JP5845674B2 - 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 106
- 239000010959 steel Substances 0.000 title claims description 106
- 238000005452 bending Methods 0.000 title claims description 42
- 238000004519 manufacturing process Methods 0.000 title claims description 20
- 229910000734 martensite Inorganic materials 0.000 claims description 26
- 238000005496 tempering Methods 0.000 claims description 26
- 229910001566 austenite Inorganic materials 0.000 claims description 19
- 230000010354 integration Effects 0.000 claims description 18
- 238000001816 cooling Methods 0.000 claims description 13
- 229910052804 chromium Inorganic materials 0.000 claims description 8
- 229910052750 molybdenum Inorganic materials 0.000 claims description 7
- 229910052720 vanadium Inorganic materials 0.000 claims description 7
- 239000012535 impurity Substances 0.000 claims description 6
- 229910052758 niobium Inorganic materials 0.000 claims description 6
- 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 20
- 238000010438 heat treatment Methods 0.000 description 16
- 239000000463 material Substances 0.000 description 15
- 229910052761 rare earth metal Inorganic materials 0.000 description 10
- 238000000034 method Methods 0.000 description 9
- 238000010276 construction Methods 0.000 description 7
- 239000000203 mixture Substances 0.000 description 7
- 238000005096 rolling process Methods 0.000 description 7
- 230000000052 comparative effect Effects 0.000 description 6
- 230000003111 delayed effect Effects 0.000 description 6
- 238000005098 hot rolling Methods 0.000 description 6
- 238000012360 testing method Methods 0.000 description 6
- 230000009466 transformation Effects 0.000 description 6
- 238000003466 welding Methods 0.000 description 6
- 238000005336 cracking Methods 0.000 description 5
- 230000006866 deterioration Effects 0.000 description 5
- 150000001247 metal acetylides Chemical class 0.000 description 5
- 150000004767 nitrides Chemical class 0.000 description 5
- 239000002245 particle Substances 0.000 description 5
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 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
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 3
- 238000002441 X-ray diffraction Methods 0.000 description 3
- 238000009825 accumulation Methods 0.000 description 3
- 239000013078 crystal Substances 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- 229910001563 bainite Inorganic materials 0.000 description 2
- 239000010953 base metal Substances 0.000 description 2
- 238000004364 calculation method Methods 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
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 230000003287 optical effect Effects 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
- 230000005855 radiation Effects 0.000 description 2
- 150000002910 rare earth metals Chemical class 0.000 description 2
- 238000003303 reheating Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 229910052719 titanium Inorganic materials 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
- 230000033228 biological regulation Effects 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
- 239000003795 chemical substances by application Substances 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
- 238000011156 evaluation Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 229910052738 indium Inorganic materials 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
- 229910052759 nickel Inorganic materials 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 238000011160 research Methods 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
- 238000009628 steelmaking Methods 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 229910052717 sulfur Inorganic materials 0.000 description 1
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Description
本発明は、主に建設機械や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板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 sheet that is suitable mainly for use in construction machinery, 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て、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, in the construction machinery and industrial machinery, ie construction industry machinery, tanks, penstock, line pipes, etc., due to the increase in the size of structures, the strength of steel materials used has increased, while the processing conditions for steel materials have increased. 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において、曲げ半径が板厚の３．０倍以上で割れが生じ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 bent at a bending radius of 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兼ね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, so various steel materials that combine high strength with 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の厚鋼板が開示されている。 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.
またさらに、特許文献４には、耐遅れ破壊特性に優れた安価な高強度厚鋼板として、板厚が６ｍｍ以上で引張強さが７８０ＭＰａ以上の高強度厚鋼板が開示されている。 Furthermore, Patent Document 4 discloses a high-strength thick steel plate having a thickness of 6 mm or more and a tensile strength of 780 MPa or more as an inexpensive high-strength thick steel plate having excellent delayed fracture resistance.
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、十分な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。
さらに、特許文献４では、曲げ加工性について考慮されておらず、また鋼の製造工程の中で焼入れのための加熱に際し、板厚中心部における平均加熱速度で５℃／ｓ以上の加熱速度を必要とするため、特殊な設備を必要とするところに問題を残していた。
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 examined this point and found that sufficient bending workability and strength cannot be achieved at the same time.
Furthermore, in Patent Document 4, bending workability is not considered, and when heating for quenching in the steel manufacturing process, an average heating rate of 5 ° C./s or more at an average heating rate in the center portion of the plate thickness is used. Because it was necessary, it left a problem where special equipment was required.
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、高強度を保持した上で、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、良好な曲げ特性を実現するためには、その集合組織を適正な範囲に制御することが好ましい。
（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. Furthermore, in order to achieve good bending characteristics, it is preferable to control the texture within an 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 components. Become.
