CC BY
25
УДК 621.771.23:669.112.227.1
POLYGONIZING CONTROLLED ROLLING STEELS FOR METAL
CONSTRUCTIONS
BOLSHAKOV V. I.1, Dr. Sc. (Tech.), Prof., LAUKHIN D. V.2* , Dr. Sc. (Tech.), Prof., SUKHOMLIN G. D.3, Dr. Sc. (Tech.), Prof, BEKETOV A. V.4, Cand. Sc. (Tech.), Ass. Prof.
1 Department of Materials Science, State Higher Educational Establishment "Prydniprovs'ka State Academy of Civil Engineering and Architecture", 24-а, Chernishevskogo str., Dnipropetrovsk 49005, Ukraine, tel. +38 (0562) 47-39-56, e-mail: bolshakov@mail.pgasa.dp.ua, ORCID ID: 0000-0003-0790-6473
2 Department of Materials Science, State Higher Educational Establishment "Prydniprovs'ka State Academy of Civil Engineering and Architecture", 24-а, Chernishevskogo str., Dnipropetrovsk 49005, Ukraine, tel. +38 (0562) 47-39-56, e-mail: 3ab120@mail.pgasa.dp.ua, ORCID ID: 0000-0002-9842-499X
3 Department of Materials Science, State Higher Educational Establishment "Prydniprovs'ka State Academy of Civil Engineering and Architecture", 24-а, Chernishevskogo str., Dnipropetrovsk 49005, Ukraine, tel. +38 (0562) 47-39-56, e-mail: g_suhomlin@mail.ru, ORCID ID: 0000-0001-5840-169X
4 Department of Materials Science, State Higher Educational Establishment "Prydniprovs'ka State Academy of Civil Engineering and Architecture", 24-а, Chernishevskogo str., Dnipropetrovsk 49005, Ukraine, tel. +38 (0562) 47-39-56, e-mail: lab120@mail.pgasa.dp.ua, ORCID ID: 0000-0003-0664-0327
Summary. It were investigated micro- and substructure processes formation with the multiple hot deformation of austenite.
Keywords: austenite, controlled rolling.
ПОЛ1ГОШЗАЦШНА КОНТРОЛЬОВАНА ПРОКАТКА СТАЛЕЙ ДЛЯ БУДШЕЛЬНИХ МЕТАЛЕВИХ КОНСТРУКЦ1Й
БОЛЬШАКОВ В. I.1, д. т. н, проф., ЛАУХ1Н Д. В.2*, д. т. н, проф., СУХОМЛИН Г. Д.3, д. т. н., проф., БЕКЕТОВ О. В.4, к. т. н., доц.
1 Кафедра матерiалознавства та обробки матерiалiв, Державний вищий навчальний заклад «Придншровська державна акадетя будiвництва та архиектури», вул. Чернишевського, 24-а, Дншропетровськ, 49005, Украша, тел. +38 (0562) 47-39-56, e-mail: bolshakov@mail.pgasa.dp.ua, ORCID ID: 0000-0003-0790-6473
2 Кафедра матерiалознавства та обробки матерiалiв, Державний вищий навчальний заклад «Придншровська державна академiя будiвництва та архггектури», вул. Чернишевського, 24-а, Дншропетровськ, 49005, Украша, тел. +38 (0562) 47-02-62, e-mail: lab120@mail.pgasa.dp.ua, ORCID ID: 0000-0002-9842-499X
3 Кафедра матерiалознавства та обробки матерiалiв, Державний вищий навчальний заклад «Придншровська державна академiя будiвництва та архггектури», вул. Чернишевського, 24-а, Днгпропетровськ, 49005, Украша, тел. +38 (0562) 46-93-72, e-mail: g_suhomlin@mail.ru, ORCID ID: 0000-0001-5840-169X
4 Кафедра матерiалознавства та обробки матерiалiв, Державний вищий навчальний заклад «Придншровська державна академТя будiвництва та архиектури», вул. Чернишевського, 24-а, Дншропетровськ, 49005, Украша, тел. +38 (0562) 46-93-72, e-mail: lab120@mail.pgasa.dp.ua, ORCID ID: 0000-0003-0664-0327
Анотащя: Досладжено процеси формування мжро- i субструктури в умовах багатократно! гарячо! деформаци аустениу.
