JP4547944B2 - Manufacturing method of high strength and high toughness thick steel plate - Google Patents

Manufacturing method of high strength and high toughness thick steel plate Download PDF

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JP4547944B2
JP4547944B2 JP2004066678A JP2004066678A JP4547944B2 JP 4547944 B2 JP4547944 B2 JP 4547944B2 JP 2004066678 A JP2004066678 A JP 2004066678A JP 2004066678 A JP2004066678 A JP 2004066678A JP 4547944 B2 JP4547944 B2 JP 4547944B2
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純二 嶋村
伸一 鈴木
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JFE Steel Corp
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本発明は、船舶、海洋構造物、建設機械、建築構造物、橋梁、タンク、パイプライン、ペンストックなどの溶接鋼構造物用として好適な、高強度高靭性厚鋼板の製造方法に係り、とくに強度−靭性バランスの向上、板厚方向特性の均質性向上に関する。なお、本発明でいう「高強度」とは、引張強さ590MPa以上の強度をいうものとする。また、本発明でいう「厚鋼板」とは、板厚2mm以上の鋼板をいうものとする。   The present invention relates to a method for producing a high-strength, high-tough steel plate suitable for welded steel structures such as ships, offshore structures, construction machines, building structures, bridges, tanks, pipelines, penstocks, etc. The present invention relates to improvement in strength-toughness balance and improvement in uniformity in the thickness direction characteristics. In the present invention, “high strength” refers to strength having a tensile strength of 590 MPa or more. The “thick steel plate” in the present invention refers to a steel plate having a thickness of 2 mm or more.

一般に、鋼板の強度が増加するにしたがい、低温靭性は低下する傾向にある。とくに、引張強さが590MPa以上の高強度鋼板において、良好な低温靭性を具備させることは容易ではない。引張強さ590MPa以上の高強度厚鋼板は、従来からオーステナイト温度域に再加熱したのち焼入れする再加熱焼入れ処理により製造されてきた。   Generally, as the strength of the steel sheet increases, the low temperature toughness tends to decrease. In particular, it is not easy to provide good low temperature toughness in a high strength steel sheet having a tensile strength of 590 MPa or more. Conventionally, high-strength thick steel plates having a tensile strength of 590 MPa or more have been manufactured by a reheating quenching process in which the steel is reheated to an austenite temperature range and then quenched.

最近、例えば、特許文献1には、熱間圧延後直ちに焼入れる直接焼入れ処理による高強度厚鋼板の製造方法が提案されている。しかしながら、良好な靭性を得るためには、再加熱焼入れ処理による場合も、直接焼入れ処理による場合もいずれも、焼入れ時に生成したマルテンサイトの硬度を低下させて靭性を回復させるための焼戻し処理を行う必要があり、生産性、製造コスト、工期等の点で問題を残していた。   Recently, for example, Patent Document 1 proposes a method for producing a high-strength thick steel plate by direct quenching that is quenched immediately after hot rolling. However, in order to obtain good toughness, both in the case of reheating quenching treatment and in the case of direct quenching treatment, tempering treatment is performed to reduce the hardness of martensite generated during quenching and restore toughness. It was necessary and left problems in terms of productivity, manufacturing cost, construction period, and the like.

これらの問題点を解決する方法として、例えば、特許文献2には、C、Si、Mn、Cr、Ti、B、Al、N量を適正量に調整した鋼素材を、900℃以下の温度域で累積圧下率50%以上の熱間圧延を施したのち、直ちにAr点以上の温度から焼入れを開始し、(Ms点+100℃)〜(Ms点−250℃)の温度域で焼入れを停止し、表層部をマルテンサイト組織とする、靭性および耐遅れ破壊性に優れた耐摩耗鋼材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、焼入れをMs点を挟む温度域の温度で停止して、表層部をマルテンサイト単相組織とし、内層部を下部ベイナイトとマルテンサイトの混合組織とするものである。特許文献2に記載された技術によれば、焼戻し処理を行うことなく、高強度で高靭性を有する厚鋼板を製造できるとしている。しかしながら、この技術では、板厚方向の硬さ分布にばらつきが生じて板厚方向材質の均質性が劣るとともに、マルテンサイト単相となる表層部の靭性が低下するなどの問題があった。 As a method for solving these problems, for example, in Patent Document 2, a steel material in which the amounts of C, Si, Mn, Cr, Ti, B, Al, and N are adjusted to appropriate amounts is set to a temperature range of 900 ° C. or lower. Immediately after hot rolling with a cumulative reduction ratio of 50% or more in, quenching is started immediately from the temperature of 3 points or more of Ar, and quenching is stopped in the temperature range of (Ms point + 100 ° C) to (Ms point -250 ° C). In addition, a method for producing a wear-resistant steel material having a martensitic surface layer and excellent toughness and delayed fracture resistance is described. The technique described in Patent Document 2 stops quenching at a temperature in the temperature range sandwiching the Ms point, and the surface layer portion has a martensite single phase structure and the inner layer portion has a mixed structure of lower bainite and martensite. is there. According to the technique described in Patent Document 2, a thick steel plate having high strength and high toughness can be manufactured without performing a tempering treatment. However, this technique has a problem in that the hardness distribution in the thickness direction varies and the homogeneity of the thickness direction material is inferior, and the toughness of the surface layer portion that becomes a martensite single phase is lowered.

また、例えば、特許文献3には、1000〜900℃の温度範囲で累積圧下率が50%以上の熱間圧延を施し、引き続き900℃未満810℃以上の未再結晶γ域で、1パスあたりの圧下率が10%未満の軽圧下圧延により累積圧下率を10〜30%としたのち、直ちに焼入れをし、その後焼戻しを行う、板厚方向の均質性に優れた溶接用超高張力鋼板の製造方法が提案されている。しかし、特許文献3に記載された技術では、板厚方向の均質性は向上するが、焼入れ後の焼戻しを必須としており、工程が複雑かつ長期化するため、生産性、製造コストの観点から問題を残していた。
特開平2−27407号公報 特開2002−80930号公報 特公平6−70248号公報
Further, for example, in Patent Document 3, hot rolling with a cumulative rolling reduction of 50% or more in a temperature range of 1000 to 900 ° C. is performed, and then in an unrecrystallized γ region of less than 900 ° C. and 810 ° C. or more per pass. An ultra-high-strength steel sheet for welding with excellent uniformity in the plate thickness direction that is immediately quenched and then tempered after light rolling with a rolling reduction of less than 10%. Manufacturing methods have been proposed. However, in the technique described in Patent Document 3, the homogeneity in the plate thickness direction is improved, but tempering after quenching is essential, and the process is complicated and prolonged, which is problematic from the viewpoint of productivity and manufacturing cost. Was leaving.
JP-A-2-27407 JP 2002-80930 A Japanese Patent Publication No. 6-70248

本発明は、上記した従来技術の問題を解決し、引張強さ590MPa以上の高強度を有し、かつ強度−靭性バランスに優れ、板厚方向特性が均質である高強度高靭性厚鋼板を、高能率でかつ安価に製造できる、高強度高靭性厚鋼板の製造方法を提案することを目的とする。   The present invention solves the above-described problems of the prior art, and has a high strength and high toughness steel plate having a high strength of tensile strength of 590 MPa or more, an excellent strength-toughness balance, and a uniform thickness direction property. The object is to propose a method for producing a high-strength, high-toughness thick steel plate that can be produced at high efficiency and at low cost.

