Direct hot rolled dual phase steel production represents a challenging route, compared with cold rolled and intercritical annealing process, due to complex and sophisticated control of the hot strip mill processing pa...Direct hot rolled dual phase steel production represents a challenging route, compared with cold rolled and intercritical annealing process, due to complex and sophisticated control of the hot strip mill processing parameters. Instead, high technology compact slab production plant offers economic advantages, adequate control and prompt use of the advanced thermomechanical controlled rolling. The current work aims to obtain different structures and tensile properties by physical simulation of direct hot rolled niobium micro alloyed dual phase low carbon steel by varying the metallurgical temperatures of hot strip mill plant. This starts with adaptation of the chemical analysis of a low carbon content to fall far from the undesired peritectic region to avoid slab cracking during casting. Thermodynamic and kinetics calculations by Thermo-Calc 2020 and JMat pro software are used to define the transformation’s temperatures Ae1 and Ae3 as well as processing temperatures;namely of reheating, finishing rolling, step cooling and coiling temperatures. The results show that the increase of finishing rolling temperature from 780<span style="white-space:nowrap;">°</span>C to 840<span style="white-space:nowrap;">°</span>C or decreasing either of step cooling duration at ferrite bay from 7 to 4 seconds, enhances yield and tensile strengths, all due to more martensite volume fraction formation. The yield and tensile strengths also increase with decreasing coiling temperature from 330<span style="white-space:nowrap;">°</span>C to 180<span style="white-space:nowrap;">°</span>C, which is explained due to the increase of dislocation densities resulted from the sudden shape change during martensite formation at the lower coiling temperature in additional to the self-tempering of martensite formed at higher coiling temperatures which soften the dual phase steel.展开更多
The effect of temperature variation owing to the cooling pattern (CP) on the microstructural evolution was investigated by establishing a thermomechanical coupled FE (finite element) model. A set of constitutive e...The effect of temperature variation owing to the cooling pattern (CP) on the microstructural evolution was investigated by establishing a thermomechanical coupled FE (finite element) model. A set of constitutive equations of phase transformation was implanted into the commercial FE solver MARC through the user defined subroutine CR- PLAW, and the temperature field was calculated by another user defined subroutine FILM. The results show that the final mierostructure is completely bainite phase for CP one, 98% of bainite phase and 2% of ferrite phase for CP two, and 55% of bainite phase, 35% of pearlite phase and 10% of ferrite phase for CP three.展开更多
The effect of delay time with constant first finishing pass temperature (800℃) has been investigated by means of multi-pass torsion tests on Ti-Nb-IF steel. All the tests have been carried out at a strain rate of 2 s...The effect of delay time with constant first finishing pass temperature (800℃) has been investigated by means of multi-pass torsion tests on Ti-Nb-IF steel. All the tests have been carried out at a strain rate of 2 s-1 with 11 passes and 0.3 strain each pass. During the final pass, dynamic recrystallization occurs to a degree that depends on the delay time. In short interpass time (1 s) and at these temperatures (T≤800℃) there is not enough time to start static recrystallization, therefore, accumulation of strain occurs and after some passes, strain reaches a critical strain for starting dynamic recrystallization. In this study, the changes of mean flow stress during each pass and also the microstructural observation confirms that dynamic recrystallization occurs after some passes in ferrite phase of this steel. The stress-strain curves with constant temperature obtained by using a kinetic model and compensation of the increasing mean flow stress with decreasing temperature. Thus, this result also confirms that dynamic recrystallization occurs in warm rolling of this IF steel.展开更多
A new theoretical thermomechanical explanation of the uneven transverse temperature distribution, along the width for thin and wide hot rolled strip was proposed. In particular, starting from the irregular pressure an...A new theoretical thermomechanical explanation of the uneven transverse temperature distribution, along the width for thin and wide hot rolled strip was proposed. In particular, starting from the irregular pressure and friction distribution which led to an uneven heat generation, a 2D mathematical model of calculating the transverse termperature distribution was presented. A physical explanation for this problem was given and the model was used as an essential basis to build a corresponding FEM simulation model, in which heat loss and generation were considered. Deformation and friction heat were described in details. For a clearer and more logical analysis, the heat generation problem was split into two parts: one for the strip centre, and one for the sides, in correspondence with the temperature peak points at 100 mm from the strip edge. Finally, the result shows that how thenew theoretical model can lead to the exact interpretation of the measured uneven temperature distribution.展开更多
Thermomechanical controlled processing (TMCP) of low carbon cold heading steel in different austenite conditions were conducted by a laboratory hot rolling mill. Effect of various processing parameters on the mechan...Thermomechanical controlled processing (TMCP) of low carbon cold heading steel in different austenite conditions were conducted by a laboratory hot rolling mill. Effect of various processing parameters on the mechanical properties of the steel was investigated. The results showed that the mechanical properties of the low carbon cold heading steel could be significantly improved by TMCP without heat treatment. The improvement of mechanical properties can be attributed mainly to the ferrite grain refinement due to low temperature rolling. In the experiments the better ultimate tensile strength and ductility are obtained by lowering finishing cooling temperature within the temperature range from 650 ℃ to 550 ℃ since the interlamellar space in pearlite colonies become smaller. Good mechanical properties can be obtained in a proper austenite condition and thermomechanical processing parameter. The ferrite morphology has a more pronounced effect on the mechanical behavior than refinement of the microstructure. It is possible to realize the replacement of medium-carbon by low-carbon for 490 MPa grade cold heading steel with TMCP.展开更多
