CN102971442A - Ultrahigh-strength cold-rolled steel sheet with excellent ductility and delayed-fracture resistance, and process for producing same - Google Patents
Ultrahigh-strength cold-rolled steel sheet with excellent ductility and delayed-fracture resistance, and process for producing same Download PDFInfo
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- CN102971442A CN102971442A CN2011800326392A CN201180032639A CN102971442A CN 102971442 A CN102971442 A CN 102971442A CN 2011800326392 A CN2011800326392 A CN 2011800326392A CN 201180032639 A CN201180032639 A CN 201180032639A CN 102971442 A CN102971442 A CN 102971442A
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/0436—Cold rolling
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
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- C21—METALLURGY OF IRON
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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Abstract
Provided is an ultrahigh-strength cold-rolled steel sheet that has a steel composition in which neither transition metal elements which considerably raise the alloy cost, such as V and Mo, nor Al, which may induce casting defects, is contained in excess and that, despite the composition, has excellent delayed-fracture resistance and a tensile strength of 1,320 MPa or higher. Also provided is a process for producing the steel sheet. The steel sheet having excellent ductility and delayed-fracture resistance is characterized by containing, in terms of mass%, 0.15-0.25% C, 1.0-3.0% Si, 1.5-2.5% Mn, up to 0.05% P, up to 0.02% S, 0.01-0.05% Al, and below 0.005% N, with the remainder comprising Fe and incidental impurities. The steel sheet is further characterized by having a metallographic structure which comprises 40-85 vol.% tempered martensite phase and 15-60 vol.% ferrite phase and by having a tensile strength of 1,320 MPa or higher.
Description
Technical field
The present invention relates generally to the intensity of material of the superstrength body structure members such as the central post that is suitable as automobile and door anti-collision joist and super high tensile cold-rolled steel plate and the manufacture method thereof of ductility balanced and excellent in delayed fracture resistance.
Background technology
In recent years, to CO
2The worry of the Global warming that the increase of output causes is set out, and in Europe, is carrying out from as CO
2The CO of automobile in mobile generation source
2Discharge quantitative limitation, strong request improves the fuel efficiency of automobile.In order to improve fuel efficiency, the lightweight of vehicle body is effectively, but also needs to guarantee occupant's security, therefore, needs to reduce tare, and guarantees that crash safety was more than in the past.Guarantee body lightening and these two requirements of crash safety in order to tackle, to by the material of using the high strength coefficient employed steel plate slimming being studied, in recent years, just constantly the high tensile steel plate of tensile strength 980MPa ~ 1180MPa level is applied to automotive structural members take central post and door anti-collision joist as representative.But, owing to the requirement of body lightening further improves, thereby be conceived to make the more light-weighted research of vehicle body by using than the more high-intensity steel plate of 1180MPa grade steel plate.
Automotive structural members is made by stamping usually, and therefore, the ductility of material affects press formability greatly.In addition, from the viewpoint of the crash safety of vehicle body, the residual ductility after stamping is very important.But the ductility of steel plate usually increases and reduces along with intensity, and therefore, intensity is higher, and the residual ductility after press formability and the shaping more reduces.In addition, surpass the high-intensity material of 980MPa for tensile strength, worries after stamping residual stress and from the caused delayed fracture of hydrogen of environment intrusion.Therefore, for high-intensity cold-rolled steel sheet is used as aforesaid automotive structural members, needing high press formability is high ductibility and good delayed fracture resistance characteristics.
For such requirement, up to the present various motions have been proposed.
For example, a kind of steel plate is disclosed in the patent documentation 1, wherein, although as the not record relevant with the component ratio of metal structure of its example, infer to have the tensile strength of 1350MPa by quenching/tempering method, and have the tempered martensite single phase structure.But the elongation at break of this steel plate is low to moderate 7%, and it is extremely difficult to make the car safety member by punch process.In addition, inferred the remarkable variation of shape that obtains this steel plate of martensite single phase structure by quick cooling.In this situation, after annealing, need the operation of shape correction, thus not preferred on making.
In addition, in the patent documentation 2, disclose and utilized retained austenite owing to the strain in the processing is phase-changed into TRIP type (phase change induction plasticity martensitic strain-induced transformation, that have high strength and high ductibility, Transformation Induced Plasticity) steel plate, in order to ensure being used for demonstrating the necessary retained austenite amount of TRIP effect, add 0.3 ~ 2% Al in quality %.But, add in a large number in the situation of Al, there is the problem that produces easily casting flaw.In addition, remain in the microstructure in order to make retained austenite, need to carry out isothermal and keep the process of cooling that begins from annealing temperature, under the temperature more than the Ms transformation temperature, manufacturing process increases.In addition, because until isothermal can cause material significantly to change when keeping the speed of cooling of temperature etc. to change when operation, therefore, and in order stably to produce the steel plate of certain quality, need to be to the tight management of operational condition, thereby not preferred on making.
