CN103201403A

CN103201403A - High-strength cold-rolled steel sheet having excellent deep-drawability and bake hardenability, and method for manufacturing same

Info

Publication number: CN103201403A
Application number: CN2011800534253A
Authority: CN
Inventors: 木村英之; 长泷康伸
Original assignee: NKK Corp
Current assignee: JFE Steel Corp; JFE Engineering Corp
Priority date: 2010-11-05
Filing date: 2011-10-28
Publication date: 2013-07-10
Anticipated expiration: 2031-10-28
Also published as: MX350226B; EP2636762A4; WO2012060294A1; BR112013011013A2; CA2814193C; KR20130055021A; CN103201403B; MX2013005011A; CA2814193A1; EP2636762A1; TW201239105A; KR101561358B1; JP5825481B2; JP2012112039A; US20130213529A1; EP2636762B1; TWI473887B

Abstract

The present invention provides a high-strength cold-rolled steel sheet having excellent deep-drawability and bake hardenability, a tensile strength of 440 MPa or greater, an average r value of 1.20 or greater, and a bake hardening value of 40 MPa or greater by a process in which a steel raw material containing, in terms of mass%, 0.010 to 0.06% C, between 0.5% and 1.5% Si, 1.0 to 3.0% Mn, 0.010 to 0.090% Nb, and 0.015 to 0.15% Ti, and having a component composition satisfying conditions of (Nb/93)/(C/12) < 0.20 and a solid-solution C content of 0.005 to 0.025%, is hot rolled and cold rolled, then annealed by heating from a temperature of 700 to 800 DEG C to a temperature of 800 to 900 DEG C at an average temperature increase rate of less than 3 DEG C/s, soaking, and then cooling at 5 DEG C/s or faster from the soaking temperature to a cooling stop temperature of 500 DEG C or below to obtain a formation which includes a ferrite phase having an area ratio of 70% or greater and a martensite phase having an area ratio of 3% or greater.

Description

Deep drawing quality and good high strength cold rolled steel plate and the manufacture method thereof of sinter-hardened property

Technical field

The present invention relates to be suitable for body of a motor car outside plate etc., tensile strength TS is that above, the average r value of 440MPa is 1.20 or more and the BH amount is deep drawing quality and good high strength cold rolled steel plate and the manufacture method thereof of sinter-hardened property more than the 40MPa.

Background technology

In recent years, from the viewpoint of the environment of preserving our planet, in order to improve the fuel efficiency of automobile, reduce CO ₂Quantity discharged, the lightweight of strong request body of a motor car.Meanwhile, the viewpoint of occupant's safety when guaranteeing to collide, also strong request improves the intensity of body of a motor car.In order to tackle above-mentioned requirements, need satisfy lightweight and the high strength of body of a motor car simultaneously, therefore, in the unquestioned scope of rigidity, make as the thickness of slab thin-walled property of the steel plate of body of a motor car material and to improve armor plate strength be effective.Therefore, in recent years, to achieve these goals, carry out the application of high tensile steel plate in automobile component just energetically.

Generally speaking, armor plate strength is got over the lightweight effect that Gao Yueneng enjoys body of a motor car.Therefore, existing recently tensile strength is the tendency that high tensile steel plate more than the 440MPa is used for body of a motor car.On the other hand, the member major part that constitutes body of a motor car is shaped by punch process, therefore, requires to have good plasticity as the steel plate of material.That is, for lightweight and the high strength of realizing body of a motor car, requiring tensile strength is that 440MPa is above and deep drawing quality is also good, particularly, requires to count high tensile steel plate more than 1.2 as the Lankford value (r value) of deep drawing quality index with average r value.

In addition, the Abdeckteil of body of a motor car also requires to have dent resistance, thereby the intensity height behind the expectation application sintering, therefore, needs sinter-hardened property (BH) also good.But the steel plate that existing BH is improved contains a large amount of solid solution C, therefore, compares with common mild steel plate, has plasticity, the particularly tendency of deep drawing quality difference.Therefore, for the lightweight that realizes body of a motor car and the raising of security, the steel plate that requires to be used for body of a motor car excellent in deep drawing characteristics and sinter-hardened property under high strength is also good.

As realizing high r value and high-intensity technology, following method is arranged: the IF (gap-free atom after in ultra-low carbon steel, to add Ti, Nb that solid solution C and solid solution N is fixing, Interstitial free) steel is as matrix, to wherein adding solution strengthening elements such as Si, Mn, P.For example, a kind of non-ageing and high tensile strength cold-rolled steel sheet of having excellent moldability that has is disclosed in the patent documentation 1, it has, and C:0.002～0.015%, Nb:C * 3～(C * 8+0.020) %, Si:1.2% are following, the one-tenth of Mn:0.04～0.8%, P:0.03～0.10% is grouped into, and tensile strength is 35～45kgf/mm ²Level (340～440MPa level).But, under the situation of this ultra-low carbon steel material, be more than the 440MPa in order to make tensile strength, need to add a large amount of alloying elements, therefore, exist the r value to reduce or cause the problem of surface texture, plating variation.In addition, owing to fix solid solution C, solid solution N with Ti, Nb, therefore also exist secondary processing brittleness to become remarkable or can't obtain guaranteeing the problem of the effective BH of dent resistance.

As the method for the raising armor plate strength beyond the method for above-mentioned interpolation solution strengthening element, the method for the build up of utilizing is arranged.For example, generally speaking, the complex tissue steel plate (DP steel plate) that comprises soft ferritic phase and hard martensitic phase is ductile good and has good strength-ductility balanced and yield ratio and hangs down such advantage.But although the complex tissue steel plate has good plasticity, but then, it is owing to the low problem that has the deep drawing quality difference of r value.We can say that this is learn to go up and not have the martensitic phase contributed and martensitic phase to form necessary solid solution C to r value can to hinder the effective { formation of 111} recrystallization texture of high r value because exist in crystalline orientation.

Technology as the r value of improving this complex tissue steel plate, for example, following technology is disclosed in the patent documentation 2: to contain C:0.05～0.15%, below the Si:1.50%, Mn:0.30～1.50%, below the P:0.030%, below the S:0.030%, the former material of steel of sol.Al:0.020～0.070%, N:0.0020～0.0080% implement predetermined condition hot rolling and cold rolling after, at recrystallization temperature～Ac ₃Implement pack annealing under the temperature of transformation temperature, AlN is separated out and improve 111} aggregation degree (Ji Plot degree), then carry out temper rolling, and then implement to be heated to 700～800 ℃ and carry out soaking, quenching, 200～500 ℃ of continuous annealings of carrying out tempering, thus, obtain that the r value is more than 1.3, intensity is 40～60kgf/mm ²The complex tissue steel plate.

In addition, following technology has been proposed: to containing below the C:0.20% in the patent documentation 3, below the Si:1.0%, Mn:0.8～2.5%, sol.Al:0.01～0.20%, N:0.0015～0.0150%, the following former material of steel of P:0.10% carries out hot rolling, after cold rolling, in 650～800 ℃ temperature range, implement pack annealing, thereby form the desirable recrystallization texture of r value and make C, the Mn atom is to the austenite phase segregation, then, enforcement is heated to more than 600 ℃ and the continuous annealing of cooling, thus, obtain comprising deep drawing quality and the good steel plate of shape of ferrite-martensite complex tissue.

In addition, following technology has been proposed in the patent documentation 4: to contain C:0.03～0.25%, Si:0.001～3.0%, Mn:0.01～3.0%, P:0.001～0.06% in quality %, below the S:0.05%, the former material of steel of N:0.001～0.030%, Al:0.005～0.3% carries out hot rolling, implement rolling rate and be more than 30% and less than 95% cold rolling, the gained steel plate implemented to be heated to 4～200 ℃/hour average rate of heating 600～800 ℃ the annealing that is up to Da Wendu, form cluster or the precipitate of Al and N, the texture that obtains expecting, and then be heated to Ac ₁Above and the ferritic-austenitic two-phase region below 1050 ℃ of transformation temperature, and cooling obtain thus having and comprise one or more the steel plate of excellent in deep drawing characteristics of tissue that adds up in 3～100% bainite, martensite, the austenite.

But, thereby the Technology Need that proposes in the above-mentioned patent documentation 2～4 is used for making the annealing operation of the flourishing r of raising of texture value and the heat treatment step that is used to form desirable tissue by the cluster or the precipitate that form Al and N.And above-mentioned annealing operation is basic with pack annealing, and the soaking hold-time also is more than 1 hour, therefore needs long-time.That is, for the technology of patent documentation 2～4, the anneal time is long, and process number is many, produces rate variance.In addition owing under the coiled material state, carry out high temperature and anneal for a long time, therefore also exist close adhesion takes place between the steel plate produce the body of heater of qualitative problem such as tempering color, annealing furnace or the producing apparatuss such as life-span reduction of inner cover on problem.