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：０．００１〜０．０２４％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３〜０．００３％
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
０．６０≦〔％Ｃｒ〕＋０．６〔％Ｍｏ〕−９．５〔％Ｖ〕≦１．３０ --- （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: 0.001 to 0.024%,
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: 0.005% or less and B: 0.0003 to 0.003%
And the balance of the following formula (1) is satisfied, the balance is composed of Fe and inevitable impurities, the volume fraction is 95% or more of the martensite structure, and the average of the prior austenite grains in the martensite structure A high-tensile steel sheet having a steel structure having a diameter equivalent to a circle of 20 μm or less and a tensile strength of 1150 MPa or more and excellent in bending workability and low-temperature toughness.
0.60 ≦ [% Cr] +0.6 [% Mo] −9.5 [% V] ≦ 1.30 (1)
However, [% 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位置での｛１１１｝面の集積度が０．８〜１．３で、かつ板厚１／４位置での｛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 integration degree at the quarter thickness position is 0.8 to 1.3, and the {100} plane integration degree at the quarter thickness position is The high-tensile steel sheet having excellent bending workability and low-temperature toughness as described in 1 above, wherein the high-tensile steel sheet is 0.7 to 1.2.
３．前記鋼板が、質量％で、さらに
Ｎｂ：０．０５％以下、
Ｔｉ：０．１％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または２に記載の高張力鋼板。
3. The steel sheet is in mass%, and Nb: 0.05% or less,
Ti: 0.1% or less,
The high-tensile steel plate according to 1 or 2 above, containing one or more selected from Cu: 2% or less and Ni: 4% or less.
４．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乃至３のいずれかに記載の高張力鋼板。
4). The steel sheet is in mass%, and further Ca: 0.01% or less,
The high-tensile steel sheet according to any one of 1 to 3 above, containing one or more selected from REM: 0.02% or less and Mg: 0.01% or less.
５．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：０．００１〜０．０２４％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３〜０．００３％
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、熱間圧延により熱延板としたのち、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したのち、３００〜６００℃の温度域で焼戻し処理を施すことを特徴とする、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板の製造方法。
記
０．６０≦〔％Ｃｒ〕＋０．６〔％Ｍｏ〕−９．５〔％Ｖ〕≦１．３０ --- （１）
但し、〔％Ｍ〕は、Ｍ元素の鋼中含有量（質量％）
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: 0.001 to 0.024%,
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: 0.005% or less and B: 0.0003 to 0.003%
And the balance of the following formula (1) is satisfied, and the balance is made of hot-rolled steel slab consisting of Fe and inevitable impurities, and then heated to a temperature of Ac 3 point (° C) or higher. It is reheated, 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 a temperature range of 300 to 600 ° C., with a tensile strength of 1150 MPa or higher. A method for producing a high-tensile steel sheet having excellent bending workability and low-temperature toughness.
0.60 ≦ [% Cr] +0.6 [% Mo] −9.5 [% V] ≦ 1.30 (1)
However, [% M] is the content of M element in steel (mass%)
６.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位置での｛１００｝面の集積度を０．７〜１．２とすることを特徴とする、前記５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。 6. 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 {100} plane is integrated at the 1/4 thickness position. 6. The method for producing a high-strength steel sheet excellent in bending workability and low-temperature toughness as described in 5 above, wherein the degree is from 0.7 to 1.2.
７．前記鋼片が、質量％で、さらに
Ｎｂ：０．０５％以下、
Ｔｉ：０．１％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５または６に記載の高張力鋼板の製造方法。
7). The steel slab is mass%, and Nb: 0.05% or less,
Ti: 0.1% or less,
7. The method for producing a high-tensile steel plate according to 5 or 6 above, which contains one or more selected from Cu: 2% or less and Ni: 4% or less.
８．前記鋼片が、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５乃至７のいずれかに記載の高張力鋼板の製造方法。
8). The steel slab is by mass%, and Ca: 0.01% or less,
8. The method for producing a high-strength steel sheet according to any one of 5 to 7 above, comprising one or more selected from REM: 0.02% or less and Mg: 0.01% or less.
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、以下の説明において％で示す単位は、特に記載がある場合を除き全て質量％である。
Ｃ：０．１０〜０．２５％
Ｃは、焼入れによってマルテンサイト主体の組織とし、強度を確保するために添加するが、含有量が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の靭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％の範囲に限定する。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 the following description, all units represented by% are mass% unless otherwise specified.
C: 0.10 to 0.25%
C is a martensite-based structure by quenching, and is added to ensure strength. However, if the content is less than 0.10%, the effect is insufficient, while if it exceeds 0.25%, the base material and The C content is limited to a range of 0.10 to 0.25% in order not only to deteriorate the toughness of the heat affected zone but also to significantly reduce the weldability. Preferably it is 0.12 to 0.25% of range.
