Hot spot quality problem of cold heading steel

The quality requirements of high-grade standard parts for raw materials are as follows: wire rod has higher plasticity index, higher area shrinkage and elongation; in cold plastic deformation, material deformation resistance is small, work hardening rate is low, material σs/ The value of σb is small; the hardness of the wire rod is appropriate, not too high; the wire rod has good surface quality, a certain surface finish, and no surface defects such as folding and cracking are allowed; the steel is densely organized and has no internal defects.

There are two main process influencing factors for the quality of cold steel in Xiangtan Iron and Steel Co., Ltd., that is, uneven cooling during the rolling process of continuous casting mold slag and wire material. The mold slag forms a large inclusion, which destroys the continuity of the steel; the uneven cooling during the wire rolling process causes the internal grain size of the wire to vary greatly. The fine grains are only 3~4μm; the coarse crystal grains can reach 50~60μm. The microhardness of the coarse and fine grain areas differs by 50%.

Taigang's precautions for defects in ultra-low carbon cold heading steel CH1T during cold heading: 1) Controlling the lower billet heating temperature of 1150~1200 °C is the key parameter for controlling ferrite grain size. At the same time, it is necessary to strictly control the wire rod spinning temperature between 910 and 930 °C, so that the wire rod finished product has a moderate grain size, preferably 6~6.5. 2) Improve the surface quality of the wire rod. The cold heading performance can be improved by adjusting the amount of drawing deformation and the corresponding annealing process in an appropriate process.

The controlled rolling and cold control results of cold-rolled steel of Jiangsu Shagang cold-rolled steel show that the heating temperature of cold-rolled steel is the same as that of low- and medium-carbon steel, but the furnace pressure is strictly controlled on the heating, so that the furnace pressure is kept at a positive pressure of 3~10Pa, and appropriate. Adjust the temperature to reduce surface oxidation and decarburization. Low-temperature rolling at 930~950°C is achieved to prevent coarsening of austenite grains. Choose an appropriate spinning temperature of 820~840 °C. Reduce the amount of iron oxide produced, coupled with a lower cooling rate of 0.8 ~ 2 ° C / s, so that the austenite decomposition transition time is longer, in order to obtain fine ferrite plus a small amount of uniform pearlite.

The most suitable cold head deformation structure is carbide spheroidized structure, which has reduced strength and hardness, and plasticity is better for cold head deformation. Maanshan Iron and Steel Co., Ltd. used ZL18A self-tapping screws for cold-twisted steel wire, and the spheroidizing annealing temperature was (680±10 °C) for 4 h. The cementite spheroidizing effect of steel wire was the best. ZL18A self-tapping screws are cracked by cold-rolling of the wire rod. The reason is that the user omits the process of spheroidizing annealing and directly causes cold rolling after drawing. Measures to meet the user's requirements: C, Mn content is controlled by the middle and lower limits; negative deviation rolling, reducing the total deformation of the drawing; improving the rolling process, reducing the strength of the wire rod, reducing or avoiding Wei Organization. ML35 and ML45 are typical medium-carbon cold-twisted steel wires. When the cold heading deformation is >60%, spheroidizing annealing should be carried out at 720 °C for 4 hours before cold heading, so that the pearlite spheroidized structure can reach 4~6 levels. Processing plasticity of steel wire.

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