Technical field
[0001]
　The present invention relates to a hot-rolled steel sheet.
BACKGROUND
[0002]
　In recent years, from increasing global awareness of environmental issues, improvement of reduction and fuel consumption of carbon dioxide emissions is a strong demand in the automotive field. For example, for such a problem it is very effective to reduce the vehicle weight, thereby reducing the weight of the vehicle body by applying high-strength steel sheet to the vehicle body. Therefore, in order to reduce carbon dioxide emissions, a conventional hot-rolled steel sheet or replaced with high-strength hot-rolled steel sheet, it has been strongly desired or further enhance the strength of the high-strength hot-rolled steel sheet.
[0003]
　Currently, the suspension parts of automobiles, high-strength hot-rolled steel sheet of a tensile strength 440 ~ 590 MPa grade is used. However, when reducing member weight by applying (member thickness) such a high-strength hot-rolled steel sheet in automobile parts, stiffness of the member is reduced.
　Further, when the applied stress is increased, the durability of the member or the fatigue characteristics are lowered member is lowered.
　Therefore, to enhance the rigidity and durability of the member by applying the structure that can reduce the load stress and stress concentration member. In this case, in order to obtain by molding a member having a complicated shape, it is necessary to extremely high moldability in the hot-rolled steel sheet.
[0004]
　In the press molding of the suspension members, burring, elongation flanging, a plurality of processing such as stretching processing is performed on the hot-rolled steel sheet, workability corresponding to these processing is required in the hot-rolled steel sheet.
　In general, the burring formability and stretch flanging property, are correlated with the hole expanding ratio measured by the hole expanding test. That is, by applying the high-strength hot-rolled steel sheet with excellent elongation and hole expansion underbody members, can be achieved improvement in the reduction and members rigid member weight by the thickness reduction of the same time, carbon dioxide emissions You can further reduce the amount.
[0005]
　Generally, as high-strength hot-rolled steel sheet for underbody members, Dual Phase steel mainly comprising a ferrite and martensite (hereinafter referred to as DP steel.) And the like. The DP steel, high strength, and excellent growth. However, the DP steel, due to the large intensity difference between the ferrite and martensite, strain and stress is concentrated on martensite in the ferrite in the vicinity during molding, cracks. Therefore, hole expansion of DP steel is low. Based on this finding, hot rolled steel sheet with increased reduced by hole expansion rate intensity differences between tissues have been developed.
[0006]
　Patent Document 1 mainly includes bainitic or bainitic ferrite, steel sheet having excellent hole expansion and high strength is disclosed. The steel sheet so substantially with a single tissue, hardly distortion or stress concentration, high hole expansion ratio. However, the steel sheet are the single steels mainly comprising bainite or bainitic ferrite, elongation deteriorates significantly. Therefore, in Patent Document 1, not possible to achieve excellent elongation and good hole expandability and at the same time.
[0007]
　In recent years, by using the ferrite excellent in elongation as a single tissue, Ti, steel plate having enhanced strength by a carbide of Mo or the like has been proposed (e.g., Patent Documents 2-4). However, the steel sheet disclosed in Patent Document 2 contains a large amount of Mo, the steel sheet disclosed in Patent Document 3 contains a large amount of V. Further, in the steel sheet disclosed in Patent Document 4, in order to refine the crystal grains, it is necessary to cool in the course of rolling. Therefore, in the prior art such as Patent Documents 2 to 4, the higher the alloy cost and manufacturing cost. Further, in the steel sheet disclosed in Patent Documents 2 to 4, the elongation because they greatly increased the strength of ferrite itself has deteriorated. Elongation of the steel sheet is higher than the growth of the single steels mainly comprising bainite or bainitic ferrite, balance between elongation and hole expansion is not necessarily sufficient.
[0008]
　Further, Patent Document 5, using the bainite instead of martensite in DP steel, steel sheet with a composite structure is disclosed in which enhanced hole expansion by the intensity difference smaller between the hard phase and the ferrite . Further, Patent Document 6 mainly includes a ferrite and tempered martensite, discloses steel sheet utilize bainite in order to increase the strength. In this steel sheet, to enhance the hole expansion by reducing the difference in hardness between the tempered martensite and ferrite. However, in Patent Documents 5 and 6, a result of enhanced area ratio of bainite in order to ensure the strength, elongation deteriorates, balance between elongation and hole expansion is not sufficient. In Patent Document 6, since it is necessary and cold rolling and subsequent annealing and cooling, the manufacturing cost is increased.
[0009]
　The member requiring good fatigue strength, conventionally, a steel sheet having enhanced fatigue strength by fine strengthening and solid solution strengthening have been used.
[0010]
　For example, Patent Documents 7 to 10 in order to obtain a steel sheet excellent in fatigue resistance, fine reinforcing is applied. Specifically, Patent Document 7 and Patent Document 8, the steel sheet was reduced ferrite grain size in average to below 2μm is disclosed. Patent Document 9, gradually becomes smaller steel sheet toward the surface average crystal grain size of polygonal ferrite from the thickness center is disclosed. Further, Patent Document 10, small steel plates the average block size of the martensite structure to 3μm or less is disclosed.
[0011]
　Further, for example, Non-Patent Document 1, fine reinforced, precipitation strengthening, increase in fatigue limit can be increased is disclosed for increasing the amount of yield strength in the order of solid solution strengthening. Non-Patent Document 2, when the Cu in the steel changes to precipitate from the solid solution product (solute), fatigue limit ratio is disclosed to be reduced. Thus, since the solid solution thereof and precipitates increases (solute) is decreased, the members that require excellent fatigue strength, limits the amount of deposit to be able to increase the fatigue strength as much as possible It was. As a result, the members that require excellent fatigue strength, steel plate having enhanced fatigue strength by solid solution strengthening have been used preferentially.
CITATION
Patent Document
[0012]
Patent Document 1: Japanese Patent 2003-193190 JP
Patent Document 2: Japanese Patent 2003-089848 JP
Patent Document 3: Japanese Patent 2007-063668 JP
Patent Document 4: Japanese Patent 2004-143518 JP
Patent Document 5: Japanese Patent 2004-204326 JP
Patent Document 6: Japanese Patent 2007-302918 JP
Patent Document 7: Japanese Patent Laid-Open 11-92859 discloses
Patent Document 8: Japanese Patent 11 -152544 JP
Patent Document 9: Japanese Patent 2004-211199 JP
Patent Document 10: Japanese Patent 2010-70789 JP
Non-patent literature
[0013]
Non-Patent Document 1: Abe Takashira: iron and steel, Vol. 70 (1984), No. 10, p. 145
Non-Patent Document 2: T. Yokoi et al: Journal of Materials Science, Vol. 36 (2001), p. 5757
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　The present invention is devised in view of the problems described above, an object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in strength and elongation and hole expansion. Another object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in strength and elongation and hole expansion and the fatigue strength.
Means for Solving the Problems
[0015]
　The present inventors have found that the chemical composition and metal structure extends to Influence the chemical composition and metal structure made extensive studies on the effect on the resistance spread hole, to optimize the chemical composition, mainly ferrite and martensite to obtain a metal structure comprising the, this metal structure and hard martensite and relatively soft martensite can mix revealed that can be enhanced elongation and hole expansion not only strength. Furthermore, we utilize Ti carbides as precipitates by controlling the particle size of the Ti carbide precipitates instead of a solid solution thereof (solid solution C and solid solution Ti) (Ti carbide) be utilized, it showed that can impart higher fatigue strength than the fatigue strength obtained by the solid solution strengthening of the steel sheet.
　That is, the gist of the present invention is as follows.
