JP5870525B2 - High strength steel plate excellent in bending workability and low temperature toughness and method for producing the same - Google Patents
High strength steel plate excellent in bending workability and low temperature toughness and method for producing the same Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 130
- 239000010959 steel Substances 0.000 title claims description 130
- 238000005452 bending Methods 0.000 title claims description 57
- 238000004519 manufacturing process Methods 0.000 title claims description 26
- 238000005496 tempering Methods 0.000 claims description 26
- 229910000734 martensite Inorganic materials 0.000 claims description 20
- 230000010354 integration Effects 0.000 claims description 18
- 229910001566 austenite Inorganic materials 0.000 claims description 16
- 238000001816 cooling Methods 0.000 claims description 16
- 239000012535 impurity Substances 0.000 claims description 9
- 229910052804 chromium Inorganic materials 0.000 claims description 7
- 229910052750 molybdenum Inorganic materials 0.000 claims description 6
- 229910052720 vanadium Inorganic materials 0.000 claims description 6
- 229910052758 niobium Inorganic materials 0.000 claims description 5
- 238000007731 hot pressing Methods 0.000 claims description 2
- 229910052742 iron Inorganic materials 0.000 claims description 2
- 238000010791 quenching Methods 0.000 description 28
- 230000000171 quenching effect Effects 0.000 description 28
- 230000000694 effects Effects 0.000 description 21
- 239000000463 material Substances 0.000 description 15
- 238000010438 heat treatment Methods 0.000 description 12
- 238000000034 method Methods 0.000 description 12
- 229910052761 rare earth metal Inorganic materials 0.000 description 9
- 238000010276 construction Methods 0.000 description 8
- 238000005096 rolling process Methods 0.000 description 7
- 230000001965 increasing effect Effects 0.000 description 6
- 230000003111 delayed effect Effects 0.000 description 5
- 238000005098 hot rolling Methods 0.000 description 5
- 238000012360 testing method Methods 0.000 description 5
- 230000009466 transformation Effects 0.000 description 5
- 238000003466 welding Methods 0.000 description 5
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 4
- 239000013078 crystal Substances 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 230000006866 deterioration Effects 0.000 description 4
- 238000011161 development Methods 0.000 description 4
- 230000018109 developmental process Effects 0.000 description 4
- 229910052739 hydrogen Inorganic materials 0.000 description 4
- 239000001257 hydrogen Substances 0.000 description 4
- 229910052751 metal Inorganic materials 0.000 description 4
- 239000002184 metal Substances 0.000 description 4
- 150000001247 metal acetylides Chemical class 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 150000004767 nitrides Chemical class 0.000 description 4
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 3
- 238000005336 cracking Methods 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 238000003303 reheating Methods 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- 229910001563 bainite Inorganic materials 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 239000003795 chemical substances by application Substances 0.000 description 2
- 230000003749 cleanliness Effects 0.000 description 2
- 238000009863 impact test Methods 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 230000003287 optical effect Effects 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 238000005498 polishing Methods 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
- 150000002910 rare earth metals Chemical class 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 238000012546 transfer Methods 0.000 description 2
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 238000009825 accumulation Methods 0.000 description 1
- 239000010953 base metal Substances 0.000 description 1
- 238000009529 body temperature measurement Methods 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 238000003776 cleavage reaction Methods 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000007405 data analysis Methods 0.000 description 1
- 238000006477 desulfuration reaction Methods 0.000 description 1
- 230000023556 desulfurization Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000002708 enhancing effect Effects 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 230000007017 scission Effects 0.000 description 1
- 230000011218 segmentation Effects 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 239000002436 steel type Substances 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
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Description
本発明は、主に建設産業機械等に供して好適な、引張強さが１１５０ＭＰａ以上で曲げ加工性および低温靱性に優れる高張力鋼材およびその製造方法に関するものである。 The present invention relates to a high-tensile steel material that is suitable mainly for use in construction industrial machinery and the like, has a tensile strength of 1150 MPa or more and is excellent in bending workability and low-temperature toughness, and a method for producing the same.
近年、建設機械や産業機械すなわち建設産業機械をはじめとして、タンクやペンストック、ラインパイプ等においては、構造物の大型化を背景とし、使用する鋼材の高強度化が進む一方、鋼材の加工条件例えば曲げ加工条件などは従来以上の厳しい条件が適用され、同時に、使用環境の過酷化に伴って優れた低温靱性をそなえることが求められている。 In recent years, construction machinery and industrial machinery, namely construction industry machinery, as well as tanks, penstock, and line pipes, have been used to increase the strength of steel materials against the backdrop of the increase in size of structures. For example, bending conditions are more severe than conventional, and at the same time, it is required to have excellent low temperature toughness as the use environment becomes severe.
例えば、代表的な建設産業機械の一つであるオールテレーンクレーンの場合、ブーム部材では、曲げ半径が板厚の３倍程度の曲げ加工が施されるため、ＪＩＳ Ｚ ２２４８に定められる曲げ試験で、曲げ半径が板厚の３．０倍以上で割れが発生しないことが要求されている。 For example, in the case of an all terrain crane, which is one of typical construction industry machines, the boom member is subjected to a bending process in which the bending radius is about three times the plate thickness. Therefore, in the bending test defined in JIS Z 2248, It is required that the bending radius is not less than 3.0 times the plate thickness and no cracking occurs.
しかしながら、鋼材の高強度化は、一般に加工性や低温靱性の劣化を伴うため、高強度と共に優れた加工性および低温靱性を備えた鋼材が種々検討されている。
例えば、特許文献１には、自動車の車体や家電に使用される薄鋼板を対象として、鋼板成分およびミクロ組織を調整することにより、引張強さが８５０ＭＰａ以上で、穴広げ性と延性を両立させた板厚が１．２ｍｍ程度の冷延鋼板が提案されている。
However, increasing the strength of steel materials generally involves deterioration of workability and low temperature toughness, and therefore various steel materials having high strength and excellent workability and low temperature toughness have been studied.
For example, in Patent Document 1, by adjusting the steel plate components and the microstructure for thin steel plates used for automobile bodies and home appliances, the tensile strength is 850 MPa or more, and both hole expandability and ductility are achieved. A cold-rolled steel sheet having a thickness of about 1.2 mm has been proposed.
また、特許文献２には、建築鋼構造物、圧力容器、その他の溶接鋼構造物に使用される厚鋼板を対象として、特定の成分下において焼入れ臨界直径Ｄｉを規定することにより、引張強さが４９０〜８００ＭＰａで、優れた冷間加工性を有する板厚が４０ｍｍの厚鋼板が記載されている。 Further, in Patent Document 2, tensile strength is specified by defining a quenching critical diameter Di under a specific component for thick steel plates used for building steel structures, pressure vessels, and other welded steel structures. Is a thick steel plate having a thickness of 40 mm and an excellent cold workability.
さらに、特許文献３には、建設産業機械に使用される溶接用鋼を対象として、特定成分の下で直接焼入れ後焼戻すことからなる、引張強さが１１５０ＭＰａ以上の厚鋼板の製造技術が提案されている。 Furthermore, Patent Document 3 proposes a technique for producing a thick steel plate having a tensile strength of 1150 MPa or more, which is made by subjecting welding steel used in construction industrial machinery to direct tempering after quenching under a specific component. Has been.
特許文献４には、自動車や建設機械などに用いられる構造用部品を対象として、特定成分の鋼板を、熱間加工ののち、冷却し焼戻す工程からなる高強度強靭鋼の製造方法が提案されている。 Patent Document 4 proposes a method for producing high-strength tough steel comprising a process of cooling and tempering a steel sheet having a specific component after hot working a structural component used in automobiles, construction machines and the like. ing.
前掲した特許文献１，２の開発により、強度と加工性に優れた鋼板が得られるようになったが、これら特許文献１，２の技術をもってしても、強度レベルがさらに上昇したり、板厚がさらに増大した場合には、加工性や低温靱性の劣化が避けられなかった。
また、特許文献３には、曲げ加工性に関して何ら言及されていないため、この点について発明者らが検討したところ、曲げ加工性と強度を両立することはできないことが判明した。
With the development of Patent Documents 1 and 2 listed above, steel sheets having excellent strength and workability have been obtained, but even with the techniques of Patent Documents 1 and 2, the strength level can be further increased, When the thickness further increased, deterioration of workability and low temperature toughness was inevitable.
