Hot-rolled steel sheet and method for producing same

Abstract

Provided is a hot-rolled steel sheet which is excellent in terms of ductility, punchability, and hole expandability and which, after heating, has excellent strength and toughness. The hot-rolled steel sheet has a specific component composition and has a steel microstructure in which the total areal proportion of ferrite and upper bainite is 87% or higher, the areal proportion of pearlite is 0-10%, the total areal proportion of martensite, lower bainite, and retained austenite is 0-3%, the pearlite band thickness is 3.0 μm or less, the average crystal grain diameter of ferrite and upper bainite is 1.0-8.0 μm, the A value is 0.30 or greater but less than 0.80, and the total content of Nb and Ti present as precipitate grains having diameters of 100 nm or larger is 0.004-0.020%.

Description

Hot-rolled steel sheet and method for manufacturing the same

[0001] This invention relates to hot-rolled steel sheets and a method for manufacturing the same.

[0002] From the perspective of improving collision safety and fuel efficiency in automobiles, high strength is required for steel plates used in automotive parts.

[0003] On the other hand, since such steel sheets are generally pressed and processed into automotive parts, excellent press-formability is also required. However, increasing the strength of the steel sheet reduces its workability, leading to significant cracking during pressing. Thus, there is a need for a steel sheet that combines the seemingly contradictory properties of strength and press-formability.

[0004] To achieve excellent press formability, steel sheets must possess superior ductility and hole-expanding properties. Furthermore, trimming is commonly performed on steel sheets during press forming. If edge cracking occurs during the trimming process, press formability deteriorates, and fatigue properties decrease. Therefore, steel sheets must also possess excellent stability at the trimmed edges, i.e., excellent punchability.

[0005] Therefore, various types of steel plates have been developed to meet these demands.

[0006] For example, Patent Document 1 proposes a high-strength steel sheet having a predetermined component composition and a steel structure with ferrite as the main phase, having a uniform elongation of 19% or more and a hole expansion ratio of 55% or more.

[0007] Furthermore, Patent Document 2 proposes a high-strength hot-rolled steel sheet having a predetermined component composition and a steel structure with bainite as the main phase, and possessing excellent press formability.

[0008] Furthermore, in recent years, a method has been proposed to improve processability by partially heating the steel sheet during press forming. In this specification, the heat treatment performed when processing the steel sheet into parts, rather than during the manufacturing of the steel sheet, may be referred to as "post-heating."

[0009] For example, Patent Document 3 discloses a technique for improving the stretch flange properties by heating the sheared end face of a steel plate prior to press working.

[0010] Japanese Patent Publication No. 2016-135925, Japanese Patent Publication No. 2015-057514, International Publication No. 2019 / 131289

[0011] However, conventional steel plates, such as those proposed in the above-mentioned Patent Documents 1 to 3, still did not perform adequately.

[0012] For example, the high-strength steel sheet proposed in Patent Document 1 has a high uniform elongation of 19% or more, but its hole expansion rate is less than 80%. Furthermore, its properties after post-heating are not considered. When manufacturing parts using post-heating, the resulting parts must have excellent strength and toughness after heating in order to exhibit superior performance.

[0013] The steel sheet proposed in Patent Document 2 is intended to improve strength by heat treatment after forming it into a part. Specifically, the heat treatment precipitates fine carbides, thereby increasing the yield strength and tensile strength. This heat treatment can also be considered a type of post-heating.

[0014] The steel sheet described in Patent Document 2 exhibits excellent strength after heating due to the effects of the heat treatment, but poor toughness after heating. Furthermore, the ductility, punchability, and hole-expanding properties of the steel sheet before heating are not considered.

[0015] While Patent Document 3 improves the stretch flange properties during forming by performing post-heating, it does not consider the characteristics of the part after the post-heating and forming. As mentioned above, for use in automotive parts and the like, excellent strength and toughness after heating are required. Furthermore, attempting to perform post-heating at numerous locations during press forming complicates control and increases costs. Therefore, the steel sheet itself is required to have excellent punchability and hole-expanding properties. However, Patent Document 3 does not adequately consider the punchability and hole-expanding properties of the steel sheet itself.

[0016] Thus, a hot-rolled steel sheet that excels in strength, ductility, punchability, and hole-expanding properties, as well as in strength and toughness after heating, had yet to be realized.

[0017] This invention has been made in view of the above circumstances, and aims to provide a hot-rolled steel sheet that is excellent in ductility, punchability, and hole-expanding properties, as well as in strength and toughness after heating.

[0018] To solve the above problems, the inventors focused on the precipitation behavior of Ti and Nb after post-heating of hot-rolled steel sheets and conducted an investigation. As a result, they found that the properties of the steel sheet after heating can be improved by controlling the amount of Ti and Nb present as coarse precipitates, and the amount of solid-solution Ti and solid-solution Nb. Furthermore, they found that the properties of the steel sheet before heating can be improved by controlling the steel structure and precipitate size of the steel sheet.

[0019] This invention was completed based on the above findings, and its gist is as follows.

[0020] 1. The composition is as follows, by mass%, C: 0.03-0.12%, Si: 0.1-1.5%, Mn: 0.5-1.8%, P: 0.050% or less, S: 0.0050% or less, Al: 1.5% or less, N: 0.0060% or less, Ca: 0-0.0050%, O: 0.0020% or less, and either or both Ti and Nb: 0.010-0.060% in total, with the remainder being Fe and unavoidable impurities; the steel structure has a total area ratio of 87% or more for ferrite and upper bainite, an area ratio of 0-10% for pearlite, a total area ratio of 0-3% for martensite, lower bainite, and retained austenite, and a pearlite band thickness of 3.0 μm or less. A hot-rolled steel sheet having an average grain size of ferrite and upper bainite of 1.0 to 8.0 μm, an A value defined by the following formula (1) of 0.30 or more and less than 0.80, and a total amount of Nb and Ti present as precipitates with a grain size of 100 nm or more of 0.004 to 0.020 mass%. A = (Amount of solid-solution Ti + Amount of solid-solution Nb) / (Total amount of Ti + Total amount of Nb) ... (1)

[0021] 2. The hot-rolled steel sheet according to item 1, wherein the mode diameter of the oxygen-containing inclusions in the particle size distribution based on the number of inclusions is 0.50 to 3.00 μm.

[0022] 3. The above component composition contains, by mass%, Ca: 0 to 0.0010%, and the number density of oxygen-containing inclusions in the mode diameter of the particle size distribution based on the number of particles is 4.0 particles / mm². ２ The hot-rolled steel sheet described in item 1 or 2 above, which is as follows:

[0023] 4. The hot-rolled steel sheet according to any one of claims 1 to 3 above, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, Cr: 1.0% or less, Cu: 0.5% or less, Ni: 1.0% or less, Mo: 0.5% or less, V: 0.2% or less, B: 0.0050% or less, REM: 0.0050% or less, Sb: 0.10% or less, and Sn: 0.10% or less.

