Hot-rolled coil

Abstract

Provided is a hot-rolled coil which is characterized by having a prescribed chemical composition and having a metallographic structure in which the average grain diameter of crystal grains is 5.0-8.0 [mu] m, the average grain diameter of precipitates is 3.0-9.5 nm, and the average grain diameter of the precipitates is 3.0-9.5 nm when a region surrounded by grain boundaries having a difference in orientation of 15 DEG or more is defined as the crystal grains in the center in the longitudinal direction. The difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction is 15.0% or less of the average particle diameter of the precipitates; in the center portion in the width direction, the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the longitudinal direction is 15.0% or less of the average particle diameter of the precipitates in the longitudinal direction.

Description

Technical Field

[0001] The present invention relates to a hot-rolled coil. Background Art

[0002] In recent years, in the automotive industry, from the viewpoint of improving fuel efficiency, weight reduction of vehicle bodies has been required. To achieve both weight reduction and collision safety of vehicle bodies, increasing the strength of the steel sheets used is one of the effective methods. Against this background, the development of high-strength steel sheets has progressed. On the other hand, in general, the workability and formability of steel sheets decrease with increasing strength. Therefore, in the development of high-strength steel sheets, it is important to increase the strength while ensuring a certain level of workability and the like.

[0003] In connection therewith, for example, Patent Document 1 discloses a high-strength hot-rolled steel sheet characterized by having a specified composition and the following structure: a ferrite phase having an area ratio of 95% or more, in which fine carbides are precipitated at 1.0×10 22 per m 3 or more, cementite particles are precipitated at 10 per 10000 μm 2 or more, the average particle diameter of the above fine carbides is 10 nm or less, and the difference ΔHV 1 / 2t between the hardness HV 1 / 4t at the center position of the sheet thickness and the hardness HV 3 / 4t at the 1 / 4 position or 3 / 4 position of the sheet thickness 0.025 is 20 HV or less. In addition, Patent Document 1 discloses that: with the above-described configuration, it is possible to easily manufacture a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, excellent workability such as ductility and hole expansion property, and an extraordinary effect in the industry. Further, Patent Document 1 points out that Ti contributes to the formation of fine carbides (Ti carbides) of 10 nm or less, and the desired high strength is ensured by precipitation strengthening, and in order to ensure the desired hole expansion property, the Ti content is set in the range of 0.070 to 0.220%.

[0004] Patent Document 2 discloses a high-strength hot-rolled steel sheet characterized by having a specified chemical composition, an average crystal grain diameter of 8 μm or less, a segregation amount of C at large-angle crystal grain boundaries with an orientation difference of 15° or more of 4 to 15 atoms / nm 2 and the number of TiC precipitates having an equivalent spherical diameter of 3 nm or more on the above large-angle crystal grain boundaries is less than 0.01 per nm 2 and the number density of TiC precipitates having an equivalent spherical diameter of 0.8 nm to 2 nm in the crystal grains is 8×10 16 per cm 3The ratio of the above grains is 10 to 70%. In addition, Patent Document 2 discloses that: according to the above configuration, it is possible to provide a high-strength hot-rolled steel sheet having excellent formability and low-temperature toughness and a tensile strength of 740 MPa or more, and the industrial contribution is extremely significant. Furthermore, Patent Document 2 indicates that: by setting the equivalent spherical diameter of TiC precipitates in the grains to 0.8 nm to 2 nm, it is possible to efficiently impart precipitation strengthening, which is effective for increasing the strength.

[0005] Patent Document 3 discloses a high-formability high-tensile steel sheet having excellent strength stability and a tensile strength of 550 MPa or more, which is characterized by containing, by weight%, C: 0.03 to 0.15%, Mn ≥ 0.2%, N ≤ 0.01%, Ti: 0.05 to 0.35%, and containing one or more selected from Mo ≤ 0.6% and W ≤ 1.5%. When Mo and W are contained individually, Mo ≥ 0.1% and W ≥ 0.2%. Ex.C represented by the formula Ex.C = C - {Ti - N×(48 / 14) - S×(48 / 32)}×(12 / 48) - Mo×(12 / 96) - W×(12 / 184) is 0.015% or less, and Mn ≤ 1.7 - 30×Ex.C is satisfied. Substantially, it contains precipitates having a size of less than 10 nm of Ti and one or more of Mo and W dispersed in the ferrite structure.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2015-063748

[0009] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2015-218352

[0010] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-321735 Summary of the Invention

[0011] Problems to be Solved by the Invention

[0012] In Patent Documents 1 and 2, in order to improve workability and the like and achieve a high strength exceeding 700 MPa, it is indicated to utilize precipitation strengthening by Ti carbides as described above. On the other hand, in the high-strength improvement of steel materials utilizing precipitation strengthening, for example, strength unevenness may occur due to different precipitation states of precipitates in the length direction and width direction of hot-rolled coils.

[0013] In relation thereto, Patent Document 3 indicates that in a steel obtained by strengthening a ferrite single-phase structure with fine precipitates containing at least one of Ti, Mo, and W, by setting the C that does not combine with Ti, Mo, and W, i.e., Ex.C, to 0.015% or less and setting Mn to 0.2 ≤ Mn ≤ 1.7 - 30×Ex.C, the material variation in the longitudinal direction within the coil, particularly the strength variation, is reduced. However, in Patent Document 3, although the reduction of strength variation has been studied mainly from the viewpoint of the chemical composition of the steel plate as described above, sufficient study has not necessarily been conducted from the viewpoint of making the precipitation state of the above-mentioned fine precipitates in the steel plate appropriate. Therefore, in the invention described in Patent Document 3, there is still room for improvement in suppressing strength unevenness.

[0014] The present invention has been made in view of such circumstances, and an object thereof is to provide a hot-rolled coil having high strength and reduced strength unevenness through a novel configuration.

[0015] Means for Solving the Problem

[0016] The inventors of the present invention conducted research to achieve the above object, and as a result, found that by achieving high strength through grain refinement strengthening and precipitation strengthening, and by appropriately controlling the particle size of precipitates contributing to the precipitation strengthening in the width direction of the central portion in the longitudinal direction and the longitudinal direction of the central portion in the width direction of the hot-rolled coil, it is possible to significantly suppress or reduce the strength unevenness in the longitudinal direction and the width direction of the hot-rolled coil, thereby completing the present invention.

[0017] The present invention that has achieved the above object is as follows.

[0018] (1) A hot-rolled coil, characterized by having the following chemical composition: by mass%:

[0019] C: 0.050 to 0.100%,

[0020] Si: 0.01 to 0.30%,

[0021] Mn: 1.30 to 2.10%,

[0022] Ti: 0.080 to 0.150%,

[0023] Nb: 0.020 to 0.050%,

[0024] Al: 0.001 to 0.050%,

[0025] P: 0.100% or less,

[0026] S: 0.050% or less,

[0027] N: 0.0050% or less,

[0028] O: 0.0050% or less,

[0029] B: 0 to 0.0050%,

[0030] Cu: 0 to 0.20%,

[0031] Ni: 0 to 0.20%,

[0032] Sn: 0 to 0.10%,

[0033] Cr: 0 to 0.40%,

[0034] Mo: 0 to 0.200%,

[0035] V: 0 to 0.100%,

[0036] As: 0 to 0.100%,

[0037] Zr: 0 to 0.100%,

[0038] Ca: 0 to 0.0050%,

[0039] Mg: 0 to 0.100%,

[0040] Bi: 0 to 0.020%,

[0041] Co: 0 to 0.20%,

[0042] W: 0 to 0.20%,

[0043] Zn: 0 to 0.20%,

[0044] REM: 0 to 0.1000%, and

[0045] The balance: consists of Fe and impurities,

[0046] having the following metallographic structure:

[0047] At the central part in the length direction, when the region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a grain, the average grain diameter of the above grains is 5.0 to 8.0 μm, the average particle diameter of the precipitates is 3.0 to 9.5 nm, and the difference between the maximum and minimum values of the particle diameter of the above precipitates in the width direction is 15.0% or less of the average particle diameter of the precipitates;

[0048] At the central part in the width direction, the difference between the maximum and minimum values of the particle diameter of the above precipitates in the length direction is 15.0% or less of the average particle diameter of the precipitates in the length direction.

[0049] (2) The hot-rolled coil according to (1) above is characterized in that the chemical composition contains at least one of the following elements by mass%:

[0050] B: 0.0001 to 0.0050%,

[0051] Cu: 0.01 to 0.20%,

[0052] Ni: 0.01 to 0.20%,

[0053] Sn: 0.01 to 0.10%,

[0054] Cr: 0.01 to 0.40%,

[0055] Mo: 0.001 to 0.200%,

[0056] V: 0.001 to 0.100%,

[0057] As: 0.001 to 0.100%,

[0058] Zr: 0.001 to 0.100%,

[0059] Ca: 0.0001 to 0.0050%,

[0060] Mg: 0.001 to 0.100%,

[0061] Bi: 0.001 to 0.020%,

[0062] Co: 0.01 to 0.20%,

[0063] W: 0.01 to 0.20%,

[0064] Zn: 0.01 to 0.20%, and

[0065] REM: 0.0001 to 0.1000%.

[0066] (3) The hot-rolled coil according to (1) or (2) above is characterized in that it has an effective Ti content of 0.070% or more.

