热轧水/油基纳米润滑剂

赵敬伟，夏文真，吴 辉，姜正义

(1.澳大利亚伍伦贡大学 机械、材料、机电和生物医学工程系，澳大利亚 伍伦贡 2522；2.德国杜塞尔多夫马普钢铁研究所 材料结构及纳米/微机械部，德国 杜塞尔多夫 40237)

Hot rolling is widely applied in the steel-making industry to manufacture steel products with required dimensions,excellent mechanical properties and essential surface finish[1-3].With the increasing concerns about energy crisis and environmental pollution in modern society,hot rolling of steels has drawn considerable attention due to its huge energy consumption and pollutant emission.The lubrication applied in hot rolling of steels has been a long-standing research topic because it can lower the friction between the work roll and workpiece,reduce rolling force,increase roll service life,improve surface quality of steel products and reduce energy consumption[4-6].Traditionally,oil-in-water (O/W) emulsions are most commonly used as lubricants in hot steel rolling due to their low cost,good cooling ability,non-flammable,good lubricating and low adverse environmental impact characteristics[7-9].The application of O/W emulsions,however,may cause significant environmental issues due to the production of smoke during hot rolling and the discharge of residual oil after rolling[10-11].

Over the past few years,nanolubrication technology for hot steel rolling has attracted an increasing interest,and has been becoming a hot area of research[12-18].Nanolubricant is a combination of nanoparticles and liquid lubricants,and it has a potential to meet emission and environmental requirements while providing the desired friction and wear performance.The nanoparticles in the base fluid can enhance lubrication efficiency and have a good thermal stability at elevated temperatures.Nanoparticles in lubricants can exhibit excellent friction reduction and anti-wear properties through specified lubrication mechanisms[19-28].For nanolubricants containing spherical nanoparticles,four lubrication mechanisms,including rolling effects,protective film,mending effect and polishing effect,may be involved,as shown in Fig.1[29].The spherical nanoparticles tend to roll at the interfaces at low shear rates and pressures,affecting the friction behaviour from the pure sliding friction to the mixed sliding-rolling friction,as shown in Fig.1a.Nanoparticles can form a protective film as a result of tribochemical reactions or tribosintering,as shown in Fig.1b.These films can avoid the contact of mating surfaces and they have better tribological performance than the substrate.Mending effect (Fig.1c) is caused by the deposit of the nanoparticles into the defects to protect the friction surface.The nanoparticles with higher hardness can polish the lubricating surface,thereby reducing the surface roughness but sometimes increasing the weight loss,as shown in Fig.1d.

Fig.1 Schematic of lubrication mechanisms of nanolubricants containing spherical nanoparticles[29] (a)—rolling effect; (b)—protective film; (c)—mending effect; (d)—polishing effect

In recent years,O/W based nanolubricants for hot rolling of steels have been of increasing interests to scholars.Nanoparticles can improve the stability of O/W emulsions,reduce friction and wear,and thereby enhance the efficiency of lubrication in the hot rolling of steels.It has the potential to reduce the concentration of oil in O/W emulsions through replacing a certain amount of oil by nanoparticles but without deteriorating the lubrication performance,which can contribute to achieving the goal of “Green Rolling” in the steel-making industry.The purpose of this article is to present the latest progress in the research on O/W based nanolubricants,in the aspects of preparation,wettability,tribological properties,and lubrication effects in practical hot rolling,and the lubrication mechanisms will be discussed.This article will be of practical significance for promoting the development of lubrication technology for hot rolling of steels.

1 Preparation methodology

Nanolubricants are generally prepared by two methods: one-step method and two-step methods.One-step method yields nanofluids directly through physical vapour deposition technique or liquid chemical method to avoid the processes of drying,storage,transportation,and dispersion of nanoparticles and to minimise the agglomeration of nanoparticles.This method combines the synthesis of nanoparticles with a dispersion of the nanoparticles in the fluid[30-32].The limitations of one-step method are that synthesis of nanofluids cannot be scaled up for great industrial functions and the cost is high.In two-step method,nanoparticles,nanofibers,nanotubes,or other nanomaterials are first synthesised in the form of dry powder by means of inert gas condensation,chemical vapour deposition,mechanical alloying or other suitable techniques.Then,the nano-sized powders are dispersed into base fluids with the aid of stirring,sonication,homogenisation or other mixing techniques along with dispersants or surfactants,in order to minimise particle aggregations and improve dispersion behaviour[30,33].The main disadvantage of two-step method is that agglomeration of nanoparticles is caused due to the high surface area and surface activity of nanoparticles,which affects the lubrication performance of the prepared nanolubricants.

