JP6275510B2 - High strength hot rolled steel sheet and method for producing the same - Google Patents
High strength hot rolled steel sheet and method for producing the same Download PDFInfo
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Description
本発明は、自動車用部材の使途に有用な、引張強さ(TS):780MPa以上の高強度と優れた表面性状および加工性(特にせん断加工性、打抜性)を兼ね備えた高強度熱延鋼板およびその製造方法に関する。 The present invention is a high-strength hot-rolling that has a high tensile strength (TS) of 780 MPa or more and excellent surface properties and workability (especially shear workability and punchability) useful for the use of automobile parts. It is related with a steel plate and its manufacturing method.
近年地球環境保全の観点から、CO2排出量の規制を目的として自動車業界全体で自動車の燃費改善が指向されている。自動車の燃費改善には、使用部品の薄肉化による自動車の軽量化が最も有効であるため、近年、自動車部品用素材としての高強度熱延鋼板の使用量が増加しつつある。 In recent years, from the viewpoint of global environmental conservation, the automobile industry as a whole has been aimed at improving the fuel efficiency of automobiles for the purpose of regulating CO 2 emissions. In order to improve the fuel efficiency of automobiles, it is most effective to reduce the weight of automobiles by reducing the thickness of parts used. In recent years, the amount of high-strength hot-rolled steel sheets used as materials for automobile parts is increasing.
ここで、鋼板を素材とする自動車部品の多くは、プレス加工やバーリング加工等によって成形される。したがって、自動車部品用鋼板には、高強度に加えて優れた加工性(打抜性や伸びフランジ性など)を有することも要求される。しかしながら、一般的に鉄鋼材料は、高強度化に伴い延性が低下して加工性が劣化する。そのため、引張強さを780MPa以上にまで高強度化した鋼板では、所望の部品形状に成形加工する際、様々な支障をきたす。例えば、引張強さ:780MPa以上の鋼板に打ち抜き加工を施すと、打ち抜き穴端面において亀裂や段差、めくれ、はがれ等の発生が顕著となり、部品の疲労特性や寸法精度が低下する。また、このような打ち抜き穴端面性状の劣化は、伸びフランジ性にも悪影響を及ぼす。 Here, many automobile parts made of steel plates are formed by pressing, burring, or the like. Therefore, the steel sheet for automobile parts is required to have excellent workability (such as punchability and stretch flangeability) in addition to high strength. However, in general, with steel materials, ductility decreases and workability deteriorates with increasing strength. Therefore, a steel sheet with a tensile strength of 780 MPa or more has various problems when it is formed into a desired part shape. For example, when a punching process is performed on a steel sheet having a tensile strength of 780 MPa or more, cracks, steps, turning, peeling, and the like are prominent on the end face of the punched hole, and the fatigue characteristics and dimensional accuracy of the component are reduced. In addition, such deterioration of the punch hole end face properties adversely affects stretch flangeability.
一方、自動車部品の多くは厳しい腐食環境下で使用されるが、鋼板を素材とする自動車部品の場合、部品の薄肉化により腐食代が少なくなる。そのため、自動車部品用鋼板は、所望の強度と耐食性を兼備することも重要となる。耐食性の付与には、鋼板にめっき処理を施して鋼板表面にめっき層を備えたり、めっき処理後に化成処理を施し、塗装することが有効である。しかしながら、引張強さ:780MPa以上の鋼板を製造するに際しては、多くの場合、所望の鋼板強度を確保する目的でSi、Mn等の固溶強化元素を多量に含有させている。このようにSi等を多量に含有させた鋼板では、鋼板表面にSi等が濃化して濡れ性が低下するため、めっき層を形成することや化成処理が困難となる。 On the other hand, many automobile parts are used in a severe corrosive environment. However, in the case of an automobile part made of a steel plate, the corrosion allowance is reduced by thinning the part. Therefore, it is also important that the steel sheet for automobile parts has desired strength and corrosion resistance. For imparting corrosion resistance, it is effective to apply a plating treatment to the steel sheet to provide a plating layer on the surface of the steel sheet, or to apply a chemical conversion treatment after the plating treatment. However, when producing a steel sheet having a tensile strength of 780 MPa or more, in many cases, a solid solution strengthening element such as Si or Mn is contained in a large amount for the purpose of securing a desired steel sheet strength. In such a steel sheet containing a large amount of Si or the like, Si or the like is concentrated on the surface of the steel sheet and wettability is lowered, so that it is difficult to form a plating layer or chemical conversion treatment.
以上の理由により、高強度熱延鋼板を自動車部品等に適用するうえでは、Si等の表面濃化を抑制した良好な表面性状と、優れた打抜性を兼ね備えた高強度熱延鋼板の開発が必須となり、現在までに様々な技術が提案されている。 For the above reasons, when applying high-strength hot-rolled steel sheets to automotive parts, etc., development of high-strength hot-rolled steel sheets that combine excellent surface properties with suppressed surface concentration such as Si and excellent punchability. Has become essential, and various technologies have been proposed to date.
例えば、特許文献1では、鋼板組成を質量%で、C:0.010〜0.200%、Si:0.01〜1.50%、Mn:0.25〜3.00%、B:0.0002〜0.0030%をそれぞれ含有し、P:0.05%以下に制限し、更に、Ti:0.03〜0.20%、Nb:0.01〜0.20%、V:0.01〜0.20%、Mo:0.01〜0.20%のうちの何れか1種又は2種以上を含有し、残部がFe及び不可避的不純物からなる組成とし、フェライトの大角結晶粒界へのCの偏析量とBの偏析量との合計を4〜10atms/nm2の範囲とする技術が提案されている。そして、特許文献1で提案された技術によると、析出強化元素であるTi、V、Nb、Moを添加して炭化物等を析出させることで鋼板強度を引張強さ:690MPa以上とし、しかも、Bおよび Cの粒界偏析量を制御することで極めて厳しい条件で打ち抜き加工を行った場合でも確実に端面の損傷を防止することができるとされている。 For example, in Patent Document 1, the steel sheet composition is mass%, C: 0.010 to 0.200%, Si: 0.01 to 1.50%, Mn: 0.25 to 3.00%, B: 0.0002 to 0.0030%, and P: 0.05% It is limited to the following, and further contains any one or more of Ti: 0.03 to 0.20%, Nb: 0.01 to 0.20%, V: 0.01 to 0.20%, Mo: 0.01 to 0.20%, and the balance Has been proposed that has a composition comprising Fe and unavoidable impurities, and the sum of the amount of C segregated and the amount of B segregated to the large-angle grain boundaries of ferrite ranges from 4 to 10 atms / nm 2 . According to the technique proposed in Patent Document 1, the steel sheet strength is increased to 690 MPa or more by adding carbides such as precipitation strengthening elements Ti, V, Nb, and Mo, and B It is said that by controlling the grain boundary segregation amount of C and C, damage to the end face can be surely prevented even when punching is performed under extremely severe conditions.
また、特許文献2では、鋼板組成を質量%で、C:0.015〜0.06%、Si:0.5%未満、Mn:0.1〜2.5%、P≦0.10%、S≦0.01%、Al:0.005〜0.3%、N≦0.01%、Ti:0.01〜0.30%、B:2〜50ppmを含有し、残部Fe及び不可避的不純物からなる組成とし、更に炭化物生成元素とCとの原子比を特定するとともに、鋼のγ/α変態温度を制御する元素であるSi、Mn、B、Moの含有量が所定の関係を満足するように規定し、フェライトとベイニティックフェライトの一方又は双方の面積率の合計が90%以上でありセメンタイトの面積率が5%以下である鋼板組織とする技術が提案されている。そして、特許文献2で提案された技術によると、伸びフランジ成形性、耐打ち抜き割れ性及び表面状態の全てが良好であり、引張強度が690MPa以上という高強度の熱延鋼板を安価に、安定して製造することができるとされている。 Moreover, in patent document 2, a steel plate composition is the mass%, C: 0.015-0.06%, Si: Less than 0.5%, Mn: 0.1-2.5%, P <= 0.10%, S <= 0.01%, Al: 0.005-0.3% , N ≦ 0.01%, Ti: 0.01 to 0.30%, B: 2 to 50 ppm, the balance consisting of Fe and unavoidable impurities, and further specifying the atomic ratio of carbide-forming elements and C, The content of Si, Mn, B, and Mo, which are elements that control the γ / α transformation temperature, is specified so as to satisfy a predetermined relationship, and the total area ratio of one or both of ferrite and bainitic ferrite is 90 %, And a cementite structure with a cementite area ratio of 5% or less has been proposed. According to the technique proposed in Patent Document 2, a high-strength hot-rolled steel sheet having excellent stretch flangeability, punching cracking resistance and surface condition, and a tensile strength of 690 MPa or more can be stably produced at low cost. It can be manufactured.
更に、特許文献3では、鋼板組成を質量%で、C:0.01〜0.1%、Si:0.01〜2%、Mn:0.05〜3%、P≦0.1%、S≦0.03%、Al:0.005〜2.0%、N≦0.01%、B:0.0005〜0.0030%を含み、さらにTi−(48/12)C−(48/14)N−(48/32)S≧0.03%を満たす範囲でTiを含有し、残部がFe及び不可避的不純物からなる組成とし、その鋼板の硬さのばらつきをその平均値で除した値を0.2以下とし、かつ、圧延方向の{110}面の面強度を1.7以下に規定する技術が提案されている。そして、特許文献3で提案された技術によると、軟質な相の生成を抑制し、硬さを均一化したうえで、集合組織の特定方向への配向を制御することで、打ち抜き穴広げ性に優れた高強度鋼板が得られるとされている。 Furthermore, in Patent Document 3, the steel sheet composition is in mass%, C: 0.01 to 0.1%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦ 0.03%, Al: 0.005 to 2.0. %, N ≦ 0.01%, B: 0.0005 to 0.0030%, and further contains Ti in a range satisfying Ti− (48/12) C− (48/14) N− (48/32) S ≧ 0.03% The balance is composed of Fe and inevitable impurities, the value obtained by dividing the variation in hardness of the steel sheet by the average value is 0.2 or less, and the surface strength of the {110} plane in the rolling direction is 1.7 or less Techniques to do this have been proposed. And according to the technique proposed in Patent Document 3, the formation of a soft phase is suppressed, the hardness is made uniform, and the orientation of the texture in a specific direction is controlled, so that the punching hole expandability can be achieved. It is said that an excellent high-strength steel sheet can be obtained.
しかしながら、特許文献1で提案された技術では、所定の粒界偏析量を有する熱延鋼板を製造するために、熱間圧延終了後、フェライト相が成長し易い600〜650℃の範囲内に急冷し、更に10℃/s以下の冷却速度で350〜600℃の範囲まで冷却して巻き取る必要がある。このような方法にしたがい製造された鋼板では、硬度が異なるフェライト相とベイナイト相が混在した組織となるため、せん断や打ち抜き時にフェライト相とベイナイト相との界面に不均一な応力集中が生じ、せん断および打ち抜き端面に割れや荒れ等の異常部が発生する。したがって、特許文献1で提案された技術では、安定して良好な打抜性が得られない。 However, in the technique proposed in Patent Document 1, in order to manufacture a hot-rolled steel sheet having a predetermined grain boundary segregation amount, after the hot rolling is completed, the steel is rapidly cooled within a range of 600 to 650 ° C. in which a ferrite phase is likely to grow. Furthermore, it is necessary to cool and wind up to a range of 350 to 600 ° C. at a cooling rate of 10 ° C./s or less. A steel sheet manufactured according to such a method has a structure in which a ferrite phase and a bainite phase having different hardnesses are mixed, so that uneven stress concentration occurs at the interface between the ferrite phase and the bainite phase during shearing or punching. In addition, abnormal portions such as cracks and roughness are generated on the punched end face. Therefore, the technique proposed in Patent Document 1 cannot stably provide good punchability.
