UDK 669.14.018.298: 620.1/.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 44(6)343(2010) THE INFLUENCE OF THE MICROALLOYING ELEMENTS OF HSLA STEEL ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES VPLIV MIKROLEGIRNIH ELEMENTOV NA MIKROSTRUKTURO IN MEHANSKE LASTNOSTI HSLA JEKLA Danijela A. Skobir1, Matjaž Godec1, Martin Balcar2, Monika Jenko1 1Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia 2ŽDAS, a. s., Strojirenska 6, 591 71 Žd'ar nad Sazavou, Czech Republic danijela.skobir@imt.si Prejem rokopisa - received: 2010-08-30; sprejem za objavo - accepted for publication: 2010-09-21 In this work, hot-forged, high-strength, low-alloy (HSLA) steels based on the chemical composition 34CrNiMo6 were investigated in order to determine the type of precipitates forming and their effect on the mechanical properties. The individual steels were microalloyed with one of the following elements or a combination of elements: niobium, titanium or zirconium and vanadium. The compositional changes in the microstructure and the various kinds of precipitates observed in the microalloyed steels were examined using energy-dispersive spectroscopy (EDS). Optical and scanning electron microscopy studies revealed that the addition of microalloying elements did not change very much the main microstructural features due to the fact that all the microstructures consisted of tempered martensite and finely dispersed carbide precipitates along martensite laths. In all the microstructures relatively large precipitates of the microalloyed elements were observed. This could be a possible reason for observing no significant improvement in the mechanical properties. Keywords: high-strength, low-alloy (HSLA) steels, microalloying elements, precipitates, mechanical properties, electron microscopy Vroče kovano jeklo HSLA kemijske sestave 34CrNiMo6 smo raziskovali z namenom, da bi določili vrsto precipitatov, ki se tvorijo, in njihov vpliv na mehanske lastnosti. Posamezna jekla so bila mikrolegirana z enim od naslednih elementov ali s kombinacijo le-teh: z niobijem, s titanom ter cirkonijem in z vanadijem. Mikrostrukturne spremembe in različne vrste precipitatov smo študirali z metodo EDS. Preiskave s svetlobnim in vrstičnim elektronskim mikroskopom so potrdile, da dodatek mikrolegirnih elementov ni bistveno spremenil osnovne mikrostrukture, saj so bile vse mikrostrukture iz popuščenega martenzita z izločenimi karbidnimi precipitati po martenzitnih letvah. Pri vseh mikrostrukturah so bili opaženi tudi precej veliki precipitati mikrolegirnih elementov, ki so po vsej verjetnosti razlog za to, da se mehanske lastnosti niso bistveno izboljšale. Ključne besede: visokotrdna malo legirana jekla (HSLA), mikrolegirni elementi, precipitati, mehanske lastnosti, elektronska mikroskopija transformation (interphase precipitation), or in super-1 INTRODUCTION saturated ferrite during the final cooling.5 , ,. , , , „ , On adding such elements to steels with 0.008-0.03 % Microalloyed high-strength,low-alloy (HSLA)stee s c and up to w = 1.5 % Mn, it became possible to proare technologically important structura^ materials.1,2 ^uce fine-grained material with yield strengths between These steels contam small amounts of alloying elements, 450 MPa and 550 MPa, and with ductile/brittle transition temperatures as low as -70 °C.6 Nowadays, microalloyed cast steels have found many applications in the manufacturing of industrial parts, such as offshore platform nodes, centrifugal cast pipes, machinery supports, ingot moulds and buckets, which all used to be produced by expensive manufacturing pro-cesses.7-9 such as titanium, niobium, vanadium or zirconium, which enhance the strength through the formation of stable carbides, nitrides or carbonitrides. Titanium also readily forms stable nitrides at high temperatures and is used to control the nitrogen content in the alloys. The microalloying elements niobium and vanadium are added to microalloyed cast steels primarily to provide grain refinement and a response to aging. Niobium, often at levels of less than 0.05 %, effectively prevents undesirable grain growth and can also contribute to precipi- 2 EXPERIMENTAL WORK tation strengthening. Vanadium, in particular at levels of less than 0.1 %, forms strengthening carbonitride pre- Hot-forged HSLA steels based on the chemical cipitates.3,4 Precipitation hardening by fine carbonitride composition 34CrNiMo6 were investigated in order to particles has been used for many years to increase the determine the type of precipitates forming and their strength of microalloyed steels. During the thermo- effect on the