[0016]
　(1) hot-rolled steel sheet according to one embodiment of the present invention, at mass%, C: less than 0.030% or more 0.075%, Si + Al: 0.08 % ~ 0.40%, Mn: 0.5 % ~ 2.0%, Ti: 0.020 % ~ 0.150%, Nb: 0% ~ 0.06%, Mo: 0% ~ 1.0%, V: 0% ~ 1.00%, W : 0% ~ 1.0%, B : 0% ~ 0.005%, Cu: 0% ~ 1.2%, Ni: 0% ~ 0.80%, Cr: 0% ~ 1.5%, Ca : 0% ~ 0.005%, REM : 0% ~ 0.050%, P: 0% ~ 0.040%, S: 0% ~ 0.0100%, N: a 0% to 0.0100% has a chemical composition the balance being Fe and impurities, having a metal structure containing ferrite and martensite, in the metal structure, in area%, ferrite 90% to 98%, the martensite % To 10%, bainite is 0% to 3%, pearlite is 0% to 3%, and in the martensite, the ratio of the number of martensite grains having a hardness of at least 10.0GPa is 10% 8 the ratio of martensite grains number N1 with a hardness of less than or 8.0 GPa 10.0 GPa to the number N2 of martensite grains having a hardness of less than .0GPa, N1 / N2 is 0.8 to 1.2.
　(2) In the hot rolled steel sheet according to (1), wherein the chemical composition at mass%, Nb: 0.005% ~ 0.06 %, Mo: 0.05% ~ 1.0%, V : 0.02% ~ 1.0%, W : 0.1% ~ 1.0%, B: 0.0001% ~ 0.005%, Cu: 0.1% ~ 1.2%, Ni: 0 .05% ~ 0.8%, Cr: 0.01% ~ 1.5%, Ca: 0.0005% ~ 0.0050%, REM: is selected from the group consisting of 0.0005% - 0.0500% it may contain at least one that.
　(3) In the hot rolled steel sheet according to (1) or (2), the mass% of Ti present as Ti carbides may be more than 40% of Tief calculated by the following formula (a).
= Tief [Ti] -48 / 14 × [N] -48 / 32 × [S]
　(a) (4) In the hot rolled steel sheet according to the above (3), 7nm ~ 20nm to the total mass of all the Ti carbide percentage of the total mass of the Ti carbide having a circle equivalent diameter of may be 50% or more.
Effect of the invention
[0017]
　Hot rolled steel sheet according to aspect (1) to (4) of the present invention, the strength is excellent in the elongation and hole expansion not only high, molded easily member even when severe working is required it can. Therefore, the hot rolled steel sheet according to the present embodiment can be widely applied to members that suspension members and other severe working in vehicle is required. Also, members obtained from hot rolled steel sheet according to the present embodiment, since it has a high durability even small plate thickness, it is possible to reduce the vehicle weight significantly. Therefore, the hot rolled steel sheet according to the present embodiment, since effectively reduce body weight through reduction in thickness, it is possible to significantly reduce carbon dioxide emissions. Moreover, the hot rolled steel sheet according to aspect (4) of the present invention, since also has excellent fatigue strength as well as excellent elongation and hole expansion and high strength, the lifetime of such a strong cyclic loading member It can also be extended further. Therefore, (4) hot-rolled steel sheet aspect, can be suitably applied to more kinds of members than hot-rolled steel sheet aspects of (1) to (3).
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
It is a diagram showing an example of the relationship between [1] and the proportion of Ti carbides 7 ~ 20 nm to the total Ti carbide and (c-YP) / YP.
2 is a diagram illustrating the size and shape of the test piece in the low cycle fatigue test.
It is a diagram showing the 3: How to determine recurring yield stress from repeated stress-strain curve.
DESCRIPTION OF THE INVENTION
[0019]
　First, the study results of the present inventors, for the new findings obtained from this study results will be described.
[0020]
　DP steel is a steel obtained by dispersing hard martensite than ferrite in the soft ferrite, elongation in addition to the strength is high. However, hole expandability of the DP steel is very low. When the DP steel is deformed, the distortion and stress is concentrated in the DP steel the intensity difference between the ferrite and martensite, voids are easily formed causing ductile fracture. However, the mechanism by which voids generated has not been investigated in detail the relationship between microstructure and ductile fracture of DP steel was not always clear.
[0021]
　Occurrence and development of cracks in the hole expansion processing, generation of voids, growth, linking caused by ductile fracture to elementary process.
　Accordingly, the present inventors used a DP steel having a variety of tissues were examined and generation mechanism and hole expansion of voids during processing in detail. As a result, many voids to break the DP steel through increased (growth) and connected, it revealed that produced by brittle fracture or ductile failure of martensite.
[0022]
　Furthermore, the present inventors have found that the internal organization and destroyed ease of martensite vicinity of ferrite martensite, i.e. to examine the relationship between the ease of formation of voids in detail. As a result, the present inventors have produced the ease of voids was found that strongly influenced the interior structure of martensite (solid solution carbon content, etc.).
[0023]
　Further, solute carbon present in supersaturated in martensite, while greatly increasing the strength of martensite was found to readily occurs brittle fracture of martensite. The solid solution carbon is the main factor to increase the hardness of the martensite, so it is very difficult to directly stably measure the solid solution carbon, in an embodiment of the present study and later, in the martensite It considers the hardness of martensite and internal structure of martensite, instead of the amount of solute carbon. If the hardness of the martensite is at least 10.0 GPa, martensite very small distortion in the initial stage of deformation voids are generated by brittle fracture. Therefore, martensite grains hardness of at least 10.0GPa is greatly inhibit hole expansion of DP steel. Therefore, in order to suppress the generation of voids, it is effective to soften the martensite.
[0024]
　The martensite in order to soften the precipitated iron carbide by heat treatment such as tempering, it is effective to reduce the solid solution carbon amount. However, martensite having reduced dissolved carbon amount by precipitation of iron carbides, strength is low, reduces the strength of DP steel. In this case, in order to compensate the reduction in strength, it is necessary to increase the area ratio of martensite. However, since the area ratio of the ferrite having a high ductility increase the area ratio of martensite is reduced, the ductility is reduced in DP steel, is not sufficient elongation and hole expansion.
[0025]
　Therefore, the present inventors have conducted extensive studies metallographic to enhance the strength and elongation and hole expansion simultaneously. As a result, the present inventors found that by controlling the amount of amount as relatively soft martensite in hard martensite by changing the internal structure of martensite, the strength and elongation and hole expansion simultaneously It revealed that it is possible to increase. Describe the findings below.
[0026]
　Martensitic grains having a hardness of less than or 8.0 GPa 10.0 GPa (hard martensite) is greatly enhance the strength of DP steel, martensite grains (very hard martensite with a hardness of at least 10.0 GPa ) higher deformability than, for no brittle fracture, it is difficult to form a relatively void. However, when martensite is considered the inventors of DP steel composed of martensite grains only having a hardness of less than 8.0 GPa 10.0 GPa, the amount of voids increases with increasing deformation amount, and finally were unable to obtain a high hole expansion by a large amount of voids.
[0027]
　On the other hand, martensitic grains having a hardness of less than 8.0 GPa (relatively soft martensite) has a very high deformability, without fracture be given a high distortion, it is difficult to form a very void . Martensitic grains having a hardness less than the 8.0 GPa even increasing the strength of DP steel, but the amount of increase in the intensity is less than the amount of increase in strength due to martensitic grains having a hardness of at least 8.0 GPa. Martensitic grains having a hardness of less than 10.0 GPa over 8.0 GPa, because the can to produce a void, there is a possibility to reduce the hole expansion, martensite having a hardness of less than or 8.0 GPa 10.0 GPa if limiting the amount of grain in a certain amount or less, since the amount of voids is small, not reduced hole expansion almost. Therefore, increasing as much as possible the strength of DP steel to increase the amount of martensite grains having a hardness of less than or 8.0 GPa 10.0 GPa to an amount that does not greatly reduced hole expansion is, hardness of less than or 8.0 GPa 10.0 GPa further increasing the deformability while increasing the martensite grains maintaining the strength of the DP steel having a hardness of less than 8.0GPa in accordance with the amount of martensite grains having, in DP steel, high strength and high hole expansion it is possible to achieve both high elongation when. That is, relatively the ratio of the amount of the amount hard martensite for the soft martensite is in the desired proportions, it is possible to achieve both high strength and high hole expandability and high elongation. In the embodiments described below, but primarily utilizing martensitic grains having a hardness of less than or 8.0 GPa 10.0 GPa in order to increase the strength, martensitic grains having a hardness of at least 10.0 GPa, the since very easily to generate voids, thereby reducing as much as possible the amount of martensite grains having a hardness of at least 10.0 GPa.
