Vpliv titana in vanadija na končne mehanske lastnosti jekla S1100QL

Foder, Jan (author), Nagode, Aleš (mentor)

Uporaba mikrolegirnih elementov, kot so titan, niobij in vanadij, je v sodobnem jeklarstvu že ustaljena praksa. Te elementi prek mehanizmov izločevalnega utrjevanja ferita in kontrole velikosti avstenitnih (in feritnih) kristalnih zrn vplivajo na končne mehanske lastnosti jekla. Jeklo z drobnimi feritnimi zrni ima tako večjo trdnost kot tudi žilavost, z izločevalnim utrjevanjem pa povečamo trdnost ob zmanjšanju žilavosti. Med široko paleto mikrolegiranih jekel sodi tudi visokotrdnostno konstrukcijsko jeklo S1100QL. Za doseganje predpisanih mehanskih lastnosti se, poleg osnovnega legirnega sistema, to jeklo mikrolegira z dodatki titana, niobija, vanadija in bora. Dodatek titana je namenjen predvsem kontroli borovega faktorja, pa tudi kontroli velikosti avstenitnih kristalnih zrn. Se pa pojavlja vprašanje, če je dodatek titana sploh nujen, saj je več raziskovalcev potrdilo, da grobi TiN delujejo kot mesta za iniciacijo krhkega cepilnega loma in tako slabšajo žilavost. V ta namen smo izdelali laboratorijske šarže jekla S1100QL s tremi različnimi mikrolegirnimi sistemi, in sicer Nb-B, V-Nb-B in Ti-Nb-B. Jeklo je bilo izdelano v indukcijski vakuumski peči, ulito v kokilo, vroče kovano, valjano (kovano-valjana izvedba) ter toplotno obdelano. Opravili smo celostno karakterizacijo vseh jekel s pomočjo katere smo določili optimalni mikrolegirni sistem s katerim dosegamo predpisane mehanske lastnosti. Sestava jekla je prirejena znotraj standarda EN 10025-6. Preiskovalne metode so zajemale kemijsko analizo, metalografsko analizo na svetlobnem mikroskopu, meritev velikosti kristalnih zrn, avtomatsko analizo nekovinskih vključkov, diferenčno termično analizo, termodinamsko analizo s Thermo-Calcom, izrisali smo CCT in popuščni diagram ter izvedli natezni preizkus in preizkus udarne žilavosti po Charpyju pri -20 °C in -40 °C. Ugotovili smo, da je za doseganje napetosti tečenja najbolj primeren mikrolegirni sistem V-Nb-B, kar smo povezali z efektom izločevalnega utrjevanja ferita z vanadijem. Hkrati dodatek titana ni bil potreben za doseganje nateznih mehanskih lastnosti. Kljub temu pa smo najboljšo žilavost dosegli z uporabo mikrolegirnega sistema Ti-Nb-B, kar je posledica vpliva titana na kontrolo začetne velikosti avstenitnih kristalnih zrn pred in med vročo predelavo ob ustrezno kontroliranem dodatku titana. Pri nizkotemperaturnem popuščanju kaljenega jekla na 200 °C smo dosegli ciljane mehanske lastnosti.

• Language: Slovenian
• Keywords: S1100QL, mikrolegiranje, titan, vanadij, niobij, bor, mehanske lastnosti
• Work type: Master's thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2019
• PID: 20.500.12556/RUL-110284
• COBISS.SI-ID: 1823839
• Publication date in RUL: 13.09.2019

Secondary language

• Language: English
• Title: Effect of titanium and vanadium addition on final mechanical properties of S1100QL steel

Abstract

The microalloying additions like titanium, niobium and vanadium are being frequently used in modern steelmaking. These microalloying elements have a major influence on the final mechanical properties of steel. Increase in strength and toughness is achieved with grain refinement, while precipitation hardening increases strength but decreases toughness. S1100QL is ultrahigh-strength microalloyed structural steel (UHSLA) used in heavy–lifting industry. Microalloying additions like titanium, boron, niobium and vanadium are being used to achieve the required mechanical properties. Titanium addition is used for boron protection as well as austenite grain refinement. However, many researchers argued that coarse TiN particles trigger cleavage fracture and thus drastically reduce the impact toughness of steels. To decide, whether the titanium addition is really needed, we fabricated experimental steels with three different microalloying additions (Nb-B, V-Nb-B and Ti-Nb-B). All three steels were vacuum melted in induction vacuum furnace, cast into ingots, hot forget, hot rolled, water quenched and tempered at 200 °C, 570 °C and 640 °C for 1 hour. The series of experiments were conducted to fully characterize and decide which microalloying addition can be used to achieve the required mechanical properties. Experimental tests included chemical analysis, metallographic analysis on light microscope, grain size measurement, automated SEM-EDS non–metallic inclusions analysis, differential thermal analysis, thermodynamic analysis using Thermo-Calc, construction of CCT and tempering diagrams, tensile tests and Charpy impact tests at -20 °C and -40 °C. V-Nb-B microalloying addition showed best response in terms of tensile properties, which can be related to precipitation hardening of ferrite with VN particles. Highest Charpy impact toughness values were achieved using Ti-Nb-B microalloyed steel. Titanium can refine the grain size of steel during reheating and hot deformation (rolling, forging…) and thus increase impact toughness, which is closely related to grain size. Mechanical properties of all three steels were achieved during low-temperature tempering of quenched samples at 200 °C.

• Keywords: S1100QL, microalloying, titanium, vanadium, niobium, boron, mechanical properties