mechanical properties. Individual steels mechanical processing, carbonitrides may nucleate in were microalloyed with one of the following elements or austenite during forging, at the y/a interface during combination of elements: niobium (steel A), titanium Table 1: Chemical compositions of the investigated steels microalloyed with niobium (steel A), titanium (steel B), vanadium and zirconium (steel C) and non-microalloyed (steel D) in mass fractions Tabela 1: Kemijska sestava jekla, mikrolegiranega z niobijem (jeklo A), s titanom (jeklo B), z vanadijem in s cirkonijem (jeklo C) in nelegi-ranega jekla (jeklo D) v masnih deležih (%) Steel C Mn Si P S Cr Ni Cu Mo Al A 0.35 0.56 0.26 0.016 0.004 1.52 1.55 0.13 0.17 0.029 Steel C Mn Si P S Cr Ni Cu Mo Al B 0.32 0.55 0.27 0.022 0.002 1.58 1.56 0.15 0.18 0.020 Steel C Mn Si P S Cr Ni Cu Mo V Al C 0.34 0.54 0.27 0.017 0.002 1.58 1.55 0.21 0.23 0.030 0.031 Nb 0.039 Ti 0.027 Zr 0.030 Steel C Mn Si P S Cr Ni Mo Al D 0.34 0.60 0.26 0.007 0.012 1.55 1.58 0.20 0.030 (steel B) or zirconium and vanadium (steel C). For comparison a non-microalloyed steel (steel D) was chosen. The chemical compositions of the steels, as specified by the manufacturer, are given in Table 1. Metallographic samples were prepared using standard polishing techniques and were then etched with 2 % Nital. The microstructure was examined in an optical microscope, a Nikon Microphot FXA. The carbide precipitates were investigated using a field-emission scanning electron microscope, a JEOL JSM 6500F, equipped with an energy-dispersive spectroscopy (EDS) analyzer (INCA X-SIGHT LN2 type detector, INCA ENERGY 450 software). The accelerating voltage during the EDS was 15 kV, the working distance 10 mm and the probe current 0.05 nA. To evaluate the mechanical properties tensile tests were conducted on a 500-kN tensile machine, an Instron 1255. As this precipitation is normally on an extremely fine scale, occurring between 850 °C and 650 °C, it is likely to be the major contribution to the dispersion strengthening. If the rate of cooling through the transformation is high, leading to the formation of supersaturated plates of ferrite, the carbides will tend to precipitate within the grains, usually onto the dislocations, which are numerous in this type of ferrite. This is the third strengthening mechanism.6 Optical microscopy of the investigated steels revealed that the addition of different microalloying elements did not considerably change the main microstructural features due to the fact that all the microstructures consisted of tempered martensite and finely dispersed carbide precipitates along the martensite laths. In all the microstructures relatively large precipitates (up to 10 pm) of microalloyed elements were observed. After etching, in the optical microscope these precipitates can 3 RESULTS AND DISCUSSION 3.1 Microstructural characterization The solubility data imply that in a microalloyed steel, carbides and carbonitrides of Nb, Ti, Zr and V will precipitate. While the primary effect of these fine dispersions is to control the grain size, dispersion strengthening will take place. The strengthening arising from this cause will depend both on the particle size and the interparticle spacing, which is determined by the volume fraction of the precipitate. These parameters will depend primarily on the type of compound that is precipitating, and this is determined by the microalloying content of the steel.6 In modern microalloyed steels there are at least three strengthening mechanisms that contribute to the final strength achieved. In the first mechanism, the precipitation takes place in the austenite and further precipitation occurs during the transformation to the ferrite. The precipitation of niobium, titanium and vanadium carbides has been shown to take place progressively as the interphase boundaries move through the steel. This is the second mechanism, called interphase precipitation. Figure 1: Optical micrographs of the steel microalloyed with niobium (steel A) at different magnifications Slika 1: Mikrostruktura jekla, mikrolegiranega z niobijem (jeklo A), pri različnih povečavah (svetlobni mikroskop) Figure 2: Optical micrographs of the steel microalloyed with titanium (steel B) at different magnifications Slika 2: Mikrostruktura jekla, mikrolegiranega s titanom (jeklo B), pri razli~nih pove~avah (svetlobni mikroskop) be seen as typically coloured (brown-reddish for the niobium and titanium precipitates and yellow for the zirconium precipitates). Representative optical micrographs of the microalloyed steels are presented in Figures 