About non-patent literature 1, non-patent literature 2, describe by embodiment.
The prior art document
Patent documentation
Patent documentation 1: TOHKEMY 2005-163055 communique
Patent documentation 2: TOHKEMY 2006-307325 communique
Non-patent literature
Non-patent literature 1: Japanese Metallkunde can Knitting: " iron Steel material ", ball be kind, 1985, p.43
Non-patent literature 2: metal Hot processes Ji Intraoperative Bian list Knitting Ji Wei STAFF meeting: " metal Hot processes Ji Intraoperative Bian list the 3rd edition ", daily magazine industry is Wen society, 1966, p.137 newly
Summary of the invention
Invent problem to be solved
The present invention In view of the foregoing finishes, its purpose is, provide the inexcessive V of containing and Mo etc. in the composition of steel make the transition metal that cost of alloy significantly increases and the Al that might bring out casting flaw, excellent in delayed fracture resistance and have super high tensile cold-rolled steel plate and the manufacture method thereof of the above tensile strength of 1320MPa.
For the method for dealing with problems
In the past, if obtain the above ultrahigh-strength steel plates of tensile strength 1320MPa, need to make microstructure become the martensite single phase structure by quench method.But, in that microstructure is become in the single-phase situation of martensite, can't obtain sufficient ductility.In addition, even wish by the tempering heat treatment after quenching ductility to be improved, but still there is following tendency: although intensity is because the Fe that separates out in the recovery of the dislocation structure in the martensitic phase and the martensitic phase
3Thickization of C carbide and reducing, but how ductility does not improve.
On the other hand, in order to demonstrate high ductibility, the TRIP steel about the strain-induced transformation that utilized the retained austenite phase has also carried out a large amount of inventions.But, in order to demonstrate the TRIP effect, need to add a large amount of alloying elements and improve austenitic stability, and need to strictly carry out isothermal when the cooling that begins from annealing temperature, under the temperature more than the Ms transformation temperature keeps, therefore, not preferred from the viewpoint of making stability and manufacturing cost.
In addition, from the viewpoint of delayed fracture resistance characteristics, the preferred hydrogen trap that brings out delayed fracture that reduces as far as possible owing to martensitic phase a large amount of dislocations that become hydrogen trap of introducing when the crystalline structure phase transformation that occurs mutually from austenite, therefore, preferably reduces as far as possible.In addition, the known retained austenite that helps to improve ductility also similarly works as hydrogen trap with dislocation, and retained austenite exists with membranaceous on crystal boundary, therefore, hydrogen brings out transgranular fracture to the intrusion in the retained austenite, might reduce delayed fracture resistance characteristics, therefore, preferably in metal structure, not comprise retained austenite.
The inventor conducts in-depth research in order to address the above problem repeatedly, the result is clear and definite by making microstructure is the tissue with tempered martensite phase and ferritic phase, the volume fraction of tempered martensite phase is changed, can restrained stretching intensity and the balance of ductility, and found by adding C and Si the hardness of tempered martensite phase and ferritic phase to be risen, reduce the not volume fraction of tempered martensite phase, thereby realize the method for the superstrength of steel plate, can access as can be known very high strength, have a steel plate of high ductibility simultaneously.
And clear and definite by in metal structure, separating out the ferritic phase that comprises hardly dislocation, make the dislocation desity in the metal structure compare significantly minimizing with the martensite single phase structure, reduce hydrogen trap, can significantly reduce thus the intrusion amount of hydrogen in the steel, can improve delayed fracture resistance characteristics as can be known.
Obtain on the other hand following opinion, in manufacturing process, effectively, when the annealing after cold rolling and cooling, suitably control annealing temperature and process of cooling afterwards, then, implementing tempering heat treatment more than 100 ℃ and in the temperature range below 300 ℃.
The present invention is based on above-mentioned opinion.
That is, purport of the present invention is constructed as follows.
[1] super high tensile cold-rolled steel plate of a kind of ductility and excellent in delayed fracture resistance, it is characterized in that, by quality ratio, contain that C:0.15 ~ 0.25%, Si:1.0 ~ 3.0%, Mn:1.5 ~ 2.5%, P:0.05% are following, S:0.02% is following, Al:0.01 ~ 0.05%, N: less than 0.005%, surplus is made of Fe and inevitable impurity, it is that 40 ~ 85% tempered martensite phase and volume fraction are 15 ~ 60% ferritic phase that metal structure contains volume fraction, and tensile strength is more than the 1320MPa.
[2] super high tensile cold-rolled steel plate of the ductility of above-mentioned [1] and excellent in delayed fracture resistance, wherein, by quality ratio, contain also that Nb:0.1% is following, Ti:0.1% following, B:5 ~ 30ppm more than one.
[3] ductility of above-mentioned [1] or [2] and the super high tensile cold-rolled steel plate of excellent in delayed fracture resistance is characterized in that, elongation at break is more than 12%.
