JP4525299B2 - High-strength hot-rolled steel sheet excellent in workability and manufacturing method thereof - Google Patents
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Description
本発明は、プレス加工、曲げ加工、伸びフランジ加工など何らかの加工が施されて使用される部材の素材に適した、ｒ値の異方性の小さい引張強度７８０ＭＰａ以上の加工性に優れた高強度熱延鋼板およびその製造方法に関する。 The present invention is suitable for a material of a member to be used after being subjected to some processing such as press processing, bending processing, stretch flange processing, etc., and has high strength excellent in workability of 780 MPa or more with small r-value anisotropy. The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof.
近年、環境問題に対する関心が高まるなか、自動車用部材をはじめとし、種々の加工品において、高強度−薄肉化による軽量化が求められている。また、複雑な加工をプレス成形により行なう傾向が強まっており、高強度でありながら加工性（特に深絞り性および伸びフランジ性）に優れた材料が必要とされている。 2. Description of the Related Art In recent years, with increasing interest in environmental issues, various processed products including automobile members have been required to be light in weight by high strength and thinning. In addition, there is an increasing tendency to perform complicated processing by press molding, and there is a need for a material that is high in strength but excellent in workability (particularly deep drawability and stretch flangeability).
従来、高強度熱延鋼板は、構造用部品に適用されることが多く、伸びフランジ性が重視されてきた。そのため、高強度と高伸びフランジ性を両立させるための多くの方法が提案されている。例えば、強度７００ＭＰａ以上でも優れた伸びフランジ性を有する鋼板およびその製造方法として、組織をアシキュラーフェライトとして、ＴｉＣまたはＮｂＣをアシキュラーフェライト組織中に析出させる方法（特許文献１）、また、組織の８５％以上をポリゴナルフェライトとし、ＴｉＣを析出させると同時に、Ｍｏを固溶させる方法（特許文献２）などが提案されている。 Conventionally, high-strength hot-rolled steel sheets are often applied to structural parts, and stretch flangeability has been emphasized. Therefore, many methods for achieving both high strength and high stretch flangeability have been proposed. For example, as a steel sheet having excellent stretch flangeability even at a strength of 700 MPa or more and a method for producing the same, a method in which the structure is acicular ferrite, TiC or NbC is precipitated in the acicular ferrite structure (Patent Document 1), A method is proposed in which 85% or more is polygonal ferrite, and TiC is precipitated, and at the same time, Mo is dissolved in solid solution (Patent Document 2).
しかしながら、上述のようにＴｉＣやＮｂＣを析出強化に利用した場合、析出物の粗大化による強度低下が避けられず、安定して強度７８０ＭＰａ以上の鋼板を得ることは困難である。 However, when TiC or NbC is used for precipitation strengthening as described above, a decrease in strength due to coarsening of the precipitates cannot be avoided, and it is difficult to stably obtain a steel plate having a strength of 780 MPa or more.
また、粗大化した析出物が割れの起点や割れ伝播経路となるため十分な伸びフランジ性を確保することは難しい。上述の問題点を解決するため、フェライト組織を微細な析出物で析出強化することにより、良好な強度−伸びフランジ性バランスを得る方法が提案されている（例えば、特許文献３〜７参照）。 Moreover, since the coarsened precipitate becomes a crack starting point and a crack propagation path, it is difficult to ensure sufficient stretch flangeability. In order to solve the above-mentioned problems, a method of obtaining a good strength-stretch flangeability balance by precipitation strengthening the ferrite structure with fine precipitates has been proposed (see, for example, Patent Documents 3 to 7).
これらの中でも特許文献６では、組織をフェライトとして、１０ｎｍ以下の超微細な析出物でフェライト組織を析出強化することにより、強度９５０ＭＰａ以上で伸びフランジ性の指標となる穴広げ率が８０〜１０５％となる方法が提案されている。図１は特許文献１〜７の方法によって製造された高強度鋼板の強度（ＴＳ）−穴広げ率（λ）バランスを示すものであり、伸びフランジ性の指標となる穴広げ率λ（％）は、強度の上昇に伴い低下する傾向を示すが、強度上昇に伴う穴広げ率λの低下の原因は明らかにされていない。そして、上記の方法は、いずれも伸びフランジ性の改善に注力したものであって、伸びフランジ性と深絞り性（特にｒ値の異方性）の両者を向上させるものではない。これは、結晶粒微細化と高強度化のために制御圧延を適用していたためと考えられる。 Among these, in Patent Document 6, the structure is ferrite, and the ferrite structure is precipitation-strengthened with ultrafine precipitates of 10 nm or less, so that the hole expansion ratio which is an index of stretch flangeability at a strength of 950 MPa or more is 80 to 105%. A method has been proposed. FIG. 1 shows the strength (TS) -hole expansion ratio (λ) balance of high-strength steel sheets produced by the methods of Patent Documents 1 to 7, and the hole expansion ratio λ (%) that serves as an index of stretch flangeability. Shows a tendency to decrease as the strength increases, but the cause of the decrease in the hole expansion ratio λ as the strength increases is not clarified. All of the above methods focus on improving the stretch flangeability and do not improve both the stretch flangeability and the deep drawability (particularly the r value anisotropy). This is thought to be because controlled rolling was applied to refine the crystal grains and increase the strength.
すなわち、制御圧延では、圧延仕上温度をＡｒ３点直上のγ粒未再結晶域とし、微細な未再結晶γからの変態を利用してα粒の微細化と鋼板の高強度化を図っていた。しかしながら、この方法では、未再結晶γからの変態に伴い、｛３１１｝〜｛２１１｝＜０１１＞方位が発生するため、ｒ値の異方性が大きくなり、深絞り性の向上は望めない。 That is, in the controlled rolling, the rolling finishing temperature is the γ-grain unrecrystallized region immediately above the Ar3 point, and the transformation from fine unrecrystallized γ is used to refine the α-grain and increase the strength of the steel sheet. . However, in this method, the {311} to {211} <011> orientations are generated along with the transformation from the non-recrystallized γ, so that the anisotropy of the r value is increased and the deep drawability cannot be improved. .
