With the rapid development of metallurgical technology and pipe-making technology in China, the technology of high-frequency electric resistance welded steel pipe has been continuously improved. Because high-frequency electric resistance welded pipe has the advantages of high efficiency, low cost, uniform wall thickness and reliable performance, it is also increasingly favored by people. 1. It uses 10~500kHz high-frequency current to generate resistance heat through the joint surface of the weldment, and is applied Or a method of welding the joint surface to the atomic bond without applying pressure. 2. Since the material near the weld reaches the molten state, the weld and the heat-affected zone structure are significantly changed, resulting in a decrease in performance. In order to improve and improve the performance of the region, the subsequent heat treatment process is indispensable, and the reasonable formulation of the heat treatment process is one of the key technologies to ensure the performance of the welded pipe.
In the production process of high-frequency electric resistance welded pipe, the welding speed can be as high as 15 fund projects: Hebei Science and Technology Support Project (09276705D); Hebei Province Natural Science Foundation Management Research. Tel: 0335 ~ 8077110, E-mail: cyddys263.com communication OF: LIAO wave (1955), male, Professor, mainly in the processed material 30m / min under conditions of such high welding speed, welding The on-line heat treatment can only be achieved by induction heating. 0. This high-speed heating is close to the “zero†insulation process, and the process is formulated differently from the traditional heat treatment process. The effect of the heating rate on the phase change must be considered. In this paper, the influence of heating rate on the phase transition temperature of X70 pipeline steel and its microstructure and properties after cooling was studied by thermal simulation tester, which was provided for the heat treatment process of high frequency resistance welded pipe. At the same time, the influence of holding time on the microstructure and properties of X70 steel under high-speed heating conditions was also studied, and it was also provided for the "zero" heat preservation heat treatment process under high-speed heating conditions.
1 Test materials and methods The materials used in this test are commercial controlled rolling and controlled cold X70 low carbon microalloyed pipeline steel, the chemical composition of which is shown in Table 1. The sample has two kinds of cylindrical samples and square samples, which are taken along the transverse direction of the steel plate. The dimensions are "10mmx100mm and 10mmX10mmx55mm. The thermal simulation test is carried out on the Gleeble-3500 thermal simulation test machine. The specific test plan is: 1 test Samples with different heating speeds (10 H瞧苒笤Ha land f.瞧苒, Lin choose fHI land flaze; hail t=t-pHé“’fls; iswtb芸 spicy è‹’, strike oil B æ½èŠ¸çž§ epilepsy tfuMa啦瞧苒110n wtb)-s. é€ é€ ä¼žé©¾çž§è‹’æ–›)-瞧苒±裆, stuffing; 葚涞苕3V01%3 loyalty; 1 æ¿1æ¿, æ–›Axiovs200MATæ½ç›Šè¦ç‚1 Choose to create a smashing umbrella.
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X70 pipeline steel controlled by rolling control, its original structure is acicular ferrite, the structure contains polygonal ferrite, quasi-polygonal ferrite, bainite, martensite / austenite island structure And various structural units such as carbon and nitrogen compounds. M. In the process of heating austenitization, different austenitizing conditions are different in the formation of austenite. After the formation of austenite, a certain diffusion time is required, and as the heating rate increases, The martensite/austenite island structure and carbides in the organizational unit are not fully dissolved, the atoms of carbon and alloying elements are not fully diffused, and the amount of undissolved carbides is increased, thus causing austenite carbon and The non-uniformity of the elements affects the phase change effect of the phase change alloying elements, resulting in an increase in the tissue inhomogeneity at room temperature after cooling (see), reducing the strength and toughness of the steel. Therefore, for the heat treatment of materials under high-speed heating conditions, it should be considered to combine the heating rate to properly control the holding time.
Under the condition of this test, under the conditions of different heating temperatures, the influence of holding time on performance is slightly different.
When the heating temperature is 930 °C, the strength reaches the maximum value when the temperature is kept for 60s, but the impact toughness is significantly reduced. However, as the holding time continues to increase, the strength and toughness are slightly decreased, and the performance change is not significant with time. When the heating temperature is 1000C, the strength is lower when the holding time is 1s, the strength and toughness of the material are lower than the performance of the material when 930C is heated, and the strength increases remarkably with the holding time increased to 60s, but the toughness does not change much; Continue to increase the holding time, the strength and toughness are significantly reduced. This is related to the difference in austenite state between different heating temperatures and holding times, and the resulting difference in tissue after cooling.
