Cold rolled steel sheet and galvanized steel sheet having strain aging hardening property and method for producing the same

Inactive Publication Date: 2003-01-23
JFE STEEL CORP
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0030] In order to achieve the objects, the inventors produced various steel sheets having different compositions under various production conditions, and experimentally evaluated various material properties. As a result, it was found that both moldability and hardenability after forming can be improved by using as a strengthening element N, which has not be positively used before in a field requiring high processability, and effectively using the great strain age hardening phenomenon manifested by the action of the strengthening element.

Problems solved by technology

However, in order to improve deep drawability, strength of the raw material sheets of the above-described steel sheets is decreased, and thus the steel sheets are not always sufficient as structural materials.
Therefore, the conventional techniques have only the effect of increasing the deformation start stress of a part, and the effect of increasing stress (tensile strength after forming) required for deformation over the entire deformation region from the deformation start to the deformation end is not said to be sufficient.
However, this technique comprises forming a fine carbide in the steel sheet by heat treatment after forming to effectively propagate a dislocation of stress applied during pressing, thereby increasing the amount of strain.
There is thus the problem of a necessary heat treatment temperature higher than general bake-hardening temperatures.
Furthermore, it is a very important problem that the body weight of an automobile is decreased in relation to the recent regulation of exhaust gases due to global environmental problems.
If the steel sheet has excessively high strength in press forming, the steel sheet causes the following problems: (1) deteriorating shape fixability; (2) deteriorating ductility to cause cracking, necking, or the like during forming; and (3) deteriorating dent resistance (resistance to a dent produced by a local compressive load) when the sheet thickness is decreased.
These problems thus inhibit the extension of application of the high-tensile-strength steel sheet to automobile bodies.
This causes the fault that the steel sheet actually less contributes to a reduction in weight of a part.
Although the technique disclosed in Japanese Unexamined Patent Application Publication No. 60-52528 greatly increases yield stress YS due to strain age hardening, the technique less increases tensile strength TS.
Also, this technique causes large variations in the increment in yield stress YS to cause large variations in mechanical properties, and thus it cannot be expected that a steel sheet can be sufficiently thinned for contributing to a reduction in weight of an automobile part, which is currently demanded.
However, in the steel sheet disclosed in Japanese Examined Patent Application Publication No. 5-24979, yield strength is increased after coating and baking to obtain a large amount of bake-hardening which is not obtained a conventional method, while tensile strength cannot be increased.
Therefore, in application to a strength member, improvements in fatigue resistance and impact resistance after forming cannot be expected.
Therefore, there is a problem in which the steel sheet cannot be used for applications greatly required to have fatigue resistance and impact resistance, etc.
Also, the yield point of the steel sheet is increased after strain aging, but TS is not increased, thereby causing the problem of limiting application to parts.
Therefore, it cannot be said that the steel sheet is sufficiently applied to parts required to have reliability.
However, this steel sheet is a hot-rolled sheet, and is thus difficult to obtain a high r value because the ferrite aggregation texture is made random due to austeniste-ferrite transformation.
Therefore, the steel sheet cannot be said to have sufficient deep drawability.
Furthermore, even if the hot-rolled steel sheet obtained by this technique is used as a starting material for cold rolling and recrystallization annealing, the increase in tensile strength obtained after forming and heat treatment is not always equivalent to a hot-rolled steel sheet, and a BH amount of as high as 80 MPa or more cannot be always obtained.

Method used

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  • Cold rolled steel sheet and galvanized steel sheet having strain aging hardening property and method for producing the same
  • Cold rolled steel sheet and galvanized steel sheet having strain aging hardening property and method for producing the same
  • Cold rolled steel sheet and galvanized steel sheet having strain aging hardening property and method for producing the same

Examples

Experimental program
Comparison scheme
Effect test

example 1

[0507] A steel slab having each of the compositions shown in Table 1 was hot-rolled to a hot-rolled sheet having a thickness of 3.5 mm, and then cold-rolled to a cold-rolled sheet having a thickness of 0.7 mm under the conditions shown in Table 2. Then, the cold-rolled sheet was recrystallized, annealed and further galvannealed in a continuous annealing line or a continuous annealing and galvanizing line. Then, the annealed sheet was temper-rolled with a rolling reduction ratio of 1.0% to produce a cold-rolled steel sheet and a galvannealed steel sheet having both sides coated with a weight of 45 g / m.sup.2 per side. In Table 2, the finisher deliver temperatures in the hot rolling process of Steel Nos. 3 and 8 are less than the Ar.sub.3 transformation point, and the finisher deliver temperatures of the others are the Ar.sub.3 transformation point or more.

