High-strength hot-rolled steel plate and its manufacturing method

Abstract

This invention provides a high-strength hot-rolled steel sheet with excellent suppression of cracking and wrinkling during bending. The high-strength hot-rolled steel sheet has the following composition by mass percent: C: 0.02–0.23%, Si: 0.10–3.00%, Mn: 0.5–3.5%, P: less than 0.100%, S: less than 0.02%, Al: less than 1.5%, with the balance being Fe and unavoidable impurities. The total area fraction of martensite and bainite is 80–100%, the maximum grain orientation density in the region 5–10 μm from the surface in the thickness direction is less than 2.5, and the maximum grain orientation density in the region 50–100 μm from the surface in the thickness direction is 2.5 or more.

Description

Technical Field

[0001] This invention relates to high-strength hot-rolled steel sheets and methods for manufacturing the same, and particularly to high-strength 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, there is a demand for high-strength steel sheets used in automotive components. Generally, increasing the strength of steel sheets reduces their machinability; therefore, steel sheets with both high strength and excellent machinability are required. Hot-rolled steel sheets with tensile strength exceeding 980 MPa are mostly used for bending processes such as frame components; therefore, excellent bending machinability is particularly important. Needless to say, suppressing severe fractures that significantly compromise crash safety during bending is crucial; from aesthetic and fatigue resistance perspectives, wrinkle suppression also becomes important. To address these issues, various types of hot-rolled steel sheets have been developed.

[0003] Patent Document 1 discloses a technology related to the manufacturing method of cold-rolled steel sheets with a tensile strength of 780 MPa or higher. This technology involves adjusting the alloying elements to an appropriate range, setting the ferrite volume fraction to 60-80%, and controlling the ratio of ferrite to the nano-hardness of the low-temperature phase transformation phase within a certain range, thereby achieving excellent elongation and bending properties. Patent Document 2 discloses a technology related to hot-dip galvanized steel sheets. This technology sets the carbon content to 0.07-0.25% by mass, adjusts other alloying elements appropriately, and then appropriately combines the area fraction of each phase in the steel sheet microstructure, the average grain size of the martensitic phase, and the deviation of Vickers hardness, thereby improving bending workability. Patent Document 3 discloses a technology related to hot-rolled steel sheets, in which excellent ductility is achieved by controlling the orientation difference within the grains.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2009-167467

[0007] Patent Document 2: International Publication No. 2016 / 129213

[0008] Patent Document 3: Japanese Patent Application Publication No. 2016-204690 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, in Patent Document 1, the yield strength (YS) is lower due to the large amount of ferrite, so it cannot be said to have effective impact resistance. Furthermore, it cannot be said that the technology in Patent Document 1 can also be applied to hot-rolled steel sheets with significantly different textures, leaving room for improvement. Patent Document 2 addresses the problem of wrinkles (stripes) caused by Mn segregation in hot-dip galvanized steel sheets, offering no inspiration for hot-rolled steel sheets. Again, it cannot be said that the technology in Patent Document 2 can also be applied to hot-rolled steel sheets with significantly different textures, leaving room for improvement. In Patent Document 3, excellent ductility can be obtained by controlling crystal orientation, but on the other hand, there is no research on wrinkles during bending processing, leaving room for improvement.

[0011] This invention addresses the aforementioned problems and aims to provide a high-strength hot-rolled steel sheet with excellent suppression of cracking and wrinkling during bending processes.

[0012] In addition, the present invention aims to provide a method for manufacturing the above-mentioned high-strength hot-rolled steel plate.

[0013] Methods for solving problems

[0014] The inventors conducted in-depth research on the conditions for wrinkle formation during bending processing. The results showed that by controlling the chemical composition and the surface texture of the steel plate within specific ranges, cracking and wrinkling during bending processing were significantly suppressed, thus completing this invention.

[0015] It should be noted that, in this invention, high strength refers to a tensile strength (TS) of 980 MPa or higher and a yield strength (YS) of 800 MPa or higher.

[0016] Furthermore, in this invention, the excellent suppression effect of cracking and wrinkling during bending processing means that the minimum bending radius R that does not cause cracking and wrinkling during bending processing, divided by the plate thickness t, has an R / t ratio of 3.0 or less.

[0017] The present invention has the following structure.

[0018] [1] A high-strength hot-rolled steel plate having a composition, by mass %, of C: 0.02-0.23%, Si: 0.10-3.00%, Mn: 0.5-3.5%, P: less than 0.100%, S: less than 0.02%, Al: less than 1.5%, with the balance being Fe and unavoidable impurities.

