CN115003842B

CN115003842B - High-tensile steel sheet excellent in base material toughness and joint toughness, and method for producing same

Info

Publication number: CN115003842B
Application number: CN202180011081.3A
Authority: CN
Inventors: 宫田亮太
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2020-04-02
Filing date: 2021-03-25
Publication date: 2024-02-27
Anticipated expiration: 2041-03-25
Also published as: KR20220141863A; CN115003842A; WO2021200572A1; JP2021161524A

Abstract

The high-tensile steel sheet excellent in base material toughness and joint toughness according to the embodiment of the present invention has a predetermined chemical composition, and the parameter PY represented by the following formula (1) is 1.300 or more and 2.500 or less, the area fraction of ferrite is 60% or more with respect to the entire metal structure, and the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less is 20% or more. The parameter py=10× ([ Nb ] +3× [ C ])× (2× [ Si ] + [ Cu ] + [ Ni ] + [ Mo ]) … (1) wherein [ (] represents the content (mass%) of each element, and the content of the element not contained is set to zero.

Description

High-tensile steel sheet excellent in base material toughness and joint toughness, and method for producing same

Technical Field

The present invention relates to a high-tensile steel sheet excellent in base material toughness and joint toughness, and a method for producing the same.

Background

Thick steel sheets used for LPG tanks and the like used in low-temperature environments are required to have both high strength and excellent toughness at low temperatures (hereinafter referred to as "low-temperature toughness"). In addition, a welded joint portion (hereinafter, simply referred to as a "joint" or a "joint portion") of the weld metal and the heat affected zone (HAZ, heat Affected Zone) is also required to have excellent low-temperature toughness (hereinafter, simply referred to as "joint toughness"). In particular, in recent years, high toughness at extremely low temperatures has been demanded from the viewpoint of safety.

Here, the addition of the alloy is effective for improving strength, but on the other hand, causes a decrease in toughness. Therefore, it is extremely difficult to achieve both strength and toughness. As a means for improving the strength and toughness together, a method of adding Ni is exemplified. However, as represented by 3.5% Ni steel and 9% Ni steel, this effect cannot be exerted to the maximum extent if Ni is not added in a large amount. Therefore, a thick steel plate having a further suppressed Ni amount and excellent base material strength and low temperature toughness of the joint portion has been studied.

For example, patent document 1 discloses a high tensile steel sheet having a yield stress of 420MPa or more and excellent low-temperature toughness at a weld heat affected zone of a multi-layer weld zone at a low-centerline energy, which is suitable for steel structures such as ships, marine structures, pressure vessels, and pressure pipes, and a method for producing the same. In patent document 1, a high tensile steel sheet excellent in low-temperature toughness in a welding heat affected zone is obtained by controlling the hardness of a center segregation portion of a steel sheet while having a predetermined composition.

Prior art literature

Patent literature

Patent document 1: japanese patent application laid-open No. 2012-184300

Disclosure of Invention

Problems to be solved by the invention

However, in the technique disclosed in patent document 1, the toughness evaluation temperature of the base material and the joint is-40 ℃, and toughness at lower temperatures may be insufficient.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-tensile steel sheet having high strength, excellent toughness of a base material at a lower temperature, and excellent toughness of a welded joint portion at the time of welding, and a method for manufacturing the same.

Means for solving the problems

Embodiment 1 of the present invention is a high-tensile steel sheet excellent in base material toughness and joint toughness, comprising:

c:0.02 to 0.06 mass%,

Si: more than 0 mass% and not more than 0.50 mass%,

Mn:0.90 to 1.60 mass%,

P: more than 0 mass% and not more than 0.03 mass%,

S: more than 0 mass% and not more than 0.01 mass%,

Al:0.020 to 0.070 mass%,

Cu:0.10 to 0.40 mass%,

Nb:0.010 mass% or more and 0.060 mass% or less,

Ni:0.40 to 0.80 mass%,

Ti:0.005 to 0.025 mass%, and

n:0.0020 to 0.0080 mass%,

the balance is composed of iron and unavoidable impurities,

the parameter PY represented by the following formula (1) is 1.300 to 2.500,

with respect to the overall metallic structure,

the area fraction of ferrite is 60% or more,

the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less is 20% or more.

