CN101191174B - Hot-rolling phase change induction plasticity steel with 750MPa-level extension strength and preparation method thereof - Google Patents
Hot-rolling phase change induction plasticity steel with 750MPa-level extension strength and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a hot rolled transformation induced plasticity steel of 750MPa in tensile strength and the manufacture method which comprises the following procedures of 1) smelting, casting and blocking, the mass percentage compositions are: 0.08 to 0.20 percent of C, no more than 0.8 percent of Si, 0.5 to 2.0 percent of Mn, 0.8 to 2.5 percent of Al, no more than 0.03 percent of Nb, no more than 0.010 percent of N, no more than 0.020 percent of P, no more than 0.008 percent of S, 0.001 to 0.005 percent of Ca and the balance iron. 2) The blooms are heated to between 1150 and 1250 DEG C; keep the temperature and the holding time is in proportion to thickness 1 to 1.5min/mm; 3) Rolling: deformation occurs between the temperature ranges for austenite recrystallization, one pass or multi passes are adopted and the blooms are rolled into inner slabs. When deformation occurs between the temperature ranges lower than the Tnr temperature without austenite recrystallization but higher than Ar3 transformation point, blooms are rolled into steel plates of final thickness, and the finishing rolling temperature 800 to 880 DEG C.4) cooling. The invention has economical components, lowersilicon contents, no Cr, Ni, Mo or other noble metal alloy elements, simple and applicable hot rolling technique, and steel slates of good surface quality, high strength and good plasticity.
Description
Technical field
The present invention relates to the manufacture method of ferrous materials, particularly tensile strength 750MPa hot-rolling phase change induction plasticity steel (TRIP) and manufacture method thereof.
Background technology
Automobile steel has a large amount in variety.Traditional low strength steel (LSS) and high-strength steel (HSS) mainly contain the part low strength rank kind of gapless atomic steel (IF), high strength gapless atomic steel (IF-HS), general carbon soft steel (MILD), isotropy steel (IS), baking hardened steel (BH), carbon manganese steel (C-Mn) and Hi-Stren steel (HSLA), dual phase steel (DP), phase change induction plasticity steel three kinds such as (TRIP).Development along with automobile industry, requirement to vehicle safety is more and more higher, and light weight also requires more and more gentlier to improve fuel economy, requirement to the intensity aspect of vehicles steel plate improves constantly, also more and more, thin more to the performance requriements division of labor of different sites steel plate simultaneously, so advanced high-strength steel (AHSS) or ultrahigh-strength steel (Ultra-HSS) arise at the historic moment.Usually tensile strength can be classified as ultrahigh-strength steel greater than the steel grade of 700MPa, and the TRIP steel has over-all propertieies such as high strength, high-ductility and good absorption energy, is an important steel grade in the advanced high-strength steel.
The TRIP effect is at room temperature to contain the part residual austenite in the steel, and residual austenite absorbs energy to martensitic transformation under the strained effect, thereby significantly improves material plasticity and work hardening capacity, is called phase-change induced plastic again.
In order to obtain this TRIP effect, early stage TRIP steel generally contains valuable alloys such as higher nickel, chromium, and carbon content is higher, and this is restricted its application.
Useful in recent years heat treatment mode produces cold rolling TRIP steel and hot-rolling method is produced the TRIP steel, its chemical ingredients scope is generally: 0.10～0.40%C, 0.50～2.50%Si, 0.50～2.50%Mn adds the alloying elements such as Nb, Ti, Ni, Cr, Mo and Cu of certain content in addition.Mainly contain low-alloy composition system, Si-Mn system and contain valuable alloy element component systems such as chromium, nickel.For low-alloy composition system, carbon content can be lower, but need to add the Nb of high level and Ti and improve the intensity of steel by strict controlled rolling, increased production cost and controlled rolling difficulty.Adding chromium and nickel element has increased the manufacturing cost of steel especially.Though Si-Mn system is a kind of economical composition system, obtain high strength, its carbon content is generally higher, and because silicone content is higher, causes product performance surface quality unstable and steel bad.