[0024] 5. A method for manufacturing a hot-rolled steel sheet according to any one of items 1 to 4 above, comprising: heating a steel slab having the above component composition at a heating temperature of 1100 to 1250°C and a holding time of 0.2 to 3.5 hours; roughly rolling the heated steel slab at a reduction ratio of 80 to 90% at 1050°C or higher to obtain a sheet bar; finishing rolling the sheet bar at a temperature T (°C) or lower defined by the following formula (2) and a reduction ratio of 25% or less per pass to obtain a hot-rolled steel sheet; allowing the hot-rolled steel sheet to cool for 0.1 s or more and less than 1.0 s; cooling the cooled hot-rolled steel sheet at an average cooling rate of 20 to 100°C / s up to 550°C; and winding the cooled hot-rolled steel sheet at a winding temperature of 400 to 550°C. T = 800 + 1000 × Ti + 2500 × Nb ... (2) Here, the element symbols in the above formula represent the content (mass %) of the element, and 0 is used if the element is not present.

[0025] 6. The method for manufacturing a hot-rolled steel sheet as described in 5 above, wherein when casting molten steel having the above-mentioned component composition, the molten steel is cast to form a steel slab by being stirred in a horizontal plane with an induction electromagnetic stirring device at a flow rate of 0.10 to 0.30 m / s relative to the mold, and the steel slab is subjected to the heating described above.

[0026] According to the present invention, it is possible to provide a hot-rolled steel sheet that is excellent in strength, ductility, punchability, and hole-expanding properties, as well as in strength and toughness after heating. By using the hot-rolled steel sheet of the present invention, it is possible to obtain products such as automobile parts that exhibit high strength and excellent toughness even after heat treatment for purposes such as improving workability and fatigue properties. Therefore, the hot-rolled steel sheet of the present invention can be used very suitably as a material for automobile parts.

[0027] In this invention, high strength refers to a tensile strength of 590 to 780 MPa. Excellent ductility refers to a uniform elongation of 10% or more. Excellent hole-expanding properties refer to a hole-expanding ratio of 80% or more. Excellent punching properties refer to the ability to secure a clearance of 5% or more in samples punched at 5% intervals with clearances of 5% to 15%, without producing cracks parallel to the plate surface.

[0028] Furthermore, superior strength after heating means that the absolute value ΔHV (= |HV0 - HV1|), which is the difference between the Vickers hardness HV0 of the hot-rolled steel sheet before heating and the Vickers hardness HV1 of the hot-rolled steel sheet after heating, is 30 or less. Superior toughness after heating means that the ductile fracture surface ratio at -100°C is 50% or more.

[0029] The embodiments of the present invention will be described in detail below. The following description illustrates preferred embodiments of the present invention, and the invention is not limited thereto.

[0030] <Hot-rolled steel sheet> The hot-rolled steel sheet in one embodiment of the present invention has a predetermined component composition, steel structure, and precipitate state. The reasons for each limitation are explained below.

[0031] [Component Composition] The hot-rolled steel sheet in 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.

[0032] C: 0.03-0.12% Carbon (C) is an element that increases tensile strength by forming and strengthening upper bainite. In addition, C suppresses the decrease in strength after heating by bonding with Ti and N to form precipitates. If the C content is less than 0.03%, the above effect cannot be sufficiently obtained, and the desired strength and strength after heating 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 0.12%, the formation of pearlite bands becomes significant, and the desired steel structure cannot be obtained. Therefore, the C content should be 0.12% or less, preferably 0.10% or less, and more preferably 0.09% or less.

[0033] Si: 0.1-1.5% Si is an effective element for solid solution strengthening of steel. Si also has the effect of suppressing the formation of pearlite bands. To obtain these effects, the Si content must be 0.1% or more. Therefore, the Si content should be 0.1% or more, preferably 0.2% or more. On the other hand, if the Si content exceeds 1.5%, excessive retained austenite is produced, and the desired steel structure cannot be obtained. Therefore, the Si content should be 1.5% or less, preferably 1.2% or less, and more preferably 0.9% or less.

[0034] Mn: 0.5-1.8% Mn is an element that increases tensile strength by generating upper bainite. Mn also has the effect of suppressing pearlite formation. If the Mn content is less than 0.5%, the above effect is insufficient, and the desired microstructure cannot be obtained. Therefore, the Mn content should be 0.5% or more, preferably 0.7% or more, and more preferably 0.8% or more. On the other hand, if the Mn content exceeds 1.8%, the formation of pearlite bands, lower bainite, martensite, and retained austenite becomes significant, and the desired steel structure cannot be obtained. Therefore, the Mn content should be 1.8% or less, preferably 1.6% or less, and more preferably 1.4% or less.

[0035] P: 0.050% or less. Since P has the effect of reducing toughness, it is desirable to reduce the P content as much as possible. In this invention, a P content of up to 0.050% is acceptable. Therefore, the P content should be 0.050% or less, preferably 0.030% or less. On the other hand, from the viewpoint of improving toughness, the lower the P content, the better, so the lower limit of the P content is not limited and may be 0%. However, excessive reduction will lead to a decrease in productivity, so it is preferable that the P content be 0.001% or more.

[0036] S: 0.0050% or less. Since sulfur (S) forms inclusions that reduce punchability, hole-expanding properties, toughness, etc., it is desirable to reduce the S content as much as possible. In this invention, an S content of up to 0.0050% is acceptable. Therefore, the S content should be 0.0050% or less, preferably 0.0030% or less, and more preferably 0.0015% or less. On the other hand, from the viewpoint of improving properties, the lower the S content, the better, so the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction leads to a decrease in productivity, so it is preferable that the S content be 0.0002% or more.

[0037] Al: 1.5% or less. Al is an element that acts as a deoxidizing agent and can be added in the deoxidation process. However, if the Al content exceeds 1.5%, a large amount of retained austenite is produced, making it impossible to obtain the desired steel structure. Therefore, the Al content should be 1.5% or less, preferably 1.0% or less, and more preferably 0.50% or less. On the other hand, the lower limit of the Al content is not particularly limited and may be 0%. However, from the viewpoint of enhancing its deoxidizing effect, it is preferable to have an Al content of 0.01% or more.