また、特許文献2で提案された技術では、延性に富むフェライトとベイニティックフェライトを主たる金属組織としているため、鋼板の打ち抜き加工時、ポンチもしくはダイスから発生した亀裂の成長が遅く、かえり量が大きくなり、亀裂伝播途中での打ち抜き端面近傍の曲げモーメントが大きくなる。そのため、打ち抜き端面に、突発的に割れ等の不具合が生じる問題がある。 Further, in the technique proposed in Patent Document 2, since the ductile ferrite and bainitic ferrite are the main metal structures, the growth of cracks generated from punches or dies when steel sheets are punched is slow, and the amount of burr is reduced. The bending moment increases in the vicinity of the punched end face during crack propagation. Therefore, there is a problem that a defect such as a crack is suddenly generated on the punched end face.
特許文献3で提案された技術では、その実施例が示すように鋼板の高強度化を図る目的でSiを多量に添加しているため、良好な表面性状を有する鋼板が得られない。なお、Si含有量が0.02%であり且つ引張強さ:795MPaである高強度鋼板も開示されているが、この高強度鋼板は鋼板強度を確保する目的でAlを多量に含有しているため、粗大かつ不均一に分布するAl窒化物やAl酸化物の影響により、良好な打抜性が得られない。 In the technique proposed in Patent Document 3, since a large amount of Si is added for the purpose of increasing the strength of the steel sheet as shown in the examples, a steel sheet having good surface properties cannot be obtained. Although a high-strength steel sheet having a Si content of 0.02% and a tensile strength of 795 MPa is also disclosed, this high-strength steel sheet contains a large amount of Al for the purpose of securing the steel sheet strength. Good punchability cannot be obtained due to the influence of coarse and unevenly distributed Al nitride and Al oxide.
以上のように、従来技術ではいずれも、熱延鋼板の強度を引張強さ780MPa以上とした場合、安定して良好な打抜性と良好な表面性状をも兼ね備えた高強度熱延鋼板は得られなかった。
本発明はかかる事情に鑑みてなされたものであって、780MPa以上の引張強さを有し、良好な表面性状を有し、打抜性にも優れた高強度熱延鋼板およびその製造方法を提供することを目的とする。
As described above, in all of the conventional techniques, when the strength of the hot-rolled steel sheet is set to a tensile strength of 780 MPa or more, a high-strength hot-rolled steel sheet having both stable and good punchability and good surface properties is obtained. I couldn't.
The present invention has been made in view of such circumstances, and has a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, good surface properties, and excellent punchability, and a method for producing the same. The purpose is to provide.
先述のとおり、Siは表面濃化することにより鋼板の表面性状の劣化を引き起こす元素である。したがって、表面性状の観点からは、熱延鋼板のSi含有量を低減することが好ましい。しかしながら、Siは固溶強化元素および焼入性元素として鋼板を高強度化する効果を有するため、Si含有量を低減すると、熱延鋼板の強度が低下する。 As described above, Si is an element that causes the surface properties of the steel sheet to deteriorate due to surface concentration. Therefore, from the viewpoint of surface properties, it is preferable to reduce the Si content of the hot-rolled steel sheet. However, since Si has an effect of increasing the strength of the steel sheet as a solid solution strengthening element and a hardenability element, when the Si content is reduced, the strength of the hot-rolled steel sheet decreases.
そこで、本発明者らは先ず、表面性状を良好にする目的で熱延鋼板のSi含有量を抑制しつつ、熱延鋼板の高強度化を図る手段について検討した。その結果、熱延鋼板組成に関し、Siを多量に含有する従来技術に代えて、焼入性の効果と粒子分散強化により高強度化に寄与するTiを多量に含有して熱延鋼板中に微細な炭化物として析出させるとともに、炭化物の析出量を増加させる目的でVを適量添加することに想到した。 Therefore, the present inventors first studied means for increasing the strength of the hot-rolled steel sheet while suppressing the Si content of the hot-rolled steel sheet for the purpose of improving the surface properties. As a result, regarding the hot-rolled steel sheet composition, instead of the conventional technology containing a large amount of Si, the hot-rolled steel sheet contains a large amount of Ti that contributes to high strength by the effect of hardenability and particle dispersion strengthening. It was conceived that an appropriate amount of V was added for the purpose of precipitating as a carbide and increasing the amount of carbide precipitation.
次に、本発明者らは、熱延鋼板の打抜性を高める手段について検討した。そして、鋼板の打ち抜き加工時の亀裂成長挙動を詳細に解析した結果、ポンチもしくはダイスから発生した亀裂を効果的に成長、合体させるには、鋼板にセメンタイトを均一に分散させることが重要であることが明らかになった。 Next, the present inventors examined means for improving the punchability of the hot-rolled steel sheet. And as a result of detailed analysis of crack growth behavior during punching of steel sheet, it is important to uniformly disperse cementite in the steel sheet in order to effectively grow and coalesce cracks generated from punches or dies. Became clear.
これらの事項を踏まえ、本発明者らは、熱延鋼板中にTiおよびVを微細な炭化物として析出させるとともに、微細なセメンタイトも均一に分散させる手段を模索した。その結果、熱延鋼板の金属組織をベイナイトにすることが最も有効な手段であることを知見した。 Based on these matters, the present inventors sought a means for precipitating Ti and V as fine carbides in the hot-rolled steel sheet and uniformly dispersing fine cementite. As a result, it has been found that the most effective means is to use bainite as the metal structure of the hot-rolled steel sheet.
熱延鋼板を製造する際、熱間圧延終了後の鋼板を急冷してベイナイト変態させると、セメンタイトの核生成サイトとなる転位密度が上昇するため、熱延鋼板中に微細なセメンタイトが均一に分散する。
また、TiやVの炭化物は、熱間圧延終了後の巻取り工程において微細に析出する。これらの炭化物の最も析出し易い温度域は、フェライト変態が生じる温度域、すなわちベイナイト変態が生じる温度域よりも高い温度域である。しかし、ベイナイト変態により、鋼板中に炭化物の核生成サイトとなる転位が導入されるため、TiやVの炭化物はベイナイト変態が生じる温度域においても微細な状態で析出する。
When manufacturing hot-rolled steel sheets, if the steel sheet after hot rolling is quenched and bainite transformed, the dislocation density, which is the nucleation site of cementite, increases, so fine cementite is uniformly dispersed in the hot-rolled steel sheet. To do.
Further, Ti and V carbides are finely precipitated in the winding process after the hot rolling is completed. The temperature range in which these carbides are most likely to precipitate is a temperature range where ferrite transformation occurs, that is, a temperature range higher than the temperature range where bainite transformation occurs. However, since bainite transformation introduces dislocations that become carbide nucleation sites in the steel sheet, Ti and V carbides precipitate in a fine state even in the temperature range where bainite transformation occurs.
以上のように、熱間圧延終了後の巻取り工程において、ベイナイト相が成長する温度域は、TiやVの炭化物の析出と、微細なセメンタイトの析出が競合する温度域となる。したがって、熱延鋼板を製造する際、熱間圧延終了後の鋼板を急冷してベイナイト変態させると、熱延鋼板中に微細なセメンタイトを均一に分散させると同時に、TiおよびVを微細な炭化物として析出させることができる。 As described above, in the winding process after the end of hot rolling, the temperature range in which the bainite phase grows is a temperature range in which precipitation of Ti and V carbides competes with precipitation of fine cementite. Therefore, when manufacturing a hot-rolled steel sheet, if the steel sheet after hot rolling is quenched and bainite transformed, fine cementite is uniformly dispersed in the hot-rolled steel sheet, and at the same time Ti and V are converted into fine carbides. It can be deposited.
そして、本発明者らは、更に検討を進めた結果、鋼素材のSi含有量を低減し、更にC含有量およびSi、Ti、V含有量の適正化を図ったうえで、熱間圧延終了後の冷却過程においてTiやVを含む炭化物が最も析出する550℃を超える温度で保持することを避け、550℃以下の温度まで急冷してベイナイト変態させることにより、微細なセメンタイトが均一に分散し、且つ、引張強さ780MPa以上の強度を達成するうえで必要となる微細な炭化物(Ti、V炭化物)も析出し、しかもSi表面濃化が抑制された熱延鋼板が得られることを知見した。 And, as a result of further investigation, the present inventors reduced the Si content of the steel material, and further optimized the C content and Si, Ti, V content, and then finished hot rolling. In the subsequent cooling process, avoid holding at a temperature exceeding 550 ° C where carbides containing Ti and V are most precipitated, and quenching to a temperature below 550 ° C and transforming to bainite, fine cementite is uniformly dispersed. In addition, it was found that fine carbides (Ti, V carbides) necessary for achieving a tensile strength of 780 MPa or more are precipitated, and that a hot rolled steel sheet with reduced Si surface concentration can be obtained. .
本発明は上記の知見に基づき完成されたものであり、その要旨は次のとおりである。
[1] 質量%で、C:0.06%以上0.13%以下、Si:0.09%以下、Mn:0.01%以上1.20%以下、P:0.03%以下、S:0.005%以下、Al:0.1%以下、N:0.01%以下、Ti:0.13%以上0.20%以下、V:0.03%以上0.13%以下を含有し、Nb:0.02%以下(0を含む)に制限し、残部がFeおよび不可避的不純物からなる組成と、ベイナイト相の面積率が80%以上、フェライト相の面積率が15%以下(0を含む)、マルテンサイト相の面積率が5%以下(0を含む)、セメンタイトの析出量が質量%で0.08%以上、該セメンタイトの平均粒子径が2μm以下であり、前記ベイナイト相の結晶粒内に平均粒子径が10nm未満である炭化物が微細分散した組織を有し、鋼板表面から0.2μmまでの深さにおけるSi濃化量が地鉄でのSi固溶量の1.3倍以下であり、引張強さが780MPa以上であることを特徴とする高強度熱延鋼板。
The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.06% to 0.13%, Si: 0.09% or less, Mn: 0.01% to 1.20%, P: 0.03% or less, S: 0.005% or less, Al: 0.1% or less, N : 0.01% or less, Ti: 0.13% or more and 0.20% or less, V: 0.03% or more and 0.13% or less, Nb: 0.02% or less (including 0), the balance consisting of Fe and inevitable impurities The area ratio of the bainite phase is 80% or more, the area ratio of the ferrite phase is 15% or less (including 0), the area ratio of the martensite phase is 5% or less (including 0), and the precipitation amount of cementite is mass%. 0.08% or more, the average particle diameter of the cementite is 2 μm or less, and has a microstructure in which carbides having an average particle diameter of less than 10 nm are finely dispersed in the crystal grains of the bainite phase, and from the steel sheet surface to 0.2 μm High strength, characterized in that the Si concentration at the depth is less than 1.3 times the amount of Si solid solution in the base iron, and the tensile strength is 780 MPa or more Hot-rolled steel sheet.
[2] 前記[1]において、前記組成に加えてさらに、質量%で、B:0.0002%以上0.0030%以下を含有することを特徴とする高強度熱延鋼板。 [2] The high-strength hot-rolled steel sheet according to [1], further including, in addition to the composition, B: 0.0002% to 0.0030% by mass.
[3] 前記[1]または[2]において、前記組成に加えてさらに、質量%で、REM、Cu、Ni、Sn、Cr、Sb、Mg、Ca、Coのいずれか1種以上を合計で0.1929%以下含有することを特徴とする高強度熱延鋼板。 [3] In the above [1] or [2], in addition to the composition, in addition to the composition, any one or more of REM, Cu, Ni, Sn, Cr, Sb, Mg, Ca, and Co in total A high-strength hot-rolled steel sheet characterized by containing 0.1929 % or less.
