JP5533729B2 - High-strength hot-rolled steel sheet with excellent local deformability and excellent ductility with less orientation dependency of formability and method for producing the same - Google Patents

High-strength hot-rolled steel sheet with excellent local deformability and excellent ductility with less orientation dependency of formability and method for producing the same Download PDF

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JP5533729B2
JP5533729B2 JP2011035776A JP2011035776A JP5533729B2 JP 5533729 B2 JP5533729 B2 JP 5533729B2 JP 2011035776 A JP2011035776 A JP 2011035776A JP 2011035776 A JP2011035776 A JP 2011035776A JP 5533729 B2 JP5533729 B2 JP 5533729B2
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千智 若林
力 岡本
貴行 野崎
幸一 佐野
展弘 藤田
学 高橋
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Description

本発明は、曲げ、伸びフランジ、バーリング加工などの局部変形能に優れ、かつその成形性の方位依存性が少なく、かつ延性に優れた熱延鋼板とその製造方法に関する。   The present invention relates to a hot-rolled steel sheet having excellent local deformability such as bending, stretch flange, burring, and the like, having less orientation dependency of formability and excellent ductility, and a method for producing the same.

自動車からの炭酸ガスの排出量を抑えるために、高強度鋼板を使用して自動車車体の軽量化が進められている。また、搭乗者の安全性確保のためにも、自動車車体には軟鋼板の他に高強度鋼板が多く使用されるようになってきている。更に自動車車体の軽量化を今後進めていくためには、従来以上に高強度鋼板の使用強度レベルを高めなければならず、例えば足回り部品に高強度鋼板を用いるにはバーリング加工のための局部変形能を改善しなければならない。   In order to reduce carbon dioxide emissions from automobiles, the weight of automobile bodies is being reduced using high-strength steel sheets. In addition, in order to ensure the safety of passengers, high strength steel plates are often used in automobile bodies in addition to mild steel plates. Furthermore, in order to further reduce the weight of automobile bodies in the future, it is necessary to increase the use strength level of high-strength steel sheets more than before. For example, to use high-strength steel sheets for undercarriage parts, local parts for burring processing The deformability must be improved.

しかしながら、一般的に鋼板を高強度化すれば成形性が低下し、絞り成形や張り出し成形に重要な均一伸びが低下する。これに対して文献1のように、鋼板にオーステナイトを残留させ均一伸びを確保する方法が開示されている。   However, generally, if the strength of the steel plate is increased, the formability is lowered, and the uniform elongation important for drawing or stretch forming is lowered. On the other hand, as disclosed in Document 1, a method is disclosed in which austenite remains in a steel sheet to ensure uniform elongation.

一方では、曲げ成形、穴拡げ加工やバーリング加工に代表される局部延性を改善する鋼板の金属組織制御法についても開示されており、介在物制御や単一組織化すること、さらには組織間の硬度差を低減すれば、曲げ性や穴広げ加工に効果的であることが文献2に開示されている。   On the other hand, a metal structure control method for a steel sheet that improves local ductility, represented by bending, hole expanding, and burring, is also disclosed. It is disclosed in Document 2 that reducing the hardness difference is effective for bendability and hole expansion.

延性と強度との両立から冷却制御により金属組織制御を行い、析出物の制御および変態組織を制御することでフェライトとベイナイトの適切な分率を得る技術も文献3に開示されている。しかし、いずれも組織制御に頼った局部変形能の改善方法で、べースの組織形成に大きく影響されてしまう。   A technique for obtaining an appropriate fraction of ferrite and bainite by controlling the metal structure by cooling control to achieve both ductility and strength, and controlling the precipitates and the transformation structure is also disclosed in Document 3. However, both are methods for improving local deformability that depend on tissue control, and are greatly influenced by the formation of the base structure.

一方、熱延鋼板の材質改善手法として、連続熱間圧延工程に於ける圧下量増加による材質改善についても開示技術がある。いわゆる、結晶粒微細化の技術であり、オーステナイト域の極力低温で大圧下を行い、未再結晶オーステナイトからフェライト変態させることで製品の主相であるフェライトの結晶粒微細化を図るもので、文献4のように細粒化により、高強度化や強靭化を狙った技術である。しかし、文献4に記載の製法では、本願発明が解決しようとする局部変形能と延性の改善については一切配慮されていない。   On the other hand, as a material improvement technique for hot-rolled steel sheets, there is a disclosed technique for improving a material by increasing the amount of reduction in a continuous hot rolling process. This is a so-called crystal grain refinement technology, which reduces the grain size of ferrite, which is the main phase of the product, by transforming ferrite from unrecrystallized austenite by performing large pressure at the lowest possible temperature in the austenite region. This is a technique aimed at increasing the strength and toughness by making the particles finer as in FIG. However, in the manufacturing method described in Document 4, no consideration is given to the improvement of local deformability and ductility that the present invention intends to solve.

:高橋、新日鉄技報(2003)No.378,p.7: Takahashi, Nippon Steel Technical Report (2003) No.378, p.7 :加藤ら、製鉄研究(1984)vol.312,p.41: Kato et al., Steel Research (1984) vol.312, p.41 :K.Sugimoto et al、(2000)Vol.40,p.920: K. Sugimoto et al, (2000) Vol.40, p.920 :中山製鋼所 NFG製品紹介: Nakayama Steel Works NFG Product Introduction

上述のように、高強度鋼板の局部延性能改善のためには主に介在物を含む組織制御を行うことが主であった。本願発明では、熱延工程の圧延工程によって結晶粒のサイズ、集合組織を制御することで、熱延鋼板の異方性についても改善できるような局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板とその製造方法を提供するものである。   As described above, in order to improve the local elongation performance of the high-strength steel sheet, it was mainly to control the structure including inclusions. In the present invention, by controlling the grain size and texture by the rolling process of the hot rolling process, it is excellent in local deformability that can improve the anisotropy of the hot rolled steel sheet, and the orientation dependency of formability. The present invention provides a hot-rolled steel sheet having excellent ductility and a method for producing the same.

従来の知見によれば、前述のように穴拡げ性や曲げ性などの改善は、介在物制御、析出物微細化、組織均質・単相化および組織間の硬度差の低減などによって行われていた。しかし、これだけでは、主な組織構成を限定せざるを得ないうえ、NbやTiなどが添加されている高強度鋼板では異方性が極めて大きい。これは、他の成形性因子を犠牲にしてしまったり、成形前のブランクの取る方向を限定してしまうなどの問題が生じてしまうこととなり、用途も限定的になってしまう。   According to the conventional knowledge, as described above, improvement of hole expansibility and bendability has been achieved by inclusion control, refinement of precipitates, homogenization / single phase structure of the structure, and reduction of hardness difference between structures. It was. However, this alone has to limit the main structural configuration, and the anisotropy is extremely large in a high-strength steel sheet to which Nb, Ti, or the like is added. This results in problems such as sacrificing other formability factors and limiting the direction in which the blank before molding is taken, and the application is also limited.

そこで本発明者らは、穴拡げ性や曲げ加工性を向上させるために、熱延の圧延工程における組織の細粒化と集合組織制御について調査・検討を行った。その結果、特定の結晶方位群の各方位の強度を制御することで、圧延方向と直角方向のrC値、圧延方向と30°のr30値がバランスしているときに局部変形能が飛躍的に向上することを明らかにしたものである。   Therefore, the present inventors have investigated and examined the refinement of the structure and the texture control in the hot rolling process in order to improve the hole expandability and the bending workability. As a result, by controlling the strength of each orientation of a specific crystal orientation group, the local deformability is dramatically improved when the rC value in the direction perpendicular to the rolling direction and the r30 value at 30 ° are balanced with the rolling direction. It is clarified that it improves.

