JP2004315902A - High strength hot rolled steel sheet excellent in fatigue property of blanking edge face and stretch-flanging property, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which is excellent in the fatigue properties of blanking edge faces and stretch-flanging properties. <P>SOLUTION: The high strength hot rolled steel sheet excellent in the fatigue properties of blanking edge faces and stretch-flanging properties has a composition comprising, by mass, 0.03 to 0.10% C, 0.05 to 1.5% Si, 1.0 to 2.2% Mn, ≤0.05% P, ≤0.01% S, 0.001 to 0.006% N, 0.06 to 0.24% Ti, 0.002 to 0.009% Ce and 0.001 to 0.006% O, and the balance Fe with inevitable impurities, and in which the concentration product of Ce and O also satisfies the inequality (1) of [Ce][O]≤3.8×10<SP>-9</SP>, and a bainitic ferrite phase is the structure with the maximum area ratio. If required, Cu, Ni, Nb, Mo and V can further be incorporated therein. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００３２】
【表２】

【００３３】
【表３】

【００３４】
表３から明らかなように、本発明の方法を用いれば、強度、延性、穴広げ性優れ、かつ打ち抜き疲労限度比に優れた鋼板を得ることができる。具体的には、強度と延性の積≧１６５００（ＭＰａ・％）、λ≧１．０、σ_Ｗ／σ_Ｂ≧０．３を満足する鋼板が得られた。
これに対して、Ａｌで脱酸した鋼７、８、９、１１および１２は伸びフランジ性と打ち抜き疲労限度比に劣り、非鉄炭化物生成元素が不足する鋼１０では伸びフランジ性が不足することが明らかとなった。また、圧延条件が不適切な鋼板（Ｎｏ．３、６、９、１０、１３および１６）ではパーライトやマルテンサイトの生成を回避できなかったため高いλが得られなかった。
【００３５】
（実施例２）
質量％にて、Ｃ：０．０３５％、Ｓｉ：１．０％、Ｍｎ：１．５％、Ｐ：０．０１％、Ｓ：０．００１％、Ｃｕ：１．２％、Ｎｉ：０．６％、Ａｌ：０．０００６％、Ｔｉ：０．１５％、Ｎｂ：０．０３％を含有し、ＣｅとＯの含有量が異なり、残部がＦｅである鋼片を製造した。これらを加熱温度１２５０℃、仕上圧延終了温度８８０℃、平均冷却速度４５℃／秒、巻取り温度４５０℃の条件で３．５ｍｍの熱延鋼板とした。このようにして得られた鋼板の強度、延性、断面組織、穴広げ性、および打ち抜き穴付き試験片の（σ_Ｗ／σ_Ｂ）を調べた。評価方法は実施例１と同じである。
【００３６】
その結果、何れの鋼も強度８００ＭＰａ以上、延性２０％以上を示し、ベイニティック・フェライトの面積率が９４％以上の鋼板となったが、λと打ち抜き疲労限度比はＣｅ濃度とＯ濃度の影響を強く受けた。図１にＣｅ濃度、およびＯ濃度を座標軸として示すように、本発明の範囲内であれば、優れた打ち抜き端面疲労特性（σ_Ｗ／σ_Ｂ≧０．３）と優れた伸びフランジ性（λ≧０．９５）を兼ね備えた高強度熱延鋼板の得られることが明らかである。
【００３７】
【発明の効果】
本発明の方法によれば、打ち抜き端面の疲労特性と伸びフランジ性に優れた高強度熱延鋼板を得ることが出来る。
【図面の簡単な説明】
【図１】鋼板の打ち抜き端面疲労特性と伸びフランジ性をＣｅ濃度およびＯ濃度を座標軸として示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a high-strength hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability of a punched end face, and a method for producing the same.
[0002]
[Prior art]
Hot-rolled steel sheets are widely used for so-called underbody parts and wheels such as frames and arms for automobiles. Since high fatigue strength is required for these components from the viewpoint of durability against vibration during traveling, it is not always easy to reduce the weight through reducing the thickness of the steel material by increasing the strength. However, several steel plates have been proposed to meet such demands for improving the so-called base metal fatigue characteristics. For example, Japanese Patent Application Laid-Open No. 11-199973 (Patent Document 1) proposes a steel sheet in which fine Cu precipitates and / or solid solutions are dispersed in a composite structure steel sheet of ferrite and martensite.
