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Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: With its high strength and hardness,wear-resistant steel has become an important material in the field of construction machinery manufacturing.Given that quenching technology is a crucial component of wear-resistant steel production,the selection of the cooling method to be used during this process is important.In this study,the feasibility of quenching wear-resistant steel by air-atomized water spray cooling was studied,and the cooling rate,microstructure,and hardness of wear-resistant steel under various cooling device structures were analyzed.The results reveal that the air-atomized water spray cooling method is an effective technique in quenching wear-resistant steel.Furthermore,martensite and uniform hardness were obtained by the air-atomized water spray cooling technique.As the space between the nozzles in each row in the device increased,the cooling rate was reduced during quenching.Meanwhile,the martensite content decreased,and more carbides were observed in the martensitic structure.A mixture comprising self-tempered martensite and bainite was formed at a large distance over a longer period of time.All these factors resulted in lower hardness and worse property uniformity.

Key words: wear-resistant steel; air-atomized water spray cooling; cooling rate; microstructure; hardness

1 Introduction

Wear-resistant steel possessing high strength,excellent hardness,moderate impact toughness,and sufficient welding properties is widely used in the field of engineering machinery[1].It is generally produced in three ways,including quenching and low-temperature tempering (QT),direct quenching and low-temperature tempering (DQT),and thermal mechanical controlled processing.The microstructure of wear-resistant steel generally consists of tempered martensite,although a small amount of bainite is sometimes introduced to improve its impact tough-ness[2].Current studies on wear-resistant steel mainly focused on alloy composition[3-5],heat treat-ment process[6-7],hot rolling process[8],and addi-tional properties[9-10],among others.In comparison,only a few studies have investigated cooling methods,focusing on the effects of laminar cooling after hot rolling,including intensive cooling,sparse cooling[11],and segmented cooling[12]on the micro-structures and properties of wear-resistant steels.In the last decades,because of its uniform cooling characteristics,the air-atomized water spray cooling technique has been investigated in cold-rolled sheet applications[13-14]but not in hot-rolled wear-resistant steels.

In accordance with the physical states of the cooling medium,cooling methods can be divided into jet impingement cooling,laminar cooling,air jet cooling,air-atomized water spray cooling,and water mist cooling[15].Jet impingement cooling includes slit jetting and pressure jetting cooling,and both the cooling methods are used in the traditional heat treatment for hot-rolled sheets due to their high cool-ing rates.However,the inhomogeneous distribution of water flow results in uneven cooling during quen-ching,thereby causing fluctuations in mechanical properties.Meanwhile,the air jet and air-atomized water spray cooling methods are applied in con-tinuous annealing for cold-rolled sheets[16].Due to the uniform cooling effect of both techniques,extraordinary uniform properties and good flatness are produced during the cooling process.To solve the problem of performance fluctuations during the heat treatment of hot-rolled sheets,it is necessary to study the effect of air-atomized water spray cooling in the application of wear-resistant steel during heat treat-ment.

In the present work,the temperature curves of wear-resistant steel under various cooling device structures were collected,by which the cooling rates were calculated during quenching.Next,the micro-structures under a variety of parameters were obser-ved,after which the effects of cooling device stru-ctures on the hardness and property uniformity of the wear-resistant steel were analyzed.The application effects of the air-atomized water spray cooling method during the heat treatment for wear-resistant steel were evaluated in terms of four aspects:cooling rate,microstructures,hardness,and property unifor-mity.

2 Materials and experiments

2.1 Experimental materials

The experimental steel was an industrially produced 4 mm hot-rolled steel sheet.The steel was prepared by smelting,continuous casting,and hot rolling.The hot rolling parameters were 1 230 ℃ (reheating temperature),900 ℃ (finish rolling temperature),and 600 ℃ (coiling temperature).The chemical composition of the sheet is listed in Table 1.Specimens with dimensions of 4 mm×200 mm×300 mm were machined from the 4 mm hot-rolled steel sheet.

2.2 Cooling device

The schematic diagram of the air-atomized water spray cooling device shown in Fig.1 includes the heat-ing,water supply,air supply,aerosol cooling,and tran-smission systems.There are several rows of nozzles on the spray box of the aerosol cooling system,and the space between the nozzles in each row is adjustable—ranging from 50 mm to 400 mm.The two adjacent rows of nozzles are staggered[14],and multiple noz-zles are installed on each row.The schematic diag-ram of each row of nozzles is shown in Fig.2.The atomization process is achieved in an appropriate air-to-water pressure ratio.

