High-strength bolt superior in delayed fracture and resistance and relaxation resistance

Abstract

Disclosed is a high-strength bolt having a tensile strength of 1,200 N/mm² or more and superior in delayed fracture resistance and relaxation resistance, prepared by wire-drawing a bolt steel containing the following elements: C: 0.5 to 1.0% (mass %, the same shall apply hereinafter), Si: 0.55 to 3%, Mn: 0.2 to 2%, P: 0.03% or less (but not 0%), S: 0.03% or less (but not 0%), and Al: 0.3% or less (but not 0%), and containing proeutectoid ferrite, proeutectoid cementite, bainite and martensite at a total areal rate of less than 20% and pearlite in balance; cold-heading the wire into a bolt shape; and then bluing the bolt in a temperature range of 100 to 500° C.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength bolt mainly for use in automobiles, and in particular to a high-strength bolt having a tensile strength (strength) of 1,200 N / mm2 or more that is superior in delayed fracture resistance and relaxation resistance. BACKGROUND ART [0002] Carbon alloy steels (such as SCM435, SCM440, and SCr440) are used for common high-strength bolts, and these bolts are given a desirable strength by quenching and tempering. However, common high-strength bolts used in automobiles and various industrial machines have a risk of generating delayed fracture when they have a tensile strength in the range above approximately 1,200 N / mm2, and thus, have restriction in use. [0003] The delayed fracture includes both phenomena occurring in corrosive and noncorrosive environments, and the reasons for its occurrence are said to be complicated by various factors, and thus, it is difficult to specify the reasons indiscriminately. The regulatory f...

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Benefits of technology

[0029] The first supplementary elements, Cr and Co, may be added in the ranges of Cr: 2.5% or less (but not 0%) and Co: 0.5% or less (but not 0%). Cr and Co are effective in preventing precipitation of proeutectoid cementite, and thus, particularly useful as the additives for the high-strength bolt according to the present invention aimed at reducing the content of proeutectoid cementite. Such an action is amplified by increase in the amounts thereof added, and thus, the Cr content is recommended to be 0.05% or more (for example, 0.1% or more and particularly preferably 0.2% or more), or the Co content, 0.01% or more (for example, 0.03% or more and particularly preferably 0.05% or more), to make the action more distinctive. An excessive addition leads to saturation of the action, and is thus uneconomical. Thus, the Cr content is 2.5% or less (preferably 2.0% or less and more preferably 1.2% or less), and the Co content is 0.5% or less (preferably 0.3% or less and more preferably 0.2% or less). Only one or both of Cr and Co may be added.

[0030] The second supplementary element Ni may be added in an amount in the range of 1.0% or less (but not 0%). Ni is not effective in improving the strength of bolt, but effective in increasing the toughness of drawn wire rod. Although the effect is amplified when the Ni content is increased, the Ni content is recommended to be preferably 0.05% or more, more preferably 0.1% or more, and particularly preferably 0.15% or more, to make the action more distinctive. However, an excessive Ni content leads to elongation of the period until completion of transformation and thus to expansion of facility and decrease in productivity. Thus, the Ni content is 1.0% or less, preferably 0.5% or less, and more preferably 0.3% or less.

[0031] The third supplementary element Cu may be added in an amount in the range of 1.0% or less (but not 0%). Cu is an element effective in improving the strength of bolt by its precipitation-hardening action. The action is amplified by increase of the Cu content, and, to make the action more distinctive, the Cu amount is recommended to be preferably 0.05% or more, more preferably 0.1% or more, and particularly preferably 0.2% or more. However, an excessive Cu content causes embrittlement of grain boundary and thus deterioration in delayed fracture resistance. Thus, the Cu amount is 1.0% or less, preferably 0.5% or less, and more preferably 0.3% or less.

[0032] The fourth supplementary elements Mo, V, Nb, Ti, W, and others may be added in a total amount of 0.5% or less (but not 0%). These elements Mo, V, Nb, Ti, and W form fine carbides / nitrides, which are effective in improving delayed fracture resistance. The action is amplified by increase of the total amount of these elements, and the total amount is preferably 0.02% or more and more preferably 0.05% or more. However, an excessive total amount of these elements leads to inhibition of delayed fracture resistance and also to deterioration in toughness. Thus, the total amount of these elements is 0.5% or less, preferably 0.2% or less, and more preferably 0.15% or less. These elements, Mo, V, Nb, Ti, W, and the like, may be added alone or in combination of two or more.

[0033] The fifth supplementary element B may be added in an amount in the range of 0.003% or less (but not 0%). B is added for improvement in quenching efficiency. The action is amplified by increase of the B content, and the B content is preferably 0.0005% or more, and more preferably 0.0010% or more, to make the action more distinctive. However, an excessive B content leads to inhibition of toughness. Thus, the B content is 0.003% or less, preferably 0.0025% or less, and more preferably 0.0020% or less.

[0034] The elements in balance include Fe and unavoidable impurities.

Problems solved by technology

However, common high-strength bolts used in automobiles and various industrial machines have a risk of generating delayed fracture when they have a tensile strength in the range above approximately 1,200 N / mm2, and thus, have restriction in use.

The delayed fracture includes both phenomena occurring in corrosive and noncorrosive environments, and the reasons for its occurrence are said to be complicated by various factors, and thus, it is difficult to specify the reasons indiscriminately.

The regulatory factors responsible for the delayed fracture include tempering temperature, structure, material hardness, grain size, various alloy elements, and others, but there is currently no effective means to prevent the delayed fracture, and various methods are studied by trial and error even now.

In these methods, the delayed fracture of high-strength bolts having a tensile strength of 1,400 N / mm2 or more is reduced by adjusting the contents of various major alloy elements, but the risk of the delayed fracture is not completely eliminated, and the application thereof still remains in extremely limited areas.

However, the pearlite steel has a problem of its own.

However, the relaxation property is not considered well in conventional high-strength bolts, except the bolt described in Patent Document 5.

Although Patent Document 5 is an invention aimed at improving relaxation resistance, it does not disclose the relationship between Si content and the relaxation resistance and claims that the Si content should be 0.5% or less because an excessive Si content leads to deterioration of the ductility of the steel material after wire-drawing as well as significant deterioration of the cold-heading efficiency.

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