また、従来、伸びフランジ性の改善は、仕上圧延温度をＡｒ３変態点〜９５０℃にすることによりなされていた（例えば、特許文献８）。これは、γ粒の未再結晶域での圧延を付加することにより、γ粒の微細化を図るためである。そして、γ粒の再結晶域あるいは部分再結晶域で仕上圧延を行なう（つまり、仕上圧延温度を９５０℃以上とする）とγ粒の粗大化が起こり、強度（ＴＳ）−穴広げ率（λ）のバランスが低下すると考えられていた。
本発明は、上記実情に鑑みてなされたものであり、ｒ値の異方性が小さく、引張強度７８０ＭＰａ以上を有し、加工性、特に深絞り性と伸びフランジ性の両者の性質に優れた高強度熱延鋼板およびその製造方法を提供することを目的とする。 The present invention has been made in view of the above circumstances, has a small anisotropy of r value, has a tensile strength of 780 MPa or more, and is excellent in both workability, particularly deep drawability and stretch flangeability. An object is to provide a high-strength hot-rolled steel sheet and a method for producing the same.
本発明者らは、強度７８０ＭＰａ以上の高強度熱延鋼板の深絞り性および伸びフランジ性の改善に関して、鋼組成および従来不利とされていた高温仕上圧延の観点から鋭意研究を重ねた。その結果、深絞り成形時のプレス割れは、ｒ値の異方性、特にｒ値の最大値（ｒｍａｘ）と最小値（ｒｍｉｎ）に依存するとの知見を得た。また、本発明で採用する鋼成分系では、伸びフランジ性の指標となる穴広げ率λもｒ値の異方性の影響を受けるとの知見を得た。そして、ｒ値の異方性を改善する方法として、適量のＺｒを含有した鋼の組織を実質的にフェライト単相とし、フェライト粒内を平均粒径１０ｎｍ以下のＴｉとＭｏを含む炭化物で析出強化することが有効であること、さらに１２００℃以上１３５０℃以下の温度域に加熱後、仕上圧延温度を９５０℃以上とし、５００℃〜７００℃の温度域で巻き取ることが有効であることを見出した。 The inventors of the present invention have made extensive studies from the viewpoint of steel composition and high-temperature finish rolling, which has been considered disadvantageous, with regard to improvement of deep drawability and stretch flangeability of high-strength hot-rolled steel sheets having a strength of 780 MPa or more. As a result, it was found that press cracks during deep drawing depend on the anisotropy of the r value, particularly the maximum value (r max ) and the minimum value (r min ) of the r value. Moreover, in the steel component system employ | adopted by this invention, the knowledge that the hole expansion ratio (lambda) used as the parameter | index of stretch flangeability was also received by the anisotropy of r value was acquired. As a method of improving the anisotropy of the r value, the steel structure containing an appropriate amount of Zr is substantially made into a ferrite single phase, and the inside of the ferrite grains is precipitated with a carbide containing Ti and Mo having an average grain size of 10 nm or less. It is effective to strengthen, and after heating to a temperature range of 1200 ° C. or higher and 1350 ° C. or lower, the finish rolling temperature is 950 ° C. or higher, and it is effective to wind up in a temperature range of 500 ° C. to 700 ° C. I found it.
従って、本発明は、以下の（１）〜（５）を提供する。
（１）質量％で、Ｃ：０．０２〜０．２０％、Ｓｉ：０．３％以下、Ｍｎ：０．５〜３．０％、Ｐ：０．０６％以下、Ｓ：０．０１％以下、Ａｌ：０．１０％以下、Ｎ：０．０２％以下、Ｍｏ：０．１〜０．８％、Ｔｉ：０．０２〜０．４０％およびＺｒ：０．０００５〜０．００５％を含有し、残部がＦｅおよび不可避不純物からなり、フェライトの面積比率が９５％以上の実質的にフェライト単相組織であり、平均粒径１０ｎｍ以下のＴｉとＭｏを含む炭化物が析出していることを特徴とする、ｒ値の異方性が小さい引張強度が７８０ＭＰａ以上の加工性に優れた高強度熱延鋼板。
Accordingly, the present invention provides the following (1) to (5).
( 1 ) By mass%, C: 0.02 to 0.20%, Si: 0.3% or less, Mn: 0.5 to 3.0%, P: 0.06% or less, S: 0.01 %: Al: 0.10% or less, N: 0.02% or less, Mo: 0.1-0.8%, Ti: 0.02-0.40% and Zr: 0.0005-0.005 %, The balance is made of Fe and inevitable impurities , the ferrite area ratio is substantially a ferrite single phase structure of 95% or more , and carbides containing Ti and Mo having an average particle size of 10 nm or less are precipitated. A high-strength hot-rolled steel sheet excellent in workability, characterized by having a low r-value anisotropy and a tensile strength of 780 MPa or more.
（２）質量％で、Ｃ：０．０２〜０．２０％、Ｓｉ：０．３％以下、Ｍｎ：０．５〜３．０％、Ｐ：０．０６％以下、Ｓ：０．０１％以下、Ａｌ：０．１０％以下、Ｎ：０．０２％以下、Ｍｏ：０．１〜０．８％、Ｔｉ：０．０２〜０．４０％およびＺｒ：０．０００５〜０．００５％を含有し、さらにＮｂ：０．００５〜０．１％、Ｖ：０．００５〜０．２％およびＷ：０．００５〜０．２％から選ばれる１種または２種以上の成分を含有し、残部がＦｅおよび不可避不純物からなり、フェライトの面積比率が９５％以上の実質的にフェライト単相組織であり、平均粒径１０ｎｍ以下のＴｉとＭｏを含む炭化物が析出していることを特徴とする、ｒ値の異方性が小さい引張強度が７８０ＭＰａ以上の加工性に優れた高強度熱延鋼板。 ( 2 ) By mass%, C: 0.02 to 0.20%, Si: 0.3% or less, Mn: 0.5 to 3.0%, P: 0.06% or less, S: 0.01 %: Al: 0.10% or less, N: 0.02% or less, Mo: 0.1-0.8%, Ti: 0.02-0.40% and Zr: 0.0005-0.005 1 or more components selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.2% and W: 0.005 to 0.2% Containing, the balance is made of Fe and inevitable impurities , the ferrite area ratio is substantially a ferrite single-phase structure of 95% or more , and carbides containing Ti and Mo having an average particle size of 10 nm or less are precipitated. A high-strength hot-rolled steel sheet characterized by excellent workability with a low r-value anisotropy and a tensile strength of 780 MPa or more.
（３）ｒ値の最大値と最小値の差（ｒｍａｘ−ｒｍｉｎ）［ただし、ｒｍａｘ、ｒｍｉｎは、それぞれ、圧延方向から０°、４５°、９０°の方向のｒ値のうちの最大値と最小値である］が、０．８以下であることを特徴とする、（１）または（２）
に記載のｒ値の異方性が小さい引張強度が７８０ＭＰａ以上の加工性に優れた高強度熱延鋼板。
( 3 ) Difference between maximum value and minimum value of r value (r max −r min ) [where r max and r min are r values in directions of 0 °, 45 °, and 90 ° from the rolling direction, respectively. The maximum value and the minimum value of] is 0.8 or less, (1) or (2)
A high-strength hot-rolled steel sheet having excellent workability with a tensile strength of 780 MPa or more with a small anisotropy of the r value described in 1.
