Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 3, 2020
Article Number 304
Number of page(s) 8
DOI https://doi.org/10.1051/metal/2020027
Published online 13 May 2020

© EDP Sciences, 2020

1 Introduction

Weathering steel, atmospheric corrosion resistant steel, as a kind of high performance steel, has been produced and applied for many years [13]. Hot-charging rolling process (HCR), as a technology saving energy and reducing cost, has been utilized in many iron and steel enterprises. Not only can HCR technology reduce the cost of energy, but also shorten the heating time and reduce the loss of iron scale [49]. However, some problems frequently occur in HCR such as crack on the surface of steels, which increases the cost and decreases production rate.

So far, some investigations have been conducted on HCR process [1014]. For example, Storck et al. [10] reported that implementation of hot charging is a lean production strategy and a waste-reduction process. Li et al. [11] studied the reasonable temperature for hot charging of continuously cast slabs and claimed that some microalloyed steels exhibit high surface crack susceptibility to hot charging temperatures due to carbonitride precipitation. The sensitive hot charging temperature for an oil pipe steel is between 625 and 775 °C.Hot charging temperature influences the transformation, microstructure and final properties of steels [15,16]. Luo et al. [16] studied the impact of hot charging conditions on microstructure of high strength microalloyed steels. They reported that the microstructure consists of fine and homogeneous lath bainite at hot charging temperature (HCT) of 1000 °C, lath bainite and granular bainite at HCT of 900 °C and as the HCT decreases to 800 °C, a small fraction of proeutectoid ferrite appears along grain boundary. However, the effect of HCT on the transformation and microstructure evolution of weathering steels has not been reported.

Therefore, the aim of the present study is to investigate microstructure and transformation in a weathering steel at different charging temperatures. The results provide the basis for optimizing the HCT of a weathering steel.

2 Experimental material and procedures

The tested steel is a commercially produced weathering steel and the chemical composition is given in Table 1. The tested steel was taken from a hot-rolled strip of 15 mm thickness. Specimens for thermal simulation experiments were machined into a cylinder of 10.0 mm diameter and 15.0 mm height. The thermal simulator experiments were conducted on a Gleeble 3500 thermo-mechanical simulator. The experimental procedures, simulating the real HCR in hot continuous rolling, are presented in Figure 1. The thermocouple was welded at the middle of the sample to measure the temperature and the variation of axial displacement was measured by extensometer for determining the deformation amount. The specimens were heated to 1180 °C at 5 °C/s and held for 10 min for primary austenitization, and then cooled to 500 ∼ 850 °C at a cooling rate of 5 °C/s, respectively, and held for 10 min. After that, all specimens were heated to 1180 °C again and held for 10 min for secondary austenitization followed by cooling to 950 °C and deformed for 50% at a strain rate of 1.0 s−1. It should be noted that in real production, the reduction (strain) is much higher than that applied in the present study. Possible recrystallization during deformation and/or between the passes may occur, which will influence the final microstructure and this should be investigated in the future work. Finally, all deformed specimens were cooled to ambient temperature at 5 °C/s. Specimen were numbered and listed in Table 2.

In addition, both temperature gradient and the deformation gradient existed during thermal simulator experiments. The temperature at the middle of the sample is higher than that that at ends of sample. The sample became drum-shape after compressive deformation, which means the deformation in the middle was higher than at the ends. In this case, the recorded dilatation change was higher than that it should be. Although possible deviation were caused by temperature and deformation gradient, the dilatation data of different samples were comparable because the deformation temperature and amount were the same for all samples.

All specimens were prepared for microstructure observation by mechanical grinding and polishing before being etched with 4% nital. A Zeiss optical microscope and a Nova400 Nano field emission scanning electron microscope (FE-SEM) with an acceleration voltage of 20 kV were used to observe the microstructures. The middle of the samples after thermal simulator experiments is selected for microstructure observation. Samples are polished and etched for optical microstructure and then the samples were polished and etched again for SEM microstructure. The average grain sizes are measured through optical micrographs and the volume fractions of phases are measured through SEM micrographs. A HV-1000 hardness tester was utilized to measure the microhardness of specimens.

Table 1

The chemical composition for experimental steel (wt.%).

thumbnail Fig. 1

Experimental procedures for thermal simulation experiment.

Table 2

Specimen number in experiments.

