Hot-rolled steel sheet and method of manufacturing the same

Abstract

Provided is a hot-rolled steel sheet having excellent elongation of flange. A hot-rolled steel sheet having a composition consisting of, in mass%, C: 0.10% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 1.5% or less, O: 0.0025% or less, and the balance consisting of Fe and inevitable impurities, a main phase being ferrite, and a maximum orientation density of crystal grains being 2.1 or less.

Description

Technical Field

[0001] This invention relates to hot-rolled steel sheets and methods for manufacturing the same, and particularly to hot-rolled steel sheets suitable as raw materials for automotive parts and methods for manufacturing the same. Background Technology

[0002] From the perspective of improving automotive crash safety and fuel efficiency, technologies such as increasing the strength and optimizing the structure of steel sheets used in automotive components are being developed. Shape optimization requires improvements in three-dimensional shape, sometimes necessitating more stringent processing than before. Therefore, automotive steel sheets require superior machinability. Particularly for components using low-strength hot-rolled steel sheets with strength grades below 780 MPa, excellent hole-expanding properties are needed to withstand stringent extended flange processing. Against this backdrop, a wide variety of hot-rolled steel sheets have been developed.

[0003] Patent Document 1 discloses a technology related to hot-rolled steel sheets, in which the addition amounts of C, Si, Mn, P, S, and Al are adjusted to appropriate ranges, and the microstructure is composed of fine polygonal ferrite and bainite. The length of cementite and grain boundary coverage are controlled, thereby achieving excellent fatigue characteristics and porosity. Patent Document 2 discloses a technology related to hot-rolled steel sheets, in which the addition amounts of C, Si, Mn, Al, and Nb are adjusted to appropriate ranges, and a microstructure composed of retained austenite and polygonal ferrite is formed and refined, thereby achieving a balance between strength and elongation, improved durability, and enhanced porosity. Patent Document 3 discloses a technology related to hot-rolled steel sheets, in which the addition amounts of C, Si, Mn, P, S, Al, N, Mg, Nb, and Ti are controlled to appropriate ranges, with ferrite as the main phase, and oxides such as MgO, Al₂O₃, Ti₂O₃, SiO₂, and MnO are controlled, thereby achieving excellent porosity and ductility.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 9-170048

[0007] Patent Document 2: Japanese Patent Application Publication No. 11-1747

[0008] Patent Document 3: Japanese Patent Application Publication No. 2001-342543 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, the technology in Patent Document 1 stops at a porosity of less than 100%, without any insights or inspirations for further improving the porosity. Furthermore, it does not investigate the amount of oxygen (O) or the crystal orientation in the steel. In Patent Document 2, 5-20% retained austenite is introduced to improve toughness and fatigue properties; however, the retained austenite transforms into martensite during punching, reducing porosity. Therefore, sufficient porosity to withstand stringent extended flange processing is not obtained. Additionally, no technology related to the amount of oxygen (O) or the crystal orientation in the steel is disclosed. In Patent Document 3, oxides are controlled, but there are no insights into improving porosity by controlling the oxygen content itself, nor is crystal orientation considered.

[0011] The present invention is an invention to solve the above-mentioned problems, and its purpose is to provide a hot-rolled steel sheet with excellent elongation flange properties.

[0012] In addition, the object of the present invention is to provide a method for manufacturing the above-mentioned hot-rolled steel sheet.

[0013] Methods for solving problems

[0014] The inventors conducted in-depth research on oxygen content and crystal orientation in steel. By controlling the amount of oxygen in steel to a certain level and randomizing the crystal orientation, the elongation flange properties can be significantly improved, thus completing this invention.

[0015] It should be noted that, in this invention, excellent elongation flange properties refer to a hole expansion rate of 100% or more.

[0016] The present invention has the following structure.

[0017] [1] A hot-rolled steel sheet having, by mass %, a composition comprising: C: less than 0.10%, Si: less than 2.0%, Mn: less than 2.0%, P: less than 0.100%, S: less than 0.02%, Al: less than 1.5%, O: less than 0.0025%, with the balance being Fe and unavoidable impurities.

[0018] The main phase is ferrite, and the maximum orientation density of the grains is below 2.1.