The parameter PY=10× ([ Nb ] +3× [ C ])× (2× [ Si ] + [ Cu ] + [ Ni ] + [ Mo ]) … (1)

Wherein [ (wt%) represents the content (wt%) of each element, and the content of the element not contained is set to zero.

Embodiment 2 of the present invention provides the high-tensile steel sheet excellent in base material toughness and joint toughness according to embodiment 1, further comprising a steel sheet obtained from

B: more than 0 mass% and not more than 0.0015 mass%,

Ca: more than 0 mass% and not more than 0.003 mass%, and

mo: more than 0 mass% and not more than 0.50 mass% of one kind selected from the group consisting of the above.

Embodiment 3 of the present invention is the method for producing a high-tensile steel sheet excellent in base material toughness and joint toughness according to embodiment 1 or 2, wherein,

comprising a step of heating the steel having the composition of mode 1 or mode 2 at a temperature of 1000 ℃ or higher and 1250 ℃ or lower, and a hot rolling step after the heating,

the hot rolling process includes:

a step of pressing at a cumulative rolling reduction of 30% or more in a temperature range of 900 ℃ or more;

a step of pressing at a cumulative rolling reduction of 20% to 80% in a temperature range of Ar3 to 900 ℃ inclusive;

and cooling the molten steel from a cooling start temperature of (Ar 3-30 ℃) to a cooling stop temperature of (the cooling start temperature-20 ℃) or higher at an average cooling rate of 1 ℃/sec or higher and 10 ℃/sec or lower.

Here the number of the elements to be processed is,

Ar3(℃)＝868－369×[C]+24.6×[Si]－68.1×[Mn]－36.1×[Ni]－20.7×[Cu]－24.8×[Cr]+29.6×[Mo]

Effects of the invention

According to the embodiment of the present invention, a high-tensile steel sheet having high strength and excellent toughness of the base material at a lower temperature and also excellent toughness of the welded joint portion at a low temperature when welding can be obtained.

Drawings

Fig. 1 is a graph showing the relationship between the MA area fraction of the joint portion and the joint toughness.

Fig. 2 is a diagram showing a relationship between parameter PY and MA area fraction of the joint portion.

Fig. 3 is a graph showing a relation between the parameter PY and the total area Fraction (FR) of ferrite having an equivalent circle diameter of 7.5 μm or less.

Fig. 4 is a graph showing a relationship between the parameter PY and the strength-toughness balance (TV).

Detailed Description

As a result of intensive studies, the inventors have found that a high-tensile steel sheet having high strength, excellent base toughness at lower temperatures than conventional ones, and excellent low-temperature toughness at welded joints when welded can be obtained by setting the parameter PY calculated from the content of a predetermined chemical component to 1.300 or more and 2.500 or less, setting the area fraction of ferrite to 60% or more, and setting the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less (hereinafter referred to as "FR") to 20% or more.

1. Chemical composition

The chemical composition of the high tensile steel sheet according to the embodiment of the present invention will be described below. First, C, si, mn, P, S, al, cu, nb, ni, ti and N, which are basic elements, will be described, and elements that can be selectively added will be described.

[ C:0.02 mass% or more and 0.06 mass% or less

C is an element contributing to the high strength of the steel sheet, and therefore is contained in an amount of 0.02 mass% or more. The C content is preferably 0.03 mass% or more. On the other hand, when C is excessively contained, MA is formed, resulting in a decrease in toughness of the base material and a decrease in HAZ toughness (i.e., toughness of the HAZ portion), and also in deterioration of weldability. Therefore, the C content is 0.06 mass% or less. The C content is preferably 0.05 mass% or less. The term "MA" is an abbreviation of martensite-austenite constituent, and is a complex (composite structure) of martensite and austenite. "MA" is also known as "island martensite".

