Abstract
Problem Statement (Relevance): This paper gives reasoning to why it is necessary to understand the effect of controlled rolling regimes on the structure and properties of pipe steel. Objectives: The objective is to understand the effect of controlled rolling of low-carbon microalloyed K60 (X70) pipe steel on the structural condition of hot-deformed austenite. Methods (Experiments) Applied: Using the facilities of the Thermodeform-NMSTU Research Centre, the physical process of controlled rolling of K60 (X70) pipe steel was simulated and analysed at various strains, temperatures and heating times. The metallographic analysis of the specimens was carried out by NMSTU’s Institute for Nanosteels through optical microscopy and using the Thixomet PRO image analyzer. Findings: The following regularities were established: An increase in the heating temperature from 1,100 to 1,240 °C and subsequent rolling with a 5 % reduction lead to a more than double growth in the average area of the austenitic grain (a 50 % reduction results in a 1.3-times growth). An increased degree of reduction won’t make up for the grain growth caused by a higher pre-rolling heating temperature. Thus, in the case of a slab preheated to 1,150 °C and rolled at a 5 % reduction, the average and maximum sizes of the austenitic grains would be less than in the case of a slab heated to 1,240 °C and deformed at a 50 % reduction. The critical reduction rate for the K60 steel grade is within 5 to 11 %, and from the perspective of obtaining a fine-grained structure, even single reductions below the critical reduction rate lead to bigger grain sizes. Practical Relevance: The data obtained can be used for designing controlled rolling regimes for pipe steel grades with enhanced performance.
Keywords
Pipe steel, microstructure, mechanical properties and performance, controlled rolling regimes, hot-deformed austenite.
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