CN101460644B

CN101460644B - High-strength steel sheet and its production method

Info

Publication number: CN101460644B
Application number: CN2007800202779A
Authority: CN
Inventors: 兴津贵隆
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 2006-06-01
Filing date: 2007-06-01
Publication date: 2011-04-20
Anticipated expiration: 2027-06-01
Also published as: JP2007321207A; CN101460644A; JP5095958B2

Abstract

A high-strength steel sheet which has a metallic structure comprising a ferrite phase and a second hard phase dispersed in the steel sheet. In the metallic structure, the areal proportion of the second hard phase is 30-70%. In the ferrite phase, the areal proportion of ferrite having a crystal grain diameter not larger than 1.2 [mu]m is 15-90% and the average grain diameter of ferrite having a crystal grain diameter not larger than 1.2 [mu]m (ds) and the average grain diameter of ferrite having a crystal grain diameter exceeding 1.2 [mu]m (dL) satisfy the relationship dL/ds>=3. The size of the ferrite crystal grains has been reduced by reducing the amount of alloying elements to be added. The steel sheet hence has an increased strength. The steel sheet has an excellent balance between strength and ductility, which is important for press molding. It is excellent in energy absorption in high-speed deformation. Also provided is a process for producing the steel sheet.

Description

High tensile steel plate and manufacture method thereof

Technical field

The present invention relates to automobile cold-rolled high tensile steel plate and manufacture method thereof, described cold rolling high strength steel plate has high strength and high ductibility concurrently, has excellent press forging formability and impact energy receptivity.In addition, the present invention relates to the manufacture method of high tensile steel plate, the manufacturing technology that particularly relates to high Strength Steel for Motor Vehicles, the high Strength Steel for Motor Vehicles of this technology manufacturing has high quiet moment (static strength and dynamic strength poor) concurrently and the ductility when being out of shape at a high speed, intensity is low during the press forging moulding, intensity height and ductility height during collision, impact energy receptivity excellence.

Background technology

In recent years, in order to tackle the requirement of the such contradiction of raising automotive crash safety and car body lightweight, the steel plate that requires to be used for car body has higher intensity and the ductility of Geng Gao simultaneously.About satisfying the high tensile steel plate of such requirement, following content is disclosed.Promptly disclose a kind of manufacture method of high tensile steel plate, wherein, the steel of the Si by will containing 0.1%～0.45% C and 0.5%～1.8% carries out hot rolling, cold rolling, annealing under prescribed condition, and producing tensile strength is 82kgf/mm ²～113kgf/mm ², tensile strength * elongation value be 2500kgf/mm ²The good high tensile steel plate (referring to patent documentation 1) of ductility that % is above.

In addition, a kind of high tensile steel plate and manufacture method thereof are also disclosed, wherein by containing 0.1%～0.4% C, improve the Mn amount with the composition system that has limited Si, and under prescribed condition, carry out 2 annealing, produce that to have tensile strength be that 811MPa～1240MPa, tensile strength * elongation are the high tensile steel plate (referring to patent documentation 2) of the above high ductibility of 28000MPa%.

In addition, also disclose a kind of manufacture method of high ductibility high strength cold rolled steel plate, wherein, for the steel that contains 0.02%～0.3% C, the austenite volume fraction during with annealing is controlled at specialized range, and producing tensile strength thus is 48kgf/mm ²～151kgf/mm ², and the value of tensile strength * elongation be 1800kgf/mm ²The high ductibility high strength cold rolled steel plate (referring to patent documentation 3) that % is above.

In addition, if improve armor plate strength, then formability reduces, so the application component of high tensile steel plate is limited to the single parts of shape.Therefore, in order to overcome such shortcoming, attempt using the high steel plate of quiet moment.

Usually, the influence of the strained speed of deformation intensity of steel plate, rate of straining is high more, and deformation stress is big more.That is, if quiet moment height, then relative intensity is lower during the press forging moulding, can guarantee formability, and during car body collision when taking place to be out of shape at a high speed, can guarantee full intensity.

For example, patent documentation 4 discloses a kind of steel sheet and manufacture method thereof, wherein, reduces C, the N that is solid-solubilized in the ferrite in the single-phase steel of ferrite, controls the reasonable quantity of cementite, makes the steel sheet of quiet moment height, shock-resistance excellence thus.In addition, patent documentation 5 discloses a kind of manufacture method of high tensile steel plate of ultra-fine grain structure, wherein, and with the 2 piece above laminating metal plates of surface through cleaningization, the accumulation ply rolling is made the high tensile steel plate of ultra-fine grain structure thus to the nano level that the miniaturization of ferrite crystal grain particle diameter is arrived less than 1 μ m.In addition, patent documentation 6 discloses a kind of high-strength high-tractility steel plate and manufacture method thereof, wherein, be that initial tissue carries out cold rolling and annealing with the martensite of ordinary low-carbon steel, generate the ultra tiny ferrite and the cementite tissue of the balance excellence of intensity and ductility thus.

Patent documentation 1: Japanese kokai publication sho 62-182225 communique (claims)

Patent documentation 2: Japanese kokai publication hei 7-188834 communique (claims)

Patent documentation 3: Japanese kokai publication sho 61-3843 communique (claims)

Patent documentation 4: Japanese kokai publication hei 7-3381 communique (claims)

Patent documentation 5: TOHKEMY 2000-73152 communique (claims)

Patent documentation 6: TOHKEMY 2002-285278 communique (claims)

As mentioned above, the impact energy the when main purpose that high tensile steel plate is applied to car body is the efficient absorption collision alleviates the impact to passenger thus, and the demand of using high tensile steel plate on more parts is arranged.But desire realizes above-mentioned application, has following 2 problems.

The 1st problem be, the intensity of steel plate is high more, and then ductility is low more, so the press forging formability is poor more, it is defined and is applied to the single parts of shape.The 2nd problem is the problem of parts fracture during collision.Promptly, head-on crash importantly in the collision of automobile, in head-on crash, parts such as front frame produce buckling at the length direction carry load of parts during collision, absorb impact energy thus, but, if the ductility of steel plate is little, then fracture appears in material during collision deformation, can not effectively absorb impact energy.So, need a kind of steel plate that has high tensile more and high ductibility with respect to existing steel.Problem to high tensile steel plate in the prior art is described below.

The complex tissue steel plate of patent documentation 1 disclosed ferrite and retained austenite has demonstrated the excellent intensity and the balance of ductility, but needs to add a certain amount of above Si, therefore has the problem of surface texture variation.In addition,, need 0.36% C in order to obtain the high strength more than the 1000MPa, so the spot welding intensity difference, and exist maximum only can obtain 113kgf/mm ²About the problem of tensile strength.

In addition, a kind of manufacture method of high tensile steel plate is disclosed in the patent documentation 2, wherein, even reduce Si, manufactured steel plates also has the good intensity and the balance of ductility, but because this method needs twice annealing, so the manufacturing cost height, and because C content height, so do not solve the problem of spot welding intensity, in addition, resulting tensile strength maximum is just about 1240MPa.Therefore, disclosing tensile strength in the patent documentation 3 is the manufacture method of the above high tensile steel plate of 1300MPa.But the elongation maximum was 12% when tensile strength was 1500MPa, and not talkative its has the good intensity and the balance of ductility.

In the above-mentioned prior art, add alloying element and form ferritic phase and hard second (martensite mutually by thermal treatment, bainite, retained austenite etc.) complex tissue steel, improve intensity thus, it is that 1300MPa is above and have a high tensile steel plate greater than 12% ductility that but these prior aries can not obtain tensile strength, even and also need a large amount of C in the scope of tensile strength 800MPa～1300MPa, Si, alloying elements such as Mn, cause the spot welding intensity difference, except these problems, from cost and again the angle of usability also have problems, so need a kind of equilibrated technology of assigning to realize good intensity and ductility with the one-tenth that has reduced these alloying elements as far as possible.

In fact, as the high strength method of steel, the inventor not only depends on above-mentioned existing method, and is conceived to the miniaturization of ferrite crystal grain.Promptly, this method is to suppress the amount of the alloying element of needs interpolation by the grain boundary area that increases the matrix ferritic phase as far as possible, keeping the method that improves the intensity of steel plate under the ferritic high purity, this method is based on the thinking that can keep the quiet moment of steel plate by keeping the ferritic phase purity of trying one's best high.

Herein, aspect the relation of size of microcrystal and intensity, Hall-Page equation is well-known, and-1/2 power of deformation intensity and size of microcrystal is proportional.According to this formula, less than 1 μ m, then intensity sharply rises as if size of microcrystal, so, in order to make the tremendous raising of intensity of steel plate by the crystal grain miniaturization, then need to form ultra tiny crystal grain following about 1 μ m.

In addition, the impurity element that the technology of control cementite proper amt is based on by reducing as far as possible in the ferrite in the patent documentation 4 improves this thinking of quiet moment, but the tensile strength that this method obtains is about 430MPa, and this tensile strength is not enough for high tensile steel plate.The tensile strength that obtains with the ferrite single phase structure is limited, therefore in order to obtain the higher steel plate of intensity, makes the complex tissue of ferrite and the two-phase of martensite grade in an imperial examination usually.But, when making complex tissue,, exist quiet moment than the also low problem of the single-phase steel of the ferrite of representing mild steel plate though tensile strength has uprised.For example, compiling " can the achievement Report book about the high speed The deformation of materials used in auto manufacturing " (2001) 174 pages of quiet moments that disclose mild steel when 5% strain takes place in Japanese iron steel association is about 210MPa, and the quiet moment of 590MPa level dual phase steel (Dual Phase Steel) is reduced to about 60MPa.Ductility, particularly uniform elongation aspect when being out of shape at a high speed on the other hand, but are that 590MPa level dual phase steel is more excellent.The angle of trolley part fracture when avoiding colliding, preferably as this dual phase steel, the high steel plate of ductility when being out of shape at a high speed.

Therefore, patent documentation 4 described technology are difficult to take into account high quiet moment and high tensile strength, also are difficult to the ductility of taking into account high quiet moment and when being out of shape at a high speed.So, need a kind of quiet moment height and have high tensile strength concurrently and the high tensile steel plate of the ductility when being out of shape at a high speed.

Herein, the method of high speed deformation test is not stdn as yet, have Hopkinson rod method (Hopkinson Bar Method), single excellent method (One-bar Method), sensor block method methods such as (SensingBlock Method) simultaneously, and the test piece shape has nothing in common with each other also.So the value of yield-point, percentage of total elongation also may have nothing in common with each other, directly the stress-strain(ed) curve that relatively obtains with different high speed tensile test methods is insecure.In other words, even for example rate of straining is identical, usually the result of general more small-sized test piece also may be different in the test piece of the JIS5 shape used of quasistatic tension test and the high speed tensile test, so, then lack accuracy if not using a testing apparatus, under the condition that only changes rate of straining, comparing with the test piece of same shape.So in the following explanation, when mentioning characteristic such as deformation stress, elongation, these data are data of measuring with method described later.That is, utilize the aigret palace to make the sensor block formula high speed material-testing machine of manufacturing and the test piece of shape shown in Figure 14 is measured under the condition that only changes rate of straining.

In addition, as the nano level method that the ferrite crystal grain particle diameter miniaturization of steel plate is arrived less than 1 μ m, can enumerate disclosed method in the above-mentioned patent documentation 5.In this method, by carrying out the accumulation ply rolling of 7 circulation passages, size of microcrystal has reached nano level ultra-fine grain structure, and tensile strength has also reached 3.1 times (870MPa) of starting material IF steel.But there are 2 following shortcomings in this method.

The 1st shortcoming, if when only being the organizing of the following ultra tiny crystal grain of size of microcrystal 1 μ m, then the ductility of material is extremely low.Its reason is for example on the books among " iron and steel " (Japanese iron steel association, the 88th 365 pages of volumes the 7th phase (2002), Fig. 6 (b)) at the contriver's of patent documentation 5 paper.According to this paper, the ferrite crystal grain particle diameter is during less than 1.2 μ m, and percentage of total elongation sharply reduces, and uniform elongation also almost reduces to zero simultaneously.Therefore, this tissue is unsuitable for the press working steel plate.

The 2nd shortcoming, when accumulating ply rolling in commercial run, production efficiency reduces, and causes production cost to increase substantially.Wherein, in order, must to give big strain, for example,, give 97% strain when being converted into rolling rate, just can carry out the ultra micro refinement of crystal grain thus by the accumulation ply rolling of 5 circulation passages with the refinement of crystal grain ultra micro.In order to adopt high common cold rolling of production efficiency to carry out this ultra micro refinement, steel plate rolling that must 32mm is thick is to 1mm, and still, this is actually impracticable.

In addition, it is that initial tissue carries out cold rolling and when annealing that patent documentation 6 discloses with martensite, can form the ultra tiny ferrite and the cementite tissue of the balance excellence of intensity and ductility, has improved the balance of intensity and ductility.In the example of patent documentation 6, the C amount is lower, is 0.13%, and the addition of alloying element is few, and it is excellent on this aspect, and still, tensile strength is that 870MPa, elongation are 21%, so characteristic is still insufficient.

Summary of the invention

In view of the foregoing, the purpose of this invention is to provide a kind of high tensile steel plate and manufacture method thereof, wherein, this steel plate has suppressed the addition of alloying element, and the miniaturization of passing through ferrite crystal grain improves intensity, and the intensity of described high tensile steel plate and the balance of ductility (this is important for the press forging moulding) excellence is absorbing excellence aspect the energy when being out of shape at a high speed.

In addition, generate in the patent documentation 6 of ultra tiny ferrite and cementite tissue, the sample of example has the tensile strength of 870MPa and 21% elongation, compares with the single-phase steel of the ferrite of patent documentation 4, and it has good ductility.But according to present inventor's research as can be known, the quiet moment of the sample of example is low, is 80MPa for the quiet moment of 5% strained.Therefore, much less single ultra micron tissue, the tissue that forms even cementite is separated out in the ultra micron tissue can not obtain enough quiet moments.

The present invention In view of the foregoing finishes, its objective is the manufacture method that a kind of high tensile steel plate is provided, wherein, described steel plate can not only improve intensity by the miniaturization of ferrite crystal grain, and quiet moment height, easily press forging moulding, and the ductility excellence when being out of shape at a high speed, it is big to absorb energy.

The inventor furthers investigate repeatedly to a kind of high tensile steel plate, this high tensile steel plate is, suppress the addition of alloying element and the miniaturization by ferrite crystal grain has improved intensity, the balance of important properties intensity and ductility is also excellent during its press forging moulding simultaneously.The result obtains following understanding: the tissue of steel plate is not made uniform ultra micron tissue, but make size of microcrystal is the following ferrite of 1.2 μ m (the application's hereafter is made " nanocrystal ") and the size of microcrystal mixed structure greater than the ferrite (the application's hereafter is made " micron crystal grain ") of 1.2 μ m, and by to the rationalization of the kind of hard second phase that contained in the steel plate and ratio and to rationalization, obtained the high tensile steel plate that intensity is high and have high ductility except the tissue of the part of hard second mutually.

Find in addition,, can access and have the above static strength of 450MPa and have good press forging formability concurrently and high tensile steel plate with characteristic of high quiet moment by above-mentioned rationalization.In addition, the inventor also finds, gives strain by utilizing methods such as skin rolling, the elongation in the time of can improving dynamic deformation.Need to prove, in the technical field of the present invention, usually, nanocrystal is meant the crystal grain of size of microcrystal below 1.0 μ m, and micron crystal grain is meant that size of microcrystal surpasses the crystal grain of 1.0 μ m, but in this application, as mentioned above, 1.2 μ m are set at the threshold value of the size of microcrystal of nanocrystal and micron intergranule.

Promptly, high tensile steel plate of the present invention presents the metal structure that is made of mutually ferritic phase and hard second in being dispersed in steel plate, it is characterized in that, at the second mutually shared area fraction of hard described in the described metal structure is 30%～70%, size of microcrystal is that the shared area fraction of ferrite below the 1.2 μ m is 15%～90% in described ferritic phase, and, in described ferritic phase, size of microcrystal is that the ferritic median size dL that following ferritic median size ds of 1.2 μ m and size of microcrystal surpass 1.2 μ m satisfies following (1) formula:

dL/ds≧3 (1)。

For such steel plate, on the cross section parallel of steel plate with rolling direction, under the situation of selecting the square square lattice of 3 μ m more than 9 arbitrarily, with the area fraction of described hard second phase in each grid be made as Ai (i=1,2,3 ...), at this moment, the mean value A (ave) of described Ai and standard deviation s satisfy following (3) formula:

s/A(ave)≦0.6 (3)。

In addition, for such steel plate, preferably contain C, and contain at least a element among Si, Mn, Cr, Mo, Ni and the B, C (ss) (refer to deduct from total C amount with Nb, Ti and V bonded C amount after the solid solution carbon amount that obtains) satisfies following (8) formula～(10) formula under the prerequisite of following (4) formula～(7) formula.Wherein, in (4) formula～(7) formula, each adds element is with composition (quality %) substitution of this interpolation element.

F ₁(Q)＝0.65Si+3.1Mn+2Cr+2.3Mo+0.3Ni+2000B (4)

F ₂(T)＝735+19Si-31Mn-12Ni+17Mo+20Cr+30V-800N (5)

F ₃(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (6)

F ₄(G)＝620+300C+0.5×F ₃(S) (7)

F ₁(Q)≧6.0 (8)

F ₂(T)≦F ₄(G)-20 (9)

0.07≦C(ss)≦0.45 (10)

Further preferably containing composition is prerequisite with following (11) formula and (12) formula, satisfies following (13) formula and (14) formula.

F ₃(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (11)

F ₅(P)＝500×Nb+1000×Ti+250×V (12)

F ₃(S)≦600 (13)

F ₅(P)≦130 (14)

Wherein, in (11) formula, (12) formula, each adds element is with composition (quality %) substitution of this interpolation element.