本発明者らは、上記した課題を達成するために、引張強さ590MPa以上の強度レベルにおいて、低温靭性に及ぼす各種要因について鋭意検討した。その結果、オーステナイト域温度から特定冷却速度で焼入れ冷却し、Ms点以下の特定温度域で焼入れ冷却を停止したのち、(焼入れ停止温度±50℃)の温度域で短時間保持すること、あるいはさらに該温度域から適正加熱速度で所定の焼戻し温度まで急速加熱する焼戻し処理を施すことにより、組織が表層部、中心部などの板厚位置によらず焼戻しマルテンサイトと下部ベイナイトの混合組織となり、強度−靭性バランスが向上し、板厚方向特性が均質化するという知見を得た。   In order to achieve the above-mentioned problems, the present inventors diligently studied various factors affecting low-temperature toughness at a tensile strength level of 590 MPa or more. As a result, after quenching and cooling at a specific cooling rate from the austenite temperature, quenching and cooling at a specific temperature range below the Ms point, and holding for a short time in the temperature range (quenching stop temperature ± 50 ° C), or further By applying a tempering treatment that rapidly heats from the temperature range to a predetermined tempering temperature at an appropriate heating rate, the structure becomes a mixed structure of tempered martensite and lower bainite regardless of the plate thickness position of the surface layer portion, center portion, etc. -The knowledge that the balance of toughness is improved and the thickness direction characteristics are homogenized was obtained.

低炭素(C:0.01〜0.20質量%)鋼では、焼入れ冷却時にオーステナイトが変態して、ラス状組織を形成する。この際、ラス間にCの濃縮が起こり未変態オーステナイト(γ)フイルムが残留しやすく、靭性に有害な針状の島状マルテンサイト(Martensite-Austenite constituent)(以下、MAともいう)を形成しやすい。   In low carbon (C: 0.01 to 0.20 mass%) steel, austenite is transformed during quenching and cooling to form a lath structure. At this time, concentration of C occurs between the laths, and untransformed austenite (γ) film tends to remain, forming acicular island martensite (hereinafter also referred to as MA) that is harmful to toughness. Cheap.

本発明者らは、Ms点以下の特定温度域で焼入れ冷却を停止し、(焼入れ停止温度±50℃)の温度域で短時間保持した後、空冷すると、焼入れ冷却時に生成したマルテンサイトが焼戻されることに加えて、下部ベイナイトが微細でかつ多量に生成するため、MAの生成量は少なくなり、靭性が向上することを見いだした。   The inventors stopped quenching and cooling in a specific temperature range below the Ms point, held for a short time in the temperature range of (quenching stop temperature ± 50 ° C.), and then cooled by air, martensite generated during quenching cooling was quenched. In addition to being returned, it was found that the amount of MA produced was reduced and the toughness was improved because the lower bainite was fine and produced in large quantities.

また、本発明者らは、Ms点以下の特定温度域で焼入れ冷却を停止し、(焼入れ停止温度±50℃)の温度域で短時間保持した後、該温度域から適正加熱速度で所定の焼戻し温度まで急速加熱する焼戻し処理を施すことにより、強度の低下が少なく、かつ靭性が向上することを知見した。   In addition, the present inventors stop quenching and cooling in a specific temperature range below the Ms point, hold for a short time in a temperature range of (quenching stop temperature ± 50 ° C.), and then perform a predetermined heating rate from the temperature range at an appropriate heating rate. It has been found that by performing a tempering treatment that rapidly heats to the tempering temperature, there is little decrease in strength and toughness is improved.

一方、焼入れ冷却をMs点より高い温度で停止すると、下部ベイナイト変態だけでなく上部ベイナイト変態が生じるため、上部ベイナイトのラス間に長く伸びた針状のMAあるいは塊状のMAが形成されやすく、そのため靭性が劣化する。   On the other hand, when quenching and cooling is stopped at a temperature higher than the Ms point, not only the lower bainite transformation but also the upper bainite transformation occurs, so that needle-like MA or massive MA extending long between the laths of the upper bainite is likely to be formed. Toughness deteriorates.

このように、本発明者らは、Ms点以下の特定温度域で焼入れ冷却を停止し、(焼入れ停止温度±50℃)の温度域で短時間保持した後、空冷するか、あるいはさらに該温度域から適正加熱速度で所定の焼戻し温度まで急速加熱する焼戻し処理を施すことにより、島状マルテンサイト相の生成をより少なくでき、さらに、微細な下部ベイナイト相を高分率で得ることができ、高強度でかつ高靭性を有する厚鋼板の製造が可能となることを知見した。本発明は、このような知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
(1)質量%で、C:0.01〜0.20%、Si:0.01〜0.60%、Mn:0.50〜2.50%、P:0.020%以下、S:0.0070%以下、sol.Al:0.001〜0.100%を含有し、残部がFeおよび不可避的不純物からなる組成を有する鋼素材を加熱したのち、圧延終了温度をAr変態点以上の温度域とする熱間圧延を施して厚鋼板とし、引き続いて、該厚鋼板をAr変態点以上の温度域から、次(1)式
log CR=2.94−0.75β ………(1)
(ここで、β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo(B≧0.0005質量%の場合)、β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo−1(B<0.0005質量%の場合)、CR:マルテンサイト生成臨界冷却速度(℃/s)、C、Si、Mn、Ni、Cr、Mo:各元素の含有量(質量%))
で定義されるマルテンサイト生成臨界冷却速度CR以上の冷却速度でマルテンサイト変態開始温度以下300℃以上の温度域の焼入れ冷却停止温度まで焼入れ冷却し、ついで焼入れ冷却停止直後から20〜60s間、該厚鋼板の温度を(焼入れ冷却停止温度±50℃)の温度域の温度に保持し、その後室温まで空冷することを特徴とする高強度高靭性厚鋼板の製造方法。
(2)(1)において、前記(焼入れ冷却停止温度±50℃)の温度域の温度に保持した後、直ちに1℃/s以上の加熱速度で450℃以上Ac変態点以下の焼戻し温度まで急速加熱したのち、空冷する焼戻し処理を施すことを特徴とする高強度高靭性厚鋼板の製造方法。
(3)(1)または(2)において、前記組成に加えてさらに、次A〜C群
A群:質量%で、Cu:0.10〜1.00%、Ni:0.10〜5.00%、Cr:0.10〜0.80%、Mo:0.01〜0.80%、B:0.0002〜0.0025%のうちから選ばれた1種または2種以上
B群:質量%で、Ti:0.03%以下、V:0.100%以下、Nb:0.050%以下のうちから選ばれた1種または2種以上
C群:質量%で、Ca:0.010%以下、REM:0.020%以下のうちから選ばれた1種または2種
のうちから選ばれた1群または2群以上を含有することを特徴とする高強度高靭性厚鋼板の製造方法。
In this way, the inventors stop quenching cooling in a specific temperature range below the Ms point, hold it for a short time in the temperature range (quenching stop temperature ± 50 ° C.), and then cool it down, or further reduce the temperature. By applying a tempering treatment that rapidly heats up to a predetermined tempering temperature at an appropriate heating rate from the region, the generation of island-like martensite phase can be reduced, and furthermore, a fine lower bainite phase can be obtained at a high fraction, It has been found that it is possible to produce a thick steel plate having high strength and high toughness. The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.60%, Mn: 0.50 to 2.50%, P: 0.020% or less, S: 0.0070% or less, sol.Al: 0.001 to 0.100% Then, after heating a steel material having a composition consisting of Fe and inevitable impurities as the balance, the steel is subjected to hot rolling with the rolling end temperature being a temperature range equal to or higher than the Ar 3 transformation point to obtain a thick steel plate. From the temperature range above the Ar 3 transformation point of the steel plate, the following formula (1)
log CR M = 2.94−0.75β (1)
(Where, β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ( For B ≧ 0.0005%), β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo -1 (B < if 0.0005 mass%), CR M: martensite Site formation critical cooling rate (℃ / s), C, Si, Mn, Ni, Cr, Mo: Content of each element (mass%)
In quenching it cooled to being defined quench cooling stop temperature of martensite critical cooling rate CR martensitic transformation starting temperature below 300 ° C. over a temperature range in M or more cooling rate, then between 20 ~60S immediately after stopping the quenching cooling, A method for producing a high-strength, high-toughness thick steel plate, characterized in that the temperature of the thick steel plate is maintained at a temperature in a temperature range of (quenching cooling stop temperature ± 50 ° C) and then air-cooled to room temperature.
(2) In (1), after maintaining the temperature in the above temperature range (quenching cooling stop temperature ± 50 ° C), immediately at a heating rate of 1 ° C / s or higher to 450 ° C or higher and the tempering temperature of Ac 1 transformation point or lower. A method for producing a high-strength, high-toughness thick steel sheet, characterized in that after rapid heating, air-cooled tempering is performed.
(3) In (1) or (2), in addition to the above composition, the following groups A to C: Group A:% by mass, Cu: 0.10 to 1.00%, Ni: 0.10 to 5.00%, Cr: 0.10 to 0.80 %, Mo: 0.01-0.80%, B: 0.0002-0.0025% or one or more selected from Group B: mass%, Ti: 0.03% or less, V: 0.100% or less, Nb: 0.050% 1 type or 2 or more types selected from the following: C group: mass%, Ca: 0.010% or less, REM: 1 group selected from 2 types or less selected from 0.020% or less Or the manufacturing method of the high intensity | strength high toughness thick steel plate characterized by containing 2 or more groups.

