Jfe Steel Corporation

Shinji Otsuka

Naho Inoue

Yoshitsugu Suzuki

Nobuyuki Ishikawa

Kohei Hasegawa

Daisuke Mizuno

Akira Matsuzaki

Yoichi Makimizu

Junichiro Hirasawa

Naoto Yoshimi

Hideki Nagano

Mai Miyata

Kentaro Hata

Kazuaki Tsuchimoto

Masataka Omoda

A method for evaluating a delayed fracture characteristic of a metal material that performs a step including a step (A) and a step (B) below once or a plurality of times to evaluate the delayed fracture characteristic of the metal material having been processed. Step (A): a step that brings an aqueous solution which includes a chloride ion into contact with a surface of the metal material to cause chloride of 1000 to 100000 mg/min chloride amount to deposit to the surface. Step (B): a step in which the metal material to which the chloride has deposited in the step (A) is left stationary in an air atmosphere at a temperature from −50 to 10° C. and at a relative humidity higher than or equal to a deliquescence humidity of the chloride.

Method for evaluating delayed fracture of metal material

A method for evaluating the delayed fracture characteristics of a metal material. The method including placing a solution-retaining material impregnated with a solution containing a chloride and having a pH of 3.5 or more on an edge face of the metal material, and maintaining a state in which the solution-retaining material is held at a deliquescence humidity of the chloride to thereby corrode the edge face.

A method for evaluating the delayed fracture characteristics of a metal material. The method including placing a solution-retaining material impregnated with a solution containing a chloride and having a pH of 3.5 or more on a stress loading part of the metal material, and maintaining a state in which the solution-retaining material is held at a deliquescence humidity of the chloride to thereby corrode the stress loading part.

A corrosion sensor is an electric resistance type corrosion sensor including a sensor portion exposed to an arbitrary environment and formed of an electric conductor, and a reference portion isolated from the arbitrary environment and formed of an electric conductor, and measuring a corrosion loss of the sensor portion based on an electric resistance value of the reference portion and an electric resistance value of the sensor portion. A width of the sensor portion is set to satisfy Formula (II): w≥(32×t) . . . (II), t: maximum corrosion loss to be measured [mm], and w: width of the sensor portion [mm].

Corrosion sensor designing method, corrosion sensor manufacturing method, and corrosion sensor

A steel sheet mainly suitable for strength members of automobiles or building materials, which has a tensile strength of 1,180 MPa or more, and which is excellent in delayed fracture resistance and primary rust prevention performance. The steel sheet includes a coating, placed on a surface of a cold-rolled steel sheet with a tensile strength of 1,180 MPa or more, containing one or more metalates selected from molybdates and tungstates and a P compound. The sum of the coating weights of the metalates in terms of Mo and W is 10 mg/mto 1,000 mg/mand is preferably 50 mg/mto 1,000 mg/m. The coating weight of the P compound in terms of P is 10 mg/mto 1,000 mg/m.

High-strength cold-rolled steel sheet

A high-strength cold-rolled steel sheet having high chemical convertibility and a tensile strength of 590 MPa or more and a method for producing such a steel sheet are provided. The steel sheet contains, in terms of percent by mass, C: 0.05 to 0.3%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.01 to 1%, N: 0.01% or less, and the balance being Fe and unavoidable impurities. The coverage ratio of reduced iron on a steel sheet surface is 40% or more. In order to produce such a steel sheet, an oxidation treatment is performed after cold rolling. Subsequently, annealing is conducted in a furnace in a 1 to 10 vol % H+balance Ngas atmosphere with a dew point of &#x2212;25&#xb0; C. or less.

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Tokyo, Japan

Shinji Otsuka