CN100510141C - Ultrahigh-strength steel sheet superior in hydrogen-embrittlement resistance - Google Patents

Abstract

The purpose of the present invention is to provide a high strength thin steel sheet that has high hydrogen embrittlement resisting property. In order to achieve the above purpose, a high strength thin steel sheet having high hydrogen embrittlement resisting property comprises: C: 0.10 to 0.25%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities; and the metal structure comprises: residual austenite; 1% by area or more in proportion to the entire structure; bainitic ferrite and martensite: 80% or more in total; and ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and the steel has tensile strength of 1180 MPa or higher.

Description

The ultrahigh-strength thin steel sheet that hydrogen-embrittlement resistance is good

Technical field

The present invention relates to the good ultrahigh-strength thin steel sheet of a kind of hydrogen-embrittlement resistance, particularly a kind of tensile strength that is suppressed at is the destructive ultrahigh-strength thin steel sheet that the hydrogen embrittlement because of placing crackle, delayed fracture and so on that exists in the above steel plate of 1180MPa causes.

Background technology

When obtaining constituting the high strength component of automobile or industrial machinery etc., require to have both good intensity and ductility for the steel plate of this processing usefulness by drawing processing or bending machining.In recent years, along with the lightweight more of automobile etc., to the demand of the above ultrahigh-strength steel plates of 1180MPa just surging, as steel plate, especially TRIP (the TRansformation Induced Plasticity that adapts to such demand; Phase change induction plasticity steel) steel plate is gazed at.

The TRIP steel plate is the residual austenite structure that has, and if make it machining deformation, then owing to stress, residual austenite (remaining γ) is brought out and becomes martensite mutually, thereby obtains the steel plate of bigger extension.Can enumerate severally as its kind, for example, well-known is parent phase with the polygon ferrite, contains the TRIP type complex tissue steel (TPF steel) of residual austenite; With the tempered martensite is parent phase, contains the TRIP type tempered martensite steel (TAM steel) of residual austenite; With the bainite ferrite is parent phase, contains the TRIP type bainitic steel (TBF steel) of residual austenite etc.Wherein the TBF steel is known very early (for example non-patent literature 1 etc.), bainite ferrite by hard is easy to obtain high strength, simultaneously in its tissue, boundary at the bainite ferrite of lath-shaped, generate tiny residual austenite easily, such tissue morphology has the feature that can bring extraordinary extension.In addition, the TBF steel has on making and carries out 1 thermal treatment (continuous annealing operation or plating process) and just can easily be made this advantage.

Yet the superstrength scope more than the 1180MPa level is well-known, and is the same in the TRIP steel plate with other High Strength Steel, also can reappear this disadvantage of delayed fracture that produces because of hydrogen embrittlement.Delayed fracture is in high-strength steel, from corrosive environment or the hydrogen that produces from atmosphere, is diffused into dislocation, pore, the defective part of crystal boundary etc. makes material embrittlement, is giving under the situation of stress, the destructive phenomenon takes place, and its result brings the disadvantage of degradation under the ductility of metallic substance and the toughness.

As everyone knows, always be used for bolt in a large number, hydrogen embrittlement (acid brittleness, plating fragility, delayed fracture etc.) when its tensile strength is 980MPa when above, because hydrogen invades, and can take place in the high-strength steel on PC steel wire and the line of pipes purposes in steel.Therefore, as the technology that improves hydrogen-embrittlement resistance, nearly all be that steel with purposes such as described bolts are object.For example in non-patent literature 2, disclose, if with the metal structure tempering based on martensite, add to show the element Cr of anti-temper softening, Mo, V, then effective to improving anti-delayed fracture.This is a kind of by alloy carbide is separated out, and uses effectively as the capture area (trap district) of hydrogen, suppresses the destructive technology thereby make the delayed fracture form transfer to intracrystalline from crystal boundary.

But, in the past, not too be utilized from the steel plate more than the viewpoint 780MPa of processibility and weldability in the steel-sheet occasion.In addition,,, can be emitted at short notice,, therefore, hydrogen embrittlement is not made positive countermeasure so hydrogen embrittlement is regarded as a problem etc. hardly even hydrogen is invaded because thickness of slab is very thin.Recently, as mentioned above, from the lightweight and the necessity that improves the collision security of automobile, to snubber, desired strengths such as the reinforcement material of impact beam etc. and seat guide rail are higher.Thereby, improving as the demand that obtains the ultrahigh-strength steel plates more than the required 980MPa of these component, the consequent is to improve the hydrogen-embrittlement resistance of this ultrahigh-strength steel plates reliably.

When seeking to improve the hydrogen-embrittlement resistance of ultrahigh-strength steel plates, can consider to use the technology of relevant described bolt steel etc., but, for example in the occasion of described non-patent literature 2, the amount of C is more than 0.4% and contains a large amount of alloying elements, if the technology of the document is used for the steel-sheet words, then can not guarantees desired processibility of steel sheet and weldability.In addition, the separating out of alloy carbide, need a few hours above separate out thermal treatment, so problem is also arranged aspect manufacturing.Therefore, need establish alone technology in order to improve the steel-sheet hydrogen-embrittlement resistance.

Up to now, as the technology of the hydrogen-embrittlement resistance that improves steel plate, in patent documentation 1, proposed by making with Ti, Mg is that the oxide compound of main body exists, and suppresses the technology of hydrogen defect.This technology be a kind of be object with the Plate Steel, consider the technology of the delayed fracture after the high heat energy welding, be not that environment for use (for example corrosive environment etc.) to the automobile component made with steel sheet etc. has been carried out the technology that takes into full account.In addition, in patent documentation 2, propose, if the oxide compound of control Mg, sulfide, compound crystalline substance goes out the dispersing morphology (from the standard deviation and the median size of median size) of thing or compound precipitate, the volume fraction of residual austenite and the mutual relationship of armor plate strength, then can improve the anti-delayed fracture characteristic after ductility and shaping are processed simultaneously., only rely on the seizure effect of bringing by the precipitate morphology Control, improve in corrosion and produce, exist the hydrogen-embrittlement resistance under the environment of hydrogen to be difficult to owing to steel plate.

In non-patent literature 3 and non-patent literature 4, the hydrogen-embrittlement resistance of TRIP steel has been discussed.Wherein, the hydrogen absorbed dose that has disclosed TBF steel particularly is very big, if observe TBF steel section, promptly destroys the thing that is suppressed because of hydrogen absorbs the quasi-cleavage crack that causes as can be known.Yet, the delayed fracture characteristic of the TBF steel that discloses in the document, also just about 1000 seconds at most time till crack because of negative electrode filling experiment was to have taken into full account such as automobile with results after being in harsh environment for use for a long time such as component hardly.In addition, because its Heating temperature is set very highly, thereby there is bad etc. the situation of the production efficiency of actual machine in the heat-treat condition of above-mentioned document, presses for exploitation also good new TBF steel on production efficiency.

[patent documentation 1]: the spy opens flat 11-293383 number

[patent documentation 2]: the spy opens 2003-166035 number

[non-patent literature 1]: NISSHIN STEEL TECHNICAL REPORT (day new system steel skill newspaper), No.43, Dec.1980, p.1-10

[non-patent literature 2]: " separating the new expansion of bright delayed fracture " (Japanese iron and steel institute, in January, 1997 distribution) p.111～120

[non-patent literature 3]: a northern bar intelligence man of virtue and ability, other 5, " hydrogen embrittlement of superstrength low-alloy TRIP steel (the 1st newspaper hydrogen absorption characteristic and ductility) ", and Japanese materialogy can be given a lecture collection of thesis by the 51st phase disquisition, and 2002, the 8th volume, p.17-18

[non-patent literature 4]: a northern bar intelligence man of virtue and ability, other 5, " the austempering treatment temp is to the influence of the hydrogen embrittlement of superstrength low-alloy TRIP steel ", and CAMP-ISIJ, 2003, the 16th volume, p.568

Summary of the invention

The present invention In view of the foregoing proposes, it is a kind of under the ductile situation of not damaging as the excellence of the feature of TRIP steel plate that its purpose is to provide, in tensile strength is superstrength zone more than the 1180MPa, can significantly improve the TRIP steel sheet of hydrogen-embrittlement resistance.

As High Strength Steel, under the situation of the tempered martensite steel that is always generally adopted and martensite+ferritic steel, because the delayed fracture that hydrogen causes, being generally considered to be hydrogen is enriched in old austenite grain boundary etc. and locates to form space etc., with this part is that starting point produces, in order to reduce the susceptibility of delayed fracture, the general solution that adopts is evenly and imperceptibly to disperse as the carbide of the capture area (trap district) of hydrogen etc., to reduce the concentration of diffustivity hydrogen.Even these carbide etc. are made it a large amount of dispersions as the capture area (trap district) of hydrogen, capturing ability also is limited, so can not suppress fully because the delayed fracture that hydrogen causes.

