Hardy Mohrbacher

KU Leuven | ku leuven · Department of Metallurgy and Materials Engineering

This research work presents a concept of extending the yield strength range of cold-rolled HSLA steels to above 500 MPa. A key aspect is to find the right balance between recovery and recrystallization of the cold rolled matrix during the annealing stage thereby mitigating the influence of solute and precipitated niobium. Additional solute draging...

The development of high strength/high toughness steel grades for thick plates requires an optimum combination of alloy design and processing conditions. In this contribution, the effect that Molybdenum and Nickel have on the static recrystallization kinetics during roughing passes is analyzed. High alloying additions, especially in the case of Mo,...

The demand for higher-strength automotive steel sheets has increased significantly for lightweight and safe body concepts. However, the increment of the steel strength is often limited by the potential occurrence of delayed fracture. This paper discusses proper microstructure control and alloy design to improve the resistance against the delayed fr...

Press hardening steel standardly relies on titanium microalloying for protecting boron from being tied up by residual nitrogen. This practice safeguards the hardenability effect of boron during die quenching. More recently, additional microalloying elements were added to press hardening steel to further improve properties and service performance. N...

Nickel is an important and widely used alloying element in carbon steels. Some of its prominent metallurgical effects in these steels are moderate solid solution strengthening, mild hardenability and, importantly, a strong promotion of toughness, especially at low temperatures. The first uses of nickel as an alloying element in commercial steel pro...

The application of press hardening steels is spreading from automobiles, where crashworthiness demands are critical, to other segments, such as the agricultural and road transport industries. However, the operational conditions to which such equipment is exposed requires the application of heavier sheet gages and adapted joining processes. In this...

The critical bending angle of high strength steel is most commonly determined according to VDA 238-100. The value of the critical bending angle generally decreases as the tensile strength of the steel increases, and for a given strength level, single-phase steels perform better than multi-phase steels. In cold forming steels, the critical bending a...

The secular trend of automotive body structure light-weighting for internal combustion engine (ICE) vehicles is constrained by simultaneous and increasingly challenging vehicle cost, fuel economy and passenger safety standards. Mass optimization via materials selection in ICE vehicles, therefore, is ultimately dependent on the normalized cost of ma...

This work investigates and compares the fatigue performance of laser-welded joints of two press hardening steels: a standard 22MnB5 and a variant modified by a combination of niobium and molybdenum (NbMo) alloying. The results indicate that joint geometry aspects, superposed to an intrusion-generated damage mechanism, were prevalent in causing a po...

Solubility of mixed niobium-titanium carbonitrides in commercially relevant High-Strength Low-Alloy (HSLA) steel was investigated by combined use of electrical resistivity measurements and APT after interrupted quenching from soaking temperatures between 950 and 1250 °C. Increasing electrical resistivity of the bulk material towards higher soaking...

This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The role of Nb and Mo during the hot deformation of low carbon steels is well known: both mainly retard austenite recrystal-lization, leading to pancaked austenite microstructures before phase transforma...

The dynamic recrystallization behavior of ultra-high strength boron-microalloyed steels optionally alloyed with niobium and molybdenum is analyzed in this paper. Multipass torsion tests were performed to simulate plate rolling conditions followed by direct quenching. The influence of alloy composition on the transformed microstructure was evaluated...

Well-established practices exist for the production of cold-rolled and continuously annealed high-strength, low-alloy (HSLA) steels as both coated and uncoated variants. Through standard processing techniques, the yield strength of cold-rolled HSLA steel is practically limited by numerous process and alloy design factors to 550 MPa and less. A prod...

Ni bonded (Nb,W,Ti)(C,N) solution cermets with overall compositions of NbC-(6–20)Ni-18WC-14 Ti(C0.7N0.3) (wt%) were prepared by pressureless vacuum sintering of compacted NbC, Ni, WC and Ti(C0.7N0.3) mixtures. The microstructure, phase constitution, room temperature mechanical properties were investigated and the phase composition in the cermets si...

