局部成形性に優れ溶接部の硬さ上昇を抑制した引張強さが780MPa以上の高強度冷延鋼板および高強度表面処理鋼板

HIGH-STRENGTH COLD ROLLED STEEL SHEET AND HIGH-STRENGTH SURFACE TREATED STEEL SHEET, EACH HAVING SUPERIOR LOCAL FORMABILITY, SUPPRESSED INCREASE IN HARDNESS IN WELD ZONE AND ≥780 MPa TENSILE STRENGTH

Abstract

【課題】局部成形性に優れ溶接部の硬さ上昇を抑制した引張強さが780MPa以上の高強度冷延鋼板および高強度表面処理鋼板を提供する。 【解決手段】重量%で、C:0.05〜0.09%、Si:0.4 〜1.3 %、Mn:2.5 〜3.2 %、P:0.001 〜0.05%、N:0.0005〜0.006 %、Al:0.005 〜0.1 %、Ti:0.001 〜0.045 %を含み、S含有量が次式で規定される範囲S≦(A式)を含む残部がFeおよび不可避不純物で、ミクロ組織が面積率でベイナイトが7%以上、残部がフェライト、マルテンサイト、焼戻しマルテンサイトおよび残留オーステナイトのいずれか1種以上で構成され、かつ下記(C)、(D)の2式を満足するものとした。 0.08 ×(Ti(%) −3.43×N(%) )+0.004 ・・・(A) 但し、(A)式のTi(%) −3.43×N(%) <0の時は0とする Mneq.=Mn(%) −0.29×Si(%) +6.24×C(%) ・・(B) 950≦(Mneq. /(C(%) −(Si(%)/75)))×ベイナイト面積率(%) ・・・(C) C(%) +(Si(%)/20)+(Mn(%)/18) ≦0.30 ・・・(D) 【選択図】 無し
<P>PROBLEM TO BE SOLVED: To provide a high-strength cold rolled steel sheet and a high-strength surface treated steel sheet, each having excellent local formability, suppressed increase in hardness in a weld zone and ≥780 MPa tensile strength. <P>SOLUTION: The steel sheets have a composition consisting of, by weight, 0.05 to 0.09% C, 0.4 to 1.3% Si, 2.5 to 3.2% Mn, 0.001 to 0.05% P, 0.0005 to 0.006% N, 0.005 to 0.1% Al, 0.001 to 0.045% Ti, S in an amount within the range specified by S≤expression (A), and the balance Fe with inevitable impurities and also have a microstructure consisting of ≥7% bainite by area ratio and the balance constituted of one or more kinds among ferrite, martensite, tempered martensite and retained austenite, and further, relationship (C) represented by 950≤(Mneq./(C(%)-(Si(%)/75)))×(bainite area ratio) and relationship (D) represented by C(%)+(Si(%)/20)+(Mn(%)/18)≤0.30 are satisfied. The above expression (A) is represented by 0.08×(Ti(%)-3.43×N(%))+0.004, where the value of (Ti(%)-3.43×N(%)) vanishes in the case of (Ti(%)-3.43×N(%))<0. Equation (B) is represented by Mneq.=Mn(%)-0.29×Si(%)+6.24×C(%). <P>COPYRIGHT: (C)2005,JPO&NCIPI

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