Therefore, present inventors have carried out the result of research with keen determination in view of above-mentioned problem, for the more excellent hydrogen-embrittlement resistance (anti-delayed fracture) of realizing considering fully the steel-sheet environment for use, find if can realize that following (a) and (b) get final product.

(a) starting point of minimizing intercrystalline failure

(b) make hydrogen innoxious by improving the hydrogen capturing ability

If will realize above-mentioned (a), as the parent phase of steel plate, preferably not the general martensite single phase structure that adopts of High Strength Steel, but contain bainite ferrite in the main body, form " bainite ferrite and martensitic two-phase structure ".This be because, under the situation of described martensite single phase structure, separate out at crystal boundary with respect to carbide (for example sheet globular cementite) and intercrystalline failure takes place easily, if form " bainite ferrite and martensitic two-phase structure " based on bainite ferrite, then this bainite ferrite is different with general (Polygons) ferrite, is tabular ferrite, the dislocation desity height, thereby identical with the monophasic situation of martensite, can easily improve and organize all intensity.In addition, also,, also have, exist, can also prevent to become the generation of the carbide of intercrystalline failure starting point by making this bainite ferrite and retained austenite described later so can improve hydrogen-embrittlement resistance because on this dislocation part, hydrogen is caught in a large number.

Also have, bainite ferrite is organized as hard, can easily obtain high strength.Also have, the dislocation desity height of parent phase, the result that hydrogen is caught in a large number on this dislocation compares with other TRIP steel, can absorb a large amount of hydrogen.In addition, also have following advantage, on the border of the bainite ferrite of lath-shaped, the retained austenite of the lath-shaped of the present invention's regulation generates easily, can access very excellent extension.In order to bring into play such effect effectively, find bainite ferrite and martensite to be added up to more than 80% be advisable with the area occupation ratio of complete relatively tissue.

Also have, above-mentioned in order to realize (b), the retained austenite that forms lath-shaped is very effective.In the past, retained austenite was considered to that hydrogen-embrittlement resistance and fatigue were had bad influence.Through present inventors' discussion as can be known, existing retained austenite is micron-sized bulk, the retained austenite of this form can have bad influence to hydrogen-embrittlement resistance and fatigue, but as if the lath-shaped of morphology Control below micron order with this retained austenite, then can bring into play the original hydrogen of retained austenite and inhale energy storage power, the a large amount of absorption captures hydrogen, thereby can significantly improve hydrogen-embrittlement resistance.Particularly, be more than 5 if establish the mean axis of retained austenite crystal grain than (major axis/minor axis), then can sharply reduce hydrogen embrittlement risk level index.This be considered to because, be elevated to more than 5 by the mean axis ratio that makes retained austenite crystal grain, retained austenite is brought into play the ability of original suction storage hydrogen fully, the hydrogen trapping ability is compared absolute change with carbide big, make the hydrogen of invading become harmless in fact, thereby realize the significant synergy of hydrogen-embrittlement resistance by so-called atomospheric corrosion.

Also have, present inventors find, steel plate of the present invention is in the scope of harmless effect of the present invention, also can have ferrite (also has, here alleged " ferrite " is meant polygonal ferrite, promptly do not have the few ferrite of dislocation desity or dislocation desity) and perlite, but these are preferably few more good more, and particularly ferrite and perlite preferably add up to and be controlled at below 9%.

That is, present inventors find to satisfy following all conditions as if being controlled to, even then inexcessive interpolation alloying element also can improve the hydrogen-embrittlement resistance of ultrahigh-strength steel plates fully, thereby finish the present invention.Promptly

Bainite ferrite and martensite total have more than 80%;

The mean axis of retained austenite crystal grain is more than 5% than (major axis/minor axis)

Ferrite and perlite are aggregated in below 9% and (comprise 0%)

Therefore, the good ultrahigh-strength thin steel sheet of the 1st hydrogen-embrittlement resistance of the present invention, it is characterized in that, by percentage to the quality, satisfy C:0.10～0.25%, Si:1.0～3.0%, Mn:1.0～3.5%, below the P:0.15%, below the S:0.02%, Al:1.5% following (not comprising 0%), all the other are made of iron and unavoidable impurities, wherein represent by area occupation ratio for whole tissue, have the residual austenite more than 1%, bainite ferrite and martensite are for amounting to more than 80%, and ferrite and perlite are for (to comprise 0%) below 9% altogether, the mean axis of described residual austenite crystal grain is more than 5 than (major axis/minor axis) simultaneously, and tensile strength is more than the 1180MPa.

Also have, present inventors find, if the average minor axis length of retained austenite crystal grain is controlled at below the 1 μ m, then the surface-area of retained austenite crystal grain (interface) becomes big, the hydrogen trapping ability increases, can improve hydrogen-embrittlement resistance reliably thus, in addition, if the intercrystalline neighbor distance of retained austenite is controlled at below the 1 μ m, the retained austenite of then fine lath-shaped is approaching, can form a large amount of dispersive states, the propagation that destroys (crackle) is suppressed, the tissue that has high resistibility for destruction can be accessed, thus, hydrogen-embrittlement resistance can be improved reliably.

Therefore, the good ultrahigh-strength thin steel sheet of the 2nd hydrogen-embrittlement resistance of the present invention, it is characterized in that, by percentage to the quality, satisfy C:0.10～0.25%, Si:1.0～3.0%, Mn:1.0～3.5%, below the P:0.15%, below the S:0.02%, Al:1.5% following (not comprising 0%), all the other are made of iron and unavoidable impurities, wherein represent by area occupation ratio for whole tissue, have the residual austenite more than 1%, the mean axis of described residual austenite crystal grain is more than 5 than (major axis/minor axis), and the average minor axis length of described residual austenite crystal grain is below the 1 μ m simultaneously, and described residual austenite is intercrystalline to be below the 1 μ m in abutting connection with spacing, and tensile strength is more than the 1180MPa.

And ultrahigh-strength thin steel sheet of the present invention also is preferably by percentage to the quality, satisfies Al:0.5% following (not comprising 0%).

If Al content surpasses 0.5%, then inclusion such as aluminum oxide increases, and the processibility deterioration if it is controlled in the above-mentioned scope, then can prevent such processibility deterioration.

And ultrahigh-strength thin steel sheet of the present invention also is preferably and contains Cu:0.003～0.5% and/or Ni:0.003～1.0% by percentage to the quality.

Make it to contain Cu:0.003～0.5% and/or Ni:0.003～1.0%, can further improve raising based on the hydrogen-embrittlement resistance of organizational controls.

In addition, ultrahigh-strength thin steel sheet of the present invention is preferably by percentage to the quality, contains to amount to 0.003～1.0% Ti and/or V.

Make it to contain by percentage to the quality 0.003～1.0% Ti and/or V altogether, then can give full play to the generation facilitation effect of the protectiveness rust of Ti and V, the effect and the grain refined effect of giving corrosion proof effect, cleaning the effect of steel, the effect that improves the hydrogen-embrittlement resistance of V, raising armor plate strength.

In ultrahigh-strength thin steel sheet of the present invention, can also contain Mo:1.0% following (not comprising 0%) by percentage to the quality, Nb:0.1% following (not comprising 0%) also can contain B:0.0002～0.01%.Be selected from by Ca:0.0005～0.005% more than one in the group that Mg:0.0005～0.01% and REM:0.0005～0.01% is formed but also can contain.

According to the present invention,, can make that hydrogen-embrittlement resistance is improved, the ultrahigh-strength steel plates of tensile strength more than 1180MPa with high productivity by making the hydrogen of invading innoxious from the outside.Thus, can provide the superstrength parts that generate delayed fracture etc. as extremely difficult, for example the trolley part of the strongthener of snubber, impact beam etc. and seat guide rail, pillar, stiffening plate (reinforce), skeleton (member) etc.

Description of drawings

Fig. 1 is the mean axis ratio of expression residual austenite crystal grain and the coordinate diagram of the relation between the hydrogen embrittlement risk level index.

Fig. 2 is that the residual austenite that schematically shows is intercrystalline in abutting connection with the figure of spacing.

Fig. 3 is the general survey stereographic map of component that is used for the withstand voltage destructive test of embodiment.

Fig. 4 is the side elevational view of the withstand voltage destructive test situation among the embodiment that schematically shows.

Fig. 5 is the general survey stereographic map of component that is used for the shock-resistant characteristic test of embodiment

Fig. 6 is the A-A sectional drawing of described Fig. 5.

Fig. 7 is the side elevational view of the shock-resistant characteristic test situation among the embodiment that schematically shows.

Fig. 8 is the tem observation photo example (multiplying power: 15,000 times) of the No.101 (example of the present invention) of embodiment 1.

Fig. 9 is the tem observation photo example (multiplying power: 15,000 times) of the No.120 (comparative example) of embodiment 1.

