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

Abstract

The present invention provides a high-strength cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more and excellent ductility, stretch flangeability, and bendability, and a method for manufacturing the same. The high-strength cold-rolled steel sheet has the following composition and steel structure. The composition contains C, Si, Mn, P, S, N, Al, Ti, Nb, and B, and the balance consists of Fe and inevitable impurities, satisfying [mol% N] / [mol% Ti] < 1. The steel structure is ferrite: 30% to 60% by area ratio, tempered martensite and bainite: 35% to 65% in total by area ratio, quenched martensite: 15% or less by area ratio, retained austenite: 1% to 10% by area ratio, the area ratio of low-Mn ferrite having an Mn concentration of 0.8 × [% Mn] or less is 5% to 40%, (the above ferrite area ratio) - (the above low-Mn ferrite area ratio): 10% or more, and the remaining structure: less than 3% by area ratio. In addition, the average 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 plated steel sheet, a method for manufacturing a high-strength cold-rolled steel sheet, a method for manufacturing a high-strength plated steel sheet, and an automotive part. Background Art

[0002] In order to balance the collision safety of an automobile and the low fuel consumption brought about by weight reduction, high-strength steel sheets are required. In addition, in order to achieve good formability in stamping, an automotive steel sheet having excellent ductility, stretch flangeability, and bendability is 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 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 Patent Application Laid-Open 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. Accordingly, there is no steel sheet that comprehensively satisfies strength, ductility, stretch flangeability, and bendability.

[0009] The present invention has been made in view of such circumstances, and an object thereof is to provide a high-strength cold-rolled steel sheet having a tensile strength (TS) of 980 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 according to JIS Z2201 is 980 MPa or more.

[0011] In addition, excellent elongation means that the elongation rate El measured according to JIS Z2201 is 12% or more.

[0012] In addition, excellent stretch flangeability means that the value of the hole expansion ratio (λ), which is an index of stretch flangeability measured according to JIS Z2256, is 40% or more.

[0013] In addition, excellent bendability means that the VDA bend angle measured according to the German Automotive Industry Association standard VDA328-100 is 90° or more.

[0014] In order to solve the above problems, the present inventors conducted repeated and in-depth studies and obtained the following findings.

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

[0016] (2) When cooling the steel sheet having undergone Mn partitioning at an appropriate cooling rate, new ferrite phase transformation occurs with low-Mn-concentration ferrite as the nucleus in the austenite. The Mn concentration in the newly formed ferrite during the phase transformation in the cooling process remains the same as that in the austenite before the phase transformation, thus forming high-Mn-concentration ferrite.

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

[0018] (4) The dispersion distribution of low-Mn-concentration ferrite can effectively improve the bendability.

[0019] The present invention was completed based on the above findings. That is, the gist of the present invention is as follows.

[0020] [1] A high-strength cold-rolled steel sheet having the following composition and steel structure: The composition contains, 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%, with the balance being Fe and unavoidable impurities, and satisfying [mol% N] / [mol% Ti] < 1,

[0021] In the steel structure, ferrite: 30% to 60% or less by area ratio, tempered martensite and bainite: 35% to 65% or less in total by area ratio, quenched martensite: 15% or less by area ratio, retained austenite: 1% to 10% by area ratio, the area ratio of low-Mn-concentration ferrite having a Mn concentration of 0.8 × [% Mn] or less is 5% to 40%, (the area ratio of the above ferrite) - (the area ratio of the above low-Mn ferrite): 10% or more, and the remaining structure: less than 3% by area ratio. In addition, the average particle size of the above low-Mn ferrite is 10 μm or less.

[0022] Here, [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.

[0023] [2] For the high-strength cold-rolled steel sheet described in the above [1], the above composition further contains, by mass%, at least one 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.

[0024] [3] A high-strength coated steel sheet having a coating on at least one side of the high-strength cold-rolled steel sheet described in the above [1] or [2].

[0025] [4] A method for manufacturing a high-strength cold-rolled steel sheet, which includes hot-rolling a steel slab having the composition described in the above [1] or [2] to form a hot-rolled sheet,

[0026] pickling the above hot-rolled sheet,

[0027] cold-rolling the pickled hot-rolled sheet to form a cold-rolled sheet,

[0028] Next, the following first heating process 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),

[0029] Next, the following second heating process is carried out: heating the above cold-rolled sheet to a second heating temperature of (the above first heating temperature + 20 °C) or more and less than the Ac 3 point at a heating rate of 10 °C / s or more, and maintaining it for 5 s to 60 s in the second heating temperature range of (the above first heating temperature + 20 °C) or more and less than the Ac 3 point,

[0030] Next, the following first cooling process 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 greater than 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 at a third cooling rate less than 10 °C / s for 10 s to 60 s to a temperature greater than the Ms point.

[0031] Next, the following second cooling process 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.

[0032] Next, the following reheating process is carried out: The above cold-rolled sheet is reheated to a reheating temperature range of the above second cooling stop temperature to 450 °C and held for 10 s to 1800 s.

[0033] [5] A method for manufacturing a high-strength plated steel sheet, after the reheating process described in the above [4], a plating treatment is performed on the above high-strength cold-rolled steel sheet to obtain a high-strength plated steel sheet.

[0034] [6] An automotive component, at least partially using the high-strength cold-rolled steel sheet described in the above [1] or [2].

[0035] [7] An automotive component, at least partially using the high-strength plated steel sheet described in the above [3].

[0036] According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility, stretch flangeability, and bendability, and a manufacturing method thereof. Detailed Embodiments

[0037] Hereinafter, embodiments of the present invention will be described. It should be noted that the present invention is not limited to the following embodiments.

[0038] 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 represents "mass%" unless otherwise specified. In addition, in this specification, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0039] [Essential Components]

[0040] C: 0.06% to 0.15%

[0041] C is contained in bainite or tempered martensite, which helps to increase the strength. In addition, the enrichment of C in austenite has the effect of stabilizing retained austenite, and this retained austenite contributes to ductility. In order to obtain such an effect, the C content is made 0.06% or more. On the other hand, when the C content is greater than 0.15%, the quenched martensite increases and the stretch flange formability decreases. Also, the bendability decreases. The C content is preferably 0.07% or more, more preferably 0.08% or more. In addition, the C content is preferably 0.14% or less, more preferably 0.11% or less.

