Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-07-23
2003-04-29
King, Roy (Department: 1742)
Metal treatment
Stock
Ferrous
C148S602000, C420S008000
Reexamination Certificate
active
06554918
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-strength hot-rolled steel sheet superior in stretch flange formability and a method for production thereof, said steel sheet being suitable for use as a raw material for automotive parts such as chassis and suspension systems (including arms and members).
2. Description of the Related Art
A recent trend in the field of automobile and industrial machine is toward the reduction in weight of parts, which is achieved by using high-strength hot-rolled steel sheet. Such steel sheet often needs good stretch flange formability (local elongation) because it undergoes pressing for hole expansion as well as shaping.
It is known that Ti-containing hot-rolled steel sheets have high strength and good workability as disclosed in Japanese Patent Laid-open Nos. 88620/1978 and 106861/1999 and Japanese Patent Publication Nos. 4450/1987, 66367/1988, and 110418/1992. However, these disclosures are not concerned at all with the structure desirable for improved stretch flange formability.
Serious attempts are being made to obtain a steel sheet having an extremely fine grained structure in which the unit grain is smaller than several micrometers in size (each unit grain being surrounded by adjacent grains whose crystal orientation is larger than 15°), as disclosed in Japanese Patent Laid-open Nos. 246931/1999 and 246932/1999. Up to date, such attempts are unsuccessful in obtaining fine-grained steel sheets having good stretch flange formability.
OBJECT AND SUMMARY OF THE INVENTION
The present invention was completed to address the above-mentioned problems. It is an object of the present invention to provide a hot-rolled steel sheet having high strength as well as good stretch flange formability. It is another object of the present invention to provide a method for producing the hot-rolled steel sheet.
The present inventors found that a hot-rolled steel sheet exhibits good stretch flange formability without its high strength being impaired if it contains 0.10-0.30% of Ti and does not substantially contain the second phase (such as martensite and bainite resulting from transformation at low temperatures) except for ferrite and has a single-phase structure of ferrite with a controlled grain size and shape. The present invention is based on this finding. The gist of the present invention resides in a high-strength hot-rolled steel sheet superior in stretch flange formability which comprises C (0.01-0.10 mass %), Si (no more than 1.0 mass %), Mn (no more than 2.5 mass %), P (no more than 0.08 mass %), S (no more than 0.005 mass %), Al (0.015-0.050 mass %), and Ti (0.10-0.30 mass %), with the remainder being substantially Fe, said hot-rolled steel sheet having a structure composed mainly of ferrite wherein the unit grain has an average particle diameter (d) no larger than 5 &mgr;m, said unit grain being defined such that adjacent grains which surround said unit grain differ from solid unit grain in orientation more than 15°.
In a preferred embodiment of the present invention, the high-strength hot-rolled steel sheet is characterized in that the unit grain adjoins its surrounding grains along a boundary whose average length (L) is such that L/d is no smaller than 4.0. This condition is necessary for improved stretch flange formability.
In another preferred embodiment of the present invention, the high-strength hot-rolled steel sheet further comprises at least one of Nb in an amount not more than 0.40 mass %, B in an amount not more than 0.0010 mass %, and Ca in an amount not more than 0.01 mass %.
The gist of the present invention resides also in a method of producing a high-strength hot-rolled steel sheet, said method comprising the steps of heating and hot-rolling a steel sheet having the above-mentioned composition and coiling the hot-rolled steel sheet in such a way that the reduction is no less than 70% at the rolling temperature of 900-840° C. and the coiling temperature is 300-500° C. or 600-750° C. The requirement for L/d no smaller than 4.0 is met when the reduction is no less than 50%, and hence the resulting steel sheet has good stretch flange formability.
The hot-rolled steel sheet according to the present invention exhibits good stretch flange formability without its high strength being impaired owing to its specific composition in which ferrite accounts for a major portion, with a Ti content being 0.10-0.30%, and also owing to its specific structure in which the ferrite unit grain has a specific particle diameter or peripheral shape to prevent crack propagation. The method of the present invention permits easy production of said high-strength hot-rolled steel sheet.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The high-strength hot-rolled steel sheet of the present invention should have the above-mentioned specific chemical composition for the reasons given below. (“%” means “mass %”.)
C: 0.01-0.10%
C is an essential element to improve strength. C in excess of 0.10% tends to form the second phase structure. Therefore, the lower limit of the C content should be 0.01%, preferably 0.02%, and the upper limit of the C content should be 0.10%, preferably 0.08%.
Si: no more than 1.0%
Si is an element to effectively increase the steel strength without deteriorating the steel ductility appreciably, although, if added in a large amount, it causes surface defects including scale defects and promotes generation of coarse ferrite grains which decreases L/d. The upper limit of the Si content should be 1.0%, preferably 0.8%.
Mn: no more than 2.5%
Mn is an element that contributes to solid-solution strengthening and in turn imparts strength to steel. It also promotes transformation, thereby forming granular bainitic ferrite and bainitic ferrite. It changes the shape of the grain boundary. It should preferably be added in an amount more than 0.5%; however, Mn added in an excess amount results in excessive hardenability, which leads to a large amount of transformation products detrimental to high stretch flange formability. Thus, the upper limit of the Mn content should be 2.5%, preferably 2.0%.
P: no more than 0.08%
P is an element that contributes to solid-solution strengthening without deteriorating ductility. However, P added in an excess amount raises the transition temperature after working. Therefore, the content of P should be no more than 0.08%.
S: no more than 0.005%
S forms sulfides (such as MnS) and inclusions detrimental to stretch flange formability. The content of S should be no more than 0.005%. The smaller, the better.
Al: 0.015-0.050%
Al is added as a deoxidizer. It produces little deoxdizing effect and promotes generation of non-metallic inclusions such as TiN by leaving much N, if its content is less than 0.015%. It forms non-metallic inclusions, such as Al
2
O
3
, detrimental to cleanliness if its content exceeds 0.050%. The content of Al should be 0.015-0.050%.
Ti: 0.10-0.30%
Ti improves hardenability and changes the particle diameter, thereby improving the stretch flange formability. The content of Ti should be no less than 0.10%, preferably no less than 0.20%, and should be no more than 0.30%, preferably no more than 0.25%. Excessive Ti is wasted without additional effects. In the hot-rolling of the steel sheet according to the present invention, Ti expands the unrecrystallized austenite region (as mentioned later) and accumulates the deformation strain energy which gives rise to fine grains and also to grains having zigzag grain boundaries both effective for stretch flange formability. This effect is produced most effectively when Ti is added. This effect is not produced when only Nb is added. If Ti content is too small, generation of ferrite is promoted and the zigzag boundaries are not obtained.
The high-strength hot-rolled steel sheet of the present invention is composed of the above-mentioned components, with the remainder being substantially Fe. It may contain, in addition to inevitable impurities, one or more of the following elements in an amount not harmful to the effect of the above-menti
Hashimoto Shunichi
Kashima Takahiro
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd).
King Roy
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wessman Andrew
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