JP4161935B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Hot-rolled steel sheet and manufacturing method thereof Download PDF

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JP4161935B2
JP4161935B2 JP2004121260A JP2004121260A JP4161935B2 JP 4161935 B2 JP4161935 B2 JP 4161935B2 JP 2004121260 A JP2004121260 A JP 2004121260A JP 2004121260 A JP2004121260 A JP 2004121260A JP 4161935 B2 JP4161935 B2 JP 4161935B2
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充 吉田
俊郎 富田
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Nippon Steel Corp
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Sumitomo Metal Industries Ltd
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Description

本発明は、自動車の足廻り部品等の延性およびバーリング加工性が求められる素材として好適な機械特性に優れる微細粒熱延鋼板およびその製造方法に関する。   The present invention relates to a fine-grain hot-rolled steel sheet excellent in mechanical properties suitable as a material for which ductility and burring workability of automobile undercarriage parts and the like are required, and a method for producing the same.

近年、地球環境保護の観点から、自動車のCO排出量の低減などを目的に自動車車体の軽量化が進められている。軽量化手段の一つとして車体部材の薄肉化がある。そのために高強度鋼板の使用が増加し、その鋼板も更なる高強度化の傾向にある。 In recent years, from the viewpoint of global environmental protection, the weight reduction of automobile bodies has been promoted for the purpose of reducing automobile CO 2 emissions. One of the means for reducing the weight is to reduce the thickness of the vehicle body member. For this reason, the use of high-strength steel sheets is increasing, and the steel sheets are also tending to have higher strength.

自動車用高強度鋼板としては、高強度であることに加えて、多様な部品形状に加工するために優れた成形性が要求される。即ち、車体軽量化の動きに伴い、車体部材に用いられる鋼板には高強度と高成形性が求められているのである。   As a high-strength steel sheet for automobiles, in addition to high strength, excellent formability is required for processing into various component shapes. That is, with the movement of the weight reduction of the vehicle body, high strength and high formability are required for the steel plate used for the vehicle body member.

鋼の強化方法には種々あるが、組織微細化は、環境保護の点から排除したい合金元素の添加を必要としない高強度化の手段として、昨今特に注目され始めている。しかし、結晶粒の微細化を進めていくと、降伏点が極端に高くなると共に、伸びが低減して、加工性が劣化するという問題がある。   Although there are various methods for strengthening steel, the refinement of the structure has recently begun to attract attention as a means for increasing the strength that does not require the addition of an alloy element that should be excluded from the viewpoint of environmental protection. However, there is a problem that as the grain refinement proceeds, the yield point becomes extremely high, the elongation decreases, and the workability deteriorates.

高強度化に伴う加工性の劣化を抑える目的で、高強度鋼における組織の微細化および第二相組織を制御する手法が提案されている。例えば、高強度化と組織微細化を同時に得られる方法として、特許文献1(特開2000−328186号公報)などに提案されているNbやTiを含ませ制御圧延する方法がある。しかしながら、この方法で得られる析出強化型高強度鋼板は、高降伏比のものになるため、プレス成形などの加工後のスプリングバックが大きく形状凍結性に劣り、さらに機械的特性の異方性も大きいという問題がある。   In order to suppress the deterioration of workability accompanying the increase in strength, a technique for refining the structure and controlling the second phase structure in the high-strength steel has been proposed. For example, as a method for simultaneously obtaining high strength and fine structure, there is a method of controlling rolling by including Nb or Ti proposed in Patent Document 1 (Japanese Patent Laid-Open No. 2000-328186). However, since the precipitation-strengthened high-strength steel sheet obtained by this method has a high yield ratio, the spring back after processing such as press molding is large and the shape freezeability is inferior, and the anisotropy of mechanical characteristics is also low. There is a problem of being big.

第二相制御による加工性に優れる熱延鋼板としては、フェライト・マルテンサイト組織またはフェライト・ベイナイト組織からなる混合組織のもの、或いはベイナイトまたはフェライト主体のほぼ単相組織のものが広く知られている。   As hot-rolled steel sheets excellent in workability by second phase control, those having a mixed structure consisting of a ferrite / martensite structure or a ferrite / bainite structure, or a substantially single-phase structure mainly composed of bainite or ferrite are widely known. .

強度と延性が共に優れる鋼材としては、例えば特許文献2(特開昭55−44551号公報)に記載されているようなDP鋼(複合組織フェライト・マルテンサイト鋼)が知られているが、この鋼は、変形の初期からマルテンサイトの周囲にミクロボイドが発生して割れを生じるため、穴拡げ性に劣る問題があり、足廻り部品等の高い穴拡げ性が要求される用途には不向きである。   As a steel material excellent in both strength and ductility, for example, DP steel (composite structure ferritic / martensitic steel) as described in Patent Document 2 (Japanese Patent Laid-Open No. 55-44551) is known. Steel has micro-voids around the martensite from the beginning of deformation, causing cracks, so there is a problem that the hole expandability is inferior, and it is not suitable for applications that require high hole expandability such as undercarriage parts. .

伸びフランジ性および穴拡げ性の改善に関して、特許文献3(特開平4−88125号公報)および特許文献4(特開平3−180426号公報)には、ベイナイトを主体とした組織を有する鋼板が開示されている。しかし、その鋼板は、ベイナイトを主体とした組織であるために、穴拡げ性は優れるものの、軟質なフェライト相が少ないので延性に劣る。   Regarding improvement of stretch flangeability and hole expandability, Patent Document 3 (Japanese Patent Laid-Open No. 4-88125) and Patent Document 4 (Japanese Patent Laid-Open No. 3-180426) disclose steel sheets having a structure mainly composed of bainite. Has been. However, since the steel sheet has a structure mainly composed of bainite, the hole expandability is excellent, but the ductility is inferior because there are few soft ferrite phases.

特許文献5(特開2000−109951号公報)には、ベイナイト単相にし、さらにその組織の微細化により、高延性と高伸びフランジ性を兼備させた熱延鋼板が開示されているが、その鋼板はTi、Nbの少なくとも一方の添加を必要としているため、降伏強度比の上昇が避けられず、形状凍結性に劣り、機械的特性の異方性も大きくなる。   Patent Document 5 (Japanese Patent Laid-Open No. 2000-109951) discloses a hot-rolled steel sheet that has a bainite single phase and further refines its structure to have both high ductility and high stretch flangeability. Since steel sheets require the addition of at least one of Ti and Nb, an increase in yield strength ratio is inevitable, shape freezeability is inferior, and anisotropy in mechanical properties increases.

特開2000−328186号公報JP 2000-328186 A

特開昭55−44551号公報JP 55-44551 A 特開平4−88125号公報JP-A-4-88125 特開平3−180426号公報Japanese Patent Laid-Open No. 3-180426 特開2000−109951号公報JP 2000-109951 A

本発明は、高い延性と伸びフランジ性を兼備するフェライトとベイナイトの複相組織の熱延鋼板とその製造方法を提供することを目的とする。   An object of the present invention is to provide a hot-rolled steel sheet having a dual phase structure of ferrite and bainite that has both high ductility and stretch flangeability, and a method for producing the same.

