TWI650434B - Steel plate - Google Patents
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Abstract
一種鋼板，具有預定化學組成，且具有以下所示金屬組織：以面積分率計，多邊形肥粒鐵：40%以下、麻田散鐵：20%以下、變韌肥粒鐵：50%~95%、且殘留沃斯田鐵：5%~50%。以面積分率計，變韌肥粒鐵中的80%以上是由長寬比為0.1~1.0且方位差角在15°以上之晶界所包圍之區域中差排密度在8×10 2(cm/cm 3)以下之變韌肥粒鐵晶粒所構成。且，以面積分率計，殘留沃斯田鐵中的80%以上是由長寬比為0.1~1.0、長軸長度為1.0μm~28.0μm且短軸長度為0.1μm~2.8μm之殘留沃斯田鐵晶粒所構成。 A steel plate having a predetermined chemical composition and having the following metal structure: in terms of area fraction, polygonal ferrite iron: 40% or less, Ma Tian loose iron: 20% or less, toughened ferrite iron: 50% to 95% And the remaining Worth Iron: 5% ~ 50%. In terms of area fraction, more than 80% of the toughened ferrite iron is 8×10 2 in the region surrounded by grain boundaries with an aspect ratio of 0.1 to 1.0 and an azimuth difference angle of 15° or more ( Cm/cm 3 ) The following toughened ferrite grains are composed of iron grains. Further, in terms of area fraction, 80% or more of the remaining Worthite iron is a residual Worstian iron crystal having an aspect ratio of 0.1 to 1.0, a long axis length of 1.0 μm to 28.0 μm, and a short axis length of 0.1 μm to 2.8 μm. Made up of grains.
Description
本發明是有關於一種適合汽車零件之鋼板。The present invention relates to a steel sheet suitable for automotive parts.
背景技術 為了抑制來自汽車之二氧化碳氣體排出量，使用有高強度鋼板之汽車車體持續向輕量化發展。例如，為了確保搭乘者的安全性，而逐漸多於車體的骨架系零件使用高強度鋼板。作為對撞擊安全性的影響較大之機械特性，可列舉拉伸強度、延性、延性-脆性過渡溫度及0.2%偏位降伏強度。例如，對於用於前側面構件的鋼板會要求優異延性。Background Art In order to suppress the amount of carbon dioxide gas emitted from automobiles, an automobile body using a high-strength steel sheet has been continuously reduced in weight. For example, in order to ensure the safety of the rider, the high-strength steel plate is gradually used more than the skeleton component of the vehicle body. Examples of mechanical properties that have a large influence on the safety of impact include tensile strength, ductility, ductility-brittle transition temperature, and 0.2% offset strength. For example, excellent ductility may be required for a steel sheet for a front side member.
另一方面，骨架系零件的形狀較複雜，而對於骨架系零件用的高強度鋼板會要求優異擴孔性及彎曲性。例如，對於用於側樑的鋼板會要求優異擴孔性。On the other hand, the shape of the skeleton component is complicated, and the high strength steel plate for the skeleton component is required to have excellent hole expandability and flexibility. For example, an excellent hole expandability is required for a steel sheet for a side sill.
不過，撞擊安全性的提升及成形性的提升難以兼顧。以往，有提案出關於撞擊安全性的提升或成形性的提升之技術(專利文獻1及2)，但即使依照該等，仍難以兼顧撞擊安全性的提升及成形性的提升。However, the improvement in impact safety and the improvement in formability are difficult to balance. In the past, there has been proposed a technique for improving the safety of impact or improving the formability (Patent Documents 1 and 2). However, even in accordance with these, it is difficult to achieve both improvement in impact safety and improvement in formability.
先前技術文獻 專利文獻 專利文獻1：日本專利特許第5589893號公報 專利文獻2：日本專利特開2013-185196號公報 專利文獻3：日本專利特開2005-171319號公報 專利文獻4：國際專利公開第2012/133563號CITATION LIST PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT PATENT DOCUMENT 2012/133563
發明概要 發明欲解決之課題 本發明之目在於提供一種可獲得優異撞擊安全性及成形性之鋼板。Disclosure of the Invention Problems to be Solved by the Invention An object of the present invention is to provide a steel sheet which can attain excellent impact safety and formability.
用以解決課題之手段 本發明人等為解決上述課題，進行了精闢研討。其結果，究明了在拉伸強度為980MPa以上的鋼板中，藉由將殘留沃斯田鐵及變韌肥粒鐵的面積分率及形態作成為預定之物，便可展現優異延伸率。並且，還究明了當多邊形肥粒鐵的面積分率低時，在鋼板內的硬度差較小，而不僅可獲得優異延伸率，亦可獲得優異擴孔性及彎曲性，並且還可獲得充分的在低溫下之耐脆化特性及0.2%偏位降伏強度。Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above problems. As a result, it has been found that in a steel sheet having a tensile strength of 980 MPa or more, an excellent elongation ratio can be exhibited by making the area fraction and morphology of the remaining Worthite iron and the tough ferrite iron into a predetermined content. Moreover, it is also clear that when the area fraction of the polygonal ferrite is low, the hardness difference in the steel sheet is small, and not only excellent elongation but also excellent hole expandability and flexibility can be obtained, and sufficient The embrittlement resistance at low temperature and the 0.2% offset fall strength.
本案之發明人根據前述知識見解進一步反覆進行精闢研討之結果，想出以下所示之發明的各種態樣。The inventors of the present invention further inferred the results of the incisive research based on the above-mentioned knowledge and thought out various aspects of the invention shown below.
(1) 一種鋼板，其特徵在於： 其具有以下所示化學組成： 以質量%計， C：0.1%~0.5%、 Si：0.5%~4.0%、 Mn：1.0%~4.0%、 P：0.015%以下、 S：0.050%以下、 N：0.01%以下、 Al：2.0%以下、 Si及Al：合計為0.5%~6.0%、 Ti：0.00%~0.20%、 Nb：0.00%~0.20%、 B：0.0000%~0.0030%、 Mo：0.00%~0.50%、 Cr：0.0%~2.0%、 V：0.00%~0.50%、 Mg：0.000%~0.040%、 REM：0.000%~0.040%、 Ca：0.000%~0.040%、且 剩餘部分：Fe及不純物；並且 具有以下所示金屬組織： 以面積分率計， 多邊形肥粒鐵：40%以下、 麻田散鐵：20%以下、 變韌肥粒鐵：50%~95%、且 殘留沃斯田鐵：5%~50%； 以面積分率計，前述變韌肥粒鐵中的80%以上是由長寬比為0.1~1.0且方位差角在15°以上之晶界所包圍之區域中差排密度在8×10 2(cm/cm 3)以下之變韌肥粒鐵晶粒所構成，且 以面積分率計，前述殘留沃斯田鐵中的80%以上是由長寬比為0.1~1.0、長軸長度為1.0μm~28.0μm且短軸長度為0.1μm~2.8μm之殘留沃斯田鐵粒晶粒所構成。 (1) A steel sheet characterized by having the chemical composition shown below: C%: 0.1% to 0.5%, Si: 0.5% to 4.0%, Mn: 1.0% to 4.0%, P: 0.015 % or less, S: 0.050% or less, N: 0.01% or less, Al: 2.0% or less, Si and Al: 0.5% to 6.0% in total, Ti: 0.00% to 0.20%, and Nb: 0.00% to 0.20%, B : 0.0000%~0.0030%, Mo: 0.00%~0.50%, Cr: 0.0%~2.0%, V: 0.00%~0.50%, Mg: 0.000%~0.040%, REM: 0.000%~0.040%, Ca: 0.000 %~0.040%, and the remainder: Fe and impurities; and have the following metal structure: in terms of area fraction, polygonal ferrite iron: 40% or less, Ma Tian loose iron: 20% or less, toughened ferrite iron: 50%~95%, and residual Worthite iron: 5%~50%; based on the area fraction, more than 80% of the above-mentioned toughened ferrite iron is from an aspect ratio of 0.1~1.0 and the azimuth difference angle is In the region surrounded by the grain boundary of 15° or more, the difference in density is 8×10 2 (cm/cm 3 ) or less, and the composition of the above-mentioned residual Worthite iron is More than 80% are from 0.1 to 1.0 in aspect ratio and 1.0 to 28.0 μm in long axis. The short-axis length is 0.1 μm to 2.8 μm of residual Worstian iron grains.
(2) 如(1)之鋼板，其中前述金屬組織以面積分率計表示為： 多邊形肥粒鐵：5%~20%、 麻田散鐵：20%以下、 變韌肥粒鐵：75%~90%、且 殘留沃斯田鐵：5%~20%。(2) The steel sheet of (1), wherein the metal structure is expressed by area fraction: polygonal ferrite iron: 5% to 20%, Ma Tian loose iron: 20% or less, toughened ferrite iron: 75%~ 90%, and residual Worth Iron: 5% to 20%.
(3) 如(1)之鋼板，其中前述金屬組織以面積分率計表示為： 多邊形肥粒鐵：大於20%且在40%以下、 麻田散鐵：20%以下、 變韌肥粒鐵：50%~75%、且 殘留沃斯田鐵：5%~30%。(3) The steel sheet according to (1), wherein the metal structure is expressed by area fraction: polygonal ferrite iron: more than 20% and less than 40%, Ma Tian loose iron: 20% or less, toughened ferrite iron: 50%~75%, and residual Worthite iron: 5%~30%.
(4) 如(1)~(3)中任一項之鋼板，其中前述化學組成中成立下述： 以質量%計， Ti：0.01%~0.20%、 Nb：0.005%~0.20%、 B：0.0001%~0.0030%、 Mo：0.01%~0.50%、 Cr：0.01%~2.0%、 V：0.01%~0.50%、 Mg：0.0005%~0.040%、 REM：0.0005%~0.040%、或 Ca：0.0005%~0.040%、 或是該等之任意組合。(4) The steel sheet according to any one of (1) to (3), wherein the chemical composition is as follows: Ti: 0.01% to 0.20%, Nb: 0.005% to 0.20%, and B: 0.0001%~0.0030%, Mo: 0.01%~0.50%, Cr: 0.01%~2.0%, V: 0.01%~0.50%, Mg: 0.0005%~0.040%, REM: 0.0005%~0.040%, or Ca: 0.0005 %~0.040%, or any combination of these.
(5) 如(1)~(4)中任一項之鋼板，其具有形成於表面上之鍍層。(5) The steel sheet according to any one of (1) to (4) which has a plating layer formed on the surface.
發明效果 根據本發明，由於殘留沃斯田鐵及變韌肥粒鐵的面積分率及形態等乃屬適切，因此可獲得優異撞擊安全性及形成性。Advantageous Effects of Invention According to the present invention, since the area fraction and morphology of the remaining Worthite iron and the toughened ferrite iron are suitable, excellent impact safety and formability can be obtained.
發明實施形態 以下說明本發明之實施形態。BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described.
首先，說明本發明實施形態之鋼板的金屬組織。本實施形態之鋼板具有以下所示金屬組織：以面積分率計，多邊形肥粒鐵：40%以下、麻田散鐵：20%以下、變韌肥粒鐵：50%~95%、且殘留沃斯田鐵：5%~50%。以面積分率計，變韌肥粒鐵中的80%以上是由長寬比為0.1~1.0且方位差角在15°以上之晶界所包圍之區域中差排密度在8×10 2(cm/cm 3)以下之變韌肥粒鐵晶粒所構成。且，以面積分率計，殘留沃斯田鐵中的80%以上是由長寬比為0.1~1.0、長軸長度為1.0μm~28.0μm且短軸長度為0.1μm~2.8μm之殘留沃斯田鐵晶粒所構成。 First, the metal structure of the steel sheet according to the embodiment of the present invention will be described. The steel sheet according to the present embodiment has the following metal structure: in terms of area fraction, polygonal ferrite iron: 40% or less, Ma Tian loose iron: 20% or less, toughened ferrite iron: 50% to 95%, and residual Wo Sita Iron: 5%~50%. In terms of area fraction, more than 80% of the toughened ferrite iron is 8×10 2 in the region surrounded by grain boundaries with an aspect ratio of 0.1 to 1.0 and an azimuth difference angle of 15° or more ( Cm/cm 3 ) The following toughened ferrite grains are composed of iron grains. Further, in terms of area fraction, 80% or more of the remaining Worthite iron is a residual Worstian iron crystal having an aspect ratio of 0.1 to 1.0, a long axis length of 1.0 μm to 28.0 μm, and a short axis length of 0.1 μm to 2.8 μm. Made up of grains.
