Copyright © 2019 O.H. Kurpe, V.V. Kukhar.This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Problems of Tribologyy, 92 (2) (2019)33-41 Problems of Tribology Website: http://tribology.khnu.km.ua/index.php/ProbTrib E-mail: tribosenator@gmail.com DOI: 10.31891/2079-1372-2019-92-2-33-41 Pilot production development of plate, steel grade EH36 applying thermo- mechanical controlled process at the rolling mill 3600 O.H. Kurpe1*, V.V. Kukhar2 1Metinvest holding, LLC, Mariupol, Ukraine, 2Department, Pryazovskyi State Technical University, Mariupol, Ukraine *E-mail: aleksandr.kurpe@gmail.com Abstract For the first time, technology has been developed, and experimental batch of shipbuilding steel plates (Grade ЕН36, 25 × 2150 × 8000 mm) has been produced according to the requirements of classification societies (Bureau Veritas Rules), at the rolling mill 3600, PJSC “AZOVSTAL IRON & STEEL WORKS”. The technol- ogy is developed using general requirements for the manufacture of rolled products with the thermo-mechanical controlled process and mathematical model of the rolling process, on the basis of which the calculations of de- formation schedules have been performed. The technical possibility of shipbuilding steel plates (Grade ЕН36, 25 × 2150 × 8000 mm) producing using thermo-mechanical controlled process (TMCP) instead of heat treatment accessing by normalization in accordance with Bureau Veritas Rules has been conformed at the rolling mill 3600. Introduction of the thermo-mechanical controlled process instead of normalization will reduce the production cost of the rolled products by eliminating natural gas consumption for heat treatment (normalization). To prepare the certification, in accordance with Bureau Veritas Rules, it is necessary to conduct an additional series of investigations for the rolled products (Grade EH36) of different thicknesses. Key words: thermo-mechanical controlled process, hot rolling plate, shipbuilding steel, rolling mill technology. Introduction Having a significant number of benefits, thermo-mechanical controlled process (TMCP) has become more widespread in the production of almost all types of rolled products. As a result, consumers have received products that meet up-to-date quality requirements, and producers have an additional opportunity to reduce their cost production thereby maintaining, and, in some cases, greatly improving their competitiveness. Nowadays, an extensive list of international standards make it possible to produce similar products in various ways, whether it be conventional hot rolling, normalizing rolling, thermo-mechanical controlled process (TMCP), or with the use of heat treatment. However, struggle for production cost, competition and product quality requirements assign clear priorities for the production method. There is a separate list of special-purpose product mix, which is still produced with the use of heat treatment only. Thus, it is very important to undertake a series of studies and master the production of rolled products using thermo-mechanical controlled process (TMCP) instead of other more cost-effective methods, if the relevant standard for the product so allows. Review of recent studies and publications The worldwide development and dissemination of thermo-mechanical controlled process (TMCP) technology began in the 1960s of the last century [1]. In our country, the technology started to be introduced from production of rolled plates 10 years later [2]. Since that time, TMCP technology has evolved and relevant equipment has undergone drastic changes [3 - 5]. Now, this production method is constantly developing and spreading to various rolled products. 34 Problems of Tribology Today, TMCP allows producing plates and coils for the design, construction, and production of pressure vessels and pipelines, the study of which is described in a large number of papers prepared by the authors from different countries around the world [3 - 18]. TMCP was predominantly used to produce low carbon steel plates [13, 14]. However, TMCP study with the use of steels, which carbon content is up to 40 %, has become more popular recently [19]. The thorough study and introduction of steels with a carbon content of 0.06 % and below was further developed with in-depth research of hardening processes and obtaining additional properties [8, 10, 15, 20, 21]. The research on the effect of various cooling rates during TMCP on the structure and properties of the rolled products is of particular interest [11, 22] and demonstrates