Obtaining high-strength weldable steels with bainite structure using thermomechanical treatment

In this paper, we present the results of investigations which form the basis for development of high-strength weldable bainite steels obtained using in-line thermomechanical treatment in the rolling mill and not requiring heat treatment with special heating. The thennomechanical treatment involves controlled rolling followed by regulated cooling. In order to achieve high strength properties than in steels with ferrite—pearlite structure, the alloying and cooling conditions for rolling in the thermomechanical treatment cycle must ensure that lowtemperature austenite transformation products be obtained with a fine original austenite grain.I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 28–33, October, 1994.     

低碳超高强度贝氏体钢的组织细化

对系列低碳、超高强度贝氏体钢(LCUHSBS),通过有效地控制相变温度、冷却速率与回火参数,贝氏体铁素体(BF)含碳量增加、组织细化、碳化物消除以及存在高稳定性、高体积分数的膜状残余奥氏体(AR).利用AFM、SEM等分析并测试了贝氏体钢的显微组织与晶粒尺寸,结果表明,板条束内含有若干大致平行的BF板条,而每一板条由许多切变单元组成;切变单元进一步又分成大量超细亚结构,其直径约为18nm.如此细化的显微组织确保了贝氏体钢在超高强度条件下,冲击吸收能成倍提高.     

Effect of austenite-decomposition temperature on bainite morphology and properties of low-carbon steel after thermomechanical treatment

Peculiarities of the bainite structure formed in low-carbon steel 07G2NDMBT during isothermal austenite decomposition, namely, the sizes of crystallites, their mutual orientation, and the presence of cementite precipitates, are considered. The temperature range of the formation of bainite with the subgrain structure was determined. The size of the austenite grain and degree of hot deformation were found to affect the transformation of bainite that occurs upon subsequent cooling and the submicrocrystalline bainite structure. We studied the structure and mechanical properties of a rolled sheet 16 mm thick, which was subjected to thermomechanical treatment (TMT) under plant conditions in accordance with optimum regimes. It was shown that the high structure dispersion of the steel subjected to TMT is due to not only the formation of bainite with the subgrain structure, but also the partial transfer of crystal-structure defects from hot-rolled austenite to the final bainite structure.     

Sulfocarbonitriding of steels with a low-carbon martensite structure

Saturation of steels with interstitial elements in a liquid phase allows creating gradient layers at relatively low temperatures and short durations of the treatment. Rather cheap and ecologically safe compositions for low-temperature nitriding and carbonitriding baths have been developed at the end of the last century. Improvement of tribotechnical characteristics can be achieved by adding a small amount of sulfur. Layers that have been formed at a low temperature after short-term treatment yield in thickness to those produced by a customary high-temperature long-term thermochemical treatment, and the necessity of subsequent quenching in liquid media creates additional technological and ecological difficulties with processing traditional steels. The use of low-carbon martensitic steels provides noticeable advantages. The activation energy of nitrogen diffusion in a low-carbon martensite is lower in comparison with that in a predominantly ferrite structure, and the quenching of low-carbon martensitic steels does not require the use of liquid cooling media. Austenitizing before quenching improves the uniformity of distribution of alloying elements in a gradient layer and increases its thickness (to several hundreds of microns) due to postnitriding.     

High-strength,ductile Mn-Mo-Nb steels with a structure of acicular ferrite

Structural Steels

High-strength, ductile Mn-Mo-Nb steels with a structure of acicular ferrite     

Low-temperature thermomechanical treatment of chromium steels

Conclusions Low-temperature thermomechanical treatment considerably improves the mechanical properties of 1Kh12NVMFA and VNS-6 steels.Low-temperature thermomechanical treatment followed by tempering at secondary hardness temperatures produces a particularly favorable combination of properties.Low-temperature thermomechanical treatment increases not only the resistance to rupture but also the resilience.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 32–35, April, 1963     

Phase and structural transformations in low-carbon martensitic steels

Effects of the composition of low-carbon steel on the phase and structural transformations during heating, under isothermal conditions, and upon continuous cooling have been investigated. The influence of alloying with carbide-forming elements on the austenite grain size has been studied. A connection between the resistance to tempering and the mechanism of formation of low-carbon austenite has been established. The influence of heating on the temperatures of phase transitions during cooling has been shown.     

Modeling of structure of double-phase low-carbon chromium steels

A physical model for determining the relative amount of phase components and the size of ferrite grains after decomposition of austenite in the process of cooling of double-phase steels is suggested. The main products of austenite transformation, i.e., polygonal ferrite, pearlite, bainite, and martensite, are considered. The driving forces of the transformation and the concentration of carbon on the phase surface are determined with the use of methods of computational thermodynamics. The model is based on equations of the classical theory of nucleation and growth. It allows for the structural features of the occurrence of γ → α transformation and contain some empirical parameters. The latter are determined using data of dilatometric measurements of the kinetics of austenite transformation and metallographic measurements of the size of ferrite grains. The model is used for predicting the kinetics of the transformation under the complex cooling conditions implemented by the VOEST-ALPINE STAHL LINZ GmbH rolling mill within the computer system for control of mechanical properties of hot-rolled strip.