Types and characteristics of high strength steel plates for automobiles
According to the application, various strengthening mechanisms are applied to automobile steel plates. The traditional automobile steel plate mainly adopts two ways: adding alloying elements to low carbon steel to form solid solution strengthened section steel and precipitation strengthened section steel with refined grains to achieve high strength of low carbon steel. Modern automobile high-strength steel plate adopts the mechanism that the metallographic structure of steel is strengthened during smelting and heat treatment, and obtains modern high-strength steel such as microstructure strengthening, composite structure strengthening, phase transformation strengthening, heat treatment strengthening, cold working hardening strengthening and aging strengthening. In recent years, these high-strength rolled steel plates have been widely used in automobile manufacturing.
(1) bake hardened steel (BH steel).
The microstructure of BH steel is based on ferrite, which is mainly strengthened in the form of solid solution. The characteristic of BH steel is that the added chemical element phosphorus can form solid solution strengthening with carbon and nitrogen during steelmaking, thus realizing solid solution strengthening. In the process of automobile body manufacturing, the density of "dislocations" in the matrix (ferrite) increases and the diffusion distance of carbon and nitrogen atoms to "dislocations" is shortened during stamping. When the car body made of BH steel is painted, the car body is heated/baked in various drying furnaces. At this time, thermal activation energy is given to the diffusion of carbon and nitrogen atoms in solid solution, so that carbon and nitrogen atoms are precipitated at "dislocations", thus enhancing the yield strength of the product, so it is called bake-hardening steel. BH steel is usually used to process parts such as doors and trunk outer panels.
(2) Duplex steel (DP steel).
DP steel has two phases: extremely soft ferrite and hard martensite. Because the metallographic structure contains a lot of ferrite (that is, the matrix of metallographic structure is ferrite), its elongation is quite high and its plasticity is quite good, which is close to the traditional high-strength steel plate formed by adding phosphorus and other elements to form a solid solution. In addition, the strain between hard martensite and soft ferrite in steel is not harmonious, but it will cause quite high work hardening when working under pressure, which will make DP steel produce quite high work hardening, make it have quite high tensile strength, limit the occurrence of "necking" during stretching, and have good ductility and formability. Therefore, it is used for sheet stamping parts with strict requirements on machinability, such as door reinforcement plates, bumpers and other parts.
(3) Transformation induced plasticity steel (TRIP steel).
The metallographic structure of TRIP steel contains a lot of bainite caused by ferrite and phase transformation, as well as residual austenite and martensite. Bainite structure can be obtained by isothermal or continuous cooling at medium temperature. Martensite and retained austenite are dispersed in the matrix (matrix) Jin Xiangzhong composed of bainite or bainite-ferrite of TRIP steel. These retained austenite will be transformed into martensite again in the process of machining, which will increase the strength of some materials, thus making the materials obtain good processability and greatly increase the impact resistance.
At present, the tensile strength of DP steel or TRIP steel produced by major iron and steel companies in the world has reached 590 ~ 980 MPa, and has entered the practical stage. The front frame longitudinal beam, lower arm of steering tie rod and body column of newly developed passenger cars are made of DP steel or TRIP steel.
(4) Multiphase composite steel (CP steel).
CP steel not only adopts measures to improve plasticity according to grain refinement mechanism, but also adopts measures to improve strength through metallographic structure hardening mechanism, and at the same time improves mechanical properties. It must be emphasized that compared with dual-phase steel DP, the yield strength of CP steel is obviously improved and greater at the same tensile strength of 800MPA. CP steel also has quite high impact resistance and energy absorption characteristics and high residual deformation capacity. Therefore, CP steel not only has quite high tensile strength and high work hardening coefficient, but also has very uniform ductility. Taking the products processed by CP steel as an example, the tensile strength can exceed 800MPA after baking and hardening during painting.
(5) Ferrite-bainite steel (FB steel).
Ferrite-bainite steel is also called tensile flanging steel or high reaming steel because it has the ability to improve the tensile strength of flange flanging or long hole. FB steel can be used to manufacture hot rolled products, and its main advantage is to improve the edge performance of flattening flanging or flanging determined by reaming flanging test. It is superior to high-strength alloy steel and dual-phase steel in these aspects. Compared with HSLA steel, FB steel also has a higher work hardening coefficient n under the same yield strength, and the total edge extension is increased. In addition, FB steel has been used to produce laser butt welded blanks (TWB) for stamping large and medium-sized body panels because of its good weldability. The important characteristics of FB steel are: good anti-collision performance and excellent fatigue resistance.