[ Si: more than 0.50% by mass and less than 0.0% by mass

Si is an element effective as a deoxidizing material and is also effective for improving the strength of the base material. Therefore, the Si content is higher than 0 mass%. The Si content is preferably 0.05 mass% or more, more preferably 0.10 mass% or more. On the other hand, if Si is excessively contained, MA is formed and the toughness and HAZ toughness of the base material are reduced, so that the Si content is 0.50 mass% or less. The Si content is preferably 0.35 mass% or less, more preferably 0.30 mass% or less.

[ Mn:0.90 mass% or more and 1.60 mass% or less ]

Mn is an element effective in stabilizing austenite and lowering the transformation temperature to refine the structure by rolling. Mn is also an element effective in increasing strength. Therefore, mn is contained in an amount of 0.90 mass% or more. The Mn content is preferably 1.10 mass% or more, more preferably 1.20 mass% or more. On the other hand, when Mn is excessively contained, mnS coarsens and the toughness of the base material deteriorates. Therefore, the Mn content is 1.60 mass% or less. The Mn content is preferably 1.55 mass% or less.

[ P: more than 0.03% by mass and not more than

P is an unavoidable impurity, and adversely affects toughness of the base material and the joint. Therefore, the P content is suppressed to 0.03 mass% or less. It is industrially difficult to achieve 0 mass% of P, and the lower limit is higher than 0 mass%.

[ S: more than 0.01% by mass and not more than 0.0% by mass

S is an element that forms MnS and deteriorates toughness of the base material. Therefore, S needs to be suppressed to 0.01 mass% or less. The S content is preferably 0.005% by mass or less. S is difficult to industrially achieve 0 mass% and the lower limit is higher than 0 mass%.

[ Al: 0.020% by mass or more and 0.070% by mass or less

Al is an element required for deoxidation. To exert this effect, al is contained in an amount of 0.020 mass% or more. The Al content is preferably 0.025 mass% or more. On the other hand, when Al is excessively contained, coarse inclusions of alumina are formed, and toughness is lowered. Therefore, the Al content is 0.070 mass% or less. The Al content is preferably 0.065 mass% or less, more preferably 0.060 mass% or less.

[ Cu:0.10 mass% or more and 0.40 mass% or less

Cu is an element effective for improving strength. In order to exert this effect, it is necessary to contain 0.10 mass% or more of Cu. The Cu content is preferably 0.15 mass% or more. On the other hand, if Cu is excessively contained, cracking is likely to occur during hot working. Therefore, the Cu content is 0.40 mass% or less. The Cu content is preferably 0.35 mass% or less.

[ Nb:0.010 mass% or more and 0.060 mass% or less

Nb is an element that suppresses recrystallization of austenite grains and refines ferrite. In order to obtain this effect, nb is contained in an amount of 0.010 mass% or more. The Nb content is preferably 0.015 mass% or more, more preferably 0.020 mass% or more. On the other hand, if Nb is excessively contained, MA is formed, and toughness is lowered. Therefore, the Nb content is 0.060 mass% or less. The Nb content is more preferably 0.055 mass% or less.

[ Ni:0.40 to 0.80 mass%

Ni is an element effective for improving the strength and low-temperature toughness of a steel sheet, and is also effective for improving the HAZ toughness. When the Ni content is less than 0.40 mass%, the effect of adding Ni is insufficient, and good low-temperature toughness of the steel sheet cannot be ensured. Therefore, ni is contained in an amount of 0.40 mass% or more. The Ni content is preferably 0.45 mass% or more, more preferably 0.50 mass% or more. On the other hand, if the Ni content is excessive, the strength-increasing effect becomes excessive as compared with the effect of suppressing ductile fracture at low temperature, and the low-temperature toughness is deteriorated. Therefore, the Ni content needs to be 0.80 mass% or less. The Ni content is preferably 0.75 mass% or less.

[ Ti:0.005 mass% or more and 0.025 mass% or less ]

Ti is a powerful nitride forming element, and TiN can be finely precipitated in a small amount, thereby exhibiting a crystal grain refinement effect. In order to exert this effect, ti is contained in an amount of 0.005% by mass or more. The Ti content is preferably 0.007 mass% or more. On the other hand, if Ti is excessively contained, the joint toughness is reduced. Therefore, the Ti content is 0.025 mass% or less. The Ti content is preferably 0.023 mass% or less.

