Mean flow stress of HSLA steel of type Ni-Cu-Cr-Mo-Nb
Střední přirozené deformační odpory HSLA oceli typu Ni-Cu-Cr-Mo-Nb
prof. Ing. Ivo Schindler, CSc., prof. Ing. Jiří Kliber, CSc., Ing. Stanislav Rusz, Ph.D., Ing. Petr Kawulok, Ing. Miroslav Legerski, Ing. Václav Šumšal, VŠB – Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Czech Republic
Ing. Radim Pachlopník, ArcelorMittal Ostrava a.s., CzechRepublic, Ing. Libor Černý, Ph.D.,ArcelorMittal Distribution Solutionss.r.o., Czech Republic
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In laboratory conditions the heat of the HSLA steel of type Ni-Cu-Cr-Mo-Nb, which should comply requirements of standard API-X100, was manufactured. After forging, the flat thickness-graded samples from this material, suitable for the determination of hot mean flow stress after recalculation from the registered roll forces, were prepared. Rolling temperatures ranged from 750 to 1150 °C, effective height reduction to the value of 0.45 and strain rate from 8 to 88 s-1. The obtained values of mean flow stress were used for set-up of the model of deformation resistance. It is difficult to describe the temperature dependence of mean flow stress by a simple equation in the whole range of experimental temperatures, even though its accuracy is from purely statistical point of view satisfactory. That is why also two additional models of mean flow stress of the investigated steel were developed, namely for temperatures below 950 °C and above this value. Accuracy of all these models was statistically compared. The deformation resistance characteristics of the given steel and the HSLA steel of type Mn-Ni-Nb-Ti, investigated before, were compared as well. In spite of the high content of various alloying elements, the deformation resistance characteristics of the investigated steel are not somehow exceptional, as compared to current HSLA steels with the higher level of microalloying elements.
Vlaboratorních podmínkách byla vyrobena tavba HSLA oceli typu Ni-Cu-Cr-Mo-Nb schemickým složením 0.06 C – 0.75 Mn – 0.77 Si – 1.53 Cu – 3.45 Ni – 0.64 Cr – 0.50 Mo – 0.036 Nb – 0.073 Al (v hm. %), jež by měla vyhovovat požadavkům API-X100. Po překování byly ztohoto materiálu vyrobeny ploché vzorky sodstupňovanou tloušťkou, vhodné pro určování hot mean flow stress po přepočtu zregistrovaných válcovacích sil. Jednotlivé vzorky byly po jednotném předehřevu na 1200 °C válcovány na laboratorní stolici duo, pro niž byl dříve vyvinut vzorec pro přesný výpočet tvářecího faktoru. Teploty tváření se pohybovaly vrozsahu 750 – 1150 °C, skutečné výškové deformace do hodnoty 0.45 a deformační rychlosti v rozsahu 8 – 88 s-1. Získané hodnoty mean flow stress posloužily ksestavení modelu deformačního odporu metodami nelineární regrese. Znovu se potvrdilo, že popsat teplotní závislost mean flow stress jednoduchou rovnicí vcelém rozsahu experimentálních teplot je ztížené, i když její přesnost z čistě statistického hlediska je vyhovující. Byly proto vyvinuty i další dva modely mean flow stress zkoumané oceli, a to pro teploty pod 950 °C a nad touto hodnotou. Statisticky byly porovnány přesnosti všech modelů. Rovněž byly srovnány deformační odpory dané oceli a dříve zkoumané HSLA oceli typu Mn-Ni-Nb-Ti. Při vysokých deformacích jsou mean flow stress values obou ocelí velmi podobné, zatímco při deformacích nízkých jsou deformační odpory oceli Ni-Cu-Cr-Mo-Nb vždy nižší. Pravděpodobně vlivem mikrolegujících prvků jsou příslušné flow curves u oceli Mn-Ni-Nb-Ti výrazně plošší, celkově s dřívějším nástupem dynamické rekrystalizace, kterou u oceli Ni-Cu-Cr-Mo-Nb účinně brzdí především molybden. Přes vysoký obsah různých legujících prvků nejsou tedy deformační odpory zkoumané oceli nijak mimořádné ve srovnání s běžnými HSLA ocelemi s vyšší hladinou mikrolegujících prvků.
Introduction
API-X100 is a newly developed HSLA steel that was designed to satisfy the increased global energy demands as well as the requirements of high pipeline pressures. It is designed to have yield strength of 100 ksi (i.e. 690 MPa).
Microalloying of plain carbon steels with small amounts of strong carbide and nitride forming elements has achieved a great improvement in their mechanical properties. The addition of small amounts of Nb, Ti and V in combination with controlled rolling and accelerated cooling has allowed production of low carbon steels with high yield stress and good toughness. These steels are known as High Strength Low Alloy (HSLA) steels [1,2]. HSLA steels are extensively used in the oil and gas transportation pipelines because they have a low price-to-yield strength ratio. The cost reduction subjects by use of HSLA steels are caused by the wall thickness reduction having a significant impact on quantity of the material, transportation and welding costs. Pipelines can operate with higher pressures, resulting in larger quantities of transported gas.
In addition to the material strength, HSLA steels provide good weldability because of their low carbon contents. The approach to generate HSLA steels involves a combination of lower carbon content and fine grain size by microalloying along with thermomechanical rolling or accelerated cooling. Thermomechanical rolling enables materials up to X70 to be produced from steels that are microalloyed with niobium and vanadium and have a reduced carbon content. An improved processing method, consisting of thermomechanical rolling plus subsequent accelerated cooling, enables to produce higher strength materials like X80, having a further reduced carbon content and thereby excellent weldability. Additions of Mo, Cu and Ni enable the strength level to be raised to that of grade X100, when the steel is processed to plate by thermomechanical rolling plus modified accelerated cooling [3].