[0019] The combined area ratio of martensite and bainite is 80-100%, the maximum orientation density of grains in the region 5-10 μm from the surface in the thickness direction is less than 2.5, and the maximum orientation density of grains in the region 50-100 μm from the surface in the thickness direction is greater than 2.5.

[0020] [2] The high-strength hot-rolled steel plate according to [1], wherein the above composition, by mass %, further contains one or more of the following: Cr: 0.005-2.0%, Mo: 0.05-2.0%, V: 0.05-1.0%, Cu: 0.05-4.0%, Ni: 0.005-2.0%, Ti: 0.005-0.20%, Nb: 0.005-0.20%, B: 0.0003-0.0050%, Ca: 0.0001-0.0050%, REM: 0.0001-0.0050%, Sb: 0.0010-0.10%, Sn: 0.0010-0.50%.

[0021] [3] A method for manufacturing a high-strength hot-rolled steel plate, wherein,

[0022] A steel billet having the composition described in [1] or [2] above is heated and rough rolled.

[0023] Next, a finishing roll is performed under the following conditions: a total reduction rate of 50% or more within a temperature range below 1000°C; a total number of passes of 3 or more within a temperature range below 1000°C; a final pass rolling temperature of 750–900°C; and a total reduction rate of 35% or less within the range from the final pass rolling temperature to +50°C.

[0024] The cooling time after the finishing rolling is set to 2.0s or less, and the cooling is carried out at an average cooling rate of 50℃ / s or more in a temperature range up to 550℃, at an average cooling rate of 100℃ / s or more in a temperature range of 300 to 400℃, and the coiling is carried out at 300℃ or less.

[0025] Invention Effects

[0026] According to the present invention, a high-strength hot-rolled steel sheet with excellent suppression effect on cracking and wrinkling during bending processing (hereinafter also referred to as "bending wrinkle resistance") can be obtained.

[0027] According to the present invention, a high-strength hot-rolled steel sheet with excellent resistance to bending and wrinkling can be obtained, making it suitable as a raw material for automotive parts. When using the high-strength hot-rolled steel sheet of the present invention, high-strength automotive parts and other products free from cracking and wrinkling caused by bending processes can be obtained. Detailed Implementation

[0028] The high-strength hot-rolled steel plate 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.

[0029] High-strength hot-rolled steel plate

[0030] The high-strength hot-rolled steel sheet of the present invention can be either a hot-rolled black sheet or a hot-rolled white sheet that has undergone further pickling after hot rolling. Preferably, the high-strength hot-rolled steel sheet of the present invention has a thickness of 0.6 mm or more. Furthermore, the high-strength hot-rolled steel sheet of the present invention preferably has a thickness of 10.0 mm or less. When the high-strength hot-rolled steel sheet of the present invention is used as a raw material for automotive parts, a thickness of 1.0 mm or more is more preferred. Furthermore, when the high-strength 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 high-strength hot-rolled steel sheet of the present invention is preferably 500 mm or more, more preferably 700 mm or more. The width of the high-strength hot-rolled steel sheet of the present invention is preferably 1800 mm or less, more preferably 1400 mm or less.

[0031] The high-strength 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.

[0032] First, the composition of the high-strength hot-rolled steel plate of the present invention will be described. It should be noted that the "%" indicating the content of the component refers to "mass %".

[0033] The high-strength hot-rolled steel plate of the present invention contains, by mass %, C: 0.02-0.23%, Si: 0.10-3.00%, Mn: 0.5-3.5%, P: less than 0.100%, S: less than 0.02%, Al: less than 1.5%, with the balance consisting of Fe and unavoidable impurities.

[0034] C: 0.02~0.23%

[0035] Carbon (C) is an element that promotes the formation and strengthening of bainite and martensite, and is effective in increasing steel strength (TS) and chromium content (YS). When the C content is less than 0.02%, this effect cannot be sufficiently obtained, and a TS of 980 MPa or higher cannot be achieved. On the other hand, when the C content exceeds 0.23%, the development of the surface texture of the steel plate becomes significant, and the desired bending wrinkle resistance is not obtained. Therefore, the C content is set to 0.02–0.23%. From the viewpoint of more stably obtaining a TS of 980 MPa or higher, the C content is preferably 0.03% or higher, and from the viewpoint of stably obtaining a TS of 1180 MPa or higher, it is preferably set to 0.06% or higher. Furthermore, from the viewpoint of bending wrinkle resistance, the C content is preferably set to 0.22% or lower, and more preferably 0.20% or lower.

