Hot-rolled steel sheet and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-10087499-B2
Application number	US-201214370158-A
Country	US
Kind code	B2
Filing date	Dec 27, 2012
Priority date	Jan 5, 2012
Publication date	Oct 2, 2018
Grant date	Oct 2, 2018

Abstract

There are provided a high-strength hot-rolled steel sheet securing low-temperature toughness and having excellent stretch flangeability by controlling a structural fraction and a hardness difference among structures, and a manufacturing method thereof. A hot-rolled steel sheet contains: C: 0.01 to 0.2%; Si: 0.001 to 2.5% or less; Mn: 0.10 to 4.0% or less; P: 0.10% or less; S: less than 0.03%; Al: 0.001 to 2.0%; N: less than 0.01%; Ti: (0.005+48/14[N]+48/32[S]) % or more and 0.3% or less; Nb: 0 to 0.06%; Cu: 0 to 1.2%; Ni: 0 to 0.6%; Mo: 0 to 1%; V: 0 to 0.2%; Cr: 0 to 2%; Mg: 0 to 0.01%; Ca: 0 to 0.01%; REM: 0 to 0.1%; and B: 0 to 0.002%, and has: an texture in which, at a central portion of a sheet thickness located between ⅜ to ⅝ thickness positions of the sheet thickness from a surface of the steel sheet, an average value of X-ray random intensity ratios of a group of {100}<011> to {223}<110> orientations of a sheet plane is 6.5 or less and an X-ray random intensity ratio of a {332}<113> crystal orientation is 5.0 or less; and a microstructure in which a total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and an average crystal grain diameter is 12.0 μm or less.

First claim

The invention claimed is:

1. A hot-rolled steel sheet comprising:
a chemical composition comprising: in mass %,
C: 0.01 to 0.2%;
Si: 0.001 to 2.5%;
Mn: 0.10 to 4.0%;
P: 0.10% or less;
S: 0.030% or less;
Al: 0.001 to 2.0%;
N: 0.01% or less;
Ti: (0.005+48/14[N]+48/32[S])%≤Ti≤0.3%;
Nb: 0 to 0.06%;
Cu: 0 to 1.2%;
Ni: 0 to 0.6%;
Mo: 0 to 1%;
V: 0 to 0.2%;
Cr: 0 to 2%;
Mg: 0 to 0.01%;
Ca: 0 to 0.01%;
REM: 0 to 0.1%; and
B: 0 to 0.002%,
with a balance being composed of Fe and impurities;
an texture in which, at a central portion of a sheet thickness that is a steel sheet portion sectioned at a ⅜ thickness position and a ⅝ thickness position of the sheet thickness from a surface of the steel sheet, an average value of X-ray random intensity ratios of a group of {100}<011> to {223}<110> orientations of a sheet plane is 6.5 or less and an X-ray random intensity ratio of a {332}<113> crystal orientation is 5.0 or less; and
a microstructure in which a total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and an average crystal grain diameter is 12.0 μm or less.

2. The hot-rolled steel sheet according to claim 1 , wherein
the chemical composition contains one or two or more selected from a group consisting of: in mass %,
Nb: 0.005 to 0.06%;
Cu: 0.02 to 1.2%;
Ni: 0.01 to 0.6%;
Mo: 0.01 to 1%;
V: 0.01 to 0.2%; and
Cr: 0.01 to 2%.

3. The hot-rolled steel sheet according to claim 1 , wherein the chemical composition contains one or two or more selected from a group consisting of: in mass %, Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and REM: 0.0005 to 0.1%.

4. The hot-rolled steel sheet according to claim 1 , wherein the chemical composition contains, in mass %, B: 0.0002 to 0.002%.

5. The hot-rolled steel sheet according to claim 1 , comprising the microstructure in which assuming that an average value of hardness is E (HV0.01) and a standard deviation is σ (HV0.01) when measuring the Vickers hardness at 100 points or more with a load of 0.098 N, σ (HV0.01)/E (HV0.01) is 0.08 or less.

6. The hot-rolled steel sheet according to claim 1 , comprising mechanical properties that an r value (rC) in a direction perpendicular to a rolling direction is 0.70 or more, and an r value (r30) in a direction 30° from the rolling direction is 1.10 or less.

7. The hot-rolled steel sheet according to claim 1 , comprising mechanical properties that an r value (rL) in a rolling direction is 0.70 or more and an r value (r60) in a direction 60° from the rolling direction is 1.10 or less.

