CN117242201A - Steel sheet and method for producing same - Google Patents

Steel sheet and method for producing same Download PDF

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Publication number
CN117242201A
CN117242201A CN202280032170.0A CN202280032170A CN117242201A CN 117242201 A CN117242201 A CN 117242201A CN 202280032170 A CN202280032170 A CN 202280032170A CN 117242201 A CN117242201 A CN 117242201A
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steel sheet
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percent
pearlite
steel
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中井启介
伊藤实
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

A steel sheet, the chemical composition of the steel sheet is C:0.030 to 0.200 percent of Si:0.050 to 0.500%, mn:0.50 to 2.00 percent of P: less than 0.030%, S: less than 0.010%, al:0.001 to 0.100 percent, N:0.0005 to 0.0080 percent, O:0.0005 to 0.0080 percent of Ti:0.001 to 0.050 percent, nb:0.001 to 0.050 percent, cu:0.01 to 0.50 percent of Mo:0.01 to 0.10 percent of Sn:0.01 to 0.30 percent, the balance: fe and impurities, wherein the total content of solid-solution Mo and solid-solution Sn in the surface layer portion of the steel sheet is 0.005% or more, and the metallographic structure at the 1/4t position is pearlite: 5-30 percent of bainite: less than 10 percent, the balance: ferrite, the metallographic structure of the 1/10t position is pearlite: 1-20 percent of bainite: less than 5 percent, the balance: ferrite, wherein the average grain size of ferrite at the 1/10t position is 5-50 μm, and the average grain size of pearlite at the 1/10t position is 30 μm or less.

Description

Steel sheet and method for producing same
Technical Field
The present invention relates to a steel sheet and a method for manufacturing the same.
Background
Steel for welded structures, which is excellent in strength and weldability, is used in oil tanks for transporting or storing crude oil (hereinafter, these are collectively referred to as "crude oil tanks"), such as crude oil vessels, or above-ground or underground crude oil tanks. In addition, steel used as a crude oil tank is required to have excellent corrosion resistance against corrosive gas components, salts, and the like contained in crude oil (for example, see patent document 1).
Patent document 1 discloses a steel for a crude oil tank for a welded structure, which is excellent in suppression of general corrosion caused by uniform corrosion on a steel plate surface and localized corrosion caused by localized concentration on the steel plate surface, and further can suppress the generation of corrosion products (sludge) containing solid S, a method for producing the steel for a crude oil tank, and an anti-corrosion method for a crude oil tank.
Prior art literature
Patent literature
Patent document 1: japanese patent application laid-open No. 2004-204344
Disclosure of Invention
Problems to be solved by the invention
The steel for a crude oil tank described in patent document 1 contains Mo and W in a solid solution state of a predetermined amount or more, and therefore has excellent corrosion resistance. However, the results of the studies conducted by the present inventors have revealed that there is room for further improvement in corrosion resistance.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a steel sheet excellent in corrosion resistance against corrosive gas components, salts, and the like contained in crude oil, and a method for producing the same.
Solution for solving the problem
The present inventors have studied the above problems in detail and as a result, have obtained the following findings.
As a method for improving the corrosion resistance of the steel sheet, it is considered to contain Cu, sn, and Mo. However, in the case of steel containing these elements, when the pearlite structure and the bainite structure are included as a mixed structure of ferrite and cementite, there is a problem that a local battery due to a difference in C concentration is formed between ferrite and cementite, and corrosion occurs.
If the metallographic structure of the steel is made to be a ferrite single phase, the above-mentioned problems do not occur, but there is a problem that sufficient strength cannot be ensured.
Accordingly, the present inventors have studied and found that by increasing the area ratio of ferrite in the surface layer region of steel, a complex phase structure including ferrite and pearlite is formed in the inner layer region of steel, and both corrosion resistance and strength can be achieved.
The present invention has been made based on the above-described findings, and is directed to a steel sheet and a method for producing the same.
(1) A steel sheet having a chemical composition in mass%
C：0.030～0.200％、
Si：0.050～0.500％、
Mn：0.50～2.00％、
P: less than 0.030 percent,
S: less than 0.010 percent,
Al：0.001～0.100％、
N：0.0005～0.0080％、
O：0.0005～0.0080％、
Ti：0.001～0.050％、
Nb：0.001～0.050％、
Cu：0.01～0.50％、
Mo：0.01～0.10％、
Sn：0.01～0.30％、
The balance: fe and impurities are mixed in the alloy,
the total content of solid-solution Mo and solid-solution Sn in the surface layer portion of the steel sheet is 0.005% by mass or more,
when the thickness of the steel sheet is set to t in the section of the steel sheet in the rolling direction,
Metallographic structure in area% at a position at a distance of 1/4t from the surface of the steel sheet
Pearlite: 5 to 30 percent,
Bainite: less than 10 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
metallographic structure in area% at a position at a distance of 1/10t from the surface of the steel sheet
Pearlite: 1 to 20 percent,
Bainite: less than 5 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
the average grain size of ferrite at a position at a distance of 1/10t from the surface of the steel sheet is 5 to 50 μm,
the pearlite at a position at a distance of 1/10t from the surface of the steel sheet has an average particle diameter of 30 [ mu ] m or less.
(2) A steel sheet having a chemical composition in mass%
C：0.030～0.200％、
Si：0.050～0.500％、
Mn：0.50～2.00％、
P: less than 0.030 percent,
S: less than 0.010 percent,
Al：0.001～0.100％、
N：0.0005～0.0080％、
O：0.0005～0.0080％、
Ti：0.001～0.050％、
Nb：0.001～0.050％、
Cu：0.01～0.50％、
Mo：0.01～0.10％、
Sn：0.01～0.30％、
W：0～0.20％、
Sb：0～0.30％、
Pb：0～0.30％、
As：0～0.30％、
Bi：0～0.30％、
Ni：0～0.50％、
Cr：0～0.10％、
V：0～0.100％、
B：0～0.0050％、
Ta：0～0.50％、
Zr：0～0.50％、
Ca：0～0.0080％、
Mg：0～0.0080％、
REM：0～0.0080％、
The balance: fe and impurities are mixed in the alloy,
the total content of solid-solution Mo and solid-solution Sn in the surface layer portion of the steel sheet is 0.005% by mass or more,
when the thickness of the steel sheet is set to t in the section of the steel sheet in the rolling direction,
metallographic structure in area% at a position at a distance of 1/4t from the surface of the steel sheet
Pearlite: 5 to 30 percent,
Bainite: less than 10 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
metallographic structure in area% at a position at a distance of 1/10t from the surface of the steel sheet
Pearlite: 1 to 20 percent,
Bainite: less than 5 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
the average grain size of ferrite at a position at a distance of 1/10t from the surface of the steel sheet is 5 to 50 μm,
the pearlite at a position at a distance of 1/10t from the surface of the steel sheet has an average particle diameter of 30 [ mu ] m or less.
