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Natural strain ageing of heavily-deformed commercial low-carbon steels
A.I. Gordienko and V.V. Krylov-Olefirenko
Physical-Technical Institute of NAS of Belarus, Minsk, 220141, Belarus
The natural ageing of samples made from heavily-deformed commercial low-carbon steels and taken after their subjecting to hot and cold modes of rolling was investigated using hardness measurements. The differences were established in mechanisms and kinetics of natural ageing of heavily-deformed samples as compared to those of natural ageing of slightly-deformed samples. The explanation of the results obtained was suggested on the basis of notions about principal distinctive features of a sample state developed after large plastic deformations. The physical mesomechanics concepts and the results obtained when investigating the effect of high strain rates on properties of alloys were also used for this purpose.
1. Introduction
The natural strain ageing is the process that proceeds in the absence of any external effects and results in varying the state and properties of a deformed alloy with time. The mechanism and kinetics of this phenomenon taken for the case of slightly-deformed low-carbon steels are described in detail in the monograph [1] on the basis of [2, 3].
Both theoretical and experimental investigations of sample strain ageing kinetics at the degree of strain from 5 to 20-30 % indicate that there is an incubation period lasting for a fairly long time. They also point to the fact that the number of impurity atoms, which take part in the process, is continuously increased and the properties of an investigated material are improved with reaching their maximum values in a certain period of time. The strain ageing process is completed with stabilizing the material properties with time [1].
When elaborating the theory of strain ageing some assumptions have been made. They state that
(i) immediately after deformation a required number of impurity atoms is located precisely in normal interstitial sites;
(ii) a necessary number of impurity atoms have no noticeable interaction with other (besides dislocations) crystal lattice defects as well as with interfaces;
(iii) dislocations initiated by deformation are uniformly distributed over the metal volume and can be considered as "isolated" ones (any marked interaction of dislocations with each other is absent);
(iv) the temperature at which interaction is analyzed accounts for a lower energy of thermal vibrations than that resulting from interaction of a dissolved impurity atom with dislocation.
The aforesaid assumptions show that they correspond to an initial deformation process stage at which the state of developed plastic deformation is not yet attained. Therefore, actually all experimental results given in [1] are, probably, obtained using the 5-20 % deformed samples which have been subjected to deformation in laboratory conditions at a low strain rate. Only few investigations have been carried out on laboratory samples with the strain degree of 30-40 %. It is naturally to suggest that principal distinctive features of a heavily-deformed alloy state may be responsible for significant differences in mechanisms and kinetics of developing the strain ageing of steels in this state and moreover after high-speed deformation processes.
This work is aimed at determining a natural ageing of commercial steels heavily-deformed at high strain rates.
2. Materials and investigation procedure
The investigations were carried out using automobile low-carbon steel with the carbon content of 0.03 to 0.05 % taken in as-cast (after continuous casting), hot-rolled and cold-rolled conditions and isotropic electrical cold- and hot-rolled steels with the content of carbon and silicon 0.040.05 % and 3 %, respectively. In all the cases the samples to be investigated were taken from materials after their subjecting to commercial processing that involved hot rolling carried out after casting with subsequent cold rolling done without using any intermediate operations. The strain degree of these cold-rolled steels was 72 to 77 %.
The natural ageing investigations were performed by means of measuring hardness and microhardness of samples at regular time intervals. The microhardness was determined
© A.I. Gordienko and V.V. Krylov-Olefirenko, 2004
using the PMT-3 device with a 50-g load. The hardness was estimated by Vickers method with a 5-kg load. The measurements were accomplished using polished sections and directly on a sheet surface. In order to prepare polished sections the samples were fixed in screw clamps, which were also used for carrying out measurements.
3. Investigation results
Figure 1 shows the results obtained when investigating a natural strain ageing of automobile low-carbon commercial steel taken in as- (continuously) cast, hot- and cold-rolled conditions. No variations in hardness of as-cast automobile steel were observed. The measurement values are in the range of 1130 to 1160 N/mm2. Thus, they are actually not changed during the measurement time (Fig. 1, curve 7). However, as to a hot-rolled condition the hardness variations are rather marked (Fig. 1, curves 2, 5). The derived hardness-storing time relationship is not monotonic and continuously increasing. There are intermediate maximums and minimums in hardness values. The hot-rolled sheet wound at the temperature of 730 °C has the maximum difference of 240 N/mm2 in hardness values at the minimum value of 1010 N/mm2. For the sheet wound at the temperature of 640 °C the maximum difference in hardness is 270 N/mm2 at the minimum value of 1110 N/mm2. The range of vibrations is 23.5 % and 24.5 %, respectively.
