Adapting AHSS Concepts to Industrial Practice

Transcription

1 Adapting AHSS Concepts to Industrial Practice R. Mostert, B. L. Ennis, D. N. Hanlon Tata Steel R&D, P.O. Box , 1970 CA IJmuiden, The Netherlands Phone: Keywords: AHSS, deformation resistance, DP800HyPerform, product development, application properties ABSTRACT There is a wealth of AHSS concepts in existence. For cold forming the many options exist for first, second and third generation AHSS. Ideas for alternative products for hot rolling and hot forming also abound. Unfortunately, a metallurgical concept does not automatically translate into a mass produced value added product. Manufacturing and application considerations can make or break a product. Two examples from Tata Steel s industrial experience are discussed in detail. The first example discusses a route towards extra wide dual phase steels by changing the chemical composition. The second example discusses overcoming the spot welding difficulties in TRIP with a new grade family, HyPerform. In both cases there were small metallurgical trade offs to accommodate process or application requirements. Both products have led to successful products that were the result of adapting rather than inventing new AHSS concepts. INTRODUCTION Metallurgy supplies new concepts that deliver improved mechanical properties. Among the concepts discussed at the 2008 International Conference on New Developments in Advanced High-Strength Sheet Steels these concepts included Q&P steels 1, TWIP steels 2, TRIP-aided ultra high strength steels 3, and flash bainite 4. Despite the advantages in mechanical properties that follow from development work, none of these new concepts are at present used in vehicle manufacture in large volumes. Economic considerations certainly play a role. New concepts on occasion demand higher alloying or more expensive alloys. In automotive applications it is often difficult to capture the full benefit of a product based on a new concept when changes are made on a running model. New concepts are not needed for running models. Metallurgy, application and process all have to work together. Without a suitable manufacturing process there will be no mass produced product, and without a suitable application there is no need for a mass produced product. In product development in industry it is necessary to get a working solution in all three areas simultaneously. Although theoretically solutions could exist that are optimum solutions in metallurgy, application and process simultaneously, in practice a degree of compromise is always needed. Two examples of product development at Tata Steel will be given to illustrate that point. The first example will involve striking a balance between mechanical properties and manufacturing issues, the second example between mechanical properties and the intended application. ADAPTATION TO FIT PROCESS For a metallurgical concept to be translated to a mass produced product, it is necessary that a suitable industrial process be identified. Every product needs a process. To keep time to market at a minimum, there is always a strong preference that a new product can be manufactured on pre-existing installations. The generally relatively small initial volumes and loading of existing installations are barriers to investment in new installations. When new installations are built for other reasons, new products, present and future, are taken into consideration. It follows that existing installations are not always ideally suited for 45

2 the production of new grades. The example that is discussed in this section is DP600 and revolves around the limitation in maximum width on rolling mills the root cause of which is the relatively high strength during hot rolling and cold rolling. The basic metallurgy of dual phase steels is known since the late seventies 5. The metallurgical principles developed then are evident in many dual phase alloying strategies, including the traditional strategy for DP600GI at Tata Steel. Some variation in composition to accommodate differences in the lay out of hot dip galvanising lines, but all are CMnCrSi compositions. Silicon was included because of the advantages it brings to strength and elongation. For dual phase steels with compositions C 0.05%-0.1%, Mn %, Si %, Messien found an increase in tensile strength of 131 MPa/%Si without a reduction in total elongation 6. Early experimental evidence suggested this trend continues below 0.2% Si. Laboratory cast alloys (Grades RefA and RefA-Si, Table 1) with Si levels of 0.01% and 0.24% were annealed with an industrial cycle with a top temperature of 830 C. The best combinations for elongation and tensile strength were found for the RefA alloy (Table 2). With the small number of laboratory trial results the statistics of the results were uncertain for the total elongation. Table 1. Composition of Reference and low Si steel grades. Grade C (wt%) Mn (wt%) Al (wt%) Si (wt%) Cr (wt%) RefA RefA-Si RefB RefB-Si Table 2. Mechanical properties of Si containing grade RefA and Si free grade RefA-Si. Grade R m (MPa) A 80 (%) R m A 80 (MPa.%) RefA RefA-Si The product of tensile strength and total elongation, which can be taken as a rough measure of the one aspect of inherent formability of a microstructure, therefore improves. In this respect higher silicon contents are preferred. As Tata Steel operates hot-dip galvanising lines for corrosion protection, the upper limit of the Si content was determined by galvanisability 7. Hot dip galvanising control becomes progressively more demanding as the Si content increases and there is an increasing risk of surface defects. Historically, the upper limit was set at 0.25%, which is a compromise between achieving highest formability and robustness of the hot dip galvanising process. A further disadvantage of higher Si contents is an increase of rolling forces in hot rolling. When plant trials were conducted with grades RefB and grade RefB-Si with reduced Si content (Table 1), it was found that rolling forces in the hot strip mill were significantly lower for the RefB-Si grade. Figure 1 shows the difference in hot band width between grades RefB-Si and RefB that can be achieved as a function of gauge on HSM No. 2 of Tata Steel in IJmuiden. A reduction in aim Si content from 0.25 to 0.05% enabled an increase in hot rolled width increasing from 50 mm at 2 mm gauge to 200 mm at 5 mm gauge. The increase cannot be explained by a large decrease in bulk deformation resistance. The increase in the deformation resistance of the substrate is estimated from the composition dependence of the mean flow stress. Following Siciliano et al. 8, we express the mean flow stress as a function of carbon content, temperature, strain and strain rate 9 multiplied with a linear function in the alloying elements. Measurements at Tata Steel lead to the conclusion that the Si contribution to the mean flow stress is approximately 16% smaller than the Mn contribution. The expected reduction in mean flow stress was 1%. The source of the difference must lie in the surface condition of the hot band. It is well known that Si containing steel produces oxides different from Si-free steels. As the difference in rolling forces cannot be explained by the bulk properties, the difference has to be ascribed to differences in friction due to differences in scale formation. 46 Intl. Symp. on New Developments in Advanced High-Strength Sheet Steels