European patent EP 748874A1 has mainly set forth a kind of production method of hot rolling TRIP steel of tensile strength 700MPa level of adding aluminium, adopts non-recrystallization district's controlled rolling and laminar flow refrigerative method to produce.This patent contains 0.12～0.30%C, and 1.2～3.5%Mn except adding Al, also can add valuable alloying elements such as Ti, Cr, Mo, Cu and Ni.Do not contain the Nb element in this hot rolling TRIP steel, carbon and manganese content are all higher, mainly rely on the effect of alloying element rather than water be used for obtain the TRIP effect, production cost is higher, its mechanical property yield tensile ratio is higher, is unfavorable for punch process; And the tensile strength grade of the steel grade of this patent is the 700MPa level.
Application No. US6797078 discloses a kind of production method of hot rolling TRIP steel of tensile strength 700MPa level of adding aluminium, adopts the method production of segmentation cooling control after the hot rolling.The alloying constituent of this patent is uneconomical and roll postcooling control and require very strict.This patent need be added Cr content, if do not add Cr, the content of its residual austenite can reduce, and can cause tensile strength to reduce.Its carbon content and manganese content are all higher in addition, bring difficulty for smelting and welding; And the tensile strength grade is the 700MPa level, is lower than strength grade of the present invention.
European patent EP 881306A1 provides a kind of good extending performance, has the composition and the production method of the heterogeneous steel of TRIP effect.It improves the intensity of steel by adding Nb, V and Ti, and does not adopt Al as the alloying element that promotes that ferrite forms.Require segmentation cooling control mode, improved the production difficulty.Its composition is 0.05～0.50%C, 0.35～0.50%Si, and 1.35～1.80%Mn also can add following several element :≤0.015%Nb ,≤0.020%V ,≤0.020%Ti.On technology, adopt the segmentation type of cooling, after finish to gauge finishes, be cooled fast to air cooling for some time after a certain temperature, and then be cooled fast to coiling temperature.This method is not easy to control.
Japan JP 2003321738A discloses a kind of composition and production method of hot rolling TRIP steel.Be organized as ferrite, bainite, paralympic (three-phase), or four phases (containing martensite).Composition is C:0.1～0.2%, Si:0.5～2.0%, and Mn:0.5～1.7%, Al :≤0.06%, N :≤0.006%, Mo:0.1～0.5%, Ti:0.05～0.2%.Silicone content is higher, and has added valuable Mo alloy.
European patent EP 295500Al discloses a kind of hot-rolled high-strength thin plate, and plasticity is good, is organized as ferrite, bainite and paralympic; Do not add other alloy, composition C:0.15～0.40%, Si:0.5～2.0%, Mn:0.5～2.0%.Though this composition is the economic sector system, silicone content is higher, in heat-processed, is easy to generate sticking and thick red iron oxide skin, is not easy de-scaling, is pressed into steel plate in rolling easily, causes surface quality bad.
Summary of the invention
The object of the present invention is to provide a kind of tensile strength 750MPa level hot-rolling phase change induction plasticity steel and manufacture method thereof, has composition economy, silicone content is less, do not contain such as valuable alloying elements such as Cr, Ni, Mo, hot rolling technology is simple, and steel plate has that surface quality is good, intensity is high and characteristics such as plasticity is good.
The present invention makes full use of " water is as alloying element " new alloy designs thought, employing is with the economic sector series of aluminium for silicon, and exploitation is used to manufacture the method for rolling the back controlled chilling of " the one-part form controlled chilling " of hot rolling TRIP steel, composition system and technology than report hot rolling TRIP steel in the document, have that carbon content is lower, composition economy, rolling technology is simple, rolls the easily characteristics of row of postcooling technological operation.The hot rolling TRIP structure of steel of producing by the inventive method is ferrite+bainite (martensite)+residual austenite, has good plasticity when strength grade is high, the good and every stable performance of product surface quality.Adopt the tensile strength of the hot rolling TRIP steel that this aspect produces to have superstrength more than or equal to 750MPa, the intensity plasticity of reflection TRIP steel important indicator is long-pending can be greater than 22000Mpa*%.
