HIGH-STRENGTH HOT-ROLLED STEEL SHEET

Abstract

A high-strength hot-rolled steel sheet: which has a predetermined chemical composition; having an average dislocation density from 1 × 10¹⁴ to 1 × 10¹⁶ m⁻², and containing at least bainitic ferrite, wherein the total area ratio of bainitic ferrite to ferrite is 70% or more and less than 90%, and wherein the total area ratio of martensite to retained austenite is 5% or more and 30% or less, wherein, in the ferrite crystal grains and in the bainitic ferrite crystal grains, the average number density of TiC precipitates is from 1 × 10¹⁷ to 5 × 10¹⁸ (precipitates / cm³), wherein the amount of Ti present as a TiC precipitate in a matrix not in the dislocations is 30% by mass or more of the total amount of Ti in the steel sheet, and wherein the tensile strength is 850 MPa or more.

Description

HIGH-STRENGTH HOT-ROLLED STEEL SHEET TECHNICAL FIELD [1] This disclosure relates to a high-strength hot-rolled steel sheet. BACKGROUND OF THE TECHNIQUE [2] As a strengthening method to increase the strength of steel, the following are effective: (1) solid-phase solution hardening by adding elements such as C, Si, and Mn; (2) precipitation hardening using precipitates such as Ti and Nb; and (3) transformation hardening using a microstructure such as a continuous-cooling transformation microstructure, in which dislocation strengthening or fine crystal grain strengthening is expressed. In particular, automotive components are being reduced in weight and improved in safety and durability, and there is a demand for increased strength in steel as a material. [3] Solid-phase solution hardening has a lesser strength-enhancing effect than precipitation hardening and transformation hardening, and thus it is difficult to increase the required strength of a material for an automotive member by solid-phase solution hardening alone. Furthermore, with regard to precipitation hardening, in recent years there has been renewed interest in developing technologies to achieve high strength while maintaining excellent deformability of the original uniform ferrite phase structure. For example, a method has been proposed that uses carbide-forming elements such as Ti, Nb, and Mo to precipitate fine carbides to strengthen the ferrite structure (e.g., Patent Documents 1 to 3). In a structure with a relatively low dislocation density, composed primarily of ferrite, fine carbides are precipitated to enhance strength through precipitation hardening. [4] According to these methods, it is necessary to form a transformed ferrite structure at a relatively high temperature in order to develop precipitation hardening. In order to develop dislocation strengthening, it is necessary to carry out a phase transformation at a low temperature, and in this way it is difficult to develop both precipitation hardening and dislocation strengthening. [5] On the other hand, a high-strength steel sheet with excellent edge formability has been proposed MA / a / ZUZZ / UI Z l zo by stretching, which includes an acicular ferrite structure transformed at a relatively low temperature and has a structure in which fine TiC and NbC carbides are precipitated (for example, Patent Document 4). [6] In general, it is known that precipitates are more likely to nucleate at defects such as dislocations and crystal grain boundaries than in defect-free portions. Consequently, conventionally, increasing the dislocation density has been used to promote precipitation at dislocations (e.g., Patent Document 5). [7] Note that Non-Patent Document 1 proposes to calculate the dislocation density using the strain of a crystal lattice obtained by measuring X-ray diffraction. [8] Patent Document 1: Japanese Patent Application Open to the Public (JP-A) No. 2003-89848 Patent Document 2: Japanese Patent Application Open to the Public (JP-A) No. 2007-262487 Patent Document 3: Japanese Patent Application Open to the Public (JP-A) No. 2007-247046 Patent Document 4: Japanese Patent Application Open to the Public (JP-A) No. H7-11382 Patent Document 5: Japanese Patent Application Open to the Public (JP-A) No. 2013-133534 MA / a / ZUZZ / UI Z l zo [9] Non-Patent Document 1: GK Williamson & RE Smallman, Dislocation densities in some annealed and coldworked metastases from measurements on X-ray Debye-Scherrer spectrum, Philosophical Magazine, Vol. 1, 1956, p. 34-46 BRIEF DESCRIPTION OF THE INVENTION Technical Problem

[10] However, in Patent Documents 4 and 5, the studies on the utilization of both precipitation hardening and dislocation strengthening have not been sufficient. In order to increase the strength of precipitation-hardened steel, one method is generally considered to be increasing the amount of precipitation hardening by increasing the content of an alloying element. However, not only can the cost increase, but workability and similar properties can also deteriorate, and an end face of a hole formed by punching a steel sheet can be damaged, for example, by peeling or lifting. There has been room for examination of further increasing the strength while suppressing the content of the alloying element.

[11] Accordingly, one object of the present disclosure is to provide a high-strength hot-rolled steel sheet that suppresses damage to a perforated edge of the steel sheet while suppressing the content of an alloying element, and has a tensile strength of 850 MPa or ML / a / ZUZZ / U 1 Z l zo more . Solution

[12] The present inventors aimed to achieve high precipitation hardening by precipitating fine TiC precipitates after phase transformation, while simultaneously increasing the dislocation density of a steel sheet through phase transformation to enhance dislocation strengthening. Accordingly, the present inventors actively utilized bainitic ferrite, which has a high dislocation density, to precipitate fine TiC precipitates after the bainitic ferrite formed. However, precipitation hardening is not effectively exhibited when the TiC precipitates precipitate into dislocations. Accordingly, the present inventors aimed to efficiently exhibit both dislocation strengthening and precipitation hardening by precipitating TiC precipitates into a non-dislocation matrix. Therefore, the present inventors have found that it is possible to suppress the content of an alloying element and obtain high tensile strength while suppressing cost by efficiently developing both dislocation strengthening due to a high dislocation density and precipitation hardening due to the formation of a TiC precipitate in a non-dislocation matrix. MA / a / ZUZZ / UI Z l zo and effectively utilizing the alloying element. Additionally, the present inventors have found that a decrease in workability due to the content of the alloying element is also suppressed, and that the occurrence of damage to a perforated edge of the steel sheet is suppressed.

[13] The present disclosure has been made on the basis of such findings, and the essence of it is as follows. (1) A high-strength hot-rolled steel sheet with a chemical composition containing, by mass: C: from 0.030 to 0.250%; Yes: from 0.01 to 1.50%; Mn: from 0.1 to 3.0%; Ti: from 0.040 to 0.200%; P: 0.100% or less; S: 0.005% or less; Al: 0.500% or less; N: 0.0090% or less; B: from 0 to 0.0030%; a total of one or more of Nb, Mo and V: from 0 to 0.040%; a total of one or more of Ca and REM: from 0 to 0.010%; and the remainder consisting of Fe and impurities, a mass ratio [Ti] / [C] of an amount of Ti to an amount of C that is from 0.16 to 3.00, and a product [Ti] × [C] of the amount of Ti and the amount of C that is from 0.0015 to 0.0160, the high hot-rolled steel sheet MA / a / ZUZZ / UI Z l zo resistance: which has an average dislocation density of 1 × 1014a 1 × 1016m-2; and containing at least bainitic ferrite, wherein a total area ratio of bainitic ferrite to ferrite is 70% or more and less than 90%, wherein a total area ratio of retained martensite to austenite is 5% or more and 30% or less, wherein, in ferrite crystal grains and in bainitic ferrite crystal grains, a mean number density of Tic precipitates is from 1 × 1017 to 5 × 1018 (precipitates / cm3), wherein an amount of Ti present as a precipitate of TiC precipitated in a matrix not in dislocations is 30% by mass or more of a total amount of Ti in the steel sheet, wherein a tensile strength is 850 MPa or more, and wherein [Ti] and [C] represent the amount of Ti and the amount of C (% by mass), respectively. (2) High-strength hot-rolled steel sheet according to (1), containing, by mass: B: 0.0001% or more and less than 0.0005%. (3) High-strength hot-rolled steel sheet according to (1) or (2), containing, by mass: the total of one or more of Nb, Mo, and V: from 0.01 to MA / a / ZUZZ / UI z / zo 0.040%. (4) High-strength hot-rolled steel sheet according to any of (1) to (3), containing, by mass: the total of one or more of Ca and REM: from 0.0005 to 0.01%. (5) High-strength hot-rolled steel sheet according to any of (1) to (4), wherein the total area ratio of bainitic ferrite to ferrite is 80% or more and less than 90%. (6) High-strength hot-rolled steel sheet according to any of (1) to (5), wherein a bainitic ferrite area ratio is 50% or more and less than 90%. Advantageous effects of the invention

