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Four different heating temperatures in the range of 770 °C -950 °C were used for laboratory heat treatment of low carbon low alloyed steel. Chemical composition of the steel was based on the most common TRIP steel concept, only the silicon content was lowered to 0.6% and it was partially replaced by 1.4 % of aluminium. The steel was further micro-alloyed by niobium. Two different ways of cooling were applied to the samples. The first set was cooled to 425 °C in a salt bath with the temperature of 200 °C, the second set was cooled to 425 °C in a salt bath heated to the temperature of 400 °C. In this way, two distinctive cooling rates were achieved for every soaking temperature. Once the samples reached 4250 °C, they were in all cases removed to the furnace for 20 minute hold at the temperature of 425 °C. The final cooling was carried out in air. Resulting microstructures were analysed by scanning electron microscopy and consisted of various amounts of ferrite, bainite and retained austenite. Tensile strength in the range of 750 -908 MPa was obtained with total elongation of 33-42%.

Two high strength low alloyed steels with 0.4%C, 0.6%Mn, 2%Si and either 1.3% of chromium or without chromium were used in this work to evaluate the effect of chromium on the final microstructure obtained by thermo-mechanical processing. Various heating temperatures, cooling rates and bainitic hold temperatures were tested. High strengths around 1700 MPa were achieved for the chromium alloyed steel, however total elongation reached only 9%. Chromiumfree steel turned out to be better suited for TRIP (transformation induced plasticity) processing. The relatively high strengths around 900 MPa were in this case accompanied by very high total elongations exceeding 30%. The final microstructure of chromium-free steel was also more typical for TRIP steel, as it consisted mainly of the mixture of bainite and polygonal ferrite.Keywords: TRIP steel, chromium, thermomechanical processing, scanning electron microscopy 1 Introduction TRIP (transformation induced plasticity) steels are advanced steel grades with high strength and enhanced total elongation and formability. Their good mechanical properties result from a complex microstructure containing ferritic matrix, carbide-free bainite and retained austenite [1,2]. Suitable microstructures with proper volume fractions, distributions and morphologies of individual phases and structural components are produced usually by thermo-mechanical treatment [1,3]. Ideal processing parameters vary according to the particular chemical composition of steel. The most conventional concept is based on C-Mn-Si alloying [1,[4][5]. Further TRIP steel variations were investigated in last decades, testing other alloying elements and their combinations. Silicon was fully or partially replaced by aluminium to improve galvanizing properties of thin sheets for automotive industry [6,7]. Micro alloying by Niobium was designed to refine the final microstructure and increase retained austenite content [8][9][10]. Chromium alloying of medium carbon TRIP steels has not been investigated very thoughtfully [11], probably due to its reputation of a carbide-forming element with a potential to increase the incubation time and decrease the transformation rate of isothermal bainite. The effect of chromium on higher hardenability is usually used in martensite-based steel, primarily to increase the strength [11][12][13]. However, chromium also retards pearlite growth rate and refines the microstructure by reducing the growth rate of prior austenite grain in steels [14][15] and improves corrosion resistance [16], which can be convenient for TRIP steels as well.

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Maraging steel is an iron-nickel steel alloy, which achieves very good material properties like high toughness, hardness, good weldability, high strength and dimensional stability during heat treatment. In this work, maraging steel 18Ni-300 was manufactured by selective laser melting. It is a method of additive manufacturing (AM) technology, which produces prototypes and functional parts. Sample of additively manufactured and conventional steel with the same chemical composition were tested after in three different states – heat treated (as-built/as-received), solution annealed and precipitation hardened. Resulting microstructures were analysed by light and scanning electron microscopy and mechanical properties were obtained by hardness measurement and tensile test. Cellular martensitic microstructures were observed in additively manufactured samples and conventional maraging steel consisted of lath martensitic microstructures. Very similar mechanical properties were obtained for both steels after the application of the same heat treatment. Ultimate tensile strengths reached 839 – 900 MPa for samples without heat treatment and heat treated by solution annealing, the samples after precipitation hardening had tensile strengths of 1577 – 1711 MPa.