(D f − D 0 ) / D 0} × 100

[0201] Where, in the above formula, D f is the hole diameter (mm) when the crack is generated, and D 0 is the initial hole diameter (mm). When it is independent of the strength of the steel sheet and the value of λ is 50% or more, it is judged that the stretch flangeability is very good.

[0202] (VDA bending test)

[0203] Take test pieces of 60 mm W × 60 mm L from cold-rolled steel sheets or plated steel sheets and calculate the VDA bending angle according to the provisions of the German Industrial Standard (VDA238-100). The bending direction is the direction perpendicular to the rolling direction, and the displacement at the maximum load of the bending test is converted into a bending angle according to the standard.

[0204]

[0205]

[0206]

[0207]

[0208]

[0209] The tensile strength of the inventive examples was all 1080 MPa or more, the tensile El was 10% or more, the hole expansion ratio λ was 50% or more, and the VDA bending angle was 85° or more. On the other hand, in the comparative examples, there were differences in one or more of the tensile strength, tensile El, hole expansion ratio λ, and VDA bending angle.

Claims

1. A high-strength cold-rolled steel sheet having the following composition and steel structure, The composition is calculated by mass %, C: 0.06% to 0.15%, Si: 0.10% to 1.8%, Mn: 2.00% to 3.50%, P: 0.050% or less, S: 0.0050% or less, N: 0.0060% or less, Al: 0.010% to 1.0%, Ti: 0.005% to 0.075%, Nb: 0.005% to 0.075% and B: 0.0002% to 0.0040%, the remainder is composed of Fe and unavoidable impurities, and satisfies [mol%N] / [mol%Ti]<1, In the steel structure, Ferrite: The area fraction is 12% or more and less than 30%, Tempered martensite and bainite: 55% to 85% in total by area fraction, Quenched martensite: less than 15% by area fraction, Retained austenite: 1%~10% by area fraction, The area fraction of low Mn ferrite having a Mn concentration of 0.8×[%Mn] or less is 5% to 20%, (Area fraction of the ferrite) - (Area fraction of the low-Mn ferrite): 10% or more, Other organizations: less than 3% by area fraction, Furthermore, the average grain size of the low-Mn ferrite is 10 μm or less. in, [mol%N] and [mol%Ti] represent the contents of N and Ti in the steel respectively, with the unit of content being mol%, and [%Mn] represents the content of Mn in the steel, with the unit of content being mass%.

2. A high-strength cold-rolled steel sheet having the following composition and steel structure, The composition is calculated by mass %, C: 0.06% to 0.15%, Si: 0.10% to 1.8%, Mn: 2.00% to 3.50%, P: 0.050% or less, S: 0.0050% or less, N: 0.0060% or less, Al: 0.010% to 1.0%, Ti: 0.005% to 0.075%, Nb: 0.005% to 0.075% and B: 0.0002% to 0.0040%, V: 0.200% or less, Cr: 0.20% or less, Mo: 0.20 % or less, Cu: 0.30% or less, Ni: 0.30% or less, Sb: 0.100% or less, Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.0200% or less, and Co: 0.100% or less, with the remainder being Fe and unavoidable impurities, and satisfying [mol%N] / [mol%Ti]<1, In the steel structure, Ferrite: The area fraction is 12% or more and less than 30%, Tempered martensite and bainite: 55% to 85% in total by area fraction, Quenched martensite: less than 15% by area fraction, Retained austenite: 1%~10% by area fraction, The area fraction of low-Mn ferrite having an Mn concentration of 0.8×[%Mn] or less is 5% to 20%. (The area fraction of the ferrite) - (the area fraction of the low-Mn ferrite): 10% or more. The remaining structure: less than 3% by area fraction. Moreover, the average crystal grain size of the low-Mn ferrite is 10 μm or less. Wherein, [mol%N] and [mol%Ti] respectively represent the contents of N and Ti in the steel, and the content unit is mol%, and [%Mn] represents the content of Mn in the steel, and the content unit is mass%.

