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The effect of cooling rate and coiling temperature on the microstructure andmechanical properties of a plain carbon steel

更新时间:2016-07-05

1 Introduction

The thermo-mechanical controlled process(TMCP) is a key technology in the hot-rolling process,and its core technology obtains fine and uniform microstructure by controlling the change of rolling temperature and cooling rate to improve the strength,plasticity,toughness,and welding perfor-mance of steel[1-2].With the development of new products and rolling technology,ultra-fast cooling(UFC) has become an important technology to control temperature rapidly and accurately after rolling.The cooling rate of UFC is much quicker than the cooling rate of traditional laminar flow cooling technology to reduce the energy loss of the cooling stage,improve the strength of the material,reduce the alloy element usage,excavate the poten-tial of steel materials and save energy resources.Thus,UFC has great significance in reducing cost,increasing efficiency,and producing green pro-ducts[3-7].At present,about 1.0%-1.7% Mn is usually added to steel to produce low alloy steel that has a strength of 345 MPa.Some manufacturers are restricted by equipment capacity,needing to add micro-alloyed elements,such as Nb,V,and Ti,which increase the cost of steel-making and have adverse effects on the subsequent rolling process and welding perfor-mance[8].Therefore,studying the UFC technology by using TMCP craft and equipment effectively is an important way to reduce the production cost,raise the bearing capacity of components,and en-hance the market competitiveness of steel.

In this paper,the tested steel is a kind of hot rolled plain carbon steel.The influence of three different cooling processes,i.e.,UFC,intensive cooling(IC),and laminar cooling(LC),on the microstructure and properties at the same coiling temperature has been studied.Moreover,the effect of different coiling temperatures during UFC process has also been studied.The study can provide experimental basis and theoretical guidance for the practical application of hot rolled UFC technology.

2 Experimental procedure

The material was produced by a 50 kg vacuum induction furnace and cast into a small ingot fol-lowed by forged and cut into 25 mm×200 mm×200 mm square billets.The chemical composition is listed in Table 1.These billets were heated to 1 100 ℃,held for 1 h,and then rolled to 8 mm thick flats through three rolling passes on the laboratory rolling mill with opening and finish rolling temperatures of 1 020 and 850 ℃,respec-tively.After rolling,separate flats were cooled to 580 ℃ through three different cooling processes,IC,LC,and UFC,and then held in the furnace for 1 h to simulate the coiling process before final air cooling to room temperature.In addition,to analyze and compare the influence of different coiling temperatures on microstructure and properties in the case of same UFC process,the specimen was cooled to 500 ℃ through UFC and held for 1 h followed by air cooling to room temperature.The finish rolling temperature,cooling type,coiling temperature and cooling rate of the samples are shown in Table 2,and the final metallographic specimens were made from the room temperature samples.After grinding,polishing,and corrosion in 4% Nital solution,the microstructure was observed and determined using a ZEISS optical microscope.

1.2.3 CT图像重建和筛选 将受试者CT扫描原始数据应用单扇区重建(Snapshot Algorithm)和回顾性心电门控技术重建R-R间期65%、70%、75%、80%相位窗上进行横切面CT图像重建并传送到SUN图像工作站,应用Advantage W indows 4.2分析软件在5个相位窗上对左、右冠状动脉及其主要分支血管进行分析,包括二维曲面重组(CR)、多平面重组(MPR)、容积再现(VR)重组、最大密度投影(MIP),筛选最佳图像用于血管腔的评价。

Table 1 Chemical composition of the tested steel %

wCwSiwMnwPwSwNb0.1400.2001.2000.0160.0040.010

Table 2 The cooling types,coiling temperatures,and cooling rates of the samples

SampleNo.CoolingtypeFinishrollingtemperature/℃Coilingtemperature/℃coolingrate/(K∙s-1)S1LC84558025S2IC84758053S3UFC845580120S4UFC843500120

3 Experimental results

3.1 Microstructure

Figs.1(a)-(d) illustrate the microstructure of samples cooled by different processes at quarter-thickness position.It can be found that sample S1 consists of ferrite and a small amount of pearlite with coarse grains.The amount of pearlite increases as the cooling rate is increased to 53 K/s,but the grain size remains almost unchanged.When the cooling rate is quick enough as 120 K/s,bainite transformation occurs,and the final microstructure is composed of ferrite,a small amount of pearlite,and bainite with refined grains.Comparing Figs.1(c) and 1(d) shows that the sample contains a large amount of bainite when the coiling temperature is reduced from 580 to 500 ℃ in the case of the cooling rate is 120 K/s.The differences between the figures show that the cooling and coiling temperatures have important effects on phase composition and grain size.

3.2 Mechanical properties

The effects of cooling rate and coiling tem-perature on microstructure and mechanical pro-perties of a plain carbon steel were investigated by combining metallography and tensile experimental results,and the conclusions can be drawn as follows:

Fig.1 Microstructure of samples under different cooling processes

Table 3 Tensile properties of samples

SampleNo.Yieldstrength/MPaTensilestrength/MPaElongation/%S138753124.3S239052724.8S352461622.3S460868215.8

4 Discussion

The plate was cooled by water cooling and air cooling after finish rolling,and the surface tem-perature rose in the air cooling process because of the higher internal temperature.Therefore,the coiling temperature is higher than the actual water cooling temperature.Figs.2(a)-(d) show the tem-perature curves of the tested steel simulated by Abaqus software,and these curves record the temperature change of surface,quarter-thickness position,and half-thickness position during the cooling process.It can be seen from Fig.2 that the out-water temperature is the lowest point of cooling process for the surface,and the values were 570 ℃ for LC and 551 ℃ for IC,which are higher than the BS temperature of the tested steel.Thus,bainite transition does not occur and the room temperature microstructure is ferrite and pearlite.Nevertheless,when cooled by UFC to 580 ℃,although the coiling temperature is high,the out-water temperature of 500 ℃ is lower than BS temperature,producing to a small amount of bainite and greatly improving the strength.When cooled by UFC to 500 ℃,the coiling temperature was lower than BS temperature,making the room temperature microstructure contain a large amount of bainite.The simulation results are consistent with the microstructure shown in Fig.1.

