Send Message

Stamping services - stamping material characteristic parameters

November 24, 2023

1. Yield strength, tensile strength and yield ratio


Tensile strength is the basic element for calculating stamping force. The so-called tensile strength is the maximum load value during the actual tensile test divided by the cross-sectional area at the beginning of the test piece. That is: b=PMAX/A(kg/mm2)


When the yield strength and tensile strength are high, the stamping forming force is large, the forming difficulty increases, and the life of the mold is also reduced. If the yield strength is high, the elastic recovery deformation will be large when the stamped parts are detached from the mold and unloaded after the stamping forming is completed, affecting the dimensional accuracy of the stamped parts. In general calculations, the shear stress of stamping materials t=0.8* tensile strength σb. Note: 1Pa=1N/m2 =1x10-6 N/mm2 =1.01972x10-7 Kg/mm2.


2. Elongation


In the material tensile test, after the sample is broken, due to the retained plastic deformation, the length of the sample changes from the original L to L1. The ratio expressed in percentage 6=(L1-L)/L is called the elongation. The uniform elongation U is the elongation when local necking occurs during the uniaxial stretching process, that is, when the stretching process becomes unstable. If the elongation of the plate is large, it is beneficial to all elongation stamping forming. When the elongation is large, the forming limits of bulging and flanging are also large. Therefore, most high-quality stamping steels have high uniform elongation.


3. Work hardening value (n)


The drawing forming of stainless steel plates requires several processes to achieve the product shape. During the drawing process, the material will harden, which is generally called work hardening. The reason for the generation of work hardening is that after the material undergoes a plastic deformation, applying a load force in the same direction will increase its yield point, thereby increasing the necessary deformation resistance to resist the recurrence of plastic deformation. The yield point is the initial point where permanent deformation occurs beyond the elastic deformation zone. According to the tensile test, it is the point where the stretching behavior continues without increasing the load.


What does the high and low work hardening coefficient mean? Materials with high n values ​​will have the following behaviors:
​(1) Continuing processing will cause the material to harden and reduce the elongation, making processing difficult.
(2) Continued processing will suppress local deformation and obtain consistent deformation.


4. Work hardening value (n)


Materials with low n values will behave as follows: continued processing will cause local deformation, and weak parts may even break. Therefore, elongation forming requires the sheet to have a larger value L.


5. Plastic strain ratio (r)


It is a parameter that represents the anisotropic properties of the sheet. Since the plate undergoes processes such as rolling and annealing during the manufacturing process, the plate forms a texture in which the crystal orientation tends to be consistent, which is anisotropic on a macro scale, that is, the performance of the plate has certain differences in different directions. difference. In production, the r value is used to represent the anisotropy of the plate. Its value is equal to the ratio of the width strain b expressed by the logarithmic strain and the thickness direction strain t, that is: r=Eb/εt=In(b/bo)/In( t/to). The r value mainly affects the drawing performance. The larger the r value of the plate, the better its drawing performance.


6. Hardness


Generally speaking, the lower the hardness, the better the plasticity. However, the hardness of the material is relatively high, and if the carbide spheroidization rate is above 90%, a good blanking surface can be obtained. On the contrary, if the hardness of the material is low but the spheroidization is insufficient, the blanking surface will be torn. Therefore, hardness is a macroscopic indicator to judge whether it is suitable for shearing, while the metal structure (uniformity and spheroidization degree of carbides) is a microscopic indicator to judge whether it is suitable for blanking.


7. Time separation


When drawing and forming certain plates (such as stainless steel plates and brass plates), due to the residual stress formed during drawing, longitudinal cracking will occur on the side wall of the cylindrical part after drawing. This cracking phenomenon may occur immediately after demoulding, or may occur after being placed for a period of time, or during the use of stamped parts, so it is called age cracking.


8. Spheroidizing


The structure of low carbon steel is composed of soft ferrite as the matrix and a small amount of pearlite. Pearlite is a fine mixture of ferrite and cementite, of which the cementite content accounts for 12%. Ferrite has good plasticity, while cementite is hard and brittle. From the perspective of materials with the same carbon content, the plasticity can be increased through the spheroidization of carbides and the quality of the blanking surface can be improved.


Note: What is spheroidizing annealing?
The purpose is to spheroidize the layered cementite in the reticular secondary cementite and pearlite into granular cementite, reduce the hardness of the material, improve the cutting processability, and prepare for quenching. Because pearlite itself is hard, and due to the presence of reticular secondary cementite, the hardness and brittleness of steel are increased. This not only brings difficulties to cutting processing, but also causes deformation and cracking during quenching.


9. Cylinder deep extension test (LDR value)


The cylinder deep extensibility test method is one of the most basic methods for evaluating the deep extensibility test of metal sheets. The purpose of this test is to find the Limit Drawing Ratio (LDR) of metal materials. The larger the LDR value, the better the drawability of the material. LDR=D/dp, where: D represents the diameter of the blank, and dp represents the diameter of the punch (extended product). The LDR value has a positive relationship with the plastic strain ratio r value, that is, materials with larger r values also have better deep extensibility.


10. Conical dish test (CCV value)


The conical dish test method is one of the most basic methods to evaluate the formability test method of metal sheets (thickness 0.5~1.6mm). The CCV value can be used as an evaluation test for the composite formability of deep drawing and stretching, and is closely related to the work hardening value system and plastic strain ratio.