Ｓｉ：０．０５〜１．５％
Ｓｉは、製鋼段階の脱酸剤および強度向上元素として有効に寄与するが、含有量が０．０５％未満ではその効果が不十分であり、一方１．５％を超えると溶接性を著しく低下させるため、Ｓｉ量は０．０５〜１．５％の範囲に限定する。好ましくは０．１５〜１．０％の範囲である。
Si: 0.05 to 1.5%
Si contributes effectively as a deoxidizer and strength improving element in the steelmaking stage, but its effect is insufficient when its content is less than 0.05%, while it significantly decreases weldability when it exceeds 1.5%. Therefore, the Si amount is limited to a range of 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組織を得て、高強度を得るために必須の元素であり、０．５％以上含有させる必要があるが、２．０％を超える添加は、靭性および溶接性を著しく低下させるため、Ｍｎ量は０．５〜２．０％の範囲に限定する。
Mn: 0.5 to 2.0%
Mn is an essential element for improving hardenability, obtaining a martensite-based structure, and obtaining high strength, and it is necessary to contain 0.5% or more, but addition exceeding 2.0% In order to significantly reduce toughness and weldability, the Mn content is limited to a range of 0.5 to 2.0%.
Ｃｒ：０．３〜２．２％
Ｃｒは、強度および靭性の向上に有用な元素である。従って、マルテンサイト主体の組織とし、高強度化する場合に積極的に添加し、特に引張強さ１１５０ＭＰａ以上の特性を得るためには０．３％以上含有させる必要がある。より好ましくは０．５５％以上である。しかしながら、Ｃｒ含有量が２．２％を超えると、溶接性が低下するので、Ｃｒ量の上限は２．２％とする。
Cr: 0.3-2.2%
Cr is an element useful for improving strength and toughness. Therefore, in order to obtain a structure mainly composed of martensite and to increase the strength, it is necessary to add 0.3% or more particularly in order to obtain a characteristic having a tensile strength of 1150 MPa or more. More preferably, it is 0.55% or more. However, if the Cr content exceeds 2.2%, weldability deteriorates, so the upper limit of the Cr content is 2.2%.
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％以上含有させる。好ましくは０．３％以上である。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に達し、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。
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 that a tensile strength of 1150 MPa or more is obtained. Therefore, 0.2% or more is contained. Preferably it is 0.3% or more. However, if the Mo content exceeds 1.4%, the effect reaches saturation, and is rather disadvantageous in terms of cost. Therefore, the upper limit of the Mo amount is set to 1.4%.
Ｖ：０．００１〜０．０２４％
Ｖ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ため、０．００１％以上含有させる。好ましくは０．００５％以上である。しかしながら、本発明の焼戻し温度では、過剰のＶ添加は母材の強度上昇への寄与が小さく、一方で溶接性の劣化を招く。また、１１５０ＭＰａ級の高張力鋼の溶接継手部において、Ｖの炭化物や窒化物は溶接時の加熱によって粗大化し、溶接部靭性を劣化させるため、Ｖ量の上限は０．０２４％とする。
V: 0.001 to 0.024%
V improves the strength as a microalloying element, and at the same time, traps diffusible hydrogen by forming carbides, nitrides, and carbonitrides, thereby improving delayed fracture resistance. % Or more. Preferably it is 0.005% or more. However, at the tempering temperature of the present invention, excessive V addition contributes little to the increase in strength of the base material, while causing deterioration of weldability. Moreover, in the welded joint portion of high-strength steel of 1150 MPa class, V carbides and nitrides are coarsened by heating during welding and deteriorate the toughness of the welded portion, so the upper limit of the V amount is 0.024%.
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、含有量が０．００５％未満の場合にはその効果が十分でなく、一方０．１％を超えて含有させると鋼板の表面疵が発生し易くなるため、Ａｌ量は０．００５〜０．１％の範囲に限定する。
Al: 0.005 to 0.1%
Al effectively contributes as a deoxidizer and as an element for refining the crystal grain size, but when the content is less than 0.005%, the effect is not sufficient, while it exceeds 0.1%. If contained, the surface flaws of the steel sheet are likely to occur, so the Al content is limited to a range of 0.005 to 0.1%.