[0042] Si: 0.10% to 1.8%

[0043] Si contributes to the increase in strength through solid solution strengthening. In addition, in order to suppress the formation of cementite and contribute to the stabilization of retained austenite, it is necessary to contain Si at 0.10% or more. On the other hand, since Si is in the ferrite-austenite two-phase region and enriches in ferrite, it will enrich in the low-Mn ferrite region. If Si enriches too much in ferrite, the slip system of dislocations changes, resulting in a decrease in bendability. Therefore, the Si content is made 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.

[0044] Mn: 2.00% to 3.50%

[0045] Mn is an important element for solid solution strengthening of ferrite by element partitioning. When the Mn content is less than 2.00%, the effect of solid solution strengthening cannot be obtained sufficiently. On the other hand, when the Mn content is greater than 3.50%, the ferrite phase transformation during cooling after the re-heating process is excessively suppressed, and ferrite with a high Mn concentration cannot be generated sufficiently. As a result, elongation and stretch flange formability deteriorate. Therefore, the Mn content is made 2.00% to 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.

[0046] P: 0.050% or less

[0047] When P is greater than 0.050%, the weldability decreases. Therefore, the P content is made 0.050% or less. The lower limit of the P content is not particularly limited and may also 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.

[0048] S: 0.0050% or less

[0049] When S is greater than 0.0050%, the stretch flangeability is reduced. Therefore, the S content is made 0.0050% or less. The lower limit of the S content is not particularly limited and may 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.

[0050] N: 0.0060% or less

[0051] When an excessive amount of N is contained, nitrides are formed and the ductility and bendability are reduced. In addition, if BN is formed by combining with B, the effect of increasing the strength by B cannot be obtained. Therefore, the N content is made 0.0060% or less. The lower limit of the N content is not particularly limited and may 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.

[0052] Al: 0.010% to 1.0%

[0053] When Al is 0.010% or more, it acts as a deoxidizer. On the other hand, if the Al content is greater than 1.0%, not only does the effect saturate, but the weldability also decreases. Therefore, the Al content is made 0.010% to 1.0%. The Al content is preferably 0.02% or more. In addition, the Al content is preferably 0.9% or less.

[0054] Ti: 0.005% to 0.075%

[0055] Ti has the effect of fixing N in steel in the form of nitride TiN. In order to obtain this effect, the Ti content is made 0.005% or more. On the other hand, when the Ti content is greater than 0.075%, an excessive amount of carbides are generated and the ductility is reduced. The Ti content is preferably 0.008% or more. In addition, the Ti content is preferably 0.05% or less.

[0056] Nb: 0.005% to 0.075%

[0057] Nb segregates to the grain boundary in a solid solution state or precipitates fine carbides having a pinning effect, and has the effect of dispersing the ferrite phase with a low Mn concentration in the first heating process in the ferrite-austenite two-phase region. In order to obtain this effect, Nb is added at 0.005% or more. On the other hand, when the Nb content is greater than 0.075%, not only does the effect saturate, but an excessive amount of carbides are generated and the ductility is reduced. Therefore, the Nb content is made 0.005% to 0.075%. The Nb content is preferably 0.008% or more. In addition, the Nb content is preferably 0.05% or less.

[0058] B: 0.0002% to 0.0040%

[0059] B is an element that not only helps to improve strength but also refines the ferrite phase with a low Mn concentration and improves the bending property effect during the first heating process in the ferrite-austenite two-phase region, and is required to contain 0.0002% or more. On the other hand, when the B content is greater than 0.0040%, the ductility decreases. Therefore, the B content is made 0.0002% to 0.0040%. The B content is preferably 0.0007% or more. In addition, the B content is preferably 0.0030% or less.

[0060] [mol% N] / [mol% Ti] < 1

[0061] Ti has the effect of fixing N in the form of TiN. However, if the molar amount of the Ti content is the same as or lower than the molar amount of the N content, the N not fixed by Ti combines with B, which will reduce or eliminate the effect of containing B.

[0062] [Arbitrary component]

[0063] In addition to the above composition, the high-strength cold-rolled steel sheet according to this 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.

[0064] V: 0.200% or less

[0065] When the V content is 0.005% or more, fine carbides are formed, which helps to improve strength. Therefore, when V is contained, the V content is preferably 0.005% or more. On the other hand, in order to further improve strength and obtain more excellent ductility by preventing the coarsening of carbides, the V content is preferably 0.200% or less. Thus, when V is 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.

[0066] Cr: 0.20% or less

[0067] When the Cr content is 0.05% or more, it helps to increase the strength through solid solution strengthening. Therefore, when Cr is contained, the Cr content is preferably 0.05% or more. On the other hand, from the viewpoint of preventing the formation of cementite and further improving ductility and stretch flangeability, when Cr is 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.

[0068] Mo: 0.20% or less

[0069] When the Mo content is 0.01% or more, it helps to increase the strength through solid solution strengthening. Therefore, when Mo is contained, the Mo content is preferably 0.01% or more. On the other hand, since the effect saturates when the Mo content is greater than 0.20%, in order to further reduce the manufacturing cost, when Mo is 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.

[0070] Cu: 0.30% or less

[0071] When the Cu content is 0.01% or more, it helps to increase the strength through solid solution strengthening. Therefore, when Cu is contained, the Cu content is preferably 0.01% or more. On the other hand, in order to obtain better ductility and stretch flangeability, when Cu is 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.

[0072] Ni: 0.30% or less

[0073] When the Ni content is 0.01% or more, it helps to increase the strength through solid solution strengthening. Therefore, when Ni is contained, the Ni content is preferably 0.01% or more. On the other hand, since the effect saturates when the Ni content is greater than 0.30%, 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.

[0074] Sb: 0.100% or less

[0075] Sn: 0.100% or less

[0076] When the contents of Sb and Sn are each 0.002% or more, it has the effect of suppressing decarburization on the surface layer of the steel sheet. Therefore, when Sb and Sn are contained, the contents of Sb and Sn are each preferably 0.002% or more. On the other hand, since the effect saturates when the contents of Sb and Sn are each greater than 0.100%. Therefore, from the viewpoint of further reducing the manufacturing cost, when Sb and Sn are contained, the contents of Sb and Sn are each preferably 0.100% or less. The contents of Sb and Sn are each more preferably 0.004% or more. In addition, the contents of Sb and Sn are each more preferably 0.05% or less.

[0077] Ca: 0.0050% or less

[0078] Mg: 0.0050% or less

[0079] REM: 0.0050% or less

[0080] When the contents of Ca, Mg and REM are each 0.0001% or more, they act as deoxidizers. Therefore, when Ca, Mg and REM are added, they are each preferably 0.0001% or more. On the other hand, from the viewpoint of further improving the stretch flangeability, when Ca, Mg and REM are contained, the contents are each preferably 0.0050% or less. The contents of Ca, Mg and REM are each more preferably 0.0002% or more. In addition, the contents of Ca, Mg and REM are each more preferably 0.0040% or less.