[4] manufacture method of the super high tensile cold-rolled steel plate of a kind of ductility and excellent in delayed fracture resistance, it is characterized in that, after will having more than the heating steel billet to 1200 ℃ of above-mentioned [1] or [2] described chemical ingredients, under the condition of finish to gauge outgoing side temperature more than 800 ℃, carry out hot rolling, then, carry out pickling and cold rolling, then, when continuous annealing, at A
C1Transformation temperature ~ A
C3Keep after 30 ~ 1200 seconds in the temperature range of transformation temperature, be cooled to 600 ~ 800 ℃ with the average cooling rate below 100 ℃/second, then, be cooled to below 100 ℃ with 100 ~ 1000 ℃/second average cooling rates, then, implement to reheat and in 100 ~ 300 ℃ temperature range, keep 120 ~ 1800 seconds temper.
The invention effect
Cold-rolled steel sheet of the present invention has high tensile strength, and the good processibility that has high ductibility and accompany with it.In addition, after shaping is processed as member, has the good delayed fracture resistance characteristics that the delayed fracture that caused by the hydrogen of invading from environment be difficult for to occur.For example, have that tensile strength 1320MPa is above, elongation at break is more than 12%, lower 100 hours of the hydrochloric environment that can easily be implemented in 25 ℃-pH3 is the delayed fracture resistance characteristics of fracture not.In addition, manufacturing method according to the invention can stably be made the cold-rolled steel sheet with aforesaid good performance.
According to the present invention, can stably make the above super high tensile cold-rolled steel plate of following tensile strength 1320MPa, its stamping for still having the good delayed fracture resistance characteristics that is difficult for occuring the delayed fracture that caused by the hydrogen of invading from environment behind the member, and when being shaped, show good processibility, thereby superstrength member, the member such as the car safety such as central post and collision prevention girders that is difficult for occuring delayed fracture can be provided.
Description of drawings
Fig. 1 is the synoptic diagram of giving the test film of stress after 180 ° of bending machining by bolted.
Embodiment
Super high tensile cold-rolled steel plate of the present invention has specific chemical ingredients and metal structure as described below.At first, the chemical ingredients of cold-rolled steel sheet described.
＜C:0.15 ~ 0.25 quality % 〉
C is the element that makes stabilization of austenite, and is the necessary element of intensity for guaranteeing steel plate.When C measures less than 0.15 quality %, in the tissue with tempered martensite phase and ferritic phase, be difficult to stably obtain more than the tensile strength 1320MPa.On the other hand, when C amount surpassed 0.25 quality %, the sclerosis of weld part and the heat affected zone that caused by welding significantly occured, and weldability reduces.Therefore, making the C amount is the scope of 0.15 ~ 0.25 quality %.The more preferably scope of 0.18 ~ 0.22 quality %.
＜Si:1.0 ~ 3.0 quality % 〉
Si is for making the effective displaced type solution strengthening of steel plate hardization element.In order to show this effect, need to contain more than the 1.0 quality %.When the Si amount surpassed 3.0 quality %, the oxide skin in the hot rolling formed and becomes significantly, and the ratio of defects during end article increases, and is not preferred economically.Therefore, making the Si amount is 1.0 ~ 3.0 quality %.
＜Mn:1.5 ~ 2.5 quality % 〉
Mn makes stabilization of austenite and to the effective element of the reinforcement of steel.But Mn is during less than 1.5 quality %, and the hardening capacity of steel is insufficient, being created in early days of the generation of the ferritic phase that produces from the cooling that annealing temperature begins and perlite and bainite begins, intensity significantly reduces, and therefore, is difficult to stably make the steel plate with target strength.On the other hand, when surpassing 2.5 quality %, segregation becomes significantly, and sometimes processibility variation, and delayed fracture resistance characteristics reduces.Therefore, to make Mn amount be 1.5 ~ 2.5 quality %, be preferably 1.5 ~ 2.0 quality %.
＜P:0.05 quality % is following 〉
P is the element that promotes the transgranular fracture that caused by grain boundary segregation, and therefore preferred low levels makes and be limited to 0.05 quality % on it, is preferably 0.010 quality %.And then, from the viewpoint of raising weldability, more preferably below the 0.008 quality %.
＜S:0.02 quality % is following 〉
S forms the inclusiones such as MnS, brings out the deteriorated of crashworthiness and delayed fracture resistance characteristics, and is therefore preferably low as far as possible, makes and be limited to 0.02 quality % on it, is preferably 0.002 quality %.
＜Al:0.01 ~ 0.05 quality % 〉
Al is for the effective element of deoxidation, in order to obtain useful deoxidation effect, need to be for more than the 0.01 quality %, but when surpassing excessive interpolations of 0.05 quality %, the inclusion in the steel plate increases, and makes the ductility reduction.Therefore, making the Al amount is 0.01 ~ 0.05 quality %.
