JP2004197114A - High strength hot-rolled steel plate excellent in stretch flanging property, and its producing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot-rolled steel plate excellent in stretch flanging property. <P>SOLUTION: This high strength hot-rolled steel plate is contained by mass% of 0.02-0.10% C, ≤0.2% Si, 0.5-2.5% Mn, ≤0.1% P, ≤0.01% S, ≤0.01% N, 0.005-2.0% Al, ≤0.005% Ti, 0.1-0.6% Nb and if necessary, 0.8-2.0% Cu, 0.4-1.0% Ni, 0.05-0.3% Mo, 0.025-0.15% V, and the balance Fe with inevitable impurities. Further, this steel plate has one or more micro-structures among polygonal ferrite, bainitic ferrite and bainite. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００３０】
（実施例２）
表２に化学成分を示す鋼のスラブを表４中から選択した条件にて熱間圧延し、厚さ３．４ｍｍの熱延板を得た。このようにして得られた帯鋼の強度σ_B、延性δ、穴広げ値λ、および各々のΔを上記実施例１と同様に調べた。その結果を鋼と条件の組み合わせ毎に表６に示す。また構成する相の同定も上記実施例１と同様に行ったが、ポリゴナル・フェライト、ベイニティック・フェライトおよびベーナイトの３相以外の相は認められなかった。
表６から明らかなように、Ｓｉを０．２％以下としてもＴｉ、Ｎｂを添加するとバラツキは大きくなるが、本発明のようにＴｉを０．００５％以下とすれば、強度、延性、および穴広げ性に優れ、何れのバラツキも４％以下と小さい鋼板を得ることができた。
【００３１】
【表２】

【００３２】
（実施例３）
表３に化学成分を示す鋼のスラブを表４中から選択した条件にて熱間圧延し、厚さ３．２ｍｍの熱延板を得た。このようにして得られた帯鋼の強度σ_B、延性δ、穴広げ値λ、および各々のΔを上記実施例１と同様に調べた。その結果を強度σ_B、延性δ、および穴広げ値λのいずれのΔも４％以下であるものと、それ以外に分け、Ｎｂ量を横軸に、巻き取り温度Ｔｃ（℃）を縦軸にして図１に示す。白丸（〇）は図中にＩ、ＩＩ、およびＩＩＩで示す直線で囲まれた領域であり、３本の直線は同図より下記のように求められた。
【００３３】
すなわち、（Ｉ）：Ｔｃ＝７０３−１０３×［Ｎｂ］
（ＩＩ）：Ｔｃ＝６１６−４１５×［Ｎｂ］
（ＩＩＩ）：Ｔｃ＝３６８＋２９０×［Ｎｂ］
である。ただしＮｂ量を［Ｎｂ］と表記した。
以上より、巻き取り温度Ｔｃを、０．１≦［Ｎｂ］≦０．３５においては、６１６−４１５×［Ｎｂ］＜Ｔｃ＜７０３−１０３×［Ｎｂ］とし、０．３５＜［Ｎｂ］≦０．６においては３６８＋２９０×［Ｎｂ］＜Ｔｃ＜７０３−１０３×［Ｎｂ］を満たす範囲にすることによって強度、延性、および穴広げ性に優れバラツキも４％以下と小さい鋼板を得ることができることが示された。
【００３４】
【表３】