[0081] Ta: 0.100% or less

[0082] Ta has the effect of forming fine carbides and increasing the strength of the steel sheet. When Ta is contained, in order to obtain such an effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content is greater than 0.100%, excessive precipitation of Ta carbides reduces the ductility. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.050% or less.

[0083] W: 0.500% or less

[0084] W has the effect of increasing the strength of the steel sheet by solid solution strengthening. When W is contained, in order to obtain such an effect, the W content is preferably 0.005% or more. On the other hand, when the W content is greater than 0.500%, excessive precipitation of W carbides reduces the ductility. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.300% or less.

[0085] Zr: 0.0200% or less

[0086] Zr can be used as a deoxidizer. In the case of containing Zr, in order to obtain such an effect, the Zr content is preferably 0.0001% or more. On the other hand, if the Zr content is greater than 0.0200%, excessive precipitation of Zr carbide occurs and the ductility decreases. Therefore, in the case of containing Zr, the Zr content is preferably 0.0200% or less. The Zr content is more preferably 0.0150% or less.

[0087] Co: 0.100% or less

[0088] Co has the effect of improving the strength of the steel sheet by solid solution strengthening. In the case of containing Co, in order to obtain such an effect, the Co content is preferably 0.005% or more. On the other hand, if the Co content is greater than 0.100%, the effect saturates. Therefore, in the case of containing Co, the Co content is preferably 0.100% or less. The Co content is more preferably 0.080% or less.

[0089] Except for the above components, the balance 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, the effects of the present invention will not be impaired. Therefore, in the case of containing these arbitrary elements less than the lower limit value, they can be regarded as inevitable impurities.

[0090] [Steel structure]

[0091] Next, the steel structure of the high-strength cold-rolled steel sheet will be described.

[0092] Ferrite: 30% - 60% by area ratio

[0093] Ferrite is effective in improving ductility. In addition, by the occurrence of ferrite phase transformation, C is enriched in the retained austenite, and thus the ductility can be improved. Therefore, the area ratio of ferrite needs to reach 30% or more. When the area ratio of ferrite is greater than 60%, the strength decreases. The area ratio of ferrite is preferably 33% or more, more preferably 35% or more. In addition, the area ratio of ferrite is preferably 54% or less, more preferably 50% or less. It should be noted that the above area ratio is the total area ratio of high-Mn-concentration ferrite and low-Mn ferrite described later.

[0094] Tempered martensite and bainite: 35% - 65% in total by area ratio

[0095] Tempered martensite and bainite have a dislocation density higher than that of ferrite and are structures containing cementite. Tempered martensite and bainite are effective in increasing strength. In order to achieve high strength, the total area ratio of tempered martensite and bainite needs to reach 35% or more. On the other hand, when the total area ratio of tempered martensite and bainite is greater than 65%, the ductility decreases. The total area ratio of tempered martensite and bainite is preferably 40% or more, more preferably 45% or more. In addition, the total area ratio of tempered martensite and bainite is preferably 60% or less.

[0096] Quenched martensite: 15% or less by area ratio

[0097] Quenched martensite also has a dislocation density higher than that of ferrite and is a very hard structure that does not contain cementite but solid-solves C. When the area ratio of quenched martensite is greater than 15%, the ductility, stretch flangeability, and bendability decrease. The area ratio of quenched martensite is preferably 13% or less, more preferably 10% or less. There is no particular limitation on the lower limit of the area ratio of quenched martensite, and it can also be 0%. However, since it is difficult to completely suppress the formation of quenched martensite, it is preferably 3% or more.

[0098] Retained austenite: 1% - 10% by area ratio

[0099] By making the retained austenite 1% or more by area ratio, the effect of transformation-induced plasticity helps to improve the ductility. On the other hand, when the retained austenite is greater than 10% by area ratio, it will cause a decrease in stretch flangeability. The area ratio of retained austenite is preferably 3% or more. In addition, the area ratio of retained austenite is preferably 8% or less.

[0100] Area ratio of the remaining structure: less than 3%

[0101] In addition to the above-mentioned ferrite, tempered martensite, bainite, quenched martensite, and retained austenite, within the range that does not damage the effects of this embodiment, the steel structure may also contain carbides such as pearlite and cementite, and other structures (the remaining structure). Since when the area ratio of the remaining structure is 3% or more, the ductility, stretch flangeability, and bendability decrease, the area ratio of the remaining structure is made less than 3%. It should be noted that the type and area ratio of the remaining structure can be confirmed and determined by, for example, SEM observation.

[0102] Low-Mn ferrite with an Mn concentration of 0.8×[%Mn] or less: 5% - 40% by area ratio

[0103] Ferrite with a low Mn concentration of 0.8×[%Mn] or less is referred to as low-Mn ferrite. Low-Mn ferrite is formed by heating and holding in the two-phase region of ferrite-austenite in the first heating process. Low-Mn ferrite is distinguished from the ferrite with a high Mn concentration formed by phase transformation in the subsequent first cooling process. Low-Mn ferrite can also remain as ferrite in the second heating process and become the nucleus when ferrite is formed by phase transformation in the first cooling process. When the area ratio of low-Mn ferrite is less than 5%, the ferrite nuclei decrease, and sufficient ferrite cannot be formed in the first cooling process, resulting in a decrease in ductility. On the other hand, when the area ratio of low-Mn ferrite is greater than 40%, the dispersion distribution of low-Mn ferrite becomes difficult, and the bendability decreases. The area ratio of low-Mn ferrite is preferably 7% or more, more preferably 15% or more. In addition, the area ratio of low-Mn ferrite is preferably 37% or less, more preferably 35% or less.

[0104] (Area ratio of ferrite) - (Area ratio of low-Mn ferrite): 10% or more

[0105] By subtracting the area ratio of low-Mn ferrite from the area ratio of all ferrite, the area ratio of the high-Mn concentration hard ferrite (hereinafter also referred to as high-Mn ferrite) formed in the first cooling process can be obtained. It should be noted that the Mn concentration of high-Mn ferrite is higher than that of low-Mn ferrite, that is, ferrite with a Mn concentration greater than 0.8×[%Mn]. When (Area ratio of ferrite) - (Area ratio of low-Mn ferrite) is less than 10%, the hard high-Mn ferrite is insufficient, and the stretch flangeability decreases. (Area ratio of ferrite) - (Area ratio of low-Mn ferrite) is preferably 12% or more, more preferably 15% or more. It should be noted that the upper limit of (Area ratio of ferrite) - (Area ratio of low-Mn ferrite) is not particularly limited, and is preferably 55% or less.