[0067] Advantages of the Invention

[0068] According to the present invention, it is possible to provide a hot-rolled coil with high strength and reduced strength non-uniformity. Brief Description of the Drawings

[0069] Figure 1 is a schematic view of a hot-rolled coil showing the state after coiling. Detailed Description of the Invention

[0070] <Hot-rolled Coil>

[0071] The hot-rolled coil according to an embodiment of the present invention is characterized by having the following chemical composition: by mass%:

[0072] C: 0.050 to 0.100%,

[0073] Si: 0.01 to 0.30%,

[0074] Mn: 1.30 to 2.10%,

[0075] Ti: 0.080 to 0.150%,

[0076] Nb: 0.020 to 0.050%,

[0077] Al: 0.001 to 0.050%,

[0078] P: 0.100% or less,

[0079] S: 0.050% or less,

[0080] N: 0.0050% or less,

[0081] O: 0.0050% or less,

[0082] B: 0 to 0.0050%,

[0083] Cu: 0 to 0.20%,

[0084] Ni: 0 to 0.20%,

[0085] Sn: 0 to 0.10%,

[0086] Cr: 0 to 0.40%,

[0087] Mo: 0 to 0.200%,

[0088] V: 0 to 0.100%,

[0089] As: 0 to 0.100%,

[0090] Zr: 0 to 0.100%,

[0091] Ca: 0 to 0.0050%,

[0092] Mg: 0 to 0.100%,

[0093] Bi: 0 to 0.020%,

[0094] Co: 0 to 0.20%,

[0095] W: 0 to 0.20%,

[0096] Zn: 0 to 0.20%,

[0097] REM: 0 to 0.1000%, and

[0098] the balance: composed of Fe and impurities,

[0099] having the following metal structure:

[0100] In the central part in the length direction, when a region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a grain, the average grain diameter of the above grains is 5.0 to 8.0 μm, the average grain diameter of the precipitates is 3.0 to 9.5 nm, and the difference between the maximum value and the minimum value of the grain diameter of the above precipitates in the width direction is 15.0% or less of the average grain diameter of the precipitates;

[0101] In the central part in the width direction, the difference between the maximum value and the minimum value of the grain diameter of the above precipitates in the length direction is 15.0% or less of the average grain diameter of the precipitates in the length direction.

[0102] As described above, in terms of the high strength of the steel sheet utilizing precipitation strengthening, for example, uneven strength may occur due to different precipitation states of precipitates in the length direction and the width direction of the hot-rolled coil. It is considered that this is because: due to different cooling rates in the hot-rolled coil after coiling, the precipitation state of precipitates changes in the length direction and / or the width direction of the hot-rolled coil. The front end and the tail end in the length direction of the hot-rolled coil correspond to the innermost circumference and the outermost circumference of the hot-rolled coil, respectively, and are thus exposed to the atmosphere. Therefore, generally, the front end and the tail end of the hot-rolled coil cool quickly, and precipitation of precipitates may not occur sufficiently. Therefore, as the precipitation state, it becomes a sub-aged state, and there is a tendency for the strength to be easily reduced. If the strength is reduced at the front end and the tail end in the length direction of the hot-rolled coil, then they are cut off and only the range with the desired strength is used as a product, resulting in a decrease in the yield and productivity. In addition, even when the front end and the tail end in the length direction of the hot-rolled coil have the desired strength, if strength unevenness occurs due to different precipitation states of precipitates in the length direction, for example, there are also problems such as poor formability such as cracking being likely to occur during pressing. On the other hand, since the central part in the length direction of the hot-rolled coil is not directly exposed to the atmosphere, it is not easily cooled and is maintained at a relatively high temperature state. Therefore, depending on the situation, the precipitates coarsen, exceeding the peak aging where a high precipitation strengthening effect can be obtained and becoming an over-aged state, and similarly, the strength may decrease. Also in this case, it is possible that the desired strength cannot be obtained, strength unevenness occurs, resulting in a decrease in productivity and poor formability during pressing.

[0103] In addition, it is considered that the precipitation states of precipitates in the length direction and the width direction of the hot-rolled coil are closely related to each other. Therefore, for example, even if only the central part in the length direction of the hot-rolled coil after coiling is appropriately cooled, without appropriately cooling the front end part and / or the tail end part, the central part will also be affected, and it will become impossible to reliably obtain the desired precipitation state of the precipitates. Similarly, for example, even if the front end part, the central part, and the tail end part in the length direction of the hot-rolled coil after coiling are appropriately cooled, without uniformly cooling in the width direction, more specifically, the cooling in the width direction from after hot rolling to before coiling and the cooling in the width direction after coiling, due to the temperature deviation in the width direction, the precipitation state of the precipitates in the length direction is also affected. As a result, it is also possible that the desired strength cannot be obtained in the finally obtained hot-rolled coil and the non-uniformity of the strength becomes significant. Since the precipitation state of the precipitates changes over the entire length (the overall length in the rolling direction of the hot-rolled coil) and the entire width (the overall length in the width direction of the hot-rolled coil) of the hot-rolled coil according to the temperature history like this, it is generally very difficult to effectively utilize the precipitation strengthening brought by the precipitates to achieve the desired strength and suppress or reduce the non-uniformity of the strength in the length direction and the width direction of the hot-rolled coil.

[0104] Here, when the "length direction" is referred to in this specification in relation to the hot-rolled coil, the so-called "length direction" is, as Figure 1 shown in [reference], the "rolling direction". Similarly, when the "width direction" is referred to in this specification in relation to the hot-rolled coil, the so-called "width direction" is, as Figure 1 shown in [reference], the "direction orthogonal to the rolling direction and the plate thickness direction".

[0105] Therefore, in addition to setting the chemical composition of the hot-rolled coil to an appropriate composition, the inventors of the present invention also specifically focused on the metallographic structure of the hot-rolled coil and conducted research. To explain in more detail, first, the inventors of the present invention found that by utilizing not only the precipitation strengthening brought about by precipitates such as Ti carbides but also the fine-grain strengthening brought about by the addition of Nb and the like, the desired high strength can be achieved. More specifically, the inventors of the present invention found that in the central portion in the length direction of the hot-rolled coil, by controlling the average particle size of precipitates such as Ti carbides within the range of 3.0 to 9.5 nm, the strength improvement effect brought about by precipitation strengthening can be fully exerted, and by controlling the average particle size of the grains within the range of 5.0 to 8.0 μm due to the addition of Nb and the like, the strength improvement effect brought about by fine-grain strengthening can be added. As a result, high strength, for example, a high strength with a tensile strength of 780 MPa or more, can be reliably achieved. In addition, as described above, since the inventors of the present invention considered that the precipitation states of precipitates in the length direction and width direction of the hot-rolled coil are closely related to each other, they specifically focused on the precipitation states of precipitates in the width direction of the central portion in the length direction and the length direction of the central portion in the width direction of the hot-rolled coil and further conducted research. As a result, the inventors of the present invention found that, as will be described in detail below for the manufacturing method of the hot-rolled coil, by making the cooling treatment in the cooling process after hot rolling and the cooling treatment in the subsequent coiling process appropriate, the unevenness of the particle sizes of precipitates in the width direction of the central portion in the length direction and the length direction of the central portion in the width direction of the hot-rolled coil can be controlled within a specified range. More specifically, the inventors of the present invention found that by making the cooling treatment in the cooling process after hot rolling and the coiling process appropriate, the difference between the maximum value and the minimum value of the particle sizes of precipitates in the width direction of the central portion in the length direction and the length direction of the central portion in the width direction of the hot-rolled coil can be controlled to be 15.0% or less of the average particle size of the precipitates. As a result, the strength unevenness in the length direction and width direction of the hot-rolled coil can be significantly suppressed or reduced.

[0106] Generally, the particle size of the precipitate is in the nanometer range, and high-precision measuring devices such as a transmission electron microscope (TEM) and a three-dimensional atom probe are required for its observation. Therefore, in order to reduce the strength non-uniformity over the entire length and width of the hot-rolled coil, for example, analyzing the particle size of the precipitate over the entire length and width of the hot-rolled coil and reflecting it in the manufacturing conditions requires a huge amount of time and cost, and is not necessarily realistic. Therefore, by making the cooling treatment in the cooling process and the coiling process after hot rolling appropriate, it is possible to control the non-uniformity of the particle size of the precipitate in the width direction of the central part in the length direction and the length direction of the central part in the width direction of the hot-rolled coil within a specified range as described above. Thus, it is extremely unexpected and surprising that the strength non-uniformity in the length direction and the width direction of the hot-rolled coil can be significantly suppressed or reduced. In addition, for the hot-rolled coil according to the embodiment of the present invention, since the strength non-uniformity in the length direction and the width direction of the hot-rolled coil is significantly suppressed or reduced as described above, the risk of forming defects during pressing can be reduced, and the productivity can also be significantly improved. Therefore, the hot-rolled coil according to the embodiment of the present invention is of course particularly useful in the automotive field and can also be very effectively used in other fields. In this specification, the so-called "hot-rolled coil" is not necessarily limited to the completely coiled hot-rolled coil as shown in Figure 1 For example, it may be partially uncoiled or completely uncoiled, and also includes the case where it at least partially has a plate shape (i.e., hot-rolled steel sheet).

[0107] Hereinafter, the hot-rolled coil according to the embodiment of the present invention will be described in more detail. In the following description, the unit of the content of each element, i.e., "%", means "% by mass" unless otherwise specified. In addition, in this specification, the "~" indicating a numerical range is used to mean including the values described before and after it as the lower limit value and the upper limit value without special notice.

[0108] [C: 0.050 to 0.100%]

[0109] C is an element effective for increasing the strength of the steel sheet. In order to fully obtain such an effect, the C content is set to 0.050% or more. The C content may also be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if C is contained excessively, the weldability may be reduced. Therefore, the C content is set to 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.

[0110] [Si: 0.01 to 0.30%]

[0111] Si is an element effective for increasing strength as a solid solution strengthening element. To fully obtain such an effect, the Si content is set to 0.01% or more. The Si content can also be 0.03% or more, 0.05% or more, 0.08% or more, 0.12% or more, or 0.15% or more. On the other hand, if Si is excessively contained, there may be surface quality defects called Si scale. Therefore, the Si content is set to 0.30% or less. The Si content can also be 0.28% or less, 0.25% or less, 0.22% or less, or 0.20% or less.

[0112] [Mn: 1.30 - 2.10%]

[0113] Mn is an element effective for increasing strength as a hardenability and solid solution strengthening element. To fully obtain these effects, the Mn content is set to 1.30% or more. The Mn content can also be 1.40% or more, 1.50% or more, 1.60% or more, or 1.70% or more. On the other hand, if Mn is excessively contained, a large amount of MnS may be generated, reducing toughness. Therefore, the Mn content is set to 2.10% or less. The Mn content can also be 2.00% or less, 1.90% or less, or 1.80% or less.

[0114] [Ti: 0.080 - 0.150%]

[0115] Ti is an element that helps to increase strength through precipitation strengthening by finely precipitating Ti carbides such as TiC in steel. To fully obtain such an effect, the Ti content is set to 0.080% or more. The Ti content can also be 0.090% or more, 0.095% or more, 0.100% or more, 0.105% or more, or 0.110% or more. On the other hand, if Ti is excessively contained, the precipitates may become coarse, and the strength increase effect brought about by precipitation strengthening cannot be fully exerted. Therefore, the Ti content is set to 0.150% or less. The Ti content can also be 0.140% or less, 0.135% or less, 0.130% or less, 0.125% or less, or 0.120% or less.

