LIAN Fuliang,LIANG Gaofei and CHEN Hongxing

Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

Effect of intercritical annealing temperature and cooling rate on the micro-structure and mechanical properties of low-carbon aluminum killed steel

LIAN Fuliang,LIANG Gaofei and CHEN Hongxing

Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

In this study,the intercritical annealing process for a typical low-carbon aluminum killed steel is investigated.A cold-rolled sheet was annealed at intercritical temperatures ranging from 730 ℃ to 770 ℃ and then cooled in air or water.The annealed steel was then baked at 210 ℃,and its mechanical properties and microstructures were analyzed in detail.It is shown that after the air-cooling process,the strength of steel decreased and ductility increased with an increase in the annealing temperature.However,after water-cooling,the strength and ductility both increased with the increase of annealing temperature.These results are attributed to the property-optimization of the steel.

aluminum killed steel; intercritical annealing; baked hardening

1 Introduction

Low-carbon aluminum killed steel contains<0.1% carbon content and excessive aluminum content.The low carbon content usually leads to the improvement of the steel formability,and the excessive aluminum content is used for deoxidation.This steel is widely used in architecture,household appliances,auto-mobiles,and the packaging industry[1].For example,it is commonly used as the exterior framework or on walls of a building,where it has already gone through a paint-application process.Low-carbon aluminum killed steel is frequently baked during the painting processes,which often have a significant effect on the final properties of steel[2-4].Therefore,the effect of annealing tem-perature on the final microstructure and mechanical properties of baked low-carbon aluminum killed steels needs to be investigated.

The annealing process for low-carbon aluminum killed steel was examined in our previous report at a cooling rate of 40 K/s[5].In this study,we investigate the annealing processes for the same steel at different cooling rates.

Two annealing processes are typically used for low-carbon aluminum killed steels: recrystallization annealing and intercritical annealing,which are distin-guished based on their annealing temperatures[6].The annealing temperature in recrystallization annealing is usually in the range of 620-700 ℃,during which equiaxed grains are generated[7].The annealing tem-perature in intercritical annealing is betweenA1(the end transformation temperature of austenite→ferrite) andA3(the initial transformation temperature of austenite→ferrite),during which ferrite and austenite are both generated during the heating process and phase transformation occurs during the cooling process[6].This paper focuses on the intercritical annealing process.

2 Experiment

Typical low-carbon aluminum killed steel was used with the chemical composition shown in Table 1.The dimensions of the final casting ingot were 200 mm (length)×60 mm(width)×30 mm(thickness).The ingot was subsequently heated to 1 180 ℃ for 40 min and then hot-rolled from a thickness of 30 mm to a thickness of 2.2 mm.The final rolling temperature was 870 ℃;the steel then was cooled via water-spraying to a coiling temperature of 580 ℃.After a 30 min holding time,the steel was cooled to 20 ℃ in air.Subsequently,the steel was acid-pickled and then cold-rolled at room temperature.The final thickness was 0.26 mm.

Table 1 Chemical composition of the low-carbon aluminum killed steel %

An optical microscope (OM) and a scanning electron microscope (SEM) were used to analyze the microstructures of the cold-rolled steel.For the OM or SEM observations,samples with a size of 10 mm(rolling direction)×15 mm(transverse direction)×0.26 mm(thickness) were used,and the surfaces corresponding to the rolling direction and transverse direction were not coating.These specimens were ground with grade 320#-2000#SiC sandpapers and polished with 1 μm diamond particles.Finally,the polished specimens were etched in an alcohol solution containing 4% of nitric acid.

The microstructure of this cold-rolled low-carbon aluminum killed steel is shown in Fig.1.Ferrite grains were severely deformed into strips.Carbide strips and particles were distributed on grain boundaries.

Fig.1 Microstructure of the low-carbon aluminum killed steel plate after cold rolling

Subsequently,the cold-rolled steel was annealed continuously.The annealing curve is shown in Fig.2.The sheet was heated to a holding temperatureTat a rate of 10 K/s,maintained at the holding tem-perature for 50 s,and then cooled in air or water to room temperature.The holding temperatures varied between 730,750,and 770 ℃.

Using the JmartPro software,theA1temperature of this steel is estimated to be 718.4 ℃ and theA3temperature is estimated to be 877.8 ℃.Therefore,when the cold-rolled steel was annealed at 730,750,and 770 ℃ ,the temperatures were all in the two-phase region.

After the annealing processes,the steel was baked at 210 ℃ for 30 min to simulate the heating process of painting.The microstructures and mechanical properties in the rolling direction were then analyzed.

3 Results

3.1 Microstructures

The microstructures of the steel after the baking pro-cesses are shown in Fig.3.As shown in Figs.3(a),(c),and (e),when the steel is annealed from 730 ℃ to 770 ℃ and subsequently air-cooled,in addition to equiaxed ferrite grains,some black zones are observed in the microstructures.The average grain sizes are approximately 7- 8 μm.The Fe3C particle-size gradually decreases as the annealing tem-perature increases.In comparison with the number of Fe3C particles distributed throughout different bands,fewer Fe3C particles were obtained at 750 ℃ than at 730 ℃.Banded-distribution was observed in the partial region,and Fe3C particles were more dispersed in the other regions.Fe3C particles completely disappear when the annealing temperature reaches 770 ℃.

