Acta Metall Sin  2019, Vol. 55 Issue (6): 783-791    DOI: 10.11900/0412.1961.2018.00485
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The Strengthening Mechanism of Cu Bearing High Strength Steel As-Quenched and Tempered and Cu Precipitation Behavior in Steel
Zhengyan ZHANG(),Feng CHAI,Xiaobing LUO,Gang CHEN,Caifu YANG,Hang SU
Department of Structural Steels, Central Iron and Steel Research Institure, Beijing 100081, China
Abstract

High strength low alloy (HSLA) steels are widely used in the construction of ship structures, oil pipelines, offshore platforms and so on because of their good strength, toughness and weldability. HSLA steel is generally designed with low carbon and Cu alloying. Tempered lath bainite or martensite and nano-precipitate phase of Cu can be obtained by quenching and ageing process after rolling to ensure the excellent matching of strength, low temperature toughness and weldability of HSLA steel. At present, increasing attention has been focused on the precipitation behavior and strengthening mechanism of Cu particles in HSLA steel which was aged at the peak hardness of ageing curve. However, in practical engineering applications, overageing heat treatment is generally used to make HSLA steel achieve a good match of strength and toughness. In this work, the microstructure and nano-sized Cu precipitates of an industrial production HSLA steel plate with thickness of 35 mm were characterized by SEM, EBSD, HRTEM and APT. Meanwhile, the strengthening mechanism of the tested steel was investigated. The results show that Cu precipitates in the tested steel processed by overageing are mainly in the range of 6~50 nm, Cu particles exhibiting short rod or spherical shape within 30 nm are 9R structure, and other particles size larger than 30 nm exhibiting long rod or spherical shape are fcc structure. The segregation of trace Mn and Ni in rod particles on the interface between Cu particles and matrix is more obvious. After ageing at a higher temperature range, the yield strength of the tested steel decreases linearly with the increase of tempering temperature. The main strengthening mechanism of the HSLA steel is fine grain strengthening, followed by dislocation strengthening and precipitation strengthening. The calculated results show that every 1%Cu added in the tested steel can produce about 90 MPa precipitation strengthening increment under the condition of overageing heat treatment. The strength difference between the surface and the center of the tested steel plate is about 40 MPa, which is mainly due to the difference of grain size and dislocation density of steel.

 ZTFLH: TG142
Fund: National Key Research and Development Program of China(No.2017YFB0304501)
Corresponding Authors:  Zhengyan ZHANG     E-mail:  zhangzhengyan@cisri.com.cn
 Fig.1  The relationship of yield strength of tested steel and tempering temperature (a) and the hardness of thickness section of tempered steel plate at 660 ℃ (b) Fig.2  SEM images of Cu bearing steel with different positions including surface (a), quarter (b) and center (c) along thickness direction Fig.3  The EBSD interface distribution maps of surface (a) and center (b) of tested steel (Where black and red lines represent the high angle grain boundaries (≥15°) and low angle grain boundaries (2°~15°), respectively), and the total grain boundary density (GBD) of surface and center of steel vs the misorientation of ferrite grain ranged 0°~60° (c) Fig.4  TEM images of particles precipitated during ageing process in surface (a) and center (b) of steel, and its EDS (c) Fig.5  HRTEM images of Cu particles with the size of ≤10 nm (a), 10~20 nm (b), 20~30 nm (c), 30~40 nm (d), >40 nm (e) precipitated in the tested steel, and the fast Fourier transformed (FFT) result of a lager particle along the $[1ˉ11]$Cu zone axis (f) Fig.6  APT image of Cu precipitates in the test steel aged at 660 ℃, where "a" expresses a spherioidal particle and "b" expresses a rod-like particle Fig.7  APT images (a, c) of Cu particles and their composition profiles (b, d) with the spherality (a, b) and shot rod-like (c, d) Fig.8  The amount of Cu precipitated in steel during ageing process calculated by Thermo-Calc software Fig.9  Analysis and comparison of strengthening mechanism for the surface and center of tested steel (σp—precipitation hardening, σd—dislocation hardening, σg—grain refinement hardening, σs—solid solution hardening, σ0—lattice friction stress)