Abstract
This paper focuses on understanding the effect of niobium content on the phase transformation behavior and resultant mechanical properties of thermomechanically rolled and direct-quenched low carbon steels containing 0.08 wt.% carbon. Investigated steels contained three different levels of niobium: 0, 0.02 and 0.05 wt.%. The continuous cooling transformation (CCT) diagrams covering cooling rates in the range 3–96 °C/s constructed based on the dilatometer studies showed only a minor effects of Nb on the phase transformation characteristics. In addition, experiments were performed for reheating and soaking the slabs at 1050–1200 °C and the results revealed that for these low-carbon steels, Nb failed to prevent the austenite grain growth during slab reheating. In the case of hot rolling trials, two different finish rolling temperatures of 820 °C and 920 °C were used to obtain different levels of pancaking in the austenite prior to direct quenching. The resultant microstructures were essentially mixtures of autotempered martensite and lower bainite imparting yield strengths in the range 940–1070 MPa. The lower finish rolling temperature enabled better combinations of strength and toughness in all the cases, predominantly due to a higher degree of pancaking in the austenite. The optimum level of Nb in the steel was ascertained to be 0.02 wt.%, which resulted not only in marginally higher strength but also without any significant loss of impact toughness.
Highlights
The use of high-strength steel is continuously growing due to several benefits achievable compared to the traditional lower strength steel grades
Blocks of ~180 mm × 80 mm × 55 mm cut from the castings were soaked at 1250 ◦ C for 2 h, thermomechanically rolled to approximately 6 mm thick plates according to the rolling schedule given in Table 2 and subsequently water quenched to room temperature at an average cooling rate of ~90 ◦ C/s
With a cooling rate of 3 ◦ C/s, bainite appeared in the final microstructure, whereas for a cooling rate of 48 ◦ C/s, the transformation shifted to lower temperatures leading to martensitic transformation
Summary
The use of high-strength steel is continuously growing due to several benefits achievable compared to the traditional lower strength steel grades. In order to decrease the environmental burden in respect of ever-increasing greenhouse gases, high strength steel offers great possibilities, especially in the transportation industry. The use of high strength steels allows lightweight constructions of vehicles leading to lower fuel consumption and higher load capacity. In terms of producing high strength steels in the steel industry, the process of direct quenching can lead to the desirable combination of strength and toughness, combined with relatively low energy consumption, compared to the traditional reheating and quenching process. The use of microalloying, such as niobium, in direct quenched steels can be an efficient way. For example, the amount of microalloying or proper processing route to achieve the highest benefit of microalloying can be challenging and needs metallurgical understanding
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