Various types of hot-rolled double-phase steels prepared by simple temperature control in hot strip mills, or by heat treatment in a continuous annealing line, were compared in this article with conventional microalloyed steels through various forming tests.
Thickness of these steels varies from 1.8 to 2.5 mm and yield strength from 300 to 520 MPa. Formation tests used include stretch, stretch, flare flanges or orifice expansion, and simulated modeling of automotive parts such as the rear axle housing and spring bracket. The behavior of these plates is discussed in these processes.
Interest in high strength steels has a long history in the steel industry. The recent development of high-strength, low-alloy steel sheet depends on the vast amount of technical information available in this field. In response to the demand of the automotive industry to reduce the total vehicle weight and thus improve fuel economy and satisfy safety and accident resistance requirements, the steel industry has developed a wide variety of steels and processes to produce hot rolled steel. and high strength cold.
STEEL PERFORMANCE AND SUITABILITY
The overall suitability of various steels for automotive body panel applications is assessed by evaluating their characteristics against performance requirements (formability, weldability, paintability, etc.). The formability of steel sheets is perhaps the most important requirement for automotive component applications.
The purpose of this text is to shed some light on the properties of steels controlled by manufacturing conditions and to recover the loss of formability that occurs as strength increases. Possible applications for automotive parts can be divided into two general categories, namely body panels and structural and safety parts.
Dual-phase steels, which have much better ductility for a given strength than conventional high-strength steels, were developed. They have microstructures consisting of two main phases: martensite and ferrite. The proper method for making these steels is to roll to the required thickness and then heat treatment in a continuous annealing line. Another method is to find out the cooling condition and steel compositions that achieve typical dual-phase properties directly from a continuous hot strip mill. These lead to the availability of hot-rolled dual-phase steels, made by two different methods and substantially different compositions.
Despite the differences between steels, it is necessary that the automotive industry have similar behavior and formation performance. Therefore, this study compares some properties of nine hot-rolled steels, two heat-treated dual-phase steels, two conventional high-strength steels and a commercial low-carbon steel with a strength of 300 to 520 MPa.
CONFORMING PROCESSES: Elongation, Deformation and Stretching
The pressing of these steels is studied to obtain an understanding of the influence of the increase in strength on formability parameters. The formability investigation is carried out through an evaluation of the steel sheet response in three deformation modes in the forming limit diagram: elongation, simple deformation and stretching.
Prints are judged acceptable if there are no obvious rips, cracks, buckles, wrinkles or necks in the finished print. In forming hot-rolled steels applied to automobile structural elements, which generally require a thicker sheet than exposed panels, it is important that the steels exhibit good stretchability and spline punching.
Tension testing is performed on parallel-sided samples with a nominal width of 25 mm. The test is performed using a constant cross velocity, and elongation to fracture, measured with a 50 mm gauge length strain gauge. Average mechanical properties are obtained from a minimum of five samples in three test directions.
The hole expansion test is carried out as follows: a 20 mm hole is drilled in the sheet before deformation and it is expanded with a conical punch. The expansion of this hole before the point of failure is referred to as the hole expansion ratio.
The stretch formation test is performed with a hemispherical, flat-bottom punch, where the 400 and 450 mm blanks are held in the die.
Simulation modeling is performed with two types of matrices. One is the spring holder from which a character is stretching and the other is a rear axle housing from which a character is drawing.
Springback is measured after a single bend in three teeth of different radius of curvature. Sample thickness is reduced to 1.7 mm per grinding surface in order to establish a constant bending stress.
Conformability parameters affect the ability of a material to be transformed from its original shape into a final shape defined by a specific forming process. Material, process and form interact in the formation of parts; therefore, they must be considered simultaneously in a formability study.
Mechanical properties such as yield strength, tensile strength, total elongation, work hardening exponent, plastic strain ratio, and strain rate sensitivity exponent, which are determined in the stress test, generally indicate the material's forming behavior. The importance of these material parameters, which all interact in the forming processes, depends on the shape of the part and the manufacturing processes. A better understanding and accurate determination of these forming parameters helps to predict the behavior of these steels in stamping operations.
HARDENING AND DEFORMATION OF STEEL
The work hardening behavior of rolled steels is often characterized by the value n, defined as the exponent in the Ludwig equation. For most hot-rolled steels, and also for highly formable interstitial free steels, the stress-strain curves do not conform to the Ludwig equation. To compare the work hardening behavior of steels, it is suggested that the most useful parameter is the instantaneous rate of work hardening normalized to yield stress. The distinct expression of the work hardening behavior is obtained by this parameter. However, it is tedious to establish normalized work hardening rate curves as a function of tensile deformation.
The rate of hole expansion is influenced by the rate of plastic deformation, the total elongation (which affects critical hole expansion), and the amount and shape of inclusions (which cause cracking). The results indicate that the hole expansion rate decreases with the increase in the total amount of inclusions.
As previously reported, shape control becomes important to achieve greater ductility along the cut edge. Without sulfide form control in these hot-rolled steels, minor expansion may occur due to breakage that starts at the perforated edge in elongated sulfide inclusions. However, even in a material with sulfide form control, there is a very important degradation of the ductility of the cut edges as the strength increases.
Note that a high strength material with an orifice expansion ratio greater than 1.5 can be considered satisfactory compared to low carbon steels. An investigation is made into the influence of the clearance between the punch and the die when a hole is drilled in the sheet. It is indicated that the clearance has a relatively small effect on the rate of orifice expansion.
FINDINGS OF THE AUTOMOTIVE INDUSTRY
For automotive components, the moldability of rolled steel is mainly determined by biaxial stretchability and deep drawing capability. The total elongation and work hardening exponent are measures of the biaxial extensibility of the sheet, and these parameters decrease as the yield strength of the steel sheet increases. As a general rule, the average rate of plastic deformation, which is a measure of deep tensile ability, also decreases as force increases. For all steels examined, the values are in a very narrow range and similar to low carbon steel.
There is a good correlation between the training index and the work hardening exponent. This test is performed parallel and transverse to the rolling direction, so that the fracture properties of the sheet in both directions can be evaluated. There is a difference in formability due to the rolling direction.
The shape of automotive sheet metal components can deviate from design configurations due to various springback effects, including kickback. Defects in the precision of the shape of finished parts are responsible for difficulties in the assembly processes. Materials should be as uniform as possible in thickness and properties in order to minimize setback after stamping.
Various types of hot-rolled double steels are examined through forming tests. Double-phase steels containing manganese and silicon are characterized by better formability. Good correlation is obtained between the hole expansion rate and the inclusion shape control.
The work hardening exponent is the main factor that determines the press performance of hot-rolled double-phase steels. In particular, n-value of 5 to 10 percent stress in the stress test is shown to have a good correlation with formability. This will allow the definition of guidelines to optimize the manufacturing conditions of these steels.
The superior properties of hot rolled steels are expected to result in significant increases in their use for automotive applications in the immediate future.