Development conditions and methods Test material: 42mm, 65Si2MnWA steel. Adopt KGPS-500/25 intermediate frequency heater, power 500kVA, frequency 2500Hz. WFHX-63 digital thermometer, heating speed 6/s, heating temperature 700723.200 medium temperature rolling mill, rolling speed V=10m/min. Air cooling after rolling The rolling specification is 29mm37mm.
Test results tissue parameters. The data in the group can be seen that the microstructure of group A is flaky pearlite, and the group B is punctate and flaky pearlite, which is finer and more uniform than group A. Group A graphite carbon 05, group B graphite carbon 05, there is no significant difference between the two. The decarburization depth of group A was 007-010mm, and the decarburization depth of group B was 006-009mm. There was no significant change in group B compared with group A. Group A hardness HB239275, group B hardness HB239285, the two are close.
It is 42mm 65Si2MnWA hot-rolled round steel, after surface grinding, five times of continuous medium frequency induction heating to 700-723, five-way rolling into the core of 22mm-28mm-32mm trapezoidal material to magnify 500 times of metallographic structure, no It is difficult to find this group of fine-grained and a small amount of pearlite 800705268 flaky pearlite 00805272 point and fine-grain pearlite weaving is extremely uniform, basically point-like and a small amount of small-grain pearlite, with excellent mechanical properties and heat treatment Craftsmanship. The layer profile is the same sample.
Analysis of the profile of the medium temperature rolling spring steel layer in the middle temperature rolling spring steel core structure and discussion of the influence of heating time on the graphitization transformation. Spring steel can obtain a higher proportion limit, mainly by carbon solid solution in austenite. If graphite is precipitated, its effect in steel is similar to inclusions due to its inability to enter solid solution, which adversely affects plasticity and fatigue strength. The free energy of carbon in high silicon steel is higher than that of non-silicon steel, which greatly improves the instability of cementite and promotes the production of graphite. According to the data <1>, long-term residence at 750-800 or 700-720 long-time thermal annealing and repeated heating annealing are prone to graphite carbon precipitation. Therefore, it is not possible to make a long-term stay in a higher temperature range, and the medium-frequency induction rapid heating adopts a speed of 6/s, and is rapidly cooled in a natural state after rolling, so that the cementite does not have sufficient heat energy to realize the graphitization conversion process. .
In the trial production, the material of Group A was rapidly heated to 700-723 in five times, and then cooled, and no precipitation of graphite carbon occurred. Effect of heating rate on the structure Before the quenching heat treatment, the spring steel has certain requirements on the original structure of the steel. To achieve good mechanical properties after heat treatment to ensure the service life of the spring, the original tissue must be spheroidized.
From the theory of metal dynamics, the faster the heating rate, the more uniform the time of grain uniformization is <2>. At the same time, according to the theory of warm rolling, the material is heated to the temperature range above Acl for medium temperature rolling, so that the material is in the rolling process. In the recrystallization range, recrystallization is accompanied by processing, and coarse crystal grains are broken and refined. The intermediate frequency rapid heating and medium temperature rolling processes based on the above theory fully confirm the above conclusions.
The comprehensive utilization of spring steel surface decarburization for medium temperature rolling can significantly reduce the mechanical properties and cause fatigue failure prematurely. The carbon content and the silicon content have a great influence on the decarburization tendency of the steel, and the decarburization is intensified as the heating temperature is increased and the time is prolonged, and the temperature influence between the two is large. The medium frequency induction rapid heating and medium temperature rolling make the material maintain good plasticity, solve the defects that spring steel is easy to crack in cold deformation, and reduce the rolling temperature from 1100 to below Acl temperature, avoiding the temperature at Acl. In the above high temperature range, the residence time in the recrystallization temperature range is also short, so that the heating surface is not easily oxidized.
Compared with the performance of hot-rolled materials, the surface of the hot-rolled trapezoidal 65Sl2MnWA steel produced by a domestic steel mill and the metallographic structure of the core, the metallographic structure of the materials rolled by the two processes is not difficult to find: (1) decarburization It has a depth of 045mm and a full decarburization layer with a decarburization depth of 009mm and semi-decarburization. (2) It is a short dense and fine-grained pearlite, which is a point-like and a small amount of small-particle pearlite. (3) At present, the service life, machining performance and heat treatment process of hot-rolled trapezoidal materials are not as good as medium-frequency induction rapid heating and medium-temperature rolling materials.
Conclusion The trapezoidal spring steel with medium frequency induction and rapid heating to 700-723 for medium temperature rolling has the superiority of the process which can not be obtained by hot working and cold working. Because the process has a fast heating speed and a relatively low temperature, the process problems of decarburization and graphite precipitation of the trapezoidal spring steel surface can be effectively solved. The medium temperature rolling is used to make the material have good plasticity in deformation, avoiding the occurrence of defects such as material cracking, optimizing the metallographic phase of the material, and improving the strength and toughness and plasticity.
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