High-strength hot-rolled steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide: a high-strength hot-rolled steel sheet having excellent toughness and punchability and excellent strength and toughness after post-heating; and a method for producing the high-strength hot-rolled steel sheet. A high-strength hot-rolled steel sheet having a component composition containing, in mass%, 0.04 to 0.18% of C, 0.1 to 3.0% of Si, 0.5 to 3.5% of Mn, more than 0% and 0.050% or less of P, more than 0% and 0.010% or less of S, more than 0% and 1.5% or less of Al, more than 0% and 0.010% or less of N, more than 0% and 0.003% or less of O, and 0.040 to 0.150% of Ti, with the remainder being Fe and unavoidable impurities; the steel structure has bainite as the main phase, the retained austenite is less than 3% by volume fraction, the amount of solid-solution Ti / the total amount of Ti is 0.30 or more and less than 0.80, and the amount of Ti present as a precipitate having a particle diameter of 100 nm or more is 0.010-0.030 mass%.

Description

Technical Field

[0001] The present invention relates to a high-strength hot-rolled steel sheet and a method for manufacturing the same, and particularly to a high-strength hot-rolled steel sheet suitable as a blank for automotive parts and a method for manufacturing the same. Background Art

[0002] From the viewpoints of improving the collision safety of automobiles and increasing fuel efficiency, high strength is required for hot-rolled steel sheets used for automotive parts. On the other hand, in high-strength hot-rolled steel sheets, the generation of cracks due to insufficient workability during pressing becomes significant, and thus it is necessary to improve the pressing method and the workability of the steel sheet. Regarding the method, research is being conducted to improve the workability by heating the steel sheet (original steel sheet). It should be noted that in this specification, the heat treatment applied when heating the steel sheet (original steel sheet) into parts, etc. is also referred to as post-heating. On the other hand, the development of steel sheets considering the characteristics of parts after post-heating and processing (after post-heating processing) is underway. In addition, if end face cracks are generated in the trimming process, which is the stage prior to post-heating, not only the press formability deteriorates, but also it becomes the starting point for various problems such as a reduction in fatigue characteristics, and thus the stability of the trimmed end face, that is, excellent blanking property, is also required. In response to such various problems, various methods and steel sheets have been developed.

[0003] Patent Document 1 discloses a technique related to a method for improving the stretch flangeability by setting the processing temperature (heating temperature of post-heating) to 400 to 1000°C. Patent Document 2 discloses a technique related to a high-strength hot-rolled steel sheet with a TS of 730 MPa or more. In the hot-rolled steel sheet disclosed in Patent Document 2, a structure mainly composed of bainite is formed and 80% or more of the total Ti amount is set as solid-solution Ti. As a result, the heat treatment hardenability in which the increase in YS (yield strength) and TS after heat treatment at a temperature in the range of 500°C to the Ac1 transformation point and holding for 60 min is 100 MPa or more is obtained. Patent Document 3 discloses a technique related to a hot-rolled steel sheet having a structure mainly composed of martensite and / or tempered martensite and excellent in toughness, blanking property, and stress corrosion cracking resistance.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-113527

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2015-57514

[0008] Patent Document 3: Japanese Patent Application Laid-Open No. 2018-188675 Summary of the Invention

[0009] However, in Patent Document 1, the strength, toughness, etc. after post-heating are not considered as the properties of the components, and there is room for improvement. The steel plate disclosed in Patent Document 2 has problems in toughness after heat treatment due to the significant increase in strength and fine precipitation of carbides after heat treatment. In addition, the blanking property of the original steel plate before heat treatment is not considered, and there is room for improvement. Patent Document 3 focuses on the characteristics of the original steel plate and does not consider the strength and toughness of the steel plate after post-heating, and there is also room for improvement.

[0010] The present invention has been completed in view of the above circumstances, and an object thereof is to provide a high-strength hot-rolled steel plate having excellent toughness and blanking property, and excellent strength and toughness after post-heating, and a method for manufacturing the same.

[0011] The inventors of the present invention focused on the precipitation behavior of Ti after post-heating of the hot-rolled steel plate, and conceived that the characteristics of the steel plate after post-heating can be improved by controlling the initial coarse Ti-containing precipitates and the amount of solid-solution Ti before post-heating. As a result, it was found that on the basis of adjusting the chemical composition, with bainite as the main phase, the residual γ amount is less than 3% by volume, the ratio of the amount of solid-solution Ti to the Ti content, that is, (amount of solid-solution Ti / total Ti amount) is 0.30 or more and less than 0.80, and the amount of Ti present as precipitates with a particle size of 100 nm or more is 0.010 to 0.030% by mass. Thus, the hot-rolled steel plate (original steel plate) has high strength, excellent blanking property and toughness, and at the same time, it also shows characteristics close to those of the original steel plate after post-heating, and excellent toughness and high strength can be obtained, thereby completing the present invention.

[0012] It should be noted that in the present invention, high strength means that the tensile strength (TS) is 780 MPa or more and less than 1320 MPa.

[0013] In addition, in the present invention, excellent toughness means that in the Charpy impact test using test pieces collected from the hot-rolled steel plate (original steel plate or hot-rolled steel plate after post-heating), the ductile fracture rate at -40 °C is 50% or more. It should be noted that the thickness of the above test piece is 0.6 to 3.0 mm. When the thickness of the hot-rolled steel plate is greater than 3.0 mm, the test piece collected from the hot-rolled steel plate is ground on the front and back surfaces to a thickness of 3.0 mm and used for the Charpy impact test.

[0014] In the present invention, excellent blanking property means that when blanking with a clearance of 5 to 30%, the clearance range where no chips or cracks are generated on the end face is 10% or more.

[0015] In the present invention, excellent strength after post-heating means that the reduction in the strength of the hot-rolled steel plate after post-heating relative to the strength of the hot-rolled steel plate (original steel plate) before post-heating is 40 or less in terms of Vickers hardness.

[0016] It should be noted that in the present invention, post-heating refers to the heat treatment of heating the hot-rolled steel sheet (original steel sheet) to a temperature above 400 °C.

[0017] The present invention has the following constitution.