the effect of substantial increase in properties with acceleration in cooling rate. If the equipment allows, state-of-the-art technology will enable the production of rolled products with a yield strength of up to 800 MPa and more, but new higher levels of properties create also new challenges that require further study [12, 15]. With the development of technology and in-depth study of its influence on new properties of the rolled products, forecasting of the technology implementation results through the simulation of the development of mi- crostructure and mechanical properties proposed by the paper authors [6, 7] became more widespread. The great list of scientific papers shows that TMCP technology mastering for further production re- quires conducting a number of studies to demonstrate the capabilities of technology, equipment and obtain ap- propriate quality in accordance with the requirements of standards and customers [23 - 29]. The development of thermo-mechanical controlled process technology to produce shipbuilding steel plates (Grade EH36, 25 × 2150 × 8000 mm) instead of heat treatment by normalization at the rolling mill 3600 is a relevant target to ensure manufacture of the rolled products in accordance with modern requirements and allow reducing production costs. Problem definition The paper objective is to develop the technology of high-strength shipbuilding steel plate production (Grade ЕН36, 25 × 2150 × 8000 mm) in strict compliance with Bureau Veritas Rules at the rolling mill 3600, PJSC “AZOVSTAL IRON & STEEL WORKS”. Statement of basic materials The existing equipment of the rolling mill 3600 currently consists of five duplicate pusher-type reheating furnaces, which are able to heat the slabs with a thickness of 130 - 350 mm, width of 1100 - 1920 mm, length of 920 - 3420 mm, and weight of 1.8-16 tonnes; descaler with a pressure of 150 atm; universal roughing stand, which provides for maximum rolling force of 11 MN with the vertical rolls and 46 MN with the horizontal rolls; duplicate carriage to transfer the rolled products with a thickness of 50.8 mm and more to further heavy plate treatment area; finishing reversing stand with a maximum rolling force of 46 MN (roughing and finishing stands are equipped with built-in 150 atm descalers on both sides); SMS Demag accelerated controlled cooling unit, including a cooling section of 25600 mm, 24 top and 24 bottom collector headers for laminar cooling (cool- ing rate is 12 - 45 °С/s); SMS Demag 9-roller hot-plate leveling machine; transverse cutting machine. Layout of the main equipment of the rolling mill 3600 is shown in Fig. 1. Fig. 1. Layout of the main equipment of the rolling mill 3600: 1 – reheating furnaces; 2 – descaler; 3 – universal roughing reversing stand; 4 – carriage for heavy plate transfer; 5 – finishing reversing stand; 6 – accelerated controlled cooling unit; 7 – roller hot-plate leveling machine; 8 – transverse cutting machine The mill produces steel plates for structural, engineering, shipbuilding purposes, as well as for construc- tion of the offshore drilling platforms, for the manufacture of large diameter high-pressure electric-welded pipes and other purposes. The rolled products have the following dimensions: thickness of 6-200 mm, width of 1500 - 3300 mm, length of 6000 - 24400 mm. Problems of Tribology 35 It should be noted that currently high-strength shipbuilding steel plates (Grade EH36) with a thickness of up to 50 mm are produced at the rolling mill 3600 using heat treatment – normalization, which is stated in the certificate No. 08458/D0 BV issued by Bureau Veritas. According to Bureau Veritas Rules, steel plates (Grade EH36) with a thickness up to 100 mm can be produced with normalization or thermo-mechanical rolling process. Following Bureau Veritas classification, NR 216 Rules, Chapter 2 (clause 1.7.3) [30], thermo- mechanical rolling, TM (thermo-mechanical controlled process, TMCP), provides for strict control of metal tem- perature and reduction during rolling. In general, we may observe larger percentage of the reduction at a tem- perature close to the temperature Ar3, which can involve rolling in the two-phase region. The following normali- zation or other types of heat treatment cannot reproduce the post-TM (TMCP) properties. The accelerated con- trolled cooling (ACC) after rolling can be used by special approval of the company, which makes it possible to improve mechanical properties through the controlled cooling with a rate greater than ambient