Figure 10 is the tem observation photo example (multiplying power: 15,000 times) of the No.201 (example of the present invention) of embodiment 2.

Figure 11 is the tem observation photo example (multiplying power: 15,000 times) of the No.220 (comparative example) of embodiment 2.

Figure 12 is the tem observation photo example (multiplying power: 15,000 times) of the No.301 (example of the present invention) of embodiment 3.

Figure 13 is the tem observation photo example (multiplying power: 60,000 times) of the No.301 (example of the present invention) of embodiment 3.

Figure 14 is the tem observation photo example (multiplying power: 15,000 times) of the No.313 (comparative example) of embodiment 3.

Among the figure: 1-withstand voltage destructive test component (experiment body), 2,5-spot welding position, 3-mould, 4-shock-resistant characteristic test component (experiment body), 6-drop hammer 7-(shock-resistant characteristic test is used) platforms

Embodiment

Below, with reference to accompanying drawing the ultrahigh-strength thin steel sheet of embodiments of the present invention on one side be described on one side.The present invention is not limited to following embodiment, can change in the scope of technology category of the present invention etc.

(embodiment 1)

The ultrahigh-strength thin steel sheet that relates to embodiments of the present invention 1, it is characterized in that, by percentage to the quality, contain C:0.10～0.25%, Si:1.0～3.0%, Mn:1.0～3.5%, below the P:0.15%, below the S:0.02%, Al:1.5% following (not comprising 0%), and contain iron and unavoidable impurities as rest part, wherein represent by area occupation ratio for whole tissue, contain the residual austenite more than 1%, amount to bainite ferrite and martensite more than 80%, and the ferrite and the perlite that (comprise 0%) below 9% altogether.The mean axis of wherein said residual austenite crystal grain is more than 5 than (major axis/minor axis), and tensile strength is more than the 1180MPa.Just stipulate that below the reason of these conditions and measuring method etc. make an explanation.

＜residual austenite (remaining γ): more than 1% 〉

Residual austenite is not only useful to the raising of full extension, but also goes far towards the raising of hydrogen-embrittlement resistance, so make it to be present in more than 1%.Be preferably more than 3%, more preferably more than 5%.

In addition,, then can not guarantee desired superstrength, so suggestion is made as 15% with its upper limit (more preferably 10%) if described residual austenite exists in a large number.

From the viewpoint of the stability of described residual austenite, recommend the C concentration in the residual austenite (C γ R) is made as more than 0.8%.In addition C γ R is controlled at more than 0.8%, also can effectively improves extension etc.Be preferably more than 1.0%, more preferably more than 1.2%.In addition, want high though wish described C γ R,, the upper limit that general consideration may be adjusted in the actually operating is approximately 1.6%.

Described residual austenite means, according to described FE-SEM/EBSP method, as the observed zone of FCC phase (face-centered cubic lattice).An object lesson as the EBSP measurement, can enumerate with described bainite ferrite and martensitic observation situation same, on the position of thickness of slab 1/4, the measured zone arbitrarily on the face parallel with calendering face (about 50 * 50 μ m measure 0.1 μ m at interval) is as measuring object.In addition, when being ground to this measurement face, in order to prevent the phase transformation of residual austenite, and use electrolytic polishing.Secondly, use described " FE-SEM that possesses the EBSP detector ", the sample irradiating electron beam in being arranged on the SEM lens barrel.Take the EBSP image that is projected on the screen with high sensitivity camera (Dage-MTI Inc. makes VE-1000-SIT), put into computer with graphic form.Carry out image analysis with computer then,, the FCC that determines is made coloured picture mutually by comparing with the figure that obtains with known crystal system [being FCC (face-centered cubic) mutually during residual austenite] simulation.Obtain the area occupation ratio that is illustrated the zone of (mapping) like this, it is decided to be " area occupation ratio of residual austenite ".In addition, among the present invention, what the hardware of described parsing and software used is the OIM of TexSEM Laboratories Inc. (Orientation Imaging MicroscopyTM) system.

(mean axis of retained austenite crystal grain than (major axis/minor axis): more than 5)

In the past, retained austenite is considered to that hydrogen-embrittlement resistance and fatigue are had bad influence, but the discussion at length that the process present inventors carry out as can be known, micron-sized blocky existing retained austenite has bad influence to hydrogen-embrittlement resistance and fatigue, if but make the shape of this retained austenite form lath-shaped, then compare and can improve the hydrogen capture ability more fully with carbide.

Fig. 1 is the mean axis ratio of expression by the retained austenite crystal grain of method mensuration described later, with graphic representation as the hydrogen embrittlement risk level exponential relation (measured by method shown in the embodiment described later, the low more expression hydrogen-embrittlement resistance of numerical value is excellent more) of the index of hydrogen-embrittlement resistance.As can be seen from Figure 1, if particularly the mean axis of retained austenite crystal grain than (major axis/minor axis) more than 5, hydrogen embrittlement risk level index sharply reduces.This be considered to because, make it more than 5 by the mean axis ratio that improves retained austenite crystal grain, inhale energy storage power and make retained austenite can bring into play original hydrogen fully, the hydrogen trapping ability is compared absolute change with carbide big, make the hydrogen of invading become harmless in fact, thereby realize the significant synergy of hydrogen-embrittlement resistance by so-called atomospheric corrosion.The mean axis ratio of above-mentioned retained austenite is preferably more than 10, more elects as more than 15.

On the other hand, from improving the viewpoint of hydrogen-embrittlement resistance, the upper limit to described mean axis ratio is not done special stipulation, but, in order to bring into play the TRIP effect effectively, then need the thickness of residual austenite to a certain degree, if consider this point, then preferably on be defined as 30, more preferably below 20.

In addition, the measurement of described mean axis ratio is to observe (1.5 ten thousand times of multiplying powers) with TEM (Transmission ElectronMicroscope), measures the major axis and the minor axis of the residual austenite crystal grain that exists in optional 3 visuals field, obtains axial ratio, calculate mean value, with it as the mean axis ratio.

(bainite ferrite (BF)+martensite (M): more than 80%)

Among the present invention, bainite ferrite and martensitic two-phase structure (based on the tissue of bainite ferrite) have been formed as mentioned above.As mentioned above, bainite ferrite is organized as hard, obtains high strength easily.Also have, the dislocation desity height of parent phase, on this dislocation, hydrogen is captured in a large number, and its result compares with other TRIP steel, has the advantage that can inhale a large amount of hydrogen of storage.In addition,, generate the retained austenite of the lath-shaped of the present invention's regulation easily, also have the advantage that can access very excellent extension on the border of the bainite ferrite of lath-shaped.In order to bring into play such effect effectively, represent according to area occupation ratio with respect to full tissue, bainite ferrite and martensite are added up to more than 80%, be preferably more than 85%, more elect as more than 90%.Also have, its upper limit by and other the balance of tissue (retained austenite) determine that when not containing in addition tissue (ferrite etc.) of retained austenite described later, its upper limit is controlled to be 99%.

In addition, the bainite ferrite described in the present invention is tabular ferrite, and the high bottom tissue of dislocation desity, has the polygon ferrite of the bottom tissue that does not have dislocation or few dislocation with what is called, by the SEM observation, is distinguished clearly as follows.

The area occupation ratio of bainite ferrite tissue is following to be obtained.Promptly, corrode steel with nital, use SEM (Scanning Electron Microscope, scanning electron microscope) to observe (multiplying power: 1500 times) in thickness of slab 1/4 position, the measured zone arbitrarily of the face parallel with calendering face (about 50 * 50 μ m) is calculated its area occupation ratio thus.

Bainite ferrite is shown as Dark grey (as SEM on the SEM photo, also have and separately to distinguish bainite ferrite, residual austenite, martensitic situation), and the polygon ferrite is black in the SEM photo, be the polygon shape, and residual austenite and martensite is not contained in inside.

Employed SEM among the present invention, be to possess " the high resolving power type FE-SEM of EBSP (electron backscattered style, Electron BackScatter diffraction Pattern) detector (FieldEmission type Scanning Electron Microscope; Philips corporate system; XL30S-FEG) ", has in the SEM viewing area advantage that can resolve by the EBSP detector.The EBSP method is made a simple explanation here, EBSP is, electron beam is shone specimen surface, the reflection electronic that produces when resolving thus and the Kikuchi pattern that obtains, determine the instrument of the crystal orientation of electron beam incident position with this, electron beam is carried out 2 dimension scannings at specimen surface, measure the crystal orientation of each institute's determining deviation, the orientation that just can measure specimen surface distributes.Utilize this EBSP to observe, can have following advantage, promptly can be identified in and be judged as homologue in the common microscopic examination according to aberration, and the tissue of the different thickness of slab direction of crystal orientation difference.