[0018] [1] A high-strength hot-rolled steel sheet having the following composition:

[0019] Containing, by mass%,

[0020] C: 0.04 to 0.18%,

[0021] Si: 0.1 to 3.0%,

[0022] Mn: 0.5 to 3.5%,

[0023] P: greater than 0% and 0.050% or less,

[0024] S: greater than 0% and 0.010% or less,

[0025] Al: greater than 0% and 1.5% or less,

[0026] N: greater than 0% and 0.010% or less,

[0027] O: greater than 0% and 0.003% or less, and

[0028] Ti: 0.040 to 0.150%,

[0029] The balance consists of Fe and unavoidable impurities;

[0030] And, the steel structure is mainly composed of bainite, and the retained austenite is less than 3% by volume fraction,

[0031] The ratio of the amount of solid-solution Ti to the Ti content, i.e., (amount of solid-solution Ti / total Ti amount), is 0.30 or more and less than 0.80,

[0032] The amount of Ti present as precipitates with a particle size of 100 nm or more is 0.010 to 0.030% by mass.

[0033] [2] The high-strength hot-rolled steel sheet according to [1], wherein the above composition further contains, by mass%, selected from

[0034] Cr: 0.005 to 2.0%,

[0035] Cu: 0.005 to 0.5%,

[0036] Ni: 0.005 to 2.0%,

[0037] Mo: 0.005 to 1.0%,

[0038] V: 0.005 to 0.5%,

[0039] B: 0.0002 to 0.0050%,

[0040] Ca: 0.0001 to 0.0050%,

[0041] REM: 0.0001 to 0.0050%,

[0042] Sb: 0.0010 to 0.10%, and

[0043] Sn: 0.0010 to 0.50%

[0044] one or more of the following.

[0045] [3] The high-strength hot-rolled steel sheet according to [1] or [2], wherein the amount of Fe present as precipitates having a particle size of 100 nm or more is greater than 0% by mass and 0.100% by mass or less.

[0046] [4] A method for manufacturing a high-strength hot-rolled steel sheet, which is the method for manufacturing the high-strength hot-rolled steel sheet according to any one of [1] to [3] above,

[0047] heating a slab having the above composition in a temperature range of 1150 to 1300 °C and holding it in this temperature range for 0.2 to 3.5 hours,

[0048] then, when performing hot rolling,

[0049] performing rough rolling with a total reduction ratio of 80 to 90% in a temperature range of 1080 °C or higher and then performing finish rolling, and in the above finish rolling, rolling is performed under the condition that the reduction ratio in one pass at a temperature of T (°C) or lower obtained by the following formula is 25% or lower. After finish rolling, air cooling is performed for 1.0 s or more.

[0050] Then, cooling is performed at an average cooling rate of 50 °C / s or higher in a temperature range up to 550 °C, and then winding is performed at a winding temperature above the Ms point (°C) and below 550 °C.

[0051] T (°C) = 800 + 1000[Ti]

[0052] wherein, [Ti] is the content of Ti (% by mass).

[0053] [5] The method for manufacturing a high-strength hot-rolled steel sheet according to [4], wherein between cooling at an average cooling rate of 50 °C / s or higher in a temperature range up to 550 °C and winding, cooling is stopped at a cooling stop temperature in a temperature range of 480 to 550 °C, held at this cooling stop temperature ±20 °C for 0.5 to 4.0 s, and then wound at the above winding temperature.

[0054] According to the present invention, there can be provided a high-strength hot-rolled steel sheet excellent in toughness and blanking property, and excellent in strength and toughness after post-heating, and a method for manufacturing the same.

[0055] According to the present invention, a high-strength hot-rolled steel sheet excellent in strength and toughness even after post-heating, which is suitable as a blank for automotive parts, can be obtained.

[0056] If the high-strength hot-rolled steel sheet of the present invention is used, even after post-heating is carried out to improve workability, fatigue characteristics, etc., products such as high-strength automotive parts showing excellent strength and toughness can be obtained. Detailed Embodiments

[0057] Hereinafter, embodiments of the high-strength hot-rolled steel sheet and the method for manufacturing the same according to the present invention will be described in detail. It should be noted that the present invention is not limited to the following embodiments.

[0058] <High-Strength Hot-Rolled Steel Sheet>

[0059] The high-strength hot-rolled steel sheet of the present invention can be either a black skin steel sheet that maintains the as-hot-rolled state or a hot-rolled steel sheet called a white skin steel sheet that is pickled after hot rolling. In addition, the target high-strength hot-rolled steel sheet of the present invention preferably has a thickness of 0.6 mm or more. In addition, the high-strength hot-rolled steel sheet of the present invention preferably has a thickness of 10.0 mm or less. When the high-strength hot-rolled steel sheet of the present invention is used as a blank for automotive parts, the thickness is more preferably 1.0 mm or more. In addition, when the high-strength hot-rolled steel sheet of the present invention is used as a blank for automotive parts, the thickness is more preferably 6.0 mm or less. In addition, the plate width of the high-strength hot-rolled steel sheet of the present invention is preferably 500 mm or more, more preferably 700 mm or more. The plate width of the high-strength hot-rolled steel sheet of the present invention is preferably 1800 mm or less, more preferably 1400 mm or less.

[0060] The high-strength hot-rolled steel sheet of the present invention has a specific composition and a specific steel structure. Here, the description will be made in the order of the composition and the steel structure.

[0061] First, the composition of the high-strength hot-rolled steel sheet of the present invention will be described. It should be noted that "%" indicating the content of the composition refers to "% by mass".

[0062] The composition of the high-strength hot-rolled steel sheet of the present invention contains, by mass%, C: 0.04 to 0.18%, Si: 0.1 to 3.0%, Mn: 0.5 to 3.5%, P: 0.050% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 1.5% or less (excluding 0%), N: 0.010% or less (excluding 0%), O: 0.003% or less (excluding 0%), Ti: 0.040 to 0.150%, and the balance is composed of Fe and inevitable impurities.

[0063] C: 0.04 to 0.18%

[0064] C is an element effective in increasing TS by generating and strengthening bainite or generating precipitates by combining with Ti, N, etc. to suppress the reduction in strength after post-heating. If the C content is less than 0.04%, such an effect cannot be obtained sufficiently, and the TS of the steel sheet (original steel sheet) of 780 MPa or more or the excellent strength after post-heating cannot be obtained. On the other hand, if the C content is greater than 0.18%, the reduction in toughness and blanking property becomes significant, and the characteristics of the present invention cannot be obtained. Therefore, the C content is 0.04 to 0.18%. The C content is preferably 0.05% or more. In addition, the C content is preferably 0.16% or less, more preferably 0.11% or less.

[0065] Si: 0.1 to 3.0%

[0066] Si is an element effective in solid solution strengthening of steel, suppressing cementite in bainite, and suppressing the reduction in strength after post-heating. In order to obtain such an effect, the content needs to be 0.1% or more. On the other hand, if the Si content is greater than 3.0%, polygonal ferrite is excessively generated and the steel structure of the present invention cannot be obtained. Therefore, the Si content is 0.1 to 3.0%. The Si content is preferably 0.2% or more. In addition, the Si content is preferably 2.0% or less, more preferably 1.5% or less.

