High toughness and high tensile strength thick steel plate with excellent material homogeneity and production method for same

Patent metadata

Field	Value
Publication number	US-10351926-B2
Application number	US-201515525672-A
Country	US
Kind code	B2
Filing date	Nov 17, 2015
Priority date	Nov 18, 2014
Publication date	Jul 16, 2019
Grant date	Jul 16, 2019

Abstract

A thick steel plate is provided by heating a continuously-cast slab, hot forging the continuously-cast slab using opposing dies having respective short sides differing such that when a short side length of a die having a shorter one of the short sides is taken to be 1, a short side length of a die having a longer one of the short sides is 1.1 to 3.0, allowing cooling to obtain a steel raw material, reheating the steel raw material, performing hot rolling of the steel raw material including at least two passes carried out, allowing cooling to obtain a thick steel plate, reheating the thick steel plate to at least the Ac3 temperature and no higher than 1050° C., rapidly cooling the thick steel plate to 350° C. or lower, and tempering the thick steel plate at at least 550° C. and no higher than 700° C.

First claim

The invention claimed is:

1. A thick steel plate having a plate thickness of 100 mm or more, having a chemical composition containing, in mass %,
C: 0.08% to 0.20%,
Si: 0.40% or less,
Mn: 0.5% to 5.0%,
P: 0.015% or less,
S: 0.0050% or less,
Ni: 5.0% or less,
Ti: 0.005% to 0.020%,
Al: 0.080% or less,
N: 0.0070% or less,
B: 0.0030% or less, and
one or more selected from
Cu: 0.50% or less,
Cr: 3.0% or less,
Mo: 1.50% or less,
V: 0.200% or less, and
Nb: 0.100% or less,
the balance being Fe and incidental impurities, wherein
a value Ceq IIW defined by formula (1) below is 0.55 to 0.80:
Ceq IIW =C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1)

where each element symbol indicates content, in mass %, of a corresponding element in the chemical composition and is taken to be 0 when the corresponding element is not contained,
a mid-thickness part of the steel plate has a yield strength of 500 MPa or more,
reduction of area in the mid-thickness part by tension in a plate thickness direction is 40% or more, and
the mid-thickness part has a low-temperature toughness at −60° C. of 70 J or more.

2. The thick steel plate of claim 1 , wherein
the chemical composition further contains, in mass %, one or more selected from
Mg: 0.0005% to 0.0100%,
Ta: 0.01% to 0.20%,
Zr: 0.005% to 0.1%,
Y: 0.001% to 0.01%,
Ca: 0.0005% to 0.0050%, and
REM: 0.0005% to 0.0200%.

3. The thick steel plate of claim 2 , wherein
in a hardness distribution in the plate thickness direction, a difference ΔHV between average hardness of a plate thickness surface (HVS) and average hardness of the mid-thickness part (HVC), where ΔHV =HVS −HVC, is 30 or less.

4. The thick steel plate of claim 1 , wherein
in a hardness distribution in the plate thickness direction, a difference ΔHV between average hardness of a plate thickness surface (HVS) and average hardness of the mid-thickness part (HVC), where ΔHV=HVS−HVC, is 30 or less.

5. A method for producing the thick steel plate of claim 1 , comprising
heating a continuously-cast slab having the chemical composition containing, in mass %,
C: 0.08% to 0.20%,
Si: 0.40% or less,
Mn: 0.5% to 5.0%,
P: 0.015% or less,
S: 0.0050% or less,
Ni: 5.0% or less,
Ti: 0.005% to 0.020%,
Al: 0.080% or less,
N: 0.0070% or less,
B: 0.0030% or less, and
one or more selected from
Cu: 0.50% or less,
Cr: 3.0% or less,
Mo: 1.50% or less,
V: 0.200% or less, and
Nb: 0.100% or less,
the balance being Fe and incidental impurities, to at least 1200° C. and no higher than 1350° C.,
then hot forging the continuously-cast slab under conditions of a temperature of 1000° C. or higher, a strain rate of 3/s or less, and a cumulative working reduction of 15% or more using opposing dies having respective short sides differing such that when a short side length of a die having a shorter one of the short sides is taken to be 1, a short side length of a die having a longer one of the short sides is 1.1 to 3.0,
then allowing cooling to obtain a steel raw material,
then reheating the steel raw material to at least an Ac 3 temperature and no higher than 1250° C.,
then performing hot rolling of the steel raw material including at least two passes carried out with a rolling reduction of 4% or more per pass,
then allowing cooling to obtain a thick steel plate,
then reheating the thick steel plate to at least the Ac 3 temperature and no higher than 1050° C.,
then rapidly cooling the thick steel plate to 350° C. or lower, and
then tempering the thick steel plate at at least 550° C. and no higher 700° C.

6. The method of claim 5 , wherein
a working reduction ratio from the continuously-cast slab prior to working to the thick steel plate obtained after the hot rolling in production of the high toughness and high tensile strength thick steel plate is 3 or less.

7. The method of claim 5 , wherein
the chemical composition further contains, in mass %, one or more selected from
Mg: 0.0005% to 0.0100%,
Ta: 0.01% to 0.20%,
Zr: 0.005% to 0.1%,
Y: 0.001% to 0.01%,
Ca: 0.0005% to 0.0050%, and
REM: 0.0005% to 0.0200%.

8. The method of claim 7 , wherein
a working reduction ratio from the continuously-cast slab prior to working to the thick steel plate obtained after the hot rolling in production of the high toughness and high tensile strength thick steel plate is 3 or less.

9. The method of claim 5 , wherein
in a hardness distribution in the plate thickness direction, a difference ΔHV between average hardness of a plate thickness surface (HVS) and average hardness of the mid-thickness part (HVC), where ΔHV =HVS −HVC, is 30 or less.

10. The method of claim 9 , wherein
a working reduction ratio from the continuously-cast slab prior to working to the thick steel plate obtained after the hot rolling in production of the high toughness and high tensile strength thick steel plate is 3 or less.

Assignees

• Jfe Steel Corp

Inventors

• Kuronuma Yota
• Ohtsubo Hirofumi
• Kitsuya Shigeki
• Ichimiya Katsuyuki
• Hase Kazukuni

Classifications

• Y10T428/12458

  having composition, density, or hardness gradient · CPC title

• C22C38/58

  with more than 1.5% by weight of manganese · CPC title

• C22C38/54

  with boron · CPC title

• C22C38/50

  with titanium or zirconium · CPC title

• C22C38/48

  with niobium or tantalum · CPC title