Research on the preparation process of pure nickel plate

The 5mm thick hot rolled pure nickel plates were used as raw materials to prepare cold rolled pure nickel plates. The study of the mechanical properties of the finished product with the trend of the annealing temperature, as well as semi-finished products’ annealing temperature, cold rolling deformation on the pure nickel plate shape, and finished product properties. It is found that there is a steep inflection point in the properties of pure nickel plates with the change of annealing temperature, which is about 580-600 ℃, and the changes of mechanical properties before and after the inflection point are small. Before the inflection point when the annealing temperature is low, the strength and cold working state properties are comparable. After the inflection point annealing temperature is higher, with the increase in temperature, the strength slowly decreases, and the elongation slowly increases. The suitable annealing temperature for finished products should be selected at 600-700℃ after the inflection point. Reduce the annealing temperature of semi-finished products and half-hard annealing can improve the shape and mechanical properties of the finished cold rolled sheet. Increasing the deformation of the single rolling process (about 80%) can improve the mechanical properties of pure nickel plates.

Pure nickel N02201 has good corrosion resistance and good mechanical properties, In the cold, and hot states has good pressure processing performance, mainly used in the chemical and electronics industries. However, the current domestic nickel plate processing level is relatively backward, mainly in process technology, equipment level, product quality, etc. cannot produce products to meet the domestic market demand; part of the high-performance, high-precision products are mainly dependent on imports, and subject to foreign constraints. Western titanium in non-ferrous metal processing technology and equipment advantages, pure nickel N02201 plate trial production.

1. Test material and test method

Test material using vacuum induction melting + vacuum self-consumption arc melting method of melting into N02201 ingots, the use of multi-fire free forging ingot forging into 260mm × 1000mm × 1100mm slabs, in 2800mm four-roll hot rolling mill on the slabs of the two fires of hot rolling, to get the 5mm-thick plate. The 5mm hot rolled sheet as raw material, the hot rolled state of the plate was heat-treated at different temperatures; the heat treatment temperatures were 700 ℃ and 550 ℃, with annealing holding time of 150min, cooling to 200 ℃ out of the air-cooled. Then, the 5mm hot rolled plates were cold rolled to 3, 2.5, and 0.6mm on a 1780mm six-roll cold rolling mill. The cold rolled state, as well as the plates heat-treated at different annealing temperatures, were subjected to room-temperature mechanical properties testing and metallographic organization observation to study the effect of single-pass rolling deformation and annealing temperature on the properties of pure nickel plates and to find the suitable cold working deformation and annealing temperature range.

2. Test results and discussion

2.1 Raw material condition

The properties of a 5mm thick plate annealed at 700℃×150min and 550℃×150min are shown in Table 1, and the metallographic organization is shown in Figure 1. As can be seen from the data in the table, 5mm hot rolled sheet by 700 ℃ × 150min annealed performance to meet the requirements of the ASTM B162 standard, by 550 ℃ × 150min annealing, tensile strength, and yield strength has a small decrease, elongation slightly increased.

Table.1 5mm thick N02201 pure nickel plate performance

State		Rm/MPa	Rp0.2 /MPa	A50(%)
R-state	Portrait	505	455	23
		500	445	22
	Transverse	510	460	17
		525	445	21
M-state (550 ° c)	Longitudinal	445	375	28
		450	385	27
	Transverse	460	390	24
		465	385	26
M-state (700 ° c)	Portrait	375	93	55
		380	97	53
	Transverse	380	95	55
		380	101	56
Standard requirements		345	80	40

Figure.1 5mm thick hot rolled N02201 nickel plate annealed state metallographic organization

As can be seen from Figure 1, after annealing at 550 ℃ × 150min, the microstructure of the hot working state still exists, and complete recrystallization did not occur. After annealing at 700℃×150min, complete recrystallization occurs, and the grain size is uniform, with an average grain size of 50μm.

