Manufacturing and processing of advanced highstrength

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1 Manufacturing and processing of advanced highstrength heavy plates Rainer Grill, Rupert Egger und Franz Mayrhofer Voestalpine Grobblech GmbH, Linz, Österreich Abstract: A short look is taken at the development of highstrength steel grades at Voestalpine Grobblech GmbH. Economy and the need of good weldability require materials with a lean alloy composition and fairly low carbon contents. The additional benefit of combining high strength and excellent toughness can only be achieved with optimized processing parameters including online-accelerated cooling. Experience gained by R&D work at Voestalpine Grobblech GmbH will be outlined in the paper. Herstellung und Verarbeitung moderner hochfester Grobbleche Zusammenfassung: In diesem Artikel wird die Herstellung hochfester Stähle bei Voestalpine Grobblech GmbH beleuchtet. Zur Sicherstellung der Wirtschaftlichkeit derartiger Stähle wird eine schlanke chemische Zusammensetzung angestrebt und dies bei niedrigen Kohlenstoffgehalten, um beste Voraussetzungen für die Schweißbarkeit zu schaffen. Die erwünschte Kombination aus Festigkeit und Zähigkeit kann nur durch optimale Prozessparameter und den Einsatz einer beschleunigten Kühlung direkt aus der Walzhitze erreicht werden. Dazu werden Ergebnisse aus der Forschungs- und Entwicklungsabteilung der Voestalpine Grobblech GmbH dargestellt. 1. Introduction The Voestalpine Grobblech GmbH (VAGB) produces about 8. t high quality heavy plates per year at the location in Linz. The share of high-strength fine-grained steel has increased enormously in the last years whereby the majority of these plates is produced without a subsequent heat treatment, so called online production, because of qualitative and economic reasons. Only at higher strengths the steel is tempered if necessary. The conventional quenching and tempering is used with bigger thickness of steel grades at a strength class of S7 or higher. The application field of these high quality heavy plates is diverse. Besides of the gas-pipeline construction (in addition to the standard steel X7 also steel grades with higher strength like X8 and X1 are used today) the application field is mechanical engineering and automotive constructions as well as steel constructions and penstock steel structures (Figure 1). In the energy sector heavy plates in bigger thicknesses up to 1mm with high requirements on the low temperature toughness for drilling and production platforms in arctic regions are needed. 2. Heavy plate production For the production of heavy plates in Linz slabs of casting thicknesses up to 355 mm from the steel plant of the Voestalpine Stahl GmbH are used. Only in special cases (thickness of plates up to 2 mm or plates with higher weight) blocks are utilized. A condition for the production of high quality heavy plates is a primary material produced by a modern steel plant with the required facilities for an improvement of the degree of purity and to produce slabs free of cracks and with little segregations 1. A vacuum treatment of the melt is state of the art and for special requirements a calcium follow-up treatment is included 2,3. The slabs (blocks) are heated by the pusher furnace (or pit furnaces) up to a temperature of 1 C to 12 C before the rolling. The production of the plates is made in two deformation steps, where the first step is the so called roughing which is rolled at higher temperatures, in order to be able to recrystallize completely. The second step is called finishing and is performed at lower temperatures, either close above the recrystallization temperature of the austenite (normalizing rolling) or below (thermomechanical rolling). In both stages of the rolling process the deforming is generated through reversed rolling in several passes. In Figure 4 the 4.2m-Quarto rolling mill (left) and the pusher furnace 1 (right) of the Voestalpine Grobblech GmbH is shown. 1

Figure 1: Application of high strength steels for drilling and production platforms (left), offshore gas pipelines (2 nd left), pressure pipelines (2 nd right) and bridge building (right) At

With increasing alloying content or rising requirements on the strength it is necessary to switch from cooling procedures at which the accelerated cooling is interrupted at higher

The accelerated cooling line of the VAGB (Figure 2) belongs to the worldwide most efficient systems and was designed in order to reach highest cooling rates for plates up to a thickness of 1

Nowadays finegrained construction steels with a yield strength of 96 MPa and if necessary even higher can be produced 4,5.

volume which has to be welded (welding and proving expenses).