[0019] [2] According to the hot-rolled steel sheet described in [1], the maximum orientation density of grains on the surface of the sheet that is inclined at 45° from the rolling right angle direction to the thickness direction is 1.5 or less.

[0020] [3] The hot-rolled steel sheet according to [1] or [2], wherein the above composition, in mass %, further contains one or more of the following: Cr: 0.005-2.0%, Ti: 0.005-0.20%, Nb: 0.005-0.20%, Mo: 0.01-2.0%, V: 0.01-1.0%, Cu: 0.01-4.0%, Ni: 0.005-2.0%, B: 0.0001-0.01%, Ca: 0.0001-0.0050%, REM: 0.0001-0.0050%, Sb: 0.0010-0.10%, Sn: 0.0010-0.10%.

[0021] [4] A method for manufacturing hot-rolled steel plate, wherein,

[0022] A steel billet having the composition described in [1] or [3] above is heated to 1100°C or higher.

[0023] Rolling is performed at a temperature range above 1000℃, with at least 6 passes and a reduction rate of at least 15% per pass.

[0024] Next, rolling is performed under the following conditions: at least three passes with a reduction rate of at least 15% per pass within a temperature range below 1000°C; a rolling time of less than 2.0 seconds within a temperature range below 1000°C; and a final pass rolling temperature of 850–940°C.

[0025] Cool to 700°C at an average cooling rate of 50°C / s or higher, and then wind up at 580–700°C.

[0026] Invention Effects

[0027] According to the present invention, a hot-rolled steel sheet with excellent elongation flange properties can be obtained.

[0028] According to the present invention, a hot-rolled steel sheet with excellent elongation flange properties can be obtained, making it suitable as a raw material for automotive parts. When using the hot-rolled steel sheet of the present invention, automotive parts and other products can be obtained with good yield. Detailed Implementation

[0029] The hot-rolled steel sheet and its manufacturing method of the present invention will be described in detail below. It should be noted that the present invention is not limited to the following embodiments.

[0030] Hot-rolled steel plate

[0031] The hot-rolled steel sheet of the present invention can be either a hot-rolled black sheet (referred to as hot-rolled black sheet) or a hot-rolled white sheet (referred to as white sheet after pickling). Preferably, the hot-rolled steel sheet of the present invention has a thickness of 0.6 mm or more. Furthermore, the hot-rolled steel sheet of the present invention preferably has a thickness of 10.0 mm or less. When the hot-rolled steel sheet of the present invention is used as a raw material for automotive parts, a thickness of 1.0 mm or more is more preferred. Furthermore, when the hot-rolled steel sheet of the present invention is used as a raw material for automotive parts, a thickness of 6.0 mm or less is more preferred. Additionally, the width of the hot-rolled steel sheet of the present invention is preferably 500 mm or more, more preferably 700 mm or more. The width of the hot-rolled steel sheet of the present invention is preferably 1800 mm or less, more preferably 1400 mm or less.

[0032] The hot-rolled steel sheet of the present invention has a specific composition and a specific steel structure. Here, it will be described in the order of composition and steel structure.

[0033] First, the composition of the hot-rolled steel sheet of the present invention will be described. It should be noted that the "%" indicating the content of the component refers to "mass %".

[0034] The hot-rolled steel plate of the present invention contains, by mass %, less than 0.10% C, less than 2.0% Si, less than 2.0% Mn, less than 2.0% P, less than 0.100% S, less than 0.02% Al, less than 1.5% O, and the balance is composed of Fe and unavoidable impurities.

[0035] C: Below 0.10%

[0036] The amount of carbon (C) is preferably minimized to obtain ferrite. In this invention, a C content of 0.10% or less is permissible. When the C content exceeds 0.10%, pearlite, bainite, etc., increase, and the microstructure of this invention cannot be obtained. Therefore, the C content is set to 0.10% or less. The C content is preferably set to 0.08% or less, and more preferably to 0.05% or less. There is no particular lower limit, but if it is less than 0.0001%, it leads to a decrease in production efficiency; therefore, the C content is preferably 0.0001% or more.

