JP2017214618A - Production method of low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness - Google Patents
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Abstract
Description
本発明は、ラインパイプに使用されるスパイラル鋼管あるいは電縫鋼管の素材として好適な、低温靭性に優れた低降伏比高強度熱延鋼板の製造方法に関する。 The present invention relates to a method for producing a low-yield-ratio high-strength hot-rolled steel sheet excellent in low-temperature toughness, which is suitable as a material for spiral steel pipes or ERW steel pipes used for line pipes.
鋼板をらせん状に巻きながら造管するスパイラル鋼管は、太径の鋼管を効率的に製造できることから、近年、原油、天然ガスを輸送するラインパイプ用として多用されるようになってきた。とくに、長距離輸送するパイプラインでは、輸送効率を高めることが要求され高圧化しており、また油井やガス井が寒冷地に多く存在することもあり、寒冷地を径由することが多い。このため、使用されるラインパイプは、高強度化、高靭性化することが要求されている。さらに、耐座屈性、耐震性の観点から、ラインパイプは、低降伏比であることが求められている。スパイラル鋼管の管長手方向の降伏比は、造管によってほとんど変化せず、素材である熱延鋼板のそれとほぼ一致する。そのため、スパイラル鋼管製のラインパイプを低降伏比化するためには、素材である熱延鋼板の降伏比を低くすることが必要となる。 In recent years, spiral steel pipes that are formed by spirally winding a steel sheet can be used to efficiently produce large-diameter steel pipes, and in recent years have come to be widely used as line pipes for transporting crude oil and natural gas. In particular, pipelines for long-distance transportation are required to have high transportation efficiency and have a high pressure, and there are many oil wells and gas wells in the cold region, which often leads to the cold region. For this reason, the line pipe used is required to have high strength and high toughness. Furthermore, from the viewpoint of buckling resistance and earthquake resistance, the line pipe is required to have a low yield ratio. The yield ratio of the spiral steel pipe in the longitudinal direction of the pipe hardly changes depending on the pipe making, and almost coincides with that of the hot-rolled steel sheet. Therefore, in order to reduce the yield ratio of a line pipe made of spiral steel pipe, it is necessary to lower the yield ratio of the hot-rolled steel sheet as the material.
このような要求に対し、例えば特許文献1には、低温靭性に優れた低降伏比高張力ラインパイプ用熱延鋼板の製造方法が記載されている。特許文献1に記載された技術では、重量%で、C:0.03〜0.12%、Si:0.50%以下、Mn:1.70%以下、Al:0.070%以下を含有し、さらに、Nb:0.01〜0.05%、V:0.01〜0.02%、Ti:0.01〜0.20%のうちの少なくとも1種を含有する鋼スラブを、1180〜1300℃に加熱した後、粗圧延終了温度:950〜1050℃、仕上圧延終了温度:760〜800℃の条件で熱間圧延を行い、5〜20℃/sの冷却速度で冷却し、670℃に至るまでの間に空冷を開始し5〜20s間保持し、次いで20℃/s以上の冷却速度で冷却し、500℃以下の温度で巻取り、熱延鋼板とするとしている。特許文献1に記載された技術によれば、引張強さ:60kg/mm2以上(590MPa以上)で85%以下の低降伏比と、破面遷移温度:−60℃以下の高靭性を有する熱延鋼板が製造できるとしている。 In response to such a demand, for example, Patent Document 1 describes a method of manufacturing a hot-rolled steel sheet for a low-yield ratio high-tensile line pipe excellent in low-temperature toughness. The technology described in Patent Document 1 contains, by weight, C: 0.03-0.12%, Si: 0.50% or less, Mn: 1.70% or less, Al: 0.070% or less, and Nb: 0.01-0.05%. , V: 0.01 to 0.02%, Ti: 0.01 to 0.20% of a steel slab containing at least one kind is heated to 1180 to 1300 ° C, followed by rough rolling finish temperature: 950 to 1050 ° C, finish rolling finish temperature : Hot rolled under conditions of 760 to 800 ° C, cooled at a cooling rate of 5 to 20 ° C / s, started air cooling until reaching 670 ° C, held for 5 to 20s, then 20 ° C / It is cooled at a cooling rate of s or higher and wound at a temperature of 500 ° C. or lower to form a hot rolled steel sheet. According to the technique described in Patent Document 1, a tensile strength: 60 kg / mm 2 or more (590 MPa or more) and a low yield ratio of 85% or less, and a fracture surface transition temperature: high toughness of −60 ° C. or less. It is said that rolled steel sheets can be manufactured.
特許文献2には、高強度低降伏比パイプ用熱延鋼板の製造方法が記載されている。特許文献2に記載された技術は、重量%で、C:0.02〜0.12%、Si:0.1〜1.5%、Mn:2.0%以下、Al:0.01〜0.10%を含有し、さらに、Mo+Cr:0.1〜1.5%を含有する鋼を1000〜1300℃に加熱し、750〜950℃の範囲で熱間圧延を終了し、冷却速度:10〜50℃/sにて巻取温度まで冷却し、480〜600℃の範囲で巻取る、熱延鋼板の製造方法である。特許文献2に記載された技術によれば、オーステナイト温度域からの急冷を行うことなく、フェライトを主体とし、面積率で1〜20%のマルテンサイトを有し、85%以下の低降伏比を有し、かつ造管後の降伏強さ低下量の少ない熱延鋼板が得られるとしている。 Patent Document 2 describes a method for producing a hot-rolled steel sheet for a high-strength, low-yield ratio pipe. The technique described in Patent Document 2 contains, by weight%, C: 0.02 to 0.12%, Si: 0.1 to 1.5%, Mn: 2.0% or less, Al: 0.01 to 0.10%, and Mo + Cr: 0.1 to Steel containing 1.5% is heated to 1000-1300 ° C, hot rolling is finished in the range of 750-950 ° C, and cooled to the coiling temperature at a cooling rate of 10-50 ° C / s, 480-600 It is a manufacturing method of a hot-rolled steel sheet which is wound up in the range of ° C. According to the technology described in Patent Document 2, without quenching from the austenite temperature range, ferrite is the main component, the area ratio is 1 to 20% martensite, and the low yield ratio is 85% or less. It is said that a hot-rolled steel sheet having a reduced yield strength after pipe making is obtained.
特許文献3には、低温靭性に優れた低降伏比電縫鋼管の製造方法が記載されている。特許文献3に記載された技術では、質量%で、C:0.01〜0.09%、Si:0.50%以下、Mn:2.5%以下、Al:0.01〜0.10%、Nb:0.005〜0.10%を含み、さらにMo:0.5%以下、Cu:0.5%以下、Ni:0.5%以下、Cr:0.5%以下のうちの1種または2種以上を、Mn、Si、P、Cr、Ni、Moの含有量の関係式であるMneqが2.0以上を満足するように含有する組成のスラブを熱間圧延し、5℃/s以上の冷却速度で500〜650℃まで冷却して巻取り、この温度範囲で10min以上滞留させてから500℃未満の温度まで冷却して熱延鋼板とし、該熱延鋼板を造管して電縫鋼管とする。特許文献3に記載された技術によれば、ベイニティックフェライトを主相とし、3%以上のマルテンサイトと、必要に応じ1%以上の残留オーステナイトを含む組織を有し、破面遷移温度が−50℃以下で、低温靭性に優れ、かつ高い塑性変形吸収能を有する電縫鋼管を製造できるとしている。 Patent Document 3 describes a method for producing a low yield ratio electric resistance welded steel pipe excellent in low temperature toughness. The technique described in Patent Document 3 includes, in mass%, C: 0.01 to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01 to 0.10%, Nb: 0.005 to 0.10%, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, the relationship between the contents of Mn, Si, P, Cr, Ni, Mo A slab having a composition containing Mneq satisfying 2.0 or more is hot-rolled, cooled to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and retained in this temperature range for 10 minutes or more. Then, it is cooled to a temperature of less than 500 ° C. to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is formed into an electric-welded steel pipe. According to the technique described in Patent Document 3, it has a structure containing bainitic ferrite as a main phase and containing 3% or more martensite and, if necessary, 1% or more retained austenite, and has a fracture surface transition temperature of It is said that an ERW steel pipe having excellent low temperature toughness and high plastic deformation absorption ability can be produced at -50 ° C or lower.
