M. Hua

On strength of microalloyed steels: an interpretive review

Abstract In the mid-1950s, hot rolled carbon steels exhibited high carbon contents, coarse ferrite pearlite microstructures, and yield strengths near 300 MPa. Their ductility, toughness and weldability were poor. Today, a half century later, hot rolled steels can exhibit microstructures consisting of mixtures of ferrite, bainite and martensite in various proportions. These structures are very fine and can show yield strengths over 900 MPa, with acceptable levels of ductility, toughness and weldability. This advancement was made possible by the combination of improved steelmaking, microalloying technology and better rolling and cooling practices. The purpose of this paper is to chronicle some of the remarkable progress in steel alloy and process design that has resulted in this impressive achievement.

New Method of Characterizing and Quantifying Complex Microstructures in Steels

From the late 1960s through the late 1970s, high strength microalloyed steels hot rolled to strip and plate exhibited ferrite-pearlite microstructures with yield strengths essentially limited to the range of 350–420 MPa. However, the advent of the energy crisis of the late 1970s led to the demand for steels of higher strengths, while maintaining acceptable levels of other properties such as weldability, toughness, and formability. Since the late 1970s, it has been recognized that the strengthening mechanisms present in ferrite-pearlite steels had reached their limit in terms of grain refinement and solute and precipitation hardening; hence the barrier of 350–420 MPa was a real restraint. At about the same time, it was also recognized that higher strengths in as-processed steels could only be achieved through the use of ferrite of lower temperature formation, i.e., non-polygonal, acicular, bainitic, or martensitic ferrite, either as a monolithic matrix microstructure or as a combination. This change in achievable microstructures was abetted by interrupted accelerated cooling, either on the runout table of a strip mill or after the finishing pass in a plate mill. Hence, high strength hot rolled or the later cold work and annealing (CRA) and/or continuous galvanizing line (CGL) processed steels, with strengths in excess of 420 MPa, now exhibit these complex microstructures. It is not uncommon today for a 490 MPa yield strength steel to exhibit a microstructure comprised of several types of microsconstituents: non-polygonal ferrite, bainite, martensite, and perhaps retained austenite. Traditional metallographic techniques are no longer capable of analyzing these complex microstructures, especially in a quantitative fashion. The inability to characterize and quantify these complex microstructures means that the true strengthening mechanisms operative in these steels may be incorrectly understood and evaluated. The recent application of the quantitative analysis of the image quality (IQ) of the Kikuchi pattern resulting from the Electron Bank Scattered Diffraction (EBSD-IQ) has led to an innovative way to quantitatively analyze complex microstructures in the higher strength steels. This article will present the technique and show where it has been used successfully in research studies involving a broad range of steels including high strength low alloy (HSLA), MA, DP, and TRIP steels.

Isothermal transformation behavior of near-gamma TiAl alloys

During the last few years near-gamma TiAl alloys have been considered as viable engineering materials for aerospace applications. A substantial amount of research effort has been directed towards the understanding of the microstructure-property relationship of these alloys. Similarly, fundamental studies have been conducted to understand the phase transformation mechanisms of single and two-phase titanium aluminides. The results of these studies have clearly established four types of microstructures: fully-lamellar, massive, equiaxed and duplex (lamellar + equiaxed). The two former types are developed through heating and cooling from the [alpha] phase region. Slow cooling produces the fully-lamellar structure and fast cooling leads to the massive microstructure. The two other types result from heating to and cooling from the ([alpha] + [gamma]) intercritical region. The development of the duplex or equiaxed structure depends on both the reheating temperature and holding time. The fully lamellar microstructure exhibits the best combination of mechanical properties at both room and high temperatures when compared to the other microstructures.

On Achieving a Better Understanding of the Polygonal Ferrite Microstructure in IF Steel Using Image Quality Analysis

The image quality of the diffraction pattern from EBSD has been used as an index distinguishing the degree of lattice imperfection. Since austenite decomposition products formed at different transformation temperatures have various dislocation or sub-grain boundary densities, this technique has been used to identify, group, and quantify the different types of ferrite. This article presents a study where the dislocation density variation, associated with different cooling rates from the austenite in a commercial IF steel, will be analyzed using this image quality analysis. Perhaps the most significant contribution of this work is the new insight into the strengthening mechanisms found in these steels.

Simulation of Line Annealing of Type 430 Ferritic Stainless Steel

The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject of this paper. The steel was cold rolled 79%, then heated at 6 °C/s to the soaking temperature of 841 °C, which is just below the Ae1 temperature. During heating, specimens were quenched from selected temperatures between 650 and 841 °C and after various times at 841 °C. These quenched samples underwent metallographic examination and micro-hardness determination. The results indicated that under the prevailing experimental conditions, the hardness appeared to correlate strongly with the extent of recrystallization. The kinetics of recrystallization appeared to originate in the cold worked state, where three kinds of grain were found: (i) smooth elongated, featureless of α-fiber orientation {001}<100>; (ii) irregular fishbone grains of the γ-fiber orientations {111}<112> plus {111}<110>; and (iii) twisted grains of the η-fiber orientation {001}<100>. It was found that the twisted grains of the η-fiber were the first to recrystallize, with the fishbone grains of the γ-fiber second, and the smooth elongated, featureless grains of the α-fiber last. It was found that the grains of the α-fiber orientation {001}<100> and the η-fiber orientation {001}<100> were replaced with grains of the γ-fiber orientations as recrystallization progressed. These results are discussed in terms of recrystallization and texture development.

Recent Development of Nb-Containing DP590, DP780 and DP980 Steels for Production on Continuous Galvanizing Lines

Dual phase steels for production on CGL, having tensile strength range from 590 to 1200 MPa, was developed in Nb-bearing Cr–Mo steels with carbon contents 0.06 and 0.15 mass%. The role of Nb in these steels, as well as the formation and transformation characteristics of austenite as a function of intercritical annealing temperature, cooling rate from intercritical annealing temperature to 460°C were investigated. The mechanical property tests of the steels treated by CGL were performed. The OIM and EBSD image quality analyses were employed in this investigation for analyzing microstructures and the phase transformation products of austenite. The results show each strength grade dual phase steel has the excellent combination of tensile strength and ductility properties. The addition of Nb can further improve the tensile strength of the Cr–Mo dual phase steel without ductility reduction. This relates to microstructure refined by the Nb in steels and the proper combination of volume fraction of recrystallized ferrite and martensite.