JP2006336065A - Low yield-ratio high tensile-strength steel, and method for producing low yield-ratio high tensile-strength steel - Google Patents
Low yield-ratio high tensile-strength steel, and method for producing low yield-ratio high tensile-strength steel Download PDFInfo
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本発明は、耐震性の観点から高靭性と低降伏比を要求される建築構造や、船舶、橋梁、各種貯留タンクに好適に用いることができ、溶接性と低温靭性に優れた低降伏比高張力鋼および低降伏比高張力鋼の製造方法に関するものである。なお、主に対象とする強度レベルは、降伏強さで325〜475MPa、引張強さで490〜640MPaの、いわゆる一般に50キロ鋼と呼ばれるクラスである。 The present invention can be suitably used for building structures, ships, bridges and various storage tanks that require high toughness and low yield ratio from the viewpoint of earthquake resistance, and has a low yield ratio and excellent weldability and low temperature toughness. The present invention relates to a method for producing a high strength steel and a low yield ratio high strength steel. In addition, the strength level which is mainly targeted is a class called a so-called 50 kilo steel generally having a yield strength of 325 to 475 MPa and a tensile strength of 490 to 640 MPa.
建築用鋼材は、弾性設計(許容応力度設計)から、1981年6月に施行された新耐震設計基準に基づく終局耐力設計への移行に伴い、低降伏比が求められている。低降伏比化を達成するため、一般に、鋼組織の二相(Dual phase)化、すなわち、降伏を支配する軟質相(通常、フェライト)と引張強さを確保するための硬質相(パーライト、ベイナイト、マルテンサイトなど)を形成させる方法が広く用いられている。具体的には、制御圧延を含む熱間圧延後の鋼または焼入後の鋼を、フェライトとオーステナイトの二相域温度に再加熱して、フェライトとCが濃化されたオーステナイトとし、その後空冷以上の冷速で冷却、さらにその後焼き戻し処理する方法が例えば特許文献1などに開示されている。 With the transition from elastic design (allowable stress design) to ultimate strength design based on the new seismic design standard enforced in June 1981, steel for construction is required to have a low yield ratio. In order to achieve a low yield ratio, the steel structure is generally made into a dual phase, that is, a soft phase (usually ferrite) governing the yield and a hard phase (pearlite, bainite) to ensure tensile strength. , Martensite, etc.) is widely used. Specifically, the steel after hot rolling including control rolling or steel after quenching is reheated to a two-phase temperature range of ferrite and austenite to obtain austenite enriched with ferrite and C, and then air-cooled. For example, Patent Document 1 discloses a method of cooling at the above-described cooling speed and further performing a tempering process thereafter.
このとき、成分的には、C量が高いほど二相組織化が容易となるばかりでなく、硬質相がより硬化し、低降伏比化が容易となる。しかし、高C化は、溶接性や低温靭性には不利となるという問題があった。それに対して、低温靭性を改善するためには、低C化や制御圧延が有効ではあるが、いずれも降伏比が上昇するため、低温靭性向上と低降伏比化とは相容れず、両立がきわめて困難であった。 At this time, as a component, as the amount of C is higher, not only the two-phase organization is facilitated, but also the hard phase is hardened and the yield ratio is easily reduced. However, the high C has a problem that it is disadvantageous for weldability and low temperature toughness. On the other hand, low C and controlled rolling are effective for improving low temperature toughness, but since both yield ratios increase, low temperature toughness improvement and low yield ratio are incompatible and compatible. It was extremely difficult.
建築用途では、従来、靭性要求レベルが低く、低降伏比化に有利な高C鋼でも特に問題となることはなかったが、阪神大震災を契機とした近年の耐震性能への要求の厳格化傾向には、必ずしも十分に対応できないという問題があった。
本発明は、上記の点に鑑みてなされたものであり、優れた溶接性、低温靭性と同時に高強度で低降伏比を得ることができる低降伏比高張力鋼および低降伏比高張力鋼の製造方法を提供することを目的とする。 The present invention has been made in view of the above points, and is an excellent weldability, low-temperature toughness and low-yield-ratio high-tensile steel and low-yield-ratio high-tensile steel that can obtain a low yield ratio at high strength at the same time. An object is to provide a manufacturing method.
本発明者は、鋭意研究することにより、溶接性、低温靭性を確保すべく鋼成分のうちC、PCMの成分をきわめて低く抑える一方、高強度を達成し得る鋼組織に制御・限定し、さらに鋼の硬さと板厚断面方向の鋼の硬さの均一性を制御・限定することにより、優れた溶接性、低温靭性と同時に高強度で低降伏比を得ることができる低降伏比高張力鋼を考案した。 The present inventor has found that by intensive study, weldability, C of the steel component to secure the low temperature toughness, while suppressing very low component of P CM, controlled-limited to steel microstructure capable of achieving high strength, Furthermore, by controlling and limiting the hardness of the steel and the uniformity of the hardness of the steel in the cross-sectional direction of the plate thickness, it is possible to obtain high strength and low yield ratio at the same time as excellent weldability and low temperature toughness. Invented steel.
すなわち、本発明の低降伏比高張力鋼は、鋼成分が質量%で、C:0.01〜0.08%、Si:0.4%以下、Mn:1.0〜2.0%、P:0.02%以下、S:0.01%以下、Nb:0.005〜0.06%、Ti:0.005〜0.025%、Al:0.06%以下、N:0.001〜0.005%、かつPCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5Bが0.1〜0.18%、残部が鉄および不可避的不純物からなり、板厚方向断面1/4厚位置の鋼組織のポリゴナルまたは擬ポリゴナルフェライトとパーライトの合計分率が50%未満であって、鋼の表面1mm下断面における荷重98Nでのビッカース硬さが300Hv以下で、前記ビッカース硬さと前記鋼の板厚断面方向に1mm間隔で測定したビッカース硬さの最小値との差の数値が、mmで表記した板厚の1.5倍の数値以下であることを特徴とする。 That is, in the low yield ratio high tensile steel of the present invention, the steel component is mass%, C: 0.01 to 0.08%, Si: 0.4% or less, Mn: 1.0 to 2.0%, P: 0.02% or less, S: 0.01% or less, Nb: 0.005 to 0.06%, Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0.00. 001 to 0.005%, and P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.1 to 0.18%, the balance is iron and inevitable impurities, and the cross section in the thickness direction When the total fraction of polygonal or pseudopolygonal ferrite and pearlite in the steel structure at the 1/4 thickness position is less than 50%, the Vickers hardness at a load 98 N in the cross section 1 mm below the surface of the steel is 300 Hv or less, the Vickers Hardness and said steel plate Numerical differences between the minimum value of Vickers hardness measured at 1mm intervals in the cross-sectional direction, and equal to or less than value of 1.5 times the plate thickness was expressed in mm.
