JP2019052341A - Non-modified low yield ratio high tension thick steel sheet excellent in flexure processability, and manufacturing method therefor - Google Patents

Non-modified low yield ratio high tension thick steel sheet excellent in flexure processability, and manufacturing method therefor Download PDF

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JP2019052341A
JP2019052341A JP2017176255A JP2017176255A JP2019052341A JP 2019052341 A JP2019052341 A JP 2019052341A JP 2017176255 A JP2017176255 A JP 2017176255A JP 2017176255 A JP2017176255 A JP 2017176255A JP 2019052341 A JP2019052341 A JP 2019052341A
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隆男 赤塚
Takao Akatsuka
隆男 赤塚
善明 村上
Yoshiaki Murakami
善明 村上
聡 伊木
Satoshi Iki
聡 伊木
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JFE Steel Corp
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Abstract

To provide a non-modified low yield ratio high tension thick steel sheet, which is non-modified only with cooling facility, and has low yield ratio of yield ratio of 80% or less, high strength of tensile strength of 590 to 740 MPa, and excellent flexure processability.SOLUTION: There is provided a non-modified low yield ratio high tension thick steel sheet having a composition containing prescribed components, satisfying 4.0≥Ti/N≥2.0, satisfying carbon equivalent Ceq of 0.35 to 0.50, and the balance Fe with inevitable impurities, a structure in a range from a steel sheet surface to 5 mm position in a sheet thickness direction consisting of a polygonal ferrite phase of 30 to 70 area%, and the balance of a bainite phase and a martensite phase, or a mixed phase, a structure in a range from the 5 mm position from the steel sheet surface in the sheet thickness direction to a sheet thickness center position consisting of the polygonal ferrite phase of 30 area% or less, and the balance of the bainite phase or a pearlite phase, or a mixed phase thereof, Vickers hardness HV at a position of 1 mm from the steel sheet surface in the sheet thickness direction of 260 HV or less, tensile strength of 590 to 740 MP, and yield ratio of 80% or less, and excellent in flexure processability.SELECTED DRAWING: None

Description

本発明は、建築等の溶接鋼構造物用として好適な、降伏比80%以下の低降伏比で、引張強さ590〜740MPaを有する非調質低降伏比高張力厚鋼板およびその製造方法に係り、とくに曲げ加工性に優れた非調質低降伏比高張力鋼板に関する。なお、ここでいう「厚鋼板」とは、板厚70mm以上である場合をいうものとする。   The present invention relates to a non-tempered low yield ratio high tensile steel plate having a tensile strength of 590 to 740 MPa and a method for producing the same, which is suitable for use in welded steel structures such as buildings, with a low yield ratio of 80% or less. In particular, the present invention relates to a non-tempered low yield ratio high-tensile steel sheet excellent in bending workability. The “thick steel plate” here refers to the case where the plate thickness is 70 mm or more.

建築、橋梁、貯蔵タンク、圧力容器などの鉄鋼構造物の製造に用いられる鋼板は、強度と靭性が優れていることはもちろん、成形時の加工性(曲げ加工性)に優れていることが要求される。しかし、通常、鋼板の強度が上がるほど曲げ加工性は低下する。また非調質で製造された鋼板は、板厚が厚くなるほど、一般的に表層硬さが高くなり、曲げ加工性が低下するという問題がある。   Steel sheets used in the manufacture of steel structures such as buildings, bridges, storage tanks, pressure vessels, etc. are required to have excellent strength and toughness as well as excellent workability (bending workability) during forming. Is done. However, bending workability usually decreases as the strength of the steel sheet increases. Moreover, the steel sheet manufactured by non-tempering has a problem that, as the plate thickness increases, the surface layer hardness generally increases and bending workability decreases.

そこで、高強度と優れた加工性とを両立させるために、鋼板表層部のみを軟化させる技術が提案されている。例えば、特許文献1には、加工性に優れた高張力鋼板の製造方法が記載されている。特許文献1は、スラブを1000〜1350℃に加熱し、製品板厚まで熱間圧延し、引続きAr変態点以上の温度から直接焼入れを行い、平均温度が400℃未満の温度まで冷却を行ったのち、400℃以上Ac変態点以下の温度で焼戻し処理を施す、高張力鋼板の製造方法であり、直接焼入れの際、鋼板表面温度が300℃以上の範囲にあるとき、0.3s以上の一時的に水冷されない時間を、1回あるいは2回以上で、合計1.5〜15sとなるように設けることを特徴としている。また、特許文献1では、鋼板板厚が厚くなると、板厚方向での冷却速度がある程度相違することになり、特に誘導加熱を用いて、鋼板内部に比べて表面温度が高くなる温度分布を与えることが好ましいとしている。これにより、表面硬さを減じることができ、板厚方向の硬度差を小さくでき、加工性が向上するとしている。 Therefore, in order to achieve both high strength and excellent workability, a technique for softening only the steel sheet surface layer portion has been proposed. For example, Patent Document 1 describes a method for producing a high-tensile steel plate having excellent workability. In Patent Document 1, a slab is heated to 1000 to 1350 ° C., hot-rolled to a product plate thickness, and then directly quenched from a temperature not lower than the Ar 3 transformation point, and cooled to a temperature having an average temperature of less than 400 ° C. After that, it is a method for producing a high-strength steel sheet that is tempered at a temperature not lower than 400 ° C. and not higher than the Ac 1 transformation point. When direct quenching, the surface temperature of the steel sheet is in the range of 300 ° C. or higher, 0.3 s or longer The time during which the water is not temporarily cooled is provided once or twice or more so as to be a total of 1.5 to 15 s. Moreover, in patent document 1, when the steel plate thickness becomes thick, the cooling rate in the thickness direction will differ to some extent. In particular, induction heating is used to give a temperature distribution in which the surface temperature becomes higher than that inside the steel plate. It is said that it is preferable. Thereby, surface hardness can be reduced, hardness difference in the plate thickness direction can be reduced, and workability is improved.

また、特許文献2には、加工性に優れた引張強度628MPa以下の高張力鋼板が記載されている。特許文献2には、質量%で、C:0.005〜0.02%、Si:0.05〜0.50%、Mn:1.0〜2.5%、Al:0.01〜0.08%、Nb:0.010〜0.060%、Ti:0.005〜0.025%、B:0.0010〜0.0040%を含み、鋼板の板厚中心部硬さが(鋼板表面部の硬さ+15HV)未満である加工性に優れた引張強度628MPa以下の高張力鋼板が開示されている。特許文献2では、誘導加熱等により、鋼板の表層部をAc変態点以上、内部をAc変態点以下に再加熱することにより、表層部を内部よりも軟らかくすることができ、高い強度と優れた加工性を兼備できるとしている。 Patent Document 2 describes a high-tensile steel plate with excellent workability and a tensile strength of 628 MPa or less. In Patent Document 2, in mass%, C: 0.005 to 0.02%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.5%, Al: 0.01 to 0 0.08%, Nb: 0.010 to 0.060%, Ti: 0.005 to 0.025%, B: 0.0010 to 0.0040%, and the center thickness hardness of the steel sheet (steel sheet) A high-tensile steel plate having a tensile strength of 628 MPa or less and excellent workability that is less than (surface portion hardness + 15 HV) is disclosed. In Patent Document 2, the surface layer portion can be made softer than the inside by reheating the surface layer portion of the steel sheet to the Ac 1 transformation point or higher and the inside to the Ac 1 transformation point or less by induction heating or the like, and has high strength. It is said that it can combine excellent workability.

特開2005−298963号公報JP 2005-298963 A 特開2011−195961号公報JP2011-195961A

しかしながら、特許文献1や特許文献2に記載された技術では、既存の設備に加え、新たな加熱設備を必要とするという問題がある。   However, the techniques described in Patent Literature 1 and Patent Literature 2 have a problem that new heating equipment is required in addition to existing equipment.

本発明は、このような従来技術の問題を解決し、冷却設備のみの非調質で、降伏比80%以下の低降伏比と、引張強さ590〜740MPaの高強度と、優れた曲げ加工性とを兼備する非調質低降伏比高張力厚鋼板を提供することを目的とする。   The present invention solves such problems of the prior art, is a non-tempered cooling equipment only, has a low yield ratio of 80% or less, a high tensile strength of 590 to 740 MPa, and an excellent bending process. An object of the present invention is to provide a non-tempered low-yield ratio high-tensile thick steel plate having both properties.