[4] 前記[1]ないし[3]のいずれかにおいて、鋼板表面にめっき層を有することを特徴とする高強度熱延鋼板。 [4] The high-strength hot-rolled steel sheet according to any one of [1] to [3], wherein the steel sheet surface has a plating layer.
[5] 前記[4]において、前記めっき層が、亜鉛めっき層であることを特徴とする高強度熱延鋼板。 [5] The high-strength hot-rolled steel sheet according to [4], wherein the plated layer is a galvanized layer.
[6] 前記[4]において、前記めっき層が、合金化亜鉛めっき層であることを特徴とする高強度熱延鋼板。 [6] The high-strength hot-rolled steel sheet according to [4], wherein the plated layer is an alloyed galvanized layer.
[7] 鋼素材を、加熱し、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻き取り、熱延鋼板とするにあたり、前記鋼素材を、質量%で、C:0.06%以上0.13%以下、Si:0.09%以下、Mn:0.01%以上1.20%以下、P:0.03%以下、S:0.005%以下、Al:0.1%以下、N:0.01%以下、Ti:0.13%以上0.20%以下、V:0.03%以上0.13%以下を含有し、Nb:0.02%以下(0を含む)に制限し、残部がFeおよび不可避的不純物からなる組成とし、前記加熱の加熱温度を1150℃以上1350℃以下とし、前記仕上げ圧延の仕上げ圧延終了温度を850℃以上とし、前記冷却を仕上げ圧延終了後3s以内に開始し、前記冷却の冷却速度を40℃/s以上とし、前記巻き取りの巻取り温度を350℃以上550℃以下とすることを特徴とするベイナイト相の面積率が80%以上、フェライト相の面積率が15%以下(0を含む)、マルテンサイト相の面積率が5%以下(0を含む)、セメンタイトの析出量が質量%で0.08%以上、該セメンタイトの平均粒子径が2μm以下であり、前記ベイナイト相の結晶粒内に平均粒子径が10nm未満である炭化物が微細分散した組織を有し、鋼板表面から0.2μmまでの深さにおけるSi濃化量が地鉄でのSi固溶量の1.3倍以下であり、引張強さが780MPa以上である高強度熱延鋼板の製造方法。 [7] A steel material is heated, subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling, cooled, wound, and made into a hot-rolled steel sheet, C: 0.06% to 0.13%, Si: 0.09% or less, Mn: 0.01% to 1.20%, P: 0.03% or less, S: 0.005% or less, Al: 0.1% or less, N: 0.01% or less, Ti: Containing 0.13% or more and 0.20% or less, V: 0.03% or more and 0.13% or less, Nb: 0.02% or less (including 0), with the balance consisting of Fe and inevitable impurities, heating temperature of the heating 1150 ° C. or more and 1350 ° C. or less, the finish rolling finish temperature of the finish rolling is 850 ° C. or more, the cooling is started within 3 s after finishing rolling, the cooling rate of cooling is 40 ° C./s or more, The area ratio of the bainite phase is 80% or more, characterized in that the winding temperature of winding is 350 ° C or higher and 550 ° C or lower. The area ratio of the cementite is 15% or less (including 0), the area ratio of the martensite phase is 5% or less (including 0), the precipitation amount of cementite is 0.08% or more by mass%, and the average particle diameter of the cementite is 2 μm or less, and having a structure in which carbides having an average particle diameter of less than 10 nm are finely dispersed in the bainite phase grains, and the Si concentration in the depth from the steel sheet surface to 0.2 μm is A method for producing a high-strength hot-rolled steel sheet that is 1.3 times or less the Si solid solution and has a tensile strength of 780 MPa or more .
[8] 前記[7]において、前記組成に加えてさらに、質量%で、B:0.0002%以上0.0030%以下を含有することを特徴とする高強度熱延鋼板の製造方法。 [8] The method for producing a high-strength hot-rolled steel sheet according to [7], further containing B: 0.0002% to 0.0030% by mass% in addition to the composition.
[9] 前記[7]または[8]において、前記組成に加えてさらに、質量%で、REM、Cu、Ni、Sn、Cr、Sb、Mg、Ca、Coのいずれか1種以上を合計で0.1929%以下含有することを特徴とする高強度熱延鋼板の製造方法。 [9] In the above [7] or [8], in addition to the composition, in addition to the composition, any one or more of REM, Cu, Ni, Sn, Cr, Sb, Mg, Ca, and Co in total A method for producing a high-strength hot-rolled steel sheet, comprising 0.1929 % or less.
[10] 前記[7]ないし[9]のいずれかにおいて、鋼板表面にめっき層を形成することを特徴とする高強度熱延鋼板の製造方法。 [10] The method for producing a high-strength hot-rolled steel sheet according to any one of [7] to [9], wherein a plating layer is formed on the steel sheet surface.
[11] 前記[10]において、前記めっき層が、亜鉛めっき層であることを特徴とする高強度熱延鋼板の製造方法。 [11] The method for producing a high-strength hot-rolled steel sheet according to [10], wherein the plated layer is a galvanized layer.
[12] 前記[10]において、前記めっき層が、合金化亜鉛めっき層であることを特徴とする高強度熱延鋼板の製造方法。 [12] The method for producing a high-strength hot-rolled steel sheet according to [10], wherein the plating layer is an galvannealed layer.
本発明によると、自動車の構造部材等の使途に好適な、引張強さ:780MPa以上であり且つ表面性状、打抜性に優れた高強度熱延鋼板が得られ、自動車部品の軽量化や自動車部品成形を可能とする等、その効果は著しい。また、高強度熱延鋼板の更なる用途展開が可能となり、産業上格段の効果を奏する。 According to the present invention, a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent surface properties and punching properties suitable for the use of structural members of automobiles can be obtained. The effect is remarkable, such as enabling molding of parts. In addition, further application development of the high-strength hot-rolled steel sheet is possible, and there are remarkable industrial effects.
以下、本発明について詳細に説明する。
まず、本発明熱延鋼板の組織および鋼板表面Si濃化量の限定理由について説明する。
本発明の熱延鋼板は、ベイナイト相の面積率が80%以上、フェライト相の面積率が15%以下(0を含む)、マルテンサイト相の面積率が5%以下(0を含む)、セメンタイトの析出量が質量%で0.08%以上、該セメンタイトの平均粒子径が2μm以下であり、前記ベイナイトの結晶粒内に平均粒子径が10nm未満である炭化物が微細分散した組織を有し、鋼板表面から0.2μmまでの深さにおけるSi濃化量が地鉄でのSi固溶量の1.3倍以下である。
Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the structure of the hot-rolled steel sheet of the present invention and the Si surface concentration of the steel sheet will be described.
The hot-rolled steel sheet of the present invention has an area ratio of bainite phase of 80% or more, an area ratio of ferrite phase of 15% or less (including 0), an area ratio of martensite phase of 5% or less (including 0), and cementite. The amount of precipitation of 0.08% or more by mass%, the average particle diameter of the cementite is 2 μm or less, and has a structure in which carbides having an average particle diameter of less than 10 nm are finely dispersed in the bainite crystal grains, The concentration of Si at a depth of 0.1 to 0.2 μm is 1.3 times or less of the amount of Si solid solution in the steel.
ベイナイト相の面積率:80%以上
ベイナイト相は、粒内に多量の転位を含むために、セメンタイトやTi、Vを含む炭化物の核生成を促進させる。また、ベイナイト相が成長する温度域ではセメンタイトとTiやVを含む炭化物の析出が競合するため、熱延鋼板の打抜性と高強度を両立させることが可能となる。これらの効果を発現させるためには、実質的にベイナイト単相組織とする必要があり、ベイナイト相を主相とし、その面積率を80%以上とする必要がある。好ましくは90%以上である。
Area ratio of bainite phase: 80% or more Since the bainite phase includes a large amount of dislocations in the grains, it promotes nucleation of carbides including cementite, Ti, and V. In addition, since cementite and precipitation of carbides including Ti and V compete in the temperature range where the bainite phase grows, both the punchability and high strength of the hot-rolled steel sheet can be achieved. In order to express these effects, it is necessary to substantially have a bainite single-phase structure, and the bainite phase is a main phase, and the area ratio is required to be 80% or more. Preferably it is 90% or more.
フェライト相の面積率:15%以下(0を含む)
フェライト相の粒内には、TiやVを含む炭化物が多量に析出するため、フェライト相の強度(硬度)はベイナイト相よりも高くなる。このように主相であるベイナイト相との硬度差を有するフェライト相が混在すると、熱延鋼板の打ち抜き加工時にフェライト相とベイナイト相との界面で不均一な応力集中が生じるため、打ち抜き端面での割れや荒れ等の異常部発生の原因となる。また、セメンタイトはフェライト相の粒内に生じ難く、フェライト粒界に優先的に析出する。そのため、フェライト相には、粒内にセメンタイトを均一に分散させることができず、打ち抜き加工時の亀裂成長を制御することが困難となる。フェライト相の面積率が15%を超えると上記悪影響が顕在化するため、フェライト相の面積率は15%以下とする。好ましくは10%以下である。なお、本発明の熱延鋼板は、フェライト相の面積率を0%としてもよく、フェライト相を含まないことが好ましい。
Area ratio of ferrite phase: 15% or less (including 0)
Since a large amount of carbides containing Ti and V is precipitated in the grains of the ferrite phase, the strength (hardness) of the ferrite phase is higher than that of the bainite phase. When a ferrite phase having a hardness difference from the bainite phase, which is the main phase, is mixed, non-uniform stress concentration occurs at the interface between the ferrite phase and the bainite phase during punching of the hot-rolled steel sheet. It may cause abnormal parts such as cracks and roughness. Moreover, cementite hardly occurs in the ferrite phase grains, and precipitates preferentially at the ferrite grain boundaries. Therefore, in the ferrite phase, cementite cannot be uniformly dispersed in the grains, and it becomes difficult to control crack growth during punching. If the area ratio of the ferrite phase exceeds 15%, the above-described adverse effect becomes obvious. Therefore, the area ratio of the ferrite phase is set to 15% or less. Preferably it is 10% or less. The hot-rolled steel sheet of the present invention may have a ferrite phase area ratio of 0% and preferably does not contain a ferrite phase.
マルテンサイト相の面積率:5%以下(0を含む)
マルテンサイト相は非常に硬質であるため、熱延鋼板の打ち抜き加工時に局所的な応力集中を生じさせ、打ち抜き端面での亀裂や割れ等の原因となり、打ち抜き端面性状を劣化させる。マルテンサイト相の面積率が5%を超えると上記悪影響が顕在化するため、マルテンサイト相の面積率は5%以下とする。好ましくは3%以下である。なお、本発明の熱延鋼板は、マルテンサイト相の面積率を0%としてもよく、マルテンサイト相を含まないことが好ましい。
Martensite phase area ratio: 5% or less (including 0)
Since the martensite phase is very hard, local stress concentration occurs during the punching process of the hot-rolled steel sheet, causing cracks and cracks at the punched end face, and deteriorating the punched end face properties. When the area ratio of the martensite phase exceeds 5%, the above-described adverse effect becomes obvious, so the area ratio of the martensite phase is set to 5% or less. Preferably it is 3% or less. The hot-rolled steel sheet of the present invention may have a martensite phase area ratio of 0% and preferably does not contain a martensite phase.
なお、本発明の熱延鋼板において、ベイナイト相、フェライト相およびマルテンサイト相以外の組織としては、パーライトや残留オーステナイト等が挙げられる。これら組織の合計面積率は3%以下とすることが好ましい。更に、マルテンサイト相を含めた合計面積率を5%以下とすることがより好ましい。 In the hot rolled steel sheet of the present invention, examples of the structure other than the bainite phase, the ferrite phase, and the martensite phase include pearlite and retained austenite. The total area ratio of these structures is preferably 3% or less. Furthermore, the total area ratio including the martensite phase is more preferably 5% or less.