[0003]
On the other hand, steel sheets used in this field require not only the fatigue properties of the base metal but also the resistance to fatigue fracture generated on the punched end face. In this document, this property will be referred to as punched end face fatigue property. In the past, finishing of the end face, which had been performed after the punching process, has been omitted in order to reduce manufacturing costs. This has become apparent as a result problem.
It is widely known that "burrs" and "small cracks" commonly found on stamped end faces are the starting points of fatigue failure, and the existence of these causes the fatigue life of parts to be predicted from the fatigue characteristics of the base metal. It is a well-known fact that the strength of the steel sheet is higher than the strength of the steel sheet. Therefore, the most reliable way to control the fatigue fracture that occurs on the punched end face is to remove "burrs" and "small cracks." It has come to be reported.
[0004]
Japanese Patent Application Laid-Open No. 9-202940 (Patent Document 2) has a main requirement that, among constituent elements, the concentration of Mn, Ti, Nb, and Cr with a predetermined weight added is within a predetermined range. Discloses a method for obtaining a steel sheet having excellent notch fatigue characteristics. Further, Japanese Patent Application Laid-Open No. 2001-40450 (Patent Document 3) discloses that the main requirement is to set the volume ratio of martensite to 3% or more and the volume ratio of pearlite to 5% or less, so that the fatigue properties of the sheared end face are reduced. A method for obtaining an excellent steel plate is described.
[0005]
[References]
(1) Patent Document 1 (Japanese Patent Application Laid-Open No. 11-199973)
(2) Patent Document 2 (Japanese Patent Laid-Open No. 9-202940)
(3) Patent Document 3 (JP-A-2001-40450)
(4) Patent Document 4 (JP-A-6-172924)
(5) Non-Patent Document 1 {"Steel Bainite Photo Book-1" (June 29, 1992, published by The Iron and Steel Institute of Japan, page 21, Fig. 2.9-d)}
[0006]
[Problems to be solved by the invention]
The properties required for a steel sheet used in this field include workability, particularly stretch flangeability represented by a hole expansion test value (λ), in addition to base metal fatigue properties and punched end face fatigue properties. In order to improve the stretch flangeability of a high-strength steel sheet, it has been considered that, in addition to miniaturization and spheroidization of inclusions, suppression of generation of a hard phase such as iron carbide (cementite) is a common means. It is described in JP-A-6-172924 (Patent Document 4) and the like that the latter can be achieved by using bainitic ferrite as a main constituent structure.
[0007]
On the other hand, in the technology disclosed in Patent Document 2, although the production of pearlite or MnS, which is a cementite-containing phase, is suppressed, and the spheroidization of MnS by addition of Ca is mentioned, the structure constituting the steel sheet is ferrite. It mainly contains a small amount of bainite and martensite, and although high notch fatigue properties can be obtained, it is not necessarily sufficient as a means for obtaining excellent stretch flangeability.
[0008]
Further, in the technology described in Patent Document 3, even though there is no specific description about the phase constituting the steel sheet, since it includes martensite and pearlite, in order to obtain excellent shear end face fatigue properties, It is presumed that the stretch flangeability is sacrificed. As described above, there is no example proposed for a steel sheet mainly composed of bainitic ferrite, which is advantageous in improving the fatigue characteristics of the punched end face and obtaining high stretch flangeability.
In view of such circumstances, the present invention has been made to obtain a steel sheet having a bainitic ferrite excellent in stretch flangeability as a main structure and excellent in fatigue characteristics of a punched end face.
[0009]
[Means for Solving the Problems]
The present inventors performed a punching fatigue test of various steel sheets and observed a process leading to fracture. First, the locations of occurrence of fatigue cracks were arranged. As a result, irrespective of the phase constituting the steel sheet, when the clearance at the time of punching is large (for example, 25% or more), cracks are generated from the burrs, but when the clearance is small or the burrs are crushed, so-called coining. It was found that the frequency of crack generation from the burr portion was greatly reduced when the surface was subjected to the heat treatment, and instead, the crack began to be generated from the inside of the punched section. When the crack and its vicinity were observed during and after the fatigue test using an electron microscope, in the case of a steel sheet containing pearlite and martensite, it is estimated that the vicinity of those hard phases is the crack initiation position. However, in the case of a steel sheet containing bainitic ferrite as a main phase, the steel sheet containing pearlite and martensite may have a uniform structure in the first place. The place of occurrence could not be identified.
