Sheet steel

Sheet steel forming process

Blanking

Blanking is a stamping process that uses a die to separate materials. Blanking, in a broad sense, is the general term for the separation process, which includes cutting, blanking, punching, trimming, cutting and other processes. But generally speaking, the blanking process mainly refers to blanking and punching out finished parts, and it can also prepare blanks for other forming processes.

After punching, the sheet is divided into two parts, namely the blanking part and the part with larvae. If the purpose of punching is to produce a certain shape of the back-flushing part, then this punching process is called blanking; if it is to produce an inner hole, it is called punching.

According to the different deformation mechanisms, blanking can be divided into two types: ordinary blanking and fine blanking.

Bending

Plastic bending is one of the most common forming methods in the press forming process. The effect of bending is manifested in the change of the radius of curvature R and the angle x of the bending deformation zone (Figure 1)

Plastic bending must first go through the stage of elastic bending. We are already well-known in "Materials Mechanics": when elastically bends, the fibers in the outer zone of the beam are under tension and the fibers in the inner zone are under compression. The tension and compression zones are bounded by the neutral layer, which just passes through the center of gravity of the section, and its stress and strain are zero. Assuming that the radius of curvature of the neutral layer is ρ and the bending angle is α (refer to Figure 1), the fiber at y from the neutral layer has a tangential strain εθ of

So the degree of deformation and stress of the material depends entirely on the distance between the fiber and the neutral layer and the radius of the neutral layer. The ratio y/ρ is independent of the size of the bending angle α. The stress and strain are the largest at the inner and outer edges of the bending deformation zone.

For a sheet with a thickness of t, when its bending radius is R, the stress (δθ)max and strain (εθ)max at the edge of the sheet are

The yield stress is δs, and the conditions for elastic bending are: The inside gradually enters plasticity and deforms.

Stretching

Deep drawing is a processing method in which the flat flange is drawn into the cavity of the cavity to form a straight wall.

The inflow of the flange can be divided into (a) compressed flange inflow, (b) parallel flange inflow, and (c) extended flange according to the deformation state of the contour shape of the die hole. Flow into three categories, (Figure 2).

Due to the different shapes and sizes of the drawn parts, the stress and strain distribution of the blank during the deformation process is also different. Therefore, when determining the process plan, process parameters and mold design, analysis and calculation should be carried out according to the specific situation to determine the reasonable blank size and the geometric parameters of each process step, mold structure parameters and equipment model, in order to obtain qualified parts .

Cold rolled steel sheet

Cold rolled steel sheet is most used in mass production. The thickness of cold-rolled sheet steel is between 0.15mm and 3.2mm, and the car body mostly adopts the thickness of 0.6mm to 0.8mm. This kind of thin steel plate has very high dimensional accuracy, smooth surface, good mechanical properties and processability. It is mainly used for exterior side panels, roof covers, engine covers, fenders, trunk covers, door panels and instrument panels. Covering parts, these parts require materials with high formability, good surface quality and welding performance.

Sheet steel for general stamping

Before the use of interstitial atomic steel (IF steel), low-carbon boiling steel (08F) and aluminum killed steel (08AL) were used for automobile panels. Punching. Boiling steel is now less used due to its poor stamping performance and easy failure. However, aluminum-killed steel plates are still widely used as steel plates for stamping, especially for body parts with low forming requirements, the use of aluminum-killed steel plates still accounts for the majority.

(1) Aluminum killed steel plate (08AL)

①Chemical composition of aluminum killed steel plate. As a traditional stamped steel plate, 08AL does not require high strength, but requires good stamping formability. For this reason, the steel plate is required to have higher plasticity, and the lower the alloy elements in the steel plate, the better.

a. Carbon. Carbon is the main element in general steel, but it is a harmful element in deep-drawn sheet. The increase in the mass fraction of carbon in the steel will increase the strength of the low-carbon steel and reduce the formability. Therefore, my country's GB/T5213-2001 stipulates that the carbon mass fraction of deep drawing steel should be less than 0.08%.

b. Manganese. 4%。 When the manganese mass fraction is high, the strength of the steel plate is increased, so the manganese mass fraction of the general aluminum killed steel should be less than 0.4%.