[0028]
　Further, the present inventors have also studied the fatigue characteristics of the steel sheet. When the ratio of the repeating yield stress for the yield strength (YP) (c-YP) is increased, the low cycle characteristics and high cycle characteristics become better. Therefore, in the embodiment described later, it defines the ratio of the repeating yield stress for the yield strength (YP) (c-YP) as the fatigue strength. Here, repetitive yield stress (c-YP) is resistance to deformation after predetermined repetition deformation which is described later, ie, a resistance to fatigue. The present inventors have found that, when the ratio of the repeating yield stress for the yield strength (YP) (c-YP) is at least 0.90, because high resistance to fatigue even at low yield stress (YP), It found that it is possible to increase the productivity of press forming without sacrificing fatigue properties of the steel sheet.
[0029]
　Further, as described above, the amount of increase in the fatigue strength by precipitation strengthening, but it is known that less than the amount of increase in the fatigue strength by solid solution strengthening, increase in tensile strength by precipitation strengthening, solid solution strengthening greater than the amount of increase in tensile strength due. Therefore, we investigated in detail how it is possible to increase the no tensile strength sacrificing fatigue strength by precipitation strengthening.
[0030]
　As a result, the inventors have, by effective use of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm as a precipitate, a higher fatigue strength than the fatigue strength even precipitation strengthening is obtained by solid-solution strengthening steel it can be imparted to, namely, been found that the proportion of the repeating yield stress (c-YP) can be increased to 0.90 or more with respect to the yield strength (YP).
[0031]
　We, Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm is the reason for increasing the fatigue strength, are considered as follows. When the circle equivalent diameter of Ti carbides are 7 nm ~ 20 nm, the dislocation will bypass the Ti carbide, to form an annular dislocation called Orowanrupu around the Ti carbides. This Ti carbide Orowanrupu proliferate whenever traversing dislocation, the dislocation density increases. Since increased yield strength dislocation density as the cyclic deformation progresses increases, fatigue strength is increased. On the other hand, when the circle equivalent diameter of Ti carbides is less than 7 nm, dislocations and shear the Ti carbide and passes through the Ti carbides. Therefore, it is impossible to prevent the movement of dislocations by Ti carbides during cyclic deformation, fatigue strength decreases. Further, the circle equivalent diameter of Ti carbides exceeds 20 nm, the number of Ti carbide (density) decreases. Therefore, it is impossible to prevent the movement of dislocations by Ti carbides during cyclic deformation, fatigue strength decreases.
[0032]
　Therefore, C and allowed to bind as much as possible the solid solution Ti increases the amount of Ti carbide, important in enhancing the fatigue strength by increasing the proportion of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm on the entire Ti carbide is there.
[0033]
　The following describes a hot rolled steel sheet according to an embodiment of the present invention.
　First, the chemical composition of the hot rolled steel sheet according to the present embodiment will be described in detail. Incidentally,% of the content of each element means mass%.
[0034]
(C: 0.030% or more and less than 0.075%)
　C is an important element to generate martensite. Further, C is, it is possible to produce a Ti carbide bonded with Ti enhance the strength of the ferrite. In order to sufficiently generate martensite, it is necessary amount of C is 0.030% or more. Preferably, it is the amount of C is 0.035% or more, or 0.040% or more. However, when the C content is 0.075% or more, and the amount of martensite is too much, hole expansion is reduced. Therefore, it is necessary amount of C is less than 0.075%. Preferably, C content 0.070% or less 0.065% or less, or is 0.060% or less.
[0035]
(Mn: 0.5% ~
　2.0%) Mn is an important element to increase the strength and hardenability of the ferrite. Enhance hardenability, in order to produce martensite, it is necessary amount of Mn is 0.5% or more. Mn content is preferably 0.6% or more, 0.7% or more, or 0.8% or more, further preferably 0.9% or more or 1.0% or more. However, when the Mn amount exceeds 2.0%, it is impossible to sufficiently generate ferrite. Therefore, the upper limit of the Mn content is 2.0%. Mn content is preferably 1.9% or less, 1.8% or less, 1.7% or less, or no more than 1.6%, even more preferably 1.5% or less or 1.4% or less .
[0036]
(P: 0% ~ 0.040 percent)
　P is an impurity element, since the weld exceeds 0.040 percent significantly embrittlement, to limit the amount of P to 0.040% or less. P amount is preferably or 0.020% or less 0.030% or, more preferably 0.015% or less. The lower limit of the P amount is not particularly defined, but, reducing the P content to less than 0.0001% is economically disadvantageous. Therefore, from the viewpoint of production cost, the P content is preferably set to 0.0001% or more.
[0037]
(S: 0% ~ 0.0100%)
　S is an impurity element, since it adversely affects the production of weldability and during casting and hot rolling, restricts the amount of S to 0.0100% or less. Further, when the steel is excessively containing S, coarse MnS is formed, hole expansion is reduced. Therefore, in order to improve the hole expansion, it is preferable to reduce the S content. From this viewpoint, S content is preferably set to less or 0.0050% 0.0060% or less, and more preferably set to 0.0040%. The lower limit of S is not particularly defined, but to reduce the S content to less than 0.0001% is economically disadvantageous. Therefore, it is preferable that the S content 0.0001% or more.
[0038]
(Si + Al: 0.08% ~
　0.40%) Si and Al, strengthening of the ferrite is an important element affecting the strength through carbide precipitation during production and martensite ferrite. To generate ferrite 90 area% or more is required total amount of Si and Al is 0.08% or more. In order to further increase the ferrite content, the total amount of Si and Al is preferably 0.20% or more, more preferably 0.30% or more. On the other hand, if the total amount of Si and Al exceeds 0.40% precipitation of iron carbides in martensite is suppressed. Therefore, to reduce the number of martensite grains hardness of less than 8 GPa, described later (N1 / N2) is greater than 1.2, hole expandability deteriorates. Thus, the total amount of Si and Al is 0.40% or less. In order to improve the hole expansion, the total amount of Si and Al is preferably 0.30% or less, and more preferably 0.20% or less. Thus, it is important that the total amount of Si and Al in the range of 0.08% to 0.40%. When reducing the steelmaking cost, preferably the Si content is 0.05% or more, when the Al amount is 0.03% or more preferred. From the above, Si content is required to be 0.40% or less, preferably is 0.37% or less. Also, Al amount is necessary to be 0.40% or less, if it is 0.35% or less preferred. In order to further improve the surface properties of the steel sheet, preferably the Si content is less than 0.20%, the Al content is not more than 0.10% preferably.
[0039]
(N: 0 Pasento ～ 0.0100 Pasento)
　N is an impurity element. When N content exceeds 0.0100%, coarse nitrides are formed, degrading the bendability and hole expansion. Therefore, to limit the N content to 0.0100% or less. Also, when the N content increases, the probability of generating blowholes increases during welding, it is preferable to reduce the N content. From this viewpoint, N amount is 0.0090% or less, 0.0080% or less, or, preferably not more than 0.0070%, 0.0060% or less, 0.0050% or less, or 0 and more preferably .0040% or less. The lower limit of the N content is not particularly defined, the N content to less than 0.0005%, the manufacturing cost increases greatly. Therefore, it is preferable that the N content less than 0.0005%.