【００３５】
【表４】

【００３６】
【表５】

【００３７】
【表６】

【００３８】
【発明の効果】
以上述べたように、本発明の方法によれば、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】鋼板の強度、延性、伸びフランジ性のバラツキをＮｂ量を横軸に、巻き取り温度を縦軸にして表したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch flangeability and a method for producing the same.
[0002]
[Prior art]
Among automotive parts, high-strength hot-rolled steel sheets are widely used especially for frames and arms called suspension systems. The characteristics required for the steel sheet used for such applications include high ductility and high stretch flangeability represented by the hole expansion test value. These two properties are contradictory with steel plates of the same strength, and steel plates are being developed to enhance the other property without significantly damaging one property. As an example of such a steel plate, Patent Literature 1 and Patent Literature 2 can be cited. Patent Document 1 has a predetermined chemical component, and by using bainitic ferrite as the main microstructure, it is possible to obtain a hot-rolled steel sheet having a tensile strength of 500 N / mm ² class or more and excellent stretch flangeability. Technology is disclosed. On the other hand, Patent Document 2 discloses a hot-rolled steel sheet having a predetermined chemical component and having a ferrite single-phase microstructure and a tensile strength of 600 N / mm ² or more and excellent stretch flangeability. Technology is disclosed.
[0003]
[Cited document]
(1) Patent Document 1 (Japanese Patent Laid-Open No. 6-172924)
(2) Patent Document 2 (Japanese Patent Laid-Open No. 7-70696)
[0004]
[Problems to be solved by the invention]
The present inventors have also studied a steel sheet having excellent stretch flangeability and a manufacturing method thereof with reference to such prior art. In the process, it has been clarified that the steel sheet based on the above-described prior art may include a portion having inferior properties, although there is a portion that exhibits excellent stretch flangeability. Although it is not remarkable in the laboratory-scale small hot-rolled sheet, it has been found that the distribution range of the characteristic is large in the commercial-scale hot-rolled coil.
[0005]
Such a problem has been accepted as an inevitable variation in characteristics when the economic situation surrounding steel plate users is booming, but is now recognized as an inevitable problem. This is because part design that has given enough room to the lower limit of the characteristics in consideration of variation is being reviewed, and the attitude to try to utilize the upper limit of the characteristic value as much as possible is becoming mainstream. Because. Under such circumstances, it is not sufficient that the characteristic value of the steel sheet is high. Rather, the smaller variation is more important than the height of the absolute value, but there are no examples that focus on this point. Absent.
[0006]
[Means for Solving the Problems]
In view of such a situation, the present inventors have intensively studied to develop a steel plate that has a stretch flangeability of a certain level or more and that has a small variation and that is also less sensitive to fluctuations in manufacturing conditions. The present invention was completed by clarifying chemical components and manufacturing methods that make these possible.
The summary is as follows. That is, (1) in mass%, C: 0.02 to 0.10%, Si: 0.2% or less, Mn: 0.5 to 2.5%, P: 0.1% or less, S: 0 0.01% or less, N: 0.01% or less, Al: 0.005 to 2.0%, Ti: 0.005% or less, Nb: 0.1 to 0.6%, with the balance being Fe and A high-strength hot-rolled steel sheet excellent in stretch flangeability, which is made of inevitable impurities and has one or more microstructures of polygonal ferrite, bainitic ferrite and bainite.
[0007]
(2) By mass%, Cu: 0.8 to 2.0%, Ni: 0.4 to 1.0%
A high-strength hot-rolled steel sheet having excellent stretch flangeability as described in (1) above.
(3) The above (1) or (1) characterized by further containing at least one of Mo: 0.05 to 0.3% and V: 0.025 to 0.15% by mass%. 2) A high-strength hot-rolled steel sheet having excellent stretch flangeability as described.
[0008]
(4) A method for producing the steel sheet according to any one of (1) to (3) above, wherein a steel material having the chemical components described therein is heated to 1200 ° C. or higher (Ar ₃ point − 50) In the step of completing the rolling at a temperature equal to or higher than 0 ° C. and cooling and winding at an average cooling rate of 10 to 50 ° C./second, the winding temperature Tc (° C.) is set to 0.1 ≦ [Nb] ≦ 0. 35, 616−415 × [Nb] <Tc <703−103 × [Nb], and in 0.35 <[Nb] ≦ 0.6, 368 + 290 × [Nb] <Tc <703−103 × [Nb] ] In a range satisfying the above, a method for producing a high-strength hot-rolled steel sheet excellent in stretch flangeability. However, [Nb] is the amount of Nb (mass%).
[0009]
DETAILED DESCRIPTION OF THE INVENTION
First, the experiment that led to the completion of the present invention will be described.
The present inventors, in mass%, C: 0.02 to 0.1%, Si: 0 (no addition) to 0.5%, Mn: 0.5 to 2.5%, P: 0.00. Steel slab containing 01 to 0.1%, S: 0.001 to 0.01%, N: 0.003 to 0.01%, Al: 0.005 to 2% and the balance being Fe These were hot-rolled to produce a steel strip (hot-rolled coil). In hot rolling, the rolling end temperature Tf (° C.), the cooling rate C _R (° C./second) after rolling, and the winding temperature Tc (° C.) were changed. While the produced steel strip was pickled, steel plates were collected from the front end, the center and the end of the steel strip. And about each of these three site | part steel plates, the test piece for characteristic evaluation and the micro structure | tissue investigation sample were extract | collected from the both ends and center part of the width direction.
[0010]
The test pieces are tensile test pieces for evaluating strength ductility (JIS No. 5, sampled so that the tensile direction is perpendicular to the rolling direction) and test pieces for measuring the hole expansion value (150 mm × 150 mm). Produced. Using these test pieces, the strength (tensile strength) σ _B (MPa), the ductility (elongation) δ (%), and the hole expansion value λ (%) were investigated. The tensile test was performed by a method based on JIS Z 2241 using an Instron type testing machine. The hole expansion value λ was measured in accordance with Japan Iron and Steel Federation Standard JFS T1001, and the clearance was 12%.
[0011]
The obtained characteristic values are arranged for five points collected from the same part, three parts in the width direction, and three positions on the steel strip, and the relationship between the distribution of characteristic values (variation) and the chemical composition of the steel sheet, and The relationship with hot rolling conditions (Tf, C _R , Tc) was examined. As a result, the variation between 5 points in the same part was extremely small in all the steel plates regardless of the chemical composition and hot rolling conditions, and the difference between the maximum value and the minimum value was within 2% of the simple average value. For this reason, it is judged that there is no problem in practical evaluation by evaluating with a simple average value of 5 points, and using this value (simple average value), the variation between three places in the width direction and between three places on the strip steel investigated. As a result, the degree of variation varies depending on the chemical composition and hot-rolling conditions, with the largest being about 16% between three locations in the width direction and about 20% between three locations on the strip. There was also.