Ｎ：０．０００５〜０．００６％
Ｎは、Ｔｉや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靭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、含有量が０．０００５％未満では組織の微細化効果が充分にもたらされず、一方０．００６％を超えると析出物および／または介在物を増加させて加工性を損なうため、Ｎ量は０．０００５〜０．００６％の範囲に限定する。
N: 0.0005 to 0.006%
N has the effect of refining the structure by forming a nitride with Ti, Nb, etc., 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 achieved. On the other hand, if it exceeds 0.006%, precipitates and / or inclusions are increased and workability is impaired. It is limited to the range of 0.0005 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、含有量が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、低温靭性や耐遅れ破壊特性を劣化させるため、Ｐ量は０．０２％以下に限定する。
P: 0.02% or less P, which is an impurity element, is easily segregated at grain boundaries such as prior austenite grain boundaries during tempering, and if the content exceeds 0.02%, the bonding strength between adjacent crystal grains is increased. In order to reduce the low temperature toughness and delayed fracture resistance, the P content 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、含有量が０．００５％を超えると、介在物の量が多くなりすぎて引張試験など延性破壊の強度が低下し、加工性を劣化させるため、Ｓ量は０．００５％以下に限定する。好ましくは０．００２％以下である。
S: 0.005% or less S, which is an impurity element, easily forms MnS, which is a non-metallic inclusion, and if the content exceeds 0.005%, the amount of inclusion increases and ductility such as a tensile test occurs. The amount of S is limited to 0.005% or less in order to reduce the strength of fracture and deteriorate the workability. Preferably it is 0.002% or less.
Ｂ：０．０００３〜０．００３％
Ｂは、焼入性を高め、強度を向上させる作用を有しているので、０．０００３％以上含有させる。しかしながら、含有量が０．００３％を超えると、靭性を劣化させるので、Ｂ量の上限は０．００３％とする。
B: 0.0003 to 0.003%
B has a function of improving hardenability and improving strength, so 0.0003% or more is contained. However, if the content exceeds 0.003%, the toughness is deteriorated, so the upper limit of the B amount is 0.003%.
０．６０≦〔％Ｃｒ〕＋０．６〔％Ｍｏ〕−９．５〔％Ｖ〕≦１．３０ --- （１）
上掲した（１）式は、特に強度に関する指標である。
本発明で規定した引張強さ１１５０ＭＰａの高強度を得るためには、（〔％Ｃｒ〕＋０．６〔％Ｍｏ〕−９．５〔％Ｖ〕）を０．６０以上とする必要があるが、１．３０を超えると溶接性および溶接部靭性の著しい低下を招くことから、（〔％Ｃｒ〕＋０．６〔％Ｍｏ〕−９．５〔％Ｖ〕）は０．６０〜１．３０の範囲に限定した。好ましくは０．７０〜１．２０の範囲である。
0.60 ≦ [% Cr] +0.6 [% Mo] −9.5 [% V] ≦ 1.30 (1)
The above-described formula (1) is an index relating to strength in particular.
In order to obtain a high strength with a tensile strength of 1150 MPa defined in the present invention, ([% Cr] +0.6 [% Mo] −9.5 [% V]) needs to be 0.60 or more. If it exceeds 1.30, the weldability and weld toughness are significantly reduced, so that [[% Cr] +0.6 [% Mo] −9.5 [% V]) is 0.60 to 1.30. It was limited to the range. Preferably it is the range of 0.70-1.20.
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。
体積分率で９５％以上がマルテンサイトからなるマルテンサイト主体組織であり、マルテンサイトの平均粒径（旧オーステナイト平均粒径）が円相当径で２０μｍ以下
鋼組織は、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％未満の場合には、鋼の強度や靱性が不足するからである。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には影響を無視することができる。具体的には、例えば、ベイナイトやパーライト、セメンタイトなどのマルテンサイト以外の組織の合計の体積分率が５％以下であれば許容できる。より好ましくは体積分率で３％以下である。
さらに、該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が円相当径で２０μｍ以下であることが必要である。この理由は、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、変態後のマルテンサイト組織における靱性が劣化するためである。前記焼入れ前のオーステナイト粒の形態は、その後の熱処理後、オーステナイト粒界を優先的に腐食する腐食液で腐食して、金属組織を観察することにより、いわゆる旧オーステナイト粒界として観察することができる。よって、この組織観察結果から線分法や画像処理などの方法により求められる、旧オーステナイト粒の円相当径を以って、前記加熱時のオーステナイト粒径を把握することができる。
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. .
The martensite structure is composed of martensite with a volume fraction of 95% or more, and the average martensite particle size (former austenite average particle size) is 20 μm or less in terms of equivalent circle diameter. The steel structure has a volume fraction of 95% or more. Must be a martensite organization. This is because the strength and toughness of steel are insufficient when the volume fraction of the martensite structure is less than 95%. It should be noted that the volume fraction of the tissue other than martensite should be as small as possible, and the effect can be ignored when the volume fraction is low. Specifically, for example, it is acceptable if the total volume fraction of structures other than martensite, such as bainite, pearlite, and cementite, is 5% or less. More preferably, the volume fraction is 3% or less.