3 Results and discussion

3.1 The first cooling process

Figure 2 gives the dilatation-temperature-time curves of specimens at different HCT. Dilatations during deformation at 950 °C and final cooling process after deformation are not plotted because of too large increase in dilatation caused by 50% strain. Specimen D500 is taken as an example to explain the dilatation curve. Specimen D500 was cooled to 500 °C after the first austenitization. During the first cooling process, transformation of austenite occurred in 669–594 °C temperature range. According to the transformation temperatures, it is assumed that ferrite, pearlite and bainite were formed, leading to the increase in dilatation (from point A to B), as shown in an enlarged image in Figure 2a. The transformation was finished at point B, and the corresponding temperature at point B was higher than the target temperature (500 °C). The specimen was continuously cooled to 500 °C, resulting in the decrease of dilatation because of no transformation from point B to C. Then, the specimen was isothermally held at 500 °C for 10 min during the first isothermal holding process. Ferrite, pearlite and bainite transformations had been already completed during the first cooling process. Thus, the diameter of specimen did not change during the thermal holding process. In addition, it is observed that the ferrite, pearlite and bainite transformations happened before cooling to isothermal temperature when the HCT was below 700 °C in Figures 2a2d.

To quantificationally compare the dilatation change caused by phase transformations during the first cooling process and isothermal holding process, the dilatation change with temperature before secondary austenitization of specimen D500 is presented in Figure 3 as an example. Specimen D500 was heated from room temperature to 1180 °C, leading to the increase in dilatation. Then, the specimen was cooled to 500 °C (the first cooling process). During the first cooling process and isothermal holding, ferrite, pearlite and bainite transformations happen, and the transformation amount can be calculated and expressed by the absolute value of dilatation change. As shown in Figure 3, ferritic, pearlite and bainite transformations begin at point C, and end at point D. Hence, the dilatation curve goes up from point C to point D. Assuming no transformation happens in the first cooling process, the dilatation would decrease from point C to point D’. Thus, the absolute value of the length of DD’ represents the transformation amount of specimen D500. Therefore, the dilatation change caused by phase transformations during first cooling process and isothermal holding of specimens at different HCT can be calculated similarly, and the results are given in Figure 4. It indicates that the transformation amount increases obviously when the HCT decreases from 850 to 700 °C, and when the HCT is further decreased to 500 °C, the transformation amount does not change significantly.

thumbnail Fig. 2

Dilatation-temperature-time curves of different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

thumbnail Fig. 3

Dilatation changing with temperature before secondary austenitization of specimen D500.

thumbnail Fig. 4

The dilatation change represents the amount of transformation during the first cooling process and isothermal holding at different HCT.

3.2 The final cooling process

Figure 5 exhibits the dilatation curves of different specimens during the final cooling process after deformation. During the final cooling process, ferritic and pearlite transformations happened. Tangent method was adopted to measure the ferrite transformation start temperature (Fs) and pearlite transformation finish temperature (Pf), and the results are given in Figure 5. It is observed that the Fs and Pf of specimens increase apparently after deformation compared to that of specimens with HCT below 700 °C in the first cooling process. This is ascribed to more nucleation sites introduced by deformation, resulting in the occurrence of ferrite transformation in advance. Moreover, Fs and Pf of all specimens during the second cooling process are about 820 and 600 °C, respectively, which are higher than that obtained in the first cooling process (about 670 and 590 °C, respectively), indicating that deformation plays a more important role on Fs and Pf of specimens.

thumbnail Fig. 5

Dilatation change with temperature during the second cooling process after deformation for different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

3.3 Microstructure observation

Figures 6 and 7 show the OM and SEM microstructures of different specimens after deformation, respectively. It can be seen that microstructure mainly consists of ferrite (F) and pearlite (P), as shown by arrows in Figures 6 and 7. When the HCT was between 500 ∼ 700 °C, which is below the dual-phase region temperature, the ferrite or ferrite and granular bainite were obtained after first cooling process and isothermal holding process. Then, the microstructure consisted of uniform fine austenite grains after secondary austenization, leading to the decomposition of nearly all austenite into ferrite in the final cooling process after deformation. Little pearlite was observed in microstructure (Fig. 6a). When the HCT was 750 °C, which belongs to the dual-phase region temperature, ferrite and austenite coexisted. The reversed transformation of ferrite to austenite happened and the pre-existing austenite became coarse during the secondary austenization. And the carbon content in fine reversed austenite was relatively low, while the coarse austenite contained higher carbon content after the second austenization. During the secondary austenitization, though the carbon distribution inside austenite grain can be homogeneous at 1180 °C for 10 min, the austenite grain boundaries are the obstacles for carbon atoms across boundaries to adjacent austenite grains, resulting in the carbon concentration gradient between austenite grains. The austenite grains with higher carbon content can easily decompose into pearlite in the final cooling process (Fig. 6f). However, the austenite grain was relatively larger at the HCT above dual-phase region temperature (800 and 850 °C) and the carbon content in grains was equally distributed. Thus pearlite transformation is difficult to proceed. In addition, ferrite is desired phase in the microstructure to ensure the plasticity of the steel. The HCT of 750 °C should be avoided in HCR process because specimen D750 contains more pearlite.

