Features and product solutions of hot rolling
- Characteristics of hot rolling
Hot rolling generally refers to rolling performed above the recrystallization temperature of the metal. During hot rolling, the deformed metal has both hardening and softening processes. Due to the influence of the deformation speed, as long as the recovery and recrystallization process is too late, the metal will produce a certain degree of work hardening with the increase of the degree of deformation. However, in the hot rolling temperature range, the softening process plays a leading role. Therefore, at the end of hot rolling, the recrystallization of the metal is often incomplete, and the hot-rolled aluminum alloy sheet and strip show the coexistence of recrystallization and deformation.
Hot rolling has the following characteristics:
(1) Hot rolling can significantly reduce energy consumption and reduce costs. During hot rolling, the metal has high plasticity and low deformation resistance, which greatly reduces the energy consumption of metal deformation.
(2) Hot rolling can improve the processing performance of metals and alloys, that is, the coarse grains in the as-cast state are broken, the micro-cracks are healed, the casting defects are reduced or eliminated, and the as-cast structure is transformed into a deformed structure, and the processing performance of the metal is improved.
(3) Hot rolling usually adopts large ingot and large reduction rolling, which not only improves the production efficiency, but also creates conditions for increasing the rolling speed and realizing the continuous and automation of the rolling process.
(4) Hot rolling cannot control the mechanical properties required by the product very precisely, and the organization and properties of hot rolled products are not uniform enough. Its strength index is lower than that of cold work hardened products but higher than fully annealed products; its plasticity index is higher than cold work hardened products and lower than fully annealed products.
(5) The thickness and size of hot-rolled products are difficult to control, and the control accuracy is relatively poor; compared with cold-rolled products, the surface of hot-rolled products is rougher, and the value of R is generally 0.5-1.5um
- Product scheme and process flow
Hot-rolled products are generally divided into two categories, one is the hot-rolled thick plate, and the other is the hot-rolled coil. Hot-rolled thick plate refers to an aluminum alloy plate with a thickness of not less than 6.0mm. The main varieties are hot-rolled plate (H112), annealed plate (0), quenched or quenched pre-stretched plate, etc. It is produced by the block method, and its typical process flow is: ingot (homogenization) → milling surface, edge milling → heating → hot rolling → shear interruption → straightening.
The aluminum alloy sheet with a thickness of less than 6.0mm is produced by the coil method. The main task is to provide billets for cold rolling. The process flow is: ingot (homogenization) → milling surface, edge milling → heating → hot rolling (blank rolling ) → hot finishing rolling (coiling rolling) → unloading.
Preparation before hot rolling
The ingots used in hot rolling are flat ingots produced by continuous, semi-continuous, iron mold and other casting methods. In modern large-scale production, semi-continuous casting methods are mostly used to produce flat ingots, and the weight of ingots can range from a few tons to dozens of tons. In order to ensure product quality and meet the performance requirements of processing technology, there are strict requirements on the selection of ingot size and shape, and the control of surface and internal quality. Ingots are surface-treated and heat-treated.
The preparatory work before hot rolling includes: selection of ingots, ingot homogenization annealing, surface treatment of ingots, heating of ingots, etc.
The selection of ingots should fully consider the inherent equipment capacity and process conditions of the factory, raw alloy varieties and quality requirements, production organization requirements and other factors, and follow the principles of high quality, high efficiency and low cost. Generally speaking, the thicker the ingot, the greater the total deformation, which is conducive to improving the performance of the final product; for the thicker product, the thicker ingot should be selected, otherwise the deformation will be insufficient, which will affect the product structure and performance. The width of the ingot is mainly determined by the width of the finished product. Considering different alloy varieties and processing characteristics, a sufficient amount of trimming should be reserved. The length of the ingot mainly depends on the rolling speed, the length of the roller table and the casting equipment.
Ingot size and shape
To determine the size of the ingot, first determine the quality of the ingot according to the production scale, and also consider the alloy process performance, production method, product specifications, equipment capacity and other conditions. Ingot size is expressed as thickness x width x length (ie HxBxL).
A. ingot thickness
Reasonable selection of ingot thickness has a great relationship with the final product quality, productivity and yield. For a product of the same thickness, the thicker the ingot, the greater the total deformation, and the deeper the hot working, which can ensure that the as-cast structure is transformed into a hot working structure and ensure product performance requirements. For products with larger thickness, ingots with larger thickness should be selected, otherwise the deformation amount of hot rolling or cold rolling will be insufficient, which will affect the structure and performance of the product. The thickness of the ingot is large, which is convenient for the continuous production process, less loss of cutting head and tail, and high productivity and yield.