[14] According to the present disclosure, it is possible to provide a high-strength hot-rolled steel sheet that has high tensile strength while suppressing the content of an alloying element, and in which damage to a perforated edge of the steel sheet is less likely to occur during perforation. BRIEF DESCRIPTION OF THE DRAWINGS

[15] FIGURE 1A shows a schematic diagram of an arrangement of TiC precipitates in dislocations. FIGURE IB shows a schematic diagram of an arrangement of TiC precipitates in an array. FIGURE 2 is a diagram showing the relationship between [Ti] χ [C] and tensile strength, between the case where a content of Ti present as a precipitate of TiC precipitated in the matrix that is not in the dislocations is 30% by mass or more and the case where the content of Ti is less than 30% of a total Ti content of a steel sheet having an average dislocation density in a range from 1 χ 1014 to 1 χ 1016m-2. DESCRIPTION OF THE MODALITIES

[16] An exemplary form of the present disclosure will now be described in detail.

[17] In the present specification, the indication of % of a content of each element of a chemical composition means % by mass. The content of each element in the chemical composition is sometimes called the amount of the element. For example, the content of C is sometimes expressed as the amount of C. A numeric range indicated using 'a' means a range that includes numeric values ​​described before and after 'a' as a lower bound value and an upper bound value. A numeric range when greater than or less than is joined to the numeric values ​​described before and after 'a' means a range that does not include these numeric values ​​as a lower bound value or an upper bound value. In the numerical ranges according to the stages here, the upper limit value according to a numerical range can be replaced with the upper limit value of any other numerical range according to the stages, and can be replaced with a value described in an Example. The lower limit value according to a numerical range can be replaced with the lower limit value of any other numerical range according to the stages, and can be replaced with a value described in an Example. The content (%) indicates that the component is an optional component and does not need to be contained. The term stage includes not only an independent stage, but also a stage that cannot be clearly distinguished from other stages, as long as the intended purpose of the stage is achieved.

[18] High-strength hot-rolled steel sheet A high-strength hot-rolled steel sheet according to the present modality (hereinafter referred to simply as steel sheet): It has predetermined chemical components, in which a mass ratio [Ti] / [C] of a Ti content to a C content is from 0.16 to 3.00, and a product [Ti] × [C] of the Ti content and the C content is from 0.0015 to 0.0160, MA a. ZUZZ U Ί ¿ l ¿o has an average dislocation density from 1 × 1014 to 1 * 1016m-2; and contains at least one bainitic ferrite. A total area ratio of bainitic ferrite to ferrite is 70% or more and less than 90%. A total area ratio of martensite to retained austenite is 5% or more and 30% or less. In ferrite crystal grains and in bainitic ferrite crystal grains, the average number density of TiC precipitates ranges from 1 × 1017 to 5 × 1018 (precipitates / cm3). A Ti content present as a TiC precipitate in a non-dislocation matrix is ​​30% by mass or more of a total Ti content of the steel sheet. A tensile strength is 850 MPa or more. [Ti] and [C] represent the amount of Ti and the amount of C (% by mass), respectively.

[19] Under the above configuration, the high-strength hot-rolled steel sheet according to the present embodiment is a high-strength hot-rolled steel sheet that has high tensile strength and in which damage to a perforated edge of the steel sheet is less likely to occur during perforation. The high-strength hot-rolled steel sheet according to the present embodiment has been found by the following ΜΛ / a / ZUZZ / U 1 Z l zo findings.

[20] In order to improve the strength of the steel sheet, it is important to control the state of Ti within it. There are three main possible states of existence: Ti as a solid solution, as a coarse TiN precipitate or a TiS precipitate, and as a TiC precipitate. TiN or TiS precipitates have a very small solubility product in iron, precipitating even in a relatively high-temperature austenite region. They become coarse and thus do not contribute to the strength of the steel sheet. The amount of TiN or TiS precipitates is almost entirely determined by the N and S content of the steel sheet. Whether the residual Ti precipitates as a TiC precipitate or remains as a solid solution atom varies greatly due to the influence of the thermomechanical treatment of the steel sheet.In the case of Ti as a solid solution, the Ti is uniformly present as a single atom in the crystal grains, and the strengthening mechanism of the steel sheet is solid-phase solution hardening, but the increase in strength is small. On the other hand, when Ti precipitates as a TiC precipitate, the amount of precipitation hardening varies greatly depending on the number density and size of the precipitate, and thus significantly affects the strength of the steel sheet. ML / a / ZUZZ / U 1 Z l zo Additionally, it has been found that the position where the Tic precipitate falls affects the strength of the steel material. The present inventors paid attention to a position where a precipitate of Tic is formed (hereafter also referred to simply as precipitate). The positions where precipitates form were considered, including cases where precipitates form at crystal grain boundaries, cases where precipitates form at dislocations within crystal grains, and cases where precipitates form uniformly within a matrix (hereafter also referred to simply as a matrix) that is not located at dislocations within the crystal grains. Normal steel with a crystal grain size of several microns or more is considered to have a low density of crystal grain boundaries, and precipitates at these boundaries do not contribute to strengthening.Precipitates have a property of preferentially nucleating in dislocations compared to the matrix, but it is considered that whether precipitates precipitate in dislocations or precipitate uniformly in the matrix depends on the hot rolling temperature and chemical composition, the degree of supercooling and diffusion length of the precipitate-forming elements, the dislocation density, and. ΜΛ / a / ZUZZ / U 1 Z l zo similar . Therefore, the present inventors considered that the position where the TiC precipitates precipitate, the number density, the ratio between the Ti and C contents in the steel sheet, and the microstructure affect the strength of the steel sheet, and they carried out studies.

[21] The present inventors melted and hot-rolled a piece of steel containing, in % by mass, C: from 0.030 to 0.250%, Si: from 0.01 to 1.50%, Mn: from 0.1 to 3.0%, Ti: from 0.040 to 0.200%, P: from 0.100% or less, S: 0.005% or less, Al: 0.500% or less, N: 0.0090% or less, B: from 0 to 0.0030%, a total of one or two or more Nb, Mo and V: from 0 to 0.040%, and a total of one or two or more Ca and REM: from 0 to 0.010%, the remainder consisting of Fe and impurities, to manufacture a steel sheet under various heat-treating conditions, and performed the following tests and studies.

[22] The average dislocation density of the obtained steel sheet was measured. The present inventors determined that a large strengthening of dislocations was obtained when the average dislocation density was in the range of 1 × 1014 to 1 × 1016 m-2, and subsequent tests were carried out on steel sheets having an average dislocation density in the range of 1 × 1014 to 1 × 1016 m-2. ML / a / ZUZZ / U 1 Z l zo

[23] First, a test piece was taken from the steel sheet, and its tensile strength was measured.

[24] Next, the microstructure was observed, the average number density of the TiC precipitates precipitated on the crystal grains was measured, and the position of formation of the TiC precipitates was observed.