文摘Direct hot rolled dual phase steel production represents a challenging route, compared with cold rolled and intercritical annealing process, due to complex and sophisticated control of the hot strip mill processing parameters. Instead, high technology compact slab production plant offers economic advantages, adequate control and prompt use of the advanced thermomechanical controlled rolling. The current work aims to obtain different structures and tensile properties by physical simulation of direct hot rolled niobium micro alloyed dual phase low carbon steel by varying the metallurgical temperatures of hot strip mill plant. This starts with adaptation of the chemical analysis of a low carbon content to fall far from the undesired peritectic region to avoid slab cracking during casting. Thermodynamic and kinetics calculations by Thermo-Calc 2020 and JMat pro software are used to define the transformation’s temperatures Ae1 and Ae3 as well as processing temperatures;namely of reheating, finishing rolling, step cooling and coiling temperatures. The results show that the increase of finishing rolling temperature from 780<span style="white-space:nowrap;">°</span>C to 840<span style="white-space:nowrap;">°</span>C or decreasing either of step cooling duration at ferrite bay from 7 to 4 seconds, enhances yield and tensile strengths, all due to more martensite volume fraction formation. The yield and tensile strengths also increase with decreasing coiling temperature from 330<span style="white-space:nowrap;">°</span>C to 180<span style="white-space:nowrap;">°</span>C, which is explained due to the increase of dislocation densities resulted from the sudden shape change during martensite formation at the lower coiling temperature in additional to the self-tempering of martensite formed at higher coiling temperatures which soften the dual phase steel.
文摘The effect of temperature variation owing to the cooling pattern (CP) on the microstructural evolution was investigated by establishing a thermomechanical coupled FE (finite element) model. A set of constitutive equations of phase transformation was implanted into the commercial FE solver MARC through the user defined subroutine CR- PLAW, and the temperature field was calculated by another user defined subroutine FILM. The results show that the final mierostructure is completely bainite phase for CP one, 98% of bainite phase and 2% of ferrite phase for CP two, and 55% of bainite phase, 35% of pearlite phase and 10% of ferrite phase for CP three.
文摘The effect of delay time with constant first finishing pass temperature (800℃) has been investigated by means of multi-pass torsion tests on Ti-Nb-IF steel. All the tests have been carried out at a strain rate of 2 s-1 with 11 passes and 0.3 strain each pass. During the final pass, dynamic recrystallization occurs to a degree that depends on the delay time. In short interpass time (1 s) and at these temperatures (T≤800℃) there is not enough time to start static recrystallization, therefore, accumulation of strain occurs and after some passes, strain reaches a critical strain for starting dynamic recrystallization. In this study, the changes of mean flow stress during each pass and also the microstructural observation confirms that dynamic recrystallization occurs after some passes in ferrite phase of this steel. The stress-strain curves with constant temperature obtained by using a kinetic model and compensation of the increasing mean flow stress with decreasing temperature. Thus, this result also confirms that dynamic recrystallization occurs in warm rolling of this IF steel.
文摘A new theoretical thermomechanical explanation of the uneven transverse temperature distribution, along the width for thin and wide hot rolled strip was proposed. In particular, starting from the irregular pressure and friction distribution which led to an uneven heat generation, a 2D mathematical model of calculating the transverse termperature distribution was presented. A physical explanation for this problem was given and the model was used as an essential basis to build a corresponding FEM simulation model, in which heat loss and generation were considered. Deformation and friction heat were described in details. For a clearer and more logical analysis, the heat generation problem was split into two parts: one for the strip centre, and one for the sides, in correspondence with the temperature peak points at 100 mm from the strip edge. Finally, the result shows that how thenew theoretical model can lead to the exact interpretation of the measured uneven temperature distribution.
基金Sponsored by National Natural Science Foundation of China (50334010)Shenyang City Application Basic Research Project (1071198-1-00)
文摘Thermomechanical controlled processing (TMCP) of low carbon cold heading steel in different austenite conditions were conducted by a laboratory hot rolling mill. Effect of various processing parameters on the mechanical properties of the steel was investigated. The results showed that the mechanical properties of the low carbon cold heading steel could be significantly improved by TMCP without heat treatment. The improvement of mechanical properties can be attributed mainly to the ferrite grain refinement due to low temperature rolling. In the experiments the better ultimate tensile strength and ductility are obtained by lowering finishing cooling temperature within the temperature range from 650 ℃ to 550 ℃ since the interlamellar space in pearlite colonies become smaller. Good mechanical properties can be obtained in a proper austenite condition and thermomechanical processing parameter. The ferrite morphology has a more pronounced effect on the mechanical behavior than refinement of the microstructure. It is possible to realize the replacement of medium-carbon by low-carbon for 490 MPa grade cold heading steel with TMCP.