[0116] [Nb: 0.020 - 0.050%]

[0117] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, and contributes to the high strength of the steel sheet through grain refinement by the pinning effect and fine grain strengthening. To fully obtain such effects, the Nb content is set to 0.020% or more. The Nb content can also be 0.025% or more, 0.028% or more, 0.030% or more, or 0.032% or more. On the other hand, if Nb is excessively contained, it is possible to form coarse carbides, etc. in the steel, reducing the toughness of the steel sheet. Therefore, the Nb content is set to 0.050% or less. The Nb content can also be 0.045% or less, 0.042% or less, 0.040% or less, or 0.038% or less.

[0118] [Al: 0.001 - 0.050%]

[0119] Al is an element that acts as a deoxidizer. To fully obtain such effects, the Al content is set to 0.001% or more. The Al content can also be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if Al is excessively contained, it is possible to form coarse oxides, reducing the toughness. Therefore, the Al content is set to 0.050% or less. The Al content can also be 0.045% or less or 0.040% or less.

[0120] [P: 0.100% or less]

[0121] If P is excessively contained, it may have an adverse effect on weldability, etc. Therefore, the P content is set to 0.100% or less. The P content can also be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and can also be 0%, but excessive reduction leads to an increase in cost. Therefore, the P content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0122] [S: 0.050% or less]

[0123] If S is excessively contained, it is possible to generate a large amount of MnS, reducing the toughness. Therefore, the Si content is set to 0.050% or less. The S content can also be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly limited and can also be 0%, but excessive reduction leads to an increase in cost. Therefore, the S content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0124] [N: 0.0050% or less]

[0125] If the content of N is excessive, there is a possibility of forming coarse nitrides, reducing the toughness. In addition, N may combine with Ti in the steel to form titanium nitride (TiN), thereby reducing the effective amount of Ti available for forming precipitates such as Ti carbides and decreasing the strength improvement effect brought about by precipitation strengthening. Therefore, the lower the N content, the more preferable it is, and it is set to 0.0050% or less. The N content can also be 0.0045% or less, 0.0040% or less, or 0.0035% or less. There is no particular limitation on the lower limit of the N content, and it can also be 0%, but excessive reduction leads to an increase in cost. Therefore, the N content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0126] [O: 0.0050% or less]

[0127] If the content of O is excessive, there is a possibility of forming coarse inclusions, reducing the toughness. Therefore, the O content is set to 0.0050% or less. The O content can also be 0.0040% or less, 0.0035% or less, or 0.0030% or less. There is no particular limitation on the lower limit of the O content, and it can also be 0%, but in order to reduce the O content to less than 0.0001%, refining requires time, resulting in a decrease in productivity. Therefore, the O content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0128] The basic chemical composition of the hot-rolled coil according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled coil may also contain at least one of the following optional elements as needed to replace a part of the remaining Fe.

[0129] [B: 0 - 0.0050%]

[0130] B is an element that improves the hardenability of the steel and contributes to the strength improvement. The B content can also be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more. The B content can also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the content of B is excessive, there is a possibility of reducing the toughness and / or weldability. Therefore, the B content is preferably 0.0050% or less. The B content can also be 0.0030% or less, 0.0015% or less, 0.0012% or less, or 0.0008% or less.

[0131] [Cu: 0 - 0.20%]

[0132] Cu is an element that contributes to the improvement of strength and / or corrosion resistance. The Cu content can also be 0%, but in order to obtain these effects, the Cu content is preferably 0.01% or more. The Cu content can also be 0.03% or more or 0.05% or more. On the other hand, if Cu is contained in excess, it may lead to deterioration of toughness and weldability. Therefore, the Cu content is preferably 0.20% or less. The Cu content can also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[0133] [Ni: 0 to 0.20%]

[0134] Ni is an element that improves the hardenability of steel and contributes to the improvement of strength and / or corrosion resistance. The Ni content can also be 0%, but in order to obtain these effects, the Ni content is preferably 0.01% or more. The Ni content can also be 0.03% or more or 0.05% or more. On the other hand, even if Ni is contained in excess, the effect saturates, resulting in an increase in manufacturing cost. Therefore, the Ni content is preferably 0.20% or less. The Ni content can also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[0135] [Sn: 0 to 0.10%]

[0136] Sn is an element effective for improving corrosion resistance. The Sn content can also be 0%, but in order to obtain such an effect, the Sn content is preferably 0.01% or more. The Sn content can also be 0.02% or more. On the other hand, if Sn is contained in excess, it may lead to a decrease in toughness. Therefore, the Sn content is preferably 0.10% or less. The Sn content can also be 0.08% or less, 0.06% or less, or 0.04% or less.

[0137] [Cr: 0 to 0.40%]

[0138] Cr is an element that improves the hardenability of steel and contributes to the improvement of strength and / or corrosion resistance. The Cr content can also be 0%, but in order to obtain these effects, the Cr content is preferably 0.01% or more. The Cr content can also be 0.05% or more or 0.10% or more. On the other hand, even if Cr is contained in excess, the effect saturates, resulting in an increase in manufacturing cost. Therefore, the Cr content is preferably 0.40% or less. The Cr content can also be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.

[0139] [Mo: 0 to 0.200%]

[0140] Mo is an element that improves the hardenability of steel and contributes to the increase in strength, and also contributes to the improvement of corrosion resistance. The Mo content can also be 0%, but in order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content can also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if Mo is contained excessively, the deformation resistance during hot working may increase and the equipment load may become large. Therefore, the Mo content is preferably 0.200% or less. The Mo content can also be 0.180% or less, 0.150% or less, 0.120% or less, 0.100% or less, or 0.080% or less.

[0141] [V: 0 to 0.100%]

[0142] V is an element that contributes to the increase in strength through precipitation strengthening and the like. The V content can also be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more. The V content can also be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if V is contained excessively, a large amount of precipitates may be generated and the toughness may be reduced. Therefore, the V content is preferably 0.100% or less. The V content can also be 0.080% or less, 0.060% or less, or 0.040% or less.

[0143] [As: 0 to 0.100%]

[0144] As is an element effective for the improvement of corrosion resistance. The As content can also be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more. The As content can also be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, even if As is contained excessively, the effect saturates and the manufacturing cost increases. Therefore, the As content is preferably 0.100% or less. The As content can also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0145] [Zr: 0 to 0.100%]

[0146] Zr is an element that can control the morphology of sulfides. The Zr content can also be 0%, but in order to obtain such an effect, the Zr content is preferably 0.001% or more. The Zr content can also be 0.005% or more, or 0.010% or more. On the other hand, even if Zr is contained excessively, the effect saturates and the manufacturing cost increases. Therefore, the Zr content is preferably 0.100% or less. The Zr content can also be 0.050% or less, 0.030% or less, or 0.020% or less.

[0147] [Ca: 0 to 0.0050%]

[0148] Ca is an element capable of controlling the morphology of sulfides. The Ca content can also be 0%, but in order to obtain such an effect, the Ca content is preferably 0.0001% or more. The Ca content can also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if Ca is excessively contained, the effect saturates, resulting in an increase in manufacturing cost. Therefore, the Ca content is preferably 0.0050% or less. The Ca content can also be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0149] [Mg: 0 to 0.100%]

[0150] Mg is an element capable of controlling the morphology of sulfides. The Mg content can also be 0%, but in order to obtain such an effect, the Mg content is preferably 0.001% or more, and can also be 0.005% or more, or 0.008% or more. On the other hand, even if Mg is excessively contained, the effect saturates, resulting in an increase in manufacturing cost. Therefore, the Mg content is preferably 0.100% or less. The Mg content can also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0151] [Bi: 0 to 0.020%]

[0152] Bi is an element effective in improving corrosion resistance. The Bi content can also be 0%, but in order to obtain such an effect, the Bi content is preferably 0.001% or more. The Bi content can also be 0.002% or more, or 0.003% or more. On the other hand, even if Bi is excessively contained, the effect saturates, resulting in an increase in manufacturing cost. Therefore, the Bi content is preferably 0.020% or less. The Bi content can also be 0.010% or less, 0.008% or less, or 0.005% or less.

[0153] [Co: 0 to 0.20%]

[0154] Co is an element contributing to the improvement of hardenability and / or heat resistance. The Co content can also be 0%, but in order to obtain these effects, the Co content is preferably 0.01% or more. The Co content can also be 0.03% or more, or 0.05% or more. On the other hand, if Co is excessively contained, there is a possibility of a decrease in hot workability and an increase in raw material cost. Therefore, the Co content is preferably 0.20% or less. The Co content can also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[0155] [W: 0 to 0.20%]

[0156] W is an element that improves the hardenability of steel and contributes to an increase in strength. The content of W may also be 0%, but in order to obtain such an effect, the content of W is preferably 0.01% or more. The content of W may also be 0.03% or more or 0.05% or more. On the other hand, if W is contained in excess, the weldability may decrease. Therefore, the content of W is preferably 0.20% or less. The content of W may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[0157] [Zn: 0 to 0.20%]

[0158] Zn is an element effective for controlling the shape of inclusions. In order to obtain such an effect, the content of Zn is preferably 0.01% or more. The content of Zn may also be 0.03% or more or 0.05% or more. On the other hand, even if Zn is contained in excess, the effect saturates, resulting in an increase in manufacturing cost. Therefore, the content of Zn is preferably 0.20% or less. The content of Zn may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[0159] [REM: 0 to 0.1000%]

[0160] REM (rare earth metals) is an element that can control the morphology of sulfides. The content of REM may also be 0%, but in order to obtain such an effect, the content of REM is preferably 0.0001% or more. The content of REM may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if REM is contained in excess, the effect saturates, resulting in an increase in manufacturing cost. Therefore, the content of REM is preferably 0.1000% or less. The content of REM may also be 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. REM in this specification is a general term for 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 among the lanthanide elements, and the content of REM is the total content of these elements.

[0161] In the hot-rolled coil of the embodiment of the present invention, the remainder other than the above elements is composed of Fe and impurities. The so-called impurities refer to components such as those mixed in through various factors in the manufacturing process represented by raw materials such as ores and scraps during the industrial manufacture of hot-rolled coils.

