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

Abstract

Provided is a high-strength hot-rolled steel sheet having a tensile strength of at least 980 MPa and excellent in bendability, hole expandability and punchability. This high-strength hot-rolled steel sheet: has a prescribed composition; has a microstructure which contains a bainite phase composed of upper bainite and granular bainite, and either / both of fresh martensite or / and a retained austenite phase, and in which the area ratio of the bainite phase is 70-97%, the total area ratio of the fresh martensite and the retained austenite phase is 3-30%, the proportion of the granular bainite in the bainite phase is 0.40 or more, and the average aspect ratio of prior austenite grains is 2.0-5.0; and has an X value of 30% or less.

Description

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

[0001] The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same.

[0002] In recent years, from the perspective of global environmental conservation, reduction of CO ２ emissions has been demanded on a global scale. In particular, improvement of fuel efficiency of automobiles is strongly desired, and weight reduction of automobile bodies is being pursued. Increasing the strength and thinning the thickness of the steel sheet used as a material for automobile members is one of the effective means for weight reduction without reducing the strength of the automobile body. In particular, steel sheets with a tensile strength of 980 MPa or more are expected as materials that can dramatically improve automobile fuel efficiency through thinning.

[0003] On the other hand, when the tensile strength of the steel sheet is increased, the ductility decreases, so the press formability deteriorates. Automobile parts, especially parts around the wheels such as lower arms, need to have a complex shape to ensure rigidity. In order to enable press forming into such a complex shape, excellent bendability is required for the steel sheet used as a material for automobile parts.

[0004] Further, when manufacturing automobile parts, especially parts around the wheels, severe burring may be performed after punching. Therefore, excellent punching property and hole expansion property are required for the steel sheet used as a material for automobile parts. However, when the strength of the steel sheet is increased, the hole expansion property deteriorates. This is because in a steel sheet with high strength, defects accompanied by unevenness occur easily on the punched end face, and cracks occur starting from these defects.

[0005] Thus, it is required that the steel sheet be excellent not only in strength but also in various characteristics. Therefore, various technologies have been proposed to improve the various characteristics of the steel sheet.

[0006] For example, in Patent Document 1, a technology related to a hot-rolled steel sheet excellent in formability, fracture characteristics, and fatigue characteristics has been proposed. Specifically, by controlling the conditions of hot rolling to make the main phase ferrite and controlling the shape and dispersion form of inclusions, the above characteristics are improved.

[0007] Patent Document 2 proposes a technology for hot-rolled steel sheets that have excellent punching fatigue characteristics and workability. Specifically, these characteristics are improved by controlling the shape and hardness of martensite in the center of the sheet thickness.

[0008] Patent Document 3 proposes a technology relating to a high-strength hot-rolled steel sheet with excellent hole-expanding and punching properties. Specifically, the aforementioned properties are improved by controlling the hot-rolling manufacturing conditions and controlling the size and number of inclusions.

[0009] Japanese Patent Publication No. 2014-031560, Japanese Patent Publication No. 2015-214718, International Publication No. 2017 / 017933

[0010] However, the prior art described in Patent Documents 1 to 3 had the following problems.

[0011] The technologies described in Patent Documents 1 and 2 cannot achieve a tensile strength of 980 MPa or higher.

[0012] Furthermore, the technology described in Patent Document 3 yields a hot-rolled steel sheet with a tensile strength of 980 MPa or more, excellent punchability, and excellent hole-expanding properties. However, bendability is not considered.

[0013] Thus, conventional technology has not established a method for producing high-strength hot-rolled steel sheets with a tensile strength of 980 MPa or higher, as well as excellent bendability, hole-expandability, and punchability.

[0014] Therefore, the present invention aims to provide a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more, and excellent bendability, hole-expandability, and punchability.

[0015] The present inventors have made this invention to solve the above-mentioned problems, and its gist is as follows.

[0016] 1. A high-strength hot-rolled steel sheet having a microstructure in mass%, containing C: 0.03-0.15%, Si: 0.5-3.0%, Mn: 1.0-3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01-1.0%, and N: 0.01% or less, with the remainder being Fe and unavoidable impurities, comprising a bainite phase consisting of upper bainite and granular bainite, and either or both of fresh martensite and retained austenite phases, wherein the area ratio of the bainite phase is 70-97%, the total area ratio of the fresh martensite and retained austenite phases is 3-30%, the proportion of granular bainite in the bainite phase is 0.40 or more, the average aspect ratio of prior austenite grains is 2.0-5.0, and the X value defined by the following formula (1) is 30% or less. X (%) = |Hv1 - Hv2| / (0.3 × TS) × 100 ... (1) Here, Hv1: Vickers hardness (Hv) at the 1 / 2 thickness position of the high-strength hot-rolled steel sheet, Hv2: Vickers hardness (Hv) at the 1 / 4 thickness position of the high-strength hot-rolled steel sheet, TS: Tensile strength (MPa) of the high-strength hot-rolled steel sheet.

[0017] 2. The high-strength hot-rolled steel sheet according to 1 above, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, V: 0.005-0.5%, Ti: 0.005-0.2%, Nb: 0.005-0.1%, Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-1.0%, Mo: 0.005-0.5%, B: 0.0002-0.005%, Sb: 0.001-0.1%, Sn: 0.001-0.1%, Ca: 0.0005-0.01%, Mg: 0.0005-0.01%, and REM: 0.0005-0.01%.

[0018] 3. A method for manufacturing a high-strength hot-rolled steel sheet as described in 1 or 2 above, wherein molten steel having the above component composition is cast into a steel material by rotating it in a horizontal plane with an induction electromagnetic stirring device at a rotation speed of 10 cm / s or more relative to a mold; the steel material is heated to a heating temperature of 1150°C or higher; the heated steel material is roughly rolled into a steel sheet; the steel sheet is finish-rolled under the conditions of a total reduction ratio of 50% or less in the temperature range of (RC + 200°C) or lower, and a finish rolling completion temperature of RC or higher and (RC + 200°C) or lower; the steel sheet after finish rolling is cooled under the conditions of a time from the end of finish rolling to the start of cooling of 2.0 s or less, an average cooling rate from the finish rolling completion temperature to Bs of 30°C / s or higher, a residence time in the temperature range of (Bs - 100°C) or higher and Bs°C or lower, and a cooling stop temperature of (Bs - 200°C) or higher and (Bs - 100°C) or lower. A method for manufacturing high-strength hot-rolled steel sheets, comprising: winding the cooled steel sheet at a winding temperature of (Bs - 200°C) or higher and (Bs - 100°C) or lower; and cooling the wound steel sheet to a cooling stop temperature of (Bs - 450°C) or lower at an average cooling rate of 1°C / s or less. Here, RC and Bs are defined by the following equations (2) and (3). RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ... (2) Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu ... (3) In equations (2) and (3) above, each element symbol represents the content (mass%) of the element, and 0 is used if the element is not contained.