(3) The steel sheet according to the above (2), wherein the chemical composition comprises, in mass%, a component selected from the group consisting of
W：0.01～0.20％、
Sb：0.03～0.30％、
Pb：0.01～0.30％、
As:0.01 to 0.30%, and
bi:0.01 to 0.30% of 1 or 2 of the group consisting of the above-mentioned Fe.
(4) The steel sheet according to the above (2) or (3), wherein the chemical composition comprises, in mass%, a metal selected from the group consisting of
Ni：0.05～0.50％、
Cr：0.01～0.10％、
V：0.010～0.100％、
B：0.0003～0.0050％、
Ta:0.005 to 0.50%, and
zr:0.005 to 0.50% of at least 1 or more of the group consisting of Fe.
(5) The steel sheet according to any one of the above (2) to (4), wherein the chemical composition contains 0.0005 to 0.0080% by mass in total of at least 1 or more selected from the group consisting of Ca, mg and REM in place of a part of the Fe.
(6) A method for manufacturing a steel sheet, comprising the steps of:
a refining step of producing molten steel;
a continuous casting step of continuously casting the molten steel to produce a billet having the chemical composition according to any one of (1) to (5) above;
A heating step of heating the obtained billet;
a hot rolling step of hot rolling the heated billet to produce a steel sheet;
a natural cooling step of naturally cooling the steel sheet after hot rolling; and
an accelerated cooling step of water-cooling the steel sheet after natural cooling,
in the heating step, the billet is heated to a heating temperature of 950 to 1300 ℃,
in the hot rolling step, the surface temperature of the slab is Ar 3 ～T rex Is subjected to the rolling within a temperature range of (2),
in the natural cooling step, the surface temperature of the billet is naturally cooled to Ar under the condition that the average cooling rate from the start of natural cooling to the end of natural cooling is 3 ℃/sec or less 3 -100～Ar 3 Natural cooling at-30 DEG CBut at the end temperature of the process,
in the accelerated cooling step, the surface temperature of the billet is water-cooled to an accelerated cooling end temperature of 350 to 650 ℃ under the condition that the average cooling rate from the start of accelerated cooling to the end of accelerated cooling is more than 3 ℃ per second and not more than 30 ℃ per second.
Wherein Ar is 3 Obtained by the following formula (i), T rex The expression (ii) below is used to determine the expression. The symbol of the element in the following formula represents the content (mass%) of each element.
Ar 3 ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (i)
T rex ＝-91900[Nb＊] 2 +9400[Nb＊]+770 (ii)
Wherein, when the solid solution Nb amount (% by mass) obtained by the following formula (iii) is taken as sol.Nb,
in the case where Nb is equal to or greater than sol.nb, [ Nb ] =sol.nb
In the case where Nb < sol.nb, [ Nb ] =nb.
sol.Nb＝(10 (-6770/(T+273)+2.26) )/(C+12×N/14) (iii)
In the above formula, T represents the heating temperature (c) of the billet.
(7) The method for producing a steel sheet according to (6) above, wherein a tempering step of heating to a temperature in the range of 350 to 650 ℃ is further performed after the accelerated cooling step.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the present invention, a steel sheet excellent in corrosion resistance against corrosive gas components, salts, and the like contained in crude oil can be obtained.
Detailed Description
Hereinafter, each feature of the present invention will be described in detail.
(A) Chemical composition
The reasons for limiting the elements are as follows. In the following description, "%" for the content refers to "% by mass". In the present specification, unless otherwise specified, "to" indicating a numerical range is used in a meaning including numerical values described before and after the numerical values as a lower limit value and an upper limit value.
C：0.030～0.200％
C is an element effective for forming pearlite and improving strength. On the other hand, when the C content is too large, it is difficult to ensure weldability and joint toughness. Therefore, the C content is set to 0.030 to 0.200%. The C content is preferably 0.050% or more, 0.070% or more, or 0.100% or more, and preferably 0.180% or less, or 0.160% or less.
Si：0.050～0.500％
Si is effective as an inexpensive deoxidizing element and strengthening element. On the other hand, when the Si content is excessive, weldability and joint toughness are deteriorated. Therefore, the Si content is set to 0.050 to 0.500%. The Si content is preferably 0.100% or more, more preferably 0.150% or more. The Si content is preferably 0.450% or less, and more preferably 0.400% or less.
Mn：0.50～2.00％
Mn is effective as an element for improving the strength and toughness of the base material. On the other hand, when the Mn content is excessive, weldability and joint toughness are deteriorated. Therefore, the Mn content is set to 0.50 to 2.00%. The Mn content is preferably 0.80% or more, more preferably 0.90% or more. The Mn content is preferably 1.60% or less, more preferably 1.50% or less.
P: less than 0.030 percent
P is an element contained in steel as an impurity, and is set to 0.030% or less in order to secure corrosion resistance. In order to secure toughness, the smaller the P content, the more preferable is 0.015% or less. The lower limit of the P content is not necessarily set, but the cost may be increased by excessively decreasing the P content to 0.003% or more.
S: less than 0.010%
S is an element contained in steel as an impurity, and is 0.010% or less to ensure corrosion resistance. In order to secure toughness, the smaller the S content is, the more preferable, and the S content is preferably 0.003% or less. The lower limit of the S content is not required, but the cost may be increased by excessively decreasing the S content to 0.001% or more.
Al：0.001～0.100％
Al is an important deoxidizing element. On the other hand, when the Al content is too large, the surface quality of the steel billet is impaired, and inclusions detrimental to toughness are formed. Therefore, the Al content is set to 0.001 to 0.100%. The Al content is preferably 0.005% or more or 0.010% or more, and preferably 0.080% or less or 0.050% or less.
N：0.0005～0.0080％
N forms a nitride with Al to improve joint toughness. On the other hand, when the N content is too large, embrittlement due to solid solution N occurs. Therefore, the N content is set to 0.0005 to 0.0080%. The N content is preferably 0.0010% or more or 0.0020% or more, preferably 0.0070% or less, more preferably 0.0060% or less.