Combined with the phase change test results, under the high-speed heating condition, the temperature of the Ac; point is higher than the temperature of 930C. In the short holding time, the material is still in the two-phase region, and some of the matrix only undergoes re-recrystallization ((a)) Therefore, the strength is low. With the prolongation of the holding time, since the temperature is higher than the phase transition temperature, the matrix will undergo a complete transformation, so the strength will increase, but due to the large amount of island-like structure ((b)) appearing in the transformed product after cooling, the toughness is lowered. As the holding time continues to increase, the heating temperature is relatively low, and the steel has a large amount of carbides formed by strong carbide forming elements such as Ti, Nb, V, etc., which can effectively inhibit austenite grain growth by 8,8 The coarsening of the tissue is not obvious ((c)), so the performance does not change much.
For the heating temperature of 1000C sample, the strength is low when the holding time is 1s. In contrast, the performance of the sample was lower than that of the sample at other heating rates. From the heating rate, the heating rate under the test conditions is higher than the heating speed in the phase change point test, resulting in poor austenite uniformity, mainly polygonal ferrite after transformation ((d)), and the strength is low. It is also consistent with the heating rate study results. With the increase of holding time, the austenite uniformity is improved, the uniformity of the structure after phase transformation is also improved ((e)), the strength is improved, and the toughness is also higher than the sample with a heating temperature of 930C. As the holding time continues to prolong, the austenite grain growth is obvious due to the high heating temperature, and the microstructure is coarsened after cooling ((f)), resulting in a decrease in the toughness of the sample.
In summary, the rapid heating method is adopted. In the process design, the influence of heating speed on the phase transition point should be considered. The austenitizing temperature can be appropriately increased according to the heating speed, and the holding time can be reasonably controlled to avoid austenitization. Uniform performance degradation; and long holding times, austenite grain growth, reduced performance and unnecessary energy waste.
The establishment of tempering characteristic function of hardened steel and energy-saving heat treatment Guo Congsheng, Long Yuming, Qi Haiwa, Zhang Shiyong (Shaanxi Institute of Technology, Hanzhong, Shaanxi 723003, China) have a correlation relationship. If a characteristic function describing this correlation is obtained, It can accurately give the hardness variation law of the tempering process of hardened steel. The hardness data of 50CrVA steel under different tempering conditions were collected by high temperature control precision test electric furnace and isothermal tempering test method. The tempering characteristic function of the steel was obtained by nonlinear regression analysis. Using this functional relationship, the tempering process parameters of 50CrVA steel can be optimized, that is, the energy-saving heat treatment of the tempering production process can be realized by adopting a process method of appropriately increasing the tempering temperature and reducing the tempering time.
Fund Project: Shaanxi Natural Science Basic Research Project (2007E2); Shaanxi Provincial Education Department Scientific Research Project (11K0818) Research on heat treatment and material damage and repair, has published more than 30 papers. Contact number: 3 Conclusion Heating rate has a significant effect on austenite transformation. In this test X70 steel, the relationship between phase transition temperature (Aq and Ac;) and heating rate (V, °C / s) are: Aq (°C) = 725.51 (± 2.76) + 0.64 (± 0.06) V; Ac3 (C) = 894.19 (± 5.19) + 0.64 (± 0.11) V. In the design of the rapid heating heat treatment process, the heating temperature should be designed in combination with the composition of the material and the heating rate.
The heating rate has a fixed influence on the microstructure and properties after cooling. Under the conditions of this test, when the heating rate is as low as 30 C/s, the heating rate has little effect on the performance; when it is higher than 30 C/s, the strength and toughness of the steel decrease remarkably with the increase of the heating rate.
When the heating temperature is 930C and 1000C, the strength of X70 steel increases with the holding time, and the strength reaches the maximum value when the temperature is kept for 60s, but the toughness decreases obviously at 930C, while the toughness does not change greatly at 1000C; When it is more than 300s, the strength and toughness tend to balance at 930C with the increase of holding time. In the field of modern machinery manufacturing, heat treatment is a widely used process technology. All important parts must be heat treated to achieve the required requirements. Mechanical properties and quality. Since the electricity consumption in the heat treatment industry accounts for about 20% of the total electricity consumption of the machinery manufacturing industry, the consumption is huge. Therefore, the strength and toughness of the heat treatment industry are significantly reduced.
(Finish)
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