[0162] The chemical composition of the hot-rolled coil according to the embodiment of the present invention can be determined by general analysis methods. For example, the chemical composition of the hot-rolled coil can be determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S can be determined by combustion-infrared absorption method, N can be determined by inert gas fusion-thermal conductivity method, and O can be determined by inert gas fusion-non-dispersive infrared absorption method.

[0163] [Effective Ti content]

[0164] In the hot-rolled coil according to the embodiment of the present invention, from the viewpoint of fully exerting the strength improvement effect brought about by precipitation strengthening, it is preferable to set the effective Ti content to 0.070% or more. The effective Ti content corresponds to the solid solution Ti content immediately before aging treatment, that is, the value obtained by subtracting the Ti fixed by N from the total Ti content in the hot-rolled coil. More specifically, since the solubility of TiN is very small, once precipitated TiN cannot be re-dissolved at the normal solution heat treatment temperature. Therefore, the effective Ti content effective for aging precipitation of precipitates such as Ti carbides is the value obtained by subtracting the amount that can be fixed as TiN from the total Ti content, and is calculated by the following formula.

[0165] Effective Ti content (%) = [Ti] - 48 / 14 [N]

[0166] Here, [Ti] and [N] are the contents (mass%) of the respective elements in the hot-rolled coil. By setting the effective Ti content to 0.070% or more, precipitates sufficient for obtaining the desired strength improvement effect brought about by precipitation strengthening can be generated. For example, the effective Ti content can also be 0.075% or more, 0.080% or more, 0.085% or more, or 0.090% or more. The upper limit is not particularly limited, but for example, the effective Ti content can also be 0.150% or less, 0.145% or less, 0.140% or less, or 0.135% or less.

[0167] [Metallographic structure]

[0168] [Average grain size of grains surrounded by grain boundaries with an orientation difference of 15° or more: 5.0 to 8.0 μm]

[0169] In the hot-rolled coil according to the embodiment of the present invention, in the central portion in the length direction, when a region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a grain, the average grain diameter of the grain is controlled within the range of 5.0 to 8.0 μm. By controlling the average grain diameter of the grain within the range of 5.0 to 8.0 μm by means of the pinning effect brought about by Nb-based carbides, nitrides, and / or carbonitrides, etc., the strength improvement effect brought about by fine grain strengthening can be fully exerted, and combined with the strength improvement effect brought about by precipitation strengthening based on precipitates such as Ti carbides described in detail below, it becomes possible to reliably achieve a high tensile strength, for example, a tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil. If the average grain diameter of the above-mentioned grain is less than 5.0 μm or exceeds 8.0 μm, such a strength improvement effect brought about by fine grain strengthening cannot be fully obtained, and the desired high strength cannot be achieved in the hot-rolled coil. The average grain diameter of the above-mentioned grain can also be, for example, 5.5 μm or more, 6.0 μm or more, or 6.5 μm or more. Similarly, the average grain diameter of the above-mentioned grain can also be, for example, 7.5 μm or less or 7.0 μm or less.

[0170] [Measurement of the average grain diameter of grains surrounded by grain boundaries with an orientation difference of 15° or more]

[0171] The average grain diameter of grains surrounded by grain boundaries with an orientation difference of 15° or more is measured by the electron backscattered diffraction method (Electron Back Scattered Diffraction, EBSD). More specifically, first, a specimen is collected from the central portion in the length direction and the central portion in the width direction of the hot-rolled coil in such a way that the plate thickness section in the direction parallel to the rolling direction and perpendicular to the plate surface becomes the observation surface. Next, at a depth position of 1 / 4 of the plate thickness from the specimen surface, EBSD analysis is performed on a region of 200 μm in the rolling direction of the specimen and 100 μm in the direction normal to the rolling plane at a measurement interval of 0.2 μm to obtain crystal orientation information. Here, the EBSD analysis is carried out using a device composed of a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) at an analysis speed of 50 to 300 points / second. Then, for the obtained crystal orientation information, a region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a grain, the equivalent circle diameter of the grain is analyzed, and their average value is obtained and determined as the average grain diameter. The grain diameter of the grain defined as above can be determined using the area average value calculated by Grain Size (diameter) in the functions provided in the software "OIM Analysis (registered trademark) Version 7.0.1" attached to the EBSD analysis device.

[0172] [Average particle size of the precipitate: 3.0 to 9.5 nm]

[0173] In the hot-rolled coil according to the embodiment of the present invention, in the central portion in the longitudinal direction, the average particle size of the precipitate is controlled within the range of 3.0 to 9.5 nm. By controlling the average particle size of the precipitate within the range of 3.0 to 9.5 nm, the strength improvement effect brought about by precipitation strengthening can be fully exerted, and combined with the strength improvement effect brought about by grain refinement described above, it becomes possible to reliably achieve a high tensile strength, for example, a tensile strength of 780 MPa or more, in the finally obtained hot-rolled coil. The precipitate may contain Ti carbide or be Ti carbide. As Ti carbide, there is no particular limitation, but for example, it may be TiC, or it may be a composite carbide containing Ti and other elements other than Ti such as Nb. If the particle size of the precipitate is small, the precipitate cannot sufficiently act as an obstacle to dislocation movement, and thus the strength improvement effect brought about by precipitation strengthening cannot be sufficiently obtained. On the other hand, even if the particle size of the precipitate is too large, it is equally possible that the desired precipitation strengthening cannot be obtained.

[0174] Although not intending to be bound by any particular theory, it is considered that this is because: as the precipitate becomes coarser, the strengthening mechanism changes in relation to dislocation movement. For example, instead of the dislocation line passing through the precipitate transversely, loops of dislocation lines remain around the coarser precipitate and pass through, resulting in a decrease in the precipitation strengthening amount. In addition, as the precipitate coarsens, the number density of the precipitate also greatly decreases, so the strength cannot be sufficiently increased by precipitation strengthening. Therefore, in order to effectively increase the strength of the hot-rolled coil by precipitation strengthening, it is important to control the average particle size of the precipitate within the range of 3.0 to 9.5 nm. The average particle size of the precipitate may also be, for example, 4.0 nm or more, 5.0 nm or more, or 6.0 nm or more. Similarly, the average particle size of the above-mentioned grains may also be, for example, 9.0 nm or less, 8.0 nm or less, or 7.0 nm or less.

[0175] [The difference between the maximum and minimum values of the particle size of the precipitate in the width direction is 15.0% or less of the average particle size of the precipitate]

[0176] In the hot-rolled coil according to the embodiment of the present invention, the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction of the central portion in the length direction is controlled to be 15.0% or less of the average particle diameter of the precipitates. By controlling the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction of the central portion in the length direction to be 15.0% or less of the average particle diameter of the precipitates, it is possible to reduce the strength unevenness in the width direction of the hot-rolled coil. As described above, it is considered that the precipitation states of the precipitates in the length direction and the width direction of the hot-rolled coil are closely related to each other. Therefore, for example, even if only the central portion in the length direction of the hot-rolled coil is appropriately cooled, without appropriately cooling the front end portion and / or the tail end portion, they will be affected, and it will also become impossible to reliably obtain the desired precipitation state of the precipitates in the central portion. Similarly, for example, even if the front end portion, the central portion, and the tail end portion in the length direction of the hot-rolled coil are appropriately cooled, without uniformly cooling in the width direction, due to the temperature deviation in the width direction, the precipitation state of the precipitates in the length direction will also be affected. Therefore, as will be described in detail below for the manufacturing method of the hot-rolled coil, it is necessary to appropriately cool in the length direction and the width direction, whereby the unevenness of the particle diameter of the precipitates in the width direction of the central portion in the length direction of the hot-rolled coil can be controlled within a specified range, and more specifically, the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction of the central portion in the length direction of the hot-rolled coil can be controlled to be 15.0% or less of the average particle diameter of the precipitates. As a result, it becomes possible to significantly suppress or reduce the strength unevenness in the width direction of the hot-rolled coil.

[0177] From the viewpoint of suppressing or reducing the strength unevenness, in the central portion in the length direction of the hot-rolled coil, the smaller the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction with respect to the average particle diameter of the precipitates, the more preferable. Therefore, in the central portion in the length direction of the hot-rolled coil, the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction is preferably 12.0% or less, 10.0% or less, or 8.0% or less of the average particle diameter of the precipitates. On the other hand, the lower limit is not particularly limited, but for example, in the central portion in the length direction of the hot-rolled coil, the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction may also be 2.0% or more, 3.0% or more, or 5.0% or more of the average particle diameter of the precipitates.

[0178] [Determination of the average particle diameter of the precipitates in the central portion in the length direction, and the ratio of the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction to the average particle diameter]

[0179] The average particle size of precipitates in the central part in the length direction of the hot-rolled coil is determined by the following operations. First, when the full width of the hot-rolled coil is set to W, samples are collected by the replication method at positions of 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W from the end in the width direction at the central part in the length direction of the hot-rolled coil. Next, 50 to 100 precipitates are observed for the collected samples using a transmission electron microscope (TEM, for example, "JEM-2100" manufactured by JEOL can be used). The particle size of each precipitate is calculated as the equivalent circle diameter, and the average value of all the calculated equivalent circle diameters is determined as the particle size of the precipitates at each width direction position. The arithmetic average of the 5 obtained particle sizes is determined as the average particle size of the precipitates in the central part in the length direction. Finally, the difference between the maximum value and the minimum value among the 5 obtained particle sizes is calculated, and by dividing the calculated value by the average particle size of the precipitates, the ratio of the difference between the maximum value and the minimum value of the particle size of the precipitates in the width direction to the average particle size is determined. The constituent elements of the precipitates can be identified by EDS analysis.

[0180] [The difference between the maximum value and the minimum value of the particle size of the precipitates in the length direction is 15.0% or less of the average particle size of the precipitates]

[0181] In the hot-rolled coil of the embodiment of the present invention, the difference between the maximum value and the minimum value of the particle size of the precipitates in the length direction in the central part in the width direction is controlled to be 15.0% or less of the average particle size of the precipitates in the length direction. By controlling the difference between the maximum value and the minimum value of the particle size of the precipitates in the length direction in the central part in the width direction to be 15.0% or less of the average particle size of the precipitates in the length direction, the strength unevenness in the length direction of the hot-rolled coil can be reduced. In association therewith, as will be described in detail below for the manufacturing method of the hot-rolled coil, it is particularly important to appropriately cool the front end part, central part, and tail end part in the length direction of the hot-rolled coil after coiling, whereby the difference between the maximum value and the minimum value of the particle size of the precipitates in the length direction in the central part in the width direction of the hot-rolled coil can be controlled to be 15.0% or less of the average particle size of the precipitates in the length direction.

