Hot-formed member and process for manufacturing same

Abstract

A hot-formed member that has both a prescribed chemical composition and a metal structure which contains 10 to 40% by area of austenite and in which the total number density of grains of the austenite and martensite is 1.0 grain / Mum2 or more and that exhibits a tensile strength of 900 to 1300MPa.

Description

technical field [0001] The present invention relates to a thermoformed member used, for example, in machine structural parts such as vehicle body structural parts and running parts, and a manufacturing method thereof. Specifically, the present invention relates to a product having a tensile strength of 900 MPa to 1300 MPa, excellent ductility with a total elongation of 15% or more in a tensile test, and an impact value of 20 J / cm in a Charpy test at 0°C. 2 The above thermoformed member with excellent impact properties and its manufacturing method. Background technique [0002] In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel materials used in vehicle bodies and reduce the weight of steel materials used. In thin steel sheets that are widely used in technical fields related to automobiles, press formability decreases with an increase in the strength of the steel sheet, making it difficult to manufacture me...

AI Technical Summary

Problems solved by technology

In the case of thin steel sheets widely used in technical fields related to automobiles, the press formability decreases with the increase in the strength of the steel sheet, making it difficult to manufacture members with complex shapes

Specifically, the following problems arise: the ductility of the steel plate decreases as the strength of the steel plate increases, and therefore, fractures occur in parts of the member where the degree of work is high, and / or the springback and wall warpage of the member become large, and the member Dimensional Accuracy Deterioration

However, in the production method using the above-mentioned technology, there is a problem that the tensile strength of the member varies significantly with the change of the quenching condition, specifically, the temperature at which the cooling is stopped.

In addition, engineering problems such as extremely difficult control of the cooling stop temperature are also unavoidable in the above-mentioned manufacturing method.

In addition, unlike conventional manufacturing methods of thermoformed members, the manufacturing method of Patent Document 5 requires a further heat treatment process called reheating.

Therefore, the production method of Patent Document 5 has significantly lower productivity than conventional production methods of thermoformed members.

In addition, as described in the examples of Patent Document 5, in the production method of Patent Document 5, it is necessary to heat the steel sheet to a high temperature, so the second phase such as martensite in the metal structure of the member tends to be sparsely distributed.

This causes the problem that the impact characteristics of the member are significantly deteriorated

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