c. Sulfur. The mass fraction of sulfur has an adverse effect on the stamping performance of thin steel sheets, so the lower the mass fraction, the better. Generally, the sulfur mass fraction specified for 08AL steel plates should be less than 0.025%.

d. Phosphorus. The role of phosphorus is to significantly increase the strength of the steel sheet. In addition to increasing its content in high-strength steel sheets, its mass fraction is limited in general cold-rolled steel sheets, and the mass fraction of phosphorus in deep-drawn sheets is less than 0.020qo.

e. Silicon. The role of silicon is mainly to improve the strength of the steel plate, but it will deteriorate the stamping performance, so the lower the mass fraction, the better, and the silicon mass fraction should be less than 0.030%.

f. Aluminum. Aluminum is added to the cold-rolled steel sheet for stamping to improve its stamping performance, so 08AL stipulates that the aluminum mass fraction should be between 0.02% and 0.07%.

08AL steel plate is a low-carbon steel with good plasticity, suitable strength, and certain aging. When used within 3-6 months, its stamping performance cannot be destroyed.

Al-killed steel has a simple structure, generally composed of ferrite, free iron carbide and a small amount of non-metallic inclusions. Free iron carbide and non-metallic inclusions have a great influence on the stamping performance of the steel plate. Among them, square inclusions have the most impact, and circular dot-shaped inclusions have the least impact, which will cause the parts to crack during the stamping process.

The size of the ferrite grains and the uniformity of the grains also have a great influence on the stamping performance. If the grains are too small, the strength will increase and the plasticity will decrease, which is detrimental to the stamping performance; the grains are too large, The surface of the parts after stamping is poor, there will be an orange peel-like surface, and the grain size is 7~8 level is the most ideal. If the crystal grains are not uniform, it will cause uneven deformation of the whole part, which will lead to cracking.

When punching complex parts, due to the different deformations of each part of the part, slip lines will appear on the surface of the part, which will seriously reduce the surface quality of the part. This is a phenomenon for automobile covering parts. It is not allowed to appear. Generally, quenching and tempering rolling is required to eliminate the slip phenomenon in stamping.

To sum up, 08AL steel plate is suitable for general parts with low stamping strength, surface quality and uniform deformation. For deep-drawn parts and complex parts, use it with caution.

(2) Interstitial atomic steel plate (IF steel plate) The car body is composed of stamped parts welded. The shape of the car body is becoming more and more complex in order to reduce air resistance and beautiful appearance, and the stamping performance of the steel plate is increasingly required. Therefore, there are certain requirements for the indicators that affect the stamping performance, such as high plastic strain ratio, appropriate strain hardening index, and high With high elongation and low yield strength, only the steel plate that meets the above requirements can punch out parts with complex shapes.

The first-generation cold-rolled steel sheet represented by ordinary boiling steel and the second-generation cold-rolled steel sheet represented by aluminum killed steel are difficult to fully meet the requirements of modern car stamping parts. The third-generation cold-rolled interstitial-free atomic steel (IF steel plate) that has been mass-produced can meet the above requirements. New steel plates derived from IF steel such as IFBH steel, IF high-strength steel, IF coated steel, etc., plus double Phase steel (DP steel) and plastic deformation induced phase change steel (TRIP steel) have brought cold-rolled steel sheets for automobiles to a new level, basically meeting the needs of car production.

After adding a sufficient amount of titanium to low-carbon steel, the carbon and nitrogen interstitial atoms in the steel are completely fixed by titanium to form a titanium-carbon compound. At this time, the steel becomes a non-interstitial steel with excellent的FORMability. Because titanium is expensive and the production cost of steel is too high, mass production is impossible. In the late 1960s, the successful application of vacuum degassing technology greatly reduced the carbon and nitrogen content in steel, and reduced the amount of titanium used, thereby reducing the production cost of IF steel. In the 1970s, continuous annealing lines were used for steel plate production, which greatly reduced production costs, and IF steels with niobium and composite additions of titanium and niobium appeared, which expanded the production and application of IF steel.