In addition, since Patent Document 3 does not mention any bending workability, the inventors have examined this point and found that it is impossible to achieve both bending workability and strength.
さらに、前掲した特許文献４に記載された方法は、厚鋼板を対象としておらず、曲げ加工性に関しても言及がない。その上、特許文献３、４に記載の技術では、所望の鋼板強度を得るために、Ｂの添加が必須であり、低温靱性が劣っているという問題があった。 Furthermore, the method described in Patent Document 4 is not intended for thick steel plates, and there is no mention of bending workability. In addition, the techniques described in Patent Documents 3 and 4 have a problem that the addition of B is essential to obtain a desired steel plate strength, and the low-temperature toughness is inferior.
上述したように、従来の技術では、建設産業機械等で要求される引張強さと板厚の鋼材に対して、十分な加工性と低温靱性を付与することができなかったため、優れた加工性と低温靱性を兼ね備える高張力鋼板の開発が要望されていた。
また、最近では、加工性の中でも、特に上述した建設産業機械への適用時に主要な成形手段である曲げ加工性が重要視されるようになってきている。すなわち、高強度を維持した上で、曲げ加工性に優れること、換言すると、割れずに鋼材を曲げることができる限界半径を従来よりも小さくすることが要求されている。
As described above, in the conventional technology, sufficient workability and low-temperature toughness could not be imparted to steel materials having the tensile strength and thickness required for construction industrial machines, etc. There has been a demand for the development of a high-tensile steel sheet having low temperature toughness.
Recently, among the workability, bending workability, which is a main forming means particularly when applied to the construction industrial machine described above, has come to be regarded as important. That is, while maintaining high strength, it is required to have excellent bending workability, in other words, to make a limit radius that can bend a steel material without cracking smaller than before.
本発明は、上記の現状に鑑み開発されたもので、引張強さが１１５０ＭＰａ以上で、板厚が７〜５０ｍｍ程度の鋼板に対して、優れた曲げ加工性と低温靱性を付与した高張力鋼板を、その有利な製造方法と共に提案することを目的とする。 The present invention has been developed in view of the above situation, and a high-tensile steel sheet having excellent bending workability and low-temperature toughness with respect to a steel sheet having a tensile strength of 1150 MPa or more and a thickness of about 7 to 50 mm. Is proposed together with its advantageous manufacturing method.
さて、発明者らは、曲げ加工性に優れた鋼材を得るため、主に製造条件が曲げ加工性に及ぼす影響について鋭意研究を重ねた結果、以下に述べる知見を得た。
（ａ）高強度を得る目的に対しては有効な手段であるとされる制御圧延と直接焼入れの組合せによる製造方法は、集合組織の発達をもたらすため、曲げ加工性に対してはむしろ劣化させる原因となる。しかしながら、直接焼入れに代えて焼入れ焼戻しによる調質処理を採用することにより、曲げ加工性を大幅に向上させることができる。さらに良好な曲げ加工性を実現するためには、その集合組織をより適正な範囲に制御することが有効である。
（ｂ）一方で、焼入れ焼戻しによる製造では、直接焼入れに比べると高強度が得難くなるが、この点については、成分組成を厳格に制御することにより、高強度と曲げ加工性の両立が可能となる。
本発明は、上記の知見にさらに検討を加えて完成されたものである。
Now, in order to obtain a steel material having excellent bending workability, the inventors have conducted extensive research mainly on the influence of manufacturing conditions on bending workability, and as a result, have obtained the following knowledge.
(A) Since the manufacturing method by the combination of controlled rolling and direct quenching, which is considered to be an effective means for the purpose of obtaining high strength, brings about the development of texture, it rather deteriorates the bending workability. Cause. However, by adopting a tempering process by quenching and tempering instead of direct quenching, bending workability can be greatly improved. In order to realize better bending workability, it is effective to control the texture to a more appropriate range.
(B) On the other hand, in the production by quenching and tempering, it is difficult to obtain high strength compared to direct quenching, but in this respect, it is possible to achieve both high strength and bending workability by strictly controlling the component composition. It becomes.
The present invention has been completed by further studying the above findings.
すなわち、本発明の要旨構成は次のとおりである。
１．質量％で、
Ｃ：０．１０〜０．２５％、
Ｓｉ：０．０５〜１．５％、
Ｍｎ：０．５〜２．０％、
Ｃｒ：０．３〜２．２％、
Ｍｏ：０．２〜１．４％、
Ｖ：０．０３〜０．１％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３％未満
を含有し、かつ下記（１）式の関係を満足し、残部はFeおよび不可避的不純物からなり、体積分率で９５％以上がマルテンサイト組織でかつ、該マルテンサイト組織における旧オーステナイト粒の平均粒径が円相当径で２０μｍ以下の鋼組織を有し、引張強さが１１５０ＭＰa以上であることを特徴とする曲げ加工性および低温靱性に優れる高張力鋼板。
記
０．８≦０．５［％Ｃｒ］＋１．２［％Ｍｏ］＋５［％Ｖ］≦２．１・・・（１） 但し、［％Ｍ］は、Ｍ元素の鋼中含有量(質量％)
That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.10 to 0.25%,
Si: 0.05 to 1.5%,
Mn: 0.5 to 2.0%
Cr: 0.3-2.2%
Mo: 0.2 to 1.4%,
V: 0.03-0.1%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: not more than 0.005% and B: less than 0.0003%, satisfying the relationship of the following formula (1), the balance is composed of Fe and inevitable impurities, and the volume fraction is 95% or more by weight. Bending workability and low-temperature toughness characterized by having a steel structure having a steel structure in which the average grain size of prior austenite grains in the martensite structure is equivalent to a circle diameter of 20 μm or less and the tensile strength is 1150 MPa or more. High-strength steel sheet with excellent resistance.
0.8 ≦ 0.5 [% Cr] +1.2 [% Mo] +5 [% V] ≦ 2.1 (1) where [% M] is the content of M element in steel ( mass%)
２．前記鋼板の鋼組織における板厚１／４位置の｛１１１｝面の集積度が０．８〜１．３であって、かつ板厚１／４位置の｛１００｝面の集積度が０．７〜１．２であることを特徴とする前記１に記載の曲げ加工性および低温靱性に優れる高張力鋼板。 2. In the steel structure of the steel sheet, the {111} plane at the 1/4 position of the thickness is 0.8 to 1.3, and the {100} plane at the 1/4 position is 0.4. The high-tensile steel sheet having excellent bending workability and low-temperature toughness as described in 1 above, which is 7 to 1.2.
３．前記鋼板が質量％で、さらにＴｉ：０．０５％以下含有することを特徴とする前記１または２に記載の曲げ加工性および低温靱性に優れる高張力鋼板。 3. 3. The high-strength steel sheet having excellent bending workability and low-temperature toughness according to 1 or 2 above, wherein the steel sheet is contained by mass% and further containing Ti: 0.05% or less.
４．前記鋼板が質量％で、さらに
Ｎｂ：０．０５％以下、
Ｃｕ：２％以下および
Ｎｉ：４％以下
のうちから選んだ一種または二種以上を含有することを特徴とする前記１〜３のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板。
4). The steel sheet is mass%, and Nb: 0.05% or less,
The high-tensile steel sheet excellent in bending workability and low-temperature toughness as described in any one of 1 to 3 above, containing one or more selected from Cu: 2% or less and Ni: 4% or less .
５. 前記鋼板が質量％で、さらに
Ｃａ：０．０１％以下、
ＲＥＭ：０．０２％以下および
Ｍｇ：０．０１％以下
のうちから選んだ一種または二種以上を含有することを特徴とする前記１〜４のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板。
5. The steel sheet is mass%, and Ca: 0.01% or less,
The bending workability and the low temperature toughness according to any one of the above 1 to 4, which contain one or more selected from REM: 0.02% or less and Mg: 0.01% or less Excellent high-tensile steel plate.