＜ferrite (F)+perlite (P): (comprise 0%) below 9% 〉

Steel plate of the present invention, can be only (promptly by described tissue, the mixed structure of bainite ferrite+martensite and residual austenite) constitutes, but in the scope of not damaging effect of the present invention, (said here " ferrite " means the polygon ferrite also can to have ferrite as other tissue, that is, do not have dislocation desity or the ferrite of few dislocation desity is arranged) and perlite.These tissues are inevitable remaining tissues in manufacturing processed of the present invention, and still, it is few more good more to be preferably, and is suppressed at below 9% among the present invention.Be preferably less than 5%, more preferably less than 3%.

As mentioned above, the invention is characterized in, particularly control retained austenite amount and form thereof, in order like this easily to control retained austenite amount and form thereof, and obtain bringing into play the steel plate of prescribed strength, be necessary that as described below being controlled to is grouped into.

(C：0.10～0.25％)

C is for guaranteeing the necessary element of high strength more than the 1180MPa.Also have, C makes austenite contain sufficient C amount in mutually, even also make the mutually residual important element of austenite of hope in room temperature, it is contained more than 0.10%.Be preferably more than 0.12%, more elect as more than 0.15%.But,, among the present invention the C amount is controlled at below 0.25% from guaranteeing the viewpoint of solidity to corrosion and weldability.Be preferably below 0.23%.

<Si：1.0～3.0％>

Si effectively suppresses the important element that residual austenite is decomposed, carbide generates.Also be the substitutional solid solution strengthening element that makes the abundant hardization of material effectively simultaneously.This effect is brought into play effectively, then be necessary to make it to contain more than 1.0%.Be preferably more than 1.2%, more preferably more than 1.5%.If but the Si amount is excessive, the formation of hot rolled oxide skin becomes significantly, will produce expense aspect the elimination of flaw in addition, and is uneconomical economically, so be suppressed at below 3.0%.Be preferably below 2.5%, more preferably below 2.0%.

<Mn：1.0～3.5％>

Mn makes stabilization of austenite, obtains the bioelement of required residual austenite.This effect is brought into play effectively, then need be made it to contain more than 1.0%.Be preferably more than 1.2%, more preferably more than 1.5%.In addition on the one hand, if the Mn amount is superfluous, segregation then takes place become obviously, the situation of processibility deterioration is arranged, so the upper limit is made as 3.5%.Be preferably below 3.0%.

＜P:0.15% following (not containing 0%) 〉

P encourages because of grain boundary segregation produces the element of intercrystalline failure, so wish that its content is low, the upper limit is made as 0.15%.Preferably control to below 0.1%, more preferably below 0.05%.

＜S:0.02% following (not containing 0%) 〉

S is the element that encourages the hydrogen absorption of steel plate under corrosive environment, so wish that its content is low, the upper limit is made as 0.02%.

＜Al:1.5% following (not containing 0%)〉(occasion of steel plate 1 of the present invention)

＜Al:0.5% following (not containing 0%)〉(occasion of steel plate 2 of the present invention)

For deoxidation Al can add to more than 0.01%.Al not only has desoxydatoin in addition, also has the element that improves the solidity to corrosion effect and improve hydrogen-embrittlement resistance.

As the mechanism of described raising solidity to corrosion effect, concrete, be commonly considered as improving mother metal himself solidity to corrosion and the effect of the generation rust that produces by atomospheric corrosion, still, the effect of inferring the latter's generation rust particularly is big.Its reason is, described generation rust is more careful and have good protectiveness than common iron rust, so atomospheric corrosion is suppressed, the hydrogen amount that its result produces this atomospheric corrosion reduces, thus hydrogen embrittlement, and promptly delayed fracture is suppressed effectively.

Improving the mechanism of hydrogen-embrittlement resistance effect about Al, it is not clear in detail, is at surface of steel plate because denseization of Al but infer, thus make hydrogen in steel the intrusion difficulty and since in the steel velocity of diffusion of hydrogen descend and make hydrogen move difficulty, be difficult to produce due to the hydrogen embrittlement.But also can think, because the interpolation of Al makes the stability of lath-shaped residual austenite increase, made contribution to improving hydrogen-embrittlement resistance.

Will bring into play the corrosion proof effect of raising of Al and the effect that improves hydrogen-embrittlement resistance effectively, then the Al amount is made as more than 0.02%, is preferably more than 0.2%, more preferably more than 0.5%.

But, seek increase, gigantism, when guaranteeing processibility at inclusiones such as inhibited oxidation aluminium, guarantee the generation of tiny residual austenite, and, suppress by containing the corrosion that the Al inclusion causes, or the increase of the cost on suppressing to make, then need the Al amount is suppressed at below 1.5%.Viewpoint from making is preferably the A3 point is adjusted to below 1000 ℃.

On the other hand, increase Al content as mentioned above, then inclusion such as aluminum oxide increases, and the processibility deterioration so will fully suppress the inclusion of described aluminum oxide etc., obtains the better steel plate of processibility, then needs be suppressed at the Al amount below 0.5%.Be preferably below 0.3%, more preferably below 0.1%.

What the present invention stipulated contains element as mentioned above, all the other compositions are actually Fe, be brought into unavoidable impurities in the steel as situation because of raw material, goods, materials and equipments, producing apparatus etc., much less allow to contain N (nitrogen) below 0.001% etc., described effect of the present invention is not being brought in the scope of bad influence, can make it to contain other following element energetically yet.

Also have, in this manual, the steel plate of Al content below 1.5% (not comprising 0%) is defined as steel plate 1 of the present invention, the steel plate of Al content below 0.5% (not comprising 0%) is defined as steel plate 2 of the present invention.

(Cu:0.003～0.5% and/or Ni:0.003～1.0%)

By making it contain Cu and/or Ni, suppress generation fully, and the hydrogen that can suppress to take place is invaded to steel plate as the hydrogen of hydrogen embrittlement reason.Its result produces synergy by the raising with the hydrogen trapping ability of the steel plate that brings of the above-mentioned tissue of control, can reduce hydrogen concentration in the steel plate fully until harmless level.

Specifically, Cu, Ni improve the solidity to corrosion of steel self, have and suppress steel plate produces hydrogen owing to corrosion effect fully.Also have these elements to have Thermodynamically stable in the middle of the rust that promotion generates in atmosphere, have the so-called ferric oxide of protectiveness: the effect of the generation of alpha-feooh; by promoting the generation of this rust; the hydrogen that can suppress to take place is to the intrusion of steel plate; under abominable corrosive environment, can improve hydrogen-embrittlement resistance fully.This effect particularly obtains easily by making Cu and Ni coexistence.

Bring into play described effect, when containing Cu, need be more than 0.003%.Be preferably more than 0.05%, more preferably more than 0.1%.If when containing Ni, need be more than 0.003%.Be preferably more than 0.05%, more preferably more than 0.1%.

But no matter be that element, when excessive containing, then processibility descends, so be controlled at below 0.5% during Cu, is controlled at below the 1.O% during Ni.

＜Ti and/or V: amount to 0.003～1.0% 〉

Ti and described Cu, Ni have the effect of the generation that promotes the protectiveness rust equally.This protectiveness rust brings the generation of the β-FeOOH of bad influence to have very effective effect to suppress particularly to generate and give solidity to corrosion (as hydrogen-embrittlement resistance consequently) under chloride environment.The formation of such protectiveness rust is especially promoted when compound interpolation Ti and V (or Zr).Ti gives very outstanding corrosion proof element, also has the advantage of cleaning steel concurrently.

In addition, V has beyond the effect that improves hydrogen-embrittlement resistance except coexisting with described Ti, still to improving armor plate strength, grain refined effective elements.

Give full play to the effect of described Ti and/or V, be preferably and it contained amount to more than 0.003% (more preferably more than 0.01%).Particularly from improving the viewpoint of hydrogen-embrittlement resistance, be preferably interpolation Ti and make it to surpass 0.03%, more preferably adding Ti is more than 0.05%.In addition on the one hand, excessive interpolations Ti, then effect becomes state of saturation, and is uneconomical economically, and excessive interpolation V, the charcoal nitride to separate out change many, cause the reduction of processibility and hydrogen-embrittlement resistance.Thereby the scope that is preferably interpolation Ti and/or V amounts to below 1.0%.More preferably below 0.5%.

<Zr：0.003～1.0％>

Zr is to improving intensity, the grain refined effective elements of steel plate, with the Ti coexistence, the effect that hydrogen-embrittlement resistance is improved being arranged.To bring into play this effect effectively, preferably contain the Zr more than 0.003%.In addition on the one hand, the excessive Zr that contains, then the charcoal nitride to separate out change many, processibility and hydrogen-embrittlement resistance reduce, so be preferably the interpolation scope below 1.0%.

＜Mo:1.0% following (not comprising 0%) 〉

It is stable that Mo has the austenite of making, and guarantees residual austenite, suppresses hydrogen and invade, and improves the effect of hydrogen-embrittlement resistance.And be to the hardenability that improves steel plate also effective elements.Further have the reinforcement crystal boundary, suppress the effect of hydrogen embrittlement.To bring into play this effect effectively, recommend to contain Mo more than 0.005%.More preferably more than 0.1%.But Mo content surpasses 1.0%, and then described effect is saturated, and is uneconomical economically.Be preferably below 0.8%, more preferably below 0.5%.