[0019] According to the present invention, a high-strength hot-rolled steel sheet can be obtained that has a tensile strength of 980 MPa or more and excellent bendability, hole-expanding properties, and punchability. When the high-strength hot-rolled steel sheet of the present invention is applied to automobile parts (for example, automobile undercarriage parts such as suspensions, structural parts, frame parts, and truck frame parts), safety can be ensured and the weight of the automobile body can be reduced. Therefore, the high-strength hot-rolled steel sheet of the present invention can be used very suitably as a material for automobile parts.

[0020] In this invention, excellent bendability means that the ratio R / t, which is the ratio of the limit bending radius R to the plate thickness t, is 3.0 or less. The limit bending radius R is the minimum bending radius at which no crack with a depth of 50 μm or more occurs on the outer side of the bend in a 90° V bending test.

[0021] Excellent hole-expanding properties refer to a hole expansion ratio λ (%) of 50% or more, as defined by the following formula. In this formula, d (mm) is the diameter of the hole when a crack penetrates the plate thickness, after punching a hole with a 10 mmΦ punch with a clearance of 12 ± 1%, and then inserting a 60° conical punch into the punch hole, pushing it upward from the punching direction, in accordance with the Japan Iron and Steel Federation standard JFST 1001. λ (%) = {(d - 10) / 10} × 100

[0022] Excellent punching performance means that when punching with a 10 mm diameter punch, punching is performed with three or more clearances within a clearance range of 10-20%, including 10% and 20%, and there are no cracks, chips, brittle fracture surfaces, or secondary shear surfaces on the end faces of each punched hole.

[0023] Embodiments of the present invention will be described in detail below. The following description is merely an example of preferred embodiments of the present invention, and the invention is not limited thereto. In this specification, the bainite phase, consisting of upper bainite and granular bainite, may be referred to as the main phase. The fresh martensite and retained austenite phase may be referred to as the hard second phase or simply the second phase.

[0024] <High-Strength Hot-Rolled Steel Sheet> The high-strength hot-rolled steel sheet in one embodiment of the present invention has a predetermined component composition and microstructure. The reasons for each limitation are explained below.

[0025] [Component Composition] The high-strength hot-rolled steel sheet according to one embodiment of the present invention has the component composition described below. In the following description, "%" as a unit of content refers to "mass%" unless otherwise specified.

[0026] C: 0.03-0.15% C is an element that promotes the formation of bainite by improving hardenability and thereby improving strength. If the C content is less than 0.03%, the above effect is insufficient, and the desired strength cannot be obtained. Therefore, the C content should be 0.03% or more, preferably 0.04% or more, and more preferably 0.05% or more. On the other hand, if the C content exceeds 0.15%, fresh martensite and retained austenite phases as hard second phases are excessively formed, and the desired hole-expanding properties cannot be obtained. Therefore, the C content should be 0.15% or less, preferably 0.14% or less, and more preferably 0.13% or less.

[0027] Si: 0.5-3.0% Si is an element that improves the strength of steel sheets through solid solution strengthening. To obtain the above effect, the Si content should be 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more. On the other hand, Si also has the effect of promoting ferrite formation. If the Si content exceeds 3.0%, ferrite is formed, resulting in insufficient area ratio of the bainite phase, and thus the desired strength cannot be obtained. Therefore, the Si content should be 3.0% or less, preferably 2.7% or less, and more preferably 2.5% or less.

[0028] Mn: 1.0-3.0% Mn is an element that stabilizes austenite. If the Mn content is less than 1.0%, the ferrite phase increases, resulting in an insufficient area ratio of the bainite phase. As a result, the desired strength and punchability cannot be obtained. Therefore, the Mn content should be 1.0% or more, preferably 1.2% or more, and more preferably 1.5% or more. On the other hand, if the Mn content exceeds 3.0%, fresh martensite and retained austenite phases as hard second phases are produced in excess, and the desired hole-expanding properties cannot be obtained. Therefore, the Mn content should be 3.0% or less, preferably 2.8% or less, and more preferably 2.5% or less.

[0029] P: 0.1% or less. Since P degrades weldability, it is desirable to reduce the P content as much as possible. In this invention, a P content of up to 0.1% is acceptable. Therefore, the P content is set to 0.1% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%. However, a P content of less than 0.03% leads to an increase in refining costs. Therefore, from the viewpoint of cost, it is preferable to have a P content of 0.03% or more, and more preferable to have a P content of 0.05% or more.

[0030] S: 0.01% or less. Since sulfur (S) degrades weldability, it is desirable to reduce the S content as much as possible. In this invention, an S content of up to 0.01% is acceptable. Therefore, the S content is set to 0.01% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, an S content of less than 0.0001% leads to a decrease in production efficiency. Therefore, from the viewpoint of production efficiency, it is preferable to have an S content of 0.0001% or more, and more preferable to have an S content of 0.0005% or more.

[0031] Al: 0.01-1.0% Al acts as a deoxidizing agent and improves the cleanliness of the steel. If the amount of Al is too low, the above effect will be insufficient. Therefore, the Al content should be 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. On the other hand, Al is an element that promotes ferrite formation. If the Al content exceeds 1.0%, ferrite will be formed, and the desired hole-expanding properties cannot be obtained. Therefore, the Al content should be 1.0% or less, preferably 0.8% or less, and more preferably 0.5% or less.

[0032] N: 0.01% or less. N is an element that precipitates as nitride and contributes to the refinement of crystal grains. However, N tends to combine with elements such as Ti at high temperatures to form coarse nitrides, and excessive content reduces shear workability. Therefore, the N content should be 0.01% or less, preferably 0.008% or less, and more preferably 0.006% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, an N content of less than 0.001% leads to a decrease in production efficiency. Therefore, from the viewpoint of production efficiency, it is preferable to have an N content of 0.001% or more.

[0033] The component composition of the high-strength hot-rolled steel sheet in one embodiment of the present invention includes the above components, with the remainder being Fe and unavoidable impurities.