６．前記１〜５のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板の製造方法であって、
質量％で、
Ｃ：０．１０〜０．２５％、
Ｓｉ：０．０５〜１．５％、
Ｍｎ：０．５〜２．０％、
Ｃｒ：０．３〜２．２％、
Ｍｏ：０．２〜１．４％、
Ｖ：０．０３〜０．１％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３％未満
を含有し、かつ下記（１）式の関係を満足し、残部はFeおよび不可避的不純物からなる鋼片を、熱間圧廷により熱延板としたのち、Ａｃ3点（℃）以上の温度に再加熱後、２℃／ｓ以上の平均冷却速度で３００℃以下の温度まで冷却し、ついで、３５０〜６００℃の範囲で焼戻し処理を施すことを特徴とする曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。
記
０．８≦０．５［％Ｃｒ］＋１．２［％Ｍｏ］＋５［％Ｖ］≦２．１・・・（１）
但し、［％Ｍ］は、Ｍ元素の鋼中含有量(質量％)
6). A method for producing a high-tensile steel sheet excellent in bending workability and low-temperature toughness according to any one of 1 to 5 ,
% By mass
C: 0.10 to 0.25%,
Si: 0.05 to 1.5%,
Mn: 0.5 to 2.0%
Cr: 0.3-2.2%
Mo: 0.2 to 1.4%,
V: 0.03-0.1%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: 0.005% or less and B: less than 0.0003%, satisfying the relationship of the following formula (1), the balance being heated by hot pressing a steel slab comprising Fe and inevitable impurities After the sheet is formed, it is reheated to a temperature of Ac 3 point (° C) or higher, then cooled to a temperature of 300 ° C or lower at an average cooling rate of 2 ° C / s or higher, and then tempered in the range of 350 to 600 ° C. A method for producing a high-strength steel sheet excellent in bending workability and low-temperature toughness, characterized by being subjected to the treatment.
0.8 ≦ 0.5 [% Cr] +1.2 [% Mo] +5 [% V] ≦ 2.1 (1)
However, [% M] is the content of M element in steel (mass%)
７．前記焼戻し処理後の鋼組織について、板厚１／４位置の｛１１１｝面の集積度を０．８〜１．３とし、かつ板厚１／４位置の｛１００｝面の集積度を０．７〜１．２とすることを特徴とする前記６に記載の曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。 7). With respect to the steel structure after the tempering treatment, the integration degree of {111} plane at the 1/4 thickness position is set to 0.8 to 1.3, and the integration degree of {100} plane at the 1/4 thickness position is 0. 7. The method for producing a high-tensile steel sheet excellent in bending workability and low-temperature toughness as described in 6 above, characterized by being from 7 to 1.2.
８．前記鋼板が質量％で、さらにＴｉ：０．０５％以下含有することを特徴とする、前記６または７に記載の曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。 8). The method for producing a high-tensile steel sheet having excellent bending workability and low-temperature toughness as described in 6 or 7 above, wherein the steel sheet is contained by mass% and further containing Ti: 0.05% or less.
９．前記鋼板が質量％で、さらに
Ｎｂ：０．０５％以下、
Ｃｕ：２％以下および
Ｎｉ：４％以下
のうちから選んだ一種または二種以上を含有することを特徴とする前記６〜８のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。
9. The steel sheet is mass%, and Nb: 0.05% or less,
The high-tensile steel sheet excellent in bending workability and low-temperature toughness according to any one of 6 to 8 above, containing one or more selected from Cu: 2% or less and Ni: 4% or less Manufacturing method.
１０.前記鋼板が質量％で、さらに
Ｃａ：０．０１％以下、
ＲＥＭ：０．０２％以下および
Ｍｇ：０．０１％以下
のうちから選んだ一種または二種以上を含有することを特徴とする前記６〜９のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。
10. The steel sheet is mass%, and further Ca: 0.01% or less,
The bending workability and low-temperature toughness according to any one of 6 to 9 above, wherein one or more selected from REM: 0.02% or less and Mg: 0.01% or less are contained. An excellent method for producing high-tensile steel sheets.
本発明によれば、引張強さが１１５０ＭＰａ以上で、しかも曲げ加工性、さらには低温靱性に優れる高張力鋼材を得ることができ、産業上極めて有用である。 According to the present invention, a high-tensile steel material having a tensile strength of 1150 MPa or more and excellent in bending workability and low-temperature toughness can be obtained, which is extremely useful industrially.
以下、本発明を具体的に説明する。
まず、本発明において、鋼材の成分組成を前記の範囲に限定した理由について述べる。なお、鋼板成分に関する％表示は、特に断らない限り何れも質量％である。
Ｃ：０．１０〜０．２５％
Ｃは、焼入れによって鋼をマルテンサイト主体の組織とし、鋼板の強度を確保するために含有するが、０．１０％未満ではその効果が不十分であり、一方、０．２５％を超えると母材および溶接熱影響部（ＨＡＺ）の靱性が劣化するだけでなく、溶接性を著しく低下させるため、Ｃ量は０．１０〜０．２５％に限定する。好ましくは０．１２〜０．２５％の範囲である。
Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, the% display regarding a steel plate component is the mass%.
C: 0.10 to 0.25%
C makes steel a martensite-based structure by quenching and is contained in order to ensure the strength of the steel sheet. However, if it is less than 0.10%, its effect is insufficient. The amount of C is limited to 0.10 to 0.25% in order to not only deteriorate the toughness of the material and the weld heat-affected zone (HAZ) but also significantly reduce the weldability. Preferably it is 0.12 to 0.25% of range.
Ｓｉ：０．０５〜１．５％
Ｓｉは、製鋼段階の脱酸材および鋼板の強度向上元素として含有するが、０．０５％未満ではその効果が不十分である。一方、１．５％を超えると溶接性を著しく低下させるため、０．０５〜１．５％に限定する。好ましくは、０．１５〜１．０％の範囲である。
Si: 0.05 to 1.5%
Si is contained as a deoxidizing material in the steelmaking stage and as an element for improving the strength of the steel sheet, but if it is less than 0.05%, its effect is insufficient. On the other hand, if it exceeds 1.5%, the weldability is remarkably lowered, so it is limited to 0.05 to 1.5%. Preferably, it is 0.15 to 1.0% of range.
Ｍｎ：０．５〜２．０％
Ｍｎは、焼入性を高め、マルテンサイト主体の鋼組織を得て、高強度を得るためには必須の元素であり、０．５％以上の添加を必要とするが、２．０％を超える添加は、靱性および溶接性を著しく低下させるため、０．５〜２．０％に限定する。
Mn: 0.5 to 2.0%
Mn is an essential element for increasing the hardenability, obtaining a martensite-based steel structure, and obtaining high strength, and requires addition of 0.5% or more. Excessive addition significantly limits toughness and weldability, so is limited to 0.5-2.0%.
Ｃｒ：０．３〜２．２％
Ｃｒは、強度および靱性の向上に有用な元素である。従って、鋼をマルテンサイト主体の組織とし、高強度化する場合には積極的に添加し、鋼板の引張強さが１１５０ＭＰａ以上の特性を得るためには、０．３％以上の添加が必要である。一方、Ｃｒ含有量が２．２％を超えると、溶接性が低下するので、含有量を２．２％以下に限定する。好ましくは、０．５５〜２．２％の範囲である。
Cr: 0.3-2.2%
Cr is an element useful for improving strength and toughness. Therefore, when steel is made mainly of martensite and is strengthened, it is added positively, and in order to obtain a characteristic in which the tensile strength of the steel sheet is 1150 MPa or more, addition of 0.3% or more is necessary. is there. On the other hand, if the Cr content exceeds 2.2%, the weldability deteriorates, so the content is limited to 2.2% or less. Preferably, it is 0.55 to 2.2% of range.
Ｍｏ：０．２〜１．４％
Ｍｏは、焼入れ性および強度の向上に有効に寄与するだけでなく、炭化物を形成することによって拡散性水素をトラップし、耐遅れ破壊特性を向上させる効果があるので、引張強さ：１１５０ＭＰａ以上を得るために０．２％以上添加することが必要である。しかしながら、Ｍｏ含有量が１．４％を超えるとその効果は飽和し、むしろコストの面で不利となるので、Ｍｏ量の上限は１．４％とする。好ましくは０．３〜１．４％の範囲である。
Mo: 0.2-1.4%
Mo not only effectively contributes to improving hardenability and strength, but also has the effect of trapping diffusible hydrogen by forming carbides and improving delayed fracture resistance, so tensile strength: 1150 MPa or more In order to obtain it, it is necessary to add 0.2% or more. However, if the Mo content exceeds 1.4%, the effect is saturated, but rather disadvantageous in terms of cost, so the upper limit of the Mo amount is set to 1.4%. Preferably it is 0.3 to 1.4% of range.
