Effect Of Rolling Parameters

Effect of Rolling Process on the Microstructure and Mechanical Properties of Low Carbon V-N Steel CAO Yan1,

HUANG Zhen-yi1, WANG Ping1, YIN Gui-quan1, LIU Xiang-hua2, WANG Guo-dong2

(1. School of Material Science and Engineering, Anhui University of Technology, Maanshan 243002, China; 2. State Key Lab of Rolling and Automation, Northeastern University, Shenyang 110004, China) Abstract: The effects of rolling process on the microstructure and mechanical properties were studied for the low carbon V-N steels with different carbon contents. The tensile strength properties and microstructural features of steels under various reduction and finish rolling temperature were determined. The results show that V-N strengthening effects were largely influenced by the rolling process parameters in the low carbon V-N steels. With the increase of carbon content in the steel and rolling reduction, the resultant ferrite grain size became smaller, and therefore caused the enhanced strength. It was observed that higher finish rolling temperature and larger deformation were suitable for the V-N steels with the higher carbon content. On the other hand, lower finish rolling temperature and larger deformation should be adopted for those with the lower carbon content. Key words: V-N microalloyed steel; controlled rolling; mechanical properties

1

Introduction

Vanadium is an effective and controllable precipitation strengthening element in low carbon steels. For low carbon V-N microalloyed steels containing a higher N content, the solubility of VN in austenite and ferrite is much lower than that of VC. As a result, precipitation of V(C, N) is usually accelerated and the precipitation strengthening effect of V causes the higher strength in the steels [1,2]. The controlled rolling has been well investigated in V-N microalloying steels. It has been found that the strengthening effects of V-N are largely determined by the thermo-mechanical control process (TMCP)[3,4]. At the beginning of the rolling, the austenitic grains are refined through austenite recrystallization after the rolling deformation[2]. There is no doubt that the finish rolling temperature and reduction also influence the growth of austenite grains and the precipitation of V(C, Table 1

N), therefore change the microstructure and mechanical properties of the rolled steels. However, scarce work has been reported on the optimization of the process parameters for the low carbon V-N steels. In this work, the effects of rolling process parameters on the microstructure and mechanical properties in the steels containing V-N （ V: 0.10-0.12%, N:100-200 ppm）were investigated in order to obtain optimal TMCP conditions for the steels.

2

Experimental Procedure

2.1 Materials Three low carbon V-N steels with different carbon contents are used in this research. The chemical compositions of the steels are shown in Table1. 2.2 Controlled rolling process The rolling experiments are carried out on a φ180 mm two-roller rolling laboratory mill. Three groups of

Chemical compositions of experimental steels

(wt，%)

Sample

C

Si

Mn

P

S

V

N

Als

1

0.19

0.45

1.35

0.01

0.0059

0.100

0.014

<0.005

2

0.11

0.41

1.32

0.01

0.0062

0.110

0.014

-

3

0.06

0.41

1.32

0.01

0.0063

0.110

0.014

-

foundation Item: Item Sponsored by State Key Lab of Rolling and Automation, Northeastern University (2003-1) Author: CAO Yan (1963 ), Femail, Associate professor, E-mail: [email protected]

blank samples (H20×B40×L200 mm) were heated in a furnace to 1150 ℃ for 30 min. The samples are then rolled at a start rolling temperature 1100 ℃, and finish rolling temperature 950 ℃ and 900 ℃ respectively. The rolling reduction is controlled to 30%, 40% and 50% respectively for different samples. The rolling reduction per pass is larger than 20% in order to avoid critical deformation and ensure refined γ grain. The rolled samples are air-cooled after rolling. 2.3 Tension test and microstructure observation Tensile specimens with a 20×20 mm section are made. The tensile test was carried out on a universal testing machine, and the yield strength and tensile strength are measured. Metallographic observation samples are prepared by mechanical polishing and subsequently etching with 4% nitric acid alcohol solution. The metallographic morphologies are observed with a Neophot-2 optical microscope. The average grain size of the ferrite microstructure is measured by the intercept method. Each sample is observed and measured for about 500 grains, and the average grain size is then calculated[5].