[0037] Si: below 2.0%

[0038] Si is an element that strengthens steel through solid solution treatment and is effective in increasing tensile strength (TS). Furthermore, Si is effective in promoting ferrite formation. In this invention, the amount of Si added can be appropriately determined considering strength and microstructure. On the other hand, excessive addition of Si can cause embrittlement of the steel or lead to aggregation of crystal orientations, reducing its elongation flangeability; therefore, the amount added should be set to 2.0% or less. Therefore, the Si content is set to 2.0% or less (including 0%). The Si content is preferably 1.0% or less, more preferably 0.6% or less. While no specific lower limit is specified, a content less than 0.001% leads to a decrease in production efficiency; therefore, the Si content is preferably 0.001% or more.

[0039] Mn: below 2.0%

[0040] Mn is preferably minimized to obtain ferrite. In this invention, an Mn content of 2.0% or less is permissible. When the Mn content exceeds 2.0%, the desired effect is not achieved, and martensite, bainite, etc., are formed, sometimes resulting in a microstructure that does not conform to this invention. Furthermore, a large amount of MnS is formed, which does not yield the desired extended flange properties. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably set to 1.6% or less, and more preferably to 1.0% or less. There is no particular lower limit, but when it is less than 0.01%, it sometimes leads to a decrease in production efficiency; therefore, the Mn content is preferably 0.01% or more.

[0041] P: below 0.100%

[0042] Polymer (P) embrittles steel and reduces the elongation of the flange, therefore its amount is preferably minimized. In this invention, the P content can be allowed down to 0.100%. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less. No specific lower limit is specified, but a P content less than 0.001% leads to a decrease in production efficiency; therefore, the P content is preferably 0.001% or more.

[0043] S: below 0.02%

[0044] Sulfur (S) embrittles steel and reduces the elongation of the flange, therefore its amount is preferably minimized. In this invention, the S content can be as low as 0.02%. Therefore, the S content is set to 0.02% or less. The S content is preferably 0.010% or less, more preferably 0.0080% or less. No particular lower limit is specified, but an S content less than 0.0002% leads to a decrease in production efficiency; therefore, the S content is preferably 0.0002% or more.

[0045] Al: below 1.5%

[0046] Regarding Al, a high Al content results in a well-developed texture, which does not produce the steel microstructure of this invention. In this invention, the Al content is allowed up to 1.5%. Therefore, the Al content is set to 1.5% or less. The Al content is preferably 0.50% or less. While no specific lower limit is specified, a content less than 0.001% can sometimes lead to a decrease in production efficiency; therefore, the Al content is preferably set to 0.001% or more.

[0047] O: below 0.0025%

[0048] O (O) is an important element in this invention, causing steel embrittlement, promoting crystallization and aggregation, and increasing inclusions, thus reducing the elongation flangeability. Therefore, it is preferable to minimize its content. To obtain the desired elongation flangeability, the O content needs to be set to 0.0025% or less. Therefore, the O content is set to 0.0025% or less. The O content is preferably 0.0020% or less. While no specific lower limit is specified, a content less than 0.0001% leads to a decrease in production efficiency; therefore, the O content is preferably 0.0001% or more.

[0049] The balance consists of Fe and unavoidable impurities. It should be noted that unavoidable impurities include N, Na, Mg, Zr, Hf, Ta, W, etc., but their total content is set to 0.020% or less. More preferably, their total content is set to 0.010% or less.

[0050] The above-mentioned components constitute the basic composition of the hot-rolled steel sheet of the present invention. In the present invention, the following elements may be further appropriately included.

[0051] Selected from one or more of the following: Cr: 0.005–2.0%, Ti: 0.005–0.20%, Nb: 0.005–0.20%, Mo: 0.01–2.0%, V: 0.01–1.0%, Cu: 0.01–4.0%, Ni: 0.005–2.0%, B: 0.0001–0.01%, Ca: 0.0001–0.0050%, REM: 0.0001–0.0050%, Sb: 0.0010–0.10%, Sn: 0.0010–0.10%.