In addition, for such high tensile steel plate,, preferably contain at least a composition among the V below Nb, the Ti below 0.13% and 0.52% below 0.26% in quality %; In quality %, preferably contain at least a composition among the Al below the P and 18% below 2%; In quality %, preferred Si is below 5%, Mn is below 5%, Cr is below 1.5%, Mo is below 0.7%, Ni is below 10% and B is below 0.003%.

In addition, count 0.007%～0.03% N with quality % by containing, the ductility variation that can not cause high tensile steel plate, and can give high paint baking hardening capacity (below be called BH amount), so can improve the load that parts produce when collision, further improve impact energy absorption characteristic.BH (Bake Hardening) is meant, if steel plate is bestowed 170 ℃ thermal treatment implementing to be equivalent to paint baking after the strain, then intrusion type solid solution element will be fixed by the dislocation that processing is introduced, thereby hinder the motion of dislocation, the phenomenon that deformation resistance improves appears in its result, therefore, contain that to count 0.007%～0.03% N with quality % be effective under the situation that has the paint baking operation as trolley part.The measuring method of BH amount is seen the attached sheet of JISG 3135 (cold rolling high-tensile steel of automobile processibility and steel band).

In addition, the inventor also furthers investigate the method that is fit to the above-mentioned high tensile steel plate of manufacturing.The result obtains following understanding: for the ultra micro refinement by common cold rolling realization crystal grain, and the crystal structure before cold rolling is made second mutually the complex tissue of soft ferrite and hard, and the content of hard second phase is adjusted to suitable scope, desired rolling rate with the interval of corresponding hard second phase is implemented cold rolling, and, can access the above micron crystal grain of stating and the mixed structure of nanocrystal thus and be parent phase and contain hard second high tensile steel plate mutually to anneal greater than the Ac1 transformation temperature and the temperature and time that can suppress grain growing.

Promptly, the feature of the 1st embodiment of the manufacture method of high tensile steel plate of the present invention is as follows: is the hot-rolled steel sheet that 30%～85% hard second constitutes mutually to metal structure by ferritic phase and content, carrying out with following (15) formula is that the degree of finish index D of prerequisite satisfies the cold rolling of following (16) formula, then, satisfy the annealing of following (17) formula and (18) formula

D＝d×t/t ₀ (15)

(d: the equispaced of hard second phase (μ m), t: the thickness of slab after cold rolling, t ₀: cold rolling preceding thickness of slab after the hot rolling)

0.50≦D≦1.0 (16)

F ₂(T)+20≦Ts≦F ₂(T)+90 (17)

F ₂(T)+20≦Ts≦F ₄(G)-1.3(ts) ^1/2 (18)

(ts: soaking time (second), Ts:: holding temperature (℃), (ts) ^1/2Be the square root of ts)

In the manufacture method of such high tensile steel plate, preferably the equispaced of hard second phase on the thickness of slab direction of described hot-rolled steel sheet is at 2.5 μ m～5 μ m.In addition, be that skin rolling 2.5% below reduces yield-point by above-mentioned high tensile steel plate being implemented elongation, the moulding load when reducing the press forging moulding, the effect of reduction elastic recovery can further be provided thus.

In addition, the feature of the 2nd embodiment of the manufacture method of high tensile steel plate of the present invention is as follows: is that 10%～85% hard second constitutes mutually and the equispaced of described hard second between mutually is the hot-rolled steel sheet of 2.5 μ m～5.0 μ m to metal structure by ferritic phase and area fraction, carrying out with following (19) formula is that the degree of finish index D of prerequisite satisfies the cold rolling of following (20) formula, then, satisfy the annealing of following (21) formula.

D＝d×t/t ₀ (19)

0.50≦D≦1.0 (20)

650-(ts) ^1/2<Ts<750-(ts) ^1/2 (21)

(ts: soaking time (second), Ts: holding temperature (℃), (ts) ^1/2Be the square root of ts).

, under the low situation of the area fraction of hard second phase, preferably accumulate ply rolling herein, promptly carry out repeatedly after rolling superimposed rolling this process of material as cold rolling.But the area fraction of hard second phase in the hot-rolled steel sheet is 30%～85% o'clock, utilizes common cold rolling getting final product.In addition, annealing back is 1%～10% processing by carrying out elongation, can improve the percentage of total elongation of steel plate when being out of shape at a high speed, the stress in the time of can reducing static deformation simultaneously.As this processing, preferred skin rolling.

According to high tensile steel plate of the present invention, rationalize and the tissue except the part of hard second is mutually rationalized by hard second ratio mutually that is contained in the steel plate of mixed structure to nanocrystal and micron crystal grain, can access high strength and have the high tensile steel plate of high ductibility.In addition, the 1st embodiment according to the manufacture method of high tensile steel plate of the present invention, crystal structure before rolling is made soft ferrite and hard second complex tissue mutually, while implements cold rolling with the necessary rolling rate at the interval of corresponding hard second phase, and then greater than transformation temperature and the temperature range that grain growing do not take place anneal, the mixed structure that can make thus with micron crystal grain and nanocrystal is parent phase and contains hard second high tensile steel plate mutually.The high tensile steel plate of the present invention that so obtains has suppressed the addition of alloying element, the miniaturization by ferrite crystal grain has improved intensity, and during its press forging moulding when the balance of important properties intensity and ductility and collision deformation important properties absorb the energy response excellence.

In addition, the 2nd embodiment according to the manufacture method of high tensile steel plate of the present invention, rationalize and the tissue except the part of hard second is mutually rationalized by hard second ratio mutually that is contained in the steel plate of mixed structure to nanocrystal and micron crystal grain, suppressed the influence of nanocrystal in the time of accessing static deformation and show as low strength but when dynamic deformation, can bring into play the high tensile steel plate of the influence of nanocrystal.

Description of drawings

Fig. 1 is the synoptic diagram of the measuring method at the interval between hard second phase that illustrates in the hot-rolled sheet.

Fig. 2 is the figure of explanation hot rolled thermal process.

Fig. 3 is the annealing temperature of explanation among the embodiment and the graphic representation of the relation of elongation.

Fig. 4 is the figure of the thermal process in 4 kinds of annealing characteristic curves of explanation.

The graphic representation of the soaking time when Fig. 5 is explanation annealing and the relation of holding temperature.

Fig. 6 is the SEM photo of tissue of the high tensile steel plate of explanation example of the present invention.

Fig. 7 is the graphic representation of size-grade distribution of the ferritic phase of explanation example of the present invention.

Fig. 8 is the tensile strength of explanation among the embodiment and the graphic representation of the relation of elongation.

Fig. 9 is the SEM photo of the tissue of high tensile steel plate after cold rolling of explanation example of the present invention.

Figure 10 is the graphic representation of size-grade distribution of the ferritic phase of explanation example of the present invention.

Figure 11 is the graphic representation of the relation of elongation under the skin rolling of explanation in the example of the present invention and percentage of total elongation.

The graphic representation of the stress-strain curve when Figure 12 is the high speed distortion that illustrates among the embodiment.

Figure 13 is the high speed anamorphic stretching rate of explanation among the embodiment and the graphic representation of the relation of quiet moment.

Figure 14 is the figure of the shape of the test piece used of explanation high speed tensile test.

Embodiment

1. the 1st embodiment

Preferred embodiment describe of the present invention below.At first, the regulation reason to the various setting formulas in the high tensile steel plate of the present invention is described.Need to prove that the content of each element that hereinafter provides is all in quality %, for convenience's sake, below % made in brief note.

Preferably the solid solution carbon amount with high tensile steel plate of the present invention is adjusted into 0.07%～0.45%.In order to improve hardening capacity and to improve hardness of steel, make at least a element that contains in this carbon steel among the 1st element set: Si, Mn, Cr, Mo, Ni and the B by solution strengthening.In addition, for miniaturization and precipitation strength by crystal grain improve hardness of steel, make at least a element that contains in this carbon steel among second element set: Nb, Ti and the V as required.And then, in order to improve hardness of steel, make at least a element that contains in this carbon steel among triad: P and the Al as required by solution strengthening.

In addition, be prerequisite with following (4) formula～(7) formula, (11) formula～(12) formula, make resulting steel satisfy whole formulas of following (8) formula～(10) formula and (13)～(14) formula.Wherein, the symbol of element in the following formula is represented the composition (quality %) of this element, and for example, " Cr " is meant the composition (quality %) of Cr.

F ₁(Q)＝0.65Si+3.1Mn+2Cr+2.3Mo+0.3Ni+2000B (4)

F ₂(T)＝735+19Si-31Mn-12Ni+17Mo+20Cr+30V-800N (5)

F ₃(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (6)

F ₄(G)＝620+300C+0.5×F ₃(S) (7)

F ₁(Q)≧6.0 (8)

F ₂(T)≦F ₄(G)-20 (9)

0.07≦C(ss)≦0.45 (10)

F ₃(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (11)

F ₅(P)＝500×Nb+1000×Ti+250×V (12)

F ₃(S)≦600 (13)

F ₅(P)≦130 (14)

Below, the implication of the symbol in the above-mentioned formula and various regulation reason are described.

＜(4) formula and (8) formula 〉

F ₁(Q) be the index of the hardening capacity of expression steel, this index is determined as (4) formula, and is calculated from each composition (quality %) of adding element.As described below, in the manufacture method of high tensile steel plate of the present invention, importantly the metal structure before cold rolling is made soft ferritic and hard second complex tissue of (in martensite, bainite and the retained austenite at least a) mutually.

These tissues can obtain by the following method: the method for carrying out chilling after hot rolling after two phase regions that slowly are cooled to be made of ferrite and austenite again; After hot rolling, first cool to room temperature heats then, is incubated at two phase regions that are made of ferrite and austenite, and then carries out the method for chilling., when obtaining these and organize, two problems appear.

The 1st problem, if the C amount is few, then hardening capacity reduces, and therefore, is difficult to obtain hard second phase.As the countermeasure of this problem, be necessary to add above-mentioned first element set that belongs to the element that makes the hardening capacity raising, be easy to obtain hard second phase.The 2nd problem under the C amount situation how, is easy to generate pearlitic transformation in the cooling after the hot rolling, is difficult to obtain necessary hard second phase.In order to address these problems,, add the element that hardening capacity is improved of necessary amount according to above-mentioned (8) formula.Need to prove that alleged C amount will be described in detail later herein, its expression from total C amount, deducted with Ti, Nb, V bonded C amount after solid solution carbon amount.

＜(5) formula～(7) formula and (9) formula 〉

(5) formula has been stipulated the Ac1 transformation temperature of steel, and according to the alloying element amount in the steel, the Ac1 transformation temperature changes shown in (5) formula like that.The coefficient of each alloying element is that the inventor determines according to experiment.(6) formula is illustrated in the amount that high tensile steel plate is reinforced under the solution strengthening effect of the 1st element set and the 2nd element set, and the unit of this amount is MPa, and it is the index of amount of reinforcement, also is simultaneously the index that solid solution element hinders the ability that the crystal boundary in the annealing moves.

Suppressing the lower limit temperature of thickization of crystal grain with the ratio of the nanocrystal of formation regulation when (7) formula is represented steel plate annealing, is above-mentioned F ₃(S) and C amount multiply by the value that adds and obtain behind separately the coefficient.The 300C on the right is the item of the effect brought of C, 0.5 * F ₃(S) be the item of the effect brought of the solid solution element beyond the C.(9) formula represents that the value of Ac1 transformation temperature+20 ℃ of steel must be at F ₄(G) below.As described later, in order to obtain metal structure of the present invention, be necessary to anneal, but this temperature is higher than F in temperature with the determined Ac1 transformation temperature of the composition of steel+more than 20 ℃ ₄When (G) promptly being higher than the ceiling temperature that can suppress thickization of crystal grain, no longer there is the annealing conditions be used for obtaining as the ultra tiny crystal grain of feature of the present invention.So the composition that will adjust steel is to satisfy (9) formula.

＜(10) formula 〉

C (ss) be meant from total C amount, deducted with the 2nd element set (Nb, Ti, V) bonded C amount after solid solution carbon amount, it calculates by following (22) formula.Need to prove that in (22) formula, each value of adding the substitution of element institute is the composition (quality %) of this interpolation element.

C (ss)=total C amount-(12/92.9 * Nb+12/47.9 * Ti+12/50.9 * V) (22)

(22) coefficient in the formula 92.9,47.9 and 50.9 is respectively the nucleidic mass of Nb, Ti, V,, * Nb+12/47.9 * Ti+12/50.9 * V formed the amount (quality %) of the C of carbide 12/92.9 representing to combine with Nb, Ti or V, total be exactly the amount of solid solution C after these are deducted from the C amount.

Next, (10) formula has been stipulated higher limit and the lower value of solid solution C, and its reason is for the metal structure before desired scope generation is cold rolling.Lower value being decided to be 0.07% herein, is because C even add the element that hardening capacity is improved, can not generate hard second phase of q.s less than 0.07% o'clock.Hard second phase contain quantity not sufficient the time, if do not adopt above-mentioned special methods such as accumulation ply rolling, just can not be with the size of microcrystal miniaturization of steel to nano level.

In addition, higher limit being set at 0.45% is because greater than 0.45% o'clock, can not obtain the ferrite and hard second complex tissue mutually of purpose.The C amount was greater than 0.45% o'clock, even add the element that hardening capacity is improved, the pearlitic transformation nose (nose) in the CCT curve will appear at the short period of time side.So, when ferritic-austenitic two phase regions carry out chilling, all can crosscut pearlitic transformation nose under any speed of cooling, the metal structure before cold rolling has become ferrite and pearlitic complex tissue or has become perlite.

Herein, perlite is that cementite shows to such an extent that be highly brittle to distortion as the cementite of the compound of C and Fe and ferritic lamellar structure, and the energy when cold rolling is consumed by the fracture of cementite.Therefore, contain in the tissue of steel under the pearlitic situation, can not give to compare big strain as the soft ferritic of the feature of manufacture method of the present invention.So, will make by interpolation the element that improves of hardening capacity and can avoid the higher limit of the C amount of pearlitic transformation to be set at 0.45%.

＜(6) formula and (11)～(13) formula 〉

(6) F of formula ₃(S) be illustrated in the amount that high tensile steel plate is reinforced under the solution strengthening effect of the 1st element set and the 3rd element set, its unit is MPa, utilizes (6) formula, calculates F according to the quality % that adds element ₃(S).(6) coefficient that multiplies each other with each element in the formula is based on following thinking, calculates according to following (23) formula.

The coefficient of each element=| r (X)-r (Fe) |/r (Fe) * M (Fe)/M (X) * 1000 (23)

Herein, r (X) is that this atoms of elements radius, r (Fe) are that iron atom radius, M (X) are that this atoms of elements amount and M (Fe) are the nucleidic mass of iron.

(23) meaning of formula is as follows.That is, the solution strengthening amount of the value that obtains divided by the iron atom radius with certain difference of adding atoms of elements radius and iron atom radius and each this interpolation element is proportional.Herein, described on duty in order to be converted into the quality % of this interpolation element with the nucleidic mass of iron and the ratio of this atoms of elements amount, in order to be MPa, multiply by 1000 again in addition with unit conversion.The coefficient of (23) formula that has provided the physical constant of each used element in the table 1 and calculated thus.

Table 1

The symbol of element	Fe	Si％	Mn％	P?％	Al?％	Cr％	Mo％	Ni％	B?％
										Atomic radius r (X)	1.24	1.17	1.12	1.09	1.43	1.25	1.36	1.25	0.9
(r(X)-r(Fe))/r(Fe)	-	0.0565	0.0968	0.1210	0.1532	0.0081	0.0968	0.0081	0.2742
										Nucleidic mass M (X)	55.8	28.1	54.9	31.0	27.0	52.0	95.9	58.7	10.8
M(Fe)/M(X)	-	1.99	1.02	1.80	2.07	1.07	0.58	0.95	5.17
										(23) coefficient of formula	-	112	98	218	317	9	56	8	1417

Then, F ₅(P) be the index of C formation carbide in expression above-mentioned the 2nd element set and the steel and its amount of reinforcement when steel being strengthened, F by precipitation strength ₅(P) as above-mentioned (12) formula, be determined.(12) meaning of formula is as follows.That is, Nb, Ti, V form carbide in steel ability is strong, for example the solubility product of the solubility product of Nb and C, Ti and C (quality %) in 700 ℃ steel ²All are 10-6 power levels, the solubility product of V and C (quality %) ²Be 10-4 power levels, so in high tensile steel plate of the present invention, Ti, Nb, V can not exist as sosoloid substantially, but as being that NbC, TiC or VC exist with C with the carbide that 1 to 1 ratio combines formation.So, can expect and the Nb, the Ti that add, the proportional precipitation strength amount of addition of V.

It should be noted that this is a situation about existing as yet not with Nb, Ti or V bonded C, further add Nb, Ti or V down, can not obtain the amount of being expected of separating out even whole C is in Nb, Ti, V bonded state.In addition, precipitation strength amount changes because of the difference of the size of precipitate.Usually, when precipitate was thick, the precipitation strength ability reduced.For high tensile steel plate of the present invention, as described later, during annealing after cold rolling, do not consider that long-time insulation is at the carbide high-temperature area of growth easily.Therefore, the carbide of Nb, Ti or V evenly disperses imperceptibly, and the precipitation strength amount only depends on the addition of these elements.Above-mentioned (12) formula has shown this phenomenon.

Herein, the

coefficient

500,1000 and 250 in (12) formula is represented the precipitation strength amount of Nb, Ti, the per 1 quality % of V respectively, and these are the numerical value of determining according to experiment.The precipitation strength amount of Nb, Ti and V and be F ₅(P) be total precipitation strength amount.Based on this understanding, (13) formula and (14) formula represent that the ferritic amount of reinforcement that solution strengthening and precipitation strength bring should be respectively below the 600MPa, below the 130MPa.