また、上記の低降伏比高張力鋼は、前記鋼成分が質量%で、Cu:0.05〜0.5%、Ni:0.05〜0.5%、Cr:0.05〜0.5%、Mo:0.05〜0.5%、V:0.01〜0.1%、B:0.0002〜0.003%、Mg:0.0002〜0.005%の範囲で1種または2種以上をさらに含有することができる。 In the above-mentioned low yield ratio high tensile steel, the steel components are in mass%, Cu: 0.05 to 0.5%, Ni: 0.05 to 0.5%, Cr: 0.05 to 0.00. 5%, Mo: 0.05 to 0.5%, V: 0.01 to 0.1%, B: 0.0002 to 0.003%, Mg: 1 in the range of 0.0002 to 0.005% A seed | species or 2 or more types can further be contained.
また、上記の低降伏比高張力鋼は、前記鋼成分が質量%で、Ca:0.0005〜0.004%、REM:0.0005〜0.008%のいずれか1種をさらに含有することができる。 Moreover, said low yield ratio high-tensile steel further contains any one of Ca: 0.0005-0.004% and REM: 0.0005-0.008%, in which the steel components are mass%. be able to.
また、本発明者は、鋭意研究することにより、鋼成分のうちC、PCMの成分をきわめて低く抑えた鋳片または鋼片を1000〜1250℃の温度に加熱し、オーステナイト未再結晶温度域での累積圧下量を30%以上として720℃以上の温度で熱間圧延を終了した後、680℃以上の温度から開始して350℃以下の温度で停止する加速冷却を行うことで、板厚方向断面1/4厚位置の鋼組織のポリゴナルまたは擬ポリゴナルフェライトとパーライトの合計分率が50%未満であって、鋼の表面1mm下断面における荷重98Nでのビッカース硬さが300Hv以下で、前記ビッカース硬さと前記鋼の板厚断面方向に1mm間隔で測定したビッカース硬さの最小値との差の数値が、mmで表記した板厚の1.5倍の数値以下であり、優れた溶接性、低温靭性と同時に高強度で低降伏比を有する低降伏比高張力鋼が容易に得られることを考案した。 Further, the present inventor has found that by intensive study, the extremely suppressed low slab or steel strip C, and components of the P CM of the steel component is heated to a temperature of 1000 to 1250 ° C., the austenite non-recrystallization temperature region After completing the hot rolling at a temperature of 720 ° C. or more with the cumulative reduction amount at 30% or more, the plate thickness is increased by starting with a temperature of 680 ° C. or more and stopping at a temperature of 350 ° C. or less. The total fraction of polygonal or pseudo-polygonal ferrite and pearlite in the steel structure at the direction cross section 1/4 thickness is less than 50%, and the Vickers hardness at a load of 98 N in the section below 1 mm of the steel surface is 300 Hv or less, The numerical value of the difference between the Vickers hardness and the minimum value of the Vickers hardness measured at intervals of 1 mm in the cross-sectional direction of the steel thickness is not more than 1.5 times the thickness expressed in mm. It was devised that low yield ratio high strength steel with high strength and low yield ratio as well as weldability and low temperature toughness can be easily obtained.
すなわち、本発明の低降伏比高張力鋼の製造方法は、板厚方向断面1/4厚位置の鋼組織のポリゴナルまたは擬ポリゴナルフェライトとパーライトの合計分率が50%未満であって、鋼の表面1mm下断面における荷重98Nでのビッカース硬さが300Hv以下で、前記ビッカース硬さと前記鋼の板厚断面方向に1mm間隔で測定したビッカース硬さの最小値との差の数値が、mmで表記した板厚の1.5倍の数値以下である低降伏比高張力鋼の製造方法であって、上記のいずれかに記載の鋼組成からなる鋳片または鋼片を、1000〜1250℃の温度に加熱し、オーステナイト未再結晶温度域での累積圧下量を30%以上として720℃以上の温度で熱間圧延を終了した後、680℃以上の温度から開始して350℃以下の温度で停止する加速冷却を行うことを特徴とする。 That is, the method for producing a low-yield-ratio high-tensile steel according to the present invention is such that the total fraction of polygonal or pseudo-polygonal ferrite and pearlite in the steel structure at the position of 1/4 thickness in the thickness direction is less than 50% The Vickers hardness at a load of 98 N in the cross section below 1 mm of the surface of the steel is 300 Hv or less, and the numerical value of the difference between the Vickers hardness and the minimum value of the Vickers hardness measured at intervals of 1 mm in the direction of the plate thickness of the steel is mm. It is a manufacturing method of low yield ratio high tensile steel that is not more than a numerical value of 1.5 times the indicated plate thickness, and a slab or steel slab made of the steel composition described above is 1000 to 1250 ° C. After heating at a temperature of 720 ° C. or more after finishing the hot rolling at a temperature of 720 ° C. or more with the cumulative reduction in the austenite non-recrystallization temperature range being 30% or more, the temperature is 350 ° C. or less. Stop And performing accelerated cooling that.
本発明の低降伏比高張力鋼によれば、優れた溶接性、低温靭性と同時に高強度で低降伏比を得ることができる。また、本発明の低降伏比高張力鋼の製造方法によれば、本発明の低降伏比高張力鋼を容易に製造できる。その結果、耐震性に優れた建築構造用、あるいは液体アンモニアとLPGなどとの混載タンク用として好適な溶接性と低温靭性に優れた低降伏比高張力鋼を大量にかつ安価に供給できるようになり、溶接鋼構造物のより一層の安全性向上に寄与できる。特に、本発明の低降伏比高張力鋼を用いることで、混載タンクの船舶への搭載が容易となる。 According to the low yield ratio high tensile steel of the present invention, it is possible to obtain a high yield and a low yield ratio simultaneously with excellent weldability and low temperature toughness. Moreover, according to the manufacturing method of the low yield ratio high tensile steel of this invention, the low yield ratio high tensile steel of this invention can be manufactured easily. As a result, low yield ratio high strength steel with excellent weldability and low temperature toughness suitable for building structures with excellent earthquake resistance or mixed tanks of liquid ammonia and LPG, etc. can be supplied in large quantities and at low cost. Therefore, it is possible to contribute to further improvement in safety of the welded steel structure. In particular, the use of the low yield ratio high tensile steel of the present invention makes it easy to mount a mixed tank on a ship.