Further, in the martensite structure, it is necessary that the average particle diameter of the prior austenite grains is 20 μm or less in terms of the equivalent circle diameter. The reason for this is that the toughness in the martensite structure after transformation deteriorates when the austenite grains, which are the structure before quenching, are coarse grains having an average particle diameter exceeding 20 μm as the equivalent-circle diameter. The form of the austenite grains before quenching can be observed as a so-called prior austenite grain boundary by corroding the austenite grain boundary with a corrosive liquid that preferentially corrodes the austenite grain boundary and observing the metal structure after the subsequent heat treatment. . Therefore, the austenite grain size at the time of the heating can be grasped from the circle observation equivalent diameter of the prior austenite grains obtained by a method such as line segmentation or image processing.
本発明では、上記した基本成分の他、所望特性に応じて、さらにＮｂ、Ｔｉ、Ｃｕ、Ｎｉ、Ｃａ、ＲＥＭおよびＭｇのうちから選んだ一種または二種以上を適宜含有させることができる。
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、溶接熱影響部の靭性劣化を招く。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。
In the present invention, in addition to the basic components described above, one or more selected from Nb, Ti, Cu, Ni, Ca, REM, and Mg can be appropriately contained according to desired characteristics.
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.
Ｔｉ：０．１％以下
Ｔｉは、圧延加熱時あるいは溶接時にＴｉＮを生成して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靭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、０．１％を超える含有は溶接熱影響部の靭性を劣化させるため、Ｔｉを含有させる場合には、０．１％以下で含有させるものとした。
Ti: 0.1% or less Ti not only improves the toughness of the base metal and the weld heat-affected zone by generating TiN during rolling heating or welding to suppress the growth of austenite grains, 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 the content exceeding 0.1% deteriorates the toughness of the weld heat affected zone, when Ti is included, the content is 0.1% 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、靭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位置での｛１１１｝面の集積度を０．８〜１．３、板厚１／４位置での｛１００｝面の集積度を０．７〜１．２に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を４％以下とすることが好ましい。
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. Moreover, when Ni is contained excessively, the integration degree of {111} planes at the 1/4 thickness position of the steel sheet is 0.8 to 1.3, and the {100} plane integration at the 1/4 thickness position. Since it becomes difficult to set the degree to 0.7 to 1.2, from the viewpoint of the texture, when Ni is contained, the content is preferably 4% or less.
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量を低減して靭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 of 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.
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位置での｛１１１｝面の集積度が０．８〜１．３で、かつ板厚１／４位置での｛１００｝面の集積度が０．７〜１．２であることが好ましい。ここで、鋼板の板厚１／４位置での｛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よるＸ線回折強度を１とした場合の相対比の値）のことを意味し、｛１００｝面の集積度についても同様である。
鋼板の板厚１／４位置での｛１１１｝面の集積度が１．３超え、あるいは板厚１／４位置での｛１００｝面の集積度が１．２超えの場合には、曲げ加工性が低下するので好ましくない。また、鋼板の板厚１／４位置での｛１１１｝面の集積度が０．８未満、あるいは板厚１／４位置での｛１００｝面の集積度が０．７未満の場合には、たとえば｛１１０｝面など、｛１１１｝面や｛１００｝面以外の面の集合組織が発達し、鋼板内の劈開破面に異方性が生じることとなり、やはり、曲げ加工性を低下させるため、好ましくない。
Next, a suitable steel structure (aggregate structure) of the steel sheet of the present invention will be described.
In the present invention, the texture is an accumulation degree of {111} planes at a position of 1/4 of the steel sheet thickness of 0.8 to 1.3 and accumulation of {100} planes at a position of 1/4 of the sheet thickness. The degree is preferably 0.7 to 1.2. Here, the accumulation degree of {111} planes at the position of the steel sheet thickness ¼ is the random intensity ratio of X-ray diffraction of {111} planes parallel to the steel sheet surface (X by a material having a random texture). (The value of the relative ratio when the line diffraction intensity is 1)), and the same applies to the degree of integration of {100} planes.
When the integration degree of {111} planes at the 1/4 position of the steel plate exceeds 1.3, or when the integration degree of {100} planes at the 1/4 position of the thickness exceeds 1.2, bending is performed. Since workability falls, it is not preferable. In addition, when the integration degree of {111} plane at the 1/4 position of the steel plate is less than 0.8, or the integration degree of {100} plane at the 1/4 position of the plate thickness is less than 0.7 For example, a texture other than the {111} plane or {100} plane, such as the {110} plane, develops and anisotropy occurs in the cleavage plane in the steel sheet, which also decreases the bending workability. Therefore, 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、３００〜６００℃の温度域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, after the hot-rolled sheet is reheated to a temperature of Ac 3 point or higher and then cooled to a temperature of 300 ° C. or lower at an average cooling rate of 2 ° C./s or higher, a temperature of 300 to 600 ° C. is applied. Apply tempering treatment in the area.
