稀有金属(英文版) 2015,34(11),764-769

收稿日期：16 May 2013

基金：financially supported by the Ministry of Science and Technology ‘‘Twelfth Five-Year’’ Plan for Science & Technology Support (No. 2011BAE22B00);

High-temperature deformation behavior of titanium clad steel plate

Shang-Wu Zeng Ai-Min Zhao Hai-Tao Jiang Xiao-Qian Yan Ji-Xiong Liu Xiao-Ge Duan

Engineering Research Institute, University of Science and Technology Beijing

Beijing Laboratory of Metallic Materials and Processing for Modern Transportation

Baoti Research Institute, Baoti Group Co., Ltd.

Abstract：

In this paper, the single-pass hot compression experiment of titanium clad steel plate was carried out by Gleeble-3500 thermal mechanics simulation test machine,and the effect of deformation temperature(T), strain rate(ε),thickness ratio(k), and friction coefficient(l) on flow pattern of the metal and stress in the deformation zone was analyzed.The results show that the metal flow behavior and the stress during compressive deformation depend strongly on the deformation temperature. At 800 and 850 °C, the bimetal can flow uniformly, while at 900 °C, the TA2 flows faster than Q235 B, and the phenomenon of TA2 wrapping Q235 B is observed. The metal flow of the bimetal material will coordinate each other through the bonding interface. It is noted that the stress increases with the increase of the ε and l and decreases when the metal flows along the contact area.

Keyword：

Titanium clad steel plate; Hot deformation simulation; Metal flow behavior; Stress;

Author： Hai-Tao Jiang e-mail: nwpujht@yahoo.com;

Received： 16 May 2013

1 Introduction

Current developments in advanced technologies are required for new materials with superior properties such as corrosion, wear resistances, high temperature for industrial applications. Therefore, numerous works were carried out to develop new materials for such purposes. However, composite materials consist of two or more different materials that not only have the features of raw materials, but also produce new performance, so it is used widely. Such as Ti_xSn_y/Ti Ni composites [1] have potential to be used as damping materials in the mechanical vibration field. Carbon nanotube reinforced Ti Ni matrix composites [2] can be used for super reinforcements. Ag–Cu com- posites [3] are used in the power equipment, telecommu- nications, household appliances, traffic, etc.

Titanium clad steel plate is one of the bimetallic mate- rials which owns excellent strength, heat transfer capabil- ity, welding performance of steel, and outstanding corrosion resistance of titanium alloy. Titanium clad steel plate has good comprehensive mechanical properties. It can save titanium and lower the cost of materials. So, it is universally used in the areas of chemical industry, ocean and electric power, and so on. Currently, explosive bonding is the main method for the production of titanium clad steel plate [4]. Hot rolling is carried out on the explosive tita- nium clad steel plate in order to produce larger size, area and to improve productivity. Today, research on titanium clad steels plate is mainly focused on heat treatment pro- cess, interface bonding performance, etc. [5–9]. The high- temperature mechanical behavior investigation of material is largely concentrated on monometal, such as the low carbon steel [10, 11], pipeline steel [12], pure titanium [13], or titanium alloys [14–16], and composite materials, for example, fiber resin [17], or metal sheets-powders [18]. But research on bimetal titanium clad steel plate (referred to as the ‘‘bimetal material’’ below) is still not enough.

Deformation temperature and strain rate are important factors that have effects on the hot deformation properties.For bimetal material, factors such as the thickness ratio (k: the ratio of thickness of titanium h and total thickness of bimetal material H, as shown in Fig. 1) and friction coef- ficient should also be considered. In this paper, the effects of the above factors on metal flow behavior and stress of bimetal material were investigated by thermal mechanics simulation test. The study can provide a valuable reference for establishing rational rolling process of bimetal material in the production.

Fig. 1 Schematic of thickness ratio

2 Experimental

Experimental material was titanium steel clad plate man- ufactured by means of explosion welding. The thickness of the specification was 4 mm (TA2) ? 36 mm (Q235B). Chemical compositions of the TA2 and Q235B are given in Table 1.