本発明者らは、上記の課題を達成するために鋼の成分および製造条件について鋭意検討を重ねた結果、下記の知見を得た。   As a result of intensive studies on steel components and production conditions in order to achieve the above-mentioned problems, the present inventors have obtained the following knowledge.

(A) フェライトとベイナイトの複相組織鋼板においては、フェライト粒の微細分散化により加工性が向上する。   (A) In a multiphase steel sheet of ferrite and bainite, workability is improved by finely dispersing ferrite grains.

(B) フェライトの微細分散化により加工時の割れの伝播を抑制することができ、伸びフランジ性が向上し、さらに、組織全体にわたってフェライト粒が一様に変形することにより高い延性が得られる。   (B) The fine dispersion of ferrite can suppress the propagation of cracks during processing, improve stretch flangeability, and obtain high ductility by uniformly deforming ferrite grains throughout the entire structure.

(C) さらに、板厚方向のフェライトの粒径分布と表層の集合組織を積極的に活用することにより、更なる加工性の向上を得られる。   (C) Furthermore, the workability can be further improved by actively utilizing the grain size distribution of ferrite in the plate thickness direction and the texture of the surface layer.

(D) 上記の微細組織をTi、Nbなどを添加せずに得る製造条件として、仕上げ圧延直後に、超急速冷却することが効果的である。また、最終圧延パスをAr3点以上の温度で終了し、その後0.6秒以内に640℃以下まで冷却することが有効である。 (D) As a manufacturing condition for obtaining the above microstructure without adding Ti, Nb, etc., it is effective to perform ultra-rapid cooling immediately after finish rolling. It is also effective to finish the final rolling pass at a temperature of Ar 3 or higher and then cool to 640 ° C. or lower within 0.6 seconds.

上記の知見を基礎とする本発明は、下記(1)〜(4)の熱延鋼板、および(5)の熱延鋼板の製造方法を要旨とする。   The gist of the present invention based on the above findings is the hot rolled steel sheet of the following (1) to (4) and the hot rolled steel sheet manufacturing method of (5).

(1)質量%で、C:0.02〜0.25%、Si:2.0%以下、Mn:0.3〜2.5%、P:0.2%以下、S:0.05%以下、Al:0.005〜0.5%およびN:0.01%以下を含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ下記の(a)と(b)の少なくとも一方の特徴を有する熱延鋼板。
(a) 鋼板の表面から板厚方向に1/8の厚さの位置におけるフェライトの平均粒径(ds)と板厚中心におけるフェライトの平均粒径(dc)との比率(ds/dc)が0.3〜0.7であること。
(b) 鋼板の表面から板厚方向に1/8の厚さの位置における{110}<111>、{110}<001>および{211}<111>の極密度の和が集合組織をもたないものの5倍以上であり、かつ、それぞれが1.5倍以上であること。
(1) By mass%, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% And the balance is Fe and impurities, the volume ratio is 40 to 95% bainite phase and the balance is ferrite. The ferrite has an average crystal grain size of 1.2 μm or more. A hot-rolled steel sheet that is less than 4 μm and has at least one of the following characteristics (a) and (b).
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each.

(2)上記(1)の成分に加えて、次の第1群の成分の中から選んだ少なくとも1種の成分を含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ前記の(a)および(b)の少なくとも一方の特徴を有する熱延鋼板。
(2) In addition to the above component (1), it contains at least one component selected from the following group 1 components, with the balance being Fe and impurities, 40 to 95% by volume It has a structure containing a bainite phase and the balance being a ferrite phase, the ferrite has an average crystal grain size of 1.2 μm or more and less than 4 μm , and at least one of the above-mentioned features (a) and (b) hot-rolled steel sheet having a.

第1群の成分
Ti:0.2%以下、Nb:0.1%以下、V:1%以下、B:0.005%以下、Cu:1.0%以下、Ni:1.0%以下、Cr:1.0%以下およびMo:1.0%以下。
Group 1 ingredients
Ti: 0.2% or less, Nb: 0.1% or less, V: 1% or less, B: 0.005% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less.

(3)上記(1)の成分に加えて、次の第2群の成分の中から選んだ少なくとも1種の成分を含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ前記の(a)および(b)の少なくとも一方の特徴を有する熱延鋼板。
(3) In addition to the above component (1), it contains at least one component selected from the following second group of components, with the balance being Fe and impurities, with a volume ratio of 40 to 95%. A structure containing a bainite phase and the balance being a ferrite phase, wherein the ferrite has an average crystal grain size of 1.2 μm or more and less than 4 μm , and at least one of the features (a) and (b) A hot-rolled steel sheet.

第2群の成分
それぞれ、または2種以上の合計で0.005%以下のCa、REMおよびMg。
Each component of the second group, or a total of two or more of 0.005% or less of Ca, REM and Mg.

(4)上記(1)の成分に加えて、前記第1群の成分から選んだ少なくとも1種および上記第2群の成分の中から選んだ少なくとも1種の成分を含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ前記の(a)および(b)の少なくとも一方の特徴を有する熱延鋼板。
(4) In addition to the component (1) above, the composition contains at least one component selected from the first group of components and at least one component selected from the second group of components, with the balance being Fe and consisting impurities, the remainder contains 40% to 95% of bainite phase by volume has a structure comprising a ferrite phase, the mean crystal grain size of the ferrite is less than 4μm above 1.2 [mu] m, and wherein the A hot-rolled steel sheet having at least one of the characteristics of (a) and (b) .

(5)上記(1)〜(4)のいずれかに記載の化学組成を有するスラブまたは鋼板に、多パスの熱間圧延を施す熱延鋼板の製造方法であって、前記熱間圧延を、Ar3点以上で「Ar3点+100℃」以下の温度域における板厚減少率が40%以上、1パス当たりの圧下率が10〜60%、最終圧延パスにおける圧延温度がAr3点以上として行い、その後0.6秒以内に640℃以下まで冷却し、600℃を超えて640℃以下の温度域における滞在時間を25秒以下とし、600℃以下350℃以上で巻き取ることを特徴とする熱延鋼板の製造方法。 (5) A method for producing a hot-rolled steel sheet that performs multi-pass hot rolling on a slab or steel sheet having the chemical composition according to any one of (1) to (4) above, The sheet thickness reduction rate in the temperature range of Ar 3 points or more and “Ar 3 points + 100 ° C.” or less is 40% or more, the rolling reduction per pass is 10 to 60%, and the rolling temperature in the final rolling pass is Ar 3 points or more. And then cooled to 640 ° C or lower within 0.6 seconds, the residence time in the temperature range from 600 ° C to 640 ° C or lower is 25 seconds or shorter, and the hot rolling is performed at 600 ° C or lower and 350 ° C or higher. A method of manufacturing a steel sheet.

上記(5)の方法においては、640℃以下までの冷却を、400℃/秒以上の冷却速度で行い、0.4秒以内に640℃以下まで冷却するのが望ましい。   In the method (5), it is desirable that the cooling to 640 ° C. or lower is performed at a cooling rate of 400 ° C./second or more, and the cooling is performed to 640 ° C. or lower within 0.4 seconds.

