High-strength steel sheet and high-strength galvanized steel sheet excellent in shape fixability, and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-9988700-B2
Application number	US-201214234938-A
Country	US
Kind code	B2
Filing date	Jul 27, 2012
Priority date	Jul 29, 2011
Publication date	Jun 5, 2018
Grant date	Jun 5, 2018

Abstract

The present invention provides a high-strength steel sheet excellent in shape fixability. The high-strength steel sheet contains C, Si, Mn, P, S, Al, N, and O with predetermined contents, in which a retained austenite phase of 5 to 20% in volume fraction is contained, an amount of solid-solution C contained in the retained austenite phase is 0.80 to 1.00% in mass %, W Siγ is 1.10 times or more W Si* , W Mnγ is 1.10 times or more W Mn* , and when a frequency distribution is measured with respect to a sum of a ratio between W Si and W Si* and a ratio between W Al and W Al* , a mode value of the frequency distribution is 1.95 to 2.05, and a kurtosis is 2.00 or more.

First claim

What is claimed is:

1. A steel sheet, comprising:
in mass %,
C: 0.075 to 0.300%;
Si: 0.30 to 2.5%;
Mn: 1.3 to 3.50%;
P: 0.001 to 0.030%;
S: 0.0001 to 0.0100%;
Al: 0.080 to 1.500%;
N: 0.0001 to 0.0100% ;
O: 0.0001 to 0.0100%; and
a balance composed of Fe and inevitable impurities, wherein:
a steel sheet structure contains a retained austenite phase of 5 to 20% in volume fraction in a range of ⅛ thickness to ⅜ thickness of the steel sheet;
an amount of solid-solution C contained in the retained austenite phase is 0.80 to 1.00% in mass %;
W Siγ defined as an amount of solid-solution Si contained in the retained austenite phase is 1.10 times or more W Si* defined as an average amount of Si in the range of ⅛ thickness to ⅜ thickness of the steel sheet;
W Mnγ defined as an amount of solid-solution Mn contained in the retained austenite phase is 1.10 times or more W Mm* defined as an average amount of Mn in the range of ⅛ thickness to ⅜ thickness of the steel sheet; and
when a frequency distribution is measured, by setting a plurality of measurement regions each having a diameter of 1 μm or less in the range of ⅛ thickness to ⅜ thickness of the steel sheet, with respect to a sum of a ratio between W Si defined as a measured value of an amount of Si in each of the plurality of measurement regions and W Si* being the average amount of Si and a ratio between W Al defined as a measured value of an amount of Al in each of the plurality of measurement regions and W Al* defined as an average amount of Al, a mode value of the frequency distribution is 1.95 to 2.05, and a kurtosis is 2.00 or more.

2. The steel sheet according to claim 1 , wherein:
the steel sheet structure further contains a ferrite phase of 10 to 75% in volume fraction, and either or both of a bainitic ferrite phase and a bainite phase of 10 to 50% in total; and
a tempered martensite phase is limited to less than 10% in volume fraction, and a fresh martensite phase is limited to 15% or less in volume fraction.

3. The steel sheet according to claim 1 , further comprising
in mass %,
one or two or more of
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005 to 0.150%,
B: 0.0001 to 0.0100%,
Mo: 0.01 to 1.00%,
W: 0.01 to 1.00%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%, and
Cu: 0.01 to 2.00%, and/or
one or two or more of Ca, Ce, Mg, Zr, Hf, and REM of 0.0001 to 0.5000% in total.

4. A galvanized steel sheet, comprising
the steel sheet according to claim 1 having a galvanized layer formed on a surface thereof.

5. The galvanized steel sheet according to claim 4 , wherein
a coating film made of a composite oxide containing a phosphorus oxide and/or phosphorus is formed on a surface of the galvanized layer.