For achieving the above object technical solution of the present invention: tensile strength 750MPa level hot-rolling phase change induction plasticity steel, its composition quality per-cent is:
C 0.10～0.20％
Si ≤0.8％
Mn 0.5～2.0％
Al 0.8～2.5％
0＜Nb≤0.03％
N ≤0.010％
P ≤0.020％
S ≤0.008％
Ca 0.001～0.005％
All the other are iron and unavoidable impurities.
Carbon: most economical, the most basic strengthening element in the steel. the height of carbon content has determined the intensity rank of steel plate largely. in all elements, the effect that carbon reduces the Ms temperature is the strongest, be that austenitic transformation becomes the requisite element of strengthening phase in the steel such as martensite, bainite. among the present invention, span of control is 0.08%～0.20%, if carbon content is too low, then in process of cooling, austenite can change martensite into and can at room temperature not stablize, and does not have the TRIP effect; Otherwise carbon content is higher than at 0.20% o'clock, and then the plasticity of steel and toughness reduce, and weldability is variation also.
Manganese: manganese is the displaced type alloying element, improves the intensity of steel by the solution strengthening crystal grain thinning.Simultaneously manganese is to enlarge one of effective elements of Fe-C phasor γ phase region, can stable austenite, reduce martensitic transition temperature, and under the suitable situation of manganese content, can increase the possibility of carbon enrichment, residual austenite content is increased.But the manganese too high levels can reduce the activity of carbon in the austenite, and promotes the formation of carbide, and too high manganese also is easy to segregation, worsens the performance of steel.Note the balance of manganese and silicon, aluminium element, adjust distribution and the volume fraction of the relative quantity of interpolation with the control phase.
Silicon: silicon plays the solution strengthening effect in steel.Silicon is to suppress the element that cementite forms in steel, can increase the activity of carbon in austenite, promotes ferrite to form, thereby carbon is diffused in the residual austenite.Silicon or analogous element serve as the ferrite function of stabilizer, not only quicken the formation of proeutectoid ferrite, and during bainite formation, hinder separating out of cementite, quicken carbon be diffused into austenite mutually in, therefore silicon helps to increase the stability of residual austenite, the interpolation of silicon also cooperates the segmentation process for cooling can promote ferrite to separate out, and helps reducing the yield strength of steel.Add silicon in the steel and can improve purity of steel and deoxidation.But too high silicon can make troubles to hot rolling heating and subsequent product coating, influences the surface quality of steel plate.So the control silicone content is not more than 0.8% among the present invention.
Aluminium: it is 1.2～2.0% that steel of the present invention adds aluminium content.Because the Si too high levels can reduce product surface quality in the Si-Mn system, and then influences the coating effect of the finished product, replaces silicon with aluminium, aluminium also is ferrite former, can promote ferritic formation, and carbon is diffused in the residual austenite.So can reduce the silicone content in the steel.Aluminium still is the main deoxidant element in the steel, helps crystal grain thinning.The aluminium that adds among the present invention is mainly used to replace the partial action of silicon, can improve the tensile strength and the unit elongation of steel, improves the surface quality of steel.
Niobium: niobium is strong carbon compound forming element.The strain inducing of niobium carbide has postponed static state and dynamic recrystallization in the thermal deformation process when hot rolling, improves non-recrystallization temperature, helps the austenitic phase-change product of refinement deformation, improves the intensity and the toughness of steel.
Sulphur and phosphorus: sulphur forms the plastic occluded foreignsubstance manganese sulfide with chemical combination such as manganese in steel, especially unfavorable to the horizontal plastic property and the toughness of steel, so the content of sulphur should be low as much as possible.Phosphorus also is the harmful element in the steel, the plasticity of grievous injury steel plate and toughness.More than or equal to 0.003% o'clock, be the detrimentally affect to lateral performance of the inclusion of avoiding the MnS class as sulphur content, the metamorphosis that can adopt the Ca treatment process to carry out sulfide is handled.