(多邊形肥粒鐵的面積分率：40%以下) 多邊形肥粒鐵為軟質組織。因此，多邊形肥粒鐵與硬質組織之麻田散鐵之間的硬度差較大，於成形時在該等之間的界面中容易發生龜裂。龜裂有時也會沿著此界面伸展。多邊形肥粒鐵的面積分率大於40%時，上述龜裂的發生及伸展容易產生，而難以獲得充分的擴孔性、彎曲性、在低溫下之耐脆化特性及0.2%偏位降伏強度。因此，多邊形肥粒鐵的面積分率是設為40%以下。(Area area of polygonal ferrite iron: 40% or less) Polygonal ferrite is a soft tissue. Therefore, the difference in hardness between the polygonal ferrite iron and the hard ground of the hard ground is large, and cracking easily occurs at the interface between the two at the time of molding. Cracks sometimes extend along this interface. When the area fraction of the polygonal fat iron is more than 40%, the occurrence and elongation of the above-mentioned cracks are likely to occur, and it is difficult to obtain sufficient hole expandability, flexibility, embrittlement resistance at low temperature, and 0.2% offset strength. . Therefore, the area fraction of the polygonal ferrite iron is set to 40% or less.
多邊形肥粒鐵的面積分率越低，C變得越不易往殘留沃斯田鐵中濃化，而擴孔性會提升但另一方面延性卻會降低。因此，當重視擴孔性更甚於延性時，多邊形肥粒鐵的面積分率宜設為20%以下，當重視延性更甚於擴孔性時，多邊形肥粒鐵的面積分率宜設為大於20%且在40%以下。為了在重視擴孔性更甚於延性時也確保延性，多邊形肥粒鐵的面積分率宜設為5%以上。The lower the area fraction of the polygonal ferrite iron, the less likely C becomes to concentrate in the remaining Worth iron, and the hole expandability will increase but the ductility will decrease. Therefore, when the hole-expanding property is more important than the ductility, the area fraction of the polygonal ferrite iron should be set to 20% or less. When the ductility is more important than the hole-expanding property, the area fraction of the polygonal ferrite iron should be set to More than 20% and below 40%. In order to ensure ductility even when the hole-expanding property is more important than the ductility, the area fraction of the polygonal ferrite iron should be set to 5% or more.
(變韌肥粒鐵的面積分率：50%~95%) 變韌肥粒鐵是較多邊形肥粒鐵以高密度含有差排，而有助於拉伸強度的提升。由於變韌肥粒鐵的硬度比多邊形肥粒鐵的硬度高，且比麻田散鐵的硬度還低，因此變韌肥粒鐵與麻田散鐵之間的硬度差會比多邊形肥粒鐵與麻田散鐵之間的硬度差還小。因此，變韌肥粒鐵也有助於擴孔性及彎曲性的提升。若變韌肥粒鐵的面積分率低於50%，便無法獲得充分的拉伸強度。因此，變韌肥粒鐵的面積分率是設為50%以上。當重視擴孔性更甚於延性時，變韌肥粒鐵的面積分率宜設為75%以上。另一方面，若變韌肥粒鐵的面積分率大於95%，殘留沃斯田鐵會不足，而無法獲得充分的成形性。因此，變韌肥粒鐵的面積分率是設為95%以下。(Area area of toughened ferrite iron: 50%~95%) The toughened ferrite iron is more dense than the polygonal ferrite iron, which contributes to the improvement of tensile strength. Since the hardness of the toughened ferrite is higher than that of the polygonal ferrite and is lower than the hardness of the Ma Tian iron, the hardness difference between the tough ferrite iron and the Ma Tian iron is higher than that of the polygonal ferrite and the Ma Tian. The difference in hardness between the loose irons is still small. Therefore, the toughened ferrite iron also contributes to the improvement of the hole expandability and the bendability. If the area fraction of the toughened ferrite is less than 50%, sufficient tensile strength cannot be obtained. Therefore, the area fraction of the toughened ferrite iron is set to 50% or more. When the hole-expanding property is more important than the ductility, the area fraction of the toughened ferrite iron should be set to 75% or more. On the other hand, if the area fraction of the toughened ferrite iron is more than 95%, the residual Worth iron will be insufficient, and sufficient formability cannot be obtained. Therefore, the area fraction of the toughened ferrite iron is set to 95% or less.
(麻田散鐵的面積分率：20%以下) 麻田散鐵包含新生麻田散鐵(未回火的麻田散鐵)及回火麻田散鐵。如上所述，多邊形肥粒鐵與麻田散鐵之間的硬度差較大，於成形時在該等之間的界面中容易發生龜裂。龜裂有時也會沿著此界面伸展。麻田散鐵的面積分率大於20%時，上述龜裂的發生及伸展容易產生，而難以獲得充分的擴孔性、彎曲性、在低溫下之耐脆化特性及0.2%偏位降伏強度。因此，麻田散鐵的面積分率是設為20%以下。(Area area of Ma Tian loose iron: 20% or less) Ma Tian loose iron contains new Ma Tian loose iron (unreturned Ma Tian loose iron) and tempered Ma Tian loose iron. As described above, the difference in hardness between the polygonal ferrite iron and the granulated iron is large, and cracking easily occurs at the interface between the two at the time of molding. Cracks sometimes extend along this interface. When the area fraction of the granulated iron is more than 20%, the occurrence and stretching of the above-mentioned cracks are likely to occur, and it is difficult to obtain sufficient hole expandability, flexibility, embrittlement resistance at low temperatures, and 0.2% eccentricity. Therefore, the area fraction of the granulated iron is set to 20% or less.
(殘留沃斯田鐵的面積分率：5%~50%) 殘留沃斯田鐵有助於成形性的提升。若殘留沃斯田鐵的面積分率低於5%，便無法獲得充分的成形性。另一方面，若殘留沃斯田鐵的面積分率大於50%，變韌肥粒鐵會不足，而無法獲得充分的拉伸強度。因此，殘留沃斯田鐵的面積分率是設為50%以下。(Area area of residual Worthfield iron: 5% to 50%) Residual Worthite iron contributes to the improvement of formability. If the area fraction of the remaining Worthite iron is less than 5%, sufficient formability cannot be obtained. On the other hand, if the area fraction of the remaining Worth iron is more than 50%, the tough ferrite iron will be insufficient, and sufficient tensile strength cannot be obtained. Therefore, the area fraction of the remaining Worth Iron is set to 50% or less.
多邊形肥粒鐵、變韌肥粒鐵、殘留沃斯田鐵及麻田散鐵之鑑定及面積分率之特定可利用例如掃描型電子顯微鏡(scanning electron microscope: SEM)觀察或穿透型電子顯微鏡(transmission electron microscope: TEM)觀察來進行。使用SEM或TEM時，係使用例如硝太蝕劑(Nital)及雷佩拉(Lepera)液腐蝕試樣後，以1000倍~100000倍的倍率觀察平行於軋延方向及厚度方向的截面(垂直於寬度方向的截面)及/或垂直於軋延方向的截面。Identification of polygonal ferrite iron, toughened ferrite iron, residual Worth iron and granulated iron and specificity of area fraction can be observed by, for example, scanning electron microscope (SEM) or transmission electron microscopy ( Transmission electron microscope: TEM) observation is carried out. When SEM or TEM is used, the sample is etched using, for example, Nital and Lepera, and the cross section parallel to the rolling direction and thickness direction is observed at a magnification of 1000 to 100,000 times (vertical A section in the width direction) and/or a section perpendicular to the rolling direction.
也可藉由結晶方位之解析或微維氏硬度測定等之微小區域的硬度測定來判別多邊形肥粒鐵、變韌肥粒鐵、殘留沃斯田鐵及麻田散鐵，前述結晶方位之解析是以使用了附屬於場發射掃描式電子顯微鏡(field emission scanning electron microscope: FE-SEM)的電子背向散射繞射(electron back scattering diffraction: EBSD)機能之結晶方位繞射(FE-SEM-EBSD)進行的。It is also possible to determine the polygonal ferrite iron, the tough ferrite iron, the residual Worthite iron, and the Ma Tian loose iron by the hardness measurement of the microscopic region such as the analysis of the crystal orientation or the micro Vickers hardness measurement, and the analysis of the crystal orientation is Crystal orientation azimuth diffraction (FE-SEM-EBSD) using electron back scattering diffraction (EBSD) function attached to a field emission scanning electron microscope (FE-SEM) ongoing.
例如，在多邊形肥粒鐵及變韌肥粒鐵的面積分率之特定中，會研磨平行於鋼板的軋延方向及厚度方向的截面(垂直於寬度方向的截面)，且以硝太蝕劑進行蝕刻。接著以FE-SEM觀察從鋼板表面起算深度為該鋼板厚度的1/8起至3/8為止之區域，以測定面積分率。以5000倍的倍率在10個視野中進行上述觀察，而可從10個視野的平均值求得多邊形肥粒鐵及變韌肥粒鐵的各面積分率。For example, in the specificity of the area fraction of the polygonal ferrite iron and the tough ferrite iron, the cross section parallel to the rolling direction and the thickness direction of the steel sheet (the cross section perpendicular to the width direction) is polished, and the nitric oxide is used. Etching is performed. Next, the area from the surface of the steel sheet from 1/8 to 3/8 of the thickness of the steel sheet was observed by FE-SEM to measure the area fraction. The above observation was carried out in 10 fields of view at a magnification of 5000 times, and the area fractions of the polygonal ferrite iron and the toughened ferrite iron were obtained from the average of 10 fields of view.
殘留沃斯田鐵的面積分率，可利用例如X射線測定來進行特定。在此方法中，是例如以機械研磨及化學研磨去除從鋼板的表面起到該鋼板厚度的1/4之部分，且使用MoKα線作為特性X射線。然後，利用下式，從體心立方晶格(bcc)相之(200)及(211)、以及面心立方晶格(fcc)相之(200)、(220)及(311)的繞射峰之積分強度比算出殘留沃斯田鐵之面積分率。在10個視野中進行上述觀察，而可從10個視野的平均值求得殘留沃斯田鐵的面積分率。 Sγ=(I 200f+I 220f+I 311f)/(I 200b+I 211b)×100 (Sγ表示殘留沃斯田鐵的面積分率，I 200f、I 220f、I 311f表示各個fcc相的(200)、(220)、(311)的繞射峰強度，I 200b、I 211b表示各個bcc相的 (200)、(211)的繞射峰強度。) The area fraction of the residual Worth iron can be specified by, for example, X-ray measurement. In this method, for example, mechanical polishing and chemical polishing are used to remove a portion from the surface of the steel sheet to a quarter of the thickness of the steel sheet, and a MoKα line is used as the characteristic X-ray. Then, using the following formula, diffraction from (200), (220), and (311) of the body-centered cubic lattice (bcc) phase and the face-centered cubic lattice (fcc) phase (200), (220), and (311) The integrated intensity ratio of the peaks is calculated as the area fraction of the remaining Worthfield iron. The above observation was carried out in 10 fields of view, and the area fraction of the residual Worthite iron was obtained from the average of 10 fields of view. Sγ=(I 200f +I 220f +I 311f )/(I 200b +I 211b )×100 (Sγ represents the area fraction of the residual Worth iron, and I 200f , I 220f , and I 311f represent the respective fcc phases (200). The diffraction peak intensities of (220) and (311), I 200b and I 211b indicate the diffraction peak intensities of (200) and (211) of each bcc phase.
麻田散鐵的面積分率可利用例如場發射掃描式電子顯微鏡(field emission-scanning electron microscope: FE-SEM)觀察及X射線測定進行特定。在此方法中，是例如以從鋼板的表面起算深度為從該鋼板厚度的1/8起至3/8為止之區域為觀察對象，且使用雷佩拉(Lepera)液進行腐蝕。由於不會被雷佩拉(Lepera)液腐蝕的組織為麻田散鐵及殘留沃斯田鐵，因此可從未被雷佩拉(Lepera)液腐蝕的區域的面積分率減去利用X射線測定所特定的殘留沃斯田鐵的面積分率Sγ，藉此來特定麻田散鐵的面積分率。麻田散鐵的面積分率也可使用例如以SEM觀察所得的電子穿隧效應對比影像(electron channeling contrast image)進行特定。在電子穿隧效應對比影像中，差排密度較高且晶粒內具有晶塊、包體等下部組織的區域為麻田散鐵。在10個視野中進行上述觀察，而可從10個視野的平均值求得麻田散鐵的面積分率。The area fraction of the granulated iron can be specified by, for example, field emission-scanning electron microscope (FE-SEM) observation and X-ray measurement. In this method, for example, a region from the surface of the steel sheet having a depth of from 1/8 to 3/8 of the thickness of the steel sheet is observed, and etching is performed using a Lepera liquid. Since the tissue not to be corroded by Lepera liquid is the granulated iron and the residual Worth iron, the area fraction of the area not corroded by Lepera can be subtracted from the X-ray measurement. The area fraction Sγ of the specific residual Worthite iron is used to specify the area fraction of the granulated iron. The area fraction of the granulated iron can also be specified using, for example, an electron channeling contrast image obtained by SEM observation. In the electron tunneling contrast image, the area where the difference in the density of the discharge is high and the lower part of the grain has crystals, inclusions, etc. is the granulated iron. The above observation was carried out in 10 fields of view, and the area fraction of the granulated iron was obtained from the average of 10 fields of view.