[0012]
When these results were first analyzed by comparing with chemical components, a relatively strong correlation was observed with the amount of Si in the constituent elements, and it was found that the variation rapidly decreased especially at 0.2% or less. .
Then, next, an experiment was conducted for the case where Si was added to 0.05 to 0.2%, the amounts of C, Mn, P, S, N, and Al were kept as described above, and Ti and Nb were added. It was. Ti was examined in the range of 0 (no addition) to 0.25%, and Nb in the range of 0.1 to 0.6%. As described in the prior art, Ti and Nb suppress the formation of cementite by forming carbides with C, so that an improvement in λ can be expected, but the effect on the variation is not clear, so the same as above The method was examined. As a result, it was found that the Nb amount has little influence on the variation, whereas the Ti amount has a strong correlation, and the variation can be rapidly reduced by making it 0.005% or less.
[0013]
Based on the above results, C: 0.02 to 0.10%, Si: 0.2%, Mn: 1.5%, P: 0.02%, S: 0.001%, Nb: 0.0. The effect of addition of Cu and Ni and Mo and V was further examined on the steel slab containing 1 to 0.6%, but these elements did not affect the variation.
From the above examination, as chemical components, C: 0.02 to 0.10%, Si: 0.2% or less, Mn: 0.5 to 2.5%, P: 0.1% or less, S: 0 .01% or less, Ti: 0.005% or less, Nb: 0.1-0.6%, Cu: 0.8-2.0%, Ni: 0.4-1. 0%, Mo: 0.05 to 0.3%, V: 0.025 to 0.15%, the balance being Fe and unavoidable impurities is effective in minimizing variation As a result, the relationship between hot rolling conditions and variation was investigated by focusing on steel sheets of this component. If the rolling end temperature Tf, the cooling rate C _R after rolling, and the winding temperature Tc are changed, the microstructure naturally changes, and the absolute values of the strength σ _B , ductility δ, and hole expansion value λ also change accordingly. However, the degree of variation was unknown, so we investigated.
[0014]
As a result, the cooling rate C _R after rolling and the rolling end temperature Tf impact on the variation is slight, the coiling temperature Tc affects the degree of dispersion, reduce variation in particular according to the change in the Nb content It has become clear that the range of the desired winding temperature Tc takes a specific range.
Although the details of the examination described above will be described in the examples, the present invention is based on such basic experiments and is a result of further earnest research, and shows a predetermined stretch flangeability with less variation. A hot-rolled steel sheet and a manufacturing method thereof are provided. In view of the fact that “less variation” is a feature demanded by steel sheet users today, a high-strength hot-rolled steel plate with little variation and excellent stretch flangeability is simply obtained by stretching flanges. It was described as a high-strength hot-rolled steel sheet with excellent properties.
[0015]
The reasons for limiting the present invention will be described below. First, the reasons for limiting chemical components will be described. All component labels are weight percent.
C is an essential element for ensuring the strength of the steel sheet, and at least 0.02% is necessary to obtain a high-strength steel sheet. However, if it is contained excessively, the formation of a phase undesirable for stretch flangeability such as cementite and martensite cannot be avoided, so the content is made 0.10% or less.
[0016]
The amount of Si is one of the most important conditions in the present invention, and needs to be 0.2% or less in order to suppress variation. Most preferably, 0%. The reason why the amount of Si strongly affects the variation is not necessarily clear, but this element fluctuates the solid solubility limit of C in the ferrite, and as a result, polygonal ferrite that may be generated in the cooling process after rolling, It seems to have a strong influence on the selection of the phase from bainitic ferrite, bainite and pearlite and the determination of its constituent fraction, and as a result, is related to the variation of the hole expansion value λ. When the Si amount is in the range of 0.2% or less, it is presumed that the variation in the C solid solubility limit is extremely small, and therefore, it is considered to work effectively for suppressing variation.
[0017]
Mn is an element effective for increasing the strength of a steel sheet and should be contained in an amount of 0.5% or more. However, if it exceeds 2.5%, the ductility deteriorates, so the upper limit is 2.5%. To do.
P is effective as a solid solution strengthening element, but it is necessary to make it 0.1% or less because there is concern about deterioration of workability due to segregation.
S should be suppressed as much as possible because it forms inclusions such as MnS and deteriorates the stretch flangeability. However, it should be 0.01% or less.
[0018]
N forms nitrides and reduces ductility and stretch flangeability. Therefore, it should be suppressed as much as possible, but is acceptable if it is 0.01% or less.
Al is used as a deoxidizer, and 0.005% is necessary to obtain an appropriate cleanliness. However, if it is excessively contained, ductility and stretch flangeability deteriorate. The upper limit is 0%.
[0019]
Ti and Nb work to improve the workability of the steel sheet (so-called scavenging effect) by fixing C, S and N as precipitates. On the other hand, when added more than necessary, they are present in the steel as solute Ti or solute Nb, and the temperature of recrystallization is increased, and the hot-worked structure tends to remain and the ductility is impaired. Careful attention is required. In particular, Ti has a strong tendency to bind to C more easily than Nb, so that slight fluctuations in the concentration greatly change the amount of C remaining in the steel, and as a result, it is assumed that the variation in the hole expansion value λ is strongly affected. Is done. Therefore, it is necessary to suppress to 0.005% or less, and 0% (no addition) is also included in the present invention.
[0020]
On the other hand, it is estimated that Nb is less likely to induce fluctuations in the amount of C due to concentration fluctuations because of its milder binding force with C than Ti. Therefore, it is necessary to add 0.1% or more in which the effect of fixing C is clearly recognized, and to avoid addition exceeding 0.6% where the remaining of the processed structure starts to affect the characteristics.
