Steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance, method of manufacturing the same, and mehod of manufacturing bolt using the same

Abstract

Provided are a high-strength, high-manganese steel wire rod having excellent cold heading quality and not requiring spheroidizing and quenching-tempering treatments during manufacturing a bolt and a method of manufacturing a bolt using the steel wire rod. The method of manufacturing a steel wire rod includes heating a steel containing 12 to 25 wt % of Mn within a temperature range of 1100° C. to 1250° C., hot rolling the heated steel within a temperature range of 700° C. to 1100° C., and cooling the hot rolled steel to a temperature of 200° C. or less and cold caliber rolling or drawing to manufacture a steel wire rod.

Description

CROSS REFERENCE[0001]This application claims foreign priority under Paris Convention and 35 U.S.C. §119 to Korean Patent Application Nos. 10-2010-0116350, filed 22 Nov. 2010, and 10-2011-0022061 filed 11 Mar. 2011, with the Korean Intellectual Property Office.BACKGROUND OF THE INVENTION[0002]1. Technical Field[0003]The present invention relates to a steel wire rod used for a bolt, and more particularly, to a high-strength, high-manganese steel wire rod, which has excellent cold heading quality and hydrogen delayed fracture resistance and is enabled to skip spheriodizing and quenching-tempering heat treatment processes, a method of manufacturing the same, and a method of manufacturing a bolt using the same.[0004]2. Description of the Related Art[0005]There is a trend towards continuously increased demand for a high-strength wire rod in order to achieve energy efficiency increase and energy consumption decrease. For example, when bolts used in an automobile engine are replaced by high...

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Benefits of technology

[0015]An aspect of the invention provides a steel wire rod promoting high strengthening of the steel wire rod as well as having excellent cold heading quality and being enabled to skip spheriodizing and quenching-tempering heat treatment processes, and a method of manufacturing the same.

[0017]Another aspect of the invention provides a steel wire rod promoting high strengthening as well as having excellent hydrogen delayed fracture resistance and a method of manufacturing the same.

[0018]According to an embodiment of the invention, there is provided a method of manufacturing a steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance including: heating a steel containing about 12 wt % to about 25 wt % of manganese (Mn) within a temperature range of about 1100° C. to about 1250° C.; hot rolling the heated steel within a temperature range of about 700° C. to about 1100° C.; and cooling the hot rolled steel to a temperature of 200° C. or less and cold caliber rolling or drawing to manufacture a steel wire rod.

[0021]According to another embodiment of the invention, there is provided a steel wire rod having excellent cold heading quality and hydrogen delayed fracture resistance including about 12 wt % to about 25 wt % of manganese (Mn), wherein a microstructure of the steel wire rod includes a face centered cubic austenitic structure and a <112>{111} twin system.

Problems solved by technology

However, an obstacle in manufacturing of high-strength bolt steel is that cold heading quality deteriorates due to the use of high carbon steel for high strengthening.

With respect to the high-strength steel wire rod having poor cold heading quality, life time of cold heading dies is decreased and this will cause an increase in production costs.

The spheroidizing heat treatment and the quenching and tempering processes before and after the cold heading cause cost increase and generate distortion during heat treatments with respect to a large bolt.

However, with respect to a general heat treatment free bolt developed to date, cold cold heading quality is excellent but high strength has not been achieved.

When a high-strength steel wire rod is used, material stability was not guaranteed due to the generation of defects in a bolt head portion and the existence of residual stress, according to cold heading of the high-strength wire rod having poor cold heading quality.

This technique may be applied to a plate, but the technique is inappropriate for manufacturing a wire rod through a caliber rolling mill or drawing which has a different deformation mode.

Also, the biggest obstacle in strengthening of a wired rod is hydrogen delayed fracture that deteriorates material stability.

As a result, intergranular fracture is generated due to the intensive weakening of bond strength by means of hydrogen at grain boundaries or the like.

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