（４）質量％で、Ｃ：０．０２〜０．２０％、Ｓｉ：０．３％以下、Ｍｎ：０．５〜３．０％、Ｐ：０．０６％以下、Ｓ：０．０１％以下、Ａｌ：０．１０％以下、Ｎ：０．０２％以下、Ｍｏ：０．１〜０．８％、Ｔｉ：０．０２〜０．４０％およびＺｒ：０．０００５〜０．００５％を含有し、残部がＦｅおよび不可避不純物からなる鋼を溶製し、スラブ加熱温度１２００℃以上１３５０℃以下、仕上圧延温度９５０℃以上、巻取温度５００℃以上７００℃以下の条件で熱間圧延を行なうことを特徴とする、ｒ値の異方性が小さい引張強度が７８０ＭＰａ以上の加工性に優れた高強度熱延鋼板の製造方法。 ( 4 ) By mass%, C: 0.02 to 0.20%, Si: 0.3% or less, Mn: 0.5 to 3.0%, P: 0.06% or less, S: 0.01 %: Al: 0.10% or less, N: 0.02% or less, Mo: 0.1-0.8%, Ti: 0.02-0.40% and Zr: 0.0005-0.005 %, With the balance being Fe and inevitable impurities , and slab heating temperature is 1200 ° C. or higher and 1350 ° C. or lower, finish rolling temperature is 950 ° C. or higher, and coiling temperature is 500 ° C. or higher and 700 ° C. or lower. A method for producing a high-strength hot-rolled steel sheet excellent in workability having a tensile strength with a small r-value anisotropy of 780 MPa or more, characterized by performing rolling.
（５）質量％で、Ｃ：０．０２〜０．２０％、Ｓｉ：０．３％以下、Ｍｎ：０．５〜３．０％、Ｐ：０．０６％以下、Ｓ：０．０１％以下、Ａｌ：０．１０％以下、Ｎ：０．０２％以下、Ｍｏ：０．１〜０．８％、Ｔｉ：０．０２〜０．４０％およびＺｒ：０．０００５〜０．００５％を含有し、さらにＮｂ：０．００５〜０．１％、Ｖ：０．００５〜０．２％およびＷ：０．００５〜０．２％から選ばれる１種または２種以上の成分を含有し、残部がＦｅおよび不可避不純物からなる鋼を溶製し、スラブ加熱温度１２００℃以上１３５０℃以下、仕上圧延温度９５０℃以上、巻取温度５００℃以上７００℃以下の条件で熱間圧延を行なうことを特徴とする、ｒ値の異方性が小さい引張強度が７８０ＭＰａ以上の加工性に優れた高強度熱延鋼板の製造方法。
( 5 ) By mass%, C: 0.02 to 0.20%, Si: 0.3% or less, Mn: 0.5 to 3.0%, P: 0.06% or less, S: 0.01 %: Al: 0.10% or less, N: 0.02% or less, Mo: 0.1-0.8%, Ti: 0.02-0.40% and Zr: 0.0005-0.005 1 or more components selected from Nb: 0.005 to 0.1%, V: 0.005 to 0.2% and W: 0.005 to 0.2% Containing steel and the remainder comprising Fe and inevitable impurities , and hot rolling under conditions of slab heating temperature 1200 ° C to 1350 ° C, finish rolling temperature 950 ° C to coiling
本発明によれば、微量のＺｒを添加することにより、優れた深絞り性と伸びフランジ性を兼ね備えた高強度熱延鋼板が提供される。
すなわち、微量のＺｒを添加した高強度鋼の母相中に、平均粒径１０ｎｍ以下のＴｉとＭｏを含む炭化物を析出させ、さらにｒ値の異方性を改善することにより、強度７８０ＭＰａ以上で、優れた深絞り性と伸びフランジ性を兼ね備えた鋼板が得られる。
According to the present invention, a high-strength hot-rolled steel sheet having both excellent deep drawability and stretch flangeability is provided by adding a small amount of Zr.
That is, by precipitating carbides containing Ti and Mo having an average particle size of 10 nm or less in the parent phase of high strength steel to which a small amount of Zr is added, and further improving the anisotropy of the r value, the strength is 780 MPa or more. A steel sheet having excellent deep drawability and stretch flangeability can be obtained.
以下、本発明について、金属組織、化学成分、および製造方法に分けて具体的に説明する。 Hereinafter, the present invention will be specifically described by dividing it into a metal structure, a chemical component, and a production method.
［金属組織］
本発明に係る高強度熱延鋼板は、実質的にフェライト単相組織であり、平均粒径１０ｎｍ以下のＴｉとＭｏを含む炭化物が析出している。以下、これらについて説明する。
[Metal structure]
The high-strength hot-rolled steel sheet according to the present invention has a substantially ferrite single-phase structure, and carbides containing Ti and Mo having an average particle size of 10 nm or less are precipitated. Hereinafter, these will be described.
・実質的にフェライト単相組織：
マトリックスを実質的にフェライト単相組織としたのは、深絞り性および伸びフランジ性の向上には、転位密度の低いフェライトが有効であり、また、伸びフランジ性の向上には、単相組織とすることが有効であり、特に、延性に富むフェライト単相組織でその効果が顕著であるためである。ただし、マトリックスは必ずしも完全にフェライト単相組織でなくともよく、実質的にフェライト単相組織であればよい。本発明において実質的にフェライト単相組織とは、本発明の炭化物以外に、微量の他の相ないしは析出物を許容することをいい、好ましくはフェライトの面積比率が９５％以上である。
・ Substantially ferrite single phase structure:
Ferrite with a low dislocation density is effective for improving the deep drawability and stretch flangeability, and the single-phase structure is used for improving the stretch flangeability. This is because the effect is particularly remarkable in a ferrite single phase structure rich in ductility. However, the matrix does not necessarily have a complete ferrite single phase structure, and may be substantially a ferrite single phase structure. In the present invention, the substantially single-phase ferrite structure means that a small amount of other phases or precipitates are allowed in addition to the carbide of the present invention, and preferably the ferrite area ratio is 95% or more.