[0040]
(Ti: 0.020 Pasento ～ 0.150
　Pasento) Ti is, carbide is formed, is an important element to strengthen the ferrite. When Ti content is less than 0.020%, the strength of the ferrite is not sufficient, the strength of the steel sheet is insufficient. Elongation decreases when increasing the area ratio of martensite to compensate for deficient strength. Therefore, it is necessary that the amount of Ti is 0.020% or more. To strengthen the ferrite, Ti amount is preferably 0.030% or more, more preferably 0.040% or more. In particular, in order to increase the tensile strength is preferentially, Ti content 0.070% or more 0.080% to 0.090% or more, or particularly preferably 0.100% or more. On the other hand, if the Ti content exceeds 0.150%, the ferrite is excessively enhanced elongation greatly reduced, to limit the amount of Ti below 0.150%. Ti content is preferably less than or equal to 0.140% or less, or 0.130%. In particular, in order to keep as much as possible the elongation, it is preferable Ti content is less than or 0.060% less than 0.070%.
[0041]
　The basic chemical composition of the hot rolled steel sheet according to the present embodiment, the above elements (essential elements), and the impurity (impurity element), and a balance of Fe. Hot rolled steel sheet according to the present embodiment may further include the following elements (optional elements). That is, the basic part of the remainder of Fe in the chemical composition, 0% to 0.06% of Nb, 0% to 1.0% of the Mo, 0% to 1.00% of the V, 0% to 1 .0% of W, 0% ~ 0.005 percent B, 0% ~ 1.2% of Cu, 0% ~ 0.80% of Ni, 0% ~ 1.5% of Cr, 0% ~ 0 it can be replaced with at least one member selected from the group consisting of .005% of Ca, 0% ~ 0.050 percent REM.
[0042]
　The hot rolled steel sheet according to the present embodiment, Nb amount may be 0% to 0.06%.
[0043]
(Nb: 0 Pasento ～ 0.06
　Pasento) Nb is an element related to precipitation strengthening of ferrite. When Nb content exceeds 0.06%, the starting temperature or the rate of the ferrite transformation significantly reduced, since the ferrite transformation does not proceed sufficiently, elongation deteriorates. Therefore, Nb content is preferably not more than 0.06%, 0.05% or less, 0.04% or less, 0.03% or less, or, more preferably not more than 0.02%. To enhance the ferrite, Nb amount thereof is preferably 0.005% or more, and more preferably 0.010% or more. Also the amount of Nb is less than 0.005% Nb do not adversely affect the steel sheet properties. Therefore, Nb amount may be 0%, may be less than 0.005%.
[0044]
　Hot rolled steel sheet according to the present embodiment, at least one selected from the group consisting of 0% to 1.0% of Mo, 0% to 1.00% of the V, 0% to 1.0% of W it may contain. That is, in the hot rolled steel sheet according to the present embodiment, Mo amount is 0% to 1.0%, the amount of V is 0% to 1.00% W content may be 0% to 1.0%.
[0045]
(V: 0 Pasento ～ 1.00 Pasento, W: 0 Pasento ～ 1.0 Pasento, Mo: 0 Pasento ～ 1.0
　Pasento) V, Mo, W is an element to increase the strength of the steel sheet. To further increase the strength of the steel sheet, steel sheet, 0.02% to 1.00% of the V, 0.05 to 1.0% of Mo, the group consisting of 0.1% to 1.0% of W preferably contains at least one selected from. V content is less than 0.02% Mo content is less than 0.05%, even the W content is less than 0.1% V, Mo, W does not adversely affect the steel sheet properties. Therefore, V content may be 0%, may be less than 0.02%. Further, Mo amount may be 0%, may be less than 0.05%. W amount may be 0%, may be less than 0.1%. However, V amount, Mo amount, the W content is excessive, there is a case where formability is deteriorated. Therefore, the amount of V is 1.00% or less, W content of 1.0% or less, it is preferable Mo amount is 1.0% or less.
[0046]
　Hot rolled steel sheet according to the present embodiment, from 0% to 0.005% of B, 0% to 1.2% of the Cu, 0% to 0.80% of the Ni, 0% to 1.5% of the Cr it may contain at least one selected from the group consisting of. That is, in the hot rolled steel sheet according to the present embodiment, B amount is 0% ~ 0.005%, Cu content of 0% ~ 1.2%, Ni content is 0% ~ 0.80%, Cr content is 0% it may be to 1.5%.
[0047]
(Cr: 0% ~ 1.5% , Cu: 0% ~ 1.2%, Ni: 0% ~ 0.80%, B: 0% ~ 0.005%)
　in order to further increase the strength of the steel sheet, steel plate, 0.01% to 1.5% Cr, 0.1% 1.2% of the Cu, 0.05% 0.80% of the Ni, 0.0001% 0.005% of the B it may contain at least one member selected from the group consisting of. Cr content is less than 0.01%, Cu content is less than 0.1%, Ni content is less than 0.05%, B content is even less than 0.0001%, Cr, Cu, Ni , B is a steel sheet Characteristics It does not adversely affect the. Therefore, Cr amount, may be 0% may be less than 0.01%. Further, Cu amount may be 0%, may be less than 0.1%. Ni content may be 0%, may be less than 0.05%. Amount B is may be 0%, may be less than 0.0001%. However, Cr content, Cu content, Ni content, the B amount is excessive, there is a case where formability is deteriorated. Therefore, Cr amount is 1.5% or less, Cu amount is 1.2% or less, Ni amount 0.80% or less, it is preferable B content is 0.005% or less.
[0048]
　Hot rolled steel sheet according to the present embodiment may contain at least one member selected from the group consisting of 0% to 0.005% Ca, 0% to 0.050% of the REM. That is, in the hot rolled steel sheet according to the present embodiment, Ca amount is 0% to 0.005%, REM amount may be 0% to 0.050%.
[0049]
(Ca: 0% ~ 0.005%, REM: 0% ~
　0.050%) Ca and REM are effective elements for control of the form of oxides and sulfides. Therefore, the steel sheet is, 0.0005% to 0.050% of the REM, may contain at least one member selected from the group consisting of 0.0005% to 0.005% of the Ca. When the Ca content and REM amount is excessive, there may impair moldability. Therefore, the upper limit of the REM content is 0.050% and the upper limit of Ca content is 0.005%. Ca content may be 0%, may be less than 0.0005%. REM content may be 0%, may be less than 0.0005%.
　In the present invention, REM refers to elements of the lanthanide series. REM is often added to the steel in at mischmetal. Therefore, it is often contain two or more steel sheet is selected from the elements of the lanthanide series, such as La and Ce. Is in the steel, the metal La and Ce may be added in place of the misch metal.
[0050]
　In the hot rolled steel sheet according to the present embodiment, the balance other than the above elements is Fe and impurities, the steel sheet within a range that does not impair the effects of the present invention may contain trace amounts of other elements.
[0051]
　It will be described in detail below metal structure of the hot rolled steel sheet according to the present embodiment.
[0052]
　Ferrite is the most important organization in ensuring the growth. The area ratio of ferrite can not achieve a high elongation is less than 90%, the area ratio of ferrite is 90% or more. Preferably, the area ratio of ferrite is 91% or more, 92% or more, or is 93% or more. However, if the area ratio of ferrite is more than 98%, the area ratio of martensite is small, can not be sufficiently increased the strength of the steel sheet by martensite. As a result, for example, compensate for the deficient strength other approaches precipitation strengthening, uniform elongation is lowered. Therefore, the area ratio of ferrite needs to be 98% or less. Preferably, the area ratio of ferrite is 97% or less, 96% or less, or 95% or less.
[0053]
　Martensite is an important organization in order to achieve high strength and high hole expansion. Since the area ratio of martensite strength is less than 2% is not sufficient, the area ratio of martensite is 2% or more. Preferably, the area ratio of martensite is 3% or more, or more than 4%. On the other hand, when the area ratio of martensite exceeds 10%, also control the internal structure of martensite, it is impossible to express the high hole expansion. Therefore, the area ratio of martensite is 10% or less. Preferably, the area ratio of martensite is less than 9% or less than 8%.
[0054]
　Further, as described above, hardness or martensite grains 10.0GPa the deformability is low, it tends to form extremely voids. Therefore, the ratio of the total martensite grain martensitic hardness than 10.0GPa for particle good The lower. Specifically, it is necessary to limit the number ratio of 10.0GPa more martensite grains to the total martensite grains (number density) to 10% or less. The number ratio of the 10.0GPa more martensite grains, preferable to be 5% or less, may be 0%.