Other technologies as the r value of improving the complex tissue steel plate, for example, following manufacture method has been proposed in the patent documentation 5: to the atomic percent that contains C:0.003～0.03%, Si:0.2～1%, Mn:0.3～1.5%, Al:0.01～0.07%, Ti:0.02～0.2% in weight % and make (effectively Ti)/(C+N) than the former material of steel that is 0.4～0.8 carry out hot rolling, cold rolling after, be implemented in Ac ₁Heat after 30 seconds～10 minutes, with the continuous annealing that the average cooling rate more than 30 ℃/second cools off more than the transformation temperature and under the temperature below 900 ℃, make the complex tissue steel plate of second phase (martensite and/or bainite) that is dispersed with predetermined amount in the ferrite.

According to this patent documentation 5, to have the former material of steel that contains the composition of C:0.012%, Si:0.32%, Mn:0.53%, P:0.03%, Al:0.03%, Ti:0.051% in weight % carry out hot rolling, cold rolling after, carry out the continuous annealing of quick cooling after being implemented in the annealing of carrying out 2 minutes under 870 ℃ as the ferritic-austenitic two-phase region, with 100 ℃/second average cooling rates, thus, obtain that the r value is 1.61, tensile strength is the complex tissue steel plate of 482MPa.

But, for the technology of above-mentioned patent documentation 5, in order to ensure 100 ℃/second speed of cooling, need possess the water quenching equipment of powerful cooling power, the problem that exists equipment cost to increase.In addition, also there is the problem of shape, the rational difference in surface in the steel plate of having implemented water quenching.In addition, the tensile strength of the steel plate that the technology by patent documentation 5 obtains does not reach 500MPa, also exist be difficult to tackle make tensile strength be 500MPa above, further be the problem of the above high tensile steel plate of 590MPa.

In addition, following technology has been proposed in the patent documentation 6: to contain C:0.01～0.08% in quality %, below the Si:2.0%, below the Mn:3.0%, Al:0.005～0.20%, below the N:0.02%, V:0.01～0.5% and carry out hot rolling, cold rolling with the former material of steel that the mode that satisfies predetermined relationship contains V and C, then at Ac ₁～Ac ₃Carry out continuous annealing (recrystallization annealing) in the temperature range of transformation temperature, thus, make to have and comprise complex tissue type high tensile strength cold-rolled steel sheet tissue, excellent in deep drawing characteristics that ferritic phase and area occupation ratio as principal phase are the martensitic phase more than 1%.

This technology is characterised in that, content by optimizing V and C and to make the C in the steel before recrystallization annealing be that the form of carbide is separated out and reduced solid solution C as far as possible and realize high r value with V, in the recrystallization annealing of then carrying out, be heated to the ferritic-austenitic two-phase region, making V is carbide dissolving and make C enrichment in austenite, after process of cooling in generation martensite and realize high strength.

But, for the technology of this patent documentation 6, making V in the ferritic-austenitic two-phase region is the carbide dissolving, but V is the dissolution rate of carbide produces deviation, therefore, need annealing temperature and annealing time in the precision highland control recrystallization annealing operation, aspect mass property stable, leave problem.

In addition, the manufacture method of following high tensile steel plate has been proposed: to containing C:0.010～0.050% in quality % in the patent documentation 7, below the Si:1.0%, Mn:1.0～3.0%, P:0.005～0.1%, below the S:0.01%, Al:0.005～0.5%, below the N:0.01%, Nb:0.01～0.3% and to satisfy (Nb/93)/(C/12): the former material of steel that 0.2～0.7 mode contains Nb and C carries out hot rolling, after cold rolling, implement to be heated to the annealing of cooling off with the average cooling rate more than 5 ℃/second in 800～950 ℃ ferritic-austenitic two-phase region temperature and the temperature range till beginning to 500 ℃ from above-mentioned annealing temperature.

The technology of this patent documentation 7 is characterised in that, by adding the miniaturization that Nb realizes the hot-rolled sheet tissue, and so that (Nb/93)/(C/12) be the content that 0.2～0.7 mode is controlled Nb and C, in the steel when making hot rolling the part of C separate out with the NbC form and reduce annealing before solid solution C, thus, promote that { crystal boundary the during self-annealing of 111} recrystal grain produces and realizes high r value, generate martensite when on the other hand, utilizing not with the cooling of the fixing solid solution C of NbC form after annealing and realize high strength.According to this patent documentation 7, can make and have that to comprise area occupation ratio be that ferritic phase more than 50% and the area occupation ratio tissue that is the martensitic phase more than 1% and average r value are the high tensile steel plate more than 1.2.

But, actively utilize the technology of the patent documentation 7 of Nb to have the following various problems of enumerating.At first, Nb is very expensive element, is disadvantageous aspect raw materials cost.In addition, Nb can significantly postpone austenitic recrystallize, therefore, and the problem that the load when having hot rolling increases.In addition, the NbC that separates out in hot-rolled sheet can make the resistance to deformation when cold rolling increase, therefore, disclosedly in the embodiment of patent documentation 7 implement under the cold rolling situation with the rolling rate of height (65%), the steel plate width that the risk that rolling load increases, breaks down increases and productivity reduces, can make is restricted etc. and has a large amount of problems aspect the stably manufactured steel plate.

The prior art document

Patent documentation

Patent documentation 1: Japanese kokai publication sho 56-139654 communique

Patent documentation 2: Japanese Patent Publication 55-10650 communique

Patent documentation 3: Japanese kokai publication sho 55-100934 communique

Patent documentation 4: TOHKEMY 2003-64444 communique

Patent documentation 5: the special fair 01-35900 communique of Japan

Patent documentation 6: TOHKEMY 2002-226941 communique

Patent documentation 7: TOHKEMY 2005-120467 communique

Summary of the invention

Invent problem to be solved

As mentioned above, for the high strength of the mild steel plate of realizing excellent in deep drawing characteristics, for the technology of utilizing solution strengthening in the past, need to add a large amount of or excessive alloying element, not only existing problems aspect r value, BH also have problems aspect raw materials cost.In addition, carry out for utilizing build up for the technology of high strength, need annealing for a long time, need after annealing, implement thermal treatment separately in order to form desirable tissue or need high speed cooling apparatus etc. making existing problems.In addition, for the technology of separating out of utilizing VC, NbC, though can access the high tensile steel plate with better processing characteristics, still also leaving room for improvement aspect quality stability, productivity and the cost.

The problem that the present invention exists in view of above-mentioned prior art and finishing, its purpose is to provide and is suitable for automotive sheet etc., has tensile strength TS is the above high strength of 440MPa and to have average r value concurrently be 1.20 or more and sinter-hardened amount (BH amount) is characteristic, deep drawing quality and the good high strength cold rolled steel plate of sinter-hardened property more than the 40MPa, and proposes its favourable manufacture method.In addition, high strength cold rolled steel plate of the present invention is except comprising that tensile strength is the cold-rolled steel sheet more than the 440MPa, also comprise tensile strength be 500MPa above, further be the above cold-rolled steel sheet of 590MPa.

For the method for dealing with problems

The contriver furthers investigate the various influences that the high strength method brings for deep drawing quality, sinter-hardened property and the industrial productivity of steel plate in order to address the above problem.Found that, has the C:0.010 of comprising～0.06 quality % in use, below the N:0.01 quality %, Nb:0.010～0.090 quality %, Ti:0.015～0.15 quality %, S:0.01 quality % is following and contain Nb and C to satisfy (Nb/93)/(C/12) less than the mode of 0.20 relation, and will be not by Nb, the solid solution C amount (C*) that Ti is fixing is adjusted to the material that the one-tenth of pre-determined range is grouped into and makes under the situation of cold-rolled steel sheet, can make to have to contain with area occupation ratio and count the ferritic phase more than 70% and count the steel plate tissue of the martensitic phase more than 3% with area occupation ratio, average r value is more than 1.20, the BH amount is for more than the 40MPa, tensile strength TS is all good high strength cold rolled steel plates of the above deep drawing quality of 440MPa and sinter-hardened property, thereby has developed the present invention.