・平均粒径１０ｎｍ以下のＴｉとＭｏを含む炭化物：
ＴｉとＭｏを含む炭化物は、微細となるため鋼を強化するのに有効である。従来は、炭化物としてＴｉＣを用いることが主流であったが、Ｔｉは炭化物形成傾向が強いため、Ｍｏを含まない場合、粗大化しやすく、強化に対する効果が低くなることから、必要な強化量を得るには、加工性を劣化させるまでの炭化物が必要となる。これに対し、ＴｉとＭｏを含む複合炭化物は、微細に析出して加工性を劣化させずに鋼を強化することができる。これは、Ｍｏの炭化物形成傾向がＴｉと比べて弱いため、安定的に微細に存在できることで強化に対する効果が高く、加工性を良好に維持できる炭化物量で必要な強化量が得られるためと考えられる。特に、この複合炭化物の平均粒径を１０ｎｍ以下とすることで、炭化物周囲の歪みが転位の移動の抵抗にとってより効果的となり、良好な鋼の強化が得られるため、平均粒径１０ｎｍ以下の複合炭化物とすることが好ましい。さらに好ましくは、平均粒径５ｎｍ以下である。
本発明の炭化物はこのようにＴｉ、Ｍｏを含有していることから、鋼中にこれらのＴｉ、Ｍｏおよび炭化物を構成するＣを以下に示す所定の範囲で含有させることにより、本発明の微細炭化物を有効に析出させることができる。炭化物が安定的に微細に存在できるためには、炭化物の組成が影響し、炭化物の組成が、原子比でＭｏ／（Ｔｉ＋Ｍｏ）≧０．２５となると、炭化物の粗大化を抑制する効果が高くなり、所望の微細炭化物を形成しやすくなる。
-Carbide containing Ti and Mo having an average particle size of 10 nm or less:
Since carbide containing Ti and Mo becomes fine, it is effective for strengthening steel. Conventionally, TiC was mainly used as a carbide, but Ti has a strong tendency to form carbides, so when Mo is not included, it is easy to coarsen and the effect on strengthening is reduced, so the necessary amount of reinforcement is obtained. For this, carbide is required until the workability is deteriorated. On the other hand, the composite carbide containing Ti and Mo can reinforce steel without precipitating finely and degrading workability. This is because Mo carbide formation tendency is weak compared to Ti, and because it can exist stably and finely, the effect on strengthening is high, and the required amount of reinforcement can be obtained with the amount of carbide that can maintain good workability. It is done. In particular, by setting the average grain size of the composite carbide to 10 nm or less, the distortion around the carbide becomes more effective for the resistance of dislocation movement, and good steel strengthening can be obtained. It is preferable to use carbide. More preferably, the average particle size is 5 nm or less.
Since the carbide of the present invention contains Ti and Mo as described above, the fine particles of the present invention can be obtained by containing the Ti, Mo and C constituting the carbide in the steel within the predetermined range shown below. Carbide can be effectively precipitated. In order for the carbide to exist stably and finely, the composition of the carbide influences. When the composition of the carbide is Mo / (Ti + Mo) ≧ 0.25 in atomic ratio, the effect of suppressing the coarsening of the carbide is high. It becomes easy to form a desired fine carbide.
化学成分：
本発明の高強度熱延鋼板には、質量％で、Ｃ：０．０２〜０．２０％、Ｍｏ：０．１〜０．８％、Ｔｉ：０．０２〜０．４０％およびＺｒ：０．０００５〜０．００５％を含有している。また、組織をより細粒化させるとともに、さらなる高強度化をはかる場合には、上記成分に加えてＮｂ：０．００５〜０．１％、Ｖ：０．００５〜０．２％およびＷ：０．００５〜０．２％から選ばれる１種または２種以上を含有する鋼板であることが好ましい。さらに、上記のＣ、Ｍｏ、Ｔｉ、Ｚｒ、Ｎｂ、Ｖ、Ｗ以外の成分については、Ｓｉ≦０．３％、Ｍｎ：０．５〜３．０％、Ｐ≦０．０６％、Ｓ≦０．０１％、Ａｌ≦０．１０％、Ｎ≦０．０２％を含み、残部が実質的にＦｅであることが好ましい。以下、これら各成分について説明する。
Chemical composition:
In the high-strength hot-rolled steel sheet of the present invention, C: 0.02 to 0.20%, Mo: 0.1 to 0.8%, Ti: 0.02 to 0.40%, and Zr: 0.0005-0.005% is contained. Moreover, in addition to the said component, Nb: 0.005-0.1%, V: 0.005-0.2%, and W: when making a structure | tissue finer and achieving further high intensity | strength. A steel sheet containing one or more selected from 0.005 to 0.2% is preferable. Furthermore, about components other than said C, Mo, Ti, Zr, Nb, V, and W, Si <= 0.3%, Mn: 0.5-3.0%, P <= 0.06%, S <= It is preferable that 0.01%, Al ≦ 0.10%, N ≦ 0.02% are included, and the balance is substantially Fe. Hereinafter, each of these components will be described.
Ｃ：０．０２〜０．２０％
Ｃ含有量は、０．０２〜０．２０％の範囲であり、好ましくは０．０３〜０．１５％の範囲である。Ｃは、ＴｉおよびＭｏを含む炭化物を形成し、フェライト母相中に微細析出することで、鋼の強度を確保する役割を担う元素である。しかし、Ｃ含有量が０．０２％未満では、他の強化機構（固溶や結晶粒の微細化）を併用しても安定して７８０ＭＰａ以上の強度を得るのは困難であり、０．２０％を超えると析出物の粗大化や第二相組織の形成により伸びフランジ性が低下する。
C: 0.02 to 0.20%
The C content is in the range of 0.02 to 0.20%, preferably in the range of 0.03 to 0.15%. C is an element that plays the role of ensuring the strength of the steel by forming carbides containing Ti and Mo and finely precipitating in the ferrite matrix. However, if the C content is less than 0.02%, it is difficult to stably obtain a strength of 780 MPa or more even if other strengthening mechanisms (solid solution and crystal grain refinement) are used in combination. If it exceeds 50%, stretch flangeability deteriorates due to coarsening of precipitates and formation of a second phase structure.