セメンタイトの平均粒子径:2μm以下
鉄系の炭化物であるセメンタイトは粗大であるため、熱延鋼板に打ち抜き加工を施すと、セメンタイトとマトリックス(主にベイナイト相)との界面に応力集中が生じ、ボイド発生の基点となる。ここで、良好な打ち抜き端面性状を得るには、熱延鋼板の板厚方向にわたって均一にボイドを発生させ、ボイドの成長、合体を促進し、亀裂成長方向を制御する必要がある。したがって、狭い粒度分布を有するセメンタイトを均一に分布させることが肝要となる。セメンタイトの平均粒子径が2μmを超えると、粒度分布が広くなり、セメンタイトとマトリックスの界面における応力集中が不均一となり、打ち抜き端面性状が劣化する。したがって、セメンタイトの平均粒子径は2μm以下とする。好ましくは1μm以下である。更に、熱延鋼板中にセメンタイトを均一に分布させるためには、セメンタイトの最近接間距離を少なくとも5μm以下にすることが好ましい。
The average particle size of cementite: 2 μm or less Since cementite, an iron-based carbide, is coarse, punching a hot-rolled steel sheet causes stress concentration at the interface between cementite and the matrix (mainly bainite phase), resulting in voids. It becomes the starting point of occurrence. Here, in order to obtain a good punched end face property, it is necessary to uniformly generate voids in the thickness direction of the hot-rolled steel sheet, promote the growth and coalescence of voids, and control the crack growth direction. Therefore, it is important to uniformly distribute cementite having a narrow particle size distribution. When the average particle diameter of cementite exceeds 2 μm, the particle size distribution becomes wide, stress concentration at the cementite / matrix interface becomes non-uniform, and the punched end face properties deteriorate. Therefore, the average particle diameter of cementite is 2 μm or less. Preferably, it is 1 μm or less. Furthermore, in order to uniformly distribute cementite in the hot-rolled steel sheet, it is preferable that the closest distance between the cementite is at least 5 μm or less.
セメンタイトの析出量(質量%):0.08%以上
上記のとおり、本発明においては、セメンタイトを微細かつ均一に分散させることで、熱延鋼板の打抜性を高めている。このような効果を発現させるためには、セメンタイトの析出量((熱延鋼板中に含まれるセメンタイトの全質量)/(熱延鋼板の質量)×100)を、質量%で0.08%以上とする必要がある。好ましくは0.10%以上である。但し、セメンタイトの析出量が過剰になると、ボイド発生起点が不均一となるために打抜性が劣化するばかりか、微細に析出するTi、V等を含む炭化物の析出量が減少するため鋼板強度が低下するおそれがある。したがって、セメンタイトの析出量は、質量%で0.18%以下とすることが好ましい。
Cementite precipitation amount (% by mass): 0.08% or more As described above, in the present invention, the punchability of the hot-rolled steel sheet is enhanced by finely and uniformly dispersing cementite. In order to exhibit such an effect, the precipitation amount of cementite ((total mass of cementite contained in the hot-rolled steel sheet) / (mass of hot-rolled steel sheet) × 100) is 0.08% or more by mass%. There is a need. Preferably it is 0.10% or more. However, if the precipitation amount of cementite is excessive, the void generation start point becomes non-uniform, so not only the punchability deteriorates, but also the precipitation amount of carbides containing finely precipitated Ti, V, etc. decreases, so May decrease. Therefore, the precipitation amount of cementite is preferably 0.18% or less by mass.
ベイナイト相の結晶粒内の炭化物
本発明の熱延鋼板では、固溶強化元素であるSiの含有量を極力低下させるため、ベイナイト相による変態強化(転位強化)のみでは引張強さ:780MPa以上の熱延鋼板を安定的に得ることは困難である。そこで、本発明の熱延鋼板では、ベイナイト相の結晶粒内に炭化物を微細析出させて、粒子分散強化により高強度化を図ることが必須となる。本発明において、ベイナイト相の結晶粒内に微細析出させる炭化物としては、Ti炭化物、V炭化物およびTiとVの複合炭化物が挙げられる。なお、これらの炭化物の多くは、熱延鋼板製造工程における仕上げ圧延終了後の巻取り過程で、ベイナイト相の結晶粒内の転位を核生成サイトとして析出する炭化物である。
Carbides in grains of bainite phase In the hot-rolled steel sheet of the present invention, the tensile strength: 780 MPa or more is obtained only by transformation strengthening (dislocation strengthening) by the bainite phase in order to reduce the content of Si as a solid solution strengthening element as much as possible. It is difficult to stably obtain a hot-rolled steel sheet. Therefore, in the hot-rolled steel sheet of the present invention, it is essential to increase the strength by finely precipitating carbides in the bainite phase crystal grains and strengthening the particle dispersion. In the present invention, examples of the carbide finely precipitated in the crystal grains of the bainite phase include Ti carbide, V carbide, and composite carbide of Ti and V. Note that most of these carbides are carbides that precipitate as dislocations in the bainite phase crystal grains in the winding process after finish rolling in the hot-rolled steel sheet manufacturing process.
炭化物の平均粒子径:10nm未満
本発明では、ベイナイト相の結晶粒内に前記したTiやVを含む炭化物を微細に分散させることで、熱延鋼板の強化を図っている。炭化物が粗大化すると、鋼板に変形が加わった際に生じる転位の運動を阻害する炭化物数が減じることから、炭化物が微細化するほど鋼板は高強度化する。引張強さ780MPa以上の高強度熱延鋼板を得るには、上記炭化物の平均粒子径を10nm未満とする必要がある。好ましくは6nm以下である。
Average particle diameter of carbide: less than 10 nm In the present invention, the above-described carbide containing Ti or V is finely dispersed in the crystal grains of the bainite phase, thereby strengthening the hot-rolled steel sheet. When the carbides are coarsened, the number of carbides that hinder the movement of dislocations that are generated when deformation is applied to the steel sheet decreases, so that the strength of the steel sheet increases as the carbides become finer. In order to obtain a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, it is necessary that the average particle diameter of the carbide is less than 10 nm. Preferably it is 6 nm or less.
鋼板表面から0.2μmまでの深さにおけるSi濃化量:地鉄でのSi固溶量の1.3倍以下
Si表面濃化層は、鋼板表面の濡れ性を著しく低下させ、めっき性や化成性を低下させる。したがって、Si濃化量が増加すると、熱延鋼板の表面性状が劣化する。表面性状の劣化を抑制するためには、鋼板表面から0.2μmまでの深さにおけるSi濃化量を、地鉄でのSi固溶量の1.3倍以下とする必要がある。好ましくは1.2倍以下である。
なお、本発明において「鋼板表面から0.2μmまでの深さにおけるSi濃化量」は、グロー放電発光分析法により鋼板表面から0.2μmまでの深さにおけるSiのスペクトル強度から得られる濃度プロファイルから測定される値とする。一方、「地鉄でのSi固溶量」は、鋼板表面から0.2μmから0.4μmまでの深さにおけるグロー放電発光分析法によるSiの濃度プロファイルから測定される値とする。
Si concentration at a depth of 0.2 μm from the steel sheet surface: 1.3 times or less of Si solid solution in the base iron
The Si surface enriched layer remarkably reduces the wettability of the steel sheet surface, and lowers the plateability and chemical conversion. Therefore, when the Si concentration increases, the surface properties of the hot-rolled steel sheet deteriorate. In order to suppress the deterioration of the surface properties, it is necessary that the Si concentration at a depth of 0.2 μm from the steel sheet surface is 1.3 times or less of the Si solid solution amount in the base iron. Preferably it is 1.2 times or less.
In the present invention, “Si concentration at a depth of 0.2 μm from the steel sheet surface” is measured from a concentration profile obtained from the spectral intensity of Si at a depth of 0.2 μm from the steel sheet surface by glow discharge optical emission spectrometry. Value. On the other hand, “the amount of Si solid solution in the base iron” is a value measured from the concentration profile of Si by glow discharge optical emission spectrometry at a depth of 0.2 μm to 0.4 μm from the steel sheet surface.
次に、本発明熱延鋼板の成分組成の限定理由について説明する。なお、以下の成分組成を表す%は、特に断らない限り質量%(mass%)を意味するものとする。
C:0.06%以上0.13%以下
Cは、TiやVと結合し炭化物として鋼板中に微細分散する。さらに、TiやVを含む炭化物の形成に関与しないCは、セメンタイトとして析出する。すなわちCは、微細な炭化物を形成して鋼板強度を上昇させるとともに、微細なセメンタイトを分散させて優れた打抜性を得るのに重要な元素である。引張強さが780MPa以上であり且つ打抜性に優れた高強度熱延鋼板を得るには、C含有量を少なくとも0.06%以上とする必要がある。一方、C含有量が0.13%を超えると、粗大なセメンタイトが析出するようになり、打抜性が劣化する。したがって、C含有量は0.06%以上0.13%とする。好ましくは0.07%以上0.11%以下である。
Next, the reason for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% (mass%) unless there is particular notice.
C: 0.06% to 0.13%
C combines with Ti and V and is finely dispersed in the steel sheet as a carbide. Furthermore, C that is not involved in the formation of carbides including Ti and V precipitates as cementite. That is, C is an important element for forming fine carbides to increase the strength of the steel sheet and dispersing fine cementite to obtain excellent punchability. In order to obtain a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent punchability, the C content needs to be at least 0.06% or more. On the other hand, when the C content exceeds 0.13%, coarse cementite starts to precipitate and the punchability deteriorates. Therefore, the C content is 0.06% or more and 0.13%. Preferably they are 0.07% or more and 0.11% or less.
また、上記したように、セメンタイトの析出量はTiやVを含む炭化物の形成に関与しないC量に比例する。所望のセメンタイト量を得るには、下記(1)式を満たす範囲でC含有量を制御することが望ましい。なお、下記(1)式において[%C]、[%Ti]、[%V]は、それぞれの元素の含有量(質量%)である。 Further, as described above, the amount of cementite deposited is proportional to the amount of C not involved in the formation of carbides including Ti and V. In order to obtain a desired amount of cementite, it is desirable to control the C content within a range satisfying the following formula (1). In the following formula (1), [% C], [% Ti] and [% V] are the contents (mass%) of the respective elements.
Si:0.09%以下
Siは、鋼板表面に濃化してSi表面濃化層を形成するが、この表面濃化層は鋼板表面の濡れ性を著しく低下させるため、めっき性や化成性を低下させる。また、Siは、鋼板表面に赤スケールを生成させ、鋼板表面の外観を著しく損ねる。さらに、この赤スケールは鋼板表面にノッチを生成させるため、鋼板の曲げ性や耐疲労性を低下させる原因となる。したがって、本発明ではSi含有量を極力低減することが望ましいが、0.09%までは許容できるため、Si含有量の上限を0.09%とする。好ましくは0.05%未満である。なお、Si含有量は不純物レベルまで低減してもよい。
Si: 0.09% or less
Si concentrates on the surface of the steel sheet to form a Si surface enriched layer. This surface enriched layer significantly reduces the wettability of the steel sheet surface, and thus lowers the plateability and chemical conversion. In addition, Si generates a red scale on the surface of the steel sheet, and remarkably deteriorates the appearance of the steel sheet surface. Furthermore, since this red scale produces notches on the steel sheet surface, it causes a decrease in the bendability and fatigue resistance of the steel sheet. Therefore, in the present invention, it is desirable to reduce the Si content as much as possible, but 0.09% is acceptable, so the upper limit of the Si content is 0.09%. Preferably it is less than 0.05%. Note that the Si content may be reduced to the impurity level.
