Method for manufacturing a high strength steel sheet

a technology of high strength steel and manufacturing method, which is applied in the direction of manufacturing tools, furnaces, heat treatment equipment, etc., can solve the problems of increasing formability, reducing press formability, and deteriorating surface properties, and achieve excellent formability and surface properties, high strength, and low cost

Inactive Publication Date: 2009-09-10
JFE STEEL CORP
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  • Abstract
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Benefits of technology

[0032]It is an object of the present invention to provide a high strength steel sheet having excellent formability (YR≦0.6) and excellent surface property through the development of a method for adequately controlling the metallic micro-structure and improving the mechanical characteristics of the steel sheet without adding excess ferrite-stabilizing elements (Si, P, and Al) which degrade the surface property, weldability, and the like, and to provide a method for manufacturing same.
[0033]The inventors of the present invention found a phenomenon which significantly enhances the fine ferrite formation compared with conventional two-stage cooling process, even without adding excess ferrite-stabilization elements, by beginning the ultra-rapid cooling at 150° C. / s or higher cooling rate within 2 seconds after the hot-rolling, followed by holding the temperatures between 750° C. and 600° C. for a specified period of time. The inventors of the present invention applied the finding to the manufacture of dual phase hot-rolled high strength steel sheet, and have perfected the present invention.
[0047]The present invention provides high strength steel sheet having excellent formability and surface property. The steel sheet manufactured by the present invention has low YR (0.6 or less) with high strength, high ductility, excellent press-formability, excellent surface property, and excellent spot weldability, thus the steel sheet can readily be applied to the automobile parts and machine structural parts. Since the high strength steel sheet can be manufactured by the conventional process for manufacturing mild steel sheet, and since the attained performance thereof is favorable without adding special elements, the manufacturing cost can be decreased. Accordingly, the high strength steel sheet according to the present invention is highly expected in practical uses in the future, and is expected to contribute to the conservation of global environment by the weight reduction of automobile and to the social development through the improvement of safety of automobile.

Problems solved by technology

Since, however, increase in the strength of a material generally decreases the press-formability, the widening of application fields of high strength steel sheets faces an important issue of increase in the formability.
Patent Documents 1 through 11, however, need to add excess Si, P, and Al, though they show favorable mechanical characteristics, thus they have problems of degradation in surface property caused by red-scale formation, degradation in coatability, and degradation in weldability.
Consequently, their applications are limited.
The steel sheets manufactured by the methods according to Patent Documents 12 through 15 contain small amount of Si, P, and Al so that the cooling method in related art cannot fully progress the transformation from austenite to ferrite on the runout table after hot-rolling.
Since Patent Document 18 is a technology to manufacture a high Si steel, the surface property of the steel sheet becomes poor.
If, however, the Si content is decreased, no adequate metallic micro-structure is obtained, and the YR characteristic becomes poor.

Method used

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  • Method for manufacturing a high strength steel sheet

Examples

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Effect test

example 1

[0081]Slabs having respective chemical compositions given in Table 1 were prepared by continuous casting. They were cooled, then heated to temperatures from 1100° C. to 1300° C., and were treated by final rolling at temperatures in a range from Ar3 point to 850° C. to obtain steel sheets having thicknesses of from 1.6 to 3.2 mm. Within 1 second after completing the final rolling, cooling began on the steel sheets to conduct the primary cooling to a temperature range from 680° C. to 720° C. at cooling rates from 300 to 500° C. / s. After that, the steel sheets were held at the temperature range for 7 to 12 seconds. Then, the steel sheets were cooled at cooling rates from 25 to 30° C. / s, and were coiled at 350° C. or lower temperature to obtain the respective hot-rolled steel sheets. As for Steel No. 4, however, the temperature to stop the primary cooling was 550° C., and Steel No. 5 was coiled at 450° C., thus adjusting the micro-structure thereof given in Table 1. The percentage of po...

example 2

[0085]Some of the slabs having the respective chemical compositions given in Table 1 were treated by hot-rolling, cooling, and coiling under the respective manufacturing conditions given in Table 3, thus obtained the respective hot-rolled steel sheets.

TABLE 3CoolingHeatingFinishbeginningPrimaryStopSecondaryCoilingSteeltemperaturetemperatureTimecoolingtemperatureHoldingcooling ratetemperatureClassificationSymbolNo.(° C.)(° C.)(s)speed (° C. / s)(° C.)time (s)(° C. / s)(° C.)ExampleA212008500.5460738445382ExampleB212508301290738421373ExampleC2123084025306441138324ComparativeD2105082052307011242144exampleComparativeE211508301507071245136exampleExampleF212508501180717640317ExampleG212008401220686724259ExampleH211008301450698620116ComparativeI611208201340800850217exampleExampleJ6115081012807301022184ExampleK6128083015306801022181ExampleL6120083012708501330347ComparativeM6113082013903501141285exampleComparativeN611808401240634—46158exampleExampleO6Hot direct8201560682833350rollingExampleP6123...

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Abstract

A method of manufacturing a high strength steel sheet. The method including casting a slab consisting essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, and a balance of Fe and inevitable impurities; hot-rolling the slab at a temperature of the Ar3 point or more; beginning cooling the hot-rolled steel sheet within 2 seconds after completing the hot-rolling at a temperature of from 750° C. to 600° C. at a cooling rate of 150° C. / s or more; holding the cooled steel sheet at a temperature between 750° C. and 600° C. for 2 to 15 seconds; cooling the steel sheet at a cooling rate of 20° C. / s or more; and coiling the cooled steel sheet at a temperature of 400° C. or less.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is a Divisional application of application Ser. No. 11 / 040,994 filed Jan. 20, 2005, the entire contents of which are hereby incorporated by reference herein.FIELD OF THE INVENTION[0002]The present invention relates to a high strength steel sheet most suitable for automobile body, reinforcements, wheels, and chassis parts and further for all kinds of machine structural parts, and to a method for manufacturing same.DESCRIPTION OF THE REED ARTS[0003]For global environmental protection and further improvement of the safety of passengers, automobile steel sheets are studied to increase the strength and to decrease the thickness. Since, however, increase in the strength of a material generally decreases the press-formability, the widening of application fields of high strength steel sheets faces an important issue of increase in the formability.[0004]A known response to the requirement is dual phase steel sheet structured by fer...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): C21D9/46C21D8/02B21B1/26B21B1/46B21B3/00C22C38/00C22C38/04C22C38/06C22C38/38C22C38/60
CPCC21D8/0226C21D8/0263C22C38/04C21D2211/008C22C38/02C21D2211/005
Inventor HASEGAWA, KOHEIMATSUOKA, SAIJIISHIGURO, YASUHIDE
Owner JFE STEEL CORP
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