[0052] Cr, Ti, Nb, Mo, and V are elements that form carbides and are effective in increasing the strength of steel. To achieve this effect, when Cr, Ti, Nb, Mo, and V are present, it is preferable to set the content of each element to the aforementioned lower limit or higher. On the other hand, when the content of each element exceeds the aforementioned upper limit, it leads to an increase in bainite and martensite, and a developed texture, sometimes resulting in the microstructure of the present invention not being obtained. Therefore, when Cr, Ti, Nb, Mo, and V are present, the content of each element is preferably set as follows: Cr: 0.005–2.0%, Ti: 0.005–0.20%, Nb: 0.005–0.20%, Mo: 0.01–2.0%, V: 0.01–1.0%. The Cr content is more preferably 0.05% or higher. Furthermore, the Cr content is more preferably 1.0% or lower. The Ti content is more preferably 0.01% or higher. Furthermore, the Ti content is more preferably 0.15% or lower. The Nb content is more preferably 0.01% or higher. Furthermore, the Nb content is more preferably 0.10% or less. The Mo content is more preferably 0.05% or more. Furthermore, the Mo content is more preferably 1.0% or less. The V content is more preferably 0.05% or more. Furthermore, the V content is more preferably 0.5% or less.

[0053] Cu and Ni are elements effective in increasing the strength of steel through solid solution strengthening. To achieve this effect, when Cu and Ni are present, it is preferable to set the content of each element to the aforementioned lower limit or above. On the other hand, when the content of Cu and Ni exceeds the aforementioned upper limit, bainite and martensite are generated in large quantities, and sometimes the steel microstructure of the present invention cannot be obtained. Therefore, when Cu and Ni are present, it is preferable to set the content of each element as follows: Cu: 0.01 to 4.0%, Ni: 0.005 to 2.0%. The Cu content is more preferably 0.05% or more. Furthermore, the Cu content is more preferably 1.0% or less. The Ni content is more preferably 0.05% or more. Furthermore, the Ni content is more preferably 1.0% or less.

[0054] Boron (B) is an effective element for strengthening grain boundaries and increasing the strength of steel. To achieve this effect, when B is present, its content is preferably set to 0.0001% or more. On the other hand, when the B content exceeds 0.01%, B-based compounds increase, and the elongation flangeability may sometimes decrease. Therefore, when B is present, the B content is preferably set to 0.0001 to 0.01%. The B content is more preferably 0.0005% or more. Furthermore, the B content is more preferably 0.0040% or less.

[0055] Ca and REM are elements that effectively improve the extended flange properties by controlling the morphology of inclusions. To achieve this effect, when Ca and REM are present, it is preferable to set their respective contents to 0.0001% or more. On the other hand, when the contents of Ca and REM exceed the above-mentioned upper limits, the amount of inclusions increases, and the extended flange properties may sometimes decrease. Therefore, when Ca and REM are present, it is preferable to set their respective contents to Ca: 0.0001 to 0.0050% and REM: 0.0001 to 0.0050%. The Ca content is more preferably 0.0005% or more. Furthermore, the Ca content is more preferably 0.0030% or less. The REM content is more preferably 0.0005% or more. Furthermore, the REM content is more preferably 0.0030% or less. It should be noted that REM is a collective term for 15 elements, including Sc, Y, and lanthanum (La) from atomic number 57 to lutetium (Lu) from atomic number 71, and the REM content described herein is the total content of these elements.

[0056] Sb and Sn are elements that effectively suppress denitrification, deboronization, and other processes that reduce the strength of steel. To achieve this effect, when Sb and Sn are present, it is preferable to set the content of each element to 0.0010% or more. On the other hand, when the content of Sb and Sn exceeds the above-mentioned upper limits, the steel becomes embrittled, sometimes leading to a decrease in elongation flangeability. Therefore, when Sb and Sn are present, it is preferable to set their respective contents to Sb: 0.0010 to 0.10% and Sn: 0.0010 to 0.10%. The Sb content is more preferably 0.005% or more. Furthermore, the Sb content is more preferably 0.015% or less. The Sn content is more preferably 0.005% or more. Furthermore, the Sn content is more preferably 0.015% or less.

[0057] It should be noted that the content of Cr, Ti, Nb, Mo, V, Cu, Ni, B, Ca, REM, Sb, and Sn is less than the aforementioned lower limit values ​​without impairing the effectiveness of the present invention. Therefore, when the content of these elements is less than the aforementioned lower limit values, these elements are treated as elements present in the form of unavoidable impurities.

[0058] Next, the steel structure of the hot-rolled steel sheet of the present invention will be described.

