Selection of mold type and size parameters, formulation of reasonable drawing procedures, etc.
In recent years, the use of computer simulation technology to replace the traditional elementary analytical method and experimental method, to obtain a more comprehensive understanding of the metal deformation process, has become an effective way to study metal pressure processing. The Steel Research Institute of the Ministry of Metallurgy used the large-scale finite element simulation software MSC.MARC to carry out more than 500 simulations of the cold drawing process of steel pipes under various process parameters, studied various existing problems, and obtained a series of practical values. In conclusion, more than 100 mathematical models of each major parameter were obtained. In the future, we will introduce these research contents in different topics. This article introduces the overall research content, research methods and main research results.
1 The determination of the research object The long-term practice has formed a variety of extraction methods for steel pipes, but there are four main types of extraction methods: the Chinese model (arc mode), the Soviet model (conical mold), and the Chinese style. Die short core rod drawing; Soviet style short core rod drawing. The four extraction methods were selected as the research object.
24 computer application cold drawn steel pipe material is divided into two categories: carbon steel and stainless steel. The application of a wider range of carbon steel is the research object. The MSC.MARC software itself has a material database, from which German ck15 steel is selected as the steel pipe material. Ck15 steel is similar to China's No. 15 steel, and the conclusions of the study apply to general carbon steel. The mechanical properties of ck15 steel are taken from the material database, taking into account the work hardening and plastic strain effects.
2 The simulation scheme is determined to realize the dynamic simulation of the drawing process on the computer, which is equivalent to the drawing experiment on the computer. With the aid of this research method, the drawing simulation under various possible process parameters is arranged as much as possible. To this end, the main influence parameters of each drawing process should be analyzed first.
The parameter analysis of the drawing process uses similar theoretical analysis methods to determine the main influence parameters of each drawing process. 111. These parameters have clear physical meanings, which not only fully reflect the main influencing factors in the drawing process, but also between them. They are independent of each other.
2.1.1 Chinese model (arc mode) The parameters of the air extraction process are divided into known parameters and unknown parameters. Known parameters are those that can be artificially specified or inherent in the equipment. Unknown parameters are those that are subject to known parameters.
The known parameters of the Chinese mold emptying process are as follows: the friction coefficient reflects the friction condition and is related to the lubrication condition, the surface condition of the mold and the material of the mold.
Steel pipe inlet diameter wall thickness ratio D / T or steel pipe outlet diameter wall thickness ratio d / t. Reflects the size characteristics of the steel pipe.
Drawing parameters; Pulling speed, take 300mm /. Steel pipe inlet diameter and die arc radius ratio D / R, or steel pipe exit diameter and die arc radius ratio d / R. Reflect the impact of the mold arc radius.
The main unknown parameters of the Chinese mold emptying process are as follows: the maximum tensile stress Ocmax in the drawing direction. It is used to study the transverse cracking and unplugging of steel pipes.
Average drawing stress a=P/FP* drawing force; F steel pipe outlet section area. Used to study steel pipe unplugging and process optimization.
The maximum equivalent plastic strain SPmax is used to study the maximum amount of deformation and microscopic quality control of steel tubes.
* Steel pipe inlet wall thickness; a steel pipe outlet wall thickness. Used to study the variation of wall thickness of steel pipes.
d steel pipe outlet diameter; o die hole diameter. Used to study the variation of steel pipe diameter.
In theory, any unknown parameter can be found from known parameters.
There are two ways to solve this problem: one is to take the inlet size of the steel pipe as a known parameter, and the solution of the steel pipe outlet size as an unknown parameter is called a forward solution; the other opposite case is called a reverse solution. Taking the average drawing stress a as an example, it can be expressed as 2.1.2 Su-type die (conical die). The parameters of the air-drawing process are analyzed. The known parameters are as follows: steel pipe inlet diameter wall thickness ratio D/T, or steel pipe outlet diameter wall The thickness ratio d/. draft angle a reflects the shape characteristics of the mold.