[0508] The thus-obtained cold-rolled steel sheets and the galvannealed steel sheets were measured with respect to tensile strength,...

example 2

[0510] A slab of steel symbol B shown in Table 1 was hot-rolled under the same production conditions as No. 2 shown in Table 2 in which the heating temperature was 1100.degree. C., and the finisher deliver temperature of hot rolling was 900.degree. C., and then coiled at coiling temperature of 550.degree. C. into a coil. The thus-obtained coil was cold-rolled with a reduction ratio of 80%, and then recrystallized and annealed at 840.degree. C. With respect to the product properties of the resultant cold-rolled steel sheet, tensile strength TS was 365 MPa, and the r value was 1.7. A test specimen of JIS No. 5 was obtained from the cold-rolled steel sheet in the rolling direction, and a tensile strain of 10% was applied by a tensile test machine. Then, the specimen was subjected to heat treatment under the heat treatment conditions (temperature and time) shown in Table 4, and a tensile test was again performed. Table 4 also shows the increase in tensile strength (.DELTA.TS) from the t...

example 3

[0512] A steel slab having each of the compositions shown in Table 6 was hot-rolled under the conditions shown Table 7 to form a hot-rolled sheet having a thickness of 3.5 mm. Each of the thus-obtained hot-rolled sheets was cold-rolled under the conditions shown in Table 7 to form a cold-rolled sheet having a thickness of 0.7 mm, and then recrystallized and annealed under the conditions shown in the same table. Some of the annealed sheets were further coated by hot-dip galvanization or alloying hot-dip galvanization under the conditions shown in the same table. The thus-obtained produced sheets were examined with respect to the amount of dissolved N, the microstructure, tensile properties, and strain age hardenability.

[0513] The results are shown in Table 8. Table 8 indicates that with the steel sheets of the present invention, TS.times.r value.gtoreq.750 MPa (in a combination with at least one of B, Nb, Ti and V, Ts.times.r value 850>MPa), BH.gtoreq.80 MPa and .DELTA.TS.gtoreq.40 M...

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Abstract

An object of the present invention is to provide a cold-rolled steel sheet and an alloyed hot-dip galvanized steel sheet in which tensile strength is effectively increased by press forming and heat treatment while maintaining excellent deep drawability in press forming. Specifically, a steel composition contains less than 0.01% of C, 0.005 to 1.0% of Si, 0.01 to 1.0% of Mn, 0.005 to 0.050% of Nb, 0.005 to 0.030% of Al, 0.005 to 0.040% of N, 0.0005 to 0.0015% of B, 0.05% or less of P, and 0.01% or less of S, the balance substantially composed of Fe, in which the following equations (1) and (2) are satisfied: <paragraph lvl="0"><in-line-formula>N %>=0.0015+14 / 93.Nb %+14 / 27.Al %+14 / 11.B % (1) < / in-line-formula><paragraph lvl="0"><in-line-formula>C %<=(12 / 93).Nb % (2) < / in-line-formula>

Description

[0001] The present invention relates to a cold-rolled steel sheet, an electro-galvanized steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet, which are suitable as raw material steel sheets for molded products such as building members, mechanical structural parts, automobile structural parts, etc., which are used at positions required to have structural strength, particularly, strength and / or stiffness in deformation, and which are subjected to heat treatment for increasing strength after processing such as pressing or the like, and a method of producing these steel sheets.[0002] 1. In the present invention, "excellent strain age hardenability" means that in aging under conditions of holding at a temperature of 170.degree. C. for 20 min. after pre-deformation with a tensile strain of 5%, the increment in deformation stress (represented by the amount of BH=yield stress after aging-pre-deformation stress before aging) after aging is 80 MPa or m...

Claims

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Application Information

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IPC IPC(8): C21D1/18C21D8/02C21D8/04C21D9/46C21D9/48C21D9/52C22C38/00C22C38/02C22C38/04C22C38/06C22C38/12
CPCC21D1/185Y10T428/12493C21D8/0236C21D8/0273C21D8/0426C21D8/0436C21D8/0473C21D9/46C21D9/48C21D2211/005C21D2211/008C22C38/001C22C38/004C22C38/02C22C38/04C22C38/06C22C38/12Y10T428/12799C21D8/0226
InventorKAMI, CHIKARATOSAKA, AKIOYAMAZAKI, TAKUYA
OwnerJFE STEEL CORP