[0036] Si: 0.10–3.00%

[0037] Si is an element effective in increasing TS and YS by strengthening steel through solid solution treatment or by suppressing the softening of martensite during tempering. Furthermore, Si is also effective in suppressing cracking and wrinkling during bending. To achieve these effects, the Si content needs to be set to 0.10% or more. On the other hand, when the Si content exceeds 3.00%, excessive polygonal ferrite is formed, and the steel microstructure of the present invention cannot be obtained. Therefore, the Si content is set to 0.10 to 3.00%. The Si content is preferably 0.20% or more. Furthermore, the Si content is preferably 2.00% or less, more preferably 1.50% or less.

[0038] Mn: 0.5–3.5%

[0039] Mn is an element that promotes the formation of martensite and bainite and is effective in increasing TS and γS. When the Mn content is less than 0.5%, this effect cannot be fully obtained, resulting in the formation of polygonal ferrite and the microstructure of the present invention. On the other hand, when the Mn content exceeds 3.5%, the development of the texture on the surface of the steel sheet becomes significant, and the desired bending wrinkle resistance is not obtained. Therefore, the Mn content is set to 0.5 to 3.5%. From the viewpoint of obtaining a more stable TS of 980 MPa or more, the Mn content is preferably set to 1.0% or more. In addition, from the viewpoint of bending wrinkle resistance, the Mn content is preferably set to 3.0% or less, and more preferably 2.3% or less.

[0040] P: below 0.100%

[0041] Polymer (P) causes steel to become brittle and promotes bending fracture; 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 (excluding 0%). The P content is preferably set to 0.030% 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.

[0042] S: below 0.02%

[0043] Sulfur (S) causes steel to become brittle and promotes bending fracture; 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 (excluding 0%). The S content is preferably set to 0.0050% or less, and more preferably 0.0030% or less. No specific 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.

[0044] Al: below 1.5%

[0045] Al acts as a deoxidizer and is preferably added during the deoxidation process. From the viewpoint of its use as a deoxidizer, the Al content is preferably 0.01% or more. On the other hand, when Al is present in large quantities, a large amount of polygonal ferrite is generated, and the steel structure of the present invention cannot be obtained. In the present invention, the Al content is allowed up to 1.5%. Therefore, the Al content is set to 1.5% or less (excluding 0%). The Al content is preferably set to 0.50% or less, more preferably 0.30% or less, and even more preferably 0.10% or less.

[0046] The balance is Fe and unavoidable impurities.

[0047] The above-mentioned components constitute the basic composition of the high-strength hot-rolled steel sheet of the present invention. In the present invention, the following elements may be further appropriately included.

[0048] Selected from one or more of the following: Cr: 0.005–2.0%, Mo: 0.05–2.0%, V: 0.05–1.0%, Cu: 0.05–4.0%, Ni: 0.005–2.0%, Ti: 0.005–0.20%, Nb: 0.005–0.20%, B: 0.0003–0.0050%, Ca: 0.0001–0.0050%, REM: 0.0001–0.0050%, Sb: 0.0010–0.10%, Sn: 0.0010–0.50%.

[0049] Cr, Mo, V, Cu, and Ni are effective elements for martensite formation and contribute to high strength. To achieve this effect, when Cr, Mo, V, Cu, and Ni are present, it is preferable to set the content of each element to the aforementioned lower limit or higher. When the content of each element exceeds the aforementioned upper limit, the surface texture of the steel sheet becomes well-developed, sometimes resulting in unsatisfactory resistance to bending and wrinkling. The Cr content is more preferably set to 0.1% or higher. Furthermore, the Cr content is more preferably set to 1.0% or lower. The Mo content is more preferably set to 0.1% or higher. Furthermore, the Mo content is more preferably set to 0.5% or lower. The V content is more preferably set to 0.1% or higher. Furthermore, the V content is more preferably set to 0.5% or lower. The Cu content is more preferably set to 0.1% or higher. Furthermore, the Cu content is more preferably set to 0.6% or lower. The Ni content is more preferably set to 0.1% or higher. Furthermore, the Ni content is more preferably set to 0.6% or lower.