[0055]
　Furthermore, the ratio of martensite grains in the number N1 is less than the hardness hardness to the number N2 of martensite grains of less than 8.0 GPa or more 8.0 GPa 10.0 GPa (N1 / N2) is at 0.8 to 1.2 There is a need. When (N1 / N2) is greater than 1.2, voids are easily generated from martensite grains, hole expansion is reduced. On the other hand, in the (N1 / N2) is smaller than 0.8, the strength increases the proportion of soft martensite is insufficient. However, in order to compensate for this lack of strength, increasing the area ratio of martensite, hole expansion and elongation decreases. To increase the hole expansion more stable, if it is (N1 / N2) is 1.1 or less preferred. To increase the strength more stable, if it is (N1 / N2) is 0.9 or more preferred.
[0056]
　Moreover, the hot rolled steel sheet according to the present embodiment, if each not more than 3% of bainite and pearlite area ratio, may contain bainite and pearlite as the balance of the metal structure. Fraction of bainite and pearlite (area ratio and area fraction), better fewer. Moreover, as will be understood from the measurement method described below, for the sum of the area ratio and the pearlite area ratio and bainite area ratio of the ferrite area ratio and martensite can be regarded as 100%, the area of ​​martensite the sum of the incidence and pearlite area ratio and bainite area ratio of is 2-10%.
[0057]
　Pearlite degrades hole expansion. Therefore, the fraction of pearlite be as small as possible, it may be 0%. However, since the influence of the area ratio of pearlite gives to 3% value, if pearlite hole expansion is small, the area ratio of pearlite is allowed up to 3%. Therefore, the area ratio of pearlite is 0% to 3%. To increase the hole expansion more reliably, it is preferable to limit the area ratio of pearlite or less or 1% or less 2%.
[0058]
　Moreover, the balance of the metal structure, there may be other bainite pearlite. Bainite increases the strength of the steel sheet and excellent in deformability, does not reduce the hole expandability of the steel sheet. However, increase in strength of the steel sheet by bainite is less than the amount of increase in the strength of the steel sheet due to martensite. Therefore, the hot rolled steel sheet according to the present embodiment need not include bainite, may area ratio of bainite be 0%. Strength when the area ratio of bainite is 3% or more is not sufficient. Therefore, the area ratio of bainite is 0% to 3%. To increase the strength and hole expansion more reliably, it is preferable to limit the area ratio of bainite than 2% or 1% or less.
[0059]
　Here, ferrite, martensite, bainite, the area ratio of pearlite observes the metal structure by an optical microscope, the ferrite in the field of view (observation area), is obtained by identifying martensite, bainite, and pearlite. The observation sample is separated 1m or more from the edge in the rolling direction of the steel sheet, and the width center parallel plate thickness cross section in the rolling direction of the steel plate from a position corresponding to the steel plate (section including the entire thickness) Surface ( are taken to have the viewing surface). Polishing the surface (observation surface) of the sample taken, nital reagent, etched with repeller reagent, to prepare 2 kinds of sample for observation. Observation region with an optical microscope, the observation plane is a region spaced in the thickness direction from the surface of the steel sheet by 1/4 of the thickness (1/4 thickness region). Ferrite by performing image processing on the image of the observation area, measuring the area fraction of pearlite and martensite. Incidentally, ferrite, regions other than the pearlite and martensite (the balance) is defined as the bainite. That is, the area ratio of bainite, 100, is calculated by subtracting the area ratio of ferrite of the area ratio of martensite, and pearlite area ratio of. Magnification of the optical microscope is 500 times, the observation area is 5 field. The area ratio of each tissue (ferrite, martensite, pearlite, bainite) is obtained by averaging the respective area ratios obtained in five fields.
[0060]
　The hardness of the martensite is measured by a nanoindentation method which can control the indentation load in μN increments. Measurement sample is taken in the same manner as the sample for the above observations. This measurement sample, the cross section parallel to the rolling direction of the steel sheet (section including the entire thickness), and chemical polishing by polishing after the colloidal silica with emery paper, further electrolytic polishing to remove the working layer. Nanoindentation method at (indentation test), using a Berkovich type indenter, indentation load is 500MyuN. Measurement region by the nanoindentation method is a region apart in the thickness direction from the surface of the steel sheet by 1/4 of the thickness (1/4 thickness region). The number of martensite grains to be measured is 30 or more. For example, the number of martensite grains to be measured is 30 to 60. The upper limit of the number of martensite grains to be measured is not particularly limited. It is increased the number result is statistically sufficiently by increasing the number of martensite measuring until no change.
　Measured were divided into three by the martensite grain hardness, martensite having a hardness of less than martensite grain number ratio and 8.0GPa having a predetermined number ratio (10.0 GPa or more the hardness of the three classes evaluating the internal structure of martensite in martensite grains ratio of the number of) having a hardness of less than or 8.0 GPa 10.0 GPa for the number of grains. For example, 40 to 50 martensite grain hardness in the region spaced ¼ thickness direction of the sheet thickness from the steel sheet surface (1/4 thickness region) is measured, these martensite grains, martensitic grains having a hardness of less than 8.0 GPa, martensite martensite grains having a hardness of less than or 8.0 GPa 10.0 GPa, classified into martensitic grains having a hardness of at least 10.0 GPa, are included in each class to count the number of site grains. Having a hardness of less than or 8.0 GPa 10.0 GPa for the number of martensite grains having a hardness less than the number ratio and 8.0 GPa in the martensitic grains having a hardness of more than 10.0 GPa from the number of martensite grains in each class calculating the ratio of the number of martensite grains.
[0061]
　Hereinafter, the hot rolled steel sheet according to a modification of this embodiment will be described in detail. This modification has met all of the requirements of the above embodiment, in this modification, further the Ti carbide metal structure is controlled as follows.
[0062]
　Ti nitride and Ti sulfide, than Ti carbide to produce at high temperature. Therefore, not all of the Ti in the steel can be effectively utilized as a Ti carbide. Therefore, as the amount of Ti that can be effectively used as a Ti carbide defines Tief (mass%) calculated by the following equation (2). In the following formula (2), [Ti], the amount of Ti (mass%), [N], the amount of N (mass%), [S] represents the amount of S (mass%).
[0063]
　Tief = [Ti] -48/14 × [N] -48/32 × [S] (2)
[0064]
　Ti carbides are important precipitates in further enhancing the fatigue strength. Therefore, to impart excellent fatigue strength steel sheet, mass% of Ti present as Ti carbide (the amount of C and bound Ti) is more than 40% of Tief calculated by the equation (2) (0. it is at least required to be 4 or more times). Therefore, in order to enhance the fatigue strength is preferably the mass% of Ti present as Ti carbide is at least 40% of Tief, more preferably a 45% or more (0.45 or higher). When the mass% of Ti present as Ti carbide is less than 40% of Tief calculated by the equation (2), Ti carbides sufficiently brought out effect on fatigue strength having circle-equivalent particle diameter of 7 nm ~ 20 nm it is not possible, it is impossible to impart excellent fatigue strength of the steel sheet.
[0065]
　Further, as described above, the Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm, increasing the fatigue strength of hot rolled steel sheet. On the other hand, Ti carbide having a circle equivalent diameter of greater than Ti carbide and 20nm with a circle-equivalent particle size of less than 7nm does not increase the fatigue strength almost. Figure 1 is a diagram showing an example of the relationship between the ratio of Ti carbides 7 ~ 20 nm to the total Ti carbide and (c-YP) / YP. The data in FIG. 1, except the ratio of Ti carbides 7 ~ 20 nm on the entire Ti carbides, meet the conditions of this modification. As shown in FIG. 1, when the ratio of the total mass of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all the Ti carbide is 50% or more, since Ti carbide enhances the fatigue strength, the proportion of the repeating yield stress for the yield strength (YP) (c-YP) can be increased to 0.90 or more. Therefore, to impart excellent fatigue strength steel sheet is also necessary that the proportion of the total weight of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all the Ti carbide is 50% or more . Therefore, it is preferable when the ratio of the total mass of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all the Ti carbide is 50% or more. When the ratio of the total mass of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all the Ti carbide is less than 50%, Ti carbide has on fatigue strength having circle-equivalent particle diameter of 7 nm ~ 20 nm since the effect is not sufficient, it is not possible to impart excellent fatigue strength of the steel sheet.
