# US 2019/0316218 A1 · Steel material for welded steel pipe, having excellent longitudinal uniform elongation, manufacturing method therefor, and steel pipe using same

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- Number: US 2019/0316218 A1
- Document ID: US20190316218A1
- Status: Published application
- Kind: A1
- Application number: 16/472,556
- Filed: 2017-12-07
- Published: 2019-10-17
- Earliest claimed: 2016-12-23
- Assignees: Posco
- Applicants: Posco
- Inventors: Hwan-Gyo JUNG
- Web page: https://patentgrep.com/patent/US20190316218A1

## CPC

- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 9/08: for tubular bodies or pipes
- C21D 9/46: for sheet metals
- C22C 38/00: Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00)
- C22C 38/02: containing silicon
- C22C 38/06: containing aluminium
- C22C 38/12: containing tungsten, tantalum, molybdenum, vanadium, or niobium
- C22C 38/14: containing titanium or zirconium
- C22C 38/16: containing copper
- C22C 38/44: with molybdenum or tungsten
- C22C 38/48: with niobium or tantalum
- C22C 38/58: with more than 1.5% by weight of manganese
- 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)
- C22C: ALLOYS (treatment of alloys C21D, C22F)

## Abstract

The present invention relates to a steel material used for a line pipe for transporting crude oil or natural gas and the like and, more specifically, to a steel material for a welded steel pipe, having excellent longitudinal uniform elongation for the pipe, a manufacturing method therefor, and a steel pipe using the same.

## Claims

### Claim 1

A steel material for a welded steel pipe having excellent longitudinal uniform elongation, the steel material comprising, by wt %, carbon (C): 0.02 to 0.07%, silicon (Si): 0.05 to 0.3%, manganese (Mn): 0.8 to 1.8%, aluminum (Al): 0.005 to 0.05%, nitrogen (N): 0.001 to 0.01%, phosphorus (P): 0.020% or less, sulfur (S): 0.003% or less, nickel (Ni): 0.05 to 0.3%, chromium (Cr): 0.05 to 0.5%, niobium (Nb): 0.01 to 0.1%, and a balance of iron (Fe) and inevitable impurities,
wherein 20 to 50% of polygonal ferrite by area fraction, a low-temperature transformation phase, and a second phase are included as a microstructure, and
the low-temperature transformation phase is acicular ferrite and bainite.

### Claim 2 (depends on 1)

The steel material of claim 1, further comprising at least one selected from the group consisting of, by wt %, molybdenum (Mo): 0.05 to 0.3%, titanium (Ti): 0.005 to 0.02%, copper (Cu): 0.3% or less, vanadium (V): 0.01 to 0.07%, and calcium (Ca): 0.0005 to 0.005%.

### Claim 3 (depends on 1)

The steel material of claim 1, which includes 20 to 40% of the acicular ferrite by area fraction.

### Claim 4 (depends on 1)

The steel material of claim 1, wherein 5% or less, including 0%, of the second phase is included by area fraction, and the second phase is at least one of martensite-austenite constituent, degenerated pearlite, and cementite.

### Claim 5 (depends on 1)

The steel material of claim 1, which has uniform elongation of 8% or more and yield strength of 600 MPa or less.

### Claim 6 (depends on 1)

A welded steel pipe, having excellent longitudinal uniform elongation, obtained by pipe making and welding a steel material for a welded steel pipe of claim 1.

### Claim 7

A method of manufacturing a steel material for a welded steel pipe having excellent longitudinal uniform elongation, the method comprising:
reheating a steel slab within a temperature range from 1100 to 1200° C., the steel slab including, by wt %, carbon (C): 0.02 to 0.07%, silicon (Si): 0.05 to 0.3%, manganese (Mn): 0.8 to 1.8%, aluminum (Al): 0.005 to 0.05%, nitrogen (N): 0.001 to 0.01%, phosphorus (P): 0.020% or less, sulfur (S): 0.003% or less, nickel (Ni): 0.05 to 0.3%, chromium (Cr): 0.05 to 0.5%, niobium (Nb): 0.01 to 0.1%, and a balance of iron (Fe) and inevitable impurities;
terminating finishing rolling of the reheated steel slab within a temperature range from Ar3 to 900° C. to manufacture a hot-rolled steel plate;
primarily cooling the hot-rolled steel plate to Bs or higher at a cooling rate of 2 to 15° C./s;
secondarily cooling the hot-rolled steel plate to a temperature of 350 to 500° C. at a cooling rate of 20 to 50° C./s after the primarily cooling; and
air-cooling the hot-rolled steel plate to a room temperature after the secondarily cooling.

### Claim 8 (depends on 7)

The method of claim 7, wherein the steel slab further includes at least one selected from the group consisting of, by wt %, molybdenum (Mo): 0.05 to 0.3%, titanium (Ti): 0.005 to 0.02%, copper (Cu): 0.3% or less, vanadium (V): 0.01 to 0.07%, and calcium (Ca): 0.0005 to 0.005%.

### Claim 9 (depends on 7)

The method of claim 7, wherein the finishing rolling is started at a temperature of 980° C. or less and is performed at a total reduction rate of 60% or more.

### Claim 10 (depends on 7)

The method of claim 7, wherein the primarily cooling is started at a temperature of Ar3-20° C. or higher.