Ｍｏ：０．１〜０．８％
Ｍｏ含有量は、０．１〜０．８％であり、好ましくは０．１５〜０．６％である。Ｍｏは、ＴｉおよびＣと結合し微細な炭化物を形成させるのに必要であり、本発明における重要な元素のひとつである。Ｍｏが０．１％未満では、形成される微細炭化物量が不十分で７８０ＭＰａ以上の強度を安定して得ることが困難となる。一方、０．８％を超えての添加は、Ｍｏ炭化物（Ｔｉを含まないＭｏの炭化物）の形成を促進し、延性の低下につながるばかりか、コストの上昇を招く。
Mo: 0.1 to 0.8%
Mo content is 0.1-0.8%, Preferably it is 0.15-0.6%. Mo is necessary for bonding with Ti and C to form fine carbides, and is one of the important elements in the present invention. If Mo is less than 0.1%, the amount of fine carbide formed is insufficient and it becomes difficult to stably obtain a strength of 780 MPa or more. On the other hand, addition exceeding 0.8% promotes the formation of Mo carbide (Mo carbide not containing Ti), leading to a decrease in ductility and an increase in cost.
Ｔｉ：０．０２〜０．４０％
Ｔｉ含有量は、０．０２〜０．４０％の範囲であり、好ましくは０．０４〜０．３０％である。ＴｉはＭｏおよびＣと結合し、微細な炭化物を形成させるのに必要であり、本発明における重要な元素の一つである。
Ｔｉが０．０２％未満では、形成される微細炭化物量が不十分で７８０ＭＰａ以上の強度を安定して得ることが困難となる。一方、０．４０％を超えての添加は、延性の低下を招く。
Ti: 0.02 to 0.40%
The Ti content is in the range of 0.02 to 0.40%, preferably 0.04 to 0.30%. Ti is necessary for bonding with Mo and C to form fine carbides, and is one of the important elements in the present invention.
If Ti is less than 0.02%, the amount of fine carbide formed is insufficient, and it becomes difficult to stably obtain a strength of 780 MPa or more. On the other hand, addition exceeding 0.40% causes a decrease in ductility.
Ｚｒ：０．０００５〜０．００５％
Ｚｒ含有量は、０．０００５〜０．００５％の範囲であり、好ましくは０．０００７〜０．００４％の範囲である。Ｚｒは、本発明における最も重要な元素である。後記の図２および図３に示すように、Ｚｒの微量添加は、仕上圧延温度の高温化に伴いｒ値の異方性を改善し、その結果として穴広げ率λの向上（すなわち、伸びフランジ性の向上）と深絞り性の向上（プレス割れを生じることなく深絞り成形できること）という効果をもたらす。Ｚｒは、本発明の効果を得るために必須の元素である。Ｚｒ含有量が、０．０００５％未満では、穴広げ率が１２０％以上という高伸びフランジ性を得ることが困難である。一方、その効果は、０．００５％を超えると飽和し、かつ、強度の低下をもたらす。０．００５％を超えるＺｒ添加による強度低下は、ＴｉとＭｏを含む微細炭化物の析出量が減少するためと考えられる。以上のことから、Ｚｒ含有量は、０．０００５〜０．００５％とする。
Zr: 0.0005 to 0.005%
The Zr content is in the range of 0.0005 to 0.005%, preferably in the range of 0.0007 to 0.004%. Zr is the most important element in the present invention. As shown in FIGS. 2 and 3 to be described later, the addition of a small amount of Zr improves the anisotropy of the r value as the finish rolling temperature is increased, and as a result, the hole expansion ratio λ is improved (that is, stretch flange) Improved drawing) and deep drawability (being capable of deep drawing without causing press cracking). Zr is an essential element for obtaining the effects of the present invention. When the Zr content is less than 0.0005%, it is difficult to obtain high stretch flangeability with a hole expansion ratio of 120% or more. On the other hand, the effect is saturated when it exceeds 0.005%, and the strength is reduced. The decrease in strength due to the addition of Zr exceeding 0.005% is considered to be due to a decrease in the amount of precipitation of fine carbides containing Ti and Mo. From the above, the Zr content is set to 0.0005 to 0.005%.
Ｎｂ、Ｖ、Ｗ：
鋼組成としては、さらに、Ｎｂ、ＶおよびＷから選ばれる１種または２種以上の成分を含有することができる。
Ｎｂ、Ｖ、Ｗの含有量は、Ｎｂが０．００５〜０．１％、Ｖ：０．００５〜０．２％、Ｗ：０．００５〜０．２％であり、これらから選ばれる１種または２種以上を添加することが好ましい。
Nb, V, W:
The steel composition can further contain one or more components selected from Nb, V, and W.
The content of Nb, V, and W is 0.005 to 0.1% of Nb, V: 0.005 to 0.2%, and W: 0.005 to 0.2%. It is preferable to add seeds or two or more.
Ｎｂ、ＶおよびＷは、加熱および熱延中のγ粒の粗大化を防止し、フェライト粒の微細化に有効であり、Ｔｉと同様にＣを含む複合炭化物を形成し、強度上昇にも寄与する。従って、これらの元素は、必要に応じて適宜添加することが好ましいが、フェライト粒微細化の効果は、添加量が０．００５％未満では現れない。一方、Ｎｂを０．１％以上、ＶおよびＷを０．２％以上添加すると延性の低下を招く。このため、Ｎｂ、Ｖ、Ｗの含有量を上記のようにした。 Nb, V, and W are effective in preventing the coarsening of γ grains during heating and hot rolling, and effective in refining ferrite grains, forming a composite carbide containing C like Ti and contributing to an increase in strength. To do. Therefore, it is preferable to add these elements as needed, but the effect of refining ferrite grains does not appear when the added amount is less than 0.005%. On the other hand, if Nb is added at 0.1% or more and V and W are added at 0.2% or more, ductility is lowered. For this reason, the contents of Nb, V, and W are set as described above.
さらに、上記以外の成分の好ましい範囲について説明する。
Ｓｉ：０．３％以下
Ｓｉ含有量は、０．３％以下であり、好ましくは、０．２％以下である。Ｓｉは固溶強化に寄与するが、０．３％を超えて添加すると粒界にセメンタイトが生成し、伸びフランジ性を低下させる傾向がある。
Furthermore, the preferable range of components other than the above will be described.
Si: 0.3% or less Si content is 0.3% or less, preferably 0.2% or less. Si contributes to solid solution strengthening, but when added over 0.3%, cementite is generated at the grain boundary and tends to lower stretch flangeability.
Ｍｎ：０．５〜３．０％
Ｍｎ含有量は、０．５〜３．０％の範囲であり、より好ましくは０．６〜２．２％の範囲である。Ｍｎは固溶強化に有効な元素であり、７８０ＭＰａ以上の強度を得るためには０．５％以上の添加が望ましい。一方、３．０％を超えて添加すると、偏析が生じやすくなり、伸びフランジ性を低下させる場合があるため、３．０％以下が好ましい。
Mn: 0.5 to 3.0%
The Mn content is in the range of 0.5 to 3.0%, more preferably in the range of 0.6 to 2.2%. Mn is an element effective for solid solution strengthening, and in order to obtain a strength of 780 MPa or more, addition of 0.5% or more is desirable. On the other hand, if added over 3.0%, segregation is likely to occur, and the stretch flangeability may be lowered, so 3.0% or less is preferable.