[0010]
On the other hand, in the case of a steel plate containing bainitic ferrite as the main phase, many observations were obtained which suggest that the crack initiation position is related to the presence of oxides and nitrides. Specifically, the oxide was an alumina-based oxide in the vicinity of the surface, and it was often thought that titanium nitride was involved inside the steel sheet.
As a result of investigations based on these observations, research was conducted on the assumption that the purpose could be achieved if a steel sheet having a structure containing bainitic ferrite as the main phase after reducing alumina-based oxides and titanium nitride as much as possible. The present invention has been completed.
[0011]
The summary is as follows. That is,
(1) In mass%, C: 0.03 to 0.10%, Si: 0.05 to 1.5%, Mn: 1.0 to 2.2%, P: 0.05% or less, S : 0.01% or less, N: 0.001 to 0.006%, Ti: 0.06 to 0.24%, Ce: 0.002 to 0.009%, O: 0.001 to 0.006% Wherein the balance consists of Fe and unavoidable impurities, the concentration product of Ce and O satisfies the formula (1), and the bainitic ferrite phase has a structure having a maximum area ratio. High strength hot rolled steel sheet with excellent fatigue properties and stretch flangeability.
[Ce] [O] ≦ 3.8 × 10 ⁻⁹ (1)
[0012]
(2) The fatigue characteristics and elongation of the punched end face according to the above (1), further containing 0.6 to 2.0% of Cu and 0.3 to 1.0% of Ni in mass%. High strength hot rolled steel sheet with excellent flangeability.
(3) In addition, one or more of Nb: 0.1 to 0.6%, Mo: 0.05 to 0.3%, and V: 0.025 to 0.15% by mass%. The high-strength hot-rolled steel sheet according to the above (1) or (2), which is excellent in fatigue characteristics and stretch flangeability of the punched end face.
[0013]
(4) The method for producing a steel sheet according to any one of (1) to (3), wherein the steel material having the chemical component according to any one of (1) to (3) is used. After heating to 1150 to 1250 ° C and rough rolling, finish rolling is completed at ₃ points of Ar to ₃ points of Ar + 100 ° C, and further cooled to 450 to 550 ° C at an average cooling rate of 20 ° C / sec or more. This is a method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability of a punched end face, which is characterized by winding at 550 ° C.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors first studied a method for suppressing an alumina-based oxide.
Generally, steel is deoxidized using ferrosilicon or aluminum, and among them, aluminum, which is excellent in efficiency and cost, is more general. Alumina-based oxides generated as a result of aluminum deoxidation easily aggregate and remain in the steel as coarse inclusions. This seems to be the starting point of the fatigue crack near the surface as described above.
[0015]
Therefore, (1) deoxidation by elements other than aluminum, and (2) oxides of alternative elements are not aggregated and coarsened like alumina-based oxides. Experimented. At this time, (3) the upper limit of the oxygen concentration that does not greatly deteriorate the yield of elements such as Ti, which is intended to fix C in steel as carbides, and (4) the starting point of fatigue cracks inside the steel sheet Research has been conducted on the fact that it is an element that also has an effect on the miniaturization of titanium nitride, which affects titanium.
[0016]
As a result, it has been found that the method using Ce is the most excellent, and that if the appropriate concentration range is satisfied together with the O concentration range, a steel sheet excellent not only in λ but also in punched end face fatigue can be obtained. It is most ideal if titanium nitride can be completely eliminated. However, as described later, Ti is an essential element in fixing C through formation of carbides and suppressing generation of cementite, and also in reducing N. Is the next best target for miniaturization due to the limitations of steelmaking technology. In addition, it is one of the reasons for selecting that Ce can be relatively easily produced in molten steel.
[0017]
Hereinafter, the reasons for limitation of the present invention will be described. First, the reasons for limiting the chemical components will be described. All component designations are in weight percent.
C is an essential element for ensuring the strength of the steel sheet, and at least 0.03% is necessary to obtain a high-strength steel sheet. However, if it is excessively contained, it is unavoidable to generate carbide due to Ti or the like and a cementite phase which is not preferable for stretch flangeability even if cooling conditions are used.
[0018]
Si is an element effective for securing strength without deteriorating the stretch flangeability, and at least 0.05% is necessary. However, if it is contained excessively, it causes scale flaws or chemical conversion treatment. The upper limit is set to 1.5% because the surface treatment property is deteriorated.
Mn is an element effective for increasing the strength of a steel sheet together with C and Si, and must be contained in an amount of 1.0% or more. However, if it exceeds 2.2%, the weldability is adversely affected. Therefore, the upper limit is set to 2.2%.