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 There are no particular restrictions on the hot-rolling conditions, and it may be carried out according to conventional methods. 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
焼入れ温度をＡｃ3点以上とし、平均冷却速度：２℃／ｓ以上で３００℃以下まで冷却する焼入れ処理によって、オーステナイトからマルテンサイトへの変態を完了させることにより、母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、直接焼入れの場合に比べて圧延集合組織の発達が抑制され、曲げ加工性が向上することに加え、オーステナイト粒径の微細化が可能となり、高い強度と優れた靭性を両立することができる。
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、目標とするマルテンサイト主体組織が得られず、鋼板の強度や靭性が十分でなく、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し、目標とするマルテンサイト主体組織が得られず、鋼板の強度や靭性が低下するという問題が生じる。なお、焼入れ温度が高くなるほど変態前のオーステナイト粒径が粗大になるため、焼入れ温度は（Ａｃ3＋９０℃）以下とすることが好ましい。さらに、冷却停止温度が３００℃を上回っていると特に鋼板内部でのマルテンサイト変態が完了せず、鋼板の強度や靭性が低下するので、冷却停止温度は３００℃以下とした。
上記した冷却条件の規定において、平均冷却速度は板厚方向での冷却速度の平均値、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点は、例えば次式によって算出することができる。
Ａｃ3＝８５４−１７９．４〔％Ｃ〕＋４４．４〔％Ｓｉ〕−１３．９〔％Ｍｎ〕
−１７．８〔％Ｎｉ〕−１．７〔％Ｃｒ〕
但し、〔％Ｍ〕は、Ｍ元素の鋼中含有量（質量％）
Quenching conditions The quenching temperature is set to Ac 3 or higher and the average cooling rate: 2 ° C / s or higher to 300 ° C or lower, and the transformation from austenite to martensite is completed, thereby improving the toughness of the base metal. To do. 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 (in this specification, simply referred to as “quenching”), the development of the rolling texture is suppressed compared to the case of direct quenching, and bending workability is improved. In addition to the improvement, the austenite grain size can be reduced, and both high strength and excellent toughness can be achieved.
Here, the quenching temperature is Ac less than 3 points, for austenitization before quenching is not sufficient, not obtained martensite mainly the target tissue is, the strength and toughness is not sufficient for the steel sheet, also the average cooling rate If it is less than 2 ° C./s, a structure that transforms at a temperature higher than the martensitic transformation temperature such as bainite is generated before martensitic transformation, and the target martensitic main structure cannot be obtained, and the strength and toughness of the steel sheet are reduced. Problems arise. In addition, since the austenite grain size before transformation becomes coarse as the quenching temperature increases, the quenching temperature is preferably set to (Ac 3 + 90 ° C.) or less. Further, when the cooling stop temperature is higher than 300 ° C., martensitic transformation is not particularly completed inside the steel sheet, and the strength and toughness of the steel sheet are lowered. Therefore, the cooling stop temperature is set to 300 ° C. or lower.
In the regulation of the cooling conditions described above, the average cooling rate is the average value of the cooling rate in the plate thickness direction, and the cooling stop temperature is the temperature of the steel sheet surface immediately after completion of recuperation. Further, the Ac 3 point can be calculated by the following equation, for example.
Ac 3 = 854-179.4 [% C] Tasu44.4 [% Si] -13.9 [% Mn]
-17.8 [% Ni] -1.7 [% Cr]
However, [% M] is the content of M element in steel (mass%)
焼戻し条件
冷却後、強度と靭性を調整するため、焼戻し処理を行う。この焼戻し処理において、焼戻し温度が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共に靭性の低下を招き、一方３００℃に満たないと本来の焼戻しの効果が得られないので、焼戻し温度は３００〜６００℃の範囲とする。なお、焼戻し温度域での保持時間は、特に規定しないが、生産性の観点から３０ｍｉｎ以下とすることが望ましい。
上記した焼戻し条件の規定において、焼戻し温度は、鋼板表面温度測定値から伝熱計算によって求めた板厚中心部の温度とする。
また、焼戻し処理の際の加熱方式としては、誘導加熱、通電加熱、赤外線輻射加熱および雰囲気加熱などいずれもが有利に適合する。
Tempering conditions After cooling, tempering is performed to adjust strength and toughness. In this tempering process, if the tempering temperature exceeds 600 ° C., the martensite structure is recovered and the strength is lowered and the toughness is lowered. On the other hand, if the tempering temperature is less than 300 ° C., the original tempering effect cannot be obtained. The temperature is in the range of 300-600 ° C. The holding time in the tempering temperature range is not particularly defined, but is preferably 30 min or less from the viewpoint of productivity.
