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CN104583439A - Thick steel plate having good ultralow-temperature toughness - Google Patents

Thick steel plate having good ultralow-temperature toughness Download PDF

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Publication number
CN104583439A
CN104583439A CN201380043453.6A CN201380043453A CN104583439A CN 104583439 A CN104583439 A CN 104583439A CN 201380043453 A CN201380043453 A CN 201380043453A CN 104583439 A CN104583439 A CN 104583439A
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steel
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CN104583439B (en
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名古秀德
伊庭野朗
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Kobe Steel Ltd
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Kobe Steel Ltd
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Abstract

A thick steel plate that contains definite components in the steel, said thick steel plate satisfying the following requirements: the average roundness (A) of inclusions present in the steel, the equivalent circle diameter of said inclusions exceeding 1.0[mu]m, is not greater than 1.8; the volume fraction (V) of the residual austenite phase at -196 DEG C is 2.0-12.0%; and value (B) represented by formula (1) is 1.3 or greater. B=V2/3/A (1)

Description

The Plate Steel of pole excellent in low temperature toughness
Technical field
The present invention relates to the Plate Steel of pole excellent in low temperature toughness, in detail, even if relate to Ni content to be reduced to about 5.0 ~ 7.5%, the Plate Steel that the toughness [particularly the toughness of plate width direction (C direction)] under the pole low temperature below-196 DEG C is also good.Below, to be described centered by the Plate Steel (representational have storage tank, transport-ship etc.) of the natural gas liquids (LNG) exposed to the open air under the low temperature of above-mentioned pole, but Plate Steel of the present invention is not limited to this meaning, but whole Plate Steels of use in purposes under being suitable for exposing to the open air the pole low temperature below-196 DEG C.
Background technology
For the LNG tank Plate Steel of the storage tank of natural gas liquids (LNG), except requiring high strength, also require the high tenacity of the pole low temperature that can tolerate-196 DEG C.Up to now, as the Plate Steel for such use, use the Plate Steel of Ni (9%Ni steel) containing about 9%, but in recent years, because the cost increase of Ni, even so under lower than the low Ni content of 9% exploitation of the Plate Steel that pole low-temperature flexibility is still excellent advanced.
Such as, non-patent literature 1 describes and to coexist the impact of territory thermal treatment on the low-temperature flexibility of 6%Ni steel about α-γ two-phase.In detail, describe and to coexist the thermal treatment (L process) of territory (between Ac1 ~ Ac3) by being applied to α-γ two-phase before temper, can give be subject to the 9%Ni steel of common Q-tempering process equal above, the pole low-temperature flexibilities of-196 DEG C; This thermal treatment also makes the toughness of C direction (plate width direction) test film improve; These effects due to a large amount of fine and still keep the existence of stable retained austenite to produce for shock load under the low temperature of pole, etc.But, according to aforesaid method, although the pole excellent in low temperature toughness of rolling direction (L direction), there is the tendency that the pole low-temperature flexibility of plate width direction (C direction) is poorer than L direction.In addition, the record of brittle rupture rate is not had.
The technology same with above-mentioned non-patent literature 1 is also documented in patent documentation 1 and patent documentation 2.Wherein, following method is described: after hot rolling is carried out for the steel be controlled in specialized range such as Ni, austenitic grain size containing 4.0 ~ 10% in patent documentation 1, be heated between Ac1 ~ Ac3, then cool, this process (being equivalent to the L process described in above-mentioned non-patent literature 1) is repeated after once or twice, carry out tempering with the temperature below Ac1 transformation temperature.In addition, in patent documentation 2, describe following method: for containing 4.0 ~ 10% Ni, make the size of the AlN before hot rolling be the steel of less than 1 μm, carry out the thermal treatment (L process → temper) same with above-mentioned patent documentation 1.Impact value (the vE of-196 DEG C that record in these methods -196) probably infer it is L direction, the above-mentioned toughness value in C direction is unclear.In addition, in these methods consideration is not given for intensity, there is no the record of brittle rupture rate.
In addition, in non-patent literature 2, the exploitation about the 6%Ni steel of the LNG storage tank above-mentioned L process (two phase region quench treatment) and TMCP combined is recorded.According to the document, although the toughness of the rolling direction of describing (L direction) demonstrates high value, there is no the toughness value in recording plate cross direction (C direction).
Describe in patent documentation 3 and a kind ofly contain the Ni of 0.3 ~ the 10% and Mg of specified amount, and make regulation particle diameter suitably disperse containing Mg oxide particle, the high tenacity high tension steel of more than 570MPa level toughness of welded zone excellence.Describe in above-mentioned patent documentation 3: make heating austenite particle diameter obtain miniaturization by the control containing Mg oxide compound, the toughness of mother metal and welding heat affected zone (HAZ) improves; For this reason, O (oxygen) amount before deoxidant element adds and the order of addition of Mg and other deoxidant element very important, be after adding Mg, Ti, Al in the steel-making of 0.001 ~ 0.02% at soluble oxygen content simultaneously, carry out casting and becoming steel billet, or when adding Mg, Ti, Al, after finally adding Al, then carry out casting and becoming steel billet.The toughness value (fracture transition temperature vTrs) in C direction is described in the embodiment of above-mentioned patent documentation 3, the above-mentioned characteristic good (fracture transition temperature vTrs≤-196 DEG C) of 9%Ni steel, but the above-mentioned characteristic of the Ni steel of 5% neighborhood is-140 DEG C, require to improve further.
And then, describe following technology in patent documentation 4: make austenitic distribution uniformity by the Ni adding 5.0 ~ 7.5%, thus obtain toughness (CTOD characteristic), stress, the unstable Plate Steel destroying rejection characteristic excellence of mother metal and welding joint.But the evaluation temperature in CTOD test is-165 DEG C, slightly high, it not the public technology towards the extremely low temperature of less than-196 DEG C.In addition, even if scrutinize patent documentation 4, the record of the brittle rupture rate in Charpy impact absorption test is not had completely yet.In addition, in order to manufacture the Plate Steel recorded in patent documentation 4, must carry out the long-time heating process at 1250 ~ 1380 DEG C under 8 ~ 50 hours this high temperature before hot rolling, be disadvantageous from the viewpoint of manufacturing cost.