1, 2 and 3. In order to observe the microstructure more closely, the precipitates of the microalloying elements were investigated using scanning electron microscopy. In the Figure 3: Optical micrographs of the steel microalloyed with vanadium and zirconium (steel C) at different magnifications Slika 3: Mikrostruktura jekla, mikrolegiranega z vanadijem in cir-konijem (jeklo C), pri razli~nih pove~avah (svetlobni mikroskop) Figure 4: SE image and X-ray maps of niobium precipitate (steel A) in the form of "Chinese script" Slika 4: SE-posnetek in porazdelitev elementov niobijevega preci-pitata v obliki kitajske pisave, dolo~ena z EDS steel A, the precipitates are very similar to the degenerated eutectic Fe-Nb(C, N) known as "Chinese script"10,11 (Figure 4) that formed because of the improper solidification process of the cast ingots. Figure 5 shows the large titanium nitride in the steel B, bounded to a complex, non-metallic inclusion composed of manganese sulphide and aluminium oxide. A similar microstructure was also observed in the steel C, where large zirconium carbonitrides were found (Figure 6). EDS analyses of all three particles were performed and the chemical composition is listed in Table 2. Typical for the Nb, Ti and Zr carbonitrides is a regular geometrical shape, which is seen as a triangular, quadrilateral shape. Their shape is related to their crystal structure and is influenced by growing along the energetically most favourable crystal planes. Such particles are not desirable in the material because they are very hard and have sharp edges, which can cause a lowering of the mechanical properties of the material. The aim of microalloying the steel is to obtain very fine Nb, Ti and Zr precipitates on the nanometre scale, which improves the material properties with relatively low costs. In our study it was found that the benefit of microalloying was suppressed by a non-optimised alloying process. We assumed that most of the micro-alloying elements were extracted from the melt as large particles instead of a finely dispersed precipitate. Carbonitride precipitation has been the subject of many investigations that have studied the different aspects of the precipitates and their effects on the mechanical properties.7 Based on these studies, the effective carbonitrides in strengthening are those formed in a fine form with an adequate distribution. As a result, the increase observed in the strength can be attributed to the precipitation of fine-scale carbonitrides in the form of interphase and random precipitates. Figure 5: SE image and X-ray maps of the titanium precipitate in the steel B Slika 5: SE-posnetek in porazdelitev elementov titanovega precipitata v jeklu B 3.2 Mechanical properties The results obtained from the tensile tests are reported in Table 3. The properties reported in this table are the average properties of two testing specimens. According to these results, the presence of different Figure 6: SE image of the zirconium precipitates in steel C Slika 6: SE-posnetek cirkonijevih precipitatov v jeklu C Table 2: Chemical composition for the analysed spectra in mass fractions Tabela 2: Kemijska sestava analiziranih spektov v masnih deležih (%) C N Ti Cr Mn Fe Ni Zr Spectrum 1 10.80 11.50 0.31 0.61 6.75 70.03 Spectrum 2 15.32 9.22 0.40 7.20 67.86 Spectrum 3 10.85 6.53 0.81 0.83 0.75 31.49 0.47 48.26 microalloying elements does not significantly change the mechanical properties. It is possible that the benefit of the fine precipitates is eliminated with the presence of large Nb, Ti and Zr particles. Table 3: Mechanical properties of the investigated steels Tabela 3: Mehanske lastnosti preizkušanih jekel Steel Re /MPa Rm /MPa A5/% Z/ % A 611 805.5 18.6 53.1 B 637 806.5 18.7 64.6 C 627 808.5 19.2 65.2 D 619 786 21.1 62.2 4 CONCLUSIONS The influence of the microalloying elements in HSLA steels on the microstructure and mechanical properties was investigated. It was found that Nb, Ti and Zr precipitate not only as fine particles but also as large particles with sharp edges. Based on the mechanical properties and the SEM/EDS analysis it is supposed that the benefit of microalloying elements is negated by these large particles and the results are almost the same mechanical properties as in the case of the non-micro-alloyed steel (steel D). 5 REFERENCES 1 T. Gladman, The physical metallurgy of microalloyed steels. London: Institute of Materials, (1997) 2Z. K. Liu, Scripta Mater. 50 (2004), 601-606 8R. C. Voigt, M. Blair, J. Rassizadehghani, Proceedings of Interna- 3S. N. Prasad, D. S. 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