The Influence of Isothermal Holding Times on Microstructural Evolution of X90 Linepipe Steel

Materials Research. 2019; 22(3): e20180605 DOI: http://dx.doi.org/10.1590/1980-5373-MR-2018-0605 The Influence of Isothermal Holding Times on Microstructural Evolution of X90 Linepipe Steel Qi Zhoua, Xian-ming Zhaob*, Zhuang Lic, Xi-jun Cuid School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, China b The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, China c College of Materials Science and Engineering, Shenyang Aerospace University, Shenyang 110136, Liaoning, China d School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, Wollongong, NSW, 2522, Australia a Received: September 04, 2018; Revised: December 18, 2018; Accepted: February 19, 2019 Microstructural phase of linepipe steels depends on different isothermal conditions. Thermal cycling testing of X90 linepipe steel was conducted using a thermomechanical simulator. The results have shown that with increased holding time, the microstructure constituents change from the martensite and bainitic ferrites to granular bainite and polygonal ferrite. There was the amount of martensite in the microstructure for isothermal holding times of 5 and 10 s at 700ºC. The effects of the martensite strengthening were weak. The changes of the hardness curve are decided by microstructural phase and the precipitation behavior. The interphase precipitation seems to begin for isothermal holding times greater than 30 s. It has a peak value for isothermal holding time of 600 s. Some fcc (Ti, Nb) (N, C) particles which belong to the MX-type precipitates were obtained at this condition. It is a major microstructural contributor to the hardness. The beneficial effects of the precipitates decreased considerably due to coarsening of the precipitates with prolonged holding, which results in the hardness quickly dropping. As a result, the hardness exhibited a low value after isothermal holding for 3600 s because the extent of precipitation strengthening was lessened and a lot of polygonal ferrites were formed. Keywords: Linepipe steel, Isothermal transformation, Hardness, Precipitation. 1. Introduction High strength low-alloy (HSLA) steels are widely used for pipelines, buildings, bridges, and ships because of their potential to obtain high strength-toughness combination1-4. Alloying elements such as Mn, Ni, Cr, and Mo are added to achieve the hardenability of austenite in these HSLA steels. In addition to those alloying elements, microalloying elements such as Nb, Ti, and V, precipitate in austenite (γ) as carbides, nitrides, or carbonitrides during hot rolling, and contribute to the mechanical properties of the microalloyed steels via grain refinement, solid solution hardening, and precipitation hardening5-9. When HSLA steels containing Nb, Ti or V are transformed from austenite (γ) into ferrite (α), alloy carbides are precipitated in parallel rows as a result of periodic nucleation at the migrating α/γ interface, which is called interphase precipitation10. Interphase precipitation is a major microstructural contributor to the hardness (strength) 11. HSLA steels are usually produced by thermomechanical control process (TMCP). TMCP consisting of controlled hot rolling followed by controlled cooling is used to maximise the benefits of the microalloy additions present in microalloyed steels 12-14. Traditional TMCP of microalloyed steels is employed to refine grain size and produce multi-phase microstructures, which provides good combinations of high strength and low temperature impact toughness. Suitable processing parameters in conjunction with micoalloying help improve strength *e-mail: through various mechanisms: microstructural refinement, solid solution hardening, precipitation strengthening, and dislocation hardening due to the modification of the resulting microstructure 15. From a commercial perspective, the formation of precipitation is of interest for applications in the steel production process. There are few studies on interphase precipitation in X90 linepipe steel using controlled thermal cycling, even though it has been conducted under different isothermal holding times 16. However, it is note worthy that the precipitation behavior develops during short isothermal holding times because it may be more important parameters in production lines. The present work will be able to clarify the variation in the hardness after the thermal cycle. Vickers hardness will first increase up to the maximum value, and then, decrease with the increase holding time. This phenomenon is attributed to the microstructural evolution and effective precipitation strengthening. The reasons which have yet to be investigated systematically would be made clear reasonably. In this work, the TMCP simulations were conducted by using a laboratory thermomechanical simulator in X90 linepipe steel. The microstructures and the precipitation behavior of the specimens during different isothermal holding time were discussed by analyzing the phases and microconstituents and measuring the hardness value. The purpose is to a better understanding of the microstructure evolution characteristics under different isothermal conditions. 2 Zhou et al. 2. Experimental X90 linepipe steel was used in this study. The chemical composition of the experimental steel (mass percent, %) is C 0.059, 0.0045N, Si 0.230, Mn 1.870, Al 0.025, S 0.0014, P 0.019, Ti+Nb+V 0.095, Ni 0.350, Cr 0.240, Cu 0.190, Mo 0.185. Cylindrical specimens taken from hot-rolled material and machined φ8×15 mm were used in this investigation. Thermal cycling experiments were performed in a Gleeble 1500 thermomechanical simulator. In order to construct continuous cooling transformation ºCCT) curves, double-pass compression test is shown in Fig. 1. In Fig. 1, specimens were electrically heated at rate of 10ºC s-1 to 1250ºC, held at this temperature for 180 s and cooled down to different deformation temperatures of 1100 and 950ºC, at 5ºC s-1, respectively. The double-pass compression test were employed with compressive strain values of 30% and 40% respectively at strain rate of 1ºC s-1, and interpass time was set 20 s. Specimens were cooled in air to room temperature at rates of 0.3 to 33 17ºC s-1 after deformation. The constituent of the microstructure at different cooling rates are presented in Fig. 2. Materials Research The thermal processing schedule is shown in Fig. 3 The thermal cycling specimens were first heated to austenitic solution temperature (1250ºC) for 600 s to dissolve the precipitates present in the initial microstructure, compressed by twice to produce grain-refined microstructure, and followed by cooling to 700ºC and isothermal holding for 5, 10, 30, 60, 600, and 3600 s. They were quenched to ambient temperature for terminating further precipitation. Finally, the specimens were tempered at 550ºC for 1 h to detect the effect of interphase precipitation on hardness. Fi (...truncated)