[25] For a steel sheet having an average dislocation density in the range from 1 × 1014 to 1 × 1016 m-2, the relationship between [Ti] × [C] and the tensile strength when the Ti content is denoted by [Ti] and the C content is denoted by [C] is shown in FIGURE 2. FIGURE 2 also shows the relationship of the number density of TiC precipitates and the relationship between the case where the Ti content present as TiC precipitates precipitated in the matrix not in the dislocations is 30% by mass or more and the case where the Ti content is less than 30% of a total Ti content of the steel sheet. It is found that, in ferrite crystal grains and bainitic ferrite crystal grains, a high strength of 850 MPa or more is achieved as a target when the number-average density of TiC precipitates is from 1 × 10¹⁷ to 5 × 10¹⁸ (precipitates / cm³), and the Ti content present as TiC precipitates in the matrix, not in dislocations, is 30% by mass or more of the total Ti content of the steel sheet. Furthermore, it was found that the [Ti] × value [C] needs to be in the range from 0.0015 to 0.0160 in order to obtain the above structure. The reason why the strength of steel sheets increases when the Ti content, present as TiC precipitates in the matrix rather than in dislocations, is high is as follows. First, in addition to the TiC precipitates in the matrix, there are coarse TiN or TiS precipitates, as previously described, Ti atoms in solid solution, and TiC precipitates in dislocations. The coarse TiN or TiS precipitates and the Ti atoms in solid solution provide a small amount of strengthening for the reasons described above.Then, when TiC precipitates exist within dislocations, the dislocations act as obstacles, and the TiC precipitates overlap in position, making it less likely that the precipitates will contribute as new obstacles to suppressing an increase in the amount of strengthening. On the other hand, when TiC precipitates precipitate within the matrix, both the dislocations and the TiC precipitates effectively act as obstacles during deformation, allowing precipitation hardening to be used more effectively.

[26] [Ti] x [C] is related to a temperature at which the TiC precipitates dissolve completely, i.e., a lower limit temperature at which TiC precipitates do not form. When the value of [Ti] χ [C] is small, the lower limit temperature at which Ti and C do not precipitate is low, and when the value of [Ti] χ [C] is large, the lower limit temperature at which Ti and C do not precipitate is high. As shown in Figure 2, when the [Ti] χ [C] value was less than 0.0015, it was not possible to increase the Ti content present as TiC precipitates in the matrix. This is considered to be due to insufficient supercooling in a cooling stage. When the [Ti] χ [C] value is small, the temperature at which TiC precipitates form is low, thus reducing the degree of supercooling. It is considered that when the degree of supercooling is small, the driving force for precipitation is weak, and the precipitation frequency at dislocations where precipitates nucleate most readily is high, so the precipitation frequency of TiC in the matrix cannot be increased. It is considered that when the [Ti] χ [C] value is 0.At 0015 or higher, the degree of supercooling of TiC precipitation increases, the driving force for precipitation increases sufficiently, and precipitation occurs in the matrix in addition to precipitation in dislocations. MA / a / ZUZZ / UI Z l zo Furthermore, even when the [Ti] χ [C] value exceeded 0.0160, and the proportion of Ti present as a TiC precipitate in the matrix increased, the strength decreased. This is thought to be because the Ti and C concentrations are too high, so the temperature at which the TiC precipitates completely dissolve becomes higher than the temperature at which the TiC precipitates are brought into solution in the austenite region, and some of the TiC is already precipitated. The TiC precipitates in the austenite region are coarse and have a low number density, and thus contribute less to precipitation hardening. That is, it is considered that when the [Ti] χ [C] value is greater than 0.0160, the concentrations of Ti and C that generate fine precipitates contributing to precipitation hardening cannot be increased, and therefore, a large tensile strength cannot be obtained. Furthermore, it is considered that, as the coarse TiC precipitates generated in the austenite region grow further during cooling, the concentrations of Ti and C that contribute to the generation of fine precipitates after the phase transformation may be reduced, or the number density may be reduced due to an increase in the size of the TiC precipitates, and that the effect of increasing strength is small.

[27] Furthermore, the content of the element is considered to be MA / a / ZUZZ / UI Z l zo alloy can be reduced and the decrease in workability caused by the alloying element can be suppressed, effectively utilizing the alloying element through the efficient development of both precipitation hardening and dislocation strengthening.

[28] In accordance with the above findings, the present inventors have found a high-strength hot-rolled steel sheet that has high tensile strength while suppressing a content of an alloying element, and in which damage to a perforated edge of the steel sheet is less likely to occur during perforation.

[29] Details of the high-strength hot-rolled steel sheet will be described below in accordance with the present modality.

[30] Chemical composition The chemical composition of the high-strength hot-rolled steel sheet according to the present modality contains the following elements.

[31] -Essential element- C: from 0.030 to 0.250%. Carbon (C) is an important element that generates fine TiC precipitates and contributes to precipitation hardening. It is also a necessary element that segregates at crystal grain boundaries to suppress damage to the perforated edge of the steel sheet. MA / a / ZUZZ / UI Z l The amount of C required to exhibit the effect is 0.030% or more, but when the amount of C is greater than 0.250%, coarse cementite is generated, thus reducing ductility, particularly local ductility. Therefore, the amount of C is from 0.030 to 0.250%, preferably from 0.040 to 0.150%.

[32] Yes: from 0.01 to 1.50% Silicon (Si) is a deoxidizing element, and the amount of Si is 0.01% or more. Si contributes to solid-phase solution hardening, but when the amount of Si exceeds 1.50%, workability deteriorates. Therefore, an upper limit for the amount of Si is set at 1.50%. Consequently, the amount of Si is from 0.01 to 1.50%, preferably from 0.02 to 1.30%.

[33] Mn: from 0.1 to 3.0%. Manganese (Mn) is an effective element for deoxidation and desulfurization and also contributes to solid-phase solution hardening; therefore, the amount of Mn is 0.1% or more. From the perspective of reducing the area ratio of polygonal ferrite, the amount of Mn is preferably 0.35% or more. On the other hand, when the amount of Mn exceeds 3.0%, segregation is likely to occur, thus deteriorating workability and increasing costs, which is undesirable. Therefore, the amount of Mn is from MA / a / ZUZZ / UI Z l zo 0.1 to 3.0%, preferably from 0.3 to 1.5%.

[34] Ti: from 0.040 to 0.200%. Titanium (Ti) is an extremely important element that precipitates fine TiC precipitates on ferrite and bainitic ferrite grains and contributes to precipitation hardening. A Ti content of 0.040% or higher is ideal because Ti precipitates within the matrix, increasing strength. However, when the Ti content exceeds 0.200%, not only does the cost increase, but the TiC precipitates also tend to become coarser, making manufacturing more difficult. To easily achieve a suitable number density of TiC precipitates, the Ti content is preferably 0.150% or less. Therefore, the Ti content ranges from 0.040% to 0.200%, preferably from 0.070% to 0.150%.

[35] P: 0.100% or less Phosphorus (P) is an impurity that impairs workability and weldability. Therefore, the amount of P is preferably as low as possible, and is limited to 0.100% or less. The amount of P is preferably limited to 0.020% or less because P segregates at grain boundaries, decreasing ductility. However, from the standpoint of cost for P removal, the amount of P is preferably 0.005% or more.

[36] S: 0.005% or less Sulfur (S) is an impurity and particularly deteriorates MA / a / ZUZZ / UI Z l zo hot workability. Therefore, the amount of S is preferably as low as possible, and is limited to 0.005% or less. In order to suppress a decrease in ductility due to an inclusion such as a sulfide, it is preferable to limit the amount of S to 0.002% or less. However, from the point of view of the cost of S removal, the amount of S is preferably 0.0005% or more.

[37] Al: 0.500% or less Aluminum (Al) is a deoxidizing agent, and the amount of Al is 0.500% or less. When Al is present in excessive amounts, a nitride forms and ductility is reduced. Therefore, the amount of Al is preferably limited to 0.150% or less. In order to sufficiently deoxidize molten steel, the amount of Al is preferably 0.002% or more.

[38] N: 0.0090% or less Nitrogen (N) forms TiN, reduces the workability of steel, and also leads to a reduction in the effective amount of Ti that forms TiC precipitates. Therefore, the amount of N is preferably as low as possible, and is limited to 0.0090% or less. However, from the standpoint of cost for N removal, the amount of N is preferably 0.0010% or more.

[39] -Optional element- The chemical composition of high-strength hot-rolled steel sheet according to the present MA / a / ZUZZ / UI Z l zo modality may contain the following optional elements in addition to the essential elements.

