Low yield ratio high tension steel plate and its producing method

Abstract

The invention provides a high tensile steel sheet having a low yield ratio in spite of its tensile strength of >=780 MPa, and having low acoustic anisotropy and excellent weldability, and to provide its production method. The steel sheet has a composition comprising, by mass, 0.010 to 0.080% C, 0.02 to 0.50% Si, 1.10 to 3.00% Mn, <=1.60% Cu, 0.40 to 2.50% Ni, <=0.030% P, <=0.010% S, <=0.200% Al, <=0.0100% N, 0.30 to 2.00% Cr, 0.10 to 1.10% Mo and 0.002 to 0.030% Ti, and the balance Fe with impurities. Further, regarding the following AS and DL, AS>=4.00 and DL<=2.80 are satisfied, the structure in the part of 1 / 4 in the sheet thickness is essentially composed of bainitic ferrite including MA by <=10 area% (excluding 0%), also, the average value of the major axis / manor axis of old austenite grains is 1.0 to 3.0, and further, the hardness ratio of the MA to the bainitic ferrite is >=1.10: AS=[Mn]+[Ni]+2*[Cu], and DL=2.5*[Mo]+30*[Nb]+10*[V]; [X] denotes the content (mass%) of the element X.

Description

technical field [0001] The invention relates to a high-strength steel plate with a tensile strength as high as 780 MPa or more, a yield ratio as low as 85 percent or less, small acoustic anisotropy, and excellent parent material toughness and weldability. Background technique [0002] For steel plates (thick steel plates) used for bridges or buildings, if there is a defect in the welded part, this part is likely to become the starting point of cracking. Therefore, it is necessary to investigate whether there is a defective part through an ultrasonic flaw detection test. repair work. However, in a steel plate whose sound velocity changes significantly in the direction of flaw detection, that is, a steel plate with high acoustic anisotropy, the ultrasonic flaw detection test cannot detect the correct position of the welding defect. Acoustic anisotropy is smaller. [0003] In the above-mentioned fields of marine structures and building structures, in order to improve the effi...

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Problems solved by technology

However, the above-mentioned techniques have a problem that when these elements are actively added, the bainite block, which has the function of resisting crack propagation, becomes coarse when the bainite transformation occurs, thereby deteriorating the toughness of the base material and the toughness of the HAZ.

[0022] In addition, the above-mentioned technology that can ensure weldability and obtain a hot-rolled steel sheet with a low yield ratio has to add a large amount of alloying elements such as C and V in order to supplement the strength reduction caused by not adding B. The problem of the decline of the property and the deterioration of the toughness of the base metal

[0023] On the other hand, in none of the techniques, the reduction of acoustic anisotropy is not taken into consideration, so there is a problem in terms of acoustic anisotropy

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