JP5476763B2 - High tensile steel plate with excellent ductility and method for producing the same - Google Patents

High tensile steel plate with excellent ductility and method for producing the same Download PDF

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JP5476763B2
JP5476763B2 JP2009080826A JP2009080826A JP5476763B2 JP 5476763 B2 JP5476763 B2 JP 5476763B2 JP 2009080826 A JP2009080826 A JP 2009080826A JP 2009080826 A JP2009080826 A JP 2009080826A JP 5476763 B2 JP5476763 B2 JP 5476763B2
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JP2010229528A (en
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智之 横田
公宏 西村
伸夫 鹿内
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Jfeスチール株式会社
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本発明は、非調質鋼板、特に非調質厚鋼板に係わり、橋梁、建築、ラインパイプ原板、造船、建設機械、産業機械、海洋構造物、ペンストック等に用いて好適な、延性に優れた高張力鋼板及びその製造方法に関する。   The present invention relates to non-tempered steel sheets, particularly non-tempered thick steel sheets, and is suitable for use in bridges, buildings, line pipe original plates, shipbuilding, construction machinery, industrial machinery, offshore structures, penstock, etc., and has excellent ductility. The present invention relates to a high-tensile steel plate and a method for producing the same.

一般に、鋼板の高強度化に伴って延性は低下する傾向にあるため、延性の改善に対する要望は強く、近年、上記技術分野で利用される570MPa級以上の高張力鋼板に対しも、さらなる延性の改善が求められている。   In general, since ductility tends to decrease with increasing strength of steel sheets, there is a strong demand for improvement in ductility. In recent years, even for high-tensile steel sheets of 570 MPa class or higher used in the above technical field, further ductility is required. There is a need for improvement.

特に、建築分野において用いられる鋼板には、高い強度と靭性に加え、鋼材の耐震性能の観点から高い塑性変形能力が求められ、貯蔵タンク、圧力容器などに用いられる鋼板では、成形後の変形回復(スプリングバック)の少ない、優れた加工性が要求される。また、ラインパイプ原板では、高い全伸び値が求められることがある。   In particular, steel sheets used in the construction field are required to have high plastic deformation ability from the viewpoint of earthquake resistance performance of steel materials in addition to high strength and toughness. Steel sheets used for storage tanks, pressure vessels, etc., recover deformation after forming. Excellent workability with little (spring back) is required. Moreover, a high total elongation value may be calculated | required by the line pipe original plate.

従来の調質高張力鋼板やTMCP鋼板などの溶接構造用鋼板は、高い強度を有するものの、強度の低い鋼板に比べ延性に劣ることがあった。引張強さ(以下、TS)が570MPa級以上の鋼板の延性向上を図る方法として、(γ+α)2相域からの焼入を含む複数の段階の熱処理によって、フェライトと硬質の第2相からなる混合組織を生成させる方法が一般的で、例えば、特許文献1では、延性の改善の方法として、引張強さに対する上降伏点の比を表す降伏比(以下、YR)の低減を指向している。   Conventional steel sheets for welded structures such as tempered high-tensile steel sheets and TMCP steel sheets have high strength, but may have poor ductility compared to steel sheets with low strength. As a method for improving the ductility of a steel sheet having a tensile strength (hereinafter referred to as TS) of 570 MPa class or more, a ferrite and a hard second phase are formed by heat treatment in a plurality of stages including quenching from a (γ + α) two-phase region. A method of generating a mixed structure is common. For example, in Patent Document 1, as a method for improving ductility, a reduction in yield ratio (hereinafter referred to as YR) representing a ratio of an upper yield point to tensile strength is aimed. .

また、特許文献2に記載されているように、圧延後から水冷開始までに鋼板を空冷する時間を設け、初析フェライトを生成させることによって低YR化を図る方法も知られている。   In addition, as described in Patent Document 2, there is also known a method for reducing YR by providing a time for air-cooling a steel sheet after rolling until the start of water cooling to generate proeutectoid ferrite.

さらに、特許文献3、4に記載されているように、Ar点以上から400〜650℃までの冷却において、1〜15℃/secの比較的ゆるやかな冷却速度に制御する方法も知られている。 Furthermore, as described in Patent Documents 3 and 4, there is also known a method of controlling to a relatively slow cooling rate of 1 to 15 ° C./sec in cooling from an Ar 3 point to 400 to 650 ° C. Yes.

また最近では、熱間圧延・冷却後の鋼板を、誘導加熱装置等を用いて加熱するに際し、鋼板内部をAc点未満に、鋼板表層をAc点以上に加熱制御することで、鋼板表層部の硬度を低下させ、これにより鋼板強度を確保しながら加工性を向上させる技術が開示されている。 In addition, recently, when heating a steel sheet after hot rolling / cooling using an induction heating device or the like, the inside of the steel sheet is controlled to be less than Ac 1 point, and the steel sheet surface layer is heated to Ac 1 point or more to control the steel sheet surface layer. A technique for improving the workability while reducing the hardness of the part and thereby securing the strength of the steel sheet is disclosed.

特許文献5には、C量が0.005%〜0.02%と比較的Cが低い成分の鋼板を熱間圧延後550℃以下まで空冷した後、板厚中心部をAc点未満に、鋼板表層をAc点以上に加熱・空冷することにより加工性の優れた鋼板が得られることが記載されている。 In Patent Document 5, a steel sheet having a relatively low C content of 0.005% to 0.02% is air-cooled to 550 ° C. or less after hot rolling, and then the center of the sheet thickness is made less than Ac 1 point. Further, it is described that a steel plate having excellent workability can be obtained by heating and air cooling the steel plate surface layer to Ac 1 point or more.