[0050] Ti and Nb are elements that form carbides and are effective in increasing the strength of steel. To achieve this effect, when Ti and Nb are present, it is preferable to set the content of each element at or above the aforementioned lower limit value. On the other hand, when the content of Ti and Nb exceeds the aforementioned upper limit value, the surface texture of the steel sheet becomes well-developed, and sometimes the desired bending wrinkle resistance is not obtained. The Ti content is more preferably set to 0.01% or more. Furthermore, the Ti content is more preferably set to 0.15% or less. The Nb content is more preferably set to 0.01% or more. Furthermore, the Nb content is more preferably set to 0.15% or less.

[0051] Boron (B) is an effective element for improving the hardenability of steel sheets, promoting martensite formation, and contributing to high strength. To achieve this effect, when B is present, it is preferable to set the B content to 0.0003% or more. On the other hand, when the B content exceeds 0.0050%, B-based compounds increase, hardenability decreases, and sometimes the steel microstructure of the present invention cannot be obtained. Therefore, when B is present, it is preferable to set the B content to 0.0003 to 0.0050%. A B content of 0.0005% or more is more preferred. Furthermore, a B content of 0.0040% or less is even more preferred.

[0052] Ca and REM are elements that effectively improve processability by controlling the morphology of inclusions. To achieve this effect, when Ca and REM are present, their respective contents are preferably set as follows: Ca: 0.0001–0.0050%, REM: 0.0001–0.0050%. When the contents of Ca and REM exceed the above upper limits, the amount of inclusions increases, and processability may sometimes deteriorate. The Ca content is more preferably set to 0.0005% or more. Furthermore, the Ca content is more preferably set to 0.0030% or less. The REM content is more preferably set to 0.0005% or more. Furthermore, the REM content is more preferably set to 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.

[0053] Sb is an element that effectively inhibits denitrification, deboronization, and other processes that reduce the strength of steel. To achieve this effect, when Sb is present, it is preferable to set the Sb content to 0.0010 to 0.10%. When the Sb content exceeds the above-mentioned upper limit, it can sometimes lead to embrittlement of the steel sheet. The Sb content is more preferably 0.0050% or more. Furthermore, the Sb content is more preferably 0.050% or less.

[0054] Sn is an element that effectively suppresses pearlite and inhibits the reduction of steel strength. To achieve this effect, when Sn is present, it is preferable to set the Sn content to 0.0010 to 0.50%. When the Sn content exceeds the above-mentioned upper limit, it can sometimes lead to embrittlement of the steel sheet. The Sn content is more preferably 0.0050% or more. Furthermore, the Sn content is more preferably 0.050% or less.

[0055] It should be noted that the content of Cr, Mo, V, Cu, Ni, Ti, Nb, B, Ca, REM, Sb, and Sn is less than the aforementioned lower limit value without impairing the effect of the present invention. Therefore, when the content of these elements is less than the aforementioned lower limit value, these elements are treated as elements present in the form of unavoidable impurities. In addition, unavoidable impurities other than these elements include N, Na, Mg, Zr, Hf, Ta, W, etc., but their total content is set to 0.020% or less.

[0056] Next, the steel structure of the high-strength hot-rolled steel sheet of the present invention will be described.

[0057] In the steel microstructure of the high-strength hot-rolled steel sheet of the present invention, the total area ratio of martensite and bainite is 80-100%, the maximum orientation density of grains in the region 5-10 μm from the surface in the thickness direction is less than 2.5, and the maximum orientation density of grains in the region 50-100 μm from the surface in the thickness direction is 2.5 or more.

[0058] The combined area ratio of martensite and bainite is 80-100%.

[0059] In this invention, a microstructure primarily comprising martensite and bainite is designed to achieve high total saturation (TS), high total saturation (YS), and excellent resistance to bending and wrinkling. When the combined area ratio of martensite and bainite is less than 80%, the desired TS, YS, and resistance to bending and wrinkling are not achieved. Therefore, the combined area ratio of martensite and bainite is set to 80-100%. Preferably, the combined area ratio is set to 90-100%, more preferably 95-100%. It should be noted that the area ratio of each phase can be determined using the methods described in the embodiments.

[0060] The maximum grain orientation density in the region 5–10 μm from the surface along the thickness direction (the surface 5–10 μm region) is less than 2.5.