[40] B: from 0 to 0.0030% Boron (B) is an optional element that may be included in the steel sheet. However, because it is an effective element that suppresses phase transformation and can increase the area ratio of bainitic ferrite while suppressing ferrite transformation as much as possible under appropriate quenching conditions, it is preferable to incorporate B as needed. Therefore, the amount of B is preferably 0.0001% or more. On the other hand, when the amount of B is greater than 0.0030%, precipitates such as BN readily form, and the effect becomes saturated. Therefore, the amount of B is set at 0.0030% or less. The amount of B is preferably 0.0020% or less. B has a very strong phase transformation suppression effect, and the amount of B is preferably less than 0.0005% to establish a total area ratio of bainitic ferrite to ferrite of 80% or more and less than 90%.

[41] Total of one or more of Nb, Mo, and V: from 0 to 0.040% Niobium (Nb), molybdenum (Mo), and vanadium (V) are optional elements contained in the steel sheet. Nb, Mo, and V are elements that precipitate carbide in MA / a / ZUZZ / UI Z l zo ferrite crystal grains are similar in shape to Ti, but the alloying cost is high and the precipitation hardening capacity is lower than that of Ti. Consequently, one or more of Nb, Mo, and V may be contained, and the total content of these is stated to be from 0 to 0.040%. Furthermore, Nb and V are effective elements for strengthening steel sheets by delaying recrystallization during hot rolling and refining the crystal grains of the steel sheet. Mo is an element that improves hardenability and is also effective in increasing the area ratio of bainitic ferrite while suppressing ferrite transformation as much as possible. To achieve these effects sufficiently, the total content of Nb, Mo, and V is preferably 0.01% or more. In the steel sheet, these elements combine with TiC precipitates and exist as (Ti, M) C. Here, M is one or more of Nb, V, and Mo.

[42] Total of one or more of Ca and REM: from 0 to 0.010% Calcium (Ca) and REM are optional elements contained in steel sheets. Ca and REM play a role in controlling the shape of inclusions, which can become fracture starting points and cause workability deterioration, by detoxifying the inclusions. One or more of Ca and REM may be contained, and a total content of the same is set at from 0 to 0.01% or less. Furthermore, in order to obtain sufficient effect from controlling the shape of the inclusions to detoxify the inclusions, the total content of one or more calcium (Ca) and REM is preferably 0.0005% or more. Note that REM refers to a total of 17 elements including Se, Y, and lanthanides. REM content means a total content of at least one of these elements. In the case of lanthanides, they are added industrially in mischmetal form.

[43] Remainder: iron (Fe) and impurities Impurities refer to components contained in a raw material or components mixed in during manufacturing and not intentionally incorporated into the steel sheet. Examples of impurities include nickel (Ni), copper (Cu), and tin (Sn), which may be mixed in from scrap. The contents of impurities such as Ni, Cu, and Sn are preferably 0.01% or less each.

[44] Mass ratio [Ti] / [C] of the amount of Ti to the amount of C A mass ratio [Ti] / [C] of the amount of Ti to the amount of C is from 0.16 to 3.00. It is important that the mass ratio [Ti] / [C] of the MA / a / ZUZZ / UI z / zo the amount of Ti to the amount of C is 3.00 or less. This value corresponds to a ratio of the number of Ti atoms to the number of C atoms of approximately 0.75 or less in terms of the ratio of the number of atoms. In conventional precipitation-hardened steel sheets, an excessive amount of Ti is incorporated relative to the amount of C in order to precipitate TiC precipitates. However, in order to allow the Ti to exist, in the steel sheet, not as a solid solution Ti atom but as a TiC precipitate as much as possible and to contribute effectively to precipitation hardening, it is necessary to prevent the amount of Ti from being excessive relative to the amount of C. Furthermore, when the mass ratio [Ti] / [C] exceeds 3.If the TiC precipitates are sufficiently precipitated, the amount of C segregated at the crystal grain boundaries is reduced, and the perforated edge of the steel sheet is likely to be damaged. A more preferable upper limit for the mass ratio [Ti] / [C] is 2.50 or less. Furthermore, since the lower limit for the amount of Ti is 0.040% and the upper limit for the amount of C is 0.250%, the lower limit for the mass ratio [Ti] / [C] is 0.16 or higher. A more preferable lower limit for the mass ratio [Ti] / [C] is 0.46 or higher.

[45] Product [Ti] x [C] of the amount of Ti and the amount of C The product of [Ti] * [C] and the amount of Ti and C ranges from 0.0015 to 0.0160. When [Ti] * [C] is less than 0.0015, the degree of supercooling for TiC precipitation is insufficient. Therefore, the Ti content present as TiC precipitates in the matrix cannot be increased, and the strength-enhancing effect is reduced. On the other hand, when [Ti] * [C] is greater than 0.0160, the TiC precipitates cannot dissolve completely when brought into solution in the austenite region, and a precipitation hardening effect corresponding to the added amount cannot be achieved in the fine precipitation after phase transformation. The product [Ti] χ [C] of the amount of Ti and the amount of C is preferably from 0.0020 to 0.0150.

[46] Microstructure The microstructure of the high-strength hot-rolled steel sheet will now be described in accordance with the present modality.

[47] -Total area ratio of bainitic ferrite to ferrite- The high-strength hot-rolled steel sheet according to the present specification contains at least bainitic ferrite. In addition, the ratio of total area of ​​bainitic ferrite to ferrite is 70% or more with respect to the entire structure.

[48] ​​When the ratio of total area of ​​bainitic ferrite to ferrite is less than 70% with respect to the whole structure, workability may deteriorate, and the drilled edge may be damaged. The ratio of total area of ​​bainitic ferrite to ferrite is preferably 80% or more with respect to the whole structure. On the other hand, when the ratio of total area of ​​bainitic ferrite to ferrite is 90% or more with respect to the entire structure, it is difficult to achieve high strength, and thus the ratio of total area of ​​bainitic ferrite to ferrite is less than 90%. From the perspective of increasing the strength of the steel sheet, the ratio of total area of ​​bainitic ferrite to ferrite is preferably 88% or less, more preferably 86% or less, and even more preferably 85% or less.

[49] -Area ratio of bainitic ferrite- In high-strength hot-rolled steel sheet according to the present modality, a ratio of bainitic ferrite area to the whole structure is preferably 50% or more, more preferably 55% or more, and even more preferably 60% or more. In the high-strength hot-rolled steel sheet according to the present modality, the ratio of bainitic ferrite area to the whole structure is preferably less than 90%, more preferably 88% or less, even more preferably 86% or less, and particularly preferably 85% or less. By establishing the bainitic ferrite area ratio within the aforementioned range, the dislocation density of the steel sheet tends to fall within a desired range, and dislocation strengthening develops more efficiently. Consequently, the steel sheet has greater tensile strength and is less likely to be damaged at the drilled edge during drilling, which is preferable.

[50] -Area ratio of the polygonal ferrite- In the high-strength hot-rolled steel sheet according to the present modality, the ratio of polygonal ferrite area to the whole structure is preferably 0% or more and 40% or less, more preferably 0% or more and 35% or less, and even more preferably 0% or more and 30% or less. When the area ratio of the polygonal ferrite is within the above range, a steel sheet is obtained that has a higher tensile strength, which is preferable.

[51] -Relationship of total martensite and retained austenite area- The high-strength hot-rolled steel sheet according to the present modality contains at least one layer of retained martensite or austenite. The ratio of total martensite to retained austenite area is 5% or more relative to the entire structure. When the ratio of total martensite to retained austenite area is less than 5%, it is difficult to achieve high strength. Therefore, the ratio of total martensite to retained austenite area is 5% or more. On the other hand, when the ratio of total martensite to retained austenite area relative to the entire structure is greater than 30%, the carbon enrichment in the martensite may be insufficient, and the contribution to strength improvement may be weakened. Therefore, the ratio of total martensite to retained austenite area is 30% or less. The ratio of total area of ​​retained martensite and austenite with respect to the whole structure is preferably 20% or less from the point of view of suppressing damage to the drilled edge.