また、特許文献6には、C量が0.02%〜0.15%の鋼板を熱間圧延後400℃未満の温度まで加速冷却した後、板厚中心部をAc点未満に、鋼板表層をAc点以上に加熱・空冷することにより、降伏比の低い、変形能に優れた鋼板が得られることが記載されている。特許文献7には、同様の処理により、耐SSC特性に優れた鋼板が得られることも記載されている。 In Patent Document 6, a steel sheet having a C content of 0.02% to 0.15% is accelerated and cooled to a temperature of less than 400 ° C. after hot rolling, and then the sheet thickness center is less than Ac 1 point. It is described that a steel sheet having a low yield ratio and excellent deformability can be obtained by heating / air cooling the surface layer to Ac 1 point or more. Patent Document 7 also describes that a steel sheet having excellent SSC resistance can be obtained by the same treatment.

特公昭59−52207号公報Japanese Patent Publication No.59-52207 特開平5−63525号公報JP-A-5-63525 特開平1−176027号公報Japanese Patent Laid-Open No. 1-176027 特開平5−214440号公報JP-A-5-214440 特開2007−2308号公報JP 2007-2308 A 特開2007−177318号公報JP 2007-177318 A 特開2007−16302号公報JP 2007-16302 A

しかしながら、特許文献1に記載の方法は、大幅な低YR化を通した延性向上が可能である一方、複数の段階の熱処理が必要となるため、製造コストが増大する。   However, the method described in Patent Document 1 can improve ductility through a significant reduction in YR, but requires a plurality of stages of heat treatment, which increases manufacturing costs.

特許文献2に記載の方法では、生産性が低下し、製造コストが増大する。特許文献3、特許文献4に記載の方法も、生産性が低下し、製造コストが増大する。また、特許文献4に記載の実施例によれば、製造される対象となる鋼板の強度は高々500MPa級にとどまる。   In the method described in Patent Document 2, productivity is reduced and manufacturing cost is increased. Also in the methods described in Patent Document 3 and Patent Document 4, productivity is reduced and manufacturing cost is increased. Moreover, according to the Example described in patent document 4, the intensity | strength of the steel plate used as the object manufactured is only 500 MPa class at most.

特許文献5、6、7に記載の方法は、誘導加熱を利用して鋼板表層部の硬度だけを低下させることで、高強度・高加工性を実現しているが、いずれの場合も、誘導加熱で表層部をAc点以上に加熱した後、空冷している。 The methods described in Patent Documents 5, 6, and 7 achieve high strength and high workability by reducing only the hardness of the steel sheet surface layer portion using induction heating. After heating the surface layer part to Ac 1 point or more by heating, it is air-cooled.

そのため、空冷される二相域に加熱された部分では、オーステナイト化したC濃化領域が冷却中に凝集粗大化して冷却後に生成する硬質第二相:パーライトが粗大になり、延性・靭性に悪影響を及ぼす懸念が残る。   Therefore, in the part heated to the two-phase region to be air-cooled, the austenitized C-concentrated region is agglomerated and coarsened during cooling, and the hard second phase formed after cooling: pearlite becomes coarse, which adversely affects ductility and toughness Concerns remain.

そこで、本発明は、複数の段階の熱処理や冷却開始温度の規制など、生産性の低下や製造コストの増大を招くことなく、高い強度と優れた延性をあわせもつ高張力鋼板及びその製造方法を提供することを目的とする。   Accordingly, the present invention provides a high-strength steel sheet having high strength and excellent ductility and a method for manufacturing the same without causing a decrease in productivity and an increase in manufacturing cost, such as heat treatment in multiple stages and regulation of cooling start temperature. The purpose is to provide.

発明者らは、直接焼入れ型高張力鋼板における延性低下の原因と、延性を向上させる方法についての研究を進め、以下の知見を得た。
1.制御冷却あるいは直接焼入れ型の高張力鋼板は、表層部(裏面側を含む両表層部)が内部に比べ硬化し、このように表層部に硬化層が存在することが延性を低下させる原因となっている。
2.表層部の硬化層が無い場合、鋼板の延性が向上し、曲げ加工性も向上する。表層部を内部よりもむしろ軟らかくすると、さらに優れた延性が得られる。
3.鋼板の成分の適正化を図った上、鋼板の表層部をAc変態点以上、内部をAc変態点未満に再加熱し、さらにその後加速冷却することで、板厚表層から板厚の5〜20%の深さまで、パーライトが分散したフェライトまたはベイナイトあるいはフェライト+ベイナイト組織となり、表層部が内部より軟らかく、延性に優れた鋼板を得られる。
The inventors have advanced the research on the cause of the ductility reduction and the method of improving the ductility in the direct quenching type high-tensile steel sheet, and obtained the following knowledge.
1. Controlled cooling or direct-quenching type high-tensile steel sheets are hardened in the surface layer part (both surface layer parts including the back side) compared to the inside, and the presence of the hardened layer in the surface layer part causes a decrease in ductility. ing.
2. When there is no hardened layer in the surface layer, the ductility of the steel sheet is improved and the bending workability is also improved. If the surface layer portion is made softer than the inside, further excellent ductility can be obtained.
3. In addition to optimizing the components of the steel sheet, the surface layer portion of the steel sheet was reheated to the Ac 1 transformation point or higher and the interior was reheated to less than the Ac 1 transformation point, and then accelerated cooling was carried out, so Up to a depth of -20%, ferrite or bainite or ferrite + bainite structure in which pearlite is dispersed is obtained, and a steel sheet having a surface layer portion that is softer than the inside and excellent in ductility can be obtained.