[0061] By randomizing the orientation of the outermost grains, cracking and wrinkling during bending can be suppressed. To achieve this effect, the maximum orientation density of grains in the region 5–10 μm from the surface in the thickness direction (the surface 5–10 μm region) needs to be set to less than 2.5. Therefore, the maximum orientation density of grains in the surface 5–10 μm region is set to less than 2.5. The maximum orientation density of grains in the surface 5–10 μm region is preferably less than 2.4, more preferably less than 2.3. Furthermore, the lower limit of the maximum orientation density of grains in the surface 5–10 μm region is preferably 1.0 or more, more preferably 1.2 or more. It should be noted that the maximum orientation density of grains in the surface 5–10 μm region can be determined by the method described in the examples.

[0062] Maximum grain orientation density in the region 50–100 μm from the surface along the thickness direction (surface 50–100 μm region): 2.5 or higher.

[0063] By developing a well-defined and non-random grain texture in the region immediately below the outermost layer, cracking and wrinkling during bending can be suppressed. While the exact details are unclear, it is speculated that because layers with different crystal orientations are adjacent in the steel sheet surface layer, which becomes the starting point for cracking and wrinkling during bending, a deformation pattern less prone to cracking and wrinkling during bending is established. To achieve this effect, the maximum grain orientation density in the 5–10 μm region of the surface layer needs to be set to less than 2.5, and the maximum grain orientation density in the region 50–100 μm from the surface in the thickness direction (the 50–100 μm region of the surface layer) needs to be set to 2.5 or higher. Therefore, the maximum grain orientation density in the 50–100 μm region of the surface layer is set to 2.5 or higher. Preferably, the maximum grain orientation density in the 50–100 μm region of the surface layer is 2.6 or higher, more preferably 2.7 or higher. Furthermore, the upper limit of the maximum orientation density of grains in the surface 50–100 μm region is preferably 6.0 or less, more preferably 5.0 or less. It should be noted that the maximum orientation density of grains in the surface 50–100 μm region can be determined using the method described in the examples.

[0064] <Manufacturing Method of High-Strength Hot-Rolled Steel Plate>

[0065] The high-strength hot-rolled steel sheet of the present invention is manufactured by the following method: a steel billet having the above-mentioned composition is heated, the steel billet is rough rolled, and finish rolled under the conditions that the total reduction rate in the temperature range below 1000°C is 50% or more, the total number of passes in the temperature range below 1000°C is 3 or more, the rolling temperature of the final pass is 750 to 900°C, and the total reduction rate from the final pass rolling temperature to the final pass rolling temperature + 50°C is 35% or less. Then, the cooling time after the finish rolling is set to 2.0 s or less, and the steel billet is cooled under the condition that the average cooling rate is 50°C / s or more in the temperature range up to 550°C, and the steel billet is cooled under the condition that the average cooling rate is 100°C / s or more in the temperature range of 300 to 400°C, and the steel billet is coiled at 300°C or less.

[0066] 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].

[0067] Steel with the above-mentioned composition is smelted using known methods such as converters, electric furnaces, or vacuum melting furnaces, and then cast using known methods such as continuous casting or ingot-bill casting to produce billets. These billets are then heated directly or temporarily after cooling and subjected to rough rolling. The conditions for rough rolling do not require special specification and can be performed using conventional methods. After rough rolling, finish rolling is performed under specified conditions.

[0068] Total reduction rate within the temperature range below 1000℃: 50% or more

[0069] In the finishing rolling of hot-rolled steel, by setting the total reduction rate within the temperature range below 1000°C to 50% or more, the maximum grain orientation density in the surface 5-10 μm and 50-100 μm regions of the present invention can be obtained. Therefore, the total reduction rate within the temperature range below 1000°C in the finishing rolling is set to 50% or more. The aforementioned reduction rate is preferably 60% or more. While there is no particular upper limit to the aforementioned reduction rate, if the reduction rate is too high, the internal texture of the steel sheet becomes well-developed, which may sometimes impair processability. Therefore, the aforementioned reduction rate is preferably set to 95% or less.

[0070] Total number of passes within a temperature range below 1000℃: 3 or more passes

[0071] By dispersing the reduction in the finishing rolling temperature range below 1000°C into multiple passes, thereby reducing the reduction rate per pass, the grains in the surface 5-10 μm region can be randomized. In this invention, by setting the total number of passes in the temperature range below 1000°C to 3 or more, the maximum grain orientation density in the surface 5-10 μm region of this invention can be obtained. The total number of passes is preferably set to 4 or more. Furthermore, although not particularly limited, the total number of passes is preferably 10 or less.