[0059] The steel microstructure of the hot-rolled steel sheet of the present invention is characterized in that the main phase is ferrite and the maximum orientation density of the grains is 2.1 or less.

[0060] Principal phase: Ferrite

[0061] In this invention, excellent elongation flange properties are achieved by making ferrite the main phase. When bainite or pearlite is the main phase, the desired elongation flange properties are not obtained. Therefore, ferrite is set as the main phase. Here, the main phase refers to a phase that accounts for more than 50% in terms of area ratio. Ferrite is preferably 90% or more in terms of area ratio, more preferably 95% or more. Furthermore, the balance other than ferrite is any one or more of martensite, bainite, pearlite, and retained austenite. The area ratio of these balances is preferably 10% or less, more preferably 5% or less. It should be noted that the area ratio of each phase (structure) can be determined by the method described in the examples.

[0062] Maximum grain orientation density: below 2.1

[0063] When the randomization of crystal orientation is insufficient, the extended flange property decreases, and when the maximum orientation density of the grains exceeds 2.1, the desired extended flange property cannot be obtained. Therefore, the maximum orientation density of the grains is set to 2.1 or less. The maximum orientation density of the grains is preferably 2.0 or less, more preferably 1.9 or less. There is no particular lower limit, but the maximum orientation density of the grains is preferably 1.0 or more. It should be noted that the maximum orientation density of the grains can be determined using the method described in the examples.

[0064] Maximum grain orientation density relative to the surface of the plate, which is inclined at 45° from the rolling right angle towards the plate thickness direction: below 1.5

[0065] By promoting the randomization of crystal orientation, the elongation flange property can be further improved. This effect is achieved by ensuring that the maximum grain orientation density in the plane inclined at 45° relative to the plate surface from the rolling right angle direction to the plate thickness direction is 1.5 or less. Therefore, the maximum grain orientation density in the plane inclined at 45° relative to the plate surface from the rolling right angle direction to the plate thickness direction is preferably set to 1.5 or less. The maximum grain orientation density in the plane inclined at 45° relative to the plate surface from the rolling right angle direction to the plate thickness direction is more preferably 1.4 or less, and even more preferably 1.3 or less. The lower limit is not specifically specified, but the maximum grain orientation density in the plane inclined at 45° relative to the plate surface from the rolling right angle direction to the plate thickness direction is preferably 1.0 or more. It should be noted that the maximum grain orientation density in the plane inclined at 45° relative to the plate surface from the rolling right angle direction to the plate thickness direction can be determined by the method described in the examples.

[0066] <Manufacturing Method of Hot-Rolled Steel Plate>

[0067] The hot-rolled steel sheet of the present invention is manufactured by the following method: a steel billet having the above-mentioned composition is heated to 1100°C or higher, rolled at a temperature range of 1000°C or higher for 6 or more passes with a reduction rate of 15% or more per pass, then rolled at a temperature range of 1000°C or lower for 3 or more passes with a reduction rate of 15% or more per pass, a rolling time of 2.0 s or less at a temperature range of 1000°C or lower, and a final rolling temperature of 850 to 940°C, then cooled to 700°C at an average cooling rate of 50°C / s or higher, and coiled at 580 to 700°C.

[0068] The following is a detailed explanation. It should be noted that the temperature mentioned above refers to the surface temperature of the central portion of the steel plate's width, and the average cooling rate mentioned above refers to the average cooling rate of the central portion of the steel plate's width. Furthermore, unless otherwise specified, the average cooling rate is set as [(cooling start temperature - cooling stop temperature) / cooling time from the cooling start temperature to the cooling stop temperature].

[0069] First, the steel with the above-mentioned composition is smelted using known methods such as converters, electric furnaces, and vacuum melting furnaces, and then cast using known methods such as continuous casting or ingot-bill casting to produce cast sheets (steel billets).

[0070] Steel billet heating temperature: above 1100℃

[0071] Heating the steel billet to 1100°C or higher allows for the solid solution of carbides and other materials, promoting randomization of crystal orientation. When the billet heating temperature is below 1100°C, this effect cannot be fully achieved, and the maximum grain orientation density of this invention cannot be obtained. Therefore, the billet heating temperature is set to 1100°C or higher. Preferably, the billet heating temperature is set to 1150°C or higher. While no specific upper limit is specified, exceeding 1350°C leads to increased electricity costs, etc. Therefore, the billet heating temperature is preferably below 1350°C.