Application and existing problems of high strength steel plate in automobile
Application of 1 high strength steel in automobile outer panel
The roof, doors, luggage and other parts require deformation stiffness and sag resistance, and BH steel plates with tensile strength of 340 ~ 390 MPa are mainly used. When baking and painting, the yield strength of BH steel plate increases, which can improve sag resistance and make the steel plate thinner without losing formability. Now some models have used 440MPA BH high strength steel plates.
Application of high strength steel plate in car body frame
With the improvement of safety standards for frontal impact and side impact, 590MPA grade high-strength steel plates are mainly used for structural members and reinforcements. Some also use 780MPA and 980MPA high-strength steel plates. There is even a method of stamping 390MPA and 440MPA grade high-strength steel plates, then carrying out high-frequency heating quenching on the strengthened parts to make the local tensile strength of the parts reach 1200 MPa, and cooling while stamping and heating the steel plates to make the overall tensile strength of the parts reach 1470 MPa. In addition, there is a tailor-welded method, that is, steel plates with different thicknesses and materials are assembled by laser, so that the material configuration is suitable for the required materials and use parts. Tailor-welded parts can be assembled, the number of parts can be reduced, and spot-welded flanges can be removed, which plays a great role in automobile lightweight. Although tailor-welded materials were only used for small parts in order to improve the utilization rate of materials at the initial stage of use, they have recently been extended to large parts such as body side panels and trunk floors. Tailor welded blanks mainly use 400 ~ 590 MPa high-strength steel plates, and sometimes 780MPA and 980MPA high-strength steel plates are used.
3 application of high strength steel plate in automobile chassis
The material used for cantilever beam has been developed from the traditional 440MPA hot-rolled plate to 780MPA, and the maximum weight loss is 30%. In recent years, the proportion of high-strength steel plates used in chassis has increased dramatically. In the future, the specific gravity of high strength steel plate and the application of higher strength steel plate are expected to be further improved.
4 Main problems in the forming of high strength steel plate
High-strength steel plate is easy to cause plastic decline and poor formability, while the increase of yield strength will cause surface distortion and rebound effect, and increase shape instability. Typical molding defects include cracking, bad shape, poor dimensional accuracy and die sticking.
(1) cracked.
Improving the strength of high-strength steel plate will easily lead to the decrease of plasticity, and will also reduce the fracture limit of bulging and stretch flanging. Steel plate has high strength and is prone to cracks. In addition, because it needs a large forming force, if the mold temperature rises during continuous processing, the mold will be stuck and cracks will be induced as a result.
(2) the shape is not good.
When the yield point rises, it is easy to wrinkle. Wrinkling will not only cause mold sticking and cracks, but also be difficult to eliminate in the later stage of forming, which often leads to bad shape. Because wrinkling can't be eliminated in the later stage of forming, stamping parts can't be formed at the bottom dead center. Moreover, the bad shape caused by elastic recovery after molding and the surface strain caused by wrinkling are also great problems.
(3) poor dimensional accuracy.
With the increase of material strength and residual stress, it is easy to cause bad shape, and the elastic recovery after molding leads to poor dimensional accuracy (springback). This is the most serious problem in the forming of high strength steel plate. Springback is a phenomenon of poor dimensional accuracy caused by the stress difference in the thickness direction, which is manifested as unqualified angle and surface warping of stamping parts. With the increase of material strength, unqualified angles and warpage become serious.
(4) mold clamping.
High-strength steel plate needs large forming force to increase the contact pressure between blank and die. In this way, it is easy to get stuck in the mold. If this phenomenon is serious, it is necessary to correct the mold. This not only requires more suitable materials for surface treatment and quenching of the mold, but also shortens the maintenance cycle of the mold and increases the maintenance cost of the mold.