本発明の熱延鋼板は、延性、伸びフランジ性等の特性に優れ、自動車の足廻り部品等の延性およびバーリング加工性が求められる素材として好適な熱延鋼板である。この鋼板は、軽量化を目指す自動車の車体用等に特に好適である。そして、本発明の製造方法によれば、上記の熱延鋼板を確実に製造することができる。   The hot-rolled steel sheet of the present invention is excellent in properties such as ductility and stretch flangeability, and is a hot-rolled steel sheet suitable as a material that requires ductility and burring workability of automobile undercarriage parts and the like. This steel plate is particularly suitable for automobile bodies and the like that aim to reduce weight. And according to the manufacturing method of this invention, said hot-rolled steel plate can be manufactured reliably.

以下、本発明で定めた熱延鋼板の化学組成および組織、ならびに製造条件の限定理由について説明する。以下の記載においては、含有量に関する「質量%」を単に「%」と記す。
熱延鋼板の化学組成
C:
Cは、ベイナイト生成による強度確保のために重要な元素であり、また、高温でのオーステナイトを安定化させることで熱延の仕上げ温度を低下させることができ、フェライト結晶粒の微細化を促進する。これらの効果を得るためには、0.02%以上の含有が必要であるが、過度に添加するとフェライトの析出不足や粗大な鉄炭化物の析出による加工性の劣化を招き、また溶接性も劣化する。従って、含有量の上限は0.25%とする。より好ましい含有量は、0.03〜0.2%である。
Hereinafter, the chemical composition and structure of the hot-rolled steel sheet defined in the present invention and the reasons for limiting the manufacturing conditions will be described. In the following description, “mass%” regarding the content is simply referred to as “%”.
Chemical composition of hot-rolled steel sheet C:
C is an important element for securing the strength due to bainite generation, and the finishing temperature of hot rolling can be lowered by stabilizing austenite at a high temperature, thereby promoting the refinement of ferrite crystal grains. . In order to obtain these effects, a content of 0.02% or more is necessary. However, if added excessively, workability deteriorates due to insufficient precipitation of ferrite and precipitation of coarse iron carbide, and weldability also deteriorates. Therefore, the upper limit of the content is 0.25%. A more preferable content is 0.03-0.2%.

Si:
Siは、鋼の延性の低下を抑制して強度を向上させる場合に添加することが有効であるが、過度の添加はAr点を上昇させ、フェライト粒微細化を困難にするばかりでなく、鋼板の表面性状および溶接性も劣化させるため上限を2.0%とする。Siは、積極的に添加しなくてもよいので、含有量の下限は不純物レベルでもよい。しかし、前記の効果を得たい場合には0.01%以上含有させるのが望ましい。
Si:
It is effective to add Si to improve the strength by suppressing the reduction of the ductility of the steel, but excessive addition raises the Ar 3 point and not only makes ferrite grain refinement difficult, In order to deteriorate the surface properties and weldability of the steel plate, the upper limit is set to 2.0%. Since Si does not need to be positively added, the lower limit of the content may be an impurity level. However, if it is desired to obtain the above effect, it is desirable to contain 0.01% or more.

Mn:
Mnは、ベイナイト組織の生成による強度確保のために重要な元素であり、また、高温でのオーステナイトを安定化させるので、熱間圧延の仕上げ温度を低下させることができ、フェライト結晶粒の微細化を促進する。この効果を得るには0.3%以上の含有を必要とするが、過度の添加は非金属介在物の多量析出により伸びフランジ性を劣化させるため、含有量の上限を2.5%とする。より好ましい含有量は0.5〜2.0%である。
Mn:
Mn is an important element for securing strength due to the formation of a bainite structure, and also stabilizes austenite at high temperatures, so that the finishing temperature of hot rolling can be lowered and the ferrite crystal grains can be refined. Promote. In order to obtain this effect, a content of 0.3% or more is required. However, excessive addition deteriorates stretch flangeability due to a large amount of nonmetallic inclusions, so the upper limit of the content is set to 2.5%. A more preferable content is 0.5 to 2.0%.

P:
Pは、固溶強化元素として働くので、高強度化のために添加してもよい。一方、偏析し易い元素でもあるため過度の添加は粒界偏析により加工性の低下を招く。従って、含有量の上限を0.2%とする。より好ましいのは0.1%以下、さらに好ましいのは0.05%以下である。なお、Pは積極的に添加しなくてもよいので、含有量の下限は不純物レベルでもよい。一般には、製鋼段階で0.005%程度は不可避に混入する。ただし、上記の高強度化の効果を得たいときには、0.01%以上含有させるのがよい。
P:
P acts as a solid solution strengthening element, so it may be added to increase the strength. On the other hand, since it is also an element that easily segregates, excessive addition causes deterioration of workability due to grain boundary segregation. Therefore, the upper limit of the content is 0.2%. More preferred is 0.1% or less, and further more preferred is 0.05% or less. In addition, since it is not necessary to add P actively, the minimum of content may be an impurity level. Generally, about 0.005% is inevitably mixed in the steelmaking stage. However, in order to obtain the effect of increasing the strength described above, it is preferable to contain 0.01% or more.

S:
Sは、硫化物系介在物を形成して加工性を低下させる不純物元素であるため、その含有量は極力少なくするのが好ましい。含有量の許容上限を0.05%とするが、一段と優れた加工性を確保するには0.008%以下とするのが好ましく、さらに好ましいのは0.003%以下である。
S:
Since S is an impurity element that forms sulfide inclusions and reduces workability, it is preferable to reduce the content thereof as much as possible. The allowable upper limit of the content is 0.05%, but 0.008% or less is preferable to further ensure excellent workability, and 0.003% or less is more preferable.

Al:
Alは、鋼の脱酸元素として必要であり、0.005%以上の含有が必要である。一方、過度の添加はAr点を上昇させ、フェライト粒微細化を困難にするばかりでなく、溶接性も劣化させるため含有量の上限は0.5%とする。
Al:
Al is necessary as a deoxidizing element for steel and must be contained in an amount of 0.005% or more. On the other hand, excessive addition not only raises the Ar 3 point and makes ferrite grain refinement difficult, but also degrades weldability, so the upper limit of the content is 0.5%.

N:
Nは、鋼の加工性を低下させる不純物元素であり、その含有量は0.01%以下に抑えることが望ましい。より好ましいのは、0.006%以下である。
N:
N is an impurity element that reduces the workability of steel, and its content is preferably suppressed to 0.01% or less. More preferred is 0.006% or less.

本発明の熱延鋼板の一つは、上記の成分のほか、残部がFeおよび不純物からなる。他の一つは、前記の第1群の成分の中の少なくとも1種を含むものである。この第1群に属する成分は、いずれも熱延鋼板の強度の向上に寄与する成分である。   One of the hot-rolled steel sheets of the present invention is composed of Fe and impurities in addition to the above components. The other includes at least one of the first group of components. The components belonging to the first group are all components that contribute to improving the strength of the hot-rolled steel sheet.

