Hot-rolled steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-9903004-B2
Application number	US-201214434835-A
Country	US
Kind code	B2
Filing date	Dec 19, 2012
Priority date	Dec 19, 2012
Publication date	Feb 27, 2018
Grant date	Feb 27, 2018

Abstract

A hot-rolled steel sheet having a tensile strength of 900 MPa or more, the hot-rolled steel sheet consisting of, in mass %, C: greater than 0.050% and less than or equal to 0.10%, Si: greater than or equal to 0.1% and less than or equal to 2.0%, Mn: greater than or equal to 1.0% and less than or equal to 3.0%, P: less than or equal to 0.1%, S: less than or equal to 0.01%, Al: greater than or equal to 0.005% and less than or equal to 0.05%, N: less than or equal to 0.01%, Ti: greater than or equal to 0.10% and less than or equal to 0.20%, Nb: greater than or equal to 0% and less than or equal to 0.06%, B: greater than or equal to 0% and less than or equal to 0.03%, Ca: greater than or equal to 0% and less than or equal to 0.005%, and the balance: Fe and impurities. An average crystal grain size is less than or equal to 7.0 μm, and an X-ray random intensity ratio in {211} <011> orientation that is parallel to a rolled surface and a rolling direction is less than or equal to 2.5.

First claim

The invention claimed is:

1. A hot-rolled steel sheet having a tensile strength of 900 MPa or more, the hot-rolled steel sheet consisting of, in mass %,
C: greater than 0.050% and less than or equal to 0.10%,
Si: greater than or equal to 0.1% and less than or equal to 2.0%,
Mn: greater than or equal to 1.0% and less than or equal to 3.0%,
P: less than or equal to 0.1%,
S: less than or equal to 0.01%,
Al: greater than or equal to 0.005% and less than or equal to 0.05%,
N: less than or equal to 0.01%,
Ti: greater than or equal to 0.10% and less than or equal to 0.20%,
Nb: greater than or equal to 0% and less than or equal to 0.06%,
B: greater than or equal to 0% and less than or equal to 0.03%,
Ca: greater than or equal to 0% and less than or equal to 0.005%, and
the balance: Fe and impurities,
wherein an average crystal grain size is less than or equal to 7.0 μm, and
wherein an X-ray random intensity ratio in {211} <011> orientation that is parallel to a rolled surface and a rolling direction is less than or equal to 2.5.

2. The hot-rolled steel sheet according to claim 1 , including one or more selected from the group consisting of, in mass %,
Nb: greater than or equal to 0.001% and less than or equal to 0.06%,
B: greater than or equal to 0.0005% and less than or equal to 0.03%, and
Ca: greater than or equal to 0.0005% and less than or equal to 0.005%.

3. The hot-rolled steel sheet according to claim 1 , further comprising:
a Zn-plating layer or an Fe—Zn-alloy-plating layer on a surface of the hot-rolled steel sheet.

4. The hot-rolled steel sheet according to claim 2 , further comprising:
a Zn-plating layer or an Fe—Zn-alloy-plating layer a surface of the hot-rolled steel sheet.

5. A method for manufacturing a hot-rolled steel sheet, the method comprising:
performing hot-rolling on a slab having a chemical composition according to claim 1 at a temperature of higher than or equal to 1200° C. and lower than or equal to 1350° C.;
ending the hot-rolling at a temperature of higher than or equal to 960° C. and lower than or equal to 1100° C.;
within 1.0 second after the ending of the hot-rolling, starting quenching;
quenching the hot-rolled steel sheet at an average cooling speed of greater than or equal to 80° C./second until the temperature becomes lower than the temperature at the ending of the hot-rolling by 50° C. to 200° C.; and
winding up the hot-rolled steel sheet at a temperature of higher than or equal to 400° C. and lower than or equal to 600° C.,
wherein a cooling after the ending of the quenching until the winding is performed at an average cooling speed of 20° C./second or more.

6. A method for manufacturing a hot-rolled steel sheet, the method comprising:
performing hot-rolling on a slab having a chemical composition according to claim 2 at a temperature of higher than or equal to 1200° C. and lower than or equal to 1350° C.;
ending the hot-rolling at a temperature of higher than or equal to 960° C. and lower than or equal to 1100° C.;
within 1.0 second after the ending of the hot-rolling, starting quenching;
quenching the hot-rolled steel sheet at an average cooling speed of greater than or equal to 80° C./second until the temperature becomes lower than the temperature at the ending of the hot-rolling by 50° C. to 200° C.; and
winding up the hot-rolled steel sheet at a temperature of higher than or equal to 400° C. and lower than or equal to 600° C.,
wherein a cooling after the ending of the quenching until the winding is performed at an average cooling speed of 20° C./second or more.

7. The method for manufacturing a hot-rolled steel sheet according to claim 5 , further comprising:
after the winding up, removing scale by acid cleaning and performing Zn-plating to form a Zn-plating layer.

8. The method for manufacturing a hot-rolled steel sheet having a tensile strength of 900 MPa or more, the method comprising:
after the winding up of the hot-rolled steel sheet manufactured by the method according to claim 5 , removing scale by acid cleaning;
heating the hot-rolled steel sheet at a temperature of higher than or equal to 500° C. and lower than or equal to 650° C. in a reduction atmosphere to activate a surface of the hot-rolled steel sheet;
immersing the hot-rolled steel sheet in a molten Zn bath at a bath temperature of higher than or equal to 430° C. and lower than or equal to 490° C. in a state in which a temperature at the time of the immersing is higher than or equal to 420° C. and lower than or equal to 500° C.; and
adjusting a deposited amount of the Zn-plating by gas wiping.

9. The method for manufacturing a hot-rolled steel sheet having a tensile strength of 900 MPa or more, the method comprising:
after the winding up of the hot-rolled steel sheet manufactured by the method according to claim 5 , removing scale by acid cleaning;
plating the hot-rolled steel sheet with Ni in greater than or equal to 0.05 g/m 2 and less than or equal to 3 g/m 2 by electrolysis;
heating the hot-rolled steel sheet at a temperature of higher than or equal to 420° C. and lower than or equal to 500° C. in a reduction atmosphere;
immersing the hot-rolled steel sheet in a molten Zn bath at a bath temperature of higher than or equal to 430° C. and lower than or equal to 490° C.; and
adjusting a deposited amount of the Zn-plating by gas wiping.

10. The method for manufacturing a hot-rolled steel sheet according to claim 8 , further comprising:
after the gas wiping, heating the hot-rolled steel sheet at a temperature of higher than or equal to 500° C. and lower than or equal to 650° C. to form an Fe—Zn-plating layer.

11. The method for manufacturing a hot-rolled steel sheet according to claim 9 , further comprising:
after the gas wiping, heating the hot-rolled steel sheet at a temperature of higher than or equal to 500° C. and lower than or equal to 650° C. to form an Fe—Zn-plating layer.

Assignees

• Nippon Steel & Sumitomo Metal Corp

Inventors

• Toyoda Takeshi
• Takahashi Takehiro
• Imai Takeshi
• Yamamoto Takeshi

Classifications

• C21D9/46

  for sheet metals · CPC title

• C23C28/022

  with at least one MCrAlX layer · CPC title

• C21D6/008

  containing Si · CPC title

• B32B15/013

  one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium · CPC title

• Y10T428/12799

  Next to Fe-base component [e.g., galvanized] · CPC title