Ключов1 слова: аустенит, контрольована прокатка
ПОЛИГОНИЗАЦИОННАЯ КОНТРОЛИРУЕМАЯ ПРОКАТКА СТАЛЕЙ ДЛЯ МЕТАЛЛИЧЕСКИХ СТРОИТЕЛЬНЫХ КОНСТРУКЦИЙ
БОЛЬШАКОВ В. И.1, д. т. н, проф., ЛАУХИН Д. В.2*, д. т. н, проф., СУХОМЛИН Г. Д.3, д. т. н., проф., БЕКЕТОВ А. В.4, к. т. н., доц.
1 Кафедра материаловедения и обработки материалов, Государственное высшее учебное заведение "Приднепровская государственная академия строительства и архитектуры", ул. Чернышевского, 24-а, Днепропетровск, 49005, Украина, тел. +38 (0562) 47-39-56, e-mail: bolshakov@mail.pgasa.dp.ua, ORCID ID: 0000-0003-0790-6473
2 Кафедра материаловедения и обработки материалов, Государственное высшее учебное заведение государственная академия строительства и архитектуры", ул. Чернышевского, 24-а, Днепропетровск, тел. +38 (0562) 47-39-56, e-mail: lab120@mail.pgasa.dp.ua, ORCID ID: 0000-0002-9842-499X
3 Кафедра материаловедения и обработки материалов, Государственное высшее учебное заведение государственная академия строительства и архитектуры», ул. Чернышевского, 24-а, Днепропетровск, тел. +38 (0562) 46-93-72, e-mail: g_suhomlin@mail.ru, ORCID ID: 0000-0001-5840-169X
4 Кафедра материаловедения и обработки материалов, Государственное высшее учебное заведение государственная академия строительства и архитектуры", ул. Чернышевского, 24-а, Днепропетровск, тел. +38 (0562) 47-39-56, e-mail: lab120@mail.pgasa.dp.ua, ORCID ID: 0000-0003-0664-0327
"Приднепровская 49005, Украина,
«Приднепровская 49005, Украина,
"Приднепровская 49005, Украина,
Анотация. Изучены процессы формирования микро- и субструктуры при многократной горячей деформации аустенита.
Ключевые слова: аустенит, контролируемая прокатка
At present, hot rolling is the most common process used in working slabs of low-carbon steels smelted with no carbide-forming additions. Although this process ensures relatively moderate strength characteristics in plates, such steels possess good weldability and plasticity at relatively low costs [1-6].
Some strengthening is connected with saturation of metals with numerous faults which in its turn results in a necessity of application of complicated processes and high production costs. For the most part, rolled product strengthening by various methods of thermomechanical treatment is economically more feasible than expensive alloying [1-6]. Specifically, an example of such leading-edge processes is controlled rolling used in making plates for the production of large-diameter pipes used in the construction of Arctic oil and gas pipelines.
This R&D work objective was improvement of mechanical properties of the steel plates produced by controlled rolling. The main problems consisted in retention of polygonized structure of hot-deformed austenite and creation of conditions for its inheritance with proeutectoid ferrite precipitated before the finish rolling step.
Temperature and deformation conditions of the controlled rolling are usually realized as follows: heating slabs in a continuous furnace to temperatures between 1100 °C and 1200 °C, homogenizing holding during 4 to 6 hours, rough rolling completed at 980-1100 °C, cooling down to 720-820 °C, finish rolling to a required thickness and slow cooling to room temperature (see Figure 1, conventional schedule). This process has its advantages but it has certain disadvantages as well.