(預定形態之變韌肥粒鐵晶粒的面積分率：相對於變韌肥粒鐵的全體為80%以上) 由於差排密度高的變韌肥粒鐵晶粒不像多邊形肥粒鐵那樣有助於提升延伸率，因此差排密度高的變韌肥粒鐵晶粒的面積分率越高，延伸率越容易降低。然後，若長寬比為0.1~1.0且方位差角在15°以上之晶界所包圍之區域中差排密度在8×10 2(cm/cm 3)以下之變韌肥粒鐵晶粒的面積分率低於80%，便難以獲得充分延伸率。因此，此種形態的變韌肥粒鐵晶粒的面積分率相對於變韌肥粒鐵全體是設為80%以上，且較宜設為85%以上。 (The area fraction of the iron grain of the toughened ferrite grain in the predetermined form: 80% or more with respect to the whole of the tough ferrite iron) The iron grain of the toughened fat grain with high difference in density is not like the polygonal ferrite It helps to increase the elongation, so the higher the area fraction of the ferrite-grained iron grains with high difference in density, the easier the elongation is. Then, if the aspect ratio is 0.1 to 1.0 and the azimuth difference angle is in the region surrounded by the grain boundary of 15° or more, the ferrite grain iron grains having a difference in density of 8×10 2 (cm/cm 3 ) or less When the area fraction is less than 80%, it is difficult to obtain a sufficient elongation. Therefore, the area fraction of the tough ferrite grains in this form is set to 80% or more with respect to the whole of the tough fat iron, and is preferably set to 85% or more.
變韌肥粒鐵的差排密度可藉由使用了穿透型電子顯微鏡(TEM)的組織觀察進行特定。可藉由例如將存在於方位差角為15°之晶界所包圍的結晶粒中之差排線的數量除以該結晶粒的面積，來特定變韌肥粒鐵的差排密度。The differential density of the tough ferrite iron can be specified by tissue observation using a transmission electron microscope (TEM). The difference in density of the ductile ferrite iron can be specified by, for example, dividing the number of the difference lines in the crystal grains surrounded by the grain boundaries having an azimuth difference angle of 15° by the area of the crystal grains.
(預定形態之殘留沃斯田鐵晶粒的面積分率：相對於殘留沃斯田鐵的全體為80%以上) 殘留沃斯田鐵在成形之際會因加工誘發變態而變態成麻田散鐵。若殘留沃斯田鐵往麻田散鐵變態，在此麻田散鐵與多邊形肥粒鐵或未變態的殘留沃斯田鐵相鄰的情況下，該等之間便會產生較大的硬度差。而如上所述，較大的硬度差會導致龜裂的發生。此種龜裂特別容易發生在應力集中的部位，且應力容易集中於由長寬比小於0.1之殘留沃斯田鐵變態而成的麻田散鐵附近。然後，若長寬比為0.1~1.0、長軸長度為1.0μm~28.0μm且短軸長度為0.1μm~2.8μm之殘留沃斯田鐵晶粒的面積分率低於80%，便容易發生伴隨應力集中而來的龜裂，而難以獲得充分延伸率。因此，此種形態的殘留沃斯田鐵晶粒的面積分率相對於殘留沃斯田鐵全體是設為80%以上，且較宜設為85%以上。在此，殘留沃斯田鐵晶粒的長寬比是指將該殘留沃斯田鐵晶粒的等值橢圓的短軸長度除以長軸長度而得之值。於圖1中顯示等值橢圓之一例。殘留沃斯田鐵晶粒1即使具有複雜的形狀，仍可從其等值橢圓2的長軸長度L1及短軸長度L2求得該沃斯田鐵晶粒的長寬比(L2/L1)。(The area fraction of the remaining Worthite iron crystal grains in the predetermined form is 80% or more with respect to the total of the remaining Worthite iron.) The residual Worthite iron is metamorphosed into a granulated iron in the form of a process-induced metamorphosis. If the Worthite iron is left to the Ma Tian bulk iron metamorphosis, in the case where the Ma Tian loose iron is adjacent to the polygonal ferrite iron or the untransformed residual Worth iron, a large hardness difference will occur between the two. As mentioned above, a large difference in hardness causes cracking to occur. Such cracks are particularly likely to occur in stress-concentrated portions, and the stress tends to concentrate in the vicinity of the granulated iron which is metamorphosed by the residual Worth iron having an aspect ratio of less than 0.1. Then, if the aspect ratio is 0.1 to 1.0, the long axis length is 1.0 μm to 28.0 μm, and the short axis length is 0.1 μm to 2.8 μm, the area fraction of the residual Worstian iron crystal grains is less than 80%, and the accompanying stress easily occurs. Concentrated cracks, and it is difficult to obtain sufficient elongation. Therefore, the area fraction of the residual Worthite iron crystal grains in this form is set to 80% or more with respect to the entire Worthfield iron, and is preferably set to 85% or more. Here, the aspect ratio of the residual Worthite iron crystal grain refers to a value obtained by dividing the short axis length of the equivalence ellipse of the residual Worthite iron crystal grain by the long axis length. An example of an equivalent ellipse is shown in FIG. Even if the residual Worstian iron crystal grain 1 has a complicated shape, the aspect ratio (L2/L1) of the Worstian iron crystal grain can be obtained from the long axis length L1 and the short axis length L2 of the equivalence ellipse 2.
接下來說明本發明實施形態之鋼板及用於製造其之鋼胚的化學組成。如上所述，本發明實施形態之鋼板是歷經熱軋延、酸洗、冷軋延、第1退火及第2退火等而製造。因此，鋼板及鋼胚的化學組成不僅考慮到鋼板特性，還考慮到該些處理。在以下說明中，鋼板及鋼胚中所含之各元素含量單位「%」，如果沒有特別指明，則是指「質量%」。本實施形態之鋼板及用於製造其之鋼胚具有以下所示化學組成：以質量％計，C：0.1%~0.5%、Si：0.5%~4.0%、Mn：1.0%~4.0%、P：0.015%以下、S：0.050%以下、N：0.01%以下、Al：2.0%以下、Si及Al：合計為0.5%~6.0%、Ti：0.00%~0.20%、Nb：0.00%~0.20%、B：0.0000%~0.0030%、Mo：0.00%~0.50%、Cr：0.0%~2.0%、V：0.00%~0.50%、Mg：0.000%~0.040%、REM(稀土族金屬：rare earth metal)：0.000%~0.040%、Ca：0.000%~0.040%，且剩餘部分：Fe及不純物。Next, the chemical composition of the steel sheet according to the embodiment of the present invention and the steel blank for producing the same will be described. As described above, the steel sheet according to the embodiment of the present invention is produced by hot rolling, pickling, cold rolling, first annealing, second annealing, or the like. Therefore, the chemical composition of the steel sheet and the steel embryo not only takes into consideration the characteristics of the steel sheet, but also considers these treatments. In the following description, the unit content "%" of each element contained in the steel sheet and the steel embryo means "% by mass" unless otherwise specified. The steel sheet according to the present embodiment and the steel blank for producing the same have the chemical composition shown below: C: 0.1% to 0.5%, Si: 0.5% to 4.0%, Mn: 1.0% to 4.0%, and mass%, P : 0.015% or less, S: 0.050% or less, N: 0.01% or less, Al: 2.0% or less, Si and Al: 0.5% to 6.0% in total, Ti: 0.00% to 0.20%, and Nb: 0.00% to 0.20% , B: 0.0000%~0.0030%, Mo: 0.00%~0.50%, Cr: 0.0%~2.0%, V: 0.00%~0.50%, Mg: 0.000%~0.040%, REM (rare earth metal: rare earth metal ): 0.000%~0.040%, Ca: 0.000%~0.040%, and the remainder: Fe and impurities.
(C：0.10%~0.5%) 碳(C)有助於鋼板強度的提升，且透過殘留沃斯田鐵的穩定性之提升而有助於延伸率的提升。若C含量低於0.10%，便難以獲得充分的強度，例如980MPa以上的拉伸強度，且殘留沃斯田鐵的穩定性會變得不充分而無法獲得充分的延伸率。因此，C含量是設為0.10%以上，且宜設為0.15%以上。另一方面，若C含量大於0.5%，由於從沃斯田鐵往變韌肥粒鐵的變態會延遲，因此預定形態的變韌肥粒鐵晶粒會不足，而無法獲得充分的延伸率。因此，C含量是設為0.5%以下，且宜設為0.25%以下。(C: 0.10% to 0.5%) Carbon (C) contributes to the improvement of the strength of the steel sheet and contributes to the improvement of the elongation by the improvement of the stability of the residual Worth iron. When the C content is less than 0.10%, it is difficult to obtain sufficient strength, for example, a tensile strength of 980 MPa or more, and the stability of the residual Worstian iron may become insufficient to obtain sufficient elongation. Therefore, the C content is set to 0.10% or more, and is preferably set to 0.15% or more. On the other hand, if the C content is more than 0.5%, since the metamorphosis from the Worthite iron to the tough ferrite iron is delayed, the iron oxide grains of the toughened ferrite in a predetermined form are insufficient, and sufficient elongation cannot be obtained. Therefore, the C content is set to 0.5% or less, and is preferably set to 0.25% or less.
(Si：0.5%~4.0%) 矽(Si)有助於鋼之強度的提升，且透過殘留沃斯田鐵的穩定性之提升而有助於延伸率的提升。若Si含量低於0.5%，便無法充分獲得該等效果。因此，Si含量是設為0.5%以上，且宜設為1.0%以上。另一方面，若Si含量大於4.0%，鋼之強度會變得過高而延伸率會降低。因此，Si含量是設為4.0%以下，且宜設為2.0%以下。(Si: 0.5% to 4.0%) Niobium (Si) contributes to the improvement of the strength of the steel and contributes to the increase in elongation by the increase in the stability of the residual Worth iron. If the Si content is less than 0.5%, such effects cannot be sufficiently obtained. Therefore, the Si content is set to 0.5% or more, and is preferably set to 1.0% or more. On the other hand, if the Si content is more than 4.0%, the strength of the steel becomes too high and the elongation decreases. Therefore, the Si content is set to 4.0% or less, and is preferably set to 2.0% or less.
(Mn：1.0%~4.0%) 錳(Mn)有助於鋼之強度的提升，且會抑制在第1退火或第2退火的冷卻途中產生的多邊形肥粒鐵變態。當進行熔融鍍鋅處理時，也會抑制在該處理的冷卻途中產生的多邊形肥粒鐵變態。若Mn含量低於1.0%，便無法充分獲得該等效果，且多邊形肥粒鐵會過度生成而使擴孔性劣化。因此，Mn含量是設為1.0%以上，且宜設為2.0%以上。另一方面，若Mn含量大於4.0%，鋼胚及熱軋鋼板的強度會變得過高。因此，要設為4.0%以下，且宜設為3.0%以下。(Mn: 1.0% to 4.0%) Manganese (Mn) contributes to the improvement of the strength of the steel and suppresses the deformation of the polygonal ferrite in the middle of the cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, the polygonal ferrite iron metamorphosis generated during the cooling of the treatment is also suppressed. If the Mn content is less than 1.0%, such effects cannot be sufficiently obtained, and the polygonal ferrite iron is excessively formed to deteriorate the hole expandability. Therefore, the Mn content is set to 1.0% or more, and is preferably set to 2.0% or more. On the other hand, if the Mn content is more than 4.0%, the strength of the steel blank and the hot-rolled steel sheet becomes too high. Therefore, it should be set to 4.0% or less, and preferably set to 3.0% or less.