＜N: less than 0.005 quality % 〉
The amount of N reaches 0.005 quality % when above, and the high temperature and the ductility under the low temperature that are caused by forming of nitride reduce.Therefore, make the N amount less than 0.005 quality %.
As required, in steel plate, can also contain more than one of Nb, Ti, B.Below, the additive effect of these three kinds of elements and its preferred addition are described.
＜Nb, Ti:0.1 quality % are following 〉
Nb and Ti have the effect that makes the crystal grain microminiaturization, are to the intensity that the makes steel plate effective element that rises, and therefore, preferably add respectively more than the 0.015 quality %.But, surpassing respectively 0.1 quality % even contain Nb, Ti, its effect is saturated, and is therefore not preferred economically.Therefore, the addition of Nb and Ti is respectively below the 0.1 quality %.
＜B:5 ~ 30 quality ppm 〉
B is the effective element that rises of the intensity to steel plate.When B measures less than 5 quality ppm, ascending effect on the intensity that can't expect to be caused by B.On the other hand, when the B amount surpassed 30 quality ppm, hot workability reduced, and therefore made not preferred.Thereby the addition that makes B is 5 ~ 30 quality ppm.
Surplus beyond above-mentioned is Fe and inevitable impurity.
Then, the metal structure of cold-rolled steel sheet described.
The inventor found that in order to improve the ductility that affects press formability and to obtain showing that also the steel plate of good delayed fracture resistance characteristics is studied after stamping, in order to bring into play high ductibility, importantly suitably controls microstructure.Particularly, finding importantly to make microstructure after the continuous annealing is that to contain volume fraction be the tempered martensite phase more than 40%, the tissue that surplus has ferritic phase.This tissue obtains by quick cooling and the quick cooled temper that begins from annealing temperature when annealing, according to the method, can make in inexcessive interpolation the transition metal such as V that cost rises and Mo and might bring out in the situation of the alloying elements such as Al of casting flaw, obtain having the super high tensile cold-rolled steel plate of high ductibility.
The hydrogen amount that Xiang Gangzhong invades is fewer, and delayed fracture resistance characteristics is better.Tempered martensite mutually since when quenching from the in opposite directions crystalline texture phase transformation of martensitic phase of austenite, introduce the dislocation of huge amount, but by in metal structure, containing in right amount ferritic phase, compare with the tempered martensite single phase structure, the dislocation as hydrogen trap of bringing out delayed fracture is significantly reduced, thereby reduce to hydrogen in steel intrusion amount.
The tensile strength of steel with tissue of tempered martensite phase and ferritic phase is along with the increase of the volume fraction of tempered martensite phase and rise.This be because, for tempered martensite phase and ferritic phase, the hardness of tempered martensite phase is higher, and the resistance to deformation when tensile deformation is born mutually by the tempered martensite as hard phase, the volume fraction of tempered martensite phase is larger, and is more approaching with the tensile strength of tempered martensite single phase structure.In composition of steel scope of the present invention, the tempered martensite volume fraction can not obtain more than the tensile strength 1320MPa less than 40% the time.Along with the tempered martensite volume fraction increases, ductility reduces, and for the volume fraction of tempered martensite phase surpasses 85% tissue, can not guarantee that in order to improve elongation at break be high ductibility 12% or more and delayed fracture resistance characteristics and the ferritic phase of necessity.In addition, the volume fraction of ferritic phase is less than 15% the time, and elongation at break is that the raising of high ductibility more than 12% and delayed fracture resistance characteristics is insufficient, on the other hand, when surpassing 60%, can't guarantee the volume fraction of the tempered martensite phase necessary for the intensity that obtains being scheduled to.
For above reason, making the volume fraction of tempered martensite phase of the metal structure of cold-rolled steel sheet of the present invention be 40 ~ 85%, making the volume fraction of ferritic phase is 15 ~ 60%.More preferably making the volume fraction of tempered martensite phase be 60 ~ 85%, making the volume fraction of ferritic phase is 15 ~ 40% metal structure.The metal structure of cold-rolled steel sheet of the present invention can serve as reasons the tempered martensite phase of volume fraction with expectation and the duplex structure that ferritic phase consists of, as the tissue beyond the described two-phase, also can contain retained austenite phase, Bainite Phases of Some, the equal formation of perlite phase.But, in the situation that Bainite Phases of Some and perlite exist mutually in a large number, bring out respectively the reduction of ductility and the reduction of intensity, therefore preferably do not contain in a large number.In addition, retained austenite mainly exists with membranaceous at crystal boundary mutually, and owing to become hydrogen trap, therefore, might become the fracture starting point of following hydrogen embrittlement, thereby preferably reduces as far as possible.Therefore, preferably make the formation phase (Bainite Phases of Some, perlite phase, retained austenite equate) beyond tempered martensite phase and the ferritic phase among the present invention, the adding up to below 1% of volume fraction.
