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Daniel Schaeffler Daniel Schaeffler
President

Formable High-Strength Steels, Part 2: Advanced High-Strength Steels

November 22, 2020


Automakers, in constant search for the ideal material for vehicle body structures, carefully consider material strength, formability, corrosion resistance, stiffness and, of course, cost. Safety regulations balanced against the need for lighter components, in order to achieve better fuel economy and reduced emissions, led, in the 1980s and 1990s, to increased use of high-strength low-alloy (HSLA) steels. Increasingly stringent safety, fuel-economy and emissions regulations continue the drive for steelmakers to develop new grades having high strength and formability.

Global commercialization of advanced high-strength steels (AHSS) started in the 2000s. Laying the foundation for application of these products were projects organized by a global consortium of steelmakers, starting in 1995 with the UltraLight Steel Auto Body. Also resulting from this consortium were the AHSS Applications Guidelines, which provide background on the many types of AHSS grades available and provide strategies for forming and joining AHSS grades in body-structure applications. The guidelines are periodically updated to reflect the latest knowledge and best practices. The 2009 version covered 32 different HSLA and AHSS grades, while the 2017 version describes 38 unique AHSS grades. This number will grow with the next release, scheduled for 2021.

The Elaborate AHSS-Production Process

HSLA steels obtain their properties from the melt chemistry, along with minor adjustments made at the steel mill to the heating and cooling cycle during rolling and annealing. The basic thermal cycle mimics that used to produce mild steels, meaning that most flat-rolled steel mills can produce HSLA grades (top graph on the accompanying image).

thermal cycle comparison between galvanized conventional steelthermal cycle comparison between galvanized conventional steel and galvanized AHSS gradesAHSS production requires new alloy compositions run on equipment able to achieve tightly controlled heating and cooling cycles (bottom graph on the accompanying image), equipment owned by relatively few steelmakers. The complexity of these cycles and overall process-control requirements further reduce the number of companies capable of producing higher-strength and more-complex AHSS grades. The elaborate production route leading to the unique microstructures and properties of AHSS explains why these grades are called “advanced.” Among the challenges in AHSS production is the desire to have zinc-coated steels available for corrosion protection. Hot-dip galvanizing, or galvannealing, is an additional heat treatment that ultimately affects the engineered microstructure and resultant properties.

Understanding Microstructure

Ferrite is the low-strength, high-ductility phase in the microstructure of most steels. Mild steels also contain in their microstructure iron carbide, which is―a low-ductility component also known as cementite. HSLA steels use microalloying elements to create carbo-nitride precipitates to increase strength. Conventional processing lines produce steels with each of these microstructural components, while lines capable of producing AHSS grades use process steps that result in the development of other microstructural phases, including martensite, bainite and austenite.

Dual-phase (DP) steels have a micro-structure consisting of ferrite and martensite, with increasing amounts of martensite leading to higher strength. Among the AHSS grades, lower-strength DP steels have the easiest mill-production routes. Initial applications in the early 2000s consisted of 590-MPa tensile-strength grades stamped into relatively simple straight parts such as rails, cross members and rocker panels. Formability of these steels exceeded that of similar-strength HSLA grades, and they absorbed more energy in a crash. Higher-strength steels always are at risk for dimensional issues such as springback, and initial countermeasure attempts evolved into the standard strategies used today: overbending, sidewall stretching and the addition of geometrical features such as darts. 

In low-carbon steels, the micro-structural phase austenite usually exists only at temperatures above 725 C, and is unstable at lower temperatures. However, with an appropriate composition and thermal profile, austenite can be retained at room temperature. This retained austenite, which provides the high ductility of many AHSS grades, is responsible for the transformation-induced plasticity (TRIP) effect, which gives TRIP steels their name. When deforming these steels, the retained austenite in the microstructure transforms into high-strength martensite in a process that delays local necking and improves ductility. 

TRIP-steel microstructures also contain bainite. The chemistry of these steels contain more alloying elements in higher amounts than dual phase steels, contributing to higher production complexity and cost. The higher alloy content results in a higher carbon equivalent, with associated welding challenges. TRIP grades with 780-MPa tensile strength exhibit formability, as measured by n-value, comparable to that of drawing steels having one-third the strength. These grades find use in applications requiring crash-energy absorption, and by the mid-2000s, some automakers began using TRIP steels in safety-critical SUV side structures for parts requiring moderate draw depths. 

The microstructure of complex-phase (CP) steels comprises many components, as the name suggests, primarily ferrite, martensite and bainite. Bainite, combined with a small hardness difference between the phases, leads to improved sheared-edge expansion. CP steels, similar to HSLA grades, rely on carbo-nitride precipitates of microalloyed elements to minimize grain size and, therefore, increase yield strength. These steels are characterized by a very high ratio of yield strength to tensile strength, suiting them for automotive applications requiring minimal deformation after a crash. Starting about 15 years ago, automakers began to address side- and rear-crash energy-management challenges with CP grades of various strength levels.

Use of AHSS grades in automotive applications continues to grow. This article describes only some of the first-generation grades; continued research led to the evolution of AHSS grades with even greater improvements in elongation, edge ductility and bendability. For example, third-generation AHSS grades entering production achieve 15-percent elongation at a tensile strength of 1200 MPa, with others reaching 20-percent elongation at greater than 1000-MPa tensile strength.

In 2007, the average vehicle body structure contained about 10-percent AHSS, the majority being DP 590. By 2018, AHSS content in vehicle body structures more than doubled, and experts project that by 2025, a typical body structure will contain more than 50-percent AHSS. MF

Industry-Related Terms: Ductility, Edge, Forming, Lines, Martensite, Run, Tensile Strength, Austenite, Corrosion Resistance, Draw, Drawing
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials

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