JP5874664B2 - High strength steel plate with excellent drop weight characteristics and method for producing the same - Google Patents

High strength steel plate with excellent drop weight characteristics and method for producing the same Download PDF

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JP5874664B2
JP5874664B2 JP2013053629A JP2013053629A JP5874664B2 JP 5874664 B2 JP5874664 B2 JP 5874664B2 JP 2013053629 A JP2013053629 A JP 2013053629A JP 2013053629 A JP2013053629 A JP 2013053629A JP 5874664 B2 JP5874664 B2 JP 5874664B2
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橋本 正幸
正幸 橋本
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本発明は、落重特性に優れた高張力鋼板およびその製造方法に関するものであって、主として風力発電用構造物、海洋構造物および船舶などの溶接構造物に適用される厚鋼板であって引張強度が780MPa以上の高張力鋼板の、特に落重特性の改善に関するものである。   The present invention relates to a high-strength steel plate having excellent drop weight characteristics and a method for producing the same, and is a thick steel plate mainly applied to a wind power generation structure, a marine structure, and a welded structure such as a ship. This particularly relates to the improvement of drop weight characteristics of high-tensile steel sheets with a strength of 780 MPa or more.
風力発電用構造物、海洋構造物および船舶など溶接構造物に適用される厚鋼板は、近年における溶接構造物の大型化設計や寒冷地環境での適用などに伴い、より厚肉(例えば50mm以上)で、高強度(例えば降伏強度:690MPa以上)、加えて低温靭性(例えば-40℃における靭性値規定)などの厳しい特性が要求される傾向にある。
殊に、大型構造物は破壊靱性に優れている必要があり、脆性破壊特性の指標として落重特性に優れていることが要求される。
Thick steel plates applied to welded structures such as wind power generation structures, offshore structures, and ships have become thicker (for example, 50 mm or more) due to recent increases in the size of welded structures and applications in cold regions. ), Strict properties such as high strength (for example, yield strength: 690 MPa or more) and low temperature toughness (for example, toughness value at -40 ° C.) tend to be required.
In particular, a large structure needs to have excellent fracture toughness, and is required to have excellent drop weight characteristics as an index of brittle fracture characteristics.
これら特性を有する厚鋼板は、通常、焼入れ・焼戻し熱処理(Q-T処理)により製造される。しかしながら、鋼板の高強度化、厚肉化に伴い、熱間圧延に引き続いて再加熱することなく焼入れを実施する直接焼入れを主体とする従来の製造方法では、低温靭性や破壊靱性、さらには落重特性を確保することが困難になっている。   Thick steel plates having these characteristics are usually produced by quenching and tempering heat treatment (Q-T treatment). However, with the strengthening and thickening of steel sheets, the conventional manufacturing method mainly consisting of direct quenching, in which quenching is performed without reheating following hot rolling, low temperature toughness, fracture toughness, and further drop. It is difficult to ensure heavy characteristics.
落重特性を改善する技術として、例えば特許文献1に示されたような技術が提案されている。この技術は、P含有量を極力低減することによって、粒界の強化を図ると共に、所定量のN添加による細粒効果や、Cr添加による靭性向上効果を図るものである。   As a technique for improving the falling weight characteristic, for example, a technique as disclosed in Patent Document 1 has been proposed. This technique aims to strengthen grain boundaries by reducing the P content as much as possible, and to achieve a fine grain effect by adding a predetermined amount of N and an effect of improving toughness by adding Cr.
また、特許文献2では、ローラクエンチ式焼入れによって、ベイナイトの生成を抑制しつつ微細フェライトを生成させることによって、良好な落重特性を達成する技術が提案されている。   Further, Patent Document 2 proposes a technique that achieves good drop weight characteristics by generating fine ferrite while suppressing the generation of bainite by roller quench quenching.
さらに、特許文献3では、適切な鋼成分の調整と、圧延後の直接冷却-再加熱焼入れ-焼戻しプロセスの適用とによって、鋼板のベイナイト組織を制御し、高強度と共に良好な落重特性を達成する技術が提案されている。   Furthermore, in Patent Document 3, the bainitic structure of the steel sheet is controlled by adjusting the appropriate steel components and applying direct cooling after rolling, reheating and quenching, and tempering processes, achieving high drop strength characteristics as well as high strength. Techniques to do this have been proposed.
加えて、特許文献4では、鋼板の適切な成分調整と圧延後の直接冷却-焼戻しプロセスの適用によって、高強度と共に良好な靭性を達成する技術が提案されている。   In addition, Patent Document 4 proposes a technique for achieving good toughness as well as high strength by adjusting the appropriate components of a steel sheet and applying a direct cooling-tempering process after rolling.
特開平2−93045号公報Japanese Patent Laid-Open No. 2-93045 特開昭60−155620号公報JP-A-60-155620 特開2011−179106号公報JP 2011-179106 A 特開平10−1720号公報JP-A-10-1720
しかしながら、特許文献1に記載の技術で得られる鋼板は、落重特性の指標となる無延性遷移温度(TNDT)を-50℃程度にできるものの、鋼板の降伏強度は620MPa程度に留まっており、高強度と共に良好な落重特性を確保することはできない。   However, the steel sheet obtained by the technique described in Patent Document 1 can have a non-ductile transition temperature (TNDT) that is an index of drop weight characteristics of about -50 ° C, but the yield strength of the steel sheet is only about 620 MPa, It is not possible to ensure good drop weight characteristics with high strength.
また、特許文献2に記載の技術においても、やはり、鋼板の高強度化は困難であり、高強度と共に良好な落重特性を確保することはできない。   Also, in the technique described in Patent Document 2, it is still difficult to increase the strength of the steel sheet, and it is impossible to ensure good drop weight characteristics with high strength.
さらに、特許文献3に記載の技術においても、鋼板の引張強度:780MPa以上の高強度化は困難であり、高強度と共に良好な落重特性を確保することができないばかりでなく、従来の製造方法に対して製造工程が増えており、製造コストの増加を招いている。   Furthermore, even in the technique described in Patent Document 3, it is difficult to increase the tensile strength of a steel plate of 780 MPa or more, and not only cannot ensure good drop weight characteristics with high strength, but also a conventional manufacturing method. On the other hand, the manufacturing process is increasing, leading to an increase in manufacturing cost.
加えて、特許文献4に記載の技術では、鋼板の特性として、NDT温度≦-50℃を安定に確保することができない。また、厚肉で所定の鋼板強度・靭性を得るために1%程度以上のNiが添加されており、製造コストの増大を招いている。   In addition, according to the technique described in Patent Document 4, it is not possible to stably secure NDT temperature ≦ −50 ° C. as a characteristic of the steel sheet. Moreover, about 1% or more of Ni is added in order to obtain a predetermined thickness and toughness of a thick steel plate, which increases the manufacturing cost.