6. A manufacturing method of the steel sheet according to claim 1 , comprising:
a hot-rolling step being a step of heating a slab containing:
in mass %,
C: 0.075 to 0.300%;
Si: 0.30 to 2.5%;
Mn: 1.3 to 3.50%;
P: 0.001 to 0.030%;
S: 0.0001 to 0.0100%;
Al: 0.080 to 1.500%;
N: 0.0001 to 0.0100%;
O: 0.0001 to 0.0100%; and
a balance composed of Fe and inevitable impurities to 1100° C. or more,
performing hot rolling on the slab in a temperature region in which a higher temperature between 850° C. and an Ar 3 temperature is set to a lower limit temperature,
performing first cooling of performing cooling in a range from a completion of rolling to a start of coiling at a rate of 10° C./second or more on average,
performing coiling in a range of coiling temperature of 600 to 750° C., and
performing second cooling of cooling the coiled steel sheet in a range of the coiling temperature to (the coiling temperature−100)° C. at a rate of 15° C./hour or less on average; and
a continuous annealing step of performing annealing on the steel sheet at a maximum heating temperature (Ac 1 +40)° C. to 1000° C. after the second cooling,
next performing third cooling at an average cooling rate of 1.0 to 10.0° C./second in a range of the maximum heating temperature to 700° C.,
next performing fourth cooling at an average cooling rate of 5.0 to 200.0° C./second in a range of 700° C. to 500° C., and
next performing retention process of retaining the steel sheet after being subjected to the fourth cooling for 30 to 1000 seconds in a range of 350 to 450° C.

7. The manufacturing method of the steel sheet according to claim 6 , further comprising
a cold-rolling step of performing pickling and then performing cold rolling at a reduction ratio of 30 to 75%, between the hot-rolling step and the continuous annealing step.

8. The manufacturing method of the steel sheet according to claim 6 , further comprising
a temper rolling step of performing rolling on the steel sheet at a reduction ratio of less than 10%, after the continuous annealing step.

9. A manufacturing method of a galvanized steel sheet, comprising
forming, after performing the retention process when manufacturing the steel sheet in the manufacturing method according to claim 6 , a galvanized layer on a surface of the steel sheet by conducting electrogalvanization.

10. A manufacturing method of a galvanized steel sheet, comprising
forming, between the fourth cooling and the retention process, or after the retention process when manufacturing the steel sheet in the manufacturing method according to claim 6 , a galvanized layer on a surface of the steel sheet by dipping the steel sheet in a galvanizing bath.

11. The manufacturing method of the galvanized steel sheet according to claim 10 , wherein
the steel sheet after being dipped in the galvanizing bath is reheated to 460 to 600° C., and retained for two seconds or more to make the galvanized layer to be alloyed.

12. The manufacturing method of the galvanized steel sheet according to claim 9 , wherein
after the galvanized layer is formed, a coating film made of a composite oxide containing either or both of a phosphorus oxide and phosphorus is given to a surface of the galvanized layer.

13. The manufacturing method of the galvanized steel sheet according to claim 11 , wherein
after the galvanized layer is alloyed, a coating film made of a composite oxide containing either or both of a phosphorus oxide and phosphorus is given to a surface of the alloyed galvanized layer.

14. The manufacturing method of a steel sheet according to claim 6 , wherein the slab comprises, in mass %,
one or two or more of
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005 to 0.150%,
B: 0.0001 to 0.0100%,
Mo: 0.01 to 1.00%,
W: 0.01 to 1.00%,
Cr: 0.01 to 2.00%,
Ni: 0.01 to 2.00%, and
Cu: 0.01 to 2.00%, and/or
one or two or more of Ca, Ce, Mg, Zr, Hf, and REM of 0.0001 to 0.5000% in total.

Assignees

• Minami Akinobu
• Kawata Hiroyuki
• Murasato Akinobu
• Yamaguchi Yuji
• Sugiura Natsuko
• Kuwayama Takuya
• Maruyama Naoki
• Suzuki Takamasa
• Nippon Steel & Sumitomo Metal Corp

Inventors

• Minami Akinobu
• Kawata Hiroyuki
• Murasato Akinobu
• Yamaguchi Yuji
• Sugiura Natsuko
• Kuwayama Takuya
• Maruyama Naoki
• Suzuki Takamasa

Classifications

• C21D8/02

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

• C21D8/0247

  characterised by the heat treatment · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C38/001

  containing N · CPC title

• C22C38/08

  containing nickel {(C22C38/105 takes precedence)} · CPC title