[0072] Final rolling temperature: 750–900℃

[0073] When the final rolling temperature (finishing end temperature) of hot rolling is below 750°C, undesirable structures such as ferrite are generated in large quantities, and the microstructure of the present invention cannot be obtained. On the other hand, when the rolling temperature exceeds 900°C, the development of the texture on the surface of the steel sheet becomes insufficient, and the maximum grain orientation density in the 50-100 μm region of the surface layer of the present invention cannot be obtained. Therefore, the final rolling temperature (finishing end temperature) is set to 750-900°C. The rolling temperature is preferably 770°C or higher. Furthermore, the rolling temperature is preferably 880°C or lower.

[0074] Total reduction rate from final pass rolling temperature to final pass rolling temperature +50°C: 35% or less

[0075] Near the final pass temperature, when the reduction rate increases, recrystallization occurs, and the development of the texture on the surface of the steel sheet becomes insufficient, failing to achieve the maximum grain orientation density in the 50-100 μm region of the surface layer according to the present invention. To obtain the maximum grain orientation density in the 50-100 μm region of the surface layer according to the present invention, the total reduction rate within the temperature range of the final pass rolling temperature to the final pass rolling temperature +50°C needs to be set to 35% or less. Therefore, the total reduction rate within the range of the final pass rolling temperature to the final pass rolling temperature +50°C is set to 35% or less. Preferably, the total reduction rate is set to 30% or less. While no specific lower limit is specified, an excessively low total reduction rate may lead to poor shape, etc. Therefore, the total reduction rate is preferably 5% or more.

[0076] Cooling time after finishing rolling: less than 2.0s

[0077] When the cooling time after finishing rolling exceeds 2.0 s, the recovery of dislocations immediately below the surface layer is promoted, and the maximum orientation density in the 50-100 μm region of the surface layer of the present invention cannot be obtained. Therefore, the cooling time after finishing rolling is set to 2.0 s or less. The cooling time is preferably set to 1.5 s or less. While there is no particular lower limit for the cooling time, when it is 0.1 s or more, the recovery of dislocations in the surface layer of the steel plate is further improved, making it easier to obtain the maximum grain orientation density in the 5-10 μm region of the surface layer of the present invention. Therefore, the cooling time is preferably set to 0.1 s or more. It should be noted that cooling refers to exposure to the atmosphere (air cooling) without active cooling (accelerated cooling) using methods such as water injection.

[0078] Average cooling rate over the temperature range up to 550℃: 50℃ / s or more

[0079] When the average cooling rate from the start of cooling after finishing rolling to 550°C is less than 50°C / s, ferrite and pearlite are formed, and the steel microstructure of the present invention cannot be obtained. Therefore, the average cooling rate from the start of cooling (the starting temperature of accelerated cooling) to 550°C is set to 50°C / s or more. The above-mentioned average cooling rate is preferably set to 80°C / s or more. There is no particular upper limit to the above-mentioned average cooling rate, but from the viewpoint of the shape stability of the steel sheet, the above-mentioned average cooling rate is preferably 1000°C / s or less. It should be noted that, as an example, the cooling start temperature is the finishing rolling end temperature (the rolling temperature of the final pass).

[0080] Average cooling rate over a temperature range of 300–400°C: 100°C / s or more

[0081] When the average cooling rate in the temperature range of 300–400°C is less than 100°C / s, bainitic and martensitic phase transformations occur with low driving force, failing to achieve the maximum grain orientation density in the surface 50–100 μm region of the present invention. Therefore, the average cooling rate in the temperature range of 300–400°C is set to 100°C / s or more. The aforementioned average cooling rate is preferably 150°C / s or more. While no specific upper limit is specified for the aforementioned average cooling rate, from the viewpoint of the shape stability of the steel plate, the aforementioned average cooling rate is preferably 1000°C / s or less.

[0082] Winding temperature: below 300℃

[0083] When the winding temperature exceeds 300°C, the driving force for the phase transformation decreases, and the maximum grain orientation density in the surface 50-100 μm region of the present invention cannot be obtained. Therefore, the winding temperature is set to 300°C or below. The winding temperature is preferably 280°C or below, more preferably 250°C or below. After winding, the temperature is cooled to room temperature, for example.