For the preparation of nanolubricants to be used in hot rolling,the two-step method,compared to the one-step method,is a better choice considering the scale and the cost of hot rolling production in the steel-making industry.Metal oxides,such as TiO2,Al2O3,and Fe3O4,with size of around tens of nanometers are commonly used as the additives.A typical preparation procedure of O/W based nanolubricants containing TiO2nanoparticles is as follows[34]: (i) TiO2nanoparticles with a particle size of 30 nm are first dispersed in distilled water under high-speed stirring for 3 min; (ii) commercial lubricant oil is dropped into the fluids and stirred for 3 min; and (iii) the mixed nanolubricants are subjected to ultrasonic vibration for 10 min.The concentration of TiO2nanoparticles can vary depending on the actual needs of research.The amount of oil is determined based on the case of O/W emulsions practically used in hot rolling.As O/W emulsions with oil concentrations mass fraction varying from 1% to 5% are in general use in steel production lines,the amount of base lubricant oil added in the nanolubricants can be 1%～5%.There are a number brands of lubricant oils,such as Quaker,commercially available in the market for hot rolling of steels.The purpose of mechanical stirring and ultrasonic vibration is to ensure the oil and nanoparticles distribute dispersedly and homogeneously in the prepared O/W based nanolubricants.

2 Wettability

Wettability is one of the most important lubricant characteristics,which reflects the affinity and adhesion of a liquid to maintain contact with a solid surface.Generally,superb wettability intends to facilitate the formation of a lubrication film which is able to separate the friction pair contacting each other.Wettability is usually evaluated by contact angle,which can be expressed by[35]:

whereθCis the equilibrium contact angle,γSGis the solid-gas interfacial energy,γSLis the solid-liquid interfacial energy,andγLGis the liquid-gas interfacial energy.The smaller the contact angle,the better the wettability.The contact angle formed when a liquid/gas interface meets a solid surface is schematically shown in Fig.2.

Fig.2 Schematic illustration of contact angle θC

Fig.3 shows the variations of contact angles when TiO2nanoparticles (particle size: 30 nm) are added into the O/W emulsion containing mass fraction 1% oil[36-37].For the contact angle tests,lubricants were dropped onto the surface of a high speed steel which was obtained directly from work rolls for hot rolling and had a surface roughness of 0.55 μm.It can be seen that the addition of TiO2nanoparticles into the O/W emulsion induces decreased contact angles.Also,the contact angle decreases gradually first,then to a minimum value at the TiO2concentration mass fraction of 4%,and finally increases slightly with the further increase of TiO2.The results indicate that the wettability between the lubricants and the work roll increases first,and then decreases with an increase of the concentration of TiO2nanoparticles.An addition of 4% TiO2mass fraction nanoparticles into O/W emulsion containing mass fraction 1% oil is the most effective for obtaining the best adhesion of nanolubricants with the work roll.An increase in the concentration of TiO2induces more TiO2nanoparticles accumulate near the solid surface,which decreases the interfacial tension between the droplet and the substrate,and thus promotes the wettability of the substrate to become more hydrophilic[38].However,the concentration of nanoparticles should be controlled within an appropriate range because nanoparticles will agglomerate with a larger particle size when the concentration of nanoparticles is high,which affect the tribological properties and lubrication performance in hot rolling.The contact angle of a nanolubricant is affected by its pH value.An increase in pH value will result in a greater contact angle which thus reduces the wettability of nanolubricants[39].

Fig.3 Variations of contact angles when TiO2 nanoparticles are added into the O/W emulsion containing 1% oil [36-37]

3 Tribological properties

Reduction in friction and wear is one of the most important objectives of tribological research,and a key indicator of lubrication performance.Due to the special physical and chemical properties of nanomaterials,a number of studies have been conducted in recent years with the purpose of improving the tribological properties of O/W based nanolubricants.We recently conducted systematic investigations on the tribological properties of TiO2nano-additive O/W based nanolubricants using a ball-on-disk tribometer.In the research,the balls made of Cr steel were used to represent work rolls in rolling,and the disks made of high strength or stainless steels were the workpieces to be rolled.The configuration of the ball-on-disk tribometer is schematically shown in Fig.4.The normal force applied to the ball holder was measured by an Fz load cell installed above a spring.The frictional force was induced by the combination of the rotating motion and the normal load,and it was recorded by an Fx load cell attached to the right point of the arm.The disk holder and disk,which were controlled by a servo motor for rotating,were located in a liquid container.