Ｖ：０．０３〜０．１％
Ｖは、炭化物や窒化物、炭窒化物を形成することによって、拡散性水素をトラップし、耐遅れ破壊特性を向上させる作用がある。本発明のようにＢが０．０００３％未満の成分の場合には、Ｖの炭化物による焼戻し軟化抵抗を高めることで強度確保することが狙いであり、これらの効果を発揮させるためには、０．０３％以上の添加が必要である。一方、過剰の添加は鋼板の強度上昇への寄与が小さく、また、溶接性を低下させるため、その含有量を０．１％以下に限定する。好ましくは０．０３〜０．０８％の範囲とする。
V: 0.03-0.1%
V acts to trap diffusible hydrogen and improve delayed fracture resistance by forming carbides, nitrides, and carbonitrides. When B is a component of less than 0.0003% as in the present invention, the aim is to ensure strength by increasing the temper softening resistance due to the carbide of V. In order to exert these effects, 0 Addition of 0.03% or more is necessary. On the other hand, excessive addition has a small contribution to the increase in strength of the steel sheet and lowers the weldability, so the content is limited to 0.1% or less. Preferably it is 0.03 to 0.08% of range.
Ａｌ：０．００５〜０．１％
Ａｌは、脱酸剤として、また結晶粒径の微細化元素として有効に寄与するが、０．００５％未満の場合にはその添加効果が十分でなく、一方、０．１％を超えて含有すると、鋼板の表面疵が発生し易くなるため、０．００５〜０．１％に限定する。
Al: 0.005 to 0.1%
Al effectively contributes as a deoxidizing agent and as an element for refining the crystal grain size, but if it is less than 0.005%, the effect of addition is not sufficient, while it contains more than 0.1%. Then, since surface flaws of the steel sheet are likely to occur, the content is limited to 0.005 to 0.1%.
Ｎ：０．０００５〜０．００６％
Ｎは、Ｎｂなどと窒化物を形成することによって組織を微細化し、母材ならびに溶接熱影響部の靱性を向上させる効果を有する。また、析出物および／または介在物の生成量に影響を及ぼし、材料の加工性に影響を及ぼす。しかしながら、０．０００５％未満の含有では組織の微細化効果が十分にもたらされず、一方、０．００６％を超える含有は析出物および／または介在物を増加させ加工性を損なうため、Ｎは０．０００５〜０．００６％に限定する。
N: 0.0005 to 0.006%
N has the effect of refining the structure by forming a nitride with Nb or the like and improving the toughness of the base material and the weld heat affected zone. In addition, the amount of precipitates and / or inclusions is affected and the workability of the material is affected. However, if the content is less than 0.0005%, the effect of refining the structure is not sufficiently brought about. On the other hand, if the content exceeds 0.006%, precipitates and / or inclusions are increased and workability is impaired. .0005 to 0.006%.
Ｐ：０．０２％以下
不純物元素であるＰは、焼戻し処理時に旧オーステナイト粒界等の結晶粒界に偏析しやすく、０．０２％を超えると隣接する結晶粒の相互の接合強度を低下させ、低温靱性や耐遅れ破壊特性を劣化させるため、Ｐは０．０２％以下に限定する。
P: 0.02% or less P which is an impurity element tends to segregate at grain boundaries such as prior austenite grain boundaries during tempering treatment, and if it exceeds 0.02%, the bonding strength between adjacent crystal grains decreases. In order to deteriorate low temperature toughness and delayed fracture resistance, P is limited to 0.02% or less.
Ｓ：０．００５％以下
不純物元素であるＳは、非金属介在物であるＭｎＳを生成しやすく、０．００５％を超えると、介在物の量が多くなるため、引張試験などの延性破壊の強度が低下し、加工性を劣化させる。そのため、Ｓは０．００５％以下に限定する。
S: 0.005% or less S, which is an impurity element, easily produces MnS, which is a non-metallic inclusion, and if it exceeds 0.005%, the amount of inclusion increases, so that ductile fracture such as a tensile test occurs. Strength decreases and processability deteriorates. Therefore, S is limited to 0.005% or less.
Ｂ：０．０００３％未満
Ｂは、焼入性を高め、強度を向上させる作用を有している。しかしながら、引張強さ１１５０ＭＰａ以上の鋼において、Ｂは靱性が低下する原因となるため、不可避に混入する場合もＢ添加量を０．０００３％未満とする。なお、本発明では、焼戻し温度を低く設定することにより、Ｂ添加量が０．０００３％未満であっても所望の引張強さを得ることができる。
B: Less than 0.0003% B has an effect of enhancing hardenability and improving strength. However, in steel having a tensile strength of 1150 MPa or more, B causes a decrease in toughness. Therefore, even when unavoidably mixed, the B addition amount is made less than 0.0003%. In the present invention, by setting the tempering temperature low, a desired tensile strength can be obtained even if the B addition amount is less than 0.0003%.
０．８≦０．５［％Ｃｒ］＋１．２［％Ｍｏ］＋５［％Ｖ］≦２．１
但し、［％Ｍ］は、Ｍ元素の鋼中含有量(質量％)を表す（以下、同じ）。
以下に示す製造条件において、引張強さが１１５０ＭＰａ以上の高強度鋼板を得るためには、０．５［％Ｃｒ］＋１．２［％Ｍｏ］＋５［％Ｖ］の値を０．８以上とする必要がある。一方、２．１を超えると溶接性および溶接部靱性が低下することから、その範囲を０．８〜２．１、好ましくは０．９５〜１．９５とする。
0.8 ≦ 0.5 [% Cr] +1.2 [% Mo] +5 [% V] ≦ 2.1
However, [% M] represents the content (mass%) of M element in steel (hereinafter the same).
In the manufacturing conditions shown below, in order to obtain a high-strength steel sheet having a tensile strength of 1150 MPa or more, the value of 0.5 [% Cr] +1.2 [% Mo] +5 [% V] is set to 0.8 or more. There is a need to. On the other hand, if it exceeds 2.1, the weldability and weld zone toughness deteriorate, so the range is set to 0.8 to 2.1, preferably 0.95 to 1.95.
本発明における基本成分は、上記したとおりであり、残部はＦｅおよび不可避的不純物である。かかる不可避的不純物としては、原料、製造設備等から不可避的に混入する不純物が挙げられる。 The basic components in the present invention are as described above, and the balance is Fe and inevitable impurities. Examples of such unavoidable impurities include impurities inevitably mixed from raw materials, production facilities, and the like.
以上、本発明の基本成分について説明したが、本発明の効果を発現させるには、成分を所定の範囲に調整するだけでは不十分で、鋼組織を以下のように調整することが重要である。
本発明では、鋼組織のうち体積分率で９５％以上がマルテンサイト組織であることが必要である。というのは、マルテンサイト組織の体積分率が９５％未満となった場合、鋼の強度や靱性が不足してしまうからである。
なお、マルテンサイト以外の組織の体積分率は少ない程良く、その体積分率が低い場合は、その影響が無視できる。具体的には、マルテンサイト以外の組織として、例えば、ベイナイト、パーライト、セメンタイトなどから選ばれる１種または２種以上の金属組織の合計の体積分率が５％以下であれば許容でき、３％以下であることが好ましい。
Although the basic components of the present invention have been described above, it is not sufficient to adjust the components within a predetermined range in order to exert the effects of the present invention, and it is important to adjust the steel structure as follows. .
In the present invention, it is necessary that 95% or more of the steel structure has a martensite structure in volume fraction. This is because when the volume fraction of the martensite structure is less than 95%, the strength and toughness of the steel are insufficient.
It should be noted that the volume fraction of the tissue other than martensite is preferably as small as possible, and when the volume fraction is low, the influence can be ignored. Specifically, as a structure other than martensite, for example, it is acceptable if the total volume fraction of one or more metal structures selected from bainite, pearlite, cementite and the like is 5% or less, and 3% The following is preferable.