本発明者らは、上記した目的を達成するため、曲げ加工性に影響を及ぼす各種要因について、鋭意検討した。その結果、曲げ加工性を向上させるためには、表層の硬い部分をできるだけ軟化することが重要であることに着目し、熱間圧延後に第一段冷却として表面温度でAr変態点以上の温度から、板厚の1/4位置の平均冷却速度2℃/s以上で、750℃以下550℃以上となるまで加速冷却し、表面温度が750℃以下600℃以上の温度範囲まで復熱させ、その後、第2段冷却として板厚の1/4位置の平均冷却速度2℃/s以上で、冷却停止後の復熱で表面温度が500℃以下になる冷却停止温度まで加速冷却することに思い至った。これにより、表層部に軟質相であるポリゴナルフェライト相を生成させることが可能となり、表層硬さを低減することができるとともに、表面下5mmから板厚中央位置(1/2t位置)の範囲の組織が、面積率が30%以下のポリゴナルフェライト相と、残部がベイナイト相かパーライト相、あるいはその混合相からなる組織とすることができ、優れた曲げ加工性と所望の高強度とを兼ね備えた厚鋼板とすることができることを知見した。 In order to achieve the above-mentioned object, the present inventors diligently studied various factors that affect bending workability. Temperature As a result, in order to improve the bending workability, focuses on the fact that the softening as possible hard portion of the surface layer is important, at a surface temperature as the first stage cooling after hot rolling than the Ar 3 transformation point And accelerated cooling until the average cooling rate is 2 ° C./s or more at ¼ position of the plate thickness until it becomes 750 ° C. or less and 550 ° C. or more, and the surface temperature is reheated to a temperature range of 750 ° C. or less and 600 ° C. or more, After that, as the second stage cooling, the cooling is accelerated to the cooling stop temperature at which the surface temperature becomes 500 ° C. or less by the recuperation after the cooling stop at an average cooling rate of 2 ° C./s or more at the 1/4 position of the plate thickness. It came. As a result, a polygonal ferrite phase, which is a soft phase, can be generated in the surface layer portion, the surface layer hardness can be reduced, and the thickness in the range from 5 mm below the surface to the center position of the plate thickness (1 / 2t position). The structure can be composed of a polygonal ferrite phase with an area ratio of 30% or less and the balance consisting of a bainite phase, a pearlite phase, or a mixed phase thereof, and has excellent bending workability and desired high strength. It was found that a thick steel plate can be obtained.

本発明は、かかる知見に基づき、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:0.05〜0.10%、Si:0.01〜0.45%、Mn:0.60〜1.80%、P:0.020%以下、S:0.003%以下、Al:0.05%以下、Ti:0.005〜0.020%、N:0.0040%以下、Nb:0.005〜0.030%、V:0.001〜0.070%、Cr:0.30%以下を含み、Ti、Nが下記(1)式を満足し、かつ下記(2)式で定義される炭素当量Ceqが0.35〜0.50を満足し、残部Feおよび不可避的不純物からなる組成であり、鋼板表面から板厚方向に5mmの位置までの範囲における組織が、面積率で30〜70%のポリゴナルフェライト相と、残部がベイナイト相またはマルテンサイト相、あるいはその混合相からなり、鋼板表面から板厚方向に5mmの位置から板厚中央位置までの範囲における組織が、面積率が30%以下のポリゴナルフェライト相と、残部がベイナイト相またはパーライト相、あるいはその混合相からなり、鋼板表面から板厚方向に1mmの位置におけるビッカース硬さHVが260HV以下、引張強さ590〜740MPa、降伏比80%以下であることを特徴とする曲げ加工性に優れた非調質低降伏比高張力厚鋼板。
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: 0.05 to 0.10%, Si: 0.01 to 0.45%, Mn: 0.60 to 1.80%, P: 0.020% or less, S: 0.003% or less, Al: 0.05% or less, Ti: 0.005-0.020%, N: 0.0040% or less, Nb: 0.005-0.030%, V: 0.001- 0.070%, Cr: 0.30% or less, Ti and N satisfy the following formula (1), and carbon equivalent Ceq defined by the following formula (2) is 0.35 to 0.50 Satisfactory composition consisting of the balance Fe and inevitable impurities, the structure in the range from the steel sheet surface to the position of 5 mm in the thickness direction is a polygonal ferrite phase with an area ratio of 30 to 70%, and the balance is the bainite phase. Or it consists of martensite phase or its mixed phase, 5m from the steel plate surface to the plate thickness direction. The structure in the range from the center position to the center position of the plate thickness is composed of a polygonal ferrite phase with an area ratio of 30% or less, and the balance is a bainite phase, a pearlite phase, or a mixed phase thereof, and 1 mm from the steel plate surface in the plate thickness direction. A non-tempered low yield ratio high tension thick steel plate excellent in bending workability, characterized in that the Vickers hardness HV at the position is 260HV or less, the tensile strength is 590 to 740 MPa, and the yield ratio is 80% or less.


4.0≧Ti/N≧2.0 ……(1)
ここで、Ti、N:各元素の含有量(質量%)
Ceq=C+Si/24+Mn/6+Cr/5+Mo/4+V/14 ……(2)
ここで、C、Si、Mn、Cr、Mo、V:各元素の含有量(質量%)
[2]前記組成に加えて、さらに質量%で、Mo:0.60%以下、Cu:0.05〜0.60%、Ni:0.05〜0.80%、B:0.0003〜0.0030%のうちから選ばれた1種または2種以上を含有することを特徴とする[1]に記載の曲げ加工性に優れた非調質低降伏比高張力厚鋼板。
[3][1]または[2]に記載の組成を有する鋼素材を加熱温度1000〜1250℃に加熱する加熱工程と、次いで、表面温度で950℃以下の温度域での累積圧下率が30%以上で、圧延終了温度が表面温度で900℃以下Ar変態点以上となる熱間圧延を行う熱間圧延工程と、次いで、第一段冷却として表面温度でAr変態点以上の温度から、板厚1/4位置の平均冷却速度が2℃/s以上で、表面温度で750℃以下550℃以上となるまで加速冷却し、次いで表面温度で750℃以下600℃以上の範囲まで復熱させ、その後、第二段冷却として板厚1/4位置の平均冷却速度が2℃/s以上で、冷却停止後の復熱で表面温度が500℃以下になる冷却停止温度まで加速冷却する二段階の冷却工程と、を施すことを特徴とする曲げ加工性に優れた非調質低降伏比高張力厚鋼板の製造方法。
[4]前記冷却工程後、さらに、焼戻温度400℃以上700℃以下の温度で焼戻工程を施すことを特徴とする[3]に記載の曲げ加工性に優れた非調質低降伏比高張力厚鋼板の製造方法。
4.0 ≧ Ti / N ≧ 2.0 (1)
Here, Ti, N: Content of each element (% by mass)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Mo / 4 + V / 14 (2)
Here, C, Si, Mn, Cr, Mo, V: Content of each element (mass%)
[2] In addition to the above-mentioned composition, it is further mass%, Mo: 0.60% or less, Cu: 0.05-0.60%, Ni: 0.05-0.80%, B: 0.0003- The non-tempered low yield ratio high-tensile steel plate excellent in bending workability according to [1], comprising one or more selected from 0.0030%.
[3] A heating step of heating the steel material having the composition described in [1] or [2] to a heating temperature of 1000 to 1250 ° C., and then a cumulative rolling reduction in a temperature range of 950 ° C. or less at a surface temperature of 30 % or more, and the hot rolling step of rolling end temperature to perform hot rolling to be 900 ° C. or less than the Ar 3 transformation point at a surface temperature, then, from the Ar 3 transformation point or more of the temperature at the surface temperature of the first stage cooling , Accelerated cooling until the average cooling rate at the position of 1/4 of the plate thickness is 2 ° C./s or more and the surface temperature is 750 ° C. or lower and 550 ° C. or higher, and then recuperates the surface temperature to 750 ° C. or lower and 600 ° C. or higher. Then, as the second stage cooling, the average cooling rate at the ¼ thickness position is 2 ° C./s or more, and accelerated cooling is performed to the cooling stop temperature at which the surface temperature becomes 500 ° C. or less by reheating after the cooling stop. And a step of cooling process, That the bending method for producing excellent formability non-heat treated low yield ratio high-strength thick steel plate.
[4] The non-tempered low yield ratio excellent in bending workability according to [3], wherein after the cooling step, a tempering step is further performed at a temperature of 400 ° C. or higher and 700 ° C. or lower. Manufacturing method of high-tensile thick steel plate.

本発明によれば、熱処理設備を導入することなく、加速冷却装置のみを利用して、非調質で、降伏比80%以下の低降伏比と、引張強さ590〜740MPaの高強度と、優れた曲げ加工性とを兼備する、板厚70mm以上の非調質低降伏比高張力厚鋼板を容易にかつ安価に製造でき、産業上格段の効果を奏する。   According to the present invention, without introducing heat treatment equipment, using only an accelerated cooling device, non-tempered, a low yield ratio of a yield ratio of 80% or less, a high strength of a tensile strength of 590 to 740 MPa, A non-tempered low yield ratio high-tensile steel plate having a thickness of 70 mm or more, which has excellent bending workability, can be easily and inexpensively produced, and has a remarkable industrial effect.