Mn:0.01%以上1.20%以下
Mnは、熱延鋼板製造時、熱間圧延終了後の冷却過程においてオーステナイト→フェライト変態点を低下させる作用があり、熱延鋼板組織を実質的なベイナイト単相組織とするのに必要な元素である。このような効果を得るには、Mn含有量を0.01%以上とする必要がある。一方、Mnは表面濃化するうえ、Siの表面濃化を助長させるため、必要以上に含有させれば熱延鋼板の表面性状を悪化させることになる。本発明においては、Mn含有量が1.20%以下であれば表面性状の劣化が問題とならないため、Mn含有量の上限を1.20%とする。好ましくは0.20%以上1.15%以下である。
Mn: 0.01% or more and 1.20% or less
Mn has the effect of lowering the austenite → ferrite transformation point in the cooling process after hot rolling at the time of hot rolled steel sheet production, and is an element necessary for making the hot rolled steel sheet structure a substantial bainite single phase structure. is there. In order to obtain such an effect, the Mn content needs to be 0.01% or more. On the other hand, Mn not only enriches the surface but also promotes the surface enrichment of Si. Therefore, if it is contained more than necessary, the surface properties of the hot-rolled steel sheet will be deteriorated. In the present invention, if the Mn content is 1.20% or less, the deterioration of the surface properties will not be a problem, so the upper limit of the Mn content is 1.20%. Preferably it is 0.20% or more and 1.15% or less.
P:0.03%以下
Pは、粒界に偏析して熱延鋼板の打ち抜き加工時に粒界割れの起点となり、打ち抜き端面の性状を劣化させる有害な元素であるため、極力低減することが好ましい。本発明では上記問題を回避すべく、P含有量を0.03%以下とする。好ましくは0.02%以下である。
P: 0.03% or less
P is a harmful element that segregates at the grain boundary and becomes the starting point of grain boundary cracking during the punching process of the hot-rolled steel sheet, and deteriorates the properties of the punched end face. Therefore, P is preferably reduced as much as possible. In the present invention, in order to avoid the above problem, the P content is set to 0.03% or less. Preferably it is 0.02% or less.
S:0.005%以下
Sは、鋼中でMnSなどの介在物として存在する。この介在物は、熱延鋼板の打ち抜き加工時に楔状に伸び変形が不均一となるため、打抜性に著しい悪影響をもたらす。したがって、本発明では、S含有量を極力低減することが好ましく、0.005%以下とする。好ましくは0.003%以下である。
S: 0.005% or less
S exists as an inclusion such as MnS in steel. This inclusion causes a remarkable adverse effect on the punchability because the deformation becomes non-uniform in a wedge shape when the hot-rolled steel sheet is punched. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and it is 0.005% or less. Preferably it is 0.003% or less.
Al:0.1%以下
Alは、脱酸剤として作用する元素である。このような効果を得るためには0.02%以上含有することが望ましいが、Al含有量が0.1%を超えるとアルミナなどの介在物による打抜性への悪影響が顕在化する。したがって、Al含有量は0.1%以下とする。好ましくは0.08%以下である。
Al: 0.1% or less
Al is an element that acts as a deoxidizer. In order to obtain such an effect, it is desirable to contain 0.02% or more, but when the Al content exceeds 0.1%, the adverse effect on punchability due to inclusions such as alumina becomes obvious. Therefore, the Al content is 0.1% or less. Preferably it is 0.08% or less.
N:0.01%以下
Nは、製鋼の段階でTiなどと結合して粗大な窒化物を形成し、微細な炭化物の形成を阻害するため熱延鋼板強度を著しく低下させる。また、粗大な窒化物は、局所的な応力集中を生じさせるため熱延鋼板の打抜性をも低下させる。したがってN含有量は極力低減することが好ましく、0.01%以下とする。好ましくは0.008%以下である。
N: 0.01% or less
N combines with Ti and the like at the steel making stage to form coarse nitrides and inhibits the formation of fine carbides, so that the strength of the hot-rolled steel sheet is significantly reduced. In addition, coarse nitrides cause local stress concentration, and thus reduce the punchability of the hot-rolled steel sheet. Therefore, the N content is preferably reduced as much as possible, and is 0.01% or less. Preferably it is 0.008% or less.
Ti:0.13%以上0.20%以下
Tiは、微細な炭化物を形成して熱延鋼板強度を上昇させるうえ、熱延鋼板製造時、熱間圧延終了後の冷却過程においてオーステナイト→フェライト変態を遅延させる焼入性元素としての作用があるため、本発明において重要な元素である。引張強さ:780MPa以上ならびに実質的なベイナイト単相組織である熱延鋼板を得るのに必要なTi含有量は0.13%以上である。好ましくは0.14%以上である。一方、Ti含有量が0.20%を超えると熱間圧延前の鋼素材の加熱工程において粗大なTi系炭化物を溶解することができず、最終的に得られる熱延鋼板中に粗大なTi炭化物が残存する。この粗大なTi炭化物は熱延鋼板の強度上昇に寄与しないため、Ti含有量が0.20%を超えると高強度化の効果が飽和する。更に、粗大なTi炭化物は局所的な応力集中を発生させるため、Ti含有量が0.20%を超えると打ち抜き端面性状が劣化する。以上の理由により、Ti含有量の上限を0.20%とする。好ましくは0.18%以下である。
Ti: 0.13% to 0.20%
Ti forms fine carbides to increase hot-rolled steel sheet strength, and also acts as a quenching element to delay the austenite → ferrite transformation in the cooling process after hot rolling at the time of hot-rolled steel sheet production. Therefore, it is an important element in the present invention. Tensile strength: 780 MPa or more and Ti content necessary for obtaining a hot rolled steel sheet having a substantial bainite single phase structure is 0.13% or more. Preferably it is 0.14% or more. On the other hand, if the Ti content exceeds 0.20%, the coarse Ti-based carbide cannot be dissolved in the heating process of the steel material before hot rolling, and the coarse Ti carbide is not obtained in the finally obtained hot-rolled steel sheet. Remains. Since this coarse Ti carbide does not contribute to the strength increase of the hot-rolled steel sheet, the effect of increasing the strength is saturated when the Ti content exceeds 0.20%. Furthermore, since coarse Ti carbides cause local stress concentration, when the Ti content exceeds 0.20%, the punched end face properties deteriorate. For the above reasons, the upper limit of Ti content is 0.20%. Preferably it is 0.18% or less.
V:0.03%以上0.13%以下
Vは、Tiと同様、Cと炭化物を形成して熱延鋼板の高強度化に寄与する元素である。Tiのみでは炭化物の析出量が不足し所望の熱延鋼板強度(引張強さ:780MPa以上)が得られないため、V含有量を少なくとも0.03%以上とする必要がある。好ましくは、0.06%以上である。一方、VはTiよりも炭化物形成能が低く鋼板中に固溶状態として残存し易い元素であるが、固溶状態にあるVは結晶格子をひずませる効果が小さいため高強度化に寄与しない。0.13%を超えるVを添加しても、炭化物の析出量は殆ど増加せず、高強度化の効果が飽和する。したがって、V含有量の上限を0.13%とする。好ましくは0.12%以下である。
V: 0.03% to 0.13%
V, like Ti, is an element that forms a carbide with C and contributes to increasing the strength of the hot-rolled steel sheet. With Ti alone, the precipitation amount of carbide is insufficient and the desired hot-rolled steel sheet strength (tensile strength: 780 MPa or more) cannot be obtained, so the V content needs to be at least 0.03% or more. Preferably, it is 0.06% or more. On the other hand, V is an element that has a lower ability to form carbides than Ti and tends to remain in a solid solution state in the steel sheet, but V in the solid solution state has little effect of distorting the crystal lattice and therefore does not contribute to an increase in strength. Even if V exceeding 0.13% is added, the precipitation amount of carbide hardly increases and the effect of increasing the strength is saturated. Therefore, the upper limit of V content is 0.13%. Preferably it is 0.12% or less.
Nb:0.02%以下(0を含む)
Nbは、鋼中への溶解量が少ないため、熱間圧延前の鋼素材の加熱で鋼に溶解させることが困難な元素である。特に、本発明のようにTi含有量が0.13%以上0.20%以下である鋼の場合、熱間圧延前の鋼素材を粗大なTi炭化物が溶解する温度域に加熱してもNbは溶解せず粗大なNb炭化物として熱延鋼板中に残存する。この粗大なNb炭化物は、熱延鋼板の打ち抜き加工時に局所的な応力集中源となり、打ち抜き端面性状を劣化させる。上記悪影響はNb含有量が0.02%を超えると顕在化するため、Nb含有量は0.02%以下に制限する必要がある。好ましくは0.01%以下に制限する。なお、Nb含有量は不純物レベルまで低減してもよく、0%としてもよい。
Nb: 0.02% or less (including 0)
Nb is an element that is difficult to be dissolved in steel by heating the steel material before hot rolling because the amount of dissolution in steel is small. In particular, in the case of steel with a Ti content of 0.13% to 0.20% as in the present invention, Nb does not dissolve even when the steel material before hot rolling is heated to a temperature range where coarse Ti carbides dissolve. It remains in the hot-rolled steel sheet as coarse Nb carbide. This coarse Nb carbide becomes a local stress concentration source during punching of the hot-rolled steel sheet, and deteriorates the punched end face properties. Since the above-mentioned adverse effect becomes apparent when the Nb content exceeds 0.02%, the Nb content needs to be limited to 0.02% or less. Preferably, it is limited to 0.01% or less. Note that the Nb content may be reduced to the impurity level or 0%.
以上が、本発明における基本組成であるが、上記した基本組成に加えてさらに、B:0.0002%以上0.0030%以下を含有してもよい。
Bは、熱延鋼板製造時、熱間圧延終了後の冷却過程においてオーステナイト→フェライト変態の進行を阻害させる元素であるため、Bを含有することにより実質的なベイナイト単相組織の熱延鋼板が製造し易くなる。上記の効果を得るには、B含有量を0.0002%以上とすることが好ましい。一方、B含有量が0.0030%を超えると、変態点を制御する上記効果が飽和する。したがって、B含有量は0.0002%以上0.0030%以下とすることが好ましい。より好ましくは0.0005%以上0.0020%以下である。
The above is the basic composition in the present invention. In addition to the above basic composition, B: 0.0002% or more and 0.0030% or less may be further contained.
B is an element that inhibits the progress of austenite → ferrite transformation in the cooling process after hot rolling during the production of hot-rolled steel sheets, so by containing B, a hot-rolled steel sheet having a substantial bainite single-phase structure can be obtained. Easy to manufacture. In order to obtain the above effects, the B content is preferably 0.0002% or more. On the other hand, when the B content exceeds 0.0030%, the above effect of controlling the transformation point is saturated. Therefore, the B content is preferably 0.0002% or more and 0.0030% or less. More preferably, it is 0.0005% or more and 0.0020% or less.
また、REM、Cu、Ni、Sn、Cr、Sb、Mg、Ca、Coのいずれか1種以上を合計で0.1929%以下含有してもよい。これら元素は、熱延鋼板の表面性状および打抜性の観点から合計で0.1929%まで許容できる。
上記以外の成分は、Feおよび不可避的不純物である。
Further, any one or more of REM, Cu, Ni, Sn, Cr, Sb, Mg, Ca and Co may be contained in a total amount of 0.1929 % or less. These elements can be tolerated in total up to 0.1929 % from the viewpoint of the surface properties and punchability of the hot-rolled steel sheet .
Components other than the above are Fe and inevitable impurities.