[0182] From the viewpoint of suppressing or reducing non-uniformity in strength, in the central portion in the width direction of the hot-rolled coil, it is more preferable that the difference between the maximum value and the minimum value of the particle diameters of the precipitates in the length direction is smaller relative to the average particle diameter of the precipitates in the length direction. Therefore, in the central portion in the width direction of the hot-rolled coil, the difference between the maximum value and the minimum value of the particle diameters of the precipitates in the length direction is preferably 12.0% or less, 10.0% or less, or 8.0% or less of the average particle diameter of the precipitates in the length direction. On the other hand, the lower limit is not particularly limited, but for example, in the central portion in the width direction of the hot-rolled coil, the difference between the maximum value and the minimum value of the particle diameters of the precipitates in the length direction may also be 1.0% or more, 2.0% or more, or 3.0% or more of the average particle diameter of the precipitates in the length direction.

[0183] [Determination of the average particle diameter of the precipitates in the length direction in the central portion in the width direction, and the ratio of the difference between the maximum value and the minimum value of the particle diameters of the precipitates in the length direction to the average particle diameter]

[0184] The average particle diameter of the precipitates in the length direction in the central portion in the width direction of the hot-rolled coil and the like are determined by the following operations. First, when the total length of the hot-rolled coil is set to L, at each of the positions at 1 / 10L, 5 / 10L, and 9 / 10L positions from the end in the length direction at the central portion in the width direction of the hot-rolled coil, samples are collected by the replica method. Then, 50 to 100 precipitates are observed for the collected samples using a transmission electron microscope (TEM), the particle diameter of each precipitate is calculated as the equivalent circle diameter, the average value of all the calculated equivalent circle diameters is determined as the particle diameter of the precipitates at each length direction position, and the arithmetic average of the three obtained particle diameters is determined as the average particle diameter of the precipitates in the length direction in the central portion in the width direction. Finally, the difference between the maximum value and the minimum value among the three obtained particle diameters is calculated, and by dividing the calculated value by the average particle diameter of the precipitates in the length direction, the ratio of the difference between the maximum value and the minimum value of the particle diameters of the precipitates in the length direction to the average particle diameter of the precipitates in the length direction is determined. The constituent elements of the precipitates can be identified by EDS analysis.

[0185] [Ferrite: 50% or more]

[0186] The metallographic structure of the hot-rolled coil according to the embodiment of the present invention is not particularly limited. For example, it may contain 50% or more of ferrite by area%. As described above, the object of the present invention is to provide a hot-rolled coil with high strength and reduced strength non-uniformity. By having a specified chemical composition, while utilizing grain refinement strengthening and precipitation strengthening, and appropriately controlling the particle size of precipitates contributing to this precipitation strengthening in the width direction at the central portion in the length direction of the hot-rolled coil and in the length direction at the central portion in the width direction, this object is achieved. Therefore, it is obvious that other features of the metallographic structure are not essential technical features in terms of achieving the object of the present invention. In fact, in the hot-rolled coil according to the embodiment of the present invention, even when the metallographic structure is composed of a soft ferrite single-phase structure, by satisfying the necessary conditions regarding the chemical composition, grain size, and precipitates described above, it is possible to reliably achieve a tensile strength of 780 MPa or more, for example. The area ratio of ferrite may also be 55% or more, 60% or more, 70% or more, 80% or more, or 90% or more, for example. The upper limit is not particularly limited. For example, the area ratio of ferrite may be 100%, or may be 95% or less. In the case where there is a remaining structure other than ferrite, the remaining structure basically becomes a structure harder than ferrite, such as martensite, bainite, pearlite, and retained austenite. Therefore, it is obvious that the specific structure of the remaining structure is not limited from the viewpoint of providing a hot-rolled coil with high strength.

[0187] The area ratio of ferrite is determined by the following operation. First, a specimen having a plate thickness cross-section in a direction parallel to the rolling direction of the hot-rolled coil and perpendicular to the plate surface is collected, and this cross-section is used as the observation surface. Next, in the electron channel contrast image obtained by using FE-SEM (field emission scanning electron microscope), it is obtained by observing a 100 μm × 100 μm area in the range of 1 / 8 to 3 / 8 of the plate thickness centered on the 1 / 4 position of the plate thickness in this observation surface. More specifically, in the above area, the portion photographed with uniform contrast is determined as ferrite, and its area ratio can be calculated by image analysis using the image analysis software Image J.

[0188] [Tensile strength]

[0189] In the hot-rolled coil according to the embodiment of the present invention, by having the chemical composition and metal structure described above, a high tensile strength can be achieved, for example, a tensile strength of 780 MPa or more. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. In the hot-rolled coil according to the embodiment of the present invention, although having such a very high tensile strength, by appropriately controlling the particle size of the precipitates in the width direction at the center in the length direction and the length direction at the center in the width direction of the hot-rolled coil, the strength unevenness in the length direction and width direction of the hot-rolled coil can also be significantly suppressed or reduced. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled coil can also be 980 MPa or less, 950 MPa or less, or 900 MPa or less. The tensile strength is determined by the following operation. First, a No. 5 tensile test piece of JIS Z2241:2011 with the test direction parallel to the rolling direction is collected from the center in the length direction and the center in the width direction of the hot-rolled coil. Then, by using this tensile test piece and performing a tensile test in accordance with JIS Z 2241:2011, the tensile strength of the hot-rolled coil according to the embodiment of the present invention is determined.

[0190] [Full width W]

[0191] The hot-rolled coil according to the embodiment of the present invention can have any full width W. Although not particularly limited, for example, the full width W can also be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more. The upper limit is not particularly limited, but for example, the full width can also be 2500 mm or less, 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.

[0192] [Thickness]

[0193] The hot-rolled coil according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 6.0 mm. For example, the thickness can also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or can also be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.

[0194] <Manufacturing method of hot-rolled coil>

[0195] Next, a preferred manufacturing method of the hot-rolled coil according to the embodiment of the present invention will be described. The following description aims to illustrate the characteristic method for manufacturing the hot-rolled coil according to the embodiment of the present invention, and does not intend to limit the hot-rolled coil to be manufactured by the manufacturing method as described below.

[0196] The manufacturing method of a hot-rolled coil according to an embodiment of the present invention is characterized by including the following steps:

[0197] A hot-rolling step, which includes heating a slab having the chemical composition described above in association with the hot-rolled coil to a temperature of 1230 to 1260 °C for rough rolling and finish rolling. The outlet temperature of the rough rolling is 1070 to 1140 °C, the inlet temperature (F0) of the finish rolling is 980 to 1050 °C, the outlet temperature (FT) of the finish rolling is 850 to 920 °C, and the total reduction ratio of the finish rolling is 85 to 95%;

[0198] A cooling step, which cools the steel plate after finish rolling at an average cooling rate of 60 to 100 °C / s in a temperature region from the outlet temperature (FT) of the finish rolling to a temperature T1 in the range of 650 to 720 °C for the first cooling, and then cools it at an average cooling rate of 5 to 10 °C / s in a temperature region from the temperature T1 to the coiling temperature CTf at the position of 1 / 10L of the total length L of the coil in the length direction for the second cooling. The upper-lower cooling ratio of the upper surface of the steel plate to the lower surface in the first cooling is 0.8 to 1.2, and the upper-lower cooling ratio of the upper surface of the steel plate to the lower surface in the second cooling is 0.8 to 1.2; and

[0199] A coiling step, which coils the steel plate after the second cooling, and then performs heat preservation treatment on the edge portions in the width direction of the coiled steel plate. The coiling temperatures CTf (°C), CTm (°C), and CTt (°C) at the positions of 1 / 10L, 5 / 10L, and 9 / 10L of the total length L of the coil in the length direction satisfy the following formulas 1 to 3.

[0200] 550 ≤ CTm ≤ 620 Formula 1

[0201] CTm + 15 ≤ CTf ≤ CTm + 30 Formula 2

[0202] CTm + 30 ≤ CTt ≤ CTm + 50 Formula 3

[0203] Hereinafter, each step will be described in detail.

[0204] [Hot-rolling step]

[0205] [Heating of slab]

[0206] First, a slab having the chemical composition described above associated with the hot-rolled coil is heated. The slab used is preferably cast by the continuous casting method from the viewpoint of productivity, but can also be manufactured by the ingot casting method or the thin slab casting method. The slab used contains relatively more alloying elements, especially Ti. Therefore, it is necessary to dissolve the alloying elements in the slab, and in particular, it is necessary to fully dissolve Ti. If Ti is not fully dissolved during slab heating, it becomes difficult to finely precipitate Ti as carbides (TiC) etc. in the steel during the coiling process to increase the strength of the steel by precipitation strengthening. Therefore, in order to fully dissolve Ti, the heating temperature of the slab needs to be set at 1230 °C or higher. On the other hand, if the heating temperature of the slab exceeds 1260 °C, the yield is reduced due to scale peeling. Therefore, the heating temperature of the slab is set at 1260 °C or lower.

[0207] [Rough rolling]

[0208] In this method, for the heated slab, rough rolling is performed before finish rolling for slab thickness adjustment etc. Regarding rough rolling, in order to ensure the desired thin slab size and adjust the total reduction ratio in the temperature range of 850 °C or higher during finish rolling to the desired range, the exit side temperature of rough rolling is set at 1070 - 1140 °C, preferably 1100 - 1140 °C. If the exit side temperature of rough rolling is lower than 1070 °C, it becomes difficult to obtain an exit side temperature of 850 °C or higher during finish rolling following rough rolling. In addition, if the exit side temperature of rough rolling exceeds 1140 °C, there is a possibility of grain coarsening and the toughness of the obtained hot-rolled coil is reduced.