上記したマルテンサイト組織において、旧オーステナイト粒の平均粒径は、円相当径で２０μｍ以下であることが必要である。というのは、焼入れ前の組織であるオーステナイト粒が、円相当径として求めた平均粒径で２０μｍを超える粗大粒であると、マルテンサイト変態後の靱性が劣化するからである。
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液で腐食させ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をすることができる。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。
In the martensite structure described above, the average particle diameter of the prior austenite grains needs to be 20 μm or less in terms of equivalent circle diameter. This is because the toughness after martensitic transformation deteriorates when the austenite grains, which are the structure before quenching, are coarse grains having an average grain size of more than 20 μm as the equivalent circle diameter.
In addition, the form of austenite grains before quenching is observed as a so-called prior austenite grain boundary by corroding the austenite grain boundary with a liquid that corrodes preferentially and observing the metal structure even after heat treatment. be able to. 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.
また、本発明では、所望する特性に応じて、さらにＴｉ、Ｎｂ、Ｃｕ、Ｎｉ、Ｃａ、ＲＥＭ、Ｍｇの一種または二種以上を含有することができる。
Ｔｉ：０．０５％以下
Ｔｉは、圧延加熱時あるいは溶接時にＴｉＮを生成し、オーステナイト粒の成長を抑制し、母材ならびに溶接熱影響部の靱性を向上させるだけでなく、炭化物や窒化物、炭窒化物を形成することによって拡散性水素をトラップし、耐遅れ破壊特性を向上させる効果がある。このような効果を発揮させるためには、０．００４％以上含有させることが好ましい。しかしながら、０．０５％を超える添加は溶接熱影響部の靱性を劣化させるため、Ｔｉを含有させる場合には、０．０５％以下で含有させるものとした。
Further, in the present invention, one or more of Ti, Nb, Cu, Ni, Ca, REM, and Mg can be further contained according to desired properties.
Ti: 0.05% or less Ti generates TiN at the time of rolling heating or welding, and suppresses the growth of austenite grains, improves not only the toughness of the base metal and the weld heat affected zone, but also carbides and nitrides, Forming carbonitride has the effect of trapping diffusible hydrogen and improving delayed fracture resistance. In order to exhibit such an effect, it is preferable to contain 0.004% or more. However, since addition exceeding 0.05% deteriorates the toughness of the weld heat affected zone, when Ti is contained, it is assumed to be contained at 0.05% or less.
Ｎｂ：０．０５％以下
Ｎｂは、マイクロアロイング元素として強度を向上させると同時に、炭化物や窒化物、炭窒化物を形成することによって拡散性水素をトラップし、耐遅れ破壊特性を向上させる作用がある。このような効果を発揮させるためには、０．００５％以上含有させることが好ましい。一方、０．０５％を超える添加は強度の向上効果が小さいことに加え、溶接熱影響部の靱性劣化を招く。従って、Ｎｂを含有させる場合には、０．０５％以下で含有させるものとした。
Nb: 0.05% or less Nb improves strength as a microalloying element, and at the same time traps diffusible hydrogen by forming carbides, nitrides, and carbonitrides and improves delayed fracture resistance There is. In order to exhibit such an effect, it is preferable to contain 0.005% or more. On the other hand, addition exceeding 0.05% causes not only a small strength improvement effect, but also causes toughness deterioration of the weld heat affected zone. Therefore, when Nb is contained, it is assumed to be contained at 0.05% or less.
Ｃｕ：２％以下
Ｃｕは、固溶強化および析出強化によって強度を向上する作用を有している。このような効果を発揮させるためには、０．０４％以上含有させることが好ましい。しかしながら、Ｃｕ含有量が２％を超えると、鋼片加熱時や溶接時に熱間での割れが生じやすくなるため、Ｃｕを含有させる場合には、２％以下で含有させるものとした。
Cu: 2% or less Cu has an effect of improving strength by solid solution strengthening and precipitation strengthening. In order to exhibit such an effect, it is preferable to make it contain 0.04% or more. However, if the Cu content exceeds 2%, hot cracking is likely to occur during heating of the steel slab or during welding. Therefore, when Cu is contained, the Cu content is 2% or less.
Ｎｉ：４％以下
Ｎｉは、靱性および焼入性を向上する作用を有している。このような効果を発揮させるためには、０．０４％以上含有させることが好ましい。しかしながら、Ｎｉは高価な元素であり含有量が４％を超えると、実用鋼としての経済性が低下するので、Ｎｉを含有させる場合には、４％以下で含有させるものとした。また、Ｎｉを過剰に含有させると、鋼板の板厚１／４位置の｛１１１｝面の集積度を０．８〜１．３の範囲とし、さらに板厚１／４位置の｛１００｝面の集積度を０．７〜１．２の範囲とすることが困難となるため、集合組織の観点からも、Ｎｉを含有させる場合にはその含有量が４％以下であることが好ましい。
Ni: 4% or less Ni has an effect of improving toughness and hardenability. In order to exhibit such an effect, it is preferable to make it contain 0.04% or more. However, Ni is an expensive element, and if the content exceeds 4%, the economical efficiency as a practical steel is lowered. Therefore, when Ni is contained, it is assumed to be contained at 4% or less. Further, when Ni is excessively contained, the degree of integration of {111} planes at the 1/4 position of the steel sheet is in the range of 0.8 to 1.3, and further {100} plane at the 1/4 thickness position. Therefore, when Ni is contained, the content is preferably 4% or less from the viewpoint of the texture.
Ｃａ：０．０１％以下
Ｃａは、硫化物系介在物の形態制御に有用な元素である。このような効果を発揮させるためには、０．０００５％以上含有させることが好ましい。しかしながら、０．０１％を超えるＣａの添加は、清浄度を低下させ、曲げ試験片の表面に微小な割れを発生させるようになるので、Ｃａを含有させる場合には、０．０１％以下で含有させるものとした。
Ca: 0.01% or less Ca is an element useful for controlling the form of sulfide inclusions. In order to exhibit such an effect, it is preferable to contain 0.0005% or more. However, the addition of Ca exceeding 0.01% lowers the cleanliness and causes micro cracks on the surface of the bending test piece. It was supposed to be included.
ＲＥＭ：０．０２％以下
ＲＥＭ（Rare Earth Metalの略、希土類）は、鋼中でＲＥＭ（Ｏ，Ｓ）として硫化物を生成することにより、結晶粒界における固溶Ｓ量を低減して靱性を改善する有用元素である。このような効果を発揮させるためには、０．０００５％以上含有させることが好ましい。しかしながら、含有量が０．０２％を超えると、沈殿晶帯にＲＥＭ硫化物が著しく集積し、材質の劣化を招くので、ＲＥＭを含有させる場合には、０．０２％以下で含有させるものとした。
REM: 0.02% or less REM (abbreviation for Rare Earth Metal, rare earth) reduces toughness by reducing the amount of solid solution S at grain boundaries by producing sulfide as REM (O, S) in steel. It is a useful element that improves In order to exhibit such an effect, it is preferable to contain 0.0005% or more. However, if the content exceeds 0.02%, the REM sulfide is remarkably accumulated in the precipitation crystal zone, leading to deterioration of the material. Therefore, when REM is included, the content should be 0.02% or less. did.
Ｍｇ：０．０１％以下
Ｍｇは、溶銑脱硫剤として使用する場合がある。その場合には、０．０００３％以上含有させることが好ましい。しかしながら、０．０１％を超える添加は、清浄度の低下を招く。従って、Ｍｇを含有させる場合には、０．０１％以下で含有させるものとした。
Mg: 0.01% or less Mg may be used as a hot metal desulfurization agent. In that case, it is preferable to contain 0.0003% or more. However, addition over 0.01% leads to a decrease in cleanliness. Therefore, when Mg is contained, it is assumed to be contained at 0.01% or less.