本発明の熱延鋼板は、720℃までの加熱処理を短時間施しても引張強さが低下することはない。そのため、鋼板に耐食性を付与する目的で、本発明の熱延鋼板にめっき処理を施し、その表面にめっき層を具えることができる。めっき処理は、720℃以下の加熱温度でも処理可能であることから、本発明の熱延鋼板にめっき処理を施しても前記した本発明の効果を損なうことはない。 Even if the hot-rolled steel sheet of the present invention is subjected to a heat treatment up to 720 ° C. for a short time, the tensile strength does not decrease. Therefore, for the purpose of imparting corrosion resistance to the steel sheet, the hot-rolled steel sheet of the present invention can be plated, and a plating layer can be provided on the surface thereof. Since the plating treatment can be carried out even at a heating temperature of 720 ° C. or less, even if the hot-rolled steel sheet of the present invention is subjected to a plating treatment, the above-described effects of the present invention are not impaired.
めっき層の種類は特に問わず、電気めっき層、無電解めっき層のいずれも適用可能である。また、めっき層の合金成分も特に問わず、溶融亜鉛めっき層、合金化溶融亜鉛めっき層などが好適な例として挙げられるが、勿論、これらに限定されない。 The type of the plating layer is not particularly limited, and any of an electroplating layer and an electroless plating layer can be applied. Further, the alloy component of the plating layer is not particularly limited, and a hot dip galvanized layer, an alloyed hot dip galvanized layer and the like can be mentioned as suitable examples, but of course, it is not limited thereto.
次に、本発明の熱延鋼板の製造方法について説明する。
本発明は、上記した組成の鋼素材(鋼スラブ)を加熱し、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻き取り、熱延鋼板とする。この際、前記加熱の加熱温度を1150℃以上1350℃以下とし、前記仕上げ圧延の仕上げ圧延終了温度を850℃以上とし、前記冷却を仕上げ圧延終了後3s以内に開始し、前記冷却の冷却速度を40℃/s以上とし、前記巻き取りの巻取り温度を350℃以上550℃以下とすることを特徴とする。
Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described.
In the present invention, the steel material (steel slab) having the above composition is heated, subjected to hot rolling consisting of rough rolling and finish rolling, cooled after completion of finish rolling, and wound into a hot-rolled steel sheet. At this time, the heating temperature of the heating is 1150 ° C. or more and 1350 ° C. or less, the finish rolling finish temperature of the finish rolling is 850 ° C. or more, the cooling is started within 3 s after the finish rolling is finished, and the cooling rate of the cooling is set. The temperature is 40 ° C./s or more, and the winding temperature of the winding is 350 ° C. or more and 550 ° C. or less.
本発明において、鋼の溶製方法は特に限定されず、転炉、電気炉等、公知の溶製方法を採用することができる。また、真空脱ガス炉にて2次精錬を行ってもよい。その後、生産性や品質上の問題から連続鋳造法によりスラブ(鋼素材)とするのが好ましいが、造塊−分塊圧延法、薄スラブ連鋳法等、公知の鋳造方法でスラブとしてもよい。なお、本発明において連続鋳造法を採用する場合には、連続鋳造時の鋼の成分偏析低減のために、電磁撹拌(EMS)、軽圧下鋳造(IBSR)等を適用することができる。電磁撹拌処理を施すことにより、板厚中心部に等軸晶を形成させ、偏析を低減させることができる。また、軽圧下鋳造を施した場合は、連続鋳造スラブの未凝固部の溶鋼の流動を防止することにより、板厚中心部の偏析を低減させることができる。これらの偏析低減処理の少なくとも1つの適用により、後述する引張り試験における伸びをより優れたレベルにすることができる。 In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, the slab (steel material) is preferably formed by a continuous casting method from the viewpoint of productivity and quality, but the slab may be formed by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. . When the continuous casting method is employed in the present invention, electromagnetic stirring (EMS), light pressure casting (IBSR), or the like can be applied to reduce the segregation of steel components during continuous casting. By performing the electromagnetic stirring treatment, equiaxed crystals can be formed in the center portion of the plate thickness, and segregation can be reduced. In addition, when light pressure casting is performed, segregation at the central portion of the plate thickness can be reduced by preventing the flow of molten steel in the unsolidified portion of the continuous cast slab. By applying at least one of these segregation reduction treatments, the elongation in a tensile test described later can be made to a more excellent level.
鋼素材の加熱温度:1150℃以上1350℃以下
上記の如く得られた鋼素材に、粗圧延および仕上げ圧延を施すが、本発明においては、粗圧延に先立ち鋼素材を加熱して実質的に均質なオーステナイト相とし、粗大な炭化物を溶解する必要がある。鋼素材の加熱温度が1150℃を下回ると、粗大な炭化物が溶解しないため、熱間圧延終了後の冷却・巻取り工程で微細分散する炭化物の量が減じることとなり、最終的に得られる熱延鋼板の強度が著しく低下する。また、粗大な炭化物が生成することにより熱延鋼板の打ち抜き端面性状が劣化する。一方、上記加熱温度が1350℃を上回ると、スケールが噛み込み、熱延鋼板の表面性状が悪化する。
Heating temperature of steel material: 1150 ° C or higher and 1350 ° C or lower The steel material obtained as described above is subjected to rough rolling and finish rolling. In the present invention, the steel material is heated prior to rough rolling to be substantially homogeneous. The austenite phase needs to be dissolved and coarse carbides need to be dissolved. When the heating temperature of the steel material is below 1150 ° C, coarse carbides do not dissolve, so the amount of carbides finely dispersed in the cooling and winding process after hot rolling is reduced, resulting in the final hot rolling. The strength of the steel sheet is significantly reduced. In addition, the formation of coarse carbides deteriorates the punched end face properties of the hot-rolled steel sheet. On the other hand, when the heating temperature exceeds 1350 ° C., the scale bites and the surface properties of the hot-rolled steel sheet deteriorate.
以上の理由により、鋼素材加熱温度は1150℃以上1350℃以下とする。好ましくは1200℃以上1300℃以下である。但し、鋼素材に熱間圧延を施すに際し、鋳造後の鋼素材が1150℃以上1350℃以下の温度域にある場合、或いは鋼素材の炭化物が溶解している場合には、鋼素材を加熱することなく直送圧延してもよい。なお、粗圧延条件については特に限定されない。 For these reasons, the steel material heating temperature is set to 1150 ° C or higher and 1350 ° C or lower. Preferably they are 1200 degreeC or more and 1300 degrees C or less. However, when hot rolling the steel material, if the steel material after casting is in the temperature range of 1150 ° C or higher and 1350 ° C or lower, or if the carbide of the steel material is dissolved, the steel material is heated. Direct rolling may be performed without any problem. The rough rolling conditions are not particularly limited.
仕上げ終了圧延温度:850℃以上
仕上げ圧延終了温度が850℃を下回ると、仕上げ圧延中にフェライト変態が開始してフェライト粒が伸展された組織となるうえ、部分的にフェライト粒が成長した混粒組織となるため、熱延鋼板の打抜性が著しく低下する。したがって、仕上げ圧延終了温度は850℃以上とする。好ましくは870℃以上である。但し、仕上げ圧延終了温度が過剰に高くなると、鋼板表面に生成するスケールが厚くなり、スケール噛み込みにより鋼板表面性状が劣化することが懸念されるため、1000℃以下とすることが好ましい。
Finishing finish rolling temperature: 850 ° C or more When the finish rolling finish temperature is lower than 850 ° C, ferrite transformation starts during finish rolling, resulting in a structure in which ferrite grains are expanded, and a mixed grain in which ferrite grains partially grow Since it becomes a structure | tissue, the punching property of a hot-rolled steel plate falls remarkably. Accordingly, the finish rolling end temperature is set to 850 ° C. or higher. Preferably it is 870 degreeC or more. However, when the finish rolling finish temperature becomes excessively high, the scale generated on the steel sheet surface becomes thick, and there is a concern that the steel sheet surface properties may deteriorate due to the bite of the scale.
仕上げ圧延終了後、強制冷却を開始するまでの時間:3s以内
仕上げ圧延直後の高温状態の鋼板においては、オーステナイト相に蓄積されたひずみエネルギーが大きいため、ひずみ誘起析出による炭化物が生じる。この炭化物は、高温で析出するため粗大化し易いことから、ひずみ誘起析出が生じると微細な析出物が得られ難くなる。したがって、本発明では、ひずみ誘起析出を抑制する目的で熱間圧延終了後速やかに強制冷却を開始する必要があり、仕上げ圧延終了後、少なくとも3s以内に強制冷却を開始する。好ましくは2s以内である。
Time until start of forced cooling after finish rolling: within 3 s In steel sheets in a high temperature state immediately after finish rolling, carbides are generated due to strain-induced precipitation because the strain energy accumulated in the austenite phase is large. Since this carbide precipitates at a high temperature and is easy to coarsen, it is difficult to obtain a fine precipitate when strain-induced precipitation occurs. Therefore, in the present invention, forced cooling needs to be started immediately after the end of hot rolling in order to suppress strain-induced precipitation, and forced cooling is started within at least 3 seconds after the end of finish rolling. Preferably it is within 2 s.
冷却速度:40℃/s以上
上記のとおり、仕上げ圧延終了後の鋼板の高温に維持される時間が長いほど、ひずみ誘起析出による炭化物の粗大化が進行し易くなり、熱延鋼板強度が低下する。また、冷却速度が40℃/s未満では、オーステナイト→フェライト変態が進行し、所望の金属組織を得ることができなくなる。そのため、本発明では仕上げ圧延終了後、後述する巻取り温度まで急冷する必要があり、上記問題を回避するには40℃/s以上の冷却速度で冷却する必要がある。好ましくは50℃/s以上である。但し、仕上げ圧延終了後の冷却速度が過剰に大きくなると、巻取り温度の制御が困難となるおそれがあるため、200℃/s以下とすることが好ましい。
なお、上記の冷却速度は、仕上げ圧延終了温度から巻取り温度までの温度域における平均冷却速度である。
Cooling rate: 40 ° C / s or more As described above, the longer the time that the steel sheet is maintained at the high temperature after finish rolling, the more easily the coarsening of carbides by strain-induced precipitation proceeds and the hot-rolled steel sheet strength decreases. . On the other hand, if the cooling rate is less than 40 ° C./s, the austenite → ferrite transformation proceeds and the desired metal structure cannot be obtained. Therefore, in the present invention, after finishing rolling, it is necessary to rapidly cool to a winding temperature described later, and to avoid the above problem, it is necessary to cool at a cooling rate of 40 ° C./s or more. Preferably, it is 50 ° C./s or more. However, if the cooling rate after finishing rolling is excessively increased, it may be difficult to control the coiling temperature.
In addition, said cooling rate is an average cooling rate in the temperature range from finish rolling completion temperature to coiling temperature.
巻取り温度:350℃以上550℃以下
550℃以下の温度域では、TiやVを含む炭化物とセメンタイトの析出が競合し、高強度かつ良好な打ち抜き端面性状を有する熱延鋼板が得られるようになる。したがって、本発明では、仕上げ圧延終了後、550℃以下の巻取り温度まで急冷する。巻取り温度が550℃を上回ると、オーステナイト→フェライト変態の進行が優先的に生じるようになり、良好な打ち抜き端面性状が得られなくなる。一方、巻取り温度が350℃を下回るとTiおよびVを含む炭化物の析出が生じ難くなるため、熱延鋼板強度が不安定になるばかりか、マルテンサイト相が生成されるため打抜性が劣化する。以上の理由により、巻取り温度の範囲は350℃以上550℃以下とする。好ましくは380℃以上530℃以下である。
Winding temperature: 350 ° C or higher and 550 ° C or lower
In the temperature range of 550 ° C. or lower, carbides containing Ti and V compete with precipitation of cementite, and a hot-rolled steel sheet having high strength and good punched end face properties can be obtained. Therefore, in the present invention, after finishing finish rolling, the steel sheet is rapidly cooled to a winding temperature of 550 ° C. or lower. When the coiling temperature exceeds 550 ° C., the austenite → ferrite transformation preferentially occurs, and good punched end face properties cannot be obtained. On the other hand, when the coiling temperature is lower than 350 ° C, it is difficult for carbides containing Ti and V to precipitate, so not only the hot-rolled steel sheet strength becomes unstable, but also the martensite phase is generated and the punchability deteriorates. To do. For the above reasons, the coiling temperature range is 350 ° C. or higher and 550 ° C. or lower. Preferably they are 380 degreeC or more and 530 degrees C or less.
