Optimization and Stability of Production of Heavy Gauge EH47 Ship Plate. Synopsis

Transcription

1 Optimization and Stability of Production of Heavy Gauge EH47 Ship Plate Douglas G. Stalheim* Synopsis The movement of products around the world via large container ships continues to be the main mode of transportation with load carrying capacity beyond the typical normal load capacity of 8000 TEU to now upwards of 15,000 TEU. As the load carrying capacity is increased the use of thicker plates up to 100 mm with higher strength, ABS EH47, with excellent low temperature toughness are required. The combination of thicker plates, higher strength and excellent low temperature toughness creates challenges for the plate producers to balance these requirements during production. Qualification of plate for these larger vessels requires passing a rigorous full thickness large plate toughness/crack arrest test called the ESSO test. To create the proper cross sectional metallurgy suitable for passing the challenging ESSO test, proper niobium microalloy technology and steelmaking/plate processing is required. Optimization and stabilization of mechanical properties of strength and especially low temperature toughness within the confines of normal variations in steel production of plates with thicknesses up to 100 mm requires a proper alloy/slab/process design. This paper will briefly describe the alloy/slab/processing strategy for optimum and stable through thickness mechanical properties in heavy gauge plate for high strength low temperature toughness large container ship applications. Keywords: Grain Size, Distribution, Niobium, Process Strategy, Metallurgy * Bachelor of Science in Metallurgical Engineering, South Dakota School of Mines and Technology, President, DGS Metallurgical Solutions, Inc., Vancouver, WA USA Consultant CBMM Technology Suisse SA, Geneva, Switzerland 1

2 Introduction In the movement of goods around the world, economy of scale, environmentally improved and energy efficiency are becoming driving forces in the design of container ships. The size of a container ship is expressed in units of TEU (twenty-foot (6.1 m) equivalent unit). One TEU is a 20-foot x 8-foot x 8-foot cargo container. Figure 1 shows 1-TEU and a 6-TEU configuration (1). 1-TEU cargo container (20 x8 x8 ) 6-TEU 2 40 foot containers (white) 4 total TEU, and 2 20 foot containers (red) 2 total for a combined 6-TEU. Fig. 1 Example of definition of cargo container ship TEU To accomplish the goals of economy of scale, environmental improvement and energy efficiency the TEU size of container ships has been increasing since the 1960 s, Figure 2 (2). Production of Ultra Large Container Vessel (ULCV) with 14,500 TEU and larger capacity has been increasing in the past 3 years. Cargo container ship sizes have been increasing since the 1960 s ULCV 18, 000 TEU Maersk Triple-E Class Container Ship Fig. 2 Examples of increasing cargo ship sizes 2

3 However, to realize the goals of economy, energy and environmental improvements of these ULCV container ships, the structural steel used in the hull designs have been modified for higher strength, thicker section sizes with improved toughness. Figure 3 shows hull design grade/thickness of structural steel for the ULCV container ships. Table 1 shows key attributes and changes in grade/thickness along with mechanical property requirements for the ULCV container ship. Fig. 3 ULCV hull design Table 1: Key Attributes, Grade/Thickness Changes and Mechanical Properties for ULCV Hull Design Requirement Large Size/Hatch Opening, Complex Loading, Longitudinal/Buckling/ Torsional Strength Yield Strength Increased to EH47, Potential Weld Joint Flaws, Plain Strain State Requirements Attribute Ensure Hull Integrity Ensure Hull Safety Grade/Thickness Change to Ensure Hull Integrity/Safety Grade: EH36/EH40 to EH40/EH47 Thickness: EH40/68 mm to EH47/85 mm Impact Toughness: Energy -40 C, center thickness location Fracture Toughness: CTOD C Crack Arrest Toughness: Kca 6000N/mm 3/2 (ESSO Test) The move to these ULCV container ships in critical hull sections as noted not only has increased the required plate thickness but also higher strength with improved cross sectional low temperature toughness requirements. Plate thicknesses from 50 mm up to 100 mm and hull construction grades of ABS EH47, Table 2 (Chemistry) Table 3 (Mechanical Properties), or equivalent societal ship standard are now required (3). Table 2: ABS EH47 Specification Chemistry Requirements C Mn P S Si Cu Ni Cr Mo V Nb Ti Als N2 B NR