本発明は、かかる従来技術の有する種々の問題に鑑みてなされたものであって、直接焼入れを鋼板に施す方法であっても、鋼板に対して特別な熱処理を付加せずに、引張強度:780MPa以上の高強度と同時にNDT温度:-50℃以下の優れた落重特性を有する厚鋼板を提供することを目的とする。   The present invention has been made in view of the various problems of the prior art, and even if it is a method of directly quenching a steel plate, the tensile strength: The purpose of the present invention is to provide a thick steel plate having high strength of 780 MPa or more and at the same time excellent drop weight characteristics of NDT temperature: -50 ° C or less.
発明者は、上記した目的を達成するために、780MPa以上の引張強度と-40℃以下の低温靭性、さらにはNDT温度(TNDT):-50℃以下の落重特性に影響する各種要因について、鋭意研究を重ねた。その結果、ある一定量以上の焼入性を有する成分系の鋼板を用いて、旧γ(オーステナイト)粒平均粒径(dγ)≦60μmの微細なベイナイトを形成したのち、さらにこれに焼戻し熱処理を施した焼戻しベイナイト組織を主体とすることによって、780MPa以上の引張強度かつ-40℃以下の低温靭性、およびNDT温度:-50℃以下の落重特性の達成が可能であることを見出した。   In order to achieve the above-mentioned object, the inventor has various factors that affect the tensile strength of 780 MPa or more and the low temperature toughness of -40 ° C or lower, and also the drop weight characteristics of NDT temperature (TNDT): -50 ° C or lower. Researched earnestly. As a result, after forming fine bainite with an old γ (austenite) grain average particle diameter (dγ) ≦ 60 μm using a steel sheet having a hardenability of a certain amount or more, a tempering heat treatment is further applied to this. It has been found that by using the tempered bainite structure as a main component, it is possible to achieve a tensile strength of 780 MPa or higher, a low temperature toughness of -40 ° C or lower, and a drop weight characteristic of NDT temperature: -50 ° C or lower.
本発明は、上記した知見に基づくものであって、本発明の要旨構成は次のとおりである。
1.質量%で、C:0.08〜0.15%、Si:0.05〜1.0%、Mn:0.50〜1.80%、P:0.020%以下、S:0.005%以下、Al:0.015〜0.080%、B:0.0005〜0.0030%およびN:0.0015〜0.0080%を含有し、かつCu:0.1〜1.0%、Ni:0.1〜1.0%、Cr:0.1〜1.0%、Mo:0.05〜1.0%、V:0.01〜0.10%、Nb:0.005〜0.05%およびTi:0.003〜0.10%のうち1種または2種以上を含み、さらに下記(1)式で求められるH値が60以上68以下であって、残部がFeおよび不可避的不純物からなる高張力鋼板において、
上記高張力鋼板の、引張強度が780MPa以上でかつ組織が、旧γ粒の平均粒径(dγ):60μm以下の焼戻しベイナイト主体組織であることを特徴とする落重特性に優れた高張力鋼板。

H値=3.0×104×0.1×(C*)0.5×(0.7×[%Si]+1)×(3.33×[%Mn]+1)×(0.35×[%Cu]+1)×(0.36×[%Ni]+1)×(2.16×[%Cr]+1)×(3.0×[%Mo]+1)/T1.5
・・(1)
ここで、 C*=[%C]-1/4×([%Ti]-48×[%N]/14)
T:鋼板板厚(mm)
[%A]はA元素の鋼中含有量(質量%)を示す。
The present invention is based on the above findings, and the gist of the present invention is as follows.
1. In mass%, C: 0.08 to 0.15%, Si: 0.05 to 1.0%, Mn: 0.50 to 1.80%, P: 0.020% or less, S: 0.005% or less, Al: 0.015 to 0.080%, B: 0.0005 to 0.0030% And N: 0.0015 to 0.0080%, and Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Cr: 0.1 to 1.0%, Mo: 0.05 to 1.0%, V: 0.01 to 0.10%, Nb: 0.005 -0.05% and Ti: One or more of 0.003-0.10% are included, and the H value obtained by the following formula (1) is 60 or more and 68 or less , and the balance consists of Fe and inevitable impurities In high-tensile steel plate,
High tensile strength steel sheet with excellent drop weight characteristics, characterized in that the high strength steel sheet has a tensile strength of 780 MPa or more and the microstructure is a tempered bainite main structure with an average grain size of old γ grains (dγ): 60 μm or less. .
H value = 3.0 × 10 4 × 0.1 × (C *) 0.5 × (0.7 × [% Si] +1) × (3.33 × [% Mn] +1) × (0.35 × [% Cu] +1) × (0.36 × [% Ni] +1) × (2.16 × [% Cr] +1) × (3.0 × [% Mo] +1) / T 1.5
(1)
Where C * = [% C] -1 / 4 × ([% Ti] -48 × [% N] / 14)
T: Steel plate thickness (mm)
[% A] indicates the content (mass%) of element A in steel.
2.質量%で、C:0.08〜0.15%、Si:0.05〜1.0%、Mn:0.50〜1.80%、P:0.020%以下、S:0.005%以下、Al:0.015〜0.080%、B:0.0005〜0.0030%およびN:0.0015〜0.0080%を含有し、かつCu:0.1〜1.0%、Ni:0.1〜1.0%、Cr:0.1〜1.0%、Mo:0.05〜1.0%、V:0.01〜0.10%、Nb:0.005〜0.05%およびTi:0.003〜0.10%のうち1種または2種以上を含み、さらに下記(1)式で求められるH値が60以上68以下であって、残部がFeおよび不可避的不純物からなる成分組成になる鋼片を、
1000〜1200℃の温度範囲に加熱したのち、900℃以下の温度域における圧下率を30%以上とし、かつ圧延仕上げ温度を780〜880℃の範囲内とした熱間圧延を施し、次いで、該熱間圧延終了後、冷却開始温度:Ar3変態点以上、冷却停止温度:350℃以下の条件で直接焼入れしたのち、Ac1変態点以下の温度に焼戻し、引張強度を780MPa以上とし、かつ鋼組織を圧延後の旧γ粒の平均粒径(dγ)が60μm以下の焼戻しベイナイト主体組織とする特徴とする落重特性に優れた高張力鋼板の製造方法。

H値=3.0×104×0.1×(C*)0.5×(0.7×[%Si]+1)×(3.33×[%Mn]+1)×(0.35×[%Cu]+1)×(0.36×[%Ni]+1)×(2.16×[%Cr]+1)×(3.0×[%Mo]+1)/T1.5
・・(1)
ここで、 C*=[%C]-1/4×([%Ti]-48×[%N]/14)
T:鋼板板厚(mm)
[%A]はA元素の鋼中含有量(質量%)を示す。
2. In mass%, C: 0.08 to 0.15%, Si: 0.05 to 1.0%, Mn: 0.50 to 1.80%, P: 0.020% or less, S: 0.005% or less, Al: 0.015 to 0.080%, B: 0.0005 to 0.0030% And N: 0.0015 to 0.0080%, and Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Cr: 0.1 to 1.0%, Mo: 0.05 to 1.0%, V: 0.01 to 0.10%, Nb: 0.005 -0.05% and Ti: One or more of 0.003-0.10% are included, and the H value obtained by the following formula (1) is 60 or more and 68 or less , and the balance consists of Fe and inevitable impurities Steel slabs with the component composition
After heating to a temperature range of 1000 to 1200 ° C., hot rolling with a rolling reduction in a temperature range of 900 ° C. or lower of 30% or more and a rolling finish temperature in a range of 780 to 880 ° C. is performed, After completion of hot rolling, after quenching directly under conditions of cooling start temperature: Ar 3 transformation point or higher and cooling stop temperature: 350 ° C or lower, tempering to a temperature below Ac 1 transformation point, tensile strength of 780 MPa or more, and steel A method for producing a high-strength steel sheet having excellent drop weight characteristics, characterized in that the structure is a tempered bainite-based structure in which the average grain size (dγ) of old γ grains after rolling is 60 μm or less.