[0067] Mn: 0.5 to 3.5%

[0068] Mn is an element effective in suppressing ferrite and generating bainite. If the Mn content is less than 0.5%, such an effect cannot be obtained sufficiently, polygonal ferrite, etc. are generated, and the microstructure of the present invention cannot be obtained. On the other hand, if the Mn content is greater than 3.5%, martensite increases and the reduction in toughness becomes significant, and the excellent toughness of the present invention cannot be obtained. Therefore, the Mn content is 0.5 to 3.5%. The Mn content is preferably 1.0% or more. In addition, the Mn content is preferably 2.7% or less.

[0069] P: greater than 0% and 0.050% or less

[0070] P reduces toughness, so it is preferred that its amount be minimized as much as possible. In the present invention, the P content can be allowed up to 0.050%. Therefore, the P content is 0.050% or less. The P content is preferably 0.030% or less. The lower limit is not particularly limited, and the P content can be greater than 0%, but if the P content is less than 0.001%, the production efficiency decreases, so the P content is preferably 0.001% or more.

[0071] S: greater than 0% and 0.010% or less

[0072] S reduces toughness, so it is preferred that its amount be minimized as much as possible, but in the present invention, the S content can be allowed up to 0.010%. Therefore, the S content is 0.010% or less. The S content is preferably 0.0050% or less, more preferably 0.0020% or less. The lower limit is not particularly limited, and the S content can be greater than 0%, but if the S content is less than 0.0002%, the production efficiency decreases, so the S content is preferably 0.0002% or more.

[0073] Al: greater than 0% and 1.5% or less

[0074] Al acts as a deoxidizer and is preferably added in the deoxidation process. The Al content can be greater than 0%, but from the viewpoint of use as a deoxidizer, the Al content is preferably 0.01% or more. On the other hand, if a large amount of Al is contained, a large amount of polygonal ferrite is generated and the steel structure of the present invention cannot be obtained. In the present invention, the Al content can be allowed up to 1.5%. Therefore, the Al content is 1.5% or less. The Al content is preferably 0.50% or less, more preferably 0.10% or less, and further preferably 0.05% or less.

[0075] N: greater than 0% and 0.010% or less

[0076] N generates TiN and hinders the precipitation of TiC, so it is preferred that its amount be minimized as much as possible. In the present invention, the N content can be allowed up to 0.010%. Therefore, the N content is 0.010% or less. The N content is preferably 0.007% or less. The lower limit is not particularly limited, and the N content can be greater than 0%, but if the N content is less than 0.0005%, the production efficiency decreases, so the N content is preferably 0.0005% or more.

[0077] O: greater than 0% and 0.003% or less

[0078] O reduces toughness, so it is preferred that its amount be minimized as much as possible. In the present invention, the O content can be allowed up to 0.003%. Therefore, the O content is 0.003% or less. The O content is preferably 0.002% or less. The lower limit is not particularly limited, and the O content can be greater than 0%, but if the O content is less than 0.0002%, the production efficiency decreases, so the O content is preferably 0.0002% or more.

[0079] Ti: 0.040 to 0.150%

[0080] Ti is the most important element in the present invention and is an element necessary to obtain excellent strength and toughness after post-heating by generating appropriate precipitates such as TiC after post-heating. If the Ti content is less than 0.040%, such an effect cannot be obtained sufficiently and excellent strength after post-heating cannot be obtained. On the other hand, if it is greater than 0.150%, the precipitates after post-heating become excessive and excellent toughness after post-heating cannot be obtained. Therefore, the Ti content is 0.040 to 0.150%. The Ti content is preferably 0.050% or more. In addition, the Ti content is preferably 0.120% or less.

[0081] The above components are the basic components of the high-strength hot-rolled steel sheet of the present invention. The high-strength hot-rolled steel sheet of the present invention can adopt a composition containing the above components and the remaining part being Fe and inevitable impurities.

[0082] In addition to the above components, the high-strength hot-rolled steel sheet of the present invention may contain one or more selected from Cr: 0.005 to 2.0%, Cu: 0.005 to 0.5%, Ni: 0.005 to 2.0%, Mo: 0.005 to 1.0%, V: 0.005 to 0.5%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, Sn: 0.0010 to 0.50%.

[0083] Cr: 0.005 to 2.0%

[0084] Cr is an element effective in suppressing ferrite and generating bainite. In order to obtain such an effect, when Cr is contained, it is preferably made such that the Cr content is 0.005% or more. On the other hand, if the content of Cr is greater than 2.0%, sometimes the decrease in corrosion resistance becomes significant. Therefore, when Cr is contained, it is preferably made such that the Cr content is 2.0% or less. The Cr content is more preferably 0.1% or more. In addition, the Cr content is more preferably 1.0% or less, and further preferably 0.8% or less.

[0085] Cu: 0.005 to 0.5%

[0086] Cu is an element effective in stabilizing austenite and forming bainite. To achieve such an effect, when Cu is contained, it is preferably that the Cu content is 0.005% or more. On the other hand, if the content of Cu is greater than 0.5%, the formation of Cu precipitates becomes significant, sometimes resulting in a reduction in toughness. Therefore, when Cu is contained, it is preferably that the Cu content is 0.5% or less. The Cu content is more preferably 0.05% or more. Additionally, the Cu content is more preferably 0.3% or less.

[0087] Ni: 0.005 - 2.0%

[0088] Ni is an element effective in suppressing ferrite and forming bainite. To achieve such an effect, when Ni is contained, it is preferably that the Ni content is 0.005% or more. On the other hand, if the content of Ni is greater than 2.0%, a large amount of martensite and residual γ are formed, sometimes resulting in a reduction in toughness. Therefore, when Ni is contained, it is preferably that the Ni content is 2.0% or less. The Ni content is more preferably 0.05% or more. Additionally, the Ni content is more preferably 0.8% or less, and further preferably 0.5% or less.

[0089] Mo: 0.005 - 1.0%

[0090] Mo is an element effective in improving the hardenability of the steel plate and forming bainite. To achieve such an effect, when Mo is contained, it is preferably that the Mo content is 0.005% or more. On the other hand, if the content of Mo is greater than 1.0%, the formation of Mo-based precipitates becomes significant, sometimes resulting in a reduction in toughness. Therefore, when Mo is contained, it is preferably that the Mo content is 1.0% or less. The Mo content is more preferably 0.05% or more. Additionally, the Mo content is more preferably 0.50% or less.