Fig.1 Schematic diagram of the air-atomized water spray cooling device

Fig.2 Schematic diagram of the nozzle arrangement

The aerosol quenching was performed in the cooling device with an air-to-water pressure ratio of 2∶1.The specimens were heated at a temperature of 925 ℃ and a holding time of 15 min in the heating furnace.The movement speed of the specimens was 5 m/min.K-type thermocouples for temperature data collection were welded on the center point of the upper surface of the steel sheet.

2.3 Cooling rate calculation method

The ferrite transformation of experimental steel started at about 780 ℃.The martensite trans-formation started at about 400 ℃ and ended at about 200 ℃.The critical cooling rate of martensite transformation is 60 K/s.In consideration of the fact that the effect of cooling rate on properties is not obvious after the martensite transformation starts,we performed the cooling rate calculation as a segmented calculation,with first and second tem-perature intervals of 400-800 ℃ and 200-400 ℃,respectively.

2.4 Microstructures

The specimens were polished and etched with a solution of 4%HNO3in deionized water.The microstructures of these specimens were observed using a Leica optical microscope (OM) and a ZEISS (EVO MA25) scanning electron microscope (SEM).

2.5 Hardness test

The Brinell hardness test was performed using a BRIN4000 Brinell hardness tester with a loading force of 3 000 kg,a spherical indenter with a diameter of 10 mm,and a holding time of 12 s.A total of 15 position points were tested for each speci-men,as shown in Fig.3.The average value was considered as the final value after removing a maxi-mum and a minimum value.Before conducting the hardness test,the top and bottom surfaces were ground up to 0.5 mm to remove the decarbonized and softened areas on the surface.

Fig.3 Diagram of the hardness testing points for the wear-resistant steel

3 Results and analysis

3.1 Effect of the cooling device on the quench-ing process

The temperature curves of the specimens during air-atomized water spray cooling under various types of device structures are shown in Fig.4.After the heating furnace door was opened and the samples were not yet out of the heating furnace,the temperature of the steel slightly decreased with a cooling rate of 2 K/s due to the cold air entering the heating furnace cavity.During the process of transferring them from the heating furnace to the cooling device,the specimens were exposed to the air,which caused the temperature to decrease at a cooling rate of 6 K/s.Finally,when the specimens were quenched in the cooling device,the tem-perature was sharply reduced to below 100 ℃.Notably,an aerosol cooling zone and an air cooling zone appeared during the spray cooling process.

When the space between the nozzles in each row is 50 mm,the specimens are almost completely in the aerosol cooling zone during quenching before quickly being cooled down to below 100 ℃.When the spacing is 150 mm and above,the specimens shift from the aerosol cooling zone into the air cool-ing zone,causing the core temperature to be higher than the surface temperature,thus leading to surface temperature reversion.Therefore,as the specimens repeatedly pass through the aerosol and air cooling zones,they demonstrate the phenomenon of surface temperature oscillation.As the space increases,the proportion of the air-cooled zone in the cooling pro-cess also increases,while the proportion of the aerosol-cooled zone decreases accordingly.Thus,the time taken to quench the steel from 925 ℃ to below 100 ℃ gradually increased.

Fig.4 Temperature curves under various types of device structures

Due to the temperature oscillations in the surface,the temperature data points are smoothed to accur-ately calculate the cooling rate.The cooling rates between different temperature zones under various device structures are shown in Fig.5.As can be seen,the cooling rates in the temperature range of 400-800 ℃ are higher than those in the temper-ature range of 200-400 ℃.The high-temperature zone is a film boiling region with low heat flux den-sity,while the middle- and low-temperature zones are nucleation boiling and transition boiling regions with high heat flux density[17].However,the mar-tensitic transformation occurs during quenching in the 200-400 ℃ zone,which generates a large amount of latent heat[18].In turn,this results in a lower cooling rate in the 200-400 ℃ zone compared with that in the 400-800 ℃ zone.