(P：0.015%以下) 磷(P)不是必需元素，而是例如作為不純物而含有於鋼中。P會偏析在鋼板厚度方向的中央部而使韌性降低，且會使熔接部脆化。因此，P含量愈低愈好。尤其，在P含量大於0.015%的情況下，韌性的降低及熔接性的脆化較為顯著。因此，P含量是設為0.015%以下，且宜設為0.010%以下。要減低P含量需花費成本，若欲減低到低於0.0001%，成本便會顯著上升。因此，P含量亦可設為0.0001%以上。(P: 0.015% or less) Phosphorus (P) is not an essential element, but is contained in steel, for example, as an impurity. P segregates in the central portion in the thickness direction of the steel sheet to lower the toughness and embrittle the welded portion. Therefore, the lower the P content, the better. In particular, when the P content is more than 0.015%, the reduction in toughness and the embrittlement of the weldability are remarkable. Therefore, the P content is set to 0.015% or less, and is preferably set to 0.010% or less. It takes cost to reduce the P content. If you want to reduce it to less than 0.0001%, the cost will increase significantly. Therefore, the P content can also be made 0.0001% or more.
(S：0.050%以下) 硫(S)不是必需元素，而是例如作為不純物而含有於鋼中。S會使鑄造及熱軋延的製造性降低，且會形成粗大的MnS而使擴孔性降低。因此，S含量愈低愈好。尤其，在S含量大於0.050%的情況下，熔接性的降低、製造性的降低及擴孔性的降低較為顯著。因此，S含量是設為0.050%以下，且宜設為0.0050%以下。要減低S含量需花費成本，若欲減低到低於0.0001%，成本便會顯著上升。因此，S含量亦可設為0.0001%以上。(S: 0.050% or less) Sulfur (S) is not an essential element but is contained in steel, for example, as an impurity. S lowers the manufacturability of casting and hot rolling, and forms coarse MnS to lower the hole expandability. Therefore, the lower the S content, the better. In particular, when the S content is more than 0.050%, the decrease in weldability, the decrease in manufacturability, and the decrease in hole expandability are remarkable. Therefore, the S content is set to 0.050% or less, and is preferably set to 0.0050% or less. It takes cost to reduce the S content. If you want to reduce it to less than 0.0001%, the cost will increase significantly. Therefore, the S content can also be made 0.0001% or more.
(N：0.01%以下) 氮(N)不是必需元素，而是例如作為不純物而含有於鋼中。N會形成粗大的氮化物，而使彎曲性及擴孔性劣化，且成為熔接時發生氣孔的原因。因此，N含量愈低愈好。尤其，若N含量大於0.01%，彎曲性及擴孔性的降低、以及氣孔的發生較為顯著。因此，N含量是設為0.01%以下。要減低N含量需花費成本，若欲減低到低於0.0005%，成本便會顯著上升。因此，N含量亦可設為0.0005%以上。(N: 0.01% or less) Nitrogen (N) is not an essential element but is contained in steel, for example, as an impurity. N forms coarse nitrides, deteriorates bendability and hole expandability, and causes pores to occur during welding. Therefore, the lower the N content, the better. In particular, when the N content is more than 0.01%, the decrease in flexibility and hole expandability and the occurrence of pores are remarkable. Therefore, the N content is set to 0.01% or less. It takes cost to reduce the N content. If you want to reduce it to less than 0.0005%, the cost will increase significantly. Therefore, the N content can also be made 0.0005% or more.
(Al：2.0%以下) 鋁(Al)雖可作為脫氧材料發揮機能，且可抑制鐵系碳化物在沃斯田鐵中的析出，但並不是必需元素。若Al含量大於2.0%，從沃斯田鐵往多邊形肥粒鐵的變態會受到促進，而多邊形肥粒鐵過度生成致使擴孔性劣化。因此，Al含量是設為2.0%以下，且宜設為1.0%以下。要減低Al含量需花費成本，若欲降低到低於0.001%時，成本便會顯著上升。因此，Al含量亦可設為0.001%以上。(Al: 2.0% or less) Aluminum (Al) functions as a deoxidizing material and suppresses precipitation of iron-based carbides in Worth iron, but is not an essential element. If the Al content is more than 2.0%, the metamorphosis from the Worthite iron to the polygonal ferrite iron is promoted, and the excessive formation of the polygonal ferrite iron causes the hole expandability to deteriorate. Therefore, the Al content is set to 2.0% or less, and is preferably set to 1.0% or less. The cost of reducing the Al content is costly, and if it is to be reduced to less than 0.001%, the cost will increase significantly. Therefore, the Al content may be 0.001% or more.
(Si及Al：合計為0.5%~6.0%) Si及Al皆會透過殘留沃斯田鐵的穩定性之提升，而有助於延伸率的提升。若Si及Al含量合計低於0.5%，便無法充分獲得該效果。因此，Si及Al含量是設為合計在0.5%以上，且宜設為1.2%以上。且可僅含有Si或Al之任一者，亦可含有Si及Al之兩者。(Si and Al: a total of 0.5% to 6.0%) Both Si and Al contribute to the improvement of the elongation by the stability of the residual Worth iron. If the total content of Si and Al is less than 0.5%, this effect cannot be sufficiently obtained. Therefore, the Si and Al contents are set to be 0.5% or more in total, and preferably 1.2% or more. It may contain only either Si or Al, and may contain both Si and Al.
Ti、Nb、B、Mo、Cr、V、Mg、REM及Ca不是必需元素，而是亦能以預定量為限度適當含有於鋼板及鋼胚中之任意元素。Ti, Nb, B, Mo, Cr, V, Mg, REM, and Ca are not essential elements, but may be appropriately contained in steel sheets and steel embryos with a predetermined amount as a limit.
(Ti：0.00%~0.20%) 鈦(Ti)是透過起因於析出強化及細粒強化之差排強化而有助於鋼之強度的提升。因此，亦可含有Ti。為了充分獲得此效果，Ti含量宜設為0.01%以上，更宜設為0.025%以上。另一方面，若Ti含量大於0.20%，Ti之碳氮化物會過度析出而使鋼板的成形性降低。因此，Ti含量是設為0.20%以下，且宜設為0.08%以下。(Ti: 0.00% to 0.20%) Titanium (Ti) contributes to the improvement of the strength of steel by the difference between the precipitation strengthening and the fine grain strengthening. Therefore, Ti may also be contained. In order to sufficiently obtain this effect, the Ti content is preferably set to 0.01% or more, and more preferably set to 0.025% or more. On the other hand, when the Ti content is more than 0.20%, the carbonitride of Ti is excessively precipitated to lower the formability of the steel sheet. Therefore, the Ti content is set to 0.20% or less, and is preferably set to 0.08% or less.
(Nb：0.00%~0.20%) 鈮(Nb)是透過起因於析出強化及細粒強化之差排強化而有助於鋼之強度的提升。因此，亦可含有Nb。為了充分獲得此效果，Nb含量宜設為0.005%以上，更宜設為0.010%以上。另一方面，若Nb含量大於0.20%，Nb之碳氮化物會過度析出而使鋼板的成形性降低。因此，Nb含量是設為0.20%以下，且宜設為0.08%以下。(Nb: 0.00% to 0.20%) Niobium (Nb) contributes to the improvement of the strength of steel by the difference between the precipitation strengthening and the fine grain strengthening. Therefore, it is also possible to contain Nb. In order to sufficiently obtain this effect, the Nb content is preferably set to 0.005% or more, and more preferably set to 0.010% or more. On the other hand, when the Nb content is more than 0.20%, the carbonitride of Nb is excessively precipitated to lower the formability of the steel sheet. Therefore, the Nb content is set to 0.20% or less, and is preferably set to 0.08% or less.
(B：0.0000%~0.0030%) 硼(B)會強化晶界，且會抑制在第1退火或第2退火的冷卻途中產生之多邊形肥粒鐵變態。當進行熔融鍍鋅處理時，也會抑制在該處理的冷卻途中產生的多邊形肥粒鐵變態。因此，亦可含有B。為了充分獲得此效果，B含量宜設為0.0001%以上，更宜設為0.0010%以上。另一方面，若B含量大於0.0030%，添加的效果會飽和，且熱軋延的製造性會降低。因此，B含量是設為0.0030%以下，且宜設為0.0025%以下。(B: 0.0000% to 0.0030%) Boron (B) strengthens the grain boundary and suppresses the deformation of the polygonal ferrite in the middle of cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, the polygonal ferrite iron metamorphosis generated during the cooling of the treatment is also suppressed. Therefore, it is also possible to contain B. In order to sufficiently obtain this effect, the B content is preferably set to 0.0001% or more, and more preferably set to 0.0010% or more. On the other hand, if the B content is more than 0.0030%, the effect of addition is saturated, and the manufacturability of hot rolling is lowered. Therefore, the B content is set to 0.0030% or less, and is preferably set to 0.0025% or less.
(Mo：0.00%~0.50%) 鉬(Mo)有助於鋼的強化，且會抑制在第1退火或第2退火的冷卻途中產生之多邊形肥粒鐵變態。當進行熔融鍍鋅處理時，也會抑制在該處理的冷卻途中產生的多邊形肥粒鐵變態。因此，亦可含有Mo。為了充分獲得此效果，Mo含量宜設為0.01%以上，更宜設為0.02%以上。另一方面，若Mo含量大於0.50%，熱軋延的製造性會降低。因此，Mo含量是設為0.50%以下，且宜設為0.20%以下。(Mo: 0.00% to 0.50%) Molybdenum (Mo) contributes to the strengthening of steel and suppresses the deformation of the polygonal ferrite in the middle of cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, the polygonal ferrite iron metamorphosis generated during the cooling of the treatment is also suppressed. Therefore, it is also possible to contain Mo. In order to sufficiently obtain this effect, the Mo content is preferably set to 0.01% or more, and more preferably set to 0.02% or more. On the other hand, if the Mo content is more than 0.50%, the manufacturability of the hot rolling is lowered. Therefore, the Mo content is set to 0.50% or less, and is preferably set to 0.20% or less.
(Cr：0.0%~2.0%) 鉻(Cr)有助於鋼的強化，且會抑制在第1退火或第2退火的冷卻途中產生之多邊形肥粒鐵變態。當進行熔融鍍鋅處理時，也會抑制在該處理的冷卻途中產生的多邊形肥粒鐵變態。因此，亦可含有Cr。為了充分獲得此效果，Cr含量宜設為0.01%以上，更宜設為0.02%以上。另一方面，若Cr含量大於2.0%，熱軋延的製造性會降低。因此，Cr含量是設為2.0%以下，且宜設為0.10%以下。(Cr: 0.0% to 2.0%) Chromium (Cr) contributes to the strengthening of steel and suppresses the deformation of the polygonal ferrite in the middle of cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, the polygonal ferrite iron metamorphosis generated during the cooling of the treatment is also suppressed. Therefore, it is also possible to contain Cr. In order to sufficiently obtain this effect, the Cr content is preferably set to 0.01% or more, and more preferably set to 0.02% or more. On the other hand, if the Cr content is more than 2.0%, the manufacturability of the hot rolling is lowered. Therefore, the Cr content is set to 2.0% or less, and is preferably set to 0.10% or less.
(V：0.00%~0.50%) 釩(V)是透過起因於析出強化及細粒強化之差排強化而有助於鋼之強度的提升。因此，亦可含有V。為了充分獲得此效果，V含量宜設為0.01%以上，更宜設為0.02%以上。另一方面，若V含量大於0.50%，V之碳氮化物會過度析出而使鋼板的成形性降低。因此，Nb含量是設為0.50%以下，且宜設為0.10%以下。(V: 0.00% to 0.50%) Vanadium (V) contributes to the improvement of the strength of steel by the difference between the precipitation strengthening and the fine grain strengthening. Therefore, it is also possible to contain V. In order to sufficiently obtain this effect, the V content is preferably set to 0.01% or more, and more preferably set to 0.02% or more. On the other hand, when the V content is more than 0.50%, the carbonitride of V is excessively precipitated to lower the formability of the steel sheet. Therefore, the Nb content is set to 0.50% or less, and is preferably set to 0.10% or less.