[0072] Number of rolling passes in the temperature range above 1000℃: 6 or more

[0073] When the number of rolling passes (rolling times) in the temperature range of 1000°C or above during hot rolling is less than 6 passes (6 times), recrystallization becomes insufficient, thereby hindering the randomization of crystal orientation and failing to obtain the maximum grain orientation density of the present invention. Therefore, the number of rolling passes in the temperature range of 1000°C or above is set to 6 passes or more. The number of rolling passes is preferably set to 8 passes or more, and more preferably 10 passes or more. There is no particular upper limit, but increasing the number of rolling passes can sometimes lead to a decrease in manufacturing efficiency, etc. Therefore, the number of rolling passes is preferably set to 20 passes or less. It should be noted that rolling in the temperature range of 1000°C or above is independent of the roughing and finishing rolling processes. Rolling in the temperature range of 1000°C or above also includes rolling of any process. The same applies to rolling in the temperature range below 1000°C, which will be described later.

[0074] Reduction per pass in rolling at temperatures above 1000℃: 15% or more

[0075] When the reduction rate per pass (one pass) of hot rolling in a temperature range of 1000°C or higher is less than 15%, recrystallization becomes insufficient, thereby hindering the randomization of crystal orientation and failing to obtain the maximum grain orientation density of the present invention. Therefore, the reduction rate per pass of rolling in a temperature range of 1000°C or higher is set to 15% or more. The above-mentioned reduction rate is preferably 18% or more, more preferably 20% or more. There is no particular upper limit, but when the above-mentioned reduction rate exceeds 80%, it sometimes leads to problems such as increased equipment load. Therefore, the above-mentioned reduction rate is preferably set to 80% or less.

[0076] Number of rolling passes in the temperature range below 1000℃: 3 or more

[0077] When the number of rolling passes (rolling times) in the hot-rolled temperature range below 1000°C is less than 3 passes (3 times), strain accumulation becomes insufficient. Consequently, the randomly oriented ferrite decreases during the subsequent ferrite phase transformation, and the maximum grain orientation density of the present invention cannot be obtained. Therefore, the number of rolling passes in the temperature range below 1000°C is set to 3 passes or more. The aforementioned number of rolling passes is preferably set to 4 passes or more. There is no particular upper limit, but increasing the number of rolling passes will lead to a decrease in the rolling temperature in the final rolling pass (finishing final pass). Therefore, the aforementioned number of rolling passes is preferably set to 8 passes or less.

[0078] Reduction per pass in rolling at temperatures below 1000°C: 15% or more

[0079] When the reduction rate per pass (one pass) in hot rolling at a temperature range below 1000°C is less than 15%, strain accumulation becomes insufficient. Consequently, the randomly oriented ferrite decreases during the subsequent ferrite phase transformation, and the maximum grain orientation density of the present invention cannot be obtained. Therefore, the reduction rate per pass in the temperature range below 1000°C is set to 15% or more. The reduction rate is preferably 17% or more, and more preferably 20% or more. There is no particular upper limit, but an increase in the reduction rate can sometimes lead to a decrease in shape stability, etc. Therefore, the reduction rate is preferably set to 50% or less.

[0080] Rolling time in the temperature range below 1000℃: less than 2.0s

[0081] When the rolling time in a temperature range below 1000°C exceeds 2.0 s, strain accumulation becomes insufficient, resulting in a reduction in randomly oriented ferrite during the subsequent ferrite phase transformation, thus failing to achieve the maximum grain orientation density of the present invention. Therefore, the rolling time in a temperature range below 1000°C is set to 2.0 s or less. Here, the rolling time in a temperature range below 1000°C refers to the time from when the same part of the rolled material changes to a temperature below 1000°C and comes into contact with the rolls of the initial rolling pass until it leaves (passes through) the rolls of the final rolling pass. The above-mentioned rolling time is preferably set to 1.7 s or less, more preferably 1.4 s or less. While no specific lower limit is specified, excessively high-speed rolling leads to a decrease in operational stability, etc., therefore, the above-mentioned rolling time is preferably set to 0.2 s or more.