In general advanced high strength steels are easily workable, but with rising strength the acceptable processing window becomes smaller.

treatment, the rolling process and the Figure 2: Accelerated cooling line for heavy plates for an online-cooling directly from the rolling heat Figure 3: Comparison of the microstructure of

2 Figure 1: Application of high strength steels for drilling and production platforms (left), offshore gas pipelines (2 nd left), pressure pipelines (2 nd right) and bridge building (right) At the most advanced procedures the finishing is performed close above the Ar 3-temperature and the plates are accelerated cooled right after the rolling process directly from the rolling heat. With increasing alloying content or rising requirements on the strength it is necessary to switch from cooling procedures at which the accelerated cooling is interrupted at higher temperatures (ACC-procedures) to a more intensive cooling with lower cooling stop temperatures and higher cooling rates (DIC-procedures). The accelerated cooling line of the VAGB (Figure 2) belongs to the worldwide most efficient systems and was designed in order to reach highest cooling rates for plates up to a thickness of 1 mm. 8 years ago the steel grade St 52 (S355) was known as high strength construction steel and slowly replaced the standard construction steel St 37 (S235). Nowadays finegrained construction steels with a yield strength of 96 MPa and if necessary even higher can be produced 4,5. Compared to the normal-strength steels the advantages of the high strength steels include not only savings of base metal and weld metal but also the reduction of the working time due to fewer volume which has to be welded (welding and proving expenses). Furthermore the costs for transport and manipulation can be reduced and in the end a lot of these constructions would not be able to be built without using high strength steels. In general advanced high strength steels are easily workable, but with rising strength the acceptable processing window becomes smaller. The challenge of the production of high strength construction steels is developing to be less about the alloying composition but about designing the production process like the slab heat treatment, the rolling process and the Figure 2: Accelerated cooling line for heavy plates for an online-cooling directly from the rolling heat Figure 3: Comparison of the microstructure of S355N (left, HNO 3 etching), and S5M (right, LePera etching) Without ohne Umformung Deformation Phi φ=.3, =,3 at bei 88 C C Hardness Härte HV HV /1 412/2 412/ Cooling Kühlrate Rate [ C/s] Cooling Kühlrate Rate [ C/s] [ C/s] Figure 4: Rolling of heavy plates on the 4.2m-Quarto rolling mill, the pusher furnace 1 can be seen on the right side Figure 5: Effect of chromium (version 412/2) and boron (412/3) on the hardness of dilatometer specimens with and without deformation depending on the cooling rate 2

Figure 7: Condition of the former austenite after conditioning (left) or a common austenitization (right) accelerated cooling procedure this way that the highest possible strength with simultaneous