air cooling rate. The slabs (220 × 1850 × 1590 - 1600 mm, 5.080 - 5.112 tonnes, heat No. 2104917) with the chemical composition given in Table 1 were used in experimental rolling. The chemical composition of the experimental slabs fully meets the Rules on Materials and Welding for the Classification of Marine Units (NR 216), Chapter 2, established for the thermo-mechanical controlled proc- ess of rolled product manufacture (Grade EH36). For comparison of chemical compositions, Table 1 demonstrates a typical chemical composition, which is currently used to produce the Grade EH36 with heat treatment – normalization. The chemical composition shown herein has a slight deviation from the composition used in the experimental rolling. However, excess of the prescribed carbon equivalent limit does not allow its use for the thermo-mechanical controlled process. Table 1 Slab parameters for experimental rolling Chemical element content, % Steel grade Heat No. СE C Mn Si S P Nb Cr Ni** Cu V** Ti** Mo Al H EH36 2104917 0. 38 0. 16 1. 28 0. 25 0. 00 40 0. 01 9 0. 03 3 0. 03 0. 03 0. 03 0. 00 3 0. 01 2 0. 00 5 0. 04 4. 9 EH36 typical* 0. 37 -0 .4 1 0. 14 -0 .1 6 1. 32 -1 .3 5 0. 22 -0 .2 6 0. 00 5- 0. 01 1 0. 00 9 - 0 .0 18 0. 03 2- 0. 03 8 0. 03 -0 .0 8 0. 02 -0 .1 3 0. 02 -0 .0 5 - - - 0. 02 8- 0. 04 0 3. 1 - 4. 7 ≤ 0. 38 ≤ 0. 18 0. 90 -1 .6 0 ≤ 0. 50 ≤ 0. 03 5 ≤ 0. 03 5 0. 02 -0 .0 5 ≤ 0. 20 ≤ 0. 40 ≤ 0. 35 0. 05 -0 .1 0 ≤ 0. 02 ≤ 0. 08 ≥ 0. 01 5 - BV Rules, NR 216, Chapter 2 Amount of Nb + V + Ti ≤ 0.12%. Carbon equivalent (CE) is calculated according to the following formula:     6 5 15 Cr Mo V Ni CuMn CE C        Notes: * typical chemical composition, which is used for the current production of the specified product mix with the use of heat treatment – normalization;** elements mentioned above are used separately or in combination. When designing technology, there have been used the principles of controlled rolling at low temperatures, in particular, formation of the necessary structure and properties of rolled products when the deformation ends in the two-phase γ - α region. The plates (25 × 2150 mm, Grade EN36), which are currently produced with the normalization, have been taken as the experimental product mix. The following target thermo-mechanical parameters of the rolling process have been calculated: deformation degree after receiving the rolled product width in the roughing stand of no less than 15 %; temperature conditions in the finishing stand; initial deformation temperature of 750 - 770 °C; final deformation temperature of 740 - 720 °C. The technology design has been based on the integrated mathematical model of the rolling mill 3600. The target rolling pattern in the roughing stand is a transverse and longitudinal pattern with an increase in deformation, which provides on average constant rolling force of 23 MN between the passes. The thermo- 36 Problems of Tribology mechanical controlled process has been introduced at the rolling mill 3600 in accordance with the developed temperature and deformation conditions (Tables 2, 3). During the study, there have been used two patterns: longitudinal-transverse-longitudinal (breakdown with pulling) for batch No. 7052 and target-transverse-longitudinal for batch No. 7053. Comparison of the calculation results with the actual rolling parameters is presented in Table 2 and 3. The actual rolling parameters, including the deformation degree after the breakdown in the roughing stand, and the temperature conditions in the finishing stand were met. The rolling force variation in the passes in the roughing and finishing stands is shown in Fig. 2 and 3. Table 2 Target and actual parameters of plate production (25 × 2150 mm, Grade ЕН36) in the roughing stand at the mill 3600 Actual parameters Target parameters batch No. 7052 batch No. 7053 Pa ss n um be r th ic kn es s, m m de fo rm at io n de gr ee , % ro lli ng fo rc e, M N × 10 0 te m pe ra tu re , ° С th ic kn es s (c lo ck fa ce ), m m th ic kn es s (r ec al cu la te d) *, m m de fo rm at io n de gr ee , % ro lli ng fo rc e, M N × 10 0 te m pe ra tu re , ° С th ic kn es s (c lo ck fa ce ), m m th ic kn es s (r ec al cu la te d) *, m m de fo rm at io n de gr ee , % ro lli ng fo rc e, M N × 10 0 te m pe ra tu re , ° С 0 T221.7 - - - 221.7 - - - - T 221.7 - - - - 1 190 14.3 2045.6 1150 200 202.2 9.6 2250 1128 180 182.7 21.3 2500 1125 2 154 18.9 2386.9 1146 180 182.3 10.9 2300 - 152 154.2 18.5 2250 - 3 T 131 14.9 2299.5 1142 145 147.7 23.4 2500 - T 115 119.3 29.3 3420 - 4 110 