Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-02-06
2003-10-14
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C148S335000, C148S336000, C148S330000, C148S332000, C148S590000, C148S909000
Reexamination Certificate
active
06632296
ABSTRACT:
TECHNICAL FIELD
This invention relates to a steel pipe, used, for example, for panels, undercarriage components and structural members of cars and the like, and a method of producing the same. The steel pipe is especially suitable for hydraulic forming (see Japanese Unexamined Patent Publication No. H10-175027).
The steel pipes according to the present invention include those without a surface treatment as well as those with a surface treatment for rust protection, such as hot dip galvanizing, electroplating or the like. The galvanizing includes plating with pure zinc and plating with an alloy containing zinc as the main component.
The steel pipe according to the present invention is very excellent especially for hydraulic forming wherein an axial compressing force is applied, and thus can improve the efficiency in manufacturing auto components when they are processed by hydraulic forming. The present invention is also applicable to high strength steel pipes and, therefore, it is possible to reduce the material thickness of the components, and encourages the global environmental conservation.
BACKGROUND ART
A higher strength of steel sheets has been desired as the need for weight reduction in cars has increased. The higher strength of steel sheets makes it possible to reduce car weight through the reduction of material thickness and to improve collision safety. Attempts have recently been made to manufacture components with complicated shapes from high strength steel pipes using hydraulic forming methods. These attempts aim at a reduction in the number of components or welded flanges, etc. in response to the need for weight and cost reductions.
The actual application of new forming technologies such as the hydraulic forming method is expected to produce great advantages such as cost reduction, the increased degree of freedom in design work and the like. In order to fully enjoy the advantages of hydraulic forming methods, new materials suitable for the new forming methods are required. The inventors of the present invention have already proposed a steel pipe excellent in formability, and having a controlled texture, in Japanese Patent Application No. 2000-52574.
DISCLOSURE OF THE INVENTION
As the issues of the global environment become more and more serious, it is considered that an increasing demand for steel pipes having higher strengths is inevitable when the hydraulic forming method is used. In that event, the formability of the higher strength materials will surely become a more serious problem than before.
Diameter reduction in the &agr;+&ggr; phase zone or the &agr; phase zone is effective for obtaining a good r-value but, in commonly used steel materials, only a small decrease in the temperature of the diameter reduction results in the problem that a deformed structure remains and an n-value lowers.
The present invention provides a steel pipe having improved formability and a method to produce the same without incurring a cost increase.
The present invention provides a steel pipe, excellent in formability for hydraulic forming or the like, by clarifying the texture of a steel material excellent in formability, for hydraulic forming or the like, and a method to control the texture and by specifying the texture.
The gist of the present invention, therefore, is as follows:
(1) A steel pipe, excellent in formability, having a chemical composition comprising, in mass,
0.0001 to 0.50% of C,
0.001 to 2.5% of Si,
0.01 to 3.0% of Mn,
0.001 to 0.2% of P,
0.05% or less of S and
0.01% or less of N,
with the balance consisting of Fe and unavoidable impurities, characterized by having: an r-value of 1.4 or larger in the axial direction of the steel pipe; and the property that the average of the ratios of the X-ray intensity in the orientation component group of {110}<110> to {332}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 3.5 or larger, and/or the ratio of the X-ray intensity in the orientation component of {110}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 5.0 or larger.
(2) A steel pipe, excellent in formability, according to the item (1) characterized by further containing 0.001 to 0.5 mass % of Al.
(3) A steel pipe, excellent in formability, having a chemical composition comprising, in mass,
0.0001 to 0.50% of C,
0.001 to 2.5% of Si,
0.01 to 3.0% of Mn,
0.001 to 0.2% of P,
0.05% or less of S,
0.01% or less of N,
0.01 to 2.5% of Al and
0.01% or less of O
in a manner to satisfy the expressions (1) and (2) below, with the balance consisting of Fe and unavoidable impurities, characterized in that: the relationship between the tensile strength (TS) and the n-value of the steel pipe satisfies the expression (3) below; the volume percentage of its ferrite phase is 75% or more; the average grain size of the ferrite is 10 &mgr;m or more; and the crystal grains of the ferrite having an aspect ratio of 0.5 to 3.0 account for, in area percentage, 90% or more of all the crystal grains composing the ferrite.
(203{square root over ( )}C+15.2Ni−44.7Si−104V−31.5Mo+30Mn+11Cr+20Cu−700P−200Al)<−20 (1)
(44.7Si+700P+200Al)>80 (2)
n
≧−0.126×ln(
TS
)+0.94 (3)
(4) A steel pipe, excellent in formability, according to the item (3), characterized by having: an r-value of 1.0 or larger in the longitudinal direction of the steel pipe; and the property that the average of the ratios of the X-ray intensity in the orientation component group of {110}<110> to {332}<110> to the random X-ray intensity is 2.0 or larger and the ratio of the X-ray intensity in the orientation component of {111}<112> to the random X-ray intensity is 1.5 or smaller on the plane at the center of the steel pipe wall thickness.
(5) A steel pipe, excellent in formability, having a chemical composition comprising, in mass,
0.0001 to 0.50% of C,
0.001 to 2.5% of Si,
0.01 to 3.0% of Mn,
0.001 to 0.2% of P,
0.05% or less of S,
0.01% or less of N,
0.2% or less of Ti and
0.15% or less of Nb
in a manner to satisfy the expression 0.5≦(Mn+13Ti+29Nb)≦5, with the balance consisting of Fe and unavoidable impurities, characterized by having the property that the ratio of the X-ray intensity in the orientation component of {111}<110> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is 5.0 or larger and the ratio of the X-ray intensity in the orientation component of {111}<112> on the plane at the center of the steel pipe wall thickness to the random X-ray intensity is below 2.0.
(6) A steel pipe, excellent in formability, according to the item (5) characterized by further containing 0.001 to 0.5 mass % of Al.
(7) A steel pipe, excellent in formability, according to the item (5) or (6), characterized in that every one of the r-values in the axial, circumferential and 45° directions is 1.4 or larger.
(8) A steel pipe, excellent in formability, according to any one of the items (1) to (7), characterized by further containing, in mass, 0.0001 to 2.5% in total of one or more of:
0.0001 to 0.5% of Zr,
0.0001 to 0.5% of Mg,
0.0001 to 0.5% of V,
0.0001 to 0.01% of B,
0.001 to 2.5% of Sn,
0.001 to 2.5% of Cr,
0.001 to 2.5% of Cu,
0.001 to 2.5% of Ni,
0.001 to 2.5% of Co,
0.001 to 2.5% of W,
0.001 to 2.5% of Mo, and
0.0001 to 0.01% of Ca.
(9) A steel pipe, excellent in formability, characterized in that the steel pipe according to any one of the items (1) to (8) is plated.
(10) A method to produce a steel pipe, excellent in formability, having a chemical composition comprising, in mass,
0.0001 to 0.50% of C,
0.001 to 2.5% of Si,
0.01 to 3.0% of Mn,
0.001 to 0.2% of P,
0.05% or less of S and
0.01% or less of N,
with the balance consisting of Fe and unavoidable impurities, characterized by heating the steel pipe, having the property that the ratio o
Fujita Nobuhiro
Shinohara Yasuhiro
Sugiura Natsuko
Takahashi Manabu
Yoshida Tohru
Nippon Steel Corporation
Yee Deborah
LandOfFree
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