次に、本願発明について詳細に説明する。
本発明は、優れた溶接性と低温靭性を得るために低降伏比高張力鋼としては画期的に低いC、PCM成分とし、その条件下で強度を確保するためにミクロ組織(鋼組織)を限定するものである。
Next, the present invention will be described in detail.
The present invention has excellent weldability and a remarkably low C, P CM component as a low yield ratio high-strength steel in order to obtain low temperature toughness, the microstructure in order to ensure the strength under these conditions (steel structure ).
本発明の低降伏比高張力鋼のミクロ組織は、板厚方向断面1/4厚位置において、ポリゴナルまたは擬ポリゴナルフェライトとパーライトの合計分率が50%未満であることを構成要素の一つとする。このようなミクロ組織は、圧延後比較的高温で変態生成する組織であり、実製造においては圧延後の冷速が比較的の遅い場合に生成する組織である。 One of the components of the microstructure of the low yield ratio high strength steel of the present invention is that the total fraction of polygonal or pseudopolygonal ferrite and pearlite is less than 50% at the position of the thickness direction cross section 1/4. To do. Such a microstructure is a structure that generates a transformation at a relatively high temperature after rolling, and is a structure that is generated when the cooling speed after rolling is relatively slow in actual production.
なお、ここでの合計分率とは、観察面積に対する面積分率(組織構成比率)を意味する。低降伏比高張力鋼のミクロ組織は、本来、高張力化を担うべきベイナイトやマルテンサイトなどと呼ばれる低温変態生成組織分率で限定すべきである。しかし、特に「ベイナイト」と総称される組織は、一般に多種多様な中間段階変態組織の総称であり、その定義は必ずしも明確ではなく、組織の規定が困難で特許上の組織の規定としては不正確さを伴うと判断される。このため、本発明では、当業者であれば、定義および組織判別上ほとんど問題が生じないと考えられ、組織の特定・識別が明確なポリゴナルまたは擬ボリゴナルフェライトとパーライトの組織分率として限定した。 In addition, the total fraction here means the area fraction (tissue composition ratio) with respect to the observation area. The microstructure of the low-yield ratio high-strength steel should be limited by the fraction of the low-temperature transformation formation structure called bainite or martensite that should be responsible for high tension. However, the organization generally referred to as “bainite” is a general term for a wide variety of intermediate-stage transformation structures, and its definition is not necessarily clear, and it is difficult to specify the organization, and it is not accurate as the organization definition in the patent. It is judged to be accompanied. For this reason, in the present invention, those skilled in the art will think that there will be almost no problem in definition and structure discrimination, and the specificity and identification of the structure is limited as a clear polygonal or pseudo-boriginal ferrite and pearlite structure fraction. .
当該組織が50%以上になると、本願発明のような低C、低PCM成分では安定した50キロ級の高張力化か困難となるため、50%未満に限定した。下限は限定するものではなく、成分や目的とする板厚や強度によっては、O%であっても優れた特性が得られる。なお、組織を本願発明の通り限定することで、多くの場合、引張試験時に明瞭な降伏点が出現せず、低降伏比が容易に得られる。 If the organization is more than 50%, low C such as in the present invention, since whether high tensile of stable 50-kilogram difficult at low P CM components, is limited to less than 50%. The lower limit is not limited, and excellent characteristics can be obtained even with O% depending on the component and the target plate thickness and strength. In many cases, by limiting the structure as in the present invention, a clear yield point does not appear during a tensile test, and a low yield ratio can be easily obtained.
合計分率を50%未満に抑えるべき組織(ポリゴナルまたは擬ポリゴナルフェライトとパーライト)は、既に述べたように圧延後比較的遅い冷速で生成するため、実製造上、圧延後強制的な加速冷却が必要となる。加速冷却条件を含む製造条件については後述するが、加速冷却を施した場合、必然的に表層は硬化し易くなる。JISなどの規定では、板厚にもよるが、板厚全厚または1/4板厚位置から引張試破片を採取することになっており、その規定の中で所定の強度を満足することは言うまでもないが、本来、板厚断面で均一であることが望ましく、曲げ加工性や穿孔性など各種加工性の観点からも板厚断面方向で強度差が小さいほうが好ましいことは言うまでもない。このため、板厚断面の硬さを以下の通り限定する。 Since the structure (polygonal or pseudopolygonal ferrite and pearlite) whose total fraction should be kept below 50% is generated at a relatively slow cold speed after rolling as already described, forced acceleration after rolling in actual production. Cooling is required. Manufacturing conditions including accelerated cooling conditions will be described later, but when accelerated cooling is performed, the surface layer is inevitably hardened. According to JIS standards, although it depends on the plate thickness, tensile test specimens are to be taken from the full thickness or 1/4 thickness position. Needless to say, it is naturally desirable that the cross section is uniform in thickness, and it is needless to say that a smaller difference in strength in the cross section direction of the thickness is preferable from the viewpoint of various workability such as bending workability and punchability. For this reason, the hardness of a plate | board thickness cross section is limited as follows.
すなわち、鋼の表面1mm下断面における荷重98Nでのビッカース硬さが300Hv以下とする。鋼の表面1mm下断面の硬さ規制は、曲げ加工時の割れ防止の観点からのものである。
また、鋼の表面1mm下断面における荷重98Nでのビッカース硬さと、前記鋼の板厚断面方向に1mm間隔で測定したビッカース硬さの最小値との差の数値が、mmで表記した板厚の1.5倍の数値以下とする。この規制は、上述した加工性やそもそも有すべき鋼材の均一性の観点から限定したもので、mmで表記した板厚の1.5倍以内であれば鋼材の加工性や均一性が使用性能上問題となることはない。
That is, the Vickers hardness at a load of 98 N in the 1 mm lower cross section of the steel surface is 300 Hv or less. The restriction of the hardness of the 1 mm lower cross section of the steel surface is from the viewpoint of preventing cracking during bending.