本発明は前述の知見に基づいて構成されており、その主旨とするところは以下の通りである。
(1)質量%で、
C:0.02%以上、0.5%以下、
Si:0.001%以上、4.0%以下、
Mn:0.001%以上、4.0%以下、
P:0.001%以上、0.15%以下、
S:0.0005%以上、0.03%以下、
N:0.0005%以上、0.01%以下、
O:0.0005%以上、0.01%以下、
Al+Si≦4.0%以下、
更に、
Ti:0.001%以上、0.2%以下、
Nb:0.001%以上、0.2%以下、
V:0.001%以上、1.0%以下、
W:0.001%以上、1.0%以下、
Cu:0.001%以上、2.0%以下、
B :0.0001%以上、0.005%以下、
Mo:0.001%以上、1.0%以下、
Cr:0.001%以上、2.0%以下、
As:0.0001%以上、0.50%以下、
Ni:0.001%以上、2.0%以下、
Co:0.0001%以上、1.0%以下、
Sn:0.0001%以上、0.2%以下、
Zr:0.0001%以上、0.2%以下、
の1種又は2種以上を含有し、残部鉄および不可避的不純物からなり、集合組織が、少なくとも鋼板の表面から5/8〜3/8の板厚における板面の{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値が4.0未満で、かつ{332}<113>の結晶方位のX線ランダム強度比が5.0以下で、さらに圧延方向と直角方向のr(rC)値が0.70以上、かつ圧延方向と30°(r30)のr値が1.10以下であり、鋼板組織として、面積率で残留オーステナイトを5%以上、30%未満、フェライトを20%以上、50%未満、ベイナイトを 10%以上、60%未満含有し、さらに、パーライト、マルテンサイトがそれぞれ20%以下である特徴とする局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板。
(2)更に、全面積のうち、20μmを超える粒の占める面積割合が10%以下であることを特徴とする上記(1)に記載の局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板。
(3)更に、残留オーステナイトとマルテンサイトの粒で、もっとも近い残留オーステナイトもしくはマルテンサイトまでの距離をLMA[μm]とし、100個以上測定した時のLMAの標準偏差が5以下である上記(1)または(2)に記載の局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板。
(4)更に、
Mg:0.0001%以上、0.010%以下、
REM:0.0001%以上、0.1%以下、
Ca:0.0001%以上、0.010%以下、
の1種又は2種以上を含有する上記(1)から()の何れかに記載の局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板。
(5)上記(1)〜()の何れかに記載の高強度鋼板を製造するに当たり、所定の鋼板成分に溶製したのち、鋼塊またはスラブに鋳造して,粗圧延を1000℃以上、1200℃以下の温度域で20%以上の圧下を少なくとも1回以上行い、オーステナイト粒径を200μm以下とし、その後、仕上圧延において式(1)にある鋼板成分により決定される温度をT1とすると、T1+30℃以上、T1+200℃以下の温度範囲において、1回は1パス30%以上の圧延を行い、かつ、該温度範囲での圧下率の合計を50%以上とし、その後のT1以上、T1+30℃未満の温度範囲における圧下率の合計を30%以下とし、更に30%以上の最終圧延終了後のT1以上、T1+200℃以下の温度域から冷却開始までの停留時間tが式(2)を満たすようにして、Ar3変態温度以上で熱間圧延を終了し、630℃以上、800℃以下の温度域に10〜100℃/secで冷却し、次いで、当該温度域において1秒以上20秒以下保持、又は、当該温度域に冷却後に20℃/sec以下の冷却速度で550℃以上の範囲内の温度まで冷却し、350〜500℃で巻き取りを行い、温度変化速度が−40℃/h以上、40℃/h以下の範囲として30〜300分保持した後、空冷させた局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板の製造方法
T1(℃)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V (1)
t≦t1×2.5 (2)
ここで、t1は式(3)で表される。
t1=0.001((Tf−T1)×P1)−0.109((Tf−T1)×P1)+3.1 (3)
ここで、Tfは30%以上の最終圧下後の温度、P1は30%以上の最終圧下の圧下率である。
(6)上記()に記載の製造方法で、仕上げ圧延終了から1秒以内に50℃/sec以上の冷却速度で冷却温度変化が40℃以上、150℃以下となる冷却を行うことを特徴とする局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板の製造方法
(7)上記()または()に記載の製造方法において、T1+30℃以上、T1+200℃以下の温度範囲における圧延の最終パスの圧延率は25%以上であることを特徴とする局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板の製造方法
The present invention is configured based on the above-mentioned knowledge, and the main points thereof are as follows.
(1) In mass%,
C: 0.02% or more, 0.5% or less,
Si: 0.001% to 4.0%,
Mn: 0.001% or more and 4.0% or less,
P: 0.001% or more, 0.15% or less,
S: 0.0005% or more, 0.03% or less,
N: 0.0005% or more, 0.01% or less,
O: 0.0005% or more, 0.01% or less,
Al + Si ≦ 4.0% or less,
Furthermore,
Ti: 0.001% or more, 0.2% or less,
Nb: 0.001% or more, 0.2% or less,
V: 0.001% or more, 1.0% or less,
W: 0.001% or more, 1.0% or less,
Cu: 0.001% or more, 2.0% or less,
B: 0.0001% or more, 0.005% or less,
Mo: 0.001% or more, 1.0% or less,
Cr: 0.001% or more, 2.0% or less,
As: 0.0001% or more, 0.50% or less,
Ni: 0.001% or more, 2.0% or less,
Co: 0.0001% or more and 1.0% or less,
Sn: 0.0001% or more, 0.2% or less,
Zr: 0.0001% or more, 0.2% or less,
{100} <011> to the surface of the plate at a thickness of 5/8 to 3/8 at least from the surface of the steel plate. The average value of the X-ray random intensity ratio of the {223} <110> orientation group is less than 4.0, the X-ray random intensity ratio of the crystal orientation of {332} <113> is 5.0 or less, and the rolling direction The r (rC) value in the direction perpendicular to the rolling direction is 0.70 or more, and the r value at 30 ° (r30) in the rolling direction is 1.10 or less. As the steel sheet structure, the retained austenite is 5% or more by area ratio, 30 %, Ferrite is 20% or more, less than 50%, bainite is contained in an amount of 10% or more and less than 60%, and pearlite and martensite are each 20% or less. Less orientation dependency Excellent hot-rolled steel sheet to sex.
(2) Furthermore, the area ratio of grains exceeding 20 μm in the total area is 10% or less, which is excellent in local deformability as described in the above (1), and has little orientation dependency of formability Hot rolled steel sheet with excellent ductility.
(3) Further, in the grains of retained austenite and martensite, the distance to the nearest retained austenite or martensite is L MA [μm], and the standard deviation of LMA when measuring 100 or more is 5 or less A hot-rolled steel sheet excellent in local deformability described in (1) or (2) and excellent in ductility with little orientation dependency of formability.
(4) Furthermore,
Mg: 0.0001% or more, 0.010% or less,
REM: 0.0001% or more, 0.1% or less,
Ca: 0.0001% or more, 0.010% or less,
A hot-rolled steel sheet excellent in local deformability according to any one of the above (1) to ( 3 ) and having excellent ductility with little orientation dependency of formability.
(5) In manufacturing the high-strength steel plate according to any one of (1) to ( 4 ) above, after melting into a predetermined steel plate component, casting into a steel ingot or slab, and rough rolling at 1000 ° C. or higher 20% or more reduction in a temperature range of 1200 ° C. or less is performed at least once, the austenite grain size is 200 μm or less, and then the temperature determined by the steel plate component in formula (1) in finish rolling is T1. In the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less, rolling is performed at 30% or more in one pass, and the total reduction ratio in the temperature range is set to 50% or more. Thereafter, T1 or more, T1 + 30 ° C. the total reduction ratio of 30% or less in the temperature range below, further more than 30% of the final rolling after the end of T1 or higher, retention time t from the temperature range of T1 + 200 ° C. or less until the start cooling formula ( In the Suyo meet 2), and terminates the hot rolled at Ar3 transformation temperature or higher, 630 ° C. or higher, cooling at 10 to 100 ° C. / sec to a temperature range of 800 ° C. or less, then 1 second in the temperature range Hold for 20 seconds or less, or cool to a temperature in the range of 550 ° C. or more at a cooling rate of 20 ° C./sec or less after cooling to the temperature range , wind up at 350 to 500 ° C., and the temperature change rate is − A method for producing a hot-rolled steel sheet having excellent ductility, which is excellent in local deformability after being held in a range of 40 ° C./h or more and 40 ° C./h or less for 30 to 300 minutes and then air-cooled, and having less orientation dependency of formability.
T1 (° C.) = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V (1)
t ≦ t1 × 2.5 (2)
Here, t1 is represented by Formula (3).
t1 = 0.001 ((Tf−T1) × P1) 2 −0.109 ((Tf−T1) × P1) +3.1 (3)
Here, Tf is the temperature after the final reduction of 30% or more, and P1 is the reduction ratio of the final reduction of 30% or more.
(6) The manufacturing method according to ( 5 ) above, wherein cooling is performed such that the cooling temperature change is 40 ° C. or more and 150 ° C. or less at a cooling rate of 50 ° C./sec or more within 1 second from the end of finish rolling. A method for producing a hot-rolled steel sheet having excellent local deformability and excellent ductility with less orientation dependency of formability.
(7) In the manufacturing method according to the above ( 5 ) or ( 6 ), the rolling ratio of the final pass of rolling in a temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. or lower is 25% or more. A method for producing a hot-rolled steel sheet that is excellent in ductility and has excellent ductility with less orientation dependency of formability.

本発明によれば、曲げ、伸びフランジ、バーリング加工などの局部変形能に優れ、かつその成形性の方位依存性の少なく、かつ延性に優れた熱延鋼板を得るものである。   According to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent local deformability such as bending, stretch flange, burring, etc., less orientation dependency of its formability, and excellent ductility.