＜Nb:0.1% following (not comprising 0%) 〉

Nb is to improving armor plate strength and organizing the very effective element of grain refined.Particularly, more can give full play to this effect with the compound interpolation of Mo.Bring into play these effects, recommend it is contained more than 0.005%.More preferably more than 0.01%.But the excessive Nb that contains, then these effects are saturated, and are uneconomical economically, so be suppressed at below 0.1%.Be preferably below 0.08%.

<B：0.0002～0.01％>

B is to improving the intensity effective elements of steel plate, bring into play this effect and be preferably and make it contain (more preferably more than 0.0005%) more than 0.0002%.In addition on the one hand, the excessive B that contains, then hot workability worsens, so preferable range is at (more preferably below 0.005%) below 0.01%.＜from Ca:0.0005～0.005%, Mg:0.0005～0.01%, select in the group that and REM:0.0005～0.01% is formed more than a kind

Ca, Mg, REM (rare earth element) are the risings of the hydrogen ion concentration of the interface atmosphere to suppressing to follow the surface of steel plate corrosion, promptly suppress the decline of pH, improve the solidity to corrosion effective elements of steel plate.Simultaneously, also effective to the form raising processibility of sulfide in the control steel, give full play to this effect, be preferably and make Ca, Mg, the amount of any one among the REM is more than 0.0005%.On the one hand, excessive containing, then the processibility deterioration is below 0.005% so preferably be controlled at Ca in addition, Mg, and REM is respectively below 0.01%.

The present invention not regulation creates conditions, but satisfy the above-mentioned tissue that steel that mentioned component forms form superstrength and bring into play excellent hydrogen-embrittlement resistance in order to use, hot rolled precision work temperature is preferably set to the low temperature of doing one's utmost that can not generate ferritic supercooled austenite regional temperature.By carrying out finish rolling in this temperature, austenite that can the refinement hot-rolled steel sheet, its as a result the tissue of end article become trickle.

Also have,, recommend to heat-treat with following main points after the hot rolling or behind the cold bundle that carries out thereafter.Promptly, with after satisfying steel that described one-tenth is grouped into temperature (T1) heating keeping 10～1800 seconds down in A3 point～(A3 point+50 ℃), the average cooling rate above with 3 ℃/s is cooled to the temperature (T2) that (Ms point-100 ℃)～Bs order, at 60～1800 seconds (t2) of this temperature province heating maintenance.

If described T1 surpasses (A3 point+50 ℃), or t1 surpasses 1800 seconds, then causes austenitic grain growing, and processibility (stretch flangeability) worsens, so can not be satisfactory.In addition on the one hand, if the temperature that described T1 is ordered less than A3, the bainite ferrite tissue that then could not obtain to stipulate.In addition, described t1 less than 10 seconds situation under, then can not realize sufficient austenitizing, cause the remaining of cementite or other alloy carbide, so can not be satisfactory.Described t1 was preferably more than 30 seconds below 600 seconds, more preferably more than 60 seconds below 400 seconds.

With the described steel plate of postcooling,,, prevent the generation of pearlitic structure in order that avoid the pearlitic transformation zone with the above average cooling rate cooling of 3 ℃/s.This average cooling rate is the bigger the better, recommend to be preferably 5 ℃/more than the s, more preferably 10 ℃/more than the s.

Secondly, be cooled to after (Ms point-100 ℃)～Bs point,, can introduce the tissue of regulation by making its constant temperature phase transformation with described speed.The heating here keeps temperature (T2) to order above Bs, and the undesirable perlite of the present invention will generate in a large number, can not guarantee the bainite ferrite tissue fully.In addition on the one hand, described T2 is lower than (Ms point-100 ℃), and then residual austenite reduces, so can not be satisfactory.

In addition, if heating hold-time (t2) surpasses 1800 seconds, then the dislocation desity except bainite ferrite diminishes, beyond the seizure quantitative change less of hydrogen, but also the residual austenite that can not obtain stipulating.In addition on the one hand, if the bainite ferrite tissue that described t2 less than 60 seconds, can not obtain stipulating.Be preferably described t2 was made as more than 90 seconds below 1200 seconds, more preferably more than 120 seconds below 600 seconds.Do not do special qualification as for the method for cooling after the heating maintenance, can carry out air cooling, chilling, air water cooling etc.

Consider actually operating, then described anneal employing continuous annealing apparatus or batch-type annealing device are easier.Plating on cold-reduced sheet in addition applies and dissolves when zinc-plated, also the plating condition can be set according to as above heat-treat condition, carries out described thermal treatment in this plating process.

Also have, do not limit hot-rolled process before described continuous annealing is handled (as required also cold rolling process) especially, can suitably select implementation condition to carry out usually.The described hot-rolled process of concrete conduct can adopt for example after the above hot rolling of Ar3 point finishes, with the about 30 ℃/s cooling of average cooling rate, the condition of batching under about 500～600 ℃ temperature.In addition, after hot rolling, during the bad situation of shape, also can be shaped as purpose and carry out cold rolling with correction.Recommend cold rolling rate to be made as 1～70% here.Its reason is, surpasses the cold rolling of cold rolling rate 70%, and the calendering load will be increased, and causes calendering to become difficulty.

The present invention is to be the invention of object with the steel sheet, do not limit the goods form especially, except the steel plate that obtains by hot rolling with further by the cold rolling steel plate that obtains, also can carry out hot rolling or the cold rolling annealing of enforcement afterwards, implement conversion processing afterwards, or implement to dissolve plating, electroplate, plating and various applications such as evaporation, application based process, organic epithelium processing etc.

As the kind of described plating, general can be zinc-plated, any of aluminizing etc.The method of plating in addition, can be dissolve plating and galvanized any, and can implement alloying thermal treatment behind the plating, can also implement the multilayer plating.And, can also implement the film lamination processing on the non-coated steel sheet and on the coated steel sheet.

When implementing described application, corresponding various uses also can be implemented the chemical conversion of phosphatizing etc. and handle electrodeposition coating.Coating can use known resin, can be with Resins, epoxy, and fluorine resin, the silicon acrylic resin, polyurethane resin, acrylic resin, vibrin, resol, Synolac, melmacs etc. use with known stiffening agent.From corrosion proof viewpoint, recommend to use Resins, epoxy especially, fluorine contains resin, the silicon acrylic resin.In addition, the known additive that adds in coating for example can add the painted pigment of using, coupler, equal paint, sensitizing agent, antioxidant, ultra-violet stabilizer, fire retardant etc.

Also have, the form of coating does not limit especially yet, can be coating according to the suitable selective solvent of purposes, coccoid coating, and aqueous coating, water-dispersed paint, purposes such as EFI coating are suitably selected.Use described coating, make steel form desirable coating, can adopt pickling process, roll-type coating method, spraying method, known method such as the mobile japanning method of curtain formula.The thickness of coating can adopt suitable known value by its purposes.

Ultrahigh-strength steel plates of the present invention, except being used for snubber and impacting a door beam, pillar, skeleton, the automobile of automobile enhancing portion materials such as member etc. can also be used for indoor component such as seat guide rail with beyond the intensity component.Component by obtaining after such shaping processing also have sufficient material characteristic (intensity), and bring into play good hydrogen-embrittlement resistance.

(embodiment 2)

The ultrahigh-strength thin steel sheet that relates to embodiments of the present invention 2, it is characterized in that, by percentage to the quality, contain C:0.10～0.25%, Si:1.0～3.0%, Mn:1.0～3.5%, below the P:0.15%, below the S:0.02%, Al:1.5% following (not comprising 0%), all the other are made of iron and unavoidable impurities, wherein represent, contain the residual austenite more than 1%, amount to bainite ferrite and martensite more than 80% according to area occupation ratio for whole tissue, and the ferrite and the perlite that (comprise 0%) below 9% altogether, and the average minor axis length of described residual austenite crystal grain is below the 1 μ m, and described residual austenite intercrystalline be below the 1 μ m in abutting connection with spacing, the mean axis of described residual austenite crystal grain is more than 5 than (major axis/minor axis), and tensile strength is more than the 1180MPa., can contain the bainite ferrite and the martensite that amount to more than 80% here, and/or the ferrite and the perlite that (comprise 0%) below 9% altogether, can not contain yet.

Just stipulate that below the reason of these conditions and measuring method etc. make an explanation.In addition, the prerequisite about identical with embodiment 1 below is omitted.