3 Figure 7: Condition of the former austenite after conditioning (left) or a common austenitization (right) accelerated cooling procedure this way that the highest possible strength with simultaneous very high toughness is reached and trying to use few alloying content. The difficulties with heavy plates compared to hot rolled strips are that bigger thicknesses (in extreme cases up to 1 mm and more) with high requirements on the strength and toughness are produced. The comparison of the microstructure of the steel S355N with an advanced thermomechanically rolled and accelerated cooled steel like S5M shows the effects of the described measures to influence the microstructure (Figure 3). The advanced steel contains a small amount of carbon (about.3%) and is microalloyed with niobium (.4%) and titanium (.1%). A further increase of the strength up to the level of a S7 is mainly achieved by an addition of boron (around 1ppm). Additionally there are little amounts of chromium and molybdenum required. In Figure 5 the hardness of three alloys (laboratory melts) in dependence of the cooling rate (cooling temperature <35 C) with and without deformation at 88 C is presented. Compared to the base alloy (heat number 412/1) the heat number 412/2 additionally contains.2% chromium and the heat number 412/3 includes additional 12ppm of boron. The simulated thermomechanical deformation (dilatometer) leads surprisingly when using little amounts of additional boron to no reduction of the hardenability. This is why boron is especially suitable for an increase of the hardenability of online-cooled thermomechanically rolled plates. In the case of an online-production the carbon content of steels in this strength class can also be reduced under Rp.2, Rp,2 und Rm Rm [MPa] S46M (C=,3 %, CE IIW =,32 %) Rp,2 Rm 3 3 lowniedrig Cooling Kühlstopptemperatur Stop Temperature hochhigh low niedrig Cooling Kühlstopptemperatur Stop Temperature hochhigh Figure 8: Effect of the cooling stop temperature on the strength of different types of steels S7M (C=,4 %, CE IIW =,42 %) Rp.2, Rp,2 und Rm Rm [MPa] [MPa] TMCP,,4,8,12, TMCP + AT Carbon Kohlenstoff Content [%] [%] Carbon Kohlenstoff Content [%] [%] Figure 6: Effect of carbon on Rp.2 and Rm after rolling (left) and tempering (right).5%. A commonly quenched and tempered steel with a yield strength of 7MPa contains a carbon content about.16%. Compared to the quenched and tempered steel (Figure 7, right) the austenite of the low carbon alloy was conditioned before the accelerated cooling (Figure 7, left). This is how it is possible to reduce the decisive austenite grain size corresponding to the deformation degree. The different procedures of the thermomechanical rolling were mainly developed for the production of large sized pipeline plates beginning of the 7s. The used steels were for the first time microalloyed with niobium, vanadium and titanium. The target was on one hand an economical production with low alloying content and without an additional heat treatment and on the other hand the assurances of the required properties like toughness, safety against brittle fracture and weldability. With the material X7 the limits of the TM-process with subsequent air cooling in terms of strength, toughness and producible plate thicknesses were reached and further developments were only reachable through accelerated cooling 6,7. In the meantime many heavy plate producers have such facilities, whereby some already master the process already very well. Besides of the process parameters, which result from the slab heating and the rolling process, also the cooling parameters, like cooling start and cooling stop temperature, have a big influence. At the same time alloys which are adequate for the process have to be designed. In Figure 8 the influence of the cooling stop temperature, where the accelerated cooling is interrupted, on the strength of the steel grades S46M and S7M is presented. If the cooling is insufficient the high strength materials show besides of a relative low strength also a low toughness because of the development of MAconstituents (high carbon martensite and retained austenite phases) 8. Even higher strengths can be reached with given hardenability through elevation of the carbon content. In Figure 6 (left) it is demonstrated that the tensile strength grows faster than the yield strength. That is why it could be necessary to temper steels with higher carbon content (Figure 6, right). The application of pipeline construction steels with a carbon content above.8-.1% are however not suitable anymore. Advanced TM-steels show very good toughness properties. 3

5 Charpy Kerbschlagarbeit V-Notch Energy [J] [J] 45 4 35 3 2 15 1 5 S96M -14-12 -1-8 -6-4 -2 Testing Prüftemperatur Temperature [ C] S7M S46M Figure 9: Typical values of the charpy V-notch impact