H value = 3.0 × 10 4 × 0.1 × (C *) 0.5 × (0.7 × [% Si] +1) × (3.33 × [% Mn] +1) × (0.35 × [% Cu] +1) × (0.36 × [% Ni] +1) × (2.16 × [% Cr] +1) × (3.0 × [% Mo] +1) / T 1.5
(1)
Where C * = [% C] -1 / 4 × ([% Ti] -48 × [% N] / 14)
T: Steel plate thickness (mm)
[% A] indicates the content (mass%) of element A in steel.
本発明によれば、鋼板の成分組成を適切に調整するとともに、加熱-圧延-冷却工程を規定することによって、直接焼入れ方法であっても、780MPa以上の引張強度と共に、NDT温度が-50℃以下の落重特性を有する厚鋼板を製造することができ、産業上格段に優れた効果が得られる。   According to the present invention, by appropriately adjusting the component composition of the steel sheet and defining the heating-rolling-cooling process, the NDT temperature is -50 ° C. with a tensile strength of 780 MPa or more even in the direct quenching method. A thick steel plate having the following drop weight characteristics can be produced, and an industrially excellent effect can be obtained.
焼入れ性指標値Hと鋼板の焼入性(鋼板強度:TS)との関係を示すグラフである。It is a graph which shows the relationship between the hardenability index value H and the hardenability (steel plate strength: TS) of a steel plate. 旧γ粒平均粒径(dγ)とTNDT(℃)との関係を示すグラフである。It is a graph which shows the relationship between an old γ grain average particle diameter (dγ) and TNDT (° C.).
以下、本発明について具体的に説明する。
まず、本発明の高張力鋼板(以下、単に鋼板とも言う)の組成を規定した理由について説明する。なお、以下の%表示は、いずれも質量%を意味する。
[C:0.08〜0.15%]
Cは、鋼板の強度を確保するために必要な元素であり、所望の強度を確保するためには、0.08%以上含有させる必要がある。一方、Cを過剰に含有させると靭性および落重特性が低下するため、上限は0.15%とする必要がある。好ましい範囲は0.09〜0.13%である。
Hereinafter, the present invention will be specifically described.
First, the reason why the composition of the high-tensile steel plate (hereinafter also simply referred to as a steel plate) of the present invention is specified will be described. In addition, all the following% display means the mass%.
[C: 0.08-0.15%]
C is an element necessary for ensuring the strength of the steel sheet, and in order to ensure the desired strength, it is necessary to contain 0.08% or more. On the other hand, if C is contained excessively, the toughness and drop weight characteristics are lowered, so the upper limit needs to be 0.15%. A preferable range is 0.09 to 0.13%.
[Si:0.05〜1.0%]
Siは、鋼板の強度を確保するのに有効な元素であり、必要により含有されるが、その量は、0.05%以上である。一方、過剰に含有させると、鋼材に硬質な島状マルテンサイト(MA)の粗大化を招き、落重特性を劣化させる。このため、その上限は1.0%とする。好ましい範囲は、0.10〜0.50%である。
[Si: 0.05-1.0%]
Si is an element effective for ensuring the strength of the steel sheet, and is contained if necessary, but its amount is 0.05% or more. On the other hand, if it is excessively contained, the steel material will become hard island-shaped martensite (MA), and the drop weight characteristic will be deteriorated. Therefore, the upper limit is 1.0%. A preferable range is 0.10 to 0.50%.
[Mn:0.50〜1.80%]
Mnは、焼入性を向上させて鋼板強度を確保する上で有効な元素であり、焼入性向上効果を発揮させるために、0.50%以上含有する必要がある。一方、Mnを過剰に含有すると、鋼板の落重特性が劣化するため、その上限は1.80%とする。好ましい範囲は、0.60〜1.30%である。
[Mn: 0.50 ~ 1.80%]
Mn is an element effective in improving the hardenability and ensuring the strength of the steel sheet, and in order to exert the effect of improving hardenability, it is necessary to contain 0.50% or more. On the other hand, if Mn is contained excessively, the drop weight characteristic of the steel sheet deteriorates, so the upper limit is made 1.80%. A preferable range is 0.60 to 1.30%.
[P:0.020%以下]、[S:0.005%以下]
PおよびSは、不純物として鋼中に存在するが、多量になると靭性を著しく劣化させるため、それらの上限をそれぞれ0.020%、0.005%とする。なお、下限は、0%であっても良い。また好ましい範囲はそれぞれ0.010%以下、0.002%以下である。
[P: 0.020% or less], [S: 0.005% or less]
P and S are present in the steel as impurities, but if the amount is large, the toughness is remarkably deteriorated, so their upper limits are made 0.020% and 0.005%, respectively. The lower limit may be 0%. The preferred ranges are 0.010% or less and 0.002% or less, respectively.
[Al:0.015〜0.080%]
Alは、鋼の脱酸および結晶粒の微細化による靭性の向上に必要な元素であり、0.015%以上の含有が必要である。一方、Alを過剰に含有すると酸化物系非金属介在物を生成して靭性を劣化させるため、その上限を0.080%とする。好ましい範囲は、0.015〜0.060%である。
[Al: 0.015-0.080%]
Al is an element necessary for improving the toughness by deoxidizing steel and refining crystal grains, and it is necessary to contain 0.015% or more. On the other hand, if an excessive amount of Al is contained, oxide-based nonmetallic inclusions are produced and the toughness is deteriorated, so the upper limit is made 0.080%. A preferred range is 0.015 to 0.060%.
