Cold forging steel for high-strength large-dimension bolt and production method thereof

Abstract

The invention discloses cold forging steel for a high-strength large-dimension bolt and a production method thereof. The production method comprises a heating process, a rolling process and a cooling process. Steel billet adopted by the production method consists of the following chemical components in percentage by weight: 0.17-0.23% of C, 0.70-1.00% of Mn, 0.30% of Si, less than or equal to 0.025% of P, less than or equal to 0.025% of S, 0.65-1.20% of Cr, 0.0008-0.0050% of B, 0.01-0.06% of Ti and the balance of Fe and inevitable impurity elements; and in the heating process, the steel billet is heated to 1000-1100 DEG C and the temperature-preserving time is 80-130 minutes. According to the production method, by optimizing components and the processes, a produced coil has relatively low strength and hardness, meets process properties of carrying out cold processing in a non-annealed mode, has good hardenability, meets large-dimension bolt hardenability requirements, and further has certain delayed fracture resistance; and a microstructure consists of uniform ferrites and pearlite, has tensile strength of 500-560 MPa, and area reduction greater than or equal to 60%, so that 1 / 3 cold upsetting is qualified, and the production cost is low. The cold forging steel has good cold processing performance and hardenability.

Description

technical field [0001] The invention relates to a cold pier steel wire rod, in particular to a cold heading steel for grade 10.9 high-strength large-size bolts and a production method thereof. Background technique [0002] Most of the traditional 10.9 high-strength bolts are produced by medium carbon steel and medium carbon alloy steel. However, this kind of steel has high carbon content and poor process performance. For large-sized bolts, the hardenability of medium carbon steel is slightly insufficient. Therefore, through the optimization of composition and controlled rolling and controlled cooling process, a kind of 10.9 grade high Cold heading steel coils for strength bolts appear to be very necessary and meaningful. [0003] At present, some manufacturers at home and abroad can produce boron-containing steel wire rods without annealing or simplified annealing, mainly by reducing the carbon content to improve the cold workability, and at the same time adding a certain a...

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Problems solved by technology

[0002] Most of the traditional 10.9 high-strength bolts are produced by medium carbon steel and medium carbon alloy steel. However, this kind of steel has high carbon content and poor process performance. For large-sized bolts, the hardenability of medium carbon steel is slightly insufficient. Therefore, through the optimization of composition and controlled rolling and controlled cooling process, a kind of 10.9 grade high Cold heading steel coils for strength bolts are very necessary and meaningful

[0003] At present, some manufacturers at home and abroad can produce boron-containing steel wire rods without annealing or simplified annealing, mainly by reducing the carbon content to improve the cold workability, and at the same time adding a certain amount of boron to improve the hardenability of the steel; but such wire rods exist The following disadvantages: it is mainly used for the production of 8.8 grade bolts, if the production of higher grade bolts needs to increase the carbon content, it will deteriorate the cold working performance of the wire rod, increase the loss of the mold, and even require spheroidizing annealing treatment

[0004] Patent No. CN201010127837.4 discloses "boron-containing cold heading steel for high-strength fasteners and its preparation process", including the chemical composition of the steel and its preparation process, which can be used to produce high-strength fasteners of grade 8.8 and above , which is mainly to add boron element on the basis of medium and low carbon steel, and the content of Cr element is as low as 0.08-0.50%. performance

[0005] Patent No. CN200910144282.1 published "10.9 grade chromium-containing non-quenched and tempered cold heading steel and its rolling method for hot-rolled wire rod", including chemical composition and rolling process, which belongs to low-carbon high-manganese content non-tempered High-quality steel, the main components are C0.08-0.14%, Mn1.80-2.30%, Cr0.20-0.40%; but this method has the following disadvantages: it will increase the mold damage in the cold working process of downstream customers, and increase the crack initiation At the same time, the production of wire rod by this method requires low-temperature rolling. At present, the equipment of most steel mills does not have the ability of low-temperature finish rolling, and the production is not easy to realize.

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