Optimization of annealing process system for 05CuPCrNi cold-rolled strip steel

Aiming at the problems of uneven performance and poor shape of 05CuPCrNi cold-rolled corrosion-resistant steel after annealing in BBN Iron and Steel Cold Rolling Plant, through laboratory experiments and production tests, the annealing process was optimized, and the annealing temperature, heating and holding time were adjusted . The properties of the produced strip are uniform and the shape of the strip is good.
Key words cold-rolled corrosion-resistant steel sheet shape annealing process

The BBN Iron and Steel Cold Rolling Plant adopts the HUGF2O0-52OHPH type imported from the German LOI company and the HICON / H 2 full-hydrogen strong convection bell-type annealing equipment imported from the Austrian EBNER company to carry out full-hydrogen strong convective recrystallization and brightening of the cold-rolled products. Annealing to improve the mechanical properties and surface finish of the coil after cold rolling . Since the annealing furnace was put into production , the main product is low-carbon deep-drawing steel . In order to expand the market and expand the variety structure , the development of 05CuPCrNi cold-rolled corrosion-resistant steel for railway passenger cars has been developed since 2001 , and the annealing process system was formulated with reference to Q235B in the initial stage of trial production . Practice shows that using this annealing process system , problems such as uneven performance and poor shape of the entire steel strip occur , and satisfactory performance and quality cannot be achieved after re-annealing , which affects the production efficiency and product quality of the cold rolling mill .
1 Optimization of annealing process system
1 . 1 Features of full hydrogen strong convection bell annealing furnace
The all-hydrogen strong convection bell annealing furnace is currently the most advanced batch annealing furnace in the world . Its main advantage is that it utilizes the characteristics of strong reducibility, low density and high thermal conductivity of hydrogen , and cooperates with the high-speed rotating hearth circulating fan to make The atmosphere in the furnace has high flow velocity and heat transfer velocity , the furnace temperature is uniform , the mechanical properties of the steel coil after annealing are excellent and uniform , the surface of the strip steel is clean and bright , and the production efficiency is high .
The bell annealing furnace is in the form of stacking steel coils , through heat radiation , convection , conduction, etc. , to heat and cool the steel coils , so as to achieve the purpose of recrystallization bright annealing . Due to the slow heat transfer , the temperature control in the process is by controlling the thermocouple ( CT ) and stovetop compression thermocouples ( BT ) To achieve , as shown in Figure 1 . The bell furnace is characterized by uneven temperature at each point of the steel coil in the furnace , and the heating and cooling time is long . By controlling the time reflected by the temperature difference between the cold spot and the hot spot , the entire annealed steel coil can meet the mechanical performance requirements . At the same time , carry out hydrogen purging in an appropriate manner to achieve light

purpose of bright annealing .
1 . 2 The original annealing process system
In the initial stage of trial production, the annealing process system of 05CuPCrNi steel was formulated with reference to Q235B steel of the same strength level. The process system is shown in Figure 2 , and the performance and structure are shown in Table 1 ( Take cold rolling number 152485-00 as an example ) .

Fig.2 Annealing process curve of 05CuPCrN i steel
Table 1 Properties and microstructure of 05CuPCrNi steel

Cold rolling number	sampling Location	Yield strength / Tensile strength / Elongation /			Organization level / level
		MPa	MPa	%
1R24RR	head	300	450	30	9
152485 -00	middle part	390	540	27	11
	tail	310	470	31	9 . 5

Grain annealing , the structure change in the annealing process includes three different processes of recovery , recrystallization , and grain growth , and these processes are carried out within a temperature range .

The recrystallization temperature range of 05CuPCrNi steel is 500~530°C ( the degree of deformation is 60% ~ 80% ) , which is slightly lower than the recrystallization temperature of ordinary low carbon steel, which is 450-500C . Therefore , the annealing temperature should be increased appropriately .
In addition, because the low-alloy high-strength corrosion-resistant steel 05CuPCrNi is different from ordinary low-carbon steel , its content of alloy elements is relatively high , and its thermal conductivity is small . Only when the heating speed is slower and the holding time is longer , can the performance of the entire strip be uniform .
In view of the problems existing in the original annealing process , considering the actual situation of our factory and the particularity of this steel type , it was decided to prolong the heating time, increase the annealing temperature , and prolong the holding time , so that the structure of the strip steel after annealing is uniform and the performance is stable . For this reason , a new annealing process system was adopted , and the performance and structure of the optimized steel strip are shown in Table 2 ( taking cold rolling number 401881-00 as an example ) ,.

After annealing, the yield strength of the whole strip steel is 300 ~ 400MPa , the tensile strength is 450 ~ 55OMPa, the elongation is 26 %~ 35 %, the maximum difference in strength is about 100MPa , and there is no rule . Figure 3 shows the metallographic structure in the middle of the steel coil . After microstructure analysis , it is considered that the grain size along the strip length direction is uneven , the grain size is 8 ~ 12 grades , and individual grains in the middle of the steel coil have not grown up completely . Therefore , the performance of the whole strip is not uniform , which affects the shape quality .
1 . 3 Optimization of annealing process system
According to the principle of metallology , annealing after cold rolling of strip steel is the process of rebonding.
The damage methods of slag to furnace lining are mainly chemical erosion and physical erosion . Chemical attack can be controlled by making the slag properties as balanced as possible with the refractory material . In addition , the corrosion process of periclase in magnesia-chrome bricks can be identified as : MgO ( brick ) ^ MgO ( slag ) ; physical scouring can be achieved by changing the amount of solid and liquid phases in the slag and controlling the viscosity of the liquid phase control . And MgO just plays the role of controlling fluidity . As the amount of MgO in the slag increases, the power of the above-mentioned process to the right decreases , that is, the chance of bricks being corroded is reduced .
From the above analysis , it can be seen that during the reduction period of the refining operation outside the furnace , the slag with high MgO , high A 2 03 and low alkalinity has less corrosion on magnesia-chrome bricks . It is suggested that MgO should be able to reach the saturated amount of balanced slag absorption . When using high-alumina slag containing more than 30 % A 2 O 3 , the basicity should be controlled below 187 , and when using Al as a reducing agent , the amount of CaF 2 should be reduced as much as possible .
3 Conclusion
( 1 ) The basicity of slag is less than 1 . In the range of 87 , the amount of erosion will increase with the increase of alkalinity ; and when the alkalinity is greater than 1 . At 87 , the sample was pulverized due to the action of C 2 S ;
( 2 ) As the amount of Al 2 O 3 and MgO in the slag increases , the erosion of the lining by the slag decreases , so the use of high-alumina slag is beneficial to slow down the erosion of the lining .

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