本発明によれば、引張強さ590MPa以上の高強度を有し、かつ強度−靭性バランスに優れ、板厚方向特性が均質である高強度高靭性厚鋼板を、高能率でかつ安価に製造でき、産業上格段の効果を奏する。また、本発明により得られた高強度厚鋼板は、熱加工精度の信頼性が高いという効果もある。   According to the present invention, a high-strength, high-toughness thick steel plate having a tensile strength of 590 MPa or more, an excellent balance between strength and toughness, and uniform properties in the thickness direction can be produced with high efficiency and at low cost. It has a remarkable industrial effect. Moreover, the high-strength thick steel plate obtained by this invention also has the effect that the reliability of a heat processing precision is high.

本発明で使用する鋼素材の組成限定理由についてまず説明する。以下、組成における質量%は単に%で記す。   The reason for limiting the composition of the steel material used in the present invention will be described first. Hereinafter, the mass% in the composition is simply expressed as%.

C:0.01〜0.20%
Cは、鋼の強度を増加する元素であり、所望の高強度とするためには、0.01%以上の含有を必要とする。一方、0.20%を超えて含有すると溶接性が劣化し、溶接割れが生じやすくなるとともに、母材靭性およびHAZ靭性が低下する。このため、Cは0.01〜0.20%の範囲に限定した。なお、好ましくは0.02〜0.16%である。
C: 0.01-0.20%
C is an element that increases the strength of steel, and in order to obtain a desired high strength, it needs to be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.20%, the weldability is deteriorated, weld cracking is likely to occur, and the base metal toughness and HAZ toughness are lowered. For this reason, C was limited to the range of 0.01 to 0.20%. In addition, Preferably it is 0.02 to 0.16%.

Si:0.01〜0.60%
Siは、脱酸剤として作用し、さらに固溶強化により鋼材の強度を増加させる元素である。このような効果を得るためには、0.01%以上の含有を必要とするが、0.60%を超える含有は、HAZ靭性を著しく劣化させる。このため、Siは0.01〜0.60%の範囲とした。なお、好ましくは、0.05〜0.20%である。
Si: 0.01-0.60%
Si is an element that acts as a deoxidizing agent and further increases the strength of the steel material by solid solution strengthening. In order to obtain such an effect, a content of 0.01% or more is required, but a content exceeding 0.60% significantly deteriorates the HAZ toughness. For this reason, Si was made into the range of 0.01 to 0.60%. In addition, Preferably, it is 0.05 to 0.20%.

Mn:0.50〜2.50%
Mnは、鋼の焼入れ性を高めるとともに、靭性を向上させる作用を有する元素であり、本発明では、0.50%以上の含有を必要とするが、2.50%を超える含有は、溶接性を劣化させる恐れがある。このため、本発明では、Mnは0.50〜2.50%の範囲に限定した。なお、好ましくは、0.80〜2.50%である。
Mn: 0.50-2.50%
Mn is an element that has the effect of enhancing the hardenability of steel and improving toughness. In the present invention, it is necessary to contain 0.50% or more, but the content exceeding 2.50% may deteriorate weldability. There is. For this reason, in this invention, Mn was limited to 0.50 to 2.50% of range. In addition, Preferably, it is 0.80 to 2.50%.

P:0.020%以下
Pは、固溶強化により強度を増加させる元素であるが、靭性、溶接性を劣化させるため、本発明ではできるだけ低減することが好ましいが、0.020%までの含有は許容できる。このため、Pは0.020%以下に限定した。なお、好ましくは0.017%以下である。また、極端なPの低減は溶製コストの高騰を招くため、本発明では、0.008%以上とすることが好ましい。
P: 0.020% or less P is an element that increases strength by solid solution strengthening. However, in order to deteriorate toughness and weldability, P is preferably reduced as much as possible in the present invention, but the content up to 0.020% is acceptable. For this reason, P was limited to 0.020% or less. In addition, Preferably it is 0.017% or less. In addition, since extreme reduction of P leads to an increase in melting cost, it is preferably 0.008% or more in the present invention.

S:0.0070%以下
Sは、鋼中では硫化物として存在し、延性を低下させる作用を示す元素である。このため、Sはできるだけ低減することが望ましいが、0.0070%までは許容できる。なお、好ましくは0.0030%以下である。極端なSの低減は溶製コストの高騰を招くため、本発明では0.0005%以上とすることが好ましい。
S: 0.0070% or less S is an element that exists as a sulfide in steel and exhibits an action of reducing ductility. For this reason, it is desirable to reduce S as much as possible, but it is acceptable up to 0.0070%. In addition, Preferably it is 0.0030% or less. Since extreme reduction of S leads to an increase in melting cost, it is preferable to make it 0.0005% or more in the present invention.

sol.Al:0.001〜0.100%
Alは、製鋼時の脱酸剤として作用し、本発明では、0.001%以上の含有を必要とするが、0.100%を超える含有は、靭性の低下を招く。このため、sol.Alは0.001〜0.100%の範囲に限定した。なお、好ましくは、0.060%以下である。
sol.Al: 0.001 to 0.100%
Al acts as a deoxidizer during steelmaking, and in the present invention, it needs to be contained in an amount of 0.001% or more. However, if it exceeds 0.100%, the toughness is lowered. For this reason, sol.Al was limited to the range of 0.001 to 0.100%. In addition, Preferably, it is 0.060% or less.