また、特許文献4には、質量%で、C:0.03〜0.10%、Si:0.01〜0.50%、Mn:1.4〜2.2%、Al:0.005〜0.10%、Nb:0.02〜0.10%、Ti:0.001〜0.030%、Mo:0.05〜0.50%、Cr:0.05〜0.50%、Ni:0.01〜0.50%を含み、Mo、Cr、Mn、Niの関係式であるMoeqが1.4〜2.2%の範囲を満足するように含有する組成の鋼素材を熱間圧延し、5℃/s〜30℃/sで600〜450℃温度域まで冷却し、巻取り工程までの間に、さらに、2℃/s以下の冷却或いは20秒以上滞留させて、コイル状に巻取り、熱延鋼板とする技術が記載されている。特許文献4に記載された技術によれば、平均粒径が10μm以下であるベイニティックフェライト主相と、面積率で1.4〜15%の、アスペクト比:5.0未満、大きさが最大で5.0μm以下、平均で0.5〜3.0μmの塊状マルテンサイトを含む組織を含有し、圧延方向から30°方向のYSが480MPa以上で、板幅方向の引張強さが600MPa以上で、vTrsが-80℃以下で、且つ降伏比が85%以下の、低温靭性に優れた低降伏比高強度鋼板を製造できるとしている。 Further, in Patent Document 4, in mass%, C: 0.03 to 0.10%, Si: 0.01 to 0.50%, Mn: 1.4 to 2.2%, Al: 0.005 to 0.10%, Nb: 0.02 to 0.10%, Ti: 0.001 -0.030%, Mo: 0.05-0.50%, Cr: 0.05-0.50%, Ni: 0.01-0.50%, Moeq which is the relational expression of Mo, Cr, Mn, Ni satisfies the range of 1.4-2.2% The steel material having a composition as described above is hot-rolled, cooled to 600 to 450 ° C at 5 ° C / s to 30 ° C / s, and further up to 2 ° C / s until the winding process. A technique is described that cools or stays for 20 seconds or more, winds in a coil shape, and forms a hot-rolled steel sheet. According to the technique described in Patent Document 4, a bainitic ferrite main phase having an average particle size of 10 μm or less, an area ratio of 1.4 to 15%, an aspect ratio of less than 5.0, and a maximum size of 5.0 μm Hereinafter, it contains a structure containing massive martensite of 0.5 to 3.0 μm on average, YS in the direction of 30 ° from the rolling direction is 480 MPa or more, tensile strength in the plate width direction is 600 MPa or more, and vTrs is -80 ° C. or less In addition, it is said that a low-yield-ratio high-strength steel sheet excellent in low-temperature toughness with a yield ratio of 85% or less can be manufactured.
しかしながら、特許文献1に記載された技術では、冷却速度、冷却停止温度等を所定の比較的速い冷却範囲内となるように制御する必要があり、とくに、厚肉の熱延鋼板を製造するためには、大掛かりな冷却設備等を必要とするという問題がある。また、特許文献1に記載された技術で得られる熱延鋼板は、軟質なポリゴナルフェライトを主とする組織を有し、所望の高強度を得にくいという問題もある。 However, in the technique described in Patent Document 1, it is necessary to control the cooling rate, the cooling stop temperature, and the like so as to be within a predetermined relatively fast cooling range, in particular, for producing a thick hot-rolled steel sheet. However, there is a problem that a large-scale cooling facility is required. Moreover, the hot-rolled steel sheet obtained by the technique described in Patent Document 1 has a problem that it has a structure mainly composed of soft polygonal ferrite and it is difficult to obtain a desired high strength.
また、特許文献2に記載された技術では、依然として造管後の降伏強さの低下が認められ、最近の鋼管強度の増加要求を満足できない場合が生じるという問題がある。 Moreover, in the technique described in Patent Document 2, a decrease in yield strength after pipe forming is still recognized, and there is a problem that a recent increase in steel pipe strength cannot be satisfied.
また、特許文献3に記載された技術では、最近の寒冷地仕様である、破面遷移温度vTrsが−80℃以下という優れた低温靭性を安定して確保できるまでには至っていないという問題がある。 In addition, the technique described in Patent Document 3 has a problem in that it has not yet been able to stably secure excellent low-temperature toughness, which is a recent cold region specification, with a fracture surface transition temperature vTrs of −80 ° C. or lower. .
また、特許文献4に記載された技術では、二次冷却を行なうための長い設備が必要となり、大掛かりな設備投資が必要となり経済面で問題がある。 Moreover, the technique described in Patent Document 4 requires a long facility for performing secondary cooling, and requires a large facility investment, which is problematic in terms of economy.
本発明は、かかる課題を解決し、複雑な熱処理を施すことなく、また、大掛かりな設備改造を行なうことなく、鋼管用素材、とくにスパイラル鋼管用として好適な、スパイラル造管後の強度低下が防止できる、低温靭性に優れた低降伏比高強度熱延鋼板を提供することを目的とする。 The present invention solves such a problem and prevents a decrease in strength after spiral pipe making, which is suitable for a steel pipe material, particularly for a spiral steel pipe, without performing complicated heat treatment and without extensive equipment modification. An object of the present invention is to provide a high yield hot-rolled steel sheet having a low yield ratio and excellent in low temperature toughness.
本発明者らは、上記した目的を達成するために、造管後の鋼管強度、および鋼管靭性に及ぼす各種要因について鋭意研究した。その結果、造管による強度の低下は、圧縮応力が作用する管内面側でのバウシンガー効果による降伏強さの低下と、引張応力が作用する管外面側での降伏伸びの消失とによって、引き起こされていることを見出した。 In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting steel pipe strength after pipe making and steel pipe toughness. As a result, the decrease in strength due to pipe making is caused by the decrease in yield strength due to the Bauschinger effect on the inner surface of the tube where compressive stress acts and the disappearance of yield elongation on the outer surface side where tensile stress acts. I found out.
そこで、本発明者らは、更なる研究を行った結果、鋼板の組織を、微細なベイニティックフェライトを主相とし、ベイニティックフェライト中に硬質な塊状マルテンサイトを微細分散させた組織とすることにより、造管後、とくにスパイラル造管後の強度低下を防止できるとともに、85%以下の低降伏比を有し、さらに優れた靭性をも兼備する鋼管とすることができることに想到した。すなわち、このような組織とすることにより、鋼管素材である鋼板の加工硬化能が向上するため、造管時における管外面側での加工硬化により十分な強度上昇が得られ、造管後、とくにスパイラル造管後、の強度低下を抑制でき、さらに塊状マルテンサイトを微細に分散させることにより、靭性が顕著に向上することを知見した。 Therefore, as a result of further research, the inventors of the present invention have a structure in which the structure of the steel sheet has fine bainitic ferrite as the main phase, and hard massive martensite is finely dispersed in bainitic ferrite. As a result, it was conceived that a steel pipe having a low yield ratio of 85% or less and also having excellent toughness can be obtained after pipe forming, in particular, after strength reduction after spiral pipe forming. That is, by making such a structure, the work hardening ability of the steel plate material steel plate is improved, so that a sufficient strength increase is obtained by work hardening on the pipe outer surface side during pipe making, especially after pipe making, It was found that after spiral pipe making, the strength reduction can be suppressed, and the toughness is remarkably improved by finely dispersing the massive martensite.
そして、Mo、 Mn、 Cr、 Niの添加バランスを適正な範囲内とした上で、巻取り後60秒以上保持し、次いでコイル全体を冷却することで、二次冷却を行なうことなく、能率的に塊状マルテンサイトを得ることができることを知見した。 And, keeping the addition balance of Mo, Mn, Cr, Ni within the proper range, holding for 60 seconds or more after winding, and then cooling the whole coil, it is efficient without performing secondary cooling It was found that massive martensite can be obtained.
