Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-02-12
2001-04-03
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Stock
Ferrous
C420S083000, C420S084000, C420S087000, C420S103000, C420S104000, C420S117000, C420S118000, C420S126000, C148S654000
Reexamination Certificate
active
06210496
ABSTRACT:
TECHNICAL FIELD
The present invention relates to cold rolled steel sheet with high strength and high formability having an excellent crushing performance which is suitable for use as a steel sheet for automobiles.
BACKGROUND ART
Under the trend of making automobiles light in weight, there has been an especially brisk demand for thin steel sheet with high strength having an excellent formability.
In addition, safety of automobiles has been thought to be important too and, accordingly, there has been a demand an improvement in crushing performance which is a yardstick for safety upon crash.
With regard to outer and inner panels for automobiles, cold rolled steel sheets are advantageous in terms of homogeneity of surface roughness and phosphatability.
Under such current circumstances, various cold rolled steel sheets with high strength have been developed already.
For example, in the Japanese Examined Patent Publication Hei-05/064215 and Laid-Open Patent Publication Hei-04/333524, there are disclosures on the method for the manufacture of high strength steel having a structure of ferrite containing not less than 3% of bainite and retained austenite (hereinafter, referred to as TRIP steel).
However, although the TRIP steel has a high elongation and a good formability (TS×El≧22000 MPa. %), there is a problem that this steel does not satisfy the current requirement for severe crushing performance.
In addition, there is another problem that the work hardening (WH) at press formability and the bake hardening (BH) at painting and baking thereafter are as low as about 70 MPa.
When those work and bake hardenings (WH+BH) are low, there is a big disadvantage in terms of ensuring the strength after forming, painting and baking.
On the other hand, the so-called dual phase steel (hereinafter, referred to as DP steel) having a dual phase of ferrite and martensite is disclosed, for example, in the Japanese Laid-Open Patent Publication Hei-09/111396 as a high strength steel sheet having an excellent crushing performance.
However, although the DP steel has an excellent crushing performance, its elongation is not sufficient and there is a problem in formability.
As mentioned above, no cold rolled steel sheet which satisfies both requirements of sufficient formability and severe safety standard has been available at present and, therefore, there has been a demand for developing it.
SUMMARY OF THE INVENTION
The present invention advantageously complies with the above requirements and its object is to offer a cold rolled steel sheet with high strength and high formability having an excellent crushing performance where the steel has both excellent formability and crushing performance (to be more specific, its tensile strength/elongation balance [TS×El] is not less than 24000 MPa % and its dynamic n-value is not less than 0.35) and, in addition, it has an excellent work hardening and bake hardening (i.e., WH+BH is not less than 100 MPa).
The term “dynamic n-value” used here has been firstly found by the present inventors as an index for crushing performance and it is now possible by the use of the dynamic n-value to evaluate the crushing performance in more precise manner than before.
Thus, in the past, crashworthiness was considered in relation with strength and it was simply believed that the higher the strength, the higher the crashworthiness. However, it has been found now that strength and crashworthiness are not always in such a simple relationship.
The present inventors have conducted an intensive investigation on this respect and found and clarified that, when automobiles are crashed, strain rate increases up to 2×10
3
/s and that, when energy upon deformation at such a high rate is to be absorbed by steel sheet as much as possible or, in other words, when crashworthiness is to be improved, it is effective that the n-value upon tension deformation of steel sheet under the condition of strain rate=2 ×10
3
/s (hereinafter, referred to as dynamic n-value) is made high.
Here, the momentary n-value when the elongation is 10% is defined as a dynamic n-value.
In the meanwhile, it has been also found that, when the dynamic n-value is made high, that is effective in improving the strength in the case of dynamic deformation as well.
Now, the history how the present invention has been achieved will be illustrated as hereunder.
Thus, in order to achieve the above-mentioned object, the present inventors have at first studied the relation between structure and characteristics in TRIP steel which is a conventional steel.
As a result, it has been found that, although production of a bainite phase has been believed to be essential for obtaining a sufficient amount of retained austenite which is advantageous for improving the formability, such a bainite phase is a cause for deteriorating the crushing performance.
Therefore, the present inventors suppressed the production of such a bainite phase, especially carbide, or, in other words, changed the minor phase other than the ferrite, (polygonal ferrite) which is a major phase, from the conventional “bainite+retained austenite” to a complex structure of “acicular ferrite+martensite+retained austenite” whereupon an unexpectedly favorable result has been achieved.
The present invention is based upon the above-mentioned finding.
Thus, the present invention relates to a cold rolled steel sheet with high strength and high formability having an excellent crushing performance which is characterized in having ferrite as a major phase and having a minor phase consisting of martensite, acicular ferrite and retained austenite.
In the present invention, it is preferred that the ratio of the minor phase in the steel structure is 3-40%. Further, it is preferred that the ratios of martensite, retained austenite and acicular ferrite in the minor phase are 10-80%, 8-30% and 5-60%, respectively.
More preferably, the steel sheet contains
0.05-0.40 mass % of C; 1.0-3.0 mass % of Si;
0.6-3.0 mass % of Mn; 0.02-1.5 mass % of Cr;
0.010-0.20 mass % of P; and 0.01-0.3 mass % of Al
and, if necessary, it may contain at least one component which is selected from:
0.005-0.25 mass % of Ti and 0.003-0.1 mass % of Nb
as component(s) for improving the strength and may further contain at least one component which is selected from:
not more than 0.1 mass % of Ca and
not more than 0.1 mass % of Rem
as component(s) for improving the formability.
REFERENCES:
patent: 5332453 (1994-07-01), Okada et al.
patent: 5470403 (1995-11-01), Yoshinaga et al.
patent: 5558727 (1996-09-01), Miura et al.
patent: 5567250 (1996-10-01), Akamatsu et al.
patent: 5618355 (1997-04-01), Koyama et al.
patent: 5634988 (1997-06-01), Kurebayashi et al.
patent: 5690755 (1997-11-01), Yoshinaga et al.
patent: 5837956 (1998-11-01), Okabe et al.
patent: 3-277743 (1991-12-01), None
patent: 4-332524 (1992-11-01), None
patent: 5-195149 (1993-08-01), None
patent: B2-5-64215 (1993-09-01), None
patent: 9-111396 (1997-04-01), None
Takagi, Shusaku, “High Strain Rate Deformation of High Strength Steels,” CAMP-ISIJ, vol. 9 (1996), pp. 1108-1111.
Furukimi Osamu
Miura Kazuya
Obara Takashi
Sakata Kei
Takagi Syusaku
Coy Nicole
Jenkins Daniel J.
Kawasaki Steel Corporation
Oliff & Berridg,e PLC
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