[0034] The component composition of the high-strength hot-rolled steel sheet in other embodiments of the present invention may further optionally contain at least one of the following components.

[0035] V: 0.005-0.5% V is an element that forms carbides and further improves the strength of steel sheets through precipitation strengthening. When V is added, the V content should be 0.005% or more, preferably 0.05% or more, and more preferably 0.1% or more, in order to obtain the above effect. On the other hand, if the V content exceeds 0.5%, the carbides may become coarser, and the punchability may deteriorate. For this reason, the V content should be 0.5% or less, preferably 0.3% or less.

[0036] Ti: 0.005-0.2% Ti is an element that forms carbides and further improves the strength of steel sheets through precipitation strengthening. When adding Ti, the Ti content should be 0.005% or more, preferably 0.01% or more, in order to obtain the above effect. On the other hand, if the Ti content exceeds 0.2%, the carbides may become coarser, and the punchability may deteriorate. For this reason, the Ti content should be 0.2% or less, preferably 0.1% or less.

[0037] Nb: 0.005-0.1% Nb is an element that forms carbides and further improves the strength of steel sheets through precipitation strengthening. When adding Nb, the Nb content should be 0.005% or more, preferably 0.01% or more, in order to obtain the above effect. On the other hand, if the Nb content exceeds 0.1%, the carbides may become coarser, and the punching performance may deteriorate. Therefore, the Nb content should be 0.1% or less, preferably 0.08% or less.

[0038] Cu: 0.005-0.5% Cu is an element that stabilizes austenite. By adding Cu, the formation of ferrite can be suppressed and the strength can be further improved. When adding Cu, in order to obtain the above effect, the Cu content should be 0.005% or more, preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the Cu content exceeds 0.5%, fresh martensite and retained austenite phase as hard second phases will be produced in excess, and the desired microstructure may not be obtained. For this reason, the Cu content should be 0.5% or less, preferably 0.3% or less.

[0039] Ni: 0.005-0.5% Ni, like Cu, is an element that stabilizes austenite. By adding Ni, the formation of ferrite can be suppressed and the strength can be further improved. When adding Ni, in order to obtain the above effect, the Ni content should be 0.005% or more, preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the Ni content exceeds 0.5%, fresh martensite and retained austenite phase as hard second phases may be excessively produced, and the desired microstructure may not be obtained. For this reason, the Ni content should be 0.5% or less, preferably 0.3% or less.

[0040] Cr: 0.005-1.0% Cr, like Cu, is an element that stabilizes austenite. By adding Cr, the formation of ferrite can be suppressed and the strength can be further improved. When adding Cr, in order to obtain the above effect, the Cr content should be 0.005% or more, preferably 0.01% or more, and more preferably 0.3% or more. On the other hand, if the Cr content exceeds 1.0%, fresh martensite and retained austenite phases as hard second phases will be excessively produced, and the desired microstructure may not be obtained. For this reason, the Cr content should be 1.0% or less.

[0041] Mo: 0.005 - 0.5% Mo, like Cu, is an element that stabilizes austenite. By adding Mo, the formation of ferrite can be suppressed, and the strength can be further improved. When adding Mo, in order to obtain the above effect, the Mo content should be 0.005% or more, preferably 0.01% or more, more preferably 0.05% or more. On the other hand, when the Mo content exceeds 0.5%, excessive fresh martensite and retained austenite phases as hard second phases may be generated, and the desired microstructure may not be obtained. Therefore, the Mo content should be 0.5% or less, preferably 0.3% or less.

[0042] B: 0.0002 - 0.005% B is an element that segregates at the austenite grain boundary and has the effect of suppressing the formation of ferrite. By adding B, the formation of upper bainite and granular bainite is promoted, and the strength of the steel sheet can be further improved. When adding B, in order to obtain the above effect, the B content should be 0.0002% or more, preferably 0.0005% or more, more preferably 0.0007% or more. On the other hand, when the B content exceeds 0.005%, the above effect becomes saturated. Therefore, the B content should be 0.005% or less, preferably 0.004% or less, more preferably 0.003% or less.

[0043] Sb: 0.001 - 0.1% Sb is an element that contributes to further improvement of the steel sheet strength by suppressing de - elementing from the steel surface when the steel material is heated. When adding Sb, in order to obtain the above effect, the Sb content should be 0.001% or more, preferably 0.005% or more. On the other hand, when the Sb content exceeds 0.1%, embrittlement of the steel sheet may occur. Therefore, the Sb content should be 0.1% or less, preferably 0.05% or less.

[0044] Sn: 0.001 - 0.1% Sn is an element that contributes to further improvement of the steel sheet strength by suppressing the formation of pearlite. When adding Sn, in order to obtain the above effect, the Sn content should be 0.001% or more, preferably 0.005% or more. On the other hand, when the Sn content exceeds 0.1%, embrittlement of the steel sheet may occur. Therefore, the Sn content should be 0.1% or less, preferably 0.05% or less.

[0045] Ca: 0.0005 to 0.01% Ca is an element that contributes to further improvement in punching property by controlling the form of inclusions. When adding Ca, in order to obtain the above effect, the Ca content should be 0.0005% or more, preferably 0.001% or more. On the other hand, when the Ca content exceeds 0.01%, the amount of inclusions increases and the punching property deteriorates. Therefore, the Ca content should be 0.01% or less, preferably 0.005% or less.

[0046] Mg: 0.0005 to 0.01% Mg is an element that contributes to further improvement in punching property by controlling the form of inclusions, similar to Ca. When adding Mg, in order to obtain the above effect, the Mg content should be 0.0005% or more, preferably 0.001% or more. On the other hand, when the Mg content exceeds 0.01%, the amount of inclusions increases and the punching property deteriorates. Therefore, the Mg content should be 0.01% or less, preferably 0.005% or less.

[0047] REM: 0.0005 to 0.01% REM (rare earth metal) is an element that contributes to further improvement in punching property by controlling the form of inclusions, similar to Ca and Mg. When adding REM, in order to obtain the above effect, the REM content should be 0.0005% or more, preferably 0.001% or more. On the other hand, when the REM content exceeds 0.01%, the amount of inclusions increases and the punching property deteriorates. Therefore, the REM content should be 0.01% or less, preferably 0.005% or less.

[0048] [Microstructure] Next, the reasons for limiting the microstructure of the hot-rolled steel sheet of the present invention will be explained. In the present invention, the microstructure at the 1 / 4 thickness position of the sheet thickness is used as the microstructure.