Cylindrical specimens of U8 mm 9 12 mm were machined from the bimetal material, then the single-pass isothermal compression experiment was carried out on a Gleeble-3500 thermal mechanics simulation test machine. The hot simulation experiment was performed according to the process as shown in Fig. 2 and Table 2. In order to compare the effect of monometal and bimetal on stress, the TA2 and Q235B were performed when T = 750 °C, e_ ? 10 s 1and tantalum sheet ? graphite as a lubricant. The experimental specimens were heated up to 750–950 °C at a rate of 10 °C s^-1. single-pass compression test was per- formed after holding this temperature for 180 s, the deformation degree of 50 %, cooling rate of 30 °C s^-1. In the deformation process, argon was used as protection gas to prevent oxidation of the specimen. At the same time, in order to keep the temperature uniform during the heating and compression processes and reduce the friction between the specimen and the press head, tantalum sheet ? graphiteor tantalum sheet was, respectively, affixed on the surfaces of the specimen contacting with the press head, and the lubricant was applied on the both ends of the specimen.

Fig. 2 Schematic of hot compression deformation

Table 2 Hot compression deformation processes of bimetal material  下载原图

Thickness ratio:m—1:9,4—1:5,.—1:3,5—1:2Lubricant:u—graphite?tantalum sheet,e—tantalum sheet

Table 2 Hot compression deformation processes of bimetal material

3 Results and discussion

3.1 Bimetal flow behavior

3.1.1 Deformation temperature

The microstructure of the as-received Q235B steel is ferrite and pearlite. The as-received microstructure of TA2 is equiaxial a-titanium. The microstructure of bimetal mate- rial will change during the heating process. The micro- structure of Q235B transforms from pearlite to austenite, then from proeutectoid ferrite to austenitic, finally all au- stenitic phases form. For the TA2, grains grow when the temperature is below the temperature T_trof a , b trans- formation (882 °C); while turning into cubic crystal (b phase), the grains grow up rapidly when temperature is above T_tr. In the process of hot compression of bimetal material, metal flow behavior of bimetal material is not consistent, owing to their different plastic deformation ability.

Table 1 Chemical compositions of as-received bimetal material (wt%)  下载原图

Table 1 Chemical compositions of as-received bimetal material (wt%)

Fig. 3 Profile morphologies after hot deformation at different deformation temperatures ( e_ ? 10 s 1, k = 1:9, tantalum sheet ? graphite): a 750 °C, b 800 °C, c 850 °C, d 900 °C, and e 950 °C

Figure 3 is the profile morphology after hot deformation at different temperatures. It is obvious that the metal flow of the bimetal material proves inhomogeneous, as shown in Fig. 3a in 1 and 2 zones. With temperature increasing, the profile morphology of bimetal material changes from ‘‘waist drum-shape’’ to ‘‘cylindrical-shape’’ gradually, and the thickness ratio after deformation decreases. The difference of plastic deformation capacity and metal fluidity is nar- rowed between TA2 and Q235B at 800 and 850 °C, as shown in Fig. 3b and c. Up to 900 and 950 °C, the phases of TA2 shift from a to b, grain grows quickly, the plastic signifi- cantly rises, and the difference of bimetal fluidity sharply rises. From Fig. 3e in 3 zone, it can be known that the Q235B side on the bonding interface is wrapped by the TA2. In hot compression process, the Q235B will make resistance to TA2 to delay extension. At the same time, the TA2 will give Q235B tension to add extension, owing to that the bimetal bonding interface has good bonding properties.

3.1.2 Thickness ratio

Figure 4 is the profile morphology after hot deformation at different thickness ratios. Figure 4 shows that the differ- ence of bimetal fluidity enlarges along with thickness ratio increasing. Based on the analysis above, we can know that the fluidity of TA2 is better than Q235B at the same temperature. The thickness ratio increases (the thickness of TA2 increases); the metal flow amount increases along the contact surface, so the TA2 after deformation becomes more non-uniform, as shown in Fig. 5b, c.

3.1.3 Friction coefficient

Figure 5 is the profile morphology after hot deformation at different temperatures when tantalum sheet was used as a lubricant. During 750–900 °C, the profile morphology after hot deformation is ‘‘waist drum-shape,’’ it is ‘‘basin-shape’’ when the temperature is at 950 °C. At lower temperature,on the one hand, frictional resistance is relatively large, on the other hand, the TA2 is in a strong three-dimensional compressive stress state and normal stress of the contact interface is high. As a result, the TA2 is hard to flow along the contact interface. It moves and accumulates along the axis direction. So the bimetal material profile morphology after hot compression deformation is similar to monometal compression test. The friction coefficient between the TA2 and the press head decreases, grain coarsens and softens with temperature rising, especially at the temperature over titanium T_tr, therefore, the TA2 flows mainly along with contact interface.