O：0.0005～0.0080％
O forms an oxide together with Ca, mg, REM described later. When the O content is too large, the oxide coarsens and toughness decreases. On the other hand, the smaller the O content, the better, but the more reduced the O content is, for example, the longer the time for the reflow operation in the RH vacuum degasser, which is not realistic. Therefore, the O content is set to 0.0005 to 0.0080%.
Ti：0.001～0.050％
Ti is contained in a small amount to improve toughness by making the microstructure of the base material and the welded portion finer. On the other hand, when the Ti content is too large, the welded portion is hardened, and toughness is significantly deteriorated. Therefore, the Ti content is set to 0.001 to 0.050%. The Ti content is preferably 0.003% or more and 0.005% or more, and preferably 0.040% or less and 0.030% or less.
Nb：0.001～0.050％
Nb is an element effective for ensuring the strength of the base material, and contributes to the fine structure by adding a small amount of Nb. On the other hand, when the Nb content is too large, the weld portion is hardened, and toughness is significantly deteriorated. Therefore, the Nb content is set to 0.001 to 0.050%. The Nb content is preferably 0.003% or more and 0.005% or more, and preferably 0.040% or less and 0.030% or less.
Cu：0.01～0.50％
Cu is an element effective for improving not only general corrosion resistance but also local corrosion resistance. Further, the effect of suppressing the generation of S from the corrosive gas component as solid S is also obtained. On the other hand, when the Cu content is too large, adverse effects such as the increase in surface cracks of the steel slab and the deterioration of joint toughness are also exhibited. Therefore, the Cu content is set to 0.01 to 0.50%. The Cu content is preferably 0.03% or more, preferably 0.40% or less, more preferably less than 0.20%.
Mo：0.01～0.10％
Mo is an element effective for improving the local corrosion resistance. On the other hand, when the Mo content is too large, the local corrosion resistance is rather lowered, and the weldability and toughness are deteriorated. Therefore, the Mo content is set to 0.01 to 0.10%. The Mo content is preferably 0.02% or more, more preferably 0.03% or more. The Mo content is preferably 0.08% or less, more preferably 0.07% or less.
Sn：0.01～0.30％
Sn has an effect of further suppressing local corrosion exacerbation. On the other hand, even if the Sn content exceeds 0.30%, the effect is saturated, and other characteristics may be adversely affected. Therefore, the Sn content is set to 0.01 to 0.30% in view of further economy. The Sn content is preferably 0.03% or more and 0.05% or more, and preferably 0.25% or less and 0.20% or less.
W：0～0.20％
W is an element effective for improving the local corrosion resistance, and thus may be contained as needed. On the other hand, when the W content is too large, the local corrosion resistance is rather lowered, and the weldability and toughness are deteriorated. Therefore, the W content is set to 0.20% or less. The W content is preferably 0.15% or less, more preferably 0.10% or less, and still more preferably less than 0.05%. When the above effect is to be obtained more reliably, the W content is preferably 0.01% or more.
Sb：0～0.30％
Sb has an effect of further suppressing the local corrosion from being intensified, and thus may be contained as needed. On the other hand, even if the Sb content exceeds 0.30%, the effect is saturated, and other characteristics may be adversely affected. Therefore, the Sb content is set to 0.30% or less in further consideration of economy. The Sb content is preferably 0.25% or less or 0.20% or less. When the above effect is to be obtained more reliably, the Sb content is preferably 0.03% or more or 0.05% or more.
Pb：0～0.30％
As：0～0.30％
Bi：0～0.30％
Pb, as and Bi have an effect of further suppressing the local corrosion from being intensified, and thus may be contained As needed. On the other hand, even if the content of either one exceeds 0.30%, the effect is saturated, and other characteristics may be adversely affected. Therefore, further considering economy, the contents of Pb, as and Bi are each set to 0.30% or less. The content of any element is also preferably 0.15% or less. When the above effect is desired, it is preferable to contain a compound selected from Pb:0.01% or more, as:0.01% or more and Bi: more than 1 of 0.01%.
Ni：0～0.50％
Ni is effective in securing strength and improving toughness, and thus may be contained as needed. On the other hand, when the Ni content is excessive, the cost increases. Therefore, the Ni content is set to 0.50% or less. When the above effect is to be obtained more reliably, the Ni content is preferably 0.05% or more.
Cr：0～0.10％
Cr improves hardenability and is effective for increasing strength, and thus can be contained as needed. On the other hand, when the Cr content is too large, the hardness of the joint may be increased and the toughness may be lowered. Therefore, the Cr content is set to 0.10% or less. When the above effect is to be obtained more reliably, the Cr content is preferably 0.01% or more.
V：0～0.100％
V contributes to an increase in strength by precipitation strengthening, and thus may be contained as needed. On the other hand, when the V content is too large, the joint toughness may be impaired. Therefore, the V content is set to 0.100% or less. When the above effect is to be obtained more reliably, the V content is preferably 0.010% or more.
B：0～0.0050％
B may be added in a small amount to improve hardenability and contribute to the improvement of the strength of the base material, and thus may be contained as needed. On the other hand, when the B content is too large, the joint toughness is deteriorated. Therefore, the B content is set to 0.0050% or less. When the above effect is to be obtained more reliably, the B content is preferably 0.0003% or more.
Ta：0～0.50％
Zr：0～0.50％
Ta and Zr are trace elements effective for improving the strength of steel, and mainly for adjusting the strength, and may be contained as needed. On the other hand, when the content of either exceeds 0.50%, the deterioration of toughness becomes remarkable. Therefore, the content of Ta and Zr is set to 0.50% or less. When the above-described effects are to be obtained, it is preferable to contain a metal selected from Ta:0.005% or more and Zr:0.005% or more of 1 or 2 kinds.
Ca：0～0.0080％
Mg：0～0.0080％
REM：0～0.0080％
Ca. Mg and REM each form sulfides to suppress the formation of coarse inclusions (extended MnS and the like) and improve toughness, and thus may be contained as needed. On the other hand, even if the content of either one exceeds 0.0080%, the effect is saturated, and coarse oxides or sulfides are formed to deteriorate toughness. Therefore, the contents of Ca, mg and REM are all set to 0.0080% or less.
When the above effect is to be obtained more reliably, the total content of these elements is preferably set to 0.0005% or more. In addition, from the viewpoint of preventing deterioration of toughness characteristics due to coarse oxides or sulfides, the total content of these elements is preferably 0.0080% or less. The total content is more preferably 0.0010% or more, and still more preferably 0.0015% or more. The total content is more preferably 0.0060% or less, and still more preferably 0.0040% or less.