3 250 max. width increase (mm) hot band gauge (mm) Figure 1. Increase in maximum width of hot band due to reduction of Si content (grade B-Si). Removal of nearly all Si from composition RefB allows the production of wider hot band and also wider galvanised coil on existing installations. There is a small effect on mechanical properties. The microstructure of grades RefB and RefB-Si are very similar and typical for industrially produced dual phase steel. Figure 2 shows a LePera etching of a typical RefB-Si sample. A small amount of martensite is dispersed in ferrite, primarily at or near the austenite grain boundaries. A small amount of darker etching phase is identified as bainite. Figure 2. LePera etching of RefB-Si steel. 47

4 There was a small, but distinct effect on mechanical properties. In Figure 3 the relative changes in yield strength (YS), tensile strength (TS), elongation (A80) and n 10-Ag (n) are shown. The tensile strength decreased by about 4% and the total elongation by about 3%. The formability of the RefB is superior to the RefB-Si grade, conforming to the rule of thumb that formability is increased when Si is added to an Advanced High Strength Steel. The yield strength and yield ratio (YS/TS) both increased, and n value decreased. 6 4 percentage change YS TS A80 n property Figure 3. Change in tensile properties due to reduction of Si (grade B-Si). As the example shows, the choice of composition is a compromise between mechanical properties and plant capabilities. If only mechanical properties needed to be taken into account in the decision, then the Si content would be chosen higher than 0.25%. When a very reliable hot dip galvanizing process is required, Si is maximized, in this case to Si=0.25% (grade RefB). When widest dual phase steel is needed Si is reduced to very low values (grade RefB-Si). Grade RefB and grade RefB-Si are different compromises. With grade B, a conventional dual phase steel grade, it was possible to find a compromise between process demands and mechanical properties. That may not always be true. The more demanding a specification is, the closer an installation needs to operate to the limits of its capabilities. The options to strike compromises are then reduced. FORMABILITY IN PRACTICE Much of the traditional development (of steels) has concentrated on improvement in mechanical properties as measured by the tensile test, but there are not many applications in which the load path is identical to that of the tensile specimen. In fact, high strength steel development is beginning to differentiate a number of classes of product based on real world applications, either in terms of making the product e.g. forming complex shapes, spot-weldability, end-user properties e.g. high strength, crash absorption and cost. The challenge in designing new steel grades should be led by the application and the customer requirements. In an ideal world, there would be a tailored solution per component, but this is neither commercially feasible nor even absolutely necessary when we group steels according to mechanical properties and formability required for the application. The first generation of advanced high strength steels as steels that possess primarily ferrite-based microstructures, 10 may be grouped according to the predominant microstructure type: dual phase (DP), complex phase (CP), transformation induced plasticity (TRIP) and martensitic (MART). The strength of DP steels is largely dependent upon the relative hardness and volume fraction of second phase (martensite) 11 and mechanical properties are characterised by low yield ratios, continuous yielding, very high initial work hardening rates and a good combination of strength and elongation. They are considered to be formable high strength steels which require comparatively low press loads due to their low yield strengths but which attain high strengths in the final pressed part. Compared to most other members of the AHSS product group DP steels tend to be relatively lightly alloyed and thus exhibit good spot weldability. 48 Intl. Symp. on New Developments in Advanced High-Strength Sheet Steels