鋼組織
次に、本発明鋼板の好適な鋼組織（集合組織）について説明する。
本発明において、集合組織は、鋼板の板厚１／４位置の｛１１１｝面の集積度を０．８〜１．３の範囲とし、板厚１／４位置の｛１００｝面の集積度を０．７〜１．２の範囲とすることが好ましい。
ここで、鋼板の板厚１／４位置の｛１１１｝面の集積度とは、鋼板表面に平行な｛１１１｝面のＸ線回折のランダム強度比（ランダムな集合組織を有する材料における｛１１１｝面のＸ線回折強度を１とした場合の相対比）を意味する。また、｛１００｝面の集積度についても同様である。
Steel structure Next, the suitable steel structure (aggregate structure) of this invention steel plate is demonstrated.
In the present invention, the texture of the {111} plane at the 1/4 position of the steel sheet is in the range of 0.8 to 1.3, and the {100} plane at the 1/4 position. Is preferably in the range of 0.7 to 1.2.
Here, the degree of integration of {111} planes at the position of 1/4 of the thickness of the steel sheet is the random intensity ratio of the X-ray diffraction of the {111} plane parallel to the steel sheet surface ({111 in a material having a random texture). } Relative ratio when the X-ray diffraction intensity of the surface is 1. The same applies to the degree of integration of {100} planes.
鋼板の板厚１／４位置の｛１１１｝面の集積度が１．３を超え、あるいは、板厚１／４位置の｛１００｝面の集積度が１．２を超えた場合には、いずれも鋼板の曲げ加工性が低下するので好ましくない。一方、鋼板の板厚１／４位置の｛１１１｝面の集積度が０．８未満、あるいは、板厚１／４位置の｛１００｝面の集積度が０．７未満の場合には、例えば｛１１０｝面などの、｛１１１｝面や｛１００｝面以外の結晶面の鋼組織が生じることで、鋼板内の劈開破面に異方性が生じてしまい曲げ加工性を低下させるため、好ましくない。 When the integration degree of the {111} plane at the 1/4 position of the steel plate exceeds 1.3, or the integration degree of the {100} plane at the 1/4 position of the plate exceeds 1.2, Any of these is not preferable because the bending workability of the steel sheet is lowered. On the other hand, when the integration degree of {111} plane at the 1/4 position of the steel plate is less than 0.8, or when the integration degree of {100} plane at the 1/4 thickness position is less than 0.7, For example, when a steel structure of a crystal plane other than {111} plane or {100} plane such as {110} plane is generated, anisotropy occurs in the cleavage plane in the steel sheet, and bending workability is lowered. It is not preferable.
次に、本発明の製造方法について説明する。
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。すなわち、熱延板を、Ａｃ3点以上の温度に再加熱し、ついで２℃／ｓ以上の平均冷却速度で３００℃以下の温度まで冷却する焼入れ処理を施したのち、３５０〜６００℃以下の範囲で焼戻し処理を施す。
以下、各製造工程の限定理由について説明する。
Next, the manufacturing method of this invention is demonstrated.
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. That is, the hot-rolled sheet is reheated to a temperature of Ac 3 point or higher, and then subjected to a quenching process of cooling to a temperature of 300 ° C. or lower at an average cooling rate of 2 ° C./s or higher, and then 350 to 600 ° C. or lower. Tempering treatment is performed in the range.
Hereinafter, the reasons for limiting each manufacturing process will be described.
（熱間圧延条件）
熱間圧延条件については特に制限はなく、常法に従って行えばよい。但し、本発明では、熱延鋼板の板厚については７〜５０ｍｍ程度の鋼板を対象とする。
(Hot rolling conditions)
The hot rolling conditions are not particularly limited and may be performed according to a conventional method. However, in this invention, about the plate | board thickness of a hot-rolled steel plate, about 7-50 mm steel plate is made into object.
（焼入れ条件）
従来公知の焼入れ装置または加速冷却装置を用いて、焼入れ処理温度をＡｃ３点以上とし、平均冷却速度を２℃／秒以上で、鋼板の温度が３００℃以下になるまで冷却することにより、オーステナイトからマルテンサイトへの相変態を完了させ、鋼材の靱性が向上する。
本発明において、焼入れ処理は、熱間圧延直後の高温状態から焼入れる直接焼入れではなく、熱間圧延後の鋼をＡｃ３点以上の温度に再加熱してから焼入れる。このいわゆる再加熱焼入れ（本明細書においては、単に「焼入れ」とも記す）処理による製造プロセスを採用することによりにより、直接焼入れに比べ圧延集合組織の発達が抑制され、曲げ加工性が向上すると共に、オーステナイト粒径の微細化が可能となり、高い強度と優れた靱性を両立することができる。
(Quenching conditions)
By using a conventionally known quenching device or accelerated cooling device, the quenching treatment temperature is set to Ac 3 points or higher, the average cooling rate is 2 ° C./second or higher, and the steel sheet is cooled to 300 ° C. or lower to austenite. Completes the phase transformation from to martensite and improves the toughness of the steel.
In the present invention, the quenching treatment is not direct quenching which is quenched from a high temperature state immediately after hot rolling, but is performed after reheating the steel after hot rolling to a temperature of Ac 3 point or higher. By adopting a manufacturing process based on this so-called reheating quenching (also simply referred to as “quenching” in this specification), the development of the rolling texture is suppressed and bending workability is improved compared to direct quenching. The austenite grain size can be made finer, and both high strength and excellent toughness can be achieved.
ここに、焼入れ温度がＡｃ３点未満では、焼入れ前のオーステナイト化が十分ではないため、目標のマルテンサイト主体組織が得られず、鋼板の強度や靱性が十分とはならない。
また、平均冷却速度が２℃／ｓ未満では、マルテンサイト変態前にベイナイトなどのマルテンサイト変態温度より高温で変態する組織が生成するため、目標のマルテンサイトを主体とした組織が得られずに、鋼板の強度や靱性が低下するという問題が生じる。また、冷却停止温度が３００℃を超えると、鋼板内部でのマルテンサイト変態が完了せず、鋼板の強度や靱性が低下するので、冷却停止温度は３００℃以下とする。
Here, if the quenching temperature is less than Ac 3 , the austenitization before quenching is not sufficient, so that the target martensite main structure cannot be obtained, and the strength and toughness of the steel sheet are not sufficient.
In addition, when the average cooling rate is less than 2 ° C./s, a structure that transforms at a temperature higher than the martensite transformation temperature such as bainite is generated before the martensite transformation, so that a structure mainly composed of the target martensite cannot be obtained. The problem that the intensity | strength and toughness of a steel plate fall arises. Further, if the cooling stop temperature exceeds 300 ° C., the martensitic transformation inside the steel plate is not completed, and the strength and toughness of the steel plate are lowered. Therefore, the cooling stop temperature is set to 300 ° C. or less.
上記した冷却条件において、平均冷却速度は鋼板の板厚方向における冷却速度の平均値とし、冷却停止温度は復熱完了直後の鋼板表面の温度とする。また、Ａｃ３温度は、例えば以下の式によって計算される。
Ａｃ３＝８５４−１７９．４［％Ｃ］＋４４．４［％Ｓｉ］−１３．９［％Ｍｎ］−１７．８［％Ｎｉ］−１．７［％Ｃｒ］
In the cooling conditions described above, the average cooling rate is the average value of the cooling rates in the plate thickness direction of the steel sheet, and the cooling stop temperature is the temperature of the steel sheet surface immediately after completion of reheating. Moreover, Ac 3 temperature is, for example, calculated by the following equation.
Ac 3 = 854-179.4 [% C] +44.4 [% Si] -13.9 [% Mn] -17.8 [% Ni] -1.7 [% Cr]
（焼戻し条件）
冷却後、強度と靱性を調整するため、焼戻し処理を行う。焼戻し温度は、６００℃を超えるとマルテンサイト組織の回復により強度の低下を招くとともに靱性が低下する。一方、３５０℃未満では、焼戻しによる靱性の向上効果が得られない。従って、３５０〜６００℃の範囲、好ましくは３５０〜５５０℃の範囲とする。
上記の焼戻し温度は、鋼板表面温度の測定値から伝熱計算によって求めた板厚中心部の温度である。また、焼戻し温度における保持時間は、特に規定しないが、生産性の観点から、３０分以下とすることが望ましい。
なお、焼戻し処理に用いる加熱装置は、誘導加熱、通電加熱、赤外線輻射加熱、雰囲気加熱等のいずれの方式でも良い。
(Tempering conditions)
After cooling, tempering is performed to adjust strength and toughness. When the tempering temperature exceeds 600 ° C., the strength is lowered due to the recovery of the martensite structure and the toughness is lowered. On the other hand, if it is less than 350 ° C., the effect of improving toughness by tempering cannot be obtained. Therefore, the range is 350 to 600 ° C, preferably 350 to 550 ° C.