In the present invention, REM means 17 elements in total of Sc, Y and lanthanoid, and the content of REM means the total content of these elements. The lanthanoid is industrially added in the form of misch metal.
In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. The term "impurities" as used herein refers to components which are mixed in by various factors of raw materials such as ores and scraps in the production process of the steel sheet in the industry, and are allowed to be contained within a range which does not adversely affect the present invention.
Total content of solid-solution Mo and solid-solution Sn in the steel sheet surface layer portion: 0.005% or more
Since Mo and Sn are present in a solid solution state, the corrosion resistance is more favorable, and therefore, the amounts of solid solution Mo and solid solution Sn in the surface layer portion of the steel sheet are ensured to be equal to or greater than a predetermined value. Specifically, the total content of solid-solution Mo and solid-solution Sn in the surface layer portion of the steel sheet is 0.005% or more by mass%. The total content of the solid-solution Mo and the solid-solution Sn in the surface layer portion of the steel sheet is preferably 0.010% or more, more preferably 0.020% or more. There is no need to set an upper limit to the total content of solid-solution Mo and solid-solution Sn, but 0.40% as an upper limit to the total content of Mo and Sn contained in the steel is a practical upper limit.
In the present invention, the steel sheet surface layer portion refers to a region extending from the surface of the steel sheet to a position of 1mm in the depth direction. In addition, the total content (mass%) of the solid-dissolved Mo and the solid-dissolved Sn was measured as follows. First, 2 test pieces 1mm thick were cut from the surface of a steel plate. Then, for one of the test pieces, the contents of Mo and Sn in the test piece were measured by using a known chemical analysis method (for example, ICP emission spectrometry).
In addition, for another test piece, 10 mass% of acetylacetone-1 mass% of tetramethylammonium chloride/methanol was used at 20mA/cm 2 0.4g of the electrolyte. The solution used in the electrolysis was filtered through a filter having a pore size of 0.2 μm, and the content of Mo and Sn in the extraction residue captured on the filter was measured by using a known chemical analysis method (for example, ICP emission spectrometry).
Mo and Sn in the test piece were considered to be Mo and Sn precipitates, and Mo and Sn in the extraction residue were considered to be Mo and Sn precipitates. Then, the difference between the Mo and Sn contents in the extraction residue and the Mo and Sn contents in the test piece was obtained, and the solid solution Mo and Sn contents were obtained.
In the chemical composition of the steel sheet according to the present invention, ceq defined by the following formula (iv) may be set to be in the range of 0.20 to 0.50%.
Ceq＝C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (iv)
The symbol of the element in the above formula represents the content (mass%) of each element contained in the steel sheet, and if not, 0 is substituted.
When the Ceq value is 0.20% or more, the strength required for the steel sheet can be easily ensured. On the other hand, when Ceq is 0.50% or less, excellent toughness can be ensured. Ceq is preferably 0.22% or more, more preferably 0.24% or more, and still more preferably 0.26% or more. The Ceq is preferably 0.48% or less, more preferably 0.46% or less, and still more preferably 0.45% or less.
(B) Metallographic structure
In the present invention, the steel sheet has the following metallographic structure at each of the inner layer position and the outer layer position. The metallographic structures of the inner layer position and the surface layer position of the steel plate are respectively described.
In the following description, "%" means "% by area". The metallographic structure at the inner layer position of the steel sheet is a structure at a position 1/4t from the surface of the steel sheet when the thickness of the steel sheet is t. The metallographic structure of the surface layer of the steel sheet means a structure at a distance of 1/10t from the surface of the steel sheet.
(B-1) metallographic Structure of inner layer position of Steel sheet
Pearlite: 5 to 30 percent
In order to secure yield stress and tensile strength as strength characteristics, the area ratio of pearlite is set to 5 to 30%. The area ratio of pearlite is preferably 10 to 20%.
Bainite: less than 10 percent
In the present invention, the metallographic structure mainly contains ferrite and includes a predetermined amount of pearlite. Even if the bainite is contained in an amount of 10% or less, the above effects are not impaired, but if the area ratio of the bainite is too large, the toughness is deteriorated. Therefore, the area ratio of bainite is 10% or less, preferably 5% or less. The bainite may be absent, i.e., the area ratio of the bainite may be 0%.
The balance: ferrite body
Ferrite is a structure excellent in toughness. The structure other than pearlite and bainite is ferrite. That is, the area ratio of ferrite is 60% or more. On the other hand, from the viewpoint of securing strength characteristics, the area ratio of ferrite is preferably 90% or less, more preferably less than 80%.
(B-2) metallographic Structure of surface layer position of Steel sheet
Pearlite: 1 to 20 percent
Bainite: less than 5%
The balance: ferrite body
Pearlite is inevitably contained in the metallographic structure. In addition, there is a possibility that bainite is mixed in. However, as described above, when the pearlite structure and the bainite structure are contained in a large amount in the surface layer region of the steel sheet in a corrosive environment, a local cell is formed between ferrite and cementite, and corrosion occurs. Therefore, it is required to reduce the area ratio of pearlite and bainite at the surface layer position. From this viewpoint, the area ratio of pearlite is 1 to 20%, and the area ratio of bainite is 5% or less.
It is desirable that the area ratio of pearlite and bainite be as low as possible. Specifically, the area ratio of pearlite is preferably 10% or less, more preferably 5% or less. The area ratio of bainite is preferably 3% or less, more preferably 1% or less. The bainite may be absent, i.e., the area ratio of the bainite may be 0%.
In the metallographic structure of the surface layer position, the balance is ferrite. That is, the area ratio of ferrite is 75% or more. The area ratio of ferrite is preferably more than 85%, and preferably more than 95%. The practical upper limit of the area ratio of ferrite is 99%.
The average grain size of ferrite is 5-50 mu m
In the metallographic structure at the surface layer position, the toughness can be improved by refining ferrite grains. Therefore, the average grain size of ferrite is 50 μm or less. Further, although finer ferrite grains are preferable, it is industrially difficult to achieve smaller than 5 μm, and therefore the lower limit is set to 5 μm. The average grain size of ferrite is preferably 40 μm or less, more preferably 30 μm or less.
The pearlite has an average particle diameter of 30 μm or less
In the metallographic structure at the surface layer position, the finer the average grain size of pearlite is, the finer cementite becomes on the cathode side, and localized corrosion is reduced. Therefore, the average particle diameter of pearlite is set to 30 μm or less.