[B:0.0005〜0.0030%]
Bは、鋼板の強度に有効な元素であり、その効果を有効に得るためには0.0005%以上の含有が必要である。一方、Bを過剰に含有させると溶接部の特性を劣化させるため、その上限を0.0030%とする。好ましい範囲は、0.0007〜0.0015%である。
[B: 0.0005-0.0030%]
B is an element effective for the strength of the steel sheet, and in order to obtain the effect effectively, it is necessary to contain 0.0005% or more. On the other hand, when B is excessively contained, the characteristics of the welded portion are deteriorated, so the upper limit is made 0.0030%. A preferable range is 0.0007 to 0.0015%.
[N:0.0015〜0.0080%]
Nは、Alと結合して窒化物を形成し、オーステナイト粒の粗大化を抑制して靭性を向上させるため、その下限を0.0015%とする。一方、過剰の添加はかえって靭性を劣化させるため、その上限を0.0080%とする。
[N: 0.0015-0.0080%]
N combines with Al to form nitrides, suppresses coarsening of austenite grains and improves toughness, so its lower limit is made 0.0015%. On the other hand, excessive addition, on the other hand, deteriorates toughness, so the upper limit is made 0.0080%.
以上の元素の他、Cu:0.1〜1.0%、Ni:0.1〜1.0%、Cr:0.1〜1.0%、Mo:0.05〜1.0%、V:0.01〜0.10%、Nb:0.005〜0.05%およびTi:0.003〜0.10%のうち、1種または2種以上を含有させることが必要である。   In addition to the above elements, Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Cr: 0.1 to 1.0%, Mo: 0.05 to 1.0%, V: 0.01 to 0.10%, Nb: 0.005 to 0.05% and Ti: It is necessary to contain 1 type or 2 types or more among 0.003 to 0.10%.
[Cu:0.1〜1.0%]
Cuは、固溶することにより焼入性を向上させる元素であり、その効果を得るためには0.1%以上の含有を必要とする。一方、過剰の含有は熱間加工性を低下させるため、その上限を1.0%とする。
[Cu: 0.1-1.0%]
Cu is an element that improves the hardenability by dissolving in a solid solution. To obtain the effect, Cu is required to be contained in an amount of 0.1% or more. On the other hand, excessive content reduces hot workability, so the upper limit is made 1.0%.
[Ni:0.1〜1.0%]
Niは、固溶することにより焼入性を向上させると同時に、靭性を向上させる元素であり、その効果を得るためには0.1%以上の含有を必要とする。一方、過剰の含有は経済性を損なうため、その上限を1.0%とする。
[Ni: 0.1-1.0%]
Ni is an element that improves hardenability and at the same time improves toughness by solid solution. To obtain the effect, Ni needs to be contained in an amount of 0.1% or more. On the other hand, excessive content impairs economic efficiency, so the upper limit is made 1.0%.
[Cr:0.1〜1.0%]
Crは、焼入れ性を向上させる元素であり、その効果を得るためには、0.1%以上の含有が必要である。一方、過剰の含有は溶接性を低下させるため、その上限を1.0%とする。
[Cr: 0.1-1.0%]
Cr is an element that improves hardenability, and in order to obtain the effect, it is necessary to contain 0.1% or more. On the other hand, excessive content reduces weldability, so the upper limit is made 1.0%.
[Mo:0.05〜1.0%]
Moは、焼入性を向上させる元素であり、その効果を得るためには、0.05%以上の含有が必要である。一方、過剰の含有は溶接性を低下させるため、その上限を1.0%とする。
[Mo: 0.05-1.0%]
Mo is an element that improves hardenability, and in order to obtain the effect, it is necessary to contain 0.05% or more. On the other hand, excessive content reduces weldability, so the upper limit is made 1.0%.
[V:0.01〜0.10%]
Vは、主として焼戻しによる析出強化に有効な元素であり、その効果を得るためには、0.01%以上の含有が必要である。一方、過剰の含有は靭性の劣化および経済性を損なうため、その上限を0.10%とする。
[V: 0.01-0.10%]
V is an element mainly effective for precipitation strengthening by tempering. In order to obtain the effect, V is required to be contained in an amount of 0.01% or more. On the other hand, excessive content impairs toughness and impairs economy, so the upper limit is made 0.10%.
[Nb:0.005〜0.05%]
Nbは、焼入性の向上の他、圧延後の組織微細化および焼戻しによる析出強化に有効な元素であり、その効果を得るためには、0.005%以上の含有が必要である。一方、過剰の含有は靭性劣化を招くため、その上限を0.05%とする。
[Nb: 0.005-0.05%]
Nb is an element effective for improving the hardenability, refining the structure after rolling, and precipitation strengthening by tempering. To obtain the effect, Nb needs to be contained in an amount of 0.005% or more. On the other hand, excessive content causes toughness deterioration, so the upper limit is made 0.05%.
[Ti:0.003〜0.10%]
Tiは、Nと結合してTiNとして鋼片加熱時のオーステナイト結晶粒を細粒化し、母材靭性の向上に有効な元素である。その効果を得るためには、0.003%以上の含有が必要である。一方、過剰な含有はかえって靭性劣化を招くため、その上限を0.10%とする。
[Ti: 0.003-0.10%]
Ti is an element effective for improving the toughness of the base metal by combining with N and making the austenite crystal grains finer when Ti is heated as TiN. In order to obtain the effect, a content of 0.003% or more is necessary. On the other hand, excessive content causes deterioration of toughness, so the upper limit is made 0.10%.
つぎに、本発明に従う高張力鋼板の特性について説明する。
[焼入れ性指標値H]
H値=3.0×104×0.1×(C*)0.5×(0.7×[%Si]+1)×(3.33×[%Mn]+1)×(0.35×[%Cu]+1)×(0.36×[%Ni]+1)×(2.16×[%Cr]+1)×(3.0×[%Mo]+1)/T1.5 ・・(1)
ここで、 C*=[%C]-1/4×([%Ti]-48×[%N]/14)
T:鋼板板厚(mm)
[%A]はA元素の鋼中含有量(質量%)を示す。
上記(1)式で求められる焼入れ性指標値Hは、図1に示すように鋼板の焼入性と関連があり、これが58を下回ると所定の強度・靭性が確保できない。従って、本発明では、安定に強度・靭性を確保するために、Hの範囲は60以上とする。
Next, the characteristics of the high-tensile steel plate according to the present invention will be described.
[Hardenability index value H]
H value = 3.0 × 10 4 × 0.1 × (C *) 0.5 × (0.7 × [% Si] +1) × (3.33 × [% Mn] +1) × (0.35 × [% Cu] +1) × ( 0.36 × [% Ni] +1) × (2.16 × [% Cr] +1) × (3.0 × [% Mo] +1) / T 1.5 (1)
Where C * = [% C] -1 / 4 × ([% Ti] -48 × [% N] / 14)
T: Steel plate thickness (mm)
[% A] indicates the content (mass%) of element A in steel.