4 Yield (Mpa) Table 3: Typical Specification Mechanical Property Requirements for EH47 or Equivalent Tensile (Mpa) Elongation % Average Longitudinal/Transverse Charpy Energy - 40 C Weld Tensile Strength (Mpa) HAZ Average Charpy -40 C -10 C (mm) ESSO Toughness, - 10 C (N/mm 3/2 ) / In general, making strength in these thick plate section sizes is not the main issue in the stable production of heavy gauge EH47. The critical issue that is difficult to control in production of these thick plate section sizes is the cross sectional low temperature toughness requirements and in particular meeting the thru thickness ESSO toughness test requirements. The thickness/width required of these plates is such that the metallurgical reduction ratio (slab thickness after dimensional rolling passes divided by final plate thickness) is between 3:1 and 5:1 for many of the world plate producers making the overall total reduction less than desirable for stable cross sectional low temperature toughness properties (4). The ESSO Test is a temperature gradient full thickness wide plate test designed to assess a structural steel fracture toughness characteristics (brittle crack arrest toughness) intended for ship hull structure applications. The test is intended for plate thicknesses > 50 mm and can test the base plate EH47 along with simulation of high heat input welding of the base plate EH47 to deck plate EH40 (duplex ESSO test). The ESSO test is a requirement that must be meet to be qualified for the construction of ULCV container ships per the societal ship codes such as Japan s Nippon Kaiji Kyokai or commonly known as Class NK (5). Examples of the ESSO test can be seen in Figure 4. Symbols used and their meanings Conceptual view of test specimen, tab and load jig. Shape and size of test specimen Necessary conditions of arrest crack position 4

5 Example of ultra-large width duplex ESSO test for welding Example brittle crack is 207 mm long Example brittle crack is 270 mm long Equation for Kca calculation Example of crack length vs. arrested temperature and Kca Metallurgical Strategy for Optimization Fig. 4 Examples of ESSO test The key to achieving optimized, stable cost effective production of heavy gauge EH47 capable of meeting the challenging cross sectional toughness requirements is to create as fine and homogenous as possible cross sectional grain size/microstructure. In addition, creating as high of a volume fraction of high angle (>15 ) grain boundaries will further enhance toughness. The desired microstructure for this grade is ferrite/acicular ferrite (low carbon bainite). To produce the desired cross sectional grain size, metallurgical reduction (slab thickness after rolling dimensional passes are completed vs. final plate thickness), should be as large as possible with an absolute minimum of 3:1. Proper generation of recrystallization behaviors in roughing and finishing by optimization of Nb metallurgy and per pass reduction strategy is an absolute critical strategy that needs to be implemented for stable/optimized cost effective production of EH47 for these ULCV applications. The critical path in the metallurgical strategy is to achieve >200 J average charpy energy at the center thickness to have any chance of passing the ESSO or CTOD testing, Figure 5 (6). If the cross sectional 5

6 grain size cannot be properly optimized to meet the toughness requirements, the only option is to add costly additions of Ni in the % to assist in the low temperature toughness performance. Table 4 is an example of a typical EH47 heavy gauge alloy design. Fig. 5 Average center thickness charpy performance required for success in ESSO testing Table 4: Example of a typical EH47 heavy gauge alloy design C Mn 1 P S Si Cu 1 Ni 2 Cr 1 Mo 1 V Nb 3 Ti 4 Als N2 B NIA.030 NIA ppm 1 Depends on mill capabilities 2 Ni is used to promote good low temperature fracture toughness performance. How much Ni is required depends on effectiveness of implementation of Nb metallurgical rolling strategy. Ni can be reduced as optimized Nb metallurgical strategy is implemented. 3 Nb needs to be optimized for a given mill s capability to create the proper recrystallization behaviors during rolling. Proper Nb optimization for cross sectional grain size/distribution the less Ni is required. 4 Ti should be sub-stoichiometric to N2 in the Ti:N range. Key metallurgical/processing strategies that should be implemented for stable/optimized production of EH47 heavy gauge plate that can meet the challenging fracture toughness requirements are as follows (7): 1. Steelmaking/Casting (8) a. Vacuum degassing for hydrogen removal to the ppm range with 4 ppm being maximum to control hydrogen cracking/embrittlement issues. Lower is always better. b. Internal slab quality centerline alloy segregation/core unsoundness rating of Mannesmann scale 2.0 or lower or equivalent. (9) i. Casting machine mechanical condition ii. Superheat C iii. Proper mold/spray chamber water cooling temperature/strategy iv. Proper casting speed c. Calcium treated with proper inclusion/steel cleanliness controls. i. Sulfur 0.003% ii. Total O2 <30 ppm, preferably <20 ppm. 6