Manufacture method of the present invention comprises the steps:
1) smelt, be cast as base by following composition, its composition quality per-cent is: C 0.10～0.20%, Si≤0.8%, Mn 0.5～2.0%, Al 0.8～2.5%, 0＜Nb≤0.03%, N≤0.010%, P≤0.020%, S≤0.008%, Ca 0.001～0.005%, all the other are iron and unavoidable impurities;
2) steel billet is heated to 1150～1250 ℃; Insulation, soaking time is by net thickness 1～1.5min/mm;
3) rolling, the temperature range internal strain of recrystallize can take place at austenite, adopt a time or multi-pass, described billet rolling is become intermediate slab; At austenite recrystallize not taking place promptly is lower than the Tnr temperature but is higher than Ar
3The temperature range internal strain of transition point becomes the steel plate of final thickness with above-mentioned steel plate rolling, and finishing temperature is between 800 ℃～880 ℃;
4) cooling.
Wherein, one-part form method of cooling (roll the first air cooling in back after strong cold method of cooling) is adopted in step 4) cooling, under the long situation of cooling roller bed after rolling, back air cooling 4-10s is rolled in employing, with speed of cooling steel plate is quickly cooled to 350～450 ℃ of bainite transformation temperature ranges then, air cooling after being incubated after 30-60 minute air cooling or coiling into coil of strip greater than 60 ℃/s.
Because aluminium and silicon all are the elements that promotes that ferrite forms, carbon and manganese content among the present invention simultaneously are all lower, and therefore, austenite is to the temperature of ferritic transformation higher (730 ℃ or above promptly enter between the phase change zone).When cooling roller bed after rolling is long, can adopt the one-part form method of cooling, be that finishing temperature is at 800 ℃～830 ℃, the air cooling stage can enter austenite and ferrite two-phase region temperature after rolling, continue air cooling to obtain the ferritic structure of enough content in this temperature, carbon is diffused in the residual austenite; Be quickly cooled to the bainite transformation temperature range and batch or be incubated, bainitic transformation takes place, silicon hinders separating out of cementite, makes carbon more than needed continue to spread to residual austenite.Because the effect of an amount of manganese and carbon has reduced martensitic transition temperature point, therefore, there is stable residual austenite to exist in the time of can making room temperature in the tissue.
Again, two-part method of cooling (cold soon-air cooling after rolling-strong cold method of cooling) is adopted in the step 4) cooling, under the short situation of cooling roller bed after rolling, employing is cooled to 650 ℃～750 ℃ medium temperature fast earlier with the cooling rate greater than 20 ℃/s and stopped 2～10 seconds, be cooled to 350～450 ℃ with the cooling rate that is not less than 50 ℃/s again, air cooling after being incubated after 30-60 minute air cooling or coiling into coil of strip.
Hot rolling TRIP steel mainly obtains high strength by phase transformation strengthening, and therefore key is in the process of cooling that requires after finish to gauge, each phase constitution that the acquisition ratio is suitable: ferrite, bainite or bainite and martensitic mixed structure and residual austenite.So in process of cooling, need following phase transition process, earlier steel is cooled to austenite and ferrite two-phase region temperature range, enough ferrite transformations take place in this temperature range slow cooling for some time, being cooled fast to the bainite transformation temperature then batches, in the coiling temperature interval, bainitic transformation can take place in part austenite, remaining austenite is because carbon containing and manganese content are higher, martensite transformation temperature is lower than room temperature, therefore be cooled in the room temperature process, martensitic transformation does not take place, exist with the form of residual austenite.Final room temperature texture is the polyphase microstructure that comprises ferrite, bainite and residual austenite.
Discover that its thermal physical property parameter of hot rolling TRIP steel of Si-Mn system causes final tissue and unstable properties easily along with variation of temperature is very sensitive.And, the control of rolling each stage of postcooling is required not to be very harsh with aluminium its stable mechanical property of steel for silicon.Can adopt the one-part form method of cooling, also can adopt the two-part method of cooling, can obtain suitable microstructure and property equally.