本発明は以上の知見に基づいてなされたものであり、以下を要旨とするものである。
[1]鋼素材に、熱延工程、冷却工程、巻取工程を施して、熱延鋼板とするにあたり、前記鋼素材を、質量%で、C:0.03〜0.10%、Si:0.10〜0.50%、Mn:1.4〜2.2%、P:0.025%以下、S:0.005%以下、Al:0.005〜0.10%、Nb:0.02〜0.10%、 Ti:0.001〜0.030%、Mo:0.05〜0.50%、 Cr:0.05〜0.50%、Ni:0.001〜1.00%を含み、残部Feおよび不可避的不純物からなる組成を有する鋼素材とし、前記熱延工程が、前記鋼素材を加熱温度:1050〜1300℃に加熱し、該加熱された鋼素材に、粗圧延を施しシートバーとし、該シートバーに、930℃以下の温度域での累積圧下率:50%以上となる仕上圧延を施し熱延鋼板とする工程とし、前記冷却工程が、仕上圧延終了後直ちに冷却を開始し、板厚中央部の平均冷却速度:5〜60℃/sで、冷却停止温度:Bs点〜450℃の温度域まで冷却する工程とし、巻取工程が、コイル状に巻取り、60秒以上600秒未満保持した後、冷却する工程とすることを特徴とする低温靭性に優れた低降伏比高強度熱延鋼板の製造方法。
[2]前記組成が、質量%で、下記(1)式で定義されるMoeq が1.4〜2.2%の範囲を満足する組成であることを特徴とする上記[1]に記載の低温靭性に優れた低降伏比高強度熱延鋼板の製造方法。
記
Moeq(%)=Mo+0.36Cr+0.77Mn+0.07Ni ‥‥(1)
ここで、Mn、Ni、Cr、Mo:各元素の含有量(質量%)
[3]前記組成に加えてさらに、質量%で、Cu:0.50%以下、V:0.10%以下、B:0.0005%以下のうちから選ばれた1種または2種以上を含有することを特徴とする上記[1]または[2]に記載の低温靭性に優れた低降伏比高強度熱延鋼板の製造方法。
[4]前記組成に加えてさらに、質量%で、Ca:0.0005〜0.0050%を含有することを特徴とする上記[1]ないし[3]のいずれかに記載の低温靭性に優れた低降伏比高強度熱延鋼板の製造方法。
This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] When a steel material is subjected to a hot rolling process, a cooling process, and a winding process to form a hot rolled steel sheet, the steel material is mass%, C: 0.03-0.10%, Si: 0.10-0.50% , Mn: 1.4 to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.02 to 0.10%, Ti: 0.001 to 0.030%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ni: 0.001 to 1.00%, a steel material having a composition consisting of the remaining Fe and inevitable impurities, the hot rolling step, the steel material is heated to a heating temperature of 1050 to 1300 ° C, The heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar is subjected to a finish rolling at a temperature range of 930 ° C. or lower and finish rolling to be 50% or more to form a hot rolled steel sheet, The cooling process starts cooling immediately after finishing rolling, and the cooling is performed at an average cooling rate of 5 to 60 ° C./s at the center of the plate thickness to a cooling stop temperature of Bs point to 450 ° C. A low-yield ratio high-strength hot-rolled steel sheet with excellent low-temperature toughness, characterized in that the winding step is a step of winding in a coil shape, holding for 60 seconds to less than 600 seconds, and then cooling. Method.
[2] Excellent low-temperature toughness as described in [1] above, wherein the composition is a mass% and the Moeq defined by the following formula (1) satisfies the range of 1.4 to 2.2%. A low yield ratio high strength hot rolled steel sheet manufacturing method.
Record
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
Here, Mn, Ni, Cr, Mo: Content of each element (mass%)
[3] In addition to the above composition, the composition further comprises one or more selected from Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less in mass%. The method for producing a low yield ratio high strength hot rolled steel sheet having excellent low temperature toughness as described in [1] or [2] above.
[4] The low yield ratio excellent in low temperature toughness according to any one of the above [1] to [3], further containing Ca: 0.0005 to 0.0050% by mass% in addition to the composition Manufacturing method of high-strength hot-rolled steel sheet.
なお、本発明において、「高強度」とは、圧延方向から30度方向の降伏強さが480MPa以上および板幅方向の引張強さが600MPa以上である場合を、また「低温靭性に優れた」とは、シャルピー衝撃試験の破面遷移温度vTrsが−80℃以下である場合を、また、「低降伏比」とは、連続降伏型の応力歪曲線を示し、降伏比が85%以下である場合を、それぞれ云うものとする。また、「鋼板」には鋼板および鋼帯を含むものとする。 In the present invention, “high strength” means a case where the yield strength in the direction of 30 ° from the rolling direction is 480 MPa or more and the tensile strength in the sheet width direction is 600 MPa or more, and “excellent in low temperature toughness”. Is a case where the fracture surface transition temperature vTrs of the Charpy impact test is −80 ° C. or lower, and “low yield ratio” indicates a continuous yield type stress-strain curve, and the yield ratio is 85% or lower. Each case shall be said. The “steel plate” includes a steel plate and a steel strip.
本発明によれば、低温靭性に優れた低降伏比高強度熱延鋼板を容易にかつ安価に製造することができる。
リールパージ法で敷設されるラインパイプや、耐震性を要求されるラインパイプ用の電縫鋼管を安価にかつ容易に製造できるという効果もある。
また、本発明の低降伏比高強度熱延鋼板を素材として用いれば、耐震性に優れた建築用部材となる高強度スパイラル鋼管杭が製造できるという効果もある。
また、本発明では、巻取機内に水冷又は空冷装置を設けるか、オフラインで水冷又は空冷設備を設けることによって、熱延設備の機長を延長したりライン内に保温カバー、誘導加熱装置等の大掛かりな設備投資を必要とせず、巻取前での一定時間保持も省略することができる。
以上のように、本発明は、産業上極めて有用な発明である。
According to the present invention, a low yield ratio high strength hot rolled steel sheet excellent in low temperature toughness can be easily and inexpensively manufactured.
There is also an effect that it is possible to easily and inexpensively manufacture a line pipe laid by the reel purge method and an electric resistance welded steel pipe for a line pipe that requires earthquake resistance.
Moreover, if the low yield ratio high-strength hot-rolled steel sheet of the present invention is used as a material, there is also an effect that a high-strength spiral steel pipe pile that becomes a building member having excellent earthquake resistance can be manufactured.
Further, in the present invention, by installing a water cooling or air cooling device in the winder, or by providing a water cooling or air cooling device offline, the machine length of the hot rolling facility is extended or a large cover such as a heat insulating cover or an induction heating device is installed in the line. It is not necessary to make a large capital investment, and holding for a certain period of time before winding can be omitted.
As described above, the present invention is an industrially extremely useful invention.
以下、本発明について具体的に説明する。
まず、本発明の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%を意味するものとする。
Hereinafter, the present invention will be specifically described.
First, 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% unless there is particular notice.
C:0.03〜0.10%
Cは、炭化物として析出し、析出強化を介し鋼板の強度増加に寄与するとともに、結晶粒微細化を介し鋼板の靭性向上にも寄与する元素である。さらに、Cは、鋼中に固溶しオーステナイトを安定化し、未変態オーステナイトの形成を促進する作用を有する。このような効果を得るためには、0.03%以上の含有を必要とする。一方、0.10%を超える含有は、結晶粒界に粗大なセメンタイトを形成する傾向が強くなり、靭性が低下する。このため、Cは0.03〜0.10%の範囲に限定した。なお、好ましくは0.04%以上である。好ましくは0.09%以下である。
C: 0.03-0.10%
C is an element which precipitates as a carbide and contributes to an increase in the strength of the steel sheet through precipitation strengthening and also contributes to an improvement in the toughness of the steel sheet through refinement of crystal grains. Further, C has an action of forming a solid solution in the steel, stabilizing austenite, and promoting the formation of untransformed austenite. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, if the content exceeds 0.10%, the tendency to form coarse cementite at the grain boundaries becomes strong, and the toughness decreases. For this reason, C was limited to the range of 0.03-0.10%. In addition, Preferably it is 0.04% or more. Preferably it is 0.09% or less.
Si:0.10〜0.50%
Siは、固溶強化を介して鋼板の強度増加に寄与するとともに、硬質第二相(例えば、マルテンサイト)の形成を介し、降伏比低減に寄与する。このような効果を得るためには、0.10%以上の含有を必要とする。一方、0.50%を超える含有は、赤スケールの生成が顕著となり、鋼板外観性状が低下する。このため、Siは0.10〜0.50%の範囲に限定した。なお、好ましくは0.20%以上である。好ましくは0.40%以下である。
Si: 0.10 to 0.50%
Si contributes to an increase in the strength of the steel sheet through solid solution strengthening and contributes to a reduction in the yield ratio through the formation of a hard second phase (for example, martensite). In order to obtain such an effect, the content of 0.10% or more is required. On the other hand, if the content exceeds 0.50%, the generation of red scale becomes remarkable, and the appearance of the steel sheet deteriorates. For this reason, Si was limited to the range of 0.10 to 0.50%. In addition, Preferably it is 0.20% or more. Preferably it is 0.40% or less.