The analogous dependence is shown by curves 4, 5 in Fig. 1 for a 73-% cold-rolled sheet. Though the curves are not exactly coincident in shape with those for a hot-rolled sheet state they have the same type. This dependence also is not continuously increasing and intermediate maximums and minimums of hardness values are not observed. The material produced from a blank subjected to hot-rolling and wound at the temperature of 640 °C has the maximum dif-
ference of 590 N/mm2 in hardness values at a minimum value of 1 730 N/mm2. As to the material produced from a blank hot-rolled and wound at the temperature of 730 °C these values are 620 N/mm2 and 1500 N/mm2, respectively. The vibration range in absolute units is 34 % and 41 %, respectively. Hence, after cold rolling the interval of varying hardness values is increased in both absolute and relative units.
The natural ageing was also investigated using isotropic electrical steel taken in as hot-rolled and cold-rolled conditions with the carbon content of 0.04 to 0.05 %. The measurements were carried out directly at the surface of sheet that had not been subjected to any treatment. The results are given in Fig. 2. As in the case of automobile steel the hardness-time relationship is not continuously increasing. It also has minima and maxima of values. For a hot-rolled condition the maximum difference is 160 N/mm at minimum hardness value of 1810 N/mm2 that is equal to 9 % in relative units. For a cold-rolled condition the analogous values are equal to 540 N/mm2, 3470 N/mm2 and 15.6 %, respectively. The characteristic feature of this material is a certain scatter in the obtained results amounting to 170 N/mm2.
The mechanical tests of these materials using standard techniques have revealed that the ultimate and yield strength values do not vary with time.
From the obtained results it follows that the natural strain ageing of heavily-deformed low-carbon commercial steels differs in both mechanism and kinetics from an analogous process proceeding in slightly-deformed samples. In the former case the difference in mechanism is associated with the presence of two competing processes such as hardening and softening. These processes are cyclically repeated. If the consideration of the material state is effected beginning with a minimum hardness value the hardening process is the first to occur with reaching the maximum intermediate hardness value and then softening starts and the minimum intermediate value is reached. The time of hardness increasing from a minimum hardness value to a maximum one is several days. Actually the same time is required for an entire
Fig. 1. Variation of hardness during natural strain ageing of automobile low-carbon steel: cast state (1); hot-rolled state, wound temperature of 640 °C (2); hot-rolled state, wound temperature of 730 °C (5); cold-rolled state, wound temperature during hot-rolling of 640 °C (4); cold-rolled state, wound temperature during hot-rolling of 730 °C (5)
Fig. 2. Variation of hardness during natural strain ageing of isotropic electrical steel with 3 % Si: hot-rolled state (1); cold-rolled state (2)
completing of a reverse process. The maximum hardness value measured by Brinell method is achieved for 5-% deformed Armco-iron samples in two months [4].
4. Discussion of results
It is known that spontaneous processes develop in non-equilibrium systems and tend to transform them in a more equilibrium state. It is evident that after being subjected to large plastic deformations the system is much more nonequi-librium than after deforming with a low degree of reduction. This is supported by the fact that stored strain energy is increasingly dependent on strain degree. The large plastic deformation nonuniformity is noted in different microscopic material volumes. It is established [5] that at a mean 66 % deformation this nonuniformity can range from -55 % (compression) to +175 % (extension) in some grains for steels with the carbon content from 0.15 % to 0.26 %. From the data given in [6] the tension tests conducted with high-purity (99.99 %) aluminum samples have shown that even within the boundaries of one grain there are areas differing in strain degree by 10 times.
The high strain rates account for increase of external effect intensity due to increasing the yield strength two, three and more times. The amount of stored energy is increased at one and the same degree of strain. At impact effect a dislocation structure undergoes a substantial change. The dislocation density is significantly increased [7].