[0049] The high-strength hot-rolled steel sheet of the present invention has a microstructure including a bainite phase composed of upper bainite and granular bainite, and one or both of a fresh martensite and a retained austenite phase.

[0050] B: 70-97% The microstructure of the high-strength hot-rolled steel sheet of the present invention includes a bainite phase (B) as the main phase. However, if the area ratio of the bainite phase is less than 70%, excellent punchability cannot be obtained. For this reason, the area ratio of the bainite phase is set to 70% or more, preferably 80% or more, and more preferably 85% or more. On the other hand, if the area ratio of the bainite phase exceeds 97%, it leads to a significant deterioration in bendability. For this reason, the area ratio of the bainite phase is set to 97% or less, preferably 95% or less, and more preferably 93% or less.

[0051] In the microstructure of the high-strength hot-rolled steel sheet of the present invention, the bainite phase consists of upper bainite (UB) and granular bainite (GB). Therefore, the area ratio of the bainite phase (B) can be determined as the sum of the area ratio of upper bainite (UB) and the area ratio of granular bainite (GB). The area ratio of the upper bainite phase can be determined by analyzing images obtained by scanning electron microscopy (SEM). The area ratio of granular bainite can be determined by electron beam reflection diffraction. More specifically, it can be determined by the method described in the examples.

[0052] FM + γ: 3-30% The microstructure of the high-strength hot-rolled steel sheet of the present invention further includes either or both of the following as a hard second phase: fresh martensite (FM) and retained austenite phase (γ). However, if the total area ratio of fresh martensite and retained austenite phase (FM + γ) is less than 3%, a strength of 980 MPa or more cannot be obtained. Therefore, the total area ratio is set to 3% or more, preferably 5% or more, and more preferably 7% or more. On the other hand, if the total area ratio exceeds 30%, the desired hole-expanding properties cannot be obtained. This is because macroscopic stress concentration occurs at the phase interface during hole-expanding tests, becoming a crack initiation point. Therefore, the total area ratio is set to 30% or less, preferably 25% or less, and more preferably 20% or less.

[0053] The area ratios of fresh martensite and retained austenite phases can be determined by electron beam reflection diffraction. More specifically, they can be determined by the method described in the examples.

[0054] The proportion of granular bainite is 0.40 or more. Granular bainite has better bendability than upper bainite. If the proportion of granular bainite in the bainite phase is less than 0.40, good bendability cannot be obtained. For this reason, the proportion of granular bainite in the bainite phase (hereinafter simply referred to as the proportion of granular bainite) is set to 0.40 or more, preferably 0.45 or more, and more preferably 0.50 or more. On the other hand, there is no particular upper limit to the proportion of granular bainite. The proportion of granular bainite may be, for example, 0.99 or less, or 0.98 or less.

[0055] The proportion of granular bainite is calculated using the following formula: Proportion of granular bainite = Area ratio of granular bainite / (Area ratio of upper bainite + Area ratio of granular bainite) As mentioned above, the area ratio of the upper bainite phase can be determined by analyzing images obtained from scanning electron microscopy (SEM) observation. The area ratio of granular bainite can be determined by electron beam reflection diffraction.

[0056] In this invention, it is sufficient that the area ratios of the bainite phase, fresh martensite, and retained austenite phase satisfy the above conditions, and the presence of other structures as the remainder is permissible. Examples of these other structures include lower bainite (LB) and ferrite (F). The total area ratio of these other structures should be low, preferably 0%. In other words, the microstructure of the hot-rolled steel sheet in one embodiment of the present invention may consist of a bainite phase comprising upper bainite and granular bainite, and either or both of the fresh martensite and retained austenite phases.

[0057] Furthermore, the microstructure of the high-strength hot-rolled steel sheet of the present invention must satisfy the following conditions regarding the average aspect ratio of prior austenite grains.

[0058] Average aspect ratio of prior austenite grains: 2.0 to 5.0 If the average aspect ratio of prior austenite grains exceeds 5.0, the dislocation density of the phase generated from the austenite grains is high, and excellent bendability cannot be obtained. Therefore, the average aspect ratio of prior austenite grains should be 5.0 or less, preferably 4.8 or less, and more preferably 4.5 or less. On the other hand, if the average aspect ratio of prior austenite grains is less than 2.0, the dislocation density of the phase generated from the austenite grains becomes significantly low, and as a result, a strength of 980 MPa or more cannot be obtained. Therefore, the average aspect ratio of prior austenite grains should be 2.0 or more, preferably 2.2 or more, and more preferably 2.5 or more. The average aspect ratio of prior austenite grains can be determined by analyzing images obtained by optical microscopy observation. More specifically, it can be determined by the method described in the examples.

[0059] Furthermore, in addition to having the above-mentioned component composition and microstructure, the high-strength hot-rolled steel sheet of the present invention must satisfy the following conditions regarding Vickers hardness.

[0060] X: When manufacturing steel materials by casting (less than 30%), some elements contained in the molten steel tend to segregate towards the center of the plate thickness. If the central segregation of elements such as Mn is excessive, a hard second phase is formed in the segregated area, causing a localized increase in hardness and consequently reducing punchability. This is because the increased hardness difference between the center of the plate thickness and other parts makes it easier for cracks and roughness to occur during punching. Therefore, in order to improve punchability, it is necessary to reduce the hardness difference in the thickness direction of the steel plate.

[0061] Specifically, if the X value defined by the following equation (1) is greater than 30%, the desired punching performance cannot be obtained. Therefore, the X value should be 30% or less, preferably 25% or less, and more preferably 20% or less. X (%) = |Hv1 - Hv2| / (0.3 × TS) × 100 … (1) Here, Hv1: Vickers hardness (Hv) at the 1 / 2 thickness position of the high-strength hot-rolled steel sheet, Hv2: Vickers hardness (Hv) at the 1 / 4 thickness position of the high-strength hot-rolled steel sheet, TS: Tensile strength (MPa) of the high-strength hot-rolled steel sheet.

[0062] The reason we use the X value mentioned above, rather than simply the difference in Vickers hardness, is that the difference in Vickers hardness tends to increase as the tensile strength of the steel plate increases. In this invention, by controlling the X value, which corrects for this effect, to 30% or less, we can reliably ensure good punching performance.

[0063] On the other hand, from the viewpoint of punching properties, a smaller X is better, so there is no particular limit to the lower limit of the X value. However, from the viewpoint of ease of manufacturing, it is preferable that X be 1% or more, more preferable that it be 2% or more, and even preferable that it be 3% or more.