Namely, the present invention relates to a kind of deep drawing quality and the good high strength cold rolled steel plate of sinter-hardened property, having following one-tenth is grouped into: contain C:0.010～0.06 quality %, Si: greater than 0.5 quality % and below 1.5 quality %, Mn:1.0～3.0 quality %, P:0.005～0.1 quality %, below the S:0.01 quality %, sol.Al:0.005～0.5 quality %, below the N:0.01 quality %, Nb:0.010～0.090 quality %, the mode of Ti:0.015～0.15 quality % and following to satisfy (1) formula and (2) formula contains C, Nb, Ti, N and S, surplus is Fe and unavoidable impurities

(Nb/93)/(C/12)＜0.20…(1)

0.005≤C*≤≤0.025…(2)

Wherein, C*=C-(12/93) Nb-(12/48) Ti-(48/14) N-(48/32) S}, the above-mentioned symbol of element in various is represented the content (quality %) of each element,

Have the tissue that comprises in the martensitic phase more than the ferritic phase and 3% of area occupation ratio more than 70%, and

Tensile strength is more than the 440MPa, and average r value is more than 1.20, and the BH amount is for more than the 40MPa.

High strength cold rolled steel plate of the present invention is characterised in that, on the basis that mentioned component is formed, also contain add up to 0.5 quality % following be selected among Mo, Cr and the V one or more.

In addition, high strength cold rolled steel plate of the present invention is characterised in that, on the basis that mentioned component is formed, also contain be selected from that Cu:0.3 quality % is following, Ni:0.3 quality % one or both in following.

In addition, high strength cold rolled steel plate of the present invention is characterised in that, on the basis that mentioned component is formed, also contains and is selected from the following and Sb:0.2 quality % of Sn:0.2 quality % one or both in following.

In addition, high strength cold rolled steel plate of the present invention is characterised in that, on the basis that mentioned component is formed, also contain Ta:0.005～0.1 quality % and following to satisfy (3) formula and replace the mode of above-mentioned (2) formula to contain C, Nb, Ta, Ti, N and S,

0.005≤C*≤0.025…(3)

Wherein, { Ti-(48/14) N-(48/32) S}, the above-mentioned symbol of element in various is represented the content (quality %) of each element to C*=C-(12/93) Nb-(12/181) Ta-(12/48).

In addition, the present invention proposes the manufacture method of the good high strength cold rolled steel plate of a kind of deep drawing quality and sinter-hardened property, the former material of steel is carried out hot rolling, cold rolling, high strength cold rolled steel plate is made in annealing, it is characterized in that, carry out above-mentioned annealing under the following conditions: so that after the average heating speed in 700～800 ℃ the temperature range is lower than the annealing temperature that 3 ℃/second mode is heated to 800～900 ℃, begin till the cooling below 500 ℃ stops temperature T c, to cool off with the average cooling rate more than 5 ℃/second from above-mentioned annealing temperature

The former material of described steel has following one-tenth and is grouped into: contain C:0.010～0.06 quality %, Si: greater than 0.5 quality % and below 1.5 quality %, Mn:1.0～3.0 quality %, P:0.005～0.1 quality %, below the S:0.01 quality %, sol.Al:0.005～0.5 quality %, below the N:0.01 quality %, Nb:0.010～0.090 quality %, the mode of Ti:0.015～0.15 quality % and following to satisfy (1) formula and (2) formula contains C, Nb, Ti, N and S, surplus is Fe and unavoidable impurities

(Nb/93)/(C/12)＜0.20…(1)

0.005≤C*≤0.025…(2)

Wherein, { Ti-(48/14) N-(48/32) S}, the above-mentioned symbol of element in various is represented the content (quality %) of each element to C*=C-(12/93) Nb-(12/48).

The former material of steel in the manufacture method of the present invention is characterised in that, on the basis that mentioned component is formed, also contain add up to 0.5 quality % following be selected among Mo, Cr and the V one or more.

In addition, the former material of the steel in the manufacture method of the present invention is characterised in that, on the basis that mentioned component is formed, also contain be selected from that Cu:0.3 quality % is following, Ni:0.3 quality % one or both in following.

In addition, the former material of the steel in the manufacture method of the present invention is characterised in that, on the basis that mentioned component is formed, also contains and is selected from the following and Sb:0.2 quality % of Sn:0.2 quality % one or both in following.

In addition, the former material of the steel in the manufacture method of the present invention is characterised in that, on the basis that mentioned component is formed, also contain Ta:0.005～0.1 quality % and following to satisfy (3) formula and replace the mode of above-mentioned (2) formula to contain C, Nb, Ta, Ti, N and S,

0.005≤C*≤0.025…(3)

In addition, manufacture method of the present invention is characterised in that the rolling rate of the final passage in the finish rolling of above-mentioned hot rolling is set at more than 10%, and the rolling rate of preceding a time of above-mentioned final passage is set at more than 15%.

In addition, manufacture method of the present invention is characterised in that, finishes to begin to cool down with interior in back 3 seconds in the finish rolling of above-mentioned hot rolling, is cooled to temperature range below 720 ℃ with the average cooling rate more than 40 ℃/second, after batching under 500～700 ℃ the temperature, carry out cold rolling with the rolling rate more than 50%.

The invention effect

According to the present invention, the content of C is set at the scope of 0.010～0.06 quality %, and the addition of Nb is restricted to (Nb/93)/(C/12) less than 0.20 in the relation with C content, thorough minimizing brings the solid solution C of disadvantageous effect to deep drawing quality unlike extremely low carbon IF steel in the past, and then will do not controlled in pre-determined range by Nb and the fixing solid solution C amount (C*) of Ti, thus, can stably make tensile strength TS is that the above and average r value of 440MPa is more than 1.20, the BH amount is the above deep drawing quality of 40MPa, sinter-hardened property is good high strength cold rolled steel plate all.

In addition, according to the present invention, by reducing expensive Nb as far as possible and utilizing Ti energetically, not only tensile strength be the above high tensile steel plate of 440MPa and also 500MPa is above, further also can be cheap for the above high tensile steel plate of 590MPa and make stably that average r value is 1.20 or more, the BH amount is deep drawing quality and the good high tensile steel plate of sinter-hardened property more than the 40MPa.

Therefore, high strength cold rolled steel plate of the present invention is being applied under the situation of trolley part, can realizing being difficult to up to now carrying out the high strength of the member of drawing, therefore, can go far towards crashworthiness and the lightweight of body of a motor car.

Embodiment

At first, basic fundamental design of the present invention is described.

Generally speaking, in order to make deep-draw with cold-rolled steel plate hight r value, namely for make the prosperity of 111} recrystallization texture, and think reduce as far as possible cold rolling before and the amount of the solid solution C before the recrystallization annealing and to make hot-rolled sheet organize miniaturization be effective.Therefore, for the complex tissue steel plate (DP steel plate) of above-mentioned prior art, because martensitic generation needs a large amount of solid solution C, therefore has { the shortcoming that 111} recrystallization texture is undeveloped, the r value is low.

But { the scope of the excellent solid solution C amount that the prosperity of 111} recrystallization texture and martensite generate that contriver's new discovery exists can be made.Namely, new discovery of the present invention: with the control of the content of C be lower than in the past be the scope of the DP steel plate of material and the C:0.010～0.06 quality % that is higher than ultra-low carbon steel plate in the past with soft steel, and add the Nb of appropriate amount and solid solution C that Ti guarantees appropriate amount amount to contain flux matched mode with this C, thus, { the prosperity of 111} recrystallization texture and improve the r value when promoting annealing, and generate the martensite of appropriate amount during the cooling after annealing and realize high strength, and then can after annealing, also guarantee high sinter-hardened amount (BH amount).

In addition, Nb has the effect that postpones recrystallize, therefore, is effective for making hot-rolled sheet organize miniaturization.In addition, Nb has high carbide and forms ability, and batching in the stage after hot rolling separated out with the NbC form in steel, therefore, can reduce cold rolling solid solution C preceding and that recrystallization annealing is preceding and measure.But Nb is expensive element, and also is the element that makes manufacturing (rolling property) variation.Therefore, among the present invention, the addition of Nb is restricted to the required MIN amount of miniaturization of hot-rolled sheet tissue, effectively utilizes in the minimizing of solid solution C to have the Ti that forms ability with the same high carbide of Nb.That is, among the present invention, make (Nb/93)/(C/12) add Nb less than 0.20 mode in the relation with C content, and will do not measured the scope that (C*) is controlled to be 0.005～0.025 quality % by Nb and the fixing solid solution C of Ti.