図1は、実施例で行った曲げ試験の要領を模式的に示す説明図である。FIG. 1 is an explanatory view schematically showing a procedure of a bending test performed in the examples.

まず、本発明の厚鋼板の組成限定理由について説明する。なお、以下の説明において、鋼成分組成の各元素の含有量の単位は「質量%」であり、以下、特に断らない限り、単に「%」で示す。   First, the reasons for limiting the composition of the thick steel plate of the present invention will be described. In the following description, the unit of the content of each element of the steel component composition is “mass%”, and hereinafter, simply indicated by “%” unless otherwise specified.

C:0.05〜0.10%
Cは、鋼の強度を増加させ、構造用鋼材として必要な強度を確保するのに有用な元素である。また、Cは、硬質相の体積率を増加させ、降伏比を低下させる作用を有する。このような効果を得るためには、0.05%以上の含有を必要とする。一方、0.10%を超える含有は、溶接性と靭性を顕著に低下させる。このため、Cは0.05〜0.10%の範囲に限定した。なお、好ましくは0.06〜0.09%である。
C: 0.05-0.10%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. Moreover, C has the effect | action which increases the volume ratio of a hard phase and reduces a yield ratio. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 0.10%, the weldability and toughness are significantly reduced. For this reason, C was limited to the range of 0.05 to 0.10%. In addition, Preferably it is 0.06 to 0.09%.

Si:0.01〜0.45%
Siは、脱酸剤として作用するとともに、鋼中に固溶して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。一方、0.45%を超える含有は、母材の靱性を低下させるとともに、溶接熱影響部(HAZとも言う)靱性を顕著に低下させる。このため、Siは0.01〜0.45%の範囲に限定した。なお、好ましくは、0.02〜0.40%である。
Si: 0.01 to 0.45%
Si is an element that acts as a deoxidizer and increases the strength of the steel by solid solution in steel. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content exceeds 0.45%, the toughness of the base metal is lowered and the toughness of the weld heat affected zone (also referred to as HAZ) is significantly lowered. For this reason, Si was limited to the range of 0.01 to 0.45%. In addition, Preferably, it is 0.02 to 0.40%.

Mn:0.60〜1.80%
Mnは、固溶して鋼の強度を増加させる作用を有する元素である。Mnは安価な元素であり、高価な他の合金元素の含有を最小限に抑えることができるという効果も有する。このようなことから、本発明では、所望の高強度(引張強さ:590MPa以上)を確保するために、0.60%以上の含有を必要とする。一方、1.80%を超える含有は、母材の靱性およびHAZ靱性を著しく低下させる。このため、Mnは0.60〜1.80%の範囲に限定した。なお、好ましくは0.80〜1.60%である。
Mn: 0.60 to 1.80%
Mn is an element having a function of increasing the strength of the steel by solid solution. Mn is an inexpensive element and has an effect that the content of other expensive alloy elements can be minimized. Therefore, in the present invention, the content of 0.60% or more is required in order to ensure a desired high strength (tensile strength: 590 MPa or more). On the other hand, if the content exceeds 1.80%, the toughness and the HAZ toughness of the base material are significantly reduced. For this reason, Mn was limited to 0.60 to 1.80%. In addition, Preferably it is 0.80 to 1.60%.

P:0.020%以下
Pは、鋼の強度を増加させる作用を有する元素であるが、靱性、とくに溶接部の靱性を低下させる。このため、できるだけ低減することが望ましい。0.020%を超えて含有すると、上記した悪影響が顕著となる。このため、Pは0.020%以下に限定した。
P: 0.020% or less P is an element having an action of increasing the strength of steel, but lowers toughness, particularly toughness of a welded portion. For this reason, it is desirable to reduce as much as possible. When the content exceeds 0.020%, the above-described adverse effects become remarkable. For this reason, P was limited to 0.020% or less.

S:0.003%以下
Sは、鋼中ではMnS等の硫化物系介在物として存在し、母材および溶接部の靱性を低下させるとともに、鋳片中央偏析部などに多量に偏在して鋳片等における欠陥を発生しやすくする傾向を有する。このような傾向は0.003%を超える含有で顕著となる。このため、Sは0.003%以下に限定した。なお、過度のS低減は、精錬コストを高騰させ、経済的に不利となるため、Sは0.001%程度以上とすることが望ましい。
S: 0.003% or less S is present in the steel as sulfide inclusions such as MnS, which lowers the toughness of the base material and the welded portion and is unevenly distributed in a large amount in the center segregating portion of the slab. There is a tendency to easily cause defects in pieces. Such a tendency becomes remarkable when the content exceeds 0.003%. For this reason, S was limited to 0.003% or less. In addition, since excessive S reduction raises refining cost and becomes economically disadvantageous, it is desirable for S to be about 0.001% or more.

Al:0.05%以下
Alは、脱酸剤として作用する元素であり、高張力鋼の溶鋼脱酸プロセスにおいては、脱酸剤として、もっとも汎用的に使われる元素である。このような効果を得るためには、0.01%以上含有することが望ましい。しかしながら、0.05%を超える含有は、母材の靱性を下させるとともに、溶接時に溶接金属に混入して溶接金属部靱性を低下させる。このため、Alは0.05%以下に限定した。なお、好ましくは0.01〜0.045%である。
Al: 0.05% or less Al is an element that acts as a deoxidizer, and is the most widely used element as a deoxidizer in a molten steel deoxidation process for high-strength steel. In order to acquire such an effect, it is desirable to contain 0.01% or more. However, the content exceeding 0.05% lowers the toughness of the base material and mixes with the weld metal during welding to lower the weld metal part toughness. For this reason, Al was limited to 0.05% or less. In addition, Preferably it is 0.01 to 0.045%.

Ti:0.005〜0.020%
Tiは、Nとの親和力が強い元素であり、凝固時にTiNとして析出し、鋼中の固溶Nを減少させ、冷間加工後の歪時効による靭性劣化を低減する作用を有する。また、Tiは、HAZの組織改善を介して、HAZ靭性の向上にも寄与する。このような効果を得るためには、Tiは0.005%以上の含有を必要とする。一方、0.020%を超えて含有すると、TiN粒子が粗大化し、上記した効果が期待できなくなる。このため、Tiは0.005〜0.020%の範囲に限定した。なお、好ましくは0.007〜0.015%である。
Ti: 0.005-0.020%
Ti is an element having a strong affinity for N, and precipitates as TiN during solidification, thereby reducing the solid solution N in the steel and reducing toughness deterioration due to strain aging after cold working. Ti also contributes to the improvement of HAZ toughness through the improvement of the HAZ structure. In order to acquire such an effect, Ti needs to contain 0.005% or more. On the other hand, if the content exceeds 0.020%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti was limited to 0.005 to 0.020% of range. In addition, Preferably it is 0.007 to 0.015%.

N:0.0040%以下
Nは、鋼中に固溶して、冷間加工後の歪時効を生起させ、靭性を劣化させる元素であり、本発明ではできるだけ低減することが望ましい。0.0040%を超える含有は、靭性の劣化が著しくなる。このため、Nは0.0040%以下に限定した。
N: 0.0040% or less N is an element that dissolves in steel, causes strain aging after cold working, and degrades toughness. In the present invention, N is desirably reduced as much as possible. When the content exceeds 0.0040%, the toughness deteriorates remarkably. For this reason, N was limited to 0.0040% or less.

Nb:0.005〜0.030%
Nbは、焼入れ性を向上させ、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、0.005%以上の含有を必要とする。一方、0.030%を超えて含有すると、HAZ靭性および母材靭性が低下する。このため、Nbは0.005〜0.030%の範囲に限定した。なお、好ましくはNb:0.010〜0.02%である。
Nb: 0.005 to 0.030%
Nb is an element that improves hardenability and increases strength. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if it exceeds 0.030%, HAZ toughness and base metal toughness are lowered. For this reason, Nb was limited to 0.005 to 0.030% of range. In addition, Preferably it is Nb: 0.010-0.02%.

V:0.001〜0.070%
Vは、析出強化を介して、強度を増加させる有効な元素である。このような効果を得るためには、0.001%以上の含有を必要とする。一方、0.070%を超えて含有すると、HAZ靭性および母材靭性が低下する。このため、Vは0.001〜0.070%の範囲に限定した。なお、好ましくは0.005〜0.060%である。
V: 0.001 to 0.070%
V is an effective element that increases the strength through precipitation strengthening. In order to acquire such an effect, 0.001% or more of content is required. On the other hand, if it exceeds 0.070%, HAZ toughness and base metal toughness are lowered. For this reason, V was limited to 0.001 to 0.070% of range. In addition, Preferably it is 0.005-0.060%.

