Multi-phase hot-rolled steel sheet having improved dynamic strength and a method for its manufacture

Patent metadata

Field	Value
Publication number	US-10041158-B2
Application number	US-201113643696-A
Country	US
Kind code	B2
Filing date	Apr 7, 2011
Priority date	Apr 28, 2010
Publication date	Aug 7, 2018
Grant date	Aug 7, 2018

Abstract

A multi-phase hot-rolled steel sheet having improved strength in an intermediate strain rate region has a chemical composition comprising, in mass percent, C: 0.07-0.2%, Si+Al: 0.3-1.5%, Mn: 1.0-3.0%, P: at most 0.02%, S: at most 0.005%, Cr: 0.1-0.5%, N: 0.001-0.008%, at least one of Ti: 0.002-0.05% and Nb: 0.002-0.05%, and a remainder of Fe and impurities. The area fraction of ferrite is 7-35%, the grain diameter of ferrite is in the range of 0.5-3.0 μm, and the nanohardness of ferrite is in the range of 3.5-4.5 GPa. A second phase which is the remainder other than ferrite contains martensite and bainitic ferrite and/or bainite. The average nanohardness of the second phase is 5-12 GPa, and the second phase contains a high-hardness phase of 8-12 GPa with an area fraction of 5-35% based on the overall structure.

First claim

The invention claimed is:

1. A multi-phase hot-rolled steel sheet characterized by:
having a chemical composition comprising, in mass percent,
C: at least 0.07% and at most 0.2%,
Si+Al: at least 0.3% and at most 1.5%,
Mn: at least 1.0% and at most 3.0%,
P: at most 0.02%,
S: at most 0.005%,
Cr: at least 0.1% and at most 0.5%,
N: at least 0.001% and at most 0.008%,
one or two of Ti: at least 0.002% and at most 0.05%, the Ti of at least 0.002% and at most 0.05% existing as Ti nitride and carbide, and Nb: at least 0.002% and at most 0.05%, the Nb of at least 0.002% and at most 0.05% existing as Nb nitride and carbide, and
a remainder of Fe and impurities,

having ferrite with an area fraction at least 7% and at most 35%, the ferrite having a grain diameter of at least 0.5 μm and at most 3.0 μm and a nanohardness of at least 3.5 GPa and at most 4.5 GPa, and
having a second phase which is the remainder other than ferrite, the second phase including martensite and at least one selected from bainitic ferrite and bainite and having an average nanohardness of at least 5 GPa and at most 12 GPa, the second phase containing a high-hardness phase of at least 8 GPa and at most 12 GPa with an area fraction of at least 5% and at most 35% with respect to the overall structure.

2. A multi-phase hot-rolled steel sheet as set forth in claim 1 characterized in that the chemical composition further contains, in mass percent, a most 0.2% of V.

3. A multi-phase hot-rolled steel sheet as set forth in claim 1 characterized in that the chemical composition further contains one or more elements selected from the group consisting of, in mass percent, Cu: at most 0.2%, Ni: at most 0.2%, and Mo: at most 0.5%.

4. A method of manufacturing the multi-phase hot-rolled steel sheet as set forth in claim 1 by continuous hot rolling of a slab having a chemical composition comprising, in mass percent, C: at least 0.07% and at most 0.2%, Si+Al: at least 0.3% and at most 1.5%, Mn: at least 1.0% and at most 3.0%, P: at most 0.02%, S: at most 0.005%, Cr: at least 0.1% and at most 0.5%, N: at least 0.001% and at most 0.008%, one or two of Ti: at least 0.002% and at most 0.05% and Nb: at least 0.002% and at most 0.05%, and a remainder of Fe and impurities to manufacture a hot-rolled steel sheet, the multi-phase hot-rolled steel sheet having ferrite with an area fraction at least 7% and at most 35%, the ferrite having a grain diameter of at least 0.5 μm and at most 3.0 μm and a nanohardness of at least 3.5 GPa and at most 4.5 GPa, and
having a second phase which is the remainder other than ferrite, the second phase including martensite and at least one selected from bainitic ferrite and bainite and having an average nanohardness of at least 5 GPa and at most 12 GPa, the second phase containing a high-hardness phase of at least 8 GPa and at most 12 GPa with an area fraction of at least 5% and at most 35% with respect to the overall structure, the method comprising the following steps:
a finish rolling step in which the slab is rolled in final finish rolling at a temperature of at least 800° C. and at most 900° C. with the length of time between passes being at least 0.15 seconds and at most 2.7 seconds to form a steel sheet;
a first cooling step including cooling the steel sheet obtained by the finish rolling step to a temperature of 700° C. or below within 0.4 seconds at a cooling rate of at least 600° C./sec;
a holding step including holding the steel sheet which passed through the cooling step in a temperature range of at least 570° C. to at most 700° C. for at least 0.4 seconds; and
a second cooling step including cooling the steel sheet which passed through the holding step to 430° C. or below at a cooling rate of at least 20° C./sec and at most 120° C./sec.

5. A method of manufacturing a multi-phase hot-rolled steel sheet as set forth in claim 4 characterized in that the chemical composition further contains, in mass percent, at least 0.2% of V.

6. A method of manufacturing a multi-phase hot-rolled steel sheet as set forth in claim 4 characterized in that the chemical composition further contains one or more elements selected from the group consisting of, in mass percent, Cu: at most 0.2%, Ni: at most 0.2%, and Mo: at most 0.5%.

Assignees

• Tanaka Yasuaki
• Tomida Toshiro
• Kawano Kaori
• Nippon Steel & Sumitomo Metal Corp

Inventors

• Tanaka Yasuaki
• Tomida Toshiro
• Kawano Kaori

Classifications

• C22C38/06

  containing aluminium · CPC title

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/40

  with nickel · CPC title

• C22C38/50

  with titanium or zirconium · CPC title

• C21D8/0226

  Hot rolling · CPC title