Prior art document
Patent documentation
Patent documentation 1: Japanese Laid-Open Patent Publication 49-135813 publication
Patent documentation 2: Japanese Laid-Open Patent Publication 51-13308 publication
Patent documentation 3: Japanese Unexamined Patent Publication 2001-123245 publication
Patent documentation 4: Japanese Patent No. 4975888 publication
Non-patent literature
Non-patent literature 1: vow open country etc., the shadow Ring of territory Hot process " the Di Wen Toughness To of 6%Ni Steel and the Pot The α-γ two-phase coexist " (α-γ two-phase coexist territory thermal treatment bring the impact of the low-temperature flexibility of 6%Ni steel), iron and steel, the 59th year (1973) No. 6, p752 ~ 763
Non-patent literature 2: Gu Gu etc., " the L N G タ Application Network development of 6%Ni Steel " (exploitation of LNG storage tank 6%Ni steel), CAMP-ISI J, Vol.23 (2010), p1322
Summary of the invention
The problem that invention will solve
As mentioned above, up to now, be in the Ni steel of about 5.0 ~ 7.5% at Ni content, although the technology of the pole excellent in low temperature toughness of-196 DEG C is suggested, the pole low-temperature flexibility in C direction is not well studied.Particularly be strongly required strength of parent high (in detail, tensile strength TS > 690MPa, yield strength YS > 590MPa) the further raising (the pole low-temperature flexibility in C direction improves) of pole low-temperature flexibility under high strength.
In addition, in the above documents, not about the research of brittle rupture rate.Brittle rupture rate is the ratio of the brittle rupture produced when representing applied load in Charpy impact test.At the position that there occurs brittle rupture, till destruction, the energy that steel absorb significantly diminishes, destroy easily development, therefore particularly in order to suppress the destruction of pole low temperature, the brittle rupture rate occurred in general Charpy impact test is suppressed in low-level (less than 10%) to be extremely important condition.But because intensity is higher, brittle rupture more easily occurs, so it is generally difficult for realizing brittle rupture rate≤10% under high strength of parent as described above.Therefore, in the high-strength steel plate that strength of parent is high, the technology possessing these two aspects not yet proposes.
The present invention makes in view of the foregoing, its object is to provide a kind of high-strength steel plate, it is in the Ni steel of about 5.0 ~ 7.5% at Ni content, and the pole low-temperature flexibility (particularly the pole low-temperature flexibility in C direction) that can realize-196 DEG C is excellent, brittle rupture rate≤10%.
For solving the means of problem
The Plate Steel of the pole of the present invention excellent in low temperature toughness of above-mentioned problem can be solved, it has following main idea: in mass % containing C:0.02 ~ 0.10%, below Si:0.40% (not containing 0%), Mn:0.50 ~ 2.0%, below P:0.007% (not containing 0%), below S:0.007% (not containing 0%), Al:0.005 ~ 0.050%, Ni:5.0 ~ 7.5%, below N:0.010% (not containing 0%), surplus is iron and inevitable impurity, the average roundness (A) of the inclusion of the equivalent circle diameter existed in steel plate more than 1.0 μm is less than 1.8, 2.0 ~ 12.0% are met at the volume fraction (V) of-196 DEG C of retained austenite phases existed, and the B value represented by following (1) formula is more than 1.3.
B＝V 2/3/A···(1)
Of the present invention preferred embodiment in, above-mentioned steel plate is:-196 DEG C exist retained austenite in volume fraction satisfied 4.0% ~ 12.0%.
In a preferred embodiment of the invention, above-mentioned steel plate is: also containing being selected from below Cu:1.00% (not containing 0%), below Cr:1.2% (not containing 0%), below Mo:1.00% (not containing 0%), below Ti:0.025% (not containing 0%), below Nb:0.10% (not containing 0%), below V:0.50% (not containing 0%), below B:0.0050% (not containing 0%), below Ca:0.0030% (not containing 0%), below REM:0.0050% (not containing 0%), at least one in below Zr:0.0050% (not containing 0%).
The effect of invention
According to the present invention, a kind of high-strength steel plate can be provided, it is reduced to the low Ni steel of 5.0 ~ 7.5% at Ni content, even if strength of parent is high (in detail, tensile strength TS > 690MPa, yield strength YS > 590MPa), pole low-temperature flexibility (particularly the pole low-temperature flexibility in C direction) below-196 DEG C is also excellent, specifically, in the Charpy impact absorption test in C direction, meet brittle rupture rate≤10% (preferably brittle rupture rate≤50% of-233 DEG C) of-196 DEG C.
Embodiment
The present inventor etc. are studied to provide following pole low-temperature flexibility to develop skill, namely, be reduced to less than 7.5% at Ni content and meet in the high-strength steel plate of tensile strength TS > 690MPa, yield strength YS > 590MPa, in the Charpy impact value in C direction, meet brittle rupture rate≤10% of-196 DEG C.It found that, about (a) the volume fraction V of retained austenite (residual γ) phase existed at-196 DEG C controlled 2.0 ~ 12.0% [preferably controlling is 4.0% ~ 12.0% (volume fraction)] and (b) for the inclusion of the equivalent circle diameter of development encouraging brittle rupture more than 1.0 μm (hereinafter sometimes referred to simply as inclusion.), the object of expectation can be reached when the average roundness A of above-mentioned inclusion being reduced to less than 1.8 and the B value shown in following (1) formula being controlled more than 1.3, thus complete the present invention.
B＝V 2/3/A···(1)
The characteristic particularly should write volumes about in the relation of aforesaid prior art is the latter (b).Below, be described for reaching process of the present invention.
The present inventor etc. are repeatedly study at the Plate Steel of the pole low-temperature flexibility of-196 DEG C also excellence in the Ni steel of less than 7.5% to be provided in Ni content.Specifically, in the present invention, from the view point of provide meet C direction brittle rupture rate≤10%, tensile strength TS > 690MPa, yield strength YS > 590MPa the high-strength steel plate of pole excellent in low temperature toughness of all characteristics, first, the method shown in document recorded in prior art is studied.
Pointing out in above-mentioned document, in order to improve the pole low-temperature flexibility of 5%Ni steel, importantly making in the retained austenite existed at-196 DEG C (residual γ) stabilization.In addition, if consider manufacture method, then it is recommended that in the steel-making stage, control the soluble oxygen content before deoxidant element interpolation, in this steel-making, finally add Al and cast, and α-γ two-phase coexist territory (between Ac1 ~ Ac3) thermal treatment (L process) after, carry out the method for temper with the temperature below Ac1 transformation temperature, enlightenment has pole low-temperature flexibility to improve thus.But, according to the result of study of the present inventor etc., although know that the pole low-temperature flexibility in L direction when utilizing aforesaid method improves, the pole low-temperature flexibility in C direction is insufficient, can not realize the above-mentioned target rank (brittle rupture rate≤10% in C direction) pointed out in the present invention.