Said tempering temperature is the temperature of the plate | board thickness center part calculated | required by the heat-transfer calculation from the measured value of the steel plate surface temperature. Further, the holding time at the tempering temperature is not particularly defined, but is preferably 30 minutes or less from the viewpoint of productivity.
Note that the heating device used for the tempering process may be any system such as induction heating, current heating, infrared radiation heating, and atmosphere heating.
本発明では、上記焼戻し処理後の鋼組織を、鋼板の板厚１／４位置の｛１１１｝面の集積度で０．８〜１．３の範囲とし、板厚１／４位置の｛１００｝面の集積度で０．７〜１．２の範囲とすることが好ましいのは、前述した鋼組織での理由のとおりである。 In the present invention, the steel structure after the tempering treatment is in the range of 0.8 to 1.3 in terms of the degree of integration of {111} planes at the 1/4 thickness position of the steel plate, and {100 at the 1/4 thickness position. } The reason why the integration degree of the plane is preferably in the range of 0.7 to 1.2 is as described above for the steel structure.
表１に示す化学成分の鋼（鋼種Ａ〜Ｒ）を溶製してスラブに鋳造し、加熱炉で加熱後、熱間圧延を行い種々の板厚の鋼板とした。これらの熱間圧延では、スラブ加熱温度を１１００〜１１５０℃、圧延仕上温度を９００〜９５０℃とし、圧延後は放冷とした。鋼Ａ〜Ｈの成分は本発明の範囲内で、鋼Ｉ〜Ｒは本発明の範囲外の比較例である。
ついで、得られた熱延鋼板に対し、表２に示す条件で焼入れ処理および焼戻し処理を行った。なお、焼入れ処理における冷却停止温度は２００℃以下、焼戻し温度での保持時間は、１０分とした。また、比較のため、直接焼入れ焼戻し処理も行った。そのときのスラブ加熱温度は１１２５℃、圧延仕上温度は８００℃、直接焼入れ開始温度は７７０℃とした。
Steels (steel types A to R) having chemical components shown in Table 1 were melted and cast into slabs, heated in a heating furnace, and then hot-rolled to obtain steel plates having various thicknesses. In these hot rolling, the slab heating temperature was 1100 to 1150 ° C., the rolling finishing temperature was 900 to 950 ° C., and the steel was allowed to cool after rolling. The components of Steels A to H are within the scope of the present invention, and Steels I to R are comparative examples outside the scope of the present invention.
Subsequently, the obtained hot-rolled steel sheet was subjected to quenching treatment and tempering treatment under the conditions shown in Table 2. The cooling stop temperature in the quenching treatment was 200 ° C. or less, and the holding time at the tempering temperature was 10 minutes. For comparison, a direct quenching and tempering treatment was also performed. The slab heating temperature at that time was 1125 ° C., the rolling finishing temperature was 800 ° C., and the direct quenching start temperature was 770 ° C.
焼戻し温度や焼入れ温度などの板厚中心部における温度は、放射温度計による表面の逐次における温度測定結果から、伝熱計算によって求めた。
金属組織は、板厚方向１／２の位置から組織観察用試験片を採取し、圧延方向断面を観察面として観察した。また、マルテンサイトの体積分率は、ナイタール液で組織を現出し、光学顕微鏡で観察した視野（２５０×２００μｍ２）について画像処理により測定した。さらに、マルテンサイト組織における旧オーステナイト粒の粒径は、同様に採取した組織観察用試験片に対してオーステナイト粒界を優先的に腐食する腐食液による腐食処理を実施後、光学顕微鏡で観察した視野（５００×４００μｍ２）について旧オ−ステナイト粒径（円相当径）の平均値を線分法にて測定した。
引張試験は、ＪＩＳ Ｚ ２２４１に準拠して行い、丸棒引張試験片により降伏強度および引張強さ（ＴＳ）を測定した。靱性は、シャルピー衝撃試験によって得られる−４０℃での吸収エネルギー値で評価した。引張強さの目標値は１１５０ＭＰａ以上、吸収エネルギーの目標値は３本の平均値が７０Ｊ以上である。
曲げ特性はＪＩＳ Ｚ ２２０４の１号曲げ試験片を用い、ＪＩＳ Ｚ ２２４８に準拠した曲げ試験を行った。このときの曲げ半径を板厚の１．５倍、曲げ角度を９０度としたときの割れの有無で評価した。
溶接部靱性は、溶接入熱：３ｋＪ／ｍｍをシミュレートした再現熱サイクル試験片を作製し、シャルピー衝撃試験によって得られる−４０℃での吸収エネルギー値で評価した。目標値は、吸収エネルギーの目標値は３本の平均値が５０Ｊ以上とした。
集合組織の測定は、鋼板の板厚１／４位置から板面に平行な面を切り出し、機械研磨後、化学研磨により試料表面の加工組織を除去した後、Ｘ線回折及びデータ解析をインバース法で実施した。なお、前述したように、｛１１１｝面の集積度とは、ランダムな集合組織を有する試料におけるＸ線回折強度を１としたときの相対的な強度比であり、｛１００｝面の集積度についても同様である。
表２に、鋼板の製造条件、組織および機械的特性を示す。
The temperature at the center of the plate thickness, such as the tempering temperature and the quenching temperature, was obtained by heat transfer calculation from the results of temperature measurement at the surface in succession by a radiation thermometer.
For the metal structure, a specimen for observing the structure was taken from a position in the plate thickness direction 1/2, and the cross section in the rolling direction was observed as an observation surface. In addition, the volume fraction of martensite was measured by image processing for a field of view (250 × 200 μm 2 ) that appeared with a nital solution and observed with an optical microscope. Furthermore, the grain size of the prior austenite grains in the martensite structure is the field of view observed with an optical microscope after performing a corrosive treatment with a corrosive liquid that preferentially corrodes the austenite grain boundaries on the specimens for structure observation similarly collected For (500 × 400 μm 2 ), the average value of the prior austenite particle size (equivalent circle diameter) was measured by the line segment method.
The tensile test was performed according to JIS Z 2241, and the yield strength and tensile strength (TS) were measured with a round bar tensile test piece. Toughness was evaluated by the absorbed energy value at −40 ° C. obtained by the Charpy impact test. The target value of tensile strength is 1150 MPa or more, and the target value of absorbed energy is an average value of three of 70 J or more.
For the bending characteristics, a bending test in accordance with JIS Z 2248 was performed using a No. 1 bending test piece of JIS Z 2204. Evaluation was based on the presence or absence of cracks when the bending radius was 1.5 times the plate thickness and the bending angle was 90 degrees.
Welded portion toughness was evaluated based on an absorption energy value at −40 ° C. obtained by a reproducible thermal cycle test piece simulating welding heat input: 3 kJ / mm and obtained by a Charpy impact test. As for the target value, the average value of the absorbed energy is set to 50 J or more.
For texture measurement, a plane parallel to the plate surface is cut out from the 1/4 thickness position of the steel plate, and after mechanical polishing, the processed structure on the sample surface is removed by chemical polishing, and then X-ray diffraction and data analysis are performed by the inverse method. It carried out in. As described above, the {111} plane integration degree is a relative intensity ratio when the X-ray diffraction intensity in a sample having a random texture is 1, and the {100} plane integration degree. The same applies to.
Table 2 shows the manufacturing conditions, structure and mechanical properties of the steel sheet.