[0209] [Finish rolling]

[0210] The slab after rough rolling is then subjected to finish rolling. As described above, since the slab used contains relatively many alloy elements, it is necessary to increase the rolling load during hot rolling. Therefore, hot rolling is carried out at high temperature and high pressure. Specifically, the entry side temperature (F0) of finish rolling is set to 980 - 1050 °C, the exit side temperature (FT) of finish rolling is set to 850 - 920 °C, and the total reduction ratio of finish rolling is set to 85 - 95%. In particular, the exit side temperature (FT) of finish rolling is important in terms of controlling the microstructure of the steel plate. More specifically, if the exit side temperature (FT) of finish rolling is low, the microstructure may become non-uniform, the formability may decrease, and / or the average grain diameter of the grains surrounded by grain boundaries with an orientation difference of 15° or more in the central part in the length direction may become less than 5.0 μm, resulting in a decrease in strength. Therefore, the exit side temperature (FT) of finish rolling is set to 850 °C or higher. On the other hand, if the exit side temperature (FT) of finish rolling exceeds 920 °C, the austenite grains coarsen, and it becomes impossible to control the average grain diameter of the grains obtained through subsequent cooling, more specifically, the grains surrounded by grain boundaries with an orientation difference of 15° or more, to 8.0 μm or less.

[0211] [Cooling process]

[0212] [Primary cooling]

[0213] In the subsequent cooling process of the steel plate after finish rolling, first, primary cooling is carried out at an average cooling rate of 60 - 100 °C / s in the temperature range from the exit side temperature (FT) of finish rolling to a temperature T1 in the range of 650 - 720 °C. By carrying out primary cooling at an average cooling rate of 60 - 100 °C / s in this temperature range, the coarsening of grains can be suppressed, and the finally obtained microstructure can be made uniform in the width direction. Associated therewith, it becomes possible to significantly suppress the non-uniformity of the particle size in the width direction of precipitates such as TiC precipitated in the subsequent coiling process. If the average cooling rate of primary cooling is less than 60 °C / s, it may not be possible to control the average grain diameter of the grains surrounded by grain boundaries with an orientation difference of 15° or more within the desired range. On the other hand, if the average cooling rate of primary cooling exceeds 100 °C / s, due to the fast cooling rate, it becomes difficult to cool the steel plate uniformly in the width direction, resulting in cooling non-uniformity (temperature deviation) in the width direction. In this case, in the finally obtained hot rolled coil, it is not possible to sufficiently suppress the non-uniformity of the particle size of precipitates such as TiC in the width direction. More specifically, it becomes impossible to control the difference between the maximum value and the minimum value of the particle size of precipitates such as TiC in the width direction to 15.0% or less of the average particle size of the precipitates. Therefore, the average cooling rate of primary cooling is set to 60 - 100 °C / s, preferably 65 - 85 °C / s.

[0214] In the first cooling, in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces. Such cooling is carried out in such a way that the up-down cooling ratio of the upper surface of the steel plate relative to the lower surface is 0.8 to 1.2, and more specifically, in such a way that the amount of cooling water sprayed onto the upper surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed onto the lower surface of the steel plate. By cooling the upper and lower surfaces of the steel plate evenly in this way, the generation of cooling unevenness can be significantly suppressed or reduced. Associated therewith, the unevenness in the particle size in the width direction of precipitates such as TiC precipitated in the subsequent coiling process can be significantly suppressed, and as a result, the strength unevenness in the length direction and width direction of the finally obtained hot-rolled coil can be significantly suppressed or reduced. If the up-down cooling ratio is less than 0.8 or exceeds 1.2, due to the generation of cooling unevenness, i.e., temperature deviation, in the width direction, it becomes difficult to perform uniform cooling in the width direction even in the subsequent coiling process. In this case, in the finally obtained hot-rolled coil, the unevenness in the particle size of precipitates such as TiC in the width direction cannot be sufficiently suppressed, that is, the difference between the maximum value and the minimum value of the particle size of precipitates such as TiC in the width direction cannot be controlled to be 15.0% or less of the average particle size of the precipitates. As a result, the strength unevenness in the length direction and / or width direction of the hot-rolled coil cannot be sufficiently suppressed or reduced.

[0215] Here, the above-mentioned up-down cooling ratio does not refer to the ratio of the total amount of cooling water on the entire upper surface to the total amount of cooling water on the entire lower surface in the temperature range of FT to T1 °C. More specifically, in this manufacturing method, the temperature range of FT to T1 °C is divided into sections of every 10 m, and the up-down cooling ratio is calculated for each of these sections from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface. The up-down cooling ratio of each section calculated in this way is all controlled within the range of 0.8 to 1.2. In the control of the up-down cooling ratio of the entire interval rather than each divided section, it is very difficult to sufficiently suppress the generation of cooling unevenness caused by local overcooling, etc., for example. However, by achieving the control of the up-down cooling ratio of each section in this way, local overcooling, etc., can be reduced and the generation of cooling unevenness can be reliably suppressed. In addition, the control of the up-down cooling ratio of each section can be carried out by any suitable means. Although not particularly limited, for example, in each section, a plurality of cooling water nozzles are arranged on the upper and lower sides of the steel plate along the advancing direction of the steel plate. Therefore, by appropriately spraying these cooling water nozzles based on on-off control, it is possible to relatively easily control the up-down cooling ratio of each section within the range of 0.8 to 1.2.

[0216] [Second cooling]

[0217] The steel sheet after the first cooling is then subjected to a second cooling at an average cooling rate of 5 to 10 °C / s in the temperature range from temperature T1 to the coiling temperature CTf at a position 1 / 10L from the front end in the full length L of the coil in the length direction. By performing the second cooling at an average cooling rate of 5 to 10 °C / s in the temperature range from temperature T1 to the coiling temperature CTf, it is possible to suitably generate grains surrounded by grain boundaries with an orientation difference of 15° or more and control their average grain diameter within a desired range. If the average cooling rate of the second cooling is less than 5 °C / s, the grains surrounded by grain boundaries with an orientation difference of 15° or more coarsen, and it becomes impossible to control their average grain diameter to 8.0 μm or less. In this case, the strength improvement effect brought about by grain refinement cannot be obtained sufficiently, so that it becomes impossible to reliably achieve the desired high strength in the finally obtained hot-rolled coil. On the other hand, if the average cooling rate of the second cooling exceeds 10 °C / s, it may not be possible to suitably generate grains surrounded by grain boundaries with an orientation difference of 15° or more, or it may not be possible to control the average grain diameter of these grains to 5.0 μm or more. In this case as well, the strength improvement effect brought about by grain refinement cannot be obtained sufficiently. In addition, if the average cooling rate of the second cooling exceeds 10 °C / s, the occurrence of strength unevenness in the length direction and / or width direction of the hot-rolled coil may become significant due to the excessive generation of hard structures.

[0218] It should be noted that in this specification, the position of X / 10L (X is a natural number from 1 to 9) in the full length L of the hot-rolled coil in the length direction means a position that is only at a distance of "X / 10L" from the front end in the length direction (rolling direction) towards the tail end in the hot-rolled coil. For example, when the full length L in the length direction of the hot-rolled coil is 1000 m, the "1 / 10L position" means a position that is only at a distance of "100 m" from the front end in the length direction towards the tail end.

[0219] In the secondary cooling, as in the case of the primary cooling, in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces. Such cooling, as in the case of the primary cooling, is carried out in such a way that the upper-lower cooling ratio of the upper surface of the steel plate relative to the lower surface is 0.8 to 1.2. More specifically, it is carried out in such a way that the amount of cooling water sprayed onto the upper surface of the steel plate is 0.8 to 1.2 times the amount of cooling water sprayed onto the lower surface of the steel plate. By cooling the upper and lower surfaces of the steel plate evenly in this way, the generation of cooling non-uniformity can be significantly suppressed or reduced. Associated therewith, the non-uniformity of the particle size in the width direction of precipitates such as TiC precipitated in the subsequent coiling process can be significantly suppressed. As a result, the strength non-uniformity in the length and width directions of the finally obtained hot-rolled coil can be significantly suppressed or reduced. If the upper-lower cooling ratio is less than 0.8 or exceeds 1.2, due to the generation of cooling non-uniformity, i.e., temperature deviation, in the width direction, it becomes difficult to carry out uniform cooling in the width direction even in the subsequent coiling process. In this case, in the finally obtained hot-rolled coil, the non-uniformity of the particle size of precipitates such as TiC in the width direction cannot be sufficiently suppressed, that is, the difference between the maximum value and the minimum value of the particle size of precipitates such as TiC in the width direction cannot be controlled to be 15.0% or less of the average particle size of the precipitates. As a result, the strength non-uniformity in the length direction and / or width direction of the hot-rolled coil cannot be sufficiently suppressed or reduced.

[0220] Here, the above-mentioned upper-lower cooling ratio does not refer to the ratio of the total amount of cooling water on the entire upper surface to the total amount of cooling water on the entire lower surface in the temperature range of T1 to CTf °C. More specifically, in this manufacturing method, the temperature range of T1 to CTf °C is divided into sections of every 10 m, and the upper-lower cooling ratio is calculated for each of these sections from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface. The upper-lower cooling ratio of each section calculated in this way is all controlled within the range of 0.8 to 1.2. In the control of the upper-lower cooling ratio of the entire interval rather than each divided section, for example, it is very difficult to sufficiently suppress the generation of cooling non-uniformity caused by local overcooling, etc. However, by achieving the control of the upper-lower cooling ratio of each section in this way, local overcooling, etc. can be reduced and the generation of cooling non-uniformity can be reliably suppressed. In addition, the control of the upper-lower cooling ratio of each section can be carried out by any suitable means. Although not particularly limited, as in the case of the primary cooling, for example, in each section, a plurality of cooling water nozzles are arranged on the upper and lower sides of the steel plate along the advancing direction of the steel plate. Therefore, by appropriately spraying these cooling water nozzles based on on-off control, the upper-lower cooling ratio of each section can be relatively easily controlled within the range of 0.8 to 1.2.

[0221] [Coiling Process]

[0222] The steel sheet after the secondary cooling is finally coiled in the coiling process, and then stress relief heat treatment is performed on the edge portions in the width direction of the coiled steel sheet. In addition, in the coiling process, the coiling temperatures CTf (°C), CTm (°C), and CTt (°C) at the 1 / 10L position, 5 / 10L position, and 9 / 10L positions respectively with respect to the total length L of the coil in the length direction need to satisfy the following formulas 1 to 3.