The hardenability index value H obtained by the above formula (1) is related to the hardenability of the steel sheet as shown in FIG. 1, and if it is below 58, the predetermined strength and toughness cannot be ensured. Therefore, in the present invention, the range of H is set to 60 or more in order to ensure the strength and toughness stably.
[旧γ粒平均粒径(dγ):60μm以下]
本発明に従う鋼板は、旧γ粒平均粒径(dγ)が60μm以下の焼戻しベイナイト主体組織とする必要がある。
旧γ粒平均粒径とは、焼入れ前の組織であるオーステナイト(γ)粒の平均粒径である。焼入れ前のγ粒径が焼入れ後の組織や材質に影響を及ぼすため、本発明では旧γ粒平均粒径を規定する。
というのは、図2に示すように、旧γ粒平均粒径(dγ)が60μmを超えると落重特性が劣化するためである。前述した鋼板の成分組成および上記の旧γ粒平均粒径の範囲を満足した組織とすることで、引張強度が780MPa以上の鋼板が得られる。また、前述した好適範囲を満足することで、さらに降伏強度が690MPa以上であり、また-40℃以下の低温靭性とNDT温度:-50℃以下の落重特性とを有する鋼板が得られる。
旧γ粒平均粒径の下限に特に限定はないが、工業的には、10μm程度である。
[Old γ grain average particle diameter (dγ): 60 μm or less]
The steel sheet according to the present invention needs to have a tempered bainite main structure with an old γ grain average particle diameter (dγ) of 60 μm or less.
The old γ grain average particle size is the average particle size of austenite (γ) grains, which is a structure before quenching. Since the γ grain size before quenching affects the structure and material after quenching, the prior γ grain average grain size is defined in the present invention.
This is because, as shown in FIG. 2, when the old γ grain average particle diameter (dγ) exceeds 60 μm, the falling weight characteristic deteriorates. A steel sheet having a tensile strength of 780 MPa or more can be obtained by making the structure satisfying the above-described component composition of the steel sheet and the above-mentioned range of the old γ grain average particle diameter. In addition, by satisfying the above-mentioned preferred range, a steel sheet having a yield strength of 690 MPa or more, low temperature toughness of −40 ° C. or less, and drop weight characteristics of NDT temperature: −50 ° C. or less can be obtained.
Although there is no particular limitation on the lower limit of the old γ grain average particle diameter, it is about 10 μm industrially.
なお、焼入れ前のオーステナイト粒の形態は、その後に熱処理を施しても、オーステナイト粒界を優先的に腐食する腐食液(たとえばピクリン酸)で腐食して、金属組織を観察することにより、いわゆる旧オーステナイト粒界として観察することができる。すなわち、この組織観察結果から線分法や画像処理などの方法を用いて旧オーステナイト粒の円相当径を求めることにより、加熱時のオーステナイト粒径を把握することができる。一例を示すと、200倍以上の倍率視野において、任意の直線上にある粒界を積算する方法により、少なくとも10個以上のγ粒を含む粒径の平均値を求めればよい。   In addition, the form of austenite grains before quenching is so-called old by observing the metal structure by corroding the austenite grain boundaries with a corrosive liquid (for example, picric acid) that preferentially corrodes the austenite grain boundaries. It can be observed as an austenite grain boundary. That is, the austenite grain size at the time of heating can be grasped by obtaining the equivalent circle diameter of the prior austenite grains by using a method such as line segmentation or image processing from the structure observation result. For example, an average value of particle diameters including at least 10 or more γ grains may be obtained by a method of integrating grain boundaries on an arbitrary straight line in a magnification field of 200 times or more.
上述したように、本発明に従う鋼板の組織は、焼戻しベイナイト主体組織であることが重要である。というのは、焼戻しベイナイト主体組織が良好な靭性を有するからである。
本発明における焼戻しベイナイト主体組織とは、鋼板の組織中体積分率で焼戻しベイナイト相が50%以上存在することである。好ましくは、80%以上である。他方、焼戻しベイナイト組織の体積分率の上限に特別の制限はないが、工業的には、マルテンサイトとの合計で98%程度であり、その際の残部組織の相の形態に制限はない。
また、本発明における厚鋼板とは、板厚:30mm以上であって、さらに板厚:50mm以上の鋼板が好ましい。他方、その上限に限定はないが100mm程度である。
As described above, it is important that the structure of the steel sheet according to the present invention is a tempered bainite-based structure. This is because the tempered bainite main structure has good toughness.
The tempered bainite main structure in the present invention means that a tempered bainite phase is present in an amount of 50% or more in the structure volume fraction of the steel sheet. Preferably, it is 80% or more. On the other hand, there is no particular restriction on the upper limit of the volume fraction of the tempered bainite structure, but industrially it is about 98% in total with martensite, and there is no restriction on the form of the phase of the remaining structure.
Further, the thick steel plate in the present invention is preferably a steel plate having a plate thickness of 30 mm or more and further a plate thickness of 50 mm or more. On the other hand, the upper limit is not limited, but is about 100 mm.
本発明では、前記した好適成分組成に調整した鋼片を、熱間圧延し、得られた熱延板に対して焼入れ焼き戻し処理を施す。
以下、各製造工程の限定理由について説明する。
なお、以下の温度は特に記載しない限り鋼板の板厚方向の平均温度を表す。板厚方向の平均温度は、板厚、表面温度および冷却条件などから、シミュレーション計算により求められる。
In the present invention, the steel slab adjusted to the above-mentioned preferred component composition is hot-rolled, and the obtained hot-rolled sheet is subjected to quenching and tempering treatment.
Hereinafter, the reason for limitation of each manufacturing process will be described.
In addition, the following temperature represents the average temperature of the steel plate thickness direction unless otherwise indicated. The average temperature in the plate thickness direction is obtained by simulation calculation from the plate thickness, surface temperature, cooling conditions, and the like.
鋼素材である鋼片の製造方法は特に限定されるものではなく、例えば、転炉で溶製させた溶鋼を連続鋳造してスラブを製造することができる。
[加熱温度:1000〜1200℃]
熱間圧延に先立ち、鋼素材である鋼片を加熱する。
合金元素の固溶を図って十分な焼入性を確保するために、加熱温度は1000℃以上とする必要がある。一方、加熱温度が1200℃を超えると、オーステナイト結晶粒の粗大化に伴い、顕著に靭性が劣化してしまう。よって、加熱温度は1000〜1200℃の範囲内とする。
The manufacturing method of the steel slab which is a steel raw material is not specifically limited, For example, the molten steel melted with the converter can be continuously cast and a slab can be manufactured.
[Heating temperature: 1000 ~ 1200 ℃]
Prior to hot rolling, the steel slab, which is a steel material, is heated.
The heating temperature needs to be 1000 ° C. or higher in order to achieve solid solution of the alloy elements and ensure sufficient hardenability. On the other hand, when the heating temperature exceeds 1200 ° C., the toughness is significantly deteriorated as the austenite crystal grains become coarse. Therefore, heating temperature shall be in the range of 1000-1200 degreeC.