(Mg：0.000%~0.040%、REM：0.000%~0.040%、Ca：0.000%~0.040%) 鎂(Mg)、稀土族金屬(REM)及鈣(Ca)是作為氧化物或硫化物存在於鋼中，且有助於擴孔性的提升。因此，亦可含有Mg、REM或Ca、或是該等之任意組合。為了充分獲得此效果，Mg含量、REM含量及Ca含量皆宜設為0.0005%以上，更宜設為0.0010%以上。另一方面，若Mg含量、REM含量或Ca含量大於0.040%，便會形成粗大氧化物而擴孔性會降低。因此，Mg含量、REM含量及Ca含量皆是設為0.040%以下，且宜設為0.010%以下。(Mg: 0.000%~0.040%, REM: 0.000%~0.040%, Ca: 0.000%~0.040%) Magnesium (Mg), rare earth metal (REM) and calcium (Ca) are present as oxides or sulfides. In steel, and contribute to the improvement of hole expandability. Therefore, it may also contain Mg, REM or Ca, or any combination of these. In order to sufficiently obtain this effect, the Mg content, the REM content, and the Ca content are preferably set to 0.0005% or more, and more preferably set to 0.0010% or more. On the other hand, if the Mg content, the REM content, or the Ca content is more than 0.040%, a coarse oxide is formed and the hole expandability is lowered. Therefore, the Mg content, the REM content, and the Ca content are all set to 0.040% or less, and preferably set to 0.010% or less.
REM(稀土族金屬)是指Sc、Y以及鑭系元素之合計17種類之元素，「REM含量」則意指該等17種元素的合計含量。REM是以例如稀土金屬合金(misch metal)的形態來添加，稀土金屬合金除La及Ce之外還含有鑭系元素。在REM的添加上，亦可使用金屬La、金屬Ce等金屬單質。REM (rare earth metal) refers to a total of 17 types of elements of Sc, Y and lanthanoid elements, and "REM content" means the total content of these 17 elements. The REM is added in the form of, for example, a rare earth metal alloy containing a lanthanoid element in addition to La and Ce. In the addition of REM, a metal element such as metal La or metal Ce can also be used.
不純物可舉例礦石或廢料等原材料所含者、於製造步驟中所含者。具體而言，P、S、O、Sb、Sn、W、Co、As、Pb、Bi及H可作為不純物來例示。O含量宜設為0.010%以下，Sb含量、Sn含量、W含量、Co含量及As含量宜設為0.1%以下，Pb含量及Bi含量宜設為0.005%以下，H含量則宜設為0.0005%以下。The impurities may be exemplified by those contained in raw materials such as ore or scrap, and included in the manufacturing steps. Specifically, P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H can be exemplified as impurities. The O content is preferably set to 0.010% or less, and the Sb content, the Sn content, the W content, the Co content, and the As content are preferably set to 0.1% or less, the Pb content and the Bi content are preferably set to 0.005% or less, and the H content is preferably set to 0.0005%. the following.
根據本實施形態，可獲得優異撞擊安全性及成形性。可獲得例如下列機械特性：擴孔性為30%以上、最小彎曲半徑(R(mm))與板厚(t(mm))的比(R/t)為0.5以下、總延伸率為21%以上、0.2%偏位降伏強度為680MPa以上、拉伸強度為980MPa以上、延性-脆性過渡溫度為-60℃以下。尤其，當多邊形肥粒鐵的面積分率為5%~20%，且變韌肥粒鐵的面積分率為75%以上時，可獲得50%以上的擴孔性，當多邊形肥粒鐵的面積分率大於20%且在40%以下時，可獲得26%以上的總延伸率。According to this embodiment, excellent impact safety and formability can be obtained. For example, the following mechanical properties can be obtained: the hole expandability is 30% or more, the ratio of the minimum bending radius (R (mm)) to the plate thickness (t (mm)) (R/t) is 0.5 or less, and the total elongation is 21%. The above 0.2% partial drop strength is 680 MPa or more, the tensile strength is 980 MPa or more, and the ductility-brittle transition temperature is -60 ° C or lower. In particular, when the area fraction of polygonal ferrite is 5% to 20%, and the area fraction of the toughened ferrite is more than 75%, more than 50% of the hole expandability can be obtained. When the area fraction is more than 20% and below 40%, a total elongation of 26% or more can be obtained.
接下來，說明本發明實施形態的鋼板之製造方法。在本發明實施形態的鋼板之製造方法中，依序進行具有上述化學組成的鋼胚之熱軋延、酸洗、冷軋延、第1退火及第2退火。Next, a method of producing a steel sheet according to an embodiment of the present invention will be described. In the method for producing a steel sheet according to the embodiment of the present invention, hot rolling, pickling, cold rolling, first annealing, and second annealing of the steel preform having the chemical composition are sequentially performed.
(熱軋延) 在熱軋延中，是進行鋼胚的粗軋延、精整軋延及捲取。鋼胚可使用例如以連續鑄造製得之鋼胚、以薄鋼胚連鑄機製作而得之鋼胚。鋼胚可在鑄造後維持於保持在1000℃以上溫度的狀態下供應到熱軋延設備，也可以在冷卻到低於1000℃的溫度後進行加熱並供應到熱軋延設備。(Hot Rolling) In the hot rolling, the rough rolling, finishing rolling and coiling of the steel blank are performed. The steel blank can be obtained, for example, from a steel preform obtained by continuous casting and a steel preform produced by a thin steel blank continuous casting machine. The steel blank may be supplied to the hot rolling equipment while being maintained at a temperature of 1000 ° C or higher after casting, or may be heated and supplied to the hot rolling equipment after cooling to a temperature lower than 1000 ° C.
粗軋延的最終道次的軋延溫度是設為1000℃~1150℃，最終道次的軋縮率是設為40%以上。若最終道次的軋延溫度低於1000℃，精整軋延後之沃斯田鐵粒徑會過度變小。此時，從沃斯田鐵往多邊形肥粒鐵的變態會過度地受到促進，且金屬組織的均一性會降低，而無法獲得充分的成形性。因此，最終道次的軋延溫度是設為1000℃以上。另一方面，若最終道次的軋延溫度高於1150℃，精整軋延後之沃斯田鐵粒徑會過度變大。此時，金屬組織的均一性也會降低，而無法獲得充分的成形性。因此，最終道次的軋延溫度是設為1150℃以下。若最終道次的軋縮率低於40%，精整軋延後之沃斯田鐵粒徑會過度變大，且金屬組織的均一性會降低，而無法獲得充分的成形性。因此，最終道次的軋縮率是設為40%以上。The rolling pass temperature of the final pass of the rough rolling is set to 1000 ° C to 1150 ° C, and the final pass rolling reduction ratio is set to 40% or more. If the rolling temperature of the final pass is lower than 1000 ° C, the particle size of the Worstian iron after the finishing rolling will be excessively small. At this time, the metamorphosis from the Vostian iron to the polygonal ferrite iron is excessively promoted, and the uniformity of the metal structure is lowered, and sufficient formability cannot be obtained. Therefore, the rolling temperature of the final pass is set to 1000 ° C or more. On the other hand, if the rolling temperature of the final pass is higher than 1150 ° C, the particle size of the Worstian iron after finishing rolling will become excessively large. At this time, the uniformity of the metal structure is also lowered, and sufficient formability cannot be obtained. Therefore, the rolling temperature of the final pass is set to 1150 ° C or less. If the final pass reduction ratio is less than 40%, the particle size of the Worstian iron after the finish rolling is excessively large, and the uniformity of the metal structure is lowered, and sufficient formability cannot be obtained. Therefore, the final pass reduction ratio is set to 40% or more.
精整軋延之軋延溫度是設為Ar 3點以上。若此軋延溫度低於Ar 3點，會變成是於熱軋鋼板的金屬組織中含有沃斯田鐵及肥粒鐵的情況，而因在沃斯田鐵與肥粒鐵之間機械特性不同，故無法獲得充分的成形性。因此，該軋延溫度是設為Ar 3點以上。當令該軋延溫度為Ar 3點以上時，可較減輕精整軋延中的軋延荷重。在精整軋延中，亦可將接合在粗軋延製得的多數片粗軋延板而成之物連續地軋延。亦可在將粗軋延板暫時捲取後，一邊捲回一邊進行精整軋延。 The rolling temperature of the finishing rolling is set to be Ar 3 or more. If the rolling temperature is lower than the Ar 3 point, it will become a case where the metal structure of the hot-rolled steel sheet contains the Worthite iron and the ferrite iron, and the mechanical properties are different between the Worthite iron and the ferrite iron. Therefore, sufficient formability cannot be obtained. Therefore, the rolling temperature is set to be Ar 3 or more. When the rolling temperature is equal to or higher than Ar 3 point, the rolling load in the finishing rolling can be reduced. In the finishing rolling, a plurality of sheets of coarsely rolled sheets which are joined by rough rolling may be continuously rolled. It is also possible to carry out the finishing rolling after winding the rough rolled sheet temporarily.
捲取溫度是設為750℃以下。若捲取溫度高於750℃，在熱軋鋼板的組織中會生成粗大肥粒鐵或波來鐵，且金屬組織的均一性會降低，而無法獲得充分的成形性。且也會有氧化物較厚地形成在表面致使酸洗性降低的情形。因此，捲取溫度是設為750℃以下。捲取溫度的下限並未特別限定，但在較室溫更低溫下會難以進行捲取。藉由將鋼胚熱軋延可製得熱軋鋼板的卷料。The coiling temperature is set to 750 ° C or lower. If the coiling temperature is higher than 750 ° C, coarse ferrite or buck iron is formed in the structure of the hot-rolled steel sheet, and the uniformity of the metal structure is lowered, and sufficient formability cannot be obtained. There is also a case where an oxide is formed thick on the surface to cause a decrease in pickling property. Therefore, the coiling temperature is set to 750 ° C or lower. The lower limit of the coiling temperature is not particularly limited, but it may be difficult to perform winding at a lower temperature than room temperature. The coil of the hot rolled steel sheet can be obtained by hot rolling the steel blank.
(酸洗) 在熱軋延後，將熱軋鋼板的卷料捲回並進行酸洗。酸洗是進行1次或2次以上。藉由酸洗可去除熱軋鋼板表面的氧化物，而可提升化學轉化處理性及鍍敷性。(Pickling) After the hot rolling, the coil of the hot rolled steel sheet is rolled back and pickled. Pickling is carried out once or twice. By pickling, the oxide on the surface of the hot-rolled steel sheet can be removed, and chemical conversion treatability and plating property can be improved.
(冷軋延) 在酸洗後進行冷軋延。冷軋延的軋縮率是設為40%~80%。若此軋縮率低於40%，會有難以將冷軋鋼板的形狀保持平坦，且無法獲得充分的延性之情形。因此，該軋縮率是設為40%以上，且宜設為50%以上。另一方面，若該軋縮率大於80%，軋延荷重會變得過大，且肥粒鐵的再結晶會過度地受到促進，而形成粗大的多邊形肥粒鐵，導致多邊形肥粒鐵的面積分率大於40%。因此，該軋縮率是設為80%以下，且宜設為70%以下。軋延道次的次數及每道次的軋縮率並未特別限定。藉由將熱軋鋼板冷軋延可製得冷軋鋼板。(Cold rolling) Cold rolling is carried out after pickling. The rolling reduction ratio of the cold rolling is set to 40% to 80%. If the rolling reduction ratio is less than 40%, it may be difficult to keep the shape of the cold-rolled steel sheet flat and it is difficult to obtain sufficient ductility. Therefore, the rolling reduction ratio is set to 40% or more, and is preferably set to 50% or more. On the other hand, if the rolling reduction ratio is more than 80%, the rolling load will become too large, and the recrystallization of the ferrite iron will be excessively promoted, and the coarse polygonal ferrite iron will be formed, resulting in the area of the polygonal ferrite iron. The fraction is greater than 40%. Therefore, the rolling reduction ratio is set to 80% or less, and is preferably set to 70% or less. The number of rolling passes and the rolling reduction rate per pass are not particularly limited. A cold rolled steel sheet can be obtained by cold rolling a hot rolled steel sheet.
(第1退火) 在冷軋延後進行第1退火。在第1退火中，進行冷軋鋼板的第1加熱、第1冷卻、第2冷卻及第1保持。第1退火可在例如連續退火生產線上進行。(First annealing) The first annealing is performed after the cold rolling. In the first annealing, the first heating, the first cooling, the second cooling, and the first holding of the cold-rolled steel sheet are performed. The first annealing can be performed, for example, on a continuous annealing line.