Ti:
Tiは、Ti(C、N)として析出し鋼の強度を増加させる。また、この析出物がオーステナイトやフェライトの粗大化を抑制して、結晶粒の微細化を促進する。これらの効果を得たい場合には添加してもよい。しかし、過度に添加すると、熱延以前の加熱時に粗大なTi(C、N)が多量に発生して伸びフランジ性の劣化を招き、また、面内異方性も大きくなるため、上限を0.2%とする。好ましいのは0.15%以下である。なお、上記の効果を得るのには、0.005%以上含有させるのがよい。
Ti:
Ti precipitates as Ti (C, N) and increases the strength of the steel. Moreover, this precipitate suppresses the coarsening of austenite and ferrite, and promotes refinement of crystal grains. If these effects are desired, they may be added. However, if added excessively, a large amount of coarse Ti (C, N) is generated during heating before hot rolling, causing deterioration of stretch flangeability and increasing in-plane anisotropy. %. Preferable is 0.15% or less. In addition, in order to acquire said effect, it is good to contain 0.005% or more.

Nb:
Nbは、Nb(C、N)として析出し、鋼の強度を増加させ、また、この析出物がオーステナイトやフェライトの粗大化を抑制して、結晶粒の微細化を促進する。これらの効果を得たい場合には添加してもよい。しかし、過度に添加すると、熱間圧延前の加熱時に粗大なNb(C、N)が多量に発生して伸びフランジ性の劣化を招き、また、面内異方性も大きくなる。従って、含有量の上限を0.1%とする。好ましいのは0.06%以下である。なお、上記の効果を得るのには、0.005%以上含有させるのがよい。
Nb:
Nb precipitates as Nb (C, N), increases the strength of the steel, and this precipitate suppresses the coarsening of austenite and ferrite and promotes the refinement of crystal grains. If these effects are desired, they may be added. However, if added excessively, a large amount of coarse Nb (C, N) is generated during heating before hot rolling, resulting in deterioration of stretch flangeability and in-plane anisotropy. Therefore, the upper limit of the content is 0.1%. Preferable is 0.06% or less. In addition, in order to acquire said effect, it is good to contain 0.005% or more.

V:
Vは、析出強化により鋼の強度を増加させ、また、この析出物がフェライトの粗大化を抑制して、結晶粒の微細化を促進する。これらの効果を得たい場合には添加してもよい。ただし、TiおよびNbと同様の理由で、延性や加工性を阻害するので、含有量の上限を1%とする。好ましいのは0.5%以下、さらに好ましいのは0.3%以下である。なお、上記の効果を得るのには、0.01%以上含有させるのがよい。
V:
V increases the strength of the steel by precipitation strengthening, and this precipitate suppresses the coarsening of ferrite and promotes the refinement of crystal grains. If these effects are desired, they may be added. However, for the same reason as Ti and Nb, ductility and workability are hindered, so the upper limit of the content is made 1%. Preferred is 0.5% or less, and more preferred is 0.3% or less. In addition, in order to acquire said effect, it is good to make it contain 0.01% or more.

B:
Bは、焼入れ性を向上させて高強度化に寄与する元素であるため、添加してもよいが、過度に添加するとその効果が飽和するだけでなく、延性が低下するので、その含有量の上限を0.005%とする。なお、上記の効果を得るのには、0.0005%以上含有させるのがよい。
B:
B is an element that contributes to high strength by improving hardenability, so it may be added, but adding too much not only saturates its effect, but also reduces ductility, so the content of The upper limit is 0.005%. In addition, in order to acquire said effect, it is good to contain 0.0005% or more.

Cu:
Cuは、固溶強化及び析出強化に有効な元素であるため添加してもよいが、過度に添加するとその効果が飽和するだけでなく、コストアップを招くので1.0%を含有量の上限とする。なお、上記の効果を得るのには、0.01%以上含有させるのがよい。
Cu:
Cu may be added because it is an effective element for solid solution strengthening and precipitation strengthening, but adding too much will not only saturate the effect but also increase the cost, so 1.0% is made the upper limit of the content . In addition, in order to acquire said effect, it is good to make it contain 0.01% or more.

Ni:
Niは、焼入れ性を向上させて所望の組織を得るために有効な元素である。また、Cuを添加する場合はスラブの粒界脆化を防止する効果もある。これらの効果を得たいときには添加してもよい。その含有量はNi≧Cu/2の範囲であることが望ましいが、過度に添加するとコストアップになるので1.0%を上限とする。Cuと併用しない場合の望ましい含有量の下限は0.01%である。
Ni:
Ni is an effective element for improving the hardenability and obtaining a desired structure. Further, when Cu is added, there is an effect of preventing grain boundary embrittlement of the slab. When these effects are desired, they may be added. The content is preferably in the range of Ni ≧ Cu / 2, but excessive addition increases the cost, so 1.0% is made the upper limit. The lower limit of the desirable content when not combined with Cu is 0.01%.

Cr:
Crは、焼入れ性を向上させて所望の組織を得るために有効な元素であるため添加してもよいが、過度に添加しても効果が飽和し、また、鋼板の製造コストを上昇させることにもなるので、その含有量の上限を1.0%とする。なお、上記の効果を得るためには、0.01%以上の含有が望ましい。
Cr:
Cr may be added because it is an effective element for improving the hardenability and obtaining the desired structure, but the effect will be saturated even if added excessively, and the production cost of the steel sheet will be increased. Therefore, the upper limit of the content is set to 1.0%. In addition, in order to acquire said effect, containing 0.01% or more is desirable.

Mo:
Moも焼入れ性を向上させて所望の組織を得るために有効な元素であるため添加してもよいが、過度に添加しても効果が飽和し、また、鋼板の製造方法コスト増加にもなるのでその上限を1.0%とする。好ましいのは0.5%以下であり、より好ましいのは0.3%以下である。なお、上記の効果を得るためには、0.005%以上の含有が望ましい。
Mo:
Mo may also be added because it is an effective element for improving the hardenability to obtain a desired structure, but if added too much, the effect is saturated, and the manufacturing cost of the steel sheet also increases. Therefore, the upper limit is set to 1.0%. Preferred is 0.5% or less, and more preferred is 0.3% or less. In addition, in order to acquire said effect, containing 0.005% or more is desirable.

本発明の熱延鋼板のさらに他の一つは、前記の第2群の成分の中の少なくとも1種を含むものである。この第2群に属する成分は、いずれも熱延鋼板の加工性の向上に寄与する成分である。なお、第2群の成分の少なくとも1種と、前記第1群の成分の少なくとも1種を併用してもよい。   Still another one of the hot-rolled steel sheets of the present invention contains at least one of the above-mentioned second group of components. All of the components belonging to the second group are components that contribute to the improvement of workability of the hot-rolled steel sheet. In addition, you may use together at least 1 sort (s) of the 2nd group component and at least 1 sort (s) of the said 1st group component.

