Bending processing technology is a type of steel processing technology widely used in various fields such as automobile manufacturing, engineering machinery, bridges, ships and construction.
Under the pressure of the upper or lower die of the press brake, the sheet metal first undergoes elastic deformation and then plastic deformation.
In the early stage of plastic bending, as the upper or lower die bends the metal sheet, the sheet gradually fits tightly to the inner surface of the V-groove of the upper or lower die, while the bending radius also gradually decreases.
As the pressure continues until the end of the stroke, the upper and lower dies come into full contact with the sheet metal, forming the V-shaped bend, which is typically processed on press brakes and rolling equipment.
Flexural cracking is a major defect in the use of steel processing. According to the location of the crack, it can be divided into corner cracks and central cracks.
Factors that cause cracking include inadequate processing technology and material quality defects, which have a negative impact on steel production companies.
Researchers analyzed, summarized and studied typical quality cases and consulted relevant materials to analyze various factors that cause flexural cracks and propose improvement measures.
Typical samples from flexural, cracking and physical-chemical tests
1.1 Samples of cracks in corners
1.1.1 Macroscopic Morphology
Corner crack is the most common type of defect in flexural cracking, and there are usually burrs, rough edges, oxygen cutting edges or plasma cutting edges at the corner crack position. If the edge of the workpiece is not sandblasted or incompletely treated during bending processing, corner cracks will occur, and the cracks in corner cracks are generally short and located in the hardened area of the corner.
Typical defects of Q235B steel and Q355B steel were selected for analysis, and the macroscopic morphology of corner cracking is shown in Figure 1.
As can be seen in Figure 4, there is cold grain deformation at the extrusion position of the grinding tool, and scratch openings can be seen at the root of longer and straighter cracks. The samples also contain clustered sulfide inclusions, central segregation, high-temperature oxidation particles, decarburization due to oxidation, and bubble characteristics.
Analysis of the causes of flexural cracking defects
2.1 Inadequate processing techniques
2.1.1 The influence of bending diameter
When bending steel, the outer layer of the bent area undergoes tension while the inner layer undergoes compression. When the thickness of the material is constant, the smaller the radius of curvature, the more severe the tensile and compressive stresses in the material. If the tensile stress at the outer corner exceeds the maximum strength of the material, cracks or fractures will occur, particularly in the middle of the part and sometimes at the corners.
2.1.2 The influence of bending tools
If the V-grooves of bending tools are rough, the part will be subjected to uneven forces as it passes through the press brake, causing surface wear or local pressure, leading to surface defects, followed by extrusion cracks. Cracks generally appear straight and long, with visible cold deformation of grains at the crack roots.
2.1.3 The influence of logistics
During transportation and loading and unloading of steel, surface scratches may occur, which destroy the continuity of the substrate surface. Cracking is likely to occur in the scratched area during bending. These cracks are generally longer and straighter, with openings visible at the root of the crack.
2.2 The influence of material defects
2.2.1 The influence of harmful elements, inclusions and gases on steel
During the casting process, the high sulfur and phosphorus content in the steel leads to a high sulfide inclusion content or, even if the overall content does not exceed the standard, these elements aggregate locally and cause severe central segregation in the inclusions. This leads to a decrease in the plasticity and toughness of the steel, making it susceptible to bending and cracking.
In addition, microcracks on the ingot surface are oxidized at high temperatures during rolling, and the high oxygen and nitrogen content in steel, especially the nitrogen element, easily forms TiN with titanium. TiN particles precipitated along grain boundaries during continuous casting can cause original cracks in the billet, which can lead to cracking during bending.
2.2.2 The influence of the surface quality of the steel
Microcracks and air holes on the steel surface are prone to cracking at the crack location under tension after bending. Multiple small cracks may be visible in the arc of curvature to the naked eye.
2.2.3 The influence of mechanical properties and anisotropy of steel
The better the plasticity of the material, the more stable the plastic deformation, and the greater the elongation at break, the better the bending performance. Even if the bending diameter is small, it is not easy to break.
In addition, the longitudinal and transverse properties of steel are different, and the longitudinal banded structure is more severe than the transverse one. This means that the longitudinal plasticity index of the steel is higher, therefore, when bending along a direction perpendicular to the rolling direction, the bending performance of the steel is better and less prone to cracking compared to bending along the transverse direction .
improvement measures
(1) Solutions to the problem of cracks caused by burrs, sharp edges and oxygen cutting in corner areas: manually sand and round the burrs and sharp edges, or use a deburring machine to automatically remove them and eliminate the area of Hardened processing to reduce the cracking rate.
Change the bending process to continuous roll forming, and then cutting after forming to avoid hard processing caused by cutting. Correct small defects through subsequent welding processes.
(2) To solve the issue of small bending radii, the angle R should be enlarged within the allowable design range to avoid too small a bending radius.
(3) Avoid surface scratches during the logistics process of transporting and unloading steel materials.
(4) In the steelmaking process, improve the purity of steel, reduce the content and aggregation of inclusions in steel. The argon blowing process must be fully utilized to ensure that larger sulfides in the steel float and separate completely.
Proper flow field must be maintained during the steel flow process to ensure adequate and stable flow field in the crystallizer, which can further remove inclusions in the steel, preventing slag entrapment contamination.
Reasonably control the casting temperature, draw rate and cooling rate during continuous casting. Proper use of light pressing technology and electromagnetic stirring technology can improve the internal quality of the billet, reduce center segregation, and prevent the formation of center line cracks.
(5) In the rolling process, strengthen the control of heating, rolling temperature and post-rolling cooling processes, prevent the formation of abnormal structures such as bainite, martensite, coarse grains and mixed crystals, and reduce the resistance within of the allowable range of product standards while improving plasticity and toughness.
