DESIGN OF DUAL PHASE STEELS WITH MPa STRENGTH. Peter Zimovčák, Ľuboš Juhar, Ivor Kučera, Juraj Graban

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1 DESIGN OF DUAL PHASE STEELS WITH MPa STRENGTH Peter Zimovčák, Ľuboš Juhar, Ivor Kučera, Juraj Graban U. S. Steel Košice, s.r.o., Vstupný areál U. S. Steel, Košice, Slovakia Abstract The presented paper deals with proposal of new dual phase steel with strength of MPa that should be an alternative to conventional microalloyed and re-phosphorized IF grades that are regularly used for car bodies. The proposed dual phase steel grades achieve the mechanical properties values that meet the requirements stated in European standards for grades HCT450X and HCT500X. The main alloying elements used to produce hot-dip galvanized steel were Mn, Cr and Mo. The unique properties of the steel predetermine it for use in automotive industry, especially in order to reduce weight, increase the car body toughness, decrease the cost of operating the vehicles and to enhance safety of passengers. The paper analyzes achieved results in respect to chemistry composition, mechanical properties and microstructure. Keywords: dual phase steels, high stregth steels, HSLA, HS-IF 1. INTRODUCTION According to ULSAB project [1], steel grades stronger than Rm = MPa are classified as high strength grades. Metallurgical conception divides this group to conventional high-strength grades and progressive high-strength grades. Amongst conventional high-strength grades are C-Mn grades, bake-hard steel grades (BH), isotropic grades (IS), high strength interstitial free steel grades (HS-IF) and micro-alloyed steel (HSLA). Amongst progressive high-strength steel grades are dual-phase steel (DP), TRIP steel, complex phase steel (CP) and martensitic steel grades. The dual-phased steel grades tend to more and more replace the abovementioned conventional high-strength steel grades [1]. Current trend in global automotive industry is aimed at increasing the passenger safety, as well as fuel economy. Dual phase steel currently offers attractive properties and thus it has gained an important share in the new concept of modern automobile [2]. The idea of producing dual phase steel grades originated in 1970s. Via innovative temperature processing of ferritic pearlitic steel a new type of steel with new properties was created. The structure of DP grades consists of soft ferritic matrix and of structure-strengthening phase of martensite. Due to its dual-phase composition the steel yield strength is relatively low this is beneficial in respect to its forming; its tensile strength is high [3]. High strength properties of DP steel grades are achieved by phase transformation. Other strengthening mechanisms are strengthening by grain boundaries and dislocation strengthening. Additional strength increase is achieved via Bake Hardening effect. The main alloys used for the production of DP grades are mostly Mn, Cr, Si and Mo. These alloys have a rather great impact on the shape of austenite isothermal transformation diagram and on critical cooling speed and annealing temperature [4]. The paper deals with proposal of new dual phase steel grades achieving strength of MPa that should offer an alternative to conventional microalloyed and re-phosphorized IF grades that are commonly used in car body production.

2 2. EXPERIMENTAL MATERIALS AND METHODS Dual phase steel grades alloyed by Mn, Cr and Mo were used to evaluate the influence of chemistry on mechanical properties and microstructure of hot-dip galvanized sheet. The results were compared to conventionally produced microalloyed and high-strength IF steel. The abovementioned steel grades were produced in the conditions of technologic process of hot-dip galvanized sheet production. Chemistry of steel sheets used in these experiments is stated in Tab. 1. Chemistry is in compliance with European standard EN10346:2009. Tab. 1 Chemical composition of the analyzed sheets [%] Sample C Mn Si P S Al N Ti Nb Mo Cr DP DP HSLA HS-IF The slabs were hot-rolled at HSM consisting of 5 four-high Roughing mill stands and 7 Finishing Mill stands. The achieved finishing temperature exceeded Ac 1 (DP1, DP2, HSLA samples) and Ac 3 (HS-IF sample) and afterwards the material was submitted to controlled cooling to coiling temperature. Coiled strips were subsequently pickled and cold-rolled at 4-stand mill. The strip was afterwards continuously annealed, hot-dip galvanized and temper rolled (skinpassed) at Hot dip galvanizing line. Tensile test was conducted in static conditions pursuant to STN EN The aim of the test was determination of basic mechanical properties values: yield strength R p0,2, tensile strength R m, elongation A 80, normal anisotropy coefficient r 90 and deformation strengthening exponent n 90. Tensile tests were conducted perpendicularly to rolling direction until the material broke. The deformation speed of the tested sample in elastic area was 0.1 s -1. Normal anisotropy coefficient r and deformation strengthening exponent n were determined using two extensometers according to these the thickness and width change was within even deformation range of %. The compressibility of the sheet was measured using the values of mechanical properties that were gathered via tension test and using formability criteria calculated using the following formulas [5]: Yield strength and tensile strength ratio: P R p0,2 R m (1) Complex formability coefficient: KUT P A 80 (2) Plasticity reserve index: Zp k ( R R A m p ) 0,2 80 (3) k = 3/4 pre materiály s nevýraznou medzou klzu k = 2/3 pre materiály s výraznou medzou klzu Formability index: IT r n 1000 (4)