[0084] There are no particular limitations other than the conditions described above for the manufacturing method, and it is preferable to adjust the conditions appropriately as follows. For example, regarding the heating temperature of the steel billet, from the viewpoint of segregation removal and precipitate solution, it is preferable to be 1100°C or higher, and from the viewpoint of energy efficiency, it is preferable to be 1300°C or lower. From the viewpoint of reducing coarse grains that lead to decreased workability, it is preferable to set the finishing rolling to 4 passes or more.

[0085] The high-strength hot-rolled steel sheet of the present invention has a tensile strength (TS) of 980 MPa or more and a yield strength (YS) of 800 MPa or more. TS is preferably 1180 MPa or more, and YS is preferably 900 MPa or more. Furthermore, although not particularly limited, TS is preferably 1570 MPa or less, and YS is preferably 1300 MPa or less. In addition, the high-strength hot-rolled steel sheet of the present invention has excellent resistance to bending and wrinkling with an R / t of 3.0 or less. R / t is preferably 2.8 or less. It should be noted that TS, YS, and R / t are each determined by the methods described in the embodiments.

[0086] Example

[0087] The steel with the composition shown in Table 1 was melted in a vacuum melting furnace. The resulting steel billet was heated to 1250°C and rough-rolled. Then, it was finished rolled, cooled, cooled (accelerated cooling), and coiled under the conditions shown in Table 2 to produce hot-rolled steel sheets. It should be noted that the total number of passes for the finish rolling was set to 7. The resulting hot-rolled steel sheets were then used to evaluate their microstructure, tensile properties, and resistance to bending wrinkles according to the following test methods.

[0088] Organizational observation

[0089] The area ratio of martensite and bainite refers to the proportion of the area of ​​each microstructure in the observed area. Regarding the area ratio of martensite, 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 position of the sheet thickness 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 (manufactured by Media Cybernetics). The average area ratio of the three fields of view was set as the area ratio of each microstructure. The image data were distinguished as follows: upper bainite was black or dark gray martensite containing carbides or with a linear interface; lower bainite was black, dark gray, gray, or light gray martensite containing carbides with consistent orientation; martensite was black, dark gray, gray, or light gray martensite containing carbides with multiple orientations, or white or light gray martensite without carbides; and retained austenite was white or light gray martensite without carbides. Martensite and retained austenite are sometimes indistinguishable. Therefore, retained austenite is determined using a method described later, excluding it from the total area ratio of martensite and retained austenite obtained from the SEM image, thus calculating the area ratio of martensite. It should be noted that in this invention, martensite can be any of the following: fresh martensite, self-tempered martensite, tempered martensite, etc. Additionally, bainite can be any of the following: upper bainite, lower bainite, tempered bainite, etc. The stronger the tempering degree of the microstructure, the more black and high-contrast the image of the substrate. Therefore, using the substrate color as a standard, in this invention, the amount of carbides, the morphology of the microstructure, etc., are comprehensively judged, including the microstructures described later, and classified into any microstructure with similar characteristics. Carbides are white dots or lines. In addition, in this invention, although it is basically not contained, ferrite is black or dark gray, and has little or no carbides or martensite with linear interfaces. Pearlite can be distinguished in the form of black and white layered or partially interrupted layered structures. Regarding the area ratio of retained austenite, for the surface after the annealed steel plate is ground to 1 / 4 + 0.1 mm of plate thickness and then further ground off 0.1 mm by chemical grinding, the integrated reflection intensity of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron (ferrite) is measured using an X-ray diffraction device with Mo Kα1 rays. The volume ratio is calculated from the intensity ratio of the integrated reflection intensity of each plane of fcc iron to the integrated reflection intensity of each plane of bcc iron, and this is taken as the area ratio of retained austenite.

[0090] Using the area ratios of each microstructure, calculate the total area ratio. The results are shown in Table 3. It should be noted that "V(M+B)" in Table 3 refers to the total area ratio of martensite and bainite, and "V(O)" refers to the total area ratio of other microstructures (microstructures other than martensite and bainite).