The paper explains how to use micro alloying for avoiding austenite grain coarsening particularly during high-temperature carburising. The influence of solubility, dissolution kinetics, particle-size distribution and segregation effects are discussed in detail. Recommendations for optimum thermo-mechanical treatment are given.

The application of direct quenching after hot rolling of plates is being employed in the production of ultra-high-strength hot rolled plates. When heavy gauge plates are produced, the complexity involve in achieving high cooling rates in the plate core is increased and the formation of undesirable soft phases within martensite is common. In the cur...

FEATURE RAW MATERIALS I n our modern world, the need for ultra-high strength steels continues to increase to meet demand for applications, including cars, trucks and cranes as well as building and construction. Steelmakers strive to produce high-end steels with improved performance as efficiently as possible, without compromising on quality. Recent...

The hot stamping steel grade 22MnB5 steel producing a target tensile strength of 1500 MPa is standardly used for press hardened components in passenger cars. The thickness of such components is usually limited to 2 mm or less. More recently, the application of press hardened components has been also considered for components of heavy commercial veh...

The synergetic effect on hardenability by combining boron with other microalloying elements (such as Nb, Mo and Nb + Mo) is widely known for high-strength medium carbon steels produced by direct quenching and subsequent tempering treatment. The improvement of mechanical properties could be reached through optimization of different mechanisms, such...

Recently, advanced thermomechanical hot rolling schedules followed by direct quenching are being developed in order to avoid reheating and quenching treatment after hot rolling to eliminate an energy and cost consuming step. The use of boron as an alloying element is a widely known practice in high strength medium carbon steels to increase the stre...

Press hardening steel (PHS) is widely applied in current automotive body design. The trend of using PHS grades with strengths above 1500 MPa raises concerns about sensitivity to hydrogen embrittlement. This study investigates the hydrogen delayed fracture sensitivity of steel alloy 32MnB5 with a 2000 MPa tensile strength and that of several alloy v...

Dual phase (DP) steels are widely applied in today's automotive body design. The favorable combination of strength and ductility in such steels is in first place related to the share of ferrite and martensite. The pronounced work hardening behavior prevents localized thinning and allows excellent stretch forming. Niobium microalloying was originall...

The microalloying with niobium (Nb) and titanium (Ti) is standardly applied in low carbon steel high-strength low-alloy (HSLA) steels and enables austenite conditioning during thermo-mechanical controlled processing (TMCP), which results in pronounced grain refinement in the finished steel. In that respect, it is important to better understand the...

Microalloying of low carbon steel with niobium (Nb) and titanium (Ti) is standardly applied in high-strength low-alloy (HSLA) steels enabling austenite conditioning during thermo-mechanical controlled processing (TMCP), which results in pronounced grain refinement in the finished steel. The metallurgical effects of microalloying elements are relate...

Considerable progress in developing flat-rolled steel grades has been made by the Chinese steel industry over the recent two decades. The increasing demand for high-performance products to be used in infrastructural projects as well as in production of consumer and capital goods has been driving this development until today. The installation of sta...

Metal casting is an important manufacturing technology for efficiently producing massive components with complex shape. A large share of industrial castings is made from iron and steel alloys, combining attractive properties and low production cost. Upgrading of properties in cast iron and steel is mainly achieved by alloying and in fewer cases by...

About 80% of all molybdenum mined in the world (not including units recovered via recycling) is used as an alloying element in iron and steel. In general, the intensity of molybdenum use in China is still lower than in more highly developed regions such as the USA and Europe. This difference is manifest in both carbon steels and stainless steels, s...

In this study, Ni or Co + Ni bonded NbC matrix cermets with secondary carbides (Mo2C and WC) as well as Ti (C0.5N0.5) were prepared by liquid phase sintering at 1450 °C in vacuum. Detailed microstructural investigation was performed by SEM, EPMA and XRD analysis. The microstructure, mechanical properties as well as the C45 (HB140) steel turning per...

Most of the automotive applications of press-hardening steel so far have focused on thin cold rolled sheets with gages typically below 2 mm. However, in trucks press-hardening steel with gages up to 8 mm has interesting potential for light-weighting and improving collision resistance. Besides, electric vehicle concepts might require increased gages...