すなわち、再加熱時の表層部と内部との到達温度を変えることにより、表層部と内部の材質をつくり分け、具体的には、表層部を軟質化させて、鋼板全体の延性改善を図ることができることを、本発明者らは知見した。   In other words, by changing the temperature reached between the surface layer and the interior during reheating, the surface layer and the internal material are created separately, specifically, the surface layer is softened to improve the overall ductility of the steel sheet. The present inventors have found that this is possible.

本発明は、得られた結果を基に、さらに検討を加えてなされたもので、すなわち、本発明は、
(1)質量%で、C:0.02〜0.15%、Si:0.01〜0.50%、Mn:0.5〜2.0%、Al:0.01〜0.08%、P:0.050%以下、S:0.010%以下、N:0.010%以下、残部Feおよび不可避的不純物からなり、板厚表層から板厚の5〜25%の深さまで、平均粒径5μm以下のパーライトが分散したフェライトまたはベイナイトあるいはフェライト+ベイナイト組織を有し、それより内部はフェライト+ベイナイト主体組織であることを特徴とする延性に優れた高張力鋼板。
(2)さらに、質量%で、Cu:1.0%以下、Ni:2.0%以下、Cr:0.5%以下、Mo:0.5%以下、V:0.1%以下、Nb:0.005〜0.045%、Ti:0.005〜0.025%、B:0.0005〜0.0030%、Ca:0.0030%以下、REM:0.02%以下、Mg:0.005%以下の1種または2種以上を含有することを特徴とする(1)に記載の延性に優れた高張力鋼板。
(3)(1)または(2)に記載の成分を有する鋼素材を、1000℃〜1300℃に加熱後、950℃以下での累積圧下率30%以上、圧延終了温度750℃以上で熱間圧延して鋼板とした後、板厚方向の平均温度が500℃以下となるまで加速冷却を行い、引き続く誘導加熱処理において、鋼板表層部がAc変態点以上、板厚中心部がAc変態点未満になるように再加熱した後、板厚方向の平均温度が500℃以下まで加速冷却することを特徴とする延性に優れた高張力鋼板の製造方法。
The present invention has been made by further study based on the obtained results. That is, the present invention
(1) By mass%, C: 0.02-0.15%, Si: 0.01-0.50%, Mn: 0.5-2.0%, Al: 0.01-0.08% , P: 0.050% or less, S: 0.010% or less, N: 0.010% or less, balance Fe and unavoidable impurities, average from thickness to 5% to 25% of thickness A high-tensile steel sheet having excellent ductility, characterized by having a ferrite, bainite or ferrite + bainite structure in which pearlite having a particle size of 5 μm or less is dispersed, and the inside being a ferrite + bainite main structure.
(2) Further, by mass%, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Nb : 0.005-0.045%, Ti: 0.005-0.025%, B: 0.0005-0.0030%, Ca: 0.0030% or less, REM: 0.02% or less, Mg: The high-tensile steel sheet having excellent ductility according to (1), containing one or more of 0.005% or less.
(3) After heating the steel material having the component described in (1) or (2) to 1000 ° C. to 1300 ° C., hot at a rolling reduction temperature of 750 ° C. or higher at a cumulative reduction ratio of 30% or higher at 950 ° C. or lower. After rolling into a steel plate, accelerated cooling is performed until the average temperature in the plate thickness direction is 500 ° C. or lower, and in the subsequent induction heat treatment, the steel plate surface layer portion is at or above the Ac 1 transformation point and the plate thickness center portion is at the Ac 1 transformation. A method for producing a high-tensile steel sheet having excellent ductility, characterized in that after reheating so as to be less than a point, the average temperature in the thickness direction is accelerated to 500 ° C. or less.

本発明によれば、表層部が内部よりも軟らかく、高い強度と優れた延性をあわせもつ高張力鋼板を、生産性の低下や製造コストの増大を招くような複数段の熱処理や冷却開始温度の規制などを施すことなく、経済的に製造することが可能で産業上極めて有用である。   According to the present invention, a high-strength steel plate having a surface layer portion that is softer than the inside and that has both high strength and excellent ductility, has multiple heat treatment and cooling start temperatures that cause a decrease in productivity and an increase in manufacturing cost. It can be manufactured economically without any restrictions and is extremely useful in industry.

本発明では、成分組成とミクロ組織を規定する。
[成分組成]
成分組成は、主に、構造用鋼としての強度、靭性を付与するために規定する。以下の説明において%で示す単位は質量%とする。
C:0.02〜0.15%
Cは、鋼の強度を向上する元素であり、本発明では誘導加熱による再加熱後も所望の強度を確保するためには0.02%以上の含有を必要とするが、0.15%を超えると、鋼板の表層部が著しく硬くなり、再加熱による軟質化の作用を妨げるため、また、靱性にも悪影響を及ぼすため、Cは、0.02〜0.15%の範囲に規定した。なお、好ましくは0.05〜0.12%である。
In the present invention, the component composition and the microstructure are defined.
[Ingredient composition]
The component composition is mainly defined for imparting strength and toughness as structural steel. In the following description, the unit represented by% is mass%.
C: 0.02-0.15%
C is an element that improves the strength of steel. In the present invention, it is necessary to contain 0.02% or more in order to ensure a desired strength even after reheating by induction heating. If it exceeds the upper limit, the surface layer portion of the steel sheet becomes extremely hard, which hinders the effect of softening due to reheating, and also adversely affects toughness. Therefore, C is specified in the range of 0.02 to 0.15%. In addition, Preferably it is 0.05 to 0.12%.