The average minor axis length of＜residual austenite crystal grain: 1 μ m is following 〉

In addition, in the present invention, the fine dispersion of above-mentioned lath-shaped residual austenite can effectively be improved hydrogen-embrittlement resistance.The concrete discovery if described strip residual austenite crystal grain is separated into (the submicron utmost point) below the 1 μ m, then can be improved hydrogen-embrittlement resistance reliably.Can think, this be because with average minor axis length short and fine residual austenite crystal grain disperse in a large number, can make the surface-area (interface) of residual austenite crystal grain become big, and the hydrogen capturing ability increases.The average minor axis length of this residual austenite crystal grain is preferably below the 0.5 μ m, more preferably below the 0.25 μ m.

In the present invention, as mentioned above, mean axis ratio and its average minor axis length by control residual austenite crystal grain, even if under the situation that allows the residual austenite of equal volume ratio exist, the hydrogen capturing ability of tiny lath-shaped residual austenite of the present invention is far longer than makes carbide dispersive situation, and can make the hydrogen of invading innoxious owing to atomospheric corrosion.

＜residual austenite is intercrystalline in abutting connection with spacing: below the 1 μ m 〉

Can know according to the present invention,, then can improve hydrogen-embrittlement resistance more if it is intercrystalline in abutting connection with spacing also to control residual austenite simultaneously.The concrete discovery, if above-mentioned residual austenite intercrystalline be below the 1 μ m in abutting connection with spacing, then can improve hydrogen-embrittlement resistance reliably.Can think that this is because because the residual austenite of above-mentioned fine lath-shaped is to form a kind of approaching mutually and a large amount of dispersive states, makes the expansion that destroys (crackle) be inhibited, thereby can obtain to have for destruction the tissue of high resistibility.Residual austenite is intercrystalline in abutting connection with spacing, is preferably below the 0.8 μ m, more preferably below the 0.5 μ m.

Below, exemplify embodiment the present invention is done more specific description, still, the present invention is not subjected to the restriction of the following example, in the scope of described aim, can suitably implement after changing before and after being fit to, and it all is contained within the technical scope of the present invention.

[embodiment 1]

Vacuum melting is grouped into the experimental steel No.A-1～Y-1 that constitutes by the one-tenth of record in the table 1, make experiment with after the slab, according to following operation (hot rolling → cold rolling → continuous annealing), obtain after the hot-rolled steel sheet of thickness of slab 3.2mm, remove surface scale by pickling, be cold-rolled to afterwards become 1.2mm thick till.＜hot-rolled process〉beginning temperature (SRT): 1150～1250 ℃ kept 30 minutes

Finish temperature (FDT): 850 ℃

Speed of cooling: 40 ℃/s

Coiling temperature: 550 ℃

＜cold rolling process〉cold rolling rate: 50%

About＜continuous annealing operation〉each experimental steel, ℃ keep after 120 seconds with A3 point+30, cool off (air cooling) in the table 2 To ℃ fast with 20 ℃/s of average cooling rate, kept for 240 seconds with this To ℃.Air water is cooled to room temperature then.

In addition, the No.116 in the table 2 in order to make the martensitic steel of high-strength steel in the past, is heated to 830 ℃ with the steel plate after cold rolling as a comparative example, keeps implementing water quenching afterwards in 5 minutes, 300 ℃ of tempering 10 minutes.No.120 in addition is the steel plate after cold rolling to be heated to 800 ℃ keep being cooled to 350 ℃ with 20 ℃/s of average cooling rate after 120 seconds, keeps for 240 seconds under this temperature.

Investigated the metal structure of each steel plate of acquisition like this according to following main points, tensile strength (TS) is extended (be full extension (El)), and hydrogen-embrittlement resistance.

(observation of metal structure)

On the position of goods thickness of slab 1/4, with (the about 50 μ m * 50 μ m of the measured zone arbitrarily on the face parallel with calendering face, measure spacing 0.1 μ m) be object, observe, photograph, measured the area occupation ratio of bainite ferrite (BF) and martensite (M), residual austenite (remaining γ) according to described method.And in optional 2 visuals field, carry out same measurement, obtain mean value.From whole tissue (100%), deduct the described shared area occupation ratio of organizing in addition, obtain other tissue (ferrite and perlite etc.).And, with the mean axis ratio of residual austenite crystal grain, measure according to described method, mean axis is satisfied prerequisite of the present invention (zero) than being evaluated as more than 5, the mean axis ratio is discontented with the prerequisite (*) that unabridged version is invented less than 5 be evaluated as.

(tensile strength (TS) and the measurement of extending (El))

Stretching experiment uses the JIS5 experiment slice to carry out, and has measured tensile strength (TS) and has extended (El).Here, the rate of straining of stretching experiment is made as 1mm/sec.And the present invention is an object with the steel plate of tensile strength more than 1180MPa of measuring according to described method, extends in being evaluated as more than 8% (extending good).

(evaluation of hydrogen-embrittlement resistance)

When measuring hydrogen-embrittlement resistance, the rectangular experiment slice that cuts out 150mm * 30mm from above-mentioned each steel plate is as experiment slice.This rectangular experiment slice is implemented bending machining, and to make the R of bend be 15mm, applies the stress of 1000Mpa then, impregnated in the time of taking place till the crackle of having measured in 5% aqueous hydrochloric acid.

And,, also carried out 4 crooked experiments of hydrogen filling about the part steel grade.As follows in detail: from the rectangular experiment slice of described each steel plate cutting 65mm * 10mm and at solution (0.5mol/H ₂SO ₄+ 0.01mol/KSCN) in dipping, carry out the cathode hydrogen filling, measure as limit rupture stress (DFL) with the maximum stress that does not rupture in 3 hours.

Described result also remembers in table 2.

(evaluation of weldability)

No.101 and No.114 as representative steel grade have been carried out the weldability experiment.Experiment uses thick the supplying of 1.2mm to try material, make experiment slice according to JISZ 3136, JISZ 3137, carry out a postwelding with following condition, shear tension experiment (measuring ultimate load) and cross tension experiment (measuring ultimate load), obtain and shear tensile strength (TSS) and cross tensile strength (CTS) with draw speed: 20mm/min with draw speed: 20mm/min.Then, be 0.2 to be evaluated as the weldability excellence when above by the represented ductility ratio (CTS/TSS) of the ratio of described shearing tensile strength (TSS) and cross tensile strength (CTS).Its result is 0.19 with respect to the ductility ratio of No.114 (existing steel), and the ductility ratio of No.101 (exploitation steel) is 0.22, the weldability excellence.

(spot welding condition)

Clamping time at initial stage: 60 circulation/60Hz, plus-pressure 450kgf (4.4kN)

Conduction time: 1 circulation/60Hz

Welding current: 8.5kA

[table 1]

[table 2]

From table 1,2 can investigate following (in addition, following note No. represents the experiment No. in the table 2).

Satisfy No.101～113 (steel plate 2 of the present invention), 121～125 (steel plates 1 of the present invention) of the prerequisite of the present invention's regulation, have the above superstrength of 1180MPa, also very good aspect the hydrogen-embrittlement resistance under harsh and unforgiving environments simultaneously.In addition, also good aspect the extension that should possess, and have good weldability as the TRIP steel plate, as the suitableeest steel plate of the reinforcement component that are placed in the automobile in the atomospheric corrosion atmosphere and obtained.Particularly No.121～125 demonstrate better hydrogen-embrittlement resistance.

In contrast, No.114～120,126 of discontented unabridged version invention regulation have following unfavorable condition respectively.

That is, No.114 is to use the example of the excessive steel grade N-1 of C amount, does not have sufficient weldability.

No.115 is because of using the steel grade O-1 of Mn quantity not sufficient, so can not guarantee enough residual austenites, the hydrogen-embrittlement resistance deterioration.And be difficult to extend fully.

It is the example of the martensitic steel of high-strength steel that No.116 is to use the steel grade P-1 of Si quantity not sufficient to obtain in the past, because almost not existence of residual austenite, so the hydrogen-embrittlement resistance deterioration.Simultaneously also fail to guarantee the desired extension of steel sheet.

No.117 is because of using the steel grade Q-1 of C quantity not sufficient, so fail to reach high strength.

No.118 contains the excessive steel grade R-1 of Nb amount because of being to use, and No.119 is to use and contains the superfluous steel grade S-1 of Mo amount, as a result all remarkable deterioration of its plasticity.In addition, No.118,119 can not carry out described processing, so can not detect the characteristic after the processing.

The steel that No.120 uses the one-tenth satisfy the present invention's regulation to be grouped into, but be not to make with the condition of recommending, so the steel plate that obtains is a TRIP steel plate in the past.Its result, the discontented foot of residual austenite mean axis ratio given to this invention, and its parent phase is not to be made of bainite ferrite and martensitic duplex structure, so there is not the desired strength level of good hydrogen-embrittlement resistance.

No.126 is because surpass the Al amount of steel plate 1 defined of the present invention, though can guarantee the residual austenite of defined amount, but, the discontented foot of this residual austenite mean axis ratio given to this invention, have or not and reach desirable parent phase, and, also generate the inclusion of AlN etc., so the hydrogen-embrittlement resistance deterioration.

