| As medium-carbon steel used for cold-heading application, SWRCH35KM are widely employed at home and abroad in the field of fastener products. However, before cold-heading treatment "two drawing one annealing" process are required such as pickling, drawing, ball annealing (or softening annealing), and Drawing. The free-annealing SWRCH35KM steel wire rod shows excellent performance during cold heading process after drawing despite of removing annealing treatment. As the wire eliminating the need for an annealing process, which saves each furnace at least eight hours of engineering-consuming-time, while saving energy consumption in the current situation of power shortage and has greatly enhanced the productivity of standard parts. Based on the needs of the domestic production of standard parts, some research was carried out in Hualing Xiangtan Iron and Steel Company to produce free-annealing XSWRCH35KM cold heading steel. The main findings are as follows:(1) The suitable chemical composition of XSWRCH35KM were determined via the analysis of composition, microstructure and performance of the same type of free-annealing 35K carbon cold heading steel used at home and abroad and the chemical composition of this alloy is as follows:0.32～0.35%C,0.13～0.15%Si,0.50～0.52% Mn, P≤0.020%, S≤0.010%,0.002～0.003%B,0.02～0.04% Als. The value of Ac1, Ac3, Ar1, Ar3 are 740℃,788.5℃,675.8℃,749℃,respectively.(2)The result of the controlled rolling and controlled cooling process carried out on the GLEEBLE1500 thermal simulation machine show the ferrite grain and pearlite appear fine and uniform pattern at the deformation temperature (760℃) and silking temperature (660℃); the ferrite grain size increased as the silking temperature increase; the lower rolling temperature is favorable to obtain a granular and short-like pearlite; the number of granular pearlite decrease and the number of flaky pearlite increased as rolling temperature increases. The microstructure of the sample rolled at temperature of 800℃and silked at temperature of 740℃consist well with that of the free-annealed cold heading steel.(3) The cold heading wire rod ofΦ6.5mm made on production line by the simulation of controlled rolling and controlled cooling process show good performance with the pass rate of 100% which meet the needs of clients free from annealing. The microstructure of wire rod were ferrite and pearlite, and the content of ferrite was 62.5%, grain size of 9.5～10.5, the rate of ball of 2.5. Micro structure analysis shows at a high temperature B has the priority to precipitate in the form of thick BN particles, thus inhibiting the precipitation of fine AlN which is beneficial to obtain a larger ferrite grain.(4) The cold heading wire rod ofΦ12mm made on production line has the pass rate of 93.3%, and the grain size distribution is uneven with grade of 10～9,meanwhile the ball of the rate was about 2. Site analysis found that as the rod diameter increased, temperature gradient between the rod core and the surface increased during the process of the finishing mill and silking, which lead to uneven rod microstructure as well as performance degradation.(5) Quenching and tempering process of XSWRCH35KM rod were carried out in the thermal simulation testing machine. Because of fast cooling rate of the surface, the quenching temperature has little effect on the microstructure of the external part, basically for the martensite. However, the surface and core of the specimen show quite different microstructure because of poor hardenability, and with the quenching temperature increased, content of martensite at the center has also increased, so increasing the quenching temperature appropriately can increase the hardenability of XSWRCH 35KM... |
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