Cu can be used as a solid solution strengthening element or a precipitation strengthening element for increasing the strength of steel sheets. Further, the fatigue strength can be further improved by the addition thereof. The effect is not manifested unless 0.8% or more is added. On the other hand, if it exceeds 2.0%, the steel sheet surface properties after hot rolling deteriorate, so 2.0% is made the upper limit.
[0021]
Ni has an effect of relieving the deterioration of hot rolled surface properties caused by Cu, and it is desirable to add about half of Cu as a guide. Therefore, the lower limit is 0.4%. On the other hand, even if added over 1.0%, the effect is saturated and only leads to an increase in cost, so 1.0% is made the upper limit.
Both Mo and V function to form carbides and increase the strength, and also to form carbonitrides combined with Nb to suppress the softening of the steel sheet due to the heat effect of welding. These effects are manifested at 0.05% and 0.025% or more, respectively. On the other hand, when the content exceeds 0.3% and 0.15%, respectively, a hard phase is formed to reduce the hole expansion value λ. Therefore, it is necessary to set these as the upper limit.
In the present invention, components other than those described above are Fe, but inevitable impurities mixed from melting raw materials such as scrap are allowed.
[0022]
Next, the structure of the steel plate will be described.
In order to obtain excellent stretch flangeability, a steel sheet having one or more microstructures selected from polygonal ferrite, bainitic ferrite and bainite is required. As the remaining structure other than these, one or more of pearlite, retained austenite, and martensite should be eliminated as much as possible, and it is desirable that the area ratio is 3% or less. On the other hand, any combination (including single) or composition ratio (area ratio) of the three phases of polygonal ferrite, bainitic ferrite and bainite may be used, and the required strength, ductility, and elongation. It can be designed based on flangeability. For example, especially in applications where high ductility is the main focus and high stretch flangeability is required, polygonal ferrite is used as the main phase or single phase, and the required strength is secured by utilizing precipitates. You can choose the method. On the other hand, for applications in which it is most important to reduce the number of constituent elements as much as possible and to use a low-priced steel sheet, a method in which bainite is the main phase or a single phase can be selected.
[0023]
Finally, each condition of heating, rolling, cooling and winding will be described.
The heating temperature is required to be 1200 ° C. or higher in order to temporarily dissolve carbonitride in the steel. By dissolving these in a solid solution, the strength of the steel sheet can be increased by finely dispersing carbonitride in the cooling process after rolling.
On the other hand, when the heating temperature exceeds 1300 ° C., oxidation of the slab surface becomes remarkable, and in particular, wedge-shaped surface defects that appear to be caused by selective oxidation of grain boundaries remain after descaling, which is the surface after rolling. Since the quality is impaired, the upper limit is 1300 ° C., preferably 1250 ° C. or less.
[0024]
The rolling end temperature Tf is important for the structure control of the steel sheet. If it is less than Ar ₃ point + 50 ° C., the crystal grain size of the surface layer is coarse, and the material in the thickness direction is not stable, which is not preferable in terms of characteristics. On the other hand, if the Ar ₃ point is higher than + 100 ° C., the austenite grain size after the rolling becomes coarse, and the composition and the fraction of the phase generated during cooling become unstable, making it difficult to operate. Is desirable.
[0025]
Cooling rate C _R after rolling is required to be 10 to 50 ° C. / sec. Polygonal ferrite by controlling the cooling rate C _R after rolling, to obtain bainitic ferrite, and the one or more configuration tissues and steel plate with its fraction selected from among the bainite I can do it. Cooling rate C _R after rolling is not possible to suppress the generation of undesirable pearlite in stretch flangeability is less than 10 ° C. / sec, whereas at the 50 ° C. / sec greater than the material dispersion in particular due to the uneven cooling in the width direction of the steel strip there are cooling rate C _R after rolling so is a concern that needs to be limited to the above range.
[0026]
The range of the coiling temperature Tc (° C.) is one of the most important conditions for obtaining the steel sheet of the present invention. In particular, when there is a certain relationship with the amount of Nb, a steel sheet with very little variation can be obtained. As will be described in the examples, when [Nb] is the amount of Nb (mass%), 616−415 × [Nb] <Tc <703—when 0.1 ≦ [Nb] ≦ 0.35. 103 × [Nb], and when 0.35 <[Nb] ≦ 0.6, the range satisfies 368 + 290 × [Nb] <Tc <703−103 × [Nb]. The reason why the variation becomes small when this relationship is satisfied is not clear at present, but it is presumed that the precipitation phenomenon of NbC is probably involved in some way. In the above relational expression, the range in which the variations of the strength σ _B , the ductility δ, and the hole expansion value λ are within 4% for each Nb amount is obtained according to an example described later.
[0027]
【Example】
Examples of the present invention will be described below together with comparative examples.
Example 1
A steel slab having chemical components shown in Table 1 was hot-rolled under the conditions selected from Table 4 to obtain a steel strip having a thickness of 3.6 mm. The strength σ _B , ductility δ, and hole expansion value λ of the above-mentioned nine steel strips thus obtained (the steel strip tip, center, tail, each widthwise both ends and center) were examined. It was. A value (Δ) obtained by dividing the difference between the maximum value and the minimum value of the nine characteristic values by their simple average value was also obtained. The results are shown in Table 5 for each combination of steel and conditions. Further, the structure formed by observing the cross-sectional structure was examined. The structure was mirror-polished and then revealed with a nital solution, and the internal position corresponding to 1/4 of the plate thickness was observed from the surface at 400 times, and the existing structure was identified with reference to the following technical document 1. As a result, the only recognized phases were polygonal ferrite, bainitic ferrite, and bainite, and no other phases were observed.
[0028]
[Technical Reference 1]
“Steel Bainite Photobook-1” (June 29, 1992, published by Japan Iron and Steel Institute, for example, page 21, FIG. 2.9)
As is apparent from Table 5, when the present invention was used, a steel sheet having excellent strength, ductility, and hole expandability, and a variation of 4% or less could be obtained.
[0029]
[Table 1]

