# US 10,870,901 B2 · Hot-rolled high-strength roll-formable steel sheet with excellent stretch-flange formability and a method of producing said steel

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- Number: US 10,870,901 B2
- Document ID: US10870901B2
- Status: Granted
- Kind: B2
- Application number: 15/761,322
- Filed: 2016-09-21
- Published: 2018-09-20
- Granted: 2020-12-22
- Earliest claimed: 2015-09-22
- Assignees: Tata Steel Ijmuiden B.V.
- Applicants: Tata Steel Ijmuiden B.V.
- Inventors: Rolf Arjan Rijkenberg; Maxim Peter Aarnts
- Web page: https://patentgrep.com/patent/US10870901B2

## CPC

- C21D 8/0263: {following hot rolling}
- B32B 15/013: {one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium}
- C21D 9/46: for sheet metals
- C22C 38/00: Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00)
- C22C 38/001: {containing N}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/12: containing tungsten, tantalum, molybdenum, vanadium, or niobium
- C22C 38/14: containing titanium or zirconium
- C22C 38/22: with molybdenum or tungsten
- C22C 38/24: with vanadium
- C22C 38/26: with niobium or tantalum
- C22C 38/28: with titanium or zirconium
- C22C 38/32: with boron
- C22C 38/38: with more than 1.5% by weight of manganese
- C21D 8/0226: {Hot rolling}
- C21D 2211/002: Bainite
- C21D 2211/004: Dispersions; Precipitations
- C21D 2211/005: Ferrite
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)
- B32B: LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- C22C: ALLOYS (treatment of alloys C21D, C22F)

## Abstract

A hot-rolled high-strength steel strip or sheet with excellent roll-forming characteristics and excellent stretch-flange formability suitable for automotive chassis part or the like and, more particularly, to a high-strength steel strip or sheet with tensile strength of 780 MPa or higher, or preferably 950 MPa or higher, with an excellent combination of total elongation, stretch-flange formability and fatigue resistance and to a method of manufacturing the steel strip or sheet, and to the use of the strip or sheet in a part.

## Claims

### Claim 1

A high-strength hot-rolled steel strip or sheet with a tensile strength of 780 MPa or higher with continuous yielding behaviour, consisting of (in wt %):
between 0.08 and 0.14% C.;
between 1.0 and 2.0% Mn;
between 0.05 and 0.45% Mo;
between 0.10 and 0.40% V;
optionally one or more of
at least 0.1 and/or at most 1.2% Cr;
at least 0.01 and/or at most 0.14% Ti;
at least 0.05 and at most 0.5% Si;
at least 0.01 and at most 0.10% Nb;
at most 0.04% P;
at most 0.008% S;
at most 0.1% Al_sol;
at most 0.020% N;
at most 0.003% B;
optionally Ca in an amount consistent with a calcium treatment for inclusion control;
balance Fe and inevitable impurities;
wherein the steel strip or sheet has a microstructure that contains a matrix consisting of a mixture of at most 60 vol. % polygonal ferrite (PF) and at least 40 vol. % bainitic ferrite (BF) and between at least 0.8 vol. % and at most 5 vol. % of Σ(martensite and retained-austenite), wherein the PF and BF is precipitation-strengthened with fine composite carbides and/or carbo-nitrides of V and/or of Mo and optionally of Ti and/or of Nb.

### Claim 2 (depends on 1)

The steel strip or sheet according to claim 1 having a tensile strength of 950 MPa or higher.

### Claim 3 (depends on 1)

The steel strip or sheet according to claim 1, wherein the steel comprises (in wt. %):
at least 0.15% Mo, and/or
at least 0.20% Cr, and/or
at least 0.02% Nb.

### Claim 4 (depends on 1)

The steel strip or sheet according to claim 1, wherein the maximum Cr content is 1.0%.