Ca、Mg、REM:
これらの元素はMnSの形態を制御して鋼の加工性を改善する効果を有するので、この効果を得たいときには添加してもよい。しかし、これらの元素の含有量が増えるとその効果は飽和する傾向があり、また、いずれも高価な元素であるため、単独の含有量または合計の含有量で0.005%を上限とする。上記の加工性改善の効果を得るには、単独または合計で0.0002%以上の含有が望ましい。なお、REMとは、希土類元素の総称で、YおよびScも含む。
Ca, Mg, REM:
Since these elements have the effect of improving the workability of steel by controlling the form of MnS, they may be added to obtain this effect. However, when the content of these elements increases, the effect tends to saturate, and since both are expensive elements, the upper limit is set to 0.005% for the single content or the total content. In order to obtain the effect of improving the workability described above, it is desirable to contain 0.0002% or more alone or in total. REM is a general term for rare earth elements and includes Y and Sc.

2.熱延鋼板の組織
熱延鋼板の伸びフランジ性向上の観点からは、金属組織が均質であることが好ましい。高強度と高伸びフランジ性を兼備させるためには、ベイナイト相分率の多いほうが好ましいが、硬質相であるベイナイトは延性に乏しく高延性が得られない。従って、延性向上の観点からはフェライトの体積率を増加させることが好ましいが、フェライトとベイナイトの界面の増加により、加工初期にき裂が発生する確率が高くなり伸びフランジ性が劣化する。従って、延性と伸びフランジ性との兼ね合いの観点から、ベイナイト相の最適量が決まる。
2. Structure of hot-rolled steel sheet From the viewpoint of improving stretch flangeability of a hot-rolled steel sheet, it is preferable that the metal structure is homogeneous. In order to combine high strength and high stretch flangeability, a higher bainite phase fraction is preferable, but bainite, which is a hard phase, has poor ductility and high ductility cannot be obtained. Therefore, from the viewpoint of improving ductility, it is preferable to increase the volume fraction of ferrite, but due to the increase in the interface between ferrite and bainite, the probability that a crack will occur at the initial stage of processing increases and stretch flangeability deteriorates. Therefore, the optimum amount of bainite phase is determined from the viewpoint of balance between ductility and stretch flangeability.

ベイナイトの体積率が40%未満では高強度が得られず、かつ伸びフランジ性に劣り、95%を超えると延性が劣る。好ましいベイナイトの体積率は50〜95%である。   When the volume fraction of bainite is less than 40%, high strength cannot be obtained and the stretch flangeability is inferior, and when it exceeds 95%, the ductility is inferior. The volume fraction of bainite is preferably 50 to 95%.

さらに、本発明の熱延鋼板において特に重要なのは、フェライトの細粒化である。フェライト粒の微細分散によりフェライト相の変形が板厚全体にわたって一様になるために延性が向上する。また、穴拡げ加工時にはフェライト相とベイナイト相の界面にき裂が発生しても、そのき裂伝播を遅延させる効果により伸びフランジ性が向上する。   Further, particularly important in the hot-rolled steel sheet of the present invention is the refinement of ferrite. Due to the fine dispersion of the ferrite grains, the deformation of the ferrite phase becomes uniform over the entire thickness, so that the ductility is improved. Further, even when a crack is generated at the interface between the ferrite phase and the bainite phase during the hole expanding process, the stretch flangeability is improved by the effect of delaying the crack propagation.

これらの効果を得るためには、鋼板全体のフェライトの平均粒径を1.2μmから4μmまでに制御する必要がある。1.2μm未満の場合は、フェライト粒の本来持つ延性を発揮できず、降伏強度比(降伏強度/最高強度)も高くなるために、延性劣化を招く。一方、4μmを超える場合には、穴拡げ加工時のき裂伝播遅延効果を得られず、伸びフランジ性が向上しない。好ましいのは1.5〜3.0μmである。   In order to obtain these effects, it is necessary to control the average grain size of ferrite in the entire steel sheet from 1.2 μm to 4 μm. If it is less than 1.2 μm, the inherent ductility of the ferrite grains cannot be exhibited, and the yield strength ratio (yield strength / maximum strength) becomes high, leading to ductility deterioration. On the other hand, if it exceeds 4 μm, the effect of delaying crack propagation at the time of hole expansion processing cannot be obtained, and stretch flangeability is not improved. Preferred is 1.5 to 3.0 μm.

また、き裂の起点となる鋼板表面の組織の微細化は、き裂発生そのものを遅らせる効果を有し、鋼板表層の微細化は伸びフランジ性の向上に効果がある。しかし、過度の微細化は鋼板表層の降伏強度比を高くし、延性劣化を招き、鋼板自体の延性が劣化する。従って、好ましいのは、板厚方向の1/8の厚さ位置でのフェライト平均粒径(ds)の範囲が、板厚中心のフェライト平均粒径(dc)に対する比、即ち、「ds/dc」で、0.3≦ds/dc≦0.7を満たす範囲にあることである。   Further, the refinement of the structure on the surface of the steel sheet, which is the starting point of the crack, has the effect of delaying the crack generation itself, and the refinement of the surface layer of the steel sheet is effective in improving the stretch flangeability. However, excessive refinement increases the yield strength ratio of the steel sheet surface layer, leading to ductility deterioration, and the ductility of the steel sheet itself deteriorates. Therefore, it is preferable that the range of the average ferrite grain size (ds) at the thickness position of 1/8 in the thickness direction is the ratio to the average ferrite grain size (dc) at the center of the thickness, that is, “ds / dc”. ”Is in a range satisfying 0.3 ≦ ds / dc ≦ 0.7.

一方、表面から板厚方向の1/8の厚さ位置における{110}<111>、{110}<001>および{211}<111>の極密度の和が集合組織をもたないものの5倍以上であり、かつ、それぞれが1.5倍以上であることも好ましい。これらの方位が発達している場合に加工性の向上が得られるからである。加工性が向上する原因は明らかではないが、次のように推測される。即ち、粒界が粒界として機能するためには、隣接粒との角度差が15度以上である粒界(大傾角粒界)であることが好ましく、前述の極密度が発達すると大傾角粒界の割合が増加して、細粒化効果が効率良く発揮されるものと考えられる。より好ましいのは、前記極密度の和が、集合組織をもたないものの6倍以上であることである。   On the other hand, the sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a thickness of 1/8 from the surface in the thickness direction has no texture. It is also preferable that the ratio is 1.5 times or more and each is 1.5 times or more. This is because workability can be improved when these orientations are developed. The reason why the workability is improved is not clear, but is presumed as follows. That is, in order for the grain boundary to function as a grain boundary, the grain boundary is preferably a grain boundary (large tilt grain boundary) having an angular difference of 15 degrees or more with adjacent grains. It is considered that the ratio of the boundary increases and the fine graining effect is efficiently exhibited. More preferably, the sum of the extreme densities is 6 times or more that of no texture.

本発明の熱延鋼板の組織は、体積率で40〜95%のベイナイトと残部がフェライトからなるものである。   The structure of the hot-rolled steel sheet of the present invention is composed of 40% to 95% bainite by volume and the balance being ferrite.