Secondly, steel grade mark A-1 with described table 1, H-1 steel plate and comparison steel plate (being the high-tensile steel of the 590MPa level of product in the past), the shaping component, as described belowly carry out withstand voltage destructive test and shock-resistant characteristic test, detected the performance (withstand voltage destructiveness and shock-resistant characteristic) of molding.

(withstand voltage destructive test)

At first, use the steel grade mark A-1 of table 1, the steel plate of H-1 and comparison steel plate make component shown in Figure 3 (experiment body, hat groove (hat channel) component) 1 respectively, have carried out following pressure destructive test.That is, on the spot welding position 2 of the component that Fig. 3 represents,, flow through electric current, carried out spot welding with the 35mm spacing as shown in Figure 3 than the low 0.5kA of sputter generation electric current from the electrode of top end diameter 6mm.Then as shown in Figure 4, begin to push mould 3, obtain maximum load from the top than the length direction central part of component 1.And obtain the absorption energy from the area of load-displacement diagram.It the results are shown in table 3.

Table 3

Find out that by table 3 component (experiment body) that use steel plate of the present invention to make show bigger than load when low in the past the steel plate of working strength, and it is also big to absorb energy, learns to have good withstand voltage destructiveness thus.

(shock-resistant characteristic test)

Use the steel grade mark A-1 of table 1, the steel plate of H-1 and comparison steel plate make component shown in Figure 5 (experiment body, hat groove (hat channel) component) 4 respectively, have carried out following shock-resistant characteristic test.In addition, Fig. 6 represents the A-A sectional drawing of component 4 among described Fig. 5.Shock-resistant characteristic test is identical with the situation of described withstand voltage destructive test, after the 5 enterprising spot weldings of the spot welding position of component 4, shown in the synoptic diagram of Fig. 7, component 4 are arranged on the pedestal 7, top from these component 4, make and drop hammer that (quality: 110kg) 6 positions from height 11m fall, and obtain the absorption energy that component 4 are deformed to 40mm (contraction of short transverse), and it the results are shown in table 4.

[table 4]

Learn that by table 4 component (experiment body) that use steel plate of the present invention to make show than absorption energy big when low in the past the steel plate of working strength to have good shock-resistant characteristic.

As a reference, enclose the tem observation photo of the experiment slice that obtains with present embodiment.Fig. 8 is the tem observation photo example of the No.101 of example of the present invention, can learn from Fig. 8, and the metal structure of ultrahigh-strength steel plates of the present invention is the dispersed state of lath-shaped residual austenite given to this invention (among Fig. 8, linear black part).In addition on the one hand, Fig. 9 is the tem observation photo example of the No.120 of comparative example, can learn from Fig. 9, in the ultrahigh-strength steel plates of No.120, have residual austenite (the black part of justifying slightly among Fig. 9), but be discontented foot blocky residual austenite given to this invention.

[embodiment 2]

The one-tenth that use is put down in writing by table 5 be grouped into constitute for examination steel No.A-2～Y-2, under the condition identical (hot-rolled process, cold mutual aid system devised for poor peasants preface and continuous annealing operation), be made into steel plate with embodiment 1.

Also have, in the No.217 of table 6, in order to make the martensitic steel of existing high-strength steel as a comparative example, make in the same manner with the No.116 of the table 2 of embodiment 1.Also have, in No.220, in the same manner steel plate is handled with the No.120 of the table 2 of embodiment 1.

Similarly to Example 1, with following main points, respectively to metal structure, the tensile strength (TS) of each steel plate of so obtaining, extend (complete extend (El)) and hydrogen-embrittlement resistance is investigated.

(evaluation of hydrogen-embrittlement resistance)

When hydrogen-embrittlement resistance is estimated, make the experiment slice identical with embodiment 1, be immersed in embodiment 1 identical aqueous hydrochloric acid in, the time till mensuration is broken.

Also have, the environment for use that imagination is actual, for the crooked experiment sheet of making in the same manner with embodiment 1, continuous once a day 30 days spraying 3%NaCl solution promotes exposure test, the number of days of mensuration till breaking.

In addition, identical for the steel grade of a part with embodiment 1, also carry out 4 pliability tests of hydrogen load, the maximum stress that did not rupture in 3 hours is measured as ultimate stress limit (DFL).Then, obtain the ratio (DFL ratio) of the DFL of the test No.203 (steel grade mark C-2) with respect to table 6.

These outcome record are in table 6.

(evaluation of weldability)

No.201 and No.215 as representative steel grade have been carried out weldability test.Test is carried out similarly to Example 1, its result, and with respect to 0.19 of the ductility ratio of N0.215 (existing steel), the ductility ratio of No.201 (invention steel) is 0.22, the weldability excellence.

[table 5]

[table 6]

From table 5,6 can investigate following (in addition, following note No. represents the experiment No. in the table 6).

Satisfy No.201～214 (steel plate 2 of the present invention), 221～225 (steel plates 1 of the present invention) of condition given to this invention, as the ultrahigh-strength steel plates more than the 1180MPa, in the promotion exposure test of the actual corrosive environment of salt spray testing and imagination, shown excellent anti-delayed fracture characteristic (hydrogen-embrittlement resistance).Also have the extension that should possess also good, in addition because also have excellent weldability, so can access the steel plate that is suitable for as the reinforcing member of the automobile that is exposed to atomospheric corrosion atmosphere etc. as the TRIP steel plate.Particularly No.221～225 have demonstrated more excellent hydrogen-embrittlement resistance.

In contrast, No.215～220,226 of discontented unabridged version invention regulation have following unfavorable condition respectively.

That is, No.215 is to use the example of the superfluous steel grade O-2 of C amount, does not have sufficient weldability.

No.216 is because of using the steel grade P-2 of Mn quantity not sufficient, so fail to guarantee enough residual austenites, hydrogen-embrittlement resistance deterioration.

It is the example of the martensitic steel of high-strength steel that No.217 is to use the steel grade Q-2 of Si quantity not sufficient to obtain in the past, because almost not existence of residual austenite, so the hydrogen-embrittlement resistance deterioration.Simultaneously also fail to guarantee the desired extension of steel sheet.

No.218 is because of using the steel grade R-2 of C quantity not sufficient, so fail to reach high strength.No.219 so fail to guarantee enough solidity to corrosions, can not guarantee the hydrogen-embrittlement resistance of level of the present invention because do not contain Cu and/or Ni.

The steel that No.220 uses the one-tenth satisfy the present invention's regulation to be grouped into, but be not to make with the condition of recommending, so the steel plate that obtains is a TRIP steel plate in the past.Its result, the discontented foot of residual austenite mean axis ratio given to this invention, and its parent phase is not to be made of bainite ferrite and martensitic duplex structure, so there is not the desired strength level of good hydrogen-embrittlement resistance.

No.226 is because surpass the Al amount of steel plate 1 defined of the present invention, though can guarantee the residual austenite of defined, but, the discontented foot of this residual austenite mean axis ratio given to this invention, have or not and reach desirable parent phase, but also the inclusion of generation AlN etc., so the hydrogen-embrittlement resistance deterioration.

Secondly, steel grade mark A-2 with described table 5, I-2 steel plate and comparison steel plate (being the high-tensile steel of the 590MPa level of product in the past) shaping component, same with described embodiment 1, and method has been carried out withstand voltage destructive test and shock-resistant characteristic test as described below, has detected the performance (withstand voltage destructiveness and shock-resistant characteristic) of molding.

(withstand voltage destructive test)

At first, use the steel grade mark A-2 of table 5, the steel plate of I-2 and comparison steel plate are obtained maximum load similarly to Example 1.And, obtain the absorption energy from the area of load-displacement diagram.It the results are shown in table 7.

Table 7

Learn that by table 7 component (experiment body) that use steel plate of the present invention to make show that in the past the steel plate load lower than working strength is big, and it is also big to absorb energy, learns to have good withstand voltage destructiveness thus.

(shock-resistant characteristic test)

Use the steel grade mark A-2 of table 5, the steel plate of I-2 and comparison steel plate similarly to Example 1, are obtained the absorption energy, and it the results are shown in table 8.

Table 8

Learn by table 8, the component (experiment body) that use steel plate of the present invention to make, big receptivity when being presented at than low in the past the steel plate of working strength has good shock-resistant characteristic.

As a reference, enclose the tem observation photo of the experiment slice that obtains with present embodiment.Figure 10 is the tem observation photo example of the No.201 of example of the present invention, can learn from Figure 10, and the metal structure of ultrahigh-strength steel plates of the present invention is the dispersed state of lath-shaped residual austenite given to this invention (among Figure 10, linear black part).In addition on the one hand, Figure 11 is the tem observation photo example of comparative example No.220, can learn from Figure 11, in the ultrahigh-strength steel plates of No.220, have residual austenite (the black part of justifying slightly among Figure 11), but be discontented foot blocky residual austenite given to this invention.