### Claim 5 (depends on 1)

The steel strip or sheet according to claim 1, wherein the sum of the intensity of the misorientation angles between 20° and 50° in a normalised MisOrientation angle Distribution (MOD) profile of the microstructure as measured with the Electron BackScatter Diffraction technique is at most 0.50.

### Claim 6 (depends on 5)

The steel strip or sheet according to claim 5, wherein the MisOrientation angle Distribution (MOD) index of the microstructure as measured with the Electron BackScatter Diffraction technique is at least 0.45.

### Claim 7 (depends on 1)

The steel strip or sheet according to claim 1, wherein Ti is at least 0.01%, and
0.2
≤
(
(
Mo
96
)
(
Nb
93
)
+
(
Ti
*
48
)
+
(
V
51
)
)
≤
2.0
⁢
,
⁢
and
0.7
≤
(
(
Nb
93
)
+
(
Ti
*
48
)
+
(
V
51
)
+
(
Mo
96
)
(
C
12
)
)
≤
2.2
,
⁢
with
⁢
⁢
Ti
*
=
Ti
total
-
(
(
N
14
×
48
)
+
(
S
32
×
48
)
)
.

### Claim 8 (depends on 1)

The steel strip or sheet according to claim 1, wherein Ti is at most 0.005%, and
Al
sol
-
(
N
14
×
27
)
≥
0
,
and
⁢
0.2
≤
(
(
Mo
96
)
(
Nb
93
)
+
(
V
51
)
)
≤
2.0
,
and
1.0
≤
(
(
Nb
93
)
+
(
V
51
)
+
(
Mo
96
)
(
C
12
)
)
≤
2.2
.

### Claim 9 (depends on 1)

The steel strip or sheet according to claim 1, wherein Ti is at most 0.005%, and
Al
sol
-
(
N
14
×
27
)
<
0
,
and
0.2
≤
(
(
Mo
96
)
(
X
51
)
)
≤
2.0
,
and
1.0
≤
(
(
X
51
)
+
(
Mo
96
)
(
C
12
)
)
≤
2.2
,
⁢
with
⁢
⁢
X
=
V
+
(
Nb
93
×
51
)
-
(
N
*
14
×
51
)
⁢
⁢
and
⁢
N
*
=
N
-
(
Al
sol
27
×
14
)
.

### Claim 10 (depends on 1)

The steel strip or sheet according to claim 1
with a yield ratio (YR) of at least 0.7 and a hole-expansion ratio (λ) of 40% or higher,
and wherein the tensile strength (TS), total elongation (El), and sheet thickness t (mm) satisfy the equation of (TS×El)/t0.2>12000.

### Claim 11

A coated high-strength steel strip or sheet consisting of:
a zinc-based coating on an upper and/or lower surface of:
a strip or sheet with a tensile strength of 780 MPa or higher with continuous yielding behaviour, consisting of (in wt %):
between 0.08 and 0.14% C;
between 1.0 and 2.0% Mn;
between 0.05 and 0.45% Mo;
between 0.10 and 0.40% V;
optionally one or more of
at least 0.1 and/or at most 1.2% Cr;
at least 0.01 and/or at most 0.14% Ti;
at least 0.05 and at most 0.5% Si;
at least 0.01 and at most 0.10% Nb;
at most 0.04% P;
at most 0.008% S;
at most 0.1% Al_sol;
at most 0.020% N;
at most 0.003% B;
optionally Ca in an amount consistent with a calcium treatment for inclusion control;
balance Fe and inevitable impurities;
wherein the steel strip or sheet has a microstructure that contains a matrix consisting of a mixture of at most 60 vol. % polygonal ferrite (PF) and at least 40 vol. % bainitic ferrite (BF) and between at least 0.8 vol. % and at most 5 vol. % of Σ(martensite and retained-austenite), wherein the PF and BF is precipitation-strengthened with fine composite carbides and/or carbo-nitrides of V and/or of Mo and optionally of Ti and/or of Nb.