As shown in Figs.3(b),(d),and (f),when the steel is annealed at temperatures between 730 ℃ and 770 ℃ and then subsequently water-cooled,equiaxed ferrite grains and black zones are observed.The average grain sizes are approximately 7- 8 μm.Fe3C particles were dispersed at 730 ℃,at which the number of these particles is much less than that observed under the air-cooling condition (Fig.3(a)).Fe3C particles completely disappear between 750 ℃ and 770 ℃.The contrast of the black zone is slightly different from that obtained under the air-cooling condition.

The black zones were again observed via SEM,as shown in Fig.4.When the steel was annealed between 730 ℃ and 770 ℃ and then air-cooled,the black zone was determined to be lamellar pearlite,which is composed of lamellar ferrite and lamellar Fe3C.The spacing between the lamellars was approximately 0.1 μm.The Fe3C particle-size was in the range of 0.1- 0.5 μm at an annealing temperature of 730 ℃ and 0.1- 0.2 μm under the water-cooling condition.The black zone obtained under the water-cooling condition was martensite.

3.2 Mechanical properties

After annealing,the steel was baked at 210 ℃ for 30 min,the resultant mechanical properties at each annealing temperature are shown in Table 2.The mechanical properties were tested according to GB/T 228—2010 using a versatile testing machine with a gauge length of 50 mm.

Based on the data shown in Table 2,the mecha-nical property-annealing temperature curves for the steel under different cooling conditions are shown in Fig.5.Under the air-cooling conditions as the an-nealing temperature increased from 730 ℃ to 760 ℃,the yield and tensile strength of the steel decreased whereas elongation increased.However,under water-cooling conditions,as the annealing temperature increased from 730 ℃ to 760 ℃,the yield strength,tensile strength,and elongation all increased.It is also clear that compared with air-cooling,water-cooling imparts better mechanical properties to the steel.

Table 2 Mechanical properties of the baked low-carbon aluminum killed steels

Fig.5 Mechanical properties of the low-carbon aluminum killed steel under different annealing temperatures and different cooling conditions

4 Analyses

4.1 Microstructure

When the cold-rolled steel is annealed at 730,750,and 770 ℃,which were all in the two-phase region,the microstructure of the steel comprises ferrite and austenite.Austenite was generated with the following reaction: ferrite + Fe3C → auste-nite;this suggests the number of Fe3C particles decreases with annealing.Using the JmartPro soft-ware,the percentages of ferrite and austenite were estimated under different annealing temperatures: 91.11% ferrite and 8.89 % austenite were obtained at 730 ℃,88.81% ferrite and 11.18% austenite at 750 ℃,and 85.75% ferrite and 14.25% austenite at 770 ℃.The reaction austenite → ferrite + pearlite occurs in the air-cooling process.It can be observed that the higher the final annealing temperatures,the more the pearlite is produced in the air-cooled micro-structure.The reaction austenite → martensite occurs in the water-cooling process.In this case,a higher final annealing temperature leads to the production of more martensite in the water-cooled microstructure.

As mentioned previously,Fe3C particles were observed in both the air-cooled and water-cooled samples after the annealing process at 730 ℃;air-cooled steel had larger particles than water-cooled steel.It is obvious that a spheroidizing annealing process occurred during the air-cooling process,i.e.,some small Fe3C particles nucleated independently and grew from austenite.Meanwhile,some other Fe3C particles nucleated and grew around the undissolved Fe3C particles.However,when the steel was cooled in water,the cooling rate was so rapid that it did not allow nucleation of the Fe3C particles;therefore,the carbon content was solid-soluted in martensite.

4.2 Mechanical properties

The variations in the mechanical properties of steel are due to the differences between the microstructures.When the steel is annealed at a higher temperature and cooled in air,recrystallization is more strongly promoted and the dislocation density and the amount of Fe3C particles decrease;consequently,its strength decreases and ductility increases with an increase in the annealing temperature.Conversely,when steel is water-cooled after the annealing process,more martensite is generated when the annealing temperature is increased;this increases its strength.Meanwhile,recrystallization during high-temperature annealing creates a ferrite matrix with a higher ductility;thus,the steel has both higher strength and higher ductility.

5 Conclusions

(1) When the low-carbon aluminum killed steel is annealed at different temperatures in the dual-phase region and then cooled in air,lamellar pearlite and ferrite are observed.In this case,the strength of the steel decreases and ductility increases with an increase in the annealing temperature.

(2) When the steel is annealed at different temperatures in the dual-phase region and cooled in water,martensite and ferrite are observed.In this case,the strength and ductility of steel both increase.

(3) High-temperature annealing in the dual-phase region with fast cooling leads to high strength and high ductility.

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LIAN Fuliang; E-mail: lianfuliang@baosteel.com

10.3969/j.issn.1674-3458.2016.04.003