{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値と板厚/最小曲げ半径の関係を示す。The relationship between the average value of the X-ray random intensity ratio of {100} <011> to {223} <110> orientation group and the thickness / minimum bending radius is shown. {332}<113>方位群のX線ランダム強度比と板厚/最小曲げ半径の関係を示す。The relationship between the X-ray random intensity ratio of the {332} <113> orientation group and the thickness / minimum bending radius is shown. 圧延方向と直角方向のr値(rC)と板厚/最小曲げ半径の関係を示す。The relationship between the r value (rC) in the direction perpendicular to the rolling direction and the sheet thickness / minimum bending radius is shown. 圧延方向の30°のr値(r30)と板厚/最小曲げ半径の関係を示す。The relationship between the r value (r30) of 30 ° in the rolling direction and the sheet thickness / minimum bending radius is shown. 粒径が20μm以上の粒の割合と板厚/最小曲げ半径の関係を示す。The relationship between the ratio of grains having a grain size of 20 μm or more and the thickness / minimum bending radius is shown. 粗圧延における40%以上の圧延回数と粗圧延のオーステナイト粒径の関係を示す。The relationship between the rolling frequency | count of 40% or more in rough rolling and the austenite grain size of rough rolling is shown. 熱延終了後の630℃以上、800℃以下での一時冷却保持時間とフェライト分率の関係を示す。The relationship between the temporary cooling holding time and ferrite fraction in 630 degreeC or more and 800 degrees C or less after completion | finish of hot rolling is shown. 巻き取り温度と残留オーステナイト分率の関係を示す。The relationship between a coiling temperature and a retained austenite fraction is shown. 巻き取り温度とベイナイト分率の関係を示す。The relationship between a coiling temperature and a bainite fraction is shown.

以下に本発明の内容を詳細に説明する。
表面から5/8〜3/8の板厚における板面の{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値、{332}<113>の結晶方位のX線ランダム強度比:
The contents of the present invention will be described in detail below.
Average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group on the plate surface at a thickness of 5/8 to 3/8 from the surface, crystal orientation of {332} <113> X-ray random intensity ratio:

この平均値は本発明で、特に重要な特性値である。表面から5/8〜3/8板厚における板面のX線回折を行い、ランダム試料に対する各方位の強度比を求めたときの、{100}<011>〜{223}<110>方位群の平均値が4.0未満であれば、直近要求される骨格部品の加工に必要な板厚/曲げ半径≧1.5を満たす。さらに穴拡げ性や小さな限界曲げ特性を必要とする場合には3.0未満が望ましい。7.0以上では鋼板の機械的特性の異方性が極めて強くなり、ひいてはある方向のみの局部変形能を改善するもののそれとは異なる方向での材質が著しく劣化し板厚/曲げ半径≧1.5を満足できなくなる。   This average value is a particularly important characteristic value in the present invention. {100} <011> to {223} <110> orientation group when X-ray diffraction of a plate surface at a thickness of 5/8 to 3/8 from the surface is performed to determine the intensity ratio of each orientation to a random sample If the average value of is less than 4.0, the thickness / bending radius ≧ 1.5 required for the machining of the most recently required skeletal part is satisfied. Furthermore, when a hole expansibility and a small limit bending characteristic are required, less than 3.0 is desirable. Above 7.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong, which improves the local deformability only in one direction, but the material in a direction different from that significantly deteriorates, and the thickness / bending radius ≧ 1. Cannot satisfy 5

この方位群に含まれる主な方位は、{100}<011>、{116}<110>、{114}<110>、{113}<110>、{112}<110>、{335}<110>および{223}<110>である。   The main orientations included in this orientation group are {100} <011>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {335} < 110> and {223} <110>.

これら各方位のX線ランダム強度比はX線回折やEBSD(Electron Back Scattering Diffraction)などの装置を用いて測定する。{110}極点図に基づきベクトル法により計算した3次元集合組織や{110}、{100}、{211}、{310}極点図のうち複数の極点図(好ましくは3つ以上)を用いて級数展開法で計算した3次元集合組織から求めればよい。   The X-ray random intensity ratio in each direction is measured using a device such as X-ray diffraction or EBSD (Electron Back Scattering Diffraction). {110} Using a three-dimensional texture calculated by the vector method based on a pole figure, or a plurality of pole figures (preferably three or more) among {110}, {100}, {211}, {310} pole figures What is necessary is just to obtain | require from the three-dimensional texture calculated by the series expansion method.

たとえば、後者の方法における上記各結晶方位のX線ランダム強度比には、3次元集合組織のφ2=45゜断面における(001)[1−10]、(116)[1−10]、(114)[1−10]、(113)[1−10]、(112)[1−10],(335)[1−10],(223)[1−10]の強度をそのまま用いればよい。   For example, the X-ray random intensity ratio of each crystal orientation in the latter method is (001) [1-10], (116) [1-10], (114) in the φ2 = 45 ° cross section of the three-dimensional texture. ) [1-10], (113) [1-10], (112) [1-10], (335) [1-10], (223) [1-10] may be used as they are.

{100}<011>〜{223}<110>方位群の平均値とは、上記の各方位の相加平均である。上記の全ての方位の強度を得ることができない場合には、{100}<011>、{116}<110>、{114}<110>、{112}<110>、{223}<110>の各方位の相加平均で代替しても良い。 The average value of {100} <011> to {223} <110> orientation group is an arithmetic average of each of the above-mentioned orientations. When the strengths of all the above directions cannot be obtained, {100} <011>, {116} <110>, {114} <110>, {112} <110>, {223} <110> Alternatively, an arithmetic average of each direction may be substituted.

さらに同様な理由から、表面から5/8〜3/8板厚における板面の{332}<113>の結晶方位のX線ランダム強度比は5.0以下でなくてはならない。望ましくは3.0以下であれば、直近要求される骨格部品の加工に必要な板厚/曲げ半径≧1.5を満たす。これが5.0超であると、鋼板の機械的特性の異方性が極めて強くなり、ひいてはある方向のみの局部変形能を改善するもののそれとは異なる方向での材質が著しく劣化し板厚/曲げ半径≧1.5を確実に満足できなくなる。一方、現行の一般的な連続熱延工程では実現が難しいが、0.5未満になると局部変形能の劣化が懸念される。   For the same reason, the X-ray random intensity ratio of the {332} <113> crystal orientation of the plate surface at a thickness of 5/8 to 3/8 from the surface must be 5.0 or less. Desirably, if it is 3.0 or less, the plate thickness / bending radius ≧ 1.5 required for the processing of the most recently required skeleton parts is satisfied. If this exceeds 5.0, the anisotropy of the mechanical properties of the steel sheet becomes extremely strong, which in turn improves the local deformability only in one direction, but the material in a direction different from that significantly deteriorates, resulting in the thickness / bending. A radius of ≧ 1.5 cannot be satisfied with certainty. On the other hand, it is difficult to realize by the current general continuous hot rolling process, but when it is less than 0.5, there is a concern about deterioration of local deformability.

X線回折に供する試料は、機械研磨などによって鋼板を所定の板厚まで表面より減厚し、次いで、化学研磨や電解研磨などによって歪みを除去すると同時に板厚の5/8〜3/8の範囲で適当な面が測定面となるように上述の方法に従って試料を調整して測定すればよい。   A sample subjected to X-ray diffraction is obtained by reducing the thickness of a steel plate from a surface to a predetermined thickness by mechanical polishing or the like, and then removing distortion by chemical polishing or electrolytic polishing, and at the same time, having a thickness of 5/8 to 3/8. What is necessary is just to adjust and measure a sample according to the above-mentioned method so that a suitable surface may become a measurement surface in the range.

当然のことであるが、上述のX線強度の限定が板厚1/2近傍だけでなく、なるべく多くの厚みについて満たされることで、より一層局延性能が良好になる。しかしながら, 鋼板の表面から3/8〜5/8の測定を行うことで概ね鋼板全体の材質特性を代表することができるためこれを規定するものとする。   As a matter of course, the above-mentioned limitation of the X-ray intensity is satisfied not only in the vicinity of the plate thickness ½ but also as much as possible, so that the spread performance is further improved. However, the measurement of 3/8 to 5/8 from the surface of the steel sheet can generally represent the material properties of the entire steel sheet, so this shall be specified.

なお、{hkl}<uvw>で表される結晶方位とは、板面の法線方向が<hkl>に平行で、圧延方向が<uvw>と平行であることを示している。   The crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw>.

圧延方向と直角方向のr値(rC):
これらの鋼板のr値は、本発明において重要である。すなわち、本発明者等が鋭意検討の結果、上述した種々の結晶方位のX線ランダム強度比だけが適正であっても、必ずしも良好な穴拡げ性や曲げ性が得られないことが判明した。また、X線ランダム強度比と同時に、rCが0.70以上であることが必須である。
R value (rC) in the direction perpendicular to the rolling direction:
The r value of these steel plates is important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that even if only the X-ray random intensity ratios of various crystal orientations described above are appropriate, good hole expandability and bendability cannot always be obtained. Moreover, it is essential that rC is 0.70 or more simultaneously with the X-ray random intensity ratio.

上述の各方向のr値の上限は特に定めないが、1.10以下であることで、よりすぐれた局部変形能を得ることができる。   The upper limit of the r value in each direction described above is not particularly defined, but when it is 1.10 or less, better local deformability can be obtained.

圧延方向の30°のr値(r30):
この鋼板のr値は、本発明において重要である。すなわち、本発明者等が鋭意検討の結果、上述した種々の結晶方位のX線強度が適正であっても、必ずしも良好な局部変形能が得られないことが判明した。図4に示すように、鋼板には上記のX線強度と同時に、r30が1.10以下であることが必須である。
R value of 30 ° in the rolling direction (r30):
The r value of this steel sheet is important in the present invention. That is, as a result of intensive studies by the present inventors, it has been found that good local deformability cannot always be obtained even when the X-ray intensities of the various crystal orientations described above are appropriate. As shown in FIG. 4, it is essential for the steel sheet that r30 is 1.10 or less simultaneously with the X-ray intensity.