上記した基本組成に加えて、本発明では、さらに次A〜C群
A群:質量%で、Cu:0.10〜1.00%、Ni:0.10〜5.00%、Cr:0.10〜0.80%、Mo:0.01〜0.80%、B:0.0002〜0.0025%のうちから選ばれた1種または2種以上
B群:質量%で、Ti:0.03%以下、V:0.100%以下、Nb:0.050%以下のうちから選ばれた1種または2種以上
C群:質量%で、Ca:0.010%以下、REM:0.020%以下のうちから選ばれた1種または2 種
のうちから選ばれた1群または2群以上を含有することもできる。
In addition to the basic composition described above, in the present invention, the following groups A to C, group A: mass%, Cu: 0.10 to 1.00%, Ni: 0.10 to 5.00%, Cr: 0.10 to 0.80%, Mo: 0.01 to One or more selected from 0.80%, B: 0.0002-0.0025% Group B: mass%, Ti: 0.03% or less, V: 0.100% or less, Nb: selected from 0.050% or less 1 type or 2 types or more Group C: By mass%, Ca: 0.010% or less, REM: 1 type or 2 types selected from 2 types or less selected from 2 types or less You can also

A群のCu、Ni、Cr、Mo、Bは、いずれも鋼の焼入れ性をより向上させる元素であり、必要に応じ選択して1種または2種以上を含有できる。   Cu, Ni, Cr, Mo, and B in group A are all elements that further improve the hardenability of the steel, and can be selected as necessary to contain one or more.

Cu:0.10〜1.00%
Cuは、焼入れ性をより向上させる作用を有する。0.10%未満の含有では、このような効果を期待することができず、一方、1.00%を超える含有は熱間脆性を引き起こす危険性が増大する。このため、Cuは0.10〜1.00%に限定することが好ましい。なお、より好ましくは、0.10〜0.30%である。
Cu: 0.10 to 1.00%
Cu has the effect of further improving the hardenability. When the content is less than 0.10%, such an effect cannot be expected. On the other hand, the content exceeding 1.00% increases the risk of causing hot brittleness. For this reason, it is preferable to limit Cu to 0.10 to 1.00%. In addition, More preferably, it is 0.10 to 0.30%.

Ni:0.10〜5.00%
Niは、鋼の焼入れ性をより向上させるとともに、靭性をも向上させる作用を有する。このような効果は、0.10%以上の含有で認められるが、5.00%を超える含有は、製造コスト上昇を招く傾向になる。このため、Niは0.10〜5.00%の範囲に限定することが好ましい。なお、より好ましくは、0.20〜2.00%である。
Ni: 0.10 to 5.00%
Ni has the effect of improving the hardenability of the steel and improving the toughness. Such an effect is recognized at a content of 0.10% or more, but a content exceeding 5.00% tends to increase the manufacturing cost. For this reason, it is preferable to limit Ni to the range of 0.10 to 5.00%. In addition, More preferably, it is 0.20 to 2.00%.

Cr:0.10〜0.80%
Crは、鋼の焼入れ性を向上させる安価な元素であり、このような効果は0.10%以上の含有で認められるが、0.80%を超える含有は、溶接性および靭性を劣化させる。このため、Crは0.10〜0.80%の範囲に限定することが好ましい。なお、より好ましくは、0.20〜0.80%である。
Cr: 0.10-0.80%
Cr is an inexpensive element that improves the hardenability of steel, and such an effect is recognized with a content of 0.10% or more. However, a content exceeding 0.80% deteriorates weldability and toughness. For this reason, it is preferable to limit Cr to the range of 0.10 to 0.80%. In addition, More preferably, it is 0.20 to 0.80%.

Mo:0.01〜0.80%
Moは、鋼の焼入れ性をより向上させる作用を有する元素であり、このような効果は0.01%以上の含有で認められるが、0.80%を超える含有は、溶接性および靭性を劣化させる。このため、Moは0.01〜0.80%の範囲に限定することが好ましい。なお、より好ましくは、0.10〜0.60%である。
Mo: 0.01-0.80%
Mo is an element that has the effect of further improving the hardenability of steel, and such an effect is recognized with a content of 0.01% or more. However, a content exceeding 0.80% deteriorates weldability and toughness. For this reason, it is preferable to limit Mo to the range of 0.01 to 0.80%. In addition, More preferably, it is 0.10 to 0.60%.

B:0.0002〜0.0025%
Bは、少量で鋼の焼入れ性を向上させる元素であり、このような効果は0.0002%以上の含有で認められるが、0.0025%を超えて含有すると、却って焼入れ性が低下する。このため、Bは0.0002〜0.0025%の範囲に限定することが好ましい。なお、より好ましくは0.0005〜0.0020%である。
B: 0.0002-0.0025%
B is an element that improves the hardenability of the steel in a small amount, and such an effect is recognized with a content of 0.0002% or more. However, when it exceeds 0.0025%, the hardenability is lowered. For this reason, it is preferable to limit B to 0.0002 to 0.0025% of range. In addition, More preferably, it is 0.0005 to 0.0020%.

B群の、Ti、V、Nbはいずれも炭化物および/または窒化物の形成を介して強度増加に影響する元素であり、必要に応じ選択して1種または2種以上を含有できる。   In the group B, Ti, V, and Nb are all elements that influence the increase in strength through the formation of carbides and / or nitrides, and can be selected as needed to contain one or more.

Ti:0.03%以下
Tiは、鋼中のNと結合しTiNを形成し、結晶粒の粗大化を抑制してHAZ靭性の向上に寄与するとともに、固溶Nを減少させ、Bの焼入れ性向上効果を確保する作用を有する。このような効果は、0.005%以上の含有で認められるが、0.03%を超えて含有すると、TiNが粗大化し、γ粒の微細化効果が消滅し、靭性が劣化する。このため、Tiは0.03%以下に限定することが好ましい。
Ti: 0.03% or less
Ti combines with N in the steel to form TiN, suppresses the coarsening of crystal grains and contributes to the improvement of HAZ toughness, and also reduces the solid solution N and ensures the effect of improving the hardenability of B. Have Such an effect is recognized when the content is 0.005% or more. However, when the content exceeds 0.03%, TiN becomes coarse, the effect of refining γ grains disappears, and toughness deteriorates. For this reason, it is preferable to limit Ti to 0.03% or less.

V:0.100%以下
Vは炭化物または窒化物として析出し、析出硬化により鋼の強度を増加させる作用を有する。このような効果は0.005%以上の含有で認められるが、0.100%を超えて含有すると、溶接性が劣化する。このため、Vは0.100%以下の範囲に限定することが好ましい。なお、より好ましくは、0.010〜0.060%である。
V: 0.100% or less V precipitates as carbide or nitride, and has the effect of increasing the strength of steel by precipitation hardening. Such an effect is recognized at a content of 0.005% or more, but if it exceeds 0.100%, the weldability deteriorates. For this reason, V is preferably limited to a range of 0.100% or less. In addition, More preferably, it is 0.010 to 0.060%.

Nb:0.050%以下
Nbは、熱間圧延時のオーステナイト粒の再結晶を抑制して、熱間圧延によるオーステナイト粒の展伸を容易にし、フェライトを微細化させて強度、靭性を向上させる作用を有する。このような効果は、0.003%以上の含有で顕著となるが、0.050%を超える含有は、溶接性およびHAZ靭性を劣化させる。このため、Nbは0.050%以下の範囲に限定することが好ましい。なお、より好ましくは、0.010〜0.030%である。
Nb: 0.050% or less
Nb has the effect of suppressing recrystallization of austenite grains during hot rolling, facilitating the expansion of austenite grains by hot rolling, and refining ferrite to improve strength and toughness. Such an effect becomes remarkable when the content is 0.003% or more, but the content exceeding 0.050% deteriorates weldability and HAZ toughness. For this reason, Nb is preferably limited to a range of 0.050% or less. In addition, More preferably, it is 0.010 to 0.030%.