Fig.4 Schematic of the ball-on-disk tribometer used for tribological tests

During hot rolling of steels,oxide scale forms inevitably on the workpiece surface and affects the tribological conditions between the work roll and the workpiece.It results in changes of the rolling forces,torques,and power consumption,as well as the wear and the surface quality of the work roll in hot rolling.For simulating the real surface of the workpiece in hot rolling,the disks for tribological tests need to be oxidised so that oxide scales are formed.Fig.5 shows the effect of TiO2nanoparticles on the coefficient of friction (COF) of O/W emulsion with the progressing of tribological testing time[40].In the figure,‘O/W’ means O/W emulsion containing mass fraction 1% oil,and ‘1,2TiO2’ means O/W nanolubricant containing mass fraction 1% oil and 2% TiO2nanoparticles.The high strength steel disks were first machined and then oxidised prior to tribological tests.It is clear that an addition of 2% TiO2mass fraction nanoparticles into O/W emulsion can lead to a significant reduction of the COF.

Fig.5 Effect of TiO2 nanoparticles on the COF of O/W emulsion with the progressing of tribological testing time[40]

Fig.6 shows the COFs measured for the O/W based nanolubricants with different oil mass fraction (0.05,0.125,0.25,0.5 and 1%) but with the same nano-TiO2addition of 2.0%.It can be seen that the reduction of oil concentration induces an increase of the COF.Serious fluctuation phenomenon of COF is found when the oil concentration mass fraction is 0.05%,but becomes better with the increase of oil,as shown in Fig.6a.Fig.6b shows the average COF values of the nanolubricants under different lubrication conditions.It is clear that an addition of 1% oil induces the lowest COF among all the lubrication conditions.When the oil concentration mass fraction is reduced to 0.5% and 0.25%,the COF values are similar,and both are slightly higher than that under O/W emulsion.When the oil concentration mass fraction is further reduced to 0.125% and 0.05%,however,significant increase in COF is caused,and the lower the oil concentration,the higher the COF.

Fig.6 (a) Dependence of COF on tribological testing time,and (b) average COF values under different lubrication conditions[40]

The TiO2nanoparticles in the O/W based nanolubricants are first distributed on the surface of the oil droplets and,when the surface of the oil droplet is saturated,TiO2nanoparticles are then distributed into the water.The TiO2nanoparticles coming from the oil droplets can remain within the oil pool,and finally reduce the COF.The extra TiO2nanoparticles in the water tend to adhere onto the surface of the ball,disk and oil pool in order to reduce free energy,and then become a nanoparticle aggregation area around the oil pool while not entering the contact area[34,40].The nanoparticles in this area can be held in the valleys to protect the surface of the oxidised disk,but rub against each other to increase the COF.For the case of ‘1,2TiO2’ lubricant,no nanoparticle aggregation area exits as all the nanoparticles can be effectively dissolved into the oil[36].With the decrease of oil,the amount of saturated nanoparticles increases,which will enter water and form a nanoparticle aggregation area.Especially when the oil concentration mass fraction is very low,such as 0.05%,significant aggregation of nanoparticles will be induced along the horizontal level.As a result,the number of nanoparticles becomes less in the oil pool but more around the oil pool.In addition,only few nanoparticles can enter the contact area due to the thinning of oil film with the reduction of oil percentage[41].Therefore,both the COF and the degree of fluctuation of the COF increase with testing time due to the reduction of oil concentration.

The oxide scale formed on the steel surface in hot rolling is generally composed of three layers of iron oxide phases,with wustite (FeO) being the closest to the steel matrix,magnetite (Fe3O4) the intermediate layer,and hematite (Fe2O3) the outermost layer.O/W based nanolubricants may exhibit different tribological properties when working on the oxide scale with different compositions.Fig.7 shows the dependences of surface roughness and COF on oxidation time of the disks used for tribological tests[42].The disks were made of high strength steels and oxidised at 1 100 ℃ for different times.The O/W based nanolubricants used in the tribological tests contained mass fraction 1% oil and 2% TiO2nanoparticles.A change of oxidation time induces the variation of the surface morphologies of oxide scale,and therefore the surface roughness of the disks.It can be seen that the surface roughness decreases sharply when the oxidation is increased to 10 min,and then decreases slightly with the further increase of oxidation time.Differently,COF decreases first,then reaches the minimum value at the oxidation time of 20 min,and finally increases when the oxidation time is further increased.Such a change of COF with oxidation time is a result of the reaction between nanoparticles and oxide scale under the mechanisms of thin film lubrication and boundary lubrication.The reduction of surface roughness results in a decrease of the COF when the lubrication regime is boundary lubrication.When the surface roughness decreases further,the main lubrication regime becomes thin film lubrication.Once the surface roughness reduces too much and some of the TiO2nanoparticles cannot go through the contact area,the COF increases[42-43].