上述の各r値はJIS5号引張試験片を用いた引張試験により評価する。引張歪みは通常高強度鋼板の場合5〜15%の範囲で、均一伸びの範囲で評価すればよい。   Each r value described above is evaluated by a tensile test using a JIS No. 5 tensile test piece. The tensile strain is usually in the range of 5 to 15% in the case of a high-strength steel plate, and may be evaluated in the range of uniform elongation.

なお、曲げ加工を施す方向は加工部品によって異なるので特に限定するものではなく、本願発明により、いずれの曲げ方向においても同様の特性が得られるものである。   The direction in which the bending process is performed is not particularly limited because it varies depending on the processed part, and the same characteristics can be obtained in any bending direction according to the present invention.

ところで、一般に集合組織とr値とは相関があることが知られているが、本発明においては、既述の結晶方位のX線強度比に関する限定とr値に関する限定とは互いに同義ではなく、両方の限定が同時に満たされなくては良好な局部変形能を得ることはできない。   By the way, it is generally known that there is a correlation between the texture and the r value, but in the present invention, the above-described limitation on the X-ray intensity ratio of the crystal orientation and the limitation on the r value are not synonymous with each other, Good local deformability cannot be obtained unless both limitations are met simultaneously.

本発明は高強度鋼板の全般に適用できるものであり、上記の限定が満たされれば組織の組み合わせに制限されることなく、高強度薄鋼板の曲げ加工性や穴広げ性などの局部成形能が飛躍的に向上する。   The present invention can be applied to high strength steel sheets in general, and if the above limitation is satisfied, the present invention is not limited to the combination of structures, and the local formability such as bending workability and hole expansibility of high strength thin steel sheets can be achieved. Improve dramatically.

また、ひずみの局部化を抑え、曲げ性を向上させるためには、全面積のうち、20μmを超える粒の占める面積割合が10%以下である必要がある。これより多いと曲げ性が劣化する。   Further, in order to suppress localization of strain and improve bendability, the area ratio of grains exceeding 20 μm needs to be 10% or less in the total area. If it exceeds this, the bendability deteriorates.

次に成分の限定条件について述べる。
Cは高強度を確保し、かつ残留オーステナイトを確保するために必須である。十分な残留オーステナイト量を得るためには、0.02%以上のC量が必要となる。一方、Cを過剰に含有すると、溶接性を損なうため、C量の上限を0.5%以下とした。
Next, the limiting conditions for the components will be described.
C is essential for securing high strength and securing retained austenite. In order to obtain a sufficient amount of retained austenite, a C amount of 0.02% or more is required. On the other hand, when C is contained excessively, weldability is impaired, so the upper limit of the C content is set to 0.5% or less.

Siは脱酸剤であり、0.001%以上の添加が好ましい。また、焼鈍時にフェライトを安定化する元素であり、かつ、巻き取り後のセメンタイト析出をおさえるためオーステナイトのC濃度を高め、残留オーステナイトの確保に寄与する。Siが高いほどその効果は大きくなるが、過剰に添加すると、表面性状、塗装性、溶接性などの劣化を招くので、上限を4.0%以下とする。   Si is a deoxidizer and is preferably added in an amount of 0.001% or more. Further, it is an element that stabilizes ferrite during annealing, and suppresses precipitation of cementite after winding, thereby increasing the C concentration of austenite and contributing to securing retained austenite. The higher the Si, the greater the effect. However, if added excessively, the surface properties, paintability, weldability and the like are deteriorated, so the upper limit is made 4.0% or less.

Mnはオーステナイトを安定化させ、焼入れ性を高める元素である。十分な焼入れ性を確保するためには、0.001%以上のMnの添加が必要である。一方、Mnを過剰に添加すると延性を損なうため、Mn量の上限を4.0%とする。   Mn is an element that stabilizes austenite and improves hardenability. In order to ensure sufficient hardenability, it is necessary to add 0.001% or more of Mn. On the other hand, if Mn is added excessively, ductility is impaired, so the upper limit of the amount of Mn is made 4.0%.

Pは不純物であり、過剰に含有すると延性や溶接性を損なう。したがって、P量の上限を0.15%以下とする。一方、Pを0.001%以下とするのは困難であるので、これを下限とする。   P is an impurity, and if contained excessively, ductility and weldability are impaired. Therefore, the upper limit of the P content is 0.15% or less. On the other hand, since it is difficult to make P 0.001% or less, this is the lower limit.

Sは不純物であり、過剰に含有すると、熱間圧延によって伸張したMnSが生成し、延性及び穴広げ性などの成形性の劣化を招く。したがって、S量の上限を0.03%以下とする。 一方、Sを0.0005%以下とするのは困難であるので、これを下限とする。   S is an impurity, and if it is contained excessively, MnS stretched by hot rolling is generated, which causes deterioration of moldability such as ductility and hole expansibility. Therefore, the upper limit of the S amount is 0.03% or less. On the other hand, since it is difficult to make S 0.0005% or less, this is the lower limit.

Oは不純物であり、過剰に含有すると、加工性を損なう。したがって、O量の上限を0.01%以下とする。一方、Oを0.0005%以下とするのは困難であるので、これを下限とする。   O is an impurity, and if it is excessively contained, workability is impaired. Therefore, the upper limit of the O amount is 0.01% or less. On the other hand, since it is difficult to make O 0.0005% or less, this is the lower limit.

Alは、Siと同様に、脱酸剤であり、焼鈍時にフェライトを安定化する元素である。しかし、過剰に含有すると溶接性が劣悪となるため、Siと合わせて4.0%以下とする。   Al, like Si, is a deoxidizer and an element that stabilizes ferrite during annealing. However, if the content is excessive, the weldability becomes poor. Therefore, the total content is 4.0% or less in combination with Si.

Nは、不純物であり、0.01%を超えると延性の劣化を招く。したがって、N量の上限を0.01%以下とする。一方、Nを0.0005%以下とするのは困難であるので、これを下限とする。   N is an impurity. When it exceeds 0.01%, ductility is deteriorated. Therefore, the upper limit of the N amount is 0.01% or less. On the other hand, since it is difficult to make N 0.0005% or less, this is the lower limit.

更に、析出強化によって強度を得る場合、微細な炭窒化物を生成させることがよい。析出強化を得るためには、Ti、Nb、V、Wの添加が有効であり、これらの1種または2種以上を含有しても構わない。   Furthermore, when obtaining strength by precipitation strengthening, fine carbonitrides are preferably generated. In order to obtain precipitation strengthening, the addition of Ti, Nb, V, and W is effective, and one or more of these may be contained.

Ti、Nb、V、W、Cuの添加でこの効果を得るためには、Tiは0.001%以上、Nbは0.001%以上、Vは0.001%以上、Wは0.001%以上、Cuは0.001%以上の添加が必要である。ただし過度な添加でも強度上昇は飽和してまうこと、加えて、熱延後の再結晶を抑制することで、結晶方位制御を困難にすることから、Tiで0.2%以下、Nbで0.2%以下、Vで1.0%以下、Wで1.0%以下、Cuで2.0%以下とする必要がある。 In order to obtain this effect by adding Ti, Nb, V, W and Cu, Ti is 0.001% or more, Nb is 0.001% or more, V is 0.001% or more, and W is 0.001%. As mentioned above, Cu needs to add 0.001% or more. However, even if it is excessively added, the strength increase will be saturated, and in addition, recrystallization after hot rolling makes it difficult to control the crystal orientation. Therefore, Ti is 0.2% or less, and Nb is 0%. 0.2% or less, V 1.0% or less, W 1.0% or less, and Cu 2.0% or less.

組織の焼き入れ性を上昇させ第二相制御を行うことで強度を確保する場合、B、Mo、Cr、Asの1種または2種以上の添加が有効である。この効果を得るためには、Bは0.0001%以上、Mo、Crは0.001%以上、Asは0.0001%以上を添加する必要がある。しかし、過度の添加は逆に加工性を劣化させるので、Bの上限を0.005%、Moの上限を1.0%、Crの上限を2.0%、Asを0.50%とする。   When ensuring the strength by increasing the hardenability of the structure and performing the second phase control, it is effective to add one or more of B, Mo, Cr and As. In order to obtain this effect, it is necessary to add 0.0001% or more of B, 0.001% or more of Mo and Cr, and 0.0001% or more of As. However, excessive addition adversely deteriorates workability, so the upper limit of B is 0.005%, the upper limit of Mo is 1.0%, the upper limit of Cr is 2.0%, and As is 0.50%. .

局部成形能を向上のため、Mg、REM、Caは介在物を無害化するため重要な添加元素である。この効果を得るためのそれぞれの下限を0.0001%とした。一方、過剰添加は清浄度の悪化につながるためMgで0.010%、REMで0.1%、Caで0.010%を上限とした。   In order to improve local forming ability, Mg, REM, and Ca are important additive elements for detoxifying inclusions. Each lower limit for obtaining this effect was made 0.0001%. On the other hand, excessive addition leads to deterioration of cleanliness, so 0.010% for Mg, 0.1% for REM, and 0.010% for Ca were made the upper limit.

