Hot rolled steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-10047416-B2
Application number	US-201314427822-A
Country	US
Kind code	B2
Filing date	Sep 11, 2013
Priority date	Sep 13, 2012
Publication date	Aug 14, 2018
Grant date	Aug 14, 2018

Abstract

A high strength hot rolled steel sheet is provided with low yield ratio and that is excellent in low-temperature toughness. The hot rolled steel sheet can be used as a raw material of a steel pipe. The steel sheet has a chemical composition containing C: 0.03% or more and 0.10% or less, Si: 0.01% or more and 0.50% or less, Mn: 1.4% or more and 2.2% or less, P: 0.025% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, Nb: 0.02% or more and 0.10% or less, Ti: 0.001% or more and 0.030% or less, Mo: 0.01% or more and 0.50% or less, Cr: 0.01% or more and 0.50% or less, and Ni: 0.01% or more and 0.50% or less, in which the condition that Moeq is 1.4% or more and 2.2% or less.

First claim

The invention claimed is:

1. A hot rolled steel sheet having a chemical composition comprising, by mass %:
C: 0.03% or more and 0.10% or less;
Si: 0.01% or more and 0.50% or less;
Mn: 1.4% or more and 2.2% or less;
P: 0.025% or less;
S: 0.005% or less;
Al: 0.005% or more and 0.10% or less;
Nb: 0.02% or more and 0.10% or less;
Ti: 0.001% or more and 0.030% or less;
Mo: 0.01% or more and 0.50% or less;
Cr: 0.01% or more and 0.50% or less;
Ni: 0.01% or more and 0.50% or less; and
the balance being Fe and inevitable impurities,
the hot rolled steel sheet comprising:
a surface layer having a microstructure that includes (i) a bainitic ferrite phase or (ii) the bainitic ferrite phase and a tempered martensitic phase, a lath thickness of the bainitic ferrite phase of the surface layer being 0.2 μm or more and 1.6 μm or less, and
an inner layer having a microstructure that includes bainitic ferrite phase as a main phase and, in terms of area fraction, 1.4% or more and 15% or less of a massive martensitic phase having an aspect ratio of less than 5.0 as a second phase, a lath thickness of the bainitic ferrite phase of the inner layer being in a range of 0.2 μm or more and 1.6 μm or less.

2. The hot rolled steel sheet according to claim 1 , wherein the chemical composition has an Moeq value in a range of 1.4% or more and 2.2% or less, the Moeq value being defined by the following formula (1),
Moeq (%)=Mo+0.36Cr+0.77Mn+0.07Ni  (1),

where Mn, Ni, Cr, and Mo respectively represent the contents of the corresponding chemical elements by mass %.

3. The hot rolled steel sheet according to claim 1 , wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of Cu: 0.50% or less, V: 0.10% or less, and B: 0.0005% or less.

4. The hot rolled steel sheet according to claim 1 , wherein the chemical composition further comprises, by mass %, Ca: 0.0005% or more and 0.0050% or less.

5. The hot rolled steel sheet according to claim 1 , wherein a maximum grain diameter of the massive martensitic phase is 5.0 μm or less and an average grain diameter of the massive martensitic phase is in a range of 0.5 μm or more and 3.0 μm or less.

6. The hot rolled steel sheet according to claim 1 , wherein the surface layer includes the tempered martensitic phase and an average grain diameter of the tempered martensitic phase in the surface layer is 3.0 μm or less and a maximum grain diameter of the tempered martensitic phase in the surface layer is 4.0 μm or less.

7. The hot rolled steel sheet according to claim 1 , wherein the surface layer is defined by a region that is within a depth of less than 2 mm in a thickness direction from an upper surface or a lower surface of a steel sheet.

8. The hot rolled steel sheet according to claim 3 , wherein the chemical composition further comprises, by mass %, Ca: 0.0005% or more and 0.0050% or less.

Assignees

• Jfe Steel Corp

Inventors

• Kami Chikara
• Goto Sota

Classifications

• C22C38/50

  with titanium or zirconium · CPC title

• C22C38/42

  with copper · CPC title

• C22C38/04

  containing manganese · CPC title

• C21D6/008

  containing Si · CPC title

• C22C38/58 Primary

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