[0038] N: 0.0060% or less. N generates inclusions such as coarse TiN, which inhibit the precipitation of TiC and NbC. Therefore, it is desirable to reduce its amount as much as possible. In this invention, an N content of up to 0.0060% is acceptable. Therefore, the N content is 0.0060% or less, preferably 0.0050% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction leads to a decrease in productivity, so it is preferable to have an N content of 0.0005% or more.

[0039] Ca: 0 to 0.0050% When the Ca content exceeds 0.0050%, inclusions excessively increase, and desired punching performance and hole expansion performance cannot be obtained. Therefore, the Ca content should be 0.0050% or less, preferably 0.0030% or less, more preferably 0.0020% or less, and even more preferably 0.0010% or less. On the other hand, Ca is an element that has the effect of reducing elongated inclusions and further improving punching performance and hole expansion performance, and can be optionally added. However, the addition of Ca is not essential, and thus the lower limit of the Ca content is 0%.

[0040] O: 0.0020% or less O combines with elements such as Si, Al, Ca, Ti, etc. to form inclusions, which reduces punching performance and hole expansion performance. Therefore, it is preferably reduced as much as possible. In the present invention, the O content can be tolerated up to 0.0020%. Therefore, the O content is 0.0020% or less, preferably 0.0015%, and more preferably 0.0010% or less. On the other hand, since the lower the O content, the better, the lower limit of the O content is not particularly limited and can be 0%. However, from the perspective of production efficiency, etc., it is preferable to set the O content to 0.0001% or more, and more preferably 0.0002% or more.

[0041] Ti, Nb: Total 0.010 - 0.060% Ti and Nb are elements that have the effect of improving strength and toughness by generating precipitates such as TiC and NbC upon heating. Also, Ti has the effect of suppressing the formation of MnS and reducing coarse inclusions. In order to exhibit these effects and obtain the desired strength and toughness after heating, it is necessary to contain either one or both of Ti and Nb in a total amount of 0.010% or more. Therefore, the total content of Ti and Nb is 0.010% or more, preferably 0.020% or more. On the other hand, when the total content exceeds 0.060%, the precipitates after heating become excessive, and good toughness cannot be obtained. Therefore, the total content of Ti and Nb is 0.060% or less, preferably 0.050% or less.

[0042] The component composition of the hot-rolled steel sheet in one embodiment of the present invention includes the above components, and the balance is Fe and inevitable impurities.

[0043] In another embodiment of the present invention, the component composition of the hot-rolled steel sheet can further optionally contain at least one of the following components.

[0044] Cr: 1.0% or less. Cr is an element that promotes the formation of upper bainite and contributes to further improvement in strength. However, when the Cr content exceeds 1.0%, lower bainite or martensite may form, and it may be impossible to obtain the desired microstructure. Therefore, when adding Cr, the Cr content should be 1.0% or less, preferably 0.5% or less. On the other hand, the lower limit of the Cr content is not particularly limited and may be 0%. However, in order to enhance the addition effect of Cr, it is preferably 0.005% or more, and more preferably 0.10% or more. [[ID=Z5]]

[0045] Cu: 0.5% or less. Cu is an element that promotes the formation of upper bainite and contributes to further improvement in strength. However, when the Cu content exceeds 0.5%, the formation of Cu precipitates becomes significant, and the toughness may decrease. Therefore, when adding Cu, the Cu content should be 0.5% or less, preferably 0.3% or less. On the other hand, the lower limit of the Cu content is not particularly limited and may be 0%. However, in order to enhance the addition effect of Cu, it is preferably 0.005% or more, and more preferably 0.05% or more.

[0046] Ni: 1.0% or less. Ni is an element that promotes the formation of upper bainite and contributes to further improvement in strength. However, when the Ni content exceeds 1.0%, lower bainite or martensite may form, and it may be impossible to obtain the desired microstructure. Therefore, when adding Ni, the Ni content should be 1.0% or less, preferably 0.5% or less. On the other hand, the lower limit of the Ni content is not particularly limited and may be 0%. However, in order to enhance the addition effect of Ni, it is preferably 0.005% or more, and more preferably 0.05% or more.

[0047] Mo: 0.5% or less Mo is an element that enhances the hardenability of steel sheets. Adding Mo promotes the formation of upper bainite, further increasing strength. However, if the Mo content exceeds 0.5%, lower bainite or martensite may form, and the desired structure may not be obtained. Therefore, when adding Mo, the Mo content should be 0.5% or less, preferably 0.3% or less. On the other hand, the lower limit of the Mo content is not particularly limited and may be 0%. However, in order to enhance the effect of adding Mo, it is preferable to have a Mo content of 0.005% or more, and more preferable to have a Mo content of 0.05% or more.

[0048] V: 0.2% or less. V is an element that enhances the hardenability of steel sheets. By adding V, the formation of upper bainite is promoted, and the strength can be further increased. However, if the V content exceeds 0.2%, lower bainite or martensite may be formed, and the desired structure may not be obtained. In addition, the formation of V-based precipitates may become significant, and toughness may decrease. For this reason, when adding V, the V content should be 0.2% or less, preferably 0.1% or less. On the other hand, the lower limit of the V content is not particularly limited and may be 0%. However, in order to enhance the effect of adding V, it is preferable to have a V content of 0.005% or more, and more preferably 0.01% or more.

[0049] B: 0.0050% or less. B is an element that enhances the hardenability of steel sheets. Adding B promotes the formation of upper bainite, further increasing strength. However, if the B content exceeds 0.0050%, the amount of B-based inclusions increases, which may reduce punchability, hole-expanding properties, and toughness after heating. Therefore, when adding B, the B content should be 0.0050% or less, preferably 0.0030% or less. On the other hand, the lower limit of the B content is not particularly limited and may be 0%. However, in order to enhance the effect of adding B, it is preferable to have a B content of 0.0001% or more, and more preferable to have a B content of 0.0005% or more.

[0050] REM: 0.0050% or less. REM (rare earth metal) is an element that further improves punchability, hole-expanding properties, and toughness by controlling the morphology of inclusions. However, if the REM content exceeds 0.0050%, the amount of inclusions increases, which may result in a decrease in punchability, hole-expanding properties, and toughness. Therefore, when adding REM, the REM content should be 0.0050% or less, preferably 0.0030% or less. On the other hand, the lower limit of the REM content is not particularly limited and may be 0%. However, in order to enhance the effect of adding REM, it is preferable to have a REM content of 0.0001% or more, and more preferably 0.0005% or more.

[0051] Sb: 0.10% or less. Sb is an element that has the effect of suppressing surface reactions such as oxidation, denitrification, and deboration. Adding Sb can improve the surface properties of steel sheets and further improve toughness. However, if the Sb content exceeds 0.10%, the steel sheet may become brittle, and its toughness may actually decrease. Therefore, when adding Sb, the Sb content should be 0.10% or less, preferably 0.030% or less. On the other hand, there is no particular lower limit to the Sb content, and it may be 0%. However, in order to enhance the effect of adding Sb, it is preferable to have an Sb content of 0.0010% or more, and more preferably 0.0050% or more.