Firstly, it is the necessity of an additional alloying to suppress austenite grain growth through the formation of particles of high-temperature carbonitrides (otherwise, the plate impact toughness can degrade) [1-6].
Secondly, this process has only proved itself well in the production of plates not thicker than 20 mm as the thicker is the rolled product the worse are tensile strength and impact toughness because of smaller total reductions.
Thirdly, it is necessary that temperature-deformation parameters of the controlled rolling process were optimized for each rolling mill and individual plate rolling schedules were corrected depending on the planned service conditions of the rolled product.
This R&D work has resulted in a new schedule for the process of polygonizing controlled rolling featuring a higher deformation fractioning in the rough stand with the final rolling temperature being 10-30 °C lower than Ac3 temperature and a shorter holding of the intermediate product at the bypass table to prevent recrystallization and maintain the rolling rate. When the temperature of start of working in the finish stand is achieved, rolling is carried out by the design schedule and the rolled product is cooled in a way ensuring retaining of subgrain boundaries in ferrite and escaping formation of special boundaries in the middle layers (see Figure 1, the proposed schedule).
The larger number of unit cycles at a constant total deformation ratio favors formation of a more developed polygonal austenite structure and the longer deformation time at a lower temperature at the end of rough rolling makes austenite subgrains fixed. The resulting deformed austenite structure saturated
with subgrain boundaries is favorable for achievement of homogeneity of the finite ferrite structure [7].
Fig. 1. Conventional and proposed controlled rolling schedules / Традиционный и предлагаемый режим контролируемой прокатки
Temperature and deformation conditions of the proposed schedule imply the temperature at the end of rolling in the rough stand to be within a range where there is no recrystallization which is a prerequisite for the formation of fine ferrite grains during cooling
in the intercritical temperature range. But if 22 mm and thicker plates are rolled, a possibility of formation of both recrystallized and non-recrystallized regions in the plate body exists.
To prevent recrystallization processes in austenite, the temperature at the end of rough rolling was shifted somewhat below the critical point Ac3 which along with the reduced time of staying at the bypass table creates conditions in which the deformed austenite is not recrystallized or rerystallized to a minute degree.
The polygonized austenite structure preserved in this way contains a large number of additional sites of heterogeneous nucleation of ferrite (polygonal boundaries, their interfaces and nodes), cf. Figures 2a and 2b. Reduction of the temperature at the end of rough rolling to the values below Ac3 results in a formation of fine ferrite nuclei fixing the polygonized substructure and preventing recrystallization and austenite grain growth (Figure 2d-f).
Fig. 2. Sequential stages of a-crystal nucleation at polygon boundaries at temperatures reduced down to the values below point Ас3; d - f: precipitation of hypoeutectoidferrite in steels which underwent austenite decomposition after cooling in
air, x800: d: from a single heating by 1050 e: after 16% hot reduction at 1000 cG;f: after 36% hot reduction at 1000 С /Последовательные этапы зарождения а-кристаллов на полигональных границах при температурах ниже точки АС3; d — f: выделение доэвтектоидного феррита в сталях после охлаждения на воздухе х800: d: нагрев 1050 С, е: после 16% горячей деформации при 1000 С; е: после 36% горячей деформации при 1000 С
Structure investigations of quenched samples have shown that cooling down to the temperatures below point Ac3 gives rise to
nucleation of new crystals of hypoeutectoid ferrite not only at large-angle boundaries but at polygonal ones as well (see Figures 2c, f). In
particular, Figure 2f shows that the internal volumes of the former austenite grains (their boundaries are seen due to the continuous ferrite fringes) are covered with ferrite nuclei of an average size 0.5-1.5 p,m.
In case of very small or zero temperature drops after rough rolling, parameters of the polygonal substructure develop in a reverse order: polygon sizes get smaller and the mean angle of orientation disorder decreases. Furthermore, the ability of polygonal boundaries to serve as the sites of ferrite crystal nucleation decreases.