[0082] Final rolling temperature: 850–940℃

[0083] When the final rolling temperature (finishing end temperature) is below 850°C, excessive strain accumulation occurs, resulting in a reduction in randomly oriented ferrite during the subsequent ferrite phase transformation, thus failing to achieve the maximum grain orientation density of the present invention. On the other hand, when the rolling temperature exceeds 940°C, strain accumulation becomes insufficient, leading to a reduction in randomly oriented ferrite during the subsequent ferrite phase transformation, again failing to achieve the maximum grain orientation density of the present invention. Therefore, the final rolling temperature is set to 850–940°C. Preferably, the rolling temperature is 860°C or higher, more preferably 870°C or higher. Furthermore, preferably, the rolling temperature is 920°C or lower, more preferably 910°C or lower.

[0084] Average cooling rate up to 700℃: 50℃ / s or more

[0085] When the average cooling rate from the final rolling temperature (finishing end temperature) to 700°C is less than 50°C / s, a portion of the rolling strain is released, and strain accumulation becomes insufficient. Consequently, the randomly oriented ferrite decreases during the subsequent ferrite phase transformation, and the maximum grain orientation density of the present invention cannot be obtained. Therefore, the average cooling rate from the final rolling temperature to 700°C is set to 50°C / s or more. The aforementioned average cooling rate is preferably 80°C / s or more. While no specific upper limit is specified, from the viewpoint of the shape stability of the steel sheet, the aforementioned average cooling rate is preferably 1000°C / s or less.

[0086] Winding temperature: 580~700℃

[0087] When the winding temperature is below 580°C, bainite and martensite increase, and the steel microstructure of the present invention cannot be obtained. On the other hand, when the winding temperature exceeds 700°C, the undercooling of the ferrite phase transformation decreases, thereby reducing the randomly oriented ferrite during the ferrite phase transformation, and the maximum grain orientation density of the present invention cannot be obtained. Therefore, the winding temperature is set to 580°C to 700°C. The winding temperature is preferably 590°C or higher. In addition, the winding temperature is preferably 680°C or lower. After winding, the steel is cooled to room temperature, for example.

[0088] There are no special limitations other than the conditions described above for the manufacturing method.

[0089] The hot-rolled steel sheet of the present invention is not particularly limited, but the tensile strength (TS) is preferably 200 MPa or more, more preferably 270 MPa or more. Furthermore, the hot-rolled steel sheet of the present invention is not particularly limited, but the tensile strength (TS) is preferably 780 MPa or less, more preferably 650 MPa or less.

[0090] Furthermore, the hot-rolled steel sheet of the present invention has excellent elongated flange properties with a hole expansion ratio of 100% or more. The hole expansion ratio is preferably 110% or more, and more preferably 120% or more. It should be noted that TS and the hole expansion ratio are obtained using the methods described in the examples.

[0091] Example

[0092] The steel with the composition shown in Table 1 was melted in a vacuum melting furnace to form steel billets, which were then heated and hot-rolled under the conditions shown in Table 2. The resulting hot-rolled steel sheets were evaluated by observing the microstructure, tensile properties, and hole expansion tests according to the following test methods.

[0093] Organizational observation

[0094] The area ratio of ferrite was calculated as follows. Samples were cut from the obtained hot-rolled steel sheet. After grinding the section parallel to the rolling direction, the sample was etched with a 3% nitric acid-ethanol solution. Three fields of view were captured at 1500x magnification at the 1 / 4 thickness position using a SEM (Scanning Electron Microscope). Based on the image data from the secondary electron images, the area ratio of each microstructure was calculated using Image-Pro (MediaCybernetics). The average area ratio of the three fields of view was set as the area ratio of each microstructure. The image data distinguished the following: Ferrite was black or dark gray with smooth grain boundaries. Carbides were white, linear or dotted. Pearlite formed a layered morphology within ferrite and carbides. Other microstructures were classified as "other".

[0095] The results are shown in Table 3. It should be noted that in Table 3, "F" refers to ferrite, "P" to pearlite, and "O" to other microstructures (any one or more of martensite and bainite). Additionally, the dominant phase refers to the phase (microstructure) that accounts for more than 50% of the total area.

