Introduction
Electrical steel sheet (also known as silicon steel sheet, silicon steel sheet) refers to a kind of extremely low carbon content (after annealing, the carbon content is Less than 0.005%) ferrosilicon soft magnetic alloy, the general silicon content is 0.5% to 4.5%, adding silicon to iron can increase its resistivity and maximum permeability. Mainly used to manufacture iron cores of various transformers, motors and generators. Electrical steel plates are divided into two categories, namely oriented electrical steel plates and non-oriented electrical steel plates.
Cold-rolled non-oriented silicon steel sheet is mainly used as the core of motors and small transformers, and it is an important soft magnetic alloy indispensable in the power, electronics and military industries. It is used the most in magnetic materials and is an important energy-saving metal functional material. The market demand for cold-rolled non-oriented silicon steel is steadily increasing.
Brief introduction of electrical steel plate cold rolling line equipment
1. Pickling. Continuous pickling unit adopts hydrochloric acid shallow tank turbulent pickling, three-stage pickling and five-stage cleaning.
2. Rolling mill. The six-high single-stand reversible rolling mill adopts a full hydraulic press-up system with a maximum rolling force of 22000 kN. It is equipped with positive and negative working rolls, positive and negative bending rolls, intermediate roll positive and negative bending, and axial movement, and roll emulsion segmented cooling. Shape control means and advanced shape and flatness control technology. The main drive adopts a fully digital AC variable frequency single motor drive.
3. Continuous annealing furnace. Equipped with alkali spray device, alkali scrubbing device, high-pressure water cleaning device, water scrubbing device, hot water spray device, the annealing furnace is mainly composed of the following furnace sections: entrance sealed chamber, preheating furnace, non-oxidizing heating furnace, radiant tube Heating furnace, soaking furnace, circulating gas injection controlled cooling section, circulating gas injection fast cooling section, outlet sealed chamber, etc.
Control of chemical composition of electrical steel sheet
In the process of electrical steel smelting, various methods are usually used to control the chemical composition within the optimal range, and to purify the steel as much as possible. The method will remove [C] below 0.006%, remove [C] below 0.02%, and further use vacuum treatment to remove [C] in steel to 0.015%, or even below 0.003%, for other elements such as Si, Mn, The content requirements of P, Al, etc. are also very strict. According to the basic composition requirements of electrical steel grades, the influence of chemical composition on the microstructure and magnetic properties is comprehensively considered.
Electrical steel sheet smelting and hot rolling process
The smelting and hot rolling process of the steel: molten iron desulfurization→top and bottom combined blowing converter→vacuum refining→continuous casting→7 rack heat Continuous rolling. The molten iron needs to undergo deep desulfurization and slag removal treatment, and the S content of the outbound molten iron is controlled below 30×10-6; lime is added when the converter is tapped to further deep desulfurization; RH refining can not only complete the alloying and Reduce C in steel to 30×10-6, and still have a small amount of desulfurization capacity; continuous casting adopts large ladle nozzle and argon seal, tundish slag covering, and immersion nozzle for full protection casting; To prevent the billet from overburning and collapse in the furnace, the heating temperature is controlled below 1200℃; a reasonable 7-stand hot tandem rolling system is formulated according to the composition, and the coiling temperature is controlled above 650℃ to ensure the structure and grain of the hot-rolled plate. Coarse and uniform.
Choice of thickness of cold-rolled billet for electrical steel sheet and its total reduction rate
The width of electrical steel required by the market is mainly 1200 mm and 1000 mm. The cold rolling mill of Tonggang Cold-rolled Plate Company It is a 1710 mm rolling mill that can roll wide gauge plates, so 1200mm wide gauge steel plates are selected for development. The main magnetic performance requirements of non-oriented silicon steel are low iron loss and high magnetic induction. Iron loss and magnetic induction belong to tissue-sensitive magnetism. In addition to chemical composition, they are also related to internal structure, steel plate thickness, surface finish, and applied stress. The use of one-time cold rolling method can increase the output and reduce the cost. The one-time cold rolling method requires a large reduction rate (≥75%) to ensure that the {110}<001> texture component is further increased and the magnetic properties are improved. According to the requirements of the overall reduction rate of electrical steel, the thickness of the hot-rolled raw material is designed to be 2.3 mm, and the total reduction rate of the cold-rolled is 78%. This not only ensures the reduction rate requirement (>75%), but also facilitates hot-rolled plate shape control.