Therefore carried out further repeatedly studying, result is known, in order to obtain the Plate Steel of the pole excellent in low temperature toughness of wishing, must substantially follow above-mentioned technology, while add feature further in Plate Steel and manufacture method thereof.In detail, i () is in Plate Steel, the volume fraction V of the residual γ phase at-196 DEG C is existed in the scope of V=2.0 ~ 12.0%, in addition, be conceived to the inclusion of equivalent circle diameter more than 1.0 μm distinguishing the development encouraging brittle rupture, the average roundness A of above-mentioned inclusion is reduced to less than 1.8, and will representing that the average roundness A of above-mentioned inclusion and the B value shown in the formula (1) of the relation of the volume fraction V (%) of-196 DEG C of above-mentioned residual γ phases existed control to be B value >=1.3, this is effective, (ii) in order to manufacture such Plate Steel, except the control of the soluble oxygen content (free O amount) before the Al interpolation in steel-making stage, beyond the temper in thermal treatment (L the process) → specified temperature region between the Ac1 ~ Ac3 after hot rolling, the further control in steel-making stage is effective, to control (to be positioned at the value of 1/2 position of the thick t of slab) below 300 seconds the cooling time (t1) of 1450 ~ 1500 DEG C during casting and the cooling time (t2) of 1300 ~ 1200 DEG C during casting be controlled (to be positioned at the value of 1/4 position of the thick t of slab) below 680 seconds effectively.
In addition find, c () is in above-mentioned (a), by the residual γ phase control-196 DEG C of existence at 4.0% ~ 12.0% (volume fraction), even if at-233 DEG C of more low temperature, also brittle rupture rate can be remained on the good level of less than 50%, (d) in order to manufacture such Plate Steel, after hot rolling, it is effective for carrying out the maintenance of specified time in thermal treatment (L process) between Ac1 ~ Ac3, thus completes the present invention.
In this manual, so-called " pole excellent in low temperature toughness ",, when measuring the brittle rupture rate in the Charpy impact absorption test in C direction (plate width direction) according to the method described in embodiment hurdle described later, meet brittle rupture rate≤10% of-196 DEG C.In embodiment described later, although the brittle rupture rate of L direction (rolling direction) cannot measure, if it is based on the brittle rupture rate in C direction below 10%, then the brittle rupture rate in L direction also inevitable thumb rule such below 10%.
In this specification sheets, " Plate Steel " refers to that the thickness of steel plate is roughly 6 ~ 50mm.
In addition in the present invention, with meet tensile strength TS > 690MPa, yield strength YS > 590MPa high-strength steel plate for object.
Below, Plate Steel of the present invention is described in detail.
As mentioned above, Plate Steel of the present invention, it is characterized in that, it is in mass % containing C:0.02 ~ 0.10%, below Si:0.40% (not containing 0%), Mn:0.50 ~ 2.0%, below P:0.007% (not containing 0%), below S:0.007% (not containing 0%), Al:0.005 ~ 0.050%, Ni:5.0 ~ 7.5%, below N:0.010% (not containing 0%), surplus is the Plate Steel of iron and inevitable impurity, the average roundness (A) of the inclusion of the equivalent circle diameter existed in steel plate more than 1.0 μm is less than 1.8, 2.0 ~ 12.0% are met at the volume fraction (V) of-196 DEG C of retained austenite phases existed, and the B value represented by following (1) formula is more than 1.3.
B＝V 2/3/A···(1)
First, composition in steel is described.
C：0.02～0.10％
C is the necessary element guaranteeing intensity and retained austenite.In order to effectively play such effect, the lower limit that C is measured is more than 0.02%.The preferred lower limit of C amount is more than 0.03%, is more preferably more than 0.04%.But, if add superfluously, then because the excessive rising of intensity causes pole low-temperature flexibility to reduce, therefore make its upper limit be less than 0.10%.The preferred upper limit of C amount is less than 0.08%, is more preferably less than 0.06%.
Below Si:0.40% (not containing 0%)
Si is as the useful element of deoxidation material.But if add superfluously, then the generation of the island-like martensite phase of hard is promoted, pole low-temperature flexibility reduces, and therefore makes its upper limit be less than 0.40%.The preferred upper limit of Si amount is less than 0.35%, is more preferably less than 0.20%.
Mn：0.50～2.0％
Mn also works as deoxidation material, is austenite (γ) stabilizing element, is the element contributing to increasing residual γ amount.In order to effectively play such effect, the lower limit that Mn is measured is 0.50%.The preferred lower limit of Mn amount is more than 0.6%, is more preferably more than 0.7%.But, if add superfluously, then cause temper embrittlement, the pole low-temperature flexibility of wishing can not be guaranteed, therefore make its upper limit be less than 2.0%.The preferred upper limit of Mn amount is less than 1.5%, is more preferably less than 1.3%.
Below P:0.007% (not containing 0%)
P is the impurity element of the reason forming intercrystalline failure, in order to ensure the pole low-temperature flexibility of hope, makes its upper limit be less than 0.007%.The preferred upper limit of P amount is less than 0.005%.P amount is more few better, but the industrial P of making amount is 0% to have any problem.
Below S:0.007% (not containing 0%)
S is also same with above-mentioned P, is the impurity element of the reason forming intercrystalline failure, in order to ensure the pole low-temperature flexibility of hope, makes its upper limit be less than 0.007%.As shown in embodiment described later, if S quantitative change is many, then brittle rupture rate increases, and can not realize the pole low-temperature flexibility (brittle rupture rate≤10% of-196 DEG C) of wishing.The preferred upper limit of S amount is less than 0.005%.If S amount is few, then more few better, but the industrial S of making amount is 0 to have any problem.
Al：0.005～0.050％
Al is deoxidant element.If Al contains quantity not sufficient, free oxygen concentration then in steel-making rises, in the process of casting cooling, the surface recombination being present in the inclusion in steel-making at script generates oxide compound or these secondary inclusion things of sulfide, the shape of inclusion changes thus, the average roundness of the inclusion of equivalent circle diameter more than 1.0 μm becomes large, therefore its lower limit is set to more than 0.005%.The preferred lower limit of Al amount is more than 0.010%, is more preferably more than 0.015%.But add if superfluous, then the aggegation of above-mentioned inclusion, zoarium are promoted, and the average roundness of this inclusion still becomes large, therefore its upper limit is set to less than 0.050%.The preferred upper limit of Al amount is less than 0.045%, is more preferably less than 0.04%.
Ni：5.0～7.5％
Ni is for guaranteeing the element necessary for the retained austenite (residual γ) that the raising of pole low-temperature flexibility is useful.In order to effectively play such effect, the lower limit that Ni is measured is more than 5.0%.The preferred lower limit of Ni amount is more than 5.2%, is more preferably more than 5.4%.But if add superfluously, then the cost causing raw material is high, its upper limit is therefore made to be less than 7.5%.The preferred upper limit of Ni amount is less than 7.0%, is more preferably less than 6.5%, more preferably less than 6.0%.
Below N:0.010% (not containing 0%)
N makes pole low-temperature flexibility reduce due to strain aging, therefore make its upper limit be less than 0.010%.The preferred upper limit of N amount is less than 0.006%, is more preferably less than 0.004%.
Plate Steel of the present invention contains mentioned component as basal component, and surplus is iron and inevitable impurity.
In the present invention, to give for the purpose of characteristic further, can containing following selection component.