About target tensile strength of the present invention and ductility, tensile strength is more than the 1320MPa, elongation at break (elongation at break in the tension test of use JIS5 tension test sheet) is more than 12%, this elongation at break can easily be realized such intensity and ductility level with can to carry out the MIN ductility of punch process to car safety members such as collision prevention girders suitable among the present invention.In addition, target delayed fracture resistance characteristics of the present invention is 100 hours performances of fracture not under 25 ℃, the hydrochloric environment of pH3, and the present invention also can easily realize such performance.
The purposes of cold-rolled steel sheet of the present invention has no particular limits, owing to have an aforesaid performance, therefore is particularly suitable for the superstrength vehicle body safety member take the door anti-collision joist of automobile and central post as representative.Need to prove, also comprise steel band as the steel plate of object of the present invention, cold-rolled steel sheet of the present invention also can effects on surface be implemented the surface treatments such as plating (electroplate etc.) or chemical conversion processing, uses as surface treated steel plate.
Then, the manufacture method of super high tensile cold-rolled steel plate of the present invention described.
Among the present invention, the steel that the melting mentioned component forms obtains slab (steel billet) by continuous casting, after more than this heating steel billet to 1200 ℃, carries out hot rolling more than 800 ℃ in finish to gauge outgoing side temperature.Below, the restriction reason of hot rolling is described.
＜billet heating temperature is more than 1200 ℃ 〉
When billet heating temperature was lower than 1200 ℃, rolling load increased, and the risk that breaks down during hot rolling increases.Therefore, making billet heating temperature is more than 1200 ℃.When Heating temperature is too high, cause following the increase of the oxide skin loss that oxidation weight increases, therefore, preferably making billet heating temperature is below 1300 ℃.
＜finish to gauge outgoing side temperature is more than 800 ℃ 〉
By finish to gauge outgoing side temperature is made as more than 800 ℃, can access uniform hot rolling parent phase tissue.When finish to gauge outgoing side temperature was lower than 800 ℃, the tissue of steel plate became inhomogeneous, and ductility reduces, and the risk increase of various unfavorable conditions occurs when being shaped.Therefore, making finish to gauge outgoing side temperature is more than 800 ℃.Need to prove that the upper limit of finish to gauge outgoing side temperature is not particularly limited, but become the reason that produces oxide skin flaw etc. when under excessive temperature, being rolled, therefore, be preferably about below 1000 ℃.
After hot rolling, batch processing.Coiling temperature is not particularly limited, but coiling temperature generates oversize particle when too high, and steel plate tissue becomes inhomogeneous, the ductility reduction.In addition, residual by the worked structure that hot rolling produces in the excessively low situation of coiling temperature, increase as the cold rolling rolling load of next operation.Therefore, preferably making coiling temperature is 600 ~ 700 ℃.Particularly preferred coiling temperature is 600 ~ 650 ℃.
After implementing hot rolling, carry out pickling and cold rolling, then, carry out continuous annealing and temper.Pickling, cold rolling condition are not particularly limited.Continuous annealing is, at A
C1Transformation temperature ~ A
C3Keep after 30 ~ 1200 seconds in the temperature range of transformation temperature, be cooled to 600 ~ 800 ℃ with the average cooling rate below 100 ℃/second, then be cooled to below 100 ℃ with 100 ~ 1000 ℃/second average cooling rate, then, implement to reheat and in 100 ~ 300 ℃ temperature range, keep 120 ~ 1800 seconds temper.Below, the restriction reason of the condition of continuous annealing and temper is described.
＜annealing temperature: at A
C1Transformation temperature ~ A
C3Kept 30 ~ 1200 seconds in the transformation temperature 〉
Annealing temperature is lower than A
C1During transformation temperature, in annealing, can not generate the austenite phase (being phase-changed into martensitic phase after the quenching) to guaranteeing predetermined strength necessity, can't obtain predetermined strength even implement after the annealing to quench yet.Annealing temperature surpasses A
C3During transformation temperature, although by being controlled at the volume fraction of the ferritic phase of from the cooling that annealing temperature begins, separating out, can access volume fraction and be the martensitic phase more than 40%, surpassing A
C3In the situation that transformation temperature is annealed, be difficult to the metal structure that obtains expecting.Therefore, annealing temperature is set as A
C1Transformation temperature ~ A
C3The scope of transformation temperature.In addition, be viewpoint more than 40% from the balance volume fraction of in this temperature range, stably guaranteeing the austenite phase, be preferably more than 760 ℃, more preferably more than 780 ℃.In addition, when the hold-time under the annealing temperature, (annealing time) was too short, microstructure is fully annealing not, becomes the inhomogeneous tissue that exists by the worked structure of cold rolling generation, and ductility reduces.On the other hand, the hold-time causes the increase of manufacturing time when long, and is not preferred on manufacturing cost.Therefore, making the hold-time is 30 ~ 1200 seconds.The particularly preferred hold-time is 250 ~ 600 seconds scope.