[0064] The high-strength hot-rolled steel sheet of the present invention has a tensile strength of 980 MPa or more, and also possesses excellent bendability, excellent hole-expanding properties, and excellent punchability. Therefore, because the high-strength hot-rolled steel sheet of the present invention has high tensile strength and excellent formability even when thinned, it can be suitably used as a material for automotive components such as trucks and passenger cars.

[0065] The mechanical properties of high-strength hot-rolled steel sheets can be evaluated by the methods described in the examples. For example, tensile strength can be measured by a tensile test in accordance with the provisions of JIS Z 2241. The upper limit of the tensile strength is not particularly limited, but may be, for example, 1400 MPa or less, or 1300 MPa or less.

[0066] The thickness of the high-strength hot-rolled steel sheet of the present invention is not particularly limited, but is typically preferably 1.5 to 6.0 mm.

[0067] [Manufacturing Method] Next, a method for manufacturing a high-strength hot-rolled steel sheet according to one embodiment of the present invention will be described. In the following description, unless otherwise specified, temperature refers to the surface temperature of the object (steel material or steel sheet).

[0068] The high-strength steel sheet of the present invention can be manufactured by sequentially carrying out the following steps (1) to (7). Each step will be described in detail below. (1) Casting (2) Heating (3) Rough rolling (4) Finish rolling (5) Cooling (first cooling) (6) Winding (7) Cooling (second cooling)

[0069] Casting: First, molten steel having the above-mentioned component composition is cast to produce steel material. The molten steel can be manufactured according to conventional methods. Scrap can be used as a raw material in this process.

[0070] Swirling speed: 10 cm / s or more In the above casting, stirring is performed using an induction electromagnetic stirring device. By casting while swirling the molten steel material by induction electromagnetic stirring, columnar crystals growing from the mold can be divided, and equiaxed crystals can be generated at the 1 / 4 to 1 / 2 position of the plate thickness. As a result, segregation of components generated during solidification can be suppressed, and the X value can be reduced. However, if the swirling speed during electromagnetic stirring is less than 10 cm / s, the columnar crystals will not be sufficiently divided, and the desired effect cannot be obtained. For this reason, the swirling speed should be 10 cm / s or more, preferably 15 cm / s or more, and more preferably 20 cm / s or more in the horizontal plane. On the other hand, from the above viewpoint, the faster the swirling speed, the better, so there is no particular upper limit to the swirling speed. However, if the swirling speed is increased excessively, the effect will saturate. For this reason, from the viewpoint of manufacturing efficiency, it is preferable to set the swirling speed to 100 cm / s or less, more preferably 50 cm / s or less, and even more preferably 30 cm / s or less.

[0071] A steel material is produced by casting under the above conditions. The casting may typically be continuous casting. The steel material may typically be a steel slab.

[0072] The resulting steel material may be subjected to a heating process after being cooled to become a hot or cold slab following the casting process. The final high-strength hot-rolled steel sheet will have the same composition as the steel material used.

[0073] Next, the steel material is heated. The heating temperature is as follows:

[0074] Heating temperature: 1150°C or higher. In steel materials, most precipitate-forming elements exist non-uniformly as coarse precipitates. If the added elements exist as coarse and non-uniform precipitates, the effect of the added elements cannot be fully obtained, and the desired microstructure cannot be obtained. Therefore, it is necessary to heat the steel material prior to hot rolling to solidify the coarse precipitates. For this reason, the heating temperature of the steel material should be 1150°C or higher, preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, there is no particular upper limit to the heating temperature. However, if the heating temperature is too high, it can lead to the occurrence of slab defects and a decrease in yield due to scale-off. For this reason, the heating temperature should preferably be 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower.

[0075] From the viewpoint of uniformizing the temperature of the steel material, it is preferable to raise the steel material to the heating temperature and then hold it at that heating temperature. The holding time at the heating temperature (holding time) is not particularly limited, but from the viewpoint of improving the uniformity of the temperature of the steel material, it is preferable to set it to 1800 seconds or more. On the other hand, if the holding time exceeds 10000 seconds, the amount of scale generated increases. As a result, scale inclusion and other problems are more likely to occur in the subsequent hot rolling, leading to a decrease in yield due to surface defects. For this reason, it is preferable to set the holding time to 10000 seconds or less, and more preferably to 8000 seconds or less.

[0076] Next, the heated steel material is roughly rolled to form a steel sheet. The conditions for rough rolling are not particularly limited, and general conditions may be used. Before performing finish rolling on the obtained steel sheet, descaling may be performed. This descaling can be performed, for example, by spraying high-pressure water onto the surface of the steel sheet (high-pressure water descaling). This descaling can be performed at the entrance of the finish rolling mill.

[0077] Next, the steel sheet is subjected to finish rolling. In the finish rolling process, the reduction ratio and the finish rolling completion temperature are controlled as follows.

[0078] Total reduction ratio below (RC + 200°C): 50% or less In the above finish rolling, the total reduction ratio in the temperature range below (RC + 200°C) shall be 50% or less. Here, RC is defined by the following equation (2) and represents the lower limit temperature of austenite recrystallization estimated from the composition of the steel. RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ... (2) In the above equation (2), each element symbol represents the content (mass%) of each element, and if the element is not contained, it shall be 0.

[0079] If the total reduction ratio exceeds 50%, transformation occurs from austenite grains with a large aspect ratio, making it impossible to maintain the aspect ratio of the prior austenite grains within the desired range. Therefore, the total reduction ratio in the temperature range below (RC + 200°C) should be 50% or less, preferably 45% or less, and more preferably 40% or less. On the other hand, the lower limit of the total reduction ratio is not particularly limited, but it is preferable to set it to 5% or more from the viewpoint of further increasing strength.

[0080] Finish rolling completion temperature: RC or higher (RC + 200°C) or lower. If the finish rolling completion temperature in the above finish rolling is lower than RC, the aspect ratio of the prior austenite grains cannot be kept within the desired range, and as a result, excellent ductility cannot be obtained. This is because strain relaxation due to recrystallization is difficult to occur. For this reason, the finish rolling completion temperature is set to be RC or higher, preferably (RC + 20°C) or higher, and more preferably (RC + 50°C) or higher. On the other hand, if the finish rolling completion temperature is higher than (RC + 200°C), the desired strength cannot be obtained. This is because the aspect ratio becomes excessively small due to the progress of recrystallization of the austenite grains. For this reason, the finish rolling completion temperature is set to be (RC + 200°C) or lower, preferably (RC + 180°C) or lower, and more preferably (RC + 150°C) or lower.