The aim of the research work was to develop the mathematical model of hot deformation resistance for the steel of type Ni-Cu-Cr-Mo-Nb, manufactured in laboratory conditions,which should comply with demands of API-X100.
Experimental procedure
After partial cooling, controlled by the thermometer, the sample was inserted for 2 minutes into the electric furnace heated to the forming temperature. The heated sample was immediately after discharging the furnace rolled in the two-high stand A of the mill Tandem (the rolls had diameter158 mm) [4]. In rolling of each sample the temperature was changed (1150 / 1050 / 950 / 850 / 750 °C), together with roll gap adjustment (and thus the total strain of individual grades of the sample) and nominal revolutions of rolls in the range of 40 – 360 rpm (and hence the strain rate values). Roll forces and the actual speed of roll rotation were computer-registered – see Fig. 1 for example.
After cooling of the rolling stock, the width and the thickness for individual grades were measured. The particular methodology of calculation of strain, strain rate and mean flow stress (MFS) from the measured variables is described e.g. in [5-7]. Here the crucial role is attributed to the model of the forming factor, which was developed for the given rolling mill stand – see [8] for example.
Fig. 1Example of measured total roll force depending on time in rolling of flat sample graded in thickness; temperature 1050 °C, nominal revolutions of rolls 50 rpm; strain and strain rate values are calculatedObr. 1Příklad měřené celkové válcovací síly v závislosti na čase při válcování plochého vzorku s odstupňovanou tloušťkou; teplota 1050 °C, nominální rychlost otáčení válců 50 min-1; deformace a deformační rychlost jsou vypočtené
Mathematical processing of experimental data
Based on the previous own experience a simple model for description of hot MFS m [MPa] of the investigated steel was chosen, in dependence on true strain – height deformation h (with taking the dynamic softening in consideration), temperature T [°C] and strain rate [s-1]:
(1)
where A – F are material constants, obtained by multiple non-linear regression of the experimental data in statistical package UNISTAT 5.6.
The entire experimental data set corresponded to temperatures 750 – 1150 °C, strains 0.03 – 0.45 and strain rates 8 – 88 s-1. By its statistical processing and applying methods of the non-linear regression the following equation was gained
(2)
describing, at first site very exactly, the initial data, when coefficient of determination R2 = 0.986. However, these parameters themselves do not implicate the situation, when an unfavourable trend of deviations between the measured and recalculated values of MFS exists, in relation to some of the independent variables. As it is obvious from Fig. 2, just this unfavourable case occurred with the deviations in dependence on the forming temperature (the relative deviation [%] is calculated as a difference between the experimentally measured and according to the pertinent equation calculated MFS value, divided by the measured MFS value).
Fig. 2Temperature relation of relative deviations of MFS, calculated according to equation (2)Obr. 2 Teplotní závislost relativních odchylek hodnot m, vypočtených dle rovnice (2)
Though the absolute values of the given deviations are not anyhow great (max. 7 %), their gradual increase for temperatures 750 – 950 °Cis evident, and vice versa decrease from positive to negative numbers for temperatures 950 – 150 °C. Therefore, the independent statistical processing of experimental data for the low-temperature and high-temperature region was chosen. The result is two following equations:
for temperatures below950 °C
(3)
Discussion of results
If compare the coefficients of determination ofequations (2-4), surprisingly the most exact is the original relation (2). For equation (3) R2 = 0.974 and for equation (4) R2 = 0.979. However, this is given by a lowered reliability of estimation of parameter F in equation (1) with a narrow temperature interval. In graphs in Fig. 3 the relative deviations in relation to the particular forming parameters are plotted – blue squares correspond to calculations of MFS according to equation (3) and red triangles according to equation (4). The absolute values of the chosen deviations somewhat raised, but the unfavourable deviation trends were virtually fully eliminated. Their values occur now, quite accidentally, in the positive and negative region of the selected coordinate system. In this way the developed models become very reliable from the viewpoint of the operational application – fast prediction of MFS and power/force parameters of forming.
Fig. 3 Relative deviations of MFS calculated according to equations (3) and (4) in relation toa)temperature
b)strain
Obr. 3 Relativní odchylky hodnot m vypočtených dle rovnic (3) a (4) v závislosti na
a)teplotě
b)deformaci
In case of application of the original model (1) we come across very often a rising and then a falling trend of the dependence = f(T) – see e.g. [9-11].However, not always it pays off to describe the deformation resistance of the specific material for more temperature regions from the point of view of accuracy. For a fast prediction of MFS of the investigated steel, also relation (2) with deviations smaller than 10 % would be sufficiently accurate.
Summary
Based on the measurement of roll forces during the laboratory rolling of flat samples of graded-in thickness, simple MFS models of the new HSLA steel of type Ni-Cu-Cr-Mo-Nb were developed. These models describe with a very good accuracy the hot deformation resistance characteristicsin the temperature range 750 – 1150 °C, effective height reductions up to 0.45 and strain ratesin the range of 8 – 88 s-1. Their extrapolation is possible especially in case of higher strain rates and hence they are suitable for the fast prediction of the power/force parameters of forming. Again, difficulty in the mathematical description of the influence of temperature on MFS in the wide range of temperature by a single equation was confirmed.
Acknowledgements
This work was realized within the projects MSMT - CZ.1.05/2.1.00/01.0040 and MSM 6198910015(Ministry of Education of the CzechRepublic) and FT-TA/091 (Ministry of Industry and Tradeof the Czech Republic).
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