[0091] V: 0.005 - 0.5%

[0092] V is an element effective in improving the hardenability of the steel plate and forming bainite. To achieve such an effect, when V is contained, it is preferably that the V content is 0.005% or more. On the other hand, if the content of V is greater than 0.5%, the formation of V-based precipitates becomes significant, sometimes resulting in a reduction in toughness. Therefore, when V is contained, it is preferably that the V content is 0.5% or less. The V content is more preferably 0.01% or more. Additionally, the V content is more preferably 0.1% or less.

[0093] B: 0.0002 - 0.0050%

[0094] B is an element effective in improving the hardenability of steel plates and forming bainite. To obtain such an effect, when B is contained, it is preferably made such that the B content is 0.0002% or more. On the other hand, if the B content is greater than 0.0050%, B-based compounds increase and the toughness sometimes decreases. Therefore, when B is contained, it is preferably made such that the B content is 0.0050% or less. The B content is more preferably 0.0005% or more. Additionally, the B content is more preferably 0.0040% or less.

[0095] Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%

[0096] Ca and REM (rare earth elements) are respectively elements effective in improving workability by controlling the morphology of inclusions. To obtain such an effect, when Ca and REM are contained, it is preferably made such that their respective contents are 0.0001% or more. On the other hand, if the contents of Ca and REM are respectively greater than 0.0050%, the influence of the increase in the amount of inclusions becomes significant and the toughness sometimes decreases. Therefore, when Ca and REM are contained, the contents of Ca and REM are respectively preferably 0.0050% or less. The Ca content is more preferably 0.0005% or more. Additionally, the Ca content is more preferably 0.0030% or less. The REM content is more preferably 0.0005% or more. Additionally, the REM content is more preferably 0.0030% or less. It should be noted that REM is a general term for 15 elements including Sc, Y, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content mentioned here is the total content of these elements.

[0097] Sb: 0.0010 to 0.10%, Sn: 0.0010 to 0.50%

[0098] Sb and Sn are respectively elements effective in suppressing surface reactions such as oxidation, denitrification, and deboronation, improving the surface properties of steel plates, and improving toughness. To obtain such an effect, when Sb and Sn are contained, it is preferably made such that the contents of Sb and Sn are respectively 0.0010% or more. On the other hand, if the content of Sb is greater than 0.10% or the content of Sn is greater than 0.50%, it will instead cause embrittlement of the steel plate and sometimes significantly reduce the toughness. Therefore, when Sb is contained, the content of Sb is preferably 0.10% or less, and when Sn is contained, the content of Sn is preferably 0.50% or less. The Sb content is more preferably 0.0050% or more. Additionally, the Sb content is more preferably 0.030% or less. The Sn content is more preferably 0.0050% or more. Additionally, the Sn content is more preferably 0.050% or less.

[0099] It should be noted that even if the contents of Cr, Cu, Ni, Mo, V, B, Ca, REM, Sb, and Sn are less than the above lower limit values, the effects of the present invention are not impaired. Therefore, when the contents of these components are less than the above lower limit values, these elements are regarded as being contained as inevitable impurities.

[0100] In addition, in the present invention, in addition to the above component composition, Mg, As, W, Ta, Pb, Zr, Hf, Te, Bi, and Se may be contained in a total amount of 0.3% or less by mass. It should be noted that the contents of these elements are preferably limited to 0.03% or less, respectively.

[0101] Next, the steel structure of the high-strength hot-rolled steel sheet of the present invention will be described.

[0102] The steel structure of the high-strength hot-rolled steel sheet of the present invention has bainite as the main phase, and the residual γ is less than 3% by volume fraction.

[0103] Main phase: Bainite

[0104] In the present invention, in order to obtain high strength and excellent toughness, a structure with bainite as the main phase is formed. If ferrite, pearlite, residual γ, etc. become the main phase, it is difficult to achieve both high strength and excellent toughness and blanking performance. In addition, if martensite becomes the main phase, the toughness and blanking performance will decrease, so it is not preferred. Therefore, the steel structure has bainite as the main phase. It should be noted that the bainite can be any one of upper bainite, lower bainite, tempered bainite, and bainite ferrite. It should be noted that in the present invention, the main phase refers to a phase that accounts for 50% or more by area fraction. The area fraction of the main phase is preferably 55% or more, more preferably 65% or more. In addition, the area fraction of the main phase is preferably 95% or less.

[0105] Amount of retained austenite (retained γ): Less than 3%

[0106] Retained austenite (retained γ) is a structure that reduces the toughness of the steel sheet and significantly reduces the strength and toughness after post-heating and transformation into pearlite. Therefore, it is preferably reduced as much as possible. In the present invention, the retained γ is allowed to be less than 3% by volume fraction. Therefore, the retained γ is less than 3% by volume fraction. The retained γ is preferably less than 2% by volume fraction, more preferably less than 1% by volume fraction. The lower limit of the volume fraction of the retained γ is not particularly limited, and the volume fraction of the retained γ can be 0%.

[0107] It should be noted that as the phases (other phases) other than bainite and retained γ, one or more of ferrite, pearlite, and martensite can be cited. The total area fraction of the other phases is preferably 40% or less. The lower limit of the area fraction of the other phases is not particularly limited, but the total area fraction of the other phases is preferably 3% or more, more preferably 5% or more, and further preferably 10% or more.

[0108] Solid solution Ti amount / total Ti amount: 0.30 or more and less than 0.80

[0109] If the ratio of the solid solution Ti amount to the Ti content, i.e., (solid solution Ti amount / total Ti amount), is less than 0.30, the amount of solid solution Ti that becomes precipitates during post-heating and offsets the strength reduction becomes insufficient, resulting in a strength reduction after post-heating, or becomes fine precipitates and causes a reduction in toughness. On the other hand, if it is 0.80% or more, the strength increase brought about by the precipitation after post-heating becomes significant, and excellent toughness after post-heating cannot be obtained. Therefore, the solid solution Ti amount / total Ti amount is 0.30 or more and less than 0.80. It is preferably 0.35 or more. Additionally, it is preferably 0.70 or less. It should be noted that the solid solution Ti amount / total Ti amount is obtained by the method described in the examples.

[0110] Ti amount present as precipitates with a particle size of 100 nm or more: 0.010 to 0.030 mass%

[0111] By containing a certain amount or more of Ti-containing precipitates with a particle size of 100 nm or more, the growth of these precipitates competes with the precipitation of new TiC during post-heating, thereby moderately suppressing the precipitation of fine TiC and suppressing excessive strength increase and toughness reduction. To obtain such an effect, it is necessary to make the Ti amount present as precipitates with a particle size of 100 nm or more 0.010 mass% or more. On the other hand, if the above Ti amount is greater than 0.030 mass%, the reduction in toughness caused by coarse precipitates becomes significant. Therefore, it is necessary to make the Ti amount present as precipitates with a particle size of 100 nm or more 0.030 mass% or less. Therefore, the Ti amount present as precipitates with a particle size of 100 nm or more is 0.010 to 0.030 mass%. It is preferably 0.013 mass% or more. Additionally, it is preferably 0.027 mass% or less. It should be noted that the Ti amount present as precipitates with a particle size of 100 nm or more is obtained by the method described in the examples.

