High-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-9512508-B2
Application number	US-201214235009-A
Country	US
Kind code	B2
Filing date	Jul 27, 2012
Priority date	Jul 27, 2011
Publication date	Dec 6, 2016
Grant date	Dec 6, 2016

Abstract

A high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability containing predetermined components and a balance being composed of iron and inevitable impurities, in which in a range of ⅝ to ⅜ in sheet thickness from the surface of the steel sheet, an average value of pole densities of the {100}<011> to {223}<110> orientation group represented by respective crystal orientations of {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110>, and {223}<110> is 6.5 or less, and a pole density of the {332}<113> crystal orientation is 5.0 or less, and a metal structure contains, in terms of an area ratio, greater than 5% of pearlite, the sum of bainite and martensite limited to less than 5%, and a balance composed of ferrite.

First claim

What is claimed is:

1. A high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability comprising:
in mass %,
C: greater than 0.01% to 0.4% or less;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.001% nor more than 4%;
P: 0.001 to 0.15% or less;
S: 0.0005 to 0.03% or less;
Al: not less than 0.001% nor more than 2%;
N: 0.0005 to 0.01% or less; and
a balance being composed of iron and inevitable impurities, wherein
in a range of ⅝ to ⅜ in sheet thickness from the surface of the steel sheet, an average value of pole densities of the {100}<011> to {223}<110> orientation group represented by respective crystal orientations of {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110>, and {223}<110> is 6.5 or less, and a pole density of the {332}<113> crystal orientation is 5.0 or less, and
a metal structure contains, in terms of an area ratio, greater than 5% of pearlite, the sum of bainite and martensite limited to less than 5%, and a balance composed of ferrite.

2. The high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 1 , wherein further, Vickers hardness of a pearlite phase is not less than 150 HV nor more than 300 HV.

3. The high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 1 , wherein further, an r value in a direction perpendicular to a rolling direction (rC) is 0.70 or more, an r value in a direction 30° from the rolling direction (r30) is 1.10 or less, an r value in the rolling direction (rL) is 0.70 or more, and an r value in a direction 60° from the rolling direction (r60) is 1.10 or less.

4. The high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 1 , further comprising:
one type or two or more types of
in mass %,
Ti: not less than 0.001% nor more than 0.2%,
Nb: not less than 0.001% nor more than 0.2%,
B: not less than 0.0001% nor more than 0.005%,
Mg: not less than 0.0001% nor more than 0.01%,
Rem: not less than 0.0001% nor more than 0.1%,
Ca: not less than 0.0001% nor more than 0.01%,
Mo: not less than 0.001% nor more than 1%,
Cr: not less than 0.001% nor more than 2%,
V: not less than 0.001% nor more than 1%,
Ni: not less than 0.001% nor more than 2%,
Cu: not less than 0.001% nor more than 2%,
Zr: not less than 0.0001% nor more than 0.2%,
W: not less than 0.001% nor more than 1%,
As: not less than 0.0001% nor more than 0.5%,
Co: not less than 0.0001% nor more than 1%,
Sn: not less than 0.0001% nor more than 0.2%,
Pb: not less than 0.001% nor more than 0.1%,
Y: not less than 0.001% nor more than 0.1%, and
Hf: not less than 0.001% nor more than 0.1%.

5. The high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 1 , wherein further, when the steel sheet whose sheet thickness is reduced to 1.2 mm with a sheet thickness center portion set as the center is punched out by a circular punch with Φ 10 mm and a circular die with 1% of a clearance, a shear surface percentage of a punched edge surface becomes 90% or more.

6. The high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 1 , wherein on the surface, a hot-dip galvanized layer or an alloyed hot-dip galvanized layer is provided.

7. A manufacturing method of a high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 1 , comprising:
on a steel billet containing:
in mass %,
C: greater than 0.01% to 0.4% or less;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.001% nor more than 4%;
P: 0.001 to 0.15% or less;
S: 0.0005 to 0.03% or less;
Al: not less than 0.001% nor more than 2%;
N: 0.0005 to 0.01% or less; and
a balance being composed of iron and inevitable impurities,
performing first hot rolling in which rolling at a reduction ratio of 40% or more is performed one time or more in a temperature range of not lower than 1000° C. nor higher than 1200° C.;
setting an austenite grain diameter to 200 μm or less by the first hot rolling;
performing second hot rolling in which rolling at a reduction ratio of 30% or more is performed in one pass at least one time in a temperature region of not lower than a temperature T1 determined by Expression (1) below +30° C. nor higher than T1+200° C.;
setting the total reduction ratio in the second hot rolling to 50% or more;
performing final reduction at a reduction ratio of 30% or more in the second hot rolling and then starting pre-cold rolling cooling in such a manner that a waiting time t second satisfies Expression (2) below;
setting an average cooling rate in the pre-cold rolling cooling to 50° C./second or more and setting a temperature change to fall within a range of not less than 40° C. nor more than 140° C.;
performing cold rolling at a reduction ratio of not less than 40% nor more than 80%;
performing heating up to a temperature region of 750 to 900° C. and performing holding for not shorter than 1 second nor longer than 300 seconds;
performing post-cold rolling primary cooling down to a temperature region of not lower than 580° C. nor higher than 750° C. at an average cooling rate of not less than 1° C./s nor more than 10° C./s;
performing retention for not shorter than 1 second nor longer than 1000 seconds under the condition that a temperature decrease rate becomes 1° C./s or less; and
performing post-cold rolling secondary cooling at an average cooling rate of 5° C./s or less;
T1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  Expression (1)

wherein C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the element (mass %);
t≦ 2.5× t 1  Expression (2)

wherein t1 is obtained by Expression (3) below;
t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+3.1  Expression (3)

wherein in Expression (3) above, Tf represents the temperature of the steel billet obtained after the final reduction at a reduction ratio of 30% or more, and P1 represents the reduction ratio of the final reduction at 30% or more.

8. The manufacturing method of the high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 7 , wherein the total reduction ratio in a temperature range of lower than T1+30° C. is 30% or less.

9. The manufacturing method of the high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 7 , wherein the waiting time t second further satisfies Expression (2a) below;
t<t 1  Expression (2a).

10. The manufacturing method of the high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 7 , wherein the waiting time t second further satisfies Expression (2b) below;
t 1≦ t≦t 1×2.5  Expression (2b).

11. The manufacturing method of the high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 7 , wherein the pre-cold rolling cooling is started between rolling stands.

12. The manufacturing method of the high-strength cold-rolled steel sheet having excellent stretch flangeability and precision punchability according to claim 7 , further comprising:
performing coiling at 650° C. or lower to obtain a hot-rolled steel sheet after performing the pre-cold rolling cooling and before performing the co

Assignees

• Shuto Hiroshi
• Fujita Nobuhiro
• Yokoi Tatsuo
• Okamoto Riki
• Nakano Kazuaki
• Watanabe Shinichiro
• Nippon Steel & Sumitomo Metal Corp

Inventors

• Shuto Hiroshi
• Fujita Nobuhiro
• Yokoi Tatsuo
• Okamoto Riki
• Nakano Kazuaki
• Watanabe Shinichiro

Classifications

• C22C38/38 Primary

  with more than 1.5% by weight of manganese · CPC title

• C22C38/04

  containing manganese · CPC title

• C22C38/005

  containing rare earths, i.e. Sc, Y, Lanthanides · CPC title

• C22C38/008

  containing tin · CPC title

• C21D9/48

  deep-drawing sheets · CPC title