# US 10,767,250 B2 · Thick steel plate for structural pipes or tubes, method of producing thick steel plate for structural pipes or tubes, and structural pipes and tubes

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- Number: US 10,767,250 B2
- Document ID: US10767250B2
- Status: Granted
- Kind: B2
- Application number: 15/560,613
- Filed: 2016-03-25
- Published: 2018-02-22
- Granted: 2020-09-08
- Earliest claimed: 2015-03-26
- Assignees: Jfe Steel Corporation
- Applicants: Jfe Steel Corporation
- Inventors: Junji Shimamura; Nobuyuki Ishikawa; Shigeru Endo; Shusaku Ota
- Web page: https://patentgrep.com/patent/US10767250B2

## CPC

- C22C 38/14: containing titanium or zirconium
- B23K 31/027: {Making tubes by soldering or welding}
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 8/0226: {Hot rolling}
- 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 38/42: with copper
- C22C 38/44: with molybdenum or tungsten
- C22C 38/46: with vanadium
- C22C 38/48: with niobium or tantalum
- C22C 38/54: with boron
- C22C 38/56: with more than 1.7% by weight of carbon
- C22C 38/58: with more than 1.5% by weight of manganese
- B23K 2101/06: Tubes
- B23K 2103/04: Steel {or steel} alloys
- C21D 2211/002: Bainite
- Y02P 10/20: Recycling
- C22C: ALLOYS (treatment of alloys C21D, C22F)
- B23K: SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- 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)
- Y02P: CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS

## Abstract

Disclosed is, as a high-strength steel plate of API X80 grade or higher with a thickness of 38 mm or more, a thick steel plate for structural pipes or tubes that exhibits high strength in the rolling direction, excellent Charpy properties at its mid-thickness part, and high material homogeneity without addition of large amounts of alloying elements. The thick steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure mainly composed of bainite; a tensile strength of 620 MPa or more; a Charpy absorption energy vE−20° C. at −20° C. at the mid-thickness part of 100 J or more; a variation of Vickers hardness in a plate thickness direction ΔHV10,t of 50 or less; and a variation of Vickers hardness in a plate widthwise direction ΔHV10,c of 50 or less.

## Claims

### Claim 1

A steel plate for structural pipes or tubes, comprising:
a chemical composition that contains, in mass %,
C: 0.030% to 0.100%,
Si: 0.01% to 0.50%,
Mn: 1.50% to 2.50%,
Al: 0.080% or less,
Mo: 0.05% to 0.50%,
Ti: 0.005% to 0.025%,
Nb: 0.005% to 0.080%,
N: 0.001% to 0.010%,
O: 0.005% or less,
P: 0.010% or less,
S: 0.0010% or less, and
a balance consisting of Fe and inevitable impurities, with the chemical composition having a carbon equivalent Ceq as defined by the following Expression (1) of 0.42 or more:
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1),
where each element symbol indicates content in mass % of the element in the steel plate and has a value of 0 if the element is not contained in the steel plate; and
a microstructure that is mainly composed of bainite, an area fraction of the bainite being 90% or more,
wherein
the steel plate has a plate thickness of 38 mm or more, and
the steel plate satisfies a set of conditions including:
a tensile strength being 705 MPa or more and 825 MPa or less;
a Charpy absorption energy vE−20° C. at 20° C. at its mid-thickness part being 100 J or more;
a variation in Vickers hardness in a plate thickness direction ΔHV10,t being 50 or less; and
a variation in Vickers hardness in a plate widthwise direction ΔHV10,c being 50 or less.

### Claim 2 (depends on 1)

The steel plate for structural pipes or tubes according to claim 1, wherein the chemical composition further contains, in mass %,
V: 0.005% to 0.100%.

### Claim 3 (depends on 2)

The steel plate for structural pipes or tubes according to claim 2, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Ca: 0.0005% to 0.0035%,
REM: 0.0005% to 0.0100%, and
B: 0.0020% or less.