[0052] Sn: 0.10% or less. Like Sb, Sn is an element that suppresses surface reactions such as oxidation, denitrification, and deboration. Adding Sn can improve the surface properties of steel sheets and further enhance toughness. However, if the Sn content exceeds 0.10%, the steel sheet may become brittle, and its toughness may actually decrease. Therefore, when adding Sn, the Sn content should be 0.10% or less, preferably 0.030% or less. On the other hand, the lower limit of the Sn content is not particularly limited and may be 0%. However, in order to enhance the effect of adding Sn, it is preferable to have a Sn content of 0.0010% or more, and more preferably 0.0050% or more.

[0053] [Structural Structure] Next, the reasons for limiting the structural structure of the hot-rolled steel sheet of the present invention will be explained. In this invention, the structural structure at the 1 / 4 position of the sheet thickness will be used as the structural structure. The area ratio of each structural structure can be measured by the method described in the examples. In this invention, tempered bainite is also included in the definition of bainite.

[0054] F + UB: 87% or more The structure of the hot-rolled steel sheet of the present invention contains either or both of ferrite (F) and upper bainite (UB) as the main phase in order to obtain desired strength, ductility, hole-expandability, and toughness after heating. Specifically, the total area ratio of ferrite and upper bainite (F + UB) is 87% or more, preferably 90% or more, and more preferably 95% or more. On the other hand, the upper limit of the total area ratio is not particularly limited and may be 100%.

[0055] P: 0-10% If the area ratio of pearlite (P) exceeds 10%, the desired punchability and toughness after heating cannot be obtained. Therefore, the pearlite area ratio should be 10% or less, preferably 7% or less, more preferably 5% or less, and even more preferably 3% or less. On the other hand, the lower the pearlite area ratio, the better, and therefore, the lower limit of the pearlite area ratio should be 0%. In other words, it is preferable that the steel structure of the hot-rolled steel sheet of the present invention does not contain pearlite.

[0056] M + LB + γ: 0-3% Martensite (M) is a structure that reduces hole-expandability and toughness after heating. Lower bainite (LB) is a structure that reduces ductility and hole-expandability. Retained austenite (γ) is a structure that reduces the hole-expandability and toughness of the steel sheet, and after heating, it transforms into pearlite, significantly reducing strength and toughness. If the total area ratio of martensite, lower bainite, and retained austenite (M + LB + γ) exceeds 3%, the desired properties cannot be obtained. Therefore, the total area ratio should be 3% or less, preferably less than 2%, and more preferably less than 1%. On the other hand, the lower the total area ratio, the better, and therefore, the lower limit of the total area ratio should be 0%. In other words, it is preferable that the steel structure of the hot-rolled steel sheet of the present invention does not contain martensite, lower bainite, and retained austenite.

[0057] Although there are different types of martensite depending on the degree of tempering, such as fresh martensite, auto-tempered martensite, and tempered martensite, in this invention, they are all treated as martensite without distinction.

[0058] A hot-rolled steel sheet in one embodiment of the present invention may have a structure in which, by area ratio, pearlite: 0 to 10%, and martensite, lower bainite, and retained austenite: 0 to 3% in total, with the remainder being either or both ferrite and upper bainite.

[0059] The area ratio of each tissue is determined by observing the cross-section of the hot-rolled steel sheet at the 1 / 4 thickness position using a scanning electron microscope (SEM), obtaining a secondary electron image, and then performing image analysis on the secondary electron image. More specifically, it can be determined by the method described in the examples.

[0060] Perlite band thickness: 3.0 μm or less. The thicker the perlite band, the lower the punching and hole-expanding properties become. If the perlite band thickness exceeds 3.0 μm, the desired punching and hole-expanding properties cannot be obtained. Therefore, the thickness of the perlite band is set to 3.0 μm or less, preferably 2.5 μm or less, and more preferably 2.0 μm or less. On the other hand, the lower limit of the perlite band thickness is not particularly limited and may be 0 μm. If no perlite band is present, the thickness of the perlite band is considered to be 0 μm.

[0061] In this invention, a pearlite band is defined as pearlite in which the ratio of the length in the rolling direction to the length in the thickness direction is 10 or more. The thickness of the pearlite band is determined by observing the position at 1 / 4 of the plate thickness in the cross-section of the hot-rolled steel sheet with a scanning electron microscope (SEM) to obtain a secondary electron image, and then performing image analysis on the secondary electron image. More specifically, it can be determined by the method described in the examples.

[0062] Average grain size of F+UB: 1.0 to 8.0 μm. If the average grain size of the main phase, ferrite, and upper bainite, increases, the toughness after heating decreases, and if the average grain size exceeds 8.0 μm, the desired toughness after heating cannot be obtained. Therefore, the average grain size of ferrite and upper bainite is set to 8.0 μm or less, preferably 7.0 μm or less, and more preferably 6.0 μm or less. On the other hand, if the average grain size is excessively small, the ductility decreases, and if the average grain size is less than 1.0 μm, the desired ductility cannot be obtained. Therefore, the average grain size is set to 1.0 μm or more, preferably 1.5 μm or more, and more preferably 1.8 μm or more.

[0063] The average grain size is determined by the sectioning method using a secondary electron image obtained by observing the cross-section of the hot-rolled steel sheet at the 1 / 4 thickness position using a scanning electron microscope (SEM). More specifically, it can be determined by the method described in the examples.

[0064] A value: 0.30 or more, less than 0.80. Solid-solution Ti and solid-solution Nb present in the steel plate precipitate upon heating, counteracting the decrease in strength due to heating. If A, defined by the following formula (1), is less than 0.30, the above effect cannot be sufficiently obtained, resulting in insufficient strength after heating. Therefore, A should be 0.30 or more, preferably 0.35 or more. On the other hand, if A is 0.80 or more, the increase in strength due to the precipitation of Ti and Nb becomes significant, resulting in the inability to obtain the desired toughness after heating. Therefore, A should be less than 0.80, preferably 0.70 or less. A = (Amount of solid-solution Ti + Amount of solid-solution Nb) / (Total amount of Ti + Total amount of Nb) ... (1)

[0065] The amounts of solid-soluble Ti and solid-soluble Nb used in calculating the A value are determined by constant-current electrolysis of the hot-rolled steel sheet in a 10% acetylacetone-based electrolyte, and then quantitative analysis of Ti and Nb in the electrolyte by ICP mass spectrometry. More specifically, they can be determined by the method described in the examples. On the other hand, the total Ti amount and total Nb amount are the Ti content and Nb content in the component composition of the hot-rolled steel sheet described above, respectively.

