Abstract Through the control of the composition and structure of MFTS non-regulated cold steel, the comprehensive performance of the wire rod is improved. After reasonable drawing process, the mechanical properties of cold heading bolts are good. According to the optimized low temperature stabilization treatment process, bolt synthesis Performance meets requirements, reducing production costs and improving the environment.
1 Introduction
Since 2005, China's fastener industry has experienced rapid development in the past ten years. From the rapid growth of fastener production in the early period to the steady improvement of the quality of varieties in recent years, with the adjustment of product structure, new technologies and new technologies have made great progress. The traditional process for producing 8.8 grade fasteners is made of medium carbon cold heading steel wire rod. The process is annealing, pickling (phosphating), pulling and cooling (twisting), quenching and tempering (tempering + high temperature tempering). Surface treatment (blackening or galvanizing, etc.) in a package. The purpose of the first process annealing is to improve the structure and properties of the material and prepare for the subsequent cold heading process. The quenching and tempering process is to obtain the mechanical properties specified by the standard after the cold rolling. It is well known that the heat treatment process in the production of fasteners is the most energy-consuming process and the most costly process. If annealing, quenching and tempering are omitted The second process will significantly reduce the production cost of 8.8 fasteners. At the same time, it can avoid the occurrence of heat treatment defects, reduce carbon emissions and improve the environment. The MFT8 hot-rolled wire rod recommended in GB/T 3098.22-2009 "Mechanical properties of fasteners, fine-grained non-tempered steel bolts, screws and studs" can eliminate the two processes of annealing, quenching and tempering and heat production. 8 grade fasteners. The main performance of the 8-8 fasteners produced according to this process are in line with the standard requirements, but when the load test is carried out, the permanent elongation is unstable and some of the results exceed the standard. In order to solve this problem, through the study of the stabilization treatment process, the problem of unstable elongation at the time of the guaranteed load test is solved, and all the indicators meet the standard requirements.
2, MFT8 steel composition control and hot rolling wire rod process
(1) Composition control
The chemical composition control of MFT8 fine-grain non-tempered cold heading steel is shown in Table 1.
(2) Hot rolled wire rod production process
Production process, blast furnace hot metal-iron water pretreatment one converter smelting one refining one continuous casting (140mm x140mm billet 1 one heating one descaling one heat machine rolling one control cold one set one package one into one.
(3) Mechanical properties of hot rolled wire rod
See Table 2 for the mechanical properties of MFT8 hot-rolled wire rods of different specifications for industrial mass production.
3, bolt trial results
The MFT8 non-regulated cold-rolled steel hot-rolled wire rod is subjected to pickling (phosphating), one drawing, one cold drawing (twisting), one stabilization process, one packaging process, and the 8.8-level bolting is made, after cold rolling (twisting) The tensile strength of the bolts for stabilization treatment is shown in Figure 1. The tensile strength of the bolts after stabilization treatment is shown in Figure 2. The results of the permanent elongation of the bolts after the stabilization treatment are shown in Figure 3. Before and after stabilization. Bolt to ensure the load test permanent elongation comparison results are shown in Figure 4. The deformation amount in the drawing process is controlled by about 25%, and the stabilization treatment process is controlled by 380 + 15 ° C x 1.5 hours.
4, discussion and analysis
MFT8 is a cold-strengthened non-quenched and tempered steel for the 88-stage bolt developed in China in recent years. The hot-rolled structure of the wire rod is ferrite + pearlite. The non-tempered steel is used to produce the bolt process in the drawing process, and the traditional quenching and tempering Compared with steel production bolts, it not only has the effect of changing the size of the wire rod, improving the surface condition of the wire rod, but also avoiding the heat treatment of the quenching and tempering while ensuring the processing performance of the bolt and the mechanical properties of the finished product, so the drawing deformation is 15%~30. Between the %, the overall performance of the cold rolled steel wire is the best. The hot rolled wire rod is hardened by drawing and cold heading, and can reach the strength, hardness and plasticity requirements of the 8.8 bolts. However, after drawing and cold rolling, the bolts are in a cold-strengthened state, and the structure is unsteady ferrite. Body + pearlite deformation structure, there are many high-density movable dislocations and other defects in this deformation structure, making it unstable. When the bolt is loaded, these movable dislocations will climb and slip. This will result in the permanent elongation of the bolt exceeding the standard during the load test. The low-temperature stabilization of the bolts after cold heading enables the microstructure after cold heading deformation to be in a stable state. The low temperature stabilization treatment is essentially different from the traditional quenching and tempering heat treatment. The material does not undergo a phase transformation, and is still a ferrite + pearlite structure, but the dislocation density and internal stress are greatly reduced, and the bolt is subjected to a certain load during service. Under the conditions, there is no permanent deformation exceeding the harmful use requirements. The process range of the stabilization treatment is relatively loose, and can be completed in the range of 280 ° C to 480 ° C and 0.5 h to 2.5 h. To reduce the cost, the stabilization treatment can be combined. Black processing or other surface treatment methods are performed simultaneously. For bolts that have not been stabilized, the permanent elongation of bolts during the load test is between 15u and 90U, which exceeds the standard value of 12.5U. After the stabilization process according to the given process, the permanent elongation of the bolt during the load-bearing test is greatly reduced to less than 5U. At the same time, the bolt tensile strength of the stabilized treatment is increased by about 60 MPa, and the overall performance of the bolt is greatly improved. During the stabilization process of the bolt, fine carbon nitrides such as Nb, V, Ti and other alloying elements are precipitated in the steel. The carbon and nitrogen atoms in the steel accumulate on the dislocations, hindering and pinning the dislocation motion, not only greatly Reducing the permanent elongation of the bolt also acts as a dispersion strengthening and solid solution strengthening, which can increase the tensile strength of the bolt.
5 Conclusion
Using MFT8 cold work to strengthen non-tempered steel to make 8.8 bolts, the mechanical properties of the wire rod and the tensile strength of the bolts meet the standard requirements, but the bolt stabilization treatment is required to ensure that the permanent elongation index of the bolt is achieved. Standard requirements (≤12.5U). Since the annealing and quenching and tempering heat treatment process is omitted, the bolt production cost is not greatly reduced, the environment is improved, the quality risk caused by the heat treatment defect is avoided, and the comprehensive performance of the 8.8 bolt is improved.(Article source: Maanshan Iron and Steel Co., Ltd. Song Qiang Sun Shan Fang Yan)
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