1. Mechanical analysis of deep drawing
The formability of deep drawing can be understood by analyzing the mechanics of cylindrical container forming (cylindrical stamping). The components of cylinder stamping are: round steel plate (blank), punch, die, and anti-wrinkle plate. Cylindrical forming is a processing method in which a material is changed from a blank to a cylindrical part. The deformation state of the blank during cylindrical forming is shown in Figure 2.
During the cylindrical forming process, the outer peripheral material of the blank flange is compressed in the circumferential direction, and compression deformation occurs. Due to the deformation, the material undergoes work hardening, creating a circumferential compressive stress in the material. In order to overcome this compressive stress, the material needs to be stretched in the inflow direction (radius direction of the billet) when deep drawing is continued. As a result, the deformation mode of the flange material is a “tension-compression” mode (Fig. 2①).
On the material that flows into the die to form the cylindrical side wall from the flange portion, a tensile force required to continue deep drawing acts. Since the diameter of the punch corresponding to the side wall of the cylinder is fixed, the material of the side wall of the cylinder cannot be stretched-compressed in the circumferential direction. Therefore, the deformation mode of the cylinder sidewall material is “unidirectional tension (plane strain) without width compression” (Fig. 2③).
The tensile force acting on the radial direction of the flange of the cylinder side wall acts on the cylinder bottom material through the punch shoulder, so that the cylinder bottom material is subjected to “stretch-stretch” stress, and the deformation mode of the cylinder bottom material It becomes “stretch-stretch” mode (Fig. 2⑤).
Therefore, the deformation modes of the bottom surface, side wall, and flange of the cylindrical stamping part are “tension-tension”, “plane strain”, and “tension-compression” mode. Theoretical and experimental studies on the formability of such thin sheets have been carried out in Japan over the past several decades, and the results of the studies have been widely recognized and used for actual forming processing and the development of forming materials. As shown in Figure 3, the punch load P and the material breaking load Pcr can be used to evaluate the deep drawability of the material.
The punch load P is the compression deformation resistance of the flange portion (Fig. 2①), the bending force of the die R portion – the bending rebound suppressing force (deformation of Fig. 2①→Fig. 2②→Fig. 2③), and the friction force between the die R portion and the flange portion Sum. In the relationship curve of punch stroke and punch load, there is a maximum punch load Pmax. Pmax and Pcr were compared, and the limit deep drawing ratio LDR (ratio of circular billet diameter to punch diameter) at the point of agreement between the two was used as an evaluation value of deep drawability.
In terms of deep drawability research, Futian’s research firstly used R.Hill’s theoretical formula of plastic anisotropy and H.W.Swift’s plastic instability condition to calculate the fracture load Pcr of the cylinder side wall. Since the deformation state of the flange portion is not uniform in the radial direction, a uniform deformation value in the radial direction is obtained by using the draw ratio obtained from the geometric dimensions of the flange. Then use the aforementioned theoretical formula of plastic anisotropy to calculate the maximum load Pmax of cylinder stamping.
The ratio of the obtained Pcr to Pmax is called the ultimate deep drawing ratio improvement factor (equivalent punching load). Then, the relationship between the limit drawing ratio improvement coefficient and the r value and the n value is obtained. The r value mentioned here is an index of thickness anisotropy of sheet deformation. The larger the r value of the steel plate, the easier it is to increase the thickness of the steel plate under the action of the tensile force. The n value is the work hardening index. Figure 4 is the effect of r value and n value on the equivalent punching load. Many experiments have proved that the deep drawability of the steel plate has a high correlation with the r value of the steel plate. In addition, since the punch load includes the frictional force between the punched sheet and the die, applying the punching lubricant and coating the die can improve the press formability.
In addition to cylinder stamping, deep-drawing has various stamping and forming modes such as square tube stamping, special-shaped stamping, truncated cone stamping, square truncated cone stamping, ball head stamping, cap stamping and so on.