[0106] Average particle size of low-Mn ferrite: 10 μm or less

[0107] By dispersing soft low-Mn ferrite in the steel sheet, the bendability can be improved. To improve the bendability, it is necessary for the low-Mn ferrite to be dispersed without connection. When the average particle size (equivalent circle diameter) of the low-Mn ferrite is greater than 10 μm, the effect of improving the bendability cannot be obtained. The average particle size of the low-Mn ferrite is preferably 8 μm or less, more preferably 6 μm or less.

[0108] Here, the area ratio of each tissue is measured as follows. First, a specimen for tissue observation is taken from the high-strength cold-rolled steel sheet. A cross-section (L-section) parallel to the rolling direction of the specimen is obtained, and from the surface of the steel sheet, it is ground along the plate thickness depth direction to a position equivalent to 1 / 4 of the plate thickness as the observation surface, and then etched with a 3% nitric acid alcohol solution. The observation surface is observed at a magnification of 2000 times using a Scanning Electron Microscope (SEM) to obtain a tissue image.

[0109] (Ferrite)

[0110] The area ratio of ferrite is obtained as follows. In the observation of the secondary electron image using SEM, high-Mn ferrite and low-Mn ferrite are observed with the same contrast and can be distinguished from other tissues. By performing image analysis on the obtained tissue image, the area ratio of ferrite and the area ratio of other tissues can be obtained.

[0111] (Quenched martensite)

[0112] The area ratio of quenched martensite is obtained as follows. Observe the same field of view as the above tissue image under SEM Electron Backscatter Diffraction (EBSD), and perform analysis using the ImageQuality map (IQ map). The area with an IQ value lower than the surrounding area is regarded as quenched martensite, and its area ratio is calculated.

[0113] (Retained austenite)

[0114] The area ratio of retained austenite is obtained as follows. A specimen is taken from the high-strength cold-rolled steel sheet. The specimen is ground and polished in the plate thickness direction (depth direction) until it reaches a position equivalent to 1 / 4 of the plate thickness from the surface of the steel sheet in the plate thickness depth direction, and this is used as the measurement surface. By performing X-ray diffraction analysis on the measurement surface, the amount of retained austenite is obtained. Calculate 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, and calculate the amount of austenite from their average value. In this method, the volume ratio of austenite can be obtained, and this value is set as the area ratio of austenite.

[0115] (Tempered martensite and bainite)

[0116] Subtract the area ratio of the above-mentioned quenched martensite and the area ratio of the above-mentioned retained austenite from the area ratio of the tissues other than the above-mentioned ferrite to obtain the total area ratio of tempered martensite and bainite.

[0117] (Low-Mn ferrite)

[0118] The area ratio and average particle size of the 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) until the position equivalent to 1 / 4 of the sheet thickness from the steel sheet surface in the sheet thickness depth direction is used as the analysis surface. The concentration of Mn in the 100×100 μm 2 area of the analysis surface is measured using an electron probe micro analyzer (EPMA). The area ratio of the low-Mn ferrite is the area ratio of the region below 0.8×[%Mn] obtained by image analysis based on the measurement result of the Mn concentration by EPMA. The average particle size (equivalent circle diameter) of the low-Mn ferrite is obtained by image analysis based on the region below 0.8×[%Mn].

[0119] 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.

[0120] In order to improve the corrosion resistance of the above high-strength cold-rolled steel sheet, a coating can be provided on at least one side. As the coating, 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 coating is not particularly limited and can be a known composition.

[0121] The composition of the hot-dip galvanized layer is not particularly limited as long as it is a normal composition. For example, the coating has the following composition: containing Fe: 20% by mass or less, Al: 0.001% by mass to 1.0% by mass, further containing one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, and the total is 0% by mass to 3.5% by mass, and the balance consists of Zn and inevitable impurities. When the coating is a hot-dip galvanized layer, for example, the content of Fe in the coating is less than 7% by mass, and when it is an alloyed hot-dip galvanized layer, for example, the content of Fe in the coating is 7% by mass to 15% by mass, and more preferably 8% by mass to 13% by mass.

[0122] The coating adhesion amount is not particularly limited, and the single-sided coating adhesion amount of the high-strength cold-rolled steel sheet is preferably 20 to 80 g / m 2 . For example, coatings are formed on both the inner and outer surfaces of the high-strength cold-rolled steel sheet.

[0123] Next, the manufacturing method of the high-strength cold-rolled steel sheet will be described. The manufacturing method of the high-strength cold-rolled steel sheet according to this embodiment is as follows: The steel billet having the above composition is hot-rolled to form a hot-rolled sheet,

[0124] the above hot-rolled sheet is pickled,

[0125] The above-mentioned hot-rolled sheet after pickling is cold-rolled to produce a cold-rolled sheet.

[0126] Next, the following first heating process is carried out: heat the above-mentioned cold-rolled sheet to a first heating temperature of Ac 1 point to (Ac 3 point - 50°C), and maintain it for more than 10 s within the first heating temperature range of Ac 1 point to (Ac 3 point - 50°C).

[0127] Next, the following second heating process is carried out: heat the above-mentioned cold-rolled sheet to a second heating temperature of (the above first heating temperature range + 20°C) or higher and less than Ac 3 point at a heating rate of 10°C / s or higher, and maintain it for 5 s to 60 s within the second heating temperature range of (the above first heating temperature range + 20°C) or higher and less than Ac 3 point.

[0128] Next, the following first cooling process is carried out: cool the above-mentioned cold-rolled sheet to a first cooling stop temperature of less than 500°C and higher than the Ms point at a first cooling rate of 10°C / s or higher, then maintain it at this first cooling stop temperature for 10 s to 60 s, or cool it from this first cooling stop temperature for 10 s to 60 s to a temperature higher than the Ms point at a third cooling rate of less than 10°C / s.

[0129] Next, the following second cooling process is carried out: cool the above-mentioned cold-rolled sheet to a second cooling stop temperature of (Ms point - 100°C) to 100°C at a second cooling rate of 10°C / s or higher.

[0130] Next, the following reheating process is carried out: reheat the above-mentioned cold-rolled sheet to a reheating temperature of the above second cooling stop temperature to 450°C, and maintain it for 10 s to 1800 s within the reheating temperature range of the above second cooling stop temperature to 450°C.