[0066]
　Therefore, it is mass% of Ti present as Ti carbides than 40% of Tief, and the proportion of the total mass of the Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all the Ti carbide 50% If it is more, the proportion of the repeating yield stress for the yield strength (YP) (c-YP) can be increased to 0.90 or more.
[0067]
　Mass% of Ti present as Ti carbide is determined by the following method. A predetermined amount of steel sheet dissolved by electrolysis, determine the total weight of Ti in the precipitates by quantifying the weight of Ti in the residue. Moreover, to calculate the total weight of the nitrogen contained in the steel sheet which had been dissolved from the mass% of nitrogen in the weight and the steel sheet of dissolved steel, the Ti in the TiN by multiplying 48/14 on the total weight of the nitrogen to determine the total weight. Steel sheet to obtain a total weight of Ti in Ti carbide by the total weight of Ti in the precipitates subtracting the total weight of Ti in Ti nitride (TiN), and dissolved the total weight of Ti in the Ti carbide mass% of Ti is calculated that from the weight present as Ti carbide.
[0068]
　Percentage of the total mass of the Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all the Ti carbide is determined by the following method. Selecting at least 20 points at the region of 10 [mu] m × 10 [mu] m from the obtained element distribution image using a 3D-AP (three-dimensional atom probe). In each region, the particles which contain Ti and C were identified as Ti carbide, measuring the circle-equivalent diameter of Ti carbide having a circle equivalent diameter of 1 nm ~ 100 nm. In measuring the circle-equivalent diameter of Ti carbide, in order to increase the accuracy, appropriately selecting the magnification of the element distribution image in accordance with the circle-equivalent particle diameter and the effective digits of Ti carbide. The resulting percentage of the weight of the Ti carbide calculated with circle-equivalent diameter of 7 nm ~ 20 nm to the weight of Ti carbide having a circle equivalent diameter of 1 nm ~ 100 nm from the density of the particle size distribution and Ti carbides, all the percentage regarded as the ratio of the total mass of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of the Ti carbide.
[0069]
　Repeat yield stress (c-YP) is determined by the following method. To obtain the relationship between the number of repetitions and the maximum stress corresponding to the number of repetitions until the test piece shown in FIG. 2 in the low cycle fatigue test is broken, strain rate of 0.4% / s, 0.2 subjecting% strain cyclic loading with amplitudes in the specimen. The low cycle fatigue test 0.3%, 0.5%, 0.8%, performed at 1.0% strain amplitude. Thereafter, obtained from the test results in each strain amplitude, to determine the maximum stress that corresponds to the number of repetitions of the number of repetitions of the half at break, the relationship between the strain amplitude and the maximum stress (repeated stress-strain curve). As shown in FIG. 3, 0.2% strain, and insert the straight line having an inclination of Young's modulus to a point stress 0 MPa, determine the intersection of the straight line and the repeated stress-strain curve. Repeated stress at this intersection determines the yield stress (c-YP).
[0070]
　On the surface of the hot rolled steel sheet according to the embodiment and its modifications described above, the organic film-forming, film laminate, organic salts / mineral salt treatment, Nonkuro process, one obtained by performing a surface treatment by plating or the like or it may have more surface layers (surface film). Even hot-rolled steel sheet have these surface layers, the effect of the present invention can be sufficiently obtained without being hindered.
[0071]
　Tensile strength of the above embodiments and hot rolled steel sheet according to the modified example, since it is desirable to increase the tensile strength in accordance with the amount of Ti in the hot-rolled steel sheet, or a tensile strength of 500MPa and (2500 × ([Ti] - preferably a 0.02) +500) MPa or more. Similarly, preferably the product of the tensile strength and elongation is at (13000 × [Ti] +15000) MPa ·% or more, preferably the product of the tensile strength and the hole expansion is at least 70000MPa%. Here, [Ti] represents the amount of Ti (mass%).
[0072]
　Hereinafter, a method for manufacturing the hot rolled steel sheet according to the above embodiment and its modifications.
[0073]
　Manufacturing method preceding the hot rolling, the molten steel chemical composition is not specifically limited except that it Steels to be within the scope of the chemical composition of the hot rolled steel sheet according to the above embodiment. That is, it is possible to first usual manner to adjust the chemical composition of the molten steel within the chemical composition described above with Steels, to produce a steel slab by casting. It is preferred to cast by continuous casting from the viewpoint of productivity.
[0074]
　Next, a steel strip having a chemical composition of the present embodiment (the slab), heated prior hot rolling. When the slab heating temperature is 1150 ° C. or higher, it is possible to sufficiently solution of Ti carbides, obtained fine Ti carbides during after finish rolling cooled, can further enhance the strength and fatigue strength. Therefore, the slab heating temperature is preferably a 1150 ° C. or higher. The upper limit of the slab heating temperature is not particularly defined. However, in order to reduce the manufacturing cost, the slab heating temperature is 1300 ° C. or less preferred. Moreover, it is not always necessary to heat the slab prior to hot rolling. For example, slabs may be sent directly hot rolled to hot rolling machine while maintaining the temperature of the cast slab above 1150 ° C..
[0075]
　After the slab heating in the hot rolling step, a rough rolling and finish rolling.
　When rough rolling termination temperature is 1000 ° C. or more, the Ti carbides without increasing the strength can be suppressed from being deposited by strain induced in the austenite region, solid solution needed to deposit in a subsequent step the Ti carbide to increase the strength Ti a sufficient amount secured. Therefore, rough rolling end temperature preferably a 1000 ~ 1300 ° C.. More preferably, the finishing temperature of the rough rolling is 1050 ° C. or more or 1080 ° C. or higher.
　The end of the final rolling temperature is 850 ~ 1000 ℃. When the finish rolling end temperature exceeds 1000 ° C., ferrite nucleation sites is reduced ferrite transformation by increasing the grain size of recrystallized austenite (gamma) is significantly delayed. As a result, the area ratio of ferrite is lowered, not enough elongation. Therefore, the finish rolling end temperature is 1000 ° C. or less. Further, in order to increase the elongation stably, finish rolling end temperature is preferably 950 ° C. or less. On the other hand, the finish rolling end temperature is less than 850 ° C., ferrite transformation starts before the next primary cooling, the driving force of ferrite transformation in the primary cooling is reduced. Therefore, even if increasing the cooling rate of the primary cooling, is not sufficient effect of the primary cooling on the concentration of carbon into austenite grains. As a result, martensite grains is reduced (N1 / N2) is less than 0.8 having a hardness of at least 8.0 GPa, the strength becomes insufficient. Therefore, the finish rolling end temperature is 850 ° C. or higher.
　Thus, in the hot rolling step, performed finish rolling after rough rolling, and ends the finish rolling in the temperature range of 850 ~ 1000 ° C..
[0076]
　After finish rolling, performing a primary cooling and the secondary cooling and tertiary cooling and quaternary cooling and winding in that order.
　After finish rolling, performing primary cooling from the final rolling end temperature to a secondary cooling start temperature. In this primary cooling, the average cooling rate from finish rolling completion temperature to a secondary cooling start temperature (primary cooling rate) is 20 ° C. / s or higher.
　In order to form a martensitic grains having various hardness in the same metal in the tissue, it is effective to control the amount of carbon contained in the martensite grains each.
　The amount of carbon in austenite before the martensitic transformation, when the austenite is transformed into ferrite, go increased by carbon moves from ferrite to austenite. When ferrite transformation proceeds, the austenite is because going isolated are separated by a ferrite, it can not move in the carbon between austenite grains. Carbon content in the austenite grains will vary with the temperature of the ferrite transformation that occurs around the austenite grains. Therefore, in the same metal structure, the ferrite transformation temperature is varied, the ferrite transformation ratio that is locally varied, austenite grains are obtained of varying the carbon content in the same metal in the organization. Martensite, because austenite is obtained by transformation, can be obtained martensite grains wide hardness range as a result.
