Effect of the hottest roughing passes on the strai

Effect of roughing passes on strain induced precipitation kinetics of Nb (C, n)

Abstract: in this paper, the effects of reheating temperature, strain and temperature of roughing passes on strain induced precipitation kinetics after finishing rolling deformation passes were studied for several niobium containing high strength low alloy steels (HSLA). Strain induced precipitation has been confirmed by the strengthening effect of the second finish rolling deformation carried out by rolling experiments or plane compression tests. Precipitation has also been observed by thin film electron microscopy (microscopy). It is found that decreasing reheating temperature/roughing temperature and increasing roughing deformation significantly accelerate precipitation, which is attributed to the accumulation of solid solution niobium

Influence of roughing rolling passes on kinetics of strain induced precipitation of Nb(C,N)

ldes and llars at the University of Sheffield

Abstract

the effects of reheating temperature and of the strain and temperature of roughing def1 can also improve the efficiency and reduce the cost of solar cells. Formation passes on the kinetics of strain induced precision after a finishing deformation pass have been investigated for severe niobium high low alloy (HSLA) steels Strict induced precision was detected via its strengthening effect on a second finishing deformation carried out either by empirical rolling or by plane strai Jinan testing machine factory testing machine analysis n compression tests Precipitates were also observed using thin foil electron microscopy. Decreasing reheating roughing temperature and increasing roughing strain were found to significantly accleerate precipitation - this acceleration is attributed to clustering of niobium in solution.

preface

as we all know, the existence of strain induced precipitation in high strength low alloy steel (HSLA) containing niobium plays a decisive role in preventing austenite recrystallization of steel plate in finishing pass controlled rolling. Therefore, the study of its precipitation kinetics has important practical value

in a recent analysis of Nb (C, n) strain induced precipitation dynamics (1), using a large number of experimental research data, it is concluded that even considering the fluctuations produced by different experimental observation techniques, there is still a significant difference in the measured precipitation time, which is mainly caused by the thermodynamic history before finishing rolling deformation during subsequent heat preservation. In particular, compared with the similar finish rolling deformation process after direct cooling from reheating temperature, the rough rolling deformation pass accelerates the precipitation after finish rolling deformation by about five times

the purpose of this study is to establish whether the thermodynamic history before finishing rolling has a significant impact on precipitation and to determine the impact of process variables

test process

the chemical composition of the steel in this study is shown in Table 1. X70 steel is a 28mm thick commercial steel plate from a previous research report by Dutta and Sellars. In order to prepare steel for experimental research, the billet of this slab was heated to 1200 ℃ in a heating furnace with exothermic protective atmosphere and homogenized for 30 minutes, and then rolled into 23.5mm in one pass or 14.3mm in two passes, and air cooled to room temperature. Using the method described by Dutta and Sellars (2), the effect of rough rolling on the strain induced precipitation kinetics of Nb (C, n) was studied

1) reheat the slab with a thickness of 14.3mm to 1200 ℃ and soaking for 30 minutes, air cool it to 950 ℃, roll it according to the compression ratio of 15%, hold it at 900 ℃ in the furnace for different times, then air cool it to 800 ℃, then roll it according to the compression ratio of 15%, and quench it to room temperature. The change of rheological stress of finishing rolling at 800 ℃ with holding time at 900 ℃ indicates the existence of strain induced precipitation

2) reheat the slab with a thickness of 23.5mm in a similar way, roughen it at 1130 ℃ or 1080 ℃ with a compression ratio of 40% (rolled to 14.3mm thick), and then carry out the above treatment according to the slab with an original thickness of 14.3mm

to record and control the rolling temperature, insert one at the centerline of all slabs φ 1.5mm chromium aluminum thermocouple with high toughness and high temperature resistance. A typical record is shown in figure 1 A shows that the peak temperature of deformation heat occurs in 3 rolling passes

other steel grades in Table 1 are experimental billets provided by swinden Laboratory of British Steel Company. First, the slab is heated to 1200 ℃, rolled into 30 mm thick steel plate and air cooled. Then, in order to study the influence of chemical composition on the flow stress as a function of rolling temperature, 30mm thick steel plate was reheated to 1200 ℃ and rolled into 10mm thick steel plate. Therefore, its thermodynamic history is similar to that of experimental rolling X70 steel. It is milled and ground to 8.50~9.85mm according to the size of the material × 50mm × 60mm plane deformation compression test specimen. To insert a thermocouple, drill a hole with a diameter of 1.6mm at the center of the plate width direction

in the plane compression test equipment, in order to prevent the oxidation of the sample during reheating, apply 5 μ M thick industrial chromium plating layer. Use water-based glass lubricant (DAG 2626, Acheson colloids company) to coat the deformation area to achieve the lubrication of the deformation area, and dry it for at least 24 hours before the test. In order to prevent the wear of coating and lubricating film, the reheating of plane deformation compression test is limited to holding at 1185 ℃ for 20 minutes

the test was carried out on a previously described hydraulic servo testing machine (3, 4) controlled by a computer at a constant strain rate of 5 s-1. Using the same principle as rolling, the kinetics of strain induced precipitation is determined. However, the final deformation of the influence of the measurement on the flow stress is carried out at 880 ℃. The furnace temperature of the test furnace surrounding the equipment is set to 880 ℃, and the sample is heated to the set temperature in the heating furnace, and then quickly transferred to the test furnace for cooling, followed by rough rolling deformation. Except for one sample without rough rolling deformation and another sample with two passes of deformation, all the other samples are subject to 15~40% single pass compression deformation within the temperature range of 1080~1180 ℃. Then all samples are subject to 15% compression deformation at 955 ℃, the samples are transferred to the third heating furnace and held at 900 ℃ for different times, and finally returned to the test furnace for 15% final compression deformation at 880 ℃, and then water quenched to room temperature. Typical temperature time records are shown in figure 1 B shows that in this figure, the rapid air cooling cycle during the transfer between heating furnaces is very obvious