[900℃以下の温度域における圧下率:30%以上]
熱間圧延を実施するに当たり、未再結晶温度域における熱間圧延、すなわち、いわゆる制御圧延を実施する。
制御圧延の条件は、旧γ粒の粗大化、ひいては鋼板の靭性に大きな影響を及ぼす要素であり、図2に示すように、旧γ粒平均粒径(dγ)が60μmを超えると落重特性が劣化するため、厳密に制御する必要がある。特に、オーステナイト未再結晶温度域の中でも低温側の温度域における圧下率を十分に確保することが重要であるので、本発明においては、900℃以下の温度域における圧下率(累積圧下率)を30%以上と規定する。900℃以下の温度域における累積圧下率が30%未満では、再結晶に有効な歪エネルギーが効果的に付与されないため結晶粒が細粒化されず、いずれの場合も靭性劣化が著しくなるからである。
なお、制御圧延より前の温度域における圧延条件は、特に規定されないが、制御圧延直前の組織が粗大化していないことが好ましいので、制御圧延実施前に、累積圧下率が50%以上の熱間圧延を実施することが好ましい。
[Rolling ratio in temperature range below 900 ℃: 30% or more]
In carrying out hot rolling, hot rolling in a non-recrystallization temperature range, that is, so-called controlled rolling is carried out.
The conditions of controlled rolling are factors that have a large influence on the coarsening of the old γ grains and, consequently, the toughness of the steel sheet. As shown in FIG. 2, when the old γ grain average particle diameter (dγ) exceeds 60 μm, the falling weight characteristics Since it deteriorates, it is necessary to strictly control it. In particular, since it is important to ensure a sufficient reduction rate in the temperature range on the low temperature side in the austenite non-recrystallization temperature range, in the present invention, the reduction rate (cumulative reduction rate) in the temperature range of 900 ° C. or lower is set. It is specified as 30% or more. If the cumulative rolling reduction in the temperature range of 900 ° C or less is less than 30%, the strain energy effective for recrystallization is not effectively applied, so the crystal grains are not refined, and in any case the toughness deteriorates significantly. is there.
Note that the rolling conditions in the temperature range before the controlled rolling are not particularly specified, but it is preferable that the structure immediately before the controlled rolling is not coarsened, so that the hot rolling with a cumulative rolling reduction of 50% or more before the controlled rolling is performed. It is preferable to perform rolling.
従って、本発明においては、900℃以下の温度域における圧下率は30%以上とする。なお、上記圧延制御温度の下限はAr3変態点であり、後述の式による求めることができる。一方、圧下率の上限は、設備的な観点から80%以下が好ましい。また、900℃以上の温度域における圧下率は特に限定されず、常法によれば良い。 Therefore, in the present invention, the rolling reduction in the temperature range of 900 ° C. or lower is set to 30% or more. Note that the lower limit of the rolling control temperature is the Ar 3 transformation point, and can be determined by the formula described below. On the other hand, the upper limit of the rolling reduction is preferably 80% or less from the viewpoint of equipment. Moreover, the rolling reduction in the temperature range of 900 ° C. or higher is not particularly limited, and may be performed by a conventional method.
[圧延仕上げ温度:780〜880℃]
熱間圧延中の圧延仕上げ温度の制御は、所定の強度・靭性に大きな影響を及ぼす要素であり、厳密に限定する必要がある。ここに、圧延仕上げ温度が880℃を超えると再結晶粒が粗大化し靭性劣化が著しくなり、一方、780℃に満たないと未再結晶粒の変形にともない焼入性が低下する。従って、熱間圧延中の圧延仕上げ温度は、780〜880℃の範囲とする。
[Rolling finishing temperature: 780 ~ 880 ℃]
Control of the rolling finishing temperature during hot rolling is a factor that greatly affects the predetermined strength and toughness, and must be strictly limited. Here, when the rolling finish temperature exceeds 880 ° C., the recrystallized grains become coarse and the toughness deteriorates remarkably. On the other hand, when the rolling finish temperature is less than 780 ° C., the hardenability decreases with the deformation of the non-recrystallized grains. Therefore, the rolling finishing temperature during hot rolling is in the range of 780 to 880 ° C.
[冷却開始温度:Ar3変態点以上]
上記圧延終了後、速やかに直接焼入れを実施する。このとき、鋼板の冷却開始温度がAr3変態点を下回っていると、フェライト相が生成して強度が著しく劣化するため、所定の強度・靭性が確保できない。従って、直接焼入れの冷却開始温度はAr3変態点以上とする。なお、Ar3変態点は次式による。
Ar3(℃)=910−310×[%C]−80×[%Mn]−20×[%Cu]−15×[%Cr]−55×[%Ni]−80×[%Mo]
ここで、[%A]はA元素の鋼中含有量(質量%)を示す。
[Cooling start temperature: Ar 3 transformation point or higher]
Immediately after the rolling, quenching is performed immediately. At this time, if the cooling start temperature of the steel sheet is lower than the Ar 3 transformation point, the ferrite phase is generated and the strength is remarkably deteriorated, so that the predetermined strength and toughness cannot be ensured. Therefore, the cooling start temperature of direct quenching is set to the Ar 3 transformation point or higher. The Ar 3 transformation point is according to the following formula.
Ar 3 (° C.) = 910−310 × [% C] −80 × [% Mn] −20 × [% Cu] −15 × [% Cr] −55 × [% Ni] −80 × [% Mo]
Here, [% A] indicates the content (mass%) of element A in steel.
[冷却停止温度:350℃以下]
Ar3点以上の温度から直接焼入れするとき、鋼板の冷却停止温度が350℃を上回ると、十分な焼入れ組織が得られない場合があり、所定の強度・靭性が確保できない。なお、その下限に特に限定はない。
[Cooling stop temperature: 350 ℃ or less]
When quenching directly from a temperature of Ar 3 or higher, if the cooling stop temperature of the steel sheet exceeds 350 ° C., a sufficient quenched structure may not be obtained, and the predetermined strength and toughness cannot be ensured. The lower limit is not particularly limited.
[焼戻し温度:Ac1変態点以下]
直接焼入れした鋼板を焼戻しするとき、その温度は所定の強度確保のためAc1変態点以下とする。好ましくは、600〜680℃の範囲である。ここで、Ac1変態点は次式による。
Ac1(℃)=751−26.6×[%C]+17.6×[%Si]−11.6×[%Mn]−22.9×[%Cu]−23.0×[%Ni]+24.1×[%Cr]+22.5×[%Mo]−39.7×[%V]−5.7×[%Ti]+233×[%Nb]−169×[%Al]−895×[%B]
ここで、[%A]はA元素の鋼中含有量(質量%)を示す。
[Tempering temperature: Ac 1 transformation point or less]
When tempering a directly quenched steel sheet, the temperature is set to the Ac 1 transformation point or less in order to secure a predetermined strength. Preferably, it is the range of 600-680 degreeC. Here, the Ac 1 transformation point is according to the following equation.
