HIGH STRENGTH STEEL PLATE HAVING LOW YIELD RATIO EXCELLENT IN TERMS OF STRAIN AGEING RESISTANCE, METHOD OF MANUFACTURING THE SAME AND HIGH STRENGTH WELDED STEEL PIPE MADE OF THE SAME

• US 20150090370A1
• Filed: 03/29/2013
• Published: 04/02/2015
• Est. Priority Date: 03/29/2012
• Status: Active Grant

• Current Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Original Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Inventors: Shimamura, Junji, Nishimura, Kimihiro
• Granted Patent: US 10,358,708 B2
• Time in Patent Office: Days
• US Class Current: 148/521
• CPC Class Codes: B23K 2101/06 Tubes
• CPC Class Codes: B23K 2101/10 Pipe-lines
• CPC Class Codes: B23K 2103/04 Steel or steel alloys
• CPC Class Codes: B23K 31/027 Making tubes with soldering or welding
• CPC Class Codes: B23K 9/025 for rectilinear seams
• CPC Class Codes: B23K 9/18 Submerged-arc welding
• CPC Class Codes: B23K 9/23 taking account of the properties of the materials to be welded
• CPC Class Codes: C21D 2211/002 Bainite
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 8/02 during manufacturing of plates or strips C21D8/12 takes precedence
• CPC Class Codes: C21D 8/0263 following hot rolling
• CPC Class Codes: C21D 8/105 of ferrous alloys
• CPC Class Codes: C21D 9/08 for tubular bodies or pipes
• CPC Class Codes: C21D 9/46 for sheet metals
• CPC Class Codes: C21D 9/50 for welded joints
• CPC Class Codes: C22C 38/001 containing N
• CPC Class Codes: C22C 38/002 containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: C22C 38/08 : containing nickel C22C38/105 takes precedence
• CPC Class Codes: C22C 38/12 : containing tungsten, tantalum, molybdenum, vanadium, or niobium
• CPC Class Codes: C22C 38/14 : containing titanium or zirconium
• CPC Class Codes: C22C 38/16 : containing copper
• CPC Class Codes: C22C 38/18 : containing chromium
• CPC Class Codes: C22C 38/26 : with niobium or tantalum
• CPC Class Codes: C22C 38/28 : with titanium or zirconium
• CPC Class Codes: C22C 38/38 : with more than 1.5% by weight of manganese
• CPC Class Codes: C22C 38/40 : with nickel
• CPC Class Codes: C22C 38/42 : with copper
• CPC Class Codes: C22C 38/44 : with molybdenum or tungsten
• CPC Class Codes: C22C 38/46 : with vanadium
• CPC Class Codes: C22C 38/48 : with niobium or tantalum
• CPC Class Codes: C22C 38/50 : with titanium or zirconium
• CPC Class Codes: C22C 38/58 : with more than 1.5% by weight of manganese

Abstract

A steel plate has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.2% or more and 3.0% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.08% or less, Nb: 0.005% or more and 0.07% or less, Ti: 0.005% or more and 0.025% or less, N: 0.010% or less, O: 0.005% or less and the balance being Fe and inevitable impurities, a structure being a dual-phase structure consisting of a bainite phase and island martensite, wherein the area fraction of the island martensite is 3% to 15%, the equivalent circle diameter of the island martensite is 3.0 μm or less, and the remainder of the structure is a bainite phase.

Claims

1. A high strength steel plate having a low yield ratio, the steel plate having a chemical composition containing, by mass %, C:

0.03% or more and 0.08% or less, Si;

0.01% or more and 1.0% or less, Mn;

1.2% or more and 3.0% or less, P;

0.015% or less, S;

0.005% or less, Al;

0.08% or less, Nb;

0.005% or more and 0.07% or less, Ti;

0.005% or more and 0.025% or less, N;

0.010% or less, O;

0.005% or less and the balance being Fe and inevitable impurities, a metallographic structure being a dual-phase structure consisting of a bainite phase and island martensite, wherein an area fraction of island martensite is 3% or more and 15% or less, wherein an equivalent circle diameter of the island martensite is 3.0 μ

m or less, and wherein the remainder of the metallographic structure is a bainite phase, a hardness variation in a thickness direction of Δ

HV30 or less in terms of Vickers hardness, a hardness variation in a width direction of Δ

HV30 or less in terms of Vickers hardness, a maximum hardness in surface portions of the steel plate of HV230 or less in terms of Vickers hardness and a yield ratio of 85% or less and an elongation of 22% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards.