The thickness of the ingot is also limited by the alloy characteristics and equipment conditions. If the capacity of the hot rolling mill is small, the production scale is not large, and the thickness of the ingot is small. However, the minimum ingot thickness is mainly limited by the minimum processing rate of the product, and is related to the casting conditions and the width of the ingot. Considering the capacity of the rolling mill, the ratio of the diameter of the roll to the thickness of the ingot is generally 5 to 7. At present, the thickness of ingots in general small and medium-sized factories in my country is less than 80mm, and the thickness of ingots in large factories is about 300mm. The maximum thickness of foreign ingots is 660mm.
B. Ingot width
The width of the ingot is mainly determined by the width of the finished product. Generally, the amount of widening and trimming during rolling is considered, and then the integer multiple of the width of the finished product is taken as the width of the ingot. The ingot width can be calculated by the following formula:
(5-1) where b is the width of the finished product, mm;
n—multiple of finished product width;
Δb—total trimming amount (related to the number of trimming or stripping), mm; ΔB—hot rolling width, mm.
Considering the limitation of the length of the roll on the width of the ingot, it should be convenient for operation and shape control, etc., depending on different rolling conditions, generally less than 80% of the length of the roll body. Ingot width is limited by casting equipment and process. In order to reduce the equipment and improve the quality of the ingot, the width of the ingot can also be determined by the thickness of the ingot. Generally, the width-thickness ratio of the ingot is 3 to 7.
In order to reduce the casting equipment, the width of the ingot should not be too many, but to meet the requirements of multiple widths, the following measures can usually be taken: (1) When the width of the finished product is larger than the width of the selected ingot, cross rolling can be used if the rolling mill capacity allows. That is, the rolling method in which the longitudinal axis of the ingot is fed into the roll parallel to the axis of the roll. Such as semi-continuous ingots, according to different width requirements, saw the corresponding lengths of ingots for cross-rolling; (2) When using the block method to produce cold-rolled sheets, cross-rolling can be performed after hot rolling or cold rough rolling; (3) When hot rolling, first longitudinally roll to the required width, and then turn to 90° cross-rolling until completion. Longitudinal rolling refers to the rolling method in which the longitudinal axis of the ingot is perpendicular to the axis of the roll and is fed into the roll. This method is usually used for sheet and strip rolling; desired width. The so-called angle rolling refers to the rolling method in which the longitudinal axis of the ingot is fed into the roll at a certain angle (15°~45°) with the axis of the roll. The two diagonals are alternately rolled for a certain number of passes to the required width and then longitudinally rolled, so that the shape of the rolled piece will not be skewed; (5) Iron mold casting, the design mold width can be adjusted, and the ingot width can also be changed.
C. Ingot length
The length of the ingot mainly depends on the rolling speed, the length of the roller table and the casting equipment. When the thickness and width of the ingot are constant, the longer and heavier the ingot, the higher the productivity and yield. If the equipment conditions permit, the longer ingot should be used as much as possible under the guaranteed finishing temperature. According to the equipment conditions, determine the length of the ingot to meet the requirements of the final length of the product, or an integer multiple of the length of the fixed length (cut off the head and tail). In the production of the block method, not only the length of the hot rolling table should be considered, but also the length of the rolling table before and after the cold rolling mill and the ease of operation, if it is directly cold, roughed after hot rolling. After the thickness and width of the ingot are determined, the ingot length can be calculated from the ingot weight.
D. Ingot shape
The ingots for the production of plates are generally rectangular flat ingots. The shape of the ingot should ensure uniform thickness along the transverse direction. The ends or sides should be regular, or arc-shaped, etc., which not only prevents defects such as cracks or “open mouth” from uneven deformation during the rolling process, but also improves bite during rolling.
a Section shape
The ingots for the production of plates are rectangular flat ingots with arc-shaped or trapezoidal cross-sections.
b The shape and treatment of the head and tail of the ingot
There are many hard spots and casting defects in the head and tail of the ingot, which have a certain impact on the product quality and rolling safety. Therefore, according to the product quality requirements and alloy characteristics, the following three treatment methods are adopted for the head and tail of the ingot before hot rolling.
(1) For products with low surface quality requirements, keep the original shape of the head and tail of the ingot, that is, do not do any treatment on the head and tail of the ingot before hot rolling to maximize the yield and reduce costs.