[0131] First, a steel billet with the above composition is hot-rolled to produce a hot-rolled sheet.

[0132] First, a steel billet with the above composition is manufactured. First, the steel is melted into molten steel with the above composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting are applicable. The obtained molten steel is solidified to produce a steel billet (slab). The method of producing a steel billet from molten steel is not particularly limited, and continuous casting, ingot casting, or thin slab casting methods can be used. The steel billet can also be cooled and heated before hot-rolling, or the cast steel billet can be continuously hot-rolled without cooling to room temperature.

[0133] Next, the hot-rolled sheet is produced by subjecting the produced steel billet to hot-rolling consisting of rough rolling and finish rolling.

[0134] In one example, the billet made as described above is temporarily cooled to room temperature and then the slab is heated and rolled. In addition, energy-saving processes can also be applied to perform hot rolling. As energy-saving processes, there can be cited direct feeding rolling in which the billet made is not cooled to room temperature but charged into a heating furnace in a warm billet state for hot rolling, or direct rolling in which the billet made is slightly heat-retained and then immediately rolled, etc.

[0135] The hot rolling start temperature is preferably 1100°C or higher. By setting the hot rolling start temperature at 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.

[0136] The finish rolling temperature is preferably above the Ar 3 point. By setting the finish rolling temperature above the Ar 3 point, the hot rolling 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 at 1000°C or lower, coarsening of the hot rolling structure can be prevented and the bendability of the high-strength cold-rolled steel sheet can be further improved.

[0137] The coiling temperature of the hot-rolled sheet after hot rolling is preferably 500°C or lower. By setting the coiling temperature at 500°C or lower, formation of a lamellar structure of ferrite - pearlite can be prevented, and connection of ferrite in the first heating process can be prevented, further improving the bendability.

[0138] Next, pickling is performed on the above hot-rolled sheet. By pickling, the scale on the surface of the hot-rolled sheet can be removed. The pickling conditions can be carried out according to conventional methods.

[0139] Next, cold rolling is performed on the pickled hot-rolled sheet to make a cold-rolled sheet. The cold rolling conditions can be carried out according to conventional methods. The reduction ratio of cold rolling is not particularly limited. For example, it can be 30% or more, or it can also be 80% or less.

[0140] Next, annealing including a first heating process, a second heating process, a first cooling process, a second cooling process, and a re-heating process is performed on the cold-rolled sheet. In one example, the cold-rolled sheet obtained as described above is fed into a continuous annealing furnace and annealing is performed. In addition, when hot-dip galvanizing or alloyed hot-dip galvanizing is performed on the surface of the high-strength cold-rolled steel sheet to form a coating, the cold-rolled sheet is fed into a continuous hot-dip galvanizing apparatus, and continuous annealing and coating treatment can also be performed.

[0141] First, the following first heating process is carried out: the cold-rolled sheet is heated to a first heating temperature of Ac 1 point to (Ac 3 point - 50°C), and at Ac1 Point ~ (Ac 3 Point - 50 °C) and maintained for 10 s or more in the first heating temperature range.

[0142] The first heating temperature and the first heating temperature range: Ac 1 Point ~ (Ac 3 Point - 50 °C)

[0143] In the first heating process, the cold-rolled sheet is heated and maintained in the two-phase region of ferrite and austenite to cause Mn partitioning, reducing the Mn concentration in the ferrite phase and increasing the Mn concentration in the austenite phase. As a result, low-Mn ferrite is formed. When the first heating temperature and the first heating temperature range are less than Ac 1 Point, Mn partitioning does not occur and the bendability decreases. On the other hand, when the first heating temperature and the first heating temperature range are greater than (Ac 3 Point - 50 °C), coarse ferrite is formed. This coarse ferrite cannot be refined even after the second heating process, and the average grain size of the low-Mn ferrite becomes greater than 10 μm, reducing the bendability. Thus, the first heating temperature and the first heating temperature range are Ac 1 Point ~ (Ac 3 Point - 50 °C). The first heating temperature range is preferably (Ac 1 Point + 10 °C) or more, more preferably (Ac 1 + 30 °C) or more. In addition, the first heating temperature and the first heating temperature range are preferably (Ac 3 Point - 60 °C) or less. It should be noted that in this application, maintaining the temperature within the specified temperature range means that the maintained temperature can vary within this temperature range and does not require isothermal holding at the specified temperature. In addition, Ac 1 and Ac 3 are calculated by the following formulas (1) and (2) respectively.

[0144] Ac 1 (°C) = 751 - 16 [%C] + 35 [%Si] - 28 [%Mn] - 5.5 [%Cu] - 16 [%Ni] + 13 [%Cr] + 3.4 [%Mo]... (1)

[0145] Ac 3 (°C) = 881 - 206 [%C] + 53 [%Si] - 15 [%Mn] - 27 [%Cu] - 20 [%Ni] - 1 [%Cr] + 41 [%Mo]... (2)

[0146] Where, [%M] represents the content of M in the steel (mass %).

[0147] The first holding time: 10 s or more

[0148] Set the holding time (the first holding time) in the first heating temperature range to 10 s or more. When the first holding time is less than 10 s, the Mn distribution is insufficient, and high-Mn ferrite cannot be sufficiently formed, resulting in a decrease in the stretch flangeability. The upper limit of the first holding time is not particularly limited, but from the perspective of production, the holding time is preferably 1800 s or less. The first holding time is preferably 20 s or more, more preferably 100 s or more. In addition, the first holding time is preferably 1500 s or less.

[0149] Next, perform the following second heating process: Heat the cold-rolled sheet at a heating rate of 10 °C / s or more to a second heating temperature of (the first heating temperature + 20 °C) to less than Ac 3 and hold it for 5 s to 60 s in the second heating temperature range of (the first heating temperature + 20 °C) to less than Ac 3 .

[0150] In the second heating process, maintain the average grain size of the low-Mn ferrite generated in the first heating process at 10 μm or less, and reduce the area ratio of the low-Mn ferrite to 5% to 40%.

[0151] Heating rate: 10 °C / s or more

[0152] When the heating rate is less than 10 °C / s, due to the growth of ferrite grains during the heating process, the average grain size of the low-Mn ferrite becomes greater than 10 μm. The heating rate is preferably 15 °C / s or more. The upper limit of the heating rate is not particularly limited, and from the perspective of production technology, it is preferably 50 °C / s or less.