Mn:1.4〜2.2%
Mnは、固溶して鋼の焼入れ性を向上させ、マルテンサイトの生成を促進させる。また、ベイニティックフェライト変態開始温度を低下させ、組織の微細化を介して鋼板靭性の向上に寄与する元素である。このような効果を得るためには、1.4%以上の含有を必要とする。一方、2.2%を超える含有は、溶接熱影響部の靭性を低下させる。このため、Mnは1.4〜2.2%の範囲に限定した。なお、塊状マルテンサイトの安定生成という観点からは、好ましくは1.6%以上である。好ましくは2.0%以下である。
Mn: 1.4-2.2%
Mn dissolves to improve the hardenability of the steel and promote the formation of martensite. Further, it is an element that lowers the bainitic ferrite transformation start temperature and contributes to improvement of steel sheet toughness through refinement of the structure. In order to obtain such an effect, the content of 1.4% or more is required. On the other hand, the content exceeding 2.2% lowers the toughness of the weld heat affected zone. For this reason, Mn was limited to the range of 1.4 to 2.2%. From the viewpoint of stable production of massive martensite, it is preferably 1.6% or more. Preferably it is 2.0% or less.
P:0.025%以下
Pは、固溶して鋼板強度の増加に寄与するが、同時に靭性を低下させる。このため、本発明では、Pは不純物として可及的に低減することが望ましいが、0.025%までは許容できる。好ましくは0.015%以下である。なお、過度の低減は精錬コストを高騰させるため、0.001%以上程度とすることが望ましい。
P: 0.025% or less P dissolves and contributes to an increase in steel sheet strength, but at the same time lowers toughness. Therefore, in the present invention, it is desirable to reduce P as an impurity as much as possible, but up to 0.025% is acceptable. Preferably it is 0.015% or less. In addition, since excessive reduction raises refining cost, it is desirable to set it as about 0.001% or more.
S:0.005%以下
Sは、鋼中ではMnS等の粗大な硫化物系介在物を形成し、スラブ等の割れを生起するとともに、鋼板の延性を低下させる。このような現象は0.005%を超える含有で顕著になる。このため、Sは0.005%以下に限定した。なお、好ましくは0.004%以下である。
S: 0.005% or less S forms coarse sulfide inclusions such as MnS in steel, causes cracking of slabs and the like, and decreases the ductility of the steel sheet. Such a phenomenon becomes remarkable when the content exceeds 0.005%. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.004% or less.
Al:0.005〜0.10%
Alは、脱酸剤として作用するとともに、歪時効の原因となるNを固定するのに有効な元素である。このような効果を得るためには、0.005%以上の含有を必要とする。一方、0.10%を超える含有は、鋼中酸化物が増加し母材および溶接部の靭性を低下させる。また、スラブ等の鋼素材、鋼板を加熱炉で加熱する際に、表層で窒化層を形成しやすく、降伏比の増加をもたらす恐れがある。このため、Alは0.005〜0.10%の範囲に限定した。なお、好ましくは0.08%以下である。
Al: 0.005-0.10%
Al is an element that acts as a deoxidizer and is effective in fixing N that causes strain aging. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.10%, the oxide in the steel increases and the toughness of the base metal and the welded portion decreases. Further, when a steel material such as a slab or a steel plate is heated in a heating furnace, a nitride layer is easily formed on the surface layer, which may increase the yield ratio. For this reason, Al was limited to 0.005 to 0.10% of range. In addition, Preferably it is 0.08% or less.
Nb:0.02〜0.10%
Nbは、鋼中に固溶し、あるいは炭窒化物として析出し、オーステナイト粒の粗大化を抑制するとともに、オーステナイト粒の再結晶を抑制する作用を有し、オーステナイトの未再結晶温度域圧延を可能とする。また、炭化物あるいは炭窒化物として微細に析出して、鋼板の強度増加に寄与する元素である。熱間圧延後の冷却中に、熱間圧延により導入された転位上に炭化物あるいは炭窒化物として析出し、γ→α変態の核として作用し、ベイニティックフェライトの粒内生成を促進し、微細な塊状の未変態オーステナイト、ひいては微細な塊状のマルテンサイトの生成に寄与する。このような効果を得るためには0.02%以上の含有を必要とする。一方、0.10%を超える過剰な含有は、熱間圧延時の変形抵抗が増大し、熱間圧延が困難となる恐れがある。また、0.10%を超える過剰な含有は、主相であるベイニティックフェライトの降伏強さの増加を招き、85%以下の降伏比を確保することが困難となる。このため、Nbは0.02〜0.10%の範囲に限定した。なお、好ましくは0.03%以上である。好ましくは0.07%以下である。
Nb: 0.02 to 0.10%
Nb dissolves in steel or precipitates as carbonitride, and has the effect of suppressing austenite grain coarsening and suppressing recrystallization of austenite grains. Make it possible. Moreover, it is an element which precipitates finely as carbide or carbonitride and contributes to an increase in strength of the steel sheet. During cooling after hot rolling, it precipitates as carbides or carbonitrides on dislocations introduced by hot rolling, acts as the core of γ → α transformation, promotes intragranular formation of bainitic ferrite, This contributes to the formation of fine massive untransformed austenite and, in turn, fine massive martensite. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, an excessive content exceeding 0.10% increases deformation resistance during hot rolling, which may make hot rolling difficult. An excessive content exceeding 0.10% leads to an increase in the yield strength of the main phase bainitic ferrite, making it difficult to ensure a yield ratio of 85% or less. For this reason, Nb was limited to 0.02 to 0.10% of range. In addition, Preferably it is 0.03% or more. Preferably it is 0.07% or less.
Ti:0.001〜0.030%
Tiは、Nを窒化物として固定し、スラブ割れの防止に寄与するとともに、炭化物として微細に析出して鋼板強度を増加させる作用を有する。このような効果を得るためには、0.001%以上の含有を必要とする。一方、0.030%を超えて多量に含有するとベイニティックフェライト変態点を過度に上昇させ、鋼板の靭性が低下する。このため、Tiは0.001〜0.030%の範囲に限定した。なお、好ましくは0.005%以上である。好ましくは0.025%以下である。
Ti: 0.001 to 0.030%
Ti fixes N as a nitride and contributes to the prevention of slab cracking, and has the effect of increasing the strength of the steel sheet by fine precipitation as a carbide. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 0.030%, the bainitic ferrite transformation point is excessively raised and the toughness of the steel sheet is lowered. For this reason, Ti was limited to the range of 0.001 to 0.030%. In addition, Preferably it is 0.005% or more. Preferably it is 0.025% or less.
Mo:0.05〜0.50%
Moは、焼入れ性向上に寄与し、ベイニティックフェライト中のCを未変態オーステナイト中に引き寄せ、未変態オーステナイトの焼入性を向上させることを介してマルテンサイト形成を促進する作用を有する。さらに鋼中に固溶し固溶強化により鋼板強度の増加に寄与する元素である。このような効果を得るためには、0.05%以上の含有を必要とする。一方、0.50%を超える含有は、必要以上にマルテンサイトを形成させ、鋼板の靭性を低下させる。また、Moは高価な元素であり、多量の含有は材料コストの高騰を招く。このようなことから、Moは0.05〜0.50%の範囲に限定した。なお、好ましくは0.10%以上である。好ましくは0.40%以下である。
Mo: 0.05-0.50%
Mo contributes to the improvement of hardenability, attracts C in bainitic ferrite to untransformed austenite, and has an action of promoting martensite formation by improving the hardenability of untransformed austenite. Furthermore, it is an element that contributes to an increase in steel sheet strength by solid solution in steel and solid solution strengthening. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.50%, martensite is formed more than necessary, and the toughness of the steel sheet is lowered. In addition, Mo is an expensive element, and a large amount thereof causes an increase in material cost. For these reasons, Mo is limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.10% or more. Preferably it is 0.40% or less.
Cr:0.05〜0.50%
Crは、γ→α変態を遅延させ、焼入れ性向上に寄与し、マルテンサイト形成を促進する作用を有する。このような効果を得るためには、0.05%以上の含有を必要とする。一方、0.50%を超える含有は、溶接部に欠陥を多発させる傾向となる。このため、Crは0.05〜0.50%の範囲に限定した。なお、好ましくは0.20%以上である。好ましくは0.45%以下である。
Cr: 0.05-0.50%
Cr delays the γ → α transformation, contributes to improving hardenability, and has an action of promoting martensite formation. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if it exceeds 0.50%, defects tend to occur frequently in the weld. For this reason, Cr was limited to the range of 0.05 to 0.50%. In addition, Preferably it is 0.20% or more. Preferably it is 0.45% or less.