Below Cu:1.00% (not containing 0%)
Cu is γ stabilizing element, is the element contributing to the increase that residual γ measures.In order to effectively play such effect, Cu is preferably made to contain more than 0.05%.But, if add superfluously, then cause the excessive raising of intensity, can not get the pole low-temperature flexibility effect of wishing, therefore preferably make its upper limit be less than 1.00%.More preferably the upper limit of Cu amount is less than 0.8%, more preferably less than 0.7%.
Be selected from least one in below Cr:1.2% (not containing 0%) and below Mo:1.00% (not containing 0%)
Cr and Mo is all that intensity improves element.These elements can add separately, also can two kinds and use.In order to effectively play above-mentioned effect, preferably making Cr measure is more than 0.05%, and it is more than 0.01% that Mo is measured.But, if add superfluously, then cause the excessive raising of intensity, the pole low-temperature flexibility of wishing can not be guaranteed, therefore the preferred upper limit of Cr amount be less than 1.2% (be more preferably less than 1.1%, more preferably less than 0.9%, be further preferably less than 0.5%), the preferred upper limit of Mo amount be less than 1.00% (be more preferably less than 0.8%, more preferably less than 0.6%).
Be selected from least one in below Ti:0.025% (not containing 0%), below Nb:0.10% (not containing 0%) and below V:0.50% (not containing 0%)
Ti, Nb and V, all as Carbonitride Precipitation, are the elements making intensity increase.These elements can add separately, also can be two or more and use.In order to effectively play above-mentioned effect, preferably making Ti measure is more than 0.005%, and it is more than 0.005% that Nb is measured, and it is more than 0.005% that V is measured.But, if add superfluously, then cause the excessive raising of intensity, the pole low-temperature flexibility of wishing can not be guaranteed, therefore the preferred upper limit of Ti amount is less than 0.025% (be more preferably less than 0.018%, more preferably less than 0.015%), the preferred upper limit of Nb amount is less than 0.10% (be more preferably less than 0.05%, more preferably less than 0.02%), the preferred upper limit of V amount be less than 0.50% (be more preferably less than 0.3%, more preferably less than 0.2%).
Below B:0.0050% (not containing 0%)
B be improved by hardenability and contribute to intensity improve element.In order to effectively play above-mentioned effect, preferably making B measure is more than 0.0005%.But, if add superfluously, then bring the excessive raising of intensity, the pole low-temperature flexibility of wishing can not be guaranteed, the preferred upper limit therefore making B measure be less than 0.0050% (be more preferably less than 0.0030%, more preferably less than 0.0020%).
Be selected from least one in below Ca:0.0030% (not containing 0%), REM (rare earth element): less than 0.0050% (not containing 0%) and below Zr:0.0050% (not containing 0%)
Ca, REM and Zr are all deoxidant elements, and by adding these elements, oxygen concn in steel reduces, the amount of oxide compound reduces, thus bring toughness and affect.These elements can add separately, also can be two or more and use.In order to effectively play above-mentioned effect, preferably making Ca measure is more than 0.0005%, and REM is measured, and (it is independent content sometimes that the following REM recorded contains separately, containing time two or more being its total amount.Below, about REM amount all with.) be more than 0.0005%, it is more than 0.0005% that Zr is measured.But, if add superfluously, then the size of oxide compound increases, pole low-temperature flexibility reduces, therefore the preferred upper limit making Ca measure is less than 0.0030% (being more preferably less than 0.0025%), the preferred upper limit of REM amount is less than 0.0050% (being more preferably less than 0.0040%), and the preferred upper limit of Zr amount is less than 0.0050% (being more preferably less than 0.0040%).
In this manual, REM (rare earth element) is that lanthanon is (in periodictable, 15 kinds of elements from the La to the Lu of atom sequence number 71 of atom sequence number 57) add the groups of elements of Sc (scandium) and Y (yttrium), it can be used alone or two or more and use.Preferred rare earth element is Ce, La.The interpolation form of REM is not particularly limited, can with the mishmetal (such as Ce: about about 70%, La: form about about 20 ~ 30%) is added, and also can add Ce, La etc. individually mainly containing Ce and La.
Above, composition in steel of the present invention is illustrated.
In addition, Plate Steel of the present invention, meets 2.0 ~ 12.0% (being preferably 4.0 ~ 12.0%) at the volume fraction V of the residual γ phase existed at-196 DEG C.
The known residual γ existing at-196 DEG C contributes to the raising of pole low-temperature flexibility.In order to effectively play such effect, the volume fraction V that in the whole tissues existed at-196 DEG C, residual γ phase is shared is made to be more than 2.0%.But residual γ is compared with matrix phase, more soft, if residual γ quantitative change obtains superfluous, then can not guarantee that YS is the value of regulation, therefore make its upper limit be 12.0% (No.43 with reference to table 2B described later).About the volume fraction V of residual γ phase, preferred lower limit is more than 4.0%, and more preferably lower limit is more than 6.0%, and preferred upper limit is less than 11.5%, and more preferably the upper limit is less than 11.0%.
In addition, the volume fraction shared by γ residual in the whole tissues existed at-196 DEG C is controlled more than 4.0%, thus, even than at-233 DEG C of above-mentioned-196 DEG C of more low temperature, also brittle rupture rate can be remained on the good level of less than 50%.Wish that more preferably lower limit when playing such effect is further more than 6.0%, the preferred upper limit is same as described above.
It should be noted that, in Plate Steel of the present invention, among-196 DEG C of tissues existed, the control of the volume fraction V of residual γ phase is very important, for the tissue of other beyond residual γ, then without any restriction, as long as the tissue usually existed in Plate Steel.As the tissue beyond residual γ, such as, bainite can be enumerated, martensite, the carbide etc. such as cementite.
And then in Plate Steel of the present invention, about the inclusion of the equivalent circle diameter existed in steel plate more than 1.0 μm, the average roundness A of above-mentioned inclusion meets A≤1.8 and B value represented by following (1) formula meets more than 1.3.
B＝V 2/3/A···(1)
Be the size being conceived to above-mentioned inclusion at this " equivalent circle diameter ", try to achieve the circular diameter of supposition and its area equation.
At this, why being conceived to the inclusion of equivalent circle diameter more than 1.0 μm in the present invention, is because distinguish that above-mentioned inclusion encourages the development of brittle rupture.Namely, in order to while guarantee specify high strength, while improve the brittle rupture rate under the low temperature of pole, need to reduce the inclusion encouraging brittle rupture, according to the result of study of the present inventor etc., if the average roundness A of above-mentioned inclusion becomes large, even if control in above-mentioned scope by the volume fraction V of the residual γ phase of-196 DEG C, the pole low-temperature flexibility (No.33,35,36 with reference to table 2B described later) of wishing can not be realized.The average roundness A of above-mentioned inclusion is the smaller the better, is preferably less than 1.7, is more preferably less than 1.5, most preferably is 1.It should be noted that, in the present invention, equivalent circle diameter is roughly less than 2.0 μm more than the mean sizes (leveled circular equivalent diameter) of the inclusion of 1.0 μm.