Ni、Co、Sn及びZrは強度を上げる元素であり、Niで0.001%以上、Coで0.0001%以上、Snで0.0001%以上、Zrで0.0001%以上の添加が有効である。しかし過剰に入れすぎると、その成形性を失ってしまうので、Niで2.0
%、Coで1.0%、Snで0.2%、Zrで0.2%を上限とした。
Ni, Co, Sn, and Zr are elements that increase the strength. Addition of 0.001% or more for Ni, 0.0001% or more for Co, 0.0001% or more for Sn, and 0.0001% or more for Zr is effective. It is. However, if too much is added, the moldability is lost, so Ni is 2.0.
%, Co 1.0%, Sn 0.2% and Zr 0.2%.

なお、本願発明の熱延鋼板に表面処理してもその局部変形能改善効果を失うものでなく、電気めっき、溶融めっき、蒸着めっき、有機皮膜形成、フィルムラミネート、有機塩類/無機塩類処理、ノンクロ処理等の何れでも本発明の効果が得られる。   Even if the hot-rolled steel sheet of the present invention is surface-treated, the effect of improving the local deformability is not lost. Electroplating, hot dipping, vapor deposition plating, organic film formation, film laminating, organic salt / inorganic salt treatment, non-chromic treatment The effect of the present invention can be obtained by any treatment.

次に本発明薄鋼板の製造方法について述べる。優れた局部変形能を実現するためには、X線ランダム強度比をもつ集合組織を形成させるための製造条件の詳細を以下に記す。   Next, a method for producing the thin steel sheet of the present invention will be described. In order to achieve excellent local deformability, details of manufacturing conditions for forming a texture having an X-ray random intensity ratio will be described below.

熱間圧延に先行する製造方法は特に限定するものではない。すなわち、高炉や電炉等による溶製に引き続き各種の2次製錬を行い、次いで、通常の連続鋳造、インゴット法による鋳造の他、薄スラブ鋳造などの方法で鋳造すればよい。連続鋳造の場合には一度低温まで冷却したのち、再度加熱してから熱間圧延しても良いし、鋳造スラブを連続的に熱延しても良い。原料にはスクラップを使用しても構わない。   The production method preceding hot rolling is not particularly limited. That is, various secondary smelting may be performed following the smelting by a blast furnace or an electric furnace, and then the casting may be performed by a method such as a thin slab casting in addition to a normal continuous casting and an ingot method. In the case of continuous casting, after cooling to low temperature once, it may be heated again and then hot rolled, or the cast slab may be continuously hot rolled. Scrap may be used as a raw material.

本発明の局部変形能に優れた高強度熱延鋼板は、以下の要件を満たす場合に得られる。
局部変形能に優れた熱延鋼板を製造するためには、仕上げ圧延前のオーステナイト粒径が小さいことが望ましく、200μm以下であれば前述の値を満足することが判明した。この200μm以下の仕上げ圧延前のオーステナイト粒径を得るためには,1000℃以上1200℃以下(好ましくは1150℃)の温度域での粗圧延で少なくとも20%以上(好ましくは40%以上)の圧下率で1回以上圧延すれば所定のオーステナイト粒径が得られることも判明した。
The high-strength hot-rolled steel sheet having excellent local deformability according to the present invention is obtained when the following requirements are satisfied.
In order to produce a hot-rolled steel sheet having excellent local deformability, it is desirable that the austenite grain size before finish rolling is small, and that the above value is satisfied if it is 200 μm or less. In order to obtain the austenite grain size before finish rolling of 200 μm or less, the rolling is reduced by at least 20% (preferably 40% or more) by rough rolling in the temperature range of 1000 ° C. to 1200 ° C. (preferably 1150 ° C.). It has also been found that a predetermined austenite grain size can be obtained by rolling at a rate one or more times.

圧下率およびその圧下の回数は大きいほど、細粒を得ることができ、この効果をより効率的に得るためには、100μm以下のオーステナイト粒径にすることが望ましく、このためには、40%以上の圧延は2回以上行うことが望ましい。ただし、70%を超える圧下や10回を超える粗圧延は温度の低下やスケールの過剰生成の懸念がある。このように、仕上げ圧延前のオーステナイト粒径を小さくすることが、後々の仕上げ圧延でのオーステナイトの再結晶促進を通じて局部変形能の改善に有効である。   Finer grains can be obtained as the rolling reduction ratio and the number of rolling reductions are increased, and in order to obtain this effect more efficiently, it is desirable to obtain an austenite grain size of 100 μm or less, and for this purpose, 40% It is desirable to perform the above rolling twice or more. However, the reduction exceeding 70% or the rough rolling exceeding 10 times may cause a decrease in temperature or excessive generation of scale. Thus, reducing the austenite grain size before finish rolling is effective in improving local deformability by promoting recrystallization of austenite in subsequent finish rolling.

これは、仕上げ圧延中の再結晶核の1つとして粗圧延後の(すなわち仕上げ圧延前の)オーステナイト粒界が機能することによると推測される。粗圧延後のオーステナイト粒径を確認するためには、仕上げ圧延に入る前の板片を可能な限り急冷することが望ましく、10℃/s以上の冷却速度で板片を冷却して、板片断面の組織をエッチングしてオーステナイト粒界を浮き立たせて光学顕微鏡にて測定する。この際、50倍以上の倍率にて20視野以上を、画像解析やポイントカウント法にて測定する。   This is presumed to be due to the function of the austenite grain boundary after rough rolling (that is, before finish rolling) as one of the recrystallization nuclei during finish rolling. In order to confirm the austenite grain size after rough rolling, it is desirable to cool the plate piece before finishing rolling as much as possible, and the plate piece is cooled at a cooling rate of 10 ° C./s or more. The structure of the cross section is etched to raise the austenite grain boundary and measured with an optical microscope. At this time, 20 fields of view or more are measured by image analysis or a point count method at a magnification of 50 times or more.

また鋼板の表面から5/8〜3/8の板厚における板面の{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値、{332}<113>の結晶方位のX線ランダム強度比を前述の値の範囲とするには、粗圧延後の仕上げ圧延で鋼板成分によって決められるT1温度
T1(℃)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V (式1)
を基準に、T1+30℃以上、T1+200℃以下の温度域で大きな圧下率による加工を行い、T1以上、T1+30℃未満で小さな圧下率による加工を行うことにより、最終製品の局部変形能を確保できる。T1+30℃以上、T1+200℃以下の温度域における大圧下と、その後のT1以上、T1+30℃未満での軽圧下は、後掲の表2、表3に見られるように、鋼板の表面から5/8〜3/8の板厚における板面の{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値、{332}<113>の結晶方位のX線ランダム強度比を制御して最終製品の局部変形能を飛躍的に改善する。T1温度自体は経験的に求めたものである。T1温度を基準として、各鋼のオーステナイト域での再結晶が促進されることを発明者らは実験により経験的に知見した。さらに良好な局部変形能を得るためには、大圧下による歪を蓄積することが重要で50%以上は必須である。さらには、70%以上の圧下を取ることが望ましく、一方で90%を超える圧下率をとることは温度確保や過大な圧延負荷を加えることとなる。
Moreover, the average value of the X-ray random intensity ratios of {100} <011> to {223} <110> orientation groups on the plate surface at a thickness of 5/8 to 3/8 from the surface of the steel plate, {332} <113> In order to make the X-ray random intensity ratio of the crystal orientation in the above-mentioned range, the T1 temperature determined by the steel plate component in the finish rolling after the rough rolling
T1 (℃) = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V (Formula 1)
Based on the above, processing with a large reduction ratio in the temperature range of T1 + 30 ° C or more and T1 + 200 ° C or less, and processing with a small reduction ratio at T1 or more and less than T1 + 30 ° C, the local deformability of the final product Can be secured. As shown in Tables 2 and 3 below, the large pressure in the temperature range of T1 + 30 ° C. or more and T1 + 200 ° C. or less and the subsequent light pressure at T1 or more and less than T1 + 30 ° C. are 5/8 from the surface of the steel plate. Average value of X-ray random intensity ratio of {100} <011> to {223} <110> orientation group of plate surface at plate thickness of ˜3 / 8, X-ray random intensity of crystal orientation of {332} <113> Control the ratio and dramatically improve the local deformability of the final product. The T1 temperature itself is determined empirically. Based on the T1 temperature, the inventors have empirically found that recrystallization in the austenitic region of each steel is promoted. In order to obtain better local deformability, it is important to accumulate distortion due to large pressure, and 50% or more is essential. Furthermore, it is desirable to take a reduction of 70% or more. On the other hand, taking a reduction ratio exceeding 90% results in securing a temperature and adding an excessive rolling load.