Thought in the past that the existence of this solid solution C can hinder that { prosperity of 111} recrystallization texture, but in the present invention, whole C are not fixed with the form of NbC or TiC, but have the required solid solution C of martensitic formation realizes high r value on this basis.The reason that can access this effect is at present also indeterminate, but think because, solid solution C amount is being set under the situation of above-mentioned scope, give with solid solution C that { 111} recrystallization texture forms the counter productive of bringing and compares, and the micronized effect of hot-rolled sheet and fine NbC and TiC separate out, accumulate strain when cold rolling near this precipitate and promote that { positive effect of the generation of 111} recrystal grain is bigger in matrix.

That is, one of the present invention is characterised in that, is grouped into by the one-tenth with steel and is controlled to be proper range, and solid solution C is measured the scope that (C*) is controlled to be 0.005～0.025 quality %, has realized the high strength that high r value, high BHization and complex tissueization are brought thus.In addition, of the present invention second is characterised in that, by (Nb/93)/(C/12) being controlled to be less than 0.20 and utilizing Ti as its surrogate energetically, significantly cut down the addition of the Nb make the costliness that hot rolling and cold rolling load increase, thus, can not cause under the situation that raw materials cost raises and productivity reduces at the industrial high strength cold rolled steel plate that has high r value and have high BH of stably making.

In addition, the present invention also finds: on the basis of the micronized effect of the hot-rolled sheet tissue that is brought by Nb, with the rolling rate control of preceding a time of the rolling rate of the final passage of the finish rolling in the hot rolling and final passage in proper range, and then with under the situation of the control of the cooling conditions after the finish rolling in proper range, the miniaturization of the crystal grain of hot-rolled sheet is carried out more significantly, cold rolling, organizing after the annealing also obtains miniaturization, and, the miniaturization of this annealing back tissue also can increase grain boundary area, the grain boundary segregation C amount of sinter-hardened property raising is also increased, therefore, can access high sinter-hardened amount (BH amount).

The present invention on above-mentioned newfound basis an enterprising step study and finish.

Next, the one-tenth of high strength cold rolled steel plate of the present invention is grouped into describes.

C:0.010～0.06 quality %

C makes steel solution strengthening and promotion comprise ferrite as principal phase and comprise martensite to realize the important element that high strength is required as the generation of the complex tissue of second phase.When C content is lower than 0.010 quality %, be difficult to guarantee the fully martensite of amount, can't obtain the above tensile strength of 440MPa of the present invention's expectation.On the other hand, when C content surpassed 0.06 quality %, the martensite volume of generation increased, the average r value (more than 1.20) that can't obtain expecting.Therefore, among the present invention, C is set at the scope of 0.010～0.06 quality %.Be preferably the scope of 0.020～0.045 quality %.

Si: greater than 0.5 quality % and below 1.5 quality %

Si promotes ferrite transformation, improves the C content in the transformed austenite not and make and comprise ferrite and the martensitic complex tissue element of formation easily, and is the good element of solution strengthening ability.Therefore, among the present invention, in order to ensure the tensile strength more than the 440MPa, add the Si greater than 0.5 quality %.On the other hand, the Si addition is during greater than 1.5 quality %, and it is oxide compound that surface of steel plate forms Si, make the chemical convertibility, application adaptation, application of goods steel plate after erosion resistance reduce.Therefore, among the present invention, Si is set at greater than 0.5 quality % and below 1.5 quality %.In addition, be more than the 500MPa in order to make tensile strength, preferred Si content is greater than 0.8 quality %, and then is more than the 590MPa in order to make tensile strength, preferably Si content is set at more than the 1.0 quality %.

Mn:1.0～3.0 quality %

Mn is the hardening capacity that improves steel, the element that promotes the martensite generation, therefore is effective elements aspect the realization high strength.When the content of Mn is lower than 1.0 quality %, be difficult to generate the martensite of desired amount, possibly can't guarantee the tensile strength that 440MPa is above.On the other hand, Mn content can cause raw materials cost to raise during greater than 3.0 quality %, and makes r value and weldability reduction.Therefore, the content of Mn is set at the scope of 1.0～3.0 quality %.In addition, be more than the 500MPa in order to make tensile strength, preferably add the above Mn of 1.2 quality %, be more than the 590MPa in order to make tensile strength, preferably add the above Mn of 1.5 quality %.

P:0.005～0.1 quality %

P is solution strengthening ability height, to the high strength effective elements of steel.But when the content of P was lower than 0.005 quality %, its effect was insufficient, also also need be in steel making working procedure dephosphorization, thereby cause manufacturing cost to raise.On the other hand, when the content of P surpassed 0.1 quality %, P caused anti-secondary processing brittleness and weldability to reduce to grain boundary segregation.In addition, P helps the C amount to grain boundary segregation of high BHization to reduce during to grain boundary segregation, therefore, possibly can't guarantee the BH amount of expecting.Therefore, the content of P is set at the scope of 0.005～0.1 quality %.In addition, in order to guarantee the BH amount reliably, preferably making P is below the 0.08 quality %, more preferably below the 0.05 quality %.

Below the S:0.01 quality %

S causes hot short harmful element, and is to exist with the form of sulfide-based inclusion in steel and reduce the harmful element of the processibility of steel plate.Therefore, preferably reduce S as far as possible, among the present invention, the upper limit of S is set at 0.01 quality %.Be preferably below the 0.008 quality %.

Sol.Al:0.005～0.5 quality %

Al is the element that adds as reductor, but because it has the solution strengthening ability, therefore high strength is played a role effectively.But, when being lower than 0.005 quality % in the Al content of sol.Al, can't obtain above-mentioned effect.On the other hand, when surpassing 0.5 quality % in the Al content of sol.Al, can cause raw materials cost to raise, and also can become the reason of the surface imperfection of bringing out steel plate.Therefore, will be set at the scope of 0.005～0.5 quality % in the content of the Al of sol.Al.Be preferably 0.005～0.1 quality %.

Below the N:0.01 quality %

When the content of N surpasses 0.01 quality %, owing in steel, generating excessive nitride ductility, toughness are reduced, and cause the surface texture variation of steel plate.Therefore, N is set at below the 0.01 quality %.

Nb:0.010～0.090 quality %

Nb is extremely important element among the present invention, and it has makes hot-rolled sheet organize miniaturization and by separating out the effect that a fixing part is present in the solid solution C in the steel with the NbC form in hot-rolled sheet, and helps high r value by these effects.In addition, that the miniaturization meeting of adding the hot-rolled sheet tissue bring by Nb makes is cold rolling, the steel plate after the annealing is organized miniaturization and increased grain boundary area, therefore, also has and makes C increase and improve the effect that BH measures to the segregation amount of crystal boundary.In order to obtain this effect, need to add the above Nb of 0.010 quality %.On the other hand, the excessive interpolation that surpasses 0.090 quality % not only can cause raw materials cost to raise, and can make hot rolling and cold rolling in rolling load increase, therefore be difficult to carry out stable manufacturing.In addition, as described later, among the present invention, in order to generate martensite in the process of cooling after annealing, need the solid solution C of predetermined amount, but the excessive interpolation of Nb can make the whole C in the steel fix with the NbC form, therefore can hinder martensitic generation.Therefore, the addition of Nb is set at the scope of 0.010～0.090 quality %.Be preferably 0.010～0.075 quality %, more preferably the scope of 0.010～0.05 quality %.

Ti:0.015～0.15 quality %

Ti is the important element among the present invention, and itself and Nb similarly separate out in hot-rolled sheet by fixation of C and with the TiC form and help high r value.In addition, though to compare effect little with Nb, make hot-rolled sheet organize the effect of miniaturization but Ti also has, thereby increase C to the segregation amount of crystal boundary by the miniaturization of steel plate tissue after cold rolling, the annealing and the increase of grain boundary area, therefore also have and improve the effect that BH measures.In order to show this effect, need to add the above Ti of 0.015 quality %.On the other hand, the excessive interpolation that surpasses 0.15 quality % can cause raw materials cost to raise, and increases the resistance to deformation when cold rolling, therefore is difficult to carry out stable manufacturing.In addition, during excessive interpolation Ti, similarly reduce solid solution C with Nb, hinder martensitic generation in the process of cooling after the annealing.Therefore, the addition of Ti is set at the scope of 0.015～0.15 quality %.

High strength cold rolled steel plate of the present invention is formed on the basis satisfying mentioned component, also need following to satisfy (1) formula and the mode of (2) formula contain C, Nb, Ti, N and S.