[ N:0.0020 to 0.0080 mass%

N is an element effective for forming AlN and TiN, and for suppressing coarsening of austenite grains during heating and welding before hot rolling, and for improving toughness of the base material and HAZ toughness. In order to exert this effect, N is contained in an amount of 0.0020 mass% or more. The N content is preferably 0.0030 mass% or more. On the other hand, if N is excessively contained, solid solution N increases, and the toughness of the base material deteriorates. Therefore, the N content is 0.0080 mass% or less. The N content is preferably 0.0070 mass% or less.

[ margin ]

The balance being iron and unavoidable impurities. As unavoidable impurities, mixing of trace elements (e.g., as, sb, sn, etc.) introduced due to the conditions of raw materials, manufacturing facilities, etc. can be allowed. For example, P and S are generally preferable as the content is smaller, and therefore are unavoidable impurities, but there are other elements defined separately as described above with respect to the composition ranges thereof. Therefore, in the present specification, the term "unavoidable impurities" constituting the balance is a concept excluding elements that are separately defined for the composition range.

Any other element may be contained as long as the characteristics of the high-tensile steel sheet according to the embodiment of the present invention can be maintained. Hereinafter, other elements that can be selectively contained in this way are exemplified.

[ from B: above 0 mass% and below 0.0015 mass%, ca: above 0 mass% and below 0.003 mass%, and Mo: more than 0 mass% and not more than 0.50 mass% of one or more selected from the group consisting of

If necessary, the composition may further contain a compound selected from the group consisting of: above 0 mass% and below 0.0015 mass%, ca: above 0 mass% and below 0.003 mass%, and Mo: more than 0 mass% and not more than 0.50 mass% of one kind selected from the group consisting of the above.

B has an effect of reducing solid solution N that adversely affects toughness by generating BN. Therefore, B may be contained in an amount of more than 0 mass%. The B content is preferably 0.0005 mass% or more. On the other hand, if the B content is too large, the precipitate of B increases, and the toughness is rather deteriorated. Therefore, when B is contained, the B content is 0.0015 mass% or less. The B content is preferably 0.0012 mass% or less.

Ca is an element effective for improving toughness of a steel sheet by inclusion control. Therefore, ca may be contained in an amount of more than 0 mass%. The Ca content is preferably 0.0005 mass% or more. On the other hand, if Ca is excessively contained, toughness is lowered. Therefore, when Ca is contained, the Ca content is 0.003 mass% or less. The Ca content is preferably 0.0025 mass% or less.

Mo is an element effective in improving strength. Therefore, mo may be contained in an amount of more than 0 mass%. The Mo content is preferably 0.10 mass% or more. On the other hand, if Mo is excessively contained, toughness is lowered. Therefore, when Mo is contained, the Mo content is 0.50 mass% or less. The Mo content is preferably 0.40 mass% or less.

[ Mg, REM, zr: total about 0.0010 mass% or less

If the element forming the oxide of Mg, REM (Rare Earth element), zr, etc. is at an unavoidable impurity level of about 0.0010 mass% or less in total, the influence on the characteristics is small, and thus it may be contained.

[ parameter PY:1.300 to 2.500 inclusive

In the embodiment of the present invention, the parameter PY represented by the following formula (1) is controlled to be 1.300 or more and 2.500 or less. Nb and C constituting the parameter PY are precipitated as NbC, thereby suppressing recrystallization of austenite grains and expanding the unrecrystallized region. Therefore, nb and C are elements that contribute to promotion of ferrite refinement by rolling. Si, cu, ni, and Mo, which constitute the parameter PY, are elements that stabilize austenite, lower the ferrite nucleation temperature, and contribute to the refinement of the structure by rolling. The inventors have made experiments in consideration of these elements contributing to ferrite refinement, and found the parameter PY based on this. When the parameter PY is less than 1.300, the balance of strength and toughness is deteriorated (i.e., 1 or more of the strength and low-temperature toughness of the base material are deteriorated). Therefore, the parameter PY is 1.300 or more. The parameter PY is preferably 1.400 or more, more preferably 1.500 or more. On the other hand, as a result of studies to reduce MA in the joint tissue, it was found that the parameter PY was related to MA fraction. That is, when the parameter PY is higher than 2.500, the MA fraction in the joint structure increases, and the low-temperature toughness of the joint deteriorates. Therefore, the parameter PY is 2.500 or less. The parameter PY is preferably 2.400 or less, more preferably 2.300 or less.