[0066] Amount of coarse precipitates: 0.004 to 0.020% If either or both of Ti and Nb are present as coarse precipitates, excessive strength increase due to heating and the resulting decrease in toughness can be suppressed. This is because, during heating, the growth of the coarse precipitates and the precipitation of new carbides (TiC, NbC) compete, and as a result, the precipitation of fine carbides is moderately suppressed. In order to obtain the above effect, the total amount of Nb and Ti present as precipitates with a particle size of 100 nm or more (hereinafter referred to as the amount of coarse precipitates) must be 0.004% or more. Therefore, the amount of coarse precipitates should be 0.004% or more, preferably 0.008% or more. On the other hand, if the amount of coarse precipitates exceeds 0.020%, the decrease in toughness due to the coarse precipitates becomes significant. Therefore, the amount of coarse precipitates should be 0.020% or less, preferably 0.017% or less.

[0067] The amount of the coarse precipitate is determined by constant-current electrolysis of a hot-rolled steel sheet in a 10% acetylacetone-based electrolyte, then decomposing the precipitate adhering to the surface of the hot-rolled steel sheet after electrolysis with acid, and quantifying the Ti and Nb contained in the resulting decomposition solution by ICP emission spectrometry. More specifically, it can be determined by the method described in the examples.

[0068] In this invention, punching properties and hole-expanding properties can be further improved by controlling the inclusions as follows.

[0069] Mode diameter: 0.50 to 3.00 μm If the mode diameter (most frequent diameter) in the particle size distribution based on the number of oxygen-containing inclusions is 0.50 μm or more, the hole-expanding properties are further improved. This is because the number of very fine inclusions dispersed in the steel is reduced. Therefore, from the viewpoint of further improving hole-expanding properties, it is preferable that the mode diameter be 0.50 μm or more, more preferably 0.70 μm or more, and even more preferably 0.80 μm or more. On the other hand, from the viewpoint of further improving punching properties, it is preferable that the mode diameter be 3.00 μm or less, more preferably 2.50 μm or less, and even more preferably 2.00 μm or less.

[0070] Number density at mode diameter: 4.0 pieces / mm ２The number density of oxygen-containing inclusions in the mode diameter is 4.0 particles / mm². ２ The following conditions can further improve the extraction and hole-expanding properties. Therefore, the number density of oxygen-containing inclusions in the mode diameter of the particle size distribution based on the number of particles should be 4.0 particles / mm². ２ Preferably, it should be 3.0 pieces / mm ２ The following is more preferable. On the other hand, the lower limit of the number density is not particularly limited, but for example, 0.3 pieces / mm ２ It may be greater than or equal to 0.6 pieces / mm ２ That's fine too.

[0071] In this invention, the mode diameter and number density of the oxygen-containing inclusions are defined as values ​​in a histogram with a class width of 0.1 μm, and can be measured by scanning electron microscopy (SEM)-energy-dispersive X-ray spectroscopy (EDX). More specifically, they can be measured by the method described in the examples.

[0072] • Plate Thickness The plate thickness of the hot-rolled steel sheet of the present invention is not particularly limited and may be any thickness. However, from the viewpoint of use as a material for automobile parts and the like, the plate thickness is preferably 0.6 mm or more, and more preferably 1.0 mm or more. From the same viewpoint, the plate thickness is preferably 10.0 mm or less, and more preferably 6.0 mm or less. The plate thickness is preferably 0.6 to 10.0 mm, and more preferably 1.0 to 6.0 mm.

[0073] - Plate width The plate width of the hot-rolled steel sheet of the present invention is not particularly limited and may be any width. However, from the viewpoint of use as a material for automobile parts and the like, the plate width is preferably 500 mm or more, and more preferably 700 mm or more. From the same viewpoint, the plate width is preferably 1800 mm or less, and more preferably 1400 mm or less. The plate width is preferably 500 to 1800 mm, and more preferably 700 to 1400 mm.

[0074] Furthermore, the hot-rolled steel sheet of the present invention may be a steel sheet in the as-hot-rolled state, or it may be a steel sheet that has been pickled. In other words, the hot-rolled steel sheet may be a black-scale material with mill scale remaining on the surface, or it may be a white-scale material from which the mill scale has been removed by pickling.

[0075] [Manufacturing Method] Next, a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention will be described. The hot-rolled steel sheet can be manufactured by hot-rolling a steel slab having the above-described component composition under predetermined conditions. The specific manufacturing procedure and conditions will be described below. In this invention, unless otherwise specified, the surface temperature at the center of the width of the steel slab or steel sheet is used. The surface temperature can be measured with a radiation thermometer or the like.

[0076] First, the steel slab having the above-mentioned component composition is heated. The heating temperature and holding time are as follows.

[0077] Heating temperature: 1100 to 1250°C. If the heating temperature is 1100°C or lower, the dissolution of Ti-containing precipitates and Nb-containing precipitates present in the steel slab will be insufficient, and as a result, the A value cannot be brought within the desired range. For this reason, the heating temperature should be 1100°C or higher, preferably 1130°C or higher, and more preferably 1160°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains will coarse, and the average crystal grain size of ferrite and upper bainite cannot be brought within the desired range. For this reason, the heating temperature should be 1250°C or lower, preferably 1230°C or lower, and more preferably 1200°C or lower.

[0078] Holding time: 0.2 to 3.5 hours If the holding time at the heating temperature is less than 0.2 hours, the dissolution of Ti-containing precipitates and Nb-containing precipitates present in the steel slab will be insufficient, and as a result, the A value cannot be brought within the desired range. For this reason, the holding time should be 0.2 hours or more, preferably 0.4 hours or more, and more preferably 0.5 hours or more. On the other hand, if the holding time exceeds 3.5 hours, the amount of coarse precipitates cannot be brought within the desired range. This is because decarburization near the surface of the steel slab becomes significant, making it difficult for Ti or Nb-containing precipitates to form near the surface. For this reason, the holding time should be 3.5 hours or less, preferably 2.5 hours or less, and more preferably 1.5 hours or less.

[0079] - Rough rolling: The heated steel slab is then roughly rolled to form a sheet bar. At this time, the reduction ratio is controlled as follows.