[52] Microstructure observation is carried out by mirror polishing a sample, subjecting the sample to nital etching, and observing the microstructure at a position 1 / 4 of a plate thickness in a plate thickness direction from its surface with an optical microscope.

[53] Here, the area ratio is measured by the following method. First, a test piece cut to obtain a cross-section parallel to the rolling direction and the sheet thickness direction of the steel sheet is mirror-polished, etched with a nital solution, and its microstructure at a position 1 / 4 of the sheet thickness is observed using an optical microscope. Martensite, retained austenite, and pearlite are identified, the area ratios of martensite, retained austenite, and pearlite are measured using a point-counting method, and the total area ratio of martensite to retained austenite is determined from the results. A value obtained by subtracting the area ratios of martensite, retained austenite, and pearlite from 100% is defined as the total area ratio of martensite to ferrite. Next, an additional electropolished test piece is used to measure the ferrite area ratio. Subsequently, using the EBSP-OIM™ method (Electron Backscatter Diffraction Pattern Orientation Imaging Microscopy), the EBSP measurement is performed under the following conditions: 2000x magnification, an area of ​​40 pm × 80 pm, and a measurement step of 0.1 pm.

[54] The EBSP-OIM™ method includes an apparatus and software for irradiating a highly inclined sample with an electron beam in a scanning electron microscope (SEM), photographing a backscattering Kikuchi pattern with a highly sensitive camera, and performing computer image processing to measure crystal orientation at an irradiation point in a short time. In the EBSP measurement, crystal orientation on a surface of a bulk sample can be quantitatively analyzed, and an analysis area is a region that can be observed by the SEM. The measurement is performed over several hours, and the regions to be analyzed are mapped into tens of thousands of points in a grid at equal intervals, so that the distribution of crystal orientation in the sample can be determined.

[55] Based on the measurement results, the ferrite area ratio is determined using the core-averaged misorientation (KAM) method. The KAM method averages the misorientation of a given pixel in the measurement data across six adjacent pixels and performs the calculation for each pixel using the value of the central pixel. By performing this calculation in a way that does not exceed crystal grain boundaries, it is possible to create a map that represents a change in orientation within the crystal grains. That is, this map represents the stress distribution based on a local change in orientation within the crystal grains.Because ferrite undergoes diffusional transformation and has a small transformation strain, crystal grains in which the average misorientation between the six pixels and the central pixel is less than 100, as determined by the KAM method, are defined here as ferrite, and their area ratio is determined. The case where the misorientation between adjacent measurement points was 15° or more was defined as the crystal grain boundary. The ratio of bainitic ferrite area to the whole structure is calculated from the difference between the ratio of total bainitic ferrite area to ferrite and the ferrite area ratio.

[56] The ratio of polygonal ferrite area to the whole structure is measured as follows. Polygonal ferrite is characterized by a low dislocation density and a particularly small disorientation across the entire region of the crystal grains. Therefore, in this embodiment, first, the average xl value of the disorientation between the six pixels and the central pixel, as determined by the KAM method, is obtained for each measurement point. Additionally, the average x2 value across all measurement points in the crystal grains is obtained from the average xl value obtained at each measurement point. Crystal grains in which the x2 value is 0.5° or less are defined as polygonal ferrite, and their area ratio is determined. In ferrite, a region not defined as polygonal ferrite is ferrite with a relatively high dislocation density, such as acicular ferrite.

[57] -Average dislocation density- The high-strength hot-rolled steel sheet according to the present modality has an average dislocation density from 1 × 1014 to 1 × 1016 m-2. When the average dislocation density is 1 × 1014m-2o or higher, dislocation strengthening is obtained. On the other hand, when the average dislocation density exceeds 1 × 1016m-2, recrystallization is likely to occur, and the strength is significantly reduced. The average dislocation density is preferably from 2 × 1014 to 2 × 1015 m-2.

[58] One method for measuring the average dislocation density is as follows. For the measurement of the average dislocation density, X-ray diffraction is used, and the measurement is performed by mirror polishing a sample so that a surface at a position 1 / 4 of the sheet thickness is horizontal to the sheet surface (laminated surface). From the stress measured by X-ray diffraction, an average dislocation density p is determined by the following equation described in Non-Patent Document 1. Equation: p = 14.4 e2 / b2en where ε is a voltage obtained from the measurement MA / a / ZUZZ / UI Z l zc of X-ray diffraction, yb is a Burgers vector (0.25 nm) .

[59] -Numerical average density of the TiC precipitate in the crystal grain- In the high-strength hot-rolled steel sheet according to the present modality, the average number density of TiC precipitates is from 1 × 1017 to 5 × 1018 (precipitates / cm3) in ferrite crystal grains and in bainitic ferrite crystal grains. The average number density of TiC precipitates on the crystal grains is preferably high to facilitate precipitation hardening. Therefore, to achieve dislocation strengthening and precipitation hardening with a tensile strength of 850 MPa or higher, the average number density of TiC precipitates on ferrite crystal grains and bainitic ferrite crystal grains is from 1 × 10¹⁷ to 5 × 10¹⁸ (precipitates / cm³), and preferably from 2 × 10¹⁷ (precipitates / cm³) to 5 × 10¹⁸ (precipitates / cm³).

[60] The average number density of TiC precipitates is measured using a three-dimensional atomic probe measurement method as follows. First, a needle-shaped sample is prepared from a sample to be measured by a cutting and electropolishing method, using a focused ion beam working method together with an electropolishing method as appropriate. MA / a / ZUZZ / UI Z l zo required, and the three-dimensional atomic probe measurement is performed on the needle-shaped sample. In the three-dimensional atomic probe measurement, the integrated data are reconstructed to obtain an image of the actual atom distribution in real space.

[61] Next, the position of formation of the TiC precipitates in the needle-shaped sample is confirmed, and the number density of the TiC precipitates precipitated on the crystal grains in the ferrite crystal grains and the bainitic ferrite crystal grains is determined from the volume of the entire stereoscopic distribution image that includes the TiC precipitates and the number of TiC precipitates. An average value obtained by performing this operation five times is defined as the mean number density of the TiC precipitates precipitated on the crystal grains.

[62] An average diameter of the TiC precipitates on the crystal grains is preferably 0.8 nm or larger from the standpoint of increasing the amount of precipitation hardening. On the other hand, when the average diameter is too large, the mean number density tends to decrease, and the amount of precipitation hardening decreases, which is not preferable. However, because it is essential that the mean number density be within the above range to increase the amount of MA / a / ZUZZ / UI Z l zo precipitation hardening, the upper limit of the average diameter is not defined.

[63] The average diameter of the TiC precipitates on the crystal grains is a diameter (spherical equivalent diameter) calculated, under the assumption that the TiC precipitates are spherical, from the number of constituent atoms of the observed TiC precipitates and the lattice constant of TiC. The diameters of 30 or more TiC precipitates are arbitrarily measured, and an average value is determined.

[64] -Amount of Ti existing as TiC precipitates in the matrix - In high-strength hot-rolled steel sheet according to the present modality, the amount of Ti present as TiC precipitates precipitated in the matrix not in dislocations (i.e., the amount of Ti contained in the TiC precipitates) is 30% by mass or more of the total amount of Ti in the steel sheet. By setting the amount of Ti present as TiC precipitates in the matrix that are not in dislocations to 30% by mass or more of the total amount of Ti in the steel sheet, the ratio of TiC precipitates in the matrix can be increased, both precipitation hardening and dislocation strengthening can be greatly developed, and a steel sheet with high tensile strength can be obtained while reducing the amount of Ti. It is preferable that the amount of Ti present as TiC precipitates in the matrix not in the dislocations be 40% or more of the total amount of Ti in the steel sheet. Furthermore, the amount of Ti present as TiC precipitates in the matrix, not in dislocations, is preferably as high as possible, but preventing the thickening of the precipitates is difficult during the manufacturing process. Therefore, the amount of Ti is preferably 90% by mass or less of the total amount of Ti in the steel sheet.