Niobium microalloying is the backbone of modern low-carbon high strength low alloy (HSLA) steel metallurgy, providing a favorable combination of strength and toughness by pronounced microstructural refinement. Molybdenum alloying is established in medium-carbon quenching and tempering of steel by delivering high hardenability and good tempering res...

Modern weldable high strength steel grades are typically based on low-carbon alloy concepts using microalloying for obtaining a good strength-toughness balance. Such steel grades having a yield strength in the range of 420 to 690 MPa are very commonly used in pipelines, heavy vehicles, shipbuilding and general structural applications. Thermomechani...

Austempered ductile iron (ADI) is a heat-treated nodular iron variant that has for a given strength level much higher elongation than conventional iron with spheroidized graphite. This paper considers the effect of niobium addition on the graphite microstructure, bainite microstructure, bainite transformation process and properties such as hardness...

Basics about physical, mechanical and tribological properties of straight NbCs, including paths to tailor these properties, were layed down in Metal Powder Report, Vol. 71(4), 2016 and Vol. 71(5), 2016. The object is now to give an up-date on the progress in the key properties, like HV30, K IC and strength, followed by machining and tribological re...

Niobium added to ferrous alloys acts as a strong carbide former. It is a widespread opinion that the solid solubility of niobium is very limited especially when the carbon content of iron is high. However, despite cast iron having very high carbon content, the solubility of niobium appears to be rather good. At sufficiently high niobium addition it...

Nb–Mo low-alloyed steels are promising advanced high strength steels (AHSS) because of the highly dislocated bainitic ferrite microstructure conferring an excellent combination of strength and toughness. In this study, the potential of precipitation strengthening during coiling for hot-strip Nb–Mo-bearing low-carbon steels has been investigated usi...

The influence of Nb addition on casting microstructures and high-temperature mechanical properties of Fe-Mn-C-Al-xNb TWIP steels was analyzed by phase-field modeling and experiments. Phase-field simulations showed that Mn, Nb, and C are enriched in inter-dendritic regions while Al is enriched in dendritic regions during solidification process of th...

NbC-xTi (C 0.7 N 0.3 )-10Ni-7.5VC (vol%) based cermets with 0, 5, 10, 15 or 25 vol% Ti (C 0.7 N 0.3 ) were prepared by conventional pressureles liquid phase sintering at 1420°C in vacuum. Detailed microstructural investigation was performed by SEM, EPMA and XRD analysis. Sintering results indicated that the partial replacement of NbC by Ti (C 0.7 N...

Weight reduction in commercial vehicles is increasingly gaining importance, following an already well-established trend in the modern car structures. While components used in automobile structures are designed especially to distribute the crash load into other structural members without collapsing, the main concern in processing thick 22MnB5 plates...

Apart by vacancies, the different pseudo-cubic phases NbC, Nb4C3 and Nb6C5, can accommodate any stoichiometry of NbCx (0.75 ≤ x ≤ 1.0) in the binary phase diagram and enable a large process window. Properties, like micro-hardness, hot hardness, sliding wear resistance, elastic modulus and toughness can be tailored by the C/Nb ratio, the addition of...

The aim of this work was to correlate the overall carbon content in NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo starting powders with the resulting microstructure, hardness, and fracture toughness of Ni-bonded NbC cermets. A series of NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo cermets with different carbon content were prepared by conventional liquid phase sintering for...

Determination of non-recrystallization temperature of Nb alloyed TWIP steels and 3D atom probe analysis of Nb precipitates in the TWIP steel

Martensitic steel is a well-established material for the equipment and tooling used in mining and processing. Martensitic steel was originally chosen for its high strength and hardness. However, the overall performance of industrial machinery requires that a number of other properties be considered, including toughness, ductile-to-brittle transitio...

The current study reports on the effect of the sintering temperature and secondary carbide (VC, Mo2C and TiC) and Mo additions on the NbC grain growth, microstructure evolution as well as concomitant Vickers hardness (HV30) and fracture toughness of Ni-bonded NbC cermets. All cermets were prepared by pressureless sintering in vacuum. Detailed micro...