C群のCa、REMはいずれも、硫化物形成元素であり、硫化物を球状化し延性を向上させる元素であり、必要に応じ選択し、1種または2種を含有できる。このような効果は、Ca:0.001%以上、REM:0.001%以上の含有で顕著となるが、Ca:0.010%、REM:0.020%を超える含有は靭性を劣化させる。このため、Ca:0.010%以下、REM:0.020%以下に限定することが好ましい。   Ca and REM in group C are both sulfide-forming elements, elements that spheroidize sulfides and improve ductility, and can be selected as necessary to contain one or two kinds. Such an effect becomes remarkable when the content is Ca: 0.001% or more and REM: 0.001% or more, but the content exceeding Ca: 0.010% and REM: 0.020% deteriorates toughness. For this reason, it is preferable to limit to Ca: 0.010% or less and REM: 0.020% or less.

上記した成分以外の残部は、Feおよび不可避的不純物からなる。   The balance other than the components described above consists of Fe and inevitable impurities.

上記した組成を有する溶鋼を、転炉、電気炉等の通常の溶製手段で溶製し、連続鋳造法または造塊−分塊法等の通常の鋳造法でスラブ等の鋼素材とすることが好ましい。なお、溶製方法、鋳造法については上記した方法に限定されるものではない。   The molten steel having the above composition is melted by a normal melting means such as a converter or an electric furnace, and is made into a steel material such as a slab by a normal casting method such as a continuous casting method or an ingot-bundling method. Is preferred. The melting method and the casting method are not limited to the methods described above.

鋼素材は、オーステナイト単相組織となる温度に加熱される。   The steel material is heated to a temperature at which it becomes an austenite single phase structure.

鋼素材の加熱温度は、鋼素材をオーステナイト化するため、好ましくは1050〜1250℃とする。鋼素材の加熱温度が1050℃未満では、熱間変形抵抗が高すぎて1回あたりの圧下率を高く採れず、生産性が低下する。また、V、Nb等の析出物形成元素を含有する場合には、これら元素が十分にオーステナイト中に固溶せず、これら元素の効果を十分に発揮することが困難となる。一方、加熱温度が1250℃を超えると、結晶粒が粗大化するとともに、スケールロス量の増加や炉の改修頻度の増加を招く。このため、鋼素材の加熱温度は1050〜1250℃の範囲に限定した。   The heating temperature of the steel material is preferably 1050 to 1250 ° C. in order to convert the steel material into austenite. If the heating temperature of the steel material is less than 1050 ° C., the hot deformation resistance is too high, so that the rolling reduction per time cannot be taken high, and the productivity is lowered. Further, when a precipitate-forming element such as V or Nb is contained, these elements are not sufficiently dissolved in austenite, and it is difficult to sufficiently exhibit the effects of these elements. On the other hand, when the heating temperature exceeds 1250 ° C., the crystal grains become coarse and the amount of scale loss and the frequency of furnace repairs increase. For this reason, the heating temperature of the steel material was limited to the range of 1050 to 1250 ° C.

加熱された鋼素材は、圧延終了温度をAr変態点以上の温度域の温度とする熱間圧延を施され、厚鋼板とされる。圧延終了温度がAr変態点未満の温度では、圧延中にフェライトが析出し、その後に焼入れ処理を行っても所望の組織が得られず、所望の強度を確保できなくなる。なお、Ar変態点以上の温度域での累積圧下率は30%以上とすることが好ましい。累積圧下率が30%未満では、十分なオーステナイト粒の微細化が達成できない。 The heated steel material is subjected to hot rolling with the rolling end temperature set to a temperature in the temperature range equal to or higher than the Ar 3 transformation point, to obtain a thick steel plate. When the rolling end temperature is lower than the Ar 3 transformation point, ferrite precipitates during rolling, and a desired structure cannot be obtained even if a quenching process is performed thereafter, so that a desired strength cannot be ensured. The cumulative rolling reduction in the temperature range above the Ar 3 transformation point is preferably 30% or more. If the cumulative rolling reduction is less than 30%, sufficient austenite grain refinement cannot be achieved.

熱間圧延終了後、厚鋼板は、Ar変態点以上の温度域から焼入れ冷却される。焼入れ冷却の開始温度が、Ar変態点未満では、焼入れ冷却開始時の組織がオーステナイト単相ではなく、一部フェライト等への変態が開始していることになり、焼入れ処理を施してもマルテンサイト量が少なく所望の強度を確保することができなくなる。 After the hot rolling, the thick steel plate is quenched and cooled from a temperature range equal to or higher than the Ar 3 transformation point. When the quenching cooling start temperature is lower than the Ar 3 transformation point, the structure at the quenching cooling start is not an austenite single phase, and a transformation to ferrite or the like has started partially. The amount of sites is small and the desired strength cannot be ensured.

また、焼入れ冷却の冷却速度は、マルテンサイト生成臨界冷却速度CR以上の冷却速度とする。なお、本発明でいうマルテンサイト生成臨界冷却速度CRは次(1)式
log CR=2.94−0.75β ………(1)
ここで、β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo(B≧0.0005質量%の場合)、
β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo−1 (B<0.0005質量%の場合)、
CR:マルテンサイト生成臨界冷却速度(℃/s)、
C、Si、Mn、Ni、Cr、Mo:各元素の含有量(質量%)
で定義される冷却速度をいう。なお、ここでいう「マルテンサイト生成臨界冷却速度CR」とは、マルテンサイト相が断面組織観察により導出した面積率で90%以上の分率で形成される冷却速度を意味する。
The cooling rate of the quenching cooling, the martensite critical cooling rate CR M or more cooling rate. Note that martensite critical cooling rate CR M in the present invention the following formula (1)
log CR M = 2.94−0.75β (1)
Here, β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo (when B ≧ 0.0005 mass%),
β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo −1 (when B <0.0005 mass%),
CR M: martensite critical cooling rate (℃ / s),
C, Si, Mn, Ni, Cr, Mo: Content of each element (mass%)
Refers to the cooling rate defined by. Here, the "martensite critical cooling rate CR M" means the cooling rate martensite phase is formed by the fraction of 90% or more in area ratio derived by the cross-sectional structure observation.

本発明の焼入れ冷却処理では、Ar変態点以上の温度域からマルテンサイト生成臨界冷却速度CR以上の冷却速度で、マルテンサイト変態開始温度(Ms点)以下300℃以上の温度域の焼入れ冷却停止温度まで冷却する。これにより、板厚方向各位置で部分的にマルテンサイトがまず生成する。 In quenching cooling process of the present invention, Ar 3 in transformation point or higher martensite temperature range of the product critical cooling rate CR M a cooling rate higher than the martensitic transformation starting temperature (Ms point) below 300 ° C. over a temperature range Quenching Cool to stop temperature. Thereby, martensite is first partially generated at each position in the plate thickness direction.

ここで、部分的にマルテンサイトを生成させることは、生成したマルテンサイトと未変態のオーステナイトとの界面にマルテンサイト変態時の膨張を利用した歪の生成を狙ったものである。この歪エネルギーにより未変態のオーステナイトが下部ベイナイトへ変態しやすくなるとともに、下部ベイナイト相を従来に比べて微細かつ多量に生成することが可能となる。   Here, the partial generation of martensite is aimed at generation of strain using expansion at the time of martensite transformation at the interface between the generated martensite and untransformed austenite. This strain energy facilitates transformation of untransformed austenite to lower bainite, and it is possible to produce a lower bainite phase in a finer amount and in a larger amount than in the prior art.

焼入れ冷却の冷却速度がマルテンサイト生成臨界冷却速度CR未満では、マルテンサイト変態前に粗大なベイナイトの生成量が増加し、上記したマルテンサイト変態による歪の生成が不十分となり、所期した効果を期待できなくなる。 The cooling rate is martensite subcritical cooling rate CR M quench cooling, the amount of coarse bainite before martensitic transformation increases, generation of distortion due to martensitic transformation described above is insufficient, the desired the effect Can not expect.