Si:0.01〜0.50%
Siは、脱酸剤として作用する元素であり、本発明では、脱酸を有効に行うための製鋼上の要請から、0.01%以上の含有を必要とするが、0.50%を超えて含有すると、靭性を低下させる。このため、Siは、0.01〜0.50%の範囲に規定した。なお、好ましくは0.20〜0.35%である。
Si: 0.01 to 0.50%
Si is an element that acts as a deoxidizing agent. In the present invention, it is necessary to contain 0.01% or more from the demand on steelmaking for effectively performing deoxidation, but it exceeds 0.50%. If contained, the toughness is reduced. For this reason, Si was prescribed | regulated in the range of 0.01 to 0.50%. In addition, Preferably it is 0.20 to 0.35%.

Mn:0.5〜2.0%
Mnは、鋼の強度を向上する元素であり、本発明では、所望の強度を得るため、0.5%以上の含有を必要とする。一方、2.0%を超える含有は、溶接部の靭性を低下させるため、0.5〜2.0%の範囲に規定した。
Mn: 0.5 to 2.0%
Mn is an element that improves the strength of steel, and in the present invention, it is necessary to contain 0.5% or more in order to obtain a desired strength. On the other hand, when the content exceeds 2.0%, the toughness of the welded portion is lowered, so the content is specified in the range of 0.5 to 2.0%.

Al:0.01〜0.08%
Alは、脱酸を有効に行うための製鋼上の要請から0.01%以上の含有を必要とするが、0.08%を超えて含有すると、靭性を低下させ、溶接した場合には、溶接金属部の靭性を低下させるため、0.01〜0.08%の範囲に規定した。なお、好ましくは、0.02〜0.04%である。
P:0.050%以下、S:0.010%以下、N:0.010%以下
Pは、含有量が0.050%を超えると延性ならびに母材および溶接部の靭性を低下させるので、0.050%以下に抑制し、Sも、0.010%を超えると延性ならびに母材および溶接部の靭性を低下させるので、0.010%以下に抑制する。Nは、0.010%を超えると延性および靭性を低下させるので、0.010%以下に抑制する。
Al: 0.01 to 0.08%
Al needs to be contained in an amount of 0.01% or more from the demand on steelmaking for effectively performing deoxidation, but if contained in excess of 0.08%, when toughness is reduced and welding is performed, In order to reduce the toughness of the weld metal part, it was specified in the range of 0.01 to 0.08%. In addition, Preferably, it is 0.02 to 0.04%.
P: 0.050% or less, S: 0.010% or less, N: 0.010% or less P, when the content exceeds 0.050%, the ductility and the toughness of the base metal and the welded portion are reduced. If it exceeds 0.010%, the ductility and the toughness of the base metal and the weld are reduced, so S is also suppressed to 0.010% or less. If N exceeds 0.010%, the ductility and toughness are reduced, so it is suppressed to 0.010% or less.

以上が本発明の基本成分組成であるが、さらに所望の特性を得るため、Cu、Ni、Cr、Mo、V、Nb、Ti、B、Ca、REM、Mgの1種または2種以上を含有することができる。これらの元素は、いずれも鋼の強度またはHAZ(溶接熱影響部)靭性を向上するのに寄与する。   The above is the basic component composition of the present invention, but contains one or more of Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, REM, and Mg in order to obtain further desired characteristics. can do. All of these elements contribute to improving the strength or HAZ (welding heat affected zone) toughness of steel.

Cu:1.0%以下
Cuは、固溶強化により鋼の強度を向上する元素であるが、1.0%を超えて含有すると、靭性が低下するため、添加する場合は、1.0%以下、より好ましくは、0.05〜1.0%とする。
Cu: 1.0% or less Cu is an element that improves the strength of the steel by solid solution strengthening, but if added over 1.0%, the toughness decreases, so if added, 1.0% Hereinafter, it is more preferably 0.05 to 1.0%.

Ni:2.0%以下
Niは、靭性を保ちつつ強度を向上する元素であるが、2.0%を超えて含有しても効果が飽和し、コスト的に不利となるため、添加する場合は、2.0%以下、より好ましくは
、0.05〜2.0%とする。
Ni: 2.0% or less Ni is an element that improves the strength while maintaining toughness, but if added over 2.0%, the effect is saturated and disadvantageous in terms of cost. Is 2.0% or less, more preferably 0.05 to 2.0%.

Cr:0.5%以下
Crは、鋼の強度を向上する元素であるが、0.5%を超えて含有するとHAZ(溶接熱影響部)靭性が低下するため.添加する場合は、0.5%以下、より好ましくは、0.05〜0.5%とする。
Cr: 0.5% or less Cr is an element that improves the strength of steel, but if it exceeds 0.5%, HAZ (welding heat affected zone) toughness is reduced. When added, it is 0.5% or less, more preferably 0.05 to 0.5%.

V:0.1%以下
Vは、V(CN)として析出強化により、鋼の強度を向上する元素であるが、0.1%を超えて含有すると、靭性を低下させるため、添加する場合は、0.1%以下、より好ましくは、0.003〜0.1%とする。
V: 0.1% or less V is an element that improves the strength of the steel by precipitation strengthening as V (CN), but if added over 0.1%, the toughness is reduced. , 0.1% or less, more preferably 0.003 to 0.1%.

