Molybdenum alloying in high-performance flat-rolled steel grades
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 state-of-the-art steel making and rolling facilities has provided the possibility of processing the most advanced steel grades. The production of high-performance steel grades relies on specific alloying elements of which molybdenum is one of the most powerful. China is nearly self-sufficient in molybdenum supplies. This paper highlights the potential and advantages of molybdenum alloying over the entire range of flat-rolled steel products. Specific aspects of steel property improvement with respect to particular applications are indicated.
KeywordsHSLA steel Quench & tempering Direct quenching Press hardening Quenching & partitioning Precipitation
Flat carbon steel is one of the most versatile materials used in a large number of industrial applications. The gages of products in this steel family range from 0.5 mm to over 100 mm. Typical products manufactured from flat steels are vehicles, pipelines, pressure vessels, machines, construction equipment, bridges, offshore platforms, ships, or containers. Most of these items involve a combination of cutting, forming and joining operations during the manufacturing process. These fabrication techniques imply particular requirements towards the steel [1, 2]. The quest for increased strength has always been a driving force for steel development over the last five decades. Higher strength allows making structures lighter by maintaining the same load bearing capacity at reduced material gage. Consequently, sustainability and efficiency in terms of raw material consumption, manufacturing time, transport weight and emissions generated along the entire production chain are subjected to continuous improvement. However, increasing the strength of steel can be in conflict with forming and welding operations applied during manufacturing. With regard to the applications, energy absorption capacity, toughness, fatigue resistance and resistance against hydrogen embrittlement are key properties for steels of increased strength. Therefore, particular attention has to be attributed to the alloy concept and microstructural design of steel. The alloy concept dominantly affects the weldability, whereby reducing the carbon content or carbon equivalent (CE) clearly has a positive impact. Steel offers a variety of microstructures, which majorly determine its properties. The combination of alloy and process design allows adjusting the microstructure. Regarding process design, thermo-mechanical controlled processing (TMCP) represents the baseline for making high strength steel and is widely used in strip as well as plate rolling. Various cooling strategies at the end of the rolling process or separate heat-treatments are further methods of influencing the microstructure and optimizing properties. For thin-gaged cold rolled products the final microstructure is adjusted by a continuous annealing process incorporating a suitable cooling strategy. Molybdenum as an alloying element has a wide spectrum of metallurgical effects. It can help achieving the desired strength level at reduced carbon equivalent, thus improving weldability. The various interactions of molybdenum during steel processing allow promoting particular microstructural features relevant for manufacturing and final properties. The present paper will give an overview on how molybdenum is used in combination with state-of-the-art processing technology for producing modern high strength steel. For particular products, the resulting benefits with regard to manufacturing processes will be highlighted.
2 Strengthening mechanisms and processing strategies for high-performance HSLA steel
Molybdenum alloying contributes to the production of such advanced steel grades in several ways. The alloyed amount of molybdenum is usually below 0.7% and more typically in the range of 0.1%–0.3%. Despite molybdenum being a carbide former, its alloy range and the low carbon content guarantee full solubility in these steels throughout the process . Thus, the metallurgical effects of molybdenum appear as direct interactions with lattice defects or indirect interactions by way of synergies with other alloying elements. Molybdenum is an atom of significantly larger size than that of iron. It has the tendency of segregating to grain boundaries or binding with other lattice defects such as vacancies or dislocations [7, 8, 9]. It also lowers the activity of carbon [10, 11], thus slowing down processes that rely on carbon diffusion. Along the processing chain of flat rolled products molybdenum alloying: enhances the solubility of the micro-alloys Nb and Ti during slab reheating , supports recrystallization delay during austenite conditioning , retards precipitation of micro-alloys during austenite conditioning , delays phase transformation from fcc to bcc , promotes non-polygonal ferrite formation with high dislocation density , controls micro-alloy precipitation in bcc to ultra-fine size and dense particle distribution .
2.1 Strengthening by microstructural refinement and dislocations
2.2 Strengthening by micro-alloy precipitation
In the case of low coiling temperature, phase transformation proceeds quickly and is likely finished before precipitation has initiated. Precipitation in this case can only occur spontaneously in the ferritic microstructure. Nucleation sites are provided by dislocation clusters present in this microstructure. In this situation molybdenum assists precipitation by providing a high dislocation density (see Fig. 2). Microalloy carbides have a cubic lattice with considerably larger lattice constant than ferrite. The precipitates nucleate with a Baker-Nutting orientation relationship MC//α-Fe, (100)MC//(100)α-Fe reducing the lattice misfit to 2.2%, 6.8% and 10.1% for VC, TiC and NbC, respectively. The formation enthalpy for the carbides is \(- \;44.2\), \(- \;78.1\) and \(- \;47.5\) kJ/mol for VC, TiC and NbC, respectively. Considering the combination of solubility in ferrite, lattice mismatch and formation enthalpy, it appears that titanium carbide should be the most efficient for precipitation strengthening. Molybdenum has a formation enthalpy of + 16.5 kJ/mol. Thus, neither the precipitation of molybdenum as such nor the partial replacement of Ti by Mo in TiC is energetically favorable. However, the formation of a (Ti, Mo) mixed carbide in the early formation stage reduces the interfacial strain energy between precipitate and ferrite matrix. Molybdenum can thus facilitate particle nucleation.