＜be cooled to 600 ~ 800 ℃ (Slow coolings) below 100 ℃/second with average cooling rate 〉
Then, begin to be cooled to the average cooling rate below 100 ℃/second 600 ~ 800 ℃ temperature (Slow cooling stops temperature) (the following description, sometimes this cooling being called " Slow cooling ") from above-mentioned annealing temperature.The Slow cooling that begins from annealing temperature, ferritic phase is separated out, can control strength-ductility balanced, but when making Slow cooling stop temperature being lower than 600 ℃, a large amount of perlite that generate in microstructure, intensity sharply reduces, and therefore can't obtain the above tensile strength of 1320MPa.In addition, be when being higher than 800 ℃ high temperature making Slow cooling stop temperature, the ferritic phase of abundant amount is separated out, can't obtain fully ductility.Therefore, making Slow cooling stop temperature is 600 ~ 800 ℃.Slow cooling on suppressing attended operation stops the viewpoint of the material change of temperature variation, and preferably making Slow cooling stop temperature is 700 ~ 750 ℃.
In addition, when the average cooling rate of Slow cooling surpasses 100 ℃/second, separating out of the abundant ferritic phase of measuring can not occur in Slow cooling, the ductility that therefore can't obtain being scheduled to.The ductility of the metal structure with tempered martensite phase and ferritic phase that is intended among the present invention, also being mixed with soft ferritic phase by the tempered martensite phase by hard exists the high work hardening capacity that shows to cause, but average cooling rate surpasses in 100 ℃/second the situation, carbon becomes insufficient to the enrichment in the austenite in Slow cooling, can't obtain the martensitic phase of hard when cooling off fast.Its result, the work hardening capacity of final tissue reduces, and can't obtain sufficient ductility.Average cooling rate when thus, making Slow cooling is below 100 ℃/second.For carbon is occured fully to the enrichment of austenite in mutually, preferably be made as the average cooling rate below 5 ℃/second.
＜be cooled to (fast cooling) below 100 ℃ with 100 ~ 1000 ℃/second of average cooling rates 〉
After the above-mentioned Slow cooling, be cooled to temperature (cooling stops temperature) below 100 ℃ (in the following description, sometimes this cooling being called " fast cooling ") with 100 ~ 1000 ℃/second average cooling rates.Quick cooling behind the Slow cooling is carried out in order to make austenite be phase-changed into martensitic phase mutually, its average cooling rate is during less than 100 ℃/second, austenite is phase-changed into ferritic phase, Bainite Phases of Some or perlite phase mutually in cooling, the intensity that therefore can't obtain being scheduled to.On the other hand, when average cooling rate surpasses 1000 ℃/second, might produce the shrinkage crack that cools off the steel plate that causes.Average cooling rate when therefore, making quick cooling is 100 ~ 1000 ℃/second.This cooling is preferably the quick cooling that utilizes water quenching.
Preferably making cooling stop temperature is below 100 ℃.When cooling stops temperature above 100 ℃, the reduction of the strength of materials that the self-tempering of the reduction of the volume fraction of the martensitic phase that brings out when quick cooling austenitic quenching fully not carry out and cause and the martensitic phase that generated by quick cooling causes, therefore not preferred on making.
＜temper: 100 ~ 300 ℃ of lower maintenances 120 ~ 1800 seconds 〉
After the above-mentioned quick cooling, for the tempering of martensitic phase, reheat and 120 ~ 1800 seconds temper of maintenance in 100 ~ 300 ℃ temperature range.This tempering makes the martensitic phase softening, and processibility improves.Be lower than in the situation of carrying out tempering under 100 ℃, martensitic softening is insufficient, can not expect the raising effect of processibility.In addition, when under surpassing 300 ℃, carrying out tempering, not only improve for the manufacturing expense that reheats, and cause significant strength decreased, thereby can't obtain useful effect.
On the other hand, make the hold-time less than in 120 seconds the situation, the martensitic softening under the maintenance temperature occurs fully, therefore can not expect the raising effect of processibility.In addition, the hold-time surpasses in 1800 seconds the situation, and martensitic softening is excessively carried out, and is thus, except intensity significantly reduces, owing to the increase that reheats the time improves manufacturing expense, therefore not preferred.
By above manufacturing process, can make super high tensile cold-rolled steel plate of the present invention.In addition, plate shape (Flatness) after the annealing of super high tensile cold-rolled steel plate of the present invention is good, therefore, not necessarily need rolling or leveling processing etc. to be used for the operation of the shape of rectification steel plate, but from regulating the viewpoint of material and surfaceness, can implement rolling with the elongation of about percentum to the steel plate after the annealing.