Above-mentioned inclusion can measure with the method described in embodiment described later.At this, the kind of " inclusion " in the inclusion of equivalent circle diameter more than 1.0 μm, is not particularly limited in the present invention.This be due to brittle rupture to make a difference maximum be not the kind of inclusion, but the size of inclusion (equivalent circle diameter).As the kind of above-mentioned inclusion, such as except can enumerating the independent particle such as oxide compound, sulfide, nitride, oxynitride, also can enumerate the mixture of the two or more compounds of these independent particle things, or the composite particles etc. that these independent particles are combined with other element.
By the average roundness A of the thick inclusion of equivalent circle diameter more than 1.0 μm being set to less than 1.8 as the present invention thus guaranteeing the intensity that specifies and the mechanism that pole low-temperature flexibility improves specifically is failed to understand, but to be presumed as follows.The hardness of inclusion is generally high than matrix, therefore easily stress concentration occurs, and consequently, has mostly played the effect of the starting point of brittle rupture.Now, because the change in shape of inclusion is more serious, exacerbate the stress concentration around inclusion more locally, and then easily bring out brittle rupture.Therefore infer, if the inclusion of distortion reduces [that is, being controlled by the average roundness A of above-mentioned inclusion below 1.8 to try one's best close to positive round (A=1)], then can avoid the generation of stress concentration, pole low-temperature flexibility then improves.
And then, in the present invention, not only need the average roundness A controlling above-mentioned inclusion, the B value represented by above-mentioned (1) formula also must be made to meet B value >=1.3.
Above-mentioned B value is the parameter of the brittle rupture rate for reducing pole low temperature, as shown in above-mentioned (1) formula, for by the average roundness A of above-mentioned inclusion and the value that calculates in the relation of the volume fraction V of-196 DEG C of retained austenites existed (residual γ) phase.Below the derivation of above-mentioned B value through and being described.
The starting point of known brittle rupture is more, larger to the less then brittle rupture rate of development resistance of brittle rupture.Generally speaking, thick inclusion easily becomes the starting point of brittle rupture, the present inventor etc. verify, and the degree that more greatly, in other words the roundness of thick inclusion departs from positive round (A=1) and be out of shape is more serious, then the easier starting point as brittle rupture works; And then residual γ more hinders the development of brittle rupture more at most.Based on these opinions, the two contribution rate to the brittle rupture rate in the low temperature region of pole utilizing experiment to obtain according to the various infrastest of quantity, the B value that found that represented by above-mentioned (1) formula is the useful parameter for evaluating pole low-temperature flexibility.As shown in aftermentioned embodiment, on the basis of the form (average roundness) of the volume fraction V and the thick inclusion of equivalent circle diameter more than 1.0 μm that guarantee residual γ phase, above-mentioned B value is controlled more than 1.3, achieves the brittle rupture rate taking into account intensity and-196 DEG C and-233 DEG C at high level thus first.
Above-mentioned B value is preferably more than 1.6, is more preferably more than 1.8.From the view point of pole low-temperature flexibility, above-mentioned B value is the bigger the better, and is not particularly limited its upper limit.But as previously mentioned, cannot guarantee that YS is the value of regulation when the volume fraction V of residual γ is excessive, be therefore 12.0% by the ceiling restriction of the volume fraction V of residual γ, now the upper limit actual upper bound of B value is made as 5.2 (=12.0 2/3/ 1) (in the calculating formula of B value, the volume fraction V=12.0% of residual γ, average roundness A=1 is substituted into).Consider intensity and tough sexual balance, more preferably B value is less than 3.0.
Below, the manufacture method for Plate Steel of the present invention is described.
The characteristic of manufacture method of the present invention, is following (A) ~ (B).
(A) in the steel-making stage, free oxygen amount [O] before being added by Al controls at below 100ppm, is controlled (to be positioned at the value of 1/2 position of the thick t of slab) below 300 seconds the cooling time (t1) of 1450 ~ 1500 DEG C during casting and is controlled (to be positioned at the value of 1/4 position of the thick t of slab) below 680 seconds the cooling time (t2) of 1300 ~ 1200 DEG C during casting.According to the method for above-mentioned (A), particularly the average roundness A of above-mentioned inclusion can be reduced to specialized range.
(B) after hot rolling, after heating in the temperature range of Ac1 ~ Ac3 point, keeping, carry out water-cooled, then carry out 10 ~ 60 minutes temper in the temperature range of 520 DEG C ~ Ac1 point, carry out air cooling or water-cooled afterwards.According to the method for above-mentioned (B), particularly suitably controlled at the volume fraction of-196 DEG C of residual γ phases existed.
It should be noted that, due to the parameter that the B value specified in the present invention is about the average roundness of above-mentioned inclusion and this two side of volume fraction of residual γ, so by suitably controlling above-mentioned (A) ~ (B), above-mentioned B value can be controlled in specialized range.
With the relation of aforesaid prior art, among the method for above-mentioned (A), maximum feature is control t1 and t2 especially.
Below, each operation is described in detail.
(about melting operation)
In the present invention, the addition means for Al pays particular attention to.This be due to, the inclusion of the equivalent circle diameter that should control in the present invention more than 1.0 μm, mainly with the Al system inclusion generated in liquation for starting point, the composition generation when the secondary inclusion thing cooling of oxide compound and sulfide etc., but above-mentioned Al system inclusion is easy coarsening due to aggegation/zoarium, and easily becomes the large deformed shape of roundness.
First, when adding the Al as deoxidation material in steel-making, the free oxygen amount before being added by Al is (sometimes referred to as soluble oxygen content, [O] amount.) control at below 100ppm.If [O] amount is more than 100ppm, then the Al system inclusion generated when Al adds increases, and roundness can exceed the scope (No.33 with reference to table 2B described later) of regulation.[O] amount is more few better, is preferably below 80ppm, is more preferably below 50ppm.It should be noted that, from the view point of the average roundness controlling above-mentioned inclusion, the lower limit that [O] measures is not particularly limited.
As controlling [O] method of measuring as described so, such as, the deoxidant element adding Mn, Si and the method for carrying out deoxidation can be set forth in steel-making.When the deoxidation material also adding Ti, Ca, REM, Zr etc. except above-mentioned element is as selection component, even if add these elements also can control [O] amount.
In order to control Al system inclusion, [O] amount importantly before control Al interpolation, and regardless of the order of addition of Al with other deoxidant element.But, if add Al under [O] measures high state, then because oxidizing reaction causes the temperature of making steel to rise, operationally form danger, therefore preferably before Al, add Si, Mn.In addition, the above-mentioned selection component of Ti etc., preferably adds after Al adds in steel-making again.
Then, casting is started.Temperature range during casting is roughly less than 1650 DEG C, but distinguish in the present invention, particularly importantly the cooling time (t1) of the temperature range of 1450 ~ 1500 DEG C is controlled to control below 680 seconds below 300 seconds and by the cooling time (t2) of 1300 ~ 1200 DEG C, thus, the average roundness of the inclusion of equivalent circle diameter more than 1.0 μm is suitably controlled.Below describe in detail.
