Effect of heating and cooling technology on microstructure of galvannealed coati

Cold Rolling Mill,Baoshan Iron & Steel Co.,Ltd.,Shanghai 200941,China

Abstract: Based on the galvannealed phase transformation theory,the phase and powder level of a galvannealed coating produced on one of the galvanizing lines at Baosteel’s cold-rolling plant were analyzed under different conditions before and after a technical innovation was introduced.After the introduction of the technical innovation,heating and cooling abilities of the galvannealing furnace were strengthened,galvanizing speed increased from 75 to 100 m/min,and the galvannealing target temperature could be reached more quickly.As such,the δ1 phase was more uniform and dense,and the coating’s anti-powdering ability was increased.Although the galvanizing time was slightly shortened,this had no negative effect on the anti-powdering ability of the galvannealed coating.

Key words: hot-dip galvannealed steel;auto-panel steel;phase transformation

1 Introduction

The steel plate with a galvannealed coating (GA coating) has attracted increasing attention and has been widely applied by industrial producers,particularly in the fields of household appliances and automobiles.This growth is caused by the excellent performance of the GA coating for environmental protection;further,it can be painted easily on to surfaces.Its formability,corrosion resistance,and weldability also make it a desirable product for various applications[1].Owing to the development of hot-dip galvanizing technology,requirements regard-ing the surface quality of galvanized plates have become stricter.First,there should be no defects on the surface of the galvanized plate to prevent the appearance of surface defects in the galvanized coating after painting.Moreover,the surface of a galvanized coating should be flat to ensure the brightness of the painting film,i.e.,freshness[2-3].In addition,the adhesion of a zinc layer is an important index for evaluating the quality of a plate with GA coating[4].If partial shedding of the coating occurs during use,especially during the stamping process,this will degrade the model and cause defects such as bright spots on the surfaces of various parts.The partial shedding of GA coating can be divided into two cases,if the size of the coating particles is less than the thickness of the coating,it is known as powdering,which is closely related to the phase structure of the GA coating.

Herein,the phases of a GA coating produced on one of the galvanizing lines at Baosteel’s cold-rolling plant were analyzed under different heating and cooling conditions before and after the introduction of the technical innovation.Using a V-bending detection method and other related means,differences in the adhesion of GA coating under different conditions were analyzed.

2 Different phases of GA coating

To obtain a GA coating containing 7%-15% Fe,a hot-dip galvanized steel plate was annealed in a galvan-nealing furnace within a temperature range of 450-550 ℃.A GA coating contains different phases,includ-ing the Γ phase (Fe3Zn10),Γ1phase (Fe5Zn21),δ1phase (FeZn8),and ζ phase (FeZn13) from the α-Fe to the outermost layer[5].Table 1 shows the structure and properties of different phases of GA coating.The powdering property of a GA coating is closely related to its microstructure and Fe-Zn intermetallic com-pounds[6].

Table 1 The structure and properties of different phases of GA coating

Before galvannealing,the steel plate was dipped into a zinc bath containing 0.10%-0.15% Al.According to the thermodynamic equilibrium condition of melted zinc,a blocking layer of Fe2Al5forms at the interface between the α-Fe and coating,which pre-vents the rapid reaction of Fe and Zn in the zinc bath that produces the ζ phase (usually referred to as an outburst).This phase is disadvantageous for the adhesion of the coating.

Subsequent galvannealing was performed at approximately 500 ℃.According to the Fe-Al-Zn ternary phase diagram proposed by Lepretre,it can be seen that Zn passes through the Fe2Al5barrier and enriches the Fe/Fe2Al5interface.When the Zn content reaches a certain concentration,the δ1phase starts to nucleate.With the growth of the δ1phase,the Fe2Al5barrier layer is destroyed.Then,with the continuous diffusion of Fe atoms in the coating,the pure Zn layer (η phase) changes to ζ phase and the ζ phase to δ1phase in turn.At the interface between the α-Fe and coating,the δ1phase continuously changes to the Γ phase.The range of the ζ phase’s Fe content is insignificant (the fluctuation of the Fe content is only 0.2%-0.5%).The composition of the ζ-phase layer is highly uniform,thereby hindering the diffusion of Fe and Zn.The diffusion of Zn to Fe through the ζ phase is blocked,which accelerates the increase of the Fe con-centration in the region between the ζ-phase layer and α-Fe,thereby promoting the formation of a δ1-phase nucleus with a high Fe content and gradual formation of the δ1-phase layer.The δ1phase grows very slowly during the initial stage and then becomes faster.

With an increase in the galvannealing temperature and prolongation of galvannealing time,a non-diffused solid-phase transition (lattice slip shear) occurs in the later stage of the galvannealing pro-cess.In this stage,the Γ and δ1phases react to generate a Γ1phase.The Γ1phase is a face-centered cubic crystal structure with a hardness value of about 500.The formation and thickening of the Γ1phase have a negative effect on the powdering index of the coating.Therefore,it is necessary to choose the appropriate heating and cooling curves to ensure the most effective phase structure of the GA coating.It is generally believed that the ideal structure of a GA coating can be obtained when the ζ phase has just disappeared,but the Γ1phase has yet to begin to form.

Fig.1 shows the typical galvannealing process.