[0153] Second heating temperature and second heating temperature range: (the first heating temperature + 20 °C) or more and less than Ac 3

[0154] When the second heating temperature and the second heating temperature range are less than (the first heating temperature + 20 °C), the area ratio of the low-Mn ferrite is greater than 40%, and in the subsequent first cooling process, hard high-Mn ferrite generated by phase transformation cannot be sufficiently obtained. On the other hand, when the second heating temperature and the second heating temperature range reach Ac 3 or more, since the ferrite disappears, the ferrite phase transformation in the first cooling process requires a nucleation process, the area ratio of the ferrite decreases, and the ductility decreases. The second heating temperature and the second heating temperature range are preferably (the first heating temperature + 30 °C) or more, more preferably (the first heating temperature + 40 °C) or more. In addition, the second heating temperature and the second heating temperature range are preferably (Ac 3 −10 °C) or less, more preferably (Ac 3 −20 °C) or less.

[0155] Second holding time: 5 s to 60 s

[0156] When the holding time (second holding time) in the second heating temperature range is less than 5 s, the area ratio of low-Mn ferrite becomes greater than 40%, and since hard high-Mn ferrite cannot be sufficiently obtained, the stretch flangeability decreases. On the other hand, when the second holding time is greater than 60 s, the ferrite-austenite interface is too stable, and in the subsequent first cooling process, the ferrite phase transformation proceeds insufficiently, and (ferrite area ratio) - (low-Mn ferrite area ratio) is less than 10%, resulting in a decrease in stretch flangeability. The second holding time is preferably 10 s or more, more preferably 20 s or more. In addition, the second holding time is preferably 40 s or less, more preferably 30 s or less.

[0157] Next, the following first cooling process 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 that is less than 500 °C and greater than the Ms point, and held at this first cooling stop temperature for 10 s to 60 s, or cooled from the first cooling stop temperature at a third cooling rate of less than 10 °C / s for 10 s to 60 s to a temperature greater than the Ms point.

[0158] First cooling stop temperature: less than 500 °C and greater than the Ms point

[0159] By holding the cold-rolled sheet cooled to the first cooling stop temperature at this first cooling stop temperature, or slowly cooling from the first cooling stop temperature at a third cooling rate of less than 10 °C / s to a temperature greater than the Ms point, hard high-Mn ferrite can be formed by ferrite phase transformation. When the first cooling stop temperature is greater than 500 °C, pearlite is formed instead of hard ferrite, and the ductility, stretch flangeability, and bendability decrease. On the other hand, when the first cooling stop temperature is below the Ms point, martensite phase transformation occurs instead of forming hard high-Mn ferrite by ferrite phase transformation, and the ductility and stretch flangeability decrease. Therefore, the first cooling stop temperature is set to be less than 500 °C and greater than the Ms point. The first cooling stop temperature is preferably 470 °C or less, more preferably 450 °C or less. In addition, the first cooling stop temperature is preferably (Ms point + 10 °C) or more, more preferably (Ms point + 20 °C) or more. It should be noted that Ms is obtained by the following formula (3).

[0160] Ms = 561 - 474[%C] - 7.5[%Si] - 33[Mn] - 17[%Ni] - 17[%Cr] - 21[%Mo]…(3)

[0161] Among them, [%M] represents the content (mass%) of M in the steel.

[0162] First cooling rate: 10 °C / s or more

[0163] When the first cooling rate upon reaching the first cooling stop temperature is less than 10 °C / s, pearlite with an area ratio of 3% or more is formed, and the ductility, stretch flange property, and bendability are reduced. The first cooling rate is preferably 15 °C / s or more. There is no particular limitation on the upper limit of the first cooling rate. From the viewpoint of production equipment, the first cooling rate is preferably 100 °C / s or less.

[0164] Holding or slow cooling time: 10 s to 60 s

[0165] By holding at the first cooling stop temperature or slowly cooling from the first cooling stop temperature to above the Ms point at a third cooling rate of less than 10 °C / s, hard high-Mn ferrite can be formed. When the holding time at the first cooling stop temperature (third holding time) or the time for slowly cooling from the first cooling stop temperature to above the Ms point is less than 10 s, (ferrite area ratio) - (low-Mn ferrite area ratio) is less than 10%, and the stretch flange property is reduced. On the other hand, when the holding time at the first cooling stop temperature or the time for slowly cooling from the first cooling stop temperature to above the Ms point is greater than 60 s, the area ratio of tempered martensite and bainite is less than 35%, and the strength decreases. Therefore, the holding time at the first cooling stop temperature or the time for slowly cooling from the first cooling stop temperature to above the Ms point is set to 10 s to 60 s. The holding time at the first cooling stop temperature or the time for slowly cooling from the first cooling stop temperature to above 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 time for slowly cooling from the first cooling stop temperature to the Ms point is preferably 50 s or less, more preferably 40 s or less.

[0166] Third cooling rate: less than 10 °C / s

[0167] When slowly cooling from the first cooling stop temperature, the slow cooling rate (third cooling rate) is set to less than 10 °C / s. When the third cooling rate is 10 °C / s or more, (ferrite area ratio) - (low-Mn ferrite area ratio) is less than 10%, and the stretch flange property is reduced. It is preferably 5 °C / s or less. If the Ms point is reached or below during the cooling process, the ferrite area ratio is less than 30%, and the ductility is reduced.

[0168] Next, the following second cooling process is carried out: The cold-rolled sheet is cooled to a second cooling stop temperature of (Ms point - 100 °C) to 100 °C at a second cooling rate of 10 °C / s or more.

[0169] Second cooling stop temperature: (Ms point - 100 °C) to 100 °C

[0170] By cooling to a second cooling stop temperature of (Ms point - 100°C) to 100°C, martensitic transformation or bainitic transformation occurs in the austenite that has not undergone a phase change. When the second cooling stop temperature is greater than (Ms point - 100°C), the quenched martensite increases and the ductility decreases. On the other hand, during the cooling to the second cooling stop temperature, since C has not been enriched in the austenite that has not undergone a phase change, if the second cooling stop temperature is set to less than 100°C, the area ratio of the retained austenite is less than 1%, and the ductility decreases. Therefore, the second cooling stop temperature is set to (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). Additionally, the second cooling stop temperature is preferably 120°C or higher, more preferably 150°C or higher.

[0171] Second cooling rate: 10°C / s or more

[0172] The second cooling rate is set to 10°C / s or more. When the second cooling rate is less than 10°C / s, the austenite that has not undergone a phase change becomes stable, and the martensitic transformation or bainitic transformation is suppressed. This austenite that has not undergone a phase change transforms into quenched martensite during the final cooling after the reheating process, resulting in a decrease in 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.

