Method for processing member having excellent chemical conversion treatability

Abstract

In a method for processing a high-strength member having a high Si content of more than 0.7% by mass, a high-strength steel material containing, on the basis of mass percent, 0.05% or more C, more than 0.7% Si, and 0.8% or more Mn is subjected to processing controlled such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain to form a member having a predetermined shape. The member, even containing more than 0.7% Si, can have markedly improved chemical conversion treatability without mechanical grinding or chemical pickling treatment.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a method for processing high-strength steel for auto body structures and, more particularly, to a method for improving the chemical conversion treatability of a member manufactured by the processing of a high-strength steel material having a high Si content of more than 0.7% by mass.BACKGROUND OF THE INVENTION[0002]In recent years, from the standpoint of global environmental protection, efforts have been made to reduce the weight of automotive bodies and improve the mileage of automobiles. The improvement in the mileage of automobiles has also been required by law. Recently, efforts have been made to use high-strength steel materials as the materials for automotive body structure to reduce the weight of automobiles by gauge down (thickness reduction). Furthermore, improvement in the stiffness of members using a closed-cross-section structure like a steel tube is under study.[0003]In general, such high-strength steel materi...

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Benefits of technology

[0040]In view of the situations described above, it is advantageous to provide a method for processing members in which a high-strength steel material having poor chemical conversion treatability used as a raw material is processed into a high-strength member having excellent chemical conversion treatability.

[0041]More particularly, it is advantageous to improve the chemical conversion treatability of a high-strength member particularly using a high-strength steel material having a high Si content of more than 0.7% by mass in which an oxide mainly composed of Si is concentrated on a surface layer as in hot-rolled sheets or cold-rolled and annealed sheets. The steel materials include steel sheets (steel strips), steel tubes, and steel bars. The concentration of an oxide mainly composed of Si, as used herein, refers to the concentration of an oxide mainly composed of Si or an oxide containing Si and another element or the concentration of a composite oxide, a eutectic oxide, a peritectic oxide, or the like containing these.

[0042]To these ends, the present inventors have performed diligent research on various factors affecting the chemical conversion treatability of high-strength steel materials having a high Si content. As a result, the present inventors have conceived the utilization of processing strain applied to a surface during the processing of steel materials. The present inventors found that in the processing of steel materials into members the chemical conversion treatability of a member manufactured from a high-strength steel material having a high Si content can be significantly improved when the processing conditions are controlled such that the processing strain applied to a surface of the steel material (the surface strain) is a predetermined value or more. The present inventors also found that the improvement in chemical conversion treatability can be well explained by employing, as the processing strain, the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing. The present inventors also found that the chemical conversion treatability of a member made of a steel material that contains more than 0.7% of Si and in which an oxide mainly composed of Si is concentrated on a surface layer can be significantly improved by processing the steel material such that the sum total of absolute surface strains in a predetermined direction each applied in individual process steps of the processing is 5% or more as nominal strain.

[0044]It has often been pointed out that an oxide mainly composed of Si in a film form is concentrated on a surface of a steel sheet having a high Si composition. In the actual production using a continuous annealing line (CAL), an oxide mainly composed of Si is concentrated mostly in a granular form, for example, by in-line light pickling. In both cases, it is assumed that a granular oxide mainly composed of Si can be very easily removed (fell out) from the surface of the steel sheet by chemical conversion treatment under the surface strain of a predetermined value or more.

[0052]In accordance with exemplary embodiments of the present invention, a member manufactured using a high-strength steel material having a high Si content of more than 0.7% on the basis of mass percent as a raw material can be a high-strength member having excellent chemical conversion treatability without performing mechanical grinding or chemical pickling treatment. Thus, the present invention can have significant industrial advantages. Also in accordance with exemplary embodiments of the present invention, a member having excellent chemical conversion treatability can be manufactured independently of the history of a steel material used as a raw material.

Problems solved by technology

Thus, it is difficult to think that steel sheets manufactured by the techniques described in PTLs 10 and 11 always have excellent chemical conversion treatability even after processing.

Since skin pass rolling results in hardening, the skin pass rolling of higher-strength materials will gradually become more difficult.

The skin pass rolling of steel materials having a tensile strength on the order of 780 MPa or more is difficult to perform at a rolling reduction of 1% or more.

Thus, the technique described in PTL 11 in which skin pass rolling is performed at a rolling reduction of 0.8% or more and 5% or less cannot be applied to high-strength materials without causing problems.

Thus, the truth of the matter is that the related art described above cannot significantly improve the chemical conversion treatability of steel materials having a high Si content of more than 0.7% by mass.

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