本発明におけるベイナイトの体積率とは、板厚方向に1/8、1/4および1/2の厚さ位置の任意の各5視野以上を、いわゆるメッシュ法にて測定した算術平均値である。フェライトの平均粒径とは、板厚方向に1/8、1/4および1/2の厚さ位置の任意の各5視野以上を切断法により測定して求めた算術平均値を、球相当直径となるように1.13倍したものであり、鋼板全体のフェライトの平均粒径とは、前記3箇所の板厚位置におけるフェライト粒径の算術平均値である。   The volume fraction of bainite in the present invention is an arithmetic average value measured by a so-called mesh method for each of five or more visual fields at thickness positions of 1/8, 1/4 and 1/2 in the thickness direction. . The average grain size of ferrite is the sphere equivalent to the arithmetic average value obtained by measuring any five or more visual fields at 1/8, 1/4 and 1/2 thickness positions in the plate thickness direction by the cutting method. The average grain size of ferrite of the entire steel sheet is an arithmetic average value of ferrite grain diameters at the three plate thickness positions.

3.製造方法
本発明の熱延鋼板の製造方法では、これまでに述べた化学組成を有する鋼の鋼片(スラブ)または鋼板を素材とする。これらの素材は、連続鋳造や鋳造・分塊により得たスラブ、ストリップキャスティングにより得た鋼板などや、必要によってはそれらに熱間加工または冷間加工を加えたものである。それらが冷材であればAc点以上の温度に再加熱して、圧延する。
3. Manufacturing method In the manufacturing method of the hot-rolled steel sheet of the present invention, a steel slab or steel sheet having the chemical composition described so far is used as a raw material. These materials are slabs obtained by continuous casting or casting / splitting, steel plates obtained by strip casting, etc., and if necessary, hot working or cold working. If they are cold materials, they are reheated to a temperature of Ac 3 point or higher and rolled.

熱間圧延の前の加熱温度はAc点以上の温度であれば特に制限はない。一般的には900〜1350℃である。TiCやNbCなどの析出物をオーステナイト中に十分に溶解させる必要がない鋼種の場合は、この範囲の中でも比較的低い温度(1100℃以下)に再加熱することが好ましい。初期のオーステナイト結晶粒が微細化し、最終のフェライトおよびベイナイトの粒径も微細化し易くなるからである。 The heating temperature before hot rolling is not particularly limited as long as it is a temperature of Ac 3 point or higher. Generally, it is 900-1350 degreeC. In the case of a steel type in which precipitates such as TiC and NbC do not need to be sufficiently dissolved in austenite, it is preferable to reheat to a relatively low temperature (1100 ° C. or lower) within this range. This is because the initial austenite crystal grains are refined, and the final ferrite and bainite grains are easily refined.

圧延は、レバースミルまたはタンデムミルを用いて、オーステナイト温度域で行う。工業的生産性の上からは、少なくとも最終の数段はタンデムミルを用いるのが好ましい。圧延温度は、圧延後にオーステナイトからフェライトへと変態させるためにAr点以上の温度とする。圧延を終了する温度はAr点に近いほど良い。これは、圧延によってオーステナイトに導入された加工歪みの蓄積効果が大きくなり、組織の微細化が促進されるからである。 Rolling is performed in the austenite temperature range using a lever mill or a tandem mill. From the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the last several stages. Rolling temperature, the Ar 3 point or more temperature in order to transformation to ferrite from austenite after rolling. The temperature at which the rolling is finished is better as it is closer to the Ar 3 point. This is because the effect of accumulating processing strain introduced into austenite by rolling increases, and the refinement of the structure is promoted.

圧下率は、細粒化の効果を得るためにAr点から「Ar点+100℃」までの温度範囲における圧下率(板厚減少率)で40%以上が必要である。より好ましいのは60%以上である。圧延は、1パスで行う必要はなく、連続した複数パスの圧延であっても良い。1パス当たりの圧下率は10〜60%の範囲で、1パス当たりの圧下率を大きくする方が、オーステナイトへの歪みの蓄積が増加して、変態によって生成するフェライトおよびベイナイトの微細化が進むので好ましい。しかし、1パス当りの圧下率が大きすぎると鋼板の板厚方向の組織差が著しくなり加工性の劣化を招く。従って、1パス当たりの好ましい圧下率は、15〜50%である。 The rolling reduction needs to be 40% or more in terms of the rolling reduction (sheet thickness reduction rate) in the temperature range from the Ar 3 point to the “Ar 3 point + 100 ° C.” in order to obtain the effect of fine graining. More preferred is 60% or more. Rolling does not have to be performed in one pass, and may be rolling in a plurality of consecutive passes. The rolling reduction per pass is in the range of 10 to 60%. Increasing the rolling reduction per pass increases the accumulation of strain in austenite, and refinement of ferrite and bainite generated by transformation progresses. Therefore, it is preferable. However, if the rolling reduction per pass is too large, the structural difference in the sheet thickness direction of the steel sheet becomes significant, leading to deterioration of workability. Therefore, the preferable rolling reduction per pass is 15 to 50%.

圧延が終了した後は、オーステナイトに導入された加工歪みを解放することなく、これを駆動力としてオーステナイトから微細なフェライトとベイナイトの複相組織に変態させる。その条件は、下記のとおりである。   After the rolling is completed, the working strain introduced into the austenite is transformed into a fine phase structure of ferrite and bainite from austenite as a driving force without releasing it. The conditions are as follows.

まず、圧延終了から640℃以下の温度まで0.6秒以内に冷却し、600℃を超えて640℃までの温度域での滞在時間を20秒以内とする。   First, it cools within 0.6 second to the temperature of 640 degrees C or less after completion | finish of rolling, and the residence time in the temperature range exceeding 600 degreeC to 640 degreeC shall be 20 seconds or less.

圧延終了から640℃以下に冷却するのに要する時間は、上記のとおり0.6秒以内とするが、より好ましいのは0.4秒以内である。この冷却は水冷が望ましく、冷却速度を400℃/秒以上とするのが望ましい。   The time required for cooling to 640 ° C. or less after the end of rolling is within 0.6 seconds as described above, but more preferably within 0.4 seconds. This cooling is preferably water cooling, and the cooling rate is preferably 400 ° C./second or more.

640℃以下の温度に冷却されるまでの時間(滞在時間)を規定する理由は、640℃以上の温度で冷却を停止したり、または640℃以下の温度になるまでの経過時間が長くなると、加工によって導入された歪みの解放や、歪みの存在形態の変化により、フェライトの核生成サイトとならずに組織が粗大化したり、フェライト変態が最も活発に起こる温度で冷却を停止させた場合はフェライトがごく短時間で急激に析出したりするため、ベイナイト体積率が不十分になるからである。   The reason for prescribing the time (dwell time) until cooling to a temperature of 640 ° C or lower is that if the cooling is stopped at a temperature of 640 ° C or higher, or if the elapsed time until reaching a temperature of 640 ° C or lower becomes longer, If the structure is coarsened due to the release of strain introduced by processing or changes in the strain's existence form, the structure becomes coarse, or the cooling is stopped at the temperature at which ferrite transformation occurs most actively, the ferrite This is because the bainite volume fraction becomes insufficient because of the rapid precipitation in a very short time.