本発明によると、以上のように鋼板組成および製造条件を適正化することで、ベイナイト相の面積率が80%以上、フェライト相の面積率が15%以下(0を含む)、マルテンサイト相の面積率が5%以下(0を含む)、セメンタイトの析出量が質量%で0.08%以上、該セメンタイトの平均粒子径が2μm以下であり、前記ベイナイト相の結晶粒内に平均粒子径が10nm未満である炭化物が微細分散した組織を有し、鋼板表面から0.2μmまでの深さにおけるSi濃化量が地鉄でのSi固溶量の1.3倍以下である引張強さ780MPa以上の熱延鋼板が得られる。 According to the present invention, by optimizing the steel plate composition and production conditions as described above, the area ratio of the bainite phase is 80% or more, the area ratio of the ferrite phase is 15% or less (including 0), the martensite phase The area ratio is 5% or less (including 0), the precipitation amount of cementite is 0.08% or more by mass%, the average particle diameter of the cementite is 2 μm or less, and the average particle diameter is less than 10 nm in the crystal grains of the bainite phase. Hot-rolled steel sheet with a tensile strength of 780 MPa or more with a microstructure in which the carbides are finely dispersed, and the Si concentration at a depth of 0.2 μm from the steel sheet surface is 1.3 times or less of the Si solid solution in the ground iron Is obtained.
本発明では、良好な表面性状と優れた打抜性を兼備した高強度熱延鋼板を得る目的で、CおよびSi、Ti、V量の最適化を図っている。良好な表面性状を得る目的でSi含有量を低減して固溶強化機構の寄与を減少させた分、粒子分散強化機構により高強度化を図らなければ、熱延鋼板の引張強さを780MPa以上とすることはできない。さらに、炭化物構成元素であるCおよびTi、Vの含有量を適切に制御しなければ、良好な打抜性が得られなくなることは上記したとおりである。 In the present invention, the amount of C, Si, Ti, and V is optimized for the purpose of obtaining a high-strength hot-rolled steel sheet having both good surface properties and excellent punchability. The tensile strength of the hot-rolled steel sheet is 780 MPa or more unless the strength of the particle dispersion strengthening mechanism is increased by reducing the Si content to reduce the contribution of the solid solution strengthening mechanism in order to obtain good surface properties. It cannot be. Furthermore, as described above, good punchability cannot be obtained unless the contents of C, Ti, and V, which are carbide constituent elements, are appropriately controlled.
なお、本発明の巻き取り後の熱延鋼板は、表面にスケールが付着した状態であっても、酸洗を行うことによりスケールを除去した状態であっても、その特性が変わることはなく、いずれの状態においても前記した優れた効果を発現する。また、本発明では、巻き取り後の熱延鋼板にめっき処理を施して、熱延鋼板表面にめっき層を形成してもよい。めっき処理の種類は特に問わず、電気めっき処理、無電解めっき処理のいずれも適用可能である。例えば、めっき処理として溶融亜鉛めっき処理を施して溶融亜鉛めっき層を形成することができる。或いは更に、上記溶融亜鉛めっき処理後、更に合金化処理を施して合金化溶融亜鉛めっき層を形成してもよい。また、溶融めっきには亜鉛の他に、アルミもしくはアルミ合金等、その他の金属や合金をめっきすることもできる。 In addition, even if the hot-rolled steel sheet after winding of the present invention is in a state in which the scale is attached to the surface, or even in a state in which the scale is removed by pickling, the characteristics do not change. In any state, the above-described excellent effects are exhibited. In the present invention, the hot-rolled steel sheet after winding may be plated to form a plating layer on the surface of the hot-rolled steel sheet. The type of the plating treatment is not particularly limited, and both electroplating treatment and electroless plating treatment are applicable. For example, a hot dip galvanizing process can be performed as a plating process to form a hot dip galvanized layer. Alternatively, after the hot dip galvanizing treatment, an alloying treatment may be further performed to form an alloyed hot dip galvanized layer. In addition to zinc, other metals and alloys such as aluminum or aluminum alloy can be plated for hot dipping.
本発明により得られる熱延鋼板は、720℃以下までの温度域に短時間保持する場合であれば、鋼板強度が低下することはない。そのため、例えば焼鈍温度を720℃以下とした連続めっきラインに通板させることができる。めっき層の付着方法としては、例えば、めっき浴に鋼板を浸漬して引き上げる方法などが挙げられる。合金化処理方法としては、例えば、めっき処理後にガス炉など、鋼板表面を加熱することができる炉内で行う方法などが挙げられる。 If the hot-rolled steel sheet obtained by the present invention is kept in a temperature range up to 720 ° C. for a short time, the steel sheet strength will not be lowered. Therefore, for example, it can be made to pass through a continuous plating line having an annealing temperature of 720 ° C. or lower. Examples of the method for attaching the plating layer include a method in which a steel sheet is immersed in a plating bath and pulled up. Examples of the alloying method include a method performed in a furnace capable of heating the steel sheet surface such as a gas furnace after plating.
表1に示す組成を有する肉厚250mmのスラブ(鋼素材)を連続鋳造法により鋳造し、該スラブに、表2に示す熱延条件で熱間圧延を施し、更に巻き取り後に酸洗を施して板厚1.4〜2.0mmの熱延鋼板とした。連続鋳造の際、表2の鋼板No.2以外のものについては、成分の偏析低減処理のため、電磁撹拌(EMS)を行った。なお、表2に記載の冷却速度は、仕上げ圧延終了温度から巻取り温度までの温度域における平均冷却速度である。また、得られた熱延鋼板の一部に対しては、焼鈍温度700℃の溶融亜鉛めっきラインに通板して460℃のめっき浴(めっき組成:Zn-0.13mass%Al)に浸漬し、溶融亜鉛めっき材(GI材)とした。また一部の鋼板は、上記溶融亜鉛めっきラインに通板してめっき浴に浸漬した後、520℃で合金化処理を施して合金化溶融亜鉛めっき材(GA材)とした。めっき付着量はGI材、GA材ともに片面当たり45〜55g/m2とした。 A 250 mm thick slab (steel material) having the composition shown in Table 1 is cast by a continuous casting method. The slab is hot-rolled under the hot rolling conditions shown in Table 2, and further pickled after winding. Thus, a hot rolled steel sheet having a thickness of 1.4 to 2.0 mm was obtained. During continuous casting, electromagnetic stirring (EMS) was performed for the components other than steel plate No. 2 in Table 2 in order to reduce segregation of components. In addition, the cooling rate described in Table 2 is an average cooling rate in a temperature range from the finish rolling finish temperature to the winding temperature. Moreover, for a part of the obtained hot-rolled steel sheet, it is passed through a hot dip galvanizing line with an annealing temperature of 700 ° C. and immersed in a 460 ° C. plating bath (plating composition: Zn-0.13 mass% Al), A hot-dip galvanized material (GI material) was used. Some steel plates were passed through the hot dip galvanizing line and immersed in a plating bath, and then alloyed at 520 ° C. to obtain alloyed hot dip galvanized materials (GA materials). The plating adhesion amount was 45 to 55 g / m 2 per side for both GI and GA materials.
上記により得られた熱延鋼板(めっき無しの酸洗材、GI材、GA材)から試験片を採取し、以下の手法にしたがい組織観察を行い、ベイナイト相、フェライト相、マルテンサイト相等の面積率、セメンタイトの析出量および平均粒子径、ベイナイト相の結晶粒内に分散した炭化物(Tiおよび/またはVを含む炭化物)の平均粒子径を求めた。
また、上記により得られた熱延鋼板から試験片を採取し、以下の手法にしたがいSi濃度分布測定を行い、鋼板表面から0.2μmまでの深さにおけるSi濃化量と地鉄でのSi固溶量を求め、地鉄でのSi固溶量に対する前記Si濃化量の比率を求めた。
更に、上記により得られた熱延鋼板から試験片を採取し、以下の手法にしたがい、引張試験、打ち抜き加工試験および外観観察を行い、降伏強度、引張強さ、伸びを求めるとともに、熱延鋼板の表面性状評価および打ち抜き端面性状評価を行った。
Samples are taken from the hot-rolled steel sheet (plating pickling material, GI material, GA material) obtained as described above, and the structure is observed according to the following method, and the area of bainite phase, ferrite phase, martensite phase, etc. The average particle size of the carbide (including Ti and / or V) dispersed in the crystal grains of the bainite phase was determined.
In addition, specimens were collected from the hot-rolled steel sheet obtained as described above, and the Si concentration distribution was measured according to the following method, and the Si concentration in the depth from the steel sheet surface to 0.2 μm and the Si solidity in the steel The dissolution amount was determined, and the ratio of the Si enrichment amount to the Si solid solution amount in the base iron was determined.
Further, a test piece was collected from the hot-rolled steel sheet obtained as described above, and according to the following method, a tensile test, a punching process test and an appearance observation were performed to obtain yield strength, tensile strength, and elongation, and hot-rolled steel sheet. The surface property evaluation and the punched end surface property evaluation were performed.
(i)組織観察
フェライト相、ベイナイト相、マルテンサイト相等の各種金属組織の面積率は以下の手法により評価した。圧延方向に平行な断面の板厚中心部について、5%ナイタールによる腐食現出組織を走査型光学顕微鏡で400倍に拡大して10視野分撮影した。各種金属組織の面積率は、画像解析によりフェライト相やベイナイト相、マルテンサイト相等に分離し、観察視野面積に対する各種金属組織が占める面積の割合を面積率として求めた。
セメンタイトの析出量(質量%)は、得られた熱延鋼板の板厚中央部から採取した電解抽出用サンプル(0.2g)を、10%AA系電解液(10vol%アセチルアセトン-1mass%塩化テトラメチルアンモニウム-メタノール)中、電流密度20mA/cm2で定電流電解した後、炭化物を抽出し、その中に含まれるFeの質量%を求め、Fe:C=3:1としてセメンタイトの質量%を求めた。
セメンタイトの平均粒子径は、上記で抽出した炭化物を、走査型電子顕微鏡(SEM)を用いて観察した写真を用いて、セメンタイトの粒子径(円相当径)を測定することにより求めた。セメンタイトの同定にはSEMに付帯するエネルギー分散型X線分析装置(EDS)によりTiもしくはVが含まれない炭化物であることを確認すればよい。
ベイナイト相の結晶粒内の炭化物の平均粒子径は、得られた熱延鋼板の板厚中央部から薄膜法によってサンプルを作製し、透過型電子顕微鏡(倍率:120000倍)で観察を行い、100点以上の炭化物粒子に対する円相当径の平均によって求めた。この炭化物粒子径を算出するうえでは、対象とする炭化物を、Ti、Vのいずれか一方または両方を含む炭化物に限定する。Tiおよび/またはVを含む炭化物の同定は、透過型電子顕微鏡に付帯するEDXによって分析すればよい。
(I) Microstructure observation The area ratios of various metal structures such as ferrite phase, bainite phase, martensite phase and the like were evaluated by the following methods. About the central part of the plate thickness with a cross section parallel to the rolling direction, the corrosion appearance structure by 5% nital was magnified 400 times with a scanning optical microscope and photographed for 10 fields of view. The area ratio of various metal structures was separated into a ferrite phase, a bainite phase, a martensite phase, and the like by image analysis, and the ratio of the area occupied by the various metal structures to the observation visual field area was obtained as the area ratio.