3 The structure was evaluated and documented via regular metallographic practices, light microscopy; polished areas treated with wet-grinding and diamond paste polishing were examined. The cuts on the polished areas were made in the rolling direction. For metallographic structure development the 3 % etching agent Nital was used. The structure of samples DP1 and DP2 bola was additionally made more visible via their reheating to 260 C for 2.5 hours without protective atmosphere. The individual phases were distinguishable because polygonal ferrite is beige, residual austenite is pink and martensite is dark-blue. 3. RESULTS AND DISCUSSION The achieved mechanical properties and calculated formability criteria of the samples are stated in Tab. 2. On Fig. 1 there are tension diagrams and Fig. 2 displays formability criteria. Tab. 2 The achieved mechanical properties Sample R eh R p0,2 R el R m A 80 [%] A g [%] r n P KUT ZP IT DP DP HSLA HS-IF Fig. 1 Stress- strain curves Fig. 2 Formability criteria The highest tensile strength R m was observed in samples DP1 and DP2. The predominant mechanism of their strengthening is the phase of martensite. During transformation of austenite to martensite the volume growth occurs and moving dislocations, along with residual stress, are generated. Due to phase transformation and martensite presence the tensile diagram course is characterized by non-significant yield strength. Due to calculated formability criteria it can be stated that DP2 sample achieved better results in all categories, despite its higher strength. The DP2 sample contains less Cr but more Mo. Irie et al. in their publication [6] analyzed the relation between critical cooling speed and the content of alloying elements necessary for creation of ferritic-martensitic structure for C-Mn chemistry of steel with the addition of Cr and Mo. The publication also shows that in order to achieve dual phase microstructure the cooling speed needs to increase as the content of alloying elements decreases, while the influence of Cr and Mo on the critical cooling speed is 1.3 and 2.7 (respectively) times higher than that of Mn. At the same time the critical cooling speed necessary to create dual-phase structure is not influenced by mutual substitution of Mn, Cr or Mo [6]. That means that there has to be a different influence that increases the plastic properties. Molybdenum

4 significantly delays transformation of austenite to ferrite and pearlite [7]. Despite Mo reduces the activity of carbon in austenite and thermodynamically supports the development of carbides, in fact the very opposite effect was observed. Due to significant solute drag effect the carbide precipitation is slowed in the presence of Mo. At the same time Mo increases the strength via strengthening the solid solution [7]. The dominant mechanism of microalloyed steel strengthening is grain refining via precipitation of NbC particles that effectively prevent grain growth during recrystallization. One of typical features of microalloyed steel grades is increased ratio of yield strength vs. tensile strength, that s why their plastic deformation ability is limited. This index was the highest in the HSLA sample all formability indexes in this sample achieved the worst ratings. Despite this disadvantage these steel grades are still widely used, especially thanks to its relatively low production cost. Recently we observe increasing demand of automobile industry for fully stabilized high strength steel grades known for its excellent deep-drawing properties that qualify them for production of the most demanding large parts by forming. HS-IF sample is a representative of re-phosphorized IF grades; it is the proper alternative between DP and HSLA steel grades. The ability of this grade to resist ageing makes it a good candidate for hot-tip galvanizing. On the other hand these steel grades are less strong, when compared to DP grades this prevents their use for parts requiring higher resistance against splitting. Via strengthening mechanisms, such as substitution reinforcement by Mn and P, such conflicting requirements with proper combination of strength and elongation can be achieved. The element that is the most efficient and also cost-effective at solid solution strengthening is phosphorus. However, phosphorus is also responsible for secondary brittleness, causing the sheet to be prone to inter-crystal fracture during secondary stamping operations involving deep drawing and/or during use. DP1 sample DP2 sample HSLA sample HS-IFsample Fig. 3 Final microstructure of samples after hot-dip galvanizing

5 The final microstructure of samples is shown on Fig. 3. The different character of ferritic grains in sample DP1 sample vs. DP2 sample shows that it is transformed ferrite that is of lighter color tone than the nontransformed one. Martensite is blue its content is higher in DP2 sample this correlates with the mechanical properties. Higher martensite content increases strength properties. On the boundaries of ferritic grains and in close proximity of martensite under high resolution we observed pink particles that were identified as residual austenite. The HSLA sample structure is ferrite-cementite-pearlite its ferritic grain size is G11. The HS-IF sample ferritic grain size is one grade smaller, i.e. G10. Since the HSLA and HS-IF samples are of similar grain size, due to their mechanical properties it can be said that substitution strengthening of Mn and P in IF steel is more important than that provided by refining grains in HSLA grades. 4. CONCLUSION The proposed dual-phase steel grades achieved mechanical properties that meet requirements stated in European standards for grades HCT450X and HCT500X. The main alloying elements for the production of hot-dip galvanized dual-phase steel were Mn, Cr and Mo. Their unique properties predestine these steel grades for use in automotive industry, especially in order to reduce car body weight and to increase the passenger safety. The proposed and investigated dual-phase steel grades are an adequate substitute to microalloyed and high-strength IF steel grades. REFERENCES [1] [2] Spindler, H., Klein, M., Rauch, R., Pichler, A., Stiaszny, P.: High strength and ultra high strenght hot rolled steel grades - products for advanced applications. "Proceedings of Super-High Strength Steels, (Rome, Italy, Associazione Italian di Metallurgica, 2005) [3] Stercz, K.: Simulovanie kontinuálneho žíhania oceľových DP plechov, Diploma thesis, Technická Univerzita v Košiciach, Hutnícka fakulta, Košice 2008 [4] [5] Spišák, E.: Matematické modelovanie a simulácia technologických procesov Ťahanie, Typo Press, Košice 2000 [6] Irie, T., Satoh, S., Hashiguchi, K., Takahashi, I., Hashimoto, O.: The metallurgical factors affecting the press formability of cold rolled, Metallurgical Factors Affecting the Formability of Cold-rolled High Strength Steel Sheets, Transactions ISIJ, Vol. 21, 1981 [7] Bleck, W., Frehn, A., Ohlert, J.: Electronic version of Niobium, STM, 2003