First the cooling time (t1) of 1450 ~ 1500 DEG C of temperature ranges was controlled below 300 seconds.When above-mentioned t1 was more than 300 seconds, the generation of the plyability of the secondary inclusion thing being core with Al system inclusion is encouraged, equivalent circle diameter deforms more than the shape of the inclusion of 1.0 μm, average roundness becomes large, or above-mentioned B value reduces, the pole low-temperature flexibility (No.34,35 with reference to table 2B described later) of hope cannot be played.If from the above point of view, t1 is ultrashort better, is preferably less than 290 seconds, is more preferably less than 280 seconds.From the above point of view, the lower limit of t1 is not particularly limited.
In addition, in the present invention, why the temperature range when casting in this, is conceived to the temperature range of 1450 ~ 1500 DEG C especially, be due to solidifying when casting in this temperature range, carry out composition and cause to the multiviscosisty of steel-making the temperature province that the growth of inclusion is promoted.
In addition, the temperature range of above-mentioned 1450 ~ 1500 DEG C refers to the temperature of the central part (t/2) of the thick t of slab.Its reason is as described below.As previously mentioned, oxide based secondary inclusion owner will in liquation composition generation, therefore must control the cooling time in liquation portion.But 1450 ~ 1500 DEG C, owing to still carrying out the temperature field solidified simultaneously, therefore according to the difference of temperature measuring position, exist in measuring the possibility of solidifying, cannot correctly measure the cooling time in liquation portion.Therefore, measure until lowest temperature also exists the cooling time of the t/2 position of liquation in the present invention.The temperature of the central part that slab is thick can measure by being inserted in mold by thermopair.
Secondly, the cooling times of 1300 ~ 1200 DEG C (t2) are controlled to be less than 680 seconds.If above-mentioned t2 was more than 680 seconds, is then mainly sulfide-based secondary inclusion thing and is encouraged to the composition generation of Al system inclusion, the average roundness of above-mentioned inclusion still becomes greatly (No.36 with reference to table 2B described later).From the above point of view, t2 is shorter, more can obtain the inclusion close to positive round, is therefore useful.Preferred t2 is less than 650 seconds, is more preferably less than 600 seconds.But if t2 is too short, cooling burden increases, therefore practical upper recommendation is set to about more than 400 seconds.
It should be noted that, the temperature range of above-mentioned 1300 ~ 1200 DEG C refers to the temperature of 1/4 (t/4) of the thick t of slab.Its reason is, the cooling time of 1300 ~ 1200 DEG C is the sulfide-based secondary inclusion thing for controlling main composition generation in solid iron, in said temperature territory, solidify and substantially complete, be therefore determined at the cooling time of the t/4 position of carrying out brittle rupture rate mensuration.The temperature in the t/4 portion that slab is thick can measure by being inserted in mold by thermopair.
In the present invention, as long as described above to controlling the cooling time (t2) of the cooling time (t1) in the temperature range of 1450 ~ 1500 DEG C and 1300 ~ 1200 DEG C of In, do not limit its means.Such as, about t1, can cool with constant speed, the average cooling rate of about less than 0.17 DEG C/sec for this temperature range, it is less than 300 seconds to make the cooling time within the scope of said temperature, or also can being cooled by different speed of cooling, is less than 300 seconds to make the cooling time of said temperature scope.T2 too.
In addition, in the present invention, for the method for cooling of temperature range during casting beyond said temperature scope without any restriction, usual method (air cooling or water-cooled) can be adopted.
In the above described manner after casting, carry out hot rolling, for thermal treatment.
At this, hot-rolled process is not particularly limited, ordinary method can be used to obtain the thickness of slab of regulation, specifically, slab can be regulated (finish to gauge) temperature, draught etc. about 1100 DEG C heating after 1 ~ 4 hour.
After hot rolling, be heated to the temperature range (TL) of Ac1 ~ Ac3 point, after maintenance, carry out water-cooled.This process is equivalent to the L process described in aforesaid prior art, thereby, it is possible at guaranteeing-196 DEG C the residual γ of stable existence in the scope of specified amount.
In detail, two phase regions [ferrite (α)-γ] temperature (TL) of Ac1 ~ Ac3 point is heated to.By being heated to this temperature province, in the γ phase generated, the alloying element of Ni etc. concentrates, and the quasi-steady that at room temperature can obtain quasi-steady existence remains γ phase.Lower than Ac 1point or more than Ac 3during point, result is the residual γ phase (No.37,38 with reference to table 2B described later) at can not fully guaranteeing-196 DEG C.Preferred Heating temperature is roughly 660 ~ 710 DEG C.
Heat-up time (hold-time, tL) at above-mentioned two phase region temperature, be preferably roughly 10 ~ 50 minutes.During lower than 10 minutes, alloying element does not fully carry out to the concentrated of γ phase, and on the other hand, during more than 50 minutes, α phase is annealed, and intensity reduces.Preferred heat-up time is roughly 15 ~ 30 minutes.
In addition by making be more than 15 minutes above-mentioned heat-up time, thus the volume fraction of residual γ phase at guaranteeing-196 DEG C is more than 4.0%, thus, be less than 50% by the brittle rupture rate at guaranteeing-233 DEG C, even and if also good toughness can be guaranteed under lower pole low temperature.It should be noted that, the upper limit of preferred heat-up time is same as described above (less than 30 minutes).
Then, after water-cooled to room temperature, temper is carried out.Temper is at 520 DEG C ~ Ac 1the temperature range (T3) of point is carried out 10 ~ 60 minutes (t3).Thus, during tempering, C is concentrated to quasi-steady and remains in γ, and the stability that quasi-steady remains γ phase increases, therefore, it is possible to obtain the residual γ phase of also stable existence at-196 DEG C.If tempering temperature T3 is lower than 520 DEG C, then the quasi-steady generated in two-phase coexists territory maintenance remains γ phase decomposition and becomes α phase and cementite phase, fully can not guarantee the residual γ phase (No.41 with reference to table 2 described later) at-196 DEG C.On the other hand, tempering temperature T3 is more than Ac 1point, or tempering time t3 lower than 10 minutes time, to remain in γ phase concentrated cannot fully carry out to quasi-steady for C, residual γ amount [No.55 (example that t3 is short) with reference to table 2 described later] at-196 DEG C that can not guarantee to wish.In addition, if tempering time t3 was more than 60 minutes, then the residual γ phase at-196 DEG C generates superfluously, can not guarantee the intensity (No.43 with reference to table 2 described later) specified.
Preferred temper condition is, tempering temperature T3:570 ~ 620 DEG C, more than tempering time t3:15 minute, less than 45 minutes (being more preferably less than 35 minutes, more preferably less than 25 minutes).
After carrying out temper as described so, be cooled to room temperature.Method of cooling is not particularly limited, air cooling or water-cooled.