This is because when the amount of reinforcement of steel plate is too high, do not manifest the high ductibility as feature of the present invention.As mentioned above, when adding a large amount of alloying elements and coming significantly reinforced ferrite, ferrite purity also reduces simultaneously, the balance variation of ferritic intensity and ductility.Ferritic purity has obtained the ductility higher than existing steel in the metal structure of high tensile steel plate of the present invention under the situation more than the prescribed value, and still ferritic purity is crossed when hanging down, and high ductibility do not occur.

The inventor has studied the ferritic purity of necessity has been carried out quantification in order to obtain high ductibility.The result finds experimentally, and it is proportional that each adds the ferrite amount of reinforcement (solution strengthening, precipitation strength) of detrimentally affect degree that element produces ductility and per unit addition (quality %).This result is furtherd investigate the back find, the higher limit of ferrite amount of reinforcement that can show high ductibility is as follows: be 500MPa in solution strengthening, be 130MPa in precipitation strength.Above-mentioned (13) formula and (14) formula are that this result is quantized.

In addition, contain an amount of N, can give steel plate high BH amount by making high tensile steel plate of the present invention.It is the influence of amount of the intrusion type solid solution element of representative that BH amount is subjected to C, N.But in the high tensile steel plate of the present invention, the increase of C amount is limited.This be because, as mentioned above, when increasing the amount of C, be easy to generate perlite in the metal structure, becoming is difficult to obtain the purpose metal structure.

Therefore, the inventor is conceived to C intrusion type solid solution element N in addition.As long as the content of N is in the scope that does not generate with the compound of iron, just can contain and can not make metal structure that big variation takes place with solid solution condition, can play a role in the strain aging phenomenon effectively, thereby can access high BH amount.In addition, high tensile steel plate of the present invention has the BH amount characteristic higher than existing steel plate.Though its reason is unclear, it is believed that this is to be subjected to ferrite crystal grain in the high tensile steel plate of the present invention mostly to be the following nanocrystal of 1.2 μ m greatly, and crystal boundary, ferrite and second mutually the very large influence of area at interface.

The crystal grain that clips crystal boundary or interface fetters distortion mutually, so the dislocation desity in when distortion is tending towards increasing at crystal boundary, near interface.In the high tensile steel plate of the present invention, this crystal boundary, interface are present in positions all in the material structure, so material monolithic has even and high dislocation desity.Therefore, it is believed that the strain aging of N appears in the whole material when paint baking is handled, thereby show high BH amount.

The qualification reason of＜each chemical ingredients 〉

Below, the qualification reason of each chemical ingredients in the high tensile steel plate of the present invention is set forth.Need to prove that the unit of the content of each element given below all is quality %, but for convenience's sake, only note is made %.In addition,, separately it is limited,, define upper lower limit value, and respectively upper lower limit value is set by (8) formula, (9) formula, (13) formula, (14) formula under the situation mostly for other elements with (10) formula for C.

＜C: solid solution C content is 0.07%～0.45% 〉

By adding C, can under the high temperature after the hot rolling, generate the mixed structure that constitutes by ferrite and austenite, by this mixed structure is carried out chilling, can form hard second phase of martensite, bainite, retained austenite.In addition, also be same in the annealing after cold rolling.Therefore, C is a most important element among the present invention.

In high tensile steel plate of the present invention, owing to can add Nb, Ti and V, so, adjust the addition of C in order to make the solid solution C except the C that separates out as carbide in this case satisfy above-mentioned (10) formula.Solid solution C is less than 0.07% o'clock, can not make hard second phase that contains q.s in the metal structure before cold rolling, and solid solution C is greater than 0.45% o'clock, and metal structure has become perlite, and this all is not suitable for the manufacture method of high tensile steel plate of the present invention.

＜the 1 element set: the Si, Mn, Cr, Mo, Ni, B 〉

Adding following element is for hardening capacity that improves steel and the intensity that improves steel by solution strengthening.Adjust addition to satisfy (8) formula, (9) formula and (13) formula.The following describes the higher limit of addition of each element and the qualification reason of lower value.

<Si：0.2％～5％>

The Si addition can not show the effect that improves hardening capacity significantly less than 0.2% o'clock.Therefore, lower value is decided to be 0.2%.In addition, the addition of Si was greater than 5% o'clock, and Si combines with Fe, and crystalline structure having occurred is the intermetallic compound Fe as stable phase of DO3 type or B2 type ₃Si, this stable phase will reduce the ductility of steel, so the upper limit of Si is set at 5%.

<Mn：0.1％～5％>

The addition of Mn can not show the effect that improves hardening capacity significantly less than 0.1% o'clock.Therefore, lower value is decided to be 0.1%.In addition, the addition of Mn was greater than 5% o'clock, even in room temperature, austenite also exists with stable phase.The austenitic intensity of stable phase is little, causes the steel integral intensity to reduce, and is not preferred therefore.Therefore, higher limit is decided to be 5%.

<Cr：0.1％～1.5％>

The addition of Cr can not show the effect that improves hardening capacity significantly less than 0.1% o'clock.Therefore, lower value is decided to be 0.1%.In addition, the addition of Cr was greater than 1.5% o'clock, and C in the steel and Cr combine and form carbide, so can not obtain the solid solution Cr of corresponding addition, can not expect to improve hardening capacity.Therefore, higher limit be set at Cr can with solid solution condition exist 1.5%.

<Mo：0.1％～0.7％>

The addition of Mo can not show the effect that improves hardening capacity significantly less than 0.1% o'clock.Therefore, lower value is decided to be 0.1%.In addition, the Mo addition was greater than 0.7% o'clock, and the C in the steel combines with Mo and forms carbide, so can not obtain the solid solution Mo of corresponding addition, can not expect to improve hardening capacity.Therefore, higher limit be set at Mo can with solid solution condition exist 0.7%.

<Ni：0.2％～10％>

The addition of Ni can not show the effect that improves hardening capacity significantly less than 0.2% o'clock.Therefore, lower value is decided to be 0.2%.In addition, the addition of Ni was greater than 10% o'clock, even in room temperature, except ferrite, austenite also exists with stable phase.Austenitic intensity is little, causes the steel integral intensity to reduce, so be not preferred.Therefore, higher limit is decided to be 10%.

<B：0.0005％～0.003％>

The addition of B can not show the effect that improves hardening capacity significantly less than 0.0005% o'clock.Therefore, lower value is decided to be 0.0005%.In addition, the solid solution limit of B in ferrite itself is very little, under the few situation of the addition of B, think that its main segregation is present in the crystal boundary of steel, but the addition of B was greater than 0.003% o'clock, and only crystal boundary is not enough as the presence bit of B, thereby had generated intermetallic compound Fe ₂B reduced the ductility of steel, so higher limit is decided to be 0.003%.

＜the 2 element set: the Nb, Ti, V 〉

The purpose of the intensity that improves steel based on miniaturization and precipitation strength by crystal grain can be added these elements as required.The following describes the higher limit of addition of each element and the qualification reason of lower value.

<Nb：0.01％～0.26％>

In order to obtain miniaturization and the precipitation strength that Nb brings, be necessary to add the Nb more than 0.01%.The addition of Nb is greater than 0.26% o'clock, and only the precipitation strength amount brought of NbC has just reached 130MPa, can not satisfy (14) formula, so the upper limit is limited at 0.26%.

<Ti：0.01％～0.13％>

The addition of Ti is greater than 0.13% o'clock, and only the precipitation strength amount brought of TiC has just reached 130MPa, can not satisfy (14) formula, so the upper limit of Ti is limited at 0.13%.

<V：0.1％～0.52％>

The addition of V can not obviously show the effect of miniaturization less than 0.1% o'clock, and the addition of V is greater than 0.52% o'clock, and only the precipitation strength amount brought of VC has just reached 130MPa, can not satisfy (14) formula, so the upper limit of V is limited at 0.52%.

＜the 3 element set: the P, Al 〉

Can add the strengthening element of these elements as required as steel.The following describes the higher limit of addition of each element and the qualification reason of lower value.

<P：0.03％～2％>

Adding P is effectively as the solution strengthening element of steel, but addition is less than 0.03% o'clock, the DeGrain of solution strengthening.Therefore, lower value is decided to be 0.03%.In addition, the addition of P generated intermetallic compound Fe greater than 2% o'clock ₃P has reduced the ductility of steel.Therefore, higher limit is decided to be 2%.

<Al：0.01％～18％>

Al is the solution strengthening element, and it also has the effect as reductor simultaneously, and adding Al is for steel is made deoxidized steel.In addition, Al combine and forms aluminum oxide with dissolved oxygen in the steel in system steel operation, and aluminum oxide floats, and by removing the aluminum oxide of come-up, can improve ductility, the toughness of steel.But the addition of Al is less than 0.01% o'clock, and it all can not embody significantly as the effect of reductor with as the effect of solution strengthening element.Therefore, lower value is decided to be 0.01%.On the other hand, the addition of Al generated intermetallic compound Fe greater than 18% o'clock ₃Al has reduced the ductility of steel.Therefore, higher limit is decided to be 18%.

<N：0.007％～0.03％>

N is the important element that is used to improve the BH amount of high tensile steel plate of the present invention, has after high tensile steel plate is shaped to parts under the situation of paint baking operation, and N effectively plays a role on the intensity that improves parts.So, suitably add N as required.The N addition was less than 0.007% o'clock, and baking hardenability is not obvious.Therefore, lower value is decided to be 0.007%.On the other hand, the N addition is greater than 0.03% o'clock, cube brilliant Fe ₄N begins to separate out, so, can not expect to improve the BH amount suitable with the N that adds.So will on be defined as 0.03%.

＜to the tissue the qualification reason

Describe the metal structure of high tensile steel plate of the present invention below in detail.

The metal structure of high tensile steel plate of the present invention satisfies following 1 simultaneously)～4) important document put down in writing.

1) metal structure is made of with second mutually (among martensite, bainite, the retained austenite more than a kind or 2 kinds) ferrite.In addition, cut out the cross section parallel of steel plate with rolling direction, with after this cross section etching, is 30%～70% from the area fraction of taking second phase that determines 2 electron images obtaining (below be called the SEM photo) with 5000 times magnification with scanning electronic microscope with nital etc.

2) in the metal structure, hard second is dispersed in the ferritic phase mutually, and satisfies following important document.Promptly, in 5000 times the SEM photo in the cross section parallel of steel plate with rolling direction, choose the square square lattice of 3 μ m more than 9 arbitrarily, when using image analysis to measure the area fraction of hard second phase in each grid, the area fraction of each hard second phase is made as Ai (i=1,2,3, ...), this moment, the mean value A (ave) of Ai satisfied following (24) formula with standard deviation s.

s/A(ave)≦0.6 (24)

3) in 5000 times the SEM photo in the cross section parallel of steel plate with rolling direction, obtain the ferrite part after from the total area of photo, deducting the area of hard second phase, the area fraction of nanocrystal is 15%～90% among this ferrite part.

4) the median size dS of nanocrystal satisfies following (25) formula with the median size dL of micron crystal grain.

dL/dS≧3 (25)

Herein, median size is meant, by image analysis the area of the whole ferrite grains in 5000 times the SEM photo in the cross section parallel with rolling direction of steel plate is measured, and the homalographic circular diameter of obtaining according to area separately.Specifically, the area of the ferrite grain that will obtain by image analysis be made as Si (i=1,2, in the time of 3...), homalographic circular diameter Di (i=1,2,3...) can calculate by following (26) formula.

Di＝2(Si/3.14) ^1/2 (26)

Set the important document 1 shown in above-mentioned)～4) the reasons are as follows.That is, separate out, can discharge solid solution element such as C mutually to second, improve ferritic purity, improve the ductility of steel thus from ferrite part by making an amount of hard second phase homodisperse.In addition, when the area fraction of hard second phase is too small, can not show the feature of complex tissue steel, so ductility is little, when the area fraction of hard second phase was too much, the material behavior of hard second phase was more obvious than ferritic material behavior, and ductility diminishes.

Herein, in the tissue of high tensile steel plate of the present invention, hard second is meant mutually in the annealing with the ferrite equilibrated and undergoes phase transition the tissue that formed phase or the residual phase that does not undergo phase transition and the annealing of these phase factors change and form in process of cooling.Specifically, hard second is any more than a kind in martensite, bainite, the retained austenite mutually.

Herein, tempering bainite is incorporated in the bainite, and tempered martensite is incorporated in the martensite.The following describes its reason.At first, tempering bainite is bainite annealed and to have improved the flexible tissue at 300 ℃～400 ℃, it is the mixed structure of high ferrite of dislocation desity and cementite, remains bainite in essence, handles so among the present invention it is incorporated in the bainite.

Tempered martensite is that martensite is annealed and reduced hardness and improved the flexible tissue, among the present invention it is incorporated in the martensite and handles.Martensitic tempering is to make supersaturation ground solid solution have the martensite of carbon to resolve into the process of ferrite and carbide.But, for example, it is such that " Corporation Japan Metallkunde can be compiled; the modern Metallkunde of lecture, a material piece of writing 4, ferrous materials; 39 pages " put down in writing, when 300 ℃～500 ℃ tempering, ferrite also has quite high dislocation desity, does not change as structures such as the pencil (packet) of the feature of lath martensite or bulks.Even so the high annealing martensite of hardness, martensitic feature does not disappear yet.

In addition, as " Corporation Japan Metallkunde can be compiled; the modern Metallkunde of lecture; a material piece of writing 4, ferrous materials, 39 pages " put down in writing like that, the C of supersaturation ground solid solution is very easy to spread in the martensite after just having quenched, so from approximately-100 ℃ just had been found that moving of C, begun preparatory stage of separating out, so only the martensite of Cui Huoing and tempered martensite are difficult to obvious differentiation.Consider above-mentioned reason, among the present invention, martensite is treated as of the same race mutually with tempered martensite.

Below the ferrite beyond second phase is partly described.By ferrite partly being made the mixed structure of different nanocrystal of size and micron crystal grain, in the time of can having high strength and the collision when the press forging moulding or after componentization concurrently high-levelly as the ductility of necessity.

In addition, with ultra tiny ferrite be defined as ferrite below the 1.2 μ m be because, Fig. 6 (b) that for example " iron and steel " (Japanese iron steel association) the 88th volume the 7th phase (2002) is 365 pages is disclosed like that, about 1.2 μ m are the boundary with the ferrite crystal grain particle diameter, and discontinuous variation takes place the material behavior of material particularly elongation.Also because ferrite crystal grain particle diameter during less than 1.2 μ m, percentage of total elongation sharply reduces, and particularly no longer shows uniform elongation in addition.

Be the qualification reason of the related various regulation reason of high tensile steel plate of the present invention, chemical ingredients and above, will the mechanism of the action effect of high tensile steel plate of the present invention be elaborated below about the qualification reason of tissue.

＜about the mechanism of the action effect of high tensile steel plate of the present invention 〉

The parent phase ferrite is made the mixed structure of nanocrystal and micron crystal grain, and further contain hard second phase, the mechanism that can give high strength and high ductibility thus is as described below.

The parent phase of high tensile steel plate of the present invention contains the very high following nanocrystal of size of microcrystal 1.2 μ m of intensity of certain limit, so deformation stress is very high.But, only be like this, as above-mentioned

patent documentation

5,6, though the intensity height, ductility is low.In order to overcome this problem, except ferrite, also introduced hard second phase of certain minute rate, the parent phase ferrite also is not only ultra micron simultaneously, also has the micron crystal grain with regular tenacity of certain limit amount.

At first, the reason of introducing hard second phase is described.Organizing of high tensile steel plate of the present invention is second mutually the complex tissue with soft parent phase and hard basically, and the consideration method of the deformational behavior of common complex tissue steel can be suitable for.Steel plate with complex tissue has taken place at first preferentially to begin to deform from soft parent phase ferrite under the situation of distortion.When further applying distortion, because the hard second that is dispersed in the tissue is difficult for deforming mutually, so the parent phase ferrite must be shared the major part of the macro-strain of material, ferrite further is out of shape, and work hardening takes place largely.

Work hardening is big, and then strain is propagated to big scope, so be difficult for producing contraction in the part, ductility becomes big as a result.The present invention is the high strength substrate that is obtained by brand-new design, it is based on the consideration method of the distortion of such complex tissue steel, have following two features: the parent phase ferrite is to have reduced alloying element as far as possible, and the ferrite that strengthened of the miniaturization by crystal grain; The parent phase ferrite is the nanocrystal and the mixed structure of the size of microcrystal with regular tenacity greater than the micron crystal grain of 1.2 μ m of the very high size of microcrystal of intensity 1.2 μ m.

At first, to setting forth as the effect that reduces alloying element of first feature as far as possible.When in the complex tissue steel, adding solution strengthening element (typical example is Mn, Si, P etc.), precipitation strength element (typical example is Nb, Ti etc.) usually, owing to improved ferritic intensity really, so the intensity of steel has also improved, but the dislocation of introducing because of distortion concentrates on the interface of interface, parent phase and the precipitate of iron in the parent phase and alloying element easily, this becomes generation destructive starting point, thereby the balance of the intensity of steel and ductility has equal extent ground to improve.

Second point, parent phase are mixed structure nanocrystalline and that micron is brilliant, and this is the big feature of the present invention.The contriver adopts the common mill-annealed of utilization described later to make the peculiar methods of nanometer crystal microstructure, the tissue of ultra tiny crystal grain with complex tissue and the relation of mechanical property have been carried out research repeatedly, found that, when the parent phase ferrite only is made of the nanocrystal below the particle diameter 1.2 μ m, though ductility is than the monophasic ultra tiny crystal grain material height of ferrite, but its effect is little, when introducing a certain amount of micron crystal grain in the parent phase, the best intensity and the balance of ductility have been presented with common intensity.