さらに、蓄積した歪の開放による均一な再結晶を促すため、T1+30℃以上、T1+200℃以下での大圧下の後、T1以上、T1+30℃未満の温度域での加工量をなるべく少なく抑えることが必要で、T1以上、T1+30℃未満での圧下率で30%以下とし、板形状からは10%以上の圧下率が望ましいが、より局部変形能を重視する場合には圧下率は0%が望ましい。また、T1以上、T1+30℃未満での圧下率が大きいとせっかく再結晶したオーステナイト粒が展伸してしまい、停留時間が短いと再結晶が十分に進まず局部変形能を劣化させてしまう。すなわち、本願発明の製造条件においては、仕上げ圧延においてオーステナイトを均一・微細に再結晶させることで熱延製品の集合組織を制御して穴拡げ性や曲げ性と言った局部変形能を改善する方法である。   Furthermore, in order to promote uniform recrystallization by releasing accumulated strain, it is necessary to minimize the amount of processing in the temperature range from T1 to less than T1 + 30 ° C after a large pressure at T1 + 30 ° C or more and T1 + 200 ° C or less. Thus, the rolling reduction at T1 or more and less than T1 + 30 ° C. is set to 30% or less, and the rolling reduction is preferably 10% or more from the plate shape. However, when the local deformability is more important, the rolling reduction is preferably 0%. In addition, if the rolling reduction at T1 or more and less than T1 + 30 ° C. is large, recrystallized austenite grains expand, and if the retention time is short, recrystallization does not proceed sufficiently and local deformability deteriorates. That is, in the manufacturing conditions of the present invention, a method for improving the local deformability such as hole expandability and bendability by controlling the texture of hot rolled products by recrystallizing austenite uniformly and finely in finish rolling. It is.

前述の規定した温度域よりも低温で圧延が行われ、大きな圧下率を取ってしまうと、オーステナイトの集合組織が発達し、最終的に得られる熱延鋼板の板面に(1)で述べた所定のX線強度レベルの各結晶方位が得られない。一方、前述の規定した温度域よりも高温で圧延が行われたり小さい圧下率を取ってしまったりすると、粗粒化や混粒となり、20μmを超える結晶粒の面積率が増大する。上述の規定した圧延が行われているか否は、圧延率は圧延荷重、板厚測定などから実績または計算により求めることができるし、温度についてもスタンド間温度計があれば実測可能で、またはラインスピードや圧下率などから加工発熱を考慮した計算シミュレーション、或いはその両方によって得ることができる。   When rolling is performed at a temperature lower than the temperature range defined above and a large reduction ratio is obtained, the austenite texture develops, and the plate surface of the finally obtained hot-rolled steel sheet is described in (1). Each crystal orientation at a predetermined X-ray intensity level cannot be obtained. On the other hand, when rolling is performed at a temperature higher than the above-mentioned temperature range or a small reduction ratio is taken, coarse grains and mixed grains are formed, and the area ratio of crystal grains exceeding 20 μm increases. Whether the rolling specified above is performed or not can be determined by the actual result or calculation from the rolling load, sheet thickness measurement, etc., and the temperature can be measured if there is an inter-stand thermometer, or the line It can be obtained by a calculation simulation considering processing heat generation from the speed and rolling reduction, or both.

熱間圧延をAr3以下で終了するとオーステナイトとフェライトに2相域圧延になってしまい{100}<011>〜{223}<110>方位群への集積が強くなり、結果として局部変形能が著しく劣化する。 When hot rolling is finished at Ar 3 or less, austenite and ferrite become two-phase rolling, and accumulation in {100} <011> to {223} <110> orientation groups becomes strong, and as a result, local deformability is increased. Deteriorates significantly.

また、T1+30℃以上、T1+200℃の温度範囲における圧延の最後の圧延スタンドで圧下後の冷却は、オーステナイトの粒径に大きな影響を与える。上記範囲での最終圧延後、0.05秒以上、5秒以内に冷却を行わないと、オーステナイト粒が粗大化するため、板厚/曲げ半径≧2.0を満たすことができない。更に、T1+30℃以上、T1+200℃未満の温度範囲のTL℃における圧下の最終パスから冷却開始までの時間(t)が、0.05秒以上、5秒以下であることが望ましい。0.05秒以下では熱延再結晶が十分に起こらず、異方性が増加すること、5秒以上ではオーステナイト粒が粗大化して強度と伸びが低下する。更に、冷却開始までの時間は、最終圧下の実施温度Tfと30%以上の最終圧延の圧延率P1に対して、式(2)を満たす必要がある。ここで、1とは式(3)で求めることのできる数値である。
t≦t1×2.5 式(2)
1=0.001((Tf−T1)×P1)−0.109((Tf−T1)×P1)+3.1 式(3)
これより低くなると、再結晶が十分に得られず、異方性が高くなる。
In addition, cooling after rolling down at the final rolling stand of rolling in a temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. greatly affects the austenite grain size. If the austenite grains are coarsened after the final rolling in the above range for 0.05 seconds or more and within 5 seconds, the austenite grains are coarsened, so that the thickness / bending radius ≧ 2.0 cannot be satisfied. Furthermore, it is desirable that the time (t) from the final pass under the reduction at TL ° C. in the temperature range of T1 + 30 ° C. or higher and less than T1 + 200 ° C. to be 0.05 second or more and 5 seconds or less. If it is 0.05 seconds or less, hot rolling recrystallization does not occur sufficiently and the anisotropy increases, and if it is 5 seconds or more, austenite grains become coarse and strength and elongation decrease. Furthermore, the time until the start of cooling needs to satisfy the formula (2) with respect to the final rolling reduction temperature Tf and the final rolling ratio P1 of 30% or more. Here, t 1 is a numerical value that can be obtained by Expression (3).
t ≦ t1 × 2.5 Formula (2)
t 1 = 0.001 ((Tf−T1) × P1) 2 −0.109 ((Tf−T1) × P1) +3.1 Formula (3)
If it is lower than this, sufficient recrystallization cannot be obtained and the anisotropy becomes high.

仕上げ圧延後、初析フェライト域のノーズ近傍である630℃以上、800℃以下の温度まで冷却し、1秒以上、20秒以下保持もしくは、該温度から該温度未満、550℃以上の温度まで20℃/sec以下で徐冷することにより、主相であるフェライトを容易に得ることができ、630℃以上、800℃以下まで冷却することで結晶粒を微細化することができる。等温保持処理の場合、保持時間が20秒を超えると、フェライト分率が高くなりすぎ、強度が低下してしまう。一方、保持時間が1秒未満では、フェライトの生成量が不足してしまう。また、徐冷を停止する温度が550℃を下回るとパーライト変態が生じてしまう可能性があるので550℃以上とする。   After finish rolling, it is cooled to a temperature of 630 ° C. or higher and 800 ° C. or lower, which is in the vicinity of the nose of the pro-eutectoid ferrite region, and is held for 1 second or longer and 20 seconds or shorter. By slowly cooling at ℃ / sec or less, ferrite as a main phase can be easily obtained, and crystal grains can be refined by cooling to 630 ° C. or more and 800 ° C. or less. In the case of isothermal holding treatment, if the holding time exceeds 20 seconds, the ferrite fraction becomes too high and the strength decreases. On the other hand, if the holding time is less than 1 second, the amount of ferrite produced is insufficient. Further, if the temperature at which the slow cooling is stopped is below 550 ° C., pearlite transformation may occur.

350〜500℃の範囲内の温度まで冷却して巻き取り、巻き取ったコイルの温度変化速度が −40℃/h以上、40℃/h以下の範囲として30〜300分保持させる。巻き取り温度が、500℃を超えると、ベイナイト変態が過度に進行し、350℃を下回るとベイナイト変態が過度に抑制され、C濃化による残留オーステナイトの安定化が十分でなく、空冷時にマルテンサイト変態を起こし、十分な残留オーステナイト量を得られなくなる。また、350℃〜500℃での保持時間が30分未満では、ベイナイト変態の進行が十分でなく、残留オーステナイト分率が不足してしまう。一方、300分を超えると、セメンタイトの析出・粗大化が進んでしまうため目的の残留オーステナイト分率が得られなくなってしまう。さらに、コイルの温度変化速度が、−40℃/h以上40℃/h以下の範囲外となり、急激な温度変化となると、コイル内での材質のばらつきが顕著になってしまう。   It cools and winds up to the temperature in the range of 350-500 degreeC, It is made to hold | maintain for 30 to 300 minutes as the temperature change rate of the coil wound up is -40 degreeC / h or more and 40 degrees C / h or less. If the coiling temperature exceeds 500 ° C., the bainite transformation proceeds excessively, and if it falls below 350 ° C., the bainite transformation is excessively suppressed, the stabilization of residual austenite due to C concentration is insufficient, and martensite during air cooling Transformation occurs and a sufficient amount of retained austenite cannot be obtained. Further, if the holding time at 350 ° C. to 500 ° C. is less than 30 minutes, the progress of the bainite transformation is not sufficient and the retained austenite fraction becomes insufficient. On the other hand, if it exceeds 300 minutes, precipitation and coarsening of cementite will proceed, and the desired retained austenite fraction cannot be obtained. Furthermore, when the temperature change rate of the coil is outside the range of −40 ° C./h to 40 ° C./h, and the temperature changes suddenly, the material variation in the coil becomes significant.