[65] The amount of Ti present as Tic precipitates precipitated in the matrix not in the dislocations is measured by the three-dimensional atomic probe measurement method as follows. First, the three-dimensional atomic probe measurement is performed in the same procedure as the mean number density measurement method described above, and the position of formation of the TiC precipitate is confirmed. Based on the spherical configuration of TiC precipitates, when TiC precipitates are arranged in a row, the TiC precipitates are determined as those precipitates in the dislocations, and, when TiC precipitates are arranged independently, the TiC precipitates are determined as those precipitates in the matrix that are not in the dislocations. Figure 1A shows a schematic diagram of an arrangement of TiC precipitates precipitated at dislocations, and Figure 1B shows a schematic diagram of an arrangement of TiC precipitates precipitated in the matrix but not at dislocations. Furthermore, there is also a case where both (A) TiC precipitates precipitated at dislocations and (B) TiC precipitates precipitated in a matrix but not at dislocations are included in the same crystal grain, and in this way, it is determined which of (A) and (B) each precipitate corresponds to.The amount of Ti present as TiC precipitate precipitated in the matrix not in the dislocations (mass ratio with respect to the total amount of Ti in the steel sheet) was calculated from the full stereoscopic distribution image volume of the TiC precipitates, the number of Ti atoms that constitute the TiC precipitates precipitated in the matrix not in the dislocations, and the Ti content of the steel sheet. In the tables and figures, this amount of Ti is referred to as the Ti precipitate-in-matrix ratio.

[66] TiC precipitates include not only carbides, but also carbonitrides in which the nitrogen is mixed within the carbides. TiC precipitates also include precipitates in which one or more of Nb, Mo, and V are dissolved as a solid solution ((Ti,M)C precipitates (M represents one or more of Nb, V, and Mo)).

[67] -Tensile strength- A tensile strength of high-strength hot-rolled steel sheet according to the present modality is 850 MPa or more. The tensile strength of high-strength hot-rolled steel sheet according to the present modality is 860 MPa or more. However, from the point of view of preventing deterioration in workability, the tensile strength of the high-strength hot-rolled steel sheet according to the present modality may be, for example, 1050 MPa or less.

[68] Tensile strength is measured as follows. First, a test piece No. 5 is taken from the steel sheet in accordance with JIS Z 2201:1998. Subsequently, a tensile test is performed in accordance with JIS Z 2241:2011, and the tensile strength is measured.

[69] Manufacturing method The following will describe an example of a manufacturing method for high-temperature hot-rolled steel sheet MA / a / ZUZZ / UI Z l zo resistance according to the present modality. The method of manufacturing high-strength hot-rolled steel sheet according to the present modality includes, for example, a hot rolling stage of heating a piece of steel that satisfies the chemical composition of high-strength hot-rolled steel sheet according to the present modality for hot rolling the same to obtain a steel sheet; a cooling stage of cooling the steel sheet obtained through the hot rolling stage; and a coiling stage of coiling the cooled steel sheet.

[70] Hot rolling stage In the hot rolling stage, a piece of steel that meets the chemical composition of high-strength hot-rolled steel sheet according to the present modality is subjected to, for example, hot rolling through rough rolling and finish rolling to obtain a hot-rolled steel sheet. As the steel part, a steel part obtained by melting and casting steel using a conventional method is used. From a productivity standpoint, the steel part is preferably manufactured by a continuous casting plant.

[71] A heating temperature in lamination in MA / a / ZUZZ / UI Z l The heating temperature is preferably 1200 °C or higher, and more preferably 1220 °C or higher, in order to sufficiently decompose and dissolve the Ti and carbon in the steel sheet. On the other hand, it is not economically preferable to set the heating temperature at an excessively high temperature, and thus it is preferable to set the heating temperature at 1300 °C or lower. After casting, the steel part can be cooled to 1200 °C or lower and then heated to 1200 °C or higher to begin rolling. When using a steel part cooled to 1200 °C or lower, it is preferable to heat the steel part to 1200 °C or higher and hold it at that temperature for 1 hour or more.

[72] A final working temperature (FT) (°C) for hot rolling is preferably 920 °C or higher, and more preferably 940 °C or higher. This is intended to suppress the generation of coarse TiC precipitates in the austenite and promote dislocation recovery by work to suppress polygonal ferrite nucleation during cooling. The final working temperature (FT) (°C) for hot rolling is more preferably 950 °C or higher in order to suppress the precipitation of TiC precipitates at a high temperature. Here, in order to suppress polygonal ferrite nucleation, the final working temperature (FT) (°C) is more preferably 940 °C or higher, but may be 920 °C or higher. ML / a / ZUZZ / U 1 zl zo more and less than 940 °C when the amount of Mn is 0.35% or more. However, from the point of view of suppressing the occurrence of fouling defects, the final working temperature FT (°C) is preferably 1050 °C or less. The final working temperature (FT) indicates a temperature at which the hot-rolled steel sheet is discharged from the final box.

[73] Cooling stage In the cooling stage, the hot-rolled steel sheet undergoes primary cooling, secondary cooling, and tertiary cooling.

[74] -Primary Cooling- In primary cooling, cooling is carried out at an average cooling rate of 30 °C / s more from the end of the hot rolling stage to a primary cooling stop temperature MT (°C). The MT (°C) primary cooling stop temperature is set within a range from 620 to 720 °C.

[75] Primary cooling preferably begins within 5.0 seconds after the end of the hot rolling stage. If this time exceeds 5.0 seconds, precipitation of TiC precipitates may occur in the austenite, which may reduce the effective precipitation in the bainitic ferrite and ferrite. ML / a / ZUZZ / U 1 Z l zo

[76] The average cooling rate of the primary cooling is preferably 30 °C / s or more. This is intended to suppress ferrite transformation during cooling, thereby suppressing a decrease in the average dislocation density and a decrease in number density that accompanies the thickening of the TiC precipitates after the phase transformation. The cooling rate of the primary cooling is preferably 35 °C / s more. An upper limit for the primary cooling rate is not particularly restricted, but is preferably 300 °C / s less in view of the cooling facility's capacity.

[77] The average cooling rate in a range from the primary cooling stop temperature MT (°C) + 50 °C to the primary cooling stop temperature MT (°C) is preferably 50 °C / s more. The reason is as follows. Through phase transformation during secondary cooling after primary cooling, the average number density of TiC precipitates can be established from 1 × 10¹⁷ to 5 × 10¹⁸ (precipitates / cm³) while the average dislocation density increases. In primary cooling, as the temperature approaches the primary cooling stop temperature (MT) (°C), the driving force of the phase transformation increases. Consequently, when the cooling rate decreases, the phase transformation begins before secondary cooling, resulting in a decrease in the average dislocation density, the average number density of precipitates, and the ratio of precipitated Ti to matrix.In order to establish the total ferrite and bainitic ferrite area ratio, which is a more preferable form of high-strength hot-rolled steel sheet according to the present method, at 80% or more, the B content is preferably less than 0.0005%. However, when the B content is less than 0.0005%, the effect of suppressing ferrite transformation is not as strong, and thus the phase transformation can begin immediately before the primary cooling stops. Consequently, the average cooling rate in the range from the primary cooling stop temperature MT (°C) + 50°C to the primary cooling stop temperature MT (°C) is preferably increased by 50°C or more. This is not necessary when the B content is from 0.0005% to 0.0030%. The average cooling rate in the range from the primary cooling stop temperature MT (°C) + 50 °C to the primary cooling stop temperature is preferably 60 °C / or more. The average cooling rate in the range from the primary cooling stop temperature MT (°C) + 50 °C to the primary cooling stop temperature is more preferably 300 °C / s less.

[78] The upper limit of the average cooling rate in a range from the start of primary cooling to the MT (°C) temperature of primary cooling stoppage + 50 °C is not particularly limited, however, it is preferably 25 °C / so more, more preferably 30 °C / so more, and even more preferably 35 °C / so more. The average cooling rate in the range from the start of primary cooling to the MT (°C) temperature of primary cooling stop + 50 °C is preferably 300 °C / s less in view of the capacity of the cooling installation.

