Manufacturing method of steel plate and continuous annealing apparatus for steel plate

Abstract

A method of manufacturing a steel sheet, which is a method of manufacturing a high-strength steel sheet having a tensile strength of 780 MPa or more, comprising heating a steel sheet having a predetermined chemical composition to a temperature range of 750° C. to 900° C. within the above temperature range In the continuous annealing process of performing continuous annealing by holding for 0 to 300 seconds, in the above continuous annealing process, hydrogen in the atmosphere in the furnace is released when the heating is carried out to the above temperature range and when the above holding is carried out in the above temperature range. The concentration is set to be lower than 10% by volume, and the log(P H2O / P H2 ) is set to be more than -3.01 and less than -0.07, and the above log(P H2O / P H2 ) is set to be more than -1.36 and less than -0.07, and the above log(P H2O / P H2 ) is set to be more than -3.01 and less than -0.53, and the dew point is set to be lower than -10°C.

Description

technical field [0001] The present invention relates to a method for manufacturing a steel plate and a continuous annealing device for a steel plate, in particular to a high-strength steel plate containing 0.050% by mass or more of C, 0.10% by mass or more of Si, and 1.20% by mass or more of Mn, and having a tensile strength of 780 MPa or more. A production method and a continuous annealing apparatus for a steel plate suitable for the production method of the steel plate. Background technique [0002] In recent years, in order to reduce fuel consumption and reduce CO 2 In the automotive field, there is an increasing demand for high-strength steel sheets for bodywork and parts, in order to reduce the weight of the car body and improve the crash safety for the purpose of displacement. Recently, high-strength steel sheets having a tensile strength of 780 MPa or more or 980 MPa or more are being used. [0003] However, if the strength of the steel sheet is increased, material ...

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Problems solved by technology

[0017] In addition, when the atmosphere in the furnace is controlled under the conditions described in Patent Document 4, although the exposure of Si oxides on the surface can be suppressed, since the dew point needs to be set to -10°C or higher, there is a risk of delamination of the steel sheet. Carbon progresses, and the tensile strength and fatigue strength of the steel plate decrease.

In particular, in high-strength steel sheets containing a large amount of C, the reduction in strength due to decarburization becomes a major problem

[0018] In addition, regarding the conditions described in Patent Documents 5 and 6, there is also a problem that it is difficult to guarantee the equations (1) and (3) due to external fluctuations in the amount of moisture in the gas and deterioration of production equipment. P H2O / P H2 Value, dew point and other appropriate conditions

However, in order to set the concentration of hydrogen in the atmosphere gas to 10% or more, special equipment is required, so a large investment in equipment is required for application.

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