2. The steel plate according to claim 1, wherein the chemical composition further comprises, by mass %, one or more selected from among Cu:

0.5% or less, Ni;

1% or less, Cr;

0.5% or less, Mo;

0.5% or less, V;

0.1% or less, Ca;

0.0005% or more and 0.003% or less and B;

0.005% or less.

3. A method of manufacturing a high strength steel plate having a low yield ratio comprising:

heating steel having the chemical composition according to claim 1 at a temperature of 1000°

C. or higher and 1300°

C. or lower,

performing hot rolling under conditions such that cumulative rolling reduction ratio is 50% or more in a temperature range of 900°

C. or lower,

finishing hot rolling at a temperature equal to or higher than the Ar₃ transformation point,

starting cooling at a temperature equal to or higher than the Ar₃ transformation point,

performing cooling at a cooling rate of 150°

C./sec. or less in terms of a surface temperature of the steel plate until the surface temperature becomes 600°

C. or lower,

performing cooling at a cooling rate of 15°

C./sec. or more in terms of the average temperature of the steel plate until the average temperature becomes 450°

C. or higher and 650°

C. or lower, and,

immediately after the cooling has been performed, performing reheating at a heating rate of 1.0°

C./sec. or more in terms of the surface temperature of the steel plate until the surface temperature becomes 550°

C. or higher and 750°

C. or lower.

4. A high strength welded steel pipe, the steel pipe being manufactured by forming the steel plate according to claim 1 into a cylindrical shape, by welding butted portions of the shaped cylinder under conditions such that single-layer welding is performed on each of the outer and inner surfaces, and by thereafter performing a pipe-expanding treatment under a condition of a pipe expanding ratio of 0.4% or more and 2.0% or less, and having a yield ratio of 90% or less and an elongation of 20% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards and, further having a yield ratio of 90% or less and an elongation of 20% or more even after a strain ageing treatment has been performed under conditions such that the temperature is 250°

C. or lower and the duration is 30 minutes or less.

5. A method of manufacturing a high strength steel plate having a low yield ratio comprising:

heating steel having the chemical composition according to claim 2 at a temperature of 1000°

C. or higher and 1300°

C. or lower,

performing hot rolling under conditions such that the cumulative rolling reduction ratio is 50% or more in a temperature range of 900°

C. or lower,

finishing hot rolling at a temperature equal to or higher than the Ar₃ transformation point,

starting cooling at a temperature equal to or higher than the Ar₃ transformation point,

performing cooling at a cooling rate of 150°

C./sec. or less in terms of a surface temperature of the steel plate until the surface temperature becomes 600°

C. or lower,

performing cooling at a cooling rate of 15°

C./sec. or more in terms of the average temperature of the steel plate until the average temperature becomes 450°

C. or higher and 650°

C. or lower, and,

immediately after the cooling has been performed, performing reheating at a heating rate of 1.0°

C./sec. or more in terms of the surface temperature of the steel plate until the surface temperature becomes 550°

C. or higher and 750°

C. or lower.

6. A high strength welded steel pipe, the steel pipe being manufactured by forming the steel plate according to claim 2 into a cylindrical shape, by welding butted portions of the shaped cylinder under conditions such that single-layer welding is performed on each of the outer and inner surfaces, and by thereafter performing a pipe-expanding treatment under a condition of a pipe expanding ratio of 0.4% or more and 2.0% or less, and having a yield ratio of 90% or less and an elongation of 20% or more in a full-thickness tensile test using a test piece having a shape in accordance with GOST standards, and, further having a yield ratio of 90% or less and an elongation of 20% or more even after a strain ageing treatment has been performed under conditions such that the temperature is 250°

C. or lower and the duration is 30 minutes or less.