[79] The average cooling rate in the range from the primary cooling stop temperature MT (°C) + 50 °C to the primary cooling stop temperature is preferably greater than the average cooling rate in the range from the start of primary cooling to the primary cooling stop temperature MT (°C) + 50 °C. This is because the nucleation of polygonal ferrite can be suppressed, the area ratio of polygonal ferrite can be reduced, and the total area ratio of bainitic ferrite to ferrite can be easily established within a range of 70% or more and less than 90%. MA / a / ZUZZ / UI Z l zo However, when the conditions are met that the average cooling rate of the primary cooling is 30 °C / s more, that the average cooling rate in the range from the MT (°C) of primary cooling stop + 50 °C to the MT (°C) of primary cooling stop is 50 °C / s more, and that the average cooling rate in the range from the start of primary cooling to the MT (°C) of primary cooling stop + 50 °C is 25 °C / s more, the average cooling rate in the range from the MT (°C) of primary cooling stop + 50 °C to the MT (°C) of primary cooling stop may be less than the average cooling rate in the range from the start of primary cooling to the MT (°C) of primary cooling stop + 50 °C.In this case, however, the difference between the average cooling rate in the range from the primary cooling stop temperature (MT) + 50 °C to the primary cooling stop temperature and the average cooling rate in the range from the start of primary cooling to the primary cooling stop temperature (MT) + 50 °C is preferably within a range of 15 °C / s or less. This can suppress polygonal ferrite nucleation, reduce the polygonal ferrite area ratio, and easily establish the total area ratio of bainitic ferrite to ferrite within the range of 70% or more and less than 90%.

[80] By setting the primary cooling rate and the primary cooling stop temperature within the ranges mentioned above, polygonal ferrite nucleation can be suppressed, and the polygonal ferrite area ratio can be reduced. By setting the primary cooling rate within the range mentioned above, the total area ratio of bainitic ferrite to ferrite can be easily set within the range of 70% or more and less than 90%.

[81] The MT (°C) temperature for stopping primary cooling is preferably from 620 °C to 720 °C in order to increase the average dislocation density associated with the phase transformation, the ratio in which TiC precipitates precipitate into the matrix after the phase transformation (matrix not into dislocations), and the number density of TiC precipitates. When the primary cooling stop temperature MT (°C) exceeds 720 °C, precipitation of TiC precipitates in dislocations is promoted, so that the size of the TiC precipitates increases, and the number density of the TiC precipitates decreases. Furthermore, when the MT (°C) temperature at which primary cooling stops is less than 620 °C, the precipitation of TiC precipitates becomes insufficient, and therefore the number density of TiC precipitates decreases.

[82] -Secondary Cooling- In secondary cooling, cooling is carried out at a cooling rate of 5 °C / s or less for from 3 to 10 seconds after the completion of primary cooling.

[83] Secondary cooling is preferably carried out at a cooling rate of 5 °C / s or less in order to promote phase transformation and precipitation of TiC precipitates. From a manufacturing cost point of view, secondary cooling is preferably done by air cooling.

[84] A cooling time for secondary cooling is preferably from 3 to 10 seconds. When the cooling time of the secondary cooling is less than 3 seconds, the phase transformation becomes insufficient, and the total area ratio of bainitic ferrite to ferrite cannot be established at 70% or more. The cooling time for secondary cooling is preferably 4 seconds or more. On the other hand, when the cooling time of the secondary quench exceeds 10 seconds, the TiC precipitates become coarse and the number density decreases; moreover, the ratio of total ferrite to bainitic ferrite area can be 90% or more. Therefore, it is preferable to set the cooling time to 10 seconds or less. The cooling time for secondary cooling is preferably 8 seconds or less. The cooling time for secondary cooling is preferably from 4 to 8 seconds.

[85] -Tertiary Cooling- E1 tertiary cooling is a cooling stage to a stopping temperature CT (°C) of less than 500 °C at a cooling rate of 30 °C / s more after the completion of secondary cooling.

[86] A tertiary cooling rate is preferably 30 °C / s more. This is aimed at preventing a decrease in number density due to the thickening of TiC precipitates generated during secondary cooling, and establishing the total ferrite and bainitic ferrite area ratio to less than 90%. The cooling rate of tertiary cooling is preferably 35 °C / s more. An upper limit for the cooling rate of tertiary cooling is not particularly restricted, but is preferably 200 °C / s less in view of the capacity of the cooling installation. MA / a / ZUZZ / UI z / zo

[87] A tertiary cooling stopping temperature CT (°C) is preferably less than 500 °C in order to establish the ferrite and bainitic ferrite area ratio less than 90%. When the tertiary cooling stopping temperature CT (°C) is 500 °C or more, the ratio of total ferrite to bainitic ferrite area increases, and it becomes difficult to obtain a desired tensile strength. The CT (°C) temperature for stopping tertiary cooling is preferably room temperature or higher from the point of view of ease of manufacture.

[88] Winding stage In the winding stage, the cooled steel sheet is wound. The winding of the steel sheet is not particularly restricted and can be carried out according to a conventional method.

[89] Other stages Rolled steel sheet can undergo well-known treatments such as 1) fit rolling in order to improve ductility by straightening the shape of the steel sheet and introducing moving dislocations, 2) pickling in order to remove the inclusions that adhere to the surface of the steel sheet, and 3) plating.

[90] Intended use High-strength hot-rolled steel sheet MA / a / ZUZZ / UI Z l zo in accordance with the present modality may be applied to various members, such as automobile parts, which are required to have a tensile strength of 850 MPa or more. EXAMPLES

[91] Hereafter, preferred forms of this disclosure will be described more specifically with reference to the Examples. However, these forms do not limit this disclosure.

[92] Steels were melted and cast with the component compositions indicated in Table 1. The component values ​​in Table 1 are chemical analysis values ​​expressed as % by mass. Next, the steel pieces were hot-rolled under the manufacturing conditions indicated in Table 2, and then the hot-rolled sheets obtained were cooled and rolled to manufacture hot-rolled steel sheets.

[93] Using the hot-rolled steel sheets obtained, the presence or absence of damage to the perforated edge was evaluated. Regarding the presence or absence of damage to the perforated edge, the hot-rolled steel sheets obtained were perforated with a 20% clearance according to the method described in the Japanese Iron and Steel Federation standard JES T 1001-1996, and the perforated edge was visually inspected for damage. When the ratio MA / a / ZUZZ / UI Z l If the ratio of the damaged portion to the perforated circumference was 30% or more, it was evaluated as an occurrence of damage (C(x)); when the ratio was 10% or more and less than 30%, it was evaluated as preferable (B(o)); and when the ratio was less than 10%, it was evaluated as more preferable (A(Θ)).

[94] In addition, for the hot-rolled steel sheets obtained, the area ratio of bainitic ferrite to ferrite, the area ratio of bainitic ferrite, the area ratio of polygonal ferrite, the total area ratio of retained martensite to austenite, the mean dislocation density, the average diameter of TiC precipitates in the crystal grains, the mean number density of TiC precipitates in the crystal grains, the amount of Ti present as TiC precipitates precipitated in a matrix not in dislocations (the amount of Ti with respect to the total amount of Ti in the steel sheet), and the tensile strength were measured according to the methods described above. The results of the evaluation are shown in Table 3.

