Research on Bell Furnace Annealing Process of Nb+Ti IF Steel Cold Rolled Sheet
Under laboratory conditions , the recrystallization temperature of Nb+TiIF steel decreases with the prolongation of holding time . The heating rate has a certain influence on the recrystallization process, and it is advisable to control it at 50 °;C/h . According to the annealing characteristics of the bell-type furnace, in order to ensure the cold spot and temperature uniformity of the steel coil, the annealing control temperature is controlled according to the upper limit of the bell-type furnace , and sufficient holding time is ensured . The results of industrial production and user use show that the product has excellent stamping performance and stable quality , which fully meets the user's requirements .
Key words: IF steel ; annealing ; recrystallization ; microstructure ; texture
Ultra-deep drawing IF steel (Interstitial-Free Steel), that is, " interstitial-free steel ", is the third generation of cold -rolled stamping steel after boiling steel and low-carbon aluminum - killed steel . Various forming requirements proposed by steel plate . According to the addition of microalloying elements, industrially produced IF steels can be divided into three categories: Ti IF steels , Nb IF steels, and Nb + Ti IF steels . The research on its metallurgical mechanism and production process is the most active development of cold-rolled steel sheets in recent years. one of the fields .
Research shows that To obtain IF steel with excellent comprehensive properties , It is necessary to start from the aspects of microalloying, ultra-low carbon, pure steel, hot rolling, cold rolling and annealing processes , etc. take the appropriate process route , Finally, strong recrystallization favorable texture and coarser ferrite grains are obtained. Among them, the cold-rolling and annealing process is the key factor in the production of IF steel ultra-deep drawing cold-rolled sheets. The author measured the recrystallization temperature of Nb + Ti IF steel under laboratory conditions , The effects of annealing temperature and heating rate on the microstructure and recrystallization texture were studied , Therefore, it provides theoretical guidance and reference data for industrial production, and has achieved good results in industrial production.
1 test material
The test material is ultra-low carbon Nb + Ti IF steel smelted in a 120 t converter . The 200 mm continuous casting slab is rough rolled and finished rolled into a 3.5 mm hot rolled coil. The heating temperature of the slab is 1220 C , and the final rolling temperature is 920 C. The coiling temperature is 730C . After pickling in a cold rolling mill and rolling with 4 stands, it is rolled into a cold - rolled coil of 0.8 mm , and the total reduction rate of cold rolling is 7714 % . The materials used in the test are taken from the cold-rolled hard coils rolled by the cold tandem rolling mill .
2 test plan
The annealing of the test steel was carried out on the Gleeble-1500 thermal simulation test machine, and the process system is shown in Figure 1 . In Figure 1(a), the heating rate ( v ) is 60 C/h; the holding temperature ( T ) is 540 C , 560 C , 580 C , 600 C , 620 C , 640 C , 660 C , 680 C , 700 C , 720 C , 740 C ; the holding time (t) is 1h and 3 h respectively; after holding the heat, spray water to cool it to room temperature . The heating rates (v ) in Figure 1(b ) are 40 C/h , 50 C/h , and 60 C/h , respectively ; The holding temperature ( T) is 600 C , 660 C , 700 C and 730 C respectively ; Except for a sample at 730 C , which was kept warm for 3 hours and then quenched by spraying water, the samples at other temperatures were cooled immediately after reaching the temperature.
Spray with water to cool to room temperature . After the treated sample is pickled, measure the hardness value ( HR30T ), draw the surface hardness-temperature relationship curve, and define the softening temperature of 50 % as the recrystallization temperature . All samples were observed metallographic structure under Axiovert200 MAT metallographic microscope . D/max-RC diffractometer was used to test the texture, and Roe system software was used for ODF analysis, ODF cross-sectional diagram was drawn and the volume fraction of 111} texture was quantitatively calculated at different annealing temperatures .
3 Test results and analysis
3.1 Recrystallization temperature
The effect of temperature on the surface hardness of Nb + Ti IF steel at different annealing temperatures for 1 h and 3 h is shown in Figure 2 . Usually, the temperature at which the surface hardness softens by 50% is used as the recrystallization temperature . It can be seen from Fig . 2(a) that under the condition of annealing and heat preservation for 1 h , when the heating temperature is in the range of 620-700 °;C , the surface hardness of the sample begins to decrease significantly, indicating that recrystallization has begun and gradually accelerated at this time, and the surface hardness has dropped to The recrystallization temperature at 50% is about 660 °; C. When the heating temperature exceeds 700 °; C , the surface hardness of the sample decreases slowly, and the recrystallization process has basically been completed at this time . It can be seen from Figure 2 (b) that under the condition of annealing and holding for 3 h , the recrystallization temperature range and recrystallization temperature are slightly lower than that of holding for 1 h , and the recrystallization temperature is about 650 °; C . The comparison of Fig. 2(a) and Fig. 2(b) shows that the length of holding time has a certain influence on the recrystallization temperature , Prolonging the holding time can reduce the recrystallization temperature .
3.2 Influence of heating rate
The effect of annealing heating rate on surface hardness is shown in Figure 3 . It can be seen that in the temperature range of 600-730 C , the hardness is obviously lower when the annealing temperature is lower and the heating rate is 40 C/h . When the annealing temperature is high, especially when the annealing temperature reaches 730 C , as the heating rate increases, the hardness value also increases. When heating to 730 C and holding for 3 h, the heating rate is 40 C/h and 50 C/h The hardness is basically the same , but the hardness is obviously higher when the heating rate is 60C/h . That is to say, the heating rate has a certain influence on the annealing process of Nb + Ti IF steel . It is beneficial to use a lower annealing heating rate , and its essence is to prolong the annealing time in the low temperature zone . From the previous measurement of the recrystallization temperature, it can be seen that: That is to say, it is equivalent to reducing the recrystallization temperature of Nb+Ti IF steel and increasing the holding time above the recrystallization temperature , which is conducive to improving the comprehensive performance of the finished cold-rolled steel plate .