[0096] Crystal orientation (maximum orientation density of grains)

[0097] For the same sample used in the above tissue observations, for a 500μm × 500μm region centered at 1 / 4 of the plate thickness in a section parallel to the rolling direction, the crystal orientation was determined by electron backscatter diffraction (EBSD). The ranges of Φ1, Φ2, and Φ were set to 0–90°, and their respective resolutions were set to 5. ODF (Orientation Distribution Function) calculations were performed to determine the maximum orientation density of the grains in this field of view. This operation was performed on the same five locations, and the average was taken as the maximum orientation density of the grains. Furthermore, for the above crystal orientation data, a coordinate transformation was performed so that the ND plane (Normal Direction) was inclined at 45° relative to the plate surface from the rolling right angle direction towards the plate thickness direction. The ranges of Φ1, Φ2, and Φ were set to 0–90°, and their respective resolutions were set to 5. ODF calculations were performed to determine the maximum orientation density of the grains in this field of view. This operation was performed on the same five locations, and the average value was taken as the maximum grain orientation density on the surface inclined at 45° from the rolling right angle to the plate thickness direction relative to the plate surface. It should be noted that the EBSD measurement was set with an accelerating voltage of 30kV and a step size of 0.5μm.

[0098] Tensile test

[0099] JIS No. 5 tensile test specimens (JIS Z2201) were cut from the obtained hot-rolled steel sheet along a direction parallel to the rolling direction, and subjected to strain rates of 10...-3 The tensile strength (TS) is determined by a tensile test according to the specifications of JIS Z 2241.

[0100] Hole Enlargement Test

[0101] Test pieces with a width of 100 mm and a length of 100 mm were cut from the obtained hot-rolled steel sheet. Five hole expansion tests were conducted according to JFST1001 (Iron and Steel Union Standard) to calculate the average hole expansion rate λ (%) and evaluate the elongation flange performance. A λ of 100% or higher was considered excellent elongation flange performance and was set as qualified.

[0102]

[0103]

[0104]

[0105] The inventive examples are all hot-rolled steel sheets with excellent elongation flange properties. On the other hand, comparative examples that deviate from the scope of the present invention do not achieve the desired elongation flange properties.

[0106] Industrial availability

[0107] According to the present invention, hot-rolled steel sheets with excellent elongation flange properties can be stably obtained. When the steel sheets of the present invention are used in automotive parts, they can greatly contribute to improving the crash safety and fuel efficiency of automobiles.

Claims

1. A hot-rolled steel sheet having, by mass percent, a composition comprising: C: less than 0.10%, Si: less than 2.0%, Mn: less than 2.0%, P: less than 0.100%, S: less than 0.02%, Al: less than 1.5%, O: less than 0.0025%, with the balance being Fe and unavoidable impurities. The main phase is ferrite, and the maximum orientation density of the grains is below 2.

1. The maximum orientation density of grains on the surface of the plate that is inclined at 45° from the rolling right angle to the plate thickness direction is less than 1.

5.

2. The hot-rolled steel plate according to claim 1, wherein, The composition, by mass%, further contains one or more of the following: Cr: 0.005–2.0%, Ti: 0.005–0.20%, Nb: 0.005–0.20%, Mo: 0.01–2.0%, V: 0.01–1.0%, Cu: 0.01–4.0%, Ni: 0.005–2.0%, B: 0.0001–0.01%, Ca: 0.0001–0.0050%, REM: 0.0001–0.0050%, Sb: 0.0010–0.10%, Sn: 0.0010–0.10%.

3. The method for manufacturing hot-rolled steel sheet as described in claim 1 or 2, wherein, A steel billet having the composition described in claim 1 or 2 is heated to above 1100°C. Rolling is performed at a temperature range above 1000℃, with at least 6 passes and a reduction rate of at least 15% per pass. Next, rolling is performed under the following conditions: at least three passes with a reduction rate of at least 15% per pass within a temperature range below 1000°C; a rolling time of less than 2.0 seconds within a temperature range below 1000°C; and a final pass rolling temperature of 850–940°C. Cool to 700°C at an average cooling rate of 50°C / s or higher, and then wind up at 580–700°C.

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