16.0 2300.4 1136 К 123 127.4 15.9 3500 - 90 93.9 27.1 3200 - 5 91 17.3 2302.9 1129 100 103.9 22.6 3200 - 72 75.2 24.9 2820 - 6 75 17.6 2202.7 1121 T 72 76.1 36.5 3300 - - - - - - Notes: T – means turning; * actual thickness of the feed recalculated with due regard to the stand stiffness Table 3 Target and actual parameters of plate production (25 × 2150 mm, Grade ЕН36) in the finishing stand at the mill 3600 Actual parameters Target parameters batch No. 7052 batch No. 7053 Pa ss n um be r th ic kn es s, m m de fo rm at io n de gr ee , % ro lli ng fo rc e, M N × 10 0 te m pe ra tu re , ° С th ic kn es s (c lo ck fa ce ), m m th ic kn es s (r ec al cu la te d) *, m m de fo rm at io n de gr ee , % ro lli ng fo rc e, M N × 10 0 te m pe ra tu re , ° С th ic kn es s (c lo ck fa ce ), m m th ic kn es s (r ec al cu la te d) *, m m de fo rm at io n de gr ee , % ro lli ng fo rc e, M N × 10 0 te m pe ra tu re , ° С 0 75 76.1 75.2 1 71 5.3 2427.1 760 66 66.8 12.2 3450 768 65 65.6 12.8 3370 767 2 65 8.5 3419.2 756 56 58.2 12.9 4300 57 58.4 11.0 3820 3 59.5 8.5 3322.9 754 48 50.3 13.6 4370 51 52.6 9.9 3950 4 54 9.2 3452.6 752 42 44.2 12.1 4320 45 46.6 11.4 3950 5 49 9.3 3345.3 750 38 39.1 11.5 3670 40 41.5 10.9 3900 6 44 10.2 3501.9 747 34 35.1 10.2 3650 36 37.3 10.1 3750 7 39.5 10.2 3393.9 744 31 31.6 10.0 3370 32 33.3 10.7 3770 8 35 11.4 3570.2 741 29 29 8.2 3020 29 30 9.9 3570 9 31 11.4 3458.9 737 29 26.6 8.3 1550 738 27 27.3 9.0 3200 10 27.5 11.3 3274.6 736 26.5 25.6 6.2 2470 721 11 25 9.1 2654.1 734 Notes: T means turning; * actual thickness of the feed recalculated with due regard to the stand stiffness Problems of Tribology 37 The rolling pattern in the roughing stand, which was used in the production of the batch No. 7053, is more appropriate, since it allows, with an increase in the deformation degree, reducing number of passes and, with an increase in rolling forces in the passes 3, 4 and 5, reducing rolling cycle. This pattern is closer to the target one. The rolling pattern used in the production of the batch No. 7052, owing to the phase changes in the longitudinal (passes 1 - 3, 6) and transverse (passes 4 - 5) rolling, with due regard to the target pattern, resulted in the rolling force growth due to increase in the rolling product width in transverse passes. When compared with the batch No. 7053, this pattern has an increased number of passes, therefore, it is longer-term and more energy-consuming. After rolling and final cooling, samples were taken from the rolled products to conduct a series of mechanical tests (main and additional) in accordance with BV Rules, NR 216, Chapter 2, and BV Rules, NR 480 Approval of the Manufacturing Process of Metallic Materials. The test results are shown in Table 4 and Table 5. Fig. 2. Rolling force variation in the passes in the roughing stand when calculating target conditions (1) and actual rolling of the batches No. 7052 (2) and No. 7053 (3) Fig. 3. Rolling force variation in the passes in the finishing stand when calculating target conditions (1) and actual rolling of the batches No. 7052 (2) and No. 7053 (3) The rolling patterns used in the roughing stand are shown in Fig. 4. In general, the implemented scheme of the thermomechanical rolling process are shown in Fig. 5. Fig. 4. Rolling pattern in the roughing stand, actually used in the batch No. 7053 (1) and actually used in the batch No.7052 (2) 38 Problems of Tribology Fig. 5. The implemented scheme of the thermomechanical rolling process in general Table 4 Main test results for the rolled products in accordance with BV Rules, NR 216, Chapter, and BV Rules, NR 480 Test results Type of tests BV requirements batch No.7052 batch No.7053 Data compared with the post heat treatment and normalization condition Rolled product thickness, mm 25 25 23-27 Yield strength, ReH, MPa, min. 355 437 450 367-426 Tensile strength, Rm, MPa 490-630 550 560 496-540 Elongation, А5, %, min. 21 30 30 25-36 Average impact energy, J КVL-40 (longitudinal test) 34 144/166/155 142/125/149 194/204 КVТ-40 (transverse test) 24 99/105/107 116/107/117 To compare the existing technology with the experimental rolling, Table 4 shows the test results for the rolled products manufactured with normalization and using TMCP technologies. Table 5 Additional test results for the rolled products in accordance with BV Rules, NR 216, Chapter, and BV Rules, NR 480 Test results Type of tests batch No. 7052 batch No. 7053 1 2 3 Tensile test (with stress relief) at 600°С (2 min/mm), min. 1 hour Yield strength, ReH, MPa, min. 