Cr:0.30%以下
Crは、焼入性向上を介し、母材の強度を増加させる元素であり、厚鋼板の高強度化に有用な元素である。このような効果を得るためには、0.05%以上含有することが望ましい。しかしながら、0.30%を超える含有は、合金コストの増加を招く。このため、Crは0.30%以下の範囲に限定した。
Cr: 0.30% or less Cr is an element that increases the strength of the base material through the improvement of hardenability, and is an element useful for increasing the strength of the thick steel plate. In order to acquire such an effect, it is desirable to contain 0.05% or more. However, the content exceeding 0.30% causes an increase in alloy cost. For this reason, Cr was limited to the range of 0.30% or less.

さらに、Ti、Nは上記した含有量の範囲内で、かつ(1)式を満足するように含有する。   Further, Ti and N are contained within the above-described content range so as to satisfy the formula (1).

4.0≧Ti/N≧2.0 ……(1)
本発明では、Tiは、N含有量(質量%)に見合う量を含有させ、固溶NをTiNとして固定する。このため、Ti含有量(質量%)とN含有量(質量%)との比、Ti/Nが2.0以上を満足するように、Ti含有量(質量%)、N含有量(質量%)を調整する。Ti/Nが2.0未満では、N含有量に比べてTi含有量が少なすぎるため、多くのNが固溶Nとして残存する。そのため、HAZ靭性が低下し、溶接部からの脆性破壊発生により部材変形性能が低下する場合がある。一方、Ti/Nが4.0を超えて大きくなると、TiN粒子が粗大化して、所望の効果を確保できなくなる。このため、Ti/Nは2.0〜4.0の範囲に限定した。なお、好ましくは、2.5〜3.5の範囲である。
4.0 ≧ Ti / N ≧ 2.0 (1)
In the present invention, Ti is contained in an amount commensurate with the N content (mass%), and solid solution N is fixed as TiN. Therefore, the ratio of Ti content (mass%) to N content (mass%), Ti content (mass%), N content (mass%) so that Ti / N satisfies 2.0 or more. ). When Ti / N is less than 2.0, the Ti content is too small compared to the N content, so that much N remains as solid solution N. Therefore, the HAZ toughness is lowered, and the member deformation performance may be lowered due to the occurrence of brittle fracture from the weld. On the other hand, when Ti / N exceeds 4.0, the TiN particles become coarse and the desired effect cannot be ensured. For this reason, Ti / N was limited to the range of 2.0 to 4.0. In addition, Preferably, it is the range of 2.5-3.5.

また、本発明では、上記した各合金元素は、上記した含有量の範囲内でかつ、炭素当量Ceqで0.35〜0.50を満足するように含有する。なお、炭素当量Ceqは、(2)式で定義される。   Moreover, in this invention, each above-mentioned alloy element is contained so that it may satisfy 0.35-0.50 by carbon equivalent Ceq within the range of above-mentioned content. The carbon equivalent Ceq is defined by the formula (2).

Ceq=C+Si/24+Mn/6+Cr/5+Mo/4+V/14 ……(2)
ここで、C、Si、Mn、Cr、Mo、V:各元素の含有量(質量%)
なお、炭素当量Ceqを算出する際に、(2)式に記載された元素で含有しないものは、零として計算するものとする。
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Mo / 4 + V / 14 (2)
Here, C, Si, Mn, Cr, Mo, V: Content of each element (mass%)
In addition, when calculating carbon equivalent Ceq, what is not contained by the element described in Formula (2) shall be calculated as zero.

炭素当量Ceqが、0.35未満では、所望の母材強度を確保できないうえ、溶接熱影響部の軟化を所望の許容限度内に抑えることができない。一方、Ceqが、0.50を超えて高くなると、溶接性が低下するとともに、母材靭性、HAZ靭性が低下する。このため、Ceqは0.35〜0.50の範囲に限定した。   When the carbon equivalent Ceq is less than 0.35, a desired base material strength cannot be ensured, and softening of the weld heat affected zone cannot be suppressed within a desired allowable limit. On the other hand, when Ceq is higher than 0.50, the weldability is lowered and the base metal toughness and the HAZ toughness are lowered. For this reason, Ceq was limited to the range of 0.35-0.50.

上記した成分が基本の成分であるが、基本の組成に加えてさらに、選択元素として、Mo:0.60%以下、Cu:0.05〜0.60%、Ni:0.05〜0.80%、B:0.0003〜0.0030%のうちから選ばれた1種または2種以上を含有できる。Mo、Cu、Ni、Bはいずれも、鋼の強度を増加させる作用を有する元素であり、必要に応じて選択して含有できる。   The above-described components are basic components. In addition to the basic composition, Mo: 0.60% or less, Cu: 0.05-0.60%, Ni: 0.05-0. One or two or more selected from 80% and B: 0.0003 to 0.0030% can be contained. Mo, Cu, Ni, and B are all elements that have an action of increasing the strength of steel, and can be selected and contained as necessary.

Mo:0.60%以下
Moは、靭性の向上と強度の増加に寄与する有効な元素である。しかし、0.60%を超えて多量に含有すると、溶接性や耐HIC性が低下する。このため、Moは0.60%以下に限定した。なお、好ましくは0.50%以下である。なお、上記の効果を得るため、Moを含有するときは、0.10%以上とすることが好ましい。
Mo: 0.60% or less Mo is an effective element that contributes to improved toughness and increased strength. However, when it contains a large amount exceeding 0.60%, weldability and HIC resistance will fall. For this reason, Mo was limited to 0.60% or less. In addition, Preferably it is 0.50% or less. In addition, in order to acquire said effect, when it contains Mo, it is preferable to set it as 0.10% or more.

Cu:0.05〜0.60%
Cu固溶強化や焼入性向上を介して、鋼板の強度を増加させ、厚鋼板の高強度化に寄与する。このような効果を得るためには、0.05%以上含有することが好ましいが、0.60%を超える含有は、合金コストの増加や熱間脆性による表面性状の劣化を招く.このため、含有する場合には、Cuは0.05〜0.60%の範囲に限定することが好ましい。なお、より好ましくは0.10〜0.40%である。
Cu: 0.05 to 0.60%
Through Cu solid solution strengthening and hardenability improvement, the strength of the steel plate is increased, which contributes to increasing the strength of the thick steel plate. In order to obtain such an effect, the content is preferably 0.05% or more. However, the content exceeding 0.60% causes an increase in alloy cost and deterioration of surface properties due to hot brittleness. For this reason, when it contains, it is preferable to limit Cu to 0.05 to 0.60% of range. In addition, More preferably, it is 0.10 to 0.40%.

Ni:0.05〜0.80%
Niは、靱性をほとんど劣化させることなく、鋼板の強度を増加させる元素である。しかも、HAZ靱性への悪影響も小さく、厚鋼板の高強度化に有用な元素である。このような効果を得るためには,0.05%以上含有することが好ましい。しかしながら、0.80%を超える多量の含有は、Niが高価な元素であるため、合金コストの増加を招く。このため、含有する場合は、Niは0.05〜0.80%の範囲に限定することが好ましい。なお、より好ましくは0.10〜0.70%である。
Ni: 0.05 to 0.80%
Ni is an element that increases the strength of the steel sheet with almost no deterioration in toughness. Moreover, it has little adverse effect on HAZ toughness and is an element useful for increasing the strength of thick steel plates. In order to acquire such an effect, it is preferable to contain 0.05% or more. However, a large content exceeding 0.80% causes an increase in alloy cost because Ni is an expensive element. For this reason, when it contains, it is preferable to limit Ni to 0.05 to 0.80% of range. In addition, More preferably, it is 0.10 to 0.70%.

B:0.0003〜0.0030%
Bは焼入れ性の向上を介し、鋼の強度増加に寄与する元素である。このような効果を得るために、0.0003%以上含有することが好ましい。しかしながら、0.0030%を超える含有は、母材やHAZの靭性を劣化させる。このため、含有する場合は、Bは0.0003%〜0.0030%の範囲に限定することが好ましい。なお、より好ましくは0.0006〜0.0020%である。
B: 0.0003 to 0.0030%
B is an element that contributes to an increase in the strength of steel through the improvement of hardenability. In order to acquire such an effect, it is preferable to contain 0.0003% or more. However, the content exceeding 0.0030% deteriorates the toughness of the base material and the HAZ. For this reason, when contained, B is preferably limited to a range of 0.0003% to 0.0030%. In addition, More preferably, it is 0.0006 to 0.0020%.