In the stipulation of the tempering conditions described above, the tempering temperature is the temperature at the center of the plate thickness obtained by heat transfer calculation from the measured value of the steel sheet surface temperature.
In addition, as a heating method in the tempering process, any of induction heating, current heating, infrared radiation heating, atmosphere heating, and the like is advantageously adapted.
表１に示す成分組成になる鋼（鋼Ａ〜Ｐ）を溶製後、スラブに鋳造し、ついで加熱炉で加熱したのち、熱間圧延に供して種々の板厚の熱延鋼板とした。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〜Ｇは、本発明の成分組成範囲を満足する適合鋼であり、鋼Ｈ〜Ｐは、本発明の成分組成範囲を逸脱した比較鋼である。
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。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℃、圧延仕上げ温度は８００℃、直接焼入れ開始温度は７７０℃とした。
Steels (steel A to P) having the composition shown in Table 1 were melted, cast into slabs, then heated in a heating furnace, and then subjected to hot rolling to obtain hot-rolled steel sheets 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. Steels A to G are compatible steels that satisfy the component composition range of the present invention, and steels H to P are comparative steels that deviate from the component composition range 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. 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.
なお、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。
鋼組織は、鋼板のミクロ組織は、板厚方向１／２の位置から圧延方向断面を観察面として、組織観察用試験片を採取して観察した。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。また、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度を測定した。靭性は、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。
また、曲げ加工性は、以下の基準により、○、×で評価した。
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°としたときの割れの有無で評価した。
○：割れなし
×：割れあり
さらに、溶接部靭性は、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、前述のように、｛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 temperature measurement results at the surface sequentially with a radiation thermometer.
As for the steel structure, the microstructure of the steel sheet was observed by collecting a test piece for observing the structure from the position in the sheet thickness direction 1/2 with the cross section in the rolling direction as the observation surface. The volume fraction of martensite was measured by image processing for a field of view (250 × 200 μm 2 ) that appeared in a nital solution and observed with an optical microscope. In addition, the grain size of the prior austenite grains in the martensite structure is a field of view observed with an optical microscope after performing a corrosive treatment with a corrosive liquid preferentially corroding 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 the tensile strength were measured using 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.
Moreover, bending workability was evaluated by ○ and × according to the following criteria.
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 °.
○: No cracking x: Cracking Furthermore, the weld zone toughness is an absorption energy value at −40 ° C. obtained by making a reproducible thermal cycle test piece simulating welding heat input: 3 kJ / mm and by Charpy impact test. evaluated. 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.
表２に示したとおり、成分組成および製造条件が本発明の適正範囲を満足するＮｏ．１〜４、６〜９、１２、１４、１５、１７に示した発明例はいずれも、強度、靭性、曲げ特性およびＨＡＺ（溶接熱影響部）靭性のいずれもが目標値を満足している。
これに対し、Ｎｏ．５の平均冷却速度が下限に満たない比較例は、マルテンサイト主体の組織が得られず、強度および靱性とも目標値に達しなかった。Ｎｏ．１０，１１，１８に示した直接焼入れ焼戻しにより製造した比較例はいずれも、曲げ特性が目標値を満足しなかった。Ｎｏ．１３の比較例は、焼戻し温度が低いため強度は高かったものの、靭性が目標値を満足しなかった。Ｎｏ．１６の比較例は、焼戻し温度が高いため、強度が目標値を満足していない。Ｎｏ．１９〜２７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、いずれかの特性が目標値から外れていた。
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、本発明で好適範囲として規定した、鋼板の板厚１／４位置での｛１１１｝面の集積度が０．８〜１．３で、かつ板厚１／４位置での｛１００｝面の集積度が０．７〜１．２を満足した場合（Ｎｏ．１〜４、６〜９、１２、１４、１５、１７、１９、２１、２３、２５）はいずれも、目標の曲げ加工性を満足した。これに対し、集合組織が上記した好適範囲を満足しなかった場合（Ｎｏ．１０、１１、１８、２７）はすべて、曲げ特性が目標を満足しなかった。
As shown in Table 2, the component composition and production conditions satisfy No. 1 in the proper range of the present invention. In all of the invention examples shown in 1-4, 6-9, 12, 14, 15, 17, the strength, toughness, bending characteristics, and HAZ (welding heat affected zone) toughness all satisfy the target values. .