The main unknown parameters of the Soviet-style mold emptying process are the same as those of the Chinese-style mold emptying process.
Taking the average drawing stress as an example, it can be expressed as 2.1.3 Chinese model short core rod drawing process parameter analysis The known parameters of the Chinese model short core rod drawing process are as follows: steel tube inlet diameter wall thickness ratio D/T, or Steel pipe outlet diameter wall thickness ratio d /. steel pipe inlet diameter and die arc radius ratio D / R, or steel pipe outlet diameter and die arc radius ratio d / R. inner die cone angle å©. Internal mold shape characteristic parameters.
The main unknown parameters of the Chinese model short core rod drawing process are as follows: average drawing stress a=P/FP* drawing force; F steel pipe exit section area.
The maximum external positive pressure PWm, ax. is used to study the wear and life of the outer mold.
The maximum internal model is positive pressure. Used to study internal mold wear and life.
Taking the average drawing stress as an example, it can be expressed as: compared with the Chinese die emptying, the number of independent known parameters is increased by two due to the existence of the mandrel.
21. Analysis of the parameters of the Soviet-style short core rod drawing process The known parameters of the Soviet-style short core rod drawing process are as follows: steel tube inlet diameter wall thickness ratio D/T or steel tube outlet diameter wall thickness ratio d/. outer mold cone angle a outer mold Shape feature parameters.
The inner die cone angle å©. Internal mold shape characteristic parameters.
The main unknown parameters of the Soviet-style short core rod drawing process are the same as those of the Chinese model short core rod drawing process.
Taking the average drawing stress a as an example, it can be expressed as the positive solution a'a compared with the Soviet model empty drawing, due to the existence of the mandrel, the number of independent known parameters plus two.
Based on the above parameters, the 22 simulation program systematically arranges simulation experiments to cover various possible actual steel pipe production conditions. In addition, in order to explore the influence law of a certain parameter, some experiments that are difficult to meet or reach in actual production are also virtually arranged. For example, the friction coefficient is as small as 0.03, as well as extremely thin walls and extremely thick wall drawing. The conclusions and models obtained in this way will have wide applicability and have practical value for almost any steel tube manufacturer. It can be noted that since the similarity theory is used in the parameter analysis, the selected parameters are dimensionless, which makes the research results not limited by the size of the steel pipe. The specific simulation scheme is shown in Table 1. In actual production, there is often one case of annealing and pulling two passes. The simulation of this connection process was also carried out. For example, in the 177 Chinese model short mandrel drawing simulation, 71 times were simulated for the continuous drawing process.
2.3 Software and hardware conditions for simulation work The simulation analysis was performed on the HP6/200 microcomputer workstation using the well-known American nonlinear finite element simulation software MSC*MARC.
3 Establishment of the simulation model In order to facilitate the establishment of the model, it is assumed that the steel pipe has been pulled out for a period, the end of the steel pipe is pulled out, and the draft is moved in the opposite direction at the drawing speed. When the core rod is drawn, the inner mold does not participate in the deformation, that is, it has not yet entered the deformation zone. At the beginning of the outer mold starting to move in the opposite direction, the inner mold does not move until the inner and outer molds reach the steady state relative position, and the inner and outer molds are simultaneously moved in the opposite direction at the drawing speed. This allows an approximate simulation of the setup phase of the drawing process.
When constructing the model, the following problems were noted: the steel tube was treated as an elastoplastic body, and the work hardening and plastic strain rate effects were considered.
The finite element mesh is refined, and the wall thickness is divided into 4~12 layers to ensure the simulation precision.
4 Analysis and processing of simulation results 4.1 Simulation results Simulation of a specific drawing process using MSC.MARC, the following results can be obtained: the animation process of the drawing process on the computer.
Various parameter field distributions at any time during the drawing process, such as stress components, strain components, plastic strain rate, unit pressure of the draft, elastic strain energy and plastic strain energy.
The distribution of residual stress inside the steel pipe after drawing.
The variation of the resultant force of the mold during the drawing process, from which the distribution of the drawing force can be obtained.