Additionally, low-angle polygonal boundaries are formed in fine ferrite grains during finish rolling which results in refining of the final structure and a simultaneous upgrade of strength and plasticity of the finished plates.
Delivery batch tests of 40 mm thick plates rolled by the proposed schedule have demonstrated simultaneous improvement of tensile strength and stabilization of viscosity as compared to the plates rolled by the conventional technology: tensile strength in Z direction being 1.5-2 times higher (230 to 480 MPa).
It is important that specification of properties in Z direction (direction of the rolled product thickness) has to be an integral part of engineering requirements to steels as the steel plasticity can fall abruptly because of an effect of tangential tensile forces, especially forces normal to the plate plane.
Percent narrowing (yZ) is the parameter most sensitive to the variation of all mechanical characteristics of thick plates in Z direction.
Actual percent narrowing in Z direction in the plates produced by the proposed schedule is 20-25 % higher than that in the plates produced by the conventional technology and almost 2 times higher than it is required by the standards for Z 35 quality rating.
Microstructure of 22 mm thick plates of microalloyed low-carbon steel 10G2FB rolled by the conventional technology and using the proposed schedule is shown in Figures 3 a, b.
Visual estimate shows that the structure in the plates rolled by the proposed schedule is more dispersed than that in the plates rolled by the conventional technology. Pearlite striation
is less pronounced than in case of an ordinary hot-worked metal.
Photographs of shadow-cast replica show that the large-angle and subgrain boundaries interact with their energies and the subgrains can be 0.5 ^m in diameter and somewhat elongated in the rolling direction (Figure 4a).
b
Fig. 3. Structure of 22 mm thick plates of low-carbon steel 10G2FB rolled by conventional technology (a) and with the use of the proposed schedule (b) / Структура пластин толщиной 22 мм из низкоуглеродистой стали 10Г2ФБ прокатанной по традиционной технологии (а) и по приведенной схеме (б)
Images of thin foils prepared from the metal rolled by the experimental schedule reveal dislocation arrangement of subgrain boundaries (Figure 4b) and networks formed by several dislocation families. They contain mostly hexagonal cells and sometimes rectangular ones. Individual dislocations are discernable if the distance between them is 3 to 5 nm, otherwise they merge into a strip with a contrast typical for the large-angle boundaries although their mean off-orientation angle does not exceed 3-6 degrees.
Fig. 4. Ткт structure in 22 mm thick plates of low-
carbon steel 10G2FB rolled by the experimental schedule: а, b, c: electron microscope image of subgrain (polygonal) boundaries; d: dispersed carbides of (Nb, ¥)С type in ferrite / Тонкая структура плит толщиной 22 мм из низкоуглеродистой стали 10Г2ФБ прокатанной по
предлагаемому режиму: а, b, c: электронномикроскопическое изображение субзеренных (полигональных) границсубзерном (полигональные) границ; d: дисперсные карбиды (Nb, ¥)С в ферритной фазе
Cementite bands part into short sections with evidently rounded edges.
Some of them take a disk or an ellipsoid shape (Figure 4c). Such changes in the pearlite component promote growth of plasticity and decrease in strength of the finished plates.
However, a different process is simultaneously taking place. Contrary to the first one, it increases strength and decreases plasticity: precipitation of excess phases. In the ferrite component, a relatively high density of disperse particles is observed. These particles have contrast typical for carbides of (Nb, V)C type [8, 9]. Figure 4d shows their uniform distribution in the entire internal volume of the ferrite grains. Some dislocations are conjugated with carbonitride particles restraining their displacement at critical loads, increasing start stresses and strengthening the metal in this way.
