Production method of carbon cold-forging steel large coil for 8.8 level high-intensity fasteners

Abstract

The invention provides a method for producing a carbon element cold forging steel big pony roll for a 8.8 grade high intensity fastening piece, which belongs to the technical field of cold forging steel. Twenty high rigidly short stress line red circle steel bar rolling mills are adopted to roll heated mid-carbon element cold forging steel billet. The billet heated to the temperature of between 1,000 and 1,150 DEG C taken out of a furnace is rough rolled by a 4+4 stander and is medially rolled by a 4 stander. A rolling piece after the mid rolling adopts the first section of water cooling to reduce the surface temperature to be between 770 and 790 DEG C, passes through the restoring section, enters four standers to be precisely rolled at a temperature of between 840 and 860 DEG C; the rolling piece after the precise rolling adopts the second water cooling to reduce the surface temperature to be between 760 and 780 DEG C, passes through the restoring section, enters four standers to be precisely rolled at a temperature of between 830 and 850 DEG C; and the rolling piece after the precision rolling enters the third section water cooling to reduce the temperature of between 830 and 850 DEG C to be curled, the curled piece enters an insulated shield to be cooled by 4,200 to 4,800 seconds at a cooling speed of between 0.08 and 0.30 DEG C per second, and the pony roll is cooled naturally to the room temperature. Phi20mm and Phi38mm pony roll strength Rm can be stably controlled to be less than or equal to 650Mpa, HBW is less than or equal to 170, and Z is more than or equal to 45 percent so that the mid-carbon element cold forging steel pony roll is softened on line and the quality of the pony roll is high.

Description

technical field [0001] The invention belongs to the technical field of cold heading steel, and in particular provides a method for producing large coils of carbon cold heading steel for grade 8.8 high-strength fasteners, especially relates to the composition design and controlled rolling method of low-strength and high-plasticity medium carbon steel . Background technique [0002] In the fastener industry, 8.8-grade fasteners above M20 mostly use Φ20-38mm large-size coil raw materials. Carbon cold heading steel with a carbon content of 0.33% to 0.45% is currently commonly used. This coil rolling method is to heat the steel billet to 1000-1150 ° C, rough rolling, intermediate rolling, pre-finish rolling after the billet comes out of the furnace, and then finish rolling at above 950 ° C, usually two stages of water cooling before and after the finish rolling, Control the final rolling temperature and coiling temperature, and slowly cool after coiling, the rolled product stru...

AI Technical Summary

Problems solved by technology

[0003] The purpose of the present invention is to provide a production method of carbon cold heading steel large coils for high-strength fasteners of grade 8.8. In order to overcome the existing rolling of large-size coils, the temperature of the rolled piece entering the finishing mill is difficult to achieve by a section of water cooling. Precise control, low process stability