[0080] Reduction ratio at 1050°C or higher: 80-90% In rough rolling, reducing the material at a temperature of 1050°C or higher promotes the formation and growth of coarse precipitates with a particle size of 100 nm or more. If the reduction ratio at 1050°C or higher is less than 80%, the above effect is insufficient, and the amount of coarse precipitates cannot be kept within the desired range. For this reason, the reduction ratio at 1050°C or higher is set to 80% or higher, preferably 81% or higher. On the other hand, if the reduction ratio exceeds 90%, the precipitation of coarse precipitates becomes calcined, and the amount of coarse precipitates cannot be kept within the desired range. For this reason, the reduction ratio at 1050°C or higher is set to 90% or lower, preferably 88% or lower.

[0081] Next, the sheet bar obtained from the rough rolling process is subjected to finish rolling to produce a hot-rolled steel sheet. At this time, the reduction ratio is controlled as follows.

[0082] Reduction ratio per pass at temperatures below T (°C): 25% or less If the reduction ratio per pass at temperatures below T (°C), as defined by equation (2) below, exceeds 25%, the formation of precipitates containing Ti or Nb is promoted, resulting in an A value of less than 0.30. Therefore, the reduction ratio per pass at temperatures below T (°C) should be 25% or less, preferably 20% or less, and more preferably 18% or less. On the other hand, the lower limit of the reduction ratio is not particularly limited. However, if the reduction ratio is 5% or less, coarse grains may be produced, so it is preferable that the reduction ratio be greater than 5%. T = 800 + 1000 × Ti + 2500 × Nb ... (2) Here, the element symbols in the above equation represent the content (mass%) of the element, and 0 is used if the element is not contained.

[0083] The number of passes in the finish rolling process is not particularly limited. However, from the viewpoint of reducing coarse grains that can lead to a decrease in workability, it is preferable to perform the process in four or more passes.

[0084] Next, the hot-rolled steel sheet is allowed to cool. The cooling time is controlled as follows.

[0085] Cooling time: 0.1 s or more, less than 1.0 s By performing a short cooling period after finish rolling, some of the strain present in the steel sheet is released. This suppresses the formation of Ti-containing precipitates and Nb-containing precipitates during the subsequent cooling process. However, if the cooling time is less than 0.1 s, the above effect is insufficient, and the desired A value cannot be achieved. Therefore, the cooling time is set to 0.1 s or more, preferably 0.2 s or more. On the other hand, if the cooling time is 1.0 s or more, the strain is released more than necessary, and as a result, pearlite bands are more likely to form during the subsequent cooling process. Therefore, the cooling time is set to less than 1.0 s, 0.8 s or less, more preferably 0.7 s or less.

[0086] After the aforementioned cooling, the hot-rolled steel sheet is cooled. During this process, the average cooling rate is controlled as follows.

[0087] Average cooling rate: 20 to 100°C / s If the average cooling rate up to 550°C is less than 20°C / s, ferrite growth is accelerated, making it impossible to achieve the desired average grain size of the main phase. Also, the formation of pearlite bands progresses, making it impossible to achieve the desired thickness of the pearlite bands. Therefore, the average cooling rate up to 550°C should be 20°C / s or higher, preferably 25°C / s or higher. On the other hand, if the average cooling rate exceeds 100°C / s, the microstructure becomes excessively refined, making it impossible to achieve the desired average grain size of the main phase. Therefore, the average cooling rate should be 100°C / s or lower, preferably 80°C / s or lower, and more preferably less than 50°C / s.

[0088] Finally, the cooled hot-rolled steel sheet is wound up. At this time, the winding temperature is controlled as follows.

[0089] Winding temperature: 400 to 550°C. If the winding temperature exceeds 550°C, Ti-containing precipitates and Nb-containing precipitates are excessively formed, making it impossible to achieve the desired A value. Therefore, the winding temperature should be 550°C or lower. On the other hand, if the winding temperature is less than 400°C, structures such as martensite, lower bainite, and retained austenite are formed, making it impossible to obtain the desired structure. Therefore, the winding temperature should be 400°C or higher, preferably 450°C or higher.

[0090] By following the above procedure, a hot-rolled steel sheet that satisfies the conditions of the present invention can be obtained.

[0091] In the present invention, it is preferable to further stir the steel slab using an induction electromagnetic stirring device when casting. By performing the stirring, the agglomeration of inclusions is suppressed, and the mode diameter of the inclusions in the final hot-rolled steel sheet can be kept within a desirable range. To obtain the above effect, it is preferable to perform casting while stirring the mold in the horizontal plane with an induction electromagnetic stirring device at a flow rate of 0.10 m / s or more. It is more preferable that the flow rate be 0.15 m / s or more. On the other hand, if the flow rate exceeds 0.30 m / s, defects may occur due to powder entrapment, etc. Therefore, it is preferable that the flow rate be 0.30 m / s or less, and more preferable that it be 0.25 m / s or less. It is preferable that the flow rate be 0.10 to 0.30 m / s, and more preferable that it be 0.15 to 0.25 m / s.

[0092] The present invention will be described in more detail below based on examples. However, the present invention is not limited to the following examples.

[0093] First, steel having the component composition shown in Table 1 was melted in a converter, and steel slabs were manufactured by continuous casting. During the continuous casting, the molten metal in the mold was stirred using an induction electromagnetic stirring device. The flow velocity in the horizontal plane relative to the mold was as shown in Tables 2 and 3.

[0094] The obtained steel slab was subjected to heating, hot rolling (rough rolling, finish rolling), cooling, and winding in sequence under the conditions shown in Tables 2 and 3 to obtain hot-rolled steel sheets.

[0095] Next, the microstructure, precipitates, and inclusions of the obtained hot-rolled steel sheets were evaluated using the following procedure. The measurement results are shown in Tables 4 and 5.

[0096] (Steel structure) The area ratio of each microstructure, the average grain size of the main phases, ferrite and upper bainite, and the thickness of the pearlite band were determined in the steel structure of the hot-rolled steel sheet using the following procedure.

[0097] - Area Ratio First, a sample was cut from the hot-rolled steel sheet, and the cross-section parallel to the rolling direction was polished. Next, the cross-section was etched with 3% nital. Using a scanning electron microscope (SEM), secondary electron images were obtained by taking three field-of-view images per sample at the 1 / 4 thickness position of the cross-section. The observation magnification was 1500x.

[0098] The area ratio of each tissue was determined by image analysis of the obtained secondary electron images. Image-Pro from Media Cybernetics was used for the image analysis. The average of the area ratios across three fields of view was used as the area ratio for each tissue. The identification of each tissue in the secondary electron images was performed using the following method.