As mentioned above, the intensity height of ultra tiny crystal grain tissue, but work hardening is little, and poor ductility does not particularly almost have uniform elongation.Even introduce hard second phase in such tissue, owing to the influence of the little parent phase of ductility is excessive, the raising degree of the ductility of steel integral body is little.It is believed that, this is because parent phase all is under the situation of ultra tiny crystal grain, the constraint of crystal boundary cause its can not with big distortion servo-actuated, even be unlike in so much that the interface of interface, iron and the solid solution element of precipitate and parent phase exists, but strain also concentrates near the crystal boundary, becomes the destructive starting point.

But by introduce the micron crystal grain of certain limit in the parent phase ferrite, ductility is greatly improved.Though its reason is unclear, but it is believed that it is owing in the parent phase ferrite, exist and help little but when having the micron crystal grain of big deformability to improving intensity, the little shortcoming of deformability of the nanocrystal part around having remedied, can with big distortion servo-actuated, and strain also concentrates on a micron crystal grain, work hardening largely takes place thus, finally suppressed to become the generation of the point that destroys starting point, and strain evenly exists in crystal structure is all.

The ratio of the nanocrystal in the parent phase ferrite is that the area fraction of 15%～90% scope and second phase of the hard in the structure of steel has shown above-mentioned phenomenon when being 30%～70% scope.Nanocrystal ratio in the parent phase ferrite was greater than 90% o'clock, and as mentioned above, ductility does not have to improve substantially, and this ratio was less than 15% o'clock, and the ferritic reinforcement that the crystal grain miniaturization brings is not enough, can not obtain tangible high strength.

In addition, the hard second in the structure of steel is less than mutually at 30% o'clock, and the feature of complex tissue becomes not obvious, relatively the superiority of the monophasic tissue of ferrite disappearance (ductility step-down).On the other hand, hard second was more than 70% o'clock, and the deformation characteristic of hard second phase is obvious, rise of yield point, and ductility reduces.

Above high tensile steel plate of the present invention is illustrated, the following describes the 1st embodiment of the method that is fit to the above-mentioned high tensile steel plate of manufacturing.Need to prove that the 1st embodiment of the manufacture method of high tensile steel plate of the present invention can be by common cold-rolled steel sheet manufacturing process, promptly the molten system of slab, hot rolling, cold rolling and each operation of annealed are made.

The molten system of＜slab 〉

The molten system of slab is undertaken by usual method with predetermined component.Industrial, directly use iron liquid or cold iron sources such as middle scrap iron that the manufacturing process of urban life scrap iron or steel is produced carry out the oxygen refining after with electric furnace, converter fusion, by casting continuously or the individual cast at intermittence is cast.In the bantam in pilot plant or laboratory etc., also iron materials such as electrolytic iron or scrap iron can be fused with process furnace in a vacuum or in the atmosphere, behind the alloying element of interpolation regulation, be injected into mold, obtain material thus.

＜hot rolling 〉

Hot rolling is first important step in the 1st embodiment of manufacture method of high tensile steel plate of the present invention.In the manufacture method of the present invention, it is principal phase that crystal structure after the hot rolling is made with the ferrite, contain the complex tissue of area fraction, and will be controlled at 2.5 μ m～5 μ m in the equispaced of hard second phase of thickness of slab direction detection in hard second phase of 30%～85% scope.Alleged herein hard second is hard second phase that the hard second from the final tissue of high tensile steel plate of the present invention has been removed cementite in mutually mutually, is at least a among martensite, bainite and the retained austenite.Cementite or perlite are under the situation of hard second phase, can not obtain the metal structure of high tensile steel plate of the present invention.

Below the reason of as above selecting hard second phase is described.

The shared area fraction of nanocrystal is 15%～90% in the ferritic phase of the metal structure of high tensile steel plate of the present invention.In order to obtain this metal structure, carry out following processing.That is, the metal structure before cold rolling is made ferrite and hard second complex tissue mutually.Next, give soft ferrite with big shear strain by cold rolling.By continuation this part is annealed at last, make the following nanocrystal of size of microcrystal 1.2 μ m.

On the other hand, hard second phase that exists before cold rolling (among martensite, bainite and the retained austenite at least a) deforms when cold rolling, but be not endowed the picture ferrite partly big shear strain.Therefore, in the annealing operation after cold rolling, do not generate nanocrystal, when separating out cementite, change to ferrite, perhaps pass through, and become micron crystal grain with micron-sized size of microcrystal based on the karyogenesis of the new ferrite grain that should tail off and the common static recrystallization process of growth.According to such mechanism, obtained the mixed structure of nanocrystal and micron crystal grain.

As mentioned above, relative matrix ferrite, hard second must be following tissue mutually: have high hardness, and will become ferrite after cold rolling and the annealing.In other words, the hard second as necessity among the present invention is not the such tissue that only is made of carbide of cementite mutually, but based on ferrite or austenite and the high tissue of hardness.

Set forth martensite, bainite and retained austenite below and have reason as the hard among the present invention second qualification mutually.Martensite is the ferrite that supersaturation ground contains C, and the lattice strain that C causes has caused high dislocation desity, so the hardness height.But, martensitic C content aspect, Fe in the Fe-C equilibrium diagram and Fe ₃The C concentration of the eutectic point of C is the most about about 0.8%, with Fe ₃The cementite of C chemical formulation is compared, and martensitic C concentration is very little.Therefore, in the annealing operation after cold rolling, change to ferrite when separating out cementite.So martensite has based on ferrite and the high tissue of hardness, has the qualification as second phase of the hard among the present invention.

Bainite is the tissue that phase transformation has taken place in a little higher than temperature that begins to form martensitic temperature, and it is the mixed structure of featheriness or acicular ferrite and fine cementite.The ferrite of bainite partly contains a large amount of dislocations, and (Corporation Japan Metallkunde can be compiled, the modern Metallkunde of lecture, a material piece of writing 4, the iron steel, 35 pages), but do not have the such degree of martensite, cementite hardness height not only, the ferrite part that dislocation desity is high also has high hardness.So bainite also is based on ferrite and the high tissue of hardness, has the qualification as second phase of the hard among the present invention.

As shown in the above description, bainite is the mixed structure of ferrite and cementite, the organized whole that the ferrite of cementite and high dislocation density partly can be combined to form regards hard second as mutually, can be obviously with the low ferrite matrix of dislocation desity in the independent cementite that exists mutually as hard second distinguish.

In addition, according to the observation of metal structure, also can know and judge the different of bainite and cementite.Grind in cross section to steel, etching, and when using observation by light microscope then, in the tissue of bainite, the acicular ferrite part seems obfuscation owing to dislocation desity is high, and the ferrite matrix that dislocation desity on every side is low seems shinny.On the other hand, cementite single organization is the spherical precipitated phase that seems burnt hair with respect to shinny ferrite matrix.

The strain phase transformation of inducting takes place in retained austenite in response to change in rolling process, become martensite, thus its have and martensitic phase with effect, and the tissue in the annealing operation after cold rolling change also with martensitic phase with.So retained austenite has the qualification as second phase of the hard among the present invention.

Be that single cementite or pearlitic situation describe mutually to hard second below.Herein, perlite is that ferrite and cementite form lamellated mixed structure, so the function of lamellated cementite performance hard second phase.Thereby hard second is that the situation of cementite is that pearlitic situation is identical in essence with hard second mutually mutually.When hard second is cementite mutually, be difficult in cold rolling, to give soft ferritic part as feature of the present invention with bigger shear strain.This be because, cementite shows to such an extent that be highly brittle to distortion, the energy when cold rolling is consumed by the fracture of cementite, can give the ferrite strain effectively for a short time.

But, be that high draft more than 85% carries out will generating nanocrystal when cold rolling with rolling rate.But in this case, the variation in the annealing process after cold rolling differs widely when being martensite, bainite, retained austenite mutually with hard second, so can not form the mixed structure as the nanocrystal and the micron crystal grain of feature of the present invention.Under high pressure in the annealing operation after rate cold rolling, annealing temperature is under the lamellated situation as the shape of the cementite of metastable phase when the Ac1 transformation temperature is following, and its shape is to spherical variation, but still left behind with the form of cementite.Therefore, the tissue after the annealing is nanocrystal ferrite and cementite, does not form the mixed structure as the feature of steel of the present invention.So,, do not demonstrate good ductility though it shows high strength.

In addition, annealing temperature is Ac1 transformation temperature when above, and the cementite part that C concentration is very high preferentially becomes austenite mutually, in the process of cooling thereafter, becomes at least a mixed structure in perlite, martensite, bainite, the retained austenite mutually.Therefore, formed the mixed structure of nanocrystal ferrite and these phase-change organizations, can not obtain high ductibility as the feature of steel of the present invention.In the final metal structure of steel of the present invention, importantly ferritic phase is the mixed structure of nanocrystal and micron crystal grain.

For ferritic phase being made this mixed structure, need suitably select on the basis of hard second phase in the hot-rolled sheet, the area fraction of hard second phase is controlled at 30%～85% scope, simultaneously its equispaced is controlled at the scope of 2.5 μ m～5 μ m.Thereafter, as described later, desired rolling rate with the interval of corresponding hard second phase is implemented cold rolling, and further annealing greater than the Ac1 transformation temperature and with temperature, the time that can suppress grain growing, to obtain above-mentioned mixed structure with micron crystal grain and nanocrystal thus be parent phase and contain hard second high tensile steel plate mutually.

Then set forth the regulation reason of the area fraction of hard second phase in the hot-rolled sheet.The area fraction of hard second phase is less than 30% o'clock or greater than 85% o'clock, can not obtain the nanocrystal of q.s after the cold rolling and annealing.EBSD (ElectronBackscatter Diffraction) method of the inventor by having utilized high resolving power SEM carried out cold rolling and the nanocrystal generation behavior of when annealing is studied to the hot-rolled sheet that is made of mutually ferrite and hard second.Found that, hard second from cold rolling microstructure mutually or the near interface of ferritic phase begin to wait shape shaft to generate the nuclear of so very little crystal grain below the 300nm, along with the passing of annealing time, these crystal grain nuclears corrode ferrite worked structure on every side and grow.Simultaneously, hard second is confirmed mutually, the situation that has its inner nuclear that generates new crystal grain is almost constant with the hard second mutually shape own and only reply the situation that is varied to equiaxial micron crystal grain by strain.In addition, carrying out the detailed parsing of crystalline orientation etc. finds, the origin of the nanocrystal in the mixed structure is that in the early stage ferrite carries out caryogenic equiaxial ferrite grain with hard second near interface mutually, and the origin of micron crystal grain is not give very much the part of shear strain and the hard at initial stage second in the early stage the ferrite mutually.

According to this understanding, hard second in hot-rolled sheet mutually after a little while, because it is also few with the interface of ferritic phase, so the karyogenesis density of nanocrystal is little, the opposite little ferritic phase of more dependent variable that but exists, so the content that finally is micron crystal grain is many, the reinforcement of the ferritic phase that nanocrystal brings is not obvious, does not have difference with common complex tissue steel.On the other hand, under the many mutually situation of hard second in the hot-rolled sheet, though to should uprising that ferritic phase is given, ferritic phase itself less, the tissue that hard second is rolled mutually, anneals and forms the i.e. content of micron crystal grain is many, causes obtaining the nanocrystal of q.s.Based on above-mentioned cognition, come cold rolling and the annealed experiment by systematically the area fraction of second phase of the hard in the hot-rolled sheet being carried out various changes, the inventor finds that the suitable scope of the area fraction of hard second phase is 30%～85%.

Herein, the measuring method to second phase of the hard in the hot-rolled steel sheet describes.Take 400～1000 times the optical microscope photograph in the cross section parallel of hot-rolled steel sheet with rolling direction.As shown in Figure 1, along thickness of slab direction, at an arbitrary position draw 3 straight lines (in same figure provided 1 straight line as representative) thereafter.Cut off hard second phase, ferrite, hard second phase on this straight line successively, with calibration gauge measure from hard second mutually and ferritic first interface pass behind the ferrite grain to the distance at next interface, unit conversion is become μ m.The whole hard second that is switched on the comparison film is implemented this operation mutually, whole measured values is averaged, as the equispaced of hard second phase.

Then, the hot-rolling method that is used to obtain the purpose tissue is described.Fig. 2 is the figure of expression hot rolled temperature course.As shown in Figure 2, at first, it is more than the Ac3 transformation temperature that slab is heated to the austenite territory, carry out roughing system after, carry out finish rolling system.The temperature of this finish rolling system is set in the temperature shortly past the Ar3 transformation temperature,, is set in cryogenic as far as possible austenite territory in the temperature range that ferrite do not separate out that is, the grain growth when suppressing rolling thus., be cooled to ferrite and austenitic two phase regions, make ferrite and austenitic mixed structure thus thereafter.At this moment, the growth of austenite grain when suppressing rolling, not only austenitic thus karyogenesis density height, and also it is also high to carry out caryogenic ferritic karyogenesis density from austenitic crystal boundary, thus can make particle diameter fine.When separating out appears in ferrite when rolling, residual unchangeably through the ferrite state of processing to room temperature, therefore, reduced the effect of utilizing phase transformation that fine ferrite is separated out.

Then, be incubated constantly, perhaps be not incubated and carry out chilling at two phase regions.Making the austenite partial phase change in this quenching process is hard second phase, and insulation makes crystal grain fine in the stage of two phase regions, and this plays effective function to the interval that dwindles hard second phase.Need to prove, be meant since the chilling of two phase regions, cool off with the speed more than the critical cooling velocity of determining according to composition of steel, that is, begin nose and the speed that reaches more than the such speed of cooling of Ms point (martensitic transformation begins temperature) is cooled off with the pearlitic transformation in the not crosscut anisothernal transformation (CCT:Continuous cooling transformationdiagram).

Do not need its be general sense chilling but, as long as be enough speed of cooling to this steel.The speed of cooling of this moment if greatly in the not crosscut CCT bainitic transformation begin nose, second is martensite just mutually.In addition, if the crosscut bainitic transformation begins nose and is cooled to below the Ms point, then second is the mixed structure of martensite and bainite mutually.In addition, if just stopping cooling and insulation shortly past the Ms point, and then cool to room temperature, then second is bainite mutually.

In addition, if increase Si, Al as the composition of high tensile steel plate, and just stopping cooling shortly past Ms point and be incubated, and then cool to room temperature, then second also contains retained austenite in mutually except bainite.In any case, importantly all to avoid pearlitic transformation, make ferrite second phase in addition not contain cementite.

In the manufacture method of such high tensile steel plate, preferably, the plate after the cross-section hot rolling parallel with rolling direction is in observed metal structure, the equispaced of hard second phase that goes out in the thickness of slab direction detection is 2.5 μ m～5 μ m, and its reason will be set forth in the back.

＜cold rolling

The equispaced of second phase of the hard in the tissue after the hot rolling is made as the thickness of slab of (before cold rolling) is made as t after d (μ m), the hot rolling ₀, when the thickness of slab after cold rolling is made as t, the degree of finish index D of representing with following (27) formula satisfy carry out under the such condition of following (28) formula cold rolling.

D＝d×t/t ₀ (27)

0.50≦D≦1.0 (28)

(28) in the formula, the D value can not give ferritic phase enough shear strains greater than 1.0 low rolling rate, and the nanocrystal of q.s after can not guaranteeing to anneal only forms the common metal structure that is made of micron crystal grain.In addition, when implementing D value less than 0.5 such high draft rolling, given uniform shear strain to all parts of ferritic phase on the contrary, the parent phase ferrite after the annealing does not form the mixed structure of nanocrystal and micron crystal grain, and integral body becomes nanocrystal.All can not give steel plate in any case is high ductibility with the feature of high tensile steel plate of the present invention.

In addition, among the present invention, above-mentioned d is set at 2.5 μ m～5 μ m.D is during greater than 5 μ m, if satisfy (28) formula, and just must be at t/t ₀Be to be rolling rate below 0.2 greater than being rolled under 80% the high pressure, but want the such high tensile steel plate of rolling the present invention, need apply big load roller mill.Use the series connection roller mill more than 4 grades,, can not in once rolling, guarantee necessary rolling rate, thereby be necessary to carry out reroll even reduce the draft of every rolling 1 passage.Therefore, in the present invention,, second in the hot-rolled sheet is defined as below the 5 μ m separately for rolling rate also obtains the nanocrystal tissue in (this rolling rate in reality can with 1 rolling realization) below 80%.

In addition, d is during less than 2.5 μ m, the hard second at initial stage is separately very little, so be rolled all and can give uniform strong strain to all parts of ferritic phase in any condition, parent phase ferrite after the annealing does not form the mixed structure of nanocrystal and micron crystal grain, and integral body becomes nanocrystal.Therefore, as mentioned above, can not give steel plate is high ductibility with the feature of high tensile steel plate of the present invention.

＜annealing 〉

Annealing is the material after cold rolling to be heat-treated to eliminate process strain, and produces the operation of purpose metal structure simultaneously.Annealing by the material after cold rolling is heated, insulation, refrigerative process constitute, holding temperature Ts (℃) and satisfy following (29) formula and (30) formula in the relation of the time ts (second) of Ts insulation.

F ₂(T)+20≦Ts≦F ₂(T)+90 (29)

F ₂(T)+20≦Ts≦F ₄(G)-1.3(ts) ^1/2 (30)

(ts: soaking time (second), Ts holding temperature (℃), (ts) ^1/2Be the square root of ts)

Fig. 3 is the graphic representation that is illustrated in the elongation of steel plate when changing annealing temperature under the same conditions before cold rolling.Need to prove that the data of Fig. 3 comprise the example that does not have record in example 1, comparative example 7,8,9,10 and the table 5.As shown in Figure 3, be equivalent to F as Ts ₂(T)+20 694 ℃ to being equivalent to F ₄(G)-1.3 (ts) ^1/2750 ℃ between the time elongation maximum.This is because Ts is lower than F ₂(T) time, being that ferrite is single-phase, is not complex tissue, and Ts is greater than F ₂(T) and be in F ₂During (T)+20 ℃ scope, be ferrite and austenitic two phase region in the annealing process, but austenitic minute rate is few, the branch rate of final hard second phase is few, so feature of the present invention is not obvious.On the other hand, annealing temperature is greater than F ₄(G)-1.3 (ts) ^1/2The time, the grain growth becomes obviously, not in the scope of suitable tissue of the present invention, so ductility reduces.