The GA coating typically comprises a thin layer of the Γ phase between the α-Fe and thick δ-phase layers.The remainder comprises scattered ζ islands on top of the δ-phase layer.

3 Comparison of different heating and cooling curves for galvannealing

Considering 0.75 mm×1 490 mm IF steel as an example,Table 2 shows a comparison of the different galvannealing heating and cooling data obtained before and after the introduction of the technical innovation on a hot-dip galvanizing line at Baosteel’s cold-rolling plant.

Table 2 Comparison of different galvannealing heating and cooling technologies

3.1 Comparison of the galvannealing heating section

Before the technical innovation,an induction heater with approximately 2 000-kW heating cap-acity and 3-m length was located in the heating section.Taking 0.75 mm×1 490 mm IF steel as an example,the annealing speed was 75 m/min,and the temperature rise of the strip in the heating section was about 50 K.

After the technical innovation,the capacity of the induction heater had increased to about 4 000 kW,and the length of the equipment was the same as before.Owing to the improvement of the heating capacity,the speed of the annealing process increased to 100 m/min to achieve the production capacity mentioned above.Under these conditions,the increase in the temperature of the strip in the heating section was about 60 K.

3.2 Comparison of the galvannealing soaking section

No change occurred in the soaking section of the galvannealing process after the technical innov-ation.The thermal resistance of the soaking furnace ensured that the strip temperature in this 30-m-long section remained constant as much as possible.

3.3 Comparison of the galvannealing cooling section

Before the technical innovation,jet cooling was used in the cooling section with the length of approximately 12 m.For 0.75 mm×1 490 mm IF steel,the rate of temperature drop of the strip in this section is approximately 262 K/s.

Following the technical innovation,the fog cooling equipment was installed to improve the cooling rate of the strip.The total length of the equipment is approxi-mately 5 m,and the active length of the fog cooling process is automatically adjustable with respect to different conditions and annealing speeds.When producing 0.75 mm×1 490 mm IF steel,the active length of the fog cooling process is approximately 3.0 and 1.5 m at 100 and 75 m/min,respectively.

4 Microstructures of the GA coating under different technology conditions

The microstructures of the GA coating before and after the technical innovation were observed using a Zeiss scanning electron microscope (SEM).The SEM images are shown in Figs.2 and 3,respectively.

At the same annealing speed,the proportion of the scattered ζ phase (rod structure) on top of the coating is basically the same before and after the technical innovation.However,the scattered ζ-phase grains are finer after the technical innovation owing to the increased cooling rate.

Fig.2SEMimageoftheGAcoatingbeforethetechnicalinnovation

After the technical innovation,the annealing speed increased.The heating rate increased by more than 60%,whereas the cooling rate increased by more than a factor of three than that before the technical innovation.In the SEM images of the GA coating in Fig.3,it can be seen that the proportion of the scattered ζ phase (FeZn13) is reduced,and the GA coating mainly comprises the δ1phase,which has better plasticity.In Fig.3(b),the rod-like ζ phase on top of the coating has completely disappeared with only the dense δ1phase visible.

To evaluate the anti-powdering ability of the GA coatings produced under different process condi-tions,a V-bending test was conducted on the IF steel (Fig.4),and the results are listed in Table 3.Grade 1 in the V-bending evaluation is the best,followed by grade 2,and so on.The higher the reflectivity,the less the zinc powder peels off after bending and the better the powdering index.In Table 3,it can be seen that the anti-powdering ability of the GA coatings was improved after the technical innovation.

Table 3 Anti-powdering level of the GA coatings before and after the technical innovation

ModeAnnealing speed/(m·min-1)Sample sizeAverage reflectivity/%Average grade of the V-bending testBefore the technical innovation75352.882.06After the technical innovation75356.761.75100359.631.42

5 Conclusions

As shown above,to improve the anti-powdering ability of the GA coating,it is necessary to have reasonable control of the phases of the coating.The ideal state is when the ζ phase on top of the GA coating has just disappeared,and the Γ1phase at the interface of the coating and the α-Fe layer has not yet formed.

(1) After the technical innovation,the heating and cooling capacities of the galvannealing furnace were strengthened and the annealing speed increased from 75 to 100 m/min.The increase in the annealing speed is beneficial for the maintenance of the Fe2Al5layer and the prevention of abnormal growth of the Γ phase at the interface.A uniform and dense δ1phase can be obtained,and the anti-powdering ability of the coating is improved.

(2) After the technical innovation,the soaking time after galvannealing decreased from 24 to 18 s.This decrease in the soaking time had no negative effect on the phase structure or anti-powdering ability of the GA coating.However,the increased speed must not shorten the soaking time to less than 12 s.If the processing speed changes,the phases and anti-powdering index of the GA coating should be tested again.

(3) After galvannealing and soaking,it is appro-priate to treat the GA coating by rapid cooling.Slow cooling promotes the transformation from the Γ phase to the Γ1phase,which thickens the brittle layer of the coating,and worsens the anti-powdering index of the coating.

Acknowledgement

The SEM images used in this paper were obtained with the assistance of Jin Xinyan,a chief researcher at Research Institute of Baosteel.Some contents of this article were directed by Lin Chuanhua,a senior engineer at cold-rolling mill of Baosteel.I would like to express my gratitude to them.