[0030]
(Example 2)
A steel slab having chemical components shown in Table 2 was hot-rolled under the conditions selected from Table 4 to obtain a hot-rolled sheet having a thickness of 3.4 mm. The strength σ _B , ductility δ, hole expansion value λ, and Δ of each of the obtained steel strips were examined in the same manner as in Example 1. The results are shown in Table 6 for each combination of steel and conditions. In addition, although the constituent phases were identified in the same manner as in Example 1, no phases other than the three phases of polygonal ferrite, bainitic ferrite and bainite were found.
As can be seen from Table 6, when Ti and Nb are added even if Si is 0.2% or less, the variation increases. However, if Ti is 0.005% or less as in the present invention, the strength, ductility, and A steel plate having excellent hole-expandability and a small variation of 4% or less in any variation could be obtained.
[0031]
[Table 2]

[0032]
(Example 3)
A steel slab having chemical components shown in Table 3 was hot-rolled under the conditions selected from Table 4 to obtain a hot-rolled sheet having a thickness of 3.2 mm. The strength σ _B , ductility δ, hole expansion value λ, and Δ of each of the obtained steel strips were examined in the same manner as in Example 1. The results are divided into those where the strength σ _B , ductility δ, and hole expansion value λ are all 4% or less, and other cases, with the Nb amount on the horizontal axis and the winding temperature Tc (° C.) on the vertical axis. As shown in FIG. White circles (◯) are regions surrounded by straight lines indicated by I, II, and III in the figure, and three straight lines were obtained from the figure as follows.
[0033]
That is, (I): Tc = 703-103 × [Nb]
(II): Tc = 616-415 × [Nb]
(III): Tc = 368 + 290 × [Nb]
It is. However, the amount of Nb was expressed as [Nb].
From the above, when the winding temperature Tc is 0.1 ≦ [Nb] ≦ 0.35, 616−415 × [Nb] <Tc <703−103 × [Nb] and 0.35 <[Nb] ≦ In 0.6, by making the range satisfying 368 + 290 × [Nb] <Tc <703−103 × [Nb], it is possible to obtain a steel sheet having excellent strength, ductility, and hole expansibility and small variation of 4% or less. It has been shown.
[0034]
[Table 3]