(1)

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The formula for calculating the bainite transfor-mation start(BS) temperature is the following[9]:

Fig.2 Cooling temperature curves of samples under different cooling processes

4.1 The effect of cooling rate on the micro-structure and mechanical properties

According to the experimental results of samples S1,S2,and S3,the microstructure and the extent of grain refinement are strongly influenced by the cooling rate,which determine the final properties of steel.For LC,coarse ferrite structure leads to low strength and high elongation.When the cooling rate was increased to IC,the amount of pearlite increased,while the grain size was still coarse,resulting in a small change in performance.Nevertheless,when the cooling rate was increased to 120 K/s,the amount of ferrite and pearlite decreased,and bainite started to form,greatly improving the strength of the experimental steel.Moreover,the precipitation of micro-alloying elements in the high temperature can be reduced with the increased cooling rate to en-hance the precipitation strengthening effect.On the other hand,the driving force was very large due to the large undercooling,leading to more initial nucle-ation sites and larger ferrite nucleation velocity,causing a very sluggish growth of grains under a very quick cooling rate that plays the role of grain refinement.Fine-grain strengthening significantly improves the strength of the material while ensuring a higher elongation.Therefore,the reasonable con-trol of the cooling rate after rolling is conducive to suppressing the grain growth,controlling internal phase change process,and obtaining an ideal microstructure to make the properties of the material meet the requirements.

4.2 The effect of coiling temperature on the microstructure and mechanical properties

(1) The amount of ferrite decreases with increased cooling rate when cooled to the same final cooling temperature.Ferrite grain size is refined,and bainite transformation occurs to ensure high strength and elongation,as the cooling rate is quick enough.

5 Conclusions

Tensile experiments were performed at room tem-perature according to the national standard GB/T 228.1-2010,and the results are shown in Table 3.Based on the results of samples S1,S2,and S3,the conventionally tested steel showed a yield strength of 387 MPa,a tensile strength of 531 MPa,and an elongation of 24.3% when the coiling temperature is 580 ℃.These properties slightly change for the IC sample.However,compared with LC,yield strength and tensile strength were improved about 137 and 85 MPa,respectively,while the elongation was decreased by only 2 percentage points for UFC.Comparing the results between sample S3 and S4 reveals that the yield strength and tensile strength of the steel plate increased to 608 and 682 MPa,and the elongation reduced to 15.8% when the coiling temperature was reduced from 580 to 500 ℃ at a cooling rate of 120 K/s through UFC.

According to the experimental results of samples S3 and S4,the coiling temperature has a great effect on the microstructure of the steel,and as the coiling temperature decreases,the microstructure of the tested steel changes from ferrite+pearlite to bainite.When the coiling temperature decreases from 580 to 500 ℃ with a cooling rate of 120 K/s,a ferrite phase transition at a high temperature is inhibited,and the abnormal phase begins to appear,such as a large amount of bainite.The appearance of an abnormal phase is the main reason of the unquali-fied elongation.Thus,when the final cooling tem-perature is between the start and finish temperature of ferrite transformation,the ferrite grain size becomes refined and the ferrite proportion becomes reduced when the cooling rate is quick enough.A small quantity of bainite appears in the insulation process after cooling to improve the strength of the steel plate.In contrast,when the final cooling temperature is lower than the final temperature of ferrite transformation when the cooling rate is quick enough,the ferrite and pearlite phase transformation is inhibited.Consequently,a larger concentration of bainite is obtained,showing a higher tensile strength and lower elongation.

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Using Equation(1),the BS temperature of the tested steel is calculated as 541 ℃.

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(2) Using the same cooling rate under UFC,yield strength and tensile strength are improved with decreased final cooling temperature,but the elongation decreases too much to meet the requirements.

(3) To fully use the advantages of UFC,the cooling rate must be sufficiently quick,and the appropriate coiling temperature should also be carefully selected to make ferrite and pearlite.In addition,bainite transformation occurs under lower temperatures,refining ferrite grain and enhancing the proportion of pearlite and bainite to improve the strength while plasticity is also ensured.Under this condition,Mn element content can be reduced to save cost or produce higher strength steels with the same chemical composition.

References

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[3] WANG B,ZHOU X G,LIU Z Y,et al.Effect of ultra fast cooling on microstructure and mechanical properties of medium carbon steels[J].Journal of Northeastern University,2011,32(1):48-51.

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[5] HU S E,SUN W H,XU Y B,et al.Effect of ultra-fast cooling on properties and microstructure of ultra-high strength steel[J].Sichuan Metallurgy,2015,37(1):29-36.

[6] ZHENG D S.Application of ultra-fast cooling technology on commercialized production of TMCP steel grade[J].Wide and Heavy Plate,2016,22(6):23-27.

[7] YUAN P J,WU D,FU C W,et al.Effect of ultra-fast cooling on steel strength[J].Iron and Steel,2012,47(10):57-60.

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ZHANG Chen,HU Xiaoping,LIU Gang and WANG Huanrong
《Baosteel Technical Research》2018年第1期文献

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