2.2 The effect of annealing temperature and deformation on cold rolled sheet properties

In the 1780mm six-roll cold rolling mill on the 700 ℃ × 150min annealed 5mm thick nickel plate was rolled, respectively, rolled into δ2.5mm × 1000mm × Lmm and δ3mm × 1000mm × Lmm two specifications, the total deformation of the rolling were 50% and 40%. From the observation of the whole plate shape after rolling, there is no obvious center wave and edge wave, but there is a large wave in the longitudinal direction of the plate, as shown in Figure 2. This is because the pure nickel by 700 ℃ × 150min annealed hardness is small and easy to deform, the rolling process plate shape is not easy to control, and there are waves.

Figure.2 Plate with longitudinal waves

In the finished plate before annealing, to find a suitable annealing temperature of pure nickel plate, annealed at 700 ℃, rolled to a 3mm thick plate annealing temperature test, the test temperature from 400-850 ℃, insulation 30min, and then air-cooled. The properties of the plates are shown in Fig.3.

Fig.3 Longitudinal mechanical properties of 3mm thick cold rolled nickel plate
It can be seen from the figure that the tensile strength and yield strength of the rolled pure nickel N02201 sheet is very small; the yield ratio is very high, but the elongation is small. After annealing, the strength of the sheet decreases, and the elongation increases. However, when annealed at 400-500 °C, the strength and elongation change little, which is very close to the performance of the Y state. When annealed at 550-580 °C, the strength decreases and elongation increases slightly. When the annealing temperature reaches 600 °C and above, the strength decreases sharply, especially the yield strength decreases more significantly, showing a very low yield ratio and a rapid increase in elongation. It can be seen that there is an inflection point (> 580-600 °C) in the annealing temperature-mechanical properties curve of pure nickel N02201 sheet. Above the inflection point temperature, the strength decreases sharply relative to the rolling state, especially the yield strength. The elongation increases sharply, but with the increase in temperature, the mechanical properties of the sheet maintain a small change in a wide range, the strength decreases slightly, and the elongation remains unchanged. Below the inflection point temperature, the properties of the plate are similar to those of the rolled state. Compared with the rolled state, the tensile strength at 580 °C decreases by 40-70MPa, the yield strength decreases by 70-80MPa, and the elongation increases by 7 % -9 %, which belongs to semi-hard annealing.

It can be seen from the above data that after annealing at 700 °C × 150min and rolling to a 3mm thick plate after a series of annealing, its performance did not meet the standard requirements. The possible reasons are as follows: the annealing temperature of a 5mm thick plate is higher; the rolling deformation is small during cold rolling 3mm plate different heat treatment temperature of the metallographic organization shown in Figure 4. Visible from the figure nickel plate after cold rolling, the grain is elongated and was “streamlined” organization. After annealing at 400, 500 ℃, the rolled state organization returns the organization with a small amount of recrystallized grains. 550 ℃ there are a small number of grains that are not recrystallized in 600 ℃, and above the temperature of the annealing, pure nickel plate fully recrystallized, but in 600-700 ℃, the grain size is not uniform, 750-800 ℃ grain size tends to be the same. Still, there is a tendency to grow up the grains. Combined with the performance of different heat treatment temperature plates, although the 3mm thick plate performance does not meet the standard requirements, from the above data, it can be seen that pure nickel N02201 plate is suitable for annealing temperature of 600-700 ℃, this temperature, the grains occur completely recrystallized, and the grains did not grow significantly.

Figure.4 Metallographic organization of 3mm thick N02201 sheet at different heat treatment temperatures
According to the 3mm thick plate organization and properties with the annealing temperature changes in the law, 5mm thick hot rolled plate rolled 2.5mm thick plate annealed at 600, 650, 700 ℃, and its performance is shown in Table 2. As seen in Table 2, the 2.5mm thick plate due to deformation greater than the 3.0mm thick plate; therefore, by (600-700)℃ × 30min annealing, its performance is higher than the performance of the 3.0mm thick plate, and the performance is qualified to meet the standard requirements. It can be seen that the deformation of pure nickel cold rolled sheet should be more than 50% to get a strong plasticity match.