In addition, the numerical value of the difference between the Vickers hardness at a load of 98 N in the 1 mm lower cross section of the steel surface and the minimum value of the Vickers hardness measured at intervals of 1 mm in the cross section direction of the steel is the thickness expressed in mm. The value is 1.5 times or less. This regulation is limited from the viewpoint of the workability mentioned above and the uniformity of the steel material that should be originally provided, and the workability and uniformity of the steel material are within the use performance as long as it is within 1.5 times the plate thickness expressed in mm. There is no problem.
次に、本発明の低降伏比高張力鋼の鋼成分について説明する。
Cは鋼材の特性に最も顕著に効くもので、下限0.01%は強度確保や溶接などの熱影響部が必要以上に軟化することのないようにするための最小量である。しかし、C量が多すぎると焼入性が必要以上に上がり、鋼材が本来有すべき強度、靭性のバランス、溶接性などに悪影響を及ぼすため、さらに、後述する比較的低温で加速冷却を停止する本願発明の製造方法において、板厚断面方向の硬さ、特に表層の硬さを本願発明の限定範囲に抑えるため、上限を0.08%とした。
Next, the steel components of the low yield ratio high tensile steel of the present invention will be described.
C is most effective for the properties of the steel material, and the lower limit of 0.01% is the minimum amount for preventing the heat-affected zone such as securing the strength and welding from being softened more than necessary. However, if the amount of C is too large, the hardenability will increase more than necessary, and it will adversely affect the strength, toughness balance, weldability, etc. that steel materials should have, and further, accelerated cooling will be stopped at a relatively low temperature, which will be described later. In the manufacturing method of the present invention, the upper limit is set to 0.08% in order to keep the hardness in the cross-sectional direction of the plate thickness, particularly the hardness of the surface layer, within the limited range of the present invention.
Siは脱酸上鋼に含まれる元素であるが、多く添加すると溶接性、HAZ靭性が劣化するため、上限を0.4%に限定した。鋼の脱酸はTi、Alのみでも十分可能であり、HAZ靭性、焼入性などの観点から低いほど好ましく、必ずしも添加する必要はない。 Si is an element contained in deoxidized upper steel, but if added in large amounts, weldability and HAZ toughness deteriorate, so the upper limit was limited to 0.4%. Deoxidation of steel can be sufficiently performed only with Ti and Al, and is preferably as low as possible from the viewpoints of HAZ toughness, hardenability, and the like, and it is not always necessary to add them.
Mnは強度、靭性を確保する上で不可欠な元素であり、その下限は1.0%である。しかし、Mn量が多すぎると焼入性が上昇して溶接性、HAZ靭性を劣化させるだけでなく、連続鋳造スラブの中心偏析を助長するので上限を2.0%とした。 Mn is an element indispensable for securing strength and toughness, and its lower limit is 1.0%. However, if the amount of Mn is too large, not only the hardenability is increased and the weldability and HAZ toughness are deteriorated, but also the center segregation of the continuously cast slab is promoted, so the upper limit was made 2.0%.
Pは本発明鋼においては不純物であり、P量の低減はHAZにおける粒界破壊を減少させる傾向があるため、少ないほど好ましい。含有量が多いと母材、溶接部の低温靭性を劣化させるため上限を0.02%とした。 P is an impurity in the steel of the present invention, and a reduction in the amount of P tends to reduce the grain boundary fracture in HAZ, so the smaller the better. If the content is large, the low temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was made 0.02%.
SはPと同様本発明鋼においては不純物であり、母材の低温靭性の観点からは少ないほど好ましい。含有量が多いと母材、溶接部の低温靭性を劣化させるため上限を0.01%とした。 S, like P, is an impurity in the steel of the present invention, and is preferably as small as possible from the viewpoint of the low temperature toughness of the base material. If the content is large, the low temperature toughness of the base metal and the welded portion is deteriorated, so the upper limit was made 0.01%.
Nbはオーステナイトの未再結晶温度を上昇させ、熱間圧延時の制御圧延の効果を最大限に発揮するのに加え、溶接や切断時の熱影響部の軟化を防止する上での必須元素で、最低0.005%の添加が必要である。また、焼入れの際の加熱オーステナイトの細粒化にも寄与する。さらに、析出硬化として、強度向上効果も有する。しかし、過剰な添加は、溶接部の靭性劣化を招くため上限を0.06%とした。 Nb is an essential element for raising the non-recrystallization temperature of austenite and maximizing the effect of controlled rolling during hot rolling, as well as preventing softening of the heat affected zone during welding and cutting. A minimum of 0.005% addition is required. It also contributes to the refinement of the heated austenite during quenching. Furthermore, it has an effect of improving strength as precipitation hardening. However, excessive addition causes deterioration of the toughness of the weld zone, so the upper limit was made 0.06%.
Tiは母材およびHAZ靭性向上のために必須である。なぜならばTiは、Al量が少ないとき(例えば0.003%以下)、Oと結合してTi2O3を主成分とする析出物を形成、粒内変態フェライト生成の核となりHAZ靭性を向上させる。また、TiはNと結合してTiNとしてスラブ中に微細析出し、加熱時のγ粒の粗大化を抑え圧延組織の細粒化に有効であり、また鋼板中に存在する微細TiNは、溶接時にHAZ組織を細粒化するためである。これらの効果を得るためには、Tiは最低0.005%必要である。しかし多すぎるとTiCを形成し、低温靭性や溶接性を劣化させるので、その上限は0.025%である。 Ti is essential for improving the base material and the HAZ toughness. This is because when Ti has a small amount of Al (for example, 0.003% or less), it combines with O to form precipitates mainly composed of Ti 2 O 3 , and becomes the nucleus of intragranular transformation ferrite formation and improves HAZ toughness. Let Ti is combined with N and finely precipitated in the slab as TiN, which suppresses the coarsening of γ grains during heating and is effective for refining the rolled structure. The fine TiN present in the steel sheet is welded. This is because sometimes the HAZ structure is refined. In order to obtain these effects, Ti needs to be at least 0.005%. However, if it is too much, TiC is formed and the low temperature toughness and weldability are deteriorated, so the upper limit is 0.025%.