[0223] 550 ≤ CTm ≤ 620 Formula 1

[0224] CTm + 15 ≤ CTf ≤ CTm + 30 Formula 2

[0225] CTm + 30 ≤ CTt ≤ CTm + 50 Formula 3

[0226] The precipitation state of the precipitates is greatly affected by the cooling history after coiling. For example, the front end and the tail end in the length direction of the hot-rolled coil correspond to the innermost periphery and the outermost periphery of the hot-rolled coil respectively, and thus are exposed to the atmosphere. Therefore, generally, the front end and the tail end of the hot-rolled coil cool quickly, and it is possible that the precipitation of the precipitates may not proceed sufficiently. Therefore, as the precipitation state, it becomes a sub-aged state, and there is a tendency for the strength to be easily reduced. In this case, it is possible that the desired strength cannot be obtained, and strength unevenness occurs due to the different precipitation states of the precipitates in the length direction. On the other hand, since the central portion in the length direction of the hot-rolled coil is not directly exposed to the atmosphere, it is not easily cooled and is maintained at a relatively high temperature. Therefore, depending on the situation, the precipitates coarsen, exceeding the peak aging where a high precipitation strengthening effect can be obtained and becoming an over-aged state, and similarly, the strength may decrease. Also in this case, it is possible that the desired strength cannot be obtained and strength unevenness occurs. In addition, when the cooling in the width direction after coiling is not uniform, due to the temperature deviation in the width direction, the precipitation state of the precipitates in the length direction is also affected. As a result, similarly, it is possible that the desired strength cannot be obtained in the finally obtained hot-rolled coil and the strength unevenness becomes significant.

[0227] Therefore, in this manufacturing method, first, by performing heat preservation treatment on the edge portions in the width direction of the coiled steel plate directly exposed to the atmosphere, it is possible to reduce the temperature deviation in the width direction of the hot-rolled coil after coiling. In the case where heat preservation treatment is not performed on the edge portions, due to the temperature deviation in the width direction, the precipitation states of the precipitates in the length direction and the width direction are also affected. As a result, it may not be possible to control the average particle size of the precipitates in the central portion in the length direction and the non-uniformity of the particle sizes of the precipitates in the central portion in the length direction within the desired range. Such heat preservation treatment of the edge portions can be carried out by any suitable means known to those skilled in the art. Although not particularly limited, for example, the heat preservation treatment of the edge portions can be carried out by arranging a plurality of hot-rolled coils adjacent to each other to prevent the cooling of the edge portions caused by the atmosphere. Here, "arranging a plurality of hot-rolled coils adjacent to each other" includes arranging them in such a way that the end faces (edge portions) in the width direction of the hot-rolled coils face each other. In addition, when the diameters of the plurality of hot-rolled coils are the same, it is preferably arranged in such a way that the central axes of the respective hot-rolled coils overlap, that is, arranged coaxially. In addition, when arranging a plurality of hot-rolled coils adjacent to each other, the distance between the above-mentioned end faces (edge portions) is preferably 200 to 800 mm, more preferably 200 to 600 mm, and further preferably 200 to 500 mm.

[0228] In addition, in this manufacturing method, appropriate cooling is also performed in the length direction of the hot-rolled coil. Specifically, in order to make the precipitation state of the precipitates in the central portion in the length direction of the hot-rolled coil appropriate, the coiling temperature CTm (°C) at the position of 5 / 10L with respect to the total length L of the coil in the length direction is controlled to satisfy the following formula 1.

[0229] 550 ≤ CTm ≤ 620 Formula 1

[0230] If CTm is lower than 550 °C, the precipitation state of the precipitates in the central portion becomes a sub-aged state. On the other hand, if CTm exceeds 620 °C, the precipitation state of the precipitates in the central portion becomes an over-aged state. In either case, it may not be possible to sufficiently obtain the strength improvement effect brought about by precipitation strengthening and / or may not be able to sufficiently suppress or reduce the strength non-uniformity in the length direction and the width direction of the hot-rolled coil. In addition, even when CTm satisfies Formula 1, if CTf does not satisfy Formula 2 and / or CTt does not satisfy Formula 3, the precipitation state of the precipitates in the central portion in the length direction is also greatly affected. In this case, it may not be possible to control the average particle size of the precipitates in the central portion in the length direction and the non-uniformity of the particle sizes of the precipitates in the width direction in the central portion in the length direction within the desired range.

[0231] Since the front end portion in the length direction of the hot-rolled coil corresponds to the innermost peripheral portion of the hot-rolled coil as described above, it is exposed to the atmosphere and is relatively easily cooled. Thus, the coiling temperature CTf (°C) at the position of 1 / 10L of the total coil length L in the length direction is controlled to be in the temperature range 15 to 30 °C higher than CTm in a manner that satisfies the following formula 2.

[0232] CTm + 15 ≤ CTf ≤ CTm + 30 Formula 2

[0233] Considering the cooling caused by the atmosphere, by controlling CTf to be in the temperature range 15 to 30 °C higher than CTm as shown in Formula 2, it becomes possible to maintain the precipitation state of the precipitates at the front end portion to be the same as that at the central portion. If CTf is lower than CTm + 15 °C, the precipitation state of the precipitates at the front end portion becomes a sub-aged state. On the other hand, if CTf exceeds CTm + 30 °C, it is possible that the precipitation state of the precipitates at the front end portion becomes an over-aged state. In either case, it may not be possible to sufficiently obtain the strength improvement effect brought about by precipitation strengthening and / or may not be possible to sufficiently suppress or reduce the strength unevenness in the length direction and width direction of the hot-rolled coil.

[0234] Since the trailing end portion in the length direction of the hot-rolled coil corresponds to the outermost peripheral portion of the hot-rolled coil as described above, it is more easily cooled than the front end portion corresponding to the innermost peripheral portion. Thus, the coiling temperature CTt (°C) at the position of 9 / 10L of the total coil length L in the length direction is controlled to be in the temperature range 30 to 50 °C higher than CTm in a manner that satisfies the following formula 3.

[0235] CTm + 30 ≤ CTt ≤ CTm + 50 Formula 3

[0236] Considering the cooling caused by the atmosphere, by controlling CTt to be in the temperature range 30 to 50 °C higher than CTm as shown in Formula 3, it becomes possible to maintain the precipitation state of the precipitates at the trailing end portion to be the same as that at the central portion. If CTt is lower than CTm + 30 °C, the precipitation state of the precipitates at the trailing end portion becomes a sub-aged state. On the other hand, if CTt exceeds CTm + 50 °C, it is possible that the precipitation state of the precipitates at the trailing end portion becomes an over-aged state. In either case, it may not be possible to sufficiently obtain the strength improvement effect brought about by precipitation strengthening and / or may not be possible to sufficiently suppress or reduce the strength unevenness in the length direction and width direction of the hot-rolled coil.

[0237] The control of the coiling temperature based on the above formulas (1) to (3) can be implemented by any suitable means without particular limitation. For example, it can be relatively easily implemented by appropriately controlling the amount of cooling water in the above secondary cooling. Conventionally, although the steel plate after hot rolling has generally been coiled within a specified temperature range, an operation such as changing the control range of the coiling temperature at the front end, central part, and tail end in the length direction of the hot-rolled coil has not been performed. Therefore, the inventors of the present invention have newly found for the first time that by making the cooling histories at the front end, central part, and tail end in the length direction of the hot-rolled coil appropriate and then performing heat retention treatment in the width direction of the hot-rolled coil, the strength unevenness due to precipitation strengthening can be significantly suppressed or reduced.

[0238] For the hot-rolled coil manufactured by the above manufacturing method, in the central part in the length direction, based on the combination of grain refinement strengthening brought about by controlling the average grain diameter of the grains surrounded by grain boundaries with an orientation difference of 15° or more to be within the range of 5.0 to 8.0 μm and precipitation strengthening brought about by controlling the average grain diameter of the precipitates to be within the range of 3.0 to 9.5 nm, it becomes possible to reliably achieve a high tensile strength, for example, a tensile strength of 780 MPa or more in the finally obtained hot-rolled coil. In addition, the difference between the maximum value and the minimum value of the grain diameter of the precipitates in the width direction of the central part in the length direction of the hot-rolled coil and in the length direction of the central part in the width direction can be controlled to be 15.0% or less of the average grain diameter of the precipitates. As a result, the strength unevenness in the length direction and width direction of the hot-rolled coil can be significantly suppressed or reduced. Therefore, although the hot-rolled coil manufactured by the above manufacturing method has a high strength, the strength unevenness can be significantly suppressed or reduced. Therefore, the risk of forming defects during the pressing process of the steel plate can be reduced, and the productivity can also be significantly improved. Therefore, this hot-rolled coil is of course particularly useful in applications in the automotive field and can also be very effectively used in other fields.

[0239] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited by any of these examples.

[0240] Examples

[0241] First, the molten steel is cast by continuous casting to form slabs with various chemical compositions shown in Table 1. These slabs are heated under the conditions shown in Table 2 and then hot-rolled. The hot rolling is carried out by rough rolling and finish rolling. The exit side temperature of the rough rolling, the entry side temperature (F0), the exit side temperature (FT), and the total reduction ratio of the finish rolling are as shown in Table 2. Next, the hot-rolled steel sheet after finish rolling is cooled once under the conditions shown in Table 2 in the temperature range from the exit side temperature (FT) of the finish rolling to a temperature T1 in the range of 650 to 720 °C, and then cooled twice in the temperature range from the temperature T1 to the coiling temperature CTf at the position of 1 / 10L of the total length L of the coil in the longitudinal direction.

[0242] In the first cooling and the second cooling, the intervals of FT to T1 °C and T1 to CTf °C are each divided into sections of every 10 m. For each of these sections, the upper and lower cooling ratios are calculated from the cooling water amounts on the upper surface and the lower surface, and the cooling is carried out in such a way that the upper and lower cooling ratios of each section calculated in this way are controlled within a specified range. The upper and lower cooling ratios in the first cooling and the second cooling in Table 2 represent the maximum absolute value of the difference from the cooling ratio 1 among the upper and lower cooling ratios of each section in the first cooling and the second cooling. Finally, the steel sheet after the second cooling is coiled, and the coiling temperatures CTf (°C), CTm (°C), and CTt (°C) at the positions of 1 / 10L, 5 / 10L, and 9 / 10L of the total length L of the coil in the longitudinal direction are as shown in Table 2. In addition, the heat preservation treatment of the edge part after coiling is carried out by arranging a plurality of hot-rolled coils adjacent to each other. The distance between the edge parts of the adjacent hot-rolled coils arranged adjacent to each other is set to 300 mm. The expression "adjacent" in Table 2 means that the heat preservation treatment is carried out after coiling. On the other hand, the expression "separate" in Table 2 means that the hot-rolled coil is air-cooled alone, that is, it means that the heat preservation treatment is not carried out on the edge part after coiling. The obtained hot-rolled coil has a plate thickness of about 2.3 to 4.0 mm, a total width W of about 800 to 1500 mm, and a total length L of about 500 to 1200 m.