4 5 Charpy Kerbschlagarbeit V-Notch Energy [J] [J] S96M Testing Prüftemperatur Temperature [ C] S7M S46M Figure 9: Typical values of the charpy V-notch impact test at different testing temperatures (DIN standard) Hardness Härte HV HV S355N S46M t 8/5 = 15s Figure 1: Physical welding simulation, effect of peak temperature on the hardness at different t 8/5-times Charpy Kerbschlagarbeit V-Notch Energy bei -6 C at -6 C [J] [J] ,4 %C,16 %C Tempering Anlasstemperatur Temperature [ C] [ C] Tempering Anlasstemperatur Temperature [ C] [ C] Figure 12: Effect of the tempering temperature on the charpy V-notch energy at -6 C (left) and the strength of a high carbon quenched and tempered steel (right) Rp,2 Rp.2, und Rm Rm [MPa] ,16 %C In Figure 9 typical fracture toughness transitions are presented. For common quenched and tempered steels (e.g. 92 C/water) tempering at a temperature between 6-66 C is necessary after hardening, on the one hand to reduce the hardness resulting from the high carbon content and on the other hand to raise the fracture toughness. For advanced high strength TM-steels tempering is only in some cases required. It leads to a small decrease of the tensile Figure 11: Influenced austenite grain size with an advanced method (right) strength and the toughness will not be improved at tempering temperatures above 5 C but to the contrary it will be negatively affected. In Figure 12 the influence of the tempering temperature on the fracture toughness (left) and the strength (right) is demonstrated. For the production of thick plates in the upper strength level the TM-process gets increasingly impractical, therefore a conventional quenching and tempering has to follow subsequently. The toughness is strongly determined by the austenitic grain size during austenitization. With new methods it could be possible to achieve an improvement through grain refinement (Figure 11) in the future. Such a treatment led to a decrease of the fracture toughness transition temperature from -6 to -14 C. When mentioning the processing properties of heavy plates the weldability would be the top priority. In addition to welding experiments it is also possible to characterize the weldability in the laboratory 9. The following investigations were performed by the physical weld simulator (HAZsimulations) of the Voestalpine in Linz. In Figure 1 the hardness of two different constructions steels in dependence of the peak temperature of several t 8/5- times (cooling time between 8 C and 5 C) are compared. On the opposite to the normalized steel S355N with.16% carbon content the advanced TM-material with.3% of carbon shows no hardness increase within the heat affected zone (HAZ, fine and coarse grained zone) and only a very low sensibility of the hardness with regard to the cooling rate. Peak temperatures above A C1 of both materials have no effect on the hardness of the base metall. The simulations make it possible to extend the heat affected zone over a big volume in order to take charpy V-notch energy and tensile samples. In Figure 13 the results of the charpy V-notch energy compared for the materials S355N and S46M are plotted. The low carbon alloyed thermomechanically rolled and accelerated cooled material exceeds the results of the normalized steel in all HAZs and also the base metal with regard to the upper shelf (testing temperature -4 C) as well as the transition temperature. 4

5 Charpy Kerbschlagarbeit V-Notch Energy [J] [J] S46M S355N Transition Übergangstemperatur Temperature [ C] [ C] Figure 13: HAZ-simulation, effect of peak temperatures on the charpy V-notch energy at -4 C (left) and the transition temperature for 5% shear fracture Tensile Zugfestigkeit Strength [MPa] [MPa] as welded S7M S69QL S46M S355N Figure 15: HAZ-simulation, effect of peak temperatures on the tensile strength after rolling (left) and after post weld heat treatment (right) Charpy Kerbschlagarbeit V-Notch Energy [J] [J] S7M S69QL PWHT (56 C) In the area of the yield strength level of 7 MPa a commonly quenched and tempered steel with.16% of carbon and an advanced thermomechanically rolled and intensive cooled steel with.4% of carbon were compared and analyzed. In the condition as welded the quenched and tempered steel shows a comparable strong hardening in the fine and coarse grained zone (CGZ), whereas the TMsteel hardens only a little at very short t 8/5-times (3s) (Figure 15, left). After a post weld heat treatment (PWHT) the high tensile strength of the quenched and tempered steel can be reduced to values below 11MPa (Figure 15, right). The strength of the TM-steel falls down to the level of the base metall or at long t 8/5-times (s) to values close below them. Both materials show a tempering and transformation softening. Lowering the PWHT temperature leads to a small Figure 17: HAZ-simulation, effect of peak temperatures on the charpy V-notch energy at -4 C (left) and the transition temperature for 5% shear fracture Übergangstemperatur [ C] S7M S69QL 5 Reduction Brucheinschnürung of Area [%] [%] CGZ, GKZ, as welded S46M S55M S69QL/QL1 S7M Testing Prüftemperatur Temperature [ C] [ C] Figure 14: Effect of the testing temperature on the reduction of area of different steels (starting condition: as welded, CGZ) strength increase at S7M and more pronounced rise at S69QL. Testing the charpy V-notch energy declared that the TMsteel shows better results in the HAZ at -4 C for 5% shear fracture (Figure 17, left) as well as for the transition temperature than the quenched and tempered steels. At the PWHT of weld seams it can occur that in the area of the coarse grained zone along the former austenite grain boundary so called relaxation cracks can be formed. A simple method of investigation for the susceptibility to cracking of different materials is to take round tensile test specimens from the physical weld simulator after a coarse grain cycle and test them at different temperatures with a hot tensile testing machine. The reduction of area is determined after the test and used as the degree of sensibility towards relaxation embrittlement. In Figure 14 it can be seen, that the susceptibility to cracking rises with increasing PWHT temperature and increasing materials toughness. The investigation of the charpy V-notch energy showed for all analyzed steels (S69QL with.16%c, S69QL1 with.9%c and S7M with.4%c) a significant impairment of the toughness in the coarse grained zone because of the PWHT. If the PWHT temperature was reduced to 5 C, the fracture toughness of the material S7M in the CGZ was significantly improved up to the level of the as welded condition. Transition Übergangstemperatur Temperature [ C] [ C] as welded S7M S69QL1 S69QL Peak Spitzentemperatur Temperature [ C] Peak Spitzentemperatur Temperature [ C] Figure 16: Physical welding simulation, effect of peak temperatures on the transition temperature for 5% shear fracture at the charpy V-notch impact test, condition: as welded (left) and after post weld heat treatment (right) PWHT (56 C) S7M S69QL1 S69QL

