科研之友
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摘要
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 350-420MPa. 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 350-420MPa 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 420MPa, now exhibit these complex microstructures. It is not uncommon today for a 490MPa 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.
- 出版日期2010