Nb:0.005〜0.045%
Nbは、鋼の強度を向上する元素であり、この効果を得るには、0.005%以上の含有を必要とする。また、再加熱時にNbCとして析出して、鋼板内部の軟質化を抑制する効果もある。一方、0.045%を超える含有は、再加熱後の靭性に悪影響を与えるため、添加する場合は、0.005〜0.045%、好ましくは0.01〜0.025%とする。
Nb: 0.005 to 0.045%
Nb is an element that improves the strength of steel. To obtain this effect, Nb needs to be contained in an amount of 0.005% or more. Moreover, it has the effect of precipitating as NbC at the time of reheating and suppressing the softening inside the steel sheet. On the other hand, the content exceeding 0.045% adversely affects the toughness after reheating. Therefore, when added, the content is 0.005 to 0.045%, preferably 0.01 to 0.025%.

Ti:0.005〜0.025%
Tiは、TiNを形成して鋼中のNを固定することによってBの効果を有効に発揮させる作用を持つ元素である。また、スラブ加熱時ならびに溶接熱影響部でのオーステナイト粒成長を抑制して組織を微細化する効果もある。これらの効果を十分に発揮させるには、0.005%以上の添加が必要であるが、0.025%を超えて含有すると、靭性を低下させるので、添加する場合は0.005〜0.025%、好ましくは0.008〜0.020%とする。
Ti: 0.005-0.025%
Ti is an element having an action of effectively exerting the effect of B by forming TiN and fixing N in the steel. In addition, there is also an effect of suppressing the austenite grain growth at the time of slab heating and at the weld heat affected zone to refine the structure. Addition of 0.005% or more is necessary to fully exhibit these effects, but if added over 0.025%, the toughness is lowered. 025%, preferably 0.008 to 0.020%.

B:0.0005〜0.0030%
Bは、微量の添加によって旧オーステナイト粒界エネルギーを低下させてフェライトの核生成を抑制する。この効果を得るには、0.0005%以上の添加が必要であるが、0.0030%を超えると、靭性を低下させるので、添加する場合は0.0005〜0.0030%とする。
B: 0.0005 to 0.0030%
B suppresses the nucleation of ferrite by lowering the prior austenite grain boundary energy by adding a small amount. In order to obtain this effect, 0.0005% or more must be added. However, if it exceeds 0.0030%, the toughness is lowered. Therefore, when added, the content is made 0.0005 to 0.0030%.

Ca:0.0030%以下
Caは、0.0003%以上の含有で、S、Oとのバランスを適切に選択することで、介在物の形態制御により、HAZ(溶接熱影響部)靭性を向上させる。一方、0.0030%を超えて含有してもその効果が飽和するため、添加する場合は、0.0030%以下、好ましくは0.0003〜0.0030%とする。
Ca: 0.0030% or less Ca is contained in 0.0003% or more, and HAZ (welding heat affected zone) toughness is improved by appropriately controlling the balance with S and O by controlling the form of inclusions. Let On the other hand, since the effect is saturated even if it contains exceeding 0.0030%, when adding, it is 0.0030% or less, Preferably it is 0.0003 to 0.0030%.

REM:0.02%以下
REMは、REM(O、S)を形成して、HAZ(溶接熱影響部)靭性を向上させる。このような効果は、0.02%を超えて含有しても、飽和するので、添加する場合は、0.02%以下、より好ましくは、その効果が得られる0.0003%以上で、0.02%以下とする。REMは希土類元素で、代表的なものは、La、Ce、Hfなどである。工業的には、ミッシュメタルなどの混合物として添加することができる。
REM: 0.02% or less REM forms REM (O, S) and improves HAZ (welding heat affected zone) toughness. Even if such an effect exceeds 0.02%, it saturates, so when added, it is 0.02% or less, more preferably, 0.0003% or more at which the effect is obtained, 0 0.02% or less. REM is a rare earth element, and typical ones are La, Ce, Hf, and the like. Industrially, it can be added as a mixture of misch metal or the like.

Mg:0.005%以下
Mgは、結晶粒の微細化により強度を向上する元素であるが、含有量が0.005%を超えるとその効果は飽和するので、添加する場合は、0.005%以下とする。
本発明において、上記成分以外の残部は、Feおよび不可避的不純物である。
[ミクロ組織]
ミクロ組織は、主に、優れた延性を付与する観点から規定し、板厚表層から板厚の5〜25%の深さまで、平均粒径5μm以下のパーライトが分散したフェライトまたはベイナイトあるいはフェライト+ベイナイト組織を有し、それより内部はフェライト+ベイナイト主体組織とする。ここで、内部のミクロ組織が「フェライト+ベイナイト主体」とは、ミクロ組織のうち、フェライトおよびベイナイトの合計体積分率が90%以上であることを意味し、残部組織として、パーライト、マルテンサイトなどを合計で10%まで含有していても本発明の効果が発現されるものである。なお、本発明では、延性を、全伸びとして評価した。
Mg: 0.005% or less Mg is an element that improves the strength by refining crystal grains. However, when the content exceeds 0.005%, the effect is saturated. % Or less.
In the present invention, the balance other than the above components is Fe and inevitable impurities.
[Microstructure]
The microstructure is mainly defined from the viewpoint of imparting excellent ductility, and ferrite, bainite, or ferrite + bainite in which pearlite having an average particle size of 5 μm or less is dispersed from the thickness layer to a depth of 5 to 25% of the thickness. It has a structure, from which the inside is a ferrite + bainite-based structure. Here, the internal microstructure “ferrite + bainite-based” means that the total volume fraction of ferrite and bainite is 90% or more of the microstructure, and the remaining structure includes pearlite, martensite, etc. Even if it contains up to 10% in total, the effect of the present invention is exhibited. In the present invention, ductility was evaluated as total elongation.