[0019]
P is effective as a solid solution strengthening element, but it is necessary to be 0.05% or less because there is a concern about deterioration of workability due to segregation.
S forms harmful effects such as forming inclusions such as MnS to deteriorate the stretch flangeability and bonding with Ti contained for the purpose of turning C into a carbide to lower the yield. Therefore, it should be suppressed as much as possible, but 0.01% or less is acceptable.
[0020]
N forms titanium nitride and deteriorates the fatigue characteristics of the punched end face. For this reason, it is preferable that the content be as low as possible, that is, 0.006% or less, and desirably 0.004% or less. On the other hand, the lower the value, the higher the production cost. Therefore, it is sufficient to set the lower limit to 0.001%.
Ti forms carbides with C to suppress the generation of cementite and contribute to the improvement of λ. On the other hand, if it is added more than necessary, it exists in the steel in a solid solution state, raises the recrystallization temperature, and the hot-worked structure tends to remain, thus impairing the ductility. The former effect is exhibited at 0.06% or more, and the latter can be avoided if it is 0.24% or less. Therefore, it is necessary to limit the effect to 0.06 to 0.24%.
[0021]
Ce and O combine to have the effect of increasing the cleanliness of the steel (function as a deoxidizing agent), and a part of the generated oxide is unlikely to coagulate and coarsen in the molten metal like alumina-based inclusions Therefore, even if it is present in the steel sheet, it does not act as a starting point of a fatigue crack or adversely affect the stretch flangeability. Further, since the fine particles are finely dispersed, they also have an effect of miniaturizing titanium nitride formed using the nuclei as a nucleus, and this is considered to have an effect of improving the fatigue characteristics of the punched end face. Such an effect is manifested when the Ce concentration is 0.002% or more, so this concentration is made the lower limit. On the other hand, if it is added in excess of 0.009%, the existing form changes from fine dispersion to agglomeration, and coarse inclusions that deteriorate λ are formed. Therefore, the upper limit is 0.009%.
[0022]
O needs to be 0.001 to 0.006% in order to exhibit the function of Ce (oxide) described above. If the content is less than 0.001%, the number of nuclei formed of titanium nitride is probably insufficient, so that the refinement of titanium nitride cannot be sufficiently achieved. On the other hand, if the content exceeds 0.006%, punching due to a decrease in cleanliness is caused. This is because deterioration of the end-face fatigue characteristics cannot be avoided.
Ce and O need to satisfy the condition of the concentration product shown by the following equation in addition to the above concentration limitation. That is, [Ce] [O] ≦ 3.8 × 10 ⁻⁹ . This conditional expression is determined based on experimental results described later as examples.
[0023]
Cu can be used as a solid solution strengthening element or a precipitation strengthening element for increasing the strength of a steel sheet. Further, the fatigue strength of the base material can be improved by the addition. However, the effect is not obtained unless 0.6% or more is added. On the other hand, if the content exceeds 2.0%, the effect is not only saturated, but also deteriorates the surface properties of the steel sheet after hot rolling. 0% is the upper limit.
Ni has an effect of mitigating the deterioration of the hot rolled surface properties due to Cu, and it is desirable to add about half of Cu as a guide. Therefore, the lower limit is 0.3%. On the other hand, even if added in excess of 1.0%, the effect is saturated and only leads to an increase in cost, so the upper limit is 1.0%.
[0024]
Nb, Mo and V combine with C similarly to Ti, thereby suppressing the generation of cementite and contributing to the improvement of λ. The effect is expressed at 0.1%, 0.05% and 0.025% or more, respectively, and saturates at 0.6%, 0.3% and 0.15%, respectively. The above can be used.
In the present invention, the component other than the above is Fe, but inevitable impurities mixed from a melting raw material such as scrap or manufacturing equipment shared with other steel sheets are allowed.
Most preferably, Al is not used as a deoxidizer, but up to 0.003% does not affect the properties, so use as a preliminary deoxidizer is acceptable within this range.
[0025]
Next, each condition of heating, rolling, cooling and winding will be described.
The heating temperature needs to be 1150 ° C. or higher to temporarily dissolve TiC, NbC, and the like in steel. By dissolving them, the formation of polygonal ferrite in the cooling process after rolling is suppressed, and a structure mainly composed of a bainitic ferrite phase preferable for improving λ can be obtained.