Fig.5 Cooling rates of the wear-resistant steel under vari-ous device structures

With the space increasing from 50 mm to 400 mm,the cooling rates in both temperature zones gradually become lower,while the different values of cooling rates between the two temperature zones become smaller.The cooling rate in the 400-800 ℃ zone and the 200-400 ℃ zone decreases from 329 K/s to 47 K/s and from 67 K/s to 14 K/s,respectively.This leads to the increase in the proportion of air-cooled parts and the corresponding decrease in the proportion of aerosol-cooled parts due to the increa-sed space between the nozzles in each row.More-over,the cooling capacity of aerosol cooling is much stronger than that of air cooling,thus leading to a decrease in cooling rate.

3.2 Effect of cooling device structures on micro-structures

The microstructures of the wear-resistant steel under various cooling device structures are shown in Fig.6.With the space between the nozzles in each row being 50 mm,the heated sheet is almost in the aerosol cooling zone,and there is no obvious temperature oscillation.Furthermore,the cooling rate over 60 K/s is greater than the critical cooling rate; hence,the quenched martensite without car-bide precipitation is obtained.

With the space increasing to 150 mm,a smaller area of the air-cooled zone appears,which makes the cooling rate decrease.On the one hand,the cooling rate in the temperature range of 400-800 ℃ is 125 K/s,which is much higher than the critical cooling rate.At this point,martensite appears.On the other hand,with the occurrence of temperature oscil-lation,the carbide precipitates inside the martensite in a phenomenon called auto-tempering.

When the space between the nozzles in each row increases further to 400 mm,the proportion of the air-cooled zone becomes larger during the cooling process.The cooling rate decreases to less than 60 K/s in the 400-800 ℃ zone,and the mixture of bainite and auto-tempering martensite is obtained.

Fig.6 OM and SEM microstructures of the wear-resistant steel under various cooling device structures

The above microstructure analysis shows that the quenched martensite can be obtained by the method of shortening the space between the nozzles in each row during quenching.This indicates that wear-resistant steel can be quenched by the air-atomized water spray cooling method.

3.3 Effect of cooling device structures on the hardness

The hardness of the wear-resistant steel under various cooling device structures is shown in Fig.7,while the hardness statistics under different cooling conditions are listed in Table 2.As can be seen,the average hardness (HB) of the top surface of the wear-resistant steel is higher 5-10 than that of the bottom surface after spray cooling.The reason is that when the aerosol falls on the specimen surface,it first vaporizes into steam and then gathers on the steel surface to form the residual water.The residual water on the top surface is difficult to drain.On the contrary,the residual water on the bottom surface drains easily due to gravity.The thermal conduc-tivity coefficient of water is larger than that of air,resulting in the cooling rate of the top surface being higher than that of the bottom.Therefore,fewer carbide precipitates,as well as higher strength and hardness,are obtained.

When the space between the nozzles in each row increases from 50 mm to 400 mm,the average hardness values (HB) of both the top and bottom surfaces gradually decrease from 462 to 405 and from 457 to 398,respectively.This can be attributed to the fact that,with the increase in the space,the cooling rate becomes lower,and the microstructures change from quenched martensite to auto-tempering martensite before finally forming a mixture of auto-tempering martensite and bainite.

As the space between the nozzles in each row increases,hardness uniformity deteriorates.As can be seen,the standard deviation of the top surface hardness increases from 2.4 to 15.1.Similarly,the standard deviation of the bottom surface hardness increases from 1.4 to 13.7.The reason is that the more times the specimens pass through the aerosol- and air-cooled zones,the greater the probability of cooling inhomogeneity because of the increase in the space between the nozzles in each row.

Fig.7 Hardness of the wear-resistant steel under various cooling device structures

In summary,the cooling rate is over 60 K/s during air-atomized water spray cooling under an appropriate cooling device structure.This leads to the creation of wear-resistant steel with quenched martensite,high strength and hardness,and uniform performance.

4 Conclusions

(1) Wear-resistant steel with quenched marten-site,uniform performance,and high strength and hardness can be prepared under air-atomized water spray cooling with an appropriate cooling device structure.

(2) During air-atomized water spray cooling,the cooling rates of wear-resistant steel in different temperature zones vary.Furthermore,the cooling rate in the 400-800 ℃ zone is higher than that in the 200-400 ℃ zone.

(3) The cooling device structure has a strong impact on the cooling rate,microstructure,and pro-perties.During air-atomized water spray cooling,the increase in the space between the nozzles in each row leads to a lower cooling rate,more carbide precipitation in quenched martensite,more bainite,lower hardness,and worse property homogeneity.