σs/MPa

600

500

950℃ 900℃ 900℃ 950℃ 900℃ 950℃

1 2 3

400 20%

30%

40%

50%

60%

Deformation

σb/MPa

750 650 550 450 20%

950℃

1

900℃ 900℃ 950℃ 900℃ 950℃

2 3

30%

40%

50%

60%

Deformation Fig. 1

Mechanical property of the three steels hot-rolled under the different rolling parameters 1 - Steel No.1;

3

2 - Steel No.2;

3 - Steel No.3

Results and Discusses

3.1 Rolling parameter optimization for the TMCP process

The mechanical properties of the three steels under the different rolling process are showed in Fig.1. The optimal rolling parameter for the steel with the higher carbon content sample (No.1) is the finish rolling temperature 950 ℃, and the reduction ratio 50%. The higher finish rolling temperature and larger reduction ratio are suitable for the carbon steel to obtain the best mechanical properties through the recrystallization control during rolling. On the other hand, the optimal rolling parameters for the lower carbon content samples (No.2 and No.3) are at a finish rolling temperature of 900 ℃, and a reduction ratio of 50% and 40%. In these steels, non-recrystallization control rolling, i.e. the lower finish rolling temperature and larger reduction ratio, is necessary for achieving best mechanical properties. With the lower carbon content, the recrystallization temperature of the steel is increased. For sample No.1 the finish rolling temperature of 950℃ is in the range of recrystallization temperature, and therefore belongs to the recrystallization controlled rolling. The recrystallization austenite grains form in the deformed austenite through some time period and repeated recrystallization occurs. The ferrite grains form at the austenite grain boundaries with the γ → α transformation processes. The refined austenite grains therefore reduce a refined ferrite microstructure with the increased austenite grain boundaries[2]. For samples No.1 the finish rolling temperature of 900℃ is lower than the recrystallization temperature of the steel. Therefore larger reduction ratio is required in order to obtain the optimized mechanical properties. For samples of No.2 and No.3 that have lower carbon contents, the finish rolling temperature of 950℃ and 900 ℃ , is in the range of non-recrystallization temperatures, and the rolling therefore belongs to non-recrystallization controlled rolling. However, VN precipitation occurs at this finish rolling temperature, and the lower finish rolling temperature causes the more obvious VN precipitation. The precipitation brings in more nucleation sites, and the finer ferrite grains are obtained after γ → α transformation. Meanwhile, more deformation bands occur at lower finish rolling temperature, which is beneficial for the formation of fine ferrite gains. The density of the deformation bands increases obviously with the increase of reduction ratio. An adequate reduction

ratio is necessary to avoid the retainment of deformation bands in the microstructure. Experiment shows that the optimal mechanical properties are obtained with lower finish rolling temperature and larger reduction ratio. As reduction ratio becomes larger, grain deformation is increased and grain refining is enhanced. Theσb is reduced relatively as the σs is ascended. 3.2 Effects of the rolling parameters on final microstructure The microstructure of the samples under several rolling processes is observed. The original austenite microstructure and the ferrite microstructure under the optimal parameters are shown in Fig.2. The microstructure under various rolling processes is composed of ferrite grains and pearlite microstructure. The ferrite grains are quite refined. As shown in Fig.2(b), finer ferrite grains are obtained in samples No.1 at the high finish rolling temperature and the larger deformation amount. And the percentage of pearlite microstructure has also increased obviously.

(a) Microstructures of initial austenite

(c) Steel No.2, finish rolling at 900℃, deformation 50%

In the other case, a mixed grain structure, has also been found. It is considered that the recrystallization grains after deformation is softer than the grains without recrystallization. And the soft grains may be easily deformed and subsequent recrystallization occurs in those grains and makes the local regions much finer. As a result, the inhomogeneous microstructure is retained in the steels. As shown in Fig.2(c), ferrite bands are clearly observed with large fraction. The lower rolling temperature causes the intensified austenite deformation bands, and ferrite grains form in the former austenite bands with the similar morphology. In contrast, with a lower rolling reduction of 40% at the finish rolling temperature of 900 ℃ homogeneous ferrite grains are observed, as shown in Fig.2(d). In this case austenite deformation bands have not developed obviously. Fig.3 shows the average grain size in the three steels under different rolling processes. A larger rolling reduction causes the decreases of ferrite grain size in all the steels.

(b) Steel No.1 , finish rolling at 950℃, deformation 50%

(d) Steel No.3 , finish rolling at 900℃, deformation 40%

Fig. 2 Microstructures of the steels under different rolling processes

16

1号钢 2号钢 3号钢

14 12

μm

10 8 6

References:

4 2 0

30%

40%

50%

----------950℃ ----------

Fig. 3

rolling parameters of higher carbon content are higher finish rolling temperature and larger reduction ratio. The better rolling parameters of lower carbon content are lower finish rolling temperature and larger reduction ratio.

30%

40%

50%

----------900℃ -----------

Average ferrite grain size in the steels with different rolling processes

4 Conclusions It was found that the rolling parameters obviously influence the microstructure and strength of hot rolled low carbon V-N steels. (1) The effect of rolling deformation is more obvious than the effect of finish rolling temperature. With the increase of rolling deformation, the grain size gets smaller, the ferrite grain becomes refiner and the yield strength is increased. (2) With the increasing of carbon content and rolling reduction, the microstructure becomes finer with the smaller grain size and increased percentage of pearlite, as well as the increased strength. (3) The optimal rolling processes for the steels with different carbon content are different. The better

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