(Nb/93)/(C/12)＜0.20…(1)

0.005≤C*≤0.025…(2)

Wherein, { Ti-(48/14) N-(48/32) S}, the symbol of element in the above-mentioned formula represent the content (quality %) of each element to C*=C-(12/93) Nb-(12/48).Wherein, Ti-(48/14) N-(48/32) S≤0 o'clock is made as Ti-(48/14) N-(48/32) S=0.

Nb compares more expensive element with Ti, and increases the rolling load of hot rolling, becomes infringement and makes one of reason of stability.In addition, as described later, among the present invention, in order to generate martensite in the process of cooling after annealing, that need guarantee predetermined amount is not measured (C*) by Nb and the fixing solid solution C of Ti.Therefore, among the present invention, from the viewpoint of raw materials cost, manufacturing stability, steel plate tissue and steel plate characteristic, need (Nb/93)/(C/12) and C* are controlled in proper range.Therefore, above-mentioned (1) formula and (2) formula for regulation (Nb/93)/(C/12) and C* is most important index in the present invention.

(Nb/93)/(C/12) be Nb with respect to the atomic ratio of C, this value is 0.20 when above, not only the amount of separating out of NbC increases and load when making hot rolling increases, and expensive Nb addition is increased, therefore, it is unfavorable also to become aspect raw materials cost.Therefore, with (Nb/93)/(C/12) be set at less than 0.20.

In addition, C* represents by Nb and the fixing solid solution C amount of Ti, and this value is during less than 0.005 quality %, can't guarantee the martensite volume be scheduled to be difficult to make tensile strength to reach more than the 440MPa.On the other hand, when C* surpasses 0.025 quality %, can hinder that { formation of 111} recrystallization texture not only can't obtain good deep drawing quality, and may increase and the BH amount that can't obtain expecting along with martensitic phase to the effective ferritic phase of high r value.Therefore, C* is set at the scope of 0.005～0.025 quality %.In addition, in order to make the BH amount for more than the 50MPa, preferably making C* is below the 0.020 quality %, and in order to make the BH amount for more than the 60MPa, preferably making C* is below the 0.015 quality %.

In the high strength cold rolled steel plate of the present invention, on the basis of above-mentioned essentially consist, can also according to desired characteristic add be selected among Mo, Cr and the V one or more and/or be selected from Cu and Ni in one or both.

Be selected among Mo, Cr and the V one or more: add up to below the 0.5 quality %

Mo, Cr and V are expensive elements, but with Mn be the element that improves hardening capacity equally, and be to making the stable effective elements that generates of martensite.This effect is that 0.1 quality % significantly shows when above at the total addition of mentioned component, therefore preferred the interpolation more than the 0.1 quality %.On the other hand, when the total addition of Mo, Cr and V surpassed 0.5 quality %, not only above-mentioned effect was saturated, and can cause raw materials cost to raise.Therefore, under the situation of adding these elements, they are added up to below the 0.5 quality %.

Be selected from the following and Ni:0.3 quality % of Cu:0.3 quality % one or both in following

Cu causes crackle and the harmful element that becomes the reason that produces surface imperfection when hot rolling.But for cold-rolled steel sheet of the present invention, the disadvantageous effect that Cu brings for the steel plate characteristic is little, therefore, can allow the content that 0.3 quality % is following.Thus, can use scrap metal etc. and effectively utilize regenerative raw materials, therefore, can realize the minimizing of raw materials cost.

The influence that Ni and Cu bring for equally the steel plate characteristic is little, but it has the effect that prevents by adding the generation surface imperfection that Cu causes.Above-mentioned effect is in 1/2 can manifesting when above of adding Cu content.But when the addition of Ni was excessive, the inhomogeneous generation that can encourage because of oxide skin produced other surface imperfection, and therefore, preferably the upper limit with the Ni addition is set at 0.3 quality %.

In the high strength cold rolled steel plate of the present invention, on the basis of mentioned component, can also add one or both and/or the Ta that are selected among Sn and the Sb.

Below the Sn:0.2 quality %, below the Sb:0.2 quality %

The decarburization in the tens of microns zones of surface of steel plate that produce for the nitrogenize that suppresses surface of steel plate, oxidation or because of oxidation can be added Sn, Sb.By suppressing this nitrogenize, oxidation, decarburization, the martensite growing amount minimizing of surface of steel plate is suppressed, thereby fatigue characteristic, surface quality are improved.In order to obtain above-mentioned effect, preferred interpolation respectively do for oneself 0.005 quality % above Sn and/or Sb.But the interpolation that surpasses 0.2 quality % may cause the toughness variation, therefore, under the situation of adding, preferably separately with 0.2 quality % as the upper limit.

Ta:0.005～0.1 quality %

Ta and Nb and Ti similarly have in hot-rolled sheet and separate out and the effect of fixation of C with the TaC form, are the elements that helps high r value therefore.In order to obtain this effect, preferably add more than the 0.005 quality %.But the interpolation that surpasses 0.1 quality % not only can increase raw materials cost, and similarly hinders martensitic formation in the process of cooling after the annealing with Nb and Ti, thereby the resistance to deformation that the TaC that perhaps separates out in hot-rolled sheet increases when cold rolling makes the manufacturing variation.Therefore, under the situation of adding Ta, be preferably set to the scope of 0.005～0.1 quality %.

In addition, add Ta situation under, following to satisfy (3) formula replaces the mode of above-mentioned (2) formula to contain C, Nb, Ta, Ti, N and S,

0.005≤C*≤0.025…(3)

C* in above-mentioned (3) formula is less than 0.005 o'clock, can't guarantee the martensite volume be scheduled to be difficult to obtain the above tensile strength of 440MPa.On the other hand, C* is greater than 0.025 o'clock, can hinder that { therefore the formation of 111} recrystallization texture, not only can't obtain good deep drawing quality, and may can't guarantee the BH amount expected along with the increase of martensitic phase to the effective ferritic phase of high r value.In addition, in order to make the BH amount for more than the 50MPa, preferably making C* is below 0.020, and in addition, in order to make the BH amount for more than the 60MPa, preferably making C* is below 0.015.

In the cold-rolled steel sheet of the present invention, the surplus beyond the mentioned component is made of Fe and unavoidable impurities.But, as long as in the scope of not damaging effect of the present invention, then can contain other compositions.But oxygen (O) can form non-metallic inclusion and bring disadvantageous effect to plate quality, therefore preferably its content is reduced to below the 0.003 quality %.

Next, the structure of steel (microstructure) to high strength cold rolled steel plate of the present invention describes.

In order to make high strength cold rolled steel plate of the present invention satisfy armor plate strength and press formability (particularly deep drawing quality), sinter-hardened property simultaneously, need have to contain with area occupation ratio with respect to steel plate organized whole and count the ferritic phase more than 70% and count the steel plate tissue of the martensitic phase more than 3% with area occupation ratio.In addition, for high strength cold rolled steel plate of the present invention, surplus tissue as beyond ferritic phase and the martensitic phase contains perlite, bainite, retained austenite and carbide etc. sometimes, and the total of these surplus tissues needs only to be counted 5% with area occupation ratio and can allow with next.

＜ferritic phase: in area occupation ratio more than 70% 〉

Ferritic phase is be used to the required soft phase of the press formability of guaranteeing steel plate, particularly deep drawing quality, among the present invention, by make ferritic phase { prosperity of 111} recrystallization texture realizes high r value.The area occupation ratio of ferritic phase is lower than at 70% o'clock, is difficult to make average r value to reach more than 1.20, can't obtain good deep drawing quality.In addition, the solid solution C amount in sinter-hardened property and the ferrite is relevant, and the area occupation ratio of ferritic phase is lower than at 70% o'clock, is difficult to make BH to measure and reaches more than the 40MPa.Therefore, be set in area occupation ratio ferritic phase more than 70%.In addition, in order further to improve average r value and BH amount, the area occupation ratio of preferred ferritic phase is more than 80%.On the other hand, the area occupation ratio of ferritic phase surpasses at 97% o'clock, and armor plate strength reduces, and is difficult to guarantee more than the tensile strength 440MPa.In addition, " ferrite " also comprises the high bainite ferrite of dislocation desity that is come by austenite phase transformation among the present invention except comprising polygonal ferrite.

＜martensitic phase: in area occupation ratio more than 3% 〉

Martensitic phase is be used to the required hard phase of the intensity of guaranteeing cold-rolled steel sheet of the present invention.The area occupation ratio of martensitic phase is lower than at 3% o'clock, and armor plate strength reduces, and is difficult to guarantee more than the tensile strength 440MPa, therefore, the area occupation ratio of martensitic phase is set at more than 3%.In addition, be more than the 500MPa or more than the 590MPa in order to make tensile strength, martensitic phase is counted more than 5% with area occupation ratio.On the other hand, the area occupation ratio of martensitic phase surpasses at 30% o'clock, and the area occupation ratio of the ferritic phase of r value and BH raising is reduced, and is difficult to guarantee good deep drawing quality, sinter-hardened property.Therefore, the area occupation ratio of martensitic phase is set at below 30%, is preferably set to below 20%.