[0112] Fe amount present as precipitates with a particle size of 100 nm or more: greater than 0 mass% and 0.100 mass% or less (preferred condition)

[0113] In the present invention, in addition to the above, the amount of Fe present as precipitates having a particle size of 100 nm or more is greater than 0 mass% and 0.100 mass% or less, whereby the blanking property can be further improved. The Fe precipitates having a particle size of 100 nm or more serve as paths for cracks when forming a fracture surface during blanking and are effective in smoothing the fracture surface. On the other hand, if there are too many such precipitates, the blanking property may sometimes be impaired. Therefore, the amount of Fe present as precipitates having a particle size of 100 nm or more is preferably greater than 0 mass% and 0.100 mass% or less. More preferably, it is 0.001 mass% or more, and still more preferably 0.004 mass% or more. It should be noted that the amount of Fe present as precipitates having a particle size of 100 nm or more is determined by the method described in the examples.

[0114] <Manufacturing method of high-strength hot-rolled steel sheet>

[0115] The high-strength hot-rolled steel sheet of the present invention is manufactured by the following operations: heating a slab having the above composition in a temperature range of 1150 to 1300 °C and holding it in this temperature range for 0.2 to 3.5 hours. Then, when performing hot rolling, rough rolling with a total reduction ratio of 80 to 90% is carried out in a temperature range of 1080 °C or higher, followed by finish rolling. And in the above finish rolling, rolling is carried out under the condition that the reduction ratio in one pass at a temperature of T (°C) or lower obtained by the following formula is 25% or less. After finish rolling, air cooling is carried out for 1.0 s or more, and then cooling is carried out at an average cooling rate of 50 °C / s or more in a temperature range up to 550 °C, and then winding is carried out at a winding temperature above the Ms point (°C) and below 550 °C.

[0116] T (°C) = 800 + 1000[Ti]

[0117] Wherein, [Ti] is the content of Ti (mass%).

[0118] In addition, the total reduction ratio in the temperature range of 1080 °C or higher is determined based on the thickness of the slab before hot rolling and is obtained by the ratio of the thickness at 1080 °C. The reduction ratio in one pass at a temperature of T (°C) or lower is determined by the ratio of the thicknesses before and after rolling in each pass at a temperature of T (°C) or lower.

[0119] Hereinafter, a detailed description will be given. It should be noted that the above temperature is the temperature on the surface at the center of the width of the steel sheet, and the above average cooling rate and cooling rate are the average cooling rate and cooling rate on the surface at the center of the width of the steel sheet, respectively. In addition, unless otherwise specified, the average cooling rate is [(cooling start temperature - cooling stop temperature) / cooling time from the cooling start temperature to the cooling stop temperature].

[0120] Heating temperature of the slab: 1150 - 1300 °C

[0121] If the heating temperature of the slab is less than 1150 °C, the dissolution of Ti-containing precipitates becomes insufficient, and it is impossible to obtain a value of not less than 0.30 and less than 0.80 for the ratio of dissolved Ti amount to total Ti amount, and a value of 0.010 to 0.030 mass% for the Ti amount present as precipitates with a particle size of 100 nm or more. On the other hand, if the heating temperature of the slab is greater than 1300 °C, the dissolution of Ti-containing precipitates becomes excessive, and it is impossible to obtain a value of not less than 0.30 and less than 0.80 for the ratio of dissolved Ti amount to total Ti amount, and a value of 0.010 to 0.030 mass% for the Ti amount present as precipitates with a particle size of 100 nm or more. Therefore, the heating temperature of the slab is 1150 to 1300 °C. The above heating temperature is preferably 1170 °C or higher, more preferably 1185 °C or higher. In addition, the above heating temperature is preferably 1280 °C or lower, more preferably 1265 °C or lower.

[0122] Holding time in the temperature range of 1150 to 1300 °C: 0.2 to 3.5 hours

[0123] If the holding time in the temperature range of 1150 to 1300 °C is less than 0.2 hours, the dissolution of Ti-containing precipitates becomes insufficient. As a result, it is impossible to obtain a value of not less than 0.30 and less than 0.80 for the ratio of dissolved Ti amount to total Ti amount, and a value of 0.010 to 0.030 mass% for the Ti amount present as precipitates with a particle size of 100 nm or more. On the other hand, if the holding time in the above temperature range is greater than 3.5 hours, decarburization near the surface becomes significant, and ferrite and residual γ are likely to form near the surface, and the structure of the present invention cannot be obtained. Therefore, the holding time of the slab in the above temperature range is 0.2 to 3.5 hours. The above holding time is preferably 0.4 hours or more. In addition, the above holding time is preferably 2.5 hours or less.

[0124] Total reduction ratio in the temperature range of 1080 °C or higher: 80 to 90%

[0125] After the above holding, hot rolling is carried out. When carrying out hot rolling, rough rolling with a total reduction ratio of 80 - 90% is carried out in a temperature range above 1080°C, and then finish rolling is carried out. By carrying out a reduction with a total reduction ratio of 80 - 90% in a temperature range above 1080°C, the generation and growth of coarse Ti-containing precipitates with a size of 100 nm or more can be promoted, and the amount of Ti present as precipitates with a size of 100 nm or more is 0.010 - 0.030 mass%. If the total reduction ratio is less than 80%, the generation of precipitates with a size of 100 nm or more becomes insufficient, and the amount of Ti present as precipitates with a size of 100 nm or more is less than 0.010 mass%. On the other hand, if the total reduction ratio is greater than 90%, the generation of precipitates with a size of 100 nm or more becomes excessive, and the amount of Ti present as precipitates with a size of 100 nm or more is greater than 0.030 mass%. Therefore, the total reduction ratio in the temperature range above 1080°C is 80 - 90%. The above total reduction ratio is preferably 81% or more. In addition, the above total reduction ratio is preferably 88% or less.