In this manual, Ac1 point and Ac3 point calculate (" 4 iron steel compiled by the Metallkunde material in lecture modern times ", from the Metallkunde meeting of Japan of Corporation) based on following formula.
Ac1 point
＝723-10.7×[Mn]-16.9×[Ni]+29.1×[Si]+16.9×[Cr]+290×[As]+6.38×[W]
Ac3 point
＝910-203×[C] 1/2-15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]
In above-mentioned formula, [] is meant to the concentration (quality %) of the alloying element in steel.It should be noted that, in the present invention, do not contain As and W as composition in steel, therefore in above formula, [As] and [W] all calculates as 0%.
Embodiment
Below, enumerate embodiment and further illustrate the present invention, but the present invention does not limit by following embodiment, can be changed in the scope that can meet aim aforementioned, described later and implement, these be all included in the scope of technology of the present invention.
Embodiment 1
Use vacuum melting stove (150kgVIF), with the melting condition shown in table 2, (surplus: iron and inevitable impurity that one-tenth shown in melting table 1 is grouped into, unit is quality %) for examination steel, after casting, by forge hot, make the ingot bar of 150mm × 150mm × 600mm.In the present embodiment, the mishmetal containing Ce about 50%, La about 25% is used as REM.In addition, the order of addition of deoxidant element, when not containing selection component, is Si, Mn (simultaneously adding) → Al; On the other hand, when the selection component containing Ti, REM, Zr, Ca, be Si, Mn (simultaneously adding) → Al → Ti → REM, Zr, Ca (simultaneously adding).It should be noted that, in the present embodiment, the time from Al is added into casting is all about 10 minutes (not showing in table).
In addition, in table 2, [O] is the soluble oxygen content (ppm) before Al adds, the cooling time (second) of 1300 ~ 1200 DEG C when cooling time (second), the t2 of 1450 ~ 1500 DEG C when t1 is casting are castings.Each temperature field be cooled through air cooling or water-cooled controlled with above-mentioned cooling time.
Then, for above-mentioned ingot bar, 1100 DEG C of heating after 1 ~ 4 hour, be rolling to thickness of slab 75mm with the temperature of more than 830 DEG C, water-cooled after being rolled with final rolling temperature 780 DEG C, obtains the Plate Steel of thickness of slab 25mm thus.After the steel plate obtained thus being heated to the temperature (in table 2, TL) shown in table 2, after heating maintenance 5 ~ 60 minutes (tL with reference to table 2), water-cooled is to room temperature.Then, as shown in table 2 carry out temper (T3=tempering temperature, t3=tempering time) after, carry out air cooling or water-cooled to room temperature.
For the Plate Steel obtained thus, evaluate the average roundness A (individual/mm of the inclusion of equivalent circle diameter more than 1.0 μm in the following manner 2), in volume fraction (%), tensile properties (tensile strength TS, yield strength YS), the pole low-temperature flexibility (the brittle rupture rate in the C direction at-196 DEG C or-233 DEG C) of the residual γ phase existed at-196 DEG C.
(1) mensuration of the average roundness A of the inclusion of equivalent circle diameter more than 1.0 μm
Mirror ultrafinish is carried out to the t/4 position (t: thickness of slab) of above-mentioned steel plate, uses opticmicroscope to take pictures for 4 visuals field with 400 times.Further, the area in each visual field is 0.04mm 2, the total area in 4 visuals field is 0.15mm 2.For viewed inclusion in these 4 visuals field, image analysis is carried out by Media Cybernetics society system " Image-ProPlus ", the roundness of equivalent circle diameter (diameter) more than the inclusion of 1.0 μm is calculated, using the average roundness A of its mean value as above-mentioned inclusion based on following formula.When the shape of inclusion is positive round, the roundness calculated by following formula is 1.The shape of inclusion is out of shape more serious, and the value of the roundness calculated by following formula is larger.
Roundness=L 2/ 4 π/S
In formula, the area (unit μm that the surrounding head (unit μm) that L is inclusion, S are inclusion 2).
It should be noted that, in the present embodiment, the inclusion of equivalent circle diameter more than 1.0 μm observes about 200 ~ 300/mm 2left and right.
(2) in the mensuration of the volume fraction of the residual γ phase existed at-196 DEG C
From the t/4 position of each steel plate, extract the test film of 10mm × 10mm × 55mm, after keeping 5 minutes with liquid nitrogen temperature (-196 DEG C), carry out X-ray diffraction mensuration with two dimension small portion X-ray diffraction device (RINT-RAPIDII) of リ ガ Network Inc..Then, about the peak value of the peak value of (110), (200), (211) of ferritic phase, each lattice plane of (220) and (111), (200), (220) of residual γ phase, each lattice plane of (311), based on the integrated intensity ratio of each peak value, calculate residual (111), (200), (220) of γ phase, the volume fraction of (311) respectively, try to achieve its mean value, it can be used as " volume fraction (%) of residual γ ".
(3) mensuration of tensile properties (tensile strength TS, yield strength YS)
From the t/4 position of each steel plate, extract No. 4 test films of JIS Z2241 abreast with C direction, carry out tension test with the method described in JIS Z2241, measure tensile strength TS and yield strength YS.In the present embodiment, TS > 690MPa, YS > 590MPa's, be evaluated as strength of parent excellence.
(4) mensuration of pole low-temperature flexibility (the brittle rupture rate in C direction)
From the t/4 position (t: thickness of slab) of each steel plate and W/4 position (W: plate is wide), and t/4 position and W/2 position, Charpy impact test sheet (the V nick-break test sheet of JIS Z2242) 3 is extracted abreast with C direction, with the method described in JIS Z2242, brittle rupture rate (%) at measuring-196 DEG C, calculates respective mean value.Then, among two mean values so calculated, adopt the mean value of (that is, brittle rupture rate is large) side of characteristic difference, this value is less than 10%, is evaluated as pole excellent in low temperature toughness in the present embodiment.
These results are recorded in table 2 in the lump.In order to reference, in table 1 and table 2, note has Ac1 point and Ac3 point in the lump.
[table 1A]
[table 1B]
[table 2A]
[table 2B]
Following investigation can be carried out by table 2.
First, No.1 ~ 32 of table 2A are the examples meeting feature of the present invention completely, even if strength of parent can be provided high, the Plate Steel that the pole low-temperature flexibility (being exactly mean value≤10% of the brittle rupture rate in C direction in detail) at-196 DEG C is still excellent.
In contrast, No.33 ~ 41,43,55 of table 2B, at least not meeting any one of preferred manufacturing condition of the present invention, is the comparative example not meeting feature of the present invention, therefore can not get the characteristic of wishing.