(B-3) relationship of metallographic Structure of inner layer position and surface layer position
As described above, in the present invention, the area ratio of ferrite is increased in the surface layer position of the steel sheet, and the complex phase structure including ferrite and pearlite is formed in the inner layer position of the steel sheet, thereby achieving both corrosion resistance and strength. If the metallographic structures of the inner layer position and the surface layer position of the steel plate respectively meet the above conditions, both corrosion resistance and strength can be achieved. Therefore, the relationship of the metallographic structure of the inner layer position and the surface layer position is not particularly limited, but in order to further improve both corrosion resistance and strength, it is preferable that the area ratio of ferrite at the surface layer position is higher than that of ferrite at the inner layer position.
Method for measuring metallographic structure of (B-4)
In the present invention, the area ratio of the metallographic structure was determined as follows. As described above, first, samples were collected from a position at a distance of 1/4t from the surface of the steel sheet and a position of 1/10t, respectively. Then, the rolling direction cross section (so-called L direction cross section) of the sample was observed. The term "rolling direction" as used herein refers to the rolling direction in finish rolling.
Specifically, the sample was etched with a nitrate alcohol solution, and after etching, the sample was observed with an optical microscope at a magnification of 500 times in a visual field of 300 μm×300 μm. Then, the obtained tissue photograph was subjected to image analysis, and the white portion was regarded as ferrite, and the black portion was regarded as pearlite, to determine the respective area ratios. In the present invention, pseudo pearlite is also included in pearlite. In the steel sheet of the present invention, since bainite is present in addition to the iron element and pearlite, the area ratio of bainite is obtained from the area ratio of the balance. In the observation under the above conditions, bainite was gray.
The average particle diameters of ferrite and pearlite at the surface layer were measured by the above microscopic observation. Specifically, the area of each crystal grain of ferrite and pearlite contained in the field of view is determined by image analysis, and the diameter of a circle equal to the area is determined to determine the crystal grain diameters of ferrite and pearlite. Then, the average diameters of all ferrite and all pearlite in the field of view are calculated, and the average particle diameters of ferrite and pearlite are obtained. When the average particle diameters of the ferrite and the pearlite are obtained, the minimum particle diameter to be analyzed is set to 1 μm.
(C) Mechanical properties
The mechanical properties are not particularly limited, but the steel sheet according to the present invention preferably has strength required for use as a crude oil tank, for example. Specifically, the Yield Stress (YS) is preferably 235MPa or more, and the Tensile Strength (TS) is preferably 400 to 620MPa. The upper limit of the preferable range of the tensile strength is set because if the tensile strength is excessive, toughness may be deteriorated.
The Tensile Strength (TS) and the Yield Stress (YS) were set according to JIS Z2241: 2011, using a tensile test piece No. 1B collected in a direction perpendicular to the rolling direction. Specifically, the Yield Stress (YS) is the conditional yield strength of the permanent elongation method at a permanent elongation of 0.2%.
(D) Method of manufacture
The conditions for producing the steel sheet according to the present invention are not particularly limited, but the steel sheet can be produced by sequentially performing a refining step, a continuous casting step, a heating step, a hot rolling step, a natural cooling step, and an accelerated cooling step, which will be described later. Each step will be described.
(a) Refining process
In the refining step, molten steel is produced. The refining step may be performed by a known method, and is not particularly limited.
(b) Continuous casting process
In the continuous casting step, molten steel is continuously cast to produce a billet having the chemical composition described above. The continuous casting step may be performed by a known method, and is not particularly limited.
(c) Heating process
In order to hot-roll a billet, the billet is heated. In the heating step, the steel billet having the chemical composition is heated to a heating temperature of 950 to 1300 ℃. The heating step is performed by a heating furnace. The heating of the billet to 950 to 1300 ℃ means heating such that the average temperature of the total thickness of the billet when the billet is drawn out from the heating furnace is in the range of 950 to 1300 ℃, and in this specification, the average temperature of the total thickness of the billet is referred to as the heating temperature of the billet. The total thickness average temperature can be calculated from the temperature in the heating furnace, the heating time, and the surface temperature of the billet.
When the heating temperature is less than 950 ℃, hot rolling is difficult to carry out. On the other hand, by setting the heating temperature to 1300 ℃ or lower, it is possible to suppress coarsening of ferrite grains and pearlite grains at the surface layer position and to optimize the area ratio of ferrite and pearlite at the inner layer position. The heating temperature is preferably 1200 ℃ or lower, more preferably 1100 ℃ or lower.
The holding time when the billet is heated is not particularly limited, and may be, for example, 120 minutes or less. The holding time is preferably 80 minutes or less or 60 minutes or less. In the present specification, the holding time refers to the total time when the heating temperature of the billet is in the temperature range of 950 to 1300 ℃.
(d) Hot rolling process
In the hot rolling step, a steel slab is hot-rolled to produce a steel sheet. At this time, the surface temperature of the billet was Ar 3 ～T rex Ending the rolling within the temperature range of (2). By at Ar 3 The rolling is finished in the above way, and the pulling can be restrainedFormation of stretched ferrite. In addition, by at T rex The rolling is completed in the non-recrystallized region below, and the area ratio of pearlite at the inner layer position can be optimized while suppressing coarsening of ferrite and pearlite grains at the surface layer position.
Here, ar is 3 The ferrite transformation start temperature at the time of cooling the steel is obtained by the following formula (i).
Ar 3 ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (i)
Wherein the symbol of the element in the above formula represents the content (mass%) of each element.
In addition, T rex The recrystallization start temperature at which the generation/growth of new grains of austenite starts is determined by the following formula (ii). The formula (ii) is an empirical formula. Since Nb which is not dissolved yet exists by heating at low temperature, the [ Nb ] in the formula (ii)]Is obtained by correcting the theoretical solid solution Nb content (mass%) calculated by using the Nb content in the steel and the heating temperature in consideration of the Nb content in the steel, and using the same [ Nb ]]Calculate T rex 。
T rex ＝-91900[Nb＊] 2 +9400[Nb＊]+770 (ii)
Wherein, regarding [ Nb ], when the solid-solution Nb amount obtained by the following formula (iii) is taken as sol.Nb,
in the case where Nb is equal to or greater than sol.nb, [ Nb ] =sol.nb
In the case where Nb < sol.nb, [ Nb ] =nb.
sol.Nb＝(10 (-6770/(T+273)+2.26) )/(C+12×N/14) (iii)
In the above formula, T represents the heating temperature (c) of the billet.