8. The hot-rolled steel sheet according to claim 1 , comprising a plating layer provided on the surface of the steel sheet.

9. A manufacturing method of a hot-rolled steel sheet according to claim 1 , which comprises: sequentially performing rough hot rolling, finish hot rolling, primary cooling and secondary cooling on a slab comprising a chemical composition comprising in mass %
C: 0.01 to 0.2%;
Si: 0.001 to 2.5%;
Mn: 0.10 to 4.0%;
P: 0.10% or less;
S: 0.030% or less;
Al: 0.001 to 2.0%;
N: 0.01% or less;
Ti: (0.005+48/14[N]+48/32[S])%≤Ti≤0.3%;
Nb: 0 to 0.06%;
Cu: 0 to 1.2%;
Ni: 0 to 0.6%;
Mo: 0 to 1%;
V: 0 to 0.2%;
Cr: 0 to 2%;
Mg: 0 to 0.01%;
Ca: 0 to 0.01%;
REM: 0 to 0.1%; and
B: 0 to 0.002%,
with a balance being composed of Fe and impurities;

and coiling a resultant slab into the hot-rolled steel sheet, wherein:
the finish hot rolling is hot rolling in which with respect to a temperature T1 defined in a following expression (1), a maximum reduction ratio per pass in a first temperature region of 1004° C. or higher and (T1+200)° C. or lower is 41% or more, a total reduction ratio in the first temperature region is 50% or more, a total reduction ratio in a second temperature region of T1° C. or higher and lower than (T1+30)° C. is 0 to 30%, and the rolling is completed in the first temperature region or the second temperature region;
the primary cooling is water cooling that satisfies a following expression (2) and achieves a cooling amount of 40° C. or higher and 140° C. or lower;
the secondary cooling is water cooling that is started within three seconds after the primary cooling and performs cooling at an average cooling rate of 30° C./sec or higher; and
the coiling is to coil the slab at a temperature CT satisfying a following expression (3),
T 1(° C.)=850+10×(C+N)×Mn+350×Nb+250×Ti+40×B+10×Cr+100×Mo+100×V  (1)

1≤ t/t 1≤2.5  (2)

CT (° C.)≤max[Ms,350]  (3)

t 1=0.001×{( Tf−T 1)× P 1/100} 2 −0.109×{( Tf−T 1)× P 1/100}+3.1   (4)

Ms(° C.)=561−474×C−33×Mn−17×Ni−21×Mo   (5)

where in the expression (1) and the expression (5), a symbol of each element is a content (mass %) of the element in the steel,
in the expression (2), t is a time period (sec) from a final reduction in the reduction in one pass at 30% or more in the first temperature region to start of the primary cooling, and t1 is a time period (sec) decided by the above expression (4),
in the expression (3), max[ ] is a function of returning a maximum value among arguments, and Ms is a temperature decided by the above expression (5), and
in the expression (4), Tf and P1 are a steel sheet temperature and a reduction ratio (%) in the final reduction in the reduction in one pass at 30% or more in the first temperature region respectively.

10. The manufacturing method of the hot-rolled steel sheet according to claim 9 , wherein the rough hot rolling achieves a maximum reduction ratio per pass in a temperature region of 1000° C. or higher and 1200° C. or lower of 40% or more, and an austenite average grain diameter of 200 μm or less.

11. The manufacturing method of the hot-rolled steel sheet according to claim 9 , wherein a maximum heat generation due to plastic deformation in a temperature region of (T1+30)° C. or higher and (T1+150)° C. or lower of the finish hot rolling is 18° C. or lower.

12. A manufacturing method of a hot-rolled steel sheet comprising: performing a plating treatment on the surface of the hot-rolled steel sheet obtained by the manufacturing method of the hot-rolled steel sheet according to claim 9 .

Assignees

• Nippon Steel & Sumitomo Metal Corp

Inventors

• Shuto Hiroshi
• Yokoi Tatsuo
• KANZAWA Yuuki
• Fujita Nobuhiro
• NIIYA Ryohta
• Saitoh Shinya

Classifications

• C21D8/0263 Primary

  following hot rolling · CPC title

• C22C38/02

  containing silicon · CPC title

• C22C38/08

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

• C21D2211/008

  Martensite · CPC title

• C22C38/008

  containing tin · CPC title