The quality of steel wire for cold heading above SWCH25K is worrying. New technology development

In view of the various problems in the quality of steel wire for cold heading above SWCH25K, Shagang adopted several technological measures to improve its main technical indicators, and developed a 35K cold heading steel non-torque controlled cold hot rolled wire rod with its own characteristics. After users use it, they give it a high rating.
In the fastener industry, the traditional process of manufacturing grade 8.8 bolts generally selects medium carbon steel to be processed and formed and then quenched and tempered to meet the performance requirements of bolts. The Japanese JISG3539?91 'Carbon Steel Wire for Cold Heading' standard clearly stipulates that the steel wire of SWCH25K and above should be produced by the DA process (that is, the wire rod is cold drawn and then spheroidized and annealed and then drawn into a steel wire for cold heading, or first The wire rod is spheroidized and annealed and then drawn into steel wire for cold heading) to ensure cold heading performance (SWCH35K cold heading steel wire requires b ≤ 620 MPa, and the corresponding reference hardness HRB ≤ 90). This process is still used in some domestic enterprises, but there is no doubt that the process consumes a lot of energy, has a long production cycle, and slow logistics, which not only increases production costs, but also slows down capital turnover, and sometimes delays timely delivery. When the raw material is spheroidized and annealed, it will also cause desorption due to various factors, excessive carbon and the loss of the original uniformity of the base material, increasing the quality loss and weakening the market competitiveness. How to produce a cold heading material to solve the above-mentioned problems faced by users of standard parts has become a focus of Shagang's research.
Since 1998, with the cooperation of Changshu Yinghu International Standard Fasteners Co., Ltd. and other units, Shagang has invested heavily in the development of 35K cold heading steel non-torque controlled cold hot rolled wire rod with its own characteristics. This product passed the expert appraisal organized by Jiangsu Economic and Trade Commission in 1999.
In recent years, Shagang has continuously improved the quality of 35K and other wire rods through a series of technological measures. These measures include:
(1) Refined material: select high-quality steel scrap (reducing impurities and harmful elements), increase the proportion of hot iron smelting in Australian mines, reduce the amount of scrap steel to about 40%, and optimize smelting raw materials;
(2) Refining: all molten steel is refined in an IJ furnace, fully degassed and exhausted;
(3) Finishing rolling: Strictly control the dimensional deviation to ensure decarburization, grain size and proper organization and performance;
Through a series of efforts, the main technical indicators of 35K wire rod have reached the following levels:
(1) The carbon content of different heats fluctuates within 0.04%, and the Mn content fluctuates within 0.10%; the P content of steel is less than 0.015%, with an average of 0.10%, and the S content is less than 0.010%, with an average of 0.07 %;
(2) Total decarburization layer ≤ 1.OD% (D is the nominal diameter);
(3) The grain size is 6-8;
(4) Tensile strength σb≤600MPa, hardness HRB≤86, Brinell hardness HB≤160 (GB/T6478 regulations: ML35, HB≤187);
(5) The cold heading performance exceeds the general requirements of GB/T6478-1986 'Technical Conditions of Cold Heading Steel
(6) The dimensional accuracy of the wire rod has reached the level above level B, and the normal coil weight is 2.4t.
Dongfeng Motor Company Truck Company gave a high evaluation of Shagang products after using 35K wire rods provided by Shagang in batches. In the past, they used the DA process to produce standard hexagonal bolts. After using Shagang SWRCH35Kgl2 wire rod, they organized the production of hexagonal bolts according to the QQ/B1235 standard. The production process is:
Φ12 wire rod peeling → pickling → phosphating → saponification → drawing (d10.98) cold heading steel wire, and then on the ZG47?12B four-station cold heading machine, the bolt is formed at one time (breaking → head Part pre-forming → head forming, rod necking → trimming → thread rolling).
The production speed reaches 90 pieces/min. No part cracks were found in the normal production of the equipment. After heat treatment (normal heating in mesh belt continuous furnace, oil quenching, tempering, water cooling), the quality of the inspection parts is stable, there is no cracking and deformation, the hardness of the parts is evenly distributed, and the finished product σbu003d835～880MPa (standard requirement ≥800MPa), The fracture morphology and position are ideal. The company believes that Shagang 35K has uniform chemical composition, low S and P content and few inclusions. It is a new material suitable for high-speed automated cold forming. The use of Shagang material can omit the spheroidizing annealing of raw materials, solving the problem of decarburization in the spheroidizing annealing of raw materials, keeping quality at the source, saving energy and reducing consumption, and the economic benefits are very obvious. Shagang's series of non-twisting controlled cold and hot-rolled wire rods for cold heading steel represented by 35K have been used in large numbers in famous standard parts factories such as Changshu Yinghu, Dongfeng Motor, Jiashan Jinyi, etc. The users are all over the country, and the sales volume of 35K is nearly close. Million tons. Shagang's 35K specification has been expanded to g6.5mm to 16mm, and g18mm products are also under research and development. More and more users have obtained good benefits from using Shagang products.
In September 2000, Shagang's non-torsion controlled cold and hot-rolled wire rod for cold heading steel was rated as a high-tech product in Jiangsu Province. In November 2001, it was awarded the title of Excellent New Product in Jiangsu Province.