Ｐ：０．０６％以下
Ｐ含有量は、０．０６％以下であり、より好ましくは０．０３％以下である。Ｐも固溶強化に有効な元素であるが、０．０６％を超えて添加すると、偏析による伸びフランジ性の低下を招く懸念があるため、０．０６％以下が好ましい。
P: 0.06% or less The P content is 0.06% or less, more preferably 0.03% or less. P is also an element effective for solid solution strengthening, but if added over 0.06%, there is a concern that stretch flangeability may be deteriorated due to segregation, so 0.06% or less is preferable.
Ｓ：０．０１％以下
Ｓ含有量は、０．０１％以下であり、好ましくは０．００５％以下である。Ｓは、ＴｉやＭｎと硫化物を形成するため、有効なＴｉやＭｎの低減を招く。このため、Ｓは、極力低減すべき元素である。
S: 0.01% or less The S content is 0.01% or less, preferably 0.005% or less. Since S forms sulfides with Ti and Mn, it causes an effective reduction of Ti and Mn. For this reason, S is an element which should be reduced as much as possible.
Ａｌ：０．１０％以下
Ａｌ含有量は、０．１０％以下であり、好ましくは、０．０６％以下である。Ａｌは脱酸材として重要な元素であるが、０．１０％を超える過度の添加は延性の低下を招くため、０．１０％以下とすることが望ましい。
Al: 0.10% or less The Al content is 0.10% or less, and preferably 0.06% or less. Al is an important element as a deoxidizing material, but excessive addition exceeding 0.10% leads to a decrease in ductility, so it is desirable to make it 0.10% or less.
Ｎ含有量：０．０２％以下
Ｎ含有量は、０．０２％以下であり、好ましくは０．０１０％以下である。ＮはＴｉと結合して比較的粗大な窒化物を形成するため、有効Ｔｉの低減につながる。このため、０．０２％以下とすることが望ましい。
N content: 0.02% or less The N content is 0.02% or less, preferably 0.010% or less. N combines with Ti to form a relatively coarse nitride, which leads to a reduction in effective Ti. For this reason, it is desirable to set it as 0.02% or less.
上記の元素以外の残部は、Ｆｅおよび不可避不純物である。なお、さらにＣｒ：０．１５％以下、Ｃｕ：０．１５％以下、Ｎｉ：０．１５％以下の１種以上を含有していても特性上問題はない。
The balance other than the above elements is Fe and inevitable impurities. In addition, there is no problem in characteristics even if one or more of Cr: 0.15% or less, Cu: 0.15% or less, and Ni: 0.15% or less are contained.
［製造方法］
次に、本発明の高強度熱延鋼板の製造方法について説明する。
本発明の高強度熱延鋼板を製造するに際し、前記鋼組成と同様の組成を有する鋼を溶製し、スラブ加熱温度１２００℃以上１３５０℃以下、仕上圧延温度９５０℃以上、巻取温度５００℃以上７００℃以下の条件で熱間圧延を行なう。以下、これらの条件について説明する。
[Production method]
Next, the manufacturing method of the high intensity | strength hot-rolled steel plate of this invention is demonstrated.
In producing the high-strength hot-rolled steel sheet of the present invention, a steel having the same composition as the steel composition is melted, and the slab heating temperature is 1200 ° C. or higher and 1350 ° C. or lower, the finish rolling temperature is 950 ° C. or higher, and the winding temperature is 500 ° C. Hot rolling is performed at a temperature of 700 ° C. or lower. Hereinafter, these conditions will be described.
スラブ加熱温度：
スラブ加熱温度は、１２００℃以上１３５０℃以下の範囲である。スラブ加熱温度は、ＣおよびＴｉの添加量に応じて、加熱時にＴｉ系炭化物が十分に溶解する温度を選択することが重要である。加熱温度が１２００℃未満では、未固溶のＴｉＣが残存し、強度不足を生じる。一方、加熱温度が１３５０℃を超えると、γ粒の異常成長により穴広げ率の低下につながる。
Slab heating temperature:
The slab heating temperature is in the range of 1200 ° C to 1350 ° C. It is important to select the slab heating temperature at a temperature at which the Ti-based carbide is sufficiently dissolved during heating, depending on the amounts of C and Ti added. When the heating temperature is less than 1200 ° C., undissolved TiC remains, resulting in insufficient strength. On the other hand, when the heating temperature exceeds 1350 ° C., it leads to a decrease in the hole expansion rate due to abnormal growth of γ grains.
仕上圧延温度：
仕上圧延温度は、９５０℃以上、より好ましくは９８０℃以上とする。
本発明の高強度熱延鋼板の製造方法の条件の中でも、仕上圧延温度はとくに重要な条件である。後記の図２および図３に示すように、仕上圧延温度の高温化とＺｒの微量添加の相乗効果によりｒ値の異方性が改善される。すなわち、ｒ値の最大値と最小値の差（ｒｍａｘ−ｒｍｉｎ）が小さくなる。仕上圧延温度が９５０℃未満では、微量のＺｒを添加したとしてもｒ値の異方性の低減効果が小さく、伸びフランジ性（穴広げ率λ）、深絞り性の向上を図ることが困難となる。そのため、仕上圧延温度は９５０℃以上とする。なお、上限については特に規定しないが、フェライト粒の粗粒化を防止する観点から１１００℃以下とすることが好ましい。
Finish rolling temperature:
The finishing rolling temperature is 950 ° C. or higher, more preferably 980 ° C. or higher.
Among the conditions for the production method of the high-strength hot-rolled steel sheet of the present invention, the finish rolling temperature is a particularly important condition. As shown in FIGS. 2 and 3 to be described later, the anisotropy of the r value is improved by a synergistic effect of increasing the finish rolling temperature and adding a small amount of Zr. That is, the difference (r max −r min ) between the maximum value and the minimum value of the r value becomes small. When the finish rolling temperature is less than 950 ° C., even if a small amount of Zr is added, the effect of reducing the anisotropy of the r value is small, and it is difficult to improve stretch flangeability (hole expansion ratio λ) and deep drawability. Become. Therefore, finishing rolling temperature shall be 950 ° C or more. In addition, although it does not prescribe | regulate especially about an upper limit, it is preferable to set it as 1100 degrees C or less from a viewpoint of preventing the coarsening of a ferrite grain.