[0081] • Cooling (First Cooling) Next, the obtained steel plate is cooled (first cooling). At this time, the time until the start of cooling, the average cooling rate, and the cooling stop temperature are controlled as follows.

[0082] Time from the end of finish rolling to the start of cooling: 2.0 s or less If the time from the end of finish rolling to the start of cooling (cooling start time) exceeds 2.0 s, the desired strength cannot be obtained. This is because recrystallization progresses, and the aspect ratio of the austenite grains becomes less than 2.0. For this reason, the cooling start time should be 2.0 s or less, preferably 1.5 s or less, and more preferably 1.0 s or less. On the other hand, the shorter the time until the start of cooling, the better. For this reason, the lower limit of the cooling start time is not particularly limited and may be, for example, 0 s or 0.1 s. A cooling start time of 0 s means that cooling starts immediately after the end of finish rolling.

[0083] Average cooling rate: 30°C / s or more If the average cooling rate from the finish rolling completion temperature to Bs is less than 30°C / s, excessive ferrite is generated, making it impossible to achieve the desired area ratio of the bainite phase. As a result, the desired strength cannot be obtained. Therefore, the average cooling rate is set to 30°C / s or more, preferably 40°C / s or more, and more preferably 50°C / s or more. On the other hand, there is no particular upper limit to the average cooling rate, but if it is too fast, it becomes difficult to control the cooling stop temperature. Therefore, the average cooling rate is preferably 500°C / s or less, more preferably 300°C / s or less, and even more preferably 150°C / s or less.

[0084] In the aforementioned cooling process, forced cooling should be performed to achieve the above-mentioned average cooling rate. The cooling method is not particularly limited, but typically it can be performed by water cooling.

[0085] Residence time: 3.0 to 20.0 s If the residence time in the temperature range of (Bs-100°C) or higher and Bs or lower during the cooling is less than 3.0 s, the proportion of granular bainite cannot be within the desired range. Therefore, the residence time is set to 3.0 s or more, preferably 4.0 s or more, and more preferably 5.0 s or more. On the other hand, if the residence time exceeds 20.0 s, an excessive amount of hard second phase is generated, and the desired hole-expanding properties cannot be obtained. Therefore, the residence time is set to 20.0 s or less, preferably 18.0 s or less, and more preferably 15.0 s or less.

[0086] Cooling stop temperature: (Bs-200°C) or higher and (Bs-100°C) or lower. If the cooling stop temperature is below (Bs-200°C), the microstructure becomes lower bainite. Lower bainite is a high-strength structure, but it does not have good bendability. For this reason, the cooling stop temperature should be (Bs-200°C) or higher, preferably (Bs-190°C) or higher, and more preferably (Bs-180°C) or higher. On the other hand, if the cooling stop temperature is higher than (Bs-100°C), an excessive amount of hard second phase is generated, and excellent hole-expanding properties cannot be obtained. For this reason, the cooling stop temperature should be (Bs-100°C) or lower, preferably (Bs-110°C) or lower, and more preferably (Bs-120°C) or lower.

[0087] Furthermore, Bs is defined by the following equation (3): Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu ... (3) In the above equation (3), each element symbol represents the content (mass %) of the element, and 0 is used if the element is not contained.

[0088] Next, the cooled steel sheet is wound up. At that time, the winding temperature is controlled as follows.

[0089] Winding temperature: (Bs-200°C) or higher and (Bs-100°C) or lower. If the winding temperature is below (Bs-200°C), lower bainite and martensite will form, and excellent punching properties cannot be obtained. Therefore, the winding temperature should be (Bs-200°C) or higher, preferably (Bs-190°C) or higher, and more preferably (Bs-180°C) or higher. On the other hand, if the winding temperature is higher than (Bs-100°C), an excessive amount of hard second phase will be formed, and excellent hole-expanding properties cannot be obtained. Therefore, the winding temperature should be (Bs-100°C) or lower, preferably (Bs-110°C) or lower, and more preferably (Bs-120°C) or lower.

[0090] • Cooling (Second Cooling) Next, the steel sheet after winding is cooled. At this time, the average cooling rate and the cooling stop temperature are controlled as follows.

[0091] Average cooling rate: 1°C / s or less If the average cooling rate from the winding temperature to the cooling stop temperature described later exceeds 1°C / s, the desired punching performance cannot be obtained. This is because the area ratio of the bainite phase becomes insufficient as a result of the formation of lower bainite and martensite. For this reason, the average cooling rate should be 1°C / s or less, preferably 0.8°C / s or less, and more preferably 0.5°C / s or less. On the other hand, there is no particular lower limit to the average cooling rate, but from the viewpoint of production efficiency, it is preferable to be 0.01°C / s or more, and more preferably 0.1°C / s or more.

[0092] Cooling stop temperature: (Bs - 450°C) or lower. If the cooling stop temperature exceeds (Bs - 450°C), the transformation to UB will not be completed, and a sufficient amount of UB will not be obtained. Therefore, the cooling stop temperature should be (Bs - 450°C) or lower. On the other hand, the lower limit is not limited, and it can be performed down to any temperature below (Bs - 450°C). From the viewpoint of productivity, it is preferable that the cooling stop temperature be 20°C or higher, more preferably 30°C or higher, and even more preferably 40°C or higher. Note that cooling can be performed in any form, for example, in the state of a wound coil.

[0093] The high-strength hot-rolled steel sheet of the present invention can be manufactured by following the above procedure. After winding and subsequent cooling, for example, temper rolling may be performed, or pickling may be performed to remove scale formed on the surface.

[0094] The present invention will be described in more detail below based on examples. However, the present invention is not limited to the following examples.

[0095] First, molten steel with the composition shown in Table 1 was melted in a converter, and steel slabs (steel materials) were manufactured by continuous casting. In the casting process, the molten steel was rotated using an induction electromagnetic stirring device at the rotation speed shown in Table 2. For convenience, the values ​​for RC, Bs, etc. in Table 1 are rounded to the first decimal place.

[0096] The obtained steel slabs were heated to the heating temperatures shown in Table 2, and then the heated steel material was subjected to hot rolling consisting of rough rolling and finish rolling to produce hot-rolled steel sheets. The total reduction ratio in the temperature range of (RC + 200°C) or lower during the finish rolling, and the finish rolling completion temperature are as shown in Table 2.