同表より、鋼板成分および製造方法が共に本発明の適性範囲内である実施例Ｎｏ．１〜８、１１、１３、１５および１７は、強度、靱性、曲げ特性およびＨＡＺ（溶接熱影響部）靱性の全ての特性について、それらの目標値をそれぞれが満足していることが分かる。
一方、Ｎｏ．９、１０および１６は、直接焼入れ焼戻しにより製造したため、曲げ特性が目標を満足しなかった。Ｎｏ．１２は、焼戻し温度が低いため強度は高いものの靱性が目標を満足しなかった。Ｎｏ．１４は、焼戻し温度が高いため、強度が目標を満足しなかった。Ｎｏ．１８〜２７の比較例はいずれも、成分組成が本発明の適正範囲外であるため、上記した特性のうち、いずれかの特性がその目標値から外れていた。
From the table, Example No. in which the steel plate components and the production method are both within the suitability range of the present invention. It can be seen that 1 to 8, 11, 13, 15, and 17 satisfy the target values for all the properties of strength, toughness, bending properties, and HAZ (welding heat affected zone) toughness.
On the other hand, no. Since 9, 10 and 16 were produced by direct quenching and tempering, the bending properties did not satisfy the target. No. In No. 12, the toughness did not satisfy the target although the strength was high because the tempering temperature was low. No. In No. 14, the tempering temperature was high, so the strength did not satisfy the target. No. In any of Comparative Examples 18 to 27, the component composition was outside the proper range of the present invention, and therefore, one of the above-described characteristics was out of the target value.
なお、集合組織と曲げ特性との関係については、次のとおりであった。
すなわち、鋼板の板厚１／４位置の｛１１１｝面の集積度が０．８〜１．３の範囲であって、かつ板厚１／４位置の｛１００｝面の集積度が０．７〜１．２の範囲を満足する、Ｎｏ．１〜８、１１、１３、１５、１７は、いずれも目標の曲げ加工性を満足した。これに対し、集合組織が上記集積度の条件を満足していない、Ｎｏ．９、１０、１６、２７は、いずれも、目標の曲げ加工性を満足しなかった。なお、Ｎｏ．１８、２０、２２、２４は、比較例ではあるが、上記集積度の条件を満足していたために、いずれも曲げ加工性には優れていた。
The relationship between texture and bending properties was as follows.
That is, the integration degree of the {111} plane at the 1/4 thickness position of the steel sheet is in the range of 0.8 to 1.3, and the integration degree of the {100} plane at the 1/4 thickness position is 0.1. No. 7 to 1.2 is satisfied. 1 to 8, 11, 13, 15, and 17 all satisfied the target bending workability. On the other hand, the texture does not satisfy the above-mentioned accumulation degree condition. None of 9, 10, 16, and 27 satisfied the target bending workability. In addition, No. Although 18, 20, 22, and 24 were comparative examples, all of them were excellent in bending workability because they satisfied the above-described conditions of integration.
Claims (10)
Ｃ：０．１０〜０．２５％、
Ｓｉ：０．０５〜１．５％、
Ｍｎ：０．５〜２．０％、
Ｃｒ：０．３〜２．２％、
Ｍｏ：０．２〜１．４％、
Ｖ：０．０３〜０．１％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３％未満
を含有し、かつ下記（１）式の関係を満足し、残部はFeおよび不可避的不純物からなり、体積分率で９５％以上がマルテンサイト組織でかつ、該マルテンサイト組織における旧オーステナイト粒の平均粒径が円相当径で２０μｍ以下の鋼組織を有し、引張強さが１１５０ＭＰa以上であることを特徴とする曲げ加工性および低温靱性に優れる高張力鋼板。
記
０．８≦０．５［％Ｃｒ］＋１．２［％Ｍｏ］＋５［％Ｖ］≦２．１・・・（１）
但し、［％Ｍ］は、Ｍ元素の鋼中含有量(質量％) % By mass
C: 0.10 to 0.25%,
Si: 0.05 to 1.5%,
Mn: 0.5 to 2.0%
Cr: 0.3-2.2%
Mo: 0.2 to 1.4%,
V: 0.03-0.1%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: not more than 0.005% and B: less than 0.0003%, satisfying the relationship of the following formula (1), the balance is composed of Fe and inevitable impurities, and the volume fraction is 95% or more by weight. Bending workability and low-temperature toughness characterized by having a steel structure having a steel structure in which the average grain size of prior austenite grains in the martensite structure is equivalent to a circle diameter of 20 μm or less and the tensile strength is 1150 MPa or more. High-strength steel sheet with excellent resistance.
0.8 ≦ 0.5 [% Cr] +1.2 [% Mo] +5 [% V] ≦ 2.1 (1)
However, [% M] is the content of M element in steel (mass%)
Ｎｂ：０．０５％以下、
Ｃｕ：２％以下および
Ｎｉ：４％以下
のうちから選んだ一種または二種以上を含有することを特徴とする請求項１〜３のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板。 The steel sheet is mass%, and Nb: 0.05% or less,
The high tension excellent in bending workability and low-temperature toughness according to any one of claims 1 to 3, comprising one or more selected from Cu: 2% or less and Ni: 4% or less steel sheet.
Ｃａ：０．０１％以下、
ＲＥＭ：０．０２％以下および
Ｍｇ：０．０１％以下
のうちから選んだ一種または二種以上を含有することを特徴とする請求項１〜４のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板。 The steel sheet is mass%, and Ca: 0.01% or less,
The bending workability and low-temperature toughness according to any one of claims 1 to 4, comprising one or more selected from REM: 0.02% or less and Mg: 0.01% or less. High-strength steel sheet with excellent resistance.
質量％で、
Ｃ：０．１０〜０．２５％、
Ｓｉ：０．０５〜１．５％、
Ｍｎ：０．５〜２．０％、
Ｃｒ：０．３〜２．２％、
Ｍｏ：０．２〜１．４％、
Ｖ：０．０３〜０．１％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３％未満
を含有し、かつ下記（１）式の関係を満足し、残部はFeおよび不可避的不純物からなる鋼片を、熱間圧廷により熱延板としたのち、Ａｃ3点（℃）以上の温度に再加熱後、２℃／ｓ以上の平均冷却速度で３００℃以下の温度まで冷却し、ついで、３５０〜６００℃の範囲で焼戻し処理を施すことを特徴とする曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。
記
０．８≦０．５［％Ｃｒ］＋１．２［％Ｍｏ］＋５［％Ｖ］≦２．１・・・（１）
但し、［％Ｍ］は、Ｍ元素の鋼中含有量(質量％) A method for producing a high-tensile steel sheet having excellent bending workability and low-temperature toughness according to claim 1,
% By mass
C: 0.10 to 0.25%,
Si: 0.05 to 1.5%,
Mn: 0.5 to 2.0%
Cr: 0.3-2.2%
Mo: 0.2 to 1.4%,
V: 0.03-0.1%
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: 0.005% or less and B: less than 0.0003%, satisfying the relationship of the following formula (1), the balance being heated by hot pressing a steel slab comprising Fe and inevitable impurities After the sheet is formed, it is reheated to a temperature of Ac 3 point (° C) or higher, then cooled to a temperature of 300 ° C or lower at an average cooling rate of 2 ° C / s or higher, and then tempered in the range of 350 to 600 ° C. A method for producing a high-strength steel sheet excellent in bending workability and low-temperature toughness, characterized by being subjected to the treatment.
0.8 ≦ 0.5 [% Cr] +1.2 [% Mo] +5 [% V] ≦ 2.1 (1)
However, [% M] is the content of M element in steel (mass%)
Ｎｂ：０．０５％以下、
Ｃｕ：２％以下および
Ｎｉ：４％以下
のうちから選んだ一種または二種以上を含有することを特徴とする請求項６〜８のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。 The steel sheet is mass%, and Nb: 0.05% or less,
9. High tension excellent in bending workability and low-temperature toughness according to any one of claims 6 to 8, characterized by containing one or more selected from Cu: 2% or less and Ni: 4% or less A method of manufacturing a steel sheet.
Ｃａ：０．０１％以下、
ＲＥＭ：０．０２％以下および
Ｍｇ：０．０１％以下
のうちから選んだ一種または二種以上を含有することを特徴とする請求項６〜９のいずれかに記載の曲げ加工性および低温靱性に優れる高張力鋼板の製造方法。 The steel sheet is mass%, and Ca: 0.01% or less,
The bending workability and low-temperature toughness according to any one of claims 6 to 9, comprising one or more selected from REM: 0.02% or less and Mg: 0.01% or less. Method for producing high-strength steel sheets with excellent resistance.
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