Process for producing steel sheet and device for continuously annealing steel sheet

Abstract

This process for producing a steel sheet is a process for producing a high-strength steel sheet having a tensile strength of 780 MPa or higher, and includes a continuous annealing step in which a steel sheet having a given chemical composition is heated to a temperature in the range of 750-900DEG C and the steel sheet is held in the temperature range for 0-300 seconds, thereby conducting continuous annealing. In the continuous annealing step, the atmosphere inside the furnace is regulated so as to have a hydrogen concentration less than 10 vol%, during the period when the heating to a temperature in the range is conducted and during the period when the holding in the temperature range is conducted. When the steel sheet has a temperature of 700DEG C or lower, the furnace-body average valueof log(PH2O / PH2) is greater than -3.01 but less than -0.07. When the steel sheet has a temperature higher than 700oC but not higher than 800oC, the furnace-body average value of log(PH2O / PH2) is greater than -1.36 but less than -0.07. When the steel sheet has a temperature exceeding 800DEG C, the furnace-body average value of log(PH2O / PH2) is greater than -3.01 but not greater than -0.53, and thedew point is lower than -10DEG 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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