3. A high-strength plated steel sheet having a plating layer on at least one side of the high-strength cold-rolled steel sheet according to claim 1 or 2.

4. A method for manufacturing a high-strength cold-rolled steel sheet, wherein a hot-rolled sheet is obtained by hot-rolling a steel slab having the composition according to claim 1 or 2. The hot-rolled sheet is pickled. The pickled hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet. Next, perform the first heating process: heat the cold-rolled sheet to the first heating temperature of Ac 1 point ~ (Ac 3 point - 50°C), and maintain it in the first heating temperature range of Ac 1 point ~ (Ac 3 point - 50°C) for more than 10 s Next, perform the second heating process: heat the cold-rolled sheet to a second heating temperature of Ac 3 point ~ (Ac 3 point + 40°C) at a heating rate of 10°C / s or more, and hold it in the second heating temperature range of Ac 3 point ~ (Ac 3 point + 40°C) for 5 s to 60 s. Then, a first cooling step is carried out: the cold-rolled sheet is cooled at a first cooling rate of 10°C / s or more to a first cooling stop temperature below 500°C and exceeding the Ms point, and then held at this first cooling stop temperature for 10 s to 60 s, or cooled from this first cooling stop temperature to exceeding the Ms point at a third cooling rate of less than 10°C / s for 10 s to 60 s. Then, a second cooling step is carried out: the cold-rolled sheet is cooled at a second cooling rate of 10°C / s or more to a second cooling stop temperature of (Ms point - 100°C) to 100°C. Then, a reheating step is carried out: the cold-rolled sheet is reheated to a reheating temperature range of the second cooling stop temperature to 450°C, and held in the reheating temperature range of the second cooling stop temperature to 450°C for 10 s to 1800 s to obtain a high-strength cold-rolled steel sheet.

5. A method for manufacturing a high-strength plated steel sheet. A hot-rolled sheet is obtained by hot-rolling a steel slab having the composition according to claim 1 or 2. The hot-rolled sheet is pickled. The pickled hot-rolled sheet is cold-rolled to obtain a cold-rolled sheet. Next, perform the first heating process: heat the cold-rolled plate to a first heating temperature of Ac 1 point ~ (Ac 3 point - 50 °C), and maintain it for more than 10 s in the first heating temperature range of Ac 1 point ~ (Ac 3 point - 50 °C). Next, perform the second heating process: heat the cold-rolled sheet to a second heating temperature of Ac 3 point ~ (Ac 3 point + 40 °C) at a heating rate of 10 °C / s or more, and hold it in the second heating temperature range of Ac 3 point ~ (Ac 3 point + 40 °C) for 5 s to 60 s. Then, a first cooling step is carried out: the cold-rolled sheet is cooled at a first cooling rate of 10°C / s or more to a first cooling stop temperature below 500°C and exceeding the Ms point, and then held at this first cooling stop temperature for 10 s to 60 s, or cooled from this first cooling stop temperature to exceeding the Ms point at a third cooling rate of less than 10°C / s for 10 s to 60 s. Then, a second cooling step is carried out: the cold-rolled sheet is cooled at a second cooling rate of 10°C / s or more to a second cooling stop temperature of (Ms point - 100°C) to 100°C. Then, a reheating step is carried out: the cold-rolled sheet is reheated to a reheating temperature range of the second cooling stop temperature to 450°C, and held in the reheating temperature range of the second cooling stop temperature to 450°C for 10 s to 1800 s to obtain a high-strength cold-rolled steel sheet. Then, the high-strength cold-rolled steel sheet is subjected to a plating treatment to obtain a high-strength plated steel sheet.

6. An automotive component, at least a part of which is made of the high-strength cold-rolled steel sheet according to claim 1 or 2.

7. An automotive component, at least a part of which is made of the high-strength plated steel sheet described in claim 3.

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