On the other hand, when the heating temperature exceeds 1250 ° C., the oxidation of the slab surface becomes remarkable, and in particular, wedge-shaped surface defects considered to be caused by selective oxidation of the grain boundaries remain after descaling, which are caused by rolling after rolling. Since the surface quality is impaired, the upper limit is set to 1250 ° C.
[0026]
The finish rolling completion temperature is important for controlling the structure of the steel sheet. When the Ar is less than ₃ points, the untransformed structure remains and the ductility of the steel sheet deteriorates, which is not preferable. On the other hand, if the temperature is higher than the Ar ₃ point + 100 ° C., a polygonal ferrite phase unfavorable for stretch flangeability is likely to be generated, so the upper limit is set to the Ar ₃ point + 100 ° C.
The reason for setting the average cooling rate to 20 ° C./sec or more and cooling to 450 to 550 ° C. is to suppress the formation of the polygonal ferrite phase and to obtain a structure mainly composed of the bainitic ferrite phase. If the cooling rate is less than 20 ° C./sec, a polygonal ferrite phase is easily formed, which is not preferable. On the other hand, it is not necessary to set an upper limit on the cooling rate in controlling the structure. However, an excessively high cooling rate may make the cooling of the steel sheet non-uniform, and it becomes difficult to control the cooling stop temperature. Is preferably used as a guide. When the cooling stop temperature is lower than 450 ° C., a martensite phase that is not favorable for stretch flangeability is generated.
[0027]
The winding temperature needs to be 450 ° C. or higher in order to suppress the formation of a martensite phase that extremely deteriorates stretch flangeability. On the other hand, if the temperature exceeds 550 ° C., the formation of the polygonal ferrite phase cannot be suppressed, and in the case of steel containing Cu, the precipitated Cu becomes coarse and the effect of solidifying the strength cannot be obtained. is there. Further, by winding at a temperature of 550 ° C. or lower, non-ferrous carbides such as TiC precipitate in the subsequent cooling process, so that the amount of solute C in the ferrite phase is significantly reduced, and the stretch flangeability is improved.
[0028]
Finally, the structure of the steel sheet will be described.
In order to obtain excellent stretch flangeability, it is necessary to form a structure having bainitic ferrite as a main phase. The term "bainitic ferrite" as used herein refers to Non-Patent Document 1 "Bainite Photograph of Steel-1" (published by the Iron and Steel Institute of Japan on June 29, 1992, page 21, FIG. 2.9). -D) It has an optical microscope structure as exemplified in｝, is a lath-like structure having a high dislocation density, and ideally does not contain cementite. However, since the formation of the structure and the formation of non-ferrous carbides by Ti and the like are in a competing and parallel relationship, C, which did not form carbides with Ti and the like, forms a solid solution or rarely forms cementite. Although it is possible, these are defined as bainitic ferrite.
[0029]
The area ratio of bainitic ferrite is most preferably 100%. However, up to 10% polygonal ferrite is acceptable. On the other hand, it is desirable to avoid martensite and pearlite as much as possible.
The area ratio of the tissue was determined by light microscopy. First, a cross section parallel to the rolling direction was polished and corroded with a nital solution, and a portion corresponding to １ ／ of the plate thickness from the surface was observed at a magnification of 200 times. Next, the field of view was cut by 20 grids vertically and horizontally, and it was determined which phase occupied the position of each grid point. The area ratio was determined by the ratio of the number of occupied phases to the total number of lattice points (400).
[0030]
【Example】
Hereinafter, examples of the present invention will be described together with comparative examples.
(Example 1)
A steel slab having the chemical composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled sheet having a thickness of 2.6 mm. The steel sheet thus obtained was examined for strength, ductility, hole expandability, punching fatigue end face characteristics, and cross-sectional structure. Table 3 shows the results for each combination of steel and conditions. The strength and ductility were determined by a tensile test of a JIS No. 5 test piece taken in a direction at 90 ° to the rolling direction. The hole-expandability was measured by measuring the hole diameter D (mm) at the time when a through-thickness crack was generated by pushing a 10 mm diameter punched hole opened in the center of a 150 × 150 mm steel plate with a 60 ° conical punch, λ = (D−10) / 10. The fatigue property of the punched end face is a value (σ _W / σ _B ) obtained by dividing the 1 × 10 ⁷ time strength (σ _W ) obtained by the method based on JIS Z 2275 by the tensile strength (σ _B ) of the steel sheet. , Hereinafter referred to as punching fatigue limit ratio). The test piece used was a No. 1 test piece (the minimum cross-section width was 30 mm and the radius of curvature was 30 mm) provided with a punched hole having a diameter of 10 mm at the center of the test piece specified in the same standard.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
[Table 3]

[0034]
As is clear from Table 3, the use of the method of the present invention makes it possible to obtain a steel sheet that is excellent in strength, ductility, hole-expandability and punching fatigue limit ratio. Specifically, a steel sheet satisfying the product of strength and ductility ≧ 16500 (MPa ·%), λ ≧ 1.0, and σ _W / σ _B ≧ 0.3 was obtained.