640℃以下の温度域は、フェライト変態が最も活発になる温度域を外れてはいるものの、600℃を超える温度域は、依然としてフェライト変態が進行する温度域である。この温度域での冷却中にフェライトが均一微細に析出する。しかし、焼入れ性の高い材料においてはフェライトの析出が不十分でベイナイト相が95%を超える場合があるため、640℃に達した後、冷却を一次停止するか、または冷却速度を低下させて、640℃以下で600℃を超える温度域での滞在時間を長めにするのがよい。ただし、滞在時間が25秒を超えるとフェライト体積率の過度の増加、フェライト粒の粗大化、およびパーライト析出などにより、「強度−伸びのバランス」および「強度−伸びフランジのバランス」の劣化が生じるので、25秒以内とするのが好ましい。   Although the temperature range below 640 ° C. is outside the temperature range where the ferrite transformation is most active, the temperature range above 600 ° C. is still the temperature range where the ferrite transformation proceeds. During cooling in this temperature range, ferrite precipitates uniformly and finely. However, in a material with high hardenability, the precipitation of ferrite may be insufficient and the bainite phase may exceed 95%, so after reaching 640 ° C, the cooling is temporarily stopped or the cooling rate is decreased, It is better to extend the residence time in the temperature range below 640 ℃ and over 600 ℃. However, if the residence time exceeds 25 seconds, deterioration of "strength-elongation balance" and "strength-elongation flange balance" occurs due to excessive increase in ferrite volume fraction, coarsening of ferrite grains, pearlite precipitation, etc. Therefore, it is preferably within 25 seconds.

捲取り温度は350℃以上、600℃以下が好ましい。350℃未満ではマルテンサイトが生成するために延性、伸びフランジ性がともに劣化する。600℃を超える温度で巻き取るとフェライト粒の粗大化、組織中のセメンタイトの粗大化などにより強度、伸びフランジ性がともに劣化する。好ましい巻取り温度は400〜550℃である。   The scooping temperature is preferably 350 ° C. or higher and 600 ° C. or lower. Below 350 ° C, martensite is formed, so both ductility and stretch flangeability deteriorate. When wound at a temperature exceeding 600 ° C, both strength and stretch flangeability deteriorate due to the coarsening of ferrite grains and the coarsening of cementite in the structure. A preferable winding temperature is 400 to 550 ° C.

表1に示す化学組成を有する鋼を溶製し、熱間鍛造によって30mm厚さにした。その後、1100〜1300℃の温度域に再加熱した後、試験用小型タンデムミルにて圧延を実施し、2mmの板厚に仕上げた。全ての圧延において、圧延の仕上げ温度は、各鋼種のAr点よりも高い温度とした。得られた鋼板の板厚断面を走査型電子顕微鏡を用いて観察して組織を調べた。機械的性質については、引張特性および伸びフランジ加工性を以下の方法で調査した。 Steel having the chemical composition shown in Table 1 was melted and made 30 mm thick by hot forging. Then, after reheating to a temperature range of 1100 to 1300 ° C., rolling was performed with a small tandem mill for testing to finish a sheet thickness of 2 mm. In all rolling, the finishing temperature of rolling was higher than the Ar 3 point of each steel type. The thickness of the obtained steel sheet was observed with a scanning electron microscope to examine the structure. Regarding mechanical properties, tensile properties and stretch flangeability were investigated by the following methods.

引張特性はJISの5号引張試験片にて行い、伸びフランジ加工性は、縦横が100mmの正方形の試験片を採取し、その中央にポンチにて直径10mmの打ち抜き穴(クリアランス:15%)をあけ、先端角60°の円錐ポンチでこの穴を拡げて、穴の縁にクラックが貫通したときの穴直径から計算される限界穴拡げ率で評価した。   Tensile properties are measured with JIS No. 5 tensile test piece. Stretch flangeability is 100 mm in length and width, and a 10 mm diameter punch hole (clearance: 15%) is punched in the center. This hole was expanded with a conical punch with a tip angle of 60 °, and the critical hole expansion rate calculated from the hole diameter when a crack penetrated the edge of the hole was evaluated.

表2に製造条件、表3に組織および機械特性の調査結果を示す。なお、表3の「ds/dc」、{110}<111>、{110}<001>および{211}<111>の極密度、ならびにそれらの和は、鋼板の表面から板厚方向に1/8の厚さの位置における、集合組織をもたないものに対する倍率である。本発明例は、いずれも優れた引張強度(TS)と伸び(EL)のバランス(TS×EL)および引張強度(TS)と穴拡げ率(HER)のバランス(TS×HER)を有している。一方、化学組成、ベイナイト体積率、およびフェライト粒径の少なくとも一つが本発明で定める範囲外である比較例は、いずれも機械特性に劣る。
Table 2 shows the manufacturing conditions, and Table 3 shows the results of the investigation of the structure and mechanical properties. It should be noted that the "ds / dc", {110} <111>, {110} <001> and {211} <111> pole densities in Table 3 and their sum are 1 in the thickness direction from the surface of the steel plate. It is a magnification with respect to a material having no texture at a position of / 8 thickness . Each of the examples of the present invention has an excellent balance of tensile strength (TS) and elongation (EL) (TS × EL) and balance of tensile strength (TS) and hole expansion rate (HER) (TS × HER). Yes. On the other hand, all of the comparative examples in which at least one of the chemical composition, the bainite volume fraction, and the ferrite particle size is outside the range defined by the present invention are inferior in mechanical properties.

Figure 0004161935
Figure 0004161935

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本発明の熱延鋼板は、延性、伸びフランジ性等の特性に優れるので、自動車の足廻り部品等の延性およびバーリング加工性が求められる素材として好適であり、自動車車体の軽量化やコストの低減などに寄与する。また、本発明の製造方法によれば、上記の熱延鋼板を確実に製造することができる。
Since the hot-rolled steel sheet of the present invention is excellent in properties such as ductility and stretch flangeability, it is suitable as a material requiring ductility and burring workability of automobile undercarriage parts, etc., and reducing the weight and cost of an automobile body. Contribute to Moreover, according to the manufacturing method of this invention, said hot-rolled steel plate can be manufactured reliably.