本発明に係る高張力鋼板の好ましい製造方法について、以下に説明する。まず、上記した組成の溶鋼を、転炉等で溶製し、連続鋳造等で鋼素材とする。
[スラブ加熱温度]
鋼素材(スラブ)を、1000〜1300℃の温度範囲に加熱し、鋼素材を完全にオーステナイト化する。加熱温度が1000℃未満では、熱間圧延を低温で行うことになり、圧延機への負荷が増大して圧延能率が低下する。一方、加熱温度が1300℃を超えると、結晶粒が粗大化して、鋼板の靱性が低下するうえ、酸化ロスが顕著となり、歩留が低下する。
The preferable manufacturing method of the high-tensile steel plate according to the present invention will be described below. First, molten steel having the above composition is melted in a converter or the like, and is made into a steel material by continuous casting or the like.
[Slab heating temperature]
The steel material (slab) is heated to a temperature range of 1000 to 1300 ° C. to completely austenite the steel material. If heating temperature is less than 1000 degreeC, it will perform hot rolling at low temperature, the load to a rolling mill will increase, and rolling efficiency will fall. On the other hand, when the heating temperature exceeds 1300 ° C., the crystal grains become coarse, the toughness of the steel sheet is lowered, the oxidation loss becomes remarkable, and the yield is lowered.

[熱間圧延]
加熱後、950℃以下での累積圧下率30%以上、圧延終了温度750℃以上で熱間圧延を行なう。950℃以下の圧延は、オーステナイト未再結晶温度域を含む圧延に相当し、オーステナイト未再結晶域で累積圧下率30%以上の熱間圧延を行うことにより、オーステナイト結晶粒界の面積を増大させ、圧延による歪エネルギーも多く蓄積させることができる。累積圧下率が30%未満では、この効果が十分には得られない。
[Hot rolling]
After the heating, hot rolling is performed at a cumulative reduction ratio of 30% or higher at 950 ° C. or lower and a rolling end temperature of 750 ° C. or higher. Rolling at 950 ° C. or lower corresponds to rolling that includes the austenite non-recrystallization temperature range, and by performing hot rolling at a cumulative reduction ratio of 30% or more in the austenite non-recrystallization region, the area of the austenite grain boundary is increased. A large amount of strain energy due to rolling can be accumulated. If the cumulative rolling reduction is less than 30%, this effect cannot be sufficiently obtained.

前記のような、圧延による歪エネルギーの蓄積により、オーステナイト粒界およびオーステナイト粒内からのフェライト変態ならびにベイナイト変態が促進されので、より高温の再結晶域での圧延によるオーステナイト粒の微細化と、オーステナイト未再結晶域での圧延との相乗効果により、生成するフェライトおよび/またはベイナイトは、粒径あるいはパケットサイズの小さい組織となり、構造用厚鋼板として良好な靭性が得られる。   As described above, the accumulation of strain energy by rolling promotes ferrite transformation and bainite transformation from within the austenite grain boundaries and austenite grains. Due to a synergistic effect with rolling in the non-recrystallized region, the ferrite and / or bainite to be formed has a structure with a small grain size or packet size, and good toughness can be obtained as a structural steel plate.

熱間圧延は、750℃以上の圧延終了温度とする。本発明では、950℃以下の圧延を規定しているため、圧延終了温度は950℃以下で、圧延終了温度が低いほど靭性は向上する。   Hot rolling is performed at a rolling end temperature of 750 ° C. or higher. In the present invention, since rolling at 950 ° C. or lower is specified, the rolling end temperature is 950 ° C. or lower, and the lower the rolling end temperature, the better the toughness.

しかし、圧延終了温度が750℃未満の場合、その効果が飽和し、圧延能率を低下させるので、圧延終了温度は750℃以上、好ましくは800〜900℃とする。熱間圧延終了後、鋼板を加速冷却する。   However, when the rolling end temperature is less than 750 ° C., the effect is saturated and the rolling efficiency is lowered. Therefore, the rolling end temperature is set to 750 ° C. or higher, preferably 800 to 900 ° C. After the hot rolling is completed, the steel sheet is accelerated and cooled.

[加速冷却]
加速冷却は冷却終了温度を500℃以下とする。冷却終了温度が500℃超えの場合には、変態が終了せず、引き続き実施する誘導加熱処理後に所望の鋼板の強度が得られない場合があるため、500℃以下とする。尚、冷却終了温度は、板厚方向の平均温度とする。
[Accelerated cooling]
In accelerated cooling, the cooling end temperature is set to 500 ° C. or lower. When the cooling end temperature exceeds 500 ° C., the transformation does not end, and the strength of the desired steel sheet may not be obtained after the subsequent induction heat treatment. The cooling end temperature is an average temperature in the thickness direction.

[再加熱処理]
加速冷却後の再加熱処理は、鋼板の表層部を鋼板内部より軟らかくするため実施する。再加熱処理は、加速冷却停止後、鋼板表層部をAc変態点以上に、板厚中心部をAc変態点以下に加熱する。
[Reheating treatment]
The reheating treatment after the accelerated cooling is performed in order to make the surface layer portion of the steel plate softer than the inside of the steel plate. In the reheating treatment, after the accelerated cooling is stopped, the steel sheet surface layer part is heated to the Ac 1 transformation point or higher and the plate thickness center part is heated to the Ac 1 transformation point or less.

鋼板表層部がAc変態点を下回る温度までしか再加熱されない場合、表層部は軟質化せず、表層部が内部より軟らかい硬さ分布が得られず、延性も低下する。 When the steel plate surface layer part is reheated only to a temperature below the Ac 1 transformation point, the surface layer part is not softened, a hardness distribution in which the surface layer part is softer than the inside is not obtained, and ductility is also reduced.