The present invention has following characteristics:
1. compare with previous patent, aluminium is not only as reductor, also replaces the effect of element silicon by the aluminium element of high level, to promote the enrichment in residual austenite of ferritic phase transformation and carbon.Better than siliceous high TRIP steel surface quality, and stable mechanical property.
2. do not add valuable alloying element, the main effect that utilizes " water is as alloying element ", in rolling back water-cooled process, obtain suitable ferrite, bainite (or bainite and martensitic mixed structure) and the residual austenite tissue of percentage composition, the economical hot rolling TRIP steel that production intensity and plasticity over-all properties are good.
3. compare with European patent EP 0748874, the both has added the aluminium element of high level, but European patent EP 0748874 carbon content height, added multiple valuable alloying element to obtain the TRIP effect, also improve intensity in addition by adding titanium elements, when obtaining high tensile strength, its yield tensile ratio is also higher, is unfavorable for punch process like this; The present invention does not add valuable alloying elements such as Cr, Mo, Cu, Ni, and alloy formula economy is simple, usually improves tensile strength by adding a spot of niobium unit.
4. the present invention in terms of existing technologies, the easier control of the online type of cooling after the finish to gauge, production control is simple and stable, be easy to produce. can adopt the one-part form cooling also can adopt the two-part cooling simultaneously. by with aluminium for the composition system of silicon with the controlled chilling after rolling, can obtain the hot rolling TRIP steel that tensile strength equals the 750MPa level, intensity plasticity is long-pending greater than 22000MPa*%, surface quality is good, be fit to be applied to automobile structure and safety member, cut down the consumption of energy, promote the manufacturing capable of circulation of iron and steel, have better market prospect.
Beneficial effect of the present invention
Composition design of the present invention is economical, helps resources effective utilization and recyclable, easy to implement and control, and technology controlling and process is simple and easy to do.The armor plate strength height has high plasticity, and intensity plasticity is long-pending to be reached more than the 22000MPa*%.After above-mentioned described composition and technological process, the tensile strength of thickness of slab 1.5～4.0mm hot-rolled steel sheet is greater than 750MPa, and unit elongation A50 (JIS5 gauge length 50mm sample) greater than 28%, can reach 34.5%.Tissue is mainly ferrite, bainite or bainite and martensitic mixed structure and residual austenite.
Description of drawings
Fig. 1 is the synoptic diagram of one-part form controlled chilling of the present invention;
Fig. 2 is the synoptic diagram of two-part controlled chilling of the present invention;
Fig. 3 a is a hot rolling TRIP steel for the C-Mn that the present invention contains Al;
Fig. 3 b is the test steel plate photo of hot rolling TRIP steel for the C-Mn of the correlated high Si of the present invention;
Fig. 4 is the typical gold phase constitution figure of the embodiment of the invention.
Embodiment
Specific embodiments of the invention are referring to table 1～3, and Fig. 4 is the typical gold phase constitution figure of embodiment.