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.

次に、本発明では、鋼板表面から板厚方向に5mmの位置までの範囲における組織が、面積率で30〜70%のポリゴナルフェライト相と、残部がベイナイト相またはマルテンサイト相、あるいはその混合相であり、鋼板表面から板厚方向に5mmの位置から板厚中央位置までの範囲における組織が、面積率が30%以下のポリゴナルフェライト相と、残部がベイナイト相またはパーライト相、あるいはその混合相からなる組織を有する。   Next, in the present invention, the structure in the range from the steel sheet surface to the position of 5 mm in the thickness direction is a polygonal ferrite phase with an area ratio of 30 to 70%, and the balance is a bainite phase or a martensite phase, or a mixture thereof. The structure in the range from the 5 mm position in the sheet thickness direction to the sheet thickness center position is the polygonal ferrite phase with an area ratio of 30% or less and the balance is the bainite phase or pearlite phase, or a mixture thereof. It has a structure consisting of phases.

本発明では、鋼板表面から板厚方向に5mmの位置までの範囲における組織が、面積率で30〜70%のポリゴナルフェライト相と、残部がベイナイト相またはマルテンサイト相、あるいはその混合相とする。鋼板表面から板厚方向に5mmの位置までの範囲において、ポリゴナルフェライト相を生成させ、表層を軟化させる。その結果、表面から板厚方向に1mmの位置におけるビッカース硬さHVを260HV以下にすることが可能となり、曲げ加工性が顕著に向上する。ポリゴナルフェライト相が30%未満では、ビッカース硬さHVを260HV以下にできず、所望の曲げ加工性の向上が期待できない。一方、ポリゴナルフェライト相が70%を超えて多量になると、所望の高強度を維持することが難しくなる。このため、鋼板表面から板厚方向に5mmの位置までの範囲におけるポリゴナルフェライト相は、30〜70%に限定した。なお、好ましくは40〜60%である。   In the present invention, the structure in the range from the steel sheet surface to the position of 5 mm in the sheet thickness direction is a polygonal ferrite phase with an area ratio of 30 to 70%, and the balance is a bainite phase or a martensite phase, or a mixed phase thereof. . In the range from the steel plate surface to the position of 5 mm in the plate thickness direction, a polygonal ferrite phase is generated and the surface layer is softened. As a result, the Vickers hardness HV at a position of 1 mm from the surface in the plate thickness direction can be made 260 HV or less, and the bending workability is remarkably improved. If the polygonal ferrite phase is less than 30%, the Vickers hardness HV cannot be reduced to 260 HV or less, and improvement in desired bending workability cannot be expected. On the other hand, when the polygonal ferrite phase is more than 70%, it becomes difficult to maintain a desired high strength. For this reason, the polygonal ferrite phase in the range from the steel plate surface to the position of 5 mm in the plate thickness direction is limited to 30 to 70%. In addition, Preferably it is 40 to 60%.

また、鋼板表面から板厚方向に5mmの位置から板厚中央位置までの範囲における組織は、面積率が30%以下のポリゴナルフェライト相と、残部がベイナイト相またはパーライト相、あるいはその混合相からなる。ポリゴナルフェライト相が30%を超えると所望の強度が得られない。このため、ポリゴナルフェライト相は30%以下に限定した。なお、好ましくは20%以下である。   Further, the structure in the range from the position of 5 mm in the sheet thickness direction to the sheet thickness center position from the surface of the steel sheet is composed of a polygonal ferrite phase with an area ratio of 30% or less and the balance from a bainite phase or a pearlite phase, or a mixed phase thereof. Become. If the polygonal ferrite phase exceeds 30%, the desired strength cannot be obtained. For this reason, the polygonal ferrite phase is limited to 30% or less. In addition, Preferably it is 20% or less.

本発明の厚鋼板は、上記した組成、組織を有し、降伏比80%以下の低降伏比と、引張強さ590〜740MPaの高強度を有する、曲げ加工性に優れた非調質低降伏比高張力厚鋼板である。   The steel plate of the present invention has the above-described composition and structure, has a low yield ratio of 80% or less, and a high strength of 590 to 740 MPa in tensile strength. It is a specific high tension steel plate.

つぎに、本発明の厚鋼板の製造方法について説明する。   Below, the manufacturing method of the thick steel plate of this invention is demonstrated.

本発明の厚鋼板の製造方法では、上記した組成の鋼素材を加熱する加熱工程と、熱間圧延を施し厚鋼板とする熱間圧延工程と、厚鋼板に第一段冷却と第二段冷却とからなる二段階の冷却工程とを施す。   In the method for producing a thick steel plate according to the present invention, a heating step for heating the steel material having the above composition, a hot rolling step for performing hot rolling to obtain a thick steel plate, first-stage cooling and second-stage cooling for the thick steel plate. And a two-stage cooling process comprising:

鋼素材の製造方法については、とくに限定する必要はないが、上記した組成の溶鋼を転炉、電気炉等の常用の溶製方法で溶製し、連続鋳造法等の常用の鋳造方法で所定寸法の鋳片(鋼素材)とすることが好ましい。なお、鋳片にさらに熱間圧延を施して、所望の寸法形状の鋼片としてもよい。また、造塊−分塊圧延法により鋼片(鋼素材)としてもよいことはいうまでもない。   The method for producing the steel material is not particularly limited, but the molten steel having the above-described composition is melted by a conventional melting method such as a converter or an electric furnace, and predetermined by a conventional casting method such as a continuous casting method. It is preferable to use a slab of a size (steel material). The cast slab may be further subjected to hot rolling to obtain a steel slab having a desired size and shape. Needless to say, the steel slab (steel material) may be obtained by ingot-making and ingot rolling.

次に、上記した組成の鋼素材を加熱する加熱工程を行う。加熱工程では、鋼素材を加熱温度:1000〜1250℃に加熱する。   Next, the heating process which heats the steel raw material of an above-described composition is performed. In the heating step, the steel material is heated to a heating temperature of 1000 to 1250 ° C.

加熱温度:1000〜1250℃
加熱温度が1000℃未満では、焼入れ性が低下し、所望の高強度を確保することができない。一方、1250℃を超えて高温となると、結晶粒が粗大化し、靭性の劣化を招く。このため、鋼素材の加熱温度は1000〜1250℃の範囲の温度に限定した。なお、好ましくは1080〜1150℃である。
Heating temperature: 1000-1250 ° C
If heating temperature is less than 1000 degreeC, hardenability will fall and desired high intensity | strength cannot be ensured. On the other hand, when the temperature exceeds 1250 ° C., the crystal grains become coarse and the toughness is deteriorated. For this reason, the heating temperature of the steel material was limited to a temperature in the range of 1000 to 1250 ° C. In addition, Preferably it is 1080-1150 degreeC.

次いで、加熱された鋼素材に、熱間圧延を行う。   Next, hot rolling is performed on the heated steel material.

熱間圧延工程は、加熱された鋼素材に、表面温度で950℃以下の温度域での累積圧下率が30%以上で、圧延終了温度が表面温度で900℃以下Ar変態点以上となる熱間圧延を行い、厚鋼板とする工程である。 In the hot rolling process, the cumulative reduction ratio in the temperature range of 950 ° C. or less at the surface temperature is 30% or more and the rolling end temperature is 900 ° C. or less and the Ar 3 transformation point or more in the surface temperature. This is a step of hot rolling to obtain a thick steel plate.

表面温度で950℃以下の温度域での累積圧下率が30%未満では、圧下量の不足により鋼組織が粗大化し、靭性が劣化するため、表面温度で950℃以下の温度域での累積圧下率は30%以上に限定した。なお、好ましくは35〜45%である。   If the cumulative reduction ratio in the temperature range of 950 ° C or less at the surface temperature is less than 30%, the steel structure becomes coarse due to insufficient reduction and the toughness deteriorates. Therefore, the cumulative reduction in the temperature range of 950 ° C or less at the surface temperature. The rate was limited to 30% or more. In addition, Preferably it is 35 to 45%.

また、圧延終了温度が表面温度で900℃を超えると、圧延終了温度が高すぎて、鋼組織が粗大化し、靭性が低下する。一方、圧延終了温度がAr変態点未満では、圧延中に粗大なフェライトが生成し、所望の高強度を確保できなくなる。 Moreover, when rolling end temperature exceeds 900 degreeC by surface temperature, rolling end temperature is too high, steel structure coarsens and toughness falls. On the other hand, if the rolling end temperature is less than the Ar 3 transformation point, coarse ferrite is generated during rolling, and a desired high strength cannot be ensured.