第1退火的退火溫度是設為750℃~900℃。若此退火溫度低於750℃，多邊形肥粒鐵的面積分率會過多，且變韌肥粒鐵的面積分率會過少。因此，該退火溫度是設為750℃以上，且宜設為780℃以上。另一方面，若該退火溫度高於900℃，沃斯田鐵晶粒會粗大化，且從沃斯田鐵往變韌肥粒鐵或回火麻田散鐵的變態會延遲。然後，變韌肥粒鐵的面積分率會因該變態的延遲而過少。因此，該退火溫度是設為900℃以下，且宜設為870℃以下。退火時間並未特別限定，是設為例如1秒以上且1000秒以下。The annealing temperature of the first annealing is set to 750 ° C to 900 ° C. If the annealing temperature is lower than 750 ° C, the area fraction of the polygonal ferrite iron will be too much, and the area fraction of the tough ferrite iron will be too small. Therefore, the annealing temperature is set to 750 ° C or higher, and is preferably set to 780 ° C or higher. On the other hand, if the annealing temperature is higher than 900 ° C, the ferrite grains will be coarsened, and the metamorphosis from the Worth iron to the tough ferrite iron or the tempered granulated iron will be delayed. Then, the area fraction of the toughened ferrite iron is too small due to the delay of the metamorphosis. Therefore, the annealing temperature is set to 900 ° C or lower, and is preferably set to 870 ° C or lower. The annealing time is not particularly limited, and is, for example, 1 second or longer and 1000 seconds or shorter.
第1冷卻的冷卻停止溫度是設為600℃~720℃，至該冷卻停止溫度為止的冷卻速度是設為1℃/秒以上且低於10℃/秒。若第1冷卻的冷卻停止溫度低於600℃，多邊形肥粒鐵的面積分率會過多。因此，該冷卻停止溫度是設為600℃以上，且宜設為620℃以上。另一方面，若該冷卻停止溫度高於720℃，殘留沃斯田鐵的面積分率會不足。因此，該冷卻停止溫度是設為720℃以下，且宜設為700℃以下。若第1冷卻的冷卻速度低於1.0℃/秒，多邊形肥粒鐵的面積分率會過多。因此，該冷卻速度是設為1.0℃/秒以上，且宜設為3℃/秒以上。另一方面，若該冷卻速度為10℃/秒以上，殘留沃斯田鐵的面積分率會不足。因此，該冷卻速度是設為低於10℃/秒，且宜設為8℃/秒以下。The cooling stop temperature of the first cooling is set to 600 ° C to 720 ° C, and the cooling rate up to the cooling stop temperature is set to 1 ° C / sec or more and less than 10 ° C / sec. If the cooling stop temperature of the first cooling is lower than 600 ° C, the area fraction of the polygonal ferrite iron is excessive. Therefore, the cooling stop temperature is set to 600 ° C or higher, and is preferably set to 620 ° C or higher. On the other hand, if the cooling stop temperature is higher than 720 ° C, the area fraction of the remaining Worth iron will be insufficient. Therefore, the cooling stop temperature is set to 720 ° C or lower, and is preferably set to 700 ° C or lower. If the cooling rate of the first cooling is less than 1.0 ° C / sec, the area fraction of the polygonal ferrite iron is excessive. Therefore, the cooling rate is set to 1.0 ° C / sec or more, and is preferably set to 3 ° C / sec or more. On the other hand, if the cooling rate is 10 ° C /sec or more, the area fraction of the remaining Worth iron will be insufficient. Therefore, the cooling rate is set to be lower than 10 ° C / sec, and preferably set to 8 ° C / sec or less.
第2冷卻的冷卻停止溫度是設為150℃~500℃，至該冷卻停止溫度為止的冷卻速度是設為10℃/秒~60℃/秒。若第2冷卻的冷卻停止溫度低於150℃，變韌肥粒鐵或回火麻田散鐵的板條寬度會變得微細，且殘存於板條間的殘留沃斯田鐵會變成微細的薄膜狀。其結果是預定形態的殘留沃斯田鐵晶粒的面積分率會過少。因此，該冷卻停止溫度是設為150℃以上，且宜設為200℃以上。另一方面，若該冷卻停止溫度高於500℃，多邊形肥粒鐵的生成會受到促進而多邊形肥粒鐵的面積分率會過多。因此，該冷卻停止溫度是設為500℃以下，且宜設為450℃以下，更宜設為室溫程度。又，該冷卻停止溫度宜按照組成設為Ms點以下。若第2冷卻的冷卻速度低於10℃/秒，多邊形肥粒鐵的生成會受到促進而多邊形肥粒鐵的面積分率會過多。因此，該冷卻速度是設為10℃/秒以上，且宜設為20℃/秒以上。另一方面，若該冷卻速度大於60℃/秒，殘留沃斯田鐵的面積分率會低於下限。因此，該冷卻速度是設為60℃/秒以下，且宜設為50℃/秒以下。The cooling stop temperature of the second cooling is set to 150 ° C to 500 ° C, and the cooling rate up to the cooling stop temperature is set to 10 ° C / sec to 60 ° C / sec. If the cooling stop temperature of the second cooling is lower than 150 ° C, the slat width of the toughened ferrite iron or the tempered granulated iron will become fine, and the residual Worth iron remaining between the slats will become a fine film. shape. As a result, the area fraction of the residual Worstian iron crystal grains in a predetermined form is too small. Therefore, the cooling stop temperature is set to 150 ° C or higher, and is preferably set to 200 ° C or higher. On the other hand, if the cooling stop temperature is higher than 500 ° C, the generation of the polygonal ferrite iron is promoted and the area fraction of the polygonal ferrite iron is excessive. Therefore, the cooling stop temperature is set to 500 ° C or lower, and is preferably 450 ° C or lower, and more preferably set to room temperature. Further, the cooling stop temperature is preferably set to be equal to or lower than the Ms point in accordance with the composition. When the cooling rate of the second cooling is less than 10 ° C / sec, the formation of the polygonal ferrite iron is promoted and the area fraction of the polygonal ferrite iron is excessive. Therefore, the cooling rate is set to 10 ° C / sec or more, and is preferably set to 20 ° C / sec or more. On the other hand, if the cooling rate is greater than 60 ° C / sec, the area fraction of the remaining Worth iron will be lower than the lower limit. Therefore, the cooling rate is set to 60 ° C / sec or less, and is preferably set to 50 ° C / sec or less.
第1冷卻及第2冷卻的冷卻方法並未限定，可進行例如輥冷卻、氣冷或水冷、或是該等之任意組合。The cooling method of the first cooling and the second cooling is not limited, and may be, for example, roll cooling, air cooling or water cooling, or any combination thereof.
在第2冷卻後，將冷軋鋼板於150℃~500℃的溫度下僅保持以下述式(1)決定之t1秒~1000秒的時間。此保持(第1保持)在例如第2冷卻後不會降溫到低於150℃的溫度而是直接進行。在式(1)中，T0為保持溫度(℃)，T1為第2冷卻的冷卻停止溫度(℃)。 t1=20×[C]+40×[Mn]-0.1×T0+T1-0.1 (1)After the second cooling, the cold-rolled steel sheet is maintained at a temperature of 150 ° C to 500 ° C for only a period of t1 second to 1000 seconds determined by the following formula (1). This holding (first holding) is performed directly, for example, without lowering the temperature to a temperature lower than 150 ° C after the second cooling. In the formula (1), T0 is a holding temperature (°C), and T1 is a cooling stop temperature (°C) of the second cooling. T1=20×[C]+40×[Mn]-0.1×T0+T1-0.1 (1)
在第1保持的期間，C往殘留沃斯田鐵中的擴散會受到促進。其結果是殘留沃斯田鐵的穩定性會提升，而能確保殘留沃斯田鐵以面積分率計在5%以上。若保持時間小於t1，C在殘留沃斯田鐵中不會充分濃化，在之後的降溫中殘留沃斯田鐵會往麻田散鐵變態，而殘留沃斯田鐵的面積分率會過少。因此，保持時間是設為t1秒以上。若保持時間大於1000秒，殘留沃斯田鐵的分解會受到促進，而殘留沃斯田鐵的面積分率會過少。因此，保持時間是設為1000秒以下。藉由將冷軋鋼板作第1退火可製得中間鋼板。During the first holding period, the diffusion of C into the remaining Worthite iron is promoted. As a result, the stability of the residual Worthite iron is improved, and it is ensured that the residual Worth iron is more than 5% by area fraction. If the holding time is less than t1, C will not be sufficiently concentrated in the residual Worthite iron. In the subsequent cooling, the Worthite iron will be metamorphosed to the Matian iron, and the area fraction of the remaining Worth iron will be too small. Therefore, the hold time is set to t1 seconds or more. If the holding time is more than 1000 seconds, the decomposition of the residual Worth iron will be promoted, and the area fraction of the remaining Worth iron will be too small. Therefore, the hold time is set to 1000 seconds or less. An intermediate steel sheet can be obtained by subjecting the cold rolled steel sheet to the first annealing.
第1保持亦可在例如降溫到低於150℃的溫度後，再加熱至150℃~500℃的溫度而進行。若再加熱溫度低於150℃，變韌肥粒鐵或回火麻田散鐵的板條寬度會變得微細，且殘存於板條間的殘留沃斯田鐵會變成微細的薄膜狀。其結果是預定形態的殘留沃斯田鐵晶粒的面積分率會過少。因此，該再加熱溫度是設為150℃以上，且宜設為200℃以上。另一方面，若該再加熱溫度高於500℃，多邊形肥粒鐵的生成會受到促進而多邊形肥粒鐵的面積分率會過多。因此，該再加熱溫度是設為500℃以下，且宜設為450℃以下。The first holding may be carried out, for example, by lowering the temperature to a temperature lower than 150 ° C and then heating to a temperature of 150 ° C to 500 ° C. If the reheating temperature is lower than 150 ° C, the slat width of the toughened ferrite iron or the tempered granulated iron will become fine, and the residual Worth iron remaining between the slats will become a fine film. As a result, the area fraction of the residual Worstian iron crystal grains in a predetermined form is too small. Therefore, the reheating temperature is set to 150 ° C or higher, and is preferably set to 200 ° C or higher. On the other hand, if the reheating temperature is higher than 500 ° C, the generation of the polygonal ferrite iron is promoted and the area fraction of the polygonal ferrite iron is excessive. Therefore, the reheating temperature is set to 500 ° C or lower, and is preferably set to 450 ° C or lower.
中間鋼板具有例如以下所示金屬組織：以面積分率計，多邊形肥粒鐵：40%以下、變韌肥粒鐵或回火麻田散鐵或該二者：合計40%~95%、且殘留沃斯田鐵：5%~60%。又，例如以面積分率計，殘留沃斯田鐵之中的80%以上是由長寬比為0.03~1.00之殘留沃斯田鐵晶粒所構成。The intermediate steel sheet has, for example, a metal structure as follows: in terms of area fraction, polygonal ferrite iron: 40% or less, toughened ferrite iron or tempered granita iron or both: 40% to 95% in total, and residual Vostian Iron: 5%~60%. Further, for example, 80% or more of the remaining Worthite iron is composed of a residual Worstian iron crystal grain having an aspect ratio of 0.03 to 1.00.
(第2退火) 在第1退火後，進行第2退火。在第2退火中，進行中間鋼板的第2加熱、第3冷卻及第2保持。第2退火可在例如連續退火生產線上進行。藉由以下述條件進行第2退火，可使變韌肥粒鐵的差排密度降低，並提高差排密度為8×10 2(cm/cm 3)以下的預定形態之變韌肥粒鐵晶粒的面積分率。 (Second Annealing) After the first annealing, the second annealing is performed. In the second annealing, the second heating, the third cooling, and the second holding of the intermediate steel sheet are performed. The second annealing can be performed, for example, on a continuous annealing line. By performing the second annealing under the following conditions, the difference in the density of the ferro-resistant ferrite iron can be lowered, and the tough ferrite-grained iron crystal having a predetermined density of 8 × 10 2 (cm/cm 3 ) or less can be obtained. Area fraction of the grain.