Ni:0.001〜1.00%
Niは、焼入れ性向上に寄与し、マルテンサイト形成を促進することに加えて、さらに靭性の向上に寄与する元素である。このような効果を得るためには、0.001%以上の含有を必要とする。一方、1.00%を超えて含有しても、効果が飽和し含有量に見合う効果が期待できないため経済的に不利となる。このため、Niは0.001〜1.00%の範囲に限定した。なお、好ましくは0.01%以上である。好ましくは0.50%以下である。
Ni: 0.001 to 1.00%
Ni is an element that contributes to the improvement of toughness, in addition to promoting the formation of martensite, and further contributing to the improvement of toughness. In order to obtain such an effect, a content of 0.001% or more is required. On the other hand, if the content exceeds 1.00%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni was limited to the range of 0.001 to 1.00%. In addition, Preferably it is 0.01% or more. Preferably it is 0.50% or less.
上記した成分が基本の成分であるが、本発明では、上記した成分を、上記した含有範囲内で、かつ、次(1)式
Moeq(%)=Mo+0.36Cr+0.77Mn+0.07Ni ‥‥(1)
(ここで、Mn、Ni、Cr、Mo:各元素の含有量(質量%))
で定義されるMoeqが、1.4〜2.2%の範囲を満足するように調整することが好ましい。
The above-described components are basic components. In the present invention, the above-described components are contained within the above-described content range, and the following formula (1)
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
(Here, Mn, Ni, Cr, Mo: Content of each element (mass%))
It is preferable to adjust so that Moeq defined by the above satisfies the range of 1.4 to 2.2%.
Moeqは、冷却工程を経た後に、鋼板中に残存する未変態オーステナイトの焼入れ性を表す指標である。Moeqが1.4%未満では、未変態オーステナイトの焼入れ性が不足し、巻取後保持中にパーライト等に変態してしまい、巻取後60秒以上保持した後に冷却しても十分なマルテンサイトを得ることができない場合がある。一方、Moeqが2.2%を超えると、必要以上にマルテンサイトが生成し、靭性が低下する場合がある。このため、Moeqは1.4〜2.2%の範囲に限定することが好ましい。Moeqが1.5%以上あれば、低降伏比となり、さらに変形能が向上する。このため、1.5%以上とすることがより好ましい。 Moeq is an index representing the hardenability of untransformed austenite remaining in the steel sheet after passing through the cooling step. If the Moeq is less than 1.4%, the hardenability of untransformed austenite is insufficient, it transforms into pearlite etc. during holding after winding, and sufficient martensite is obtained even after cooling for 60 seconds or more after winding. It may not be possible. On the other hand, when Moeq exceeds 2.2%, martensite is generated more than necessary, and the toughness may be lowered. For this reason, Moeq is preferably limited to a range of 1.4 to 2.2%. If Moeq is 1.5% or more, the yield ratio is low and the deformability is further improved. For this reason, it is more preferable to set it as 1.5% or more.
上記以外の残部はFeおよび不可避的不純物である。なお、不可避的不純物としては、O(酸素):0.005%以下、Mg:0.003%以下、Sn:0.005%以下が許容できる。 The balance other than the above is Fe and inevitable impurities. Inevitable impurities include O (oxygen): 0.005% or less, Mg: 0.003% or less, and Sn: 0.005% or less.
以上が本発明の熱延鋼板の基本成分であるが、本発明では、上記した成分の範囲に加えて、さらに必要に応じて、選択元素として、Cu:0.50%以下、V:0.10%以下、B:0.0005%以下のうちから選ばれた1種または2種以上、および/または、Ca:0.0005〜0.0050%を含有することができる。 The above is the basic component of the hot-rolled steel sheet of the present invention. In the present invention, in addition to the above-described component range, if necessary, as optional elements, Cu: 0.50% or less, V: 0.10% or less, B: One or two or more selected from 0.0005% or less and / or Ca: 0.0005 to 0.0050% can be contained.
Cu:0.50%以下、V:0.10%以下、B:0.0005%以下のうちから選ばれた1種または2種以上
Cu、V、Bはいずれも、鋼板の高強度化に寄与する元素であり、必要に応じて選択して含有できる。
Cu、Vは、固溶強化、あるいは析出強化を介して、また、Bは、結晶粒界に偏析して焼入れ性向上を介し、鋼板の高強度化に寄与する。このような効果を得るためには、Cu:0.01%以上、V:0.01%以上、B:0.0001%以上含有することが好ましい。一方、Cu:0.50%を超える含有は熱間加工性を低下させる。V:0.10%を超える含有は溶接性を低下させる。B:0.0005%を超える含有は鋼板の靭性を低下させる。このため、含有する場合には、Cu:0.50%以下、V:0.10%以下、B:0.0005%以下に限定することが好ましい。
One or more selected from Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less
Cu, V, and B are all elements that contribute to increasing the strength of the steel sheet, and can be selected and contained as necessary.
Cu and V contribute to the strengthening of the steel sheet through solid solution strengthening or precipitation strengthening, and B segregates at the grain boundary and improves hardenability. In order to obtain such an effect, it is preferable to contain Cu: 0.01% or more, V: 0.01% or more, and B: 0.0001% or more. On the other hand, if the Cu content exceeds 0.50%, the hot workability is lowered. V: Containing more than 0.10% reduces weldability. B: Content exceeding 0.0005% lowers the toughness of the steel sheet. For this reason, when it contains, it is preferable to limit to Cu: 0.50% or less, V: 0.10% or less, B: 0.0005% or less.
Ca:0.0005〜0.0050%
Caは、粗大な硫化物を球状の硫化物とする硫化物の形態制御に寄与する元素であり、必要に応じて含有できる。このような効果を得るためには、Ca:0.0005%以上含有することが好ましい。一方、Ca:0.0050%を超える含有は、鋼板の清浄度を低下させる。このため、含有する場合にはCa:0.0005〜0.0050%の範囲に限定することが好ましい。
Ca: 0.0005 to 0.0050%
Ca is an element that contributes to the control of the morphology of sulfides in which coarse sulfides are spherical sulfides, and can be contained as required. In order to acquire such an effect, it is preferable to contain Ca: 0.0005% or more. On the other hand, the content exceeding Ca: 0.0050% reduces the cleanliness of the steel sheet. For this reason, when it contains, it is preferable to limit to Ca: 0.0005 to 0.0050% of range.
次に、本発明の熱延鋼板のミクロ組織について説明する。 Next, the microstructure of the hot rolled steel sheet of the present invention will be described.
本発明熱延鋼板は、上記した組成を有し、さらに、ベイニティックフェライトを主相とし、主相と第二相とからなる組織を有する。 The hot-rolled steel sheet of the present invention has the above-described composition, and further has a structure composed of bainitic ferrite as a main phase and a main phase and a second phase.
ここで、主相とは、面積率で50%以上の占有面積を有する相をいう。主相であるベイニティックフェライトは、転位密度が高い下部組織を有する相であり、針状フェライト、アシキュラーフェライトを含む。なお、ベイニティックフェライトには、転位密度が極めて低いポリゴナルフェライトや、細かいサブグレン等の下部組織をともなう準ポリゴナルフェライトは含まれない。なお、所望の高強度を確保するために、主相であるベイニティックフェライトには、微細な炭窒化物が析出していることが必要となる。 Here, the main phase refers to a phase having an occupied area of 50% or more in area ratio. The main phase bainitic ferrite is a phase having a substructure with a high dislocation density, and includes acicular ferrite and acicular ferrite. The bainitic ferrite does not include polygonal ferrite having a very low dislocation density or quasi-polygonal ferrite with a substructure such as fine subgrains. In order to secure a desired high strength, it is necessary that fine carbonitride is precipitated in the bainitic ferrite as the main phase.
そして、第二相として、塊状マルテンサイトを分散させた組織を有する。本発明でいう塊状マルテンサイトとは、圧延後の冷却過程で未変態オーステナイトから旧γ(オーステナイト)粒界、あるいは旧γ粒内に生成したマルテンサイトである。本発明では、このような塊状マルテンサイトを、旧γ粒界、あるいは主相であるベイニティックフェライト粒とベイニティックフェライト粒の間に分散させる。マルテンサイトは、主相と比べ硬質であり、加工時にベイニティックフェライト中に可動転位を多量に導入することができ、降伏挙動を連続降伏型とすることができる。また、マルテンサイトはベイニティックフェライトより高い引張強さを有するため、低降伏比を達成できることになる。 And it has a structure in which massive martensite is dispersed as the second phase. The massive martensite referred to in the present invention is martensite produced from untransformed austenite in the prior γ (austenite) grain boundary or in the old γ grain in the cooling process after rolling. In the present invention, such massive martensite is dispersed between the old γ grain boundaries or the bainitic ferrite grains as the main phase and the bainitic ferrite grains. Martensite is harder than the main phase, and a large amount of movable dislocations can be introduced into the bainitic ferrite during processing, and the yield behavior can be a continuous yield type. Moreover, since martensite has a higher tensile strength than bainitic ferrite, a low yield ratio can be achieved.
