A cold-rolled steel sheet having a tensile strength of 780...

Details A cold-rolled steel sheet having a tensile strength of 780... A cold-rolled steel sheet having a tensile strength of 780...

C - Chemistry, Metallurgy

22

C

C22C 38/04 (2006.01) C21D 9/46 (2006.01) C22C 38/14 (2006.01)

Patent

CA 2526488

The present invention provides a high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase and being characterized by: said steel sheets containing, in weight, C: 0.05 to 0.09%, Si: 0.4 to 1.3%, Mn: 2.5 to 3.2%, P: 0.001 to 0.05%, N: 0.0005 to 0.006%, Al: 0.005 to 0.1%, Ti: 0.001 to 0.045%, and S in the range stipulated by the following expression (A), with the balance consisting of Fe and unavoidable impurities; the microstructures of said steel sheets being composed of bainite of 7% or more in terms of area percentage and the balance consisting of one or more of ferrite, martensite, tempered martensite and retained austenite; and said components in said steel sheets satisfying the following expressions (C) and (D) when Mneq. is defined by the following expression (B); S <= 0.08 x (Ti(%) - 3.43 x N(%)) + 0.004... (A), where, when a value of the member Ti(%) - 3.43 x N(%) of said expression (A) is negative, the value is regarded as zero, Mneq. = Mn(%) - 0.29 x Si(%) + 6.24 x C(%) ... (B), 950 <= (Mneq./(C(%) - (Si(%)/75))) x bainite area percentage (%)... (C), C(%) + (Si(%)/20) + (Mn(%)/18) 5 0.30... (D).

L'invention concerne une tôle d'acier laminée à froid et une tôle d'acier dont la surface traitée offre une résistance à la traction élevée d'au moins 780 Mpa, qui présente une excellente formabilité locale et un accroissement supprimé de la dureté de soudage. Lesdites tôle d'acier contiennent, en poids, 0,05 à 0,09 % de C, 0,4 à 1,3 % de Si, 2,5 à 3,2 % de Mn, 0,001 à 0,05 % de P, 0,0005 à 0,006 % de N, 0,005 à 0,1 % de Al, 0,001 à 0,001 à 0,045 % de Ti, et S dans la plage stipulée dans l'expression suivante (A), le restant étant constitué de Fe et d'impuretés inévitables. Les microstructures desdites tôle d'acier sont composées de bainite de 7 % en termes de surface, le restant étant constitué d'au moins une ferrite, un martensite, un martensite revenu et une austénite résiduelle. Lesdits composants desdites tôles d'acier satisfont aux expressions suivantes (C) et (D) lorsque Mneq. est définie par l'expression suivante (B) ; S<=0,08 x (Ti(%) 3,43 x N(N%) + 0,004... (A), dans laquelle, si une valeur de l'élément Ti(%) 3,43 x N(%) de ladite expression (A) est négative, la valeur est considérée comme étant zéro, Mneq. = Mn(%) 0,29 x Si(%) + 6,24 x C(%) ... (B), 950 <= (Mneq./(C(%) (Si(%)/75))) x bainite en (%) de surface ... (C), C(%) + (Si(%)/20) + (Mn(%)/18)?5¿0,30 ... (D).

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