### Claim 12 (depends on 1)

The method of manufacturing the high strength hot-rolled steel strip or sheet according to claim 1 with tensile strength of 780 MPa or higher with continuous yielding behaviour, the method comprising the step of casting a thick or thin slab, consisting of (in wt %):
between 0.08 and 0.14% C;
between 1.0 and 2.0% Mn;
between 0.05 and 0.45% Mo;
between 0.10 and 0.40% V;
optionally one or more of
at least 0.1 and/or at most 1.2% Cr;
at least 0.01 and/or at most 0.14% Ti;
at least 0.05 and at most 0.5% Si;
at least 0.01 and at most 0.10% Nb;
at most 0.04% P;
at most 0.008% S;
at most 0.1% Al_sol;
at most 0.020% N;
at most 0.003% B;
optionally Ca in an amount consistent with a calcium treatment for inclusion control;
balance Fe and inevitable impurities;
followed by the step of reheating the solidified slab to a temperature of between 1050 and 1250° C., hot rolling the steel slab and finishing said hot rolling at a finish hot rolling temperature at the Ar3-temperature or higher, cooling the hot-rolled steel strip or sheet to the coiling temperature at an average cooling rate of at least 10° C./s and at most 150° C./s, and coiling the hot rolled steel strip or sheet in the temperature range between 450 and 620° C. to produce a microstructure that consists of a matrix consisting of a mixture of at most 60 vol. % polygonal ferrite (PF) and at least 40 vol. % bainitic ferrite (BF) and between at least 0.8 vol. % and at most 5 vol. % of Σ(martensite and retained-austenite), wherein the PF and BF is precipitation-strengthened with fine composite carbides and/or carbo-nitrides of V and/or of Mo and optionally of Ti and/or of Nb.

### Claim 13 (depends on 12)

The method according to claim 12, wherein the coiled hot-rolled steel strip or sheet is subjected to cooling by immersing the coil into a water basin or by actively cooling the coil with a spray of water.

### Claim 14 (depends on 1)

The part produced from the high-strength hot-rolled steel strip or sheet according to claim 1, wherein the steel strip has a YR of at least 0.7 and a λ of 40% or higher and wherein tensile strength (TS), total elongation (El), and sheet thickness t (mm) satisfy the equation of (TS×El)/t0.2>12000.

### Claim 15 (depends on 1)

The steel strip or sheet according to claim 1, wherein the MisOrientation angle Distribution (MOD) index of the microstructure as measured with the Electron BackScatter Diffraction technique is at least 0.45.

### Claim 16 (depends on 1)

The steel strip or sheet according to claim 1, with a YR of at least 0.8 and a λ of 40% or higher, and wherein the tensile strength (TS), total elongation (El), and sheet thickness t (mm) satisfy the equation of (TS×El)/t0.2>12000.

### Claim 17 (depends on 1)

The steel strip or sheet according to claim 1, with a YR of at least 0.7 and a λ of 50% or higher, and wherein the tensile strength (TS), total elongation (El), and sheet thickness t (mm) satisfy the equation of (TS×El)/t0.2>12000.

### Claim 18 (depends on 1)

The steel strip or sheet according to claim 1, with a YR of at least 0.8 and a λ of 50% or higher, and wherein the tensile strength (TS), total elongation (El), and sheet thickness t (mm) satisfy the equation of (TS×El)/t0.2>12000.

### Claim 19 (depends on 1)

The steel strip or sheet according to claim 1, with a YR of at least 0.7 and a λ of 60% or higher, and wherein the tensile strength (TS), total elongation (El), and sheet thickness t (mm) satisfy the equation of (TS×El)/t0.2>12000.

### Claim 20 (depends on 1)

The steel strip or sheet according to claim 1, with a YR of at least 0.8 and a λ of 60% or higher, and wherein the tensile strength (TS), total elongation (El), and sheet thickness t (mm) satisfy the equation of (TS×El)/t0.2>12000.