[0126] Reduction ratio in one pass at or below T (°C): 25% or less

[0127] After carrying out the above rough rolling, finish rolling is carried out. In finish rolling, if a reduction greater than 25% is carried out in one pass at or below the T (°C) obtained by the following formula, Ti-containing precipitates are generated, and a value of 0.30 or more and less than 0.80 of the solid solution Ti amount / total Ti amount cannot be obtained. Therefore, the reduction ratio in one pass at or below T (°C) is 25% or less. The above reduction ratio is preferably 20% or less, more preferably 18% or less. The lower limit of the above reduction ratio is not particularly limited, but if the above reduction ratio is 5% or less, coarse grains sometimes occur, so the above reduction ratio is preferably greater than 5%. The above reduction ratio is more preferably 7% or more. It should be noted that in the present invention, rolling at or below T (°C) (no passes at or below T (°C)) may not be carried out. In addition, in finish rolling, the reduction ratio in one pass above T (°C) is not particularly limited.

[0128] It should be noted that T (°C) is obtained by the following formula.

[0129] T (°C) = 800 + 1000[Ti]

[0130] Wherein, [Ti] is the content of Ti (mass%).

[0131] Air cooling for 1.0 s or more

[0132] By performing air cooling after the above-mentioned finish rolling, strain can be partially released, and the formation of Ti-containing precipitates during subsequent cooling can be suppressed. To achieve such an effect, the air cooling time after finish rolling needs to be 1.0 s or more. The above-mentioned air cooling time is preferably 1.5 s or more, more preferably 2.0 s or more, and further preferably 2.2 s or more. The upper limit of the above-mentioned air cooling time is not particularly limited, but if the above-mentioned air cooling time is 5.0 s or less, it is easy to perform subsequent hot rolling control. Therefore, the above-mentioned air cooling time is preferably 5.0 s or less. It should be noted that air cooling means exposure to the atmosphere (air cooling) rather than active cooling (accelerated cooling) such as water injection.

[0133] Cool at an average cooling rate of 50 °C / s or more in the temperature range up to 550 °C

[0134] After the above-mentioned air cooling, cool at an average cooling rate of 50 °C / s or more in the temperature range up to 550 °C. If the average cooling rate up to 550 °C is less than 50 °C / s, excessive formation of ferrite and Ti-containing precipitates will occur, and the phase structure and precipitates of the present invention cannot be obtained. Therefore, the average cooling rate in the temperature range from the cooling start temperature to 550 °C after the above-mentioned air cooling is 50 °C / s or more. The above-mentioned average cooling rate is preferably 70 °C / s or more. The upper limit of the above-mentioned average cooling rate is not particularly limited, but if the above-mentioned average cooling rate is 500 °C / s or more, it may sometimes cause deterioration of the steel plate shape. Therefore, the above-mentioned average cooling rate is preferably less than 500 °C / s, and more preferably 300 °C / s or less.

[0135] Coiling temperature: above the Ms point (°C) and below 550 °C

[0136] If the coiling temperature is greater than 550 °C, excessive formation of ferrite and Ti-containing precipitates will occur, and the phase structure and precipitates of the present invention cannot be obtained. On the other hand, if it is less than the Ms point, excessive formation of martensite will occur and the structure of the present invention cannot be obtained. Therefore, the coiling temperature is above the Ms point (°C) and below 550 °C. It is preferably above (Ms point + 20) °C. In addition, it is preferably 530 °C or less. It should be noted that the Ms point (°C) is the start temperature of martensite transformation and can be obtained using a thermomechanical simulation tester (Processing Formaster), etc. When obtaining the Ms point (°C) using a thermomechanical simulation tester, for example, the sample can be heated to 1250 °C, held at the above temperature for 300 s, and then cooled at a cooling rate of 100 °C / s. The temperature at which the size of the sample changes from contraction to expansion is obtained as the Ms point (°C).

[0137] Holding time at ±20 °C of the cooling stop temperature in the temperature range of 480 - 550 °C: 0.5 - 4.0 s (preferred condition)

[0138] In the present invention, it is further preferably cooled at an average cooling rate of 50°C / s or more in the temperature range from to 550°C before winding, and the cooling is stopped at the cooling stop temperature in the temperature range of 480 - 550°C, and held at the cooling stop temperature ±20°C for 0.5 - 4.0 s. Thereby, the blanking property can be further improved. By holding for 0.5 s or more, Fe precipitates can be formed, and the blanking property can be further improved. On the other hand, by making the above holding time 4.0 s or less, it is easy to make the amount of Fe present as precipitates with a particle size of 100 nm or more 0.100 mass% or less, and it is easy to suppress the disappearance of the blanking property improvement effect. Therefore, the holding time at the cooling stop temperature ±20°C is preferably 0.5 - 4.0 s. More preferably, it is 0.5 - 2.0 s.

[0139] There are no particular limitations other than the conditions of the above manufacturing method, but it is preferably manufactured by appropriately adjusting the conditions as follows. For example, from the viewpoint of reducing coarse grains that cause a decrease in workability, the finish rolling is preferably 4 passes or more.

[0140] The high-strength hot-rolled steel sheet of the present invention has excellent strength and toughness after post-heating. Here, as the heating temperature for post-heating, 400°C or more can be cited. In addition, there is no particular limitation on the upper limit of the heating temperature for post-heating, but as an example, the heating temperature for post-heating can be cited as 1150°C or less. The post-heating time (holding time at the above heating temperature) is not particularly limited, but as an example, it can be cited as more than 0 s. In addition, the above heating time can be cited as 3600 s or less as an example.

[0141] Examples

[0142] The steel having the composition shown in Table 1 was melted by a converter to form a slab, and then the slab was heated and hot-rolled under the conditions shown in Table 2 to manufacture a hot-rolled steel sheet (original steel sheet). Using the obtained hot-rolled steel sheet, the following test methods were used to evaluate the microstructure observation, analysis of solid-solution Ti and Ti-containing precipitates, analysis of Fe-containing precipitates, tensile properties, hardness, blanking property, and toughness. Furthermore, the hot-rolled steel sheet was subjected to post-heating shown in Table 2, and using the hot-rolled steel sheet after post-heating, the hardness, toughness, and stress corrosion cracking resistance were evaluated according to the following test methods. The post-heating was carried out under the conditions that the post-heating temperature was 400°C or more at which an improvement in the stretch flangeability was observed, and the post-heating time was 3600 s or less from the viewpoint of productivity. It should be noted that "-" in Table 2 indicates that the treatment was not carried out. In addition, the Ms point (°C) in Table 2 was determined by a test using a thermomechanical simulator.

