Cold rolling of steel strips with metal ‐ working

The efficiency of cold rolling of steel strip in the main depends on the quality of technological lubricant and its cost. In this regard, it is important to develop new compositions of effective metalworking coolants that are low cost and provide maximum reduction in the friction coefficient. We developed and tested the new compositions of metalworking coolants on the basis of vegetable oil and chicken fat. The metalworking coolants were tested in Donbas State Engineering Academy (DSEA) on a laboratory rolling mill, 100 × 100, in cold rolling of 08Kp steel. The efficiency of the coolants was determined by the stretch ratio λ and the friction coefficient μ in the deformation zone, which was found by the forward slip method. We found the metalworking coolant with 100% concentration of boric acid esters of mono- and diglycerides is the most effective in the rolling of thin steel strips. Thus, the new metalworking coolants (MWC) on the basis of boric acid esters of mono- and diglycerides, synthesized on the basis of sunflower oil, can be recommended for use in the rolling of structural steels on account of its availability, high efficiency and low cost.


Introduction
The efficiency of cold sheet and strip rolling of steels and non-ferrous metals in the main depends on the quality of the technological lubricant [1][2][3][4].The research results presented in this article is the continuation of the cycle of our works [5][6][7][8][9][10] related to the search of new metal-working coolants (MWC) which can effectively reduce a friction level on the contact surfaces of metal and tool in metal forming.First of all it relates to production of cold-rolled plates and strips of ferrous and nonferrous metals and as well as the production of long products by metal drawing and pressing in a cold state.
Earlier we found the MWC containing 30% of boric acid esters of mono-and diglycerides based on sunflower oil are the most effective [10].We also noted the enough high efficiency, availability, and low cost of the MWC with chicken fat.

Materials and Methods
The task of the first stage was to study the MWC with the increased content of boric acid esters of mono-and diglycerides based on sunflower oil (from 30% up to 60%) (Table 1), and the new coolant compositions with chicken fat (Table 2).
The test results are shown in Table 3.  Notes.* The diethanolamide of sunflower oil acids was used as emulsifier.** Antibacterial additive evolves formalin because it is a condensation product of formalin and monoethanolamine and its formula is *** We used the lubricant SP-3 (GOST 5702-75) as MWC comparison.
All tests with MWC we carried out on industrial and laboratory rolling mill in Donbas State Engineering Academy (Figure 1) as well as in [10].
Annealed 08Kp steel rolling was carried out for samples with initial thickness of h0 = 1.88…1.91mm and width of b0 = 39.90 mm.The rolling rate was 0.2 m/s.We used two cobbing levels: 1 st level with εavg ≈ 18% and 2 st level with εavg ≈ 46.5%.Before rolling we laid MWC on the strip.Before each pass we thoroughly cleaned rollers.
To compare the results obtained for new MWC we carried out rolling without MWC (dry samples in dry rollers), with MWC № 5 and № 17 which was studied in work [10], and with MWC № 25 used in Ilyich Iron and Steel Works (Mariupol, Ukraine) as MWC for cold rolling of steel strips.Working rolls from 9X steel had diameter of 260 mm.A hardness of forming surfaces of roll body was 80HS, a roughness was 0.63 microns.
The efficiency of MWC was determined by comparing the reduction ratios in rolling of the same samples without MWC (dry) and with MWC when initial roll opening S0 was fixed [10; 11].It is known the more effective MWC promotes the decrease of pressing force and stand springing.Therefore in this case when the initial roll opening S0, thickness h0, and width b0 of samples are invariable the metal redaction increases.That let us estimate an efficiency of MWC.In thin flat-sample rolling the widening is practically absent (it is less than 1%), and the ratio of the thicknesses h0 and h1 is equal to reduction ratio λ for the strip in pass, that is h0/h1 = λ.Therefore, to improve the accuracy we estimated the redaction level in each case with reduction ratio λ calculated as a ratio of the distance between the initial marks previously applied on the strip before rolling (l0 = 200 mm) and the distance between the same marks after rolling (l1), that is λ = l1/l0 [10].
In this work to determine the friction coefficient μ for the tested MWC we used the forward slip method [12].
It is known the speed of forward motion of the strip exiting from the rollers is higher than the surface speed of rollers.This phenomenon is called the forward slip [11].
The forward slip is determined by the formula: where V1 is the start strip speed and Vf is the surface speed of rollers.
In practice the forward slip is determined by kern method.For this the small indentation is punched on the body of roller on the distance lb along circular arc.These marks leave the tracks on the strip, and the distance between these tracks l1 is longer from forward slip.
Hence it appears where l1 is the distance between the tracks on the strip and lb is the distance between marks on the body of roller along circular arc.
Indeed, for a time t lb = Vf t, and l1 = V1 t, so Vf = lb/t, and V1 = l1/t.Theoretically the forward slip in symmetrical rolling can be determined considering the position of the neutral section.In thin strip rolling when the roller radius Rf is much bigger than the strip thickness h1 in exit from deformation zone the forward slip can be calculated by the Dresden formula [11]: where γ is neutral angle.If we know the forward slip and use the forward slip method we can approximately determine the average value of the friction coefficient.
In this case the value of forward slip S is determined experimentally as an indicator of relative metal sliding on roller surface in exit from deformation zone in steady rolling.After this the value of neutral angle γ is calculated with help of Fink formula [12]: where Df is roller diameter.
For the same purpose it can be used the Dresden formula (3) simplified in the form: The angle γ found in this way is substituted into known Ekelund-Pavlov formula [12]: which is used to find the friction angle β: The friction coefficient is determined by the equation ( 7) in consideration of equation (8) .tq   (8) It should be taken into account the formulas (4-6) were obtained under the assumption of uniform height distribution of speed in deformation zone, that is, the hypothesis of ʺflat sectionsʺ was taken in their derivation.It is now established that the longitudinal velocities of metal particles have non-uniformly height distribution in deformation zone.Full decreasing of nonuniformity of speeds occurs only after exit of metal from zone of contact with the rollers.In this regard, the neutral section has generally curved form [13].This important fact is not reproduced in the formulas (4-6).Therefore, the use of formulas (4)(5)(6) gives satisfactory results in relatively thin strip rolling when l/havg > 3…4 at plane deformation, that is b0/l  5, where l is the horizontal projection of the contact arc length of metal and rollers, havg is average height of strip in the deformation zone, b0 is initial width of strip.
In this case, the use of these dependencies was justified.