7 iii. Total LMF time average of 45 minutes iv. Low flow argon rinse 3-5 minutes prior at the end of the LMF cycle. A two-step low flow argon rinse with the first rinse of 5-8 minutes prior to calcium treatment followed by the final 3-5 minute rinse after calcium treatment is optimum for total O2 control and cleanliness. 2. Proper slab reheat temperature for metallurgy/rolling, typically C for this application. 3. Type I Static Recrystallization behavior > 50% total deformation after any dimensional width rolling passes and Type II No-recrystallization behavior (pancaking) > 30%. (10) 4. Slab thickness/width design needs to assure that the metallurgical reduction ratio is >3:1. Preferably >5:1. This means that in some cases only straight-away (as-cast slab width equals the final plate width) rolling can be used to maintain the minimum 3:1 metallurgical reduction ratio and the proper percentage balance of Type I and Type II recrystallization behaviors. 5. Proper roughing/finishing transfer thickness and per pass reduction strategy to achieve the desired recrystallization behavior responses and corresponding thru thickness grain size/distribution, Figure 6 (11). Example of establishing the proper transfer thickness for stable/optimum metallurgy in straight-away rolling Example of establishing the proper transfer thickness for stable/optimum metallurgy in width dimension/broadside rolling Example of key per pass reductions in roughing/finishing for optimum Nb performance and corresponding recrystallization behavior and cross sectional grain size/distribution. Example of importance of the last roughing pass per pass reduction on cross sectional grain size homogenization. Fig. 6 Examples of determining proper transfer thickness and rolling per pass reduction strategy for stable/optimum EH47 cross sectional toughness performance. 7

8 6. Proper use of valid equations for determining key metallurgical temperatures/processing parameters such as Nb solubility, RST (Recrystallization Stop Temperature), Ar3 (austenite to ferrite start temperature) and Bs (bainite start temperature). Valid equations for determining key metallurgical temperatures/ processing parameters are as follows (12,13,14,15, 16, 17): a. NbCN Solubility (Irvine): C = (-6770/( LOG10(Nb*(C+12/14*N2)))) b. NbC Solubility (Nordberg): C = (-7510/(-2.96+LOG10(Nb*(C))) c. RST/Tnr (Bai): C = (174log(Nb*(C+12/14*N))+1444( C))-75 ( C) d. Ar3 (Ouchi Modified): C=( *(C+(Mn+Mo)/ Cu/15.5+Cr/20.67+Ni/5.636)+16*((FPT)-0.315)-32)*5/9 e. Bs (Bodnar): C = *C-63*Mn-16*Ni-78*Cr f. Bs (Kirkaldy): C = *C 35*Mn 75*Si 15*Ni 34*Cr 41*Mo 7. Implementation of mean flow stress analysis of actual per pass rolling mill data to evaluate if the proper Nb metallurgy/recrystallization behaviors are occurring. 8. Post rolling cooling parameters need to be optimized to achieve the following key points for optimized cross sectional toughness properties (18): a. Proper microstructure of polygonal ferrite/acicular ferrite. b. High volume fraction of high angle grain boundaries (HAGB > 15), c. Fine/homogenous cross sectional gain size. d. These three points come from proper control of the cooling rate and final cooling temperature. A relatively high cooling rate and lower cooling stop temperature are needed to create the balance of microstructure, HAGB and cross sectional grain size. Results Mills that have implemented the alloy/process/metallurgical strategies have successfully produced heavy gauge EH47. Examples of 50 mm EH47 plate with various Nb levels and two different Ni levels effect on cross sectional strength and toughness can be seen in Figure 7 (19). It can be seen the positive effect of implementation of the proper amount of Nb and the proper processing on the center thickness strength and toughness. This shows that a Nb level of 0.040% is a minimum amount for cost effective stable mechanical property performance. In addition, as can be seen the Ni can be reduced by 50% and still achieve excellent low temperature center thickness toughness. Further work shows that with optimized Nb metallurgy further reduction of Ni can be realized from 0.44% to 0.25%, Figure 8. Again, a Nb level of 0.045% is optimum as seen by the increase in elongation and the ability to reduce Ni with no adverse effect on charpy toughness, CTOD and Kca fracture toughness. CTOD performance of 50 mm and 90 mm EH47 plate with optimized Nb/Ni at -10 C averaged 1.02 mm and 1.03 mm respectively. Optimizing the processing as illustrated in Figure 6 in rolling to achieve the proper recrystallization behaviors and thru thickness austenite grain size is a key parameter. Figure 9 shows optimized and non-optimized rolling and the corresponding thru thickness charpy and ESSO test performance on 65 mm EH47. Not only having the proper/optimized rolling process, but having sufficient Nb to accommodate any processing deviations that may occur in production is also very important. 8