[0143] Microstructure observation

[0144] The area ratio of bainite refers to the proportion of the area of bainite in the observed area. For the area ratio of bainite, samples are cut from the obtained hot-rolled steel plate, the plate thickness section parallel to the rolling direction is ground, etched with a 3% nitric acid ethanol etching solution, and three fields of view are respectively photographed at a magnification of 1500 times using SEM (scanning electron microscope) at the 1 / 4 position of the plate thickness. According to the image data of the obtained secondary electron images, the area ratio of each tissue is obtained using Image-Pro manufactured by Media Cybernetics, and the average area ratio of the three fields of view is used as the area ratio of each tissue. The determination of the tissue can be carried out by general classification. For example, it can be determined as follows. In the image data, bainite is distinguished as black or dark gray including carbide or martensite with a linear interface, and lower bainite is distinguished as black or dark gray or gray or light gray including carbide with consistent orientation. Martensite is a black to light gray tissue including regular but multiple-oriented carbides. Or it is observed as white or light gray without carbide. Retained γ is observed as white or light gray without carbide. Since sometimes a part of martensite and retained γ cannot be distinguished, the retained γ is obtained by the method described below, and subtracted from the total area ratio of martensite and retained γ obtained from the SEM image to obtain the area ratio of martensite. It should be noted that in the present invention, martensite includes fresh martensite, self-tempered martensite, tempered martensite, etc. according to its tempering degree, but can be any one of them. In addition, bainite can also be any one of upper bainite, lower bainite, tempered bainite, etc., but upper bainite or tempered bainite is more preferred. The stronger the tempering degree of the tissue, the blacker the matrix is with a strong contrast image. Therefore, based on the color of the above matrix, the amount of carbide, tissue morphology, etc. are comprehensively judged in the present invention, and classified into a certain tissue with similar characteristics including the tissues described below. Carbide is white dot-like or linear. In addition, as tissues other than the above, ferrite is a tissue that is black or dark gray and does not have lower tissues such as carbide and lath inside, and pearlite can be distinguished as a black and white layered or partially discontinuous layered tissue. The amount of retained γ is obtained as follows. After grinding the hot-rolled steel plate to 1 / 4 + 0.1 mm of the plate thickness, it is further ground by 0.1 mm by chemical polishing, and the obtained surface is used as the measurement surface. For the above measurement surface, the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (γ) and the (200), (211), and (220) planes of bcc iron (ferrite) are measured using the Kα1 ray of Mo in an X-ray diffractometer. Then, the volume ratio is obtained from the intensity ratio of the integrated reflection intensities of each plane from fcc iron to the integrated reflection intensities of each plane from bcc iron, and used as the amount of retained γ.

[0145] The structures of the matrix phase, in which the area ratio of each of the obtained structures is 50% or more, and other structures are shown in Table 3. It should be noted that in Table 3, B represents bainite, γ represents retained austenite, and O represents other phases. The other phases include one or more of ferrite, pearlite, and martensite.

[0146] Analysis of solid-solution Ti and Ti-containing precipitates

[0147] Specimens with a width of 30 mm and a length of 30 mm were collected from the obtained hot-rolled steel sheet and subjected to constant-current electrolysis in a non-aqueous solvent-based electrolyte (10% AA-based electrolyte: 10 Vol% acetylacetone - 1 mass% tetramethylammonium chloride - methanol). The current density was 20 mA / cm 2 , and the electrolysis amount was approximately 0.2 g. The electrolyzed electrolyte was used as an analytical solution, and the concentrations (mass%) of Ti and Fe, which was used as a comparative element, in the solution were measured by ICP mass spectrometry. Based on the obtained concentrations, the concentration ratio of Ti to Fe was calculated, and then multiplied by the content (mass%) of Fe in the specimen to obtain the amount of solid-solution Ti (mass%). It should be noted that the content (mass%) of Fe in the specimen was obtained by subtracting the total content (mass%) of components other than Fe from 100 mass%. Using the obtained amount of solid-solution Ti (mass%), the ratio to the amount of Ti contained (mass%) was calculated. On the other hand, the specimen with precipitates attached to the surface after electrolysis was taken out of the electrolyte and immersed in a sodium hexametaphosphate aqueous solution (500 mg / L) (hereinafter referred to as SHMP aqueous solution). Then, ultrasonic vibration was applied to peel the precipitates from the specimen and extract them into the SHMP aqueous solution. Next, the SHMP aqueous solution containing the precipitates was filtered through a filter with a pore size of 100 nm. Then, the precipitates trapped on the 100-nm filter were acid-decomposed, and the decomposition solution was analyzed using an ICP emission spectroscopic analyzer to measure the absolute value of Ti in the decomposition solution. The absolute value of Ti obtained was divided by the electrolysis mass to obtain the amount of Ti contained in the precipitates with a particle size of 100 nm or more (mass% when the total composition of the specimen is 100 mass%). Next, the obtained amount of Ti (mass%) was divided by the amount of Ti contained (mass%) in the specimen to obtain the amount of Ti (mass%) present as Ti-containing precipitates with a particle size of 100 nm or more. It should be noted that the electrolysis mass was obtained by measuring the mass of the specimen after the precipitates were peeled off and subtracting it from the mass of the specimen before electrolysis.

[0148] Analysis of Fe-containing precipitates

[0149] Specimens with a width of 30 mm and a length of 30 mm were collected from the obtained hot-rolled steel sheet and subjected to constant-current electrolysis in a non-aqueous solvent-based electrolyte (10% AA-based electrolyte: 10 Vol% acetylacetone - 1 mass% tetramethylammonium chloride - methanol). The current density was 20 mA / cm2 , the electrolysis amount was about 0.2 g. The test piece with deposits adhering to the surface after electrolysis was taken out of the electrolytic solution, immersed in an SHMP aqueous solution, ultrasonic vibration was applied, the deposits were peeled off from the test piece, and extracted into the SHMP aqueous solution. Then, the SHMP aqueous solution containing the deposits was filtered using a filter with a pore size of 100 nm. Next, the deposits trapped by the 100-nm filter were acid-decomposed, and the decomposition solution was analyzed using an ICP emission spectrometer to measure the absolute value of Fe in the decomposition solution. The absolute value of Fe obtained was divided by the electrolysis mass to obtain the amount of Fe contained in the deposits with a particle size of 100 nm or more (mass % when the total composition of the test piece is set to 100 mass %). Next, the obtained amount of Fe (mass %) was divided by the amount of Fe contained in the test piece (mass %) to obtain the amount of Fe (mass %) present as Fe-containing deposits with a particle size of 100 nm or more. It should be noted that the electrolysis mass was obtained by measuring the mass of the test piece after peeling off the deposits and subtracting it from the mass of the test piece before electrolysis.