Results and Discussion
On the base of analysis of the experiments (see Table 3) it should be noted that in the MWC group based on boric acid esters (№ 5, 19, 20, and 21) the MWC № 21 has shown the best results in reduction ratios λ and friction coefficients μ for 1 st and 2 st levels of cobbing (for εavg ≈ 17.82 % μ = 0.0481, and for εavg ≈ 46.84 % μ = 0.0548).This coolant contained 60% of boric acid esters of monoand diglycerides based on sunflower oil, that is, with an increase in the percentage of boric acid esters the MWC efficiency increased.
Boric acid ester of monoglyceride based on sunflower oil has the structural formula: This is stable compound as opposed to boric acid ester of diglyceride.In this regard, in the product having a name ʺboric acid esters of mono-and diglycerides based on sunflower oilʺ the active components are the boric acid ester of monoglyceride based on sunflower oil and diglyceride based on sunflower oil with structural formula: The feedstock in the synthesis of boric acid esters of mono-and diglycerides based on a sun-oil is a mixture of diglyceride and monoglyceride: OH In previous study [10] the prospects of use of chicken fat as MWC was noted (MWC № 17 with 100% chicken fat).
In this work the compositions № 22, 23, and № 24 based on of chicken fat and containing very important components -an emulsifier and an antibacterial additive, that provide high performance, have shown good results (see Table 3), but slightly worse than composition № 21 based on boric esters.
In consideration of results described above we decided to increase the concentrations of boric acid esters of mono-and diglycerides based on sunflower oil up to 80% and 100% in MWC № 27 and 28.Also we decided to estimate the possibility of use of the feedstock (a mixture of monoglyceride and diglyceride) as MWC in steel rolling.This is MWC № 29.
Results of experiments are shown in Table 4.As expected, MWC № 28 with 100% concentration of boric acid esters showed the highest results.It is indicate that the chlorinated paraffin wax ХР-470 and calcium stearate, widely used in various compositions of MWC for steel rolling, have not influence on properties of MWC with boric acid esters of mono-and diglycerides based on sunflower oil.
It should be noted that on the 2 st level of cobbing for the coolant № 28 the absolute reduction is practically maximum (Δh = 1.01 mm, εavg = 53.72 %) due to the high efficiency of this MWC and as a result the low value of friction coefficient (μ = 0.0450).This is evidenced by the negative value of the forward slip (S = -0.79%).
It indicates that the reserve of friction forces in deformation zone for rolling with this cobbing is exhausted due to the high quality of MWC because the lag zone extended to all length of deformation zone.
MWC № 29 with 100% concentration of mono-and diglycerides based on sunflower oil showed relatively high results.Therefore, the both diglyceride and boric acid ester of monoglyceride raise the reduction ratio of steel  and reduce the friction coefficient .
In the first level of cobbing we observed the smallest forward slip (S = 1.43%), that is the smallest coefficient of friction, for mono-and diglycerides based on rapeseed oil.In the second level of cobbing (εavg ≈ 42.8%) we found the insignificant reduction in the coefficient of friction for MWC № 30 and № 31, and more significant reduction in the coefficient of friction for MWC № 32 and № 33 in comparison with rolling without MWC.But these results cannot be called positive.
Due to the fact that MWC based on rapeseed oil (MWC № 32 and 33) reduced the friction coefficient, we also investigated 100% emulsion MWC based on boric acid esters of mono-and diglycerides of rapeseed oil (MWC № 34) and aqueous emulsions with MWC concentration of 30% (№ 35), 20% (№ 36), 10% (№ 37), 5% (№ 38), and 3% (№ 39) (see  In all cases we obtained negative results: a decrease in the friction coefficient was not observed.For MWC № 34, 35, and 36 we observed even an insignificant increase in the coefficient of friction in comparison with rolling without MWC.That may indicate a chemical interaction of coolant components with the metal surface of the rolls and rolled steel.

Conclusions
In steel rolling the metal-working coolant with 100 % concentration of boric acid esters of mono-and diglycerides based on sunflower oil is the most effective.It has been found that for the 2 st level of cobbing (εavg=53.72%) the absolute reduction is practically maximum (h=1.01mm) when initial thickness of strip ho is 1.88 mm and roller diameter Df is equal 260 mm.That is due to the high

Figure 1 .
Figure 1.A working stand of the industrial and laboratory rolling duo-mill of 260x200 in DSEA

Table 1 .
Compositions of new metal-working coolants (MWC) on the base of boric acid esters for metal rolling Notes.* These components were synthesized in Ukrainian State University of Chemical Technology (Dnipro, Ukraine).** As MWC comparison we used coolant «Universal-1TC» developed and used in Ilyich Iron and Steel Works (Mariupol, Ukraine), TU U23.2-31023384.002-2004.

Table 2 .
Compositions of new metal-working coolants (MWC) with chicken fat

Table 4 .
Experimental data of 08Kp steel cold rolling with MWC with high concentrations of boric acid esters of mono-and diglycerides based on sunflower oil (l0 = 200 mm, lb = 150 mm, Df = 260 mm)