(2) For products with high surface quality requirements, such as 3004 (3104) can-making materials, the round head part at the bottom of the ingot should be cut off. The length of the cut head varies according to the alloy characteristics and product quality requirements, but at least Cut off the non-parallel straight line part (that is, the entire arc part), the general cutting amount is 200-250mm
(3) The round head at the bottom of the ingot and the shrinkage part of the gate must be cut off in the following five cases: 1) Products with extremely high surface requirements; 2) Alloys that are easy to open and delaminate during hot rolling, such as 5x xx series alloys; 3) Alloys that need to be rolled with aluminum; 4) Plates rolled laterally to ensure the width and size; 5) According to the process conditions, the length of the ingot is adjusted to ensure that the quality of the finished product meets the requirements.
The length of the cut part varies depending on the product quality requirements and alloy characteristics, but at least it should be ensured: (1) The non-parallel straight line part should be cut off from the bottom of the ingot, generally 200 ~ 250mm; (2) The casting gate of the cast key should be cut. The distance from the bottom of the shrinkage mouth to the center of the ingot should not be less than 100mm. Generally, the overall cutting amount of the pouring mouth is 200mm.
E. Ingot Specifications
The ingot specification should be determined according to the alloy, variety, specification and technical requirements of the product, as well as the plant’s equipment capacity and production batch. The figure below lists some of the ingot sizes produced by a domestic factory using a fixed mold.
Ingot quality requirements
The quality of the ingot has a great influence on the process performance and final quality of the product processing. In addition to the size and shape of the ingot, the quality of the ingot should meet the requirements, but also the chemical composition, surface and internal quality of the ingot should meet the technical standards.
A. chemical composition
The chemical composition of the ingot does not meet the technical standards, or the chemical composition is uneven, which not only deteriorates the technological performance of the processing process, resulting in processing difficulties, but also the final microstructure and properties of the product will not meet the technical requirements. Therefore, the chemical composition of the ingot must meet the standards to ensure uniform composition.
B. Ingot surface quality
The surface of the ingot should be free of defects such as cold insulation, cracks, pores, segregation nodules and slag inclusions, and the surface should be smooth and smooth. Otherwise, cold isolation will cause rough surface after hot rolling, or peeling and cracking; cracks will oxidize the inside of the ingot casting, cracking and peeling during hot rolling; pores cannot be pressed together, which will cause surface peeling or blistering; segregation tumor It can cause hot brittleness, cracking or delamination, etc. Therefore, the surface of the ingot is usually milled (milled), cleaned, or scraped to eliminate the above defects as much as possible.
C. Ingot internal mass
Internal defects of the ingot, uneven composition, and organization, have a great impact on the processing process and product quality and even cause a large number of waste products. Common defects in the cast key include segregation, shrinkage, cracks, pores, and non-metallic inclusions.
The phenomenon of the uneven chemical composition of the ingot is called segregation. Intragranular segregation is generally eliminated by heat treatment and processing methods; grain boundary segregation is the accumulation of low melting point substances at the grain boundary, which increases the tendency of hot cracking of the ingot, and the product is prone to grain boundary corrosion. Such as sodium brittleness in high-magnesium aluminum alloys; macrosegregation, that is, the internal and external components of the ingot are inconsistent, which makes the structure and performance of the cast lock and processed products uneven. For example, the reverse segregation of tin and copper in duralumin ingots causes hot brittleness and easy cracking. Macrosegregation cannot be eliminated by homogenization annealing, and special attention should be paid to preventing its occurrence.
Shrinkage holes often appear in the middle and head of the ingot. Small and scattered shrinkage holes (shrinkage porosity) can generally be pressed during rolling; concentrated shrinkage holes with large volume and accumulated gas and non-metallic inclusions cannot be pressed, but can only be elongated, and hot rolling causes the ingot to grow along the Shrinkage cavities, cracking or delamination, or peeling, blistering and other waste products appear during annealing.
The internal cracks in the ingot reduce the plasticity, make it easy to crack, or reduce the performance of the product. During rolling, the pores can be flattened, and it is difficult to press together, and skinning and blistering are often generated during the rolling and heat treatment process. For aluminum and aluminum alloys, porosity is an important defect that is often encountered in the production of ingots and difficult to completely eliminate, especially in wet weather. Slag inclusions are metal and non-metallic inclusions in ingots, light metals are mostly internal slag inclusions, and heavy metals are mostly surface slag inclusions. Slag inclusion has a great influence on the mechanical properties of the product. When some slag inclusions are rolled, they are elongated and flattened along the extension direction of the metal, so that the transverse strength of the metal is about 50% lower than the longitudinal direction, the elongation is about 90% lower, and peeling or delamination occurs.
It can be seen that improving the internal quality of the ingot is of great significance to improving the processing performance, product quality and yield. In order to ensure the quality of the ingot, in addition to the composition analysis, low or high magnification structure inspection, and non-destructive testing of the cast chain, the ingot surface treatment and homogenization annealing should be carried out before hot rolling.