Next, the manufacture method to high strength cold rolled steel plate of the present invention describes.

High strength cold rolled steel plate of the present invention is made through following operation successively: will be adjusted to steel that above-mentioned chemical ingredients forms and utilize converter etc. to carry out melting and make the steel making working procedure of the former material of steel (steel billet) by continuous casting etc.; Above-mentioned steel billet is made the hot-rolled process of hot-rolled sheet by the hot rolling that is constituted by roughing and finish rolling; Carry out cold rolling to above-mentioned hot-rolled sheet and make the cold rolling process of cold-reduced sheet; And above-mentioned cold-reduced sheet annealed and obtain the annealing operation of predetermined strength and deep drawing quality, sinter-hardened property.

(steel making working procedure)

In the manufacture method of the present invention, the melting method of steel can adopt secondary refinings the such as for example molten steel that utilizes converter or electric furnace etc. to obtain being carried out the vacuum outgas processing and obtain being predetermined to be the known melting technology that is grouped into, and is not particularly limited.In addition, consider from problems such as segregations, molten steel is made the method for steel billet and preferably used continuous metal cast process, but also can make steel billet by methods such as ingot casting-split rolling method method, sheet blank continuous casting methods.

(hot-rolled process)

＜steel billet reheat 〉

Then, preferably the steel billet that obtains is in the above described manner carried out reheat, hot rolling.From by make thickization of precipitate such as TiC make thereby the prosperity of 111} recrystallization texture improves the viewpoint of deep drawing quality, the reheat temperature of above-mentioned steel billet is more low more preferred.But when Heating temperature was lower than 1000 ℃, the rolling load in the hot rolling increased, and may produce rolling fault, therefore, preferably billet heating temperature is set at more than 1000 ℃.In addition, from the viewpoint that the oxide skin loss that suppresses to be caused by oxidation increases, preferably the upper limit with Heating temperature is set at about 1300 ℃.In addition, when steel billet is carried out hot rolling, generally steel billet being packed into, reheat is rolled to preset temperature in the process furnace, but under the steel billet behind the continuous casting is situation more than the preset temperature, can adopt and steel billet not carried out reheat and directly be rolled (direct sending is rolling), in the process furnace of perhaps under the condition of high temperature, packing into and omit the method (warm sheet is packed into) of a part of reheat.

＜roughing 〉

Steel billet behind the reheat is under these conditions carried out roughing and the base that laminates.At this, the condition of roughing gets final product according to conventional methods, does not have special stipulation.In addition, under the situation that has reduced billet heating temperature, from the hot-rolled temperature of guaranteeing to be scheduled to or the viewpoint that prevents rolling fault, can effectively utilize the thin slab well heater to realize the intensification of thin slab, this is self-evident.

＜finish rolling 〉

Then, the thin slab after the above-mentioned roughing is carried out finish rolling and makes hot-rolled sheet, but in the present invention, preferably with the rolling rate control of preceding a time of the final passage of above-mentioned finish rolling and final passage in proper range.That is, preferably the rolling rate with the final passage of finish rolling is set at more than 10%, introduces a plurality of shear zones in original austenite grain, increases the nucleation site of ferrite transformation, thereby realizes the miniaturization of hot-rolled sheet tissue.{ the preferential nucleation site of 111} recrystallization texture increased when the miniaturization meeting of this hot-rolled sheet tissue made annealing after cold rolling, therefore, not only effective to improving the r value, and can make the steel plate after the annealing organize miniaturization, increase grain boundary area and grain boundary segregation C amount is increased, therefore, also effective to improving sinter-hardened property.On the other hand, the final rolling rate of passage is lower than at 10% o'clock, and thickization takes place ferrite crystal grain, therefore, may can not get above-mentioned high r value, the effect of high BHization.Therefore, preferably the rolling rate with final passage is set at more than 10%, more preferably more than 13%.

In addition, in order further to improve the effect of high r value, high BHization, on the basis of the rolling rate of controlling above-mentioned final passage, preferably the rolling rate with preceding a time of final passage is set at more than 15%.The rolling rate of the preceding a time by controlling this final passage, the Strain Accumulation effect further improves, and introduces a plurality of shear zones in original austenite grain, therefore, the nucleation site of ferrite transformation further increases, and hot-rolled sheet is organized further miniaturization, thereby r value and BH are further improved.The rolling rate of preceding a time of final passage is lower than at 15% o'clock, and the micronized effect of hot-rolled sheet tissue becomes insufficient, possibly can't fully obtain above-mentioned r value, the raising effect of BH.Therefore, preferably the rolling rate with preceding a time of final passage is set at more than 15%, more preferably more than 18%.

In addition, from the viewpoint of rolling load, preferably the upper limit of the rolling rate of these two passages of preceding a time of above-mentioned final passage and final passage is set at respectively and is lower than 40%.

In addition, be not particularly limited for the rolling temperature of final passage with preceding a time of final passage, but the rolling temperature of final passage is preferred more than 800 ℃, more preferably more than 830 ℃.In addition, the rolling temperature of preceding a time of final passage is preferred below 980 ℃, more preferably below 950 ℃.

When the rolling temperature of final passage is lower than 800 ℃, increase to ferritic phase transformation from the non-recrystallization austenite, the steel plate tissue after cold rolled annealed is subjected to the influence of hot-rolled sheet tissue and forms the heterogeneous structure of extending along rolling direction, thereby makes the processibility reduction.

In addition, this is because when the rolling temperature of preceding a time of final passage surpassed 980 ℃, owing to answer makes the Strain Accumulation effect become insufficient, therefore, the hot-rolled sheet tissue was difficult to miniaturization, may obtain the effect of high r value, high BHization.

Cooling conditions after the＜hot rolling and coiling temperature 〉

From utilizing the crystal grain miniaturization to realize that the r value improves, the viewpoint of BH raising, the hot-rolled sheet that preferably will finish after the above-mentioned hot rolling finishes to begin to cool down with interior in back 3 seconds in finish rolling, be cooled to temperature range below 720 ℃ with the average cooling rate more than 40 ℃/second, under 500～700 ℃ temperature, batch the rolling material.

Surpass 3 seconds, average cooling rate until the time that begins to cool down and be lower than 40 ℃/second or cooling and stop under the situation that temperature is higher than 720 ℃, hot-rolled sheet tissue becomes thick, the effect of the r value that can't be improved sometimes, raising BH.

In addition, when coiling temperature surpassed 700 ℃, thickization may take place and worry the intensity reduction in hot-rolled sheet tissue, and may hinder high r value, high BHization after cold rolled annealed.On the other hand, when coiling temperature was lower than 500 ℃, separating out of NbC, TiC became difficult, and solid solution C increases, and is therefore still unfavorable to high r value.

(cold rolling process)

Then, the steel plate to above-mentioned hot rolling carries out pickling, cold rolling according to conventional methods, makes cold-reduced sheet.The cold rolling rolling rate of this moment is preferably set to 50～90% scope, but in order to realize high r value, more preferably sets the cold rolling rate higher.This be because, rolling rate is lower than at 50% o'clock, ferritic phase { the insufficient prosperity of 111} recrystallization texture may obtain good deep drawing quality.On the other hand, rolling rate surpasses at 90% o'clock, and cold rolling load increases, and logical plate fault may take place.

(annealing operation)

Then, above-mentioned steel plate after cold rolling is annealed, give desired intensity and deep drawing quality, sinter-hardened property.For this reason, as described belowly carry out above-mentioned annealing, the average heating speed of 700～800 ℃ of temperature ranges is set at is lower than 3 ℃/second, be heated to 800～900 ℃ annealing temperature, after carrying out soaking, till stopping temperature T c from above-mentioned annealing temperature (soaking temperature) beginning to the cooling below 500 ℃, cool off with the average cooling rate more than 5 ℃/second.As the method for annealing that satisfies above-mentioned condition, preferably use continuous annealing.