[0035]
[Table 4]

[0036]
[Table 5]

[0037]
[Table 6]

[0038]
【The invention's effect】
As described above, according to the method of the present invention, a high-strength hot-rolled steel sheet excellent in stretch flangeability can be obtained.
[Brief description of the drawings]
FIG. 1 is a graph showing variations in strength, ductility, and stretch flangeability of steel sheets, with the Nb amount on the horizontal axis and the winding temperature on the vertical axis.

Claims (4)

In mass%
C: 0.02-0.10%,
Si: 0.2% or less,
Mn: 0.5 to 2.5%
P: 0.1% or less,
S: 0.01% or less,
N: 0.01% or less,
Al: 0.005 to 2.0%,
Ti: 0.005% or less,
Nb: 0.1 to 0.6%
And the balance consists of Fe and inevitable impurities, and has one or more microstructures of polygonal ferrite, bainitic ferrite and bainite, and is excellent in stretch flangeability Strength hot-rolled steel sheet. % By mass, further Cu: 0.8-2.0%,
Ni: 0.4-1.0%
The high-strength hot-rolled steel sheet having excellent stretch flangeability according to claim 1. % By mass, Mo: 0.05 to 0.3%,
V: 0.025 to 0.15%
The high-strength hot-rolled steel sheet excellent in stretch flangeability according to claim 1 or 2, characterized by containing at least one of the above. 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 1200 ° C or higher, and (Ar _{In the step} of completing rolling at a temperature of ₃ points-50) ° C. or higher and cooling at an average cooling rate of 10-50 ° C./second and winding up, the winding temperature Tc (° C.) is set to 0.1 ≦ [Nb] When ≦ 0.35, 616−415 × [Nb] <Tc <703−103 × [Nb], and when 0.35 <[Nb] ≦ 0.6, 368 + 290 × [Nb] <Tc <703−103 X A method for producing a high-strength hot-rolled steel sheet excellent in stretch flangeability, characterized by being in a range satisfying [Nb].
However, [Nb] is the amount of Nb (mass%).

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