Choice of cold rolling pass and reduction rate of electrical steel sheet
According to the requirement of total reduction rate of electrical steel sheet, one-time cold rolling method is adopted to obtain a good structure and Performance. Considering the equipment capacity and the characteristics of silicon steel, the design selects 5 passes, on the one hand, it gives full play to the equipment capacity while taking into account the metal plasticity of silicon steel.
After the total reduction rate and passes are determined, it is necessary to allocate the reduction rate for each pass. The number of rolling passes is related to the thickness of the blank, equipment capacity, work roll diameter, and product quality requirements. The specific distribution principle is: the reduction rate of the first pass should not be too large, and the original plate shape of the hot rolling is mainly considered. The second and third passes make full use of the plasticity and dimensional conditions of the rolled piece and adopt the largest pass reduction rate to minimize the thickness of the rolled piece. The fourth pass reduces the reduction as the degree of work hardening increases.
The reduction in the 5th pass is further reduced, mainly to control the plate shape and product dimensional accuracy. Try to make the rolling force distribution of each pass balanced, which is conducive to stabilizing the rolling process and can give full play to the equipment potential. The rolling force and motor load of each pass cannot exceed the allowable rated value of the equipment to ensure the safe operation of the equipment. During rolling, the deformation should be made uniform, and the size, performance, shape, surface, etc. should all meet the technical standard requirements.
Electrical steel plate tension
In the actual production process, because the coil is thicker in the first pass, a larger front tension is allowed, and the back tension is restricted by the pickling tension. Large, so as not to cause interlayer slippage. After several passes, the tension is greater than the front tension, and the tension difference between the front and back is controlled below 30 kN, and the specific tension is less than 60%σS. In the rolling process, try to use large tension to ensure the stability of the rolling process, to obtain a good plate shape and coil shape, and to reduce the unit rolling pressure. In actual production, the back tension is greater than the front tension and it is not easy to break, and the rolling force is obviously reduced, but the back tension is too large and it is easy to slip. The tension setting of the last pass matches the uncoiling tension of the continuous annealing line to ensure the smooth uncoiling of the continuous unwinding line.
Control of roll shape and shape of electrical steel sheet
Work roll bending is an effective means to compensate the bending deformation of work rolls. The change in the deflection of the work roll caused by the bending force will inevitably affect the pressure distribution between the rolls and between the work roll and the rolled piece, which in turn affects the elastic deformation of the work roll and the edge drop of the rolled piece. During the operation, a positive bending roll is used, and the first pass is set to about 280kN, and the subsequent passes decrease gradually, and the bending force of the roll should be adjusted appropriately according to the actual plate shape. The middle roll bending roll adopts a positive bending roll, which cooperates with the work roll bending roll to control the shape. In the first pass, the middle roll bending force is set to 72 kN, and the subsequent passes decrease sequentially. The horizontal tandem movement of the intermediate rolls adjusts the pressure distribution between the rolls, which in turn affects the pressure distribution between the work roll and the rolled piece, so as to change the elastic deformation of the work roll and the edge drop of the rolled piece. Tight-edge rolling is adopted for flatness control. The flatness roll of the first pass is set to 15I, and the finished product pass is controlled at 3I.
Multi-stage cooling adopts Lekler precision spray beams to cool the work rolls in sections, which can effectively control the thermal crown of the rolls and have a good ability to correct local flatness defects. The roll shape design of the rolls is all flat rolls, with high grinding accuracy, and the roundness, roundness and taper errors of the rolls are controlled within 3μm/m.
Choice of emulsion concentration for electrical steel sheet
Emulsion concentration is an important process parameter in the cold rolling process, and different varieties and specifications have different moderate values during rolling. If the concentration is high, the amount of separated water spread increases, and the amount of oil film introduced increases, which leads to over-lubrication, which will cause slippage between the rolling piece and the roll, which is prone to scratches, mill chatter, and poor plate shape. When the concentration is low, the amount of separated water spread is reduced, and the amount of oil film introduced is reduced, resulting in insufficient lubrication, which is prone to thermal scratches, rolling mill chatter, and increasing rolling load and rolling force. In actual production, the concentration of the emulsion is controlled within the range of 3% to 3.5%.
Continuous annealing of electrical steel sheet
Using full hydrogen protection continuous annealing, the temperature of the radiant tube heating furnace is controlled at 800～850℃, and the temperature of the resistance band heating furnace is controlled at 880～900℃. Steel annealing and heat preservation for 3 to 4 minutes meet the process requirements.