2.3 Two-step processing with secondary heat treatment
Effects of heat treatment at 600 °C on mechanical properties of 0.05%C-0.2%Si-1.7%Mn-0.18%Cr-0.016%Ti-0.08%Nb (mass fraction) alloys added with 0.1% and 0.3%Mo (mass fraction) (CST = 450 °C)
2.4 Application examples of high-performance HSLA steels
China has been the first country installing large-scale pipeline systems applying X80 grade pipe steel . This steel is mostly produced as hot-rolled strip for spiral welded pipe. However, hot-rolled plate is used for longitudinal welded pipe when particularly heavy wall gage is needed. Due to toughness requirements, a carbon content of 0.04%–0.06% and a homogeneous, fine-grained acicular ferritic microstructure is preferred . Alloy concepts using 0.07%–0.1% Nb and 0.2%–0.3% Mo have been established as robust solution for hot-rolled strip production. Severe austenite pancaking followed by accelerated cooling to a low coiling temperature are key process characteristics. Initially, plate steel alloy designs did not use Mo-alloying due to the more efficient cooling after finish rolling. More recent modifications of plate steel X80 alloys, however, considered Mo additions of around 0.2%. This modification provides a better process robustness and also improves toughness properties especially when heavier gages of around 30 mm have to be produced. Recent pipe steel developments are targeting the strength levels of X90 and X100. A cost-effective approach to X-100 with a minimum of 6% uniform elongation to meet strain-based design requirements is using 0.08% Nb and 0.25% Mo base chemistry produced into a dual-phase microstructure of ferrite and a carbon rich second phase . Such multi-phase processing requires holding in the inter-critical temperature regime before quenching and thus needs the superior capabilities of a modern hot rolling mill.
Light-weighting of frame and chassis structures for commercial vehicles including trailers has been a major driving force for developing hot strip grades with minimum yield strength of 700 MPa in the gage range of 5–10 mm [1, 2]. Recent developments in China introduced truck frame steel grades with 600 MPa yield strength and higher. The alloy design is based on low carbon content and Nb-Ti dual microalloying . With this concept either polygonal ferritic or bainitic microstructure with precipitation strengthening can be produced depending on the run-out table conditions in the hot-strip mill. The presence of pearlite or other hard phases in the microstructure is rather avoided as these have a negative impact on the bendability. Furthermore, such hard particles also lead to edge damage after mechanical cutting and cause increased wear on cutting tools. Pearlite-free steels intrinsically feature good toughness and fatigue resistance due to fine grain size and low carbon content. For reaching 700 MPa yield strength most concepts are based on 0.06%C-1.8%Mn-0.06%Nb-Ti-Mo alloying. This concept exhibits a remarkable robustness against temperature variations on the run-out table of the hot-strip mill . The resulting narrow scattering of strength is beneficial to forming operations in manufacturing, especially with respect to compensating spring-back occurring during forming of high strength steel. Good weldability is a result of reduced CE and particularly a low absolute carbon content. The CE (IIW) is typically in the range of 0.35–0.45 so that harden ability is rather low. The carbon content being typically clearly below 0.1 reduces peak hardness in particular, thus minimizing the risk of cold cracking.
Besides strength, many applications require other properties such as resistance against weathering or increased temperature. Substitution of traditional SPA-H steel (min(yield strength) = 355 MPa) by higher strength steel (yield strength > 550 MPa) enables weight reduction of 15%–20% in the construction of shipping containers or truck cargo boxes resulting in significantly lower fuel consumption. The reduced gage of such containers requires, however, a relatively better weathering resistance to guarantee sufficient lifetime. Typical alloying elements in weathering steels are Cr, Cu and Ni. Molybdenum additionally contributes to weathering resistance and simultaneously increases strength. A practical alloy concept for 700 MPa weathering steel uses around 0.2% Mo added to a low-carbon Cr-Cu-Ni-Nb-Ti base alloy. The microstructure of finished steel ideally consists of acicular ferrite and granular bainite, while keeping the share of MA-phase on a low level for guaranteeing high toughness and good cold bending properties. Other weather resistant steel applications for bridges or transmission line towers requiring yield strength in the range of 460–550 MPa are produced as heavy plate or angles. Molybdenum addition in the range of 0.2%–0.4% to these products consisting of a Cr-Cu-Ni-Nb base alloy showed appreciable improvement of weathering resistance .
