Plasticity Behavior of the Steel Depending on the Cooling Regimes in the Case of a Non-Conventional Treatments

CARMEN PENELOPI PAPADATU1*, ANDREI VICTOR SANDU2,3, MARIAN BORDEI1, ION SANDU2,4 1Dunarea de Jos University of Galati, Faculty of Engineering, 47 Domneasca Str., 800008, Galati, Romania 2Gheorghe Asachi Technical University of Iasi, Faculty of Material Science and Engineering, 61A D. Mangeron Blvd., 700050 Iasi, Romania 3Romanian Inventors Forum, 3 Sf. Petru Movila, Str. Bl. L11, Sc. A, Et. III, Ap.3, 700089 Iasi, Romania 4 Alexandru Ioan Cuza University of Iasi, ARHEOINVEST Interdisciplynary Platform, 22 Carol I Blvd., Corp G, 700506 Iasi, Romania

The plasticity of the steel depends on many factors. For example, it depends on deformability of the steel. According to the literature, the deformability of the steel characterizes the ability to be deformed permanently without breaking the inner linkages. Deformable metals and alloys and some types of steels can be considered elasto-plastic-viscous bodies [1,2].
To study the plasticity of a steel with a low carbon content, must be considered the influence of the temperature which is an important factor influencing the physico-chemical state of the alloys. After deformation at higher temperature (lamination, for example), the speed of cooling modifies the properties of the plasticity of the steel by increasing and it is possible to appear micro-cracks in the structure, if this cooling speed isn't controlled very carefully.
In the Literature, Hanning 's and Boulger's deformability variations depends on the temperature of the deformation. It can be seen that the temperature of deformability around (250 ... 300) °C is due to the formation of precipitates that * email: papadatu.carmen@yahoo.com favor the fragility of the tempering to blue. The increasing rapidly of the deformability between 600 and 800 °C is due to inter-crystalline displacements and re-crystallization of steel. At the same time, the deformability of the steels depends on the carbon content [1][2][3]5].
Studying the microstructure of steels with 0.1% C, 0.48% Mn, 0.24% Si the mechanical properties of the steels are influenced by the sizes and the shapes of pearlitic (P) and ferritic (F) grains which form its microstructure. Ferrite has a low mechanical strength but it has good plasticity properties. Cementite (Cem) in steels has a good hardness but it is fragile, that's why Cementite can cause cracks. Cementite can break the inner linkages. Therefore, with the increasing of the carbon content, the hardness and the strength of the steels increase and the plasticity properties decrease [1, 3, 5-9,].
The steels with low carbon content have high plasticity properties below 550 C and the thermal stresses will not leads to the formation of cracks [1]. The paper focuses on directed fast cooled steels strips for automotive application after hot laminated process [3].

Lamination of large thickness sheets before nonconventional treatment
In table 1 was considered a scheme of hot lamination of steel sheet. The samples had the following dimensions: 10 x 150 x 200 mm and came from a slab with the dimensions: 150 x 1500 x 2500 mm. For a thick sheet rolling it is necessary to have one system type quarto (two pairs of rolls -four Rollers Cylinders), with a sheet width maximum of 3000 mm and a vertical rolling system, placed in front of the first system. The hot lamination scheme is shown in table 1 [1].

Experimental part
Twelve experiments have been carried outconsidering three groups of samples and each group supported three types of non-conventional treatments, taking in consideration four values of the initial temperatures.
There were considered three groups of samples of steel and each group had four samples.
The samples were heating at the different temperatures: T1 = 850ºC (for the first batch of samples), T2 = 900 o C (for the second batch of samples), T3 = 950 o C (for the third batch of samples) and T4 = 1000 o C (for the last batch of samples). Three cooling regimes [1,3] have been used: (1) cooling regime in normal conditions; (2) cooling regime in metallic box; (3) cooling in air flow (using a jet of cold air). If the cooling mediums differ, the speeds of the cooling differ too.

Results and discussions
In  (1) where t = the initial temperature corresponding to each cooling regime, [ o C]; t o = the final temperature which was approximate to 20 o C; ∆τ = the cooling duration, [min.].
Flow Limit or High Deformation Limit is very important for steels used for metal construction or for armatures. The resistance of the metallic bridge or the resistance of the wall in construction depends on the flow limit of the steel with a low content of Carbon.
In figures 1 to 3, the evolution of the Flow Limit depending on the cooling speed from the initial temperature corresponding to the steel analyzed, have been presented.   When the steel is cooling in a metal box, the cooling rate is very low. A good resilience measured at -20 o C was obtained when cooling under normal conditions from 850 o C. A good stretch has been achieved by cooling in a metal box, especially from an initial temperature of 1000 o C. However, its value is lower than that of cooling under normal conditions.
A very good Flow Limit of steel was obtained by cooling in a cool air jet from the initial temperature of 850 o C. Therefore, a quick cooling in cold air jet from an initial temperature of 850 o C is recommended for obtaining an optimum flow limit, improving the resistance of steel to plastic deformation. The behavior of steel in the case of plastic deformation is the best when the steel is heated at If the samples are cooling in a metallic box from an initial temperature of 1000°C, a good resilience is obtained, measured at ambient temperature (+20°C).
The temperature of the rolling of steel influences the characteristics of the steel. If it is applied a heating after lamination and it is studied the influence of the speed of cooling on some characteristics of that steel, it can be obtained changes of the plasticity evolution.
A fine structure of the hot-laminated steel is observed in the case of the cooling in metallic recipient (box) and in the case of the cooling in cold jet of air. The basic structure is compound by Ferrite (F) and lamellae of Pearlite (P).The micro-structures in strings are specific to laminated structures. Manganese (Mn) content more than 2.75% determines the increasing of the tenacity and plasticity of the steel, comparing to simple casted steel.

Conclusions
The flowing limit -the highest value -was obtained in the case of the cooling in cold jet air, especially in the case of the cooling from 950°C. In all cases, a high Flow limits were recorded at the cooling in jet of air from the temperatures of 950, 900 and 850°C. After non-conventional heating process and cooling in different mediums, the steel flowing limits have exceeded the values of the same steel after hot lamination (σ c = 360 daN/mm 2 ).
The lowest resilience value determined at negative temperatures (-20°C) was recorded for air-jet cooling from 1000°C. The resilience represents the tenacity of the steel. A good tenacity represents a good plasticity of the steel.