仕上げ圧延終了後には、1秒以内に50℃/sec以上の冷却速度で冷却温度変化が40℃以上、150℃以下となる冷却を行う必要がある。冷却までの時間が1秒より長くなると、再結晶したオーステナイト粒が高温で保持されるために粒成長し、強度・延性が低くなってしまう。また、この温度変化が40℃未満であるとやはり再結晶したオーステナイト粒が粒成長してしまう。一方、150℃超ではAr3変態点温度以下までオーバーシュートする恐れがありその場合再結晶オーステナイトからの変態であってもバリアント選択の先鋭化の結果やはり集合組織が形成され等方性が低下してしまい加工性の方位依存性が大きくなってしまう。この冷却での冷却速度が50℃/sec未満であるとやはり再結晶したオーステナイト粒が粒成長してしまう。一方、冷却速度の上限は特に定めないが板形状の観点から200℃/sec以下が妥当と思われる。   After finishing rolling, it is necessary to perform cooling at a cooling rate of 50 ° C./sec or more within 1 second so that the change in cooling temperature is 40 ° C. or more and 150 ° C. or less. If the time until cooling is longer than 1 second, the recrystallized austenite grains are held at a high temperature, so that the grains grow and the strength and ductility are lowered. If the temperature change is less than 40 ° C., recrystallized austenite grains will grow. On the other hand, if it exceeds 150 ° C, there is a risk of overshooting below the Ar3 transformation point temperature. In this case, even if the transformation is from recrystallized austenite, as a result of sharpening of the variant selection, a texture is also formed and isotropicity is lowered. As a result, the orientation dependency of workability increases. If the cooling rate in this cooling is less than 50 ° C./sec, recrystallized austenite grains will grow. On the other hand, the upper limit of the cooling rate is not particularly defined, but 200 ° C./sec or less is considered appropriate from the viewpoint of the plate shape.

さらに、更に,熱延板の均質性を高め,伸び,局部延性を極限まで高めるためには,T1+30℃以上、T1+200℃以下の温度域での圧延のうち,最終パスの圧延率は25%以上である必要がある.但し,より高い加工性が要求される場合は最終の2パスを25%以上とする必要がある.   Furthermore, in order to further improve the homogeneity of the hot-rolled sheet and increase the elongation and the local ductility, the rolling rate of the final pass is 25 in the temperature range of T1 + 30 ° C. or higher and T1 + 200 ° C. or lower. % Or more. However, if higher workability is required, the final two passes must be 25% or more.

次に、本発明の鋼板のミクロ組織について説明する。
本発明の鋼板のミクロ組織は、フェライトとベイナイト、焼戻しマルテンサイト、残留オーステナイトおよびパーライト、マルテンサイトからなる。
Next, the microstructure of the steel sheet of the present invention will be described.
The microstructure of the steel sheet of the present invention comprises ferrite and bainite, tempered martensite, retained austenite, pearlite, and martensite.

フェライトとベイナイトは残留オーステナイトにCを濃化させ、TRIP効果による延性向上に必須である。開発の狙いの強度レベルにより,フェライトとベイナイトの分率を変化させることが可能であるが、フェライト20%以上50%未満、ベイナイト10%以上60%未満とすることによって、優れた延性を得ることができる。   Ferrite and bainite concentrate C in retained austenite and are essential for improving ductility by the TRIP effect. It is possible to change the fraction of ferrite and bainite depending on the strength level targeted for development, but obtaining excellent ductility by setting ferrite to 20% or more and less than 50% or bainite to 10% or more and less than 60%. Can do.

残留オーステナイトは、変態誘起塑性によって延性、特に一様伸びを高める組織であり、面積率で、5%以上必要である。また、加工によってマルテンサイトに変態するため、強度の向上にも寄与する。残留オーステナイトの面積は高いほど好ましいが、面積率で30%超の残留オーステナイトを確保するためには、C、Si量を増加させる必要があり、溶接性や表面性状を損なう。したがって、残留オーステナイトの面積率の上限を30%以下とする。   Residual austenite is a structure that enhances ductility, particularly uniform elongation, by transformation-induced plasticity, and the area ratio is required to be 5% or more. Moreover, since it transforms into martensite by processing, it contributes to the improvement of strength. The higher the area of retained austenite, the better. However, in order to ensure retained austenite with an area ratio of more than 30%, it is necessary to increase the amount of C and Si, which impairs weldability and surface properties. Therefore, the upper limit of the area ratio of retained austenite is set to 30% or less.

また、パーライトと、マルテンサイトはそれぞれ20%含んでもよいものとする。更に粒径は、全組織で、20μmを超える粒の占める面積率が10%以下とする。粒径の大きな粒が増えると、引張強度が小さくなり、局部変形能も低下する。したがって、なるべく細粒にすることが好ましい。   Further, each of pearlite and martensite may be contained by 20%. Further, the particle size is such that the area ratio occupied by grains exceeding 20 μm is 10% or less in the entire structure. As the number of large grains increases, the tensile strength decreases and the local deformability also decreases. Therefore, it is preferable to make it as fine as possible.

また、曲げ、伸びフランジ、バーリング加工などの局部変形能を向上させるために、残留オーステナイトやマルテンサイト等硬質組織は分散していたほうが好ましい。そのため残留オーステナイトとマルテンサイトの粒で、もっとも近い残留オーステナイトもしくはマルテンサイトまでの距離をLMA[μm]とし、100個以上測定したときのLMAの標準偏差が5以下とする。
[実施例]
In order to improve local deformability such as bending, stretch flange, and burring, it is preferable that hard structures such as retained austenite and martensite are dispersed. Therefore, the distance to the nearest retained austenite or martensite is L MA [μm] among the grains of retained austenite and martensite, and the standard deviation of LMA when 100 or more are measured is 5 or less.
[Example]

本発明の実施例を挙げながら、本発明の技術的内容について説明する。
実施例として、表1に示した成分組成を有するaからjまでの本発明の請求項の成分を満たす鋼、aからdの比較鋼を用いて検討した結果について説明する。これらの鋼は、鋳造後、そのままもしくは一旦室温まで冷却された後に再加熱し、900℃〜1300℃の温度範囲に加熱され、その後、表2の条件で熱間圧延が施され、2〜5mm厚の熱延鋼板とした。
The technical contents of the present invention will be described with reference to examples of the present invention.
As examples, the results of studies using steels satisfying the components of claims of the present invention from a to j having the composition shown in Table 1 and comparative steels a to d will be described. After casting, these steels are reheated as they are or once cooled to room temperature, heated to a temperature range of 900 ° C. to 1300 ° C., and then hot-rolled under the conditions shown in Table 2 to 2 to 5 mm. A thick hot-rolled steel sheet was used.

Figure 0005533729
Figure 0005533729

表2及び表3に各製造条件とそれぞれの組織形成と機械的特性を示す。局部変形能の指標としては最終製品の穴拡げ率および60°V字曲げによる限界曲げ半径を用いた。曲げ試験はC方向曲げと45°方向曲げを行いその比率を使って成形性の方位依存性の指標とした。なお、引っ張り試験および曲げ試験はJIS Z 2241およびZ2248 (Vブロック90°曲げ試験)に、穴拡げ試験は鉄連規格JFS T1001にそれぞれ準拠した。X線ランダム強度比は、前述のEBSDを用いて圧延方向に平行な断面の5/8〜3/8の領域を0.5μmピッチで測定した。また、各方向のr値について前述した方法により測定した。   Tables 2 and 3 show the production conditions, the structure formation, and the mechanical properties. As indices of local deformability, the hole expansion rate of the final product and the critical bending radius by 60 ° V-bending were used. In the bending test, C direction bending and 45 ° direction bending were performed, and the ratio was used as an index of orientation dependency of formability. The tensile test and the bending test were compliant with JIS Z 2241 and Z2248 (V-block 90 ° bending test), and the hole expansion test was compliant with the ironwork standard JFS T1001. The X-ray random intensity ratio was measured at a 0.5 μm pitch in the 5/8 to 3/8 region of the cross section parallel to the rolling direction using the aforementioned EBSD. Further, the r value in each direction was measured by the method described above.

Figure 0005533729
Figure 0005533729

Figure 0005533729
Figure 0005533729

本願規定を満たす鋼板を用いたもののみが、表2に示すように優れた穴拡げ性と、曲げ性と成形異方性の少なさを併せ持つことができることがわかる。さらに、望ましい製造条件範囲にあるものは、より優れた、穴拡げ率および曲げ性、成形性の異方性レスを示すことがわかる。   As shown in Table 2, it can be seen that only those using a steel sheet satisfying the requirements of the present application can have both excellent hole expansibility and low bendability and molding anisotropy. Furthermore, it can be seen that those in the desired production condition range exhibit a superior hole expansion rate, bendability, and anisotropy of formability.