[0099] Ferrite is distinguished as a structure with curved interfaces, black or dark gray in color, and without internal substructures such as carbides or lath. Perlite can be distinguished as a black and white layered or partially discontinuous layered structure. Upper bainite is distinguished as a black or dark gray structure containing carbides or martensite with linear interfaces. Lower bainite is distinguished as a black to light gray structure containing carbides with only one orientation. Martensite is distinguished as a black to light gray structure containing regular but multiple orientation carbides, or as a white or light gray structure without carbides. Retained austenite is observed as a white or light gray structure without carbides. Note that some martensite and retained austenite may be indistinguishable, but in this invention, there is no particular need to distinguish them, so they are collectively considered as retained austenite.

[0100] The stronger the tempering, the more the base material appears as a high-contrast black image. Therefore, the above base material color is merely a guideline. In this invention, the amount of carbides, the microstructure morphology, etc., are considered comprehensively, and the material is classified into one of the similarly characterized microstructures, including those described later. The carbides are white, dot-like or linear.

[0101] The measurement results are shown in Tables 4 and 5. The following values ​​are shown in Tables 4 and 5: • Total area ratio of ferrite and upper bainite (F + UB) • Area ratio of pearlite (P) • Total area ratio of martensite, lower bainite, and retained austenite (M + LB + γ)

[0102] - Average grain size of the main phase: The average grain size of the main phase, ferrite and upper bainite, was determined by sectioning using the same field of view as the microstructure observation field. The number of sections was set to 10 vertically and 10 horizontally per field of view, and the average of the grain size in the vertical and horizontal directions was taken as the average grain size of the main phase.

[0103] - Perlite band thickness: In the above microstructure observation, the average thickness of all pearlite bands within the observation field was calculated. The total average pearlite band thickness obtained for each observation field was divided by the number of observation fields to determine the pearlite thickness. In calculating the pearlite band thickness, pearlite with a ratio of length in the rolling direction to length in the thickness direction of 10 or more was considered a pearlite band. The thickness of a single pearlite band was determined by dividing the area of ​​that pearlite by its length in the rolling direction. Connected pearlite was considered as a single pearlite.

[0104] (Value A) Next, in order to determine the value A defined by equation (1) above, the amount of solid-soluble Ti and solid-soluble Nb were measured by the following method.

[0105] A test specimen measuring 30 mm in width and 30 mm in length was taken from the obtained hot-rolled steel sheet, and this specimen was subjected to constant-current electrolysis in a non-aqueous solvent electrolyte. The non-aqueous solvent electrolyte used was a 10% acetylacetone (AA)-based electrolyte (10 vol% acetylacetone - 1 mass% tetramethylammonium chloride - methanol). The constant-current electrolysis was performed at a current density of 20 mA / cm². ２ The test was conducted under the condition of an electrolytic charge of approximately 0.2 g.

[0106] Using ICP mass spectrometry, the concentrations (mass%) of Ti, Nb, and Fe (as a comparative element) in the electrolyte after electrolysis were measured. From the obtained concentrations, the concentration ratios of Ti and Nb to Fe were calculated, and these were then multiplied by the Fe content (mass%) in the sample to obtain the solid-solution Ti amount (mass%) and solid-solution Nb amount (mass%). The Fe content (mass%) in the sample was determined by subtracting the total content (mass%) of the components other than Fe from 100 mass%. The values ​​for the content of each component are listed in Table 1.

[0107] The obtained solid-solution Ti amount (mass%) and solid-solution Nb amount (mass%) were substituted into equation (1) to calculate the A value. The total Ti amount (mass%) and total Nb amount (mass%) used were the values ​​listed in Table 1.

[0108] (Amount of coarse precipitates) The total amount of Nb and Ti present as precipitates with a particle size of 100 nm or larger was measured using the following procedure.

[0109] In the measurement of the A value described above, the test specimens with precipitates attached to their surfaces after electrolysis were removed from the electrolyte and immersed in an aqueous solution of sodium hexametaphosphate (SHMP) (500 mg / l). While in this state, ultrasonic vibrations were applied to detach the precipitates from the test specimens and extract them into the SHMP aqueous solution.

[0110] Next, the SHMP aqueous solution containing the precipitate was filtered using a filter with a pore size of 100 nm. The precipitate collected on the filter was acid-decomposed, and the weights of Ti and Nb contained in the resulting decomposition solution were measured using an ICP emission spectrometer.

[0111] The measured masses of Ti and Nb were divided by the electrolyte amount to obtain the amount of Ti and Nb (mass%) contained in precipitates with a particle size of 100 nm or larger. Here, "electrolyte amount" refers to the mass of the test specimen reduced by the electrolysis, and was determined by subtracting the mass of the test specimen after precipitate removal from the mass of the test specimen before electrolysis.

[0112] - Mode diameter and number density at the mode diameter Next, the mode diameter of the inclusions containing oxygen and the number density at the mode diameter were determined by the following procedure. First, a backscattered electron image was obtained in the range of the total plate thickness × 2 mm in width on the cross-section where the above-mentioned microstructure observation was performed. In the obtained backscattered electron image, a region with a different composition from the matrix was determined as an inclusion. Next, for each inclusion, component analysis by EDX was performed, and those with an O content of 15 wt% or more were judged as inclusions containing oxygen.

[0113] The equivalent circle diameter was determined from the area of each of the inclusions containing oxygen, and a particle size distribution based on the number was created. The particle size distribution was a histogram with a class width of 0.1 μm. The mode diameter (the particle size with the largest number) was determined from the obtained histogram.

[0114] Further, the value obtained by dividing the number at the mode diameter by the evaluation area (plate thickness × 2 mm) was defined as the number density at the mode diameter.

[0115] Next, the strength, ductility, punching property, and hole expansion property of the obtained hot-rolled steel sheet were evaluated by the following procedure. The evaluation results are shown in Tables 4 and 5.

[0116] (Strength, ductility) A tensile test was conducted to evaluate the strength and ductility of the above hot-rolled steel sheet. Specifically, first, a JIS No. 5 tensile test piece (JIS Z 2241) was taken from the hot-rolled steel sheet in a direction parallel to the rolling direction. Next, using the tensile test piece, a tensile test was conducted in accordance with the provisions of JIS Z 2241 to obtain the tensile strength TS and the uniform elongation U.El. The strain rate in the tensile test was 10 －３ / s. The uniform elongation was evaluated in a state where unloading was not performed.

[0117] In the present invention, for strength, it was judged as acceptable if the tensile strength was 590 to 780 MPa. For ductility, it was judged as acceptable if the uniform elongation U.El was 10% or more.

[0118] (Hole Expansion Properties) Hole expansion tests were conducted to evaluate the hole expansion properties of the hot-rolled steel sheet. Specifically, a test piece measuring 100 mm in width and 100 mm in length was first taken from the hot-rolled steel sheet. Using this test piece, a hole expansion test was performed five times in accordance with JFST1001 (Japan Iron and Steel Federation standard), and the average hole expansion ratio λ (%) was determined. In this invention, a hole expansion ratio λ of 80% or more was considered acceptable.