また、本発明の熱延鋼板は、板厚が8mm以上25mm以下であることが好ましい。 The hot-rolled steel sheet of the present invention preferably has a thickness of 8 mm or more and 25 mm or less.
次に、本発明の熱延鋼板の製造方法について説明する。 Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described.
本発明では、上記した組成を有する鋼素材に、熱延工程、冷却工程、巻取工程を施して、熱延鋼板とする。 In the present invention, a hot-rolled steel sheet is obtained by subjecting a steel material having the above composition to a hot-rolling process, a cooling process, and a winding process.
なお、使用する鋼素材の製造方法はとくに限定する必要はなく、上記した組成の溶鋼を、転炉、電気炉等の通常公知の溶製方法を用いて、溶製し、連続鋳造法等の通常公知の溶製方法により、スラブ等の鋼素材とすることが好ましい。 In addition, the manufacturing method of the steel raw material to be used is not particularly limited, and the molten steel having the above-described composition is melted using a generally known melting method such as a converter or an electric furnace, and a continuous casting method or the like is used. It is preferable to use a steel material such as a slab by a generally known melting method.
得られた鋼素材には、熱延工程を施す。
熱延工程は、上記した組成を有する鋼素材を加熱温度:1050〜1300℃に加熱し、加熱された鋼素材に、粗圧延を施しシートバーとし、シートバーに、930℃以下の温度域での累積圧下率:50%以上となる仕上圧延を施し熱延鋼板とする工程とする。
The obtained steel material is subjected to a hot rolling process.
In the hot rolling step, the steel material having the above composition is heated to a heating temperature of 1050 to 1300 ° C., and the heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar has a temperature range of 930 ° C. or less. Cumulative rolling reduction: It is set as the process which gives the finish rolling used as 50% or more, and makes it a hot-rolled steel plate.
加熱温度:1050〜1300℃
本発明で使用する鋼素材は、上記したようにNb、Tiを必須で含有する。析出強化により所望の高強度を確保するためには、これらの粗大な炭化物、窒化物等を一旦溶解させて、その後微細析出させることが必要となる。そのため、鋼素材の加熱温度は1050℃以上とする。1050℃未満では、各元素が未固溶のままとなり、所望の鋼板強度が得られない。一方、1300℃を超えて高温になると、結晶粒の粗大化が生じ、鋼板靭性が低下する。このため、鋼素材の加熱温度は1050〜1300℃に限定した。
Heating temperature: 1050-1300 ° C
The steel material used in the present invention essentially contains Nb and Ti as described above. In order to secure a desired high strength by precipitation strengthening, it is necessary to dissolve these coarse carbides, nitrides and the like once and then finely precipitate them. Therefore, the heating temperature of the steel material is 1050 ° C. or higher. If it is less than 1050 degreeC, each element will remain undissolved and desired steel plate strength will not be obtained. On the other hand, when the temperature exceeds 1300 ° C., the crystal grains become coarse and the steel sheet toughness decreases. For this reason, the heating temperature of the steel material was limited to 1050 to 1300 ° C.
上記した加熱温度に加熱された鋼素材は、粗圧延を施されてシートバーとされる。粗圧延の条件はとくに限定する必要はなく、所望の寸法形状のシートバーが確保できる条件であればよい。 The steel material heated to the above heating temperature is subjected to rough rolling to form a sheet bar. The conditions for rough rolling need not be particularly limited as long as a sheet bar having a desired size and shape can be secured.
得られたシートバーは、次いで仕上圧延され、所望の寸法形状の熱延鋼板とされる。仕上圧延は、930℃以下の温度域での累積圧下率が50%以上の圧延とする。 The obtained sheet bar is then finish-rolled to obtain a hot-rolled steel sheet having a desired size and shape. Finish rolling is a rolling with a cumulative reduction of 50% or more in a temperature range of 930 ° C. or lower.
930℃以下の温度域での累積圧下率:50%以上
ベイニティックフェライトの微細化、および塊状マルテンサイトの微細分散のために、930℃以下の温度域での累積圧下率を50%以上とする。930℃以下の温度域での累積圧下率が50%未満では、圧下量が不足し、主相である微細なベイニティックフェライトを確保できない。また、γ→α変態の核生成を促進するNbC等の析出サイトとなる転位が不足し、ベイニティックフェライトの粒内生成が不足し、塊状マルテンサイトを形成するための塊状の未変態γを微細かつ多数分散して残留させることができなくなる。このため、仕上圧延における930℃以下の温度域での累積圧下率を50%以上に限定した。なお、好ましくは累積圧下率は80%以下である。圧下率が80%を超えて大きくしても、効果が飽和し、さらにセパレーションの発生が著しくなり、シャルピ−吸収エネルギーの低下を招く場合がある。
Cumulative rolling reduction in the temperature range below 930 ° C: 50% or more Cumulative rolling reduction in the temperature range below 930 ° C is 50% or more for finer bainitic ferrite and fine dispersion of massive martensite. To do. If the cumulative rolling reduction in the temperature range of 930 ° C or lower is less than 50%, the rolling amount is insufficient, and fine bainitic ferrite as the main phase cannot be secured. In addition, dislocations that become precipitation sites such as NbC that promote nucleation of γ → α transformation are insufficient, intragranular formation of bainitic ferrite is insufficient, and massive untransformed γ to form massive martensite It cannot be dispersed finely and in large numbers. For this reason, the cumulative rolling reduction in the temperature range of 930 ° C. or lower in finish rolling is limited to 50% or more. The cumulative rolling reduction is preferably 80% or less. Even if the rolling reduction exceeds 80%, the effect is saturated, the occurrence of separation becomes significant, and the Charpy absorbed energy may be reduced.
なお、仕上圧延の圧延終了温度は、鋼板靭性、鋼板強度、圧延負荷等の観点から、850〜750℃とすることが好ましい。仕上圧延の圧延終了温度が850℃を超えて高温となると、930℃以下の温度域での累積圧下率を50%以上とするために、1パス当たりの圧下量を大きくする必要があり、圧延荷重の増加を招く。一方、750℃未満と低温となると、圧延中にフェライトが生成し、組織、析出物の粗大化を招き、低温靭性、強度が低下する。 In addition, it is preferable that the rolling completion temperature of finish rolling shall be 850-750 degreeC from viewpoints of steel plate toughness, steel plate strength, rolling load, etc. When the finishing temperature of finish rolling exceeds 850 ° C and becomes high, it is necessary to increase the reduction amount per pass in order to increase the cumulative reduction rate in the temperature range of 930 ° C or less to 50% or more. Increases load. On the other hand, when the temperature is lower than 750 ° C., ferrite is generated during rolling, resulting in coarsening of the structure and precipitates, and low temperature toughness and strength are reduced.
得られた熱延鋼板は、次いで冷却工程を施される。 The obtained hot-rolled steel sheet is then subjected to a cooling step.
冷却工程は、仕上圧延終了後直ちに冷却を開始し、板厚中央部の平均冷却速度:5〜60℃/sで、冷却停止温度:Bs点〜450℃の温度域まで冷却する工程とする。 The cooling process is a process in which cooling is started immediately after finishing rolling, and cooling is performed at an average cooling rate of 5 to 60 ° C./s at the center of the plate thickness to a cooling stop temperature: Bs point to 450 ° C.