The high-magnification image patch in the upper left corner of Figure 4d shows a characteristic 20 mm diameter ring-shaped contrast formed by diffracting electrons. Such contrast reveals itself due to the elastic stresses arising around the carbonitride particles [5]. These particles themselves have smaller sizes, not larger than 3-7 nm. Their diameter is smaller than the light spots in the centre of the ring-shaped images.
Based on the foregoing, the following conclusions can be drawn:
- the proposed hot plate rolling schedule is based on a creation of a polygonized austenite structure being formed during hot working and forcibly kept stable up to the temperatures of the upper part of the intercritical range. The further multiple nucleation of preuetectoid ferrite at both large-angle and polygonal boundaries improves dispersity of ferrite grains in the metal entering the finish rolling stand, therefore a more dispersed final ferrite structure is formed in the finished plates and accordingly better mechanical properties are achieved;
- the proposed plate rolling schedule can be implemented with no capital investments at the existing equipment of Ukrainian metallurgical works;
- the proposed plate rolling schedule promotes gain in and stabilization of plasticity and viscosity at sub-zero temperatures and reduction of plate rejections over unsatisfactory mechanical properties;
- the results of comprehensive studies allow to recommend plates of steel grades 10G2FB and S355j2 for their use as a material
for the production of large-diameter oil and gas rise buildings and large-span floors. line pipes and construction of frames for high-
ВИКОРИСТАН1 ДЖЕРЕЛА
1. Влияние длительности аустенитизации и деформации на структуру и свойства малоуглеродистых сталей 09Г2С и 10Г2ФБ / В. И. Большаков, Г. Д. Сухомлин, Л. Н. Лаухина, Д. В. Лаухин // Theoretical foundations of civil engineering. Proc. 13th Polish-Ukrainian Transactions (conference), Warsaw, June, 2005 / е± by W. Szczesniak. - Warsaw ; Dnepropetrovsk, 2005. - № 13. - P. 83-88.
2. Бернштейн М. Л. Структура деформированного металла / М. Л. Бернштейн - Москва : Металлургия, 1977. - 432 с. - Режим доступа: http://www.twirpx.com/file/675269/.
3. Гриднев В. Н. Прочность и пластичность холоднодеформированной стали / В. Н. Гриднев, В. Г. Гаврилюк,
Ю. Я. Мешков - Киев : Наук. думка, 1974. - 231 с.
4. Yokota T. Formation of nanostructured steel by phase transformation / Yokota T., Garica Mateo C., Bhadeshia H. K. D. H. // Scripta Materialia. -2004. - Vol. 51. - Р. 767-770. - Available at: http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=57EFEDCE91137938E3698483210A9C48?do i=10.1.1.216.2904&rep=rep 1 &type=pdf.
5. Большаков В. И. Термомеханическая обработка конструкционной стали / В. И. Большаков. - 3-е изд., перераб. и доп. - Канада : Базилиан Пресс, 1998. - 316 с.
6. Langford G. Subgrain strengthening of materials / Langford G., Cohen M. // Transactions of the American Society for Metals. - 1969. - Vol. 62 - Р. 823-835. - Режим доступа: http://www.springer.com.
7. Большаков В. И. Полигонизация аустенита при контролируемой прокатке : моногрaфiя / В. И. Большаков, Д. В. Лаухин. - Днепропетровск : ПГАСА, 2011. - 268 с.
8. Утевский Л. М. Дифракционная электронная микроскопия в материаловедении / Л. М. Утевский. - Москва : Металлургия, 1973. - 584 с. - Режим доступа: http://www.twirpx.com/file/687682/.
9. Электронная микроскопия тонких кристаллов : пер. с англ. / П. Хирш, А. Хови, Р. Николсон, Д. Пэшли, М. Уэлан. - Москва : Мир, 1968. - 574 с. - Режим доступа: http://www.twirpx.com/file/911407.
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Рецензент: д-р. т. н. Даншевський В. В.
Надшшла до редкзолегп: 17.03.2016 р. Прийнята до друку:31.03.2016 р.