Most of the industrial products are stamped and formed products combined with these forming modes. The press formability of the various modes depends on the breaking load of the part material at the punch shoulder and the deformation resistance and friction resistance of the part flange.
2. Steel plate for deep drawing
The following is a brief introduction to the deep-drawn thin steel sheets that are most used in industries such as automotive parts, containers, building parts, and motor housings. So far, many thin steel sheets have been developed to meet the performance requirements (workability, weldability, corrosion resistance, and decorative properties) of various industries. The development of sheet steel is closely related to the automobile manufacturing industry. In order to suppress the occurrence of cracks and wrinkles during sheet steel forming and to meet the decorative requirements of formed products, the development of sheet steel with improved press formability, especially deep drawing formability, has been carried out.
On the basis of the results of the forming test, a technology has been developed to optimize the chemical composition and manufacturing process, to control the grain orientation of the steel sheet, and to improve the deep drawability of the thin steel sheet. in. Continuous annealing plays an important role in the development and production of high-quality deep-drawing steel sheets.
Mild steel plate includes cold-rolled mild steel plate and hot-rolled mild steel plate. Hot-rolled mild steel sheets are graded according to total elongation, and cold-rolled mild steel sheets are graded according to total elongation and r value. Difficult-to-form parts with complex shapes mainly use deep drawing methods.
The stamping material used is a thin steel sheet with excellent elongation and deep drawing formability. A representative product of such a steel sheet with excellent deep drawing formability is IF steel in which the interstitial elements C and N are reduced to the limit. In the production of IF steel, the vacuum degassing technology of steelmaking is used to reduce the C content to less than 50ppm to become ultra-low carbon steel molten steel, and then single or compound addition of carbide-forming elements such as Ti and Nb is used to combine with all C, Nb and other carbide-forming elements in the steel. N forms a precipitate and fixes C and N. In this way, in the recrystallization process of the annealing stage after cold rolling of the steel sheet, the grain orientation of the steel sheet is preferentially formed to improve the deep drawability, so that the steel sheet has a high r-value deep drawing formability. At present, various iron and steel enterprises have been able to mass-produce IF-based cold-rolled steel sheets and galvanized steel sheets with excellent deep drawability with a C content of 20 ppm or less. In addition, the TRIP high-strength steel sheet having transformation-induced plasticity is not a steel sheet for deep drawing, but has a high n value, so it has excellent bulge formability and deep drawing formability. The reason is that when the retained austenite in the steel is processed to undergo martensitic transformation, the steel sheet has a dependence on the deformation mode. The retained austenite in the compressively deformed part of the flange of the component is difficult to undergo martensitic transformation, so the deformation resistance is small.
The tensile deformation of the flange of the part promotes martensitic transformation due to the TRIP effect, which causes the material to harden significantly. That is to say, the deformation resistance of the flange of the component is small, and the fracture resistance of the side wall of the component is large, and this stress state is conducive to the deep drawing process.
3. Deep drawing
The stamping process by heating includes: warm stamping and hot stamping technology of stainless steel sheet. Warm press forming is a technology in which the stamping die and the anti-wrinkle platen are heated with a heating device, and the punch is cooled with a coolant to perform deep drawing processing. The metal material softens at high temperature, the deformation resistance of the flange decreases, and the relative strength of the punch is higher, thereby improving the deep drawability. Using the warm stamping forming method greatly improves the stamping depth of the stainless steel sheet.
The hot stamping forming method is to heat the steel plate with hardenability to a temperature above the austenite transformation point in a heating furnace, and form the material in a state of low material strength. The forming material is quenched to produce high-strength parts. The advantage of the hot stamping method is that the strength of the product can be as high as 1500MPa, and the shape freezing of the product is good. However, in hot stamping, the flange material is cooled in contact with the die, resulting in an increase in deformation resistance and a decrease in deep drawability. To this end, we have developed technologies to improve the formability of hot stamping, such as lubrication utilization technology, die shape optimization technology, and forming speed enhancement technology.