仕上圧延終了後、直ちに、好ましくは15s以内に冷却を開始する。冷却速度は、板厚中央部で、冷却停止温度までの平均の冷却速度で、5〜60℃/sの範囲とする。平均冷却速度が5℃/s未満では、ポリゴナルフェライト主体の組織となり、所望のベイニティックフェライトを主相とする組織を確保することが難しくなる。一方、平均冷却速度が60℃/sを超える急冷とすると、未変態オーステナイトへの合金元素の濃縮が不十分となり、その後の冷却で所望量の塊状マルテンサイトを微細分散することができなくなり、所望の低降伏比、所望の優れた低温靭性を有する熱延鋼板とすることが困難となる。また平均冷却速度が60℃/sを超える急冷では、表層部がマルテンサイト単相組織となり、その後、焼戻されて焼戻マルテンサイト単相組織となり降伏比が高くなる。このため、仕上圧延終了後の冷却速度を板厚中央部の平均で、5〜60℃/sの範囲に限定した。なお、好ましくは5〜25℃/sである。 Immediately after finishing rolling, cooling is preferably started within 15 s. The cooling rate is an average cooling rate up to the cooling stop temperature at the center of the plate thickness, and is in the range of 5 to 60 ° C./s. When the average cooling rate is less than 5 ° C./s, the structure is mainly composed of polygonal ferrite, and it becomes difficult to secure a structure having a desired bainitic ferrite as a main phase. On the other hand, if the average cooling rate exceeds 60 ° C./s, the concentration of the alloy elements to the untransformed austenite becomes insufficient, and the desired amount of massive martensite cannot be finely dispersed by the subsequent cooling. It is difficult to obtain a hot-rolled steel sheet having a low yield ratio and a desired excellent low-temperature toughness. Further, when the average cooling rate exceeds 60 ° C./s, the surface layer portion has a martensite single phase structure, and then is tempered to become a tempered martensite single phase structure, resulting in a high yield ratio. For this reason, the cooling rate after finishing rolling was limited to a range of 5 to 60 ° C./s in average at the center of the plate thickness. In addition, Preferably it is 5-25 degreeC / s.
上記した冷却の冷却停止温度はBs点(ベイナイト変態開始温度)〜450℃の範囲の温度とする。冷却停止温度が上記した温度範囲より高温では、所望のベイニティックフェライトを主相とする組織を確保することが難しくなる。一方、冷却停止温度が上記した温度域より低温では、未変態γがほぼ変態を完了して所望量の塊状マルテンサイトを確保できなくなる。 The cooling stop temperature of the above cooling is a temperature in the range of Bs point (bainite transformation start temperature) to 450 ° C. When the cooling stop temperature is higher than the above temperature range, it is difficult to secure a structure having a desired bainitic ferrite as a main phase. On the other hand, when the cooling stop temperature is lower than the above temperature range, the untransformed γ is almost completely transformed and a desired amount of massive martensite cannot be secured.
巻取工程は、コイル状に巻取り、60秒以上600秒未満保持した後、冷却する工程とする。 The winding process is a process of winding in a coil shape and holding it for 60 seconds or more and less than 600 seconds and then cooling.
本発明では、コイル状に巻取り、巻取りが完了してから60秒以上600秒未満保持した後、冷却する。巻取り完了から冷却まで一定時間保持するのは、その後のコイル冷却工程において所望のマルテンサイト量を得ることができるように未変態オーステナイトの焼入性を増加させるためである。保持時間が60秒未満だと未変態オーステナイトへの炭素濃化が十分に進行しておらず、その後コイル冷却を施しても所望のマルテンサイト量を得ることができない。また、600秒以上保持しても未変態オーステナイトの焼入性はそれ以上増加しなくなる。このため、保持時間は600秒未満に限定した。
上記、巻取り後の保持中に残留した未変態オーステナイトをマルテンサイトに変態させるためには、冷却するにあたっての平均冷却速度は5℃/s以上とすることが好ましい。冷却速度の上限は特に規定する必要はないが、冷却速度を大きくすればするほど、冷却ムラが生じやすく、コイル形状が崩れその後の運搬に支障が出る恐れがあるため、形状が崩れない程度の冷却に留めることが好ましい。なお、熱延コイル外周面を冷却する具体的な手段としては、特に制限はないが、冷却空気、冷却フォグ、冷却水、冷却槽あるいはこれらを適宜組み合わせた冷却方法であれば好適に用いることができる。
In the present invention, winding is performed in a coil shape, and after the winding is completed, the coil is held for 60 seconds or more and less than 600 seconds, and then cooled. The reason for holding for a certain time from the completion of winding to cooling is to increase the hardenability of untransformed austenite so that a desired amount of martensite can be obtained in the subsequent coil cooling step. If the holding time is less than 60 seconds, carbon concentration to untransformed austenite does not proceed sufficiently, and the desired amount of martensite cannot be obtained even after coil cooling. Further, the hardenability of the untransformed austenite does not increase any longer even if it is held for 600 seconds or longer. For this reason, the holding time was limited to less than 600 seconds.
In order to transform the untransformed austenite remaining during the holding after winding into martensite, the average cooling rate for cooling is preferably 5 ° C./s or more. The upper limit of the cooling rate does not need to be specified, but the larger the cooling rate, the more likely it is that cooling unevenness will occur and the coil shape may collapse and hinder subsequent transportation. It is preferable to keep cooling. The specific means for cooling the outer peripheral surface of the hot-rolled coil is not particularly limited, but it can be suitably used as long as it is a cooling air, a cooling fog, a cooling water, a cooling tank, or a cooling method appropriately combining these. it can.
以下、さらに実施例に基づいて本発明を詳細に説明する。
表1に示す組成の溶鋼を、連続鋳造法でスラブ(肉厚220mm)とし、鋼素材とした。次いで、これら鋼素材を表2に示す加熱温度に加熱して、粗圧延を行い、シートバーとしたのち、該シートバーに、表2に示す条件で仕上圧延を行い熱延鋼板(板厚:8〜25mm)とする熱延工程を施した。得られた熱延鋼板に、仕上圧延終了後直ちに(表2に示す時間内に)冷却を開始し、表2に示す平均冷却速度で、表2に示す冷却停止温度(巻取温度)まで冷却する冷却を行う冷却工程を施した。冷却工程後、表2に示す巻取温度で、コイル状に巻き取った後、表2に示すコイル冷却速度でコイル全体を冷却する巻取工程を施した。Bs点は加工フォーマスタ試験により求めた。加工フォーマスタ試験は、φ8mm×12mmHの円筒試験片を1200℃で5分加熱した後、970〜800℃の間で65%の圧下を与え、その後15℃/sで室温まで連続冷却した。連続冷却中の試料の変態による膨張を検出することによって変態点を(Bs点)として求めた。
Hereinafter, the present invention will be described in detail based on examples.
The molten steel having the composition shown in Table 1 was made into a slab (thickness: 220 mm) by a continuous casting method to obtain a steel material. Next, these steel materials are heated to the heating temperature shown in Table 2 and subjected to rough rolling to form a sheet bar, and then the sheet bar is subjected to finish rolling under the conditions shown in Table 2 to obtain a hot rolled steel sheet (sheet thickness: A hot rolling step of 8 to 25 mm) was performed. The obtained hot-rolled steel sheet is cooled immediately (within the time shown in Table 2) after finishing rolling, and cooled to the cooling stop temperature (winding temperature) shown in Table 2 at the average cooling rate shown in Table 2. A cooling process for performing cooling was performed. After the cooling step, the coil was wound into a coil shape at the winding temperature shown in Table 2, and then the winding step for cooling the entire coil at the coil cooling rate shown in Table 2 was performed. The Bs point was obtained by a machining for master test. In the processing for master test, a φ8 mm × 12 mmH cylindrical test piece was heated at 1200 ° C. for 5 minutes, then subjected to 65% reduction between 970 and 800 ° C., and then continuously cooled to room temperature at 15 ° C./s. The transformation point was determined as (Bs point) by detecting expansion due to transformation of the sample during continuous cooling.
