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High-strength cold rolled steel sheet having excellent formability, and plated steel sheet

a formability technology, applied in the field of high-strength cold rolled steel sheet having excellent formability and a plated steel sheet, can solve the problem that the satisfaction of stretch-flangeability characteristics has never been achieved, and achieve the effect of high formability, good tensile strength and elongation, and good tensile strength

Inactive Publication Date: 2005-11-10
KOBE STEEL LTD
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0031] According to the present invention, there can be provided a high-strength cold rolled steel sheet that satisfies that the product of tensile strength [TS (MPa)] and elongation [E1 (%)] is 20,000 or more and the product of tensile strength [TS (MPa)] and stretch-flangeability [A (%)] is 40,000 or more, and has well-balanced tensile strength and elongation (total elongation) as well as well-balanced tensile strength and stretch-flangeability, and a plated steel sheet. These cold-rolled steel sheets can be used with high formability in the manufacture of automobile parts and industrial machine parts that require high strength.

Problems solved by technology

In a conventional TBF steel, however, satisfactory characteristics have never been attained in stretch-flangeability.

Method used

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  • High-strength cold rolled steel sheet having excellent formability, and plated steel sheet
  • High-strength cold rolled steel sheet having excellent formability, and plated steel sheet
  • High-strength cold rolled steel sheet having excellent formability, and plated steel sheet

Examples

Experimental program
Comparison scheme
Effect test

example 1

Investigation on Composition

[0099] In this example, steel specimens A to J having the compositions shown in Table 1 (rest of the composition consists of Fe and inevitable impurities) was made by melting to obtain a slab that was subjected to hot rolling. The slab was hot rolled at SRT of 1300° C. and FDT of 900° C. and then wound up at 500° C. to obtain a hot rolled steel sheet having a thickness of 2.4 mm. The hot rolled steel sheet was pickled to remove scales and then cold rolled (rolling reduction: 50%) to obtain a cold rolled steel sheet having a thickness of 1.2 mm.

[0100] The resulting cold rolled sheet was subjected to heat treatment by using a CAL simulator. Specifically, the steel sheet was maintained in a temperature range of about 900° C. (T1) for a duration of 60 seconds, cooled forcibly at a cooling rate (CR) of 20° C. / s to about 400° C. (T2), maintained in a temperature range of about 400° C. (T2) for about 4 minutes (240 seconds), and was then cooled down to the roo...

example 2

Investigation on Heat Treatment Conditions

[0118] In this example, an influence of heat treatment conditions on the structure and mechanical characteristics was investigated in cold rolled steel sheets (Nos. 11 to 17) made of a steel of type F (steel of type that satisfies the scope of the present invention) in the same manner as in the method of Example 1, except that some of the heat treatment conditions does not satisfy the requirements of the present invention. The heat treatment conditions are the same as described in Example 1, except for alterations shown in Table 5. Specifically, No. 11 is an example made by changing the slab temperature SRT at the starting of hot rolling and Nos. 12 to 17 are examples made by changing the heat treatment conditions on annealing.

[0119] The results are also shown in Table 5. For reference, results of No. 6 made of a steel of type F in Table 1 are also shown.

TABLE 5AnnealingExperimentSteelHot rollingHeating temperatureCooling rateAustemperin...

example 3

Investigation on Other Manufacturing Methods

[0124] This reference example was carried out so as to demonstrates that, unlike Example 1 described above, even when using a Nb-free steel containing no Nb added therein (provided that the essential components in the steel satisfies the scope of the present invention), the mean number of the residual γ blocks can be controlled to 20 or more) and thus a high-strength cold rolled steel sheet having excellent formability can be obtained (by the way, the cold rolling reduction increases in this reference example).

[0125] Specifically, a steel material (steel type C in Table 1, that satisfies the components in the steel in the present invention) was subjected to a hot rolling step (SRT: 1150° C., FDT: 800° C., winding up temperature: 600° C.), a cold rolling step (cold rolling reduction: 80%) and an annealing step [of maintaining in a temperature range of about 900° C. for a duration of 120 seconds, cooling forcibly at a mean cooling rate of ...

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Abstract

To provide a high-strength cold rolled steel sheet that has well-balanced tensile strength and elongation as well as well-balanced tensile strength and stretch-flangeability, and a plated steel sheet manufactured by plating the steel sheet. The high-strength cold rolled steel sheet contains: 0.10 to 0.28% of C, 1.0 to 2.0% of Si, 1.0 to 3.0% of Mn, and 0.03 to 0.10% of Nb in terms of % by mass, wherein the content of Al is controlled to 0.5 or less, the content of P is controlled to 0.15% or less, and the content of S is controlled to 0.02% or less, and wherein residual austenite accounts for 5 to 20%, bainitic ferrite accounts for 50% or more, and polygonal ferrite accounts for 30% or less (containing 0%), of the entire structure, and wherein a mean number of residual austenite blocks is 20 or more as determined when the random area (15 μm×15 μμm) is observed by EBSP (electron back scatter diffraction pattern).

Description

BACKGROUND OF THE INVENTION [0001] 1. Technical Field [0002] The present invention relates to a high-strength cold rolled steel sheet having excellent formability, and a plated steel sheet. More particularly, it relates to a high-strength cold rolled steel sheet that has “excellent formability” in such a sense that it has well-balanced tensile strength and elongation (total elongation) as well as well-balanced tensile strength and stretch-flangeability, and a plated steel sheet manufactured by plating the steel sheet. More specifically, the high-strength cold rolled steel sheet or plated steel sheet of the present invention satisfies that the product of tensile strength [TS (MPa)] and elongation [E1 (%)] is 20,000 or more and the product of tensile strength [TS (MPa)] and stretch-flangeability [λ (%)] is 40,000 or more. [0003] The steel sheet described above can be utilized in wide fields of industry including automobile, electric apparatuses and machinery. Description that follows ...

Claims

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Application Information

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IPC IPC(8): C21D8/02C22C38/12
CPCC22C38/02C22C38/12C22C38/04
Inventor IKEDA, SHUSHIMUKAI, YOICHIAKAMIZU, HIROSHIMAKII, KOICHISUGIMOTO, KOICHIHASHIMOTO, SHUNICHISAITO, KENJI
Owner KOBE STEEL LTD
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