Fig. 4 Profile morphologies after hot deformation at different thickness ratios ( e_ ? 10 s 1, T = 750 °C, tantalum sheet ? graphite): a k = 1:5, b k = 1:3, and c k = 1:2

By comparing Fig. 5 with Fig. 3, when using graph- ite ? tantalum sheet as a lubricant, the bimetal material flows mainly along the contact interface. However, when using tantalum sheet as a lubricant, the bimetal material flows mainly along the axis direction. The results show that friction coefficient is an important factor for bimetal flow behavior.

Through the above analysis, bimetal material flow rule can be summarized as follows. Figure 6 is the schematic diagram of bimetal material hot compression deformation process. The whole deformation process can be pided into three stages: the initial, the mid, and the final. S₁is the contact area of press head and TA2, S₂stands for the connection area of Q235B and TA2 (that is bonding interfacial), S₃is for contact area of press head and Q235B. Stress r₁, r₂, r₃corresponds to S₁, S₂, S₃.

(1) Deformation initial period: S₁= S₂= S₃, σ₁=σ₂=σ₃.

At this moment, all the positions of specimens bear the same small load, and no plastic deformation occurs.

(2) Deformationmidperiod:S₁＞ S₂= S₃,_σ1＜σ2= σ3.

With the increase of load, stress on TA2 side which is caused by the press head exceeds yield strength, so the TA2 will undergo plastic deformation at first, and flow on the S₁. Meanwhile, the stress per unit area reduces with the increase of S₁.

(3) Deformationfinalperiod:S₁＞ S₂＞ S₃,σ₁＜σ₂＜σ₃.

As the load continued increasing, the Q235B will also undergo plastic deformation. At the moment, the plastic deformation of TA2 side on the bonding interface is easierto occur; and metal flows along the radial direction and makes S₂gradually increase. Contrast with TA2, the Q235B is hard to deform, and will hinder S₂enlarging. So S₂becomes large, and it is less than S₁, stress would also change.

Fig. 5 Profile morphologies after hot deformation at lubricant for tantalum sheet ( e_ ? 10 s 1, k = 1:9, tantalum sheet): a 750 °C, b 800 °C, c 850 °C, d 900 °C, and e 950 °C

Fig. 6 Schematic diagrams of bimetal hot compression deformation process: a initial period, b mid period, and c final period

When the temperature is lower than 900 °C, bimetal material hot compressive deformation is the result of the constantly coordinated deformation above mentioned. However, when temperature is higher than 900 °C, the metal in TA2 side is softening quickly, and flows to the edge, then the metal in Q235B side starts to plastic deformation.

3.2 Bimetal stress–strain curves

In the process of hot compression deformation, on the one hand, the material undergoes plastic deformation because of compression, and work hardening forms with the increase of dislocation density. On the other hand, owing to the thermal activation, the dislocation dipole is cancelled; the cell wall becomes a regular arrangement, forming sub- grains and sub-grains of coalescence in dynamic recovery or recrystallization process.

The stress–strain curves of bimetal material isothermal compressed under different deformation temperatures, strain rates, thickness ratio, and friction coefficient are shown in Fig. 7a–d, respectively.

3.2.1 Deformation temperature

As shown in Figs. 7a and 8, the changing rule of stress along with the deformation temperatures under the different strain rates is given. It can be seen from Figs. 7a and 8 that under the same deformation degree, the stress is reduced along with the increase of the deformation temperatures. However, there is a rising stage in 850 °C, and the stage height decreases with the decrease of the strain rate.

