An overview of the hot rolling process of stainless steel
1 Overview
The hot rolling process of stainless steel is rolling above the recrystallization temperature, while cold rolling is rolling below the recrystallization temperature. Simply put, hot rolling is a process in which steel billets are heated and rolled through several rounds and then trimmed into steel plates. It can significantly reduce energy consumption and costs. That is to say, since the metal has high plasticity and low deformation resistance during hot rolling, the energy consumption of metal deformation can be greatly reduced. Hot rolling of stainless steel can improve the processing properties of metals and alloys, that is, the as-cast coarse grains are broken, cracks are healed, casting defects are reduced or eliminated, and the as-cast structure is transformed into a deformed structure to improve the processing properties of the alloy.
2. Concept and characteristics of hot rolling
(1) Concept
Hot rolling refers to rolling performed above the metal recrystallization temperature.
Recrystallization means that when the annealing temperature is high enough and the time is long enough, unstressed new grains (recrystallization cores) are produced in the fiber structure of the deformed metal or alloy; the new grains continue to grow until the original deformed structure completely disappears. Metal or alloy properties also undergo significant changes. This process is called recrystallization. The temperature at which new grains begin to form is called the initial recrystallization temperature. The temperature at which the microstructure is completely occupied by new grains is called the final recrystallization temperature. The recrystallization temperature is usually called the arithmetic mean of the initial recrystallization temperature and the final recrystallization temperature. Generally, the recrystallization temperature is mainly affected by factors such as alloy composition, degree of deformation, original grain size, and annealing temperature.
(2) Features
① Low energy consumption, good plastic processing, low deformation resistance, not obvious work hardening, easy to roll, and reduce metal deformation energy consumption.
② Hot rolling usually uses large ingots and high-pressure reduction rates. The production speed is fast and the output is large, paving the way for large-scale production.
③The as-cast structure is transformed into a processed structure through hot rolling, and the structure transformation greatly improves the plasticity of the material.
④The characteristics of the rolling method determine the anisotropy of the rolled plate. First, there are obvious performance differences in the longitudinal, transverse and high directions of materials. Secondly, the existence of deformation texture and recrystallization texture makes the stamping performance have obvious directionality.
3. Advantages and disadvantages of hot rolling
(1) Advantages
①Rolling can significantly reduce energy consumption and reduce costs. The metal has high plasticity and low deformation resistance during hot rolling, which greatly reduces the energy consumption of metal deformation.
② Hot rolling can improve the processing properties of metals and alloys, that is, the as-cast coarse grains are broken, cracks are healed, casting defects are reduced or eliminated, and the as-cast structure is transformed into a deformed structure, thereby improving the processing properties of the alloy.
③ Hot rolling usually uses large ingots and large reductions, which not only improves production efficiency, but also creates conditions for increasing rolling speed and achieving continuous and automated rolling.
(2) Disadvantages
① After hot rolling, the non-metallic inclusions in the steel (mainly sulfides, oxides and silicates) are pressed into thin sheets and stratified (interlayered). Delamination severely deteriorates the tensile properties of the steel through the thickness and also creates the risk of interlaminar tearing when the weld shrinks. The local strain caused by the shrinkage of the weld usually reaches several times the yield point strain, which is much higher than the strain caused by the load.
② Residual stress caused by uneven cooling. Residual stress is internal self-balancing stress without external forces. Hot-rolled steel of various profiles has this kind of residual stress. Generally speaking, the larger the cross-sectional size of the section steel, the greater the residual stress. Although residual stress is self-balancing, it has a certain impact on the performance of steel components under the action of external forces, such as adverse effects on deformation, stability, fatigue, etc.
③Hot rolling cannot control the required mechanical properties of the product very accurately. The structure and properties of hot-rolled products are difficult to be uniform. The strength index is lower than cold work hardened products but higher than fully annealed products. The plasticity index is higher than that of cold work hardened products, but lower than that of fully annealed products.
④ It is difficult to control the thickness of hot-rolled products and the control accuracy is poor; the hot-rolled surface roughness Ra value is usually 0.5 to 1.5 μm higher than the cold-rolled surface roughness Ra value. Therefore, hot-rolled products are often used as cold-rolled blanks.
4. Application of hot rolled products
1) General structural steel, engineering hot-rolled steel plates and steel strips;
2) Welded pipes, cold rolled materials, bicycle parts and important welded, riveted and bolted components.
3) Cold rolled and deep drawn products.
4) Stamping parts and structural parts for automobiles, tractors, engineering cranes, and small light industrial machinery.
5) Containers and bare metal structural parts.
6) Pipelines transporting oil and natural gas.
7) Frame and beams.
8) Special steel belt for automobile wheels.
9) Industrial skateboards, stairs, ladder treads, etc.