[0243]

[0244]

[0245] The characteristics of the obtained hot-rolled coil are measured and evaluated by the following method.

[0246] [Tensile strength of hot-rolled coil]

[0247] First, from the central part in the length direction and the central part in the width direction of the hot-rolled coil, collect No. 5 tensile test pieces of JIS Z 2241:2011 with the direction parallel to the rolling direction as the test direction. Then, by using these tensile test pieces and conducting a tensile test in accordance with JIS Z 2241:2011, determine the tensile strength of the hot-rolled coil.

[0248] [Strength non-uniformity in the length direction]

[0249] First, at the central part in the width direction of the hot-rolled coil, collect No. 5 tensile test pieces of JIS Z 2241:2011 with the direction parallel to the rolling direction as the test direction from the positions of 1 / 10L, 5 / 10L, and 9 / 10L of the full length L of the coil with respect to the length direction. Then, by using these tensile test pieces and conducting a tensile test in accordance with JIS Z 2241:2011, obtain three values of tensile strength. Then, calculate the difference between the maximum value and the minimum value among them, and determine the obtained value as the value of strength non-uniformity in the length direction. Evaluate the case where the value of this strength non-uniformity is 15.0 MPa or less as qualified for strength non-uniformity in the length direction, and evaluate the case where it exceeds 15.0 MPa as unqualified for strength non-uniformity in the length direction.

[0250] [Strength non-uniformity in the width direction]

[0251] First, at each of the positions of 1 / 10W, 3 / 10W, 5 / 10W, 7 / 10W, and 9 / 10W from the end in the width direction at the position of 1 / 10L of the full length L of the coil with respect to the length direction of the hot-rolled coil, collect No. 5 tensile test pieces of JIS Z 2241:2011 with the direction parallel to the rolling direction as the test direction. Then, by using these tensile test pieces and conducting a tensile test in accordance with JIS Z 2241:2011, obtain five values of tensile strength. Then, calculate the difference between the maximum value and the minimum value among them, and determine the obtained value as the value of strength non-uniformity in the width direction. Conduct tensile tests in the same manner for the positions of 5 / 10L and 9 / 10L of the full length L of the coil with respect to the length direction of the hot-rolled coil, and determine the values of strength non-uniformity in the width direction at each position. Evaluate the case where all the values of strength non-uniformity at the positions of 1 / 10L, 5 / 10L, and 9 / 10L in the length direction are 15.0 MPa or less as qualified for strength non-uniformity in the width direction, and evaluate the case where it exceeds 15.0 MPa as unqualified for strength non-uniformity in the width direction.

[0252] Evaluate the hot-rolled coil with a tensile strength of 780 MPa or more and qualified strength non-uniformity in both the length direction and the width direction as a high-strength and reduced-strength non-uniformity hot-rolled coil. Show the results in Table 3.

[0253] [Table 3]

[0254]

[0255] Referring to Tables 1 to 3, it can be seen that in Comparative Example 16, since the coiling temperatures CTf and CTt were low and post-coiling heat treatment was not performed, the average particle size of the precipitates in the central portion in the length direction, the non-uniformity of the particle sizes of the precipitates in the width direction in the central portion in the length direction, and the non-uniformity of the particle sizes of the precipitates in the length direction in the central portion in the width direction could not be controlled within the desired ranges. As a result, the strength non-uniformity in the length direction and the width direction became significant. In Comparative Example 17, since post-coiling heat treatment was not performed, the average particle size of the precipitates in the central portion in the length direction and the non-uniformity of the particle sizes of the precipitates in the width direction in the central portion in the length direction could not be controlled within the desired ranges. As a result, the strength non-uniformity in the width direction became significant. In Comparative Example 18, since the coiling temperatures CTf and CTt were low, the non-uniformity of the particle sizes of the precipitates in the width direction in the central portion in the length direction and the non-uniformity of the particle sizes of the precipitates in the length direction in the central portion in the width direction could not be controlled within the desired ranges. As a result, the strength non-uniformity in the length direction became significant. In Comparative Example 19, since the average cooling rate during the first cooling was high and the upper-lower cooling ratio during the first cooling was inappropriate, the non-uniformity of the particle sizes of the precipitates in the width direction in the central portion in the length direction could not be controlled within the desired ranges due to the occurrence of cooling non-uniformity. As a result, the strength non-uniformity in the width direction became significant. In Comparative Example 20, since the Nb content was low, the grain refinement was insufficient, and the strength improvement effect brought about by fine grain strengthening could not be obtained sufficiently. As a result, the desired tensile strength could not be achieved. It is considered that in Comparative Example 21, since the exit side temperature (FT) during finish rolling was high, the austenite grains coarsened. As a result, even through subsequent cooling, the average particle size of the grains could not be refined sufficiently, and the desired tensile strength could not be achieved. In Comparative Example 22, since the coiling temperatures CTf and CTt were high, the non-uniformity of the particle sizes of the precipitates in the length direction in the central portion in the width direction could not be controlled within the desired ranges. As a result, the strength non-uniformity in the length direction became significant. In Comparative Example 23, since the coiling temperature CTm was low, the average particle size of the precipitates in the central portion in the length direction became small, and the desired tensile strength could not be achieved.

[0256] In Comparative Example 24, since the temperature (FT) at the exit side of finish rolling was low, the average grain diameter of the grains in the central portion in the length direction became small, and the desired tensile strength could not be achieved. In Comparative Example 25, since the average cooling rate in the first cooling was low, the average grain diameter of the grains in the central portion in the length direction could not be controlled within the desired range, and similarly, the desired tensile strength could not be achieved. In Comparative Example 26, since the average cooling rate in the first cooling was high, due to the occurrence of cooling unevenness, the unevenness in the particle diameter of the precipitates in the width direction of the central portion in the length direction could not be controlled within the desired range. As a result, the strength unevenness in the width direction became significant. In Comparative Examples 27 and 28, since the upper-lower cooling ratio in the first cooling was inappropriate, due to the occurrence of cooling unevenness, the unevenness in the particle diameter of the precipitates in the width direction of the central portion in the length direction could not be controlled within the desired range. As a result, the strength unevenness in the width direction became significant. In Comparative Example 29, since the coiling temperature CTm was high, the precipitates in the central portion in the length direction coarsened and the average grain diameter became large, and the desired tensile strength could not be achieved.

[0257] In contrast, for the hot-rolled coils of all the inventive examples, by having a specified chemical composition and further appropriately controlling each condition in the manufacturing method, in the central portion in the length direction, based on the combination of grain refinement strengthening brought about by controlling the average grain diameter of the grains surrounded by grain boundaries with an orientation difference of 15° or more to be within the range of 5.0 to 8.0 μm and precipitation strengthening brought about by controlling the average particle diameter of the precipitates to be within the range of 3.0 to 9.5 nm, a tensile strength of 780 MPa or more can be achieved. In addition, the difference between the maximum value and the minimum value of the particle diameter of the precipitates in the width direction of the central portion in the length direction and in the length direction of the central portion in the width direction of the hot-rolled coil can be controlled to be 15.0% or less of the average particle diameter of the precipitates ("width direction unevenness of precipitate particle diameter" and "length direction unevenness of precipitate particle diameter" in Table 3). As a result, the strength unevenness in the length direction and width direction of the hot-rolled coil can be significantly suppressed or reduced. In addition, the metallographic structure of the obtained hot-rolled coil was analyzed, and as a result, in the hot-rolled coils of all the inventive examples, the area ratio of ferrite was 90% or more.

Claims

1. A hot-rolled coil, characterized in that, it has the following chemical composition: by mass%: C: 0.050 - 0.100%, Si: 0.01 - 0.30%, Mn: 1.30 - 2.10%, Ti: 0.080 - 0.150%, Nb: 0.020 - 0.050%, Al：0.001～0.050％、 P: below 0.100%, S: below 0.050%, N: below 0.0050%, O: below 0.0050%, B：0～0.0050％、 Cu: 0 - 0.20%, Ni: 0 - 0.20%, Sn: 0 - 0.10%, Cr：0～0.40％、 Mo: 0 - 0.200%, V：0～0.100％、 As: 0 - 0.100%, Zr：0～0.100％、 Ca: 0 - 0.0050%, Mg: 0 - 0.100%, Bi: 0 - 0.020%, Co: 0 - 0.20%, W：0～0.20％、 Zn: 0 - 0.20%, REM: 0 - 0.1000%, and the balance: consists of Fe and impurities, it has the following metal structure: At the central part in the length direction, when defining the region surrounded by grain boundaries with an orientation difference of 15° or more as grains, the average grain size of the grains is 5.0 - 8.0 μm, the average particle size of the precipitates is 3.0 - 9.5 nm, and the difference between the maximum and minimum values of the particle size of the precipitates in the width direction is 15.0% or less of the average particle size of the precipitates; At the central part in the width direction, the difference between the maximum and minimum values of the particle size of the precipitates in the length direction is 15.0% or less of the average particle size of the precipitates in the length direction.

2. The hot-rolled coil according to claim 1, characterized in that, the chemical composition contains at least one of the following elements by mass%: B：0.0001～0.0050％、 Cu: 0.01 - 0.20%, Ni: 0.01 - 0.20%, Sn: 0.01 - 0.10%, Cr：0.01～0.40％、 Mo: 0.001 - 0.200%, V：0.001～0.100％、 As: 0.001 - 0.100%, Zr：0.001～0.100％、 Ca: 0.0001 - 0.0050%, Mg: 0.001 - 0.100%, Bi: 0.001 - 0.020%, Co: 0.01 - 0.20%, W：0.01～0.20％、 Zn: 0.01 - 0.20%, and REM: 0.0001 - 0.1000%.

3. The hot-rolled coil according to claim 1 or 2, characterized in that, it has an effective Ti content of 0.070% or more.

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