[0077]
　By controlling the primary cooling rate over 20 ° C. / s, it is possible to obtain a martensite grains of different hardness. Throughout this primary cooling, ferrite transformation occurs in a wide temperature range, the carbon content in the austenite grains by the temperature range, i.e. carbon into austenite grains are varying amounts of concentrate. As a result, the austenite grains obtained containing various amounts of carbon, it is possible from these austenite grains obtain martensite grains of different hardness.
　If the primary cooling rate is less than 20 ° C. / s, ferrite transformation proceeds only at high temperatures. As a result, since the driving force of ferrite transformation is small slow down ferrite transformation, most of the austenite grains is occupied by a low austenite grains of carbon content. Therefore, martensite grains is reduced (N1 / N2) is less than 0.8 having a hardness of at least 8.0 GPa, the strength becomes insufficient.
　In order to increase the strength of the steel sheet, when increasing the amount of martensite grains of 8.0 ~ 10.0 GPa, when the primary cooling rate is 30 ° C. / s or higher or 40 ° C. / s or more preferred.
[0078]
　After the primary cooling, performing secondary cooling in some sections of 600 ~ 750 ° C.. That is, the secondary cooling start temperature (the primary cooling stop temperature) is 600 ° C. Ultra and 750 ° C. or lower. Beyond 750 ° C. secondary cooling start temperature is decreased driving force of ferrite transformation, the area ratio of ferrite is less than 90%, the elongation is lowered. In order to make the mass% of Ti present as Ti carbide more than 40% of Tief, it is necessary that the secondary cooling start temperature is 750 ° C. or less. On the other hand, if the secondary cooling start temperature is 600 ° C. or less, or greater than the area ratio of 3% bainite, the area ratio of ferrite may become less than 90%, the elongation is lowered. Further, as the secondary cooling start temperature is low, the equivalent circle diameter of the Ti carbide is reduced, the amount of fine Ti carbides increases. Therefore, increasing the proportion of the total weight of Ti carbide having a circle equivalent diameter of 7nm ~ 20 nm to the total mass of all the Ti carbides limit the amount of Ti carbides having a circle-equivalent mean diameter of less than 7nm to more than 50% in order, the secondary cooling start temperature is required to be 670 ° C. or higher. Therefore, in order to obtain an excellent fatigue strength, preferably secondary cooling start temperature is 670 ℃ ~ 750 ℃. Incidentally, the secondary cooling end temperature (tertiary cooling start temperature) is below 600 ° C. or higher and the secondary cooling start temperature.
[0079]
　Average cooling rate in the secondary cooling is less 10 ° C. / s, secondary cooling time is 2-10 seconds. Or exceed an average cooling rate of 10 ° C. / s, the secondary cooling time when or less than 2 seconds, the area ratio of ferrite is deteriorated elongation decreases. In order to make the mass% of Ti present as Ti carbide more than 40% of Tief, it is necessary that the secondary cooling time is 2 seconds or more. On the other hand, the secondary cooling time exceeds 10 seconds, hole expansion area ratio of pearlite is increased to deteriorate. To get the stretch more stable, if the secondary cooling time is more than 3 seconds or 5 seconds or more preferred. To get more stable the hole expansion, when the secondary cooling time is less than 9 seconds or 7 seconds preferred. Secondary cooling end temperature is the temperature at the time has elapsed secondary cooling time from the start of secondary cooling, calculated from the secondary cooling start temperature and the average cooling rate and secondary cooling time of the secondary cooling It is.
[0080]
　Incidentally, hot rolled as described above, only by controlling the primary cooling and the secondary cooling can not be obtained a desired metal structure. That is, it is possible to obtain the desired metal structure by further controlling the cooling after the secondary cooling (tertiary cooling, quaternary cooling).
[0081]
　It performs three primary cooling after the secondary cooling. In this tertiary cooling, the steel sheet is cooled to a temperature range of up to 400 ° C. from the secondary cooling end temperature at 80 ° C. / s greater average cooling rate of martensite is generated from a low carbon content austenitic. In this temperature range, the diffusion rate of carbon is large, the average cooling rate is less than 80 ° C. / s, generated in a short time carbides grow, martensite is significantly softened. As a result, N1 / N2 is lowered to less than 0.8, the strength is not sufficient. The upper limit of the tertiary cooling rate is not particularly limited. To increase the accuracy of the cooling stop temperature is preferably a tertiary cooling rate 200 ° C. / s or less.
[0082]
　After three primary cooling, the four primary cooling is carried out. This quaternary cooling, the temperature range of the steel sheet from 400 ° C. to 100 ° C. and cooled at an average cooling rate of 30 ~ 80 ℃ / s. In the scope of this 100 ~ 400 ° C., martensite is generated from a high carbon content austenite. In this low temperature range, the average cooling rate exceeds 80 ° C. / s, it is impossible to sufficiently generate carbides. Therefore, hardness of the ratio of the number of martensite grains or 10.0GPa becomes 10% or more, the void is easily formed, hole expansion is reduced. On the other hand, if four primary cooling rate is less than 30 ° C. / s, excessive carbides are precipitated, because the martensite grains are softened, N1 / N2 is lowered to less than 0.8, the strength is not sufficient. The hardness increase more stably hole expansion as more limited amount of more martensite grains 10.0GPa, when quaternary cooling rate is less than 70 ° C. / s preferred. Furthermore, the hardness increase more elevated stronger the amount of martensite and less than 8.0 GPa 10.0 GPa, when quaternary cooling rate is 50 ° C. / s or more preferred. After four primary cooling, take up the hot-rolled steel sheet. Therefore, the coiling temperature is 100 ° C. or less.
[0083]
　By the method for producing a hot rolled steel sheet according to the above embodiment can produce hot-rolled steel sheet according to the above embodiment.
[0084]
　If necessary, an organic film-forming, film laminate, organic salts / mineral salts treatment may be subjected to a surface treatment using Nonkuro treatment.
Example
[0085]
　Hereinafter, while examples of the present invention, further describes the technical contents of the present invention. The conditions in the embodiment shown below is an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. Further, the present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention may employ various conditions.
[0086]
　Dissolving a steel having the chemical components shown in Table 1, were cast to obtain steel slabs. In hot rolling, the resultant steel slab was subjected to rough rolling and finish rolling after heating to 1150 ° C.. Rough rolling termination temperature is 1000 ° C., the finish rolling end temperature (FT) was temperature indicated in Tables 2-4. Then, (cooling from the finishing rolling end temperature to a secondary cooling start temperature) primary cooling (cooling from the start of secondary cooling until the elapse of the secondary cooling time) secondary cooling, tertiary cooling (secondary cooling perform termination temperature from up to 400 ° C. cooling) and quaternary cooling (cooling from 400 ° C. to 100 ° C.) under the conditions shown in tables 2-4, wound steel plates. The thickness of the hot-rolled steel sheet was 3.2mm. In Table 2-4, "primary cooling rate" shows the average cooling rate in the temperature range from finish rolling end temperature (FT) to the secondary cooling start temperature. "Secondary cooling rate" shows the average cooling rate from the start of secondary cooling until the elapse of the secondary cooling time. "Mitsugihiyasoku" indicates the average cooling rate in the temperature range up to 400 ° C. from the secondary cooling end temperature. "Quaternary cooling rate" shows the average cooling rate in the temperature range up to 100 ° C. from 400 ° C.. In Table 1, in the column that does not satisfy the essential conditions indicated in the above embodiments, underlined is granted. In Tables 2-4, the column that does not satisfy the essential conditions indicated in the above-described manufacturing method, underlined is granted.