(e) Natural cooling process
In the natural cooling step, the rolled steel sheet is naturally cooled. At this time, the surface temperature of the billet is naturally cooled to Ar under the condition that the average cooling rate from the start of natural cooling to the end of natural cooling is 3 ℃/sec or less 3 -100～Ar 3 -30 ℃ natural cooling end temperature. By setting the average cooling rate to 3 ℃/sec or lessIn the surface layer position of the steel sheet, pearlite transformation and bainite transformation can be suppressed. In the present invention, the surface temperature of the billet at the end of natural cooling is managed as the natural cooling end temperature.
In addition, the surface temperature of the billet is naturally cooled to Ar 3 The area ratio of ferrite can be sufficiently ensured in the metallographic structure at the surface layer position at 30 ℃ below zero. On the other hand, by setting the natural cooling end temperature in the natural cooling step to Ar 3 At a temperature of above 100 ℃ below zero, can prevent the temperature of the inner layer position of the steel plate from being lower than Ar in the natural cooling process 3 A predetermined pearlite is generated in the metallographic structure at the inner layer position.
(f) Accelerated cooling process
In the accelerated cooling step, the naturally cooled steel sheet is water-cooled. At this time, the water is cooled to an accelerated cooling completion temperature of 350 to 650 ℃ under the condition that the average cooling rate from the start of accelerated cooling to the completion of accelerated cooling is more than 3 ℃/sec and 30 ℃/sec or less. By cooling the steel sheet to an accelerated cooling finish temperature of 350 to 650 ℃ at an average cooling rate of more than 3 ℃ per second and 30 ℃ per second or less, pearlite having a predetermined area% can be produced in the metallographic structure at the inner layer position. In the present invention, the surface temperature of the billet at the time when water cooling is completed and the surface temperature of the billet is completed for reheating is managed as the accelerated cooling termination temperature.
(g) Tempering process
After the accelerated cooling step, a tempering step of heating to a temperature range of 350 to 650 ℃ may be further provided. In the case where the cooling stop temperature in the accelerated cooling step is high, the tempering step may not be performed because the self-tempering effect is obtained.
The present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
Examples
Steel sheets having a sheet thickness of 5 to 50mm were produced by trial using billets having the chemical compositions shown in table 1 according to the production conditions shown in table 2.
TABLE 1
TABLE 2
TABLE 2
The metallographic structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, when the thickness of the steel sheet is t in the section of the steel sheet in the rolling direction, test pieces for observing the metallographic structure are cut from the position at a distance of 1/4t and the position at a distance of 1/10t from the surface of the steel sheet.
Then, the section of the test piece in the rolling direction (so-called L-direction section) was etched with a nitrate alcohol solution, and after etching, the test piece was observed in a visual field of 300 μm×300 μm at a magnification of 500 times using an optical microscope. The area ratios of ferrite, pearlite, and bainite were obtained by image analysis of the obtained structure photograph. More specifically, the white structure was regarded as ferrite, the black structure was regarded as pearlite, the area ratios of the respective structures were obtained, and the area ratio of bainite was obtained from the area ratio of the remaining portion.
Further, the average particle diameters of ferrite and pearlite at the surface layer position were measured by the following procedure. The area of each crystal grain of ferrite and pearlite contained in the field of view is determined by image analysis, and the diameter of a circle equal to the area is determined to determine the crystal grain diameters of ferrite and pearlite. Then, the average diameters of all ferrite and all pearlite in the field of view are calculated, and the average particle diameters of ferrite and pearlite are obtained. When the average particle diameters of the ferrite and the pearlite are obtained, the minimum particle diameter to be analyzed is set to 1 μm.
Further, the total content (mass%) of the solid-solution Mo and the solid-solution Sn in the surface layer portion of the steel sheet was measured in accordance with the following procedure. First, 2 test pieces having a thickness of 1mm were cut out from the surface of a steel sheet, and for one of the test pieces, the contents of Mo and Sn in the test piece were measured by using ICP emission spectrometry.
In addition, for another test piece, 10% acetylacetone-1% tetramethylammonium chloride/methanol was used at 20mA/cm 2 The resulting solution was filtered through a filter having a pore size of 0.2. Mu.m, and the content of Mo and Sn in the extracted residue was measured by ICP emission spectrometry with respect to the extracted residue trapped on the filter.
Then, the difference between the Mo and Sn contents in the extraction residue and the Mo and Sn contents in the test piece was obtained, and the solid solution Mo and Sn contents were obtained.
Further, the Tensile Strength (TS) and the Yield Stress (YS) are based on JIS Z2241: 2011. The test piece was measured using a tensile test piece No. 1B collected with a direction perpendicular to the rolling direction (width direction) as a longitudinal direction. The Yield Stress (YS) is the conditional yield strength of the permanent elongation method at a permanent elongation of 0.2%.
Then, in order to evaluate corrosion resistance of the steel sheet, 3 corrosion tests shown below were performed. For corrosion tests 1 and 2, the test was performed according to Resolution MSC.289 (87) of IMO of International maritime organization.
< Corrosion test 1 >
Test pieces having a rolling direction length of 60mm, a width direction length of 25mm and a thickness direction length of 5mm were collected from the surfaces of the steel plates. All surfaces of 6 surfaces of the test piece were polished with steel grit polishing paper No. 600 to prepare a test piece in which all surfaces of the test piece were exposed to the iron matrix. The test piece was immersed in a 10 mass% aqueous NaCl solution adjusted to pH0.85 with hydrochloric acid. The impregnation conditions were carried out at a liquid temperature of 30℃and an impregnation time of 72 hours. The test solution was replaced with a new one every 24 hours. The volume of the test liquid was 25cc/cm in terms of the surface area ratio of the test piece 2 。
And (5) measuring corrosion weight loss and evaluating corrosion speed. The composition of the corrosive liquid simulates the environmental condition when the local corrosion occurs in the actual steel structure, and as the corrosion speed in the corrosion test is reduced, the aggravation speed of the local corrosion in the corresponding actual environment is also reduced. The corrosion weight loss was determined by subtracting the mass of the test piece after the corrosion test from the mass of the test piece before the corrosion test, and removing the corrosion product by acid washing.
< Corrosion test 2 >
Test pieces having a length of 60mm in the rolling direction, a length of 25mm in the width direction, and a length of 5mm in the thickness direction were collected from the surface of the steel sheet. The surface of the test piece was polished with steel grit polishing paper No. 600. A test piece having a cross section (except the surface) coated with a paint was produced in a form of 60mm X25 mm and having an iron matrix exposed only on the surface of the steel sheet. Test pieces were prepared for measurement after 21 cycles, 49 cycles, 77 cycles and 98 cycles, respectively.