Alは、一般に脱酸上鋼に含まれる元素であるが、脱酸はSiまたはTiだけでも十分であり、本発明鋼においては、その下限は限定しない。しかし、Al量が多くなると鋼の清浄度が悪くなるだけでなく、溶接金属の靭性が劣化するので上限を0.06%とした。 Al is an element generally contained in deoxidized upper steel, but Si or Ti is sufficient for deoxidation, and the lower limit is not limited in the steel of the present invention. However, when the amount of Al increases, not only the cleanliness of the steel deteriorates but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.
Nは、不可避的不純物として鋼中に含まれるものであるが、Nbと結合して炭窒化物を形成して強度を増加させ、また、TiNを形成して前述のように鋼の性質を高める。このため、N量として最低0.001%必要である。しかしながら、N量の増加はHAZ靭性、溶接性にきわめて有害であり、本発明鋼においてはその上限は0.005%である。 N is contained in the steel as an unavoidable impurity, but combines with Nb to form carbonitride to increase the strength, and TiN to increase the properties of the steel as described above. . For this reason, the N amount is required to be at least 0.001%. However, the increase in the amount of N is extremely harmful to the HAZ toughness and weldability, and the upper limit is 0.005% in the steel of the present invention.
次に必要に応じて含有することができるCu、Ni、Cr、Mo、V、B、Mgの添加理由について説明する。 Next, the reason for adding Cu, Ni, Cr, Mo, V, B, and Mg that can be contained as necessary will be described.
基本となる成分に、さらにこれらの元素を添加する主たる目的は、本発明鋼の優れた特徴を損なうことなく、強度、靭性などの特性を向上させるためである。したがってその添加量は自ずと制限されるべき性質のものである。 The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should naturally be limited.
CuはNiとほぼ同様の効果、現象を示し、上限の0.5%は溶接性劣化に加え、過剰な添加は熱間圧延時にCu−クラックが発生し製造困難となるため規制される。下限は実質的な効果が得られるための最小量とすべきで0.05%である。これは後述するCr、Moについても同様である。 Cu exhibits substantially the same effects and phenomena as Ni, with the upper limit of 0.5% being restricted in terms of weldability deterioration and excessive addition because Cu-cracks are generated during hot rolling, making production difficult. The lower limit should be the minimum amount for obtaining a substantial effect, and is 0.05%. The same applies to Cr and Mo described later.
Niは過剰に添加しなければ、溶接性、HAZ靭性に悪影響を及ぼすことなく母材の強度、靭性を向上させる。これら効果を発揮させるためには、少なくとも0.05%以上の添加が必須である。一方、過剰な添加は高価なだけでなく、溶接性に好ましくない。また、Niを多く添加すると液体アンモニア中で応力腐食割れ(SCC)を誘起する可能性が指摘されている。発明者らの実験によれば、1%までの添加は溶接性や液体アンモニア中でのSCCを大きく劣化させず、強度、靭性向上効果の方が大きいため、上限を1.0%とした。 If Ni is not added excessively, it improves the strength and toughness of the base material without adversely affecting the weldability and HAZ toughness. In order to exert these effects, addition of at least 0.05% is essential. On the other hand, excessive addition is not only expensive, but is not preferable for weldability. Further, it has been pointed out that the addition of a large amount of Ni may induce stress corrosion cracking (SCC) in liquid ammonia. According to the experiments by the inventors, addition up to 1% does not significantly deteriorate weldability and SCC in liquid ammonia, and the effect of improving the strength and toughness is larger, so the upper limit was set to 1.0%.
Cr、Moは、母材の強度、靭性をともに向上させるため、それぞれ0.05%以上必要である。しかし添加量が多すぎると母材、溶接部の靭性および溶接性を劣化を招き、経済性も失するためそれぞれ上限を0.5%とした。 Cr and Mo are required to be 0.05% or more in order to improve both the strength and toughness of the base material. However, if the added amount is too large, the base material, the toughness and weldability of the welded portion are deteriorated, and the economical efficiency is also lost.
Vは、Nbとほぼ同様の作用を有するものであるが、Nbに比べてその効果は小さい。また、Vは焼入れ性にも影響を及ぼし、上記元素と同様組織制御の観点から添加するものである。Nbと同様の効果は0.01%未満では効果が少なく、上限は0.10%まで許容できる。 V has substantially the same action as Nb, but its effect is smaller than that of Nb. V also affects the hardenability, and is added from the viewpoint of controlling the structure in the same manner as the above elements. The effect similar to Nb is less if it is less than 0.01%, and the upper limit is acceptable up to 0.10%.
Bは、オーステナイト粒界に偏析し、フェライトの生成を抑制することを介して、焼入性を向上させ、強度向上に寄与する。この効果を享受するため、最低0.0002%以上必要である。しかし、多すぎる添加は焼入性向上効果が飽和するだけでなく、靭性上有害となるB析出物を形成する可能性もあるため、上限を0.003%とした。なお、タンク用鋼などとして、応力腐食割れが懸念されるケースでは、母材および溶接熱影響部の硬さの低減がポイントとなることが多く(例えば、硫化物応力腐食割れ(SCC)防止のためにはHRC≦22(HV≦248)が必須とされる)、そのようなケースでは焼入性を増大させるB添加は好ましくない。 B segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby improving hardenability and contributing to strength improvement. In order to enjoy this effect, at least 0.0002% is necessary. However, too much addition not only saturates the effect of improving hardenability but also may form B precipitates that are harmful to toughness, so the upper limit was made 0.003%. In cases where stress corrosion cracking is a concern, such as for tank steel, reduction of the hardness of the base metal and the weld heat affected zone is often the point (for example, prevention of sulfide stress corrosion cracking (SCC)). Therefore, HRC ≦ 22 (HV ≦ 248) is essential), and in such a case, B addition for increasing the hardenability is not preferable.
Mgは、溶接熱影響部においてオーステナイト粒の成長を抑制し、細粒化する作用があり、溶接部の強靭化が図れる。このような効果を享受するためには、Mgは0.0002%以上必要である。一方、添加量が増えると添加量に対する効果代が小さくなるため、コスト上得策ではないので上限は0.005%とした。 Mg suppresses the growth of austenite grains in the weld heat-affected zone and has the effect of making the grains finer, so that the weld zone can be strengthened. In order to enjoy such an effect, Mg needs to be 0.0002% or more. On the other hand, since the effect cost for the added amount decreases as the added amount increases, the upper limit is set to 0.005% because this is not a cost effective measure.