5 TRIP steels are a more recent development 12 than DP and were developed in order to further improve the formability and crash performance. TRIP steels consist of a ferritic and bainitic microstructure with retained austenite. The bainite offers strength, whilst the ferrite offers formability. The formability of these steels is increased by the retained austenite that transforms during forming operations into martensite whilst the martensitic transformation product leads to increased strength via composite hardening. Although some TRIP steels have been applied successfully in the automotive industry, their application is limited due to issues with weldability, crash performance and cost. 13 Due to the high alloy content, particularly silicon 14, it is not possible to coat these steels in a continuous annealing and galvanising line with a conventional process. The alloying additions differ depending on coating type: hot dip galvanised and uncoated are primarily high Al and electrogalvanised products are primarily high Si chemistries. Both elements play a key role in TRIP metallurgy since they suppress the formation of carbides and, in combination with modified annealing practice, enable the retention of a fraction of metastable retained austenite in the final microstructure. Si is more effective in this respect and in addition provides solid solution strengthening, however the use of Al is necessitated in the case of hot dip coated products since Si severely reduces the adhesion of zinc. Although research is being carried out in coating high silicon steels, these currently still have to be electrogalvanised, making the resulting TRIP products extremely expensive. Furthermore, whilst the dual phase and TRIP steels have excellent plane strain formability and stretchability due to extended work hardening/increased work hardening capacity, they do have a number of critical disadvantages: high edge cracking sensitivity (flangeabilty and hole expansitivity are generally low) and, as is becoming increasingly apparent, they exhibit a tendency to undergo shear fracture before the onset of local necking. The apparently low damage tolerance of DP and TRIP steels is due to the fact that mixtures of hard and soft phases tend to exhibit poor local elongations (these properties correlate with reduction in area rather than gauge length elongation). Metallographic observations 15 have demonstrated that the nucleation of micro-voids is reduced when the martensite co-deforms with the ferrite matrix. It is apparent then that the closer the phases are in strength, the more likely it will be that they will co-deform. Since the strength of martensite is proportional to the carbon content of the austenite from which it is formed (either during annealing or as a result of strain induced transformation), it is important to control the carbon content of the retained austenite during overaging in order to promote co-deformation with the ferrite matrix, either by reducing the bulk carbon content or by controlling the amount of austenite (and thence martensite) during annealing. In developing a steel with an improved formability over DP steels, but without the disadvantages of poor spot-weldability and coatability, the first step was to examine the requirements for formability. Table 3 shows the most important mechanical properties for common types of formability, from which it can be deduced that the n-value is indicated as a primary or secondary influencing factor in each type Requirement Plane Strain Formability Table 3. Mechanical properties required for different types of formability Materials and Product Parameters 1st order 2nd order Low importance n-value r min thickness (stiffness) Deep Drawability r min n-value Spring-back Stretch Flangeability/ Hole expansitivity E-modulus YS thickness (stiffness) n-value r min n-value r min YS thickness (stiffness) E-modulus YS E-modulus YS - E-modulus Although the tensile response is a key indicator for crash performance, most real world structures are (spot) welded and therefore it is essential that steels in crash applications are spot-weldable. When higher strength advanced high strength steels are resistance spot welded, problems occur concerning their failure modes and sometimes the appearance of hot cracks. The problems arise when welding TRIP steels: increased strength levels and the need to stabilise high fractions of RA require increased carbon levels which, in combination with other alloying elements and tramp elements such as phosphorus and sulphur, lead to the production of very hard and embrittled martensitic microstructures spot-welded structures. 16 As a result of this, the weld failure mode has become a very important factor in spot weldability. Many automotive manufacturers require a stringent spot weld performance taking into account strength and failure mode obtained, especially when a steel grade is welded to itself. An acceptable failure mode means a ductile fracture failure occurs in the HAZ or parent material and not in the weld zone (full plug failure). 49