On the other hand, Ts is greater than F ₂O'clock (T)+90, the area fraction of hard second phase in the metal structure is excessive, and the deformation characteristic of hard second phase is that little this feature of high yield strength, ductility becomes obviously, so the ductility of steel plate reduces.From the above, the scope that (29) formula and (30) formula are represented is decided to be the proper range of annealing temperature.

Hard second phase in the metal structure after the annealing can corresponding chemical ingredients obtain various hard second mutually with the annealing characteristic curve.Fig. 4 is the figure of the various annealing characteristic curves of explanation.Among Fig. 4, rational curve the 1, the 2nd, the situation of CAL (continuous annealing line), rational curve 3 is situations of CGL (molten zinc plating line), rational curve 4 is situations of pack annealing.At first, annealing temperature Ts is set in the scope of satisfied (29) formula and (30) formula as described above, when suppressing grain growing, forms ferrite and austenitic two-phase structure thus.Thereafter in the process of cooling of cool to room temperature, austenite is changed to various phases, by selecting suitable cooling conditions, can form the complex tissue that the hard second by ferritic phase and suitable amount and kind constitutes mutually.Herein, as mentioned above, hard second is meant martensite, bainite and retained austenite mutually.

Below the method for cooling after the annealing is elaborated.Cooling can adopt any means in the following method to carry out: the method for using gas, use the method for jet of water or make the gentle body of water mixing jetting method or in tank, quench (WQ) or carry out contact cooling with roller.Alleged herein gas can adopt the arbitrary gas in mixed gas, helium or the argon gas of air, nitrogen, hydrogen, nitrogen and hydrogen.

About cooled temperature course, according to the formation of annealed wire, 4 specific character curves shown in Figure 4 all can be used.After annealing zone, in the line of the such formation of rational curve 1, can near the temperature of regulation, stop cooling with cooling zone and overaging band thereafter, directly carry out overaging and handle, also can cool off the back reheat and carry out overaging and handle.Rational curve 2 is that the line with overaging band constitutes, and rational curve 3 is the rational curves that are equivalent to CGL (molten zinc plating line), and the refrigerative terminal temperature is limited at the temperature that fused zinc is bathed, and in addition, it is identical with rational curve 1.Rational curve 4 is pack annealing.

In the above-mentioned process of cooling, speed of cooling is crossed when slow, and austenite phase transformation is perlite, ferrite, so speed of cooling to a certain degree is necessary.This speed of cooling is so long as can avoid that the critical cooling velocity (it depends on composition of steel) of pearlitic transformation, ferrite transformation is above to get final product.If speed of cooling is fast, ferrite transformation nose in the not crosscut CCT curve and bainitic transformation nose just are cooled to below the Ms point, then obtain martensite as hard second phase.Have in the rational curve 1 and 3 of overaging band, say that strictly martensite becomes tempered martensite.But as mentioned above, tempered martensite is not treated with the martensite difference among the present invention.

In addition, cooling off with the such speed of cooling of crosscut bainitic transformation nose, and cooling termination temperature is decided to be the Ms point when following, hard second is the complex tissue that is made of martensite and bainite mutually, in rational curve 1 and 3 with overaging band, if just stop cooling and directly carry out overaging and handle shortly past the Ms point, the mixed structure that then forms bainite or form retained austenite and bainite as hard second mutually.Whether generated retained austenite, the austenitic stability when depending on annealing.That is,, perhaps promote C to the austenite enrichment, make austenite stable, obtain retained austenite thus by the prolongation overaging treatment time by increasing the amount of alloying element (Si, Al).

Be specifically described respectively, the hard second that rational curve 1 obtains be mutually among martensite, bainite, the retained austenite at least a more than.Then, shown in annealing characteristic curve 2,, below the annealing postcooling to 100 ℃, finish annealing for the line of the formation that does not have the overaging band.At this moment, hard second be mutually among martensite, the bainite at least a more than.

Then, annealing characteristic curve 3 is the annealed rational curves that are equivalent to CGL (molten zinc plating line).Carry out chilling from annealing temperature, make zinc be attached to the surface with the fused zinc bath.Thereafter, can reheat, make the zinc coating alloying, also can omit reheat, do not make the zinc coating alloying.Carry out under the situation of reheat, the kind of hard second phase that obtains is identical with rational curve 1, does not carry out under the situation of reheat, and the kind of hard second phase that obtains is identical with rational curve 2.

Annealing characteristic curve 4 is pack annealing.Usually do not take out the roll coil of strip after finishing annealing, in stove, cool off, therefore be defined as the very high composition of hardening capacity that yet can obtain purpose hard second phase in the cold condition of stove from body of heater.Specifically, adopt annealing characteristic curve 4 in the example 6.

At last, after cold rolling and annealing, can implement elongation to high tensile steel plate of the present invention is skin rolling below 2.5%.Skin rolling is the operation of using usually based on correcting final this main purpose of shape in the manufacturing process of cold-rolled steel sheet.In the 1st embodiment of the manufacture method of high tensile steel plate of the present invention, except shape correction, can further give following effect: reduce yield-point, the moulding load when reducing the press forging moulding, further reduce elastic recovery.But elongation can not be ignored the problem of the ductility reduction of material greater than 2.5% o'clock, presented reverse effect, so higher limit is decided to be 2.5%.

2. the 2nd embodiment

The high tensile steel plate of manufacturing of the present invention presents the metal structure that is made of mutually ferritic phase and hard second in being dispersed in above-mentioned ferritic phase, the hard second mutually shared area fraction is 3%～30% in the metal structure, the shared area fraction of nanocrystal is 15%～90% in the ferritic phase, in the ferritic phase, the median size dS of nanocrystal and the median size dL of micron crystal grain satisfy following (31) formula.

dL/dS≧3 (31)

Such high tensile steel plate is preferably on the cross section parallel with rolling direction of steel plate, under the situation of selecting the square square lattice of 3 μ m more than 9 arbitrarily, with the area fraction of described hard second phase in each grid be made as Ai (i=1,2,3 ...), at this moment, the mean value A (ave) of described Ai and standard deviation s satisfy following (32) formula, simultaneously, the median size dp of hard second phase and whole ferritic median size df satisfy following (33) formula.

s/A(ave)≦0.6 (32)

df/dp≧3 (33)

In addition, also contain at least a among Si, Mn, Cr, Mo, Ni and the B when such high tensile steel plate preferably contains C, and C (ss) (refer to deduct with Nb, Ti and V bonded C amount after the solid solution carbon amount that obtains) is with following (34) formula formula～(37) formula that is that prerequisite satisfies following (35) from total C amount.Need explanation, the element that respectively adds in the formula is with composition (quality %) substitution of this interpolation element.

F ₁(Q)＝0.65Si+3.1Mn+2Cr+2.3Mo+0.3Ni+2000B (34)

F ₁(Q)≧-40C+6 (35)

F ₁(Q)≧25C-2.5 (36)

0.02≦C(ss)≦0.3 (37)

In addition, such high tensile steel plate preferably contains composition with following (38) formula, (39) formula formula that is that prerequisite satisfies following (40).In addition, need explanation, each adds element is with composition (quality %) substitution of this interpolation element.

F ₂(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (38)

F ₃(P)＝500×Nb+1000×Ti+250×V (39)

F ₂(S)+F ₃(P)≦360 (40)

In addition, for such high tensile steel plate,, preferably contain Nb, the Ti below 0.36% below 0.72%, at least a composition among the V below 1.44% in quality %; In quality %, preferably contain at least a composition among the Al below the P and 18% below 2%; In quality %, extremely preferred Si is below 5%, Mn is below 3.5%, Cr is below 1.5%, Mo is below 0.7%, Ni is below 10% and B is below 0.003%.

In addition, in such high tensile steel plate, count 0.007%～0.03% N with quality % by also containing, can not make the ductility deterioration of high tensile steel plate, give high baking hardenability, therefore, when collision, produce high load after being shaped to parts, can further improve impact energy absorption characteristic.Need explanation, baking hardenability is meant, is shaped to when having the operation of paint baking behind the parts, in the thermal treatment of this roasting procedure, intrusion type solid solution element will be fixed by the dislocation that forming process is introduced, and when parts deform thus, show the phenomenon of big deformation resistance.This characteristic is known as BH (Bake Hardening), has provided the method for measuring its amount (BH amount) in the attached sheet of JIS G3135.In the following explanation, also baking hardenability is called BH, the baking hardening amount is called the BH amount.

In addition, the inventor furthers investigate the method that is fit to the above-mentioned high tensile steel plate of manufacturing.The result has obtained following understanding: for the ultra micro refinement with common cold rolling realization crystal grain, crystal structure before rolling is made soft ferrite and hard second complex tissue mutually, simultaneously to implement cold rolling with the corresponding desired rolling rate in hard second interval mutually, further anneal, can access the high tensile steel plate of the mixed structure of above-mentioned micron crystal grain and nanocrystal thus with temperature, the time that can suppress grain growing.

In addition, according to the present invention, when the crystal structure before rolling is made mutually complex tissue of soft ferrite and hard second, to implement cold rolling with the rolling rate of the corresponding necessity in hard second interval mutually, further anneal, can make the high tensile steel plate of the above-mentioned mixed structure that constitutes by micron crystal grain and nanocrystal thus in the temperature range that does not produce grain growing.Addition by suppressing alloying element also makes the ferrite crystal grain miniaturization, the intensity of the high tensile steel plate that so obtains is improved, and the intensity of this steel plate and the balance of ductility (this is important when the press forging moulding) excellence, quiet moment is more than the 170MPa.

Below, with reference to accompanying drawing preferred implementation of the present invention is described.At first, the regulation reason to the various setting formulas in the high tensile steel plate of the present invention describes.Need to prove that the content of each element that hereinafter provides is all in quality %, for convenience's sake, below % made in brief note.

The raw material of high tensile steel plate of the present invention uses carbon steel, but as described later, need to adjust from total C amount, deduct with Ti, Nb, V bonded C after solid solution carbon amount C (ss) be 0.02%～0.3%.In order to improve hardening capacity and to improve hardness of steel, make at least a element that contains in this carbon steel among the 1st element set: Si, Mn, Cr, Mo, Ni and the B by solution strengthening.In addition, for miniaturization and precipitation strength by crystal grain improve hardness of steel, make at least a element that contains in this carbon steel among second element set: Nb, Ti and the V as required.And then, in order to improve hardness of steel, make at least a element that contains in this carbon steel among triad: P and the Al as required by solution strengthening.

In addition, resulting steel satisfies all formulas of following (34) formula～(40) formula.But the symbol of element in the following formula is represented the composition (quality %) of this element, and for example, " Cr " is meant the composition (quality %) of Cr.

F ₁(Q)＝0.65Si+3.1Mn+2Cr+2.3Mo+0.3Ni+2000B (34)

F ₁(Q)≧-40C+6 (35)

F ₁(Q)≧25C-2.5 (36)

0.02≦C(ss)≦0.3 (37)

F ₂(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (38)

F ₃(P)＝500×Nb+1000×Ti+250V (39)

F ₂(S)+F ₃(P)≦360 (40)

Herein, the meaning of the symbol in these formulas and various regulation reason are described.

The regulation reason of＜(34) formula～(36) formula 〉

F ₁(Q) be the index of the hardening capacity of expression steel, it defines shown in (34) formula like that, calculates according to each composition (quality %) of adding element.

As described later, in the manufacture method of high tensile steel plate of the present invention, importantly the metal structure before cold rolling is made soft ferritic and hard second complex tissue of (in martensite, bainite and the retained austenite mutually at least a) mutually.These tissues can obtain by the following method: the method for carrying out chilling after the hot rolling from two phase regions that are made of ferrite and austenite; First cool to room temperature after the hot rolling, and the method that heats with the state under this room temperature; Perhaps carry out earlier after the hot rolling cold rolling, heating then, and, carry out the method for chilling then in the insulation of two phase regions that constitute by ferrite and austenite.But, when obtaining these and organize, have two problems.

First problem, if the C amount is few, then hardening capacity is low, therefore, is difficult to obtain hard second phase.As the countermeasure of this problem, be necessary to add above-mentioned first element set as the element that hardening capacity is improved, be easy to obtain hard second phase.But necessary hardening capacity and C amount are inversely proportional to, so C measures for a long time, the addition of the element of hardening capacity raising are got final product less.Above-mentioned formula (35) is represented this relation.Add the element that hardening capacity is improved of necessary amount according to above-mentioned (35) formula.Need explanation, alleged herein C amount (C) will be described in detail later, its expression from total C amount, deduct with Nb, Ti, V bonded C amount after solid solution carbon amount.

Pearlitic transformation under the C amount situation how, takes place in second problem easily from the process of cooling that two phase regions that are made of ferrite and austenite begin, be difficult to obtain necessary hard second phase.For fear of this problem, it is effective adding the 1st element set.That is, by the element that interpolation improves hardening capacity, the pearlitic transformation in the anisothernal transformation (Continuouse cooling transformation diagram is designated hereinafter simply as " CCT curve ") begins nose to long-time side shifting.Therefore, avoid occurring perlite, can form ferrite and hard second complex tissue mutually.Under the C amount situation how, pearlitic transformation takes place easily, so need a large amount of elements that hardening capacity is improved.Above-mentioned (36) formula is represented this relation.Add the element that hardening capacity is improved of necessary amount according to above-mentioned (36) formula.Need explanation, alleged herein C amount also is above-mentioned C.

The regulation reason of the explanation of＜C and (37) formula 〉

C be meant from total C amount, deduct with the 2nd element set (Nb, Ti, V) bonded C after the solid solution carbon amount that obtains, it is the value that calculates by following (41) formula.Need to prove, in (41) formula, add the composition (quality %) that element place's substitution should be added element at each.

C (ss)=total C amount-(12/92.9 * Nb+12/47.9 * Ti+12/50.9 * V) (41)

(41) coefficient in the formula 92.9,47.9 and 50.9 is respectively the nucleidic mass of Nb, Ti and V, (expression of 12/92.9 * Nb+12/47.9 * Ti+12/50.9 * V) and Nb, Ti or V be combined into are the C amount (quality %) of carbide, total be exactly solid solution C after it is deducted from the C amount.

(37) formula has been stipulated higher limit and the lower value of solid solution C, and its reason is for the metal structure before desired scope generation is cold rolling.Lower value being decided to be 0.02% herein, is because C even add the element that hardening capacity is improved, also do not generate hard second phase, thereby it was single-phase to become ferrite less than 0.02% o'clock.Single-phase for ferrite, have only and adopt above-mentioned special methods such as accumulation ply rolling, the size of microcrystal miniaturization of steel could be arrived than the also little nano level of 1 μ m.

In addition, higher limit being set at 0.3% is because greater than 0.3% o'clock, can not obtain the ferrite and hard second complex tissue mutually of purpose.C was greater than 0.3% o'clock, even add the element that hardening capacity is improved, the pearlitic transformation nose in the CCT curve also rests on the short period of time side.Thus, when two phase regions that are made of ferrite and austenite carry out chilling, all can crosscut pearlitic transformation nose under any speed of cooling, cause the metal structure before cold rolling to become ferrite and pearlitic complex tissue.

Herein, perlite is that cementite shows to such an extent that be highly brittle to distortion as the cementite of the compound of C and Fe and ferritic lamellar structure, and the energy when cold rolling is consumed by the fracture of cementite.Therefore, in the tissue of steel, contain under the pearlitic situation, can not give soft ferritic as the feature of manufacture method of the present invention with big strain.So, can avoid the C amount higher limit of pearlitic transformation to be set at 0.3% the element that hardening capacity is improved.

The regulation reason of＜(38) formula～(40) formula 〉

F ₂(S) be that the solution strengthening of expression by first element set and triad is used for amount that high tensile steel plate is strengthened, the unit of this amount is MPa, and this amount is to calculate according to the quality % of (38) formula from the interpolation element.(38) coefficient that multiplies each other with each element in the formula is based on following consideration method and calculates from following (42) formula.

The coefficient of each element=| r (X)-r (Fe) |/r (Fe) * M (Fe)/M (X) * 1000 (42)

Herein, r (X) is that this atoms of elements radius, r (Fe) are that atomic radius, the M (X) of iron is that this atoms of elements amount, M (Fe) are the nucleidic mass of iron.

(42) meaning of formula is as follows.That is, the solution strengthening amount of the value that obtains divided by the iron atom radius with certain difference of adding atoms of elements radius and iron atom radius and each this interpolation element is proportional.Herein, described on duty in order to be converted into the quality % of this interpolation element with the nucleidic mass and this ratio that adds the atoms of elements amount of iron, in order to be MPa, multiply by 1000 again in addition with unit conversion.The coefficient of (42) formula that has provided the physical constant of each used element in the table 2 and calculated thus.

Table 2

The symbol of element	Fe?	Si％	Mn％	P％	Al％	Cr％	Mo％	Ni％	B％
										Atomic radius r (X)	1.24	1.17	1.12	1.09	1.43	1.25	1.36	1.25	0.9
(r(X)-r(Fe))/r(Fe)	-	0.0565	0.0968	0.1210	0.1532	0.0081	0.0968	0.0081	0.2742
										Nucleidic mass M (X)	55.8	28.1	54.9	31.0	27.0	52.0	95.9	58.7	10.8
M(Fe)/M(X)	-	1.99	1.02	1.80	2.07	1.07	0.58	0.95	5.17
										(42) coefficient of formula	-	112	98	218	317	9	56	8	1417

F ₃(P) be the index of C formation carbide in expression above-mentioned the 2nd element set and the steel and its amount of reinforcement when steel being strengthened, F by precipitation strength ₃(P) as above-mentioned (39) formula, be determined.