また、焼入れ冷却停止温度が、Ms点を超える温度では、マルテンサイトの生成による歪生成効果が期待できず、下部ベイナイト相への変態促進が不十分となるうえ、その後の
(焼入れ冷却停止温度±50℃)の温度域での短時間保持により、靭性に有害な島状マルテンサイト量が増加する。一方、焼入れ冷却停止温度が300℃未満では、Cの拡散が不十分となり、亀裂伝播抵抗に有効な炭化物がベイニティックフェライト内部に析出しない。このようなことから、焼入れ冷却停止温度はMs点以下300℃以上の温度域の温度とした。なお、好ましくは、Ms点以下350℃以上の温度範囲である。
Further, when the quenching and cooling stop temperature exceeds the Ms point, the strain generation effect due to the formation of martensite cannot be expected, the transformation to the lower bainite phase is insufficiently promoted, and the subsequent (quenching and cooling stop temperature ± The amount of island martensite harmful to toughness is increased by holding in a temperature range of 50 ° C for a short time. On the other hand, if the quenching and cooling stop temperature is less than 300 ° C., the diffusion of C becomes insufficient, and carbide effective for crack propagation resistance does not precipitate inside the bainitic ferrite. For this reason, the quenching and cooling stop temperature was set to a temperature in the temperature range of 300 ° C. or higher below the Ms point. In addition, Preferably, it is a temperature range below 350 degreeC above Ms point.

上記した範囲の焼入れ冷却停止温度で焼入れ冷却を停止した後、該焼入れ冷却の停止直後から20s以上60s以下の間、厚鋼板の温度を(焼入れ冷却停止温度±50℃)の温度域の温度に保持し、その後室温まで空冷する。 After stopping the quenching cooling in quench cooling stop temperature range described above, while immediately after stopping the該焼insertion cooling over 60s below 20s, the temperature of the temperature range of the temperature of the steel plate (quenching cooling stop temperature ± 50 ° C.) Hold and then air cool to room temperature.

(焼入れ冷却停止温度±50℃)の温度域の温度で1〜60s間保持することにより、マルテンサイトが自己焼鈍されるとともに、未変態オーステナイトの下部ベイナイトへの変態が促進され、焼戻しマルテンサイトと下部ベイナイトとの混合組織を得ることができる。本発明では、厚鋼板の温度は、(焼入れ冷却停止温度±50℃)の温度域の一定温度としても、また、冷却停止による復熱、あるいは高周波加熱装置等による加熱等により該温度域内で任意に変動させてもいずれでもよい。なお、保持温度が、(焼入れ冷却停止温度+50℃)を超えると、ミクロ組織の粗大化が顕著になり、強度、靭性が低下する。一方、(焼入れ冷却停止温度−50℃)未満となると、下部ベイナイトへの変態が促進されない。このため、保持温度は(焼入れ冷却停止温度±50℃)の温度域の温度とした。   By maintaining the temperature in the temperature range of (quenching cooling stop temperature ± 50 ° C.) for 1 to 60 s, martensite is self-annealed and the transformation of untransformed austenite to lower bainite is promoted, and tempered martensite and A mixed structure with lower bainite can be obtained. In the present invention, the temperature of the thick steel plate may be set to a constant temperature in the temperature range of (quenching cooling stop temperature ± 50 ° C.), or may be arbitrarily set within the temperature range by recuperation by cooling stop or heating by a high-frequency heating device. Any of these may be used. If the holding temperature exceeds (quenching cooling stop temperature + 50 ° C.), the coarsening of the microstructure becomes remarkable, and the strength and toughness are lowered. On the other hand, when it becomes less than (quenching cooling stop temperature −50 ° C.), the transformation to lower bainite is not promoted. For this reason, the holding temperature was set to a temperature range of (quenching cooling stop temperature ± 50 ° C.).

また、保持時間が1s未満では上記した効果が不十分であり、一方、60sを超えて長くすると、転位の回復が起こり、強度、靭性が低下する。このため、(焼入れ冷却停止温度±50℃)の温度域の温度での保持時間は1〜60sとし、20〜60sに限定した。なお、保持時間は好ましくは30〜60sである。保持後、室温まで空冷することにより、未変態オーステナイトが下部ベイナイトへ変態しやすくなる。 Further, when the holding time is less than 1 s, the above-described effect is insufficient. On the other hand, when the holding time is longer than 60 s, dislocation recovery occurs, and the strength and toughness decrease. For this reason, the holding time in the temperature range of (quenching cooling stop temperature ± 50 ° C.) was set to 1 to 60 seconds, and was limited to 20 to 60 seconds . The holding time is preferably 30 to 60 s. After holding, the untransformed austenite is easily transformed into lower bainite by air cooling to room temperature.

また、本発明では、(焼入れ冷却停止温度±50℃)の温度域の温度に保持した後、空冷せずに、直ちに1℃/s以上の加熱速度で450℃以上Ac変態点以下の焼戻し温度まで急速加熱し、該焼戻し温度に到達したのち直ちに、空冷する焼戻し処理を厚鋼板に施してもよい。これにより、強度をほとんど低下させることなく、靭性を顕著に向上させることができる。 Further, in the present invention, after maintaining the temperature in the temperature range of (quenching cooling stop temperature ± 50 ° C), tempering at 450 ° C or more and Ac 1 transformation point or less immediately at a heating rate of 1 ° C / s or more without air cooling. The steel plate may be subjected to a tempering treatment in which air is cooled immediately after rapid heating to a temperature and the tempering temperature is reached. Thereby, toughness can be remarkably improved without substantially reducing the strength.

焼戻し処理の加熱速度が1℃/s未満では、靭性は向上するが強度の劣化が著しくなる。なお、加熱速度の上限はとくに限定する必要はなく、設備能力で決定されるが、概ね5℃/s以上である。また、焼戻し温度が450℃未満では、上記した効果がほとんど認められず、一方、Ac変態点を超えて高くなると、強度の低下が顕著となる。 When the heating rate of the tempering treatment is less than 1 ° C./s, the toughness is improved, but the strength is remarkably deteriorated. The upper limit of the heating rate is not particularly limited and is determined by the equipment capacity, but is generally 5 ° C./s or more. Further, when the tempering temperature is less than 450 ° C., the above-mentioned effect is hardly recognized, whereas when the temperature is higher than the Ac 1 transformation point, the strength is significantly reduced.

上記した製造条件で得られる厚鋼板は、上記した組成を有しかつ、板厚方向位置に拠らず、焼戻しマルテンサイトと下部ベイナイトの混合組織を有する。焼戻しマルテンサイトと下部ベイナイトの組織分率は、体積率で80%以上となる。なお、下部ベイナイトの組織分率で全体の50%以上となることが好ましい。また、焼戻しマルテンサイトと下部ベイナイト以外の相としては、体積率で20%以下の上部ベイナイトやフェライトの混在が許容できる。なお、ここでいう「焼戻しマルテンサイト」とは、炭化物が析出あるいは球状化したマルテンサイトをいうものとする。また、ここでいう「下部ベイナイト」は、炭化物が析出あるいは球状化した焼戻し下部ベイナイトをも含むものとする。   The thick steel plate obtained under the manufacturing conditions described above has the above-described composition and has a mixed structure of tempered martensite and lower bainite regardless of the position in the plate thickness direction. The structural fraction of tempered martensite and lower bainite is 80% or more by volume. In addition, it is preferable that it becomes 50% or more of the whole by the structure fraction of a lower bainite. In addition, as phases other than tempered martensite and lower bainite, mixing of upper bainite and ferrite having a volume ratio of 20% or less is acceptable. Here, “tempered martensite” refers to martensite in which carbides are precipitated or spheroidized. The “lower bainite” here also includes tempered lower bainite in which carbides are precipitated or spheroidized.