[0173] Next, the following reheating process is carried out: The cold-rolled sheet is reheated to a reheating temperature of the second cooling stop temperature to 450°C and held within the reheating temperature range of the second cooling stop temperature to 450°C for 10 s to 1800 s.

[0174] Reheating temperature and reheating temperature range: Second cooling stop temperature to 450°C

[0175] Tempering of martensite or bainite is carried out by reheating to improve the ductility and stabilize the retained austenite by C partitioning to the austenite that has not undergone a phase change, thereby further improving the ductility. When the reheating temperature and reheating temperature range are greater than 450°C, the supersaturated C in martensite or bainite precipitates as cementite, inhibiting the C enrichment in the retained austenite and 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 to reach the reheating temperature.

[0176] Fourth holding time: 10 s to 1800 s

[0177] When the holding time (the fourth holding time) within the reheating temperature range is less than 10 s, C partitioning to retained austenite does not occur, 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. If the fourth holding temperature is greater than 1800 s, the retained austenite decomposes into pearlite, and the area ratio of pearlite reaches 3% or more, resulting in a decrease in ductility, stretch flangeability, and bendability. The holding time of the reheating temperature is more preferably 1500 s or less.

[0178] It should be noted that the manufacturing conditions other than the above conditions can be carried out according to conventional methods.

[0179] Next, a method for manufacturing a high-strength plated steel sheet will be described.

[0180] The method for manufacturing a high-strength plated steel sheet according to the present embodiment is a method for manufacturing a high-strength plated steel sheet by subjecting the above high-strength cold-rolled steel sheet to a plating treatment after the above reheating process.

[0181] The plating treatment can be carried out under known conditions. The plating treatment is preferably hot-dip galvanizing, alloyed hot-dip galvanizing, or electrogalvanizing.

[0182] [Automobile parts]

[0183] The present invention can provide an automobile part that uses at least partially the above high-strength steel sheet or high-strength plated steel sheet. In one example, the above high-strength steel sheet or high-strength plated steel sheet can be formed into a target shape by stamping and used as an automobile part. It should be noted that the automobile part can also contain a steel sheet other than the high-strength steel sheet or high-strength plated steel sheet according to the present embodiment as a material. According to the present embodiment, a high-strength steel sheet with a TS of 980 MPa or more and having ductility, stretch flangeability, and bendability can be provided. Therefore, it is suitable as an automobile part that contributes to the weight reduction of the vehicle body. The high-strength steel sheet or high-strength plated steel sheet of the present invention can be applied to automobile parts, particularly all parts used as frame structure parts or reinforcement parts.

[0184] Examples

[0185] A steel material having the composition shown in Table 1 and the remaining part consisting of Fe and inevitable impurities is melted into a steel slab. After reheating the steel slab, hot rolling is carried out to obtain a hot-rolled sheet, and the hot-rolled sheet is pickled and then 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 reduction 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, the second heating process, the first cooling process, the second cooling process, and the reheating process, except for Example No. 42, isothermal holding is carried out at the first heating temperature, the second heating temperature, the first cooling stop temperature, the second cooling stop temperature, and the reheating temperature. For Example No. 42, in the first cooling process, it is slowly cooled from the first cooling stop temperature for 35 s to 415 °C.

[0186] It should be noted that for a part of the cold-rolled steel sheet, after the reheating process, hot-dip galvanizing treatment is further carried out to form a hot-dip galvanized layer on the surface to obtain a hot-dip galvanized steel sheet (GI). The hot-dip galvanizing treatment uses a continuous hot-dip galvanizing production line. According to needs, the cold-rolled annealed sheet after annealing is reheated in the temperature range of 430-480 °C and immersed in a hot-dip galvanizing bath (bath temperature: 470 °C) to adjust the single-sided coating adhesion amount to 45 g / m 2 . It should be noted that when manufacturing the hot-dip galvanized steel sheet, the composition of the hot-dip galvanizing bath contains 0.18% by mass of Al, and the remaining part consists of Fe and inevitable impurities. In addition, for a part of the hot-dip galvanized steel sheets, the composition of the hot-dip galvanizing bath contains 0.18% by mass of Al, and the remaining part consists of Fe and inevitable impurities. After the plating treatment, an alloying treatment is carried out at 520 °C to alloy the hot-dip galvanized layer to obtain an alloyed hot-dip galvanized steel sheet (GA). It should be noted that the Fe concentration in the alloyed hot-dip galvanized layer is 9% to 12% by mass. In addition, after the annealing process of a part of the cold-rolled steel sheet, electro-galvanizing treatment is further carried out using an electro-galvanizing production line to make the single-sided adhesion amount 30 g / m 2 , to obtain an electro-galvanized steel sheet (EG).

[0187] Specimens are taken from the obtained high-strength cold-rolled steel sheet, and microstructure observation is carried out according to the above method. In addition, tensile tests, hole expansion tests, and VDA bending tests are carried out according to the following methods. The results are shown in Tables 3-1 and 3-2.

[0188] (Tensile test)

[0189] The tensile test is carried out using a No. 5 test piece conforming to JIS Z2201, and the tensile strength and elongation are measured according to JIS Z2201. The test piece is cut out with the direction perpendicular to the rolling direction as the length direction.

[0190] (Hole expansion test)

[0191] A test piece of 100 mmW×100 mL is taken from the cold-rolled steel sheet or the plated steel sheet, and the hole expansion test is carried out according to JIS Z2256(2010). The test piece is blanked with a clearance of 12±1%. The hole is punched, and a conical punch with a vertex angle of 60° is pressed into the hole. When a crack appears in the thickness direction of the plate, the rise of the punch is stopped, and the hole expansion rate λ is calculated from the hole diameter after the crack occurs and the hole diameter before the test according to the following formula.

[0192] Ultimate hole expansion rate: λ(%) ={(D f -D 0 ) / D 0}×100

[0193] Among them, in the above formula, D f is the hole diameter (mm) when the crack occurs, and D 0 is the initial hole diameter (mm). Regardless of the strength of the steel sheet, when the value of λ is 40% or more, it is judged that the stretch flange property is good.

[0194] (VDA bending test)

[0195] A test piece of 60 mmW×60 mmL is taken from the cold-rolled steel sheet or the plated steel sheet, and the VDA bending angle is obtained according to the provisions of the German Industrial Standard (VDA238-100). The bending direction is perpendicular to the rolling direction, and the displacement at the maximum load in the bending test is converted into the bending angle according to the standard.