435 459 Tensile strength, Rm, MPa 548 556 Elongation, А5, %, min. 30 31 Impact test on the specimens without ageing, J КVL0 171/192/187 149/177/182 КVL-20 185/187/192 173/188/146 КVТ-20 143/135/132 132/142/136 КVL-40 144/166/155 142/125/149 КVТ-40 99/105/107 116/107/117 КVL-60 113/127/117 104/112/115 КVТ-60 80/72/81 77/96/86 Impact test on the specimens with deformation aging, J КVL-20 158/153 184/166 КVL-40 107/113 140/116 T em pe ra tu re Time Problems of Tribology 39 Table continuation 5 1 2 3 КVL-60 48/10 70/84 Fiber-60, % 40/0 90/100 Non-metallic inclusions (average / maximum rating) Brittle silicates СХ, points 2.1/2.5 0.8/1.5 Undeformed silicates, СН, points 2.0/2.5 1.5/2.0 Sulfur prints In the axial zone, there is sulfur accumulation in the form of spots and individual short irregular segregation lines Microstructural examination Ferrite-pearlitic, with an increase in the perlite component fraction in the axial zone Size distribution 9; 10 9; 10 Through-thickness tensile testing, Z, % 61/67 60/52 Thus, the results of the main mechanical tests of rolled products manufactured according to thermo- mechanical rolling conditions, fully meet the requirements contained in BV Rules. In general, additional tests also demonstrated positive results. The only exceptions were impact test results for the samples taken from the batch No. 7052 with deformation aging and assessment of the fiber share in the fracture at -60 °C, which showed a lower level of properties. These results require additional testing and a microstructure study. Slight increase in the properties obtained during tensile tests of the batch No. 7053 can be attributed to the use of more intense rolling schedule in the roughing stand. Thermo-mechanical rolling allowed obtaining a finer ferrite grain (10; 9 points) in comparison with normalization (8; 9 points) for the comparable product mix. When comparing with the tensile strength test results on the comparable product mix manufactured with normalization, thermo-mechanical rolling provides a higher level of yield strength and tensile strength, so it could be said that it is possible to optimize the technology and chemical composition of steel. Conclusions 1. The technical possibility of producing shipbuilding steel plate (Grade ЕН36) using thermo- mechanical controlled process (TMCP) at the rolling mill 3600, PJSC “AZOVSTAL IRON & STEEL WORKS”, has been confirmed. 2. For the first time, at the rolling mill 3600, PJSC “AZOVSTAL IRON & STEEL WORKS”, an experimental batch of the rolled products (Grade EH36) has been produced with TMCP in strict compliance with Bureau Veritas Rules, a complex of main and additional tests has been carried out, which demonstrates the possibility of further mastering of the technology and product certification. 3. Introduction of the thermo-mechanical controlled process instead of normalization will reduce the production cost of the rolled products by eliminating natural gas consumption for heat treatment (normalization). 4. 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Дослідне освоєння виробництва товстолистового прокату з марки сталі EH36 із застосуванням термомеханічного контрольованого процесу на стані 3600 Вперше розроблена технологія, та виготовлена дослідна партія товстолистового прокату з судносталі підвищеної міцності марки ЕН36, розмірами 25×2150×8000 мм. Дослідна партія виготовлена відповідно до вимог класифікаційного товариства Бюро Верітас на прокатному стані 3600 ПАТ «МК «АЗОВСТАЛЬ». Технологія розроблена з використанням загальних вимог до виготовлення прокату з термомеханічного контрольованого процесу та з використанням математичної моделі процесу прокатки, на підставі якої виконано розрахунки режимів деформації. Підтверджена технічна можливість виготовлення товстолистового прокату з судносталі підвищеної міцності марки ЕН36 розмірами 25×2150×8000 мм з використанням термомеханічного контрольованого процесу (ТМСР) замість термічної обробки шляхом нормалізації, відповідно до правил Бюро Верітас на прокатному стані 3600. Застосування термомеханічного контрольованого процесу, замість нормалізації, дозволить знизити собівартість прокату за рахунок виключення споживання природного газу для термічної обробки (нормалізації). Для підготовки сертифікації, згідно Правил Бюро Верітас, необхідно провести додаткову серію досліджень для прокату марки ЕН36 у різних товщинах. Ключові слова: термомеханічний контрольований процес, товстолистовий гарячий прокат, сталь для суднобудування, технологія прокатки