| Dual-phase steel is widely used in the automotive industry owing to high tensile strength,low yield strength,high initial work hardening rate and excellent strength-toughness match.However,to be used in automotive exposed parts galvanizing is essential to provide corrosion protection.Galvanizing of DP steels poses two major challenges:On the one hand,the heat treatment used to obtain the DP steel microstructure and mechanical properties is not necessarily compatible with the higher temperatures required for hot-dip galvanizing;On the other hand selective oxidation of the alloying elements used in DP steels,especially Si,can result in bare spot defects and unacceptable coatings.Based on this,in the present paper the design idea of chemical composition for substitution of Si by Al was proposed,which was composited of(0.10～0.12)C-(1.75～1.80)Mn-(0.38～0.42)Al-(035～0.38)Cr-(0.15～0.18)Mo-(0.03～0.05)Nb(in wt.%).The intention is to enhance the anti-oxygenic ability of the steel and keep desired mechanical properties.Hot rolling process,cold rolling and continuous annealing process,continuous annealing process of hot-dip galvanizing DP steel was researched deeply and systematically.Moreover,the surface selective oxidation of the experimental steel also was studied.The main works are present as follows:1)A new composition system for substitution of Si by Al was designed.The static and dynamic CCT curves were determined by thermal dialation testing.Then phase transformation during intercritical annealing was studied in a cold-rolled sample.The results showed that under fast heating condition,the recrystallization stage is suppressed during heating stage,the austenite formation and its distribution are influenced by the overlapping of the processes of ferrite recrystallization and austenitization.The volume fraction of austenite increases with the increase of heating rates,intercritical annealing temperatures and holding times,while,the hardenability of austenite decreases.While,a smaller amount of banite is observed for the higher intercritical annealing temperatures.2)The effect mechanism of parameters for hot-rolling process on the microstructure and mechanical properties of experimental steel was investigated,to provide theoretical basis for making hot rolling process of the substitution of Si by Al and niobium microalloying DP steel.Ferrite-martensite DP microstructures was obtained when the tested steels were hot rolled in 7 passes from 60 mm to about 4 mm(60→40→30→16→10→7→5→4 mm),the rolling temperature in recrystallized austenite region was controlled over 1000 ℃,strat rolling temperature in the non-recrystallized austenite region was 950 ℃,finishing temperature in the non-recrystallized austenite region and coiling temperature were controlled in the range of 850～790 ℃ and 200～300 ℃.With a decrease in finish rolling temperature,the volume fractions of ferrite increase and matensite decrease.Moreover,the nano scale Nb based carbides have played a crucial role in achieving microstructure refinement of hot-rolled DP steel.When the finish rolling temperature is 790 ℃ and coiling temperature is 200 ℃,the steel has excellent mechanical properties,the product of strength and ductility(Rm×TEL)is 20264 MPa·%.The improvement of strength and plasticity can be attributed to the proportion,morphology and distribution of ferrite and martensite,which is also the consequence of coordination and combination action of each factor.3)To explore the effect of annealing process on microstructure and mechanical properties of cold-rolled DP steel.The interactional relationship and control principle among annealing parameters,microstructure and properties were given.Among the different heat treatment scheduced,intermediate quenching(IQ)sample clearly yields most attractive combination of strength and ductility compared to intercritical annealing(IA)and step quenching(SQ)samples,which can be explained by the fine and fibrous martensite morphologies,precipitation of fine carbide and a small amount of residual austenite.Rapid heating of continuous annealing process can improve recrystallization temperature further by inhibitting the recovery,thus the recrystallization kinetics was greatly promoted and the ultra-fine microstructure of the DP steels was achieved.When the heating rate is 300 ℃/s,fine and uniform microstructure was obtained,thus the tested steel has excellent mechanical properties.With the increase of intercritical annealing temperature,martensite morphology and distribution showed significant change and resulted in significant changes in the mechanical properties of tested steels.When the intercritical annealing temperature was 780 ℃,the tested steel had excellent mechanical properties.Rm=1043.5 MPa,TEL=18.7%,the product of strength and ductility(Rm×TEL)is 19513 MPa·%.In addition,ultrafine grained dual phase steel with uniform microstructures and excellent properties was obtained through hot-rolled,cold-rolled and ultra-rapid heat treatment.The average ferrite and martensite grain size was refined to 1.5 μm and 1.3 μm,respectively.The new developed UFG-DP steel showed superior strength-uniform elongation balance(Rm×UEL=13645MPa·%)in comparison with the UFG-DP steels mentioned in references.4)To explore the effect of hot-dip galvanizing annealing process on microstructure and mechanical properties of cold-rolled DP steel.The interactional relationship and control principle among annealing parameters,microstructure and properties were given.On one hand,the increase in intercritical annealing temperature increases austenite volume fraction.On the other hand,carbon content of the austenite phase decreases with increasing volume fraction of the austenite.Therefore,with the increase of intercritical annealing temperature,the microstructure and mechanical properties of tested steel showed significant change.The process parameters of hot-dip continuous galvanizing line such as the cooling rate after inter-critical annealing influenced the mechanical properties.An increase of the cooling rate after inter-critical annealing is benefit for the mechanical properties of tested steel.When the 460 ℃isothermal soaking time is less than 20 s,the microstructure and properties of tested steel had no obvious impacts with the varied of hot-dip galvanizing times,this illustrates the test steel has a wide galvanized process window.When the cooling rate was 50 ℃/s and 460 0C isothermal soaking times were 3～20 s,the tested steel has excellent mechanical properties:Rm=946.8～962.3 MPa,TEL=20.5～21.0%,the product of strength and ductility(Rm×TEL)is 19727～19882 MPa-%.When the heating rate was 300 ℃/s,the strength increased with a prolongation in intercritical annealing time,but the elongation rate first increased and then decreased with the increase of holding time.The value of strength×elongation(Rm×TEL)characterized on combined mechanical properties for strength and plasticity reached up to 20000 MPa·%when the intercritical annealing time was 60 s.5)Thermodynamic simulations show that temperature and H2 component has a little influence in the oxide.Dew point has the most impact in oxide.The influences of dew point on selective oxidation have been investigated by experiments.The results show that as dew point raised the internal oxidation of alloying elements takes place,and consequently the mass of external oxides decrease and thus improve the galvanisability of steel sheets.However,the present work also found that the dew point has an impact on the microstructure of the experimental steel.With the increase of dew point,the decarburization layer on surface increased.Compared with industrial steel,the experimental steel showed excellent galvanisability under the same annealing processing. |
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