A glass container with distilled water in the lower 1/3 portion was prepared, and the open upper end of the glass container was sealed with an acrylic lid having a gas supply port for mounting the collected test piece on the lower surface. Next, the sealed glass container was set in a constant temperature bath, and 4-level distilled water at 30 ℃ and test piece temperature at 50 ℃ x 19 hours, cooling x 1 hour, 25 ℃ x 3 hours, heating x 1 hour were applied at 21, 49, 77 and 98 cycles. At this time, a gas having the following composition was blown from the gas supply port into the gas phase portion in the glass container. The composition of the gas used was CO 2 : 13% by volume, H 2 S：500ppm、O 2 :4 vol%, SO 2 ：100ppm、N 2 : the balance.
Then, corrosion weights after 21 cycles, 49 cycles, 77 cycles and 98 cycles were measured, respectively, and corrosion rates were evaluated based on the relationships thereof. The composition of the corrosive liquid simulates the environmental condition when the general corrosion occurs in the actual steel structure, and along with the reduction of the corrosion speed in the corrosion test, the aggravation speed of the general corrosion in the corresponding actual environment is also reduced. The corrosion weight loss was determined by subtracting the mass of the test piece after the corrosion test from the mass of the test piece before the corrosion test, and removing the corrosion product by acid washing.
< Corrosion test 3 >
Test pieces having a length of 40mm in the rolling direction, a length of 40mm in the width direction and a length of 4mm in the thickness direction were collected from the surfaces of the steel plates. The cross section (except the surface) was coated with a paint, and the surface was subjected to wet grinding of 600 # to remove iron oxide (scale) on the surface of the steel sheet, thereby producing a test piece having a thickness of 40mm×40mm in which only the surface of the steel sheet was exposed to the iron matrix. Then, using the test piece, the corrosion rate and the rate of formation of sludge mainly composed of solid S were evaluated in the following steps.
First, before the corrosion test, the amount of NaCl attached was set to 1000mg/m 2 In the above manner, an aqueous NaCl solution was applied to the surface of the test piece and dried, and the resultant was horizontally placed on a constant temperature heating plate in a test chamber. Then, the gas adjusted to a constant dew point (30 ℃) was fed into the test chamber. The gas used has CO 2 :12 vol%, H 2 S：500ppm、O 2 : 5% by volume of N 2 : the balance of the composition.
Then, a total of 2 hours/cycle of temperature cycles of 20 ℃ x 1 hour and 40 ℃ x 1 hour was applied so that dry-wet reciprocation was generated on the test piece surface. After 720 cycles, the corrosion rate was evaluated based on the corrosion weight loss, and the sludge formation rate was evaluated based on the mass of the product formed on the surface of the test piece. The product was confirmed to be iron oxyhydroxide (rust) and solid S by a preliminary test using chemical analysis and X-ray analysis. The product mass was obtained from the difference between the mass before and after the removal of the corrosion product by acid washing. The corrosion weight loss was obtained by subtracting the mass of the test piece after pickling from the mass of the test piece before the corrosion test.
Based on the measurement results of the corrosion tests 1, 2 and 3, the relative values of the test numbers were obtained for each corrosion test, with the corrosion rate and sludge formation rate of the test number 45 being set to 100. That is to say,
Relative corrosion rate= (corrosion rate of each test number/corrosion rate of test number 45) ×100
Relative sludge generation speed= (sludge generation speed of each test number/sludge generation speed of test number 45) ×100.
The relative corrosion rates and relative sludge formation rates for each corrosion test are shown in table 3. In this example, when the relative corrosion rate and the relative sludge formation rate were both 40% or less, it was determined that the corrosion resistance was excellent.
TABLE 3
TABLE 3 Table 3
As is clear from table 3, in the examples of the present invention (test numbers 1 to 26) satisfying the regulations of the present invention, the results had appropriate strength, and excellent corrosion resistance was exhibited in all corrosion tests.
In contrast, in the comparative examples, test nos. 28, 31 to 38 and 40, the corrosion resistance was poor. Specifically, in test No.28, the area ratio of pearlite exceeds the predetermined range due to the excessive C content, and the corrosion resistance is deteriorated. In test nos. 31 and 32, corrosion resistance was deteriorated due to excessive contents of P and S, respectively. In test No.33, the corrosion resistance was deteriorated due to excessive Mo content. In test No.34, since Sn and Sb were not contained, corrosion resistance was deteriorated.
In test No.35, since the heating temperature in the heating step was too high, ferrite grains and pearlite grains at the surface layer position coarsened, and the area ratio of ferrite and pearlite at the inner layer position was outside the prescribed range. In test No.36, since the rolling end temperature in the hot rolling step was too low, ferrite transformation occurred before dislocation was sufficiently introduced, and ferrite grains and pearlite grains at the surface layer position could not be refined. On the other hand, in test No.37, since the rolling end temperature in the hot rolling step is too high, the recrystallization dislocation is reduced, and sufficient dislocation cannot be ensured at the time of ferrite transformation, and ferrite grains and pearlite grains at the surface layer position coarsen.
In test No.38, the average cooling rate in the natural cooling step was too high, and in test No.40, the natural cooling end temperature in the natural cooling step was too high, so that the area ratio of pearlite and bainite in the surface layer was too large.
In the comparative examples, the contents of C, si and Mn in test nos. 27, 29 and 30 were lower than the predetermined values, respectively. In test No.39, the natural cooling end temperature in the natural cooling step was too low, and in test No.41, the average cooling rate in the accelerated cooling step was too low, and in test No.44, the accelerated cooling end temperature in the accelerated cooling step was too high, so that the area ratio of pearlite was insufficient at the inner layer position. Therefore, in these examples, although the corrosion resistance as the subject of the present invention is good, the tensile strength is low.
On the other hand, in test No.42, since the average cooling rate in the accelerated cooling step was too high, in test No.43, since the accelerated cooling end temperature in the accelerated cooling step was too low, the area ratio of bainite was too large in the inner layer position. Therefore, in these examples, although the corrosion resistance as the subject of the present invention is good, the strength is excessive and does not satisfy the appropriate conditions.
Industrial applicability
According to the present invention, a steel sheet excellent in corrosion resistance against corrosive gas components, salts, and the like contained in crude oil can be obtained. Therefore, the steel sheet according to the present invention can be suitably used as a crude oil tank.