第2退火的退火溫度是設為760℃~800℃。若該退火溫度低於760℃，多邊形肥粒鐵的面積分率會過多，且變韌肥粒鐵晶粒的面積分率或殘留沃斯田鐵的面積分率或該二者會過少。因此，該退火溫度是設為760℃以上，且宜設為770℃以上。另一方面，若該退火溫度高於800℃，沃斯田鐵的面積分率會隨著沃斯田鐵變態而變高，而變韌肥粒鐵的面積分率會過少。因此，該退火溫度是設為800℃以下，且宜設為790℃以下。The annealing temperature for the second annealing is set to 760 ° C to 800 ° C. If the annealing temperature is lower than 760 ° C, the area fraction of the polygonal ferrite iron may be excessive, and the area fraction of the ferro-resistant ferrite iron crystal grains or the area fraction of the residual Worth iron or both may be too small. Therefore, the annealing temperature is set to 760 ° C or higher, and is preferably set to 770 ° C or higher. On the other hand, if the annealing temperature is higher than 800 ° C, the area fraction of the Worth iron will become higher as the Worth iron deforms, and the area fraction of the tough ferrite iron will be too small. Therefore, the annealing temperature is set to 800 ° C or lower, and is preferably set to 790 ° C or lower.
第3冷卻的冷卻停止溫度是設為600℃~750℃，至該冷卻停止溫度為止的冷卻速度是設為1℃/秒~10℃/秒。若該冷卻停止溫度低於600℃，多邊形肥粒鐵的面積分率會過多。因此，該冷卻停止溫度是設為600℃以上，且宜設為630℃以上。另一方面，若該冷卻停止溫度高於750℃，麻田散鐵的面積分率會過多。因此，該冷卻停止溫度是設為750℃以下，且宜設為730℃以下。若第3冷卻的冷卻速度低於1.0℃/秒，多邊形肥粒鐵的面積分率會過多。因此，該冷卻速度是設為1.0℃/秒以上，且宜設為3℃/秒以上。另一方面，若該冷卻速度大於10℃/秒，變韌肥粒鐵的面積分率會過少。因此，該冷卻速度是設為10℃/秒以下，且宜設為8℃/秒以下。The cooling stop temperature of the third cooling is set to 600 ° C to 750 ° C, and the cooling rate up to the cooling stop temperature is set to 1 ° C / sec to 10 ° C / sec. If the cooling stop temperature is lower than 600 ° C, the area fraction of the polygonal ferrite iron may be excessive. Therefore, the cooling stop temperature is set to 600 ° C or higher, and is preferably set to 630 ° C or higher. On the other hand, if the cooling stop temperature is higher than 750 ° C, the area fraction of the granulated iron is excessive. Therefore, the cooling stop temperature is set to 750 ° C or lower, and is preferably set to 730 ° C or lower. If the cooling rate of the third cooling is less than 1.0 ° C / sec, the area fraction of the polygonal ferrite iron is excessive. Therefore, the cooling rate is set to 1.0 ° C / sec or more, and is preferably set to 3 ° C / sec or more. On the other hand, if the cooling rate is more than 10 ° C / sec, the area fraction of the tough ferrite iron will be too small. Therefore, the cooling rate is set to 10 ° C / sec or less, and is preferably set to 8 ° C / sec or less.
當重視擴孔性更甚於延性時，該冷卻停止溫度宜設為710℃以上，且更宜設為720℃以上。因為這較容易使多邊形肥粒鐵的面積分率為20%以下。當重視延性更甚於擴孔性時，該冷卻停止溫度宜設為低於710℃，且更宜設為690℃以下。因為這較容易使多邊形肥粒鐵的面積分率為大於20%且在40%以下。When the hole expansibility is more important than the ductility, the cooling stop temperature is preferably set to 710 ° C or higher, and more preferably set to 720 ° C or higher. This is because it is easier to make the area ratio of the polygonal ferrite iron to 20% or less. When the ductility is more important than the hole expandability, the cooling stop temperature is preferably set to be lower than 710 ° C, and more preferably set to 690 ° C or lower. This is because it is easier to make the area fraction of the polygonal ferrite iron more than 20% and below 40%.
在第3冷卻後，將鋼板冷卻到150℃~550℃的溫度，且保持在該溫度1秒以上。在此保持(第2保持)的期間，C往殘留沃斯田鐵中的擴散會受到促進。若保持時間小於1秒，C在殘留沃斯田鐵中不會充分濃化，而殘留沃斯田鐵的穩定性會降低，且殘留沃斯田鐵的面積分率會過少。因此，保持時間是設為1秒以上，且宜設為2秒以上。若保持溫度低於150℃，C在殘留沃斯田鐵中不會充分濃化，而殘留沃斯田鐵的穩定性會降低，且殘留沃斯田鐵的面積分率會過少。因此，保持溫度是設為150℃以上，且宜設為200℃以上。另一方面，若保持溫度高於550℃，從沃斯田鐵往變韌肥粒鐵的變態會延遲，故C往殘留沃斯田鐵中的擴散不會進展，而殘留沃斯田鐵的穩定性會降低，且殘留沃斯田鐵的面積分率會過少。因此，保持溫度是設為550℃以下，且宜設為500℃以下。After the third cooling, the steel sheet was cooled to a temperature of 150 ° C to 550 ° C and maintained at this temperature for 1 second or longer. During this period of retention (second hold), the diffusion of C into the remaining Worthite iron is promoted. If the holding time is less than 1 second, C will not be sufficiently concentrated in the residual Worth iron, and the stability of the residual Worth iron will be lowered, and the area fraction of the remaining Worth iron will be too small. Therefore, the holding time is set to 1 second or longer, and is preferably set to 2 seconds or longer. If the temperature is kept below 150 ° C, C will not be sufficiently concentrated in the residual Worth iron, and the stability of the residual Worth iron will be lowered, and the area fraction of the remaining Worth iron will be too small. Therefore, the holding temperature is set to 150 ° C or higher, and is preferably set to 200 ° C or higher. On the other hand, if the temperature is kept above 550 °C, the metamorphosis from the Worthite iron to the tough ferrite iron will be delayed, so the diffusion of C into the residual Worthite iron will not progress, and the stability of the residual Worthite iron will remain. Will decrease, and the area fraction of the remaining Worth Iron will be too small. Therefore, the holding temperature is set to 550 ° C or lower, and is preferably set to 500 ° C or lower.
如此一來，便可製造本發明實施形態之鋼板。In this way, the steel sheet according to the embodiment of the present invention can be produced.
在到此為止所述之本發明實施形態中，是藉由將第1次退火的1次冷卻速度控制在1℃/s以上且低於10℃/s，而使沃斯田鐵的一部分變態成肥粒鐵。隨著肥粒鐵的生成，Mn會往未變態的沃斯田鐵中擴散並濃化。藉由Mn在沃斯田鐵中濃化，在第2退火中的第2保持中，沃斯田鐵的降伏應力會上升，且有利於緩和隨著往變韌肥粒鐵的變態而發生之變態應力的結晶方位會優先生成。因此，可減低導入至變韌肥粒鐵內部的應變，而能將差排密度控制在8×10 2(cm/cm 3)以下。藉由將變韌肥粒鐵的差排密度控制在8×10 2(cm/cm 3)以下，便能提高塑性變形時的加工效果能力，因此可獲得優異的延性。藉由減低變韌肥粒鐵的差排密度而提升延性的機制是如以下。若TRIP鋼因加工誘發變態而從殘留沃斯田鐵生成麻田散鐵，差排會被導入到鄰接的變韌肥粒鐵而加工硬化。只要變韌肥粒鐵的差排密度低的話，在應變大的區域中仍可將加工硬化率維持得較高，因此均勻延伸率會提升。 In the embodiment of the present invention described so far, a part of the Worthite iron is metamorphosed by controlling the primary cooling rate of the first annealing to 1 ° C/s or more and less than 10 ° C/s. Become fat iron. With the formation of ferrite iron, Mn will diffuse and concentrate into the untransformed Worth Iron. By Mn is concentrated in the Worthite iron, in the second hold in the second annealing, the lodging stress of the Worthite iron rises, and it is advantageous to alleviate the metamorphic stress that occurs with the transformation of the fermented ferrite iron. The crystal orientation will be generated preferentially. Therefore, the strain introduced into the interior of the tough ferrite can be reduced, and the difference in density can be controlled to be 8 × 10 2 (cm/cm 3 ) or less. By controlling the difference density of the tough ferrite iron to 8 × 10 2 (cm/cm 3 ) or less, the processing ability at the time of plastic deformation can be improved, and thus excellent ductility can be obtained. The mechanism for increasing ductility by reducing the differential density of the toughened ferrite iron is as follows. If TRIP steel is produced by the residual Worthite iron due to processing induced metamorphism, the difference is introduced into the adjacent toughened ferrite iron and work hardened. As long as the difference in density of the tough ferrite is low, the work hardening rate can be maintained high in the region where the strain is large, so the uniform elongation is increased.
可對鋼板進行電鍍處理、蒸鍍處理等鍍敷處理，亦可進一步在鍍敷處理後進行合金化處理。且亦可對鋼板進行有機皮膜的形成、薄膜積層、有機鹽類/無機鹽類處理、無鉻處理等表面處理。The steel sheet may be subjected to a plating treatment such as a plating treatment or a vapor deposition treatment, or may be further subjected to an alloying treatment after the plating treatment. Further, the steel sheet may be subjected to surface treatment such as formation of an organic film, lamination of a film, treatment with an organic salt/inorganic salt, and treatment without a chromium.
作為鍍敷處理而對鋼板進行熔融鍍鋅處理時，是例如將鋼板的溫度加熱或冷卻到比鋅鍍敷浴的溫度低40℃的溫度以上且比鋅鍍敷浴的溫度高50℃的溫度以下之溫度後，使鋼板通過鋅鍍敷浴。藉由熔融鍍鋅處理，可製得於表面具備有熔融鍍鋅層的鋼板亦即熔融鍍鋅鋼板。熔融鍍鋅層具有例如以下所示化學組成：Fe：7質量%以上且15質量%以下、且剩餘部分：Zn、Al及不純物。When the steel sheet is subjected to a hot-dip galvanizing treatment as a plating treatment, for example, the temperature of the steel sheet is heated or cooled to a temperature lower than the temperature of the zinc plating bath by 40° C. and higher than the temperature of the zinc plating bath by 50° C. After the following temperature, the steel sheet was passed through a zinc plating bath. By hot-dip galvanizing, a steel sheet having a hot-dip galvanized layer on its surface, that is, a hot-dip galvanized steel sheet can be obtained. The hot-dip galvanized layer has, for example, the chemical composition shown below: Fe: 7 mass% or more and 15 mass% or less, and the remainder: Zn, Al, and impurities.
在熔融鍍鋅處理後進行合金化處理時，是例如將熔融鍍鋅鋼板加熱到460℃以上且600℃以下的溫度。若該溫度低於460℃，會有合金化不足的情形。若該溫度高於600℃，則會有合金化過度而使耐蝕性劣化的情形。藉由合金化處理，可製得於表面具備有合金化熔融鍍鋅層的鋼板亦即合金化熔融鍍鋅鋼板。When the alloying treatment is performed after the hot-dip galvanizing treatment, for example, the hot-dip galvanized steel sheet is heated to a temperature of 460 ° C or more and 600 ° C or less. If the temperature is lower than 460 ° C, there is a case where the alloying is insufficient. If the temperature is higher than 600 ° C, there is a case where the alloying is excessive and the corrosion resistance is deteriorated. By alloying, a steel sheet having an alloyed hot-dip galvanized layer on the surface, that is, an alloyed hot-dip galvanized steel sheet can be obtained.
再者，上述實施形態均僅是用於表示實施本發明時的具體化之例者，並非用以透過其等而限定解釋本發明之技術範圍者。亦即，本發明只要沒有脫離其技術思想或其主要特徵，即能以各種形式實施。It is to be understood that the above-described embodiments are merely illustrative of the specific embodiments of the invention, and are not intended to limit the scope of the invention. That is, the present invention can be implemented in various forms without departing from the technical idea or its main features.
實施例 接下來，說明本發明之實施例。實施例中之條件是為了確認本發明之可實施性以及效果而採用的一個條件例，本發明並不受限於此一條件例。只要能在不脫離本發明之宗旨下達成本發明之目的，本發明可採用各種條件。EXAMPLES Next, examples of the invention will be described. The conditions in the examples are a conditional example employed to confirm the workability and effects of the present invention, and the present invention is not limited to this condition example. The present invention can adopt various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention.
(第1試驗) 在第1試驗中，製造了具有表1~表3所示化學組成之鋼胚。表1~表3中之空欄表示該元素之含量低於檢測極限，且剩餘部分為Fe及不純物。表1~表3中的底線表示該數值超出本發明的範圍外。(First Test) In the first test, steel slabs having the chemical compositions shown in Tables 1 to 3 were produced. The blank columns in Tables 1 to 3 indicate that the content of the element is below the detection limit, and the remainder is Fe and impurities. The bottom line in Tables 1 to 3 indicates that the value is outside the scope of the present invention.
