Research On High Heat Input Welding Alkaline Metals Micro-alloyed High Strength Low-alloyed Steels

High heat input welding non-quenched and tempered high strength low-alloyed (HSLA) steels with tensile strength of 490MPa grade were designed by composite-micro-alloying with single, double and tri-oxide-forming elements of alkaline earth metal elements of calcium(Ca), magnesium(Mg) and niobium(Nb), vanadium(V), titanium(Ti) in this paper. Mechanical properties, morphological distribution of inclusions and microstructure characteristics of the as-rolled steel plates and the heat-affected zone (HAZ) of 30kJ/cm, 60kJ/cm, 100kJ/cm heat input welding thermal simulation were studied systematically. It was found that:The mechanical properties of the as-rolled steel plates met the target requirements. For improving the strength and toughness of the materials, Nb composite-micro-alloying with Ca, Mg, Ca-Mg and Ti-Ca-Mg was superior to that of V and Nb-V. All of the experiment steels could bear 100kJ/cm high heat input welding, indicating that in addition to the traditional elements of Ti, the selected oxide-forming elements of the alkaline earth metal elements Ca, Mg could improve the strength and toughness of HAZ. The tri-oxide-forming elements of Ti-Ca-Mg micro-alloying was more favorable for improving the strength and toughness of 100kJ/cm HAZ than single oxide-forming element of Ti, Ca or Mg micro-alloying as well as double oxide-forming elements of Ti-Ca, Ti-Mg, Ca-Mg micro-alloying.The microstructures of the as-rolled steel plates were ferrite with sub-structures and a small amount of pearlite, and the microstructures of HAZ changed from bainite (or bainite and a small amount of intragranular acicular ferrite--IAF) to bainite and IAF, and further changed to a small amount of grain boundary ferrite and IAF as the welding heat input increased.The compositions of the composite precipitates analysised with scanning electron microscopy(SEM) and transmission electron microscopy(TEM) both in the impact fracture of the as-rolled steel plates and in different heat input welding HAZ were consistent with the corresponding micro-alloying steel series. The cleanliness index of Ca, Mg, Ca-Mg and Ti-Ca-Mg steels were significantly lower compared to that of Ti steel, Ti-Ca-Mg steel had the smallest of cleanliness index and the highest degree of cleanliness, and the average particle size of inclusions in Ti-Ca-Mg steel was the smallest. Ca, Mg micro-alloying had significant spheroidizing capabilities of the inclusions, and Ti-Ca-Mg micro-alloying had the best spheroidizing effect of the inclusions. Alkaline earth metal elements Ca, Mg appeared in the form of composite oxides, and the majority of composite precipitates were rich in manganese(Mn) and sulphur(S). The nuclei of IAF were the non-metallic oxides inclusions containing Ca, Mg with the size of about 500nm to 3μm. The number of effective IAF nuclei were inconsistent for different microalloying series, resulting the corresponding changes of the volume fraction of IAF in HAZ and strength and toughness of HAZ. The larger the number of IAF nuclei in HAZ of the micro-alloying series, the higher the volume fraction and the relatively finer of IAF became in HAZ, the increase of IAF inhibited the growing up and coarsening of grain boundary ferrite. The relative amount of IAF and the morphology of grain boundary ferrite in HAZ were the decisive factors affecting the strength and toughness of HAZ.Based on their features of steel purification of and inclusion modification, Ca and Mg micro-alloying improved strength and toughness of the as-rolled steel plates, and formed small-size precipitates containing MnS on its surface which promoting the nucleation of IAF in HAZ and then improved the toughness of HAZ. The composite micro-alloying of Ti with alkaline earth metal elements Ca, Mg or Ca-Mg could promote the MnS precipitating on the surface of precipitates containing Ca, Mg or Ca-Mg, enhanced IAF nucleation ability of precipitates, and further improved the strength and toughness of HAZ. The IAF nucleation capability of precipitates in HAZ increased in the order of Ca(O,S)-MnS or Mg(O,S)-MnS, Ti-MnS or CaMg(O,S)-MnS, TiCa(O,S)-MnS or TiMg(O,S)-MnS, TiCaMg(O,S)-MnS.The IAF nucleation mechanism in HAZ of alkaline earth metal Ca, Mg micro-alloyed steels including two aspects: on the one hand, as almost all of the surface of precipitates in HAZ containing different amounts of MnS, the MnS could appropriatly increase the lattice matching between precipitates and the ferrite, and also appropriatly reduce average coefficient of thermal expansion differences between precipitates and austenite, thus inhibited the high temperature formation of proeutectoid ferrite, and improved the IAF nucleation capability and its volume fraction; on the other hand, the certain amount precipitates with the size of 500nm to 3μm in HAZ could increase the strain energy around the precipitates, and increased the driving force of IAF transformation. The dominant nucleation mechanism of IAF in HAZ of different micro-alloyed series was not the same as each other.The chemical compositions of a new steel were optimized, and the internal controlled smelting chemical compositions ranges and controlled rolling production processes of the new steel were suggested in combination with the equipment and technological process of a steel enterprises. The industrial production processes and delivery terms of the new steel were also established combining with the technical standards related to the new steel, and a national invention patent for the new steel were declared. All of the above works laid the foundation for the commercialization of the new steel.