Engineering codes for the construction of high-rise buildings sometimes demand fire resistant steel to be used in vertical columns . Such steels are intended to resist accelerated creep, or thermally activated deformation when exposed to high temperature for a relatively short time. It has been shown that additions 0.5% Mo and 0.02% Nb to a standard grade 50 steel increased the elevated temperature strength in a synergetic way. Molybdenum by itself reduces softening and enhances creep resistance at elevated temperature. It also prevents niobium precipitates from coarsening and thus helps maintaining that strengthening mechanism during elevated temperature exposure .
3 Alloy concepts and processing routes for martensitic steels
Increasingly many applications in manufacturing of machinery, agricultural, hoisting or transportation equipment as well as vehicle structures require material strength clearly above 800 MPa [21, 22, 23]. Such a strength level is difficult to achieve with the low-carbon HSLA steel approach described in Sect. 2 of this paper. Martensitic steel can readily fulfil demands for ultra-high strength and hardness. However, unless it is sufficiently tempered, martensite has lower ductility and toughness than ferritic or bainitic steels. When the strength is passing the threshold of roughly 1 000 MPa, steels also become increasingly sensitive to hydrogen embrittlement . The presence of diffusible hydrogen in martensitic steel can severely lower fracture toughness or cause so-called “delayed cracking” which unexpectedly destroys an initially sound component . Such critical issues require metallurgical optimization, which is related to the alloy concept and processing.
3.1 Process variants for producing martensitic steels
3.2 Molybdenum alloying effects in martensitic steels
Using molybdenum as alloying element to martensitic steel not only offers superior harden ability but brings about additional benefits such as microstructural refinement, tempering resistance, delayed cracking resistance, synergy effects with other alloying elements.
The sensitivity to hydrogen embrittlement is a consequence of three conditions acting together: microstructure, diffusible hydrogen and stress. Diffusible hydrogen present in a martensitic steel can originate from different sources such as steel making, intermediate treatments, welding or corrosion occurring during operational conditions. Pronounced hydrogen embrittlement is observed when the amount of diffusible hydrogen in the steel is high. In this case hydrogen aggregates at prior austenite grain boundaries present in the martensitic microstructure. Hydrogen lowers grain boundary cohesion so that in severe cases the material fails far below the actual yield strength with practically zero ductility. The resulting fracture pattern is showing intergranular cracking along the prior austenite grain boundaries. In less severe case of hydrogen embrittlement, the fracture pattern changes to the quasi-cleavage type. If the diffusible hydrogen content remains below a critical threshold value, ductile fracture occurs showing the typical dimple type pattern. The challenge towards alloy and process design is to appropriately modify the steel so that it can sustain the typical amount of hydrogen being picked up by the material during all phases of production and over the component service life. The microstructural cornerstones of such a modification are : (i) Refinement of the prior austenite grain size before final quenching to lower the hydrogen concentration per unit grain boundary area. (ii) Introduction of hydrogen trapping sites such as nano-sized precipitates to fix hydrogen or to at least reduce its diffusivity. (iii) Reinforcement of the grain boundary cohesion to counteract the deleterious effect of hydrogen.
These effects impede the underlying mechanisms of hydrogen-induced crack initiation and propagation, which will not be discussed in detail in the present context.
3.3 Application examples of martensitic steels
Martensitic steels are particularly interesting for application in hoisting equipment. Crane booms demand the lowest possible dead weight for lifting heavy loads with a long reach. For that purpose, steel grades with minimum yield strength of 960–1 300 MPa are nowadays being used. Taking a conventional grade 50 steel (355 MPa, yield strength) as a reference, substitution by a 960 MPa grade allows reducing the material gage and thus the component weight by more than 70%. Regarding a typical butt-welding situation with a V-shape bevel, the weld seam volume will be reduced by over 90% . Accordingly, component handling and manufacturing become much more efficient. However, it must be mentioned that the weldability of ultra-high strength steels is more demanding in terms of filler material as well as skill level of the welder. Another important consideration relates to the heat-affected zone properties after welding . The heat input by the welding process leads to peak temperatures of over 1 300 °C close to the fusion line and induces significant modification of the original microstructure. This results in soft zones having a yield strength lower than that of the base material as well as the fusion line. Such soft zones bear the risk of localized premature yielding under service load conditions. An appropriate alloy concept using molybdenum and niobium reduces or completely avoids heat-affected zone softening. This is related to the tempering resistance provided by these alloying elements as was demonstrated by Fig. 14. For that reason, molybdenum is added up to 0.7% in such steels while niobium is usually limited to 0.05%. The carbon equivalent (CE IIW) of such steels is typically in the range of 0.5–0.6, while the absolute carbon content remains below 0.2%. Components used for equipment in the mining and mineral processing industry typically require high resistance against wear. Correspondingly, the steel should have high hardness which is achieved by raising the carbon content. Toughness is not only needed due to likely high-impact loading under service conditions, but also to further improve wear resistance [41, 42]. Molybdenum alloying, often in combination with niobium, provides the required property spectrum of strength, hardness and toughness. The production of thin-gage martensitic steel has been steadily increasing over the last years due to the intensive use of press-hardened components in car bodies. Car models utilize 1 500 MPa press hardening steel (22MnB5) to an amount of up to 40% of the total body weight. For enabling further weight saving or crash resistance, steelmakers are developing press-hardening steel in the 2 000 MPa class (34MnB5). Qualification tests by carmakers confirmed that sufficient delayed cracking resistance required alloying of molybdenum. First 2 000 MPa class products in the market indeed rely on a combined molybdenum and niobium addition to standard 34MnB5 steel .