Fig.7 Dependences of surface roughness and COF on oxidation time[42]

4 Lubrication effects in hot rolling

The developed O/W based nanolubricants are to be finally applied to practical hot rolling of steels.Hot rolling tests are therefore essential to be conducted to evaluate the lubrication effects of the lubricants.Fig.8 shows the effects of O/W based nanolubricants on rolling force during one-pass hot rolling[44].In the figure,‘Dry’ means no lubrication,‘O/W’ means O/W emulsion containing mass fraction 1% oil,and ‘0.5TiO2’,‘1TiO2’,‘1.5TiO2’,‘2TiO2’,‘4TiO2’ and ‘6TiO2’ mean nanolubricants containing mass fraction 0.5%,1%,1.5%,2%,4% and 6% TiO2nanoparticles,respectively,in addition to 1% oil.The specimens used were 304 stainless steels,and the hot rolling temperature was around 1050 °C.It can be seen from Fig.8 that nanolubricants can effectively reduce the rolling force comparing to the conventional O/W emulsion.In addition,the rolling force decreases first,and then reaches the minimum value at the TiO2mass fraction of 1.5%,and finally increase when the concentration of TiO2is further increased.

Fig.8 Effects of lubrication conditions on rolling force during one-pass hot rolling[44]

There is a positive relationship between the rolling force and COF during rolling process.The high temperature circumstance during hot rolling has a significant influence on the lubrication mechanism,but the contacting time between the lubricant oil and hot workpiece is not enough for lubricant oil to reach its burning point,and thus the oil/water mixture still keeps in liquid[45].The oil working in the form of liquid in hot rolling process can become a thin oil film in hot rolling process,which reduce the COF and therefore the rolling force.The lubrication mechanism of the O/W based nanolubircants in the hot rolling process is schematically shown in Fig.9[44].

Fig.9 Schematic diagram of (a) side and (b) top cross-section views of lubrication mechanism of the nanolubricant during hot rolling[44]

Fig.10 shows the effects of TiO2concentration mass fraction in O/W emulsion (1% oil) on rolling force under different hot rolling conditions[42].In order to produce two kinds of surface morphologies of oxide scale,one group of specimens was heated to 1100 °C for 1 h under protection of nitrogen(first condition) and the other group was heat treated without any protection (second condition).The start rolling temperature was about 1050 °C.It can be seen that the rolling force under the first condition decreases slightly when thew(TiO2) increases up to 4%,and then increases remarkably when the w(TiO2) is over 4%.In case of the second condition,the rolling force decreases continuously with the increase ofw(TiO2).Also,the rolling force under the second condition is lower than that under the first condition.

Fig.10 Effects of TiO2 concentration mass fraction on rolling force under different hot rolling conditions[42]

In case of the first rolling condition,the addition of TiO2nanoparticles in the O/W emulsion can reduce the COF when the mass fraction of TiO2nanoparticles is below 4%.This is because enormous craters on the surface of the workpiece within the contact area can hold a mass of nanoparticles,and therefore provide better lubrication.Once the nanoparticles enter the contact area,they can take effect to reduce COF.When the mass fraction of nanoparticles is over 4%,the interface between the oil droplet and water will be saturated by nanoparticles,and some of the nanoparticles will remain in the water and concentrate to around the contact area.This causes the nanoparticles to rub against each other and the surface of work roll,and increases the COF.For the second rolling condition,the addition of 6% concentration mass fraction of TiO2nanoparticles induces continuous reduction of COF,because cracks occur more easily on the surface during deformation of thick scale and can hold a large amount of nanoparticles including that exist in the water,so that more nanoparticles enter the contact area to further reduce the COF.As an increase in the thickness of oxide scale contributes to the decrease of COF,the rolling force under the second condition is lower than that under the first condition[46].The lubrication mechanisms of the O/W based nanolubricant containing 6% TiO2nanoparticles under different hot rolling conditions are schematically illustrated in Fig.11[42].Such mechanisms can also be used to explain the positive effect of TiO2nanoparticles on reducing the rolling force in the second pass of hot rolling[43].

Fig.11 Lubrication mechanisms of the O/W based nanolubricant containing 6% TiO2 nanoparticles under different hot rolling conditions of (a,b) first condition and (c,d) second condition corresponding to Fig.10[42]. Note: (a,c) and (b,d) are respectively the side and top cross-section views

5 Conclusions

O/W emulsions widely used as lubricants in hot rolling of steels may cause significant environmental issues due to the production of smoke and the discharge of residual oil after rolling.The addition of nanoparticles in O/W emulsions can enhance the efficiency of lubrication,and has a potential to reduce the concentration of oil in O/W emulsions through replacing a certain amount of oil but without deteriorating the lubrication performance in hot rolling of steels.This paper examines the latest progress made in the research on O/W based nanolubricants for hot rolling of steels,focusing on the preparation methodology,wettability,tribological properties and lubrication effects in hot rolling.As a new research area with great practical significance,much more work on O/W based nanolubricants still deserve to be done until the developed lubricants can be finally applied in practical hot rolling process of steels in the steel-making industry.