前記加速冷却までを終了した鋼をAc変態点以上に再加熱すると、加速冷却で生成した硬質のベイナイト組織がAc変態点以上、Ac変態点未満の二相域温度に加熱されて、一部がオーステナイトに変態する。 When the steel after the accelerated cooling is reheated to the Ac 1 transformation point or higher, the hard bainite structure generated by the accelerated cooling is heated to a two-phase region temperature of the Ac 1 transformation point or higher and lower than the Ac 3 transformation point, Part is transformed into austenite.

板厚中心部は、Ac変態点未満に加熱されるため、オーステナイトに変態することなく、焼戻し効果により、強度低下が抑制されつつ延靭性が回復し、鋼板全体として所望の強度を確保しながら、延性を向上させることができる。板厚中心部分がAc変態点以上に加熱されると、鋼板全体としての強度が大幅に低下する。
なお、Ac点および後述のAc点は、例えば下式で求めることができる。
Ac(℃)=854−180C+44Si−14Mn−17.8Ni−1.7Cr
Ac1(℃)=723+22Si−14Mn−14.4Ni+23.3Cr
ただし、各元素記号は質量%表示の含有量とする。
Since the center of the plate thickness is heated to less than the Ac 1 transformation point, the toughness is restored while suppressing the strength reduction by the tempering effect without transformation to austenite, while ensuring the desired strength as a whole steel plate. Ductility can be improved. When the plate thickness center portion is heated to the Ac 1 transformation point or higher, the strength of the entire steel plate is greatly reduced.
In addition, Ac 3 point and Ac 1 point mentioned later can be calculated | required by the following Formula, for example.
Ac 3 (° C.) = 854-180C + 44Si-14Mn-17.8Ni-1.7Cr
Ac 1 (° C.) = 723 + 22Si-14Mn-14.4Ni + 13.3Cr
However, each element symbol is a content in mass%.

鋼板の板厚方向に上記の温度分布を与えるため、誘導加熱装置の利用が望ましい。誘導加熱装置は、所望の特性を得るためには、厚板圧延ラインにおける搬送経路上であれば、オンライン、オフラインのいずれでも良い。   In order to provide the above temperature distribution in the thickness direction of the steel sheet, it is desirable to use an induction heating device. In order to obtain the desired characteristics, the induction heating device may be either on-line or off-line as long as it is on the conveying path in the thick plate rolling line.

但し、エネルギーコストの観点からは、圧延、冷却直後に加熱が可能なオンラインとすることが好ましい。   However, from the viewpoint of energy cost, it is preferable to be on-line capable of heating immediately after rolling and cooling.

また、誘導加熱装置により加熱する場合、板厚中心部がAc変態点を超えて加熱されないようにするための、板厚に応じた加熱時間の関係を、モデル式や数表にて予め決めておき、再加熱対象の被圧延材がくるごとに、その関係から決まる加熱時間だけ、手動または自動で再加熱するようにするのが好ましい。 In addition, when heating with an induction heating device, the relationship of the heating time according to the plate thickness is determined in advance using a model formula or numerical table so that the plate thickness center portion is not heated beyond the Ac 1 transformation point. It is preferable that each time a material to be reheated is reheated manually or automatically for a heating time determined from the relationship.

[第2段目の加速冷却]
再加熱処理後、更に、第2段目の加速冷却を冷却停止温度:板厚方向の平均温度で500℃以下として行う。500℃以下まで加速冷却することで、再加熱処理後の、硬質のベイナイト組織でオーステナイトに変態した一部が微細な硬質第二相(主として微細パーライト)となり、フェライトまたはベイナイトあるいはフェライト+ベイナイト組織に微細分散する。第2段目の加速冷却の停止温度が500℃超えであると、粒状パーライトが凝集粗大化するため、靱性が低下する。
[Second stage accelerated cooling]
After the reheating treatment, the second-stage accelerated cooling is performed at a cooling stop temperature: an average temperature in the thickness direction of 500 ° C. or less. By accelerated cooling to 500 ° C. or less, a part of the hard bainite structure transformed into austenite after reheating treatment becomes a fine hard second phase (mainly fine pearlite), and becomes ferrite or bainite or ferrite + bainite structure. Finely disperse. If the stop temperature of the second stage accelerated cooling exceeds 500 ° C., the granular pearlite is agglomerated and coarsened, so that the toughness is lowered.

なお、第二段目の加速冷却の冷却速度は5℃/s以上であることが望ましい。5℃/sを下回ると炭素の拡散による粒状パーライトの凝集粗大化が生じるため、靭性が低下する。   Note that the cooling rate of the second stage accelerated cooling is desirably 5 ° C./s or more. When the temperature is less than 5 ° C./s, aggregation and coarsening of granular pearlite due to carbon diffusion occurs, and thus toughness decreases.

以上の製造方法の説明において、鋼板の温度は、特にことわらない限り、鋼板の表面の温度とする。   In the above description of the manufacturing method, the temperature of the steel plate is the temperature of the surface of the steel plate unless otherwise specified.

表1に示す各組成の溶鋼を、ラボ真空溶解炉で溶製し、分塊圧延により100mm厚としたものを圧延素材(スラブ)とし(鋼記号A〜P)、種々の圧延条件にて板厚20mmに熱間圧延・冷却した後、種々の条件で誘導加熱装置による再加熱を行い、No.1〜27の供試鋼を得た。鋼板の製造において、板厚表面の温度、中心温度ともに、板側面に挿入した熱電対により実測した。   The molten steels having the respective compositions shown in Table 1 were melted in a laboratory vacuum melting furnace and made 100 mm thick by ingot rolling as a rolling material (slab) (steel symbols A to P) and subjected to various rolling conditions. After hot rolling and cooling to a thickness of 20 mm, reheating was performed with an induction heating device under various conditions. 1-27 test steels were obtained. In the production of the steel sheet, both the temperature of the plate thickness surface and the center temperature were measured by a thermocouple inserted on the side of the plate.