(39) the formula meaning is as follows.That is, Nb, Ti, V form carbide in steel ability is strong, for example the solubility product of the solubility product of Nb and C, Ti and C (quality %) in 700 ℃ steel ²All are 10-6 power levels, the solubility product of V and C (quality %) ²Be 10-4 power levels, so in high tensile steel plate of the present invention, Ti, Nb, V can not exist as sosoloid substantially, but as being that NbC, TiC or VC exist with C with the carbide that 1 to 1 ratio combines formation.So, can expect and the Nb, the Ti that add, the proportional precipitation strength amount of addition of V.It should be noted that certainly this is a situation about existing as yet not with Nb, Ti or V bonded C, further add Nb, Ti, V down, can not obtain the amount of being expected of separating out even whole C is in Nb, Ti, V bonded state.In addition, precipitation strength amount changes because of the difference of the size of precipitate.

Usually, when precipitate was thick, the precipitation strength ability reduced.For high tensile steel plate of the present invention, as described later, during annealing after cold rolling, do not consider that long-time insulation is at the carbide high-temperature area of growth easily.Therefore, the carbide of Nb, Ti or V evenly disperses imperceptibly, and the precipitation strength amount only depends on the addition of these elements.Above-mentioned (39) formula has shown this phenomenon.

Herein, the

coefficient

500,1000 and 250 in (39) formula is represented the precipitation strength amount of Nb, Ti, the per 1 quality % of V respectively, and these are the numerical value of determining according to experiment.The precipitation strength amount of Nb, Ti and V and be F ₃(P) be total precipitation strength amount.

Based on this understanding, (40) formula represents that ferritic total amount of reinforcement that solution strengthening and precipitation strength bring should be below 360MPa.This is because when the amount of reinforcement of steel plate is too high, do not manifest high quiet moment as the feature of wood invention (dynamic strength and static strength poor).As mentioned above, when adding a large amount of alloying elements and coming significantly reinforced ferrite, ferrite purity also reduces simultaneously, and ferritic deformation stress diminishes to the dependency of rate of straining.In the metal structure of high tensile steel plate of the present invention, ferritic purity has obtained the quiet moment higher than existing steel under the situation more than the prescribed value, and still ferritic purity is crossed when hanging down, and high quiet moment do not occur.

The contriver has attempted the ferritic purity of necessity is carried out quantification in order to obtain high quiet moment.The result finds experimentally, and it is proportional that each adds the ferrite amount of reinforcement (solution strengthening, precipitation strength) of detrimentally affect degree that element produces ferritic quiet moment and per unit addition (quality %).Further investigate based on this result, result of study shows that the higher limit that can show the ferrite amount of reinforcement of high quiet moment is 360MPa.Above-mentioned (40) formula quantizes to this result.

In addition, contain an amount of N, can give steel plate high BH by making high tensile steel plate of the present invention.The C that high tensile steel plate of the present invention contains with solid solution condition be present in hard second mutually in, in the carbide of Nb, Ti, V, in the ferrite parent phase.Wherein, the solid solution C in the ferrite parent phase helps BH.But, even the C that increases in the chemical ingredients measures, because C is assigned to hard second phase, carbide, so the increase that the increase of C amount may not bring the BH amount.Therefore, giving aspect the high BH, the inventor is conceived to as the intrusion type solid solution element N beyond the C.The content of N can play a role in the strain aging phenomenon effectively, thereby can access high BH amount as long as in the scope that does not generate with the compound of iron, just can contain N and can not make metal structure that big variation takes place with solid solution condition.

In addition, the high tensile steel plate that obtains of the present invention has its BH amount characteristic higher than existing steel plate.Though its reason is unclear, but the complex tissue of the parent phase ferrite of the high tensile steel plate of the present invention micron crystal grain that to be high nanocrystal of intensity and intensity lower, it is carried out press forging moulding etc. add man-hour, because the constraint of the high nanocrystal of intensity, in the near interface generation a large amount of dislocation of the lower micron crystal grain of intensity with nanocrystal.It is believed that these a large amount of dislocations promoted the strain aging of N when paint baking was handled, thereby show high BH amount.

The qualification reason of＜each chemical ingredients 〉

Then, the qualification reason of each chemical ingredients in the high tensile steel plate of the present invention is set forth.Need to prove that the unit of the content of each element given below all is quality %, but for convenience's sake, only note is made %.In addition, for C, separately it is limited with (37) formula, for other elements, define lower value by (35) formula, (36) formula respectively under the situation mostly, define higher limit, and set higher limit respectively by (40) formula, (43) formula, (44) formula.

Cr≦1.5 (43)

Mo≦0.7 (44)

＜C: solid solution C content is 0.02%～0.3% 〉

By adding C, can generate the mixed structure that constitutes by ferrite and austenite at high temperature, by this mixed structure is carried out chilling, can form hard second phase of martensite, bainite, retained austenite.Therefore, C is a most important element among the present invention.

In high tensile steel plate of the present invention, owing to can add Nb, Ti, V, so, adjust the addition of C in order to make the solid solution C except the C that separates out as carbide in this case satisfy above-mentioned (37) formula.Solid solution C was less than 0.02% o'clock, and cold rolling preceding metal structure is a ferrite, and solid solution C was greater than 0.3% o'clock, and metal structure becomes the complex tissue that is made of ferrite and perlite, and this all is not suitable for the manufacture method of high tensile steel plate of the present invention.

＜the 1 element set: the Si, Mn, Cr, Mo, Ni, B 〉

Adding these elements is for hardening capacity that improves steel and the intensity that improves steel by solution strengthening.Adjust addition above-mentioned to satisfy (35) formula, (36) formula, (40) formula, (43) formula and (44) formula.The following describes the higher limit of addition of each element and the qualification reason of lower value.

<Si：0.2％～5％>

The Si addition can not show the effect that improves hardening capacity significantly less than 0.2% o'clock.Therefore, lower value is decided to be 0.2%.In addition, the Si addition was greater than 5% o'clock, and Si combines with Fe, and crystalline structure having occurred is the intermetallic compound Fe of DO3 type or B2 type ₃Si, this will reduce the ductility of steel, so the upper limit of Si is set at 5%.

<Mn：0.1％～3.5％>

The addition of Mn can not show the effect that improves hardening capacity significantly less than 0.1% o'clock.Therefore, lower value is decided to be 0.1%.In addition, the addition of Mn was greater than 3.5% o'clock, even in room temperature, except ferrite, austenite also exists with stable phase.Austenitic intensity is little, causes the steel integral intensity to reduce, so be not preferred.Therefore, higher limit is decided to be 3.5%.

<Cr：0.1％～1.5％>

The addition of Cr can not show the effect that improves hardening capacity significantly less than 0.1% o'clock.Therefore, lower value is decided to be 0.1%.In addition, the addition of Cr was greater than 1.5% o'clock, and the C in the steel combines with Cr and forms carbide, so can not obtain the solid solution Cr of corresponding addition, can not expect to improve hardening capacity.Therefore, higher limit be set at Cr can with solid solution condition exist 1.5%.

<Mo：0.1％～0.7％>

The addition of Mo can not show the effect that improves hardening capacity significantly less than 0.1% o'clock.Therefore, lower value is decided to be 0.1%.In addition, the addition of Mo was greater than 0.7% o'clock, and the C in the steel combines with Mo and forms carbide, so can not obtain the solid solution Mo of corresponding addition, can not expect to improve hardening capacity.Therefore, higher limit be set at Mo can with solid solution condition exist 0.7%.

<Ni：0.2％～10％>

<B：0.0005％～0.003％>

The addition of B can not show the effect that improves hardening capacity significantly less than 0.0005% o'clock.Therefore, lower value is decided to be 0.0005%.In addition, the solid solution limit of B in ferrite itself is very little, under the few situation of the addition of B, think that its main segregation is present in the crystal boundary of steel, but the addition of B was greater than 0.003% o'clock, and only crystal boundary is not enough as the presence bit of B, thereby had generated intermetallic compound Fe ₂B has reduced the ductility of steel.Therefore higher limit is decided to be 0.003%.

＜the 2 element set: the Nb, Ti, V 〉

<Nb：0.01％～0.72％>

The addition of Nb is less than 0.01% o'clock, the DeGrain of miniaturization and precipitation strength.Therefore, lower value is decided to be 0.01%.In addition, by above-mentioned (39) formula as can be known, the addition of Nb is greater than 0.72% o'clock, and only the precipitation strength amount brought of NbC has just reached 360MPa, can not satisfy above-mentioned (40) formula, so the higher limit of Nb is limited at 0.72%.

<Ti：0.01％～0.36％>

The addition of Ti is less than 0.01% o'clock, the DeGrain of miniaturization and precipitation strength.Therefore, lower value is decided to be 0.01%.In addition, by above-mentioned (39) formula as can be known, the addition of Ti is greater than 0.36% o'clock, and only the precipitation strength amount brought of TiC has just reached 360MPa, can not satisfy above-mentioned (40) formula, so the higher limit of Ti is limited at 0.36%.

<V：0.1％～1.44％>

The addition of V is less than 0.1% o'clock, the DeGrain of miniaturization and precipitation strength.Therefore, lower value is decided to be 0.1%.In addition, by above-mentioned (39) formula as can be known, the addition of V is greater than 1.44% o'clock, and only the precipitation strength amount brought of VC has just reached 360MPa, can not satisfy (40) formula, so the upper limit of V is limited at 1.44% naturally.

<N：0.007％～0.03％>

In the high tensile steel plate of the present invention, N is an important element of giving baking hardenability, exists under the situation of paint baking operation after high tensile steel plate is shaped to parts, is utilizing the strain aging phenomenon to improve aspect the yield strength of parts that it plays a role effectively.So, suitably add N as required.The addition of N was less than 0.007% o'clock, and baking hardenability is not obvious.Therefore, the lower value of N is decided to be 0.007%.On the other hand, the addition of N is greater than 0.03% o'clock, cube brilliant Fe ₄N begins to separate out, so, can not expect to improve the BH amount suitable with the N that adds.So will be defined as 0.03% on the N.

＜the 3 element set: the P, Al 〉

<P：0.03％～2％>

<Al：0.01％～18％>

Al is the solution strengthening element, and it also has the effect as reductor simultaneously, can make steel become so-called " deoxidized steel ".In addition, Al combine and forms aluminum oxide with dissolved oxygen in the steel in system steel operation, and aluminum oxide floats, and by removing the aluminum oxide of come-up, can improve ductility, the toughness of steel.Therefore, can add Al as required.But the addition of Al is less than 0.01% o'clock, and it all can not embody significantly as the effect of reductor with as the effect of solution strengthening element.Therefore, lower value is decided to be 0.01%.On the other hand, the addition of Al generated intermetallic compound Fe greater than 18% o'clock ₃Al has reduced the ductility of steel.Therefore, higher limit is decided to be 18%.

＜to the tissue the qualification reason

The metal structure of high tensile steel plate of the present invention satisfies following 1 simultaneously)～5) important document put down in writing.

1) metal structure by ferrite and hard second mutually (among cementite, martensite, bainite, the retained austenite at least a) constitute.In addition, cut out the cross section parallel of steel plate with rolling direction, with after this cross section etching, the area fraction of hard second phase of measuring from the SEM photo of taking with 5000 times magnification with scanning electronic microscope is 3%～30% with nital etc.

2) in the metal structure, hard second is dispersed in the ferritic phase mutually, and satisfies following important document.Promptly, in 5000 times the SEM photo in the cross section parallel of steel plate with rolling direction, choose the square square lattice of 3 μ m more than 9 arbitrarily, when using image analysis to measure the area fraction of hard second phase in each grid, the area fraction of each hard second phase is made as Ai (i=1,2,3, ...), this moment, the mean value A (ave) of Ai satisfied following (32) formula with standard deviation s.

s/A(ave)≦0.6 (32)

4) the median size dS of nanocrystal satisfies following (31) formula with the median size dL of micron crystal grain.

dL/dS≧3 (31)

5) the median size dp of hard second phase and whole ferritic median size df satisfy following (33) formula.

df/dp≧3 (33)

Herein, median size is meant, by image analysis the area of the whole ferrite grains in 5000 times the SEM photo in the cross section parallel with rolling direction of steel plate is measured, and the homalographic circular diameter of obtaining according to area separately.Specifically, the area of the ferrite grain that will obtain by image analysis be made as Si (i=1,2, in the time of 3...), homalographic circular diameter Di (i=1,2,3...) can calculate by following (45) formula.

Di＝2(Si/3.14) ^1/2 (45)

Set the important document 1 shown in above-mentioned)～5) the reasons are as follows.That is, separate out, can discharge solid solution elements such as C mutually to hard second, improve ferritic purity, can in the ductility that improves steel, improve quiet moment thus from the ferrite part by making an amount of hard second phase homodisperse.Under the uneven situation of dispersion state of hard second phase, the purity step-down of the little ferrite of the density of hard second phase part can not show high ductility, high quiet moment.

In addition, the area fraction of hard second phase is defined in 3%～30% the reasons are as follows.That is, the area fraction of hard second phase was less than 3% o'clock, and ferritic high purity deficiency is not so quiet moment increases.On the other hand, the area fraction of hard second phase was greater than 30% o'clock, and it is big that the detrimentally affect of hard second phase that low-purity and quiet moment are low becomes, and the quiet moment of material monolithic does not improve.

In addition, need the hard second mutually fine and homodisperse, what represent this demand is formula (32) and formula (33).Formula (32) shows that the standard deviation of the area fraction of measuring in the so very little scope of the square square lattice of 3 μ m is little, and promptly the dispersion state of second phase is even, and (33) formula shows that the ratio of the second phase particle diameter and the size of ferrite grain is less than certain value.

Herein, in the tissue of high tensile steel plate of the present invention, hard second be meant mutually with the ferrite equilibrated mutually and this equilibrium phase phase transformation formed tissue, the further tissue that forms because of annealing changes of these tissues have taken place in process of cooling.Specifically, hard second is any more than a kind in cementite, martensite, bainite, the retained austenite mutually.Cementite is to exist mutually with the ferrite balance in the steel, and martensite, bainite, retained austenite are the phase-change organizations of equilibrium phase.Herein, retained austenite is meant that only the austenite that at high temperature exists as equilibrium phase has remained into the austenite of the not phase transformation of room temperature, though in fact do not undergo phase transition, but retained austenite is meant the tissue that austenite obtains through cooling and in room temperature, so it is included in the phase-change organization.In addition, perlite is not suitable as hard second phase in the metal structure of high tensile steel plate of the present invention.This be because, second is that pearlitic situation is limited to annealing temperature after cold rolling in ferrite and austenite two phase regions or austenite one phase territory mutually, and the situation that speed of cooling thereafter is slow, but in this case, the grain growth of ferrite crystal grain can not be suppressed, nanocrystal can not be contained as the area fraction of the regulation of the feature of high tensile steel plate of the present invention.

Except these phases, tissue, the tissue that also has tempering bainite, tempered martensite, troostite, sorbite and make the balling of cementite part by pearlitic annealing., these tissues are treated as any one the tissue in mutually that is included in hard second phase that exemplifies concrete title.Need explanation,, in the 1st embodiment, be illustrated for above-mentioned tempering bainite, tempered martensite etc.

In addition, troostite is the term that not too uses now, and in JIS G0201 iron and steel term (thermal treatment), troostite is classified into tempering troostite and quenched troostite.Tempering troostite is the tissue that produces when martenaging martempering, and it is the tissue that is made of fine ferrite and cementite, but actual be tempered martensite.In addition, quenched troostite is the fine pearlite tissue that produces when quenching, and treats as perlite in the lump in the present invention.

In addition, sorbite also not too uses now, and in JIS G0201 iron and steel term (thermal treatment), sorbite is classified into tempered sorbite and quenched sorbite.Tempered sorbite is the cementite and the ferritic mixed structure of separating out growth after martenaging martempering in pelletized form, but is actually tempered martensite.Quenched sorbite is the fine pearlite tissue that generates when quenching, and treats as perlite in the lump in the present invention.

Need explanation, making organizing of cementite part balling by pearlitic annealing also is the mixed structure of ferrite and cementite, and in other words, hard second is cementite mutually.

Then, the ferrite beyond hard second phase is partly described.The tissue of ferrite part is the nanocrystal that varies in size and the mixed structure of micron crystal grain.Therefore, intensity is lower during the press forging moulding, and is excellent on the balance of intensity and ductility, on the other hand, after making goods, the excellent intensity of performance during high speed distortion when collision etc.Therefore, adopt the tissue of this ferrite part, can on high level, obtain formability and shock absorption energy simultaneously.

Need explanation, set forth the application below the size of microcrystal of nanocrystal is defined as reason below the 1.2 μ m.That is, this be because, for example " iron and steel " (Fig. 6 (b) that Japanese iron steel association, the 88th volume the 7th phase (2002) are 365 pages) are disclosed like that, are the boundary with the scope of the about 1.2 μ m of ferrite crystal grain particle diameter, discontinuous variation takes place in material behavior particularly ductility.Specifically, the ferrite crystal grain particle diameter is during less than 1.2 μ m, and percentage of total elongation sharply reduces, and can not show uniform elongation.

In addition, by high tensile steel plate of the present invention being implemented the processing of dependent variable 1%～10%, can improve the ductility when being out of shape at a high speed., method for processing is not particularly limited, common cold-rolled steel sheet manufacturing process comprises skin rolling herein, thus the simplest be exactly to utilize this point, implement skin rolling with the elongation of above-mentioned scope.Certainly can also be other method, for example give tensile and carry out method for processing with levelling arm simultaneously; Cut into tabular back and give the method for strain etc. with stretching.