Table 1
Unit: mass percent
| Embodiment | C | Si | Mn | Al | Nb | P | S | N | Ca | Fe |
| 1 | 0.08 | 0.80 | 2.00 | 1.70 | - | 0.004 | 0.004 | 0.0029 | 0.0010 | Surplus |
| 2 | 0.08 | 0.80 | 1.95 | 0.85 | 0.030 | 0.012 | 0.008 | 0.0032 | 0.005 | Surplus |
| 3 | 0.10 | 0.60 | 1.98 | 1.20 | - | 0.005 | 0.002 | 0.0035 | 0.0026 | Surplus |
| 4 | 0.10 | 0.60 | 1.95 | 1.10 | 0.015 | 0.020 | 0.003 | 0.0030 | 0.0034 | Surplus |
| 5 | 0.12 | 0.08 | 1.15 | 2.30 | 0.028 | 0.014 | 0.007 | 0.0040 | 0.0035 | Surplus |
| 6 | 0.15 | 0.06 | 0.95 | 2.50 | - | 0.005 | 0.002 | 0.0034 | 0.0048 | Surplus |
| 7 | 0.19 | 0.10 | 0.85 | 2.30 | - | 0.004 | 0.003 | 0.0026 | 0.0037 | Surplus |
| 8 | 0.20 | 0.10 | 1.70 | 2.35 | - | 0.006 | 0.003 | 0.0039 | 0.0022 | Surplus |
| 9 | 0.15 | 0.12 | 1.20 | 2.30 | 0.019 | 0.014 | 0.002 | 0.0090 | 0.0023 | Surplus |
| Embodiment | C | Si | Mn | Al | Nb | P | S | N | Ca | Fe |
| 10 | 0.16 | 0.08 | 0.50 | 2.40 | 0.025 | 0.006 | 0.002 | 0.01 | 0.0025 | Surplus |
The processing parameter that table 2 embodiment is adopted
| Embodiment | Heating temperature ℃ | Finishing temperature ℃ | Finished product thickness of slab mm | Roll postcooling technology | Cold ℃ eventually | Remarks |
| 1 | 1200 | 800 | 1.5 | Roll the cold 5s in back, 70 ℃/s is as cold as coiling temperature | 350 | The one-part form control is cold |
| 2 | 1220 | 800 | 1.5 | Roll the cold 5s in back, 70 ℃/s is as cold as coiling temperature | 350 | The one-part form control is cold |
| 3 | 1200 | 830 | 2.0 | Roll the cold 6s in back, 80 ℃/s is as cold as coiling temperature | 380 | The one-part form control is cold |
| 4 | 1220 | 830 | 3.0 | Roll the cold 4s in back, 85 ℃/s is as cold as coiling temperature | 350 | The one-part form control is cold |
| 5 | 1220 | 830 | 3.0 | 24 ℃/s of leading portion.714 ℃ stopped 50 ℃/s of back segment 4 seconds | 350 | The two-part control is cold |
| 6 | 1170 | 850 | 2.0 | 670 ℃ stopped 65 ℃/s of back segment 8 seconds | 450 | The two-part control is cold |
| 7 | 1180 | 830 | 3.0 | Roll the cold 4s in back, 70 ℃/s is as cold as coiling temperature | 400 | The one-part form control is cold |
| Embodiment | Heating temperature ℃ | Finishing temperature ℃ | Finished product thickness of slab mm | Roll postcooling technology | Cold ℃ eventually | Remarks |
| 8 | 1180 | 850 | 3.0 | 40 ℃/s of leading portion.700 ℃ stopped 50 ℃/s of back segment 8 seconds | 400 | The two-part control is cold |
| 9 | 1220 | 880 | 4.0 | 34 ℃/s of leading portion.650 ℃ stopped 60 ℃/s of back segment 6 seconds | 400 | The two-part control is cold |
| 10 | 1220 | 830 | 4.0 | Roll the cold 5s in back, 70 ℃/s is as cold as coiling temperature | 400 | The one-part form control is cold |
The mechanical property of the foregoing description sees Table 3.By table as seen, wherein 7 tensile strength are all greater than 760MPa among 10 embodiment, and yield strength 505-580Mpa, unit elongation A50 be greater than 28%, up 34.5%, and strength and ductility product is greater than 22000MPa*%, up 26220MPa*.The intensity of not adding Nb element comparative example is on the low side.
Table 3 embodiment mechanical property
| Embodiment | Yield strength/MPa | Tensile strength/MPa | Unit elongation A 50％ | Strength and ductility product/MPa*% |
| 1 | 505 | 710 | 33.0 | 23430 |
| 2 | 575 | 760 | 34.5 | 26220 |
| 3 | 530 | 680 | 32.0 | 21760 |
| 4 | 580 | 760 | 29.0 | 22040 |
| 5 | 585 | 810 | 30.0 | 24300 |
| 6 | 520 | 780 | 28.0 | 21840 |
| 7 | 510 | 850 | 29.5 | 25075 |
| 8 | 530 | 880 | 28.5 | 25080 |
| 9 | 580 | 870 | 29.0 | 25230 |
| 10 | 550 | 890 | 28.5 | 25365 |
Fig. 1, Fig. 2 are respectively that the refrigerative dual mode is the synoptic diagram of one-part form controlled chilling, two-part controlled chilling in the technology of the present invention.