### Claim 4 (depends on 3)

A method of producing a steel plate for structural pipes or tubes, comprising at least:
heating a steel raw material having the chemical composition as recited in claim 3 to a heating temperature of 1100° C. to 1300° C.;
hot-rolling the heated steel raw material, with a cumulative rolling reduction ratio at 850° C. or lower being set to 70% or more, to obtain a hot-rolled steel plate;
subjecting the hot-rolled steel plate to first cooling, whereby the hot-rolled steel plate is subjected to accelerated cooling under a set of conditions including, in terms of a surface temperature of the steel plate, a cooling start temperature Ts,1 being no lower than Ar3, a cooling end temperature Te,1 being 500° C. or lower, and an average cooling rate V1 being 20° C./s or higher and 100° C./s or lower, so that the following Expression (2) is satisfied:
3≤(700−Te,1)/V1 (2),
where units are ° C. for Te,1 and ° C./s for V1;
subjecting the steel plate to second cooling after the first cooling, whereby the steel plate is subjected to accelerated cooling under a set of conditions including, in terms of an average temperature of the steel plate, a cooling end temperature Te,2 being 500° C. or lower and an average cooling rate V2 being 5° C./s or higher, and
thereby producing the steel plate having a plate thickness of 38 mm or more,
the steel plate having a microstructure that is mainly composed of bainite, an area ratio of the bainite being 90% or more, and
the steel plate satisfying a set of conditions including:
a tensile strength being 705 MPa or more and 825 MPa or less;
a Charpy absorption energy vE−20° C. at −20° C. at its mid-thickness part being 100 J or more;
a variation in Vickers hardness in a plate thickness direction ΔHV10,t being 50 or less; and
a variation in Vickers hardness in a plate widthwise direction ΔHV10,c being 50 or less.

### Claim 5 (depends on 4)

The method producing a steel plate for structural pipes or tubes according to claim 4, further comprising, immediately after the second cooling, reheating the steel plate to a temperature range of 400° C. to 700° C. at a heating rate from 0.5° C./s to 10° C./s.

### Claim 6 (depends on 3)

A structural pipe or tube formed from the steel plate for structural pipes or tubes as recited in claim 3.

### Claim 7 (depends on 3)

A structural pipe or tube obtainable by forming the steel plate as recited in claim 3 into a tubular shape in its longitudinal direction, and then joining butting faces by welding from inside and outside to form at least one layer on each side along the longitudinal direction.

### Claim 8 (depends on 2)

A method of producing a steel plate for structural pipes or tubes, comprising at least:
heating a steel raw material having the chemical composition as recited in claim 2 to a heating temperature of 1100° C. to 1300° C.;
hot-rolling the heated steel raw material, with a cumulative rolling reduction ratio at 850° C. or lower being set to 70% or more, to obtain a hot-rolled steel plate;
subjecting the hot-rolled steel plate to first cooling, whereby the hot-rolled steel plate is subjected to accelerated cooling under a set of conditions including, in terms of a surface temperature of the steel plate, a cooling start temperature Ts,1 being no lower than Ar3, a cooling end temperature Te,1 being 500° C. or lower, and an average cooling rate V1 being 20° C./s or higher and 100° C./s or lower, so that the following Expression (2) is satisfied:
3≤(700−Te,1)/V1 (2),
where units are ° C. for Te,1 and ° C./s for V1;
subjecting the steel plate to second cooling after the first cooling, whereby the steel plate is subjected to accelerated cooling under a set of conditions including, in terms of an average temperature of the steel plate, a cooling end temperature Te,2 being 500° C. or lower and an average cooling rate V2 being 5° C./s or higher, and
thereby producing the steel plate having a plate thickness of 38 mm or more,
the steel plate having a microstructure that is mainly composed of bainite, an area ratio of the bainite being 90% or more, and
the steel plate satisfying a set of conditions including:
a tensile strength being 705 MPa or more and 825 MPa or less;
a Charpy absorption energy vE−20° C. at −20° C. at its mid-thickness part being 100 J or more;
a variation in Vickers hardness in a plate thickness direction ΔHV10,t being 50 or less; and
a variation in Vickers hardness in a plate widthwise direction ΔHV10,c being 50 or less.

### Claim 9 (depends on 8)

The method producing a steel plate for structural pipes or tubes according to claim 8, further comprising, immediately after the second cooling, reheating the steel plate to a temperature range of 400° C. to 700° C. at a heating rate from 0.5° C./s to 10° C./s.

### Claim 10 (depends on 2)

A structural pipe or tube formed from the steel plate for structural pipes or tubes as recited in claim 2.

### Claim 11 (depends on 2)

A structural pipe or tube obtainable by forming the steel plate as recited in claim 2 into a tubular shape in its longitudinal direction, and then joining butting faces by welding from inside and outside to form at least one layer on each side along the longitudinal direction.

### Claim 12 (depends on 1)

The steel plate for structural pipes or tubes according to claim 1, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
Cu: 0.50% or less,
Ni: 0.50% or less,
Cr: 0.50% or less,
Ca: 0.0005% to 0.0035%,
REM: 0.0005% to 0.0100%, and
B: 0.0020% or less.