なお、実操業においては、鋼板の温度管理は、鋼板表面温度により行われ、リアルタイムで鋼板全体の平均温度を計算して、この平均温度に基づいて温度制御や速度制御を行うのが一般的であるため、本発明でいう「温度」は鋼板全体の平均温度、「冷却速度」は鋼板全体の平均冷却速度、「加熱速度」は鋼板全体の平均加熱速度を意味するものとする。   In actual operation, the temperature of the steel sheet is generally controlled by the surface temperature of the steel sheet, and the average temperature of the entire steel sheet is calculated in real time, and temperature control and speed control are generally performed based on this average temperature. Therefore, “temperature” in the present invention means the average temperature of the whole steel sheet, “cooling rate” means the average cooling rate of the whole steel sheet, and “heating rate” means the average heating rate of the whole steel sheet.

また、本発明では、Ar変態点、Ms点、Ac変態点は、各鋼素材(厚鋼板)中の各元素の含有量に基づいた、次(2)、(3)、(4)式
Ar=910−273C−74Mn−56Ni−16Cr−9Mo−5Cu ………(2)
Ms =517−300C−11Si−33Mn−22Cr−17Ni−11Mo ………(3)
Ac=751−26.6C+17.6Si−11.6Mn−169Al−23Cu−23Ni+24.1Cr+22.5Mo−39.7V+233Nb−5.7Ti−895B ………(4)
(ここで、C、Si、Mn、Al、Cu、Ni、Cr、Mo、V、Nb、Ti、B:各元素の含有量(質量%)
を用いて計算して得られる値を用いるものとする。なお、含有しない元素は零として計算するものとする。
In the present invention, the Ar 3 transformation point, the Ms point, and the Ac 1 transformation point are the following (2), (3), (4) based on the content of each element in each steel material (thick steel plate). Formula Ar 3 = 910-273C-74Mn-56Ni-16Cr-9Mo-5Cu (2)
Ms = 517-300C-11Si-33Mn-22Cr-17Ni-11Mo (3)
Ac 1 = 751-26.6C + 17.6Si-11.6Mn -169Al-23Cu-23Ni + 24.1Cr + 22.5Mo-39.7V + 233Nb-5.7Ti-895B ......... (4)
(Here, C, Si, Mn, Al, Cu, Ni, Cr, Mo, V, Nb, Ti, B: Content of each element (mass%)
The value obtained by calculating using is used. It should be noted that elements not contained are calculated as zero.

表1に示す組成の溶鋼を転炉で溶製し、連続鋳造法で230mm厚のスラブとし、さらにこれらスラブに熱間圧延を施し肉厚100mmの鋼素材とした。ついで、これら鋼素材に、表2に示す条件の仕上熱間圧延とそれに続いて焼入れ冷却処理を施して、厚鋼板(板厚:16〜40mm)とした。   Molten steel having the composition shown in Table 1 was melted in a converter, and slabs having a thickness of 230 mm were formed by a continuous casting method, and these slabs were hot-rolled to obtain a steel material having a thickness of 100 mm. Next, these steel materials were subjected to finish hot rolling under the conditions shown in Table 2, followed by quenching and cooling treatment to obtain thick steel plates (plate thickness: 16 to 40 mm).

得られた厚鋼板について、板厚方向1/2の位置からJIS Z 2201の規定に準拠して、4号引張試験片を採取して、JIS Z 2241の規定に準拠して引張試験を実施し、0.2%耐力YSおよび引張強さTSを求めた。なお、表層部とは、表面から1mmの位置をいうものとする。   With respect to the resulting thick steel plate, a No. 4 tensile test piece was taken from the position in the thickness direction 1/2 in accordance with the provisions of JIS Z 2201, and a tensile test was conducted in accordance with the provisions of JIS Z 2241. 0.2% proof stress YS and tensile strength TS were determined. The surface layer portion means a position 1 mm from the surface.

また、得られた厚鋼板の板厚方向1/2の位置からJIS Z 2202の規定に準拠して、Vノッチシャルピー衝撃試験片(厚さ:10mm)を採取して、JIS Z 2241の規定に準拠して衝撃試験を実施し、−40℃でのシャルピー衝撃吸収エネルギーvE-40(J)を求めた。 In addition, V-notch Charpy impact test specimens (thickness: 10 mm) were sampled from the position in the plate thickness direction 1/2 of the obtained thick steel plate in accordance with JIS Z 2202, and specified in JIS Z 2241. An impact test was carried out in accordance with this, and Charpy impact absorption energy vE- 40 (J) at −40 ° C. was determined.

また、得られた厚鋼板から、硬さ測定試験片を採取し、圧延方向に直交する断面(C断面)で板厚方向に表面から裏面まで連続的にビッカース硬度計を用いて硬さを測定した。得られた板厚方向の硬さ分布から、最高値と最低値の差、ΔHVを求め、厚鋼板の板厚方向の均質性を評価した。   In addition, a specimen for hardness measurement is taken from the obtained thick steel plate, and the hardness is continuously measured using a Vickers hardness tester from the front surface to the back surface in the plate thickness direction at a cross section (C cross section) orthogonal to the rolling direction. did. From the obtained hardness distribution in the thickness direction, the difference between the maximum value and the minimum value, ΔHV, was obtained, and the uniformity in the thickness direction of the thick steel plate was evaluated.

また、得られた厚鋼板から、組織観察用試験片を採取し、走査型電子顕微鏡および透過型電子顕微鏡により板厚方向1/2の位置の組織観察を行い、組織の同定、および各組織の組織分率を求めた。なお、焼戻しマルテンサイトと下部ベイナイトは炭化物の析出形態により判別した。各組織の組織分率は、走査型電子顕微鏡を用いて線分法により平均オーステナイト(γ)粒径を測定し、その平均的なγ粒径の粒をランダムに10個選び、そのγ粒内の各組織の領域をそれぞれ断面面積率として求め、10個の断面面積率をその鋼板各位置の組織分率とした。なお、表2中の鋼板温度、冷却速度は、平均温度、平均冷却速度を用いて表示した。   In addition, from the obtained thick steel plate, a specimen for tissue observation was collected, and the structure was observed at a position in the plate thickness direction 1/2 with a scanning electron microscope and a transmission electron microscope. The tissue fraction was determined. In addition, tempered martensite and lower bainite were discriminated by the precipitation form of carbides. The structure fraction of each structure is determined by measuring the average austenite (γ) particle size by a line segment method using a scanning electron microscope, and randomly selecting 10 particles having an average γ particle size. The area of each structure was determined as the cross-sectional area ratio, and the 10 cross-sectional area ratios were taken as the structure fractions at each position of the steel sheet. In addition, the steel plate temperature in Table 2 and the cooling rate were displayed using average temperature and average cooling rate.

得られた結果を表3に示した。   The obtained results are shown in Table 3.