なお、Ar変態点は、下記式を用いて算出した値を用いるものとする。 As the Ar 3 transformation point, a value calculated using the following formula is used.

Ar変態点(℃)=900−332C+6Si−77Mn−20Cu−50Ni−18Cr−68Mo
ここで、C、Si、Mn、Cu、Ni、Cr、Mo:各元素の含有量(質量%)
ついで、熱間圧延工程を経た厚鋼板に、冷却工程を施す。
Ar 3 transformation point (° C.) = 900-332C + 6Si-77Mn-20Cu-50Ni-18Cr-68Mo
Here, C, Si, Mn, Cu, Ni, Cr, Mo: Content of each element (mass%)
Next, a cooling step is performed on the thick steel plate that has undergone the hot rolling step.

冷却工程は、第一段冷却と、冷却を停止し復熱させる過程と、第二段冷却とからなる二段階の冷却工程である。本発明では、第一段冷却で、表層部を過冷却したのち復熱させ、第二段冷却の開始までの時間(冷却停止時間)を利用して、表層部のフェライト変態を進行させて所望の表層ミクロ組織を得る。   The cooling process is a two-stage cooling process consisting of a first stage cooling, a process of stopping cooling and recovering heat, and a second stage cooling. In the present invention, in the first stage cooling, the surface layer part is supercooled and then reheated, and the time until the start of the second stage cooling (cooling stop time) is used to advance the ferrite transformation of the surface layer part, and this is desired. To obtain a surface microstructure.

第一段冷却は、表面温度でAr変態点以上の温度から冷却を開始し、板厚1/4位置の平均冷却速度が2℃/s以上で加速冷却し、表面温度が750℃以下550℃以上となる時点で、加速冷却を停止する。 In the first stage cooling, the cooling is started from the surface temperature at the Ar 3 transformation point or higher, the average cooling rate at the thickness 1/4 position is 2 ° C./s or higher, and the surface temperature is 750 ° C. or lower 550 ° C. Accelerated cooling is stopped when the temperature exceeds ℃.

第一段冷却の開始温度:表面温度でAr変態点以上
第一段冷却の開始温度が、Ar変態点未満では、加速冷却開始前に鋼板全体に粗大なポリゴナルフェライトが生成し、所望の強度を満足できなくなる。このため、第一段冷却の開始温度をAr変態点以上に限定した。
First stage cooling start temperature: Ar 3 transformation point or higher at the surface temperature If the first stage cooling start temperature is less than the Ar 3 transformation point, coarse polygonal ferrite is generated on the entire steel plate before the start of accelerated cooling. Can no longer satisfy the strength. For this reason, the start temperature of the first stage cooling was limited to the Ar 3 transformation point or higher.

第一段冷却の冷却速度:板厚1/4位置の平均冷却速度が2℃/s以上
板厚1/4位置の平均冷却速度が2℃/s未満では、冷却が遅く、ベイナイト、パーライト、マルテンサイト等の硬質相が不十分となるため、所望の強度を満足できない。このため、板厚1/4位置の平均冷却速度が2℃/s以上に限定した。ここでいう「板厚1/4位置の平均冷却速度」とは、板厚1/4位置における加速冷却開始から終了までの平均の冷却速度をいう。
Cooling rate of the first stage cooling: The average cooling rate at the thickness 1/4 position is 2 ° C./s or more. When the average cooling rate at the thickness 1/4 position is less than 2 ° C./s, the cooling is slow, bainite, pearlite, Since the hard phase such as martensite becomes insufficient, the desired strength cannot be satisfied. For this reason, the average cooling rate at the position of the plate thickness ¼ is limited to 2 ° C./s or more. The “average cooling rate at the position of the plate thickness ¼” here means an average cooling rate from the start to the end of the accelerated cooling at the position of the plate thickness ¼.

第一段冷却の冷却停止温度:表面温度で750℃以下550℃以上
本発明における第一段冷却では、表層部(鋼板表面から板厚方向に5mmの位置までの範囲)とそれより内部との温度差が大きくなるように冷却し、第一段冷却停止後の復熱と、復熱後、第二段冷却を開始するまでの時間に、表層部にポリゴナルフェライトを生成させる。冷却を停止する温度が、表面温度で750℃を超える温度では、その後の復熱温度が高すぎて、表層部におけるポリゴナルフェライト生成が不十分となる。一方、冷却を停止する温度が表面温度で550℃未満では、表層部の温度が低温となりすぎて、冷却中にベイナイトやマルテンサイトなどの硬質相への相変態がほぼ完了してしまい、表層部において所定量のポリゴナルフェライト組織が得られない。
Cooling stop temperature of the first stage cooling: 750 ° C. or less and 550 ° C. or more at the surface temperature In the first stage cooling in the present invention, the surface layer portion (range from the steel plate surface to the position of 5 mm in the plate thickness direction) and the inside thereof Cooling is performed so that the temperature difference becomes large, and polygonal ferrite is generated in the surface layer portion at the time of recuperation after stopping the first-stage cooling and after the recuperation until starting the second-stage cooling. When the temperature at which the cooling is stopped exceeds the surface temperature of 750 ° C., the subsequent recuperation temperature is too high, and the formation of polygonal ferrite in the surface layer becomes insufficient. On the other hand, if the temperature at which cooling is stopped is less than 550 ° C. at the surface temperature, the temperature of the surface layer portion becomes too low, and the phase transformation to a hard phase such as bainite or martensite is almost completed during cooling, and the surface layer portion Thus, a predetermined amount of polygonal ferrite structure cannot be obtained.

このようなことから、第一段冷却の冷却停止温度を、表面温度で750℃以下550℃以上の範囲に限定した。なお、好ましくは700〜560℃である。   For this reason, the cooling stop temperature of the first stage cooling was limited to a range of 750 ° C. or lower and 550 ° C. or higher as the surface temperature. In addition, Preferably it is 700-560 degreeC.

第一段冷却停止後の復熱温度:表面温度で750℃以下600℃以上
本発明では、上記した第一段冷却を停止したのち、復熱し、厚鋼板の表面温度が所定の温度まで達した後、第二段冷却を開始する。復熱は、表面温度で750℃以下600℃以上となる時点まで行う。
Recuperation temperature after stopping the first stage cooling: 750 ° C. or less and 600 ° C. or more at the surface temperature In the present invention, after the first stage cooling is stopped, the heat is recovered and the surface temperature of the thick steel plate reaches a predetermined temperature. Then, the second stage cooling is started. Reheating is performed until the surface temperature reaches 750 ° C. or lower and 600 ° C. or higher.

本発明では、復熱中あるいは復熱後、第二段冷却を開始するまでの時間に、鋼板表層部にポリゴナルフェライトを生成させるため、復熱後の鋼板温度、すなわち第二段冷却の冷却開始温度が、ポリゴナルフェライトの生成量を制御する観点から重要な因子となる。   In the present invention, in order to generate polygonal ferrite in the steel sheet surface layer portion during recuperation or after reheating until the start of second-stage cooling, the steel plate temperature after reheating, that is, cooling of the second-stage cooling is started. The temperature is an important factor from the viewpoint of controlling the amount of polygonal ferrite produced.

復熱温度が、表面温度で600℃未満では、表層部において強度および降伏比が比較的高い針状フェライト(アシキュラーフェライト)やベイナイトが生成してしまい、表層部の硬度が上昇し、曲げ特性の低下を招く。また、復熱温度が、表面温度で750℃を超えると、鋼板表層部におけるポリゴナルフェライトの生成量が不十分となる。このようなことから、復熱温度、すなわち第二段冷却の冷却開始温度を、表面温度で750℃以下600℃以上の範囲に限定した。   If the recuperation temperature is less than 600 ° C. at the surface temperature, acicular ferrite (acicular ferrite) and bainite having relatively high strength and yield ratio are formed in the surface layer portion, the hardness of the surface layer portion is increased, and bending characteristics are increased. Cause a decline. On the other hand, when the recuperation temperature exceeds 750 ° C. as the surface temperature, the amount of polygonal ferrite produced in the steel sheet surface layer becomes insufficient. For this reason, the recuperation temperature, that is, the cooling start temperature of the second stage cooling, was limited to the range of 750 ° C. or lower and 600 ° C. or higher as the surface temperature.

ここで、本発明では、第一段冷却から復熱までの工程を複数回繰り返す冷却としてもよい。加速冷却を複数回に分割することにより、表層と内部との温度差を、過度に大きくすることなく、目的の温度まで冷却することが可能となる。これにより、冷却温度制御の選択肢が拡大でき、冷却温度の制御が容易となる。   Here, in the present invention, the process from the first stage cooling to the recuperation may be repeated a plurality of times. By dividing the accelerated cooling into a plurality of times, it is possible to cool to the target temperature without excessively increasing the temperature difference between the surface layer and the inside. Thereby, the choice of cooling temperature control can be expanded and control of cooling temperature becomes easy.