[表1] [Table 1]
[表2] [Table 2]
[表3] [table 3]
接著，在暫時冷卻後或不進行冷卻而直接將鋼胚加熱到1100℃~1300℃，並以表4~表7所示條件進行熱軋延而製得熱軋鋼板。之後，進行酸洗，並以表4~表7所示條件進行冷軋延而製得冷軋鋼板。表4~表7中的底線表示該數值超出適於製造本發明之鋼板的範圍外。Next, the steel slab was directly heated to 1100 ° C to 1300 ° C after the temporary cooling or without cooling, and hot rolled steel sheets were produced under the conditions shown in Tables 4 to 7 to obtain a hot rolled steel sheet. Thereafter, pickling was carried out, and cold rolling was carried out under the conditions shown in Tables 4 to 7, to obtain a cold rolled steel sheet. The bottom line in Tables 4 to 7 indicates that the value is outside the range of the steel sheet suitable for the production of the present invention.
[表4] [Table 4]
[表5] [table 5]
[表6] [Table 6]
[表7] [Table 7]
接著，以表8~表11所示條件進行冷軋鋼板的第1退火而製得中間鋼板。表8~表11中的底線表示該數值超出適於製造本發明之鋼板的範圍外。Next, the first annealing of the cold-rolled steel sheet was carried out under the conditions shown in Tables 8 to 11, to obtain an intermediate steel sheet. The bottom line in Tables 8 to 11 indicates that the value is outside the range suitable for the steel sheet of the present invention.
[表8] [Table 8]
[表9] [Table 9]
[表10] [Table 10]
[表11] [Table 11]
接著，觀察中間鋼板的金屬組織。在此觀察中，測定多邊形肥粒鐵的面積分率(PF)、變韌肥粒鐵或回火麻田散鐵的面積分率(BF-tM)及殘留沃斯田鐵的面積分率(殘留γ)，並且更從殘留沃斯田鐵的形狀算出預定形態之殘留沃斯田鐵晶粒的面積分率。於表12~表15中顯示其等之結果。表12~表15中的底線表示該數值超出適於製造本發明之鋼板的範圍外。Next, the metal structure of the intermediate steel sheet was observed. In this observation, the area fraction (PF) of the polygonal ferrite iron, the area fraction of the tough ferrite iron or the tempered granulated iron (BF-tM), and the area fraction of the residual Worth iron (residue) were determined. γ), and the area fraction of the remaining Worthite iron crystal grains in a predetermined form is calculated from the shape of the residual Worth iron. The results of these are shown in Tables 12 to 15. The bottom line in Tables 12 to 15 indicates that the value is outside the range suitable for the steel sheet of the present invention.
[表12] [Table 12]
[表13] [Table 13]
[表14] [Table 14]
[表15] [Table 15]
之後，以表16~表19所示條件進行中間鋼板的第2退火而製得鋼板試樣。在製造No.150及No.151中，於第2退火後進行了鍍敷處理，在製造No.151中，於鍍敷處理後進行了合金化處理。作為鍍敷處理是進行熔融鍍鋅處理，且合金化處理的溫度是設為500℃。表16~表19中的底線表示該數值超出適於製造本發明之鋼板的範圍外。Thereafter, the second annealing of the intermediate steel sheet was carried out under the conditions shown in Tables 16 to 19 to obtain a steel sheet sample. In the manufacturing No. 150 and No. 151, the plating treatment was performed after the second annealing, and in the manufacturing No. 151, the alloying treatment was performed after the plating treatment. The plating treatment was performed by a hot-dip galvanizing treatment, and the temperature of the alloying treatment was set to 500 °C. The bottom line in Tables 16 to 19 indicates that the value is outside the range suitable for the steel sheet of the present invention.
[表16] [Table 16]
[表17] [Table 17]
[表18] [Table 18]
[表19] [Table 19]
接下來，觀察鋼板試樣的金屬組織。在此觀察中，測定多邊形肥粒鐵的面積分率(PF)、變韌肥粒鐵的面積分率(BF)、殘留沃斯田鐵的面積分率(殘留γ)及麻田散鐵的面積分率(M)，並且更從殘留沃斯田鐵及變韌肥粒鐵的形狀算出預定形態之殘留沃斯田鐵晶粒的面積分率及預定形態之變韌肥粒鐵晶粒的面積分率。於表20~表23中顯示其等之結果。表20~表23中的底線表示該數值超出本發明的範圍外。Next, the metal structure of the steel sheet sample was observed. In this observation, the area fraction (PF) of the polygonal ferrite iron, the area fraction (BF) of the tough ferrite iron, the area fraction of the residual Worth iron (residual γ), and the area of the granulated iron The fractional rate (M), and the area fraction of the residual Worthite iron grains in a predetermined form and the area fraction of the tough ferrite grains in a predetermined form are calculated from the shape of the residual Worthite iron and the tough ferrite iron. . The results of these are shown in Tables 20 to 23. The bottom line in Tables 20 to 23 indicates that the value is outside the scope of the present invention.
[表20] [Table 20]
[表21] [Table 21]
[表22] [Table 22]
[表23] [Table 23]
接著，測定鋼板試樣的機械特性(總延伸率、0.2%偏位降伏強度、拉伸強度(最大拉伸強度)、擴孔值、彎曲半徑與板厚之比R/t以及延性-脆性過渡溫度)。總延伸率、0.2%偏位降伏強度及拉伸強度的測定，是從鋼板試樣採取以垂直於軋延方向之方向(板寬方向)為長邊方向之JIS5號試驗片，並進行以JIS Z 2242為準據的拉伸試驗。擴孔值的測定是進行JIS Z 2256的擴孔試驗。比R/t的測定是進行JIS Z 2248的試驗。延性-脆性過渡溫度的測定是進行JIS Z 2242的試驗。於表24~表27中顯示其等之結果。表24~表27中的底線表示該數值超出理想範圍外。Next, the mechanical properties (total elongation, 0.2% offset strength, tensile strength (maximum tensile strength), hole expansion value, ratio of bending radius to plate thickness R/t, and ductile-brittle transition of the steel sheet sample were measured. temperature). The JIS No. 5 test piece in which the total elongation, the 0.2% eccentricity, and the tensile strength were measured from the steel sheet sample in the direction perpendicular to the rolling direction (plate width direction) was carried out in JIS No. 5 Z 2242 is the standard tensile test. The hole expansion value was measured by performing a hole expansion test of JIS Z 2256. The measurement of the ratio R/t was carried out in the test of JIS Z 2248. The ductile-brittle transition temperature was measured by JIS Z 2242. The results of these are shown in Tables 24 to 27. The bottom line in Tables 24 to 27 indicates that the value is outside the ideal range.
[表24] [Table 24]
[表25] [Table 25]
[表26] [Table 26]
[表27] [Table 27]
如表24~表27所示，在本發明範圍內的試驗No.1及No.4等發明例，獲得了優異的延伸率、0.2%偏位降伏強度、拉伸強度、擴孔值、比R/t及延性-脆性過渡溫度。As shown in Tables 24 to 27, in the inventive examples No. 1 and No. 4 within the scope of the present invention, excellent elongation, 0.2% offset strength, tensile strength, hole expansion value, and ratio were obtained. R/t and ductile-brittle transition temperatures.
另一方面，在製造No.2及No.3等比較例中，延伸率、擴孔值及比R/t較低，前述製造No.2及No.3等是多邊形肥粒鐵的面積分率過多、變韌肥粒鐵的面積分率不足、殘留沃斯田鐵的面積分率不足，且預定形態的殘留沃斯田鐵晶粒的比率不足、預定形態的變韌肥粒鐵晶粒的比率不足。在製造No.5及No.6等比較例中，延伸率、擴孔值及比R/t較低，前述製造No.5及No.6等是變韌肥粒鐵的面積分率不足、麻田散鐵的面積分率過多，且預定形態的殘留沃斯田鐵晶粒的比率不足、預定形態的變韌肥粒鐵晶粒的比率不足。在製造No.30及No.37等比較例中，延伸率較低，前述製造No.30及No.37等是預定形態的殘留沃斯田鐵晶粒的比率不足。在製造No.70及No.85等比較例中，延伸率、擴孔值及比R/t較低，前述製造No.70及No.85等是變韌肥粒鐵的面積分率不足、麻田散鐵的面積分率過多，且預定形態的殘留沃斯田鐵晶粒的比率不足、預定形態的變韌肥粒鐵晶粒的比率不足。On the other hand, in the comparative examples such as No. 2 and No. 3, the elongation, the hole expansion value, and the ratio R/t were low, and the above-mentioned production No. 2 and No. 3 were the areas of the polygonal ferrite iron. The rate is too large, the area fraction of the toughened ferrite iron is insufficient, the area fraction of the residual Worth iron is insufficient, and the ratio of the residual Worthite iron grains in the predetermined form is insufficient, and the ratio of the tough ferrite grains in the predetermined form is determined. insufficient. In Comparative Examples such as No. 5 and No. 6, the elongation, the hole expansion value, and the ratio R/t were low, and the production No. 5 and No. 6 were insufficient in the area ratio of the tough ferrite iron. The area fraction of the granulated iron is too large, and the ratio of the residual Worthite iron crystal grains in the predetermined form is insufficient, and the ratio of the tough ferrite iron crystal grains in the predetermined form is insufficient. In Comparative Examples such as No. 30 and No. 37, the elongation was low, and the ratios of the remaining Worstian iron crystal grains in the predetermined form, No. 30 and No. 37, were insufficient. In Comparative Examples such as No. 70 and No. 85, the elongation, the hole expansion value, and the ratio R/t were low, and the production No. 70 and No. 85 were insufficient in the area ratio of the tough ferrite iron. The area fraction of the granulated iron is too large, and the ratio of the residual Worthite iron crystal grains in the predetermined form is insufficient, and the ratio of the tough ferrite iron crystal grains in the predetermined form is insufficient.
產業上之可利用性 本發明可利用於例如與適用於汽車零件的鋼板相關之產業。Industrial Applicability The present invention can be utilized, for example, in an industry related to steel sheets suitable for automobile parts.
1‧‧‧殘留沃斯田鐵晶粒1‧‧‧Residual Vostian iron grain
2‧‧‧等值橢圓 2‧‧‧ equivalent ellipse
L1‧‧‧長軸長度 L1‧‧‧Long shaft length
L2‧‧‧短軸長度 L2‧‧‧ short shaft length
圖1是顯示殘留沃斯田鐵晶粒的等值橢圓(equivalent ellipse)之例的圖。Fig. 1 is a view showing an example of an equivalent ellipse of residual Worthite iron crystal grains.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2013227654A (en) * | 2012-03-29 | 2013-11-07 | Kobe Steel Ltd | High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet and high-strength hot-dip galvannealed steel sheet, having excellent formability and shape fixability, and process for manufacturing same |
| JP2016180140A (en) * | 2015-03-23 | 2016-10-13 | 株式会社神戸製鋼所 | High-strength steel sheet with excellent formability |
| TW201641708A (en) * | 2015-02-24 | 2016-12-01 | 新日鐵住金股份有限公司 | Cold rolled steel sheet and manufacturing method thereof |
| TW201708563A (en) * | 2015-08-24 | 2017-03-01 | Nippon Steel & Sumitomo Metal Corp | Steel plate particularly having excellent impact-resistant characteristic for automobiles |
| CN107250409A (en) * | 2015-02-17 | 2017-10-13 | 杰富意钢铁株式会社 | High strength cold-rolled sheet metal and its manufacture method |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2013227654A (en) * | 2012-03-29 | 2013-11-07 | Kobe Steel Ltd | High-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet and high-strength hot-dip galvannealed steel sheet, having excellent formability and shape fixability, and process for manufacturing same |
| CN107250409A (en) * | 2015-02-17 | 2017-10-13 | 杰富意钢铁株式会社 | High strength cold-rolled sheet metal and its manufacture method |
| TW201641708A (en) * | 2015-02-24 | 2016-12-01 | 新日鐵住金股份有限公司 | Cold rolled steel sheet and manufacturing method thereof |
| JP2016180140A (en) * | 2015-03-23 | 2016-10-13 | 株式会社神戸製鋼所 | High-strength steel sheet with excellent formability |
| TW201708563A (en) * | 2015-08-24 | 2017-03-01 | Nippon Steel & Sumitomo Metal Corp | Steel plate particularly having excellent impact-resistant characteristic for automobiles |
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