On the other hand, steels 7, 8, 9, 11, and 12 deoxidized with Al are inferior in stretch flangeability and punching fatigue limit ratio, and steel 10 in which non-ferrous carbide forming elements are insufficient lacks stretch flangeability. It became clear. In addition, in steel sheets with inappropriate rolling conditions (Nos. 3, 6, 9, 10, 13, and 16), generation of pearlite and martensite could not be avoided, and thus high λ could not be obtained.
[0035]
(Example 2)
In mass%, C: 0.035%, Si: 1.0%, Mn: 1.5%, P: 0.01%, S: 0.001%, Cu: 1.2%, Ni: 0 A slab containing 0.6%, Al: 0.0006%, Ti: 0.15%, and Nb: 0.03%, having different contents of Ce and O, and the balance being Fe was produced. These were formed into a 3.5 mm hot-rolled steel sheet under the conditions of a heating temperature of 1250 ° C., a finish rolling end temperature of 880 ° C., an average cooling rate of 45 ° C./sec, and a winding temperature of 450 ° C. Strength of the steel sheet obtained in this manner was examined ductility, the cross-sectional structure, hole expansion, and perforations with the test piece (σ _{W /} σ _B). The evaluation method is the same as in the first embodiment.
[0036]
As a result, all the steels showed a strength of 800 MPa or more and a ductility of 20% or more, and a steel sheet having an area ratio of bainitic ferrite of 94% or more. Affected strongly. As shown in FIG. 1 as the coordinate axes of the Ce concentration and the O concentration, within the range of the present invention, excellent punched end face fatigue characteristics (σ _W / σ _B ≧ 0.3) and excellent stretch flangeability (λ) It is clear that a high-strength hot-rolled steel sheet having (≧ 0.95) can be obtained.
[0037]
【The invention's effect】
According to the method of the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having excellent punching end face fatigue properties and stretch flangeability.
[Brief description of the drawings]
FIG. 1 is a graph showing punch end face fatigue characteristics and stretch flangeability of a steel sheet using Ce concentration and O concentration as coordinate axes.

Claims (4)

In mass%,
C: 0.03 to 0.10%,
Si: 0.05 to 1.5%,
Mn: 1.0 to 2.2%,
P: 0.05% or less,
S: 0.01% or less,
N: 0.001 to 0.006%,
Ti: 0.06 to 0.24%,
Ce: 0.002 to 0.009%,
O: contains 0.001 to 0.006%,
Fatigue characteristics and elongation of the punched end face, characterized in that the balance consists of Fe and unavoidable impurities, the concentration product of Ce and O satisfies the formula (1), and the bainitic ferrite phase has a structure with the maximum area ratio. High strength hot rolled steel sheet with excellent flangeability.
[Ce] [O] ≦ 3.8 × 10 ⁻⁹ (1) In further mass%,
Cu: 0.6-2.0%,
Ni: 0.3 to 1.0%
The high-strength hot-rolled steel sheet according to claim 1, which is excellent in fatigue characteristics and stretch flangeability of a punched end face. In further mass%,
Nb: 0.1 to 0.6%,
Mo: 0.05-0.3%,
V: 0.025 to 0.15%
The high-strength hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has excellent fatigue characteristics and stretch flangeability. A method for producing a steel sheet according to any one of claims 1 to 3, wherein the steel material having the chemical component according to any one of claims 1 to 3 is heated to 1150 to 1250 ° C to obtain a steel sheet. After rolling, finish rolling is completed at Ar ₃ points to Ar ₃ points + 100 ° C., further cooled to 450 to 550 ° C. at an average cooling rate of 20 ° C./sec or more, and wound at 450 to 550 ° C. For producing high-strength hot-rolled steel sheets with excellent fatigue properties and stretch flangeability at the punched end faces.

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