[0087]
[Table 1]

[0088]
[Table 2]

[0089]
[table 3]

[0090]
[Table 4]

[0091]
　Microstructure was identified as follows: using an optical microscope. The resulting sample was taken from the hot-rolled steel sheet (No.A-1 ~ No.O-1 and No.a-1 ~ n-1) was, polished parallel plate thickness cross section in the rolling direction, the polishing surface It was etched in the reagent. The reagent, using a nital reagent and repeller reagent, and the sample was etched an abrasive surface at nital reagent was prepared and etched samples were polished surface at repeller reagent. 1/4 thickness region in the etched samples was observed by an optical microscope at 500 times magnification at nital reagent and photographed five regions (field of view). The area ratio of the ferrite by an image analysis of this photograph, was determined area ratio of pearlite. Further, the polishing surface 1/4 thickness region in the etched samples was observed by an optical microscope at 500 times magnification at repeller reagent, it took a photo of five regions (field of view). To determine the area ratio of the martensite by the image analysis of this photo. Area ratio of bainite, from 100, was determined by subtracting the area ratio of ferrite of a pearlite area ratio of the area ratio of martensite.
[0092]
　Further, in the obtained hot-rolled steel sheet (No.A-1 ~ No.O-1 and No.a-1 ~ No.n-1), was evaluated the following characteristics.
[0093]
　Yield stress (YP), tensile strength (TS), elongation (El), to the No. 5 test pieces disclosed in JIS Z 2201, were evaluated subjected to tensile test according to JIS Z 2241. Specimen, the longitudinal direction of the test piece to match the direction perpendicular to the rolling direction (sheet width direction), were taken from 1/4 of the distance away of the plate width from the edge in the plate width direction of the steel sheet . The tensile strength (TS) is the is and above 500MPa (2500 × ([Ti] -0.02) +500) MPa or more, the strength of the steel sheet was evaluated to be sufficient. In Table 8-10, the column strength of the steel sheet is evaluated as not sufficient, underlined is granted. If the product of the tensile strength (TS) elongation and (El) (TS × El) is at (13000 × [Ti] +15000) MPa ·% or more was evaluated as the elongation of the steel sheet is sufficient. In Table 8-10, the column elongation of the steel sheet is evaluated as not sufficient, underlined is granted.
[0094]
　It performs a hole expanding test in accordance with the hole expansion test method according to the Japan Iron and Steel Federation standard JFST1001-1996, was to evaluate the hole expansion value (λ). The product of the tensile strength (TS) hole expansion value (λ) (TS × λ) is the is more 70000MPa%, was evaluated as hole expansion of the steel sheet is sufficient. In Table 8-10, the column hole expansion was evaluated as not sufficient for steel, underline is granted.
[0095]
　In this embodiment, the hardness of martensite grains was determined by the nanoindentation method. Specifically, the sheet thickness cross section parallel to the rolling direction of the sample steel, polished with emery paper, and chemical polishing by colloidal silica, and electrolytic polishing in order to further remove processing layer. The nanoindentation method, using a Berkovich type indenter, the indentation load on the polished surface was 500MyuN. Indentation size was below a diameter 0.1 [mu] m.
　In this embodiment, 1/4 was measured 40-50 pieces of martensite grains in the thickness region, and hardness range of less than 8.0 GPa These martensite grain, less than 8.0 GPa 10.0 GPa (8.0 ~ and hardness range of 10.0 GPa), were classified into three categories of more hardness range 10.0 GPa. From the number of martensite grains classified in each segment, and the ratio of the number of martensite grains having a hardness of at least 10.0 GPa (number density) (%), the number of martensite grains having a hardness of less than 8.0 GPa N2 It was calculated and the ratio of martensite grains number N1 with a hardness of less than or 8.0 GPa 10.0 GPa for. In Tables 5 to 10, "> 10 GPa" represents the ratio of the number of martensite grains having a hardness of at least 10.0GPa (percent). Further, "number ratio N1 / N2" represents the ratio of martensite grains number N1 with a hardness of less than or 8.0 GPa 10.0 GPa to the number N2 of martensite grains having a hardness of less than 8.0 GPa .
[0096]
　In this example, it dissolved in a predetermined amount electrolyte samples taken from 1/4 of the distance away of the plate width from the edge in the plate width direction of the steel sheet by electrolysis. The residue was the total amount recovered from the electrolyte, to determine the total weight of Ti in the precipitates was determined by weight chemical analysis of Ti in the residue. Moreover, to calculate the total weight of the nitrogen contained in the steel sheet which had been dissolved from the mass% of nitrogen in the weight and the steel sheet of dissolved steel, the Ti in the TiN by multiplying 48/14 on the total weight of the nitrogen the total weight were determined. Steel sheet to obtain a total weight of Ti in Ti carbide by the total weight of Ti in the precipitates subtracting the total weight of Ti in Ti nitride (TiN), and dissolved the total weight of Ti in the Ti carbide from the weight calculated mass% of Ti present as Ti carbide.
[0097]
　Further, the needle-like sample taken from a distance away 1/4 of the plate width from the edge in the plate width direction of the steel sheet was analyzed by 3D-AP, to give an element distribution image. The element particles include Ti and C in the region of 10 [mu] m × 10 [mu] m of the distribution image is identified as Ti carbides was measured circle-equivalent diameter of Ti carbide having a circle equivalent diameter of 1 nm ~ 100 nm. The measurement was carried out with respect to a total of 20 regions, to obtain a particle size distribution of Ti carbides, give the percentage of the total weight of Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all Ti carbides.
[0098]
　Tissue and mechanical properties of the resulting steel sheet by the above method are shown in Tables 5-10. In Table 5-7, the column that does not satisfy the essential conditions indicated in the above embodiments, underlined is granted.

The scope of the claims
[Requested item 1]
　At
mass%, C: less than 0.030% or more%
0.075,
Si + Al: 0.08% ~ 0.40%, Mn:
0.5% ~ 2.0%, Ti: 0.020% ~ 0
Pasento .150,
Nb: 0
Pasento ～ 0.06 Pasento, Mo: 0 Pasento ～ 1.0 Pasento,
V: 0 Pasento ～ 1.00 Pasento, W: 0 Pasento ～ 1.0
Pasento, B: 0 Pasento ～ 0
%
.005,
Cu: 0% ~ 1.2%, Ni: 0% ~
0.80%, Cr: 0% ~ 1.5%, Ca: 0% ~
0.005%, REM: 0% ~ 0
% .050,
P: 0% ~ 0.040%, S:
0% ~ 0.0100%, N: 0% ~ 0.0100%
is has a chemical composition the balance being Fe and impurities,
　ferrite and it has a metal structure including a martensite,
　in the metal structure, in area%, ferrite 90% to 98%, martensite is 2% to 10%, bainite % To 3%, pearlite is 0% to 3%,
　and in the martensite, the ratio of the number of martensite grains having a hardness of at least 10.0GPa is
　10% martensite having a hardness of less than 8.0GPa the ratio of martensite grains number N1 with a hardness of less than or 8.0 GPa 10.0 GPa to the number N2 of sites grains, N1 / N2 is 0.8 to 1.2
hot-rolled steel sheet, characterized in that.
[Requested item 2]
　The chemical composition, by
mass%,
Nb: 0.005% ~ 0.06%,
Mo: 0.05% ~ 1.0%, V: 0.02% ~
1.0%, W: 0.
%
~ 1.0 1%, B: 0.0001%
~ 0.005%, Cu: 0.1% ~ 1.2%, Ni:
0.05% ~ 0.8%, Cr: 0.01%
1.5%
~,
containing at least one selected from the group consisting of
claim 1, wherein the hot-rolled steel sheet according to.
[Requested item 3]
　Mass% of Ti present as Ti carbide is not less than 40% of Tief calculated by the following formula (1)
hot-rolled steel sheet according to claim 1 or 2, characterized in that.
Tief = [Ti] -48 / 14 × [N] -48 / 32 × [S] (1)
[Requested item 4]
　Percentage of the total mass of the Ti carbide having a circle equivalent diameter of 7 nm ~ 20 nm to the total mass of all the Ti carbide is 50% or more
hot rolled steel sheet according to claim 3, characterized in that.