①The chemical composition of IF steel. The chemical composition of steel is the basis of its performance. For IF steel, the first is to reduce the mass fraction of carbon and nitrogen that are harmful to the forming performance, while adding titanium and niobium.

The role of carbon, silicon and other elements in IF steel is the same as that of 08A1 steel. Titanium and niobium are important elements in IF steel. The interstitial carbon and nitrogen in the solid solution can be removed by the treatment of titanium and niobium. Thereby removing the adverse effects of interstitial atoms. If the ultra-low carbon steel in industrial production is not processed by adding titanium and niobium to eliminate interstitial atoms, its plastic strain ratio is not high, and its forming performance is not good.

There are three types of IF steel produced in industry, namely Ti-IF steel with a single addition of Ti, a single Nb-IF steel with a single addition of Nb, and a composite Ti-Nb-IF steel with simultaneous addition of Ti and Nb.

The characteristics of the above three IF steels are as follows:

a. Ti-IF steel. In terms of process parameters, low heating temperature, high coiling temperature, high annealing temperature and large cold rolling reduction rate are beneficial to the improvement of the forming performance of Ti-IF steel. The influence of alloy composition on the elongation of Ti-IF steel is not as sensitive as that of Nb-IF steel, and the elongation of titanium stabilized steel is generally higher. High coiling temperature and annealing temperature will get coarse TiC particles, so the strength level is low. Precipitates such as TiS and Ti4C2S2 that have an important impact on the properties of Ti-IF steel generally begin to precipitate during the heating process and the initial stage of hot rolling. Therefore, the process parameters are not very sensitive to the effect of Ti-IF steel, and the process is highly operability. Stable performance. However, the steel plane of this composition system has large anisotropy and poor powdering resistance of the coating, which is not suitable for galvanized sheets.

b. Nb-IF steel. Compared with Ti-IF steel, due to the precipitation of fine NbC particles, the strength of steel can be improved, so Nb-IF steel has a higher strength level. The addition of Nb improves the weave structure of the steel, reduces the anisotropy value and increases the average value of the plastic strain ratio (r); Nb segregates to the grain boundary, which can prevent cold working brittleness, improve the plateability and resistance to powdering. However, since the precipitation process of Nb-IF steel occurs in the hot rolling cooling stage or annealing stage, the mechanical properties are more sensitive to process parameters, and Nb-IF steel has a higher recrystallization temperature, and its mechanical properties are not as good as Ti-IF steel.

c. (Nb +Ti) -IF steel. The elongation rate of niobium-titanium stabilized IF steel is lower than that of Ti-IF steel, but r. The value of r45 0 is relatively high, and it has strong formability. Niobium-titanium-stabilized IF steel has better coating adhesion than titanium-stabilized steel, has good alloying and hot-dip galvanized steel sheet resistance to powdering, and its mechanical properties are not sensitive to the process, and the performance of the entire roll is uniform. It is suitable for use in The production of high-strength steel and hot-dip galvanized steel under continuous annealing process is also the best choice for electro-galvanized IF steel and hot-dip galvanized IF steel substrates.

②The characteristics of IF steel plate. Compared with the widely used aluminum-killed low-carbon deep-drawing steel, the most obvious performance feature of IF steel plate is its excellent formability and never aging.

a. Excellent formability.

b. No timeliness. Generally, steel plates are time-sensitive. The so-called aging is the change of the yield stress and elongation of the steel plate over time. After a period of time, an obvious yield platform will appear on the drawing performance curve of the steel plate, and the formability of the steel plate will decrease at this time. The aging of the steel sheet is caused by the existence of interstitial solid solution atoms such as carbon and nitrogen in the steel sheet. Although aluminum sedation is used in deep drawing steel plates, its timeliness is not obvious, but its existence is still an objective fact. Therefore, there is still such a rule that the aging period of the deep-drawn steel sheet after rolling is 3 months, that is to say, the deep-drawn steel sheet after rolling must be used up within 3 months, otherwise the formability will be reduced. Down.