[0091] Maximum orientation density of grains

[0092] For the same sample used in the above microstructure observations, the crystal orientation of the plate section perpendicular to the rolling direction was determined by electron backscatter diffraction (EBSD) in the regions of 5–10 μm and 50–100 μm from the steel plate surface. 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 grain orientation density in that field of view. This operation was performed on five locations in each of the 5–10 μm and 50–100 μm regions from the steel plate surface, and the average values ​​were taken as the maximum grain orientation density for each of the surface 5–10 μm and 50–100 μm regions. It should be noted that the five locations in the 5–10 μm distance from the steel plate surface mentioned above refer to five locations taken from a measurement area centered at a distance of 7.5 μm from the steel plate surface, with the center of the measurement area in the thickness direction of 5 μm × 1000 μm in the width direction, and spaced 1000 μm apart from the center of the measurement area in the width direction. Similarly, the five locations in the 50–100 μm distance from the steel plate surface mentioned above refer to five locations taken from a measurement area centered at a distance of 75 μm from the steel plate surface, with the center of the measurement area in the thickness direction of 5 μm × 1000 μm in the width direction, and spaced 10 μm apart from the center of the measurement area in the vertical direction of the thickness (i.e., the center of each measurement area is 55 μm, 65 μm, 75 μm, 85 μm, and 95 μm from the steel plate surface). It should also be noted that the EBSD measurement was performed under an accelerating voltage of 30 kV and a step size of 0.05 μm.

[0093] Tensile test

[0094] 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 test according to JIS Z 2241 is used to determine TS and YS (0.2% endurance). It should be noted that in this invention, TS of 980 MPa or higher and YS of 800 MPa or higher are set as qualified.

[0095] Bending test (resistance to bending and wrinkling)

[0096] Test pieces with a width of 30 mm and a length of 100 mm were cut from the obtained hot-rolled steel sheet and bent using a 90°V bending punch. The outer surface of the bend was visually inspected or examined with a magnifying glass to confirm the presence or absence of cracks and wrinkles. The minimum bending radius R at which no cracks or wrinkles were observed in test number 3 (N=3) was calculated, and then divided by the plate thickness t to obtain R / t. Table 3 shows the values ​​obtained by dividing the minimum bending radius R at which no wrinkles were observed by the plate thickness t (i.e., "wrinkle R / t") and the minimum bending radius R at which no cracks were observed by the plate thickness t (i.e., "crack R / t"). In this invention, R / t of 3.0 or less is considered acceptable (both "wrinkle R / t" and "crack R / t" shown in Table 3 are considered acceptable if they are 3.0 or less). It should be noted that cracks originating from the end face are excluded from the judgment.

[0097]

[0098]

[0099]

[0100] The inventive examples all use high-strength steel plates with excellent resistance to bending and wrinkling. On the other hand, comparative examples that deviate from the scope of the present invention did not achieve more than one of the desired strength or resistance to bending and wrinkling.

[0101] Industrial availability

[0102] According to the present invention, a high-strength hot-rolled steel sheet with a TS of 980 MPa or higher, a YS of 800 MPa or higher, and excellent resistance to bending and wrinkling can be obtained. When the high-strength steel sheet of the present invention is used in automotive parts, it can greatly contribute to improving the collision safety and fuel efficiency of automobiles.

Claims

1. A high-strength hot-rolled steel sheet, comprising, by mass%, 0.02-0.23% C, 0.10-3.00% Si, 0.5-3.5% Mn, less than 0.100% P, less than 0.02% S, less than 1.5% Al, with the balance being Fe and unavoidable impurities. The combined area ratio of martensite and bainite is 80-100%, the maximum orientation density of grains in the region 5-10 μm from the surface in the thickness direction is less than 2.5, and the maximum orientation density of grains in the region 50-100 μm from the surface in the thickness direction is greater than 2.

5.

2. The high-strength 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%, Mo: 0.05–2.0%, V: 0.05–1.0%, Cu: 0.05–4.0%, Ni: 0.005–2.0%, Ti: 0.005–0.20%, Nb: 0.005–0.20%, B: 0.0003–0.0050%, Ca: 0.0001–0.0050%, REM: 0.0001–0.0050%, Sb: 0.0010–0.10%, and Sn: 0.0010–0.50%.

3. A method for manufacturing a high-strength hot-rolled steel plate, wherein, A steel billet having the composition described in claim 1 or 2 is heated and rough rolled. Then, it is finished rolled under the following conditions: a total reduction rate of 50% or more in a temperature range below 1000°C, a total number of passes of 3 or more in a temperature range below 1000°C, a final pass rolling temperature of 750 to 900°C, and a total reduction rate of 35% or less from the final pass rolling temperature to the final pass rolling temperature + 50°C. Then, the cooling time after the finishing rolling is set to 2.0 s or less, and the billet is cooled at an average cooling rate of 50°C / s or more in a temperature range up to 550°C, cooled at an average cooling rate of 100°C / s or more in a temperature range of 300 to 400°C, and coiled at 300°C or below.

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