6 3. Outlook The most advanced production method for high strength heavy plates is the thermomechanical rolling with subsequent intensive cooling. In comparison to the commonly quenched and tempered steel this method gives the opportunity to produce high strength materials with excellent toughness properties using lean alloy compositions and low carbon contents. The investigations show that when using advanced TM-steels the PWHT temperature can be decreased or the subsequent treatment can totally be left out since the hardenability is relatively low. This would lead to a definite improvement of the HAZ toughness in the CGZ and result in a reduction of relaxation cracking at high strength steels. A further development of weld metal for advanced high strength construction steels was necessary since the strength of the weld metal was not possible to be increased to the desired extend through alloying because of the base metal having a lean alloy composition. 4. References 1 Jungreithmeier, A., R. Grill und R. Schimböck: Metallurgie und Walztechnik zur Erzeugung sauergasbeständiger Röhrenbleche bei Voestalpine in Linz. BHM, 148 (23) 2 Yahya, O.M.L., et al.: Statistical modelling of intergranular brittle fracture in a low alloy steel, Centres des Matériaux de l Ecole des Mines de Paris; Fatigue & Fracture of Engineering Materials & Structures, Wilson, A.D.: Calcium treatment of plate steels and its effect on fatigue and toughness properties, Offshore Technology Conference, 3 April-7 May 1979, Houston, Texas 4 Listhuber, F.E., F. Wallner, F.M. Oberhauser und G. Aigmüller: Mikrolegierte Feinkornbaustähle. BHM 114 (1969) 5 Akihide, N., I. Takayuki und O. Tadashi: Development of YP 96 and 11 MPa Class Ultra High Strength Steel Plates with Excellent Toughness and High Resistance to Delayed Fracture for Construction and Industrial Machinery, JFE technical report, No. 11, 28 6 Grill, R.: Thermomechanical Rolling of Heavy Plates, PhD. Thesis, University of Vienna, 27 7 Kayano, R., Y. Nitta und S. Sato: Development of High Toughness X7 Grade Clad Steel Pipe for Natural Gas Transportation Pipelines, the Japan Steel Works, Ltd., 28 8 Felfer, P.: Microscopical and Sub-microscopical Characterization of a Heavy Plate made of a Micro-alloyed HSLA-Steel, Montanuniversität Leoben, 28 9 Cerjak, H. and M. Pudar: Untersuchung der Schweißbarkeit des Werkstoffes ALDUR 7QL1, Institut für Werkstoffkunde und Schweißtechnik, Technische Universität Graz, 27 6

High-strength heavy plates. Weight savings combined with excellent weldability. voestalpine Grobblech GmbH

High-strength heavy plates. Weight savings combined with excellent weldability. voestalpine Grobblech GmbH High-strength heavy plates Weight savings combined with excellent weldability voestalpine Grobblech GmbH www.voestalpine.com/grobblech High-strength and wear-resistant steels voestalpine Grobblech quantity