③Application of IF steel plate. Because IF steel plate has excellent formability and unique non-aging properties, it has been widely used in the automobile industry, especially the car industry. In summary, its application has the following aspects.

a. Used for stamping parts with complex shapes. Some stamping parts have complex shapes and large drawing, even if the best aluminum killed deep drawing steel is used, it is difficult to fully meet the requirements. For example, the oil pan of the engine has high requirements on the ductility of the steel plate. In the past, imported deep-drawn steel plates have been used, but the effect is not satisfactory. Since the use of domestic deep-drawing IF steel plate, this problem has been solved. Using domestic Ⅲ steel plate to replace imported deep-drawing steel plate to produce some more complicated-shaped stamping parts, the effect is very good, and it is worthy of further promotion.

b. It is used to manufacture high-strength automobile body covering parts. In order to save energy and meet increasingly stringent environmental protection requirements, high-strength steel plates are being widely used in the automotive industry. For example, many outer coverings of car bodies widely used high-strength steel plates in the original design, reducing the thickness of the steel plates, thereby reducing the quality of the car body, reducing the fuel consumption of the car, and reducing the harmful emissions of the car.

IF steel plate has excellent formability. On this basis, the high-strength IF steel plate developed through solid solution strengthening and other methods has obtained higher strength while still retaining higher formability.

In the case of the same strength level, the comprehensive level of high-strength IF steel plate's formability index is significantly better than that of traditional high-strength steel plates. In addition, this type of IF steel plate is not time-sensitive, and the stamping process There is no slip line on the surface that affects the appearance quality of the panel, so it is very suitable to use this kind of steel plate to produce automobile outer coverings.

c. As a substrate for high-performance galvanized steel sheet. Galvanized steel sheets are used more and more widely in automobiles. For example, in European models such as Audi and some models in North America, even the entire body is made of galvanized steel. Galvanized steel sheets are widely used because firstly, in cold-climate countries such as Northern Europe and North America, spraying salt water on the road after snow in winter will cause serious corrosion to body parts. Secondly, in order to improve the service life of automobiles, countries have repeatedly extended the period of time when automobile plates are not corroded. , The general bare steel plate can not meet these requirements, so the galvanized steel plate has received attention.

Generally, deep-drawn steel sheets are used as substrates to produce galvanized sheets. Since hot-dip galvanizing needs to be heated to a high temperature of about 500C, the formability of the substrates is deteriorated due to severe aging, so that the galvanized sheets The application has limitations and cannot be used for more complicated stamping parts. The advent of IF steel plate successfully solved this problem. Due to the high formability of the IF steel sheet itself and the lack of aging properties, it can still maintain good formability after hot-dip galvanizing. Table 3.7 shows the measured properties of two steel plates produced with ZTE steel plates as substrates. We can see from them that these two steel plates basically maintain the characteristics of their original plates and have good formability.

In addition, due to the large anisotropy in the plane of the micro-alloyed IF steel plate using titanium, the galvanized steel plate produced with this steel plate as the substrate may be affected by various directions of the steel plate during the stamping process. The degree of strain is different and pulverization occurs. When niobium is used as a microalloying element, the situation is much better, so in actual production, IF steel plates with Nb or Ti+Nb added are generally used as the original plates of galvanized plates.

High-strength steel plates for automobiles

In order to control CO2 emissions that cause global warming, automobile manufacturers are actively promoting the design of lightweight automobile bodies for the purpose of reducing fuel consumption. In the early 1990s, the proportion of high-strength steel plates used in car bodies increased to 30%, reducing the weight of the car. In the late 1990s, due to collision safety considerations, the use of reinforcements was increased in the car body, thereby increasing the weight of the car. In order to ensure car body collision safety and lightweight, the strength of high-strength steel plates used for car bodies has been increased from 440MPa to 590MPa, and 980MPa ultra-high-strength steel plates are used for body structural parts, and the proportion of use is also more than 40%. From the perspective of improving the body's corrosion inhibition performance, the alloyed hot-dip galvanized steel sheet has the same high-strength characteristics as the cold-rolled steel sheet.

High-strength cold-rolled steel sheets for automobiles are mainly used for body parts, which can be roughly divided into three categories, namely, inner and outer panels, structural parts, and reinforcements. Table 3-8 lists the strength and characteristics of the steel plate required for each component and the problems to be solved.