Application of New Hot-Rolled High-Strength Sheet Steels

Several types of hot-rolled dual-phase sheet steels prepared by simple temperature control in hot strip mill or by heat treatment on a continuous annealing line have been compared in this article with conventional micro alloyed steels through various forming tests.

Thickness of these steels ranges from 1.8 to 2.5 mm and yield strength from 300 to 520 MPa. Forming tests employed include stretching, drawing, flange stretching or hole expansion, and simulative model forming of automotive parts such as rear axle housing and spring support, and the behavior of these sheets is discussed.

Interest in high-strength steel has a lengthy history in the steel industry. Recent development of high-strength low-alloy (HSLA) sheet steel depends upon the large amount of technical information available in this field. In response to the automobile industry’s demand to reduce overall vehicle weight and thereby improve fuel economy, and to satisfy safety and crash-worthiness requirements, the steel industry has developed a large variety of steels and processes for producing high-strength hot-rolled and cold-rolled steel sheets.

The overall suitability of various steels for automobile body panel applications is assessed by evaluating their characteristics with regard to the performance requirements (formability, weldability, paintability, etc.). The formability of the steel sheets is perhaps the most important requirement for automotive component applications.

The aim of this article is to shed some light on the properties of steels which are controlled by the manufacturing conditions, and to recover the loss of formability that occurs as strength increases. The possible applications to automotive parts can be divided into two general categories, namely body panels and structural and safety-related parts. This article describes the formability of hot-rolled high-strength sheet steels for the latter category and the principal material properties which become the indication when producing such materials.

Dual phase steels, which have much better ductility for a given strength than conventional high-strength steels, have been developed. They have microstructures consisting of two major phases: martensite and ferrite. The suitable method of making these steels is to roll to the required thickness and then make use of heat treatment on a continuous annealing line. Another method is to find out the cooling condition and steel compositions which 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 the steels, it is necessary for the automotive industry that they should have similar forming behavior and performance. This study therefore compares some of the properties for nine as- hot-rolled dual phase steels, two continuously heat treated dual phase steels, two conventional high-strength steels, and a commercial low carbon steel with yielding strength of 300 to 520 MPa.

The press forming of these steels is studied to gain an understanding of the influence of increasing strength on formability parameters. The formability investigation is performed through an evaluation of the response of the sheet steel in three deformation modes in the forming limit diagram: stretching, plain strain, and drawing.

Stampings are judged acceptable if there are no obvious tears, cracks, buckles, wrinkles, or necks in the finished stamping. In the forming of hot-rolled steels applied to the frame members of automobiles, which generally require thicker sheet than that of exposed panels, it is important that the steels exhibit good stretch flanging and punch stretching ability.

Tension testing is performed on parallel-sided specimens with a nominal width of 25 mm. Testing is carried out using a constant cross head speed, and elongation to fracture measured with a 50-mm gage length extensometer. Average mechanical properties are obtained from a minimum of five specimens in three test directions.

Hole expansion testing is carried out as follows: a 20-mm hole is punched into the sheet before deformation and is expanded with a conical punch. The expansion of this hole prior to the point of failure is referred to as the ratio of hole expansion.

The stretch forming test is performed with a hemispherical and flat bottom punch in which 400 and 450 mm square blanks are held in the die.

Simulative model forming is carried out with two types of dies. One is a spring support of which a character is stretching, and the other is a rear axle housing of which a character is drawing.

Springback is measured after a simple bending over three dies of different radius of curvature. Thickness of specimen is reduced to 1.7 mm by surface grinding in order to establish a constant strain of bending.

Formability parameters affect the ability of a material to be transformed from its original shape into a defined final shape by a specific forming process. Material, process, and shape interact in forming 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 tension test, generally indicate the forming behavior of the material. The importance of these material parameters, which all interact in forming processes, depends upon the shape of the part and the manufacturing processes. Better understanding and accurate determination of these forming parameters help to predict the behavior of these steels in stamping operations.

The work hardening behavior of sheet steels is often characterized by the n-value, defined as the exponent in the Ludwig’s equation. For most dual phase steels, and also for highly formable interstitial free steels, the stress-strain curves do not conform to the Ludwig’s equation. To compare the work hardening behavior of the steels, it is suggested that the most useful parameter is the instantaneous work hardening rate normalized with respect to the flow stress. The distinct expression of the work hardening behavior is obtained by this parameter. However, it is tedious to establish the curves of the normalized work hardening rate in the function of the tensile strain.

Hole expansion ratio is influenced by the plastic strain ratio, by total elongation (which affects the critical hole expansion), and by quantity and shape of inclusions (which cause cracks). Results indicates that the hole expansion ratio decreases with the increase of total quantity of inclusions.

As reported previously, sulfide shape control becomes important in achieving a higher ductility along the sheared edge. Without sulfide shape control in these hot-rolled steels, lower expansion can occur due to the tearing which initiates on the punched edge at elongated sulfide inclusions. However, even in a material with sulfide shape control, there is a rather important degradation of sheared edge ductility as strength increases.

It is noted that a high-strength material which has a hole expansion ratio of more than 1.5 may be considered satisfactory, compared with the low-carbon steels. An investigation is made of the influence of the clearance between punch and die when a hole is punched into the sheet. It is indicated that the clearance has relatively little effect on the hole expansion ratio.

For automotive components the formability of sheet steel is determined principally by biaxial stretchability and deep draw ability. The total elongation and work hardening exponent are measures of the biaxial stretchability of sheet, and these parameters decrease as the yield strength of the sheet steel increases. As a general rule, the average plastic strain ratio, which is a measure of deep draw ability, also decreases as strength increases. For all the steels examined, the values are in a very narrow range and similar to those for low-carbon steel.

There is a good correlation between the forming index and work hardening exponent. This test is performed both parallel and transverse to the rolling direction, so 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 components is apt to deviate from the design configurations because of various elastic recovery effects including springback. Defects in shape precision of finished parts are responsible for difficulties in assembly processes. Materials must be as uniform as possible with regard to thickness and properties in order to minimize springback after stamping.

Various types of hot rolled dual phase steels are examined by forming tests. Dual phase steels containing manganese and silicium are characterized by improved formability. Good correlation is obtained between the hole expansion ratio and inclusion shape control.

The work hardening exponent is the principal factor determining the press performance of hot-rolled dual-phase steels. In particular, n-value from 5 to 10 percent strain in tension testing is shown to have a good correlation with formability. This will allow the setting of guidelines for optimizing manufacturing conditions for these steels.

It is expected that the superior properties of dual phase steels will result in significant increases in their use for automotive applications in the immediate future.