[0097] Next, the obtained hot-rolled steel sheet was cooled under the conditions of cooling start time, average cooling rate, and cooling stop temperature shown in Table 2 (first cooling). The cooled hot-rolled steel sheet was wound at the winding temperature shown in Table 2, and the wound steel sheet was cooled at the average cooling rate shown in Table 2 (second cooling) to obtain a high-strength hot-rolled steel sheet. After the above cooling, temper rolling was performed, followed by pickling. The pickling was carried out using a 10% by mass hydrochloric acid aqueous solution at a temperature of 85°C.

[0098] (Microstructure) The microstructure of each of the obtained high-strength hot-rolled steel sheets was evaluated using the procedure described below.

[0099] From the obtained high-strength hot-rolled steel sheet, a specimen for microstructural observation was taken so that the cross-section parallel to the rolling direction served as the observation surface. The surface of the obtained specimen was polished, and the microstructure was revealed by further etching the surface with an etching solution (3% nital solution). Next, the microstructure at the 1 / 4 thickness position was observed using a scanning electron microscope (SEM), and an SEM image was obtained. The observation was performed at a magnification of 5000x and in 10 fields of view. The obtained SEM images were analyzed by image processing, and the area percentages of upper bainite (UB), polygonal ferrite (F), lower bainite (LB), martensite (M), and fresh martensite and / or retained austenite phase (FM+γ) were quantified.

[0100] Fresh martensite (FM) has a white contrast in SEM images, while martensite (M) is a self-tempered phase with a relatively dark contrast. Since fresh martensite and martensite differ in shape and hardness, they will be treated separately here.

[0101] Furthermore, since granular bainite (GB), fresh martensite (FM), and retained austenite (γ) are difficult to distinguish using SEM, they were identified using electron beam reflection diffraction, and their respective area fractions were determined. The measured area fractions of each microstructure are shown in Table 3.

[0102] The upper bainite phase is an aggregate of bainite ferrites, and typically has a structure containing Fe-based carbides and / or retained austenite phases between the bainite ferrites. However, in the present invention, the upper bainite phase is also included in cases where there are no Fe-based carbide retained austenites between the bainite ferrites.

[0103] Lath-like bainitic ferrite differs from lamellar (layered) ferrite and polygonal ferrite in pearlite in that it has a lath-like shape and a relatively high dislocation density within it. Therefore, the two can be distinguished using SEM (scanning electron microscope) or TEM (transmission electron microscope). If retained austenite is present between the laths, only the lath-like bainitic ferrite portion is considered upper bainite and is distinguished from the retained austenite. Fresh martensite is martensite that does not contain Fe-based carbides.

[0104] Furthermore, granular bainite is a structure composed of granular bainitic ferrite, with Fe-based carbides and / or retained austenite phases between the granular bainitic ferrites. However, granular bainite is also included in cases where Fe-based carbides and / or retained austenite are not present between the granular bainitic ferrites. In this invention, granular bainite is defined as grains with an aspect ratio of 2 or less and grain boundaries that do not have the orientation relationships of <15 16 18>-60°, <6 15 15>-56°, <6 7 7>-50°, <1 0 1>-60°, and <1 2 2>-17° in the <axial>-rotation angle relationship. The crystal orientation is determined using electron backscatter diffraction (EBSD) method.

[0105] Fresh martensite and / or retained austenite phases have low clarity (IQ value). Since the prior austenite grains have a large aspect ratio and large grain size, in this study, grains with an aspect ratio of 2 or more and a size of 10 μm or larger were considered prior austenite grains. Therefore, grain boundaries with an orientation difference of 15° or more that have relationships with upper bainite grains were excluded from the recognition of grain boundaries, and among the remaining grains, those with a high IQ value and an aspect ratio of 2 or less and a size of 10 μm or less were judged to be granular bainite.

[0106] Furthermore, the fresh martensite and / or retained austenite phase exhibits brighter contrast in SEM images compared to the upper bainite phase, lower bainite phase, martensite phase, and polygonal ferrite phase. Therefore, the fresh martensite and / or retained austenite phase can be distinguished from these tissues using SEM.

[0107] Although the fresh martensite phase and the retained austenite phase have similar contrast in SEM, they can be distinguished from each other using electron beam reflection diffraction.

[0108] Furthermore, from the obtained high-strength hot-rolled steel sheet, test specimens for microstructural observation were taken so that the cross-section parallel to the rolling direction served as the observation surface. The surface of the obtained test specimens was polished, and the prior austenite structure was revealed using an etching solution (an aqueous solution containing picric acid, a surfactant, and oxalic acid). Next, the microstructure at the 1 / 4 thickness position was observed using an optical microscope, and microscopic images were obtained. The observation was performed at a magnification of 500x and in 5 fields of view. The obtained microscopic images were analyzed by image processing, and the aspect ratio of the prior austenite grains was calculated. In the above analysis, the prior austenite grains were approximated as ellipses. That is, the longest part of the prior austenite grain was defined as the major axis, and the shortest part as the minor axis, and the aspect ratio (major axis / minor axis) of each prior austenite grain was determined. The arithmetic mean of the obtained aspect ratios of each prior austenite grain was taken as the average aspect ratio.

[0109] The measurement results are shown in Table 3. Table 3 also includes the area percentage of the bainite phase (UB + GB), consisting of upper bainite and granular bainite, and the total area percentage of fresh martensite and retained austenite (FM + γ).

[0110] Next, the Vickers hardness of the obtained hot-rolled steel sheets was measured using the following procedure, and the X value was determined. The evaluation results are shown in Table 3.

[0111] From the obtained hot-rolled steel sheet, a sample for hardness measurement was taken so that the thickness cross-section parallel to the rolling direction would be the hardness measurement cross-section. The Vickers hardness Hv1 at the 1 / 2 thickness position and the Vickers hardness Hv2 at the 1 / 4 thickness position of the sample were measured. In the measurement, the Vickers hardness was measured at five points with measurement intervals of 250 μm or more in the region within ±1 / 20t in the thickness direction centered on the measurement position, and the average value was taken as the Vickers hardness at the measurement position. The measurement conditions for Vickers hardness were a load of 100 g and a holding time of 10 s. Hereinafter, t is the thickness of the hot-rolled steel sheet.