巻取温度：
巻取温度は、５００〜７００℃の範囲であり、好ましくは５７０〜６５０℃である。
組織を実質的にフェライト単相とするため、および、平均粒径１０ｎｍ以下のＴｉとＭｏを含む炭化物の析出量を十分に確保するため、巻取温度は、５００〜７００℃とした。巻取温度が５００℃未満では、ＴｉとＭｏを含む炭化物の析出量が不十分となり、強度低下をまねく。一方、巻取温度が７００℃を超えると、析出した炭化物の粗大化が起こり、強度低下を招く。また、パーライトが生成しやすくなり、伸びフランジ性の低下を招く。
Winding temperature:
The coiling temperature is in the range of 500 to 700 ° C, preferably 570 to 650 ° C.
The coiling temperature was set to 500 to 700 ° C. in order to make the structure substantially a ferrite single phase and to ensure a sufficient amount of precipitation of carbide containing Ti and Mo having an average particle size of 10 nm or less. When the coiling temperature is less than 500 ° C., the amount of precipitation of carbides including Ti and Mo becomes insufficient, resulting in a decrease in strength. On the other hand, when the coiling temperature exceeds 700 ° C., the precipitated carbide is coarsened, resulting in a decrease in strength. Moreover, it becomes easy to produce | generate pearlite and causes the fall of stretch flangeability.
本発明において、鋼の溶製方法は特に限定されず、公知の溶製方法の全てを適用することができる。例えば、溶製方法としては、転炉、電気炉等で溶製し、真空脱ガス炉にて２次精錬を行なう方法が好適である。鋳造方法は、生産性、品質上の観点から、連続鋳造法が好ましい。また、鋳造後、直ちに、または補熱を目的とする加熱を施した後に、そのまま熱間圧延を行なう直送圧延を行なっても、本発明の効果に影響はない。さらに、粗圧延後に、仕上圧延前で、圧延材を加熱してもよく、粗圧延後に圧延材を接合して行なう連続熱延を行なっても、さらには、圧延材の加熱と連続圧延を同時に行なっても、本発明の効果は損なわれない。
そして、本発明鋼板は、黒皮ままでも、酸洗ままでもその特性に差違はない。調質圧延についても、通常行なわれるものであれば特に制限はない。また、溶融亜鉛めっき、電気めっきも可能であり、化成処理を施してもよい。
In the present invention, the steel melting method is not particularly limited, and all known melting methods can be applied. For example, as a melting method, a method of melting in a converter, electric furnace or the like and performing secondary refining in a vacuum degassing furnace is preferable. The casting method is preferably a continuous casting method from the viewpoint of productivity and quality. Further, the effect of the present invention is not affected even if the direct feed rolling, in which the hot rolling is performed as it is, immediately after casting or after heating for the purpose of supplementary heating is performed. Furthermore, after rough rolling and before finish rolling, the rolled material may be heated, or even when continuous rolling is performed by joining the rolled material after rough rolling, and further, heating and continuous rolling of the rolled material are performed simultaneously. Even if it carries out, the effect of this invention is not impaired.
And this invention steel plate does not have a difference in the characteristic, whether it is black skin or pickling. The temper rolling is not particularly limited as long as it is usually performed. Moreover, hot dip galvanization and electroplating are also possible, and chemical conversion treatment may be performed.
以下、実施例を挙げ、本発明をさらに詳細に説明するが、本発明はこれによって制約されるものではない。
（参考試験例）
まず、参考試験例として、本発明の基礎となった実験結果について、述べる。
高周波真空溶解でＣ：０．０８％、Ｓｉ：０．０２％、Ｍｎ：０．８％、Ｐ：０．００４％、Ｓ：０．０００６％、Ａｌ：０．０４２％、Ｎ：０．００３８％、Ｍｏ：０．３６％、Ｔｉ：０．１８２％を含み、残部が実質的にＦｅからなる鋼に微量のＺｒ：０〜０．００６７％（０は無添加を示す）を添加した鋼塊を溶製し、分塊圧延により板厚２７ｍｍの実験室熱延用シートバーとした。シートバーを１２７０℃に加熱後、７パスの熱間圧延により板厚２．０ｍｍの熱延鋼板を作製した。このとき、仕上圧延温度は８５０〜１０００℃の範囲で変化させた。熱延後は直ちに冷却速度７０〜１００℃／ｓで冷却し、巻取り処理として６００℃で１時間保持を行なった。
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further in detail, this invention is not restrict | limited by this.
(Reference test example)
First, as a reference test example, an experimental result on which the present invention is based will be described.
C: 0.08%, Si: 0.02%, Mn: 0.8%, P: 0.004%, S: 0.0006%, Al: 0.042%, N: 0.0. A small amount of Zr: 0 to 0.0067% (0 indicates no addition) was added to the steel containing 0038%, Mo: 0.36%, Ti: 0.182%, and the balance being substantially Fe. A steel ingot was melted, and a sheet bar for laboratory hot rolling with a thickness of 27 mm was obtained by split rolling. After heating the sheet bar to 1270 ° C., a hot-rolled steel sheet having a thickness of 2.0 mm was produced by 7-pass hot rolling. At this time, the finish rolling temperature was changed in the range of 850 to 1000 ° C. Immediately after hot rolling, the steel sheet was cooled at a cooling rate of 70 to 100 ° C./s, and held at 600 ° C. for 1 hour as a winding process.
（１）穴広げ率λに対するＺｒ添加量、仕上圧延温度の影響
こうして得られた熱延鋼板のうち、強度レベルが近い強度９８５〜１０２０ＭＰａの熱延鋼板について、Ｚｒ添加量および仕上圧延温度が穴広げ率λに及ぼす影響を調べた。その結果を図２に示す。なお、伸びフランジ性の指標となる穴広げ率λは、熱延鋼板から１３０ｍｍ角の板を切り出しドリルによって１０ｍｍφの切削穴を空けた後、６０°円錐ポンチを下から押し上げ、亀裂が鋼板を貫通した時点で穴径ｄを測定し、穴広げ率λ（％）を次式より算出した。
(1) Effect of Zr addition amount and finish rolling temperature on hole expansion ratio λ Among hot-rolled steel plates thus obtained, Zr addition amount and finish rolling temperature are holes in hot rolled steel plates having strengths of 985 to 1020 MPa, which are close in strength level. The effect on the spreading factor λ was investigated. The result is shown in FIG. The hole expansion ratio λ, which is an index of stretch flangeability, is obtained by cutting a 130 mm square plate from a hot-rolled steel plate and drilling a 10 mmφ hole with a drill, then pushing up a 60 ° conical punch from below, and a crack penetrates the steel plate. At that time, the hole diameter d was measured, and the hole expansion ratio λ (%) was calculated from the following equation.