以上により得られた熱延鋼板から、試験片を採取し、組織観察、引張試験、衝撃試験を実施した。試験方法はつぎのとおりである。
(1)組織観察
得られた熱延鋼板から、圧延方向断面(L断面)が観察面となるように、組織観察用試験片を採取した。試験片を研磨し、ナイタール腐食して、光学顕微鏡(倍率:500倍)または電子顕微鏡(倍率:2000倍)を用いて、組織観察を行い、撮像して、画像解析装置を用いて、組織の種類、各相の組織分率(面積率)を測定した。なお、アスペクト比が5.0未満のマルテンサイト粒を塊状マルテンサイトと定義する。マルテンサイト粒のアスペクト比は、各粒における長手方向の長さ(長辺)とそれに直角な方向の長さ(短辺)との比、(長辺)/(短辺)、で算出するものとする。
(2)引張試験
得られた熱延鋼板から、引張方向が、圧延方向と直角方向(板幅方向)および圧延方向から30度方向となるように、それぞれ引張試験片(API−5Lに定める全厚試験片;GL50mm、幅38.1mm)を採取し、ASTM A 370の規定に準拠して、引張試験を実施し、引張特性(降伏強さYS(MPa)、引張強さTS(MPa)、一様伸びuEl(%))を測定した。得られた結果から、YR(降伏比)(=(板幅方向YS/板幅方向TS)×100)(%)を算出し、評価した。
(3)衝撃試験
得られた熱延鋼板から、試験片長手方向が、圧延方向に直角方向となるように、Vノッチ試験片を採取し、ASTM A 370の規定に準拠して、シャルピー衝撃試験を実施し、破面遷移温度vTrs(℃)を求めた。
(4)造管での引張試験
得られた熱延鋼板を管素材として、スパイラル造管工程により、スパイラル鋼管(外径:1067mmφ)を製造した。得られた鋼管から、引張方向が管周方向となるように、引張試験片(APIに定める試験片)を採取し、ASTM A 370の規定に準拠して、引張試験を実施し、引張特性(降伏強さYS(MPa)、引張強さTS(MPa))を測定した。得られた結果から、鋼管YR(降伏比)(=(鋼管YS/鋼管TS)×100)(%)、ΔYS(=鋼管YS−鋼板YS(圧延方向から30°方向YS))(MPa)を算出し、造管による強度低下の程度を評価した。ΔYS=0 MPa以上を良好と判断した。
得られた結果を表3に併記した。
Test pieces were collected from the hot-rolled steel sheet obtained as described above, and subjected to structure observation, tensile test, and impact test. The test method is as follows.
(1) Microstructure observation From the obtained hot-rolled steel sheet, a microstructural specimen was taken so that the cross section in the rolling direction (L cross section) became the observation surface. The specimen is polished, subjected to Nital corrosion, and the structure is observed using an optical microscope (magnification: 500 times) or an electron microscope (magnification: 2000 times). The type and the structure fraction (area ratio) of each phase were measured. In addition, martensite grains having an aspect ratio of less than 5.0 are defined as massive martensite. The aspect ratio of the martensite grain is calculated by the ratio of the length in the longitudinal direction (long side) and the length in the direction perpendicular to the grain (short side) (long side) / (short side). And
(2) Tensile test From the obtained hot-rolled steel sheet, the tensile test pieces (all specified in API-5L) were adjusted so that the tensile direction was perpendicular to the rolling direction (sheet width direction) and 30 degrees from the rolling direction. Thick specimens (GL50mm, width 38.1mm) are collected and subjected to a tensile test in accordance with ASTM A 370. Tensile properties (yield strength YS (MPa), tensile strength TS (MPa), one Elongation uEl (%)) was measured. From the obtained results, YR (yield ratio) (= (plate width direction YS / plate width direction TS) × 100) (%) was calculated and evaluated.
(3) Impact test V-notch test specimens were taken from the obtained hot-rolled steel sheet so that the longitudinal direction of the specimen was perpendicular to the rolling direction, and Charpy impact test was performed in accordance with ASTM A 370 regulations. And the fracture surface transition temperature vTrs (° C.) was determined.
(4) Tensile test in pipe making Using the obtained hot-rolled steel sheet as a pipe material, a spiral steel pipe (outer diameter: 1067 mmφ) was manufactured by a spiral pipe making process. From the obtained steel pipe, take a tensile test piece (test piece specified in API) so that the tensile direction is the pipe circumferential direction, conduct a tensile test in accordance with the provisions of ASTM A 370, and obtain tensile properties ( Yield strength YS (MPa), tensile strength TS (MPa)) were measured. From the obtained results, steel pipe YR (yield ratio) (= (steel pipe YS / steel pipe TS) x 100) (%), ΔYS (= steel pipe YS-steel sheet YS (30 ° direction YS from rolling direction)) (MPa) The degree of strength reduction due to pipe making was calculated. ΔYS = 0 MPa or more was judged good.
The obtained results are also shown in Table 3.
本発明例はいずれも、特別な熱処理を施すこともなく、圧延方向から30度方向の降伏強さが480MPa以上で、板幅方向の引張強さが600MPa以上で、破面遷移温度vTrsが−80℃以下で、かつ降伏比が85%以下の低降伏比を有する、低温靭性に優れた低降伏比高強度熱延鋼板が得られている。
一方、本発明の範囲を外れる比較例は、降伏強さが不足しているか、引張強さが低下しているか、靭性が低下しているか、低降伏比が確保できていないか、いずれかの特性が劣っており所望の特性を有する熱延鋼板が得られていない。
In all of the examples of the present invention, no special heat treatment was performed, the yield strength in the direction of 30 ° from the rolling direction was 480 MPa or more, the tensile strength in the sheet width direction was 600 MPa or more, and the fracture surface transition temperature vTrs was − A low-yield-ratio high-strength hot-rolled steel sheet excellent in low-temperature toughness having a low yield ratio of 80 ° C. or lower and a yield ratio of 85% or lower has been obtained.
On the other hand, the comparative example out of the scope of the present invention is either one in which the yield strength is insufficient, the tensile strength is reduced, the toughness is reduced, or the low yield ratio is not secured. The property is inferior and the hot-rolled steel plate which has a desired characteristic is not obtained.
さらに本発明例はいずれも、造管され鋼管となったのちも、造管による強度低下もすくなく、スパイラル鋼管あるいは電縫鋼管用素材として、好適な熱延鋼板となっている。 Furthermore, all of the examples of the present invention, after being piped to become a steel pipe, are not susceptible to a decrease in strength due to the pipe making, and are suitable hot rolled steel sheets as materials for spiral steel pipes or ERW steel pipes.
Claims (4)
前記鋼素材を、質量%で、C:0.03〜0.10%、Si:0.10〜0.50%、Mn:1.4〜2.2%、P:0.025%以下、S:0.005%以下、Al:0.005〜0.10%、Nb:0.02〜0.10%、 Ti:0.001〜0.030%、Mo:0.05〜0.50%、 Cr:0.05〜0.50%、Ni:0.001〜1.00%を含み、残部Feおよび不可避的不純物からなる組成を有する鋼素材とし、
前記熱延工程が、前記鋼素材を加熱温度:1050〜1300℃に加熱し、該加熱された鋼素材に、粗圧延を施しシートバーとし、該シートバーに、930℃以下の温度域での累積圧下率:50%以上となる仕上圧延を施し熱延鋼板とする工程とし、
前記冷却工程が、仕上圧延終了後直ちに冷却を開始し、板厚中央部の平均冷却速度:5〜60℃/sで、冷却停止温度:Bs点〜450℃の温度域まで冷却する工程とし、
巻取工程が、コイル状に巻取り、60秒以上600秒未満保持した後、冷却する工程とすることを特徴とする低温靭性に優れた低降伏比高強度熱延鋼板の製造方法。 The steel material is subjected to a hot rolling process, a cooling process, and a winding process to make a hot rolled steel sheet.
The steel material is, in mass%, C: 0.03-0.10%, Si: 0.10-0.50%, Mn: 1.4-2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005-0.10%, Nb : 0.02 to 0.10%, Ti: 0.001 to 0.030%, Mo: 0.05 to 0.50%, Cr: 0.05 to 0.50%, Ni: 0.001 to 1.00%, and a steel material having a composition comprising the balance Fe and inevitable impurities ,
In the hot rolling step, the steel material is heated to a heating temperature of 1050 to 1300 ° C., the heated steel material is subjected to rough rolling to form a sheet bar, and the sheet bar has a temperature range of 930 ° C. or less. Cumulative rolling reduction: It is a process to make a hot-rolled steel sheet by applying finish rolling to 50% or more,
The cooling step starts cooling immediately after finishing rolling, and is a step of cooling to a temperature range of cooling stop temperature: Bs point to 450 ° C. at an average cooling rate of the central portion of the plate thickness: 5 to 60 ° C./s,
A method for producing a high-strength hot-rolled steel sheet having a low yield ratio and excellent in low-temperature toughness, wherein the winding step is a step of cooling after winding in a coil shape and holding for 60 seconds or more and less than 600 seconds.
記
Moeq(%)=Mo+0.36Cr+0.77Mn+0.07Ni ‥‥(1)
ここで、Mn、Ni、Cr、Mo:各元素の含有量(質量%) 2. The low yield ratio excellent in low temperature toughness according to claim 1, wherein the composition is a composition satisfying a Moeq defined by the following formula (1) in a range of 1.4 to 2.2% by mass%. Manufacturing method of high-strength hot-rolled steel sheet.
Record
Moeq (%) = Mo + 0.36Cr + 0.77Mn + 0.07Ni (1)
Here, Mn, Ni, Cr, Mo: Content of each element (mass%)
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