CaおよびREMは、MnSの形態を制御し、母材の低温靭性を向上させるほか、湿潤硫化水素環境下での水素誘起割れ(HIC、SSC、SOHIC)感受性を低減させる。これらの効果を発揮するためには、最低0.0005%必要である。しかし、多すぎる添加は、鋼の清浄度を逆に高め、母材靭性や湿潤硫化水素環境下での水素誘起割れ(HIC、SSC、SOHIC)感受性を高めるため、Caの添加量の上限は0.004%、REMの添加量の上限は0.008%に限定した。CaとREMは、ほぼ同等の効果を有するため、いずれか1種を上記範囲で添加すればよい。 Ca and REM control the morphology of MnS, improve the low temperature toughness of the base material, and reduce the susceptibility to hydrogen induced cracking (HIC, SSC, SOHIC) in a wet hydrogen sulfide environment. In order to exert these effects, 0.0005% is necessary at least. However, too much addition increases the cleanliness of the steel, and increases the base metal toughness and susceptibility to hydrogen induced cracking (HIC, SSC, SOHIC) in a wet hydrogen sulfide environment, so the upper limit of the Ca addition amount is 0. 0.004%, and the upper limit of the amount of REM added was limited to 0.008%. Since Ca and REM have substantially the same effect, any one of them may be added in the above range.
鋼の個々の成分を限定しても、成分系全体が適切でないと優れた特性は得られない。このため、PCMの値を0.10〜0.18%の範囲に限定する。下記式により規定されるPCMは溶接性を表す指標で、低いほど溶接性は良好である。本発明鋼においては、PCMが0.25%以下であれば、優れた溶接性の確保が可能である。下限の0.10%は、広い板厚範囲で安定して強度を確保するための必要量であり、上限の0.18%は、後述する比較的低温で加速冷却を停止しても、強度が過剰となったり、鋼材の表層硬さや板厚断面方向硬さ差が必要以上に大きくならないように限定したものである。
PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B
Even if the individual components of the steel are limited, excellent properties cannot be obtained unless the entire component system is appropriate. Thus, limiting the value of P CM in the range of from 0.10 to 0.18%. The P CM defined by the following formula in an index representing the weldability, the lower the weldability is good. In the present invention steel, as long as P CM is 0.25% or less, it is possible to ensure excellent weldability. The lower limit of 0.10% is a necessary amount for ensuring the strength stably over a wide thickness range, and the upper limit of 0.18% is the strength even when accelerated cooling is stopped at a relatively low temperature, which will be described later. It is limited so that the surface layer hardness of the steel material and the difference in hardness in the plate thickness cross-section direction do not become larger than necessary.
P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
優れた溶接性と低温靭性を確保しつつ、上述した組織を安定して得るためには、以下に示す通り製造条件を限定することがきわめて有効である。以下、その理由について説明する。 In order to stably obtain the above-mentioned structure while ensuring excellent weldability and low temperature toughness, it is extremely effective to limit the production conditions as shown below. The reason will be described below.
圧延に先立つ加熱温度を1000〜1250℃に限定した理由は、加熱時のオーステナイト粒を小さく保ち、圧延組織の微細化を図るためである。1250℃は加熱時のオーステナイトが極端に粗大化しない上限温度であり、加熱温度がこれを超えるとオーステナイト粒が粗大混粒化し、変態後の組織も粗大化するため鋼の靭性が著しく劣化する。一方、加熱温度が低すぎると、後述する圧延終了温度の確保が困難となるばかりでなく、オーステナイトの未再結晶温度を上昇させ、熱間圧延時の制御圧延の効果を最大限に発揮させたり、析出硬化を発現させるためのNbの溶体化の観点から下限を1000℃に限定した。 The reason for limiting the heating temperature prior to rolling to 1000 to 1250 ° C. is to keep the austenite grains during heating small and to refine the rolled structure. 1250 ° C. is an upper limit temperature at which the austenite during heating is not excessively coarsened. When the heating temperature is exceeded, the austenite grains are coarsely mixed and the structure after transformation is also coarsened, so that the toughness of the steel is remarkably deteriorated. On the other hand, if the heating temperature is too low, it is difficult not only to secure the rolling end temperature described later, but also to increase the non-recrystallization temperature of austenite and maximize the effect of controlled rolling during hot rolling. The lower limit was limited to 1000 ° C. from the viewpoint of solution of Nb for causing precipitation hardening.
上述のような条件で加熱した鋳片または鋼片を、オーステナイト未再結晶温度域での累積圧下量を30%以上とし、720℃以上で熱間圧延を終了した後、680℃以上の温度から加速冷却する。 The slab or steel slab heated under the above-mentioned conditions is set to a cumulative reduction amount of 30% or more in the austenite non-recrystallization temperature range, and after hot rolling is completed at 720 ° C. or higher, from a temperature of 680 ° C. or higher. Accelerate cooling.
オーステナイト未再結晶温度域での圧延を行うことによって、オーステナイト粒を顕著に細粒化するため、少なくとも30%以上の累積圧下量が必要である。圧延終了温度が720℃を下回ると、フェライトが変態析出し、フェライトを加工(圧延)する恐れがあり、低降伏比化や低温靭性確保の点で好ましくない。このため、圧延終了温度は、720℃以上に限定する。 By rolling in the austenite non-recrystallization temperature range, the austenite grains are remarkably refined, so that a cumulative reduction amount of at least 30% or more is required. If the rolling end temperature is lower than 720 ° C., ferrite may be transformed and precipitated, and the ferrite may be processed (rolled), which is not preferable in terms of reducing the yield ratio and securing low temperature toughness. For this reason, rolling end temperature is limited to 720 degreeC or more.
720℃以上で熱間圧延を終了した後、加速冷却を行う。放冷すると強度が低下するだけでなく、転移の回復に伴って低降伏比化が困難となる(現象論的には、明瞭な降伏点が出現する)。 After the hot rolling is finished at 720 ° C. or higher, accelerated cooling is performed. When allowed to cool, not only does the strength decrease, but it becomes difficult to reduce the yield ratio as the transition recovers (phenomenologically, a clear yield point appears).