Table 1 Cold drawing process of steel pipe computer finite element simulation scheme Drawing type parameter symbol name name simulation range simulation number Chinese model empty drawing extension coefficient friction coefficient steel pipe population diameter wall thickness steel pipe outlet diameter wall thickness ratio drawing speed coefficient steel pipe inlet diameter and The radius of the die arc is smaller than the diameter of the steel pipe outlet and the radius of the die arc. The coefficient of the air-cutting coefficient of the Soviet-style die is the coefficient of the wall thickness of the steel pipe. The wall thickness of the steel pipe is smaller than the thickness of the steel pipe. The drawing speed coefficient is the outer die cone angle. Coefficient of friction coefficient steel tube population diameter wall thickness ratio steel pipe outlet diameter wall thickness ratio drawing speed coefficient steel pipe diameter and die arc radius ratio steel pipe outlet diameter and die arc radius ratio inner die cone angle reduction ratio and reduction ratio Su-type short core rod drawing elongation coefficient friction coefficient steel tube population diameter wall thickness ratio steel tube diameter diameter ratio drawing coefficient coefficient outer mold cone angle inner mold cone angle reduction ratio and reduction ratio ratio 42 analysis results analysis The above simulation results were analyzed and compared, and the following problems were mainly studied: the shape of the deformation zone of the steel pipe and the dimensions after the drawing, and the wall thickness of the steel pipe was straight. Variation law; metal flow law under various process parameters, optimization of process parameters; influence of mold shape on deformation process, optimization of mold shape; distribution of force energy parameters, discussion of transverse crack mechanism and control method; residual stress distribution law, discussion of longitudinal crack Mechanism and control method; compare four drawing methods to provide a basis for reasonable selection.
4.3 Processing of simulation results Through the simulation of the drawing process, the deformation parameters of the force can be obtained under various process conditions. Mathematical processing of these results yields a mathematical model of each parameter.
Taking the average drawing stress during the emptying of the Chinese model as an example, there are (1) and (2) formulas. The specific form of the function is unknown, but it can always be expanded into a series of forms. For example, you can take the first 86 items of the series expansion. Of course, such a complicated formula is not advisable, and a stepwise regression method is introduced for this purpose.
The idea of ​​stepwise regression is to introduce the most significant quantity from each of the independent variables into the regression equation. Each time a quantity is introduced, the quantity introduced has been checked, and those insignificant quantities are eliminated, until all the regression equations are included. A significant amount, without an insignificant amount, yields the best regression equation. The regression program is developed on its own.
Based on the simulation results of the Chinese model emptying 72 times, the first 86 items of the (1) and (2) series expansions are subjected to stepwise regression, and the following mathematical model is obtained.
A~A7 is the coefficient. The significance test value of the regression equation is F=1295, and the correlation coefficient r=0.996. B~B7 is the coefficient. The significance of the regression equation is F=1156, and the correlation coefficient is r=0.996. From equations (9) and (10), the average drawing stress under any process conditions can be calculated, and the pull-out force can be obtained. In the same way, the mathematical models of the main parameters in the four extraction modes can be obtained, totaling more than 100. For example, the wall thickness change rate model, the maximum draw stress model, the maximum circumferential stress model, the maximum draft pressure model, and the maximum residual stress model are all solved in both positive and negative directions.
4.4 Steel pipe cold drawing process CAD system based on a large number of simulation studies, developed a steel pipe cold drawing process CAD system 121. The CAD system is suitable for each steel pipe cold drawing enterprise, helping enterprises to quickly and effectively develop optimized drawing workers B.
Conclusion The finite element simulation can replace the traditional experimental method to realize the computer virtual steel tube cold drawing experiment, and can obtain more abundant results.
The similar theoretical analysis is used to obtain the comprehensive and independent influence parameters of the four common extraction methods.
The common problems in the cold drawing process of steel pipes were analyzed.
Using the stepwise regression method, more than 100 mathematical models of important parameters were obtained.
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