得られた供試鋼について、全厚のJIS1B号引張試験片を試験片長手方向が圧延方向と平行になるよう採取し、引張試験を行い、降伏点(YS)、引張強さ(TS)ならびに全伸び(tEl)を測定した。また、板厚中心部よりJIS4号衝撃試験片を試験片長手方向が圧延方向と平行になるよう採取し、シャルピー試験を行って、破面遷移温度(vTrs)を求めた。本発明範囲は、TS>570MPa、tEl≧15%、vTrs≦−20℃とした。鋼板のミクロ組織は光学顕微鏡にて行い、粒状パーライト径は画像解析によりもとめた。   About the obtained test steel, a full thickness JIS No. 1B tensile test piece was taken so that the longitudinal direction of the test piece was parallel to the rolling direction, a tensile test was conducted, yield point (YS), tensile strength (TS) and Total elongation (tEl) was measured. Further, a JIS No. 4 impact test piece was collected from the center of the plate thickness so that the longitudinal direction of the test piece was parallel to the rolling direction, and a Charpy test was performed to determine the fracture surface transition temperature (vTrs). The scope of the present invention was TS> 570 MPa, tEl ≧ 15%, and vTrs ≦ −20 ° C. The microstructure of the steel sheet was measured with an optical microscope, and the granular pearlite diameter was determined by image analysis.

表2に圧延条件、誘導加熱条件とともに得られた試験結果を示す。化学成分、製造条件が本発明の範囲内であるNo.1、4〜6、14〜19の鋼板は、機械的性質が本発明範囲を満足した。一方、化学成分あるいは製造条件が本発明の範囲から外れるNo.2、3、7〜13、20〜27の鋼板は、強度(TS)、延性(tEl)、靭性(vTrs)のいずれかが本発明範囲から外れていた。   Table 2 shows the test results obtained together with rolling conditions and induction heating conditions. The chemical composition and production conditions are within the scope of the present invention. In the steel sheets of 1, 4 to 6, and 14 to 19, the mechanical properties satisfied the scope of the present invention. On the other hand, the chemical composition or the production conditions deviate from the scope of the present invention. In the steel sheets of 2, 3, 7 to 13, 20 to 27, any of strength (TS), ductility (tEl), and toughness (vTrs) was out of the scope of the present invention.

Claims (3)

質量%で、C:0.02〜0.15%、Si:0.01〜0.50%、Mn:0.5〜2.0%、Al:0.01〜0.08%、P:0.050%以下、S:0.010%以下、N:0.010%以下、残部Feおよび不可避的不純物からなり、板厚表層から板厚の5〜25%の深さまで、平均粒径5μm以下のパーライトが分散したフェライトまたはベイナイトあるいはフェライト+ベイナイト組織を有し、それより内部はフェライト+ベイナイト主体組織であることを特徴とする延性に優れた高張力鋼板。   In mass%, C: 0.02 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, Al: 0.01 to 0.08%, P: 0.050% or less, S: 0.010% or less, N: 0.010% or less, balance Fe and unavoidable impurities, average thickness 5 μm from surface thickness layer to depth of 5-25% of thickness A high-tensile steel sheet having excellent ductility, characterized in that it has a ferrite, bainite or ferrite + bainite structure in which the following pearlite is dispersed, and the inside is a ferrite + bainite main structure. さらに、質量%で、Cu:1.0%以下、Ni:2.0%以下、Cr:0.5%以下、Mo:0.5%以下、V:0.1%以下、Nb:0.005〜0.045%、Ti:0.005〜0.025%、B:0.0005〜0.0030%、Ca:0.0030%以下、REM:0.02%以下、Mg:0.005%以下の1種または2種以上を含有することを特徴とする、請求項1に記載の延性に優れた高張力鋼板。   Further, in terms of mass%, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, V: 0.1% or less, Nb: 0.00%. 005 to 0.045%, Ti: 0.005 to 0.025%, B: 0.0005 to 0.0030%, Ca: 0.0030% or less, REM: 0.02% or less, Mg: 0.005 % High-tensile steel sheet excellent in ductility according to claim 1, wherein the high-tensile steel sheet contains 1 type or 2 types or less. 請求項1または2に記載の成分を有する鋼素材を、1000℃〜1300℃に加熱後、950℃以下での累積圧下率30%以上、圧延終了温度750℃以上で熱間圧延して鋼板とした後、板厚方向の平均温度が500℃以下となるまで加速冷却を行い、引き続く誘導加熱処理において、鋼板表層部がAc変態点以上、板厚中心部がAc変態点未満になるように再加熱した後、板厚方向の平均温度が500℃以下まで加速冷却することを特徴とする延性に優れた高張力鋼板の製造方法。 A steel material having the component according to claim 1 or 2 is heated to 1000 ° C. to 1300 ° C., and then hot-rolled at a cumulative reduction ratio of 30% or more at 950 ° C. or less and a rolling end temperature of 750 ° C. or more to obtain a steel plate After that, accelerated cooling is performed until the average temperature in the sheet thickness direction is 500 ° C. or less, and in the subsequent induction heat treatment, the steel sheet surface layer part is at least the Ac 1 transformation point and the sheet thickness center part is less than the Ac 1 transformation point. A method for producing a high-tensile steel sheet having excellent ductility, characterized in that after the reheating, the average temperature in the thickness direction is accelerated to 500 ° C. or less.
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