9 For mechanical property stability, primarily toughness, during the production a sufficient Nb level in the % can help in compensating for production processing deviations, especially in post rolling cooling. Figure 10 shows Nb vs. quarter and center thickness charpy performance when post rolling cooling production processing deviations occur. It can be easily seen that the increasing Nb content helps to maintain cross sectional charpy stability to -60 C. Cost effective welding procedures for heavy gauge plate have been successfully developed on all the steels shown in the results. Yield strength vs. ¼ and center thickness vs. Nb content. Tensile strength vs. ¼ and center thickness vs. Nb content. Note that at 0.040% Nb, the center thickness and ¼ thickness strength are similar suggesting good homogenization of cross sectional grain size. Example of various Nb and Ni levels vs. center thickness TCVN. A minimum average of C (red and black lines) is required to pass the ESSO testing. Fig. 7 Nb and Ni vs. strength and toughness for 50 mm EH47 plate 9

10 Strength vs. Nb/CE Elongation vs. Nb/CE, note the improvement in elongation with the higher Nb content. Average TCVN charpy performance vs. Nb/CE Kca fracture toughness for 0.045% Nb and 0.25% Ni alloy design vs. plate thickness Fig. 8 Examples of optimized Nb/Ni content for cost effective stable EH47 heavy gauge (85 mm) structural plate performance. Example of the importance of optimizing the rolling schedule to improve cross sectional toughness performance. Corresponding ESSO test results for optimized rolling schedule 1. Fig mm EH47 optimized vs. non-optimized rolling schedule and corresponding charpy and ESSO test performance. 10

11 Fig. 10 Example of Nb vs. cross sectional charpy performance during production process deviations. Conclusions With the proper understanding and implementation of the alloy/process/metallurgical design discussed, optimum cross sectional toughness can be achieved in heavy gauge EH47 plate up to 100 mm. In addition, with implementation of proper Nb metallurgy ( %), costly alloy additions of nickel can be minimized from 0.90% to 0.25%. Examples have been given of various alloy, processing and plate thickness and successful production of cost effective stable/optimized mechanical property performance of EH47 heavy gauge plate intended for ULCV applications. References and 3. American Bureau of Shipping, Application of Higher-Strength Hull Structural Thick Steel Plates in Container Carriers, February 2009 (Updated February 2017). 4. Stalheim, D., Slab and Level 2 Automation Design Guidelines for Optimum Metallurgy and Productivity for Plate and Steckel Mills, Proceedings and Presentation of HSLA2010 Conference, Beijing, China, Nippon Kaiji Kyokai, Guidelines on Brittle Crack Arrest Design, Stalheim, D., Basic Discussion of Successful Development of Heavy Gauge ABS Eh40/EH47 Structural Ship Plate, DGS Metallurgical Solutions, Inc. internal training presentation, Stalheim, D., Processing of High Strength Nb-microalloyed Steel Plates for Offshore and Marine Applications, DGS Metallurgical Solutions, Inc. internal training presentation, SEAISI Continuous Casting Training, Understanding Metallurgy Involved in Continuous Casting and How This Can Improve Product Quality, 2016 SEAISI Conference & Exhibition, Hanoi, Vietnam,

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