λ［％］＝１００・（ｄ−１０）／１０ λ [%] = 100 · (d−10) / 10
図２から明らかなように、仕上圧延温度が１０００℃の場合、Ｚｒ添加量が０．０００５％以上になると穴広げ率λが１２０％を超えることが判るが、仕上圧延温度が８５０℃の場合、λの改善効果はほとんど認められなかった。さらに、穴広げ試験終了直後の穴広がり部を観察すると、穴広がり部は板厚が不均一に変形し、板厚減少の大きい部分で割れが発生していることが確認された。すなわち、穴広げ率λにはｒ値の異方性が大きく影響していることが判明した。 As can be seen from FIG. 2, when the finish rolling temperature is 1000 ° C., the hole expansion ratio λ exceeds 120% when the Zr addition amount is 0.0005% or more, but the finish rolling temperature is 850 ° C. The improvement effect of λ was hardly recognized. Further, when observing the hole expanding portion immediately after the end of the hole expanding test, it was confirmed that the plate thickness of the hole expanding portion was deformed non-uniformly, and cracking occurred in the portion where the plate thickness decrease was large. That is, it has been found that the anisotropy of the r value greatly affects the hole expansion ratio λ.
（２）プレス割れに対する仕上圧延温度、ｒ値の異方性の影響：
次に、プレス割れに及ぼすｒ値の異方性と仕上圧延温度の影響を図３に示す。仕上圧延温度が１０００℃の場合、ｒｍａｘ−ｒｍｉｎが０．８以下ではプレス割れは発生しなかった（５サンプル中割れ発生ゼロ）。一方、仕上圧延温度が８５０℃の場合、異方性自体の改善は図られず、かつ、ｒｍａｘ−ｒｍｉｎが０．８程度の場合でも５サンプル中２サンプルで割れが発生した。
(2) Effect of finish rolling temperature and r value anisotropy on press cracking:
Next, the influence of r-value anisotropy and finish rolling temperature on press cracking is shown in FIG. When the finish rolling temperature was 1000 ° C., no press cracks occurred when r max −r min was 0.8 or less (zero cracks generated in 5 samples). On the other hand, when the finish rolling temperature was 850 ° C., the anisotropy itself was not improved, and cracking occurred in 2 of 5 samples even when r max −r min was about 0.8.
なお、ｒ値の測定は、圧延方向から０度、４５度および９０度の方向からＪＩＳ ５号引張り試験片を採取し、最もｒ値の高いものをｒｍａｘ、最も低いものをｒｍｉｎと定義した。 In the measurement of the r value, JIS No. 5 tensile test specimens were taken from directions of 0, 45 and 90 degrees from the rolling direction, and the highest r value was defined as r max and the lowest one was defined as r min. did.
また、プレス加工性（プレス割れ）の評価は、板厚２．０ｍｍ、ブランク径１３５ｍｍの板を直径６５ｍｍの円筒ポンチで深絞り加工し、割れ発生の有無を調査することにより行なった。各条件毎に５サンプル試験し、５サンプルの全てに割れ発生がない場合を、プレス加工性（深絞り性）良好と判定した。 The evaluation of press workability (press cracking) was performed by deep drawing a plate having a plate thickness of 2.0 mm and a blank diameter of 135 mm with a cylindrical punch having a diameter of 65 mm and investigating whether or not cracks occurred. Five samples were tested for each condition, and when no cracks occurred in all five samples, it was determined that press workability (deep drawability) was good.
（本発明例および比較例）
表１に示す化学組成のスラブを種々の温度に加熱した後、熱間圧延により板厚２．０ｍｍの熱延鋼板とした。この際、加熱温度、仕上圧延温度、冷却速度、および巻取温度を変化させた。これら熱延鋼板に酸洗処理を施した後、１３０ｍｍ角の穴広げ用試験片および圧延方向から０、４５および９０度の方向からＪＩＳ ５号引張試験片を採取し、穴広げ試験、引張試験、ｒ値の測定及び組織観察、プレス加工性の評価を行なった。また、これらの鋼板から作製した薄膜を透過型電子顕微鏡（ＴＥＭ）によって観察し、炭化物平均粒径を測定した。炭化物の組成は、ＴＥＭに装備されたＥＤＸ分析から決定した。なお、穴広げ率、ｒ値、プレス加工性の試験条件は、前記の（参考試験例）の場合と同様である。
(Invention Example and Comparative Example)
A slab having the chemical composition shown in Table 1 was heated to various temperatures, and then hot rolled into a hot rolled steel sheet having a thickness of 2.0 mm. At this time, the heating temperature, finish rolling temperature, cooling rate, and winding temperature were changed. These hot-rolled steel sheets were pickled, and 130 mm square hole-expanding specimens and JIS No. 5 tensile specimens from 0, 45, and 90 degrees from the rolling direction were sampled, hole-expanding test, and tensile test. The r value was measured, the structure was observed, and the press workability was evaluated. Moreover, the thin film produced from these steel plates was observed with the transmission electron microscope (TEM), and the carbide | carbonized_material average particle diameter was measured. The carbide composition was determined from EDX analysis equipped with TEM. Note that the test conditions for the hole expansion ratio, r value, and press workability are the same as in the above (Reference Test Example).
実験結果は表２に示すとおりである。本発明例の鋼板は、いずれも比較例の鋼板に比べて優れたプレス成形性（深絞り性）および伸びフランジ性（穴広げ率λ）を有することが示された。すなわち、いずれか１つ以上の条件において本発明の範囲から外れる比較例１〜９の鋼板は、強度、プレス成形性および伸びフランジ性の全ての特性を同時に満足するものではなかった。 The experimental results are shown in Table 2. It was shown that all the steel plates of the present invention had excellent press formability (deep drawability) and stretch flangeability (hole expansion ratio λ) as compared with the steel plate of the comparative example. That is, the steel plates of Comparative Examples 1 to 9 that deviate from the scope of the present invention under any one or more conditions did not satisfy all the properties of strength, press formability, and stretch flangeability at the same time.
本発明は、例えば自動車用鋼板など、種々の用途に加工されて使用される高強度熱延鋼板として好適である。 The present invention is suitable as a high-strength hot-rolled steel sheet that is processed and used in various applications such as a steel sheet for automobiles.
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