本発明はいずれも、引張強さ590MPa以上の高強度と、各強度に応じた高靭性を有し、しかも板厚方向のΔHVも小さく板厚方向特性の均質性に優れた厚鋼板となっている。とくに、鋼板No.13、14、15と鋼板No.8、10、12をそれぞれ比較することにより、焼入れ冷却停止後に、急速加熱焼戻し処理を施すことにより、強度が低下することなく靭性がさらに向上していることがわかる。   Each of the present invention is a steel plate having a high strength with a tensile strength of 590 MPa or more and a high toughness according to each strength, and also has a small ΔHV in the thickness direction and excellent uniformity in the thickness direction properties. Yes. In particular, by comparing steel plates Nos. 13, 14, and 15 with steel plates Nos. 8, 10, and 12, respectively, rapid quenching and tempering after quenching and cooling have further improved toughness without reducing strength. You can see that

一方、本発明の範囲を外れる比較例は、いずれも下部ベイナイト量が少なく、強度、靭性、あるいは均質性のうちいずれかが低下している。とくに、下部ベイナイト量の増加は靭性向上に大きく寄与していることがわかる。   On the other hand, all of the comparative examples outside the scope of the present invention have a small amount of lower bainite, and any one of strength, toughness, and homogeneity is lowered. In particular, it can be seen that the increase in the amount of lower bainite greatly contributes to the improvement of toughness.

例えば、焼入れ冷却開始温度が本発明の範囲を外れる比較例(鋼板No.21)では同一鋼種の発明例に比べてYSが低下している。一方、焼入れ冷却の冷却速度が本発明範囲を外れる比較例(鋼板No.22、No.23)では下部ベイナイト量が50%以下と少なく、靭性が劣化している。また、焼入れ冷却停止温度がMs点より高い比較例(鋼板No.26)では靭性が劣化している。また、焼入れ冷却停止温度が本発明範囲より低めに外れる比較例(鋼板No.24、No.25)でも、同様に靭性が劣化している。   For example, in the comparative example (steel plate No. 21) in which the quenching cooling start temperature is out of the range of the present invention, YS is lower than that in the inventive example of the same steel type. On the other hand, in comparative examples (steel plates No. 22 and No. 23) in which the cooling rate of quenching cooling is out of the range of the present invention, the amount of lower bainite is as small as 50% or less, and the toughness is deteriorated. Further, in the comparative example (steel plate No. 26) in which the quenching cooling stop temperature is higher than the Ms point, the toughness is deteriorated. Further, in the comparative examples (steel plates No. 24 and No. 25) in which the quenching and cooling stop temperature deviates lower than the range of the present invention, the toughness is similarly deteriorated.

また、急速加熱焼戻し処理における、加熱速度が、本発明の範囲を外れる比較例(鋼板No.29)では、靭性は優れているものの、その合金量に見合ったレベルの強度に達しておらず、本発明例に比べて強度の低下が大きくなっている。また、化学組成が本発明の範囲を外れる比較例(鋼板No.16〜No.20)はいずれも、本発明例に比べて板厚方向均質性、引張強度、靭性のいずれかが大きく劣化している。 Further, in the rapid thermal tempering, heat speed pressure is, in comparative examples out of the scope of the present invention (steel No .2 9), but toughness is excellent, reach the intensity level commensurate with the alloy amount In addition, the decrease in strength is greater than that of the example of the present invention. In addition, any of the comparative examples (steel plates No. 16 to No. 20) whose chemical composition deviates from the scope of the present invention is greatly deteriorated in any of the thickness direction homogeneity, tensile strength, and toughness as compared with the present invention example. ing.

Claims (3)

質量%で、
C:0.01〜0.20%、 Si:0.01〜0.60%、
Mn:0.50〜2.50%、 P:0.020%以下、
S:0.0070%以下、 sol.Al:0.001〜0.100%
を含有し、残部がFeおよび不可避的不純物からなる組成を有する鋼素材を加熱したのち、圧延終了温度をAr変態点以上の温度域とする熱間圧延を施して厚鋼板とし、引き続いて、該厚鋼板をAr変態点以上の温度域から、下記(1)式で定義されるマルテンサイト生成臨界冷却速度CR以上の冷却速度でマルテンサイト変態開始温度以下300℃以上の温度域の焼入れ冷却停止温度まで冷却し、ついで焼入れ冷却停止直後から20〜60s間、該厚鋼板の温度を(焼入れ冷却停止温度±50℃)の温度域の温度に保持し、その後室温まで空冷することを特徴とする高強度高靭性厚鋼板の製造方法。

log CR=2.94−0.75β ‥‥‥(1)
ここで、β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo(B≧0.0005質量%の場合)
=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo−1(B<0.0005質量%の場合)
CR:マルテンサイト生成臨界冷却速度(℃/s)
C、Si、Mn、Ni、Cr、Mo:各元素の含有量(質量%)
% By mass
C: 0.01-0.20%, Si: 0.01-0.60%,
Mn: 0.50-2.50%, P: 0.020% or less,
S: 0.0070% or less, sol.Al: 0.001 to 0.100%
After heating a steel material having a composition consisting of Fe and the inevitable impurities in the balance, the steel is subjected to hot rolling with a rolling end temperature in the temperature range equal to or higher than the Ar 3 transformation point to obtain a thick steel plate, quenching the thick steel plate from Ar 3 transformation point or more temperature range, the following (1) of the martensite critical cooling rate CR martensitic transformation starting temperature below 300 ° C. over a temperature range in M or more cooling rate defined by the formula It is cooled to the cooling stop temperature, then the temperature of the thick steel plate is maintained in the temperature range of (quenching cooling stop temperature ± 50 ° C) for 20 to 60 seconds from immediately after quenching cooling stop, and then air cooled to room temperature. A method for producing a high-strength, high-tough steel plate.
Record
log CR M = 2.94-0.75β ‥‥‥ (1 )
Here, β = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo (when B ≧ 0.0005 mass%)
= 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo −1 (when B <0.0005 mass%)
CR M: martensite critical cooling rate (° C. / s)
C, Si, Mn, Ni, Cr, Mo: Content of each element (mass%)
前記(焼入れ冷却停止温度±50℃)の温度域の温度に保持した後、直ちに1℃/s以上の加熱速度で450℃以上Ac変態点以下の焼戻し温度まで急速加熱したのち、空冷する焼戻し処理を施すことを特徴とする請求項1に記載の高強度高靭性厚鋼板の製造方法。 Tempering after holding at a temperature in the above temperature range (quenching cooling stop temperature ± 50 ° C), immediately heating to 450 ° C or more and a tempering temperature of less than Ac 1 transformation point at a heating rate of 1 ° C / s or more, and then air-cooling. 2. The method for producing a high-strength, high-toughness thick steel plate according to claim 1, wherein the treatment is performed. 前記組成に加えてさらに、下記A〜C群のうちから選ばれた1群または2群以上を含有することを特徴とする請求項1または2に記載の高強度高靭性厚鋼板の製造方法。

A群:質量%で、Cu:0.10〜1.00%、Ni:0.10〜5.00%、Cr:0.10〜0.80%、Mo:0.01〜0.80%、B:0.0002〜0.0025%のうちから選ばれた1種または2種以上
B群:質量%で、Ti:0.03%以下、V: 0.100%以下、Nb:0.050%以下のうちから選ばれた1種または2種以上
C群:質量%で、Ca:0.010%以下、REM:0.020%以下のうちから選ばれた1種または 2種
The method for producing a high strength and high toughness thick steel plate according to claim 1 or 2, further comprising one group or two or more groups selected from the following groups A to C in addition to the composition.
Group A: Mass%, Cu: 0.10 to 1.00%, Ni: 0.10 to 5.00%, Cr: 0.10 to 0.80%, Mo: 0.01 to 0.80%, B: 0.0002 to 0.0025% Alternatively, two or more types B group: mass%, Ti: 0.03% or less, V: 0.100% or less, Nb: 0.050% or less selected from one or more types C group: mass%, Ca: 0.010 % Or less, REM: 1 or 2 types selected from 0.020% or less
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