Abstract
Isothermal treatment affects the microstructural evolution and the precipitation behavior of high-strength low alloy (HSLA) steels. In this regard, thermal simulation of different isothermal treatment temperatures was adopted by using a thermomechanical simulator. The results showed that hardness reached the maximum value at 600 °C holding temperature, which was related to a finer grain structure and granular bainite. The strengthening effect of precipitates was remarkable due to the combination of small particle size and small interparticle spacing. It is presumed that the precipitation started after 600 s at 600 °C. Precipitation strengthening continued to exist, even though coarsening of ferrite grains led to softening phenomena when the specimen was isothermally held at 750 °C, which led to relatively high hardness. The precipitates were fcc (Ti, Nb) (N, C) particles, and belonged to MX-type precipitates. Average size of precipitates increased from 3.14 to 4.83 nm when the specimens were isothermally held between 600 °C and 800 °C. Interparticle spacing of precipitates also increased with increasing isothermal treatment temperatures. These led to a reduction in precipitation strengthening. At the same time the polygonal ferrite content increased and ferrite grain size got larger, such that the hardness decreased continuously.
Highlights
High-strength low alloy (HSLA) steels present a good combination of high strength and ductility obtained through the addition of microalloying elements, thermo-mechanical controlled processing (TMCP), and processes capable of producing complex microstructures that improve the mechanical properties of steels [1]
This paper analyzed the formation of bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF), and MA islands and the distribution of precipitates after different isothermal treatments through the isothermal process experiments of X90 pipeline steel, and established the effect mechanism of isothermal process on the evolution and precipitation behavior of the microstructure of experimental materials
The strengthening effect of precipitates was remarkable due to the combination of small particle radius and small interparticle spacing, and this was the prime reason of hardness variation
Summary
High-strength low alloy (HSLA) steels present a good combination of high strength and ductility obtained through the addition of microalloying elements, thermo-mechanical controlled processing (TMCP), and processes capable of producing complex microstructures that improve the mechanical properties of steels [1]. HSLA steels containing niobium and/or titanium as the microalloying elements are widely used for construction, line pipe, pressure vessel, engineering, automobile, naval, and defense applications [2,3,4]. Over the past few decades, the need for improved combinations of high strength, toughness and weldability on an industrial scale at affordable prices has driven the development of steel for production lines [5,6,7]. In order to improve transportation efficiency and safety under high pressure conditions, linepipe steel has become thicker and larger in diameter. High-grade pipeline steels have been used for the practical application [9,10,11,12].
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