Analysis of stamping process characteristics of high strength plate
Mold manufacturing is mainly divided into five steps: process design, structure design, processing, clamping and debugging. It should be said that the design of process scheme is the most critical factor affecting the final quality of parts. The process scheme directly determines the appearance and function of the product, and also affects the production cost of the mold. In addition, the rationality of the mold structure will directly affect the processing performance of the mold and the convenience of operation and maintenance. Material selection and heat treatment are very important for beam high strength steel plate. It is difficult to solve the problems of parts springback and drawing die galling. In order to solve the above two problems, the whole process must be controlled. Based on the above analysis, the key control points of the whole project are determined as follows:
Control product input (review product stamping process);
Optimum stamping process scheme design (different schemes are adopted for plates with different strength);
Reasonable mold structure (easy to adjust processability and springback);
Wear and scratch of insert materials (select appropriate materials and heat treatment methods).
1 formability prediction of high strength plate products
Eliminate the abrupt boss
Avoid local bulging;
Avoid the rapid change of section line length;
Anti-rebound measures;
Fillet: The fillet of the part must be large enough, in principle, not less than 8, otherwise the drawing will be easily broken and the flange of the part will be upturned after molding.
2 high strength steel plate defect control technology
High-strength steel plates are prone to stamping defects such as cracking, poor shape and poor dimensional accuracy.
(1) technology to prevent cracking and bad shape (wrinkling).
The high strength of steel plate easily leads to the decline of formability. Therefore, it is very difficult to punch parts with complex shapes. However, this is an effective way to smooth the shape of the part. With the improvement of computer processing speed and software function, simulation molding technology has developed rapidly, which can predict bad shape defects such as cracks and wrinkles with high accuracy. Now, the simulation forming technology has been applied to the prediction of cracking and wrinkling, the setting of suitable blank holder force, the setting of reasonable blank allowance and the optimization of the shape of rigid rib punch.
(2) Techniques to prevent poor dimensional accuracy (springback).
With the rapid development of high-strength materials for automobile structural parts, in recent years, the voice of developing anti-springback technology for stamping has become increasingly strong. Therefore, the following measures are taken in the die structure: ① reverse bending near the die shoulder is used to reduce the surface warping that often occurs in stamping bending; (2) Set the pressing rib on the die to press the convex part near the bottom dead center into the steel plate, so as to reduce the residual stress generated in the thickness direction of the plate and prevent the surface from warping. In addition, there is a bending stamping method, that is, most of the whole forming process is completed in one forming process, and then the rib pressing process is carried out for secondary forming, and high-pressure blank holder force is applied to prevent the blank from flowing in from the blank holder ring.
On the other hand, the springback of blank is predicted during die design. The method of predicting die shape has been widely used as a measure to prevent springback. Simulation molding method is one of the prediction methods, and its function is more and more obvious, which can predict cracks and bad shapes with high accuracy. This method has been applied to practical engineering.
(3) Measures to prevent mold sticking (affecting productivity).
A new film processing method, diamond-like carbon (DLC) film processing method, can solve the phenomenon of die sticking that affects productivity. In addition, there is a treatment method of DLC-Si film, which can improve the service life of the die 1 times compared with the current TIC and TIN treatment. From the point of view of controlling mold cost, a method of predicting the parts that need surface treatment in advance by simulation molding is also put forward. With the increase of forming force, the insufficient capacity of stamping equipment is also a problem worth worrying about. Simplifying the shape of stamping parts and using tailor-welded materials are effective ways to solve this problem. This is also effective for increasing the number of forming processes and re-examining the division of stamping parts and processes, but it involves the increase of costs and needs careful study.
Key points of die structure for high strength plate parts
In order to facilitate the mold processing and prevent the die casting from being damaged by accidental double pressure when the mold is reworked, a brand-new structure is adopted. The mold fixing seat is divided into two pieces, the middle of which is connected by M24 studs, the two sides are connected by locking plates, and the bottom lies in the bottom bracket. When processing, the left and right parts are separated, which is convenient for the processing of the middle groove, and the bolt and lock plate are damaged when encountering work accidents, thus ensuring the safety of the casting.
For the particularity of high-strength plate and beam parts, although the springback was corrected and compensated after several rounds of simulation in the early stage. However, it is inevitable to modify the mold in kind. Because the die inserts are all CR 12MOV, it is difficult to weld the die surface, and the hardness deviation of local materials after repair welding is easy to cause the surface roughening defect of the parts. Secondly, the heat-treated CR 12MOV insert is difficult to process and the material cost is high. Therefore, in order to avoid the scrap loss of the insert, it is decided to use HT300 as the punch insert first, and then replace it after the parts are debugged.
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