Ac 1 (° C.) = 751−26.6 × [% C] + 17.6 × [% Si] −11.6 × [% Mn] −22.9 × [% Cu] −23.0 × [% Ni] + 24.1 × [% Cr ] + 22.5 × [% Mo] −39.7 × [% V] −5.7 × [% Ti] + 233 × [% Nb] −169 × [% Al] −895 × [% B]
Here, [% A] indicates the content (mass%) of element A in steel.
以下、表1および2を参照して本発明の種々の実施例について説明する。
表1に成分組成、板厚を示す。表1中の1〜10は本発明に従う鋼種であり、11〜15は成分組成が本発明の範囲外となる鋼種(比較鋼)を示している。
これらの組成を有するスラブを表2に記載の各種条件によって製造して、表2の鋼番1-1〜15の鋼板を得た。また、表2に、表1に示した組成の諸性質について調べた結果をまとめたものを併記する。具体的には、表中の、スラブ加熱温度、制御温度、圧下率、圧延仕上げ温度、冷却開始温度、冷却停止温度および焼戻し温度で製造したときの、それぞれの鋼板における、引張特性(降伏強度、引張強度)、靭性(破面遷移温度)および落重特性(NDT温度)を示している。また、鋼番1-1と1-2は、表1の鋼種1と同じ組成の鋼板について制御圧延開始温度を本発明の範囲内および範囲外に変化させて、また鋼番2-1と2-2は、表1の鋼種2と同じ組成の鋼板について制御圧延の圧下率を本発明の範囲内および範囲外に変化させて、さらに鋼番3-1と3-2は、表1の鋼種3と同じ組成の鋼板について圧下率と圧延仕上げ温度を本発明の範囲内および範囲外に変化させて、また鋼番4-1,4-2および4-3は、表1の鋼種4と同じ組成の鋼板について、それぞれ冷却開始温度や、冷却停止温度を本発明の範囲内および範囲外に変化させて製造したものである。
Hereinafter, various embodiments of the present invention will be described with reference to Tables 1 and 2.
Table 1 shows the component composition and plate thickness. 1 to 10 in Table 1 are steel types according to the present invention, and 11 to 15 indicate steel types (comparative steels) whose composition is outside the scope of the present invention.
Slabs having these compositions were produced under the various conditions shown in Table 2, and steel plates 1-1 to 15 in Table 2 were obtained. Table 2 also summarizes the results of investigations on the properties of the compositions shown in Table 1. Specifically, the tensile properties (yield strength, yield), slab heating temperature, control temperature, rolling reduction, rolling finish temperature, cooling start temperature, cooling stop temperature, and tempering temperature in each steel sheet in the table are shown. Tensile strength), toughness (fracture surface transition temperature) and drop weight characteristics (NDT temperature). Steel numbers 1-1 and 1-2 were obtained by changing the controlled rolling start temperature within the range of the present invention and outside the range of the present invention for steel plates having the same composition as steel type 1 in Table 1. -2 changes the rolling reduction ratio of the controlled rolling for the steel plate of the same composition as steel type 2 in Table 1 within and outside the range of the present invention, and steel numbers 3-1 and 3-2 show the steel types in Table 1. The steel plate Nos. 4-1, 4-2 and 4-3 are the same as the steel type 4 in Table 1 by changing the rolling reduction and rolling finish temperature within the range of the present invention and outside the range of the steel plate having the same composition as No. 3. About the steel plate of a composition, it manufactures by changing cooling start temperature and cooling stop temperature in the range of this invention, and the outside of the range, respectively.
以下、諸性質および結果について順に説明する。
(1) 引張特性
降伏強度YPおよび引張強度TSは、各鋼の板厚中心部からJIS Z 2201の規定に準拠してJIS 5号試験片を採取し、JIS Z 2241の規定に準拠して引張試験を実施して求めた。そして、引張強度が780MPa以上となるものを合格とした。
発明鋼では、降伏強度と引張強度のいずれもが合格であったが、比較鋼の鋼番3-2では圧延仕上げ温度が低いため、また鋼番4-3では冷却停止温度が高いため、また鋼番11〜13では各組成の適正範囲外のため、さらに鋼番14、15ではH値が60未満のために、いずれも強度不良が認められた。

Hereinafter, various properties and results will be described in order.
(1) Tensile properties Yield strength YP and tensile strength TS were obtained by collecting JIS No. 5 test pieces from the center of the thickness of each steel in accordance with JIS Z 2201 and pulling in accordance with JIS Z 2241. Determined by conducting a test. And the thing whose tensile strength becomes 780MPa or more was set as the pass.
The invention steel, none of the yield strength tensile strength were acceptable, since the rolling finishing temperature in the Steel No. 3-2 of the comparative steel is low, or the cooling stop temperature in the Steel No. 4-3 is high, Steel Nos. 11 to 13 were outside the proper range of each composition, and Steel Nos. 14 and 15 had an H value of less than 60.

(2) 靭性
破面遷移温度:vTsは、各鋼の板厚中心部から、JIS Z 2201の規定に準拠してJIS 4号試験片を採取し、JIS Z 2241の規定に準拠して衝撃試験を実施して求めた。なお、vTsが-40℃以下となるものを合格とした。
発明鋼はいずれも合格であったが、比較鋼の鋼番1-2では制御圧延開始温度が高すぎて900℃以下における累積圧下率が低くなったため、また鋼番2-2では制御圧延の圧下率が低いため、いずれも靭性不良が認められた。
(2) Toughness Fracture surface transition temperature: For vTs, JIS No. 4 specimens were collected from the center of each steel plate thickness according to JIS Z 2201, and impact tests were conducted according to JIS Z 2241. Was carried out. In addition, the thing in which vTs becomes -40 degrees C or less was set as the pass.
The invention steels were all passed, but the control rolling start temperature was too high for comparative steel No. 1-2, and the cumulative rolling reduction at 900 ° C. or lower was low. Since the rolling reduction was low, all had poor toughness.
(3) 落重特性
NDT温度は、各鋼の板厚中心部から試験片を採取し、NRL落重試験を実施して求めた。なお、NDT温度が-50℃以下となるものを合格とした。
発明鋼はいずれも合格であったが、比較鋼の鋼番1-2では制御圧延開始温度が高すぎて900℃以下における累積圧下率が低くなったため、また鋼番2-2では制御圧延の圧下率が低いために、いずれも靭性不良が認められた。
(3) Drop weight characteristics
The NDT temperature was obtained by collecting a test piece from the center of the thickness of each steel and performing an NRL drop weight test. In addition, what passed NDT temperature below -50 degreeC was set as the pass.