The micrographs of D600 specimen are given as an example to calculate the ferrite and pearlite amounts. The microstructure is composed of ferrite and pearlite and it can be identified according to morphology. As shown in Figure 8c, the regions belong to pearlite are circled. And the other regions are automatically colored in pink by Image-Pro Plus software, as shown in Figure 8b. The area percentage of pink regions can be automatically calculated by the software, and the result is termed as A1. Second, the area percentage of the circled regions (labeled as A2) can be obtained by A2 = 100 - A1. In this example, A1 is calculated to be 87.1%, thus A2 is 12.9%. Therefore, the ferrite and pearlite amounts in the D600 specimen are measured to be 87.1% and 12.9%. Similarly, the ferrite and pearlite amounts in other specimens are determined in the same way. To improve the accuracy of statistical results, three typical scanning electron microscopy (SEM) micrographs of each specimen were calculated, and the average value was obtained as the final result. The volume fractions of ferrite and pearlite in different specimens are listed in Table 3. It reveals that the ferrite amount decreases first and then increases, whereas the pearlite amount appears the opposite change trend.

Due to the unclear lamellar structure of pearlite, the grain size of pearlite cannot be measured, and only ferrite grain sizes in different specimens are determined statistically by Nano Measurer software. To improve the accuracy of statistical results, the grain sizes of three optical micrographs of each specimen were calculated, and the average value was obtained as the final result. The results of average ferrite grain size (FGS) are listed in Table 3. It shows that FGS of ferrite at 650 and 750 °C are larger than that in other samples.

thumbnail Fig. 6

OM microstructure of different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

thumbnail Fig. 7

SEM microstructure of different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

thumbnail Fig. 8

The example to show the method of calculating the volume fractions of ferrite and pearlite in D600 sample: a: the original micrograph; b: the ferrite regions in Figure 8a are colored pink; c: the pearlite regions are manually marked.

Table 3

The ferrite and pearlite amounts and ferrite grain sizes in different specimens.

3.4 Vickers microhardness

The microhardness tester was used to measure the microhardness of ferrite and pearlite of different specimens at the room temperature, and a force of 10 g was used. Five values were measured for each sample, and the maximum and one minimum values were all removed. Thus the average value of three intermediate values was obtained and the results are given in Table 4 and the pictures of imprints in samples D700 are given in Figure 9 as an example. It is seen that the Vickers hardness of ferrite is between 170 and 189 HV, and that of pearlite is between 201 and 221 HV.

Table 4

The microhardness of ferrite and pearlite in different specimens (HV).

thumbnail Fig. 9

The image of patches in sample D700: a: ferrite; b: pearlite.

4 Conclusions

The effect of HCT on transformation and microstructure evolution of a weathering steel was studied by metallography and dilatometry. The following conclusions are obtained:

  1. When the HCT is between 500 ∼ 700 °C, which is below the dual-phase region temperature, the ferrite, perilite and granular bainite are obtained after first cooling process and isothermal holding process. The microstructure consists of uniform fine austenite grains after secondary austenization, leading to the decomposition of nearly all austenite into ferrite in the final cooling process after deformation.

  2. When the HCT is at 750 °C, which belongs to the dual-phase region temperature, ferrite and austenite coexist. The reversed transformation of ferrite to austenite happens and the pre-existing austenite becomes coarse during the secondary austenization.

  3. The austenite grain is relatively larger at the HCT above dual-phase region temperature (800 and 850 °C), and the carbon content in grain is equally distributed. Thus, it is difficult for pearlite formation.

  4. To ensure the ductility of the tested weathering steel, the HCT of about 750 °C should be avoided in the industrial production.

Acknowledgment

The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (NSFC) (Nos. 51874216 and 51704217), The Major Projects of Technology Innovation of Hubei Province (No. 2017AAA116), The Cooperation Project of Hebei Iron and Steel Group (No. 2019313).