Table.2 2.5mm thick cold rolled N02201 pure nickel plate mechanical properties

State		Rm /MPa	Rp0.2/MPa	A50(%)
600°Cx30min	Transverse	430	96	43
		430	95	44
	Portrait	435	100	46
		440	105	45
650°Cx30min	Transverse	430	91	53
		430	98	55
	Portrait	435	96	50
		430	98	50
700°Cx30min	Transverse	425	84	50
		425	84	51
	Portrait	425	88	52
		430	89	52
Standard requirements		≥345	≥80	≥40

A 2.5mm thick plate metallographic organization is shown in Figure 5. As can be seen from the figure, the plate after annealing for the recrystallization of the organization, the grain size is generally 30-50μm. 2.5mm thick plate grain relative to the 3.0mm thick plate grain to be small.

Figure.5 2.5mm thick finished plate after annealing metallography

To verify the influence of a semi-finished annealing system on the performance of the finished cold rolled sheet, a 5mm thick plate annealed at 550 ℃/150min, after annealing and surface treatment, cold rolled to 3mm thick, rolled, and found that the plate shape of the plate is better than the 700 ℃ × 150min annealed and then cold rolled to 3mm thick plate shape is good. Its plate photo is shown in Figure 6. Annealed state properties are shown in Table 3.

Figure.6 550 ℃ annealed nickel plate cold rolled to 3mm thick plate

Table.3 550 ℃ annealed nickel plate cold rolled into 3mm thick plate mechanical properties

Technology		Rm/MPa	Rp0.2/MPa	A50(%)
650°Cx90 min	Portrait	395	136	50
		400	130	50
	Transverse	390	121	53
		395	118	55
Standard requirements		≥345	≥80	≥40

As seen from Table 3, reducing the annealing temperature of semi-finished products can be appropriate to maintain the strength level of semi-finished products and ultimately improve the mechanical properties of the finished product, especially the yield strength.

To further find out the maximum deformation of cold rolling single pass, after 550 ℃ × 150min annealed 3mm thick plates continue to cold rolled to 0.6mm, single pass deformation of 80%, rolled plate shape is good, as shown in Figure 7, its properties as shown in Table 4, metallographic organization as shown in Figure 8.

Figure.7 0.6mm thick cold rolled nickel plate

Table.4 Properties of 0.6mm thick pure nickel plate after annealing

Technology		Rm/MPa	Rp0.2/MPa	A50(%)
650°Cx90 min	Portrait	455	146	54
		450	143	55
	Transverse	440	141	53
		445	138	55
Standard requirements		≥345	≥80	≥40

Figure.8 0.6mm thick cold rolled nickel plate metallographic organization
From the above data, it can be seen that the 0.6mm thick plate, due to cold working deformation of 80%, the grain gets a greater degree of broken, strong plasticity higher than 3.0mm thick and 2.5mm thick cold rolled plate performance.

3. Conclusion

• (1) The performance of pure nickel plate with the change of annealing temperature there is a steep inflection point, about 580-600 ℃, before and after the inflection point, the mechanical properties of small changes. Before the inflection point, when the annealing temperature is low, the strength and cold working state properties are comparable. After the inflection point annealing temperature is higher, with the increase in temperature, the strength slowly decreases, and the elongation slowly increases. Suitable annealing temperature should be selected after the inflection point, 600-700 ℃.
• (2) By reducing the annealing temperature of semi-finished products, half-hard annealing can improve the shape and mechanical properties of the finished cold rolled sheet.
• (3) Increasing the deformation of a single rolling process can improve the mechanical properties of the final product; the finished plate can reach more than 80% of the deformation of a single rolling process.

Author: Xi Jinhui