680℃以上の温度から加速冷却を開始するのは、変態域の冷速を早めることで組織を微細化し、強度と靭性を同時に向上させるためである。また、組織を微細化することは、C濃縮相であるマルテンサイト−オーステナイト混合相(M−A constituents)が生成する可能性があるが、その場合でも微細に分散生成することになるため、母材靭性への悪影響を抑えることにも寄与する。加速冷却を開始の温度が680℃未満であると、粗大なフェライトが析出し始め、強度低下や靭性を劣化させるため、680℃以上からの加速冷却に限定した。この加速冷却は、350℃以下の温度で停止しなければならない。350℃を超える温度では、加速冷却停止後の放冷が実質上の焼き戻しとなり、強度低下とともに転位の回復に伴って低降伏比化が困難となるからである。 The reason why the accelerated cooling is started from a temperature of 680 ° C. or more is to make the structure finer by increasing the cooling speed in the transformation region and simultaneously improve the strength and toughness. Further, refinement of the structure may generate a martensite-austenite mixed phase (MA constituent) which is a C-enriched phase, but even in that case, it is finely dispersed and produced. It also contributes to reducing adverse effects on toughness. If the temperature at which accelerated cooling is started is less than 680 ° C., coarse ferrite starts to precipitate and deteriorates strength and toughness. Therefore, the accelerated cooling is limited to 680 ° C. or higher. This accelerated cooling must be stopped at a temperature below 350 ° C. When the temperature exceeds 350 ° C., the cooling after stopping the accelerated cooling is substantially tempered, and it is difficult to reduce the yield ratio as the strength decreases and the dislocation recovers.
なお、加速冷却時の冷速は、鋼成分や意図する降伏比、低温靭性レベルによっても変わるため一概には言えないが、板厚1/4厚位置の加速冷却開始温度から350℃以下となるまでの平均冷速で、少なくとも3℃/秒以上とすることが望ましい。
また、本発明の低降伏比高張力鋼の製造方法は、加熱・圧延後、加速冷却を施し、焼き戻しを行わない、いわゆる非調質型とするものである。
Although the cooling speed during accelerated cooling varies depending on the steel components, the intended yield ratio, and the low temperature toughness level, it cannot be generally stated, but it is 350 ° C. or less from the accelerated cooling start temperature at the ¼ thickness position. It is desirable that the average cooling rate is at least 3 ° C./second.
Moreover, the manufacturing method of the low yield ratio high-tensile steel of the present invention is a so-called non-tempered type in which accelerated cooling is performed after heating and rolling and tempering is not performed.
転炉−連続鋳造−厚板工程で表1および表3に示す種々の鋼成分の鋼板(厚さ15〜80mm)を表2および表4に示す条件で製造し、諸特性について調べた。その結果を表2および表4に示す。 In the converter-continuous casting-thick plate process, steel plates (thickness 15 to 80 mm) having various steel components shown in Tables 1 and 3 were manufactured under the conditions shown in Tables 2 and 4, and various properties were examined. The results are shown in Tables 2 and 4.
表1〜表4において、上述した好ましい範囲未満または好ましい範囲を越える数値については、下線を付して示した。なお、表2および表4に示す項目における好ましい範囲について以下に示す。 In Tables 1 to 4, numerical values that are less than the above-described preferable range or exceed the preferable range are shown with an underline. In addition, about the preferable range in the item shown in Table 2 and Table 4, it shows below.
降伏強さ:325〜475MPa(50キロ鋼クラス)
引張強さ:490〜640MPa(50キロ鋼クラス)
降伏比(YR):80%未満
靭性(vTrs):−40℃以下
板厚方向断面1/4厚位置の鋼組織のα+P分率(α=ポリゴナルまたは擬ポリゴナルフェライト、P=パーライト):50%未満
鋼の表面1mm下断面における荷重98Nでのビッカース硬さ:300Hv以下
最小硬さとの差(鋼の表面1mm下断面における荷重98Nでのビッカース硬さと、鋼の板厚断面方向に1mm間隔で測定したビッカース硬さの最小値との差の数値):mmで表記した板厚の1.5倍の数値以下
Yield strength: 325-475MPa (50kg steel class)
Tensile strength: 490-640 MPa (50 kg steel class)
Yield ratio (YR): less than 80% Toughness (vTrs): −40 ° C. or less α + P fraction of steel structure at thickness position of ¼ thickness (α = polygonal or pseudopolygonal ferrite, P = pearlite): 50 Less than% Vickers hardness at a load of 98 N at a cross section of 1 mm below the surface of the steel: 300 Hv or less Difference from the minimum hardness (Vickers hardness at a load of 98 N at a cross section of 1 mm below the surface of the steel and at 1 mm intervals in the plate thickness cross section direction of the steel The numerical value of the difference from the measured minimum value of Vickers hardness): less than 1.5 times the plate thickness expressed in mm
表1〜表4において、実験例1〜実験例10は、本発明の製造方法によって製造した本発明の鋼板であり、表1および表2に示すように、すべての諸特性が良好であった。
これに対し、表1〜表4において、実験例11〜実験例23は、本発明によらない比較例であり、表1〜表4に示すように、いずれかの特性が劣る結果となった。
In Tables 1 to 4, Experimental Examples 1 to 10 are steel plates of the present invention manufactured by the manufacturing method of the present invention, and as shown in Tables 1 and 2, all the various properties were good. .
On the other hand, in Tables 1 to 4, Experimental Examples 11 to 23 are comparative examples not according to the present invention, and as shown in Tables 1 to 4, any of the characteristics was inferior. .
より詳細には、実験例11は、C量が高いため表面硬さや最小硬さとの差が大きく、また、γ未再結晶温度域における累積圧下量も小さいために靭性に劣る。
実験例12は、鋼成分としては本願発明範囲内にあるもののPCMが高いため、強度が高く、靭性にも劣る。また、断面の硬さ差も大きい。
実験例13は、鋼成分としてNbが添加されておらず、加速冷却停止温度も高いため、降伏比は高めで強度が低く、靭性にも劣る。
実験例14は、Tiが添加されておらず、プロセス的には圧延温度が低く、それに連動して加速冷却開始温度も低いため、組織が適正でなく降伏比は高めで強度は低く、靭性もやや劣る。
実験例15は、C量が低いため、組織が適正でなく強度が低い。
More specifically, in Example 11, the difference between the surface hardness and the minimum hardness is large because the C amount is high, and the toughness is inferior because the cumulative reduction amount in the γ non-recrystallization temperature range is also small.