＜average heating speed 〉

Among the present invention, TiC, NbC are separated out in steel, therefore, the recrystallization temperature of the steel plate after cold rolling reaches higher temperature.Therefore, when cold-reduced sheet is heated, make from promoting recrystallize that effectively { viewpoint of 111} recrystallization texture prosperity need be in 700～800 ℃ of temperature ranges be lower than 3 ℃/second low speed with average heating speed and heats to high r value.Average heating speed is more than 3 ℃/second the time, and { 111} recrystallization texture flourishing insufficient may be difficult to carry out high r value.In addition, from the viewpoint of boosting productivity, preferably average heating speed is set at more than 0.5 ℃/second.

＜annealing temperature 〉

Comprise the ferritic phase of expecting area occupation ratio and the complex tissue of martensitic phase for the steel plate tissue after the annealing that makes steel plate of the present invention forms, annealing temperature (soaking temperature) need be set at the two-phase region temperature of ferritic phase and austenite phase.Therefore, among the present invention, annealing temperature is set at 800～900 ℃ temperature range.When annealing temperature is lower than 800 ℃, the martensite volume that can not get expecting after the cooling after the annealing, and recrystallize can't fully be finished in annealing, therefore, ferritic phase { 111} recrystallization texture is undeveloped, possibly can't guarantee the average r value more than 1.20.On the other hand, when annealing temperature surpassed 900 ℃, the solid solution C amount in the ferrite reduced, and possibly can't guarantee the BH amount that 40MPa is above.In addition, when annealing temperature surpassed 900 ℃, according to cooling conditions afterwards, second phase (martensitic phase, bainite phase, perlite phase) increased to more than necessity, can not get expecting the ferritic phase of area occupation ratio, possibly can't obtain good r value.In addition, also existence causes problems such as productivity reduction, energy cost increase.Therefore, annealing temperature is set at 800～900 ℃ scope, is preferably 820～880 ℃ scope.

In addition, from make the viewpoint that elements such as C fully carry out to the enrichment the austenite and fully promote ferritic phase { viewpoint of the prosperity of 111} recrystallization texture, the soaking hold-time in preferably will annealing was set at more than 15 seconds.On the other hand, not only thickization took place and can not get high BH amount in crystal grain when surpassing 300 seconds the soaking hold-time, and intensity reduced or surface of steel plate proterties variation etc. brings disadvantageous effect for each characteristic of steel plate.Therefore, the soaking hold-time in preferably will annealing is set at 15～300 seconds scope.15～200 seconds scope more preferably.

＜speed of cooling 〉

Then, the steel plate after recrystallize is finished in the above-mentioned annealing need be stopped temperature T c from the cooling that annealing temperature (soaking temperature) beginning is cooled to below 500 ℃ with the average cooling rate more than 5 ℃/second.When average cooling rate is lower than 5 ℃/second, be difficult to guarantee that martensitic phase counts more than 3% with area occupation ratio with respect to steel plate organized whole, may can not get desired intensity (more than the tensile strength 440MPa).In addition, when cooling stops temperature and surpasses 500 ℃, still possibly can't guarantee to count martensitic phase more than 3% with area occupation ratio.In addition, average cooling rate is preferably more than 8 ℃/second, and more preferably more than 10 ℃/second, in addition, cooling stops the scope that temperature T c is preferably 400～450 ℃.In addition, when average cooling rate surpasses 100 ℃/second, need specific installations such as water-cooled, may cause manufacturing cost to increase or cause steel plate shape variation, therefore, the upper limit of average cooling rate is set at about 100 ℃/second.

In addition, among the present invention, stopping the later cooling conditions of temperature T c for cooling is not particularly limited, suitably carry out from the tempering that makes martensitic phase and recover the viewpoint of ductility and toughness, preferably the average cooling rate with 0.2～10 ℃/second cools off in the temperature range that stops from cooling till temperature T c begins to 200 ℃.That is, when the average cooling rate of said temperature scope was lower than 0.2 ℃/second, the tempering of martensitic phase was excessively carried out, and may can not get desired intensity.Be because when the average cooling rate of said temperature scope surpassed 10 ℃/second, the tempering of martensitic phase is insufficient to be carried out, and can not expect very much the recovery effects of ductility and toughness on the other hand.Preferred average cooling rate is 0.5～6 ℃/second scope.

Then, can be that purpose is implemented temper rolling, leveling processing etc. to the cold-rolled steel sheet of making in the above described manner of the present invention with shape correction or surfaceness adjusting etc.In addition, under the situation of carrying out temper rolling, preferably elongation is set at about 0.3%～about 1.5%.

Embodiment 1

By through the known melting technology of converter, vacuum outgas processing etc. the steel with A～V that one-tenth is grouped into shown in the table 1 being carried out melting, make the steel billet that thickness is 260mm by continuous casting.After these steel billets are heated to 1220 ℃, carry out hot rolling, make the hot-rolled sheet that thickness of slab is 3.8mm.In addition, average cooling rate and coiling temperature that cooling after finishing with the rolling temperature of preceding a time of the final passage in the finish rolling of above-mentioned hot rolling and final passage and rolling rate, from finish rolling begins till 720 ℃ are shown in table 2, and the time set after will finishing from finish rolling till begin to cool down is in 3 seconds.

Then, above-mentioned hot-rolled sheet is carried out pickling, carry out cold rolling under the conditions shown in Table 2 and after making the cold-reduced sheet of thickness of slab 1.2mm, carry out continuous annealing under the conditions shown in Table 2, then, implement the temper rolling of elongation 0.5%, make cold-rolled steel sheet (goods).

Cut specimen material from each cold-rolled steel sheet that obtains in the above described manner, carry out structure observation, tension test by following method, carry out the evaluation of steel plate tissue and area occupation ratio, tensile strength, elongation, average r value and the sinter-hardened amount (BH amount) of mensuration ferritic phase and martensitic phase.

＜structure observation 〉

Cut the structure observation test film from above-mentioned specimen material, mechanical mill is carried out in L cross section (vertical cross-section parallel with rolling direction) to test film, after the nital corrosion, utilize scanning electronic microscope (SEM) to take with 2000 times multiplying power and obtain organizing photo (SEM photo), organize photo to carry out the evaluation of steel plate tissue and the area occupation ratio of mensuration ferritic phase and martensitic phase according to gained.In addition, utilize the above-mentioned photo of organizing to identify when steel plate is organized, ferrite is the zone of black contrast slightly, and perlite is the zone that carbide is generated as stratiform, bainite is the zone that carbide is generated as the point range shape, and martensite and residual austenite (remaining γ) are for having the particle of white contrast.In addition, after 250 ℃ * 4 hours temper of above-mentioned test film enforcement, obtain organizing photo in the same way, the zone that carbide is generated as stratiform is pearlitic zone before as thermal treatment, the zone that carbide is generated as the point range shape is bainite or martensitic zone before as thermal treatment, obtain its area occupation ratio again, still the particulate of residual white contrast is measured as remaining γ, obtained the area occupation ratio of martensitic phase by the difference of the area occupation ratio of the particle that has white contrast (martensite and residual austenite) before itself and the temper.In addition, for the area occupation ratio of each phase, press each phase layering at transparent OHP sheet, and make it painted, obtain carrying out binaryzation behind the image, utilize image analysis software (Microsoft makes, Digital Image-Pro Plus ver.4.0) to obtain area occupation ratio.

The mensuration of＜tension test, sinter-hardened amount (BH amount) 〉

Cut to be that 90 ° direction (C direction) is the JIS5 tension test sheet (JIS Z2201) of draw direction with respect to rolling direction from above-mentioned specimen material, carry out tension test according to the regulation of JIS Z2241, measure tensile strength TS, percentage of total elongation El.

In addition, for sinter-hardened amount (BH amount), after giving 2% stretching prestrain, implement the thermal treatment suitable with 170 ℃ * 20 minutes application sintering condition, then, carry out tension test again, the nominal stress that the upper yield point after the thermal treatment deducts prestrain when giving, obtain difference, it is measured as BH.

The mensuration of＜average r value 〉

Cut with the direction (L direction) that with respect to rolling direction is 0 °, be 45 ° direction (D direction), be that 90 ° direction (C direction) is as the JIS5 tension test sheet of draw direction with respect to rolling direction with respect to rolling direction from above-mentioned specimen material, width true strain and the thickness direction true strain of each test film when mensuration is given 10% unilateral stretching strain to these test films calculate average r value (average plastic strain ratio) by these measured values according to the regulation of JIS Z2254.

With the results are shown in the table 3 of said determination.