4 Alloy concepts and processing routes for multi-phase steels
HSLA steels offer a very good compromise between strength, toughness and weldability. However, some applications require higher elongation than that HSLA steel can provide at comparable strength level. Other applications need higher strength than that offered by HSLA steels yet having more ductility than martensitic steels. This product range of steels having tensile strengths from 600 MPa up to 1 200 MPa and high ductility is covered by so-called multi-phase steels. In this concept a particular combination of strong phases and ductile phases is adjusted by specific thermo-mechanical treatment.
4.1 Dual-phase steel design and applications
Microstructural design consists of a ductile matrix, typically ferrite, containing a dispersed second phase of usually martensite and/or bainite has been existing for several decades and is most prominently represented by the dual-phase steel family, which can reach tensile strength up to about 1 000 MPa. The vast majority of dual-phase steel production is cold-rolled to gages below 2 mm and used for car body components. Hot-rolled dual-phase steel in the gage range of 3–6 mm is being used almost exclusively for manufacturing of vehicle wheels. More recently, however, heavier-gaged hot-rolled dual-phase steel gained interest in pipeline applications. While the current top grade for pipe manufacturing is X80, as discussed in Sect. 2, future projects investigate the feasibility of grade X90 or X100. Enhancing strength further based on the established X80 alloy platform would result in rather low elongation of higher strength steel and thus not allow the implementation of strain-based design concepts to pipeline construction. A dual-phase microstructure can solve this conflict providing sufficient elongation at X90–X100 strength level. The challenge for the steel producer is to rather precisely adjust the two microstructural phases, being ferrite and martensite, in terms of volume fraction, grain size and spatial distribution under the available processing conditions.
4.2 Multi-phase steel containing retained austenite
Steel processed to a microstructure comprising a bainitic matrix containing retained austenite has been commercialized for automotive applications under the name TBF (TRIP Bainitic-Ferrite) steel. Due to the combination of ultra-high strength and good elongation TBF steel has high energy absorption capacity and is well-suited for crash-resistant components in the lateral structure of the car body.
Further developments aim at combining a martensitic matrix with retained austenite by employing a process called quenching & partitioning (Q&P) . Here, austenite is immediately quenched to below the martensite-start temperature, yet cooling is interrupted before full martensite transformation. Fast reheating to a higher temperature enables partitioning of carbon from the initially formed martensite to enrich still existing austenite. That austenite, depending on the level of carbon enrichment, forms fresh martensite upon final cooling or remains as metastable retained austenite. This approach is principally limited to thin-gage strip products and requires specific adaptations of continuous annealing lines or galvanizing lines to allow fast reheating. Alloying concepts, including the benefits of molybdenum alloying, are similar to those for TBF steel . Like martensitic steels, TBF as well as Q&P steels are sensitive to hydrogen embrittlement. Hence, the same beneficial effects of molybdenum alloying preferably in combination with niobium micro-alloying apply.
Mechanical properties of C-Si-Mn-Ni-Cu-Mo-Nb steel before and after M3 heat treatment cycle
As hot rolled
M3 heat treated
The use of modern high-performance steels enables significant improvements in design of structures, efficiency of manufacturing, durability of equipment, safety, cost reduction and lifecycle sustainability. The production of such steels requires state-of-the-art steel rolling and heat treatment facilities in combination with suitable alloying concepts. Molybdenum was shown to be one of the most effective and versatile alloying elements by enabling robust processing of such steel grades as well as by providing superior application properties. Particularly the remarkable effect of molybdenum, ideally in synergy with niobium, in improving the resistance against hydrogen-induced cracking in ultra-high strength steels is an issue of intensive ongoing research activities.
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