[0196]

[0197]

[0198]

[0199]

[0200]

[0201] In the examples of the present invention, the tensile strength is 980 MPa or more, the elongation rate El is 12% or more, the hole expansion rate λ is 40% or more, and the VDA bending angle is 90° or more. On the other hand, in the comparative examples, there is a difference in one or more of the tensile strength, elongation rate El, hole expansion rate λ, and VDA bending angle.

Claims

1. A high-strength cold-rolled steel sheet having the following composition and steel structure: The composition contains, by mass%, C: 0.06% - 0.15%, Si: 0.10% - 1.8%, Mn: 2.00% - 3.50%, P: below 0.050%, S: below 0.0050%, N: below 0.0060%, Al: 0.010% - 1.0%, Ti: 0.005% - 0.075%, Nb: 0.005% - 0.075%, and B: 0.0002% - 0.0040%. The balance consists of Fe and inevitable impurities. And, [mol%N] / [mol%Ti] < 1, The steel structure is ferrite: 30% to 60% by area ratio, tempered martensite and bainite: a total of 35% to 65% by area ratio, quenched martensite: 15% or less by area ratio, retained austenite: 1% to 10% by area ratio, and the area ratio of low-Mn ferrite with an Mn concentration of 0.8×[%Mn] or less is 5% to 40%, (the ferrite area ratio) - (the low-Mn ferrite area ratio): 10% or more, and the remaining structure: less than 3% by area ratio, and the average particle size of the low-Mn ferrite is 10 μ m or less, wherein, [mol%N] and [mol%Ti] respectively represent the contents of N and Ti in the steel by mol%, and [%Mn] represents the content of Mn in the steel by mass%.

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

3. A high-strength coated steel sheet having a coating 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, which subjects a steel billet having the following composition to hot rolling to form a hot-rolled sheet, subjects the hot-rolled sheet to pickling, subjects the pickled hot-rolled sheet to cold rolling to form a cold-rolled sheet, Next, the following first heating process is carried out: heating the cold-rolled sheet to a first heating temperature of Ac 1 point to (Ac 3 point - 50°C), and maintaining it for more than 10 s within the first heating temperature range of Ac 1 point to (Ac 3 point - 50°C). Next, the following second heating process is carried out: heating the cold-rolled plate at a heating rate of 10 °C / s or more to a second heating temperature that is (the first heating temperature + 20 °C) or more and less than Ac 3 point, and maintaining it for 5 s to 60 s within the second heating temperature range that is (the first heating temperature + 20 °C) or more and less than Ac 3 point then, performs the following first cooling process: cooling the cold-rolled sheet at a first cooling rate of 10°C / s or more to a first cooling stop temperature below 500°C and higher than the Ms point, and then holding at this first cooling stop temperature for 10 s - 60 s, or cooling from this first cooling stop temperature to above the Ms point at a third cooling rate of less than 10°C / s for 10 s - 60 s, then, performs the following second cooling process: cooling the 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) - 100°C, then, performs the following reheating process: reheating the cold-rolled sheet to a reheating temperature range of the second cooling stop temperature - 450°C and holding for 10 s - 1800 s; The composition of the steel slab contains, by mass%, C: 0.06% to 0.15%, Si: 0.10% to 1.8%, Mn: 2.00% to 3.50%, P: below 0.050%, S: below 0.0050%, N: below 0.0060%, 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 balance is composed of Fe and inevitable impurities. And, [mol%N] / [mol%Ti] < 1, wherein, [mol%N] and [mol%Ti] respectively represent the contents of N and Ti in the steel in mol%.

5. The manufacturing method of the high-strength cold-rolled steel sheet according to claim 4, wherein, the composition of the steel slab further contains, by mass%, at least one element selected from V: below 0.200%, Cr: below 0.20%, Mo: below 0.20%, Cu: below 0.30%, Ni: below 0.30%, Sb: below 0.100%, Sn: below 0.100%, Ca: below 0.0050%, Mg: below 0.0050%, REM: below 0.0050%, Ta: below 0.100%, W: below 0.500%, Zr: below 0.0200%, and Co: below 0.100%.

6. A manufacturing method of a high-strength plated steel sheet, which performs hot rolling on a steel slab having the following composition to make a hot-rolled sheet, performs pickling on the hot-rolled sheet, performs cold rolling on the pickled hot-rolled sheet to make a cold-rolled sheet, Next, perform the following first heating process: heat the cold-rolled sheet to a first heating temperature of Ac 1 point to (Ac 3 point - 50°C), and maintain it for more than 10 s within the first heating temperature range of Ac 1 point to (Ac 3 point - 50°C). Next, the following second heating process is carried out: heating the cold-rolled sheet at a heating rate of 10 °C / s or more to a second heating temperature that is above (the first heating temperature + 20 °C) and less than Ac 3 point, and maintaining it for 5 s to 60 s within the second heating temperature range that is above (the first heating temperature + 20 °C) and less than Ac 3 point then, performs the following first cooling process: cools the cold-rolled sheet at a first cooling rate of 10°C / s or more to a first cooling stop temperature below 500°C and higher than the Ms point, and then holds it at this first cooling stop temperature for 10 s to 60 s, or cools 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, then, performs the following second cooling process: cools the 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, then, performs the following reheating process: reheats the cold-rolled sheet again to a reheating temperature range of the second cooling stop temperature to 450°C and holds it for 10 s to 1800 s, after the reheating process, performs plating treatment on the cold-rolled sheet to obtain a high-strength plated steel sheet; The composition of the steel billet contains, by mass%, C: 0.06% to 0.15%, Si: 0.10% to 1.8%, Mn: 2.00% to 3.50%, P: below 0.050%, S: below 0.0050%, N: below 0.0060%, 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 balance is composed of Fe and inevitable impurities, and [mol%N] / [mol%Ti] < 1. Wherein, [mol%N] and [mol%Ti] respectively represent the contents of N and Ti in the steel in mol%.

7. The method for manufacturing a high-strength plated steel sheet according to claim 6, Wherein, The composition of the steel billet further contains, by mass%, at least one element selected from V: below 0.200%, Cr: below 0.20%, Mo: below 0.20%, Cu: below 0.30%, Ni: below 0.30%, Sb: below 0.100%, Sn: below 0.100%, Ca: below 0.0050%, Mg: below 0.0050%, REM: below 0.0050%, Ta: below 0.100%, W: below 0.500%, Zr: below 0.0200%, and Co: below 0.100%.

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

9. An automotive part, at least a part of which uses the high-strength plated steel sheet according to claim 3.

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