6 Based on the above insights an alloying strategy has been generated in order to deliver enhancement of n-value whilst at the same time conforming to strict boundary conditions to maintain spot weldability (primarily governed by Carbon Equivalent) and galvanisability (governed by Si content and Si-Mn ratio). Alloying is employed to retain some austenite in the DP structure after galvanising which results in a similar work hardening capacity to DP800 but delays the onset of local necking. Due to the emphasis on optimisation of n-value, the uniform elongation is enhanced (for stretch forming), while total elongation to standard DP800 exhibits only a modest improvement with respect to conventional DP800. This is the development philosophy behind Tata Steel's enhanced DP based steel grade, DP800HyPerform (DP800HpF) which delivers a coatable high strength alloy (UTS of 800MPa) with increased stretchability and hole-expansitivity, while retaining a good spot weld performance, with failure mode as the main priority, as shown in Figure 4. Figure 4. Partial plug failure in cross tensile testing of TRIP800 (left) and base metal failure in cross tensile testing of DP800HyPerform (right). The mechanical response of DP800, DP800HpF and TRIP800 can best be illustrated by the work hardening profile, as shown in Figure 5. The DP type of work hardening behaviour shows an initially high rate work-hardening due to the low yield ratio, but with a rapid decay and steep fall off in the post uniform region due to damage intolerance. DP800HpF also exhibits a DP type of work-hardening behaviour, but due to the presence of retained austenite, there is increased resistance to strain localisation i.e. necking. The presence of some bainite in the microstructure imparts increased damage tolerance (holeexpansitivity) over standard DP. TRIP material shows a slightly lower initial rate of work hardening, but continues to increase due to transformation induced plasticity allowing for much larger strains before necking than either DP800 or DP800HpF. Figure 5. Continuous work-hardening profile of DP800, DP800HpF and TRIP800 Crash performance is a mixture of a number of properties: tensile strength, formability and spot-weldability. The strength is a primary indicator of and, all other factors being equal, is linearly related to the amount of energy which can be absorbed during crash. Formability, primarily stretchability, is also vital in crash as the material begins to deform to accommodate the deformation as a result of dynamic loading i.e. during a frontal collision. If cracking should occur, either in the base material or in (spot)-welds on the assembly, then the full crash resistance of the material will not be realised. TRIP materials are traditionally better than DP for crash applications because there is residual ductility after forming into complex shapes. The disadvantage is in spot-welding which requires either extra passes to relieve strain, or re-work i.e. increasing cost of 50 Intl. Symp. on New Developments in Advanced High-Strength Sheet Steels

7 manufacture. Due to the enhanced uniform elongation of DP800HpF, there is still residual ductility for crash after forming complex shapes and combined with the spot-welding performance of DP800 makes it a cost effective alternative to TRIP for crash. Figure 6 shows the results of a simulated pole impact test using a drop weight of 145kg at 40km/h on a bumper assembly. DP800HpF performs equally well as the TRIP800 grade normally used as the cross member for this application. DP800HyPerform TRIP800 TRIP800 Figure 6. Comparison of crash performance of DP800HpF and TRIP800 in pole impact test CP grades are relative newcomers to the steel developers palette and address the need for increased bendability, holeexpansitivity and spring-back resistance. The classification covers a broad range of microstructures, but basically they can be defined as a mixed ferrite/bainite structure with little or no martensite and/or retained austenite exhibiting high strength (800MPa+), a high yield ratio and moderate ductility. Due to the close match in mechanical response of the constituent phases, these microstructures are highly damage tolerant, which is essential for (edge) cracking resistance. Table 4. Choice of steel grade based on formability application for a given strength level (800MPa) Grade Stretching Deep drawing Holeexpansion Bending resistance weldability Roll-forming/ Spring-back Spot- DP800 +Z DP800HpF +Z /+ 0/+ CP800 +Z TRIP800 +EZ Table 4 shows the relative performance of steel grades of the same strength level for different types of formability applications together with spot-weldability. The baseline is DP800 and superior performance is indicated with (+), inferior performance with (-). This illustrates the limitations of specifying a steel grade purely on the tensile response alone. Figure 7 shows a comparison of DP800HpF with similar grades of DP and TRIP steels. 51

8 DP800HpF DP600 DP800 TRIP800 EZ Mass saving Increase strength Improved press performance w.r.t. DP800 Cost saving Improved weldability Figure 7. Design considerations of DP800HyPerform compared with other AHSS grades IMPLICATION FOR PRODUCT DEVELOPMENT Successful industrial product development sits at the intersection of metallurgical possibilities, process limitations and application requirements (Figure 8). To some extent and as illustrated above, a successful product is nearly always a compromise between two or three of the elements. The initial idea for a new product can be a metallurgy, process or application idea, but already at a very early stage of the development is it necessary to align the three. The sooner this is done, the sooner it is clear that whatever is developed can be produced and is needed. Figure 8. Successful product development (Product) sits at the intersection of successful metallurgy, process and application. Without alignment scarce resources are wasted and fewer successful products are brought to market. For an integrated steelmaker, alignment with manufacturing is an in-house activity. Alignment with the application side of product development requires alignment with customers, which involves effectively managing an additional interface. In the end that can be solved, because all stakeholders want the steel industry to develop what is needed, and does not want the steel industry to develop what is not needed. ACKNOWLEDGMENTS The authors would like to express their thanks to Dr. Kees Bos for his contribution to determining the deformation resistance of silicon free steels and to the Tata Steel R&D Product Application Centre, and in particular Dr. Eisso Atzema, for crash testing and applications knowledge during the development of DP800HpF. 52 Intl. Symp. on New Developments in Advanced High-Strength Sheet Steels

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