the quenched specimen was cut into pieces along the longitudinal direction, and the specimen for optical metallographic study of austenite structure was prepared by mechanical polishing and erosion in saturated picric acid solution containing wetting agent (teepol thipol surfactant). In order to remove the residue left by acid pickling, conduct regular scrubbing and acid pickling at 60~80 ℃ for 5~7 minutes. The austenite grain size is measured by the line intercept method, 400 or more grains are calculated longitudinally, and the relative confidence limit of ± 6.5% is given in the thickness direction

for electron microscopic observation, the quenched sample is cut into φ 3mm Round Bar, and prepare thin discs with a mechanical grinder. Then, on a Struers tenupol device, spray polishing was carried out with glacial acetic acid and 10% perchloric acid electrolyte at 12V voltage and about 20mA current. These films were examined by Philips 301 transmission electron microscope (TEM) at 100kV voltage

results

◇ austenite microstructure

when initially studying the austenite grain structure of X70 Steel during rolling, the slab was quenched at various stages. The results are shown in Figure 2 and table 2. The reheated austenite grains are equiaxed before and after rough rolling and cooling to 950 ℃. However, the austenite grains become smaller after rough rolling, indicating that complete static recrystallization has occurred. First, 15% rolling deformation is carried out at 950 ℃, then the temperature is maintained at 900 ℃ for 240 seconds, and finally quenching is carried out. There is no obvious Recrystallization at all, and the length width ratio of the elongated grain (Table 2) is completely consistent with the expected value of 1.38 when 15% plane deformation is carried out. After continuous rolling deformation at 800 ℃, the length width ratio of elongated grains further increases, as shown in Figure 2 B. Delayed quenching after rolling results in ferrite decoration on austenite grain boundary

when carrying out the plane deformation compression deformation test, check the sample that is kept at 900 ℃ for the longest time and quenched after the final deformation. The reheated microstructure data can be obtained from the undeformed area of the sample, and the final microstructure data can be obtained from the deformation area of the same sample. Figure 3 shows an example, and the inspection results are shown in Table 3. In all samples, the grains of reheated samples were equiaxed. However, the average grain size varies greatly among steel grades. The final microstructure shows that there is no recrystallization during finish rolling deformation, and the ratio of longitudinal and thick grain size (dl/dt) is consistent with the ratio of grain size of equiaxed grains after rough rolling after 2 passes of 15% finish rolling deformation

◇ flow stress

for the rolling experiment of X70 steel, measure the rolling load at the center of the slab length of the final pass, and then convert the average flow stress according to the Sims equation (2) previously described. The temperature change of the final pass between slabs is ± 5K, and all flow stresses are accurately corrected to 800 ℃ according to the method described in Appendix 1. Its flow stress is a function of isothermal holding time at 900 ℃, as shown in Figure 4. As described in Appendix 2, the isothermal time is calculated based on the actual holding time and the air cooling time from 950 ℃ to 800 ℃ (Figure 1). For comparison, the results of previous research work on this X70 steel are included in Figure 4. All curves show that the flow stress has a peak value in a short time when the roughing reduction is increased or the roughing temperature is reduced

due to the limited number of each test steel available, other steel types are only studied under the condition of plane deformation compression test. For different steel grades, check the combination of different roughing variables. The typical form of stress-strain curve of 15% single pass rough rolling deformation and finish rolling deformation at 955 ℃ and 880 ℃ is shown in Figure 5. In order to directly compare with the rolling results, the average stress of the final deformation at 800 ℃ is derived and corrected according to the description in Appendix 1. For the final deformation at 880 ℃, this correction simply adds 24mnm-2, so the effect of this correction on the time of the peak of convective strain stress is not observed. Figure 6 A shows the results of X70 steel, including the results of early work 2. Figure 6 The form of a is the same as that in Figure 4. The flow stress has a similar value, but the peak value tends to narrow. For comparable rough rolling deformation conditions (40%/1080 ℃ and 30%/1130 ℃), the peak value of rheological stress occurs in a slightly shorter time than that under actual rolling conditions. The shortest time is due to the process system containing 2 passes of rough rolling deformation. Figure 6 b. Figure 6 c. Figure 6 d. Figure 6 E shows that it has similar behavior in other steel grades, and the peak value of the final pass flow stress occurs in a short time when the roughing reduction is increased or the roughing temperature is reduced. Comparing figures 6a to 6e shows that for most steel grades and rough rolling deformation conditions, the stress increase at the peak value Δσ The range is 50~70mnm-2, and there is no obvious systematic change with the holding time at the peak. For 40% rough rolling deformation at 1080 ℃, a direct comparison between time and peak stress can be made from Fig. 6a~d. Here, we give 46 seconds for X70 steel, 55 seconds for 3039a steel, 66 seconds for 3036 steel and 28 seconds for 3038 steel. According to figures 6D and 6e, for 30% rough rolling deformation at 1080 ℃, the time of 3038 steel and 3039 steel is 46 seconds and 33 seconds respectively. Therefore, for a given rough rolling deformation condition, the equivalent and isothermal holding time to reach the peak flow stress is 3039a steel and 3038 steel