No.3～13 and 16～22 steel plate are that composition of steel is formed and the example according to the invention of creating conditions, form have that tensile strength TS is that above, the average r value of 440MPa is 1.20 or more, the BH amount be more than the 40MPa characteristic, satisfy the cold-rolled steel sheet of intensity, deep drawing quality and sinter-hardened property simultaneously.Wherein, solid solution C amount (C*) is that the BH amount of the following No.8,12 of 0.020 quality %, 13 and 22 steel plate is more than the 50MPa, in addition, C* is that No.3～7 below 0.015% and the BH amount of 16～20 steel plate reach more than the 60MPa, has high sinter-hardened amount.

Relative therewith, the C of the steel plate of comparative example No.1, Si content and C* depart from the scope of the invention, and in addition, the Mn content of the steel plate of comparative example No.2 departs from the scope of the invention, therefore, the martensite volume that can not get expecting, tensile strength is lower than 440MPa.In addition, the C* of comparative example No.14,15 steel plate exceeds the scope of the invention, and is therefore low to the area occupation ratio of high r value, the effective ferritic phase of high BHization, and average r value is measured less than 1.20, BH and also is lower than 40MPa.

Embodiment 2

After the steel billet that becomes to be grouped into that will have the described steel D of table 1, G and a L is heated to 1220 ℃, carries out hot rolling and make the hot-rolled sheet of thickness of slab 3.8mm.In addition, finish rolling condition, cooling conditions, the coiling temperature in the hot rolling is shown in Table 4.In addition, the time set that finishes to begin to cooling from finish rolling is in 3 seconds.Then, above-mentioned hot-rolled sheet is carried out pickling, carry out cold rolling under the conditions shown in Table 4 and make the cold-reduced sheet of thickness of slab 1.2mm, then, carry out continuous annealing equally under the conditions shown in Table 4, implement the temper rolling of elongation 0.5% and make cold-rolled steel sheet (goods).

From the cold-rolled steel sheet that obtains in the above described manner, cut test film similarly to Example 1, carry out structure observation, tension test, measure area occupation ratio, tensile strength, elongation, average r value and the sinter-hardened amount of ferrite, martensite etc.

With the results are shown in the table 5 of said determination.By this table as can be known, satisfy the steel plate of the example of the No.23 that creates conditions of the present invention～29,31,32,35,36,38 and 39, to form tensile strength TS and be above, the average r value of 440MPa be 1.20 or more and the BH amount is more than the 40MPa, satisfy the steel plate of intensity and deep drawing quality, sinter-hardened property simultaneously.Wherein, average cooling rate after to utilize hot-rolled sheet organizing miniaturization to realize that high r value, high BH turn to purpose finish rolling being finished be set at No.25 more than 40 ℃/second, 26 with 29 steel plate with finish rolling is finished after average cooling rate be set at other steel plates that are lower than 40 ℃/second and compare, obtained higher average r value, BH measures.

Relative therewith, for the steel plate of comparative example No.30, the rolling rate of preceding a time of the rolling rate of final passage in the finish rolling and final passage is lower than the scope of the invention, therefore, can not get organizing the high r value that miniaturization brings, the effect of high BHization by hot-rolled sheet, average r value is less than 1.20, and the BH amount is lower than 40MPa.

In addition, the annealing temperature of the steel plate of the No.33 of comparative example is lower than the scope of the invention, therefore, the martensite volume that can not get expecting, tensile strength is lower than 440MPa, and recrystallize do not finish, therefore, effectively { prosperity of 111} recrystallization texture becomes insufficient, and average r value is less than 1.20 to high r value.

In addition, the annealing temperature of the steel plate of comparative example No.34 exceeds the scope of the invention and anneals in the austenite one phase district, therefore, after process of cooling in do not generate high r value, the effective ferritic phase of high BHization, average r value is less than 1.20, and the BH amount is lower than 40MPa.

In addition, for the steel plate of comparative example No.37, begin to be lower than the scope of the invention to the average cooling rate that cooling stops till the temperature T c from annealing temperature, therefore, the martensite volume that can not get expecting, tensile strength is lower than 440MPa.In addition, for the steel plate of comparative example No.40, the average heating speed in 700～800 ℃ of whens heating annealing exceeds the scope of the invention, therefore, ferritic phase { prosperity of 111} recrystallization texture becomes insufficient, and average r value is also less than 1.20.

Utilizability on the industry

The purposes of high strength cold rolled steel plate of the present invention is not limited to the automobile member, can be suitable for requiring other purposes of high strength and deep drawing quality and sinter-hardened property.Therefore, also be suitable as the material of household electrical appliances parts and steel pipe etc.

Claims

1. a deep drawing quality and the good high strength cold rolled steel plate of sinter-hardened property,

Having following one-tenth is grouped into: contain C:0.010～0.06 quality %, Si: greater than 0.5 quality % and below 1.5 quality %, Mn:1.0～3.0 quality %, P:0.005～0.1 quality %, below the S:0.01 quality %, sol.Al:0.005～0.5 quality %, below the N:0.01 quality %, Nb:0.010～0.090 quality % and Ti:0.015～0.15 quality %, and the mode of following to satisfy (1) formula and (2) formula contains C, Nb, Ti, N and S, surplus is Fe and unavoidable impurities

Tensile strength is more than the 440MPa, and average r value is more than 1.20, and the BH amount is more than the 40MPa,

(Nb/93)/(C/12)＜0.20…(1)

0.005≤C*≤0.025…(2)

2. high strength cold rolled steel plate as claimed in claim 1 is characterized in that, on the basis that described one-tenth is grouped into, also contain add up to 0.5 quality % following be selected among Mo, Cr and the V one or more.

3. high strength cold rolled steel plate as claimed in claim 1 or 2 is characterized in that, on the basis that described one-tenth is grouped into, also contain be selected from that Cu:0.3 quality % is following, Ni:0.3 quality % one or both in following.

4. as each described high strength cold rolled steel plate in the claim 1～3, it is characterized in that, on the basis that described one-tenth is grouped into, also contain and be selected from the following and Sb:0.2 quality % of Sn:0.2 quality % one or both in following.

5. as each described high strength cold rolled steel plate in the claim 1～4, it is characterized in that, on the basis that described one-tenth is grouped into, also contain the mode that Ta:0.005～0.1 quality % and above-mentioned to replace (2) formula satisfy following (3) formula and contain C, Nb, Ta, Ti, N and S

0.005≤C*≤0.025…(3)

6. the manufacture method of the high strength cold rolled steel plate that a deep drawing quality and sinter-hardened property are good, the former material of steel is carried out hot rolling, cold rolling, high strength cold rolled steel plate is made in annealing, it is characterized in that, carry out described annealing under the following conditions: so that after the average heating speed in 700～800 ℃ the temperature range is lower than the annealing temperature that 3 ℃/second mode is heated to 800～900 ℃, begin till the cooling below 500 ℃ stops temperature T c, to cool off with the average cooling rate more than 5 ℃/second from described annealing temperature

(Nb/93)/(C/12)＜0.20…(1)

0.005≤C*≤0.025…(2)

7. the manufacture method of high strength cold rolled steel plate as claimed in claim 6 is characterized in that, on the basis that described one-tenth is grouped into, also contain add up to 0.5 quality % following be selected among Mo, Cr and the V one or more.

8. as the manufacture method of claim 6 or 7 described high strength cold rolled steel plates, it is characterized in that, on the basis that described one-tenth is grouped into, also contain be selected from that Cu:0.3 quality % is following, Ni:0.3 quality % one or both in following.

9. as the manufacture method of each described high strength cold rolled steel plate in the claim 6～8, it is characterized in that, on the basis that described one-tenth is grouped into, also contain and be selected from the following and Sb:0.2 quality % of Sn:0.2 quality % one or both in following.

10. as the manufacture method of each described high strength cold rolled steel plate in the claim 6～9, it is characterized in that, on the basis that described one-tenth is grouped into, also contain Ta:0.005～0.1 quality % and following to satisfy (3) formula and replace the mode of above-mentioned (2) formula to contain C, Nb, Ta, Ti, N and S

0.005≤C*≤0.025…(3)

11. the manufacture method as each described high strength cold rolled steel plate in the claim 6～10, it is characterized in that, the rolling rate of the final passage in the finish rolling of described hot rolling is set at more than 10%, and the rolling rate of preceding a time of described final passage is set at more than 15%.

12. the manufacture method as each described high strength cold rolled steel plate in the claim 6～11, it is characterized in that, finish to begin to cool down with interior in back 3 seconds in the finish rolling of described hot rolling, be cooled to temperature range below 720 ℃ with the average cooling rate more than 40 ℃/second, after batching under 500～700 ℃ the temperature, carry out cold rolling with the rolling rate more than 50%.