Claims (7)

1. A steel sheet, the chemical composition of the steel sheet is C:0.030 to 0.200 percent,
Si：0.050～0.500％、
Mn：0.50～2.00％、
P: less than 0.030 percent,
S: less than 0.010 percent,
Al：0.001～0.100％、
N：0.0005～0.0080％、
O：0.0005～0.0080％、
Ti：0.001～0.050％、
Nb：0.001～0.050％、
Cu：0.01～0.50％、
Mo：0.01～0.10％、
Sn：0.01～0.30％、
The balance: fe and impurities are mixed in the alloy,
the total content of solid-solution Mo and solid-solution Sn in the surface layer portion of the steel sheet is 0.005% by mass or more,
when the thickness of the steel sheet is set to t in the section of the steel sheet in the rolling direction,
metallographic structure in area% at a position at a distance of 1/4t from the surface of the steel sheet
Pearlite: 5 to 30 percent,
Bainite: less than 10 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
metallographic structure in area% at a position at a distance of 1/10t from the surface of the steel sheet
Pearlite: 1 to 20 percent,
Bainite: less than 5 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
the average grain size of ferrite at a position at a distance of 1/10t from the surface of the steel sheet is 5 to 50 μm,
the pearlite at a position at a distance of 1/10t from the surface of the steel sheet has an average particle diameter of 30 [ mu ] m or less.
2. A steel sheet, the chemical composition of the steel sheet is C:0.030 to 0.200 percent,
Si：0.050～0.500％、
Mn：0.50～2.00％、
P: less than 0.030 percent,
S: less than 0.010 percent,
Al：0.001～0.100％、
N：0.0005～0.0080％、
O：0.0005～0.0080％、
Ti：0.001～0.050％、
Nb：0.001～0.050％、
Cu：0.01～0.50％、
Mo：0.01～0.10％、
Sn：0.01～0.30％、
W：0～0.20％、
Sb：0～0.30％、
Pb：0～0.30％、
As：0～0.30％、
Bi：0～0.30％、
Ni：0～0.50％、
Cr：0～0.10％、
V：0～0.100％、
B：0～0.0050％、
Ta：0～0.50％、
Zr：0～0.50％、
Ca：0～0.0080％、
Mg：0～0.0080％、
REM：0～0.0080％、
The balance: fe and impurities are mixed in the alloy,
the total content of solid-solution Mo and solid-solution Sn in the surface layer portion of the steel sheet is 0.005% by mass or more,
when the thickness of the steel sheet is set to t in the section of the steel sheet in the rolling direction,
metallographic structure in area% at a position at a distance of 1/4t from the surface of the steel sheet
Pearlite: 5 to 30 percent,
Bainite: less than 10 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
metallographic structure in area% at a position at a distance of 1/10t from the surface of the steel sheet
Pearlite: 1 to 20 percent,
Bainite: less than 5 percent,
The balance: the ferrite phase of the steel is a ferrite phase,
the average grain size of ferrite at a position at a distance of 1/10t from the surface of the steel sheet is 5 to 50 μm,
the pearlite at a position at a distance of 1/10t from the surface of the steel sheet has an average particle diameter of 30 [ mu ] m or less.
3. The steel sheet according to claim 2, wherein the chemical composition contains, in mass%, a composition selected from the group consisting of
W：0.01～0.20％、
Sb：0.03～0.30％、
Pb：0.01～0.30％、
As:0.01 to 0.30%, and
bi:0.01 to 0.30% of 1 or 2 of the group consisting of the above-mentioned Fe.
4. A steel sheet according to claim 2 or claim 3, wherein the chemical composition contains, in mass%, a composition selected from the group consisting of
Ni：0.05～0.50％、
Cr：0.01～0.10％、
V：0.010～0.100％、
B：0.0003～0.0050％、
Ta:0.005 to 0.50%, and
zr:0.005 to 0.50% of at least one kind selected from the group consisting of Fe and Fe.
5. The steel sheet according to any one of claims 2 to 4, wherein the chemical composition contains 0.0005 to 0.0080% in total of at least one or more selected from the group consisting of Ca, mg and REM in mass% instead of a part of Fe.
6. A method for manufacturing a steel sheet, comprising the steps of:
a refining step of producing molten steel;
a continuous casting step of continuously casting the molten steel to produce a billet having the chemical composition according to any one of claims 1 to 5;
a heating step of heating the obtained billet;
a hot rolling step of hot rolling the heated billet to produce a steel sheet;
a natural cooling step of naturally cooling the steel sheet after hot rolling; and
an accelerated cooling step of water-cooling the steel sheet after natural cooling,
in the heating step, the billet is heated to a heating temperature of 950 to 1300 ℃,
In the hot rolling step, the surface temperature of the slab is Ar 3 ～T rex Is subjected to the rolling within a temperature range of (2),
in the natural cooling step, the surface temperature of the billet is naturally cooled to Ar under the condition that the average cooling rate from the start of natural cooling to the end of natural cooling is 3 ℃/sec or less 3 -100～Ar 3 A natural cooling end temperature of-30 ℃,
in the accelerated cooling step, the surface temperature of the billet is cooled to an accelerated cooling end temperature of 350 to 650 ℃ under the condition that the average cooling speed from the start of accelerated cooling to the end of accelerated cooling is more than 3 ℃ per second and less than 30 ℃ per second,
wherein Ar is 3 Obtained by the following formula (i), T rex The content (mass%) of each element is determined from the following formula (ii),
Ar 3 ＝910-310×C+65×Si-80×Mn-20×Cu-55×Ni-15×Cr-80×Mo (i)
T rex ＝-91900[Nb＊] 2 +9400[Nb＊]+770 (ii)
wherein, when the solid solution Nb amount (% by mass) obtained by the following formula (iii) is taken as sol.Nb,
in the case where Nb is equal to or greater than sol.nb, [ Nb ] =sol.nb
In the case of Nb < sol.nb, [ Nb ] =nb
sol.Nb＝(10 (-6770/(T+273)+2.26) )/(C+12×N/14) (iii)
In the above formula, T represents the heating temperature (c) of the billet.
7. The method for producing a steel sheet according to claim 6, wherein a tempering step of heating to a temperature range of 350 to 650 ℃ is further performed after the accelerated cooling step.
CN202280032170.0A 2021-07-27 2022-07-11 Steel sheet and method for producing same Pending CN117242201A (en)

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