第一段冷却工程から復熱までの工程を複数回繰り返す場合は、2回以上繰り返すことが好ましい。第一段冷却工程から復熱までの工程を1回もしくは複数回繰り返した後、第二段冷却を開始する。   When the steps from the first stage cooling step to the recuperation are repeated a plurality of times, it is preferable to repeat the steps two or more times. After the process from the first stage cooling process to the recuperation is repeated once or a plurality of times, the second stage cooling is started.

本発明では、第一段冷却後、表面温度で750℃以下600℃以上に復熱したのち、第二段冷却を開始する。第二段冷却は、板厚1/4位置の平均冷却速度が2℃/s以上で加速冷却し、加速冷却を停止した後の復熱で表面温度が500℃以下になるまで行う。   In the present invention, after the first stage cooling, after the surface temperature is reheated to 750 ° C. or lower and 600 ° C. or higher, the second stage cooling is started. The second-stage cooling is performed until the average cooling rate at the ¼ thickness position is 2 ° C./s or higher, and the surface temperature is reduced to 500 ° C. or lower by reheating after the accelerated cooling is stopped.

第二段冷却により、第一段冷却後に未変態のオーステナイト組織を、パーライト、ベイナイト、マルテンサイト等の硬質相に変態させることができ、所望の高強度、低降伏比を実現できる。冷却停止後の復熱で表面温度が500℃を超えると、所望の強度を得ることができない。   By the second stage cooling, the untransformed austenite structure can be transformed into a hard phase such as pearlite, bainite, martensite, etc. after the first stage cooling, and a desired high strength and low yield ratio can be realized. If the surface temperature exceeds 500 ° C. due to recuperation after cooling stops, the desired strength cannot be obtained.

第二段冷却の冷却速度:板厚1/4位置の平均冷却速度が2℃/s以上
未変態のオーステナイト組織を硬質相とするために、第二段冷却では、板厚1/4位置の平均冷却速度が2℃/s以上、好ましくは8℃/s以上で加速冷却する。板厚1/4位置の平均冷却速度が2℃/s未満では、硬質相への変態量が低下し、所望の高強度、低降伏比を実現できなくなる。
Cooling rate of the second stage cooling: The average cooling rate at the thickness 1/4 position is 2 ° C./s or more. In order to make the untransformed austenite structure a hard phase, Accelerated cooling is performed at an average cooling rate of 2 ° C./s or higher, preferably 8 ° C./s or higher. If the average cooling rate at the 1/4 position of the plate thickness is less than 2 ° C./s, the amount of transformation to the hard phase decreases, and the desired high strength and low yield ratio cannot be realized.

第二段冷却の冷却停止温度:加速冷却を停止した後の復熱で表面温度が500℃以下になる温度
第二段冷却の冷却停止温度が、第二段冷却の冷却停止後の復熱で表面温度が500℃超えとなるような温度では、硬質相への変態量が低下したり、自己焼戻しによって強度が低下し、所望の高強度を確保できなくなる。このため、第二段冷却の冷却停止温度は、冷却を停止した後の復熱で表面温度が500℃以下になる温度に限定した。
Cooling stop temperature of the second stage cooling: The temperature at which the surface temperature becomes 500 ° C or less by recuperation after stopping the accelerated cooling. The cooling stop temperature of the second stage cooling is the reheat after the cooling stop of the second stage cooling. If the surface temperature exceeds 500 ° C., the amount of transformation to the hard phase decreases, or the strength decreases due to self-tempering, and the desired high strength cannot be ensured. For this reason, the cooling stop temperature of the second stage cooling is limited to a temperature at which the surface temperature becomes 500 ° C. or lower by recuperation after stopping the cooling.

なお、上記した冷却工程後、必要に応じて、強度および靭性の調整を目的として、焼戻工程を施してもよい。焼戻工程は、400℃以上700℃以下の焼戻温度で行うことが好ましい。焼戻温度が400℃未満では、所望の効果を期待できない。一方、700℃を超える焼戻温度では、強度低下が著しくなる。   In addition, you may give a tempering process for the purpose of intensity | strength and toughness adjustment as needed after the above-mentioned cooling process. The tempering step is preferably performed at a tempering temperature of 400 ° C. or higher and 700 ° C. or lower. If the tempering temperature is less than 400 ° C., the desired effect cannot be expected. On the other hand, at a tempering temperature exceeding 700 ° C., the strength is significantly reduced.

以下、実施例に基づき、さらに本発明について説明する。   Hereinafter, based on an Example, this invention is demonstrated further.

表1に示す組成で得られた鋼素材を、表2に示す加熱温度に加熱する加熱工程と、表2に示す条件の熱間圧延を施す熱間圧延工程と、表2に示す条件の加速冷却を施す冷却工程と、を順次施し、表2に示す板厚の非調質厚鋼板とした。なお、表2に記載の累積圧下率は表面温度で950℃以下の温度域での累積圧下率である。   A heating process for heating the steel material obtained with the composition shown in Table 1 to the heating temperature shown in Table 2, a hot rolling process for performing hot rolling under the conditions shown in Table 2, and acceleration of the conditions shown in Table 2 A cooling process for performing cooling was sequentially performed to obtain a non-tempered thick steel plate having a thickness shown in Table 2. In addition, the cumulative reduction rate described in Table 2 is the cumulative reduction rate in a temperature range of 950 ° C. or less at the surface temperature.

得られた各厚鋼板から、試験片を採取し、組織観察、引張試験、曲げ試験、硬さ試験を実施した。試験方法は次のとおりである。
(1)組織観察
得られた厚鋼板から、観察面が圧延方向に垂直な断面(板厚方向断面)となるように組織観察用試験片を採取し、鏡面となるまで研磨した後、腐食液(3%硝酸メタノール溶液)で腐食し、光学顕微鏡(倍率:400倍)を用いて、鋼板表面から板厚方向に板厚中央位置まで観察し、ミクロ組織が連続するように撮像した。得られた組織写真を用い、画像解析により、相の同定、および組織分率を算出した。
(2)引張特性
得られた厚鋼板の板厚1/4位置から、引張方向が圧延方向に対して90°方向(C方向)となるように、JIS 4号引張試験片を採取し、JIS Z 2241の規定に準拠してクロスヘッド速度10mm/minで引張試験をおこない、引張特性(降伏強さYS、引張強さTS)を測定した。なお、引張特性から、降伏比YR(=YS/TS×100%)を算出した。
(3)曲げ試験
得られた厚鋼板から、曲げ試験片(幅50mm×長さ500mm)を採取し、図1に示す要領で90°曲げ試験を実施し、必要とした荷重Pe(kN)を求めた。得られた荷重Peと、厚鋼板の引張強さTSから、次式
Pc=1.6×鋼板TS(MPa)×W×t×1/L×1/1000
(ここで、W:曲げ試験片幅(=50mm、図1の紙面垂直方向)、t:板厚(mm)、L:支点距離(=400mm))
を用いて算出した90°曲げに必要な必要荷重Pc(kN)を求め、Pe/Pcで曲げ加工性を評価した。なお、Pe/Pcが0.80以下である場合を曲げ加工性に優れると評価した。
(4)硬さ試験
得られた厚鋼板から、硬さ試験片を採取し、鋼板表面から板厚方向に1mmの位置において、ビッカース硬度計(荷重:98.07N)を用いて、ビッカース硬さHVを測定した。なお、硬さ測定は、1mm間隔で5点以上測定し、その平均値をビッカース硬さHVとした。
Test pieces were sampled from each of the obtained thick steel plates and subjected to structure observation, tensile test, bending test, and hardness test. The test method is as follows.
(1) Microstructure observation A specimen for microstructural observation was collected from the obtained thick steel plate so that the observation surface had a cross section perpendicular to the rolling direction (cross section in the plate thickness direction), polished to a mirror surface, and then corroded. The sample was corroded with (3% nitric acid methanol solution), and observed using an optical microscope (magnification: 400 times) from the steel plate surface to the plate thickness center position in the plate thickness direction, and imaged so that the microstructure was continuous. Using the obtained tissue photograph, phase identification and tissue fraction were calculated by image analysis.
(2) Tensile properties JIS No. 4 tensile test specimens were sampled from the position of 1/4 thickness of the obtained thick steel plate so that the tensile direction was 90 ° direction (C direction) with respect to the rolling direction. A tensile test was performed at a crosshead speed of 10 mm / min in accordance with the provision of Z2241, and tensile properties (yield strength YS, tensile strength TS) were measured. The yield ratio YR (= YS / TS × 100%) was calculated from the tensile properties.
(3) Bending test A bending test piece (width 50 mm x length 500 mm) was sampled from the obtained thick steel plate, subjected to a 90 ° bending test in the manner shown in Fig. 1, and a required load Pe (kN) was obtained. Asked. From the obtained load Pe and the tensile strength TS of the thick steel plate, the following formula: Pc = 1.6 × steel plate TS (MPa) × W × t 2 × 1 / L × 1/1000
(W: bending test specimen width (= 50 mm, vertical direction in FIG. 1), t: plate thickness (mm), L: fulcrum distance (= 400 mm))
The required load Pc (kN) required for the 90 ° bending calculated using was calculated, and the bending workability was evaluated by Pe / Pc. In addition, it evaluated that the case where Pe / Pc was 0.80 or less was excellent in bending workability.
(4) Hardness test A hardness test piece was taken from the obtained thick steel plate, and the Vickers hardness was measured using a Vickers hardness meter (load: 98.07 N) at a position of 1 mm from the steel plate surface in the thickness direction. HV was measured. In addition, hardness measurement measured 5 or more points at 1 mm intervals, and made the average value Vickers hardness HV.

