Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-07-28
2003-04-08
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Ferrous
C148S541000, C148S602000, C148S603000
Reexamination Certificate
active
06544354
ABSTRACT:
TECHNICAL FIELD
The present invention relates to press formable, high strength hot rolled and cold rolled steel sheets having high flow stress during dynamic deformation, which can be used for automobile members and the like to provide assurance of safety for passengers by efficiently absorbing the impact energy of a collision, as well as a method for producing the same.
BACKGROUND ART
In recent years, protection of passengers from automobile collisions has been acknowledged as an aspect of utmost importance for automobiles, and hopes are increasing for suitable materials exhibiting excellent high-speed deformation resistance. For example, by applying such materials to front side members of automobiles, the energy of frontal collisions may be absorbed as the materials are crushed, thus alleviating the impact on passengers.
Since the strain rate for deformation undergone by each section of an automobile upon collision reaches about 10
3
(l/s), consideration of the impact absorption performance of a material requires knowledge of its dynamic deformation properties in a high strain rate range. Because it is also essential to consider at the same time such factors as energy savings and CO
2
exhaust reduction, as well as weight reduction of the automobile, requirements for effective high-strength steel sheets are therefore increasing.
For example, in CAMP-ISIJ Vol. 9 (1996), pp.1112-1115 the present inventors have reported on the high-speed deformation properties and impact energy absorption of high-strength thin steel sheets, and in that article it was reported that the dynamic strength in the high strain rate range of about 10
3
(l/s) is drastically increased in comparison to the static strength in the low strain rate of 10
−3
(l/s), that the strain rate dependence for deformation resistance varies based on the strengthening mechanism for the material, and that TRIP (transformation induced plasticity) steel sheets and DP (ferrite/martensite dual phase) steel sheets possess both excellent formability and impact absorption properties compared to other high strength steel sheets.
Furthermore, Japanese Unexamined Patent Publication No. 7-18372, which provides retained austenite-containing high strength steel sheets with excellent impact resistance and a method for their production, discloses a solution for impact absorption simply by increasing the yield stress brought about by a higher deformation rate; however, it has not been demonstrated what other aspects of the retained austenite should be controlled, apart from the amount of retained austenite, in order to improve impact absorption.
Thus, although understanding continues to improve with regard to the dynamic deformation properties of member constituent materials affecting absorption of impact energy in automobile collisions, it is still not fully understood what properties should be maximized to obtain steel materials for automotive members with more excellent impact energy absorption properties, and on what criteria the selection of materials should be based. Steel materials for automotive members are formed into the required part shapes by press molding and, after usually undergoing painting and baking, are then incorporated into automobiles and subjected to actual instances of impact. However, it is still not clear what steel-strengthening mechanisms are suitable for improving the impact energy absorption of steel materials against collisions subsequent to such pre-deformation and baking treatment.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide high-strength steel sheets with high impact energy absorption properties as steel materials for shaping and forming into such parts as front side members which absorb impact energy upon collision, as well as a method for their production. First, the high-strength steel sheets exhibiting high impact energy absorption properties according to the present invention include:
(1) The press formable high-strength steel sheets with high flow stress during dynamic deformation, characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5×10
−4
~5×10
−3
(l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the tensile deformation strength ad when deformed at a strain rate of 5×10
2
~5×10
3
(l/s) after the aforementioned pre-deformation, i.e. &sgr;d−&sgr;s, is at least 60 MPa, and the work hardening coefficient between 5% and 10% of a strain is at least 0.130; and
(2) The press formable high-strength steel sheets with high flow stress during dynamic deformation, characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength &sgr;s when deformed in a strain rate range of 5×10
−4
~5×10
−3
(l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength ad when deformed at a strain rate of 5×10
2
~5×10
3
(l/s) after the aforementioned pre-deformation, i.e. &sgr;d−&sgr;s, is at least 60 MPa, the difference between the average value &sgr;dyn (MPa) of the flow stress at an equivalent strain in the range of 3~10% when deformed in a strain rate range of 5×10
2
~5×10
3
(l/s) and the average value &sgr;st (MPa) of the flow stress at an equivalent strain in the range of 3~10% when deformed in a strain rate range of 5×10
−4
~5×10
−3
(l/s) satisfies the inequality: (&sgr;dyn−&sgr;st)≧−0.272×TS+300 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5×10
−4
2~5×10
−3
(l/s), and the work hardening coefficient between 5% and 10% of a strain is at least 0.130.
They further include:
(3) The press formable high-strength steel sheets with high flow stress during dynamic deformation, characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5×10
−4
~5×10
−3
(l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength ad when deformed at a strain rate of 5×10
2
~5×10
3
(l/s) after the aforementioned pre-deformation, i.e. &sgr;d−&sgr;s, is at least 60 MPa, the difference between the average value &sgr;dyn (MPa) of the flow stress at an equivalent strain in the range of 3~10% when deformed in a strain rate range of 5×10
2
~5×10
3
(l/s) and the average value &sgr;st (MPa) of the flow stress at an equivalent strain in the range of 3~10% when deformed in a strain rate range of 5×10
−4
~5×10
−3
(l/s) satisfies the inequality: (&sgr;dyn−&sgr;st)≧−0.272×TS+300 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5×10
−4
~5×10
−3
(l/s), the value (M) determined by the solid solution [C] in the retained austenite and the average Mn equivalents of the steel material {Mn eq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equat
Kawano Osamu
Kuriyama Yukihisa
Mabuchi Hidesato
Okamoto Riki
Sakuma Yasuharu
Combs Morillo Janelle
Nippon Steel Corporation
Wyszomierski George
LandOfFree
High-strength steel sheet highly resistant to dynamic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High-strength steel sheet highly resistant to dynamic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-strength steel sheet highly resistant to dynamic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3054755