Wire rod having superior hydrogen delayed fracture resistance, method for manufacturing same, high strength bolt using same and method for manufacturing bolt

Abstract

The present invention relates to a wire rod used in bolts for automobile engines, for example, and more specifically to a wire rod having an improved resistance to hydrogen delayed fracture, to a manufacturing method for same, to a bolt using same and a method for manufacturing the bolt. Provided are a high strength wire rod having a superior resistance to hydrogen delayed fracture and a method for manufacturing same, a high strength bolt using the wire rod and a method for manufacturing same, wherein.

the wire rod comprises, 0.3-0.7 wt % C, 0.05-2.0 wt % Si, 0.7-1.5 wt % Mn, 0.01-0.1 wt % Ni, and 30-70 ppm La, and the remainder thereof is comprised by Fe and inevitable impurities.

Description

TECHNICAL FIELD [0001]The present disclosure relates to a wire rod used for automobile engine bolts and the like, and more particularly, to a wire rod having improved hydrogen delayed fracture resistance, a method for manufacturing the same, a high strength bolt using the same, and a method for manufacturing the bolt.BACKGROUND ART[0002]in accordance with the recent trend for weight reductions and high functionalization in automobiles, requirements for driving parts, in particular, engine parts such as bolts and the like, to have high strength, have increased in order to reduce energy consumption. Currently used high strength bolts are manufactured to have a tensile strength of 1200 MPa through quenching and tempering processes, using alloyed steels such as SCM435, SCM440, and the like. However, in the bolts having a tensile strength of 1200 MPa or greater, since delayed fractures may be easily caused due to hydrogen, the uses of wire rods for manufacturing ultrahigh strength bolts ...

AI Technical Summary

Benefits of technology

[0018]The wire rod according to the present disclosure may be a high strength wire rod used for the coupling of automobile components or used in such automobile components, and the method of manufacturing the wire rod may be advantageous in that a wire rod having high strength of 1200 MPa to 2000 MPa and superior hydrogen delayed fracture resistance, even in a case in which a tiny amount of lanthanum and nickel is added or even in a case in which a martensite microstructure is present after the final heat treatment, may be manufactured with low manufacturing costs.

[0019]In accordance with the development of a wire rod for bolts having superior hydrogen delayed fracture resistance and high strength, the stability of a steel structure may be increased due to a reinforcement of coupling force and a reduction of vacancies in a coupling part at the time of coupling the bolts, and an amount of steel used may be reduced due to a decrease in the number of coupled bolts. In addition, in terms of automobile components, the development of the wire rod for bolts as described above may contribute to lightening of the automobile components. Due to the lightening of automobile components, various automobile assembling device designs may be enabled and compactness of automobile assembling devices may be allowed.

Problems solved by technology

However, in the bolts having a tensile strength of 1200 MPa or greater, since delayed fractures may be easily caused due to hydrogen, the uses of wire rods for manufacturing ultrahigh strength bolts remain inadequate.

However, the wire rod as a raw material needs to have as little strength as possible in order to facilitate bolt-forming.

In order to highly strengthen a steel having a single-phase structure composed of tempered martensite, the addition of alloying elements, in particular, carbon elements, has been known as the most effective method; however, the addition of carbon may rapidly increase a ductile to brittle transition temperature (DETT) of a wire rod as well as increasing strength of the wire rod, and remarkably deteriorate hydrogen delayed fracture resistance.

In addition thereto, work hardening may be increased, causing disadvantages in bolt-forming and a separate softening heat treatment may be required.

A main factor hindering the high strengthendng of the basic material may be a degradation in delayed fracture resistance due to the introduction of hydrogen, and it has been known because the introduced hydrogen may deteriorate the strength of grain boundaries.

Thus, in order to achieve the high strengthening of bolts, improvements in delayed fracture resistance may be unavoidably required to increase critical delayed fracture strength, and to this end, a method of generating precipitates capable of trapping diffusible hydrogen or controlling microstructure by adding certain elements while maximally suppressing phosphorus (P) and sulfide (S) brominating austenitic grain boundaries, and the like may be present.

However, most inventions created domestically and internationally may have disadvantages such as high manufacturing costs and complex processes required therefor, and require excessively precise rolling and cooling conditions at the time of manufacturing steel.

By way of example, in order to improve delayed fracture characteristics of a high strength wire rod having a tensile strength of 1600 MPa, technologies of adding 0.5 wt % of titanium (Ti), niobium (Nb), and vanadium (V), which are grain refinement elements, and then, adding corrosion resistance elements such as molybdenum (Mo), nickel (Ni), copper (Cu), cobalt (Co), and the like and carbide elements are present, but production costs required therefor may be significantly high, Furthermore, a method of improving hydrogen brittleness using ferrite structures extracted from grain boundaries is present, but the method does not include a chemical combination, and a product manufacturing cost may also increase due to the addition of a considerable amount of molybdenum (Mo).

Thus, such a technology may have disadvantages such as high manufacturing costs and complex processes and have limitations such as the requirement for excessively precise rolling and cooling conditions at the time of manufacturing steel.

However, since the technology basically aims at improving hydrogen delayed fracture resistance by adding a great quantity of molybdenum (Mo), it may be disadvantageous in terms of high manufacturing costs.

As described above, limitations to a decrease in hydrogen delayed fracture resistance as compared to an improvement in tensile strength in heat-treated and non heat-treated carbon steels having a tensile strength of 1200 MPa or greater have not yet been overcome, the securing of price competitiveness may not be available due to the addition of expensive alloying elements, and in particular, the stable securing of data regarding delayed fracture characteristics due to hydrogen may be defective.

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