What is rolling?
A pressure processing method in which a metal blank is passed through the gap between a pair of rotating rolls (various shapes), then the cross-section of the material decreases and the length increases as a result of compression by the rolls.
This is the most common production method used to produce steel, which is mainly used to produce profiles, plates and tubes.
Types of rolling
According to the movement of rolled parts, rolling can be divided into longitudinal rolling, cross- rolling and skew rolling.
The longitudinal rolling process is a process in which metal passes between two rolls that rotate in opposite directions and produces plastic deformation between them.
The movement direction of the rolled piece after deformation is consistent with the roll axis direction.
The rolling piece moves in a spiral, the rolling piece and the roll axis are not with a special angle.
It can destroy the casting structure of the steel ingot, refine the grain of the steel, and eliminate the defects of the microstructure, so that the steel structure is dense and the mechanical properties are improved.
This improvement is mainly reflected in the rolling direction so that the steel is no longer isotropic to a certain extent; bubbles, cracks and looseness formed during casting can also be welded under high temperature and pressure.
1. After rolling, the non-metallic inclusions (mainly sulfides, oxides and silicates) inside the steel are pressed into thin sheets, and the delamination (interlayer) phenomenon occurs.
Delamination greatly deteriorates the tensile properties of the steel in the thickness direction, and it is possible that interlayer tearing occurs when the weld shrinks.
The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain caused by the load.
2. Residual stresses due to uneven cooling.
Residual stress is no external force in the internal self-phase equilibrium under the stress.
Various cross-section of hot-rolled steel has this kind of residual stress.
In general, the larger the section size of the beam, the greater the residual stress.
Although the residual stress is self-phase equilibrium, the steel component under the action of external forces or have some influence on the performance.
For example, it may have adverse effects on deformation, stability and fatigue resistance.
3. Hot-rolled steel products are not easy to control in terms of thickness and edge width.
We are familiar with the thermal expansion and contraction, due to the length and thickness are up to the standard at the beginning, but there will still be a certain negative difference after cooling.
The wider the negative difference, the more obvious the thicker the thickness.
Therefore, it is not possible to be too precise about the width, thickness, length, angles and edge lines of large steels.
Forging & Pressing
It is a processing method that uses forging and pressing machinery to exert pressure on metal billets to produce plastic deformation in order to obtain forgings with certain mechanical properties and certain shapes and sizes.
Forging can eliminate the metal in the smelting process, such as casting loose defects, optimization of microstructure.
At the same time, due to the preservation of the integrity of the metal flow lines, the mechanical properties of the forgings are generally better than the same material castings.
The important parts with high load and severe working conditions in the relevant machinery, in addition to the simpler shape of the available rolled plate, profile or welded parts, are mostly forgings.
Types of forging
Forging can be divided into free forging, die forging, flashless die forging
1. Free forging
Use impact or pressure to deform the metal between the upper and lower iron (anvil) to produce deformation to obtain the required forgings, which is mainly divided into manual forging and mechanical forging.
2. Die forging
Die forging is divided into open-die forging and flashless forging.
The metal blank is compressed and deformed in the forging die with a certain shape to obtain forgings, which can be divided into cold heading, roll forging, radial forging and extrusion etc.
3. Since there is no flash in the flashless die forging and closed upsetting forging, the material utilization rate is high.
It is possible to complete the finishing of complex forgings with one process or several processes.
Because there is no flash, the force-bearing area of the forging is reduced, and the required load is also reduced.
However, it should be noted that blanks cannot be completely restricted.
For this reason, the volume of the blanks should be strictly controlled, the relative position of the forging dies and the measurement of the forgings should be controlled, and efforts should be made to reduce the wear of the forging dies.
Compared with castings, forging metal can improve its structure and mechanical properties after forging.
The casting structure is deformed and recrystallized by the metal after hot working through the forging method, which makes the original coarse dendrites and columnar grains become an equiaxed recrystallized structure with finer grains and uniform size.
The original segregation, porosity, porosity and slag inclusion etc. in the steel ingot are compacted and welded, the structure becomes tighter, which improve the plasticity and mechanical properties of the metal.
The mechanical properties of castings are lower than the mechanical properties of the forgings of the same material.
In addition, the forging process can ensure the continuity of the metal fiber tissue, so that the fiber tissue of the forgings and the shape of the forgings to maintain consistency and metal flow integrity.
It can ensure that the parts have good mechanical properties and long service life by using precision die forging, cold extrusion, temperature extrusion and other processes to produce forgings, which are unmatched by casting.
Forging vs Rolling
(1) The difference in axial and radial mechanical properties of forgings is smaller than that of rolled products.
That is to say, the isotropy of forgings is much higher than that of rolled products, so the life of forgings is much longer than that of rollings.
The following figure shows the metallographic diagram of the eutectic carbides in different directions of the Cr12MoV rolled sheet.
(2) In terms of the degree of transformation, the degree of deformation of the forging is much greater than that of the rolling, which means that the effect of breaking eutectic carbide through forging is better than that of rolling.
(3) In terms of processing cost, the cost of forging is much higher than the cost of rolling.
For some key parts, workpieces that are subject to large loads or impacts, and workpieces with complex shapes or very strict requirements, forging must be used for processing.
(4) Forgings have complete metal flow lines.
The integrity of the metal streamline is destroyed by mechanical work after rolling, which greatly shortens the life of the workpiece.
The following picture shows the metal flow lines of casting, machining and forging workpieces.