The invention steels were all passed, but the control rolling start temperature was too high for comparative steel No. 1-2, and the cumulative rolling reduction at 900 ° C. or lower was low. Due to the low rolling reduction, poor toughness was observed in all cases.

Claims (2)

  1. 質量%で、C:0.08〜0.15%、Si:0.05〜1.0%、Mn:0.50〜1.80%、P:0.020%以下、S:0.005%以下、Al:0.015〜0.080%、B:0.0005〜0.0030%およびN:0.0015〜0.0080%を含有し、かつCu:0.1〜1.0%、Ni:0.1〜1.0%、Cr:0.1〜1.0%、Mo:0.05〜1.0%、V:0.01〜0.10%、Nb:0.005〜0.05%およびTi:0.003〜0.10%のうち1種または2種以上を含み、さらに下記(1)式で求められるH値が60以上68以下であって、残部がFeおよび不可避的不純物からなる高張力鋼板において、
    上記高張力鋼板の、引張強度が780MPa以上でかつ組織が、旧γ粒の平均粒径(dγ):60μm以下の焼戻しベイナイト主体組織であることを特徴とする落重特性に優れた高張力鋼板。

    H値=3.0×104×0.1×(C*)0.5×(0.7×[%Si]+1)×(3.33×[%Mn]+1)×(0.35×[%Cu]+1)×(0.36×[%Ni]+1)×(2.16×[%Cr]+1)×(3.0×[%Mo]+1)/T1.5
    ・・(1)
    ここで、 C*=[%C]-1/4×([%Ti]-48×[%N]/14)
    T:鋼板板厚(mm)
    [%A]はA元素の鋼中含有量(質量%)を示す。
    In mass%, C: 0.08 to 0.15%, Si: 0.05 to 1.0%, Mn: 0.50 to 1.80%, P: 0.020% or less, S: 0.005% or less, Al: 0.015 to 0.080%, B: 0.0005 to 0.0030% And N: 0.0015 to 0.0080%, and Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Cr: 0.1 to 1.0%, Mo: 0.05 to 1.0%, V: 0.01 to 0.10%, Nb: 0.005 -0.05% and Ti: One or more of 0.003-0.10% are included, and the H value obtained by the following formula (1) is 60 or more and 68 or less , and the balance consists of Fe and inevitable impurities In high-tensile steel plate,
    High tensile strength steel sheet with excellent drop weight characteristics, characterized in that the high strength steel sheet has a tensile strength of 780 MPa or more and the microstructure is a tempered bainite main structure with an average grain size of old γ grains (dγ): 60 μm or less. .
    H value = 3.0 × 10 4 × 0.1 × (C *) 0.5 × (0.7 × [% Si] +1) × (3.33 × [% Mn] +1) × (0.35 × [% Cu] +1) × (0.36 × [% Ni] +1) × (2.16 × [% Cr] +1) × (3.0 × [% Mo] +1) / T 1.5
    (1)
    Where C * = [% C] -1 / 4 × ([% Ti] -48 × [% N] / 14)
    T: Steel plate thickness (mm)
    [% A] indicates the content (mass%) of element A in steel.
  2. 質量%で、C:0.08〜0.15%、Si:0.05〜1.0%、Mn:0.50〜1.80%、P:0.020%以下、S:0.005%以下、Al:0.015〜0.080%、B:0.0005〜0.0030%およびN:0.0015〜0.0080%を含有し、かつCu:0.1〜1.0%、Ni:0.1〜1.0%、Cr:0.1〜1.0%、Mo:0.05〜1.0%、V:0.01〜0.10%、Nb:0.005〜0.05%およびTi:0.003〜0.10%のうち1種または2種以上を含み、さらに下記(1)式で求められるH値が60以上68以下であって、残部がFeおよび不可避的不純物からなる成分組成になる鋼片を、
    1000〜1200℃の温度範囲に加熱したのち、900℃以下の温度域における圧下率を30%以上とし、かつ圧延仕上げ温度を780〜880℃の範囲内とした熱間圧延を施し、次いで、該熱間圧延終了後、冷却開始温度:Ar3変態点以上、冷却停止温度:350℃以下の条件で直接焼入れしたのち、Ac1変態点以下の温度に焼戻し、引張強度を780MPa以上とし、かつ鋼組織を圧延後の旧γ粒の平均粒径(dγ)が60μm以下の焼戻しベイナイト主体組織とすることを特徴とする落重特性に優れた高張力鋼板の製造方法。

    H値=3.0×104×0.1×(C*)0.5×(0.7×[%Si]+1)×(3.33×[%Mn]+1)×(0.35×[%Cu]+1)×(0.36×[%Ni]+1)×(2.16×[%Cr]+1)×(3.0×[%Mo]+1)/T1.5
    ・・(1)
    ここで、 C*=[%C]-1/4×([%Ti]-48×[%N]/14)
    T:鋼板板厚(mm)
    [%A]はA元素の鋼中含有量(質量%)を示す。
    In mass%, C: 0.08 to 0.15%, Si: 0.05 to 1.0%, Mn: 0.50 to 1.80%, P: 0.020% or less, S: 0.005% or less, Al: 0.015 to 0.080%, B: 0.0005 to 0.0030% And N: 0.0015 to 0.0080%, and Cu: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Cr: 0.1 to 1.0%, Mo: 0.05 to 1.0%, V: 0.01 to 0.10%, Nb: 0.005 -0.05% and Ti: One or more of 0.003-0.10% are included, and the H value obtained by the following formula (1) is 60 or more and 68 or less , and the balance consists of Fe and inevitable impurities Steel slabs with the component composition
    After heating to a temperature range of 1000 to 1200 ° C., hot rolling with a rolling reduction in a temperature range of 900 ° C. or lower of 30% or more and a rolling finish temperature in a range of 780 to 880 ° C. is performed, After completion of hot rolling, after quenching directly under conditions of cooling start temperature: Ar 3 transformation point or higher and cooling stop temperature: 350 ° C or lower, tempering to a temperature below Ac 1 transformation point, tensile strength of 780 MPa or more, and steel A method for producing a high-strength steel sheet having excellent drop weight characteristics, characterized in that the microstructure is a tempered bainite-based structure in which the average grain size (dγ) of old γ grains after rolling is 60 μm or less.
    H value = 3.0 × 10 4 × 0.1 × (C *) 0.5 × (0.7 × [% Si] +1) × (3.33 × [% Mn] +1) × (0.35 × [% Cu] +1) × (0.36 × [% Ni] +1) × (2.16 × [% Cr] +1) × (3.0 × [% Mo] +1) / T 1.5
    (1)
    Where C * = [% C] -1 / 4 × ([% Ti] -48 × [% N] / 14)
    T: Steel plate thickness (mm)
    [% A] indicates the content (mass%) of element A in steel.
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