[embodiment 3]

The one-tenth that use is put down in writing by table 9 be grouped into constitute for examination steel No.A-3～R-3, under the condition identical (hot-rolled process, cold mutual aid system devised for poor peasants preface and continuous annealing operation), be made into steel plate with embodiment 1.

Also have, in the No.312 of table 10, in order to make the martensitic steel of existing high-strength steel as a comparative example, make in the same manner with the No.116 of the table 2 of embodiment 1.Also have, in No.313, in the same manner steel plate is handled with the No.120 of the table 2 of embodiment 1.

Similarly to Example 1, with following main points, respectively to metal structure, the tensile strength (TS) of each steel plate of so obtaining, extend (complete extend (El)) and hydrogen-embrittlement resistance (hydrogen embrittlement risk level index) is investigated.

(evaluation of hydrogen-embrittlement resistance)

Use the dull and stereotyped experiment slice of thickness of slab 1.2mm, with rate of straining 1 * 10 ^-4The low rate of straining of/sec carries out stretching experiment method (SSRT), obtains by the defined hydrogen embrittlement risk level of following formula index (%), has estimated hydrogen-embrittlement resistance.

Hydrogen embrittlement risk level index (%)=100 * (1-E1/E0)

Here, the extension when in fact E0 represents in steel the experiment slice of hydrogeneous state fracture, the extension when E1 is illustrated in steel (experiment slice) fracture that electrochemically makes the hydrogen filling in the sulfuric acid.Here, described hydrogen filling is at H with steel (experiment slice) ₂SO ₄(0.5mol/L) and in the mixing solutions of KSCN (0.01mol/L) flood, at room temperature and certain electric current (100A/m ²) condition under carry out.

Described hydrogen embrittlement risk level index surpasses 50%, and the danger that in use causes hydrogen embrittlement is arranged, so in the present invention will be in that to be evaluated as hydrogen-embrittlement resistance below 50% good.The described table 10 that the results are shown in.

(evaluation of weldability)

No.301 and No.311 as representative steel grade have been carried out weldability test.Test is carried out similarly to Example 1, its result, and with respect to 0.19 of the ductility ratio of NO.311 (existing steel), the ductility ratio of No.301 (invention steel) is 0.22, the weldability excellence.

[table 9]

[table 10]

From table 9,10 can investigate following (in addition, following note No. represents the experiment No. in the table 10).

When No.301～310 (steel plate 2 of the present invention), 314～318 (steel plates 1 of the present invention) that satisfy the prerequisite of the present invention regulation show the above superstrength of 1180MPa, also demonstrate and have good hydrogen-embrittlement resistance.The extension that should possess as the TRIP steel plate is also good in addition, and has good weldability, as the suitableeest steel plate of the reinforcement component that are placed in the automobile in the atomospheric corrosion atmosphere and obtained.

In contrast, No.311～313,319 of discontented unabridged version invention regulation have following unfavorable condition respectively.

That is, No.311 is to use the example of the superfluous steel grade k-3 of C amount, does not have sufficient weldability.

It is the example of the martensitic steel of high-strength steel that No.312 is to use the steel grade L-3 of Si quantity not sufficient to obtain in the past, because almost not existence of residual austenite, so the hydrogen-embrittlement resistance deterioration.

No.313, use be to satisfy the steel that one-tenth given to this invention is grouped into, but because under the condition of recommending, do not make, so the steel plate of gained becomes existing TRIP steel plate.That is,, become bulk, also have parent phase also not form bainite ferrite and martensitic two-phase structure, so undercapacity because retained austenite does not satisfy the mean axis ratio of the present invention's regulation.

No.319 is because surpass the Al amount of steel plate 1 defined of the present invention, though can guarantee the residual austenite of defined amount, but, the discontented foot of this residual austenite mean axis ratio given to this invention, and do not reach desirable parent phase, and, also generate the inclusion of AlN etc., so the hydrogen-embrittlement resistance deterioration.

Secondly, steel grade mark A-3 with described table 9, D-3 steel plate and comparison steel plate (being the high-tensile steel of the 590MPa level of product in the past) shaping component, same with described embodiment 1, and method has been carried out withstand voltage destructive test and shock-resistant characteristic test as described below, has detected the performance (withstand voltage destructiveness and shock-resistant characteristic) of molding.

(withstand voltage destructive test)

At first, use the steel grade mark A-3 of table 9, the steel plate of D-3 and comparison steel plate are obtained maximum load similarly to Example 1.And, obtained the absorption energy from the area of load-displacement diagram.It the results are shown in table 11.

Table 11

Find out that by table 11 component (experiment body) that use steel plate of the present invention to make show bigger than load when low in the past the steel plate of working strength, and it is also big to absorb energy, learns to have good withstand voltage destructiveness thus.

(shock-resistant characteristic test)

Use steel plate and the comparison steel plate of steel grade mark A-3, the D-3 of table 9, similarly to Example 1, obtained the absorption energy, it the results are shown in table 12.

Table 12

Learn by table 12, the component (experiment body) that use steel plate of the present invention to make, big absorption energy when showing than low in the past the steel plate of working strength has good shock-resistant characteristic.

As a reference, enclose the tem observation photo of the experiment slice that obtains with present embodiment.Figure 12 is the tem observation photo example (15,000 times of multiplying powers) of the No.301 of example of the present invention, and Figure 13 is the tem observation photo example (60,000 times of multiplying powers) that the part of described Fig. 3 photo has been amplified.From Figure 12, Figure 13 can learn, the metal structure of ultrahigh-strength steel plates of the present invention is the slightly dispersed state of residual austenite (Figure 12, among Figure 13, straight line black part).The shape of this residual austenite satisfies the lath-shaped of prerequisite given to this invention.In addition on the one hand, Figure 14 is the tem observation photo example of the No.313 of comparative example, can learn from Figure 14, in the ultrahigh-strength steel plates of No.313, have residual austenite (the black part of justifying slightly among Figure 14), but be discontented foot blocky residual austenite given to this invention.

Claims (9)

1. a tensile strength is the above steel sheet of 1180MPa, it is characterized in that: by percentage to the quality, satisfy C:0.10～0.25%, Si:1.0～3.0%, Mn:1.0～3.5%, below the P:0.15%, below the S:0.02%, Al:1.5% is following but do not comprise 0%, and all the other are made of iron and unavoidable impurities, wherein represent by area occupation ratio for whole tissue

Have the residual austenite more than 1%, bainite ferrite and martensite are to amount to more than 80%,

Ferrite and perlite are for amounting to below 9% and comprising 0%, simultaneously

The mean axis ratio of described residual austenite crystal grain is that major axis/minor axis is more than 5.

2. tensile strength as claimed in claim 1 is the above steel sheet of 1180MPa, and wherein, the average minor axis length of described residual austenite crystal grain is below the 1 μ m, and described residual austenite intercrystalline be below the 1 μ m in abutting connection with spacing.

3. a tensile strength is the above steel sheet of 1180MPa, it is characterized in that: by percentage to the quality, satisfy C:0.10～0.25%, Si:1.0～3.0%, Mn:1.0～3.5%, below the P:0.15%, below the S:0.02%, Al:1.5% is following but do not comprise 0%, and all the other are made of iron and unavoidable impurities, wherein represent by area occupation ratio for whole tissue

Have the residual austenite more than 1%,

The mean axis ratio of described residual austenite crystal grain is that major axis/minor axis is more than 5, simultaneously

The average minor axis length of described residual austenite crystal grain is below the 1 μ m, and described residual austenite intercrystalline be below the 1 μ m in abutting connection with spacing.

4. be steel sheet more than the 1180MPa as each described tensile strength in the claim 1 to 3, wherein, by percentage to the quality, it is following but do not comprise 0% to satisfy Al:0.5%.

5. be the above steel sheet of 1180MPa as claim 1 or 3 described tensile strength, wherein, by percentage to the quality, also contain Cu:0.003～0.5% and/or Ni:0.003～1.0%.

6. be the above steel sheet of 1180MPa as claim 1 or 3 described tensile strength, wherein, by percentage to the quality, also contain and amount to 0.003～1.0% Ti and/or V.

7. be the above steel sheet of 1180MPa as claim 1 or 3 described tensile strength, wherein, by percentage to the quality, it is following but do not comprise 0% also to contain Mo:1.0%, and Nb:0.1% is following but do not comprise 0%.

8. be the above steel sheet of 1180MPa as claim 1 or 3 described tensile strength, wherein, by percentage to the quality, also contain B:0.0002～0.01%.

9. be the above steel sheet of 1180MPa as claim 1 or 3 described tensile strength, wherein, by percentage to the quality, also contain and be selected from, more than one in the group that Mg:0.0005～0.01% and REM:0.0005～0.01% is formed by Ca:0.0005～0.005%.

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