[0119] (Punching Performance) Punching tests were conducted to evaluate the punching performance of the hot-rolled material. Specifically, a test piece with a width of 50 mm and a length of 50 mm was taken from the hot-rolled steel sheet. Punching was performed on the test piece using a φ10 mm punch. Punching was performed three times for each clearance range of 5 to 15%. Tables 4 and 5 show the clearance ranges in which no chipping or cracking occurred on the end face after three punchings. In this invention, a clearance range of 5% or more was considered acceptable.

[0120] Furthermore, the strength and toughness of the obtained hot-rolled steel sheets after heating were evaluated using the following procedure. The evaluation results are shown in Tables 4 and 5. The heating described above was based on post-heating in the actual manufacturing of automotive components, and was carried out at the temperatures and holding times shown in the "Post-heating" column of Tables 2 and 3. In other words, the conditions for post-heating in the actual manufacturing of automotive components cannot be uniquely determined due to various equipment constraints. For example, when using furnace heating, heating is performed at a relatively low temperature for a long time. On the other hand, when using partial heating such as laser or induction heating (IH), heating is performed at a relatively high temperature for a short time. Therefore, in this embodiment, in order to show that the desired strength and toughness can be achieved even under such diverse post-heating conditions, evaluations were performed under various conditions, from heating at a relatively low temperature for a long time to heating at a relatively high temperature for a short time, as shown in Tables 2 and 3.

[0121] (Strength after heating) Vickers hardness test The change in strength due to heating was evaluated by performing a Vickers hardness test. Specifically, samples were first cut from both the hot-rolled steel sheet before heating and the hot-rolled steel sheet after heating. After polishing the thickness cross section of the sample parallel to the rolling direction, a Vickers hardness test was performed with a load of 5 kg and 5 measurement points. A sample was considered acceptable if the absolute value ΔHV (= |HV0 - HV1|) of the difference between the Vickers hardness HV0 of the hot-rolled steel sheet before heating and the Vickers hardness HV1 of the hot-rolled steel sheet after heating was 30 or less.

[0122] (Toughness after heating) Charpy impact test The toughness after heating was evaluated by performing a Charpy impact test. Specifically, first, after post-heating the obtained hot-rolled steel sheet, a test piece with a width of 10 mm and a length of 55 mm was taken. From the test piece, a Charpy impact test piece was prepared with a V-notch with a tip angle of 45°, a tip radius of 0.25 mm, and a length of 2 mm. A Charpy impact test was performed using the Charpy impact test piece to measure the ductile fracture surface ratio at -100°C. The measurement was performed five times, and the average value of the obtained ductile fracture surface ratio is shown in Tables 4 and 5. A value of 50% or more was considered acceptable. The notch direction was parallel to the rolling direction.

[0123]

[0124]

[0125]

[0126]

[0127]

[0128] As can be seen from the results shown in Tables 4 and 5, the hot-rolled steel sheets that satisfy the conditions of the present invention exhibited excellent strength, ductility, punchability, and hole-expanding properties, as well as excellent strength and toughness after heating. Therefore, the hot-rolled steel sheets of the present invention can be used very suitably as a material for automobile parts and can greatly contribute to improving the collision safety and fuel efficiency of automobiles. In contrast, the hot-rolled steel sheets that do not satisfy the conditions of the present invention were inferior in at least one property.

Claims

1. The composition is as follows, by mass%, C: 0.03-0.12%, Si: 0.1-1.5%, Mn: 0.5-1.8%, P: 0.050% or less, S: 0.0050% or less, Al: 1.5% or less, N: 0.0060% or less, Ca: 0-0.0050%, O: 0.0020% or less, and either or both Ti and Nb: 0.010-0.060% in total, with the remainder being Fe and unavoidable impurities; the steel structure has a total area ratio of 87% or more for ferrite and upper bainite, an area ratio of 0-10% for pearlite, a total area ratio of 0-3% for martensite, lower bainite, and retained austenite, and a pearlite band thickness of 3.0 μm or less. A hot-rolled steel sheet having an average grain size of ferrite and upper bainite of 1.0 to 8.0 μm, an A value defined by the following formula (1) of 0.30 or more and less than 0.80, and a total amount of Nb and Ti present as precipitates with a grain size of 100 nm or more of 0.004 to 0.020 mass%. A = (Amount of solid-solution Ti + Amount of solid-solution Nb) / (Total amount of Ti + Total amount of Nb) ... (1) 2. The hot-rolled steel sheet according to claim 1, wherein the mode diameter of the oxygen-containing inclusions in the particle size distribution based on the number of inclusions is 0.50 to 3.00 μm.

3. The above component composition contains, by mass%, Ca: 0 to 0.0010%, and the number density of oxygen-containing inclusions in the mode diameter of the particle size distribution based on the number of particles is 4.0 particles / mm². ２ The hot-rolled steel sheet according to claim 1 or 2, which is as follows:

4. The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the component composition further comprises at least one selected from the group consisting of, in mass%, Cr: 1.0% or less, Cu: 0.5% or less, Ni: 1.0% or less, Mo: 0.5% or less, V: 0.2% or less, B: 0.0050% or less, REM: 0.0050% or less, Sb: 0.10% or less, and Sn: 0.10% or less.

5. A method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 4, comprising: heating a steel slab having the above-mentioned component composition at a heating temperature of 1100 to 1250°C and a holding time of 0.2 to 3.5 hours; roughly rolling the heated steel slab at a reduction ratio of 80 to 90% at 1050°C or higher to obtain a sheet bar; finishing rolling the sheet bar at a temperature T (°C) or lower defined by the following formula (2) and a reduction ratio of 25% or less per pass to obtain a hot-rolled steel sheet; allowing the hot-rolled steel sheet to cool for 0.1 s or more and less than 1.0 s; cooling the cooled hot-rolled steel sheet at an average cooling rate of 20 to 100°C / s up to 550°C; and winding the cooled hot-rolled steel sheet at a winding temperature of 400 to 550°C. T = 800 + 1000 × Ti + 2500 × Nb ... (2) Here, the element symbols in the above formula represent the content (mass %) of the element, and 0 is used if the element is not present.

6. The method for producing a hot-rolled steel sheet according to claim 5, wherein when casting molten steel having the above-mentioned component composition, the molten steel is cast into a steel slab by being stirred in a horizontal plane with an induction electromagnetic stirring device at a flow rate of 0.10 to 0.30 m / s relative to the mold, and the steel slab is subjected to the heating process.

Images