[95] In Table 1, it means that the component is not added intentionally. The underlines in Tables 1 to 3 mean that the underlined values ​​are outside the scope of the preferred modalities of this disclosure. The details of the abbreviations in Tables 2 and 3 are as follows. Final temperature of hot rolling: final working temperature FT (°C) MT of primary cooling: MT temperature (°C) at primary cooling stop Tertiary cooling CT: tertiary cooling stopping temperature (°C) • TiC precipitate diameter: average diameter of TiC precipitates in ferrite crystal grains and bainitic ferrite crystal grains • TiC precipitate density: number mean density of TiC precipitates in ferrite crystal grains and bainitic ferrite crystal grains • Ti precipitate-in-matrix ratio: percentage ratio obtained by dividing the amount of Ti present as TiC precipitates in the matrix (not in dislocations) by the amount of Ti in the steel sheet • Bainitic ferrite to ferrite area ratio: ratio of total bainitic ferrite to ferrite area Ratio of martensite to retained austenite area: ratio of total martensite to retained austenite area Dislocation density: dislocation density MA / a / ZUZZ / UI Z l zo media Board MA / a / ¿U¿¿ / U1 ¿ l ¿o ML / a / ZUZZ / U 1 ζ ι ζο ινΐΛ / a / zuzz / ui ¿ / ¿o MA / a / ZUZZ / UI Z l zo ML / a / ZUZZ / U 1 ζ ι ζο

[101] Based on the above results, Tests No. 1, 3, 5, 7, 8, 10, 11, 14, 18, 19, 20, 26, 27, 28, 29, 30, and 31 are examples in which the chemical composition, microstructure, and manufacturing conditions for the steel sheet were within the scope of the preferred modalities of this disclosure. They had high strength and no damage at the perforated edge.

[102] On the other hand, test No. 2 is an example in which the primary cooling rate was low. This is an example in which the average dislocation density, average number density of precipitates, precipitate-to-matrix Ti ratio, and tensile strength decreased with phase transformation at high temperature. Test No. 4 is an example where the primary cooling stop temperature was low. This is an example where the TiC precipitates precipitated insufficiently, and the average number density of the precipitates, the Ti precipitate-to-matrix ratio, and the tensile strength decreased. Test No. 6 is an example where the tertiary cooling arrest temperature was high. This is an example where the total ferrite to bainitic ferrite area ratio increased and the tensile strength decreased. Test No. 9 is an example in which the temperature MA / a / ZUZZ / UI Z l The final heat of hot rolling was low. This is an example in which coarse TiC precipitates precipitated onto austenite, ferrite transformation was promoted at a high temperature, and the average dislocation density, average number density of TiC precipitates, ratio of precipitated Ti to matrix, and tensile strength decreased. Test No. 12 is an example where the cooling start time after hot rolling was long. This is an example where the precipitation of coarse TiC precipitates in the austenite progressed, and the number-average density of the TiC precipitates, the ratio of precipitated Ti to matrix, and the tensile strength decreased.

[103] Test No. 13 is an example in which the cooling rate in the range from (MT + 50) °C to (MT) °C during primary cooling was low. This is an example in which the precipitation of TiC precipitates was promoted in the dislocations, and the mean number density, the ratio of precipitated Ti to matrix, and the tensile strength decreased. Test No. 15 is an example where the primary cooling arrest temperature was high. This is an example where the average dislocation density was low, and, furthermore, the precipitation of TiC precipitates on the dislocations was promoted, decreasing the Ti precipitate-in-matrix ratio, the average number density of TiC precipitates, and the tensile strength. Test No. 16 is an example where the tertiary cooling rate was low. This is an example where the average number density of the TiC precipitates and the tensile strength decreased. Test No. 17 is an example where the secondary cooling rate was high and the cooling time was short. This is an example where the TiC precipitates were insufficiently precipitated, and the average number density of the precipitates, the Ti precipitate-to-matrix ratio, and the tensile strength decreased. Test No. 21 is an example in which the value of [Ti] x [C] was less than 0.0015. This is an example in which the ratio of Ti precipitated in matrix and tensile strength decreased.

[104] Test No. 22 is an example in which the amount of C was small. The average number density of the TiC precipitates and the tensile strength decreased. In addition, this is an example in which the [Ti] / [C] ratio was high and damage occurred to the perforated edge. Test No. 23 is an example in which the Ti content was small and the [Ti] χ [C] value was less than 0.0015. This is an example in which the average number density of TiC precipitates, the ratio of precipitated Ti to matrix, and the tensile strength decreased. Test No. 24 is an example where the [Ti] / [C] ratio was high. This is an example where damage occurred to the perforated edge. Test No. 25 is an example in which the [Ti] χ [C] value was greater than 0.0160. This is an example in which coarse TiC precipitates were precipitated at a high temperature, and the average number density of the TiC precipitates and the tensile strength decreased. Test No. 32 is an example in which the Ti content was small and the [Ti] / [C] ratio was less than 0.16. This is an example in which the average number density of TiC precipitates, the ratio of precipitated Ti to matrix, and the tensile strength decreased. Test No. 33 is an example where the cooling rate in the range from (MT + 50) °C to (MT) °C during primary cooling was less than the average cooling rate in the range from the start of primary cooling to the primary cooling stop temperature MT (°C) + 50 °C. This is an example where the area ratio of the polygonal ferrite increased, and furthermore, the precipitation of TiC precipitates in the dislocations was promoted, and the mean number density of the TiC precipitates, the Ti precipitate-in-matrix ratio, and the tensile strength decreased. Test No. 34 is an example where the cooling rate in the range from (MT + 50) °C to (MT) °C during primary cooling was less than the average cooling rate in the range from the start of primary cooling to the primary cooling stop temperature MT (°C) + 50 °C. This is an example where the area ratio of the polygonal ferrite increased, and furthermore, the precipitation of TiC precipitates in the dislocations was promoted, and the mean number density of the TiC precipitates, the Ti precipitate-in-matrix ratio, and the tensile strength decreased.

[105] The preferred embodiments and examples of this disclosure have been described above, but this disclosure is not limited to such examples. It is obvious that those skilled in the art can conceive of various variations or modifications within the scope of the idea set forth in the claims, and it is understood that such variations or modifications naturally fall within the technical scope of this disclosure.

[106] The disclosure of Japanese Patent Application No. 2020-074180 filed on April 17, 2020 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each document, patent application, and technical standard were specifically and individually indicated to be incorporated by reference.

Claims

1. A high-strength hot-rolled steel sheet having a chemical composition comprising, by mass MA / a / ZUZZ / UI Z l zo C: from 0.030 to 0.250%; Si: from 0.01 to 1.50%; Mn: from 0.1 to 3.0%; Ti: from 0.040 to 0.200%; P: 0.100% or less; S: 0.005% or less; Al: 0.500% or less; N: 0.0090% or less; B: from 0 to 0.0030%; a total of one or more of Nb, Mo, and V: from 0 to 0.040%; a total of one or more of Ca and REM: from 0 to 0.010%; and the remainder consisting of Fe and impurities, a mass ratio [Ti] / [C] of an amount of Ti to an amount of C that is from 0.16 to 3.00, and a product [Ti] χ [C] of the amount of Ti and the amount of C that is from 0.0015 to 0.0160, the high-strength hot-rolled steel sheet: having an average dislocation density from 1 × 1014 to 1 × 1016 m-2; and comprising at least bainitic ferrite, wherein a total area ratio of bainitic ferrite to ferrite is 70% or more and less than 90%, wherein a total area ratio of retained martensite to austenite is 5% or more and 30% or less, wherein, in the ferrite crystal grains and in the bainitic ferrite crystal grains, a mean number density of TiC precipitates is from 1 × 1017 to 5 × 1018 (precipitates / cm3), wherein an amount of Ti present as a TiC precipitate precipitated in a matrix not in dislocations is 30% by mass or more of a total amount of Ti in the steel sheet, wherein a tensile strength is 850 MPa or more, and wherein [Ti] and [C] represent the amount of Ti and the amount of C (% by mass), respectively.

2. The high-strength hot-rolled steel sheet according to claim 1, comprising, by mass: B: 0.0001% or more and less than 0.0005%.

3. The high-strength hot-rolled steel sheet according to claim 1 or 2, comprising, by mass: the total of one or more of Nb, Mo, and V: from 0.01 to 0.040%.

4. The high-strength hot-rolled steel sheet according to claim 1 or 2, comprising, by mass: MA / a / ZUZZ / UI Z l zo the total of one or more of Ca and REM: from 0.0005 to 0.01%.

5. The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the total area ratio of the bainitic ferrite to the ferrite is 80% or more and less than 90%.

6. The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the area ratio of the bainitic ferrite is 50% or more and less than 90%.