Embodiment
One-tenth by record in the table 1 is grouped into the tested steel A ~ M that consists of and carries out vacuum melting, make steel billet after, in table 2, carry out hot rolling under the condition of record, obtain the hot-rolled steel sheet of thickness of slab 3.4mm.This hot-rolled steel sheet is carried out cleanup acid treatment, remove surface scale, it is thick to be cold-rolled to afterwards 1.4mm.Then, in table 2, implement continuous annealing and temper under the condition of record.Need to prove the A of each steel grade
C1Transformation temperature is the value of obtaining about transformation temperature and the dependent relational expression of alloying constituent by record in the non-patent literature 1, A
C3Transformation temperature is the value of obtaining about transformation temperature and the dependent relational expression of alloying constituent (2 following formulas) by record in the non-patent literature 2.
A
C1[℃]=723-10.7 * (quality %Mn)+29.1 * (quality %Si) ... (1)
A
C3[℃]=910-203 * (quality %C)
1/2+ 29.1 * (quality %Si)-30 * (quality %Mn)+700 * (quality %P)+400 * (quality %Al)+400 * (quality %Ti) ... (2)
Table 1
Table 2
Cut test film from the steel plate that obtains by above manufacturing process, implement observation (mensuration) and the tension test of metal structure.In addition, a part of steel grade is implemented the delayed fracture test.These be the results are shown in table 3.
The observation of metal structure (mensuration) and performance test are following carries out.
(1) observation of metal structure
Cut test film from resulting cold-rolled steel sheet, after the section parallel with rolling direction carried out mirror ultrafinish, carry out etching by nital, use opticmicroscope or sem observation and take microtissue, identify that tempered martensite phase and ferrite equate to consist of the kind of phase, and use image analysis apparatus will organize the photo binaryzation, obtain thus the volume fraction of tempered martensite phase and ferritic phase.Need to prove, might in resulting cold-rolled steel sheet, there be the retained austenite phase, therefore, attempts carrying out the mensuration of retained austenite phase by X ray (Mo-K alpha-ray) assay method for example, but its amount almost is zero, therefore is not included in the surplus of table 3.
(2) tension test
From resulting cold-rolled steel sheet, cut JIS5 tension test sheet with rolling direction with meeting at right angles, regulation according to JIS-Z-2241 is carried out tension test, obtains tensile properties (0.2% stress (YS), tensile strength (TS), elongation at break (EL).
(3) delayed fracture evaluating characteristics test
Use with the rolling direction of resulting cold-rolled steel sheet as length, cut into 30mm * 100mm and end face ground the test film of cutting after the processing, test film is implemented 180 ° of bending machining with the radius-of-curvature 10mm of the end of rushing to summit.The elastic recovery that test film after implementing this bending machining produces is measured as shown in Figure 1, fastening in the mode that the clear width of test film 1 reaches 20mm by bolt 2, behind test film 1 stress application, impregnated in 25 ℃, the hydrochloric acid of pH3, till the longest 100 hours, measure until the time of fracture.It is qualified that 100 hours situations with interior not fracture are set as.
Table 3
Confirm to 3 according to table 1, the example of condition according to the invention obtains that tensile strength 1320MPa is above, elongation at break such high strength and ductility balanced 12% or more, in the delayed fracture evaluating characteristics is tested, 100 hours fractures not have good delayed fracture resistance characteristics.
Annealing time is set as 10 seconds No.24 outside the scope of the present invention, the pearlitic structure that generates after the hot rolling is still remaining after annealing operation, and fully do not remove the impact of following cold rolling processing strain, thus, do not obtain intensity and the ductility of being scheduled to.In addition, annealing temperature is set as A
C3Separating out of ferritic phase can't occur in the No.25,29 that point is above in Slow cooling, become the martensite single phase structure, although the intensity that obtains being scheduled to does not obtain the ductility of being scheduled to.The No.26 and 27 of composition of steel outside scope of the present invention be not even continuous annealing and the temper stipulated among enforcement the present invention obtain the intensity of being scheduled to yet.It is 500 ℃ No.30 that Slow cooling is stopped Temperature Setting, separates out a large amount of ferritic phases, and generates the perlite phase, the intensity that therefore can not obtain being scheduled to.The average cooling rate of quick refrigerating work procedure is set as 20 ℃/second No.31 outside the scope of the present invention, can not obtains the martensitic phase of predetermined amount, thus the intensity that can not obtain being scheduled to.Tempering temperature is set as 400 ℃ No.32, because it is softening that the overtempering of martensitic phase occurs, the intensity that can not obtain being scheduled to.
The example No.1 of condition according to the invention ~ 23, in delayed fracture evaluating characteristics test, 100 hours fractures not confirm that the cold-rolled steel sheet that obtains by the present invention has sufficient delayed fracture resistance characteristics.But metal structure is the comparative example No.25,29 outside single-phase, the of the present invention scope of tempered martensite, cracked with interior at 100 hours, so the delayed fracture resistance characteristics test-results is defective.
