STEEL PLATE THAT EXHIBITS EXCELLENT LOW-TEMPERATURE TOUGHNESS IN BASE MATERIAL AND WELD HEAT-AFFECTED ZONE AND HAS SMALL STRENGTH ANISOTROPY, AND MANUFACTURING METHOD THEREOF

• US 20110036469A1
• Filed: 10/01/2009
• Published: 02/17/2011
• Est. Priority Date: 10/01/2008
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel Corporation
• Inventors: Saitoh, Naoki, Furuya, Hitoshi, Okushima, Motohiro, Takahashi, Yasunori
• Granted Patent: US 7,967,923 B2
• Time in Patent Office: Days
• US Class Current: 148/645
• CPC Class Codes: C21D 2211/001 Austenite
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0263 following hot rolling
• CPC Class Codes: C21D 9/50 for welded joints
• CPC Class Codes: C22C 38/001 containing N
• 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 38/44 with molybdenum or tungsten
• CPC Class Codes: C22C 38/46 with vanadium

Abstract

The present invention provides a steel plate that exhibits excellent low-temperature toughness in a base material and a weld heat-affected zone and has small strength anisotropy, wherein the steel includes, by mass, C: 0.04%-0.10%; Si: 0.02%-0.40%; Mn: 0.5%-1.0%; P: 0.0010%-0.0100%; S: 0.0001%-0.0050%; Ni: 2.0%-4.5%; Cr: 0.1%-1.0%; Mo: 0.1%-0.6%; V: 0.005%-0.1%; Al: 0.01%-0.08%; and N: 0.0001%-0.0070%, with the balance including Fe and inevitable impurities, a Ni segregation ratio at a portion located at one-fourth of a thickness of the steel plate in a steel-plate thickness direction from a surface of the steel plate is 1.3 or lower, a degree of flatness of a prior austenite grain is in a range from 1.05 to 3.0, an effective diameter of crystal grain is 10 μm or lower, and a Vickers hardness number is in a range of 265 HV to 310 HV.

Claims

1. A steel plate that exhibits excellent low-temperature toughness in a base material and a weld heat-affected zone and has small strength anisotropy, wherein

the steel plate includes, by mass,

C:

0.04%-0.10%;

Si;

0.02%-0.40%;

Mn;

0.5%-1.0%;

P;

0.0010%-0.0100%;

S;

0.0001%-0.0050%;

Ni;

2.0%-4.5%;

Cr;

0.1%-1.0%;

Mo;

0.1%-0.6%;

V;

0.005%-0.1%;

Al;

0.01%-0.08%; and

N;

0.0001%-0.0070%,

with a balance including Fe and inevitable impurities,

a Ni segregation ratio at a portion located at one-fourth of a thickness of the steel plate in a steel-plate thickness direction from a surface of the steel plate is 1.3 or lower,

a degree of flatness of a prior austenite grain is in a range from 1.05 to 3.0,

an effective diameter of crystal grain is 10 μ

m or lower, and

a Vickers hardness number is in a range of 265 HV to 310 HV.

2. The steel plate that exhibits excellent low-temperature toughness in the base material and the weld heat-affected zone and has small strength anisotropy according to claim 1, wherein

the steel plate further includes at least one or two components of, by mass,

Nb:

0.005%-0.03%;

Ti;

0.005%-0.03%;

Cu;

0.01%-0.7%%;

B;

0.0002%-0.05%;

Ca;

0.0002%-0.0040%; and

REM;

0.0002%40.0040%,

with a balance including Fe and inevitable impurities.

3. A manufacturing method of a steel plate that exhibits excellent low-temperature toughness in a base material and a weld heat-affected zone and has small strength anisotropy, the steel plate including, by mass,

C:

0.04%-0.10%;

Si;

0.02%-0.40%;

Mn;

0.5%-1.0%;

P;

0.0010%-0.0100%;

S;

0.0001%-0.0050%;

Ni;

2.0%-4.5%;

Cr;

0.1%-1.0%;

Mo;

0.1%-0.6%;

V;

0.005%-0.1%;

Al;

0.01%-0.08%; and

N;

0.0001%-0.0070%,

with a balance including Fe and inevitable impurities, wherein the method includes;

heating a casting slab having a thickness 5.5 times to 50 times thicker than a final plate thickness, to a temperature ranging from 1250°

C. to 1380°

C., and maintaining the temperature for eight hours or more;

applying a first hot rolling to the casting slab at a reduction ratio of 1.2 to 10.0, and a temperature before a final rolling pass of 800°

C. to 1250°

C. to obtain a steel strip;

air-cooling the steel strip to 300°

C. or lower, and then heating the steel strip to a temperature ranging from 900°

C. to 1270°

C.;

applying a second hot rolling to the steel strip at a reduction ratio of 2.0 to 40.0, and a temperature before a final rolling pass of 680°

C. to 1000°

C.;

starting water-cooling within 100 seconds after the second hot rolling, and cooling the steel strip to a surface temperature of 200°

C. or lower; and

,

applying tempering to the steel strip at a temperature of 550°

C. to 720°

C.

4. The manufacturing method of the steel plate that exhibits excellent low-temperature toughness in the base material and the weld heat-affected zone and has small strength anisotropy according to claim 3, the steel plate further including at least one or two components of, by mass,

Nb:

0.005%-0.03%;

Ti;

0.005%-0.03%;

Cu;

0.01%-0.7%%;

B;

0.0002%-0.05%;

Ca;

0.0002%-0.0040%; and

REM;

0.0002%-0.0040%,

with a balance including Fe and inevitable impurities.