Fig. 3 a is a hot rolling TRIP surface of steel plate photomacrograph for the C-Mn that the present invention contains Al; Fig. 3 b is a hot rolling TRIP surface of steel plate photomacrograph for the C-Mn of the correlated high Si of the present invention;
As seen from the figure, the present invention is more smooth for the hot rolling TRIP surface of steel plate of Si with Al, and the red iron oxide skin is few, and surface quality is good; And the hot rolling TRIP steel surface red iron scale of high Si is very many, and is embedded into the steel plate the inside, also is difficult to clean by pickling.Show in the composition design of hot rolling TRIP steel, by improving plate surface quality for Si with Al.
Fig. 4 is the typical gold phase constitution figure of the embodiment of the invention.Tissue is mainly ferrite (shown in the arrow A), bainite or bainite and martensitic mixed structure (shown in the arrow B) and residual austenite (shown in the arrow C), meets the requirement of organizing of TRIP steel.
In sum, the present invention is a kind of excellent comprehensive performance that has, and has high-intensity composition design and the manufacture method that has the hot rolling TRIP steel of good plasticity simultaneously.By the composition design of reasonable economy and the process for cooling after the special strict hot rolling of controlling, the back one-part form is rolled in employing or the sectional type process for cooling reaches more than or equal to 750MPa the tensile strength of the hot-rolled steel sheet of thickness of slab 1.5mm～4.0mm, yield strength is more than or equal to 500MPa, unit elongation A50 (JIS5 gauge length 50mm sample) is up to 34.5%, and has good work hardening ability.The structural part, stiffener and the safety member that can be used for automobile.
Claims (2)
1. tensile strength 750MPa level hot-rolling phase change induction plasticity steel, its composition quality per-cent is:
C 0.10～0.20％
Si ≤0.8％
Mn 0.5～2.0％
A1 0.8～2.5％
0＜Nb≤0.03％
N ≤0.010％
P ≤0.020％
S ≤0.008％
Ca 0.001～0.005％
All the other are iron and unavoidable impurities.
2. the manufacture method of tensile strength 750MPa level hot-rolling phase change induction plasticity steel as claimed in claim 1 comprises the steps:
1) smelt, be cast as base by following composition, its composition quality per-cent is: C 0.10～0.20%, Si≤0.8%, Mn 0.5～2.0%, Al 0.8～2.5%, 0＜Nb≤0.03%, N≤0.010%, P≤0.020%, S≤0.008%, Ca 0.001～0.005%, all the other are iron and unavoidable impurities;
2) steel billet is heated to 1150～1250 ℃; Insulation, soaking time is by net thickness 1～1.5min/mm;
3) rolling, the temperature range internal strain of recrystallize can take place at austenite, adopt a time or multi-pass, described billet rolling is become intermediate slab; At austenite recrystallize not taking place promptly is lower than the Tnr temperature but is higher than Ar
3The temperature range internal strain of transition point becomes the steel plate of final thickness with above-mentioned steel plate rolling, and finishing temperature is between 800 ℃～880 ℃;
4) cooling, adopt the one-part form method of cooling or, the two-part method of cooling;
Adopt the one-part form method of cooling, under the long situation of cooling roller bed after rolling, back air cooling 4～10s is rolled in employing, with the speed of cooling greater than 60 ℃/s steel plate is quickly cooled to 350～450 ℃ of bainite transformation temperature ranges then, air cooling after being incubated after 30～60 minutes air cooling or coiling into coil of strip;
Adopt the two-part method of cooling, under the short situation of cooling roller bed after rolling, employing is cooled to 650 ℃～750 ℃ medium temperature fast earlier with the cooling rate greater than 20 ℃/s and stopped 2～10 seconds, be cooled to 350～450 ℃ with the cooling rate that is not less than 50 ℃/s again, air cooling after being incubated after 30～60 minutes air cooling or coiling into coil of strip.
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