### Claim 13 (depends on 12)

A method of producing a steel plate for structural pipes or tubes, comprising at least:
heating a steel raw material having the chemical composition as recited in claim 12 to a heating temperature of 1100° C. to 1300° C.;
hot-rolling the heated steel raw material, with a cumulative rolling reduction ratio at 850° C. or lower being set to 70% or more, to obtain a hot-rolled steel plate;
subjecting the hot-rolled steel plate to first cooling, whereby the hot-rolled steel plate is subjected to accelerated cooling under a set of conditions including, in terms of a surface temperature of the steel plate, a cooling start temperature Ts,1 being no lower than Ar3, a cooling end temperature Te,1 being 500° C. or lower, and an average cooling rate V1 being 20° C./s or higher and 100° C./s or lower, so that the following Expression (2) is satisfied:
3≤(700−Te,1)/V1 (2),
where units are ° C. for Te,1 and ° C./s for V1;
subjecting the steel plate to second cooling after the first cooling, whereby the steel plate is subjected to accelerated cooling under a set of conditions including, in terms of an average temperature of the steel plate, a cooling end temperature Te,2 being 500° C. or lower and an average cooling rate V2 being 5° C./s or higher, and
thereby producing the steel plate having a plate thickness of 38 mm or more,
the steel plate having a microstructure that is mainly composed of bainite, an area ratio of the bainite being 90% or more, and
the steel plate satisfying a set of conditions including:
a tensile strength being 705 MPa or more and 825 MPa or less;
a Charpy absorption energy vE−20° C. at −20° C. at its mid-thickness part being 100 J or more;
a variation in Vickers hardness in a plate thickness direction ΔHV10,t being 50 or less; and
a variation in Vickers hardness in a plate widthwise direction ΔHV10,c being 50 or less.

### Claim 14 (depends on 13)

The method producing a steel plate for structural pipes or tubes according to claim 13, further comprising, immediately after the second cooling, reheating the steel plate to a temperature range of 400° C. to 700° C. at a heating rate from 0.5° C./s to 10° C./s.

### Claim 15 (depends on 12)

A structural pipe or tube formed from the steel plate for structural pipes or tubes as recited in claim 12.

### Claim 16 (depends on 12)

A structural pipe or tube obtainable by forming the steel plate as recited in claim 12 into a tubular shape in its longitudinal direction, and then joining butting faces by welding from inside and outside to form at least one layer on each side along the longitudinal direction.

### Claim 17 (depends on 1)

A method of producing a steel plate for structural pipes or tubes, comprising at least:
heating a steel raw material having the chemical composition as recited in claim 1 to a heating temperature of 1100° C. to 1300° C.;
hot-rolling the heated steel raw material, with a cumulative rolling reduction ratio at 850° C. or lower being set to 70% or more, to obtain a hot-rolled steel plate;
subjecting the hot-rolled steel plate to first cooling, whereby the hot-rolled steel plate is subjected to accelerated cooling under a set of conditions including, in terms of a surface temperature of the steel plate, a cooling start temperature Ts,1 being no lower than Ar3, a cooling end temperature Te,1 being 500° C. or lower, and an average cooling rate V1 being 20° C./s or higher and 100° C./s or lower, so that the following Expression (2) is satisfied:
3≤(700−Te,1)/V1 (2),
where units are ° C. for Te,1 and ° C./s for V1;
subjecting the steel plate to second cooling after the first cooling, whereby the steel plate is subjected to accelerated cooling under a set of conditions including, in terms of an average temperature of the steel plate, a cooling end temperature Te,2 being 500° C. or lower and an average cooling rate V2 being 5° C./s or higher, and
thereby producing the steel plate having a plate thickness of 38 mm or more,
the steel plate having a microstructure that is mainly composed of bainite, an area ratio of the bainite being 90% or more, and
the steel plate satisfying a set of conditions including:
a tensile strength being 705 MPa or more and 825 MPa or less;
a Charpy absorption energy vE−20° C. at −20° C. at its mid-thickness part being 100 J or more;
a variation in Vickers hardness in a plate thickness direction ΔHV10,t being 50 or less; and
a variation in Vickers hardness in a plate widthwise direction ΔHV10,c being 50 or less.

### Claim 18 (depends on 17)

The method producing a steel plate for structural pipes or tubes according to claim 17, further comprising, immediately after the second cooling, reheating the steel plate to a temperature range of 400° C. to 700° C. at a heating rate from 0.5° C./s to 10° C./s.

### Claim 19 (depends on 1)

A structural pipe or tube formed from the steel plate for structural pipes or tubes as recited in claim 1.

### Claim 20 (depends on 1)

A structural pipe or tube obtainable by forming the steel plate as recited in claim 1 into a tubular shape in its longitudinal direction, and then joining butting faces by welding from inside and outside to form at least one layer on each side along the longitudinal direction.