Microstructural Controlling And Process Development Of Economical X70 Grade Pipeline Steel

The world energy resources consumption increases gradually and crude oil and nature gas will be 2/3 of all the sources in several decades. However pipeline is the effective way to transport crude oil and nature gas by long distance. Rigorous transporting environment need that pipeline steels must have excellent mechanical properties such as toughness and fragility etc.. To satisfy the requirement, pipeline steels are added with Mo and Nb etc. microalloyed element. However this way not only need low rolling temperaute, but also need high cost. HTP(High Temperature Processing) technique is a new technique to produce pipeline steels. The chemical compositions include less Mo or replacing Mo with Nb, which can save cost.To realize this purpose, the state key lab of rolling technology and automation of north-eastern university and Benxi iron and steel group limited company make a cooperation to develop pipeline with high strength and toughness and weldability. The purpose is decreasing cost by adding high Nb content and getting rid of Mo element to produce X70 grade pipeline steel. The research content of this thesis including: dynamic recrystallization, static recrystallization, precipitation action and phase transformation rules of HTP steels. Furthermore, hot rolling experiments in lab and industry experiments have also been done. Hot rolling results show that Nb elements instead of Mo in chemical composition of X70 grade pipeline steel can satisfy the mechanical properties requirement. The main research content can be described as followed:(1) Dynamic recrystallization action of high Nb micro-alloyed steels has been simulated by Gleeble-2000 thermal/mechanical simulation tester. Results show that peak strain and peak stress will decrease with the increasing of deformation temperature and the decreasing of strain rate; for a fixed steel, activation energy of peak value state is higher than that of stable state; for different steels, activation energy will decrease with the increasing of Nb content; for no obvious peak value point strain-stress curves, the existence of dynamic recrystallization can be determined by analyzing the relationship between strain-harden rate and strain.(2) Static recrystallization has been investigated for two different Nb content steels. Results show that static recrystallization will be difficult with the decreasing of deformation temperagture; with the increasing of strain, deformation storage energy will increase and recrystallization rate will be accelerated. By comparing PPT figure and recrystallization curves, the Tnr of A steel with high Nb content is 970℃and the Tnr of B steel with low Nb content is about 925℃. The end temperature of recrystallization is 880℃and 850℃respectively. With the increasing of original austenite grain size, the end temperature of recrystallization will increase.(3) Precipitation action of high Nb HTP steels has been studied. Results show that PTT curves of experimental steels after deformation is "C" shape, the nose temperature is about 800℃. For a fixed steel, the start time and the end time of strain induced precipitation will decrease with the increasing of strain. Furthermore, compared these two steels, the start time of precipitation of A steel is shorten than that of B steel.(4) According to thermal dilatation curves and metallographic examination, phase transformation during continuous cooling has been investigated. Results show that phase transformation production including massive ferrite, granular ferrite, bainite ferrite and some island microstructure. TEM examination shows that island microstructure is M/A island, some pearlite and degenerative pearlite. For a fixed steel, when the reheating temperature is same, hot deformation enlarged ferrite transformation zone and increased the ferrite transformation start temperature(Ar3). When the strain is same, with the increasing of reheating temperature, ferrite zone will be diminished and ferrite transformation start temperature(Ar3) will decrease.(5) Control rolling and control cooling experiments have been done at experimental rolling mills. The effect of reheating temperature, final rolling temperature, reduction amount and cooling mode on microstructure and mechanical properties for Nb bearing steel has been investigated. Results show that for A steel, when the non recrystallization temperature equals to 950℃and 920℃, mechanical properties(including strength and toughness) satisfied the standard of X70 grade pipeline steel. For E steel, mechanical properties satisfied the standard of X80 grade pipeline steel, and have good comprehensive property. (6) HTP steels have been produced at 1700mm rolling mills. Results show that mechanical properties satisfied the standard of X70 grade pipeline steel. Suitable Nb content has been added in X70 grade pipeline steel. The ideal temperature scheme for producing X70 grade pipeline steel is that enter fine rolling temperature is about 1030℃; final rolling temperature is about 810℃; coiling temperature is about 560℃.30,000 ton pipeline steels have been provided for India etc international market.