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High-strength hot-rolled steel sheet and method for producing the same (as amended)

Inactive Publication Date: 2016-03-10
JFE STEEL CORP
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

The present invention provides a strong hot-rolled steel sheet that can withstand severe stamping in mass production and has good punchability in mass production. This steel sheet can be used for automotive parts and truck frame parts, as well as underbody parts of automobiles. The use of this steel sheet helps to reduce the weight of parts.

Problems solved by technology

In general, higher-strength steel sheets have lower workability.
However, the technique described in Patent Literature 1 only prevents the intergranular segregation of P and improves hole expansion formability.

Method used

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Examples

Experimental program
Comparison scheme
Effect test

first embodiment

[0049]The reason for limiting the compositions of a high-strength hot-rolled steel sheet according to a first embodiment will be described below. Unless otherwise specified, “%” refers to “% by mass”. The term “high-strength” in the embodiment refers to a tensile strength TS of 900 MPa or more.

C: More than 0.07% and 0.2% or Less

[0050]C is an element that effectively contributes to increase the strength of steel sheets and is a useful element that promotes bainite transformation and contributes to the formation of a bainite phase. An adequate C content results in increased carbide in bainite lath grains and improved punchability in mass production. These effects require a C content of more than 0.07%. An excessive C content of more than 0.2% results in poor workability and weldability. Thus, the C content is limited to more than 0.07% and 0.2% or less, preferably 0.079% or more, more preferably 0.10% or more, and preferably 0.19% or less.

Si: 2.0% or Less

[0051]Si is an element that in...

example 1

[0091]A steel slab having a composition listed in Table 1 was subjected to heating, finish rolling, and cooling after rolling listed in Table 2 to produce a hot-rolled steel sheet. In continuous casting, hot-rolled steel sheets other than a hot-rolled steel sheet No. 1′ of steel Al in Tables 1 to 3 described below were subjected to an electro-magnetic stirrer (EMS) to reduce segregation of components of the steel. Table 1 also lists the Ar3 transformation temperature of each steel slab determined from a thermal expansion curve. After pickling, some of the hot-rolled steel sheets were passed through a continuous hot-dip galvanizing line and were subjected to annealing treatment under the conditions listed in Table 2 and hot-dip galvanizing treatment to produce hot-dip galvanized steel sheets (GI). In the hot-dip galvanizing treatment, each of the hot-rolled steel sheets subjected to the annealing treatment was immersed in a galvanizing bath (0.1% by mass Al—Zn) at 480° C., and 45 g / m...

second embodiment

[0101]The reason for limiting the compositions of a high-strength hot-rolled steel sheet according to a second embodiment will be described below. Unless otherwise specified, “%” refers to “% by mass”. The term “high-strength” in the second embodiment refers to a tensile strength TS in the range of 700 to 900 MPa.

C: 0.05% to 0.15%

[0102]C is an element that effectively contributes to increase the strength of steel sheets and is a useful element that promotes bainite transformation and contributes to the formation of a bainite phase. An adequate C content results in increased carbide in bainite lath grains and improved punchability in mass production. These effects require a C content of 0.05% or more. An excessive C content of more than 0.15% results in poor workability and weldability. Thus, the C content is limited to the range of 0.05% to 0.15%, preferably 0.071% or more, more preferably 0.080% or more, and 0.14% or less.

Si: 1.5% or Less

[0103]Si is an element that increases the st...

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Abstract

A steel slab having a composition containing C: more than 0.07% and 0.2% or less, Si: 2.0% or less, Mn: 1.0% to 3.0%, Al: 0.1% or less, Ti: 0.05% to 0.3%, and V: 0.05% to 0.3% on a mass percent basis is heated to 1100° C. or more and is subjected to rough rolling and finish rolling. In the finish rolling, the total rolling reduction of two final passes is 30% or more, and the finish rolling temperature ranges from (Ar3 transformation temperature) to (Ar3 transformation temperature+120° C.). Cooling is started within 2 seconds after the finish rolling. Coiling is performed at an average cooling rate of 40° C. / s or more at a coiling temperature in the range of 300° C. to 500° C. The resulting high-strength hot-rolled steel sheet has a microstructure in which a bainite phase constitutes more than 90% by volume, the average lath interval of bainite is 0.45 μm or less, and the ratio of Fe-based carbide precipitated in bainite lath to all Fe-based carbide is 10% or more, has a high tensile strength of 900 MPa or more, and has significantly improved punchability in mass production.

Description

TECHNICAL FIELD[0001]The present invention relates to a high-strength hot-rolled steel sheet suitable for, for example, structural parts, such as members and frames, and underbody parts, such as suspensions, of automobile bodies, as well as automotive body components, such as truck frame parts. In particular, the present invention relates to improvement of punchability during mass production (hereinafter also referred to as punchability in mass production).BACKGROUND ART[0002]In recent years, from a global environmental conservation standpoint, there has been a strong demand for improved vehicle fuel efficiency. Thus, high-strength steel sheets are actively utilized as automotive body components in order to reduce the weight of automobile bodies. High-strength steel sheets are utilized not only in automobile structural parts but also in underbody parts and truck frame parts. In general, higher-strength steel sheets have lower workability. In particular, because automotive parts are ...

Claims

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Application Information

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IPC IPC(8): C22C38/58B32B15/01C21D6/00C21D8/02C22C38/00C22C38/02C22C38/04C22C38/06C22C38/08C22C38/12C22C38/14C22C38/16C22C38/24C22C38/28C22C38/32C22C38/38C22C38/44C22C38/46C22C38/50C23C2/02C23C2/40
CPCC22C38/58B32B15/013C23C2/02C21D6/004C21D6/005C21D6/008C21D8/0205C21D8/0226C22C38/50C22C38/46C22C38/44C22C38/38C22C38/32C22C38/28C22C38/24C22C38/16C22C38/14C22C38/12C22C38/08C22C38/06C22C38/04C22C38/02C22C38/008C22C38/005C22C38/002C22C38/001C23C2/40C21D9/46C22C18/04C22C38/00C22C38/18C21D8/0263C21D8/0426C21D8/0463C21D2211/002C21D2211/003C22C38/42C22C38/48C22C38/54C21D1/20C21D9/48C21D1/28C23C2/28C23C2/06C23C2/024C23C2/022B21D22/022C21D1/84C23F17/00
Inventor NAKAJIMA, KATSUMIYAMAZAKI, KAZUHIKOKAMI, CHIKARA
Owner JFE STEEL CORP
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