Experimental Example 12, since P CM is high but as the steel component is within the present invention range, high strength, inferior in toughness. Moreover, the hardness difference of a cross section is also large.
In Experimental Example 13, Nb is not added as a steel component and the accelerated cooling stop temperature is high, so the yield ratio is high, the strength is low, and the toughness is also poor.
In Experimental Example 14, since Ti is not added, the rolling temperature is low in the process, and the accelerated cooling start temperature is low in conjunction with it, the structure is not appropriate, the yield ratio is high, the strength is low, and the toughness is also low. Somewhat inferior.
In Experimental Example 15, since the amount of C is low, the structure is not appropriate and the strength is low.
実験例16は、実験例11と同一でC量が高いため、製造条件は本発明の限定範囲内であるが、鋼の表面1mm下断面におけるビッカース硬さ、最小硬さとの差、靭性に劣る。
実験例17は、本発明の鋼板である実験例4と同一成分であるが、加速冷却開始温度が低いため、板厚方向断面1/4厚位置の鋼組織のα+P分率が大きく、引張強さに劣る。
実験例18は、本発明の鋼板である実験例4と同一成分であるが、加速冷却停止温度が高いため、降伏比が高く、引張強さもやや劣る。
Since Experimental Example 16 is the same as Experimental Example 11 and has a high C content, the manufacturing conditions are within the limited range of the present invention. .
Experimental Example 17 is the same component as Experimental Example 4 which is a steel sheet of the present invention, but because the accelerated cooling start temperature is low, the α + P fraction of the steel structure at the 1/4 thickness position in the thickness direction is large, and the tensile strength is high. Inferior.
Experimental Example 18 is the same component as Experimental Example 4 which is a steel sheet of the present invention, but has a high yield ratio and a slightly inferior tensile strength because the accelerated cooling stop temperature is high.
Claims (4)
C:0.01〜0.08%、
Si:0.4%以下、
Mn:1.0〜2.0%、
P:0.02%以下、
S:0.01%以下、
Nb:0.005〜0.06%、
Ti:0.005〜0.025%、
Al:0.06%以下、
N:0.001〜0.005%、かつ
PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5Bが0.1〜0.18%、残部が鉄および不可避的不純物からなり、
板厚方向断面1/4厚位置の鋼組織のポリゴナルまたは擬ポリゴナルフェライトとパーライトの合計分率が50%未満であって、
鋼の表面1mm下断面における荷重98Nでのビッカース硬さが300Hv以下で、
前記ビッカース硬さと前記鋼の板厚断面方向に1mm間隔で測定したビッカース硬さの最小値との差の数値が、mmで表記した板厚の1.5倍の数値以下であることを特徴とする低降伏比高張力鋼。 Steel component is mass%,
C: 0.01 to 0.08%,
Si: 0.4% or less,
Mn: 1.0-2.0%,
P: 0.02% or less,
S: 0.01% or less,
Nb: 0.005 to 0.06%,
Ti: 0.005 to 0.025%,
Al: 0.06% or less,
N: 0.001 to 0.005%, and P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B is 0.1 to 0.18%, the balance is iron and inevitable impurities,
The total fraction of polygonal or pseudopolygonal ferrite and pearlite in the steel structure at the position of 1/4 thickness in the thickness direction cross section is less than 50%,
The Vickers hardness at a load of 98 N on the 1 mm lower cross section of the steel surface is 300 Hv or less,
The difference between the Vickers hardness and the minimum value of the Vickers hardness measured at intervals of 1 mm in the cross-sectional direction of the steel thickness is not more than 1.5 times the thickness expressed in mm. Low yield ratio high tensile steel.
Cu:0.05〜0.5%、
Ni:0.05〜0.5%、
Cr:0.05〜0.5%、
Mo:0.05〜0.5%、
V:0.01〜0.1%、
B:0.0002〜0.003%、
Mg:0.0002〜0.005%の範囲で1種または2種以上をさらに含有することを特徴とする請求項1に記載の低降伏比高張力鋼。 The steel component is mass%,
Cu: 0.05 to 0.5%,
Ni: 0.05 to 0.5%,
Cr: 0.05 to 0.5%,
Mo: 0.05-0.5%
V: 0.01 to 0.1%
B: 0.0002 to 0.003%,
The low yield ratio high-tensile steel according to claim 1, further comprising one or more of Mg: 0.0002 to 0.005%.
Ca:0.0005〜0.004%、
REM:0.0005〜0.008%のいずれか1種をさらに含有することを特徴とする請求項1または請求項2に記載の低降伏比高張力鋼。 The steel component is mass%,
Ca: 0.0005 to 0.004%,
The low yield ratio high strength steel according to claim 1 or 2, further comprising any one of REM: 0.0005 to 0.008%.
請求項1〜請求項3のいずれか1項に記載の鋼成分からなる鋳片または鋼片を、1000〜1250℃の温度に加熱し、
オーステナイト未再結晶温度域での累積圧下量を30%以上として720℃以上の温度で熱間圧延を終了した後、
680℃以上の温度から開始して350℃以下の温度で停止する加速冷却を行うことを特徴とする低降伏比高張力鋼の製造方法。
The total fraction of polygonal or pseudopolygonal ferrite and pearlite in the steel structure at the position of 1/4 thickness in the thickness direction cross section is less than 50%, and the Vickers hardness at a load of 98 N in the cross section below 1 mm of the steel surface is 300 Hv or less Thus, the low yield value where the numerical value of the difference between the Vickers hardness and the minimum value of the Vickers hardness measured at intervals of 1 mm in the thickness direction of the steel is not more than 1.5 times the thickness expressed in mm A method for producing high-tensile steel,
The slab or steel slab comprising the steel component according to any one of claims 1 to 3 is heated to a temperature of 1000 to 1250 ° C,
After finishing the hot rolling at a temperature of 720 ° C. or more with the cumulative reduction amount in the austenite non-recrystallization temperature range being 30% or more,
A method for producing a low-yield ratio high-strength steel, characterized by performing accelerated cooling that starts at a temperature of 680 ° C or higher and stops at a temperature of 350 ° C or lower.
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