3.3 Metallographic structure
Nb+TiIF steel at different annealing temperatures and holding times is shown in Figure 4. It can be seen that when the annealing temperature is 660 °;C for 1 h , the grains of Nb+TiIF steel protrude in a jagged shape, and regeneration has occurred. Nucleation and growth of crystallization - When the annealing temperature is 660C for 3 hours and 680C for 1 hour , the sample has basically completed recrystallization . That is to say, the annealing temperature of 660 C must be matched with a long annealing time, and the annealing temperature of 680 C can shorten the annealing time . From the perspective of large-scale industrial production , the use of high temperature (680 C) short-time annealing process is beneficial to reduce costs and increase productivity .
3.4 Texture
Nb+Ti IF steel at different annealing temperatures is shown in Figure 5, and the relationship between annealing temperature and {111} texture volume fraction is shown in Figure 6 . It can be seen that the texture of the test steel is mainly {114} , {335} and {332} , and the texture of {111} is not particularly prominent, which is similar to the characteristics of the steel used for cold rolling and stamping series in industrial production . The {111} texture of Nb + Ti IF steel is strengthened with the increase of annealing temperature. When the annealing temperature is lower than 660 C , the volume fraction of {111} texture is basically consistent. When the annealing temperature exceeds 660 C , { 111} Texture volume fraction increases with the increase of annealing temperature, and this critical temperature is basically consistent with the recrystallization temperature measured above .
(a) 660 °;C ; (b) 680 £ ; (c) 700 *C f (d) 720X?
Fig.5 Texture of Nb+Ti IF steel
3.5 Formulation of bell annealing process
The recrystallization temperature, metallographic structure and texture measurement results show that the heating rate has a certain influence on the recrystallization process of Nb+Ti IF steel, and the heating rate should not be too fast and should be controlled at 50 C/h .
Nb+ TiIF steel can complete the recrystallization process at 660 C and 680 C , but the holding time is different . The research shows that the main factor affecting the finished properties of Nb+ TiIF steel cold-rolled sheet is the annealing temperature, followed by the holding time . With the increase of annealing temperature, the n value and r value of Nb+Ti IF steel increase significantly . As the holding time increases, the yield strength and tensile strength of the steel decrease, and the n value , value and elongation increase, but this trend is largely affected by the annealing temperature . When the annealing temperature is low, the holding time is prolonged, the yield strength decreases and the n value increases more obviously, while the elongation, n value and r value closely related to the structure and grain orientation change little [2] o Therefore, Nb+ The annealing temperature of 680 C should be adopted for Ti IF steel cold-rolled sheet .
According to the annealing characteristics of the bell furnace, the core temperature is lower than the control temperature, especially the cold spot temperature of the steel coil is lower . In order to ensure the performance of cold-rolled coils, it is necessary to ensure that the annealing temperature of the steel coils in the entire bell annealing furnace is above 680 °;C . At the same time, in order to ensure the uniform performance of the entire steel coil , It is necessary to ensure that its temperature is uniform , Since the annealing temperature rise rate of IF steel is faster than that of general deep drawing steel and the heating time is short , when the annealing control temperature reaches the set temperature , the winding core temperature is much lower than the control temperature , so a longer annealing holding time should be adopted . To ensure that the temperature of the cold spot reaches the required annealing temperature ( 680 C), and ensure that the temperature of the steel coil is uniform . Therefore, the annealing control temperature of Nb + Ti IF steel is controlled according to the upper limit temperature of the bell furnace , and sufficient holding time is ensured .
3.6 Application effect
According to the laboratory research results, the Nb+TiIF steel annealing process is used for industrial production . The routine mechanical properties ( R eL , R m , A s . ), n value and r value of the cold-rolled finished products have all met the requirements of the product standard , as shown in Table 2 . The practice of making automobile , motorcycle oil tank body , instrument shell and other parts shows that its stamping performance is excellent, the quality is stable, and it can fully meet the requirements of users .
Table 2 Mechanical properties of Nb+Ti IF steel cold rolled products
Table 2 Mechanical properties of cold rolled
and annealed Nb+Ti IF steel
Mechanical properties R eL/ MPa R m/ MPa 4 8°;/ % n r
Range 136 ~ 174 275 ~ 328 42 ~ 52 0.23 ~ 0.29 1.92 ~ 2. 64
Mean 146 292 45.4 0.271 2.17
4 Epilogue
- The recrystallization temperature of Nb + Ti IF steel is 660 C for 1 h , and 650 C for 3 h . The length of the holding time has a certain influence on the recrystallization temperature, and extending the holding time can reduce the recrystallization temperature .
- The heating rate has a certain influence on the recrystallization process of Nb+Ti IF steel. The heating rate should not be too fast and should be controlled at 50 C/h .
- According to the annealing characteristics of the bell-type furnace, in order to ensure that the temperature of the cold spot reaches the required annealing temperature (680 C), and to ensure the uniformity of the steel coil temperature, the annealing control temperature of Nb+Ti IF steel is controlled according to the upper limit of the bell-type furnace . And ensure enough heat preservation time . The annealing process formulated with reference to laboratory research results has achieved good results in the production of Nb+TiIF steel .
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