In contrast, no. In the comparative example in which the average cooling rate of 5 was less than the lower limit, a martensite-based structure was not obtained, and the strength and toughness did not reach the target values. No. In all of the comparative examples manufactured by direct quenching and tempering shown in 10, 11 and 18, the bending characteristics did not satisfy the target value. No. In Comparative Example 13, although the tempering temperature was low and the strength was high, the toughness did not satisfy the target value. No. Since the comparative example of 16 has a high tempering temperature, the strength does not satisfy the target value. No. In any of Comparative Examples 19 to 27, the component composition was outside the appropriate range of the present invention, and therefore, any of the characteristics was out of the target value.
The relationship between texture and bending properties was as follows. That is, the degree of integration of {111} planes at the 1/4 thickness position of the steel plate defined as a preferred range in the present invention is 0.8 to 1.3 and {100} at the 1/4 thickness position. When the integration degree of the surface satisfies 0.7 to 1.2 (No. 1 to 4, 6 to 9, 12, 14, 15, 17, 19, 21, 23, 25), all of the target bending Satisfied processability. On the other hand, when the texture did not satisfy the above-described preferred range (No. 10, 11, 18, 27), the bending characteristics did not satisfy the target.
Claims (8)
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：０．００１〜０．０２４％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３〜０．００３％
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
０．６０≦〔％Ｃｒ〕＋０．６〔％Ｍｏ〕−９．５〔％Ｖ〕≦１．３０ --- （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: 0.001 to 0.024%,
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: 0.005% or less and B: 0.0003 to 0.003%
And the balance of the following formula (1) is satisfied, the balance is composed of Fe and inevitable impurities, the volume fraction is 95% or more of the martensite structure, and the average of the prior austenite grains in the martensite structure A high-tensile steel sheet having a steel structure having a diameter equivalent to a circle of 20 μm or less and a tensile strength of 1150 MPa or more and excellent in bending workability and low-temperature toughness.
0.60 ≦ [% Cr] +0.6 [% Mo] −9.5 [% V] ≦ 1.30 (1)
However, [% M] is the content of M element in steel (mass%)
Ｎｂ：０．０５％以下、
Ｔｉ：０．１％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または２に記載の高張力鋼板。 The steel sheet is in mass%, and Nb: 0.05% or less,
Ti: 0.1% or less,
The high-tensile steel sheet according to claim 1 or 2, comprising one or more selected from Cu: 2% or less and Ni: 4% or less.
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乃至３のいずれかに記載の高張力鋼板。 The steel sheet is in mass%, and further Ca: 0.01% or less,
The high-tensile steel sheet according to any one of claims 1 to 3, comprising one or more selected from REM: 0.02% or less and Mg: 0.01% or less.
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：０．００１〜０．０２４％、
Ａｌ：０．００５〜０．１％、
Ｎ：０．０００５〜０．００６％、
Ｐ：０．０２％以下、
Ｓ：０．００５％以下および
Ｂ：０．０００３〜０．００３％
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、熱間圧延により熱延板としたのち、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したのち、３００〜６００℃の温度域で焼戻し処理を施すことを特徴とする、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板の製造方法。
記
０．６０≦〔％Ｃｒ〕＋０．６〔％Ｍｏ〕−９．５〔％Ｖ〕≦１．３０ --- （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: 0.001 to 0.024%,
Al: 0.005 to 0.1%,
N: 0.0005 to 0.006%,
P: 0.02% or less,
S: 0.005% or less and B: 0.0003 to 0.003%
And the balance of the following formula (1) is satisfied, and the balance is made of hot-rolled steel slab consisting of Fe and inevitable impurities, and then heated to a temperature of Ac 3 point (° C) or higher. It is reheated, 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 a temperature range of 300 to 600 ° C., with a tensile strength of 1150 MPa or higher. A method for producing a high-tensile steel sheet having excellent bending workability and low-temperature toughness.
0.60 ≦ [% Cr] +0.6 [% Mo] −9.5 [% V] ≦ 1.30 (1)
However, [% M] is the content of M element in steel (mass%)
Ｎｂ：０．０５％以下、
Ｔｉ：０．１％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５または６に記載の高張力鋼板の製造方法。 The steel slab is mass%, and Nb: 0.05% or less,
Ti: 0.1% or less,
The method for producing a high-strength steel sheet according to claim 5 or 6, comprising one or more selected from Cu: 2% or less and Ni: 4% or less.
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５乃至７のいずれかに記載の高張力鋼板の製造方法。 The steel slab is by mass%, and Ca: 0.01% or less,
The method for producing a high-tensile steel plate according to any one of claims 5 to 7, comprising one or more selected from REM: 0.02% or less and Mg: 0.01% or less. .
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