Because low-strength ratio steel possesses good forming property, it is commonly believed that the lower the yield-strength ratio of steel is, the better the plasticity of steel is. However, it was found through the controlled rolling and controlled cooling experiment of Mn-B steel that the ductility of the low-strength ratio steel is lower than that of high-strength ratio steel. The research indicates that steel possessing low-strength ratio only represents that it possesses relatively high homogeneous elongation. But the ductility of steel is determined together by homogeneous elongation and local concentration elongation after necking.

The industrial contrast experiment of X70 pipeline steel produced by hot tandem mill and reversing mill was carried out in a domestic certain iron and steel corporation.The results show that in comparison with pipeline steel produced by hot tandem mill,that produced by reversing mill has lower yield strength,higher tensile strength and lower yield-strength ratio.The interval in pass during finish rolling is an important reason to affect the mechanical properties of X70 pipeline steel.

Low-carbon bainitic steel is a new type steel with high strength,high toughness and multiple uses,its emergency is an inevitable result of social demand and the development of modern metallurgical technology.At present,the low-carbon bainitic steel is extensively used in construction machinery.However,the low-carbon bainitic steel is usually composed of several kinds of microstructures,so the corresponding relation between the mechanical performances of the steel and bainite structure cannot be directly reflected.In view of this situation,controlled rolling and cooling experiments of low-carbon Mn-B-Cr-Mo-Nb steel are carried out in a domestic iron and steel corporation.By the control of final cooling temperature,three low-carbon bainitic steels with different microstructure are obtained.They are respectively composed of granular bainite,lath bainite,and granular bainite + lath bainite.After tensile and impact tests,the results indicate that under the condition of identical chemical composition,the granular bainitic steel is the lowest in strength and is the best in toughness and plasticity,the lath bainitic steel is the highest in strength and is the weakest in toughness and plasticity,and the steel composed of granular bainite and lath bainite is medium in strength,toughness and plasticity.So it can be known that the final cooling temperature plays an important role in microstructure and mechanical performances of hot rolled plate.Accordingly,the comprehensive mechanical performances of low-carbon bainitic steel can be further improved by the control of medium-temperature transformation microstructure.

Weldability is one of main index to value the performance of construction steel. Besides earthquake-resistant and fire-resistant capabilities, high-rise building should possess good weldability. The welding experiments of 490MPa grade high-rise constructional steel produced by a domestic steel corporation were tested. The results indicate that the yield strength and tensile strength of the steel before welding are similar to those of steel after welding, so it can meet the requirement of engineering. The research can provide experimental basis to further investigate the weldability of high-rise constructional steel.

To increase the strength of low-carbon alloy steel,the hot rolling experiment in(α+γ) dual-phase region with finish rolling temperature of 710℃,and the hot rolling experiment in γ single-phase region with finish rolling temperature of 800℃ were carried out.The results indicate that in comparison with that of the plate rolled in γ region,no significant increase of strength of the steel plate rolled in(α+γ) region is observed.Further more,the polygonal ferrite rolled in(α+γ) region exhibits two characteristics: grain boundary allotriomorphic ferrite forms,some ferrite grains are stretched along rolling direction,and texture structure also appears to some extent but it is not very obvious.

The effect of granular bainite,ferrite-pearlite on the strength,ductility and toughness of low-carbon steel was investigated.The results indicate that the strength and toughness of low-carbon granular bainite steel are higher than that of ferrite-pearlite steel.And for granular bainite steel,not only grain refinement and dislocation strengthening,but also dispersion strengthening are effective.

Niobium is an important alloying element for the steel. To know further the effect of Nb in the steel, the contrast experiments on the austenite grain growth of the 0.015%Nb and free Nb steels were carried out using Gleeble 1500 thermomechanical simulator. The experimental results indicate that the austenite grain of 0.015%Nb steel is finer than that of Nb free steel at 1150–1230°C. And when the heating temperature arrives the critical temperature 1240°C, the austenite grain of Nb steel suddenly grows up, while the austenite grain of Nb free steel changes little. Finally, the austenite grain of Nb steel is obviously coarser than that of Nb free steel. By transmission electron microscopy (TEM) using a carbon extraction replica technique, the precipitates of Nb(C,N) were not observed in the 0.015%Nb steel. It is concluded that the grain-boundary internal adsorption of Nb atoms leads to the result.