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

本発明例はいずれも、所望の組織を有し、鋼板表層の硬さが低く、80%以下の低降伏比で、所望の高強度(TS:590〜740MPa)を有し、かつ鋼板表層の硬さが260HV以下と低く、Pe/Pcが0.8以下で曲げ加工性に優れた非調質低降伏比高張力厚鋼板となっている。一方、本発明の範囲から外れる比較例は、所望の高強度が得られていないか、表層の硬さが高いため、あるいは所望の組織が得られていないため、曲げ加工性が低下している。   Each of the inventive examples has a desired structure, the steel sheet surface layer has a low hardness, a low yield ratio of 80% or less, a desired high strength (TS: 590 to 740 MPa), and the steel sheet surface layer It is a non-tempered low yield ratio high-tensile thick steel plate having a low hardness of 260 HV or less, Pe / Pc of 0.8 or less and excellent bending workability. On the other hand, in comparative examples that are out of the scope of the present invention, the desired high strength is not obtained, the hardness of the surface layer is high, or the desired structure is not obtained, so the bending workability is lowered. .

1 鋼材サンプル
2 押金具
1 Steel material sample 2 Press fitting

Claims (4)

質量%で、C:0.05〜0.10%、Si:0.01〜0.45%、Mn:0.60〜1.80%、P:0.020%以下、S:0.003%以下、Al:0.05%以下、Ti:0.005〜0.020%、N:0.0040%以下、Nb:0.005〜0.030%、V:0.001〜0.070%、Cr:0.30%以下を含み、Ti、Nが下記(1)式を満足し、かつ下記(2)式で定義される炭素当量Ceqが0.35〜0.50を満足し、残部Feおよび不可避的不純物からなる組成であり、
鋼板表面から板厚方向に5mmの位置までの範囲における組織が、面積率で30〜70%のポリゴナルフェライト相と、残部がベイナイト相またはマルテンサイト相、あるいはその混合相からなり、鋼板表面から板厚方向に5mmの位置から板厚中央位置までの範囲における組織が、面積率が30%以下のポリゴナルフェライト相と、残部がベイナイト相またはパーライト相、あるいはその混合相からなり、鋼板表面から板厚方向に1mmの位置におけるビッカース硬さHVが260HV以下、引張強さ590〜740MPa、降伏比80%以下であることを特徴とする曲げ加工性に優れた非調質低降伏比高張力厚鋼板。

4.0≧Ti/N≧2.0 ……(1)
ここで、Ti、N:各元素の含有量(質量%)
Ceq=C+Si/24+Mn/6+Cr/5+Mo/4+V/14 ……(2)
ここで、C、Si、Mn、Cr、Mo、V:各元素の含有量(質量%)
In mass%, C: 0.05 to 0.10%, Si: 0.01 to 0.45%, Mn: 0.60 to 1.80%, P: 0.020% or less, S: 0.003 %: Al: 0.05% or less, Ti: 0.005-0.020%, N: 0.0040% or less, Nb: 0.005-0.030%, V: 0.001-0.070 %, Cr: 0.30% or less, Ti and N satisfy the following formula (1), and the carbon equivalent Ceq defined by the following formula (2) satisfies 0.35 to 0.50, The composition consists of the balance Fe and inevitable impurities,
The structure in the range from the steel sheet surface to the position of 5 mm in the sheet thickness direction is composed of a polygonal ferrite phase with an area ratio of 30 to 70%, and the balance is a bainite phase or a martensite phase, or a mixed phase thereof. The structure in the range from the 5 mm position in the sheet thickness direction to the center position of the sheet thickness is composed of a polygonal ferrite phase with an area ratio of 30% or less, and the balance is a bainite phase, a pearlite phase, or a mixed phase thereof, Vickers hardness HV at a position of 1 mm in the thickness direction is 260 HV or less, tensile strength is 590 to 740 MPa, and yield ratio is 80% or less. steel sheet.
4.0 ≧ Ti / N ≧ 2.0 (1)
Here, Ti, N: Content of each element (% by mass)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 + Mo / 4 + V / 14 (2)
Here, C, Si, Mn, Cr, Mo, V: Content of each element (mass%)
前記組成に加えて、さらに質量%で、Mo:0.60%以下、Cu:0.05〜0.60%、Ni:0.05〜0.80%、B:0.0003〜0.0030%のうちから選ばれた1種または2種以上を含有することを特徴とする請求項1に記載の曲げ加工性に優れた非調質低降伏比高張力厚鋼板。   In addition to the above-mentioned composition, it is further mass%, Mo: 0.60% or less, Cu: 0.05-0.60%, Ni: 0.05-0.80%, B: 0.0003-0.0030 The non-tempered low yield ratio high-tensile thick steel plate excellent in bending workability according to claim 1, comprising one or more selected from%. 請求項1または2に記載の組成を有する鋼素材を加熱温度1000〜1250℃に加熱する加熱工程と、次いで、表面温度で950℃以下の温度域での累積圧下率が30%以上で、圧延終了温度が表面温度で900℃以下Ar変態点以上となる熱間圧延を行う熱間圧延工程と、次いで、第一段冷却として表面温度でAr変態点以上の温度から、板厚1/4位置の平均冷却速度が2℃/s以上で、表面温度で750℃以下550℃以上となるまで加速冷却し、次いで表面温度で750℃以下600℃以上の範囲まで復熱させ、その後、第二段冷却として板厚1/4位置の平均冷却速度が2℃/s以上で、冷却停止後の復熱で表面温度が500℃以下になる冷却停止温度まで加速冷却する二段階の冷却工程と、を施すことを特徴とする曲げ加工性に優れた非調質低降伏比高張力厚鋼板の製造方法。 A heating step of heating the steel material having the composition according to claim 1 or 2 to a heating temperature of 1000 to 1250 ° C, and then rolling at a cumulative reduction rate of 30% or more in a temperature range of 950 ° C or less at the surface temperature. From the hot rolling step of performing hot rolling at an end temperature of 900 ° C. or less at the surface temperature and the Ar 3 transformation point or higher, and then from the temperature at the surface temperature of the Ar 3 transformation point or higher as the first stage cooling, the plate thickness 1 / The average cooling rate at 4 positions is 2 ° C./s or more, accelerated cooling is performed until the surface temperature becomes 750 ° C. or less and 550 ° C. or more, and then the surface temperature is reheated to a range of 750 ° C. or less and 600 ° C. or more. A two-stage cooling process in which the average cooling rate at the position of 1/4 of the plate thickness is 2 ° C./s or more as two-stage cooling, and accelerated cooling to a cooling stop temperature at which the surface temperature becomes 500 ° C. or less by reheating after the cooling stop; Bending characterized by applying Excellent non-heat treated low yield ratio method for manufacturing a high-tensile steel plates in processability. 前記冷却工程後、さらに、焼戻温度400℃以上700℃以下の温度で焼戻工程を施すことを特徴とする請求項3に記載の曲げ加工性に優れた非調質低降伏比高張力厚鋼板の製造方法。   The non-tempered low yield ratio high tensile thickness excellent in bending workability according to claim 3, wherein after the cooling step, a tempering step is further performed at a temperature of 400 ° C to 700 ° C. Manufacturing method of steel sheet
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WO2021054344A1 (en) 2019-09-20 2021-03-25 Jfeスチール株式会社 Thick steel sheet, and method for producing thick steel sheet
KR20220047363A (en) 2019-09-20 2022-04-15 제이에프이 스틸 가부시키가이샤 Thick steel plate and manufacturing method of thick steel plate

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