Below, be the importance of the ductility of concrete example illustrative material, particularly uniform elongation with the vehicle frame class in cap shape cross section.At first, compare with the size in cross section, the broken box parts such as (Network ラ Star シユボ Star Network ス) of part length weak point are because of the reason of its shape, and the buckling during Shock Compression is stable, so regardless of the work hardening characteristic of material, it can have good impact energy receptivity.But, for comparing with the size in cross section, loading direction during the long or collision of part length and parts axially may not be consistent parts such as bogie side frame, the instability that becomes easily of the buckling during Shock Compression is necessary aspect material behavior it to be remedied.That is, the unsettled parts of preferred buckling originally use the material of the big characteristic of the uniform elongation that has when being out of shape at a high speed, the material that promptly has the big characteristic of work hardening.High tensile steel plate of the present invention satisfies such characteristic requirement, so by implement the processing of the dependent variable of specialized range in cold rolled annealed back, can improve the work hardening uniform elongation when being out of shape at a high speed.

More than be the qualification reason of the related various regulation reason of high tensile steel plate of the present invention, chemical ingredients and about the qualification reason of tissue, below, the mechanism about the action effect of high tensile steel plate of the present invention is elaborated.

＜about the mechanism 1 of the action effect of high tensile steel plate of the present invention 〉

By ferrite being made the mixed structure of nanocrystal and micron crystal grain, the mechanism that can give high quiet moment is as follows.That is, high tensile steel plate of the present invention is to be that the nanocrystal of size of microcrystal below 1.2 μ m and the size of microcrystal with common intensity surpass a kind of complex tissue steel plate that the micron crystal grain of 1.2 μ m constitutes by the very high part of intensity.The static deformation behavior of high tensile steel plate of the present invention is identical with the deformational behavior of general complex tissue steel plate, during static deformation, at first begin distortion from the most yielding part of material, specifically, inner or micron is intragranular begins distortion with near interface nanocrystal from micron crystal grain.Then, distortion is slowly carried out, and micron crystal grain is the main body of distortion.Therefore, be out of shape with stress, and the balance quality of intensity and ductility is also general with the situation equal extent that micron crystal grain is only arranged.

On the other hand, be under the situation of the high speed distortion about 1000/s at rate of straining, different with the behavior of general steel plate.The distortion that it is main body that when Deformation velocity is about static deformation 100,000 times, this speed only depend on soft micron crystal grain is difficult to catch up with.Therefore, not only the micron crystal grain, distortion also must be born in the inside of nanocrystal.Therefore, the influence of the nanocrystal that intensity is very high becomes significantly, thereby needs high deformation stress.

This phenomenon can be in the ratio of nanocrystal in 15%～90% the scope and occur.When the ratio less than 15% of nanocrystal, the influence of nanocrystal is little, and in static deformation or dynamic deformation, soft micron crystal grain can both fully be shared distortion, thereby quiet moment is not high.On the other hand, surpass at 90% o'clock in the ratio of nanocrystal owing to be nanocrystal nearly all, therefore, when static deformation the influence of nanocrystal very remarkable, cause the intensity height but ductility is low, therefore, be unsuitable for the press forging moulding.So,, all can not obtain excellent high speed deformation intensity and impact energy receptivity and excellent processibility simultaneously in the ratio less than 15% of nanocrystal with above 90% o'clock.

More than high tensile steel plate of the present invention is illustrated, below, the method that is suitable for making above-mentioned high tensile steel plate is described.Need explanation, except hot rolling and annealing operation, the 2nd embodiment of the manufacture method of high tensile steel plate of the present invention is identical with the 1st embodiment, therefore only these hot rollings and annealing operation is described below.

＜hot rolling 〉

In the 2nd embodiment of the manufacture method of high tensile steel plate of the present invention, crystal structure after the hot rolling need be made with the ferrite is principal phase, contain the complex tissue of area fraction, and will be controlled at 2.5 μ m～5 μ m in the equispaced of hard second phase of thickness of slab direction detection in hard second phase of 10%～85% scope.Thereafter, as described later, desired rolling rate with the interval of corresponding hard second phase is implemented cold rolling, by annealing with temperature, the time that can suppress grain growing, to obtain above-mentioned mixed structure with micron crystal grain and nanocrystal thus be parent phase and contain hard second high-strength plate mutually.

Identical in hard second in the 2nd embodiment and the 1st embodiment, its definition, formation etc. have been illustrated in the 1st embodiment, so following to being that the regulation reason of hard second area fraction mutually in the hot-rolled sheet describes with the 1st embodiment difference.The area fraction of hard second phase is less than 10% o'clock or greater than 85% o'clock, can not obtain the nanocrystal of q.s after the cold rolling and annealing.

Herein, the area fraction of hard second phase is more than 10% and less than 30% o'clock in lower scope, needs big rolling rate, is difficult to common cold rolling being rolled.But, when carrying out above-mentioned accumulation ply rolling, can implement big rolling rate, and, compare when being material with the single-phase steel of ferrite, can realize the ultra micro refinement of tissue with few circulation road number of times.The area fraction of hard second phase is 30%～85% o'clock, can implement with the cold rolling rate below 80% (this cold rolling rate utilization common cold rolling also can fully implement) rolling, so the area fraction of preferred hard second phase is 30%～85%.

＜annealing 〉

Then annealing is described.Annealing is the material after cold rolling to be heat-treated to eliminate process strain, and produces the operation of purpose metal structure simultaneously.Annealing by the material after cold rolling is heated, insulation, refrigerative process constitute, holding temperature Ts (℃) and satisfy following (46) formula in the relation of the time ts (second) of Ts insulation.

650-(ts) ^1/2<Ts<750-(ts) ^1/2 (46)

(ts: soaking time (second), Ts: holding temperature (℃), (ts) ^1/2Be the square root of ts)

Fig. 5 is the graphic representation of the proper range of above-mentioned holding temperature of explanation and soaking time.Annealing temperature Ts is higher than 750-(ts) ^1/2The time, the area fraction of nanocrystal is greater than higher limit 90%, so be not preferred.On the other hand, annealing temperature Ts is lower than 650-(ts) ^1/2The time, the area fraction of nanocrystal is lower than lower value 15%, so be not preferred.

For hard second phase in the metal structure after the annealing, can corresponding annealing characteristic curve and obtain various hard second phases.Fig. 4 is the figure of the various annealing characteristic curves of explanation.In figure, rational curve the 1, the 2nd, the situation of CAL (continuous annealing line), rational curve 3 is situations of CGL (molten zinc plating line), rational curve 4 is situations of pack annealing.In addition, table 3 guide look shows the tissue that obtains with each annealing characteristic curve shown in Figure 4.

Table 3

M: martensite, B: bainite, A: retained austenite, C: cementite

At first annealing temperature is described.Annealing temperature Ts is set in the Ac1 transformation temperature when following, can accesses the complex tissue that constitutes by ferrite and cementite.In addition, annealing temperature Ts and chilling are begun temperature T _QBe set in the Ac1 transformation temperature when above, can make the mixed structure that constitutes by as 1 at least a (hard second mutually) among the annealed structure after the ferrite of matrix and austenitic phase-change organization or the annealing of this phase-change organization.

Herein, austenitic phase-change organization is meant martensite, bainite and retained austenite.Herein, in fact retained austenite does not undergo phase transition, but retained austenite is meant the tissue that austenite obtains through cooling and in room temperature, so it is included in the phase-change organization.In addition, the annealed structure after phase-change organization's annealing is the annealed structure of above-mentioned phase-change organization, but as explained above, it is treated as the tissue that is included in above-mentioned certain phase-change organization.

Illustrate that under the situation that heat-up rate is big, soaking time is short, annealing temperature Ts and chilling begin temperature T _QEven more than the Ac1 transformation temperature, the C in the steel is also not enough to austenitic enrichment, has the possibility of residual oversaturated C in the ferrite, and this residual oversaturated C separates out as cementite when cooling sometimes.So, in this case, form the mixed structure that constitutes by as at least a (hard second is mutually) among the annealed structure after the ferrite of matrix and austenitic phase-change organization or the annealing of these phase-change organizations sometimes, and in ferrite, contain cementite.

In addition, the Ac1 transformation temperature depends on the composition and the rate of heating of material, but among the present invention, they are probably between 600 ℃～750 ℃.

Below the method for cooling after the annealing is described.Cooling can adopt any means in the following method to carry out: the method for using gas, use the method for jet of water or make the gentle body of water mixing jetting method or in tank, quench (WQ) or carry out contact cooling with roller.Arbitrary gas in mixed gas, helium or the argon gas that alleged herein gas is air, nitrogen, hydrogen, nitrogen and hydrogen.

In above-mentioned process of cooling, if speed of cooling is slow excessively, then can not ignore the generation of ferrite crystal grain, its result causes the area fraction of nanocrystal to reduce, therefore, with the speed of cooling of plate temperature in the scope more than 600 ℃ be decided to be 10 ℃/more than the s.The limiting plate temperature is because when 600 ℃ of plate temperature less thaies, the growth of crystal grain is very slow, therefore, can ignore the influence of speed of cooling basically in the reason of the scope more than 600 ℃ only.

Then, describe,, can use 4 specific character curves shown in Figure 4 according to the formation of annealed wire for cooled annealing characteristic curve.After annealing zone, in the line of formation, can adopt near specified temperature, to stop cooling with cooling zone and overaging band subsequently, carry out the rational curve 1 that overaging is handled.In addition, rational curve 3 is the rational curves that are equivalent to CGL (molten zinc plating line), and its refrigerative terminal temperature is limited at the temperature that fused zinc is bathed, and is in addition, identical with rational curve 1.

Just as already described, annealing temperature Ts is when the Ac1 transformation temperature is following, and the hard that obtains second only is cementite mutually, therefore, regards to annealing temperature Ts down and chilling begins temperature T _QSituation more than the Ac1 transformation temperature is elaborated.If speed of cooling soon, ferrite transformation nose or bainitic transformation nose in the not crosscut CCT curve just be cooled to below the Ms point, then obtain martensite as hard second phase.In rational curve 1 with overaging band and rational curve 3, say that strictly martensite becomes tempered martensite.But as mentioned above, tempered martensite is owing to still keep high dislocation density, thereby has high rigidity and go far towards the reinforcement of steel, and therefore, in the present invention, tempered martensite and martensite are not treated with a certain discrimination.

In addition, cooling off with the such speed of cooling of crosscut bainitic transformation nose, and cooling termination temperature is decided to be the Ms point when following, hard second is the complex tissue for being made of martensite and bainite mutually, in rational curve 1 with overaging band and rational curve 3, if just stop cooling and directly carry out overaging and handle in the temperature of ordering shortly past Ms, the mixed structure that then forms bainite or form retained austenite and bainite as hard second mutually.Whether generate the austenitic stability that retained austenite depends on when annealing.That is,, perhaps promote C to the austenite enrichment, austenite is stablized, obtain retained austenite thus by the prolongation overaging treatment time by increasing the amount of alloying element (Si, Al).

In addition, during, crosscut pearlitic transformation nose slow, also contain perlite in mutually at hard second in speed of cooling.But perlite is not suitable as the tissue of high tensile steel plate of the present invention.This be because, can generate under the pearlitic so slow speed of cooling, produced growing of ferrite grain simultaneously, so be difficult to retain a large amount of nanocrystals in the ferritic structure, can not show high quiet moment.

Make a concrete analysis of respectively, for rational curve 1, annealing temperature Ts and chilling begin temperature T _QBe Ac1 transformation temperature when above, hard second is at least a among martensite, bainite, the retained austenite mutually.Annealing temperature Ts is during less than the Ac1 transformation temperature, and hard second only is a cementite mutually.

Then, shown in annealing characteristic curve 2,, after the annealing, be cooled to finish below 100 ℃ for the line of the formation that does not have the overaging band.In this case, annealing temperature Ts and chilling begin temperature T _QWhen the Ac1 transformation temperature was above, hard second was at least a in martensite, the bainite mutually.When annealing temperature Ts was lower than the Ac1 transformation temperature, hard second only was a cementite mutually.

Then, annealing characteristic curve 3 is the annealed rational curves that are equivalent to CGL (molten zinc plating line).Carry out chilling from annealing temperature, make zinc be attached to the surface with the fused zinc bath.Thereafter, as shown in the figure, can reheat, make the zinc coating alloying, also can omit reheat, do not make the zinc coating alloying.Carry out under the situation of reheat, the kind of hard second phase that obtains is identical with rational curve 1, does not carry out under the situation of reheat, and the kind of hard second phase that obtains is identical with rational curve 2.

At last, annealing characteristic curve 4 is pack annealing.After annealing in pack annealing finishes, from body of heater, take out the roll coil of strip, force cooling can satisfy the above speed of cooling condition of 10 ℃/s, in this case, to annealing temperature without limits, but after annealing finishes, from body of heater, do not take out the roll coil of strip usually, but in stove, cool off, therefore not satisfying speed of cooling is the above condition of 10 ℃/s, is lower than 600 ℃ so be necessary annealing temperature is defined as.Thereafter, continuing to implement with the skin rolling with the dependent variable of 1%～10% scope is the processing of representative.

＜about the mechanism 2 of the action effect of high tensile steel plate of the present invention 〉

Below further set forth by the common cold rolling mechanism that obtains the tissue of nanocrystal.

At first, the accumulation ply rolling for the conduct trial of describing in beginning all the time describes.The accumulation ply rolling gives tabular sample with bigger strain, is the effective ways that obtain having the tissue of nanocrystal.For example, in Japanese plastic working association will (the 40th volume, the 467th phase, 1190 pages), provided the example of aluminium.Under the situation about being rolled after Rolling roller is lubricated, have to put in place, and under the situation of lubrication and rolling roller not, obtain nanocrystal to the little subgrain tissue of difference.

The reason that this phenomenon produces be because, by unlubricated generation shearing strain the time, can import than the lubricated bigger strain of situation, and by circulative accumulation ply rolling repeatedly, make that the part as the top layer becomes material internal in the last circulation, shear strain as a result is directed to material internal.That is, if in accumulation ply rolling process,, then can not realize the ultra micro refinement of crystal grain by carrying out the unlubricated rolling material internal that can not give with bigger shear strain.

Even the contriver also can study with the method for shear strain by the rolling material internal of giving of common oil lubrication for not carrying out the low accumulation ply rolling of production efficiency or applying the unlubricated rolling of bigger load to Rolling roller.The result recognizes: as long as the tissue before rolling is made the complex tissue of soft section and hard part.That is, cold rolling by the steel plate of the complex tissue that is made of mutually soft ferritic and hard second is implemented, be clipped in the ferrite zone of hard second between mutually and shearing strain take place owing to the constraint of hard second phase.Like this, shear strain can be imported in the scope of material internal broad.

In addition, the contriver obtains following understanding on the basis that studies in great detail: even the hard before rolling second is different separately, if the hard second that is rolled after rolling reaches certain value separately, then shearing strain can similarly import to material internal.That is, shown in equispaced second phase of the hard in the tissue after the hot rolling be made as d (μ m), with hot rolling after the thickness of slab of (cold rolling before) be made as t ₀, when the thickness of slab after cold rolling is made as t, carries out the degree of finish index D and satisfy such the cold rolling of condition of following (47) formula and get final product.

D＝d×t/t ₀≦1 (47)

Last reason is to utilize processing that the uniform elongation when being out of shape is at a high speed increased, but it is believed that what bring effect is that the dislocation desity of ferrite grain is adjusted to more than the certain value.The inventor tests the cold-rolled steel sheet of above-mentioned manufacture method manufacturing, has changed the rolling rate of skin rolling in experiment, utilizes its inside of film tem observation, has observed dislocation structure thus in detail.In the tem observation, cut the cross section parallel of steel plate, make film, utilize TEM to take bright-field image,, measure the dislocation number in this circle, measure dislocation desity by this method at this photo upper drawing circle by mechanical mill and electropolishing with rolling direction.

Consequently, under the state after cold rolled annealed, the dislocation desity of the ferrite intragranular in the high tensile steel plate of the present invention is very low, and only 10 ¹³/ m ²About.And when carrying out high-speed stretch, after the initial stage showed big deformation intensity, load reduced, and ruptures, and does not demonstrate tangible work hardening.After the high-speed stretch, when observing dislocation with identical method, have dislocation though find the ferrite intragranular, its distribution is more irregular.Above-mentioned iron steel association compiles " about the high speed The deformation meeting achievement Report book of materials used in auto manufacturing " 171 pages and has provided the dislocation structure of IF steel after being out of shape at a high speed, even it does not present tangible dislocation modular construction yet under 20% strain, be identical therewith phenomenon.That is to say that owing to be difficult to work hardening, so work hardening is not obvious, thereby uniform elongation is little, causes the percentage of total elongation value difference.

On the other hand, when skin rolling more than 1% was implemented in cold rolled annealed back, dislocation desity was increased to about 5 times～30 times, has found the blocking of dislocation under 5% skin rolling.It has been carried out after the high-speed stretch, and on the dislocation basis that utilizes skin rolling to introduce, dislocation desity increases, and it is more obvious that modular construction becomes.That is, think when having introduced dislocation to a certain degree by skin rolling that its initial stage dislocation also becomes effective dislocation source when being out of shape at a high speed, promoted the growth of dislocation, forms the dislocation modular construction easily, work hardening is that uniform elongation is improved.In fact, variation occurred in the stress-strain(ed) curve under the high-speed stretch, demonstrated work hardening after the surrender, uniform elongation is improved.But when carrying out elongation greater than 10% skin rolling, during static deformation, the ductility of material obviously reduces, and therefore, the ductility when being out of shape at a high speed also reduces on the contrary.Based on this understanding, the present invention is defined in 1%～10% as obviously showing effect that uniform elongation improves and the condition that does not make the extreme deterioration of ductility of material with the proper range of dependent variable.