Specifically, although No.33 employs the No.33 that composition in steel meets the table 1B of feature of the present invention, soluble oxygen content [O] amount before Al adds is many, is therefore the example that the average roundness A of above-mentioned inclusion increases.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
Although No.34 employs the No.34 that composition in steel meets the table 1B of feature of the present invention, the cooling time (t1) of 1500 ~ 1450 DEG C during casting is long, is therefore the example of B value lower than the scope of regulation.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.35 is because of the No.35 using P to measure many table 1B, and the cooling time (t1) of 1500 ~ 1450 DEG C during casting is long, so be the example of the average roundness A increase of above-mentioned inclusion.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.36 is because employ the No.36 of the many table 1B of C amount, and the cooling time (t2) of 1300 ~ 1200 DEG C during casting is long, so be the example of the average roundness A increase of above-mentioned inclusion.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
Although No.37 employs the No.37 that composition in steel meets the table 1B of feature of the present invention, but with the heating temperatures lower than two phase region temperature (TL), so be the example of residual γ quantity not sufficient.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.38 measures the No.38 of many table 1B because of employing Si, and, with the heating temperatures more than two phase region temperature (TL), so be the example of residual γ quantity not sufficient.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
Although No.39 employs the No.39 that composition in steel meets the table 1B of feature of the present invention, the heated hold time (tL) at two phase region temperature (TL) is short, is therefore the example of residual γ quantity not sufficient.Consequently, brittle rupture rate also increases, and can not realize the pole low-temperature flexibility of wishing.
Although No.40 employs the No.40 that composition in steel meets the table 1B of feature of the present invention, long at the heated hold time (tL) of two phase region temperature (TL), be therefore that residual γ measures the example increased.Consequently, yield strength YS and tensile strength TS reduces, and can not guarantee the strength of parent of wishing.
Although No.41 employs the No.41 that composition in steel meets the table 1B of feature of the present invention, tempering temperature (T3) is low, is therefore the example of residual γ quantity not sufficient.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
Although No.43 employs the No.43 that composition in steel meets the table 1B of feature of the present invention, tempering time (t3) is long, so be that residual γ measures the example increased.Consequently, yield strength YS reduces, and can not guarantee the strength of parent of wishing.
Although No.55 employs the No.55 that composition in steel meets the table 1B of feature of the present invention, tempering time (t3) is short, is therefore the example of residual γ quantity not sufficient.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.42,44 ~ 54 is the use of only has composition in steel to depart from, the comparative example manufactured with the inventive method.
In detail, No.42 is the use of the No.42 of the many table 1B of Mn amount, therefore the example of residual γ quantity not sufficient.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.44 employs the No.44 that Mn measures few table 1B, therefore the example of residual γ quantity not sufficient.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.45 is the example using S to measure the No.45 of many table 1B.Therefore, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.46 is the No.46 that C measures less, Al amount is many, Ni measures few table 1B, and therefore the average roundness A of above-mentioned inclusion increases, remains the example of γ quantity not sufficient.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.And then TS also reduces.
No.47 uses the No.47 that Al measures less, N measures many table 1B, the example of the therefore average roundness A increase of above-mentioned inclusion.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.48 is the example of the No.48 of the table 1B that Cu measures and Ca amount is many employed as selection component.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.49 is the example of the No.49 of the table 1B that Cr measures and Zr amount is many employed as selection component.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.50 is the example of the No.50 of the table 1B that Nb measures and REM amount is many employed as selection component.Consequently, brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.51 is owing to employing the No.51 as the many table 1B of the Mo amount of selection component, and therefore brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.52 is owing to employing the No.52 as the many table 1B of the Ti amount of selection component, and therefore brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.53 is owing to employing the No.53 as the many table 1B of the V amount of selection component, and therefore brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
No.54 employs the No.54 as the many table 1B of the B amount of selection component, and therefore brittle rupture rate increases, and can not realize the pole low-temperature flexibility of wishing.
Embodiment 2
In the present embodiment, for the data (being example of the present invention) of the part for above-described embodiment 1, the brittle rupture rate of-233 DEG C is evaluated.
Specifically, for the No. (the corresponding aforesaid table 1 of No. of table 3 and the No. of table 2) described in table 3, from t/4 position and test film 3 is extracted in W/4 position, implement the Charpy impact test of-233 DEG C with method as described below, evaluate the mean value of brittle rupture rate.In the present embodiment, above-mentioned brittle rupture rate≤50%, the brittle rupture rate being evaluated as-233 DEG C is excellent.
" high pressure gas ", the 24th volume 181 pages, " the pole low temperature impact test of austenite stainless steel cast steel "
These the results are shown in table 3.
Table 3
[table 3]
The No.3,4,6,15,19 and 24 of table 3, is all the examples (with reference to table 2A) controlled by the heated hold time (tL) at two phase region temperature more than 15 minutes, can guarantees that residual γ phase is more than 4.0%.Consequently, not only at-196 DEG C, more the brittle rupture rate of-233 DEG C of low temperature is also good, can reach very excellent pole low-temperature flexibility.
In detail and describe the present invention with reference to particular implementation, but those skilled in the art are known, can in addition various changes and modifications in the scope not departing from the spirit and scope of the present invention.
The Japanese patent application (Patent 2012-184593) that the application proposed based on August 23rd, 2012, its content is as with reference to introducing here.
Usability in industry
Plate Steel pole of the present invention excellent in low temperature toughness, the steel particularly as the storage tank and transport-ship etc. of natural gas liquids (LNG) are useful.

Claims (3)

1. a Plate Steel for pole excellent in low temperature toughness, is characterized in that,
Described Plate Steel contains in mass %:
C：0.02～0.10％、
Below Si:0.40% but containing 0%,
Mn：0.50～2.0％、
Below P:0.007% but containing 0%,
Below S:0.007% but containing 0%,
Al：0.005～0.050％、
Ni：5.0～7.5％、
Below N:0.010% but not containing 0%,
Surplus is iron and inevitable impurity,
The average roundness A of the inclusion of the equivalent circle diameter existed in steel plate more than 1.0 μm is less than 1.8,
Meet 2.0 ~ 12.0% at the volume fraction V of-196 DEG C of retained austenite phases existed, and the B value represented by following (1) formula is more than 1.3,
B＝V 2/3/A···(1)。
2. Plate Steel according to claim 1, wherein, counts 4.0 ~ 12.0% at-196 DEG C of retained austenites existed with volume fraction.
3. Plate Steel according to claim 1 and 2, wherein,
In mass % also containing being selected from
Below Cu:1.00% but containing 0%,
Below Cr:1.2% but containing 0%,
Below Mo:1.00% but containing 0%,
Below Ti:0.025% but containing 0%,
Below Nb:0.10% but containing 0%,
Below V:0.50% but containing 0%,
Below B:0.0050% but not containing 0%
Below Ca:0.0030% but containing 0%,
Below REM:0.0050% but containing 0%,
Below Zr:0.0050% but containing at least one in 0%.
CN201380043453.6A 2012-08-23 2013-08-19 The steel plate of pole excellent in low temperature toughness Expired - Fee Related CN104583439B (en)

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