We investigated the effect of Nb micro-alloying in the range of 0.01 to 0.l wt.-% on the microstructures and mechanical properties of Fe17Mn0.6C1.5Al (wt. -%) TWIP steel. EBSD analysis shows that the Nb addition retards recrystallization in both the hot-rolled and cold-rolled and annealed TWIP steels. The Nb addition in the cold-rolled and annealed...

Both the tooth root and tooth flank load carrying capacity are characteristic parameters that decisively influence gear size, as well as gearbox design. The principal requirements towards all modern gearboxes are to comply with the steadily-increasing power density and to simultaneously offer a high reliability of their components. With increasing...

Hydrogen originating from steelmaking, coating or welding processes as well as corrosion processes is known to cause severe problems in steel. These problems augment with increasing strength of steel and become definitely a serious concern when yield strength exceeds 1000 MPa. Features related to hydrogen in such steel are hydrogen-induced cracking...

The current study reports on the influence of the addition of 5–15 vol% VC or/and Mo2C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified...

This chapter examines how NbC can be synthesized by carbothermal conversion of Nb2O5 or be metallurgically grown and leached out, provides comparably low friction in many relevant tribo-contacts and displays low wear. Niobium carbide can be synthesized by various practical methods, including a carbo-thermal reduction of pure niobium oxide and melt...

The effects of niobium addition up to 0.11 wt. % on the microstructure and tensile properties of as-cast ductile iron (ACDI) were investigated. Metallographic analyses by both optical microscopy (OM) and scanning electron microscopy (SEM) indicated that niobium (Nb) promoted the formation of pearlite, reduced pearlite lamellar spacing and decreased...

Steel has been the dominant construction material in passenger and commercial vehicles for decades. Recently, the position of steel in vehicle construction has been challenged by low-density materials such as aluminum, plastics, and magnesium in either body-in-white as well as chassis. This challenge motivated the steel industry to develop new stee...

Global energy demand continues to increase, especially in countries with fast-growing economies. Renewable and low carbon sources can cover only a limited share of the total energy need. Fossil fuels, particularly coal, stand in for more than 60% of the global power generation. The efficiency of a thermal power plant is represented by the ratio bet...

This chapter demonstrates the key characteristics of niobium carbide (NbC) and discusses its sustainability and reliable value chain through an experiment. Due to its low solubility, solid NbC particles can be readily dispersed into liquid cast iron preserving its original morphology and without significantly disturbing the carbon balance in the al...

This chapter focuses on optimizing special steel alloys used in power train applications, presenting a modification of the well-established gear steel 20MnCr5 capable of accepting substantially higher loads. The general requirements for high performance gear components are a hard case providing wear resistance and a tough core preventing brittle fa...

The microstructure and mechanical properties of 316 L and 430 L stainless steel bonded NbC cermets were assessed. NbC starting powder mixtures with 15 and 30 vol% steel binder were pressureless vacuum sintered for 1 h at 1420 °C. The liquid forming temperature and shrinkage behaviour of the green powder compacts were investigated by differential sc...

Carburized steel grades are widely used in applications where high surface near hardness is required in combination with good core toughness as well as high strength and fatigue resistance. The process of carburizing lower to medium carbon containing steel can generally provide this combination of properties and has been practiced for several decad...

The current study reports on the effect of the addition of Al metal and secondary refractory carbides on the microstructure and mechanical properties of Ni bonded NbC matrix cermets. Powder mixtures were pressurelessly sintered for 1h at 1420°C in vacuum. Microstructural and elemental mapping of the sintered cermets were performed by electron probe...

The origin of the intrinsic wear resistance of NbC based materials is investigated through an assessment of the tribological performance of fully dense, crack-free spark plasma sintered Nb2O5 (here as a reduced polymorph: monoclinic Nb12O29 or NbO2.416). The most likely wear mechanism on NbC is the tribo-oxidation to Nb2O5. The unlubricated (dry) f...