[0150] Tensile test

[0151] From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece (JIS Z2241: 2011) was collected in the direction parallel to the rolling direction, and a tensile test was carried out in accordance with JIS Z 2241: 2011 with a strain rate of 10 -3 / s to obtain TS. It should be noted that in the present invention, when TS is 780 MPa or more and less than 1320 MPa, it is rated as qualified.

[0152] Vickers hardness test

[0153] Samples were cut out from the obtained hot-rolled steel sheet and the hot-rolled steel sheet after post-heating. After grinding the cross-section parallel to the rolling direction of the plate thickness, at the 1 / 4 position of the plate thickness, a Vickers hardness test was carried out with a load of 5 kg and 5 measurement points, and the average (arithmetic mean) was taken as the Vickers hardness of the steel sheet. When the difference in hardness (ΔHV) before and after post-heating was 40 or less, it was judged that the strength after post-heating was excellent and rated as qualified.

[0154] Blanking test

[0155] A test piece with a width of 50 mm and a length of 50 mm was collected from the obtained hot-rolled steel sheet, and a blanking punch was used to perform blanking three times for each gap in the range of 5 to 30% of the gap, and the gap range where there were no chips or cracks on the end face in all three times was obtained. When the gap range was 10% or more, it was rated as qualified.

[0156] Charpy impact test

[0157] Test pieces with a width of 10 mm and a length of 55 mm were collected from the obtained hot-rolled steel sheet and the post-heated hot-rolled steel sheet, respectively, and Charpy impact test pieces with a V-notch having a front end angle of 45°, a front end radius of 0.25 mm, and a depth of 2 mm were fabricated. Then, according to JIS Z 2242:2018, five Charpy impact tests were conducted at -40°C to evaluate the ductile fracture rate. When the average value of the five ductile fracture rates was 50% or more, it was judged that the toughness was excellent and rated as qualified. It should be noted that the plate thickness was 2.9 mm, and the notch direction was parallel to the rolling direction.

[0158]

[0159]

[0160]

[0161] The inventive examples all had a TS of 780 MPa or more and less than 1320 MPa, excellent blanking property and toughness, and excellent strength and toughness after post-heating. On the other hand, the comparative examples deviating from the scope of the present invention did not have one or more of the desired strength, blanking property, and toughness, or did not obtain one or more of the desired strength and toughness after post-heating.

[0162] Industrial availability

[0163] According to the present invention, a high-strength hot-rolled steel sheet with a TS of 780 MPa or more and less than 1320 MPa, excellent blanking property and toughness, and excellent strength and toughness after post-heating can be obtained. When the high-strength steel sheet of the present invention is used for automotive part applications, it can greatly contribute to improving the collision safety of automobiles and increasing fuel efficiency.

Claims

1. A high-strength hot-rolled steel sheet having the following composition: Containing by mass % C：0.04～0.18％、 Si: 0.1 - 3.0%, Mn: 0.5 - 3.5%, P: More than 0% and 0.050% or less, S: More than 0% and 0.010% or less, Al: More than 0% and 1.5% or less, N: More than 0% and 0.010% or less, O: More than 0% and 0.003% or less, and Ti: 0.040 - 0.150%, The balance consists of Fe and inevitable impurities; And, the steel structure has bainite as the main phase, and the retained austenite is less than 3% by volume fraction, The ratio of the amount of solid-solution Ti to the Ti content, i.e., solid-solution Ti amount / total Ti amount, is 0.30 or more and less than 0.80, The amount of Ti present as precipitates with a particle size of 100 nm or more is 0.010 - 0.030 mass %.

2. The high-strength hot-rolled steel sheet according to claim 1, wherein, The above composition further contains, by mass %, one or more selected from Cr：0.005～2.0％、 Cu: 0.005 - 0.5%, Ni: 0.005 - 2.0%, Mo: 0.005 - 1.0%, V：0.005～0.5％、 B：0.0002～0.0050％、 Ca: 0.0001 - 0.0050%, REM: 0.0001 - 0.0050%, Sb: 0.0010 - 0.10%, and Sn: 0.0010 - 0.50% of one or more kinds.

3. The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein The amount of Fe present as precipitates with a particle size of 100 nm or more is more than 0 mass % and 0.100 mass % or less.

4. A method for manufacturing a high-strength hot-rolled steel sheet, which is the method for manufacturing the high-strength hot-rolled steel sheet according to claim 1 or 2, The slab having the above composition is heated in a temperature range of 1150 - 1300 °C and held in this temperature range for 0.2 - 3.5 hours, Then, when performing hot rolling, After rough rolling with a total reduction ratio of 80 - 90% in a temperature range of 1080 °C or higher, finish rolling is carried out, and in the finish rolling, rolling is performed under the condition that the reduction ratio in one pass below T calculated by the following formula is 25% or less. After finish rolling, air cooling is carried out for 1.0 s or more, Next, cool in the temperature range up to 550°C at an average cooling rate of 50°C / s or more, and then wind at a winding temperature above the Ms point and below 550°C, where The unit of the Ms point is °C, T = 800 + 1000[Ti] where [Ti] is the content of Ti by mass %, and the unit of T is °C.

5. A method for manufacturing a high-strength hot-rolled steel sheet, which is the method for manufacturing the high-strength hot-rolled steel sheet according to claim 3, The slab having the above composition is heated in a temperature range of 1150 - 1300 °C and held in this temperature range for 0.2 - 3.5 hours, Then, when performing hot rolling, After rough rolling with a total reduction ratio of 80 - 90% in a temperature range of 1080 °C or higher, finish rolling is carried out, and in the finish rolling, rolling is performed under the condition that the reduction ratio in one pass below T calculated by the following formula is 25% or less. After finish rolling, air cooling is carried out for 1.0 s or more, Next, it is cooled in a temperature range up to 550°C at an average cooling rate of 50°C / s or more, and then wound at a winding temperature above the Ms point and below 550°C, where The unit of the Ms point is °C, T = 800 + 1000[Ti] where [Ti] is the content of Ti by mass %, and the unit of T is °C.

6. The manufacturing method of the high-strength hot-rolled steel sheet according to claim 4, wherein, Between cooling from a temperature range up to 550°C to winding at an average cooling rate of 50°C / s or more, cooling is stopped at a cooling stop temperature in the temperature range of 480 to 550°C, held at the cooling stop temperature ±20°C for 0.5 to 4.0 s, and then wound at the winding temperature.

7. The manufacturing method of the high-strength hot-rolled steel sheet according to claim 5, wherein, Between cooling from a temperature range up to 550°C to winding at an average cooling rate of 50°C / s or more, cooling is stopped at a cooling stop temperature in the temperature range of 480 to 550°C, held at the cooling stop temperature ±20°C for 0.5 to 4.0 s, and then wound at the winding temperature.

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