Embodiment

1. the 1st embodiment

The slab of the chemical constitution shown in the melting table 4 (invention slab 1～8 and comparison slab 1～9).Then, use these slabs, make hot-rolled sheet, then, carry out cold rolling and annealing, obtain having the steel plate of the annealed structure shown in the table 7 etc. with each condition shown in the table 6 with each condition shown in the table 5.Then, become the mode of tensile axis with the direction parallel with rolling direction, the tensile test specimen that downcuts shape shown in Figure 14 from each steel plate is to carry out tension test.From the nominal stress nominal strain graphic representation that obtains, try to achieve yield-point, tensile strength, baking hardening amount (BH amount) and percentage of total elongation, tensile strength calculated * elongation is long-pending simultaneously.These results are recorded in the table 5 in the lump.Need explanation, comparative example 4 is identical with comparative example 3, and mechanical workout is carried out on the two sides of hot-rolled sheet, is machined to thickness of slab 1mm, carries out 3 circulation passages accumulation ply rolling (ARB), and total rolling rate is equivalent to 88%.In this case, the nanocrystal ratio height after common cold rolling of the nanocrystal ratio after cold rolled annealed shows good characteristic.This second phase area mark that has shown hot-rolled sheet also can obtain the purpose tissue by ARB to about 10% less.

Table 5

P: perlite

C: cementite

M: martensite

B: bainite

A: retained austenite

Table 6

Figure DEST_PATH_G47349482150138000D000011

WQ: water quenching

Table 7

P: perlite

C: cementite

M: martensite

B: bainite

A: retained austenite

Cut out the cross section parallel from the steel plate of example 11, it with 1% nital etching, is utilized the SEM tissues observed with rolling direction.This tissue is seen Fig. 6.Tissue shown in Figure 6 is a ferrite (black part) and the hard that is made of retained austenite and the bainite two-phase structure of (white part) mutually, is nanocrystal mostly in the ferritic phase.In addition, measure the size-grade distribution of the ferritic phase of example 11, it the results are shown in Figure 7.

The tensile strength in example and the comparative example and the relation of elongation have been provided among Fig. 8.As shown in Figure 8, compare with comparative example, the intensity of example and the balance of ductility are good.3 points (comparative example 16,17,18) identical with the invention level among the comparative example are the steel plates that utilizes prior art to draw, but it is to have realized the good intensity and the balance of ductility by adding the alloying element of Duoing than example of the present invention.

Comparative example 16 is to use C amount is 0.16% steel, makes by existing method (patent documentation 1), and it has the tensile strength of 800MPa level, but, example 1 shows, according to the present invention, is that about 0.1% material just can obtain equal intensity and ductility with the C amount.

Comparative example 17,18 is to use C amount is 0.36% steel, makes by existing method (patent documentation 1), and it has the tensile strength of 1100MPa level, but, example 3,5 shows, according to the wood invention, is that 0.2% or 0.15% material just can obtain equal intensity and ductility with the C amount.

Example 7,8,9,10,11 shows, by method of the present invention, can make the steel plate that has the elongation more than the above tensile strength of 1300MPa and 15% concurrently that prior art can't be made.

C measures when high, solder joint hardness significantly rises during spot welding, notch sensitivity increases, even steel is carried out high strength, compare with the rising ratio of TSS (tensile shear strength), CTS (cross tensile strength) rises fewly, carries out under the situation of thin-walled property so use high tensile steel plate, needs to increase welding and gets ready.It is the example 1,5,11 of 1mm and the spot welding intensity of comparative example 16,17,18 that table 8 has provided thickness of slab.

Table 8

?	TS?MPa?	Thickness of slab mm	Spot size mm	Tensile shear strength TSS kN	Cross tensile strength CTS kN	CTS/TSS?
							Example 1	788	1	6.2	16	8	0.50
Example 5	1122	1	6.4	21.5	9.2	0.43
							Example 11	1539	1	6.3	26.3	9.1	0.35
Comparative example 16	806	1	6.1	16.1	7.1	0.44
							Comparative example 17	1098	1	6.2	20.7	7.5	0.36
Comparative example 18	1124	1	6.3	21.3	7.7	0.36

Using the D type electrode (footpath 6mm) of JISC 9304 in welding, is that the condition in 18 cycles is implemented with pressurization 350kgf, energising cycle, measures the spot size in cross sections according to JISZ 3139.In addition, TSS and CTS measure according to JISZ 3136.3137.Compare with the comparative example with intensity, example is because the C amount is low, so the CTS height has shown good spot welding intensity.But shown in example 11, if the C amount also is 0.4%, then TSS rises with the rising of tensile strength, but CTS does not rise substantially.Promptly, spot welding still is difficult to improve the cross tensile strength, so in this case, engage by adopting the little docking laser welding of notch geometry or flushing friction stir, or adopt the methods such as rivet combination, ca(u)lk joint, point-like friction stir joint though notch geometry is arranged but be difficult for producing quenching structure, can guarantee good bond strength.

Need explanation, steel plate of the present invention is except being used as cold-rolled steel sheet, also can utilize above-mentioned CGL (molten zinc plating line) to implement molten zinc plating or hot dip alloyed zinc-plated, perhaps utilize and electroplate zinc wire enforcement electro-galvanizing, perhaps further handle after improving erosion resistance and use at enforcement organic envelope on the coating.

2. the 2nd embodiment

The slab of the chemical constitution of the composition shown in the melting table 9 (invention slab 9～19 and comparison slab 10～14).Need explanation, " invention slab " is meant the material that belongs to above-mentioned preferable range, and " relatively slab " is meant the material that does not belong to above-mentioned preferable range.

Next, use these slabs, make hot-rolled sheet with each condition shown in the table 10, then, carry out cold rolling, annealing and skin rolling, the steel plate (example 12～29 and comparative example 23～41) that obtains having annealed structure shown in the table 12 etc. with each condition shown in the table 11.Need explanation, the elongation of the skin rolling in table 6 and the table 11 is following calculating: draw 2 at interval 200mm and the vertical line of rolling direction steel plate in advance, carry out skin rolling then, calculate the elongation of described skin rolling with (line after rolling at interval)-(line before rolling at interval)/(the line interval before rolling).For example 12, mechanical workout is carried out on the two sides of hot-rolled sheet, being machined to thickness of slab is 1.2mm, carries out the accumulation ply rolling of 3 circulation passages.The total rolling rate of this accumulation ply rolling is equivalent to 88%.

Table 10

P: perlite

C: cementite

M: martensite

B: bainite

A: retained austenite

Table 11

Figure DEST_PATH_G47349482150138000D000021

WQ: water quenching

Table 12

P: perlite

C: cementite

M: martensite

B: bainite

A: retained austenite

And then cut out the cross section parallel with rolling direction from the steel plate of example 18, it with 1% nital etching, is utilized the SEM tissues observed.This tissue is seen Fig. 9.In addition, measure the size-grade distribution of the ferritic phase of example 18, it the results are shown in Figure 10.

By Fig. 9 and Figure 10 as can be known, contain cementite as hard second phase, remaining is the mixed structure that is made of nanocrystal and micron crystal grain.

In addition, the tensile test specimen that downcuts shape shown in Figure 14 from each steel plate makes that the direction parallel with rolling direction is tensile axis, carries out tension test.Tension test is to adopt the aigret palace to make the high speed material-testing machine TS-2000 of institute and with the rate of straining enforcement of 0.01/s and 1000/s.From the nominal stress nominal strain graphic representation that obtains, try to achieve static tensile strength (TS), baking hardenability (BH), percentage of total elongation and quiet moment.Need to prove the following calculating of quiet moment: the nominal strain under the rate of straining 1000/s is that to deduct nominal strain under the rate of straining 0.01/s be that 3%～5% average nominal stress is calculated described quiet moment for 3%～5% average nominal stress.These results are recorded in the table 12 in the lump.

＜to the investigation of example 12～29 〉

For example 12～29, each steel plate all demonstrates each excellent material behavior as can be known, and particularly quiet moment is bigger.Therefore, for the steel plate of each example,, therefore can be used for the car body of automobile etc. owing to can have higher high speed deformation intensity and impact energy absorption characteristic and high working property concurrently.In the example 12, the area fraction of hard second phase of hot-rolled sheet is 12%, near lower value, but owing to carried out the accumulation ply rolling, so the nanocrystal ratio after cold rolled annealed becomes greatly, has shown good characteristic.This result shows, even the area fraction of the hard of hot-rolled sheet second phase is low to about 10%, also can obtain the purpose tissue by the accumulation ply rolling.

＜to the investigation of comparative example 23～41 〉

Relative therewith, the quiet moment of each steel plate of comparative example 27～41 is all less as can be known.Therefore, for the steel plate of these comparative examples, can not have higher high speed deformation intensity and impact energy absorption characteristic and high working property concurrently, therefore, not be preferred for the car body of automobile etc.Need explanation, for comparative example 23 etc., although obtained the above quiet moment of 170MPa, other characteristics such as elongation are than example difference.Particularly comparative example 23, owing to do not implement skin rolling, so though quiet moment height, the percentage of total elongation when being out of shape at a high speed is low.

＜about distortion of the present invention 〉

Except the manufacture method shown in above, can obtain hot-dip galvanized steel sheet and alloy galvanized steel plate by the actual coating that adheres to when the annealing among the present invention.In addition, in order to improve erosion resistance, after having implemented molten zinc plating, can also further adopt plating line to carry out plating iron.And then, after steel of the present invention is implemented annealing, adopt plating line that plating is implemented on the surface, thus, can obtain electrogalvanizing steel plate or alloying (Ni-Zn) electrogalvanizing steel plate.In addition, in order to improve erosion resistance, can also implement organic envelope and handle.

Need explanation,, in table 13, provided the characteristic of commercially available material for relatively.According to table 13 as can be known, compare with each example of table 12, the quiet moment of each commercially available material 1～5 is all little.So, can confirm, to compare with existing commercially available material, the steel plate of each example has had high speed deformation intensity and impact energy absorption characteristic and processibility concurrently with higher level.

Table 13

F: ferrite C: cementite M: martensite

Figure 11 is the elongation of explanation under the skin rolling and the figure of the relation of percentage of total elongation, and table 14 has been summed up example shown in Figure 11.Along with the increase of the elongation of skin rolling, the percentage of total elongation during static deformation reduces, and is about 5% but the percentage of total elongation when being out of shape at a high speed is increased to elongation.This shows that the absorption energy when being out of shape at a high speed increases.Absorption energy when being out of shape at a high speed is 1%～10% o'clock height at elongation, so this scope is suitable.

Table 14

Figure 12 has provided the stress-strain curve that has carried out the material of skin rolling and do not carried out the material of skin rolling.As shown in Figure 12, by carrying out skin rolling, yield-point reduces, and distortion has just taken place under more weak stress.Figure 13 is the elongation of explanation when being out of shape at a high speed and the figure of the relation of quiet moment.As shown in Figure 13, the elongation of example and quiet moment are all than commercially available material, comparative example excellence.

Utilizability on the industry

According to the present invention, a kind of high-strength steel sheet and manufacture method thereof are provided, wherein said high-strength steel sheet has been owing to the miniaturization of ferrite crystal grain has improved intensity, and the critical nature intensity during its press forging moulding and the balance excellence of ductility, the absorbed energy excellence when being out of shape at a high speed. In addition, also provide a kind of high-strength steel sheet according to the present invention, this high-strength steel sheet is not only because the miniaturization of ferrite crystal grain has improved intensity, and because quiet moment height, so the press forging moulding is easy, and its ductility excellence when being out of shape at a high speed, absorbed energy is big. Therefore, the present invention is expected to be used for to have the automobile body of the characteristic of demanding high speed deformation intensity and impact energy absorption characteristic and high processability.

Claims

1. high tensile steel plate, it is characterized in that, this steel plate presents the metal structure that is made of mutually ferritic phase and dispersive hard second, described hard second is martensite mutually, mutually at least a in bainite and the residual austenite, at the second mutually shared area fraction of hard described in the described metal structure is 30%～70%, size of microcrystal is that the shared area fraction of ferrite below the 1.2 μ m is 15%～90% in described ferritic phase, and, in described ferritic phase, size of microcrystal is that the ferritic median size dL that following ferritic median size ds of 1.2 μ m and size of microcrystal surpass 1.2 μ m satisfies following (1) formula:

dL/ds≥3 (1)。

2. high tensile steel plate according to claim 1, it is characterized in that, on the cross section parallel of described steel plate with rolling direction, select the square square lattice of 3 μ m more than 9 arbitrarily, in this case, with the area fraction of described hard second phase in each grid be made as Ai and i=1,2,3 ... the time, the mean value A (ave) of described Ai and standard deviation s satisfy following (3) formula:

s/A(ave)≤0.6 (3)。

3. high tensile steel plate according to claim 1, it is characterized in that, in described steel plate, contain C, and contain at least a element among Si, Mn, Cr, Mo, Ni and the B, C (ss) be meant from total C amount, deduct with Nb, Ti and V bonded C amount after the solid solution carbon amount that obtains, with following (4) formula～following (7) formula is prerequisite, and described C (ss) satisfies following (8) formula～(10) formula

F ₁(Q)＝0.65Si+3.1Mn+2Cr+2.3Mo+0.3Ni+2000B (4)

F ₂(T)＝735+19Si-31Mn-12Ni+17Mo+20Cr+30V-800N (5)

F ₃(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (6)

F ₄(G)＝620+300C+0.5×F ₃(S) (7)

F ₁(Q)≥6.0 (8)

F ₂(T)≤F ₄(G)-20 (9)

0.07≤C(ss)≤0.45 (10)

Wherein, in (4) formula, (5) formula, (6) formula and (7) formula, each adds element is that described composition is represented with quality % with the composition substitution of this interpolation element.

4. high tensile steel plate according to claim 3 is characterized in that, is prerequisite with following (11) formula, (12) formula, and the composition that contains of described steel plate satisfies following (13) formula and (14) formula,

F ₃(S)＝112Si+98Mn+218P+317Al+9Cr+56Mo+8Ni+1417B (11)

F ₅(P)＝500×Nb+1000×Ti+250×V (12)

F ₃(S)≤600 (13)

F ₅(P)≤130 (14)

Wherein, in (11) formula, (12) formula, each adds element is that described composition is represented with quality % with the composition substitution of this interpolation element.

5. high tensile steel plate according to claim 3 is characterized in that, in quality %, contains the Nb below 0.26%, at least a composition among the V below the Ti below 0.13% and 0.52% in described steel plate.

6. high tensile steel plate according to claim 4 is characterized in that, in quality %, contains at least a composition among the Al below the P and 18% below 2% in described steel plate.

7. high tensile steel plate according to claim 3 is characterized in that, in quality %, contains 0.007%～0.03% N in described steel plate.

8. high tensile steel plate according to claim 3 is characterized in that, in quality %, in described steel plate, Si is below 5%, Mn is below 5%, Cr is below 1.5%, Mo is below 0.7%, Ni is below 10% and B is below 0.003%.

9. the manufacture method of a high tensile steel plate, this manufacture method is to make the method for the described high tensile steel plate of claim 1, it is characterized in that, is the hot-rolled steel sheet that 30%～85% hard second constitutes mutually to metal structure by ferritic phase and area fraction, carrying out with following (15) formula is that the degree of finish index D of prerequisite satisfies the cold rolling of following (16) formula, then, satisfy the annealing of following (17) formula and (18) formula

D＝d×t/t ₀ (15)

(15) d in the formula represents the equispaced of described hard second phase, and unit is μ m, and t represents the thickness of slab after cold rolling, t ₀Cold rolling preceding thickness of slab after the expression hot rolling,

0.50≤D≤1.0 (16)

F ₂(T)+20≤Ts≤F ₂(T)+90 (17)

F ₂(T)+20≤Ts≤F ₄(G)-1.3(ts) ^1/2 (18)

Ts in the following formula represents soaking time, and unit is second, and Ts represents holding temperature, and unit is ℃, (ts) ^1/2It is the square root of ts.

10. the manufacture method of high tensile steel plate according to claim 9 is characterized in that, the equispaced of described hard second phase on the thickness of slab direction of described hot-rolled steel sheet is 2.5 μ m～5 μ m.

11. the manufacture method of a high tensile steel plate, this manufacture method is to make the method for the described high tensile steel plate of claim 1, it is characterized in that, is that 10%～85% hard second constitutes mutually and the equispaced of described hard second between mutually is the hot-rolled steel sheet of 2.5 μ m～5.0 μ m to metal structure by ferritic phase and area fraction, carrying out with following (19) formula is that the degree of finish index D of prerequisite satisfies the cold rolling of following (20) formula, then, satisfy the annealing of following (21) formula

D＝d×t/t ₀ (19)

(19) d in the formula represents the equispaced of described hard second phase, and unit is μ m, and t represents the thickness of slab after cold rolling, t ₀Cold rolling preceding thickness of slab after the expression hot rolling,

0.50≤D≤1.0 (20)

650-(ts) ^1/2＜Ts＜750-(ts) ^1/2 (21)

(21) ts in the formula represents soaking time, and unit is second, and Ts represents holding temperature, and unit is ℃, (ts) ^1/2It is the square root of ts.

12. the manufacture method of high tensile steel plate according to claim 11 is characterized in that, described cold rolling be the accumulation ply rolling of carrying out repeatedly after rolling superimposed rolling this process of material.

13. the manufacture method of high tensile steel plate according to claim 11 is characterized in that, the area fraction of described hard second phase in the described hot-rolled steel sheet is 30%～85%.

14. the manufacture method of high tensile steel plate according to claim 11 is characterized in that, carries out elongation and be 1%～10% processing after described annealing.

15. the manufacture method of high tensile steel plate according to claim 14 is characterized in that, described processing is skin rolling.