Claims (6)

質量%で、C:0.02〜0.25%、Si:2.0%以下、Mn:0.3〜2.5%、P:0.2%以下、S:0.05%以下、Al:0.005〜0.5%およびN:0.01%以下を含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ下記の(a)と(b)の少なくとも一方の特徴を有する熱延鋼板。
(a) 鋼板の表面から板厚方向に1/8の厚さの位置におけるフェライトの平均粒径(ds)と板厚中心におけるフェライトの平均粒径(dc)との比率(ds/dc)が0.3〜0.7であること。
(b) 鋼板の表面から板厚方向に1/8の厚さの位置における{110}<111>、{110}<001>および{211}<111>の極密度の和が集合組織をもたないものの5倍以上であり、かつ、それぞれが1.5倍以上であること。
In mass%, C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 0.3 to 2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005 to 0.5% and N: 0.01% or less And the balance is composed of Fe and impurities, has a volume ratio of 40 to 95% bainite phase and the balance is composed of a ferrite phase, and the average crystal grain size of the ferrite is 1.2 μm or more and less than 4 μm. A hot-rolled steel sheet having at least one of the following characteristics (a) and (b):
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each.
質量%で、C:0.02〜0.25%、Si:2.0%以下、Mn:0.3〜2.5%、P:0.2%以下、S:0.05%以下、Al:0.005〜0.5%およびN:0.01%以下、ならびにTi:0.2%以下、Nb:0.1%以下、V:1%以下、B:0.005%以下、Cu:1.0%以下、Ni:1.0%以下、Cr:1.0%以下およびMo:1.0%以下の中から選んだ1種以上を含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ下記の(a)と(b)の少なくとも一方の特徴を有する熱延鋼板。
(a) 鋼板の表面から板厚方向に1/8の厚さの位置におけるフェライトの平均粒径(ds)と板厚中心におけるフェライトの平均粒径(dc)との比率(ds/dc)が0.3〜0.7であること。
(b) 鋼板の表面から板厚方向に1/8の厚さの位置における{110}<111>、{110}<001>および{211}<111>の極密度の和が集合組織をもたないものの5倍以上であり、かつ、それぞれが1.5倍以上であること。
% By mass, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% or less, and Ti: 0.2% or less, Nb: 0.1% or less, V: 1% or less, B: 0.005% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less It contains one or more selected components, the balance is composed of Fe and impurities, has a volume ratio of 40 to 95% bainite phase and the balance is composed of a ferrite phase, and the average crystal grain size of the ferrite is A hot-rolled steel sheet that is 1.2 μm or more and less than 4 μm and has at least one of the following characteristics (a) and (b).
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each.
質量%で、C:0.02〜0.25%、Si:2.0%以下、Mn:0.3〜2.5%、P:0.2%以下、S:0.05%以下、Al:0.005〜0.5%およびN:0.01%以下、ならびにCa、REMおよびMgのうちの1種以上を合計で0.005%以下含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ下記の(a)と(b)の少なくとも一方の特徴を有する熱延鋼板。
(a) 鋼板の表面から板厚方向に1/8の厚さの位置におけるフェライトの平均粒径(ds)と板厚中心におけるフェライトの平均粒径(dc)との比率(ds/dc)が0.3〜0.7であること。
(b) 鋼板の表面から板厚方向に1/8の厚さの位置における{110}<111>、{110}<001>および{211}<111>の極密度の和が集合組織をもたないものの5倍以上であり、かつ、それぞれが1.5倍以上であること。
% By mass, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% or less, and Containing one or more of Ca, REM and Mg in a total amount of 0.005% or less, the balance being Fe and impurities, the volume fraction of 40% to 95% bainite phase and the balance being ferrite phase A hot-rolled steel sheet having an average crystal grain size of 1.2 μm or more and less than 4 μm and having at least one of the following characteristics (a) and (b):
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each.
質量%で、C:0.02〜0.25%、Si:2.0%以下、Mn:0.3〜2.5%、P:0.2%以下、S:0.05%以下、Al:0.005〜0.5%およびN:0.01%以下、ならびにTi:0.2%以下、Nb:0.1%以下、V:1%以下、B:0.005%以下、Cu:1.0%以下、Ni:1.0%以下、Cr:1.0%以下およびMo:1.0%以下の中から選んだ1種以上、さらにCa、REMおよびMgのうちの1種以上を合計で0.005%以下を含有し、残部がFeおよび不純物からなり、体積率で40〜95%のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が1.2μm以上で4μm未満であって、かつ下記の(a)と(b)の少なくとも一方の特徴を有する熱延鋼板。
(a) 鋼板の表面から板厚方向に1/8の厚さの位置におけるフェライトの平均粒径(ds)と板厚中心におけるフェライトの平均粒径(dc)との比率(ds/dc)が0.3〜0.7であること。
(b) 鋼板の表面から板厚方向に1/8の厚さの位置における{110}<111>、{110}<001>および{211}<111>の極密度の和が集合組織をもたないものの5倍以上であり、かつ、それぞれが1.5倍以上であること。
% By mass, C: 0.02-0.25%, Si: 2.0% or less, Mn: 0.3-2.5%, P: 0.2% or less, S: 0.05% or less, Al: 0.005-0.5% and N: 0.01% or less, and Ti: 0.2% or less, Nb: 0.1% or less, V: 1% or less, B: 0.005% or less, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less One or more selected, further one or more of Ca, REM, and Mg contain 0.005% or less in total, the balance consists of Fe and impurities, and contains a bainite phase of 40 to 95% by volume. A hot-rolled steel sheet having a structure in which the balance is composed of a ferrite phase, the ferrite having an average crystal grain size of 1.2 μm or more and less than 4 μm , and having at least one of the following characteristics (a) and (b):
(a) The ratio (ds / dc) of the average grain size (ds) of ferrite at the position of 1/8 thickness from the surface of the steel plate to the average grain size (dc) of ferrite at the center of the plate thickness is 0.3 to 0.7.
(b) The sum of the polar densities of {110} <111>, {110} <001> and {211} <111> at a position of 1/8 thickness from the surface of the steel plate to the thickness direction has a texture. It must be at least 5 times that of the other and 1.5 times each.
請求項1から4までのいずれかに記載の化学組成を有するスラブまたは鋼板に、多パスの熱間圧延を施す熱延鋼板の製造方法であって、前記熱間圧延を、Ar3点以上で「Ar3点+100℃」以下の温度域における板厚減少率が40%以上、1パス当たりの圧下率が10〜60%、最終圧延パスにおける圧延温度がAr3点以上として行い、その後640℃以下まで0.6秒以内に冷却し、600℃を超えて640℃以下の温度域における滞在時間を25秒以下とし、600℃以下350℃以上で巻き取ることを特徴とする熱延鋼板の製造方法。 A method for producing a hot-rolled steel sheet in which multi-pass hot rolling is performed on a slab or steel sheet having the chemical composition according to any one of claims 1 to 4, wherein the hot rolling is performed at an Ar point of 3 or more. The thickness reduction rate in the temperature range below “Ar 3 point + 100 ° C.” is 40% or more, the rolling reduction per pass is 10 to 60%, the rolling temperature in the final rolling pass is Ar 3 point or more, and then 640 ° C. A method for producing a hot-rolled steel sheet, which is cooled within 0.6 seconds until the following, wherein the residence time in the temperature range from 600 ° C. to 640 ° C. is 25 seconds or less, and winding is performed at 600 ° C. or less and 350 ° C. or more. 640℃以下までの冷却を、400℃/秒以上の冷却速度で行い、0.4秒以内に640℃以下まで冷却する請求項5に記載の熱延鋼板の製造方法。 The method for producing a hot-rolled steel sheet according to claim 5 , wherein cooling to 640 ° C or lower is performed at a cooling rate of 400 ° C / second or more, and cooling to 640 ° C or lower within 0.4 seconds.
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