Claims (7)

質量%で、
C:0.02%以上、0.5%以下、
Si:0.001%以上、4.0%以下、
Mn:0.001%以上、4.0%以下、
P:0.001%以上、0.15%以下、
S:0.0005%以上、0.03%以下、
N:0.0005%以上、0.01%以下、
O:0.0005%以上、0.01%以下、
Al+Si≦4.0%以下、
更に、
Ti:0.001%以上、0.2%以下、
Nb:0.001%以上、0.2%以下、
V:0.001%以上、1.0%以下、
W:0.001%以上、1.0%以下、
Cu:0.001%以上、2.0%以下、
B :0.0001%以上、0.005%以下、
Mo:0.001%以上、1.0%以下、
Cr:0.001%以上、2.0%以下、
As:0.0001%以上、0.50%以下、
Ni:0.001%以上、2.0%以下、
Co:0.0001%以上、1.0%以下、
Sn:0.0001%以上、0.2%以下、
Zr:0.0001%以上、0.2%以下、
の1種又は2種以上を含有し、残部鉄および不可避的不純物からなり、集合組織が、少なくとも鋼板の表面から5/8〜3/8の板厚における板面の{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値が4.0未満で、かつ{332}<113>の結晶方位のX線ランダム強度比が5.0以下で、さらに圧延方向と直角方向のr(rC)値が0.70以上、かつ圧延方向と30°(r30)のr値が1.10以下であり、鋼板組織として、面積率で残留オーステナイトを5%以上、30%未満、フェライトを20%以上、50%未満、ベイナイトを 10%以上、60%未満含有し、さらに、パーライト、マルテンサイトがそれぞれ20%以下である特徴とする局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板。
% By mass
C: 0.02% or more, 0.5% or less,
Si: 0.001% to 4.0%,
Mn: 0.001% or more and 4.0% or less,
P: 0.001% or more, 0.15% or less,
S: 0.0005% or more, 0.03% or less,
N: 0.0005% or more, 0.01% or less,
O: 0.0005% or more, 0.01% or less,
Al + Si ≦ 4.0% or less,
Furthermore,
Ti: 0.001% or more, 0.2% or less,
Nb: 0.001% or more, 0.2% or less,
V: 0.001% or more, 1.0% or less,
W: 0.001% or more, 1.0% or less,
Cu: 0.001% or more, 2.0% or less,
B: 0.0001% or more, 0.005% or less,
Mo: 0.001% or more, 1.0% or less,
Cr: 0.001% or more, 2.0% or less,
As: 0.0001% or more, 0.50% or less,
Ni: 0.001% or more, 2.0% or less,
Co: 0.0001% or more and 1.0% or less,
Sn: 0.0001% or more, 0.2% or less,
Zr: 0.0001% or more, 0.2% or less,
{100} <011> to the surface of the plate at a thickness of 5/8 to 3/8 at least from the surface of the steel plate. The average value of the X-ray random intensity ratio of the {223} <110> orientation group is less than 4.0, the X-ray random intensity ratio of the crystal orientation of {332} <113> is 5.0 or less, and the rolling direction The r (rC) value in the direction perpendicular to the rolling direction is 0.70 or more, and the r value at 30 ° (r30) in the rolling direction is 1.10 or less. %, Ferrite is 20% or more, less than 50%, bainite is contained in an amount of 10% or more and less than 60%, and pearlite and martensite are each 20% or less. Less orientation dependency Excellent hot-rolled steel sheet to sex.
更に、全面積のうち、20μmを超える粒の占める面積割合が10%以下であることを特徴とする請求項1に記載の局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板。   Furthermore, it is excellent in the local deformability of Claim 1 characterized by the area ratio which the particle | grains exceeding 20 micrometers occupy among the total area, and it was excellent in ductility with little orientation dependence of a moldability Hot rolled steel sheet. 更に、残留オーステナイトとマルテンサイトの粒で、もっとも近い残留オーステナイトもしくはマルテンサイトまでの距離をLMA[μm]とし、100個以上測定した時のLMAの標準偏差が5以下である請求項1または2に記載の局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板。 Further, the grain of the residual austenite and martensite, the distance to the closest retained austenite or martensite and L MA [μm], the standard deviation of the L MA when measured more than 100 is 5 or less claim 1 or A hot-rolled steel sheet having excellent local deformability described in 2 and excellent ductility with less orientation dependency of formability. 更に、
Mg:0.0001%以上、0.010%以下、
REM:0.0001%以上、0.1%以下、
Ca:0.0001%以上、0.010%以下、
の1種又は2種以上を含有する請求項1からの何れか1項に記載の局部変形能に優れ、
成形性の方位依存性の少ない延性に優れた熱延鋼板。
Furthermore,
Mg: 0.0001% or more, 0.010% or less,
REM: 0.0001% or more, 0.1% or less,
Ca: 0.0001% or more, 0.010% or less,
Excellent local deformability according to any one of claims 1 to 3, containing one or more of,
Hot-rolled steel sheet with excellent ductility and less orientation dependency of formability.
請求項1〜の何れか1項に記載の高強度鋼板を製造するに当たり、所定の鋼板成分に溶製したのち、鋼塊またはスラブに鋳造して,粗圧延を1000℃以上、1200℃以下の温度域で20%以上の圧下を少なくとも1回以上行い、オーステナイト粒径を200μm以下とし、その後、仕上圧延において式(1)にある鋼板成分により決定される温度をT1とすると、T1+30℃以上、T1+200℃以下の温度範囲において、1回は1パス30%以上の圧延を行い、かつ、該温度範囲での圧下率の合計を50%以上とし、その後のT1以上、T1+30℃未満の温度範囲における圧下率の合計を30%以下とし、更に30%以上の最終圧延終了後のT1以上、T1+200℃以下の温度域から冷却開始までの停留時間tが式(2)を満たすようにして、Ar3変態温度以上で熱間圧延を終了し、630℃以上、800℃以下の温度域に10〜100℃/secで冷却し、次いで、当該温度域において1秒以上20秒以下保持、又は、当該温度域に冷却後に20℃/sec以下の冷却速度で550℃以上の範囲内の温度まで冷却し、350〜500℃で巻き取りを行い、温度変化速度が−40℃/h以上、40℃/h以下の範囲として30〜300分保持した後、空冷させた局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板の製造方法
T1(℃)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V (1)
t≦t1×2.5 (2)
ここで、t1は式(3)で表される。
t1=0.001((Tf−T1)×P1)−0.109((Tf−T1)×P1)+3.1 (3)
ここで、Tfは30%以上の最終圧下後の温度、P1は30%以上の最終圧下の圧下率である。
In producing the high-strength steel sheet according to any one of claims 1 to 4 , after melting into a predetermined steel plate component, casting into a steel ingot or slab, rough rolling is performed at 1000 ° C or more and 1200 ° C or less. T1 + 30 ° C. or more, assuming that the reduction of 20% or more is performed at least once in the temperature range, the austenite grain size is 200 μm or less, and then the temperature determined by the steel sheet component in formula (1) in finish rolling is T1. In the temperature range of T1 + 200 ° C. or less, rolling is performed at 30% or more in one pass, and the total rolling reduction in the temperature range is set to 50% or more, and then the temperature range of T1 or more and less than T1 + 30 ° C. and the total reduction ratio of 30% or less in an additional 30% or more of the final rolling after the end of T1 or more, satisfy the residence time t is the formula to the start cooling from the temperature range of T1 + 200 ° C. or less (2) In the Suyo, exit the hot rolled at Ar3 transformation temperature or higher, 630 ° C. or higher, the temperature range of 800 ° C. or less and cooled at 10 to 100 ° C. / sec, then 20 seconds or less for more than one second in the temperature range Hold or cool to the temperature range, cool to a temperature in the range of 550 ° C. or higher at a cooling rate of 20 ° C./sec or less, take up at 350 to 500 ° C., and the temperature change rate is −40 ° C./h As described above, a method for producing a hot-rolled steel sheet having excellent ductility, which is excellent in local deformability after being held in a range of 40 ° C./h or less for 30 to 300 minutes and then air-cooled and has less orientation dependency of formability.
T1 (° C.) = 850 + 10 × (C + N) × Mn + 350 × Nb + 250 × Ti + 40 × B + 10 × Cr + 100 × Mo + 100 × V (1)
t ≦ t1 × 2.5 (2)
Here, t1 is represented by Formula (3).
t1 = 0.001 ((Tf−T1) × P1) 2 −0.109 ((Tf−T1) × P1) +3.1 (3)
Here, Tf is the temperature after the final reduction of 30% or more, and P1 is the reduction ratio of the final reduction of 30% or more.
請求項に記載の製造方法で、仕上げ圧延終了から1秒以内に50℃/sec以上の冷却速度で冷却温度変化が40℃以上、150℃以下となる冷却を行うことを特徴とする局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板の製造方法6. The method according to claim 5 , wherein the cooling is performed so that the change in cooling temperature is 40 ° C. or more and 150 ° C. or less at a cooling rate of 50 ° C./sec or more within 1 second from the end of finish rolling. A method for producing a hot-rolled steel sheet having excellent ductility and excellent ductility with less orientation dependency of formability. 請求項またはに記載の製造方法において、T1+30℃以上、T1+200℃以下の温度範囲における圧延の最終パスの圧延率は25%以上であることを特徴とする局部変形能に優れ、成形性の方位依存性の少ない延性に優れた熱延鋼板の製造方法The manufacturing method according to claim 5 or 6 , wherein the rolling rate of the final pass of rolling in a temperature range of T1 + 30 ° C or higher and T1 + 200 ° C or lower is 25% or more, and has excellent local deformability and has formability. A method for producing a hot-rolled steel sheet having excellent ductility with little orientation dependency.
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