The amount of cementite deposited (% by mass) was obtained by using a 10% AA electrolyte solution (10vol% acetylacetone-1mass% tetramethyl chloride) from a sample for electrolytic extraction (0.2g) collected from the center of the thickness of the obtained hot-rolled steel sheet. (Ammonium-methanol) After constant-current electrolysis at a current density of 20 mA / cm 2 , the carbide was extracted, and the mass% of Fe contained therein was determined, and the mass% of cementite was determined as Fe: C = 3: 1. It was.
The average particle size of cementite was determined by measuring the particle size (equivalent circle diameter) of cementite using a photograph obtained by observing the extracted carbide using a scanning electron microscope (SEM). For identification of cementite, it is only necessary to confirm that the carbide is free of Ti or V using an energy dispersive X-ray analyzer (EDS) attached to SEM.
The average particle size of carbides in the bainite phase grains was measured by a transmission electron microscope (magnification: 120,000 times) by preparing a sample by the thin film method from the center of the thickness of the obtained hot rolled steel sheet. It calculated | required by the average of the equivalent circle diameter with respect to the carbide | carbonized_particle particle | grains more than a point. In calculating the carbide particle diameter, the target carbide is limited to a carbide containing one or both of Ti and V. Identification of carbide containing Ti and / or V may be analyzed by EDX attached to a transmission electron microscope.
(ii)Si濃度分布測定
グロー放電発光分析法により濃度プロファイルを測定し、熱延鋼板表層のSi濃化量および地鉄のSi固溶量を求めた。
熱延鋼板表面(GI材、GA材の場合は、酸洗によってめっき層を剥離した後の表面)から深さ0.2μmまでのSi積算値と、深さ0.2μmから0.4μmまでのSi積算値を求め、熱延鋼板表面から深さ0.2μmまでの濃度ピーク(Si濃化量)に深さ0.2μmから0.4μmまでの平均強度(Si固溶量)を割り付けることで、鋼板表面から0.2μmまでの深さにおけるSi濃化量と地鉄でのSi固溶量の比(Si濃化量/Si固溶量)を求めた。
(Ii) Si concentration distribution measurement The concentration profile was measured by glow discharge optical emission spectrometry, and the Si concentration of the hot-rolled steel sheet surface layer and the Si solid solution of the ground iron were determined.
Si integrated value from the hot-rolled steel sheet surface (in the case of GI and GA materials, the surface after the plating layer is peeled off by pickling) to a depth of 0.2 μm and Si integrated value from a depth of 0.2 μm to 0.4 μm By assigning the average strength (Si solid solution amount) from 0.2 μm to 0.4 μm in depth to the concentration peak (Si enrichment amount) from the hot rolled steel plate surface to 0.2 μm depth, 0.2 μm from the steel plate surface. The ratio of the Si concentration in the depth up to the Si solid solution amount in the base iron (Si concentration / Si solid solution amount) was determined.
(iii)引張試験
得られた熱延鋼板から圧延方向と垂直方向にJIS13B号引張試験片を作製し、JIS Z 2241(2011)の規定に準拠した引張試験を5回行い、平均の降伏強度(YS)、引張強さ(TS)、全伸び(El)を求めた。引張試験のクロスヘッドスピードは10mm/minとした。
(Iii) Tensile test JIS13B tensile test piece was produced from the obtained hot-rolled steel sheet in the direction perpendicular to the rolling direction, the tensile test was conducted 5 times in accordance with the provisions of JIS Z 2241 (2011), and the average yield strength ( YS), tensile strength (TS), and total elongation (El) were determined. The crosshead speed in the tensile test was 10 mm / min.
(iv)打ち抜き加工試験(打ち抜き端面性状評価)
得られた熱延鋼板の各々について、鋼板長手方向に50点の打ち抜き加工を行い、その打ち抜き端面の欠陥の有無を目視により観察した。なお、打ち抜き加工は、鋼板長手方向に直径:10mmφの穴を100mm間隔(穴の中心同士の間隔)で50点、打ち抜き加工(クリアランス:試験片板厚の13%)することにより行った。打ち抜き端面に亀裂や段差、めくれ、はがれなど端面に異常部が観察された場合には評価を“不良(×)”、これらの異常部が観察されない場合には評価を“良好(○)”とした。
(Iv) Punching test (evaluation of punched end face properties)
Each of the obtained hot-rolled steel sheets was punched at 50 points in the longitudinal direction of the steel sheet, and the presence or absence of defects on the punched end face was visually observed. The punching was performed by punching 50 holes with a diameter of 10 mmφ in the longitudinal direction of the steel plate at 100 mm intervals (intervals between the centers of the holes) (clearance: 13% of the test piece plate thickness). If abnormal parts are observed on the end face, such as cracks, steps, turning, peeling, etc. on the punched end face, the evaluation is “defect (×)”. If these abnormal parts are not observed, the evaluation is “good (◯)”. did.
(v)外観観察
得られた熱延鋼板の各々について、表面積が1.5m2以上の領域の鋼板表面を目視観察し、表面性状の優劣を判断した。めっき無しの酸洗材については、熱延鋼板表面に赤スケールや赤スケール由来の凹凸が認められた場合の評価を“不良”とし、上記赤スケールおよび上記凹凸がいずれも認められなかった場合の評価を“良好”とした。めっき材(GI材、GA材)については不めっき部が認められた場合の評価を不良として以下の表3中に“不めっき”と記す一方、不めっき部が認められなかった場合の評価を“良好”とした。
得られた結果を表3に示す。
(V) Appearance observation For each of the obtained hot-rolled steel sheets, the surface of the steel sheet having a surface area of 1.5 m 2 or more was visually observed to determine whether the surface properties were superior or inferior. For pickling materials without plating, the evaluation when red scale or unevenness derived from red scale is observed on the surface of the hot-rolled steel sheet is “bad”, and the above red scale and unevenness are not recognized. The evaluation was “good”. For plating materials (GI materials, GA materials), the evaluation when the non-plating part is recognized is regarded as a failure, and “non-plating” is indicated in Table 3 below, while the evaluation when the non-plating part is not recognized “Good”.
The obtained results are shown in Table 3.
本発明例はいずれも、引張強さTS:780MPa以上であり且つ表面性状および打抜性(打ち抜き端面性状)にも優れた熱延鋼板となっている。一方、本発明の範囲を外れる比較例は、良好な表面性状もしくは打抜性が得られていない。 All of the inventive examples are hot-rolled steel sheets having a tensile strength TS of 780 MPa or more and excellent surface properties and punching properties (punching end surface properties). On the other hand, in the comparative example which is out of the scope of the present invention, good surface properties or punchability is not obtained.
Claims (12)
C :0.06%以上0.13%以下、 Si:0.09%以下、
Mn:0.01%以上1.20%以下、 P :0.03%以下、
S :0.005%以下、 Al:0.1%以下、
N :0.01%以下、 Ti:0.13%以上0.20%以下、
V :0.03%以上0.13%以下
を含有し、
Nb:0.02%以下(0を含む)
に制限し、残部がFeおよび不可避的不純物からなる組成と、ベイナイト相の面積率が80%以上、フェライト相の面積率が15%以下(0を含む)、マルテンサイト相の面積率が5%以下(0を含む)、セメンタイトの析出量が質量%で0.08%以上、該セメンタイトの平均粒子径が2μm以下であり、前記ベイナイト相の結晶粒内に平均粒子径が10nm未満である炭化物が微細分散した組織を有し、鋼板表面から0.2μmまでの深さにおけるSi濃化量が地鉄でのSi固溶量の1.3倍以下であり、引張強さが780MPa以上であることを特徴とする高強度熱延鋼板。 % By mass
C: 0.06% or more and 0.13% or less, Si: 0.09% or less,
Mn: 0.01% or more and 1.20% or less, P: 0.03% or less,
S: 0.005% or less, Al: 0.1% or less,
N: 0.01% or less, Ti: 0.13% or more and 0.20% or less,
V: contains 0.03% or more and 0.13% or less,
Nb: 0.02% or less (including 0)
With a balance of Fe and inevitable impurities, a bainite phase area ratio of 80% or more, a ferrite phase area ratio of 15% or less (including 0), and a martensite phase area ratio of 5% Below (including 0), the precipitation amount of cementite is 0.08% or more by mass%, the average particle diameter of the cementite is 2 μm or less, and the carbide having an average particle diameter of less than 10 nm is fine in the crystal grains of the bainite phase. It has a dispersed structure, the Si concentration at a depth of 0.2 μm from the steel sheet surface is 1.3 times or less of the Si solid solution amount in the base iron, and the tensile strength is 780 MPa or more. High strength hot rolled steel sheet.
C :0.06%以上0.13%以下、 Si:0.09%以下、
Mn:0.01%以上1.20%以下、 P :0.03%以下、
S :0.005%以下、 Al:0.1%以下、
N :0.01%以下、 Ti:0.13%以上0.20%以下、
V :0.03%以上0.13%以下
を含有し、
Nb:0.02%以下(0を含む)
に制限し、残部がFeおよび不可避的不純物からなる組成とし、前記加熱の加熱温度を1150℃以上1350℃以下とし、前記仕上げ圧延の仕上げ圧延終了温度を850℃以上とし、前記冷却を仕上げ圧延終了後3s以内に開始し、前記冷却の冷却速度を40℃/s以上とし、前記巻き取りの巻取り温度を350℃以上550℃以下とすることを特徴とするベイナイト相の面積率が80%以上、フェライト相の面積率が15%以下(0を含む)、マルテンサイト相の面積率が5%以下(0を含む)、セメンタイトの析出量が質量%で0.08%以上、該セメンタイトの平均粒子径が2μm以下であり、前記ベイナイト相の結晶粒内に平均粒子径が10nm未満である炭化物が微細分散した組織を有し、鋼板表面から0.2μmまでの深さにおけるSi濃化量が地鉄でのSi固溶量の1.3倍以下であり、引張強さが780MPa以上である高強度熱延鋼板の製造方法。 The steel material is heated, subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled, wound, and made into a hot-rolled steel sheet, the steel material in mass%,
C: 0.06% or more and 0.13% or less, Si: 0.09% or less,
Mn: 0.01% or more and 1.20% or less, P: 0.03% or less,
S: 0.005% or less, Al: 0.1% or less,
N: 0.01% or less, Ti: 0.13% or more and 0.20% or less,
V: contains 0.03% or more and 0.13% or less,
Nb: 0.02% or less (including 0)
The balance is composed of Fe and inevitable impurities, the heating temperature of the heating is 1150 ° C. or higher and 1350 ° C. or lower, the finish rolling finish temperature of the finish rolling is 850 ° C. or higher, and the cooling is finished. The area ratio of the bainite phase is 80% or more , starting within 3 s later, the cooling rate of the cooling is 40 ° C./s or more, and the winding temperature of the winding is 350 ° C. or more and 550 ° C. or less The area ratio of the ferrite phase is 15% or less (including 0), the area ratio of the martensite phase is 5% or less (including 0), the precipitation amount of cementite is 0.08% or more by mass%, the average particle diameter of the cementite Is 2 μm or less, and has a microstructure in which carbides having an average particle diameter of less than 10 nm are finely dispersed in the crystal grains of the bainite phase, and the Si concentration at a depth of 0.2 μm from the steel sheet surface is Less than 1.3 times the amount of Si solid solution and tensile strength is 780MPa Process for producing a high strength hot rolled steel sheet which is above.
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