The works of V.E. Panin with coworkers and V.V. Rybin with coworkers discuss the varying of deformation mechanism during transition to high plastic deformations. They also describe a principal difference of a state finely obtained in the latter case from that observed after low degrees of deformation. The deformation is characterized by originating not only point defects but also defects of mesoscale level, in particular, disclinations. The deformation mechanism is varied with increasing the strain degree and a rotational component appears. The shift resulting from moving single carries of shearing deformation and dislocations alternates with rotation of separate structure elements. The concept of structure and scale deformation levels is introduced. With enhancing the degree of strain increasingly complicated structure and scale levels are involved in the deformation process.
The stored energy enhancement that occurs with increasing the strain rate is indicative of a higher level of stresses acting in the material. The deformation nonuniformity existing in the sample volume, formation of mesodefects, high density of dislocations, occurrence of a rotational component in the deformation mechanism and the fact that increasingly complicated structure and scale deformation levels are involved in the progressing deformation process change the mechanisms and kinetics of natural ageing. It is possible to suggest that the distribution of stresses in the structure is more complicated as compared to that of single dislocation
stress fields or dislocation pileups and very high gradients of stress are present.
In this connection the differences in mechanisms responsible for natural ageing of heavily- and slightly-deformed investigated steels can be explained in the following way. According to the theory of strain ageing the hardening is conditioned by segregating the atmospheres of atoms which compensate the action of stress fields at dislocations. A softening process can be attributed to diffusion flow of interstitial atom that tends to uniformly distribute the alloy component concentration over its volume after nitrogen and carbon atom segregation at dislocations. The possibility of proceeding of back diffusion during strain ageing was already noted in work [8]. It has been suggested that a segregation process is completed on establishing a specific dynamic equilibrium when the rate of drifting the interstitial atoms becomes equal to the rate of back diffusion under the action of concentration gradient. B.Ya. Lubov [9] also has pointed to the interaction of two diffusion components.
As is follows from the aforesaid data a dynamic equilibrium between these processes is not established for the case of heavily-deformed hot- and cold-rolled commercial steels and their driving forces are not compensated. It is known that diffusion flows of alloy components generally exist when a chemical potential of this component is nonzero. Its additive components are gradients of concentration, temperature and pressure. When the temperature of natural ageing is constant the gradient of temperature is absent. On the basis of alternating the oppositely directed diffusion flows of atoms caused by the action of concentration and pressure gradients it is possible to conclude that a driving force of drift diffusion gradually diminishes and a driving force of concentration diffusion gradually increases. With their definite relationship a total flow changes its direction. In this case the gradient of chemical potential is retained considerably high for a long time in order to maintain this alternation.
5. Conclusion
The investigation has been made of spontaneous varying the properties of heavily-deformed low-carbon commercial alloys taken in hot-rolled and cold-rolled conditions during their natural ageing in the absence of any external effect.
It is established the following:
1. Microhardness and hardness values of heavily-deformed materials in hot and cold conditions are not stabilized in the process of natural ageing even after long storing.
2. For a low-carbon steel a state instability is observed following commercial hot rolling.
3. The range of the obtained values is 9-41 % of the minimum value.
The strain ageing of heavily-deformed low-carbon commercial steels differs from an analogous process proceeding in slightly-deformed samples both in mechanism and kine-
tics. In the former case the difference in mechanism is associated with the existence of two competing processes such as hardening and softening. These processes are cyclically repeated. The time required for reaching maximum hardness values is several times longer than that required for a slightly-deformed steel.
The softening process can be conditioned by the occurrence of a diffusion flow of interstitial atoms that tends to uniformly distribute the concentration of alloy components over its volume subsequent to segregation of nitrogen and carbon atoms at dislocations. In the case of heavily-deformed hot- and cold-rolled commercial samples the dynamic equilibrium between these processes is not attained and their driving forces are not mutually compensated.
References
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[3] A.H. Cottrel and B.A. Bilby, Proc. Phys. Soc., A62 (1949) 49.
[4] E.S. Davenport and E.C. Bain, TASM, V. 35, 1047.
[5] B.B. Chechulin, Investigation of microinhomogeneity of steel plastic deformation, Physics of Metals and Metallography, No. 1 (1955).
[6] C.I. Gubkin, Plastic Deformation of Metals, V. 2, Metallugizdat, 1960.
[7] H. Kressel and N. Brown, J. Appl. Phys., 38 (1967) 138.
[8] R. Billough and R.C. Newmann, Acta Metallurgica, 10, No. 11 (1962) 971.
[9] B.Ya. Lubov, Diffusion Changes of Solid Body Defect Structure, Metallurgia, Moscow, 1985.