Utilizability on the industry
The present invention is the quenching that mainly is fit in the purposes of the superstrength vehicle body safety member take the door anti-collision joist of automobile and central post as representative etc., the temper steel sheet, when making the automotive part that uses above-mentioned steel plate, form by suitably controlling steel, rolling condition and annealing conditions, have that to comprise volume fraction be more than 40% and 85% following tempered martensite phase and volume fraction be 15% or more and 60% below the tissue of ferritic phase, more than the tensile strength 1320MPa, elongation at break is more than 12%, have good strength-ductility balancedly, and delayed fracture resistance characteristics is also good.When using super high tensile cold-rolled steel plate of the present invention, can carry out the punch process of the car safety members such as collision prevention girders, this car safety member demonstrates good delayed fracture resistance characteristics.
Label declaration
1 test film
2 bolts
Claims (4)
1. the super high tensile cold-rolled steel plate of a ductility and excellent in delayed fracture resistance, it is characterized in that, by quality ratio, contain that C:0.15 ~ 0.25%, Si:1.0 ~ 3.0%, Mn:1.5 ~ 2.5%, P:0.05% are following, S:0.02% is following, Al:0.01 ~ 0.05%, N: less than 0.005%, surplus is made of Fe and inevitable impurity, it is that 40 ~ 85% tempered martensite phase and volume fraction are 15 ~ 60% ferritic phase that metal structure contains volume fraction, and tensile strength is more than the 1320MPa.
2. the super high tensile cold-rolled steel plate of ductility as claimed in claim 1 and excellent in delayed fracture resistance wherein, by quality ratio, contains also that Nb:0.1% is following, Ti:0.1% is following, among B:5 ~ 30ppm more than one.
3. the super high tensile cold-rolled steel plate of ductility as claimed in claim 1 or 2 and excellent in delayed fracture resistance is characterized in that, elongation at break is more than 12%.
4. the manufacture method of the super high tensile cold-rolled steel plate of a ductility and excellent in delayed fracture resistance, it is characterized in that, after will having more than the heating steel billet to 1200 ℃ of claim 1 or 2 described chemical ingredientss, under the condition of finish to gauge outgoing side temperature more than 800 ℃, carry out hot rolling, then, carry out pickling and cold rolling, then, when continuous annealing, at A
C1Transformation temperature ~ A
C3Keep after 30 ~ 1200 seconds in the temperature range of transformation temperature, be cooled to 600 ~ 800 ℃ with the average cooling rate below 100 ℃/second, then, be cooled to below 100 ℃ with 100 ~ 1000 ℃/second average cooling rates, then, implement to reheat and in 100 ~ 300 ℃ temperature range, keep 120 ~ 1800 seconds temper.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2010148531A JP5668337B2 (en) | 2010-06-30 | 2010-06-30 | Ultra-high-strength cold-rolled steel sheet excellent in ductility and delayed fracture resistance and method for producing the same |
| JP2010-148531 | 2010-06-30 | ||
| PCT/JP2011/065135 WO2012002520A1 (en) | 2010-06-30 | 2011-06-24 | Ultrahigh-strength cold-rolled steel sheet with excellent ductility and delayed-fracture resistance, and process for producing same |
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| CN2011800326392A Pending CN102971442A (en) | 2010-06-30 | 2011-06-24 | Ultrahigh-strength cold-rolled steel sheet with excellent ductility and delayed-fracture resistance, and process for producing same |
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| Country | Link |
|---|---|
| US (1) | US20130087257A1 (en) |
| EP (1) | EP2589674A4 (en) |
| JP (1) | JP5668337B2 (en) |
| KR (1) | KR101540507B1 (en) |
| CN (1) | CN102971442A (en) |
| WO (1) | WO2012002520A1 (en) |
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- 2011-06-24 CN CN2011800326392A patent/CN102971442A/en active Pending
- 2011-06-24 EP EP11800982.8A patent/EP2589674A4/en not_active Withdrawn
- 2011-06-24 KR KR1020127034013A patent/KR101540507B1/en active IP Right Grant
- 2011-06-24 WO PCT/JP2011/065135 patent/WO2012002520A1/en active Application Filing
- 2011-06-24 US US13/805,144 patent/US20130087257A1/en not_active Abandoned
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| JP2010090432A (en) * | 2008-10-08 | 2010-04-22 | Jfe Steel Corp | Super high-strength cold-rolled steel sheet excellent in ductility, and producing method of the same |
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Also Published As
| Publication number | Publication date |
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| JP5668337B2 (en) | 2015-02-12 |
| EP2589674A4 (en) | 2017-06-21 |
| KR101540507B1 (en) | 2015-07-29 |
| KR20130037208A (en) | 2013-04-15 |
| WO2012002520A1 (en) | 2012-01-05 |
| EP2589674A1 (en) | 2013-05-08 |
| US20130087257A1 (en) | 2013-04-11 |
| JP2012012642A (en) | 2012-01-19 |
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