It is universally acknowledged that the deformation temperatures are the most significant factor among all the facts affecting the stress value. The reason is that when the temperature rises, the energy for thermal motion of the atoms increases, which will facilitate easier dislocation glide and climb, generate new slip system or other plastic defor- mation mechanism, eliminate part of the dislocation pile-up due to the deformation, and reduce the dislocation motion resistance. At the same time, higher temperature facilitates easier development of the recovery and recrystallization process, and softens the work hardening which results in plastic deformation and thus reduces the stress. There are two possible reasons for the increase of stress at 850 °C. First, the Q235B has two phases coexisting at this temper- ature, which causes non-uniform deformation, produces additional stress, and makes material plastic reduction, so it is with TA2 [19]. Second, steel easily comes out the hot brittle phenomenon when sulfur content is over 80 9 10^-6[20], which has poor plasticity and high stress. The Q235B might take place hot brittle at 850 °C because the sulfur content is more than 80 9 10^-6. The higher the strain rate is, the larger the non-uniform deformation is, so the stress rises obviously when the strain rate is 10 s^-1.

3.2.2 Strain rate

As shown in Fig. 7b, the effects of the strain rate on the stress under the different strain rates and the same defor- mation temperature were studied. It can be seen that the stress increases significantly with the strain rate increasing, indicating the sensitivity of stress to the variations of strain rate.

When the bimetal material was deformed at a relatively slow strain rate (for example, at strain rate of 0.01 s^-1). Figure 7a shows that when the strain is less than 0.1, the stress increases rapidly with the increase of strain. This is because that the increase of dislocation density leads to the rapid increase of work hardening rate, and more than the recrystallization softening. When the strain is more than0.10 at 750 °C, the stress value becomes stable gradually because that the work hardening and the recrystallization softening reach a balance at this time. At 900 and 950 °C, the stress value shows increasing–decreasing–increasing wavy trend. This is because the bimetal material repeatedly undergoes intermittent dynamic recrystallization during the deformation process.

Fig. 7 Stress–strain curves in isothermal compression of bimetal material: a temperatures, b strain rate, c thickness rate, and d friction coefficient

Fig. 8 Relation of deformation temperature and stress (k = 1:9)

When the bimetal material was deformed at a relatively fast strain rate, the heat by deformation will have not enough time to be dissipated. This will increase the deformation temperature and thus assist the disorderly arranged dislocations produced during the deformation to be rearranged into certain low-energy structure, which is beneficial to the merging of unlike dislocations and decreases the dislocation density [11], so the stress rising rate decreases.

3.2.3 Thickness ratio

The curves in Fig. 7c represent the relationship between the stress and thickness ratio of the bimetal material. The stress rises along with the decrease of the thickness ratio. When thickness ratio is bigger (TA2 takes a large part of the bimetal material whole volume), it will cause the overall stress to decline and the stress–strain curve of bimetal is closer to monometal TA2. In the same way, when thickness ratio is small, the stress–strain curve of bimetal is closer to monometal Q235B. For the industry using of titanium clad steel plate, the thickness ratio can be reduced appropriately in order to improve strength.

3.2.4 Friction coefficient

As is shown in Fig. 7d, it can be seen clearly the rela- tionship between stress and friction coefficient. When strain is less than 0.3 and other conditions are the same, the stress of using tantalum sheet as lubricant is greater than graphite ? tantalum sheet. This is due to that the friction coefficient between the tantalum sheet and metal is greater than the graphite. Metal flow is mainly along with axis and resulting in larger stress while using tantalum sheet. However, when using the graphite ? tantalum sheet, metal flow is mainly along with contact area and leads to small stress. When strain is larger than 0.3, whether using tan- talum or graphite ? tantalum sheet as a lubricant, the stress is basically consistent. At the moment, the stress mainly depends on metal internal defects (dislocation density) at this deformation degree.

Figure 7 also shows that the stress–strain curves of bimetal material have slight serrated oscillation, especially high strain rate. This may be explained by that the plas- ticities of TA2 and Q235B are not the same. At the same time, the bonding interface of bimetal material shows the excellent bonding properties, which plays a part in coor- dination deformation during the deformation process. Another reason may be the low natural frequency of ther- mal simulation test machine.

4 Conclusion

In this study, hot compression deformation of titanium clad steel plate with different technological parameters was carried out. Experimental results show that the bimetal material flow behavior is greatly affected by deformation temperature, thickness ratio, and friction coefficient. The whole deformation process can be pided into three stages. The stress–strain curve of bimetal material has slight ‘‘serration’’ shape.