[0112] The X value was calculated from the obtained Vickers hardness and the tensile strength TS (MPa) of the hot-rolled steel sheet measured by the tensile test described later, using the following formula (1). For convenience, Table 3 shows the obtained X value rounded to two decimal places. X (%) = |Hv1 - Hv2| / (0.3 × TS) × 100 …(1)

[0113] Furthermore, the strength, bendability, hole-expandability, and punchability of the obtained hot-rolled steel sheets were evaluated using the following procedure. The evaluation results are shown in Table 3.

[0114] (Strength) The strength of the hot-rolled steel sheet was evaluated by performing a tensile test. Specifically, first, a JIS No. 5 tensile test specimen (JIS Z 2241) was taken from the hot-rolled steel sheet in a direction parallel to the rolling direction. Next, a tensile test was performed using the tensile test specimen in accordance with the provisions of JIS Z 2241 to determine the tensile strength. The strain rate in the tensile test was 10 －３ The value was set to / s. A tensile strength of 980 MPa or higher was considered acceptable.

[0115] (Bendability) The bendability of the hot-rolled steel sheet was evaluated by performing a bending test. Specifically, the hot-rolled steel sheet was first sheared, and a bending test specimen was taken so that the longitudinal direction of the specimen was perpendicular to the rolling direction. The dimensions of the bending test specimen were 35 mm in width and 100 mm in length. Using the specimen with a sheared end face, a V-block 90° bending test was performed in accordance with the push bending method specified in JIS Z 2248. For each hot-rolled steel sheet, the test was performed using three specimens, and the minimum bending radius at which no crack occurred in any of the specimens was determined as the limit bending radius R (mm). The R / t value was calculated by dividing the limit bending radius R by the thickness t (mm) of the hot-rolled steel sheet. If R / t was 3.0 or less, it was judged to be acceptable.

[0116] (Hole Expansion Properties) The hole expansion properties of the hot-rolled steel sheet were evaluated by conducting a hole expansion test. Specifically, first, a test piece for the hole expansion test was taken from the hot-rolled steel sheet. The dimensions of the test piece were thickness: t × width 100 mm × length 100 mm. Using the test piece, a punch hole was made in the center of the test piece with a 10 mmφ punch, in accordance with the Japan Iron and Steel Federation standard JFST 1001, with a clearance of 12% ± 1%. Then, a 60° conical punch was inserted into the punch hole, pushing upward from the punching direction, and the hole diameter d (mm) at the point when the crack penetrated the thickness of the sheet was determined. From the hole diameter, the hole expansion rate λ (%) was calculated using the following formula. A hole expansion rate λ of 50% or more was considered acceptable. λ (%) = {(d - 10) / 10} × 100

[0117] (Punching Performance) The punching performance of the hot-rolled steel sheet was evaluated by performing punching. Specifically, a test piece was first taken from the hot-rolled steel sheet. The dimensions of the test piece were thickness: t × width 30 mm × length 30 mm. A 10 mm diameter cylindrical punch was used to punch out a hole in the center of the test piece with three or more clearances, including 10% and 20%, within a clearance range of 10-20%. The clearance values ​​are expressed as a percentage of the sheet thickness of the test piece. If there were no cracks, chips, brittle fracture surfaces, or secondary shear surfaces on the end face of the punched hole, it was judged to be acceptable.

[0118]

[0119]

[0120]

[0121] As can be seen from the results shown in Table 3, the hot-rolled steel sheets that met the conditions of the present invention had a tensile strength of 980 MPa or more and excellent bendability, hole-expanding ability, and punchability. In contrast, the hot-rolled steel sheets that did not meet the conditions of the present invention were inferior in at least one characteristic.

Claims

1. The composition is as follows, by mass%, containing C: 0.03-0.15%, Si: 0.5-3.0%, Mn: 1.0-3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01-1.0%, and N: 0.01% or less, with the remainder being Fe and unavoidable impurities; comprising a bainite phase consisting of upper bainite and granular bainite, and either or both of fresh martensite and retained austenite phases; the area ratio of the bainite phase is 70-97%, the total area ratio of the fresh martensite and retained austenite phases is 3-30%, the proportion of granular bainite in the bainite phase is 0.40 or more, and the average aspect ratio of prior austenite grains is 2.0-5.0; A high-strength hot-rolled steel sheet having an X value of 30% or less, as defined by the following equation (1): X (%) = |Hv1 - Hv2| / (0.3 × TS) × 100 … (1) where, Hv1: Vickers hardness (Hv) at the 1 / 2 thickness position of the high-strength hot-rolled steel sheet, Hv2: Vickers hardness (Hv) at the 1 / 4 thickness position of the high-strength hot-rolled steel sheet, TS: Tensile strength (MPa) of the high-strength hot-rolled steel sheet.

2. The high-strength hot-rolled steel sheet according to claim 1, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, V: 0.005 to 0.5%, Ti: 0.005 to 0.2%, Nb: 0.005 to 0.1%, Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 1.0%, Mo: 0.005 to 0.5%, B: 0.0002 to 0.005%, Sb: 0.001 to 0.1%, Sn: 0.001 to 0.1%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.01%.

3. A method for manufacturing a high-strength hot-rolled steel sheet according to claim 1 or 2, comprising: casting molten steel having the above-mentioned component composition with an induction electromagnetic stirring device while rotating it in a horizontal plane relative to a mold at a rotational speed of 10 cm / s or more to obtain a steel material; heating the steel material to a heating temperature of 1150°C or higher; roughly rolling the heated steel material to obtain a steel sheet; and finish rolling the steel sheet under the conditions of a total reduction ratio of 50% or less in a temperature range of (RC + 200°C) or less, and a finish rolling completion temperature of RC or higher and (RC + 200°C) or lower. A method for manufacturing a high-strength hot-rolled steel sheet, comprising: cooling the steel sheet after finish rolling under the following conditions: time from the end of finish rolling to the start of cooling: 2.0 s or less, average cooling rate from the end temperature of finish rolling to Bs: 30°C / s or more, residence time in the temperature range of (Bs-100°C) or above to Bs: 3.0 to 20.0 s, and cooling stop temperature: (Bs-200°C) or above to (Bs-100°C); winding the cooled steel sheet under the condition: winding temperature: (Bs-200°C) or above to (Bs-100°C); and cooling the wound steel sheet to a cooling stop temperature of (Bs-450°C) or below at an average cooling rate of 1°C / s or less. Here, RC and Bs are defined by the following equations (2) and (3). RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ... (2) Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu ... (3) In equations (2) and (3) above, each element symbol represents the content (mass%) of the element, and 0 is used if the element is not contained.

Images