References

  1. T. Nishimura, Corros. Sci. 50, 1306–1312 (2008) [Google Scholar]
  2. J.H. Yang, Q.Y. Liu, X.D. Wang, L.I. Xiangyang, D. Sun, J. Chin. Soc. Corros. Protect. 27, 367–372 (2009) (in Chinese) [Google Scholar]
  3. L. Hao, S. Zhang, J. Dong, W. Ke, Metall. Mater. Trans. A 43, 1724–1730 (2012) [CrossRef] [Google Scholar]
  4. B.H. Liu, J.H. Jung, H.H. Lee, K.Y. Lee, J.Y. Lee, J. Alloys Compd. 245, 132–141 (1996) [Google Scholar]
  5. J.H. Jung, H.H. Lee, D.M. Kim, B.H. Liu, K.Y. Lee, J.Y. Lee, J. Alloys Compd. 5, 253–254 (1997) [Google Scholar]
  6. G. Wang, L. Zhao, H. Liu, Z. Liu, L. Sun, L.I. Qiang, J. Iron Steel Res. 13, 15–19 (2001) (in Chinese) [Google Scholar]
  7. Y.L. Sun, D.Y. Liu, J.H. Cui, Iron Steel 38, 27–29 (2003) (in Chinese) [Google Scholar]
  8. K. Puttkammer, M.G. Wichmann, T.S. Spengler, Hot strip mill scheduling under consideration of energy consumption, Operations Res. Proc., (2013) [Google Scholar]
  9. H. Bruns, R. Kaspar, Steel Res. 68, 364–367 (1997) [CrossRef] [Google Scholar]
  10. J. Storck, B. Lindberg, A lean production strategy for hot charge operation of a steel mill, Iet Int. Conf. Agile Manuf. IET, (2007) [Google Scholar]
  11. Y. Li, X. Chen, K. Liu, J. Wang, W. Jin, Metall. Mater. Trans. A 44, 5354–5364 (2013) [CrossRef] [Google Scholar]
  12. H.T. Ma, Gansu Metall. 38, 38–40 (2016) (in Chinese) [Google Scholar]
  13. E.J. Fang, F.X. Cui, W. Kang, X.W. Liao, Angang Technol. 1, 56–59 (2015) (in Chinese) [Google Scholar]
  14. Y.Z. Luo, J.M. Zhang, C. Xiao, S.Z. Wu, Steelmaking 4, 74–78 (2011) (in Chinese) [Google Scholar]
  15. J.H. Lee, W.J. Kwak, C.G. Sun, K.H. Kim, K.H. Ko, S.M. Hwang, Ironmak. Steelmak. 31, 153–168 (2004) [CrossRef] [Google Scholar]
  16. Y.Z. Luo, J.M. Zhang, C. Xiao, W. Song, S. Wang, Steel Res. Int. 83, 1214–1220 (2012) [CrossRef] [Google Scholar]

Cite this article as: Man Liu, Guang Xu, Guanghui Chen, Zhoutou Wang, Study on the transformation and microstructure evolution during hot-charging rolling process of a weathering steel, Metall. Res. Technol. 117, 304 (2020)

All Tables

Table 1

The chemical composition for experimental steel (wt.%).

Table 2

Specimen number in experiments.

Table 3

The ferrite and pearlite amounts and ferrite grain sizes in different specimens.

Table 4

The microhardness of ferrite and pearlite in different specimens (HV).

All Figures

thumbnail Fig. 1

Experimental procedures for thermal simulation experiment.

In the text
thumbnail Fig. 2

Dilatation-temperature-time curves of different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

In the text
thumbnail Fig. 3

Dilatation changing with temperature before secondary austenitization of specimen D500.

In the text
thumbnail Fig. 4

The dilatation change represents the amount of transformation during the first cooling process and isothermal holding at different HCT.

In the text
thumbnail Fig. 5

Dilatation change with temperature during the second cooling process after deformation for different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

In the text
thumbnail Fig. 6

OM microstructure of different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

In the text
thumbnail Fig. 7

SEM microstructure of different specimens: a: D500; b: D550; c: D600; d: D650; e: D700; f: D750; g: D800; h: D850.

In the text
thumbnail Fig. 8

The example to show the method of calculating the volume fractions of ferrite and pearlite in D600 sample: a: the original micrograph; b: the ferrite regions in Figure 8a are colored pink; c: the pearlite regions are manually marked.

In the text
thumbnail Fig. 9

The image of patches in sample D700: a: ferrite; b: pearlite.

In the text

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