The reason for setting the parameter PY will be described in more detail with reference to fig. 1 to 4.

The present inventors have examined the relationship between the low-temperature toughness of a joint and the structure of the joint in order to secure the joint toughness at low temperatures. As shown in examples described later, in order to evaluate the joint toughness of the welded article obtained by welding, the impact absorption work vE at-62℃was measured _－62℃ . FIG. 1 shows this vE _－62℃ A graph showing the relationship between the MA (island martensite) fraction of the joint. In the embodiments of the present invention, it was found that in order to achieve the targeted vE _－62℃ As shown in fig. 1, the MA fraction of the joint structure needs to be suppressed to 4 area% or less, which is excellent in low-temperature toughness of 27J or more.

The present inventors have studied means for suppressing the MA fraction in the tissue of the joint. Fig. 2 is a graph showing a relationship between the MA fraction of the joint and the parameter PY. The MA fraction in fig. 1 and 2 is obtained by observing the structure of the joint in the welded article after welding in examples and the like described later. As shown in fig. 2, it was found that if the parameter PY was suppressed to 2.500 or less, the MA fraction in the tissue of the above joint could be suppressed to 4 area% or less.

On the other hand, the embodiment of the present invention aims to achieve an excellent balance between the strength and toughness of the base material. Therefore, the present inventors have further studied that MA may remain in the base material structure even if the parameter PY is satisfied as described above to ensure the joint toughness. As a result, the strength and toughness balance of the base material is also deteriorated. Therefore, the present inventors considered that the influence of the MA residue can be reduced by increasing the area ratio of fine ferrite having an equivalent circle diameter of 7.5 μm or less, and studied the effect more intensively. FIG. 3 is a graph showing the relationship between the area ratio (FR) of ferrite having an equivalent circle diameter of 7.5 μm or less and the parameter PY. As a result of the intensive studies, it was found that, as shown in fig. 3, by setting the parameter PY to 1.300 or more, the area ratio of fine ferrite having an equivalent circle diameter of 7.5 μm or less can be increased to 20% or more.

The inventors of the present invention confirmed that the increase in the area ratio of the fine ferrite has an effect on the balance of the strength and toughness of the base material. In the embodiment of the present invention, the parameter TV calculated from the tensile strength and fracture morphology transition temperature of the base material is used as an index of balance of strength and toughness. If the parameter TV is-4000 or less, it can be said that the strength and toughness are well balanced. Details of the parameter TV will be described later. Fig. 4 is a diagram showing a relationship between parameter PY and parameter TV. As shown in fig. 4, it was confirmed that the strength and toughness were well balanced by setting the parameter PY to 1.300 or more and the parameter TV to-4000 or less. As described above, the inventors have found that by setting the parameter PY to 1.300 or more and 2.500 or less, the strength-toughness balance of the base material can be improved together with the low-temperature toughness of the joint portion.

2. Metal structure of base material

The following describes the details of the metallic structure of the high-tensile steel sheet according to the embodiment of the present invention.

Area fraction of ferrite: 60% or more ]

In order to achieve an improvement in the balance of strength and toughness due to fine ferrite, which will be described later, the area fraction of ferrite with respect to the entire metal structure is 60% or more. The area fraction of ferrite is preferably 70% or more, more preferably 78% or more. The area fraction of ferrite is preferably 99% or less, more preferably 98% or less, if the chemical composition of the steel sheet and the manufacturing method according to the embodiment of the present invention are considered. The method for measuring the area fraction of ferrite is described below.

[ total area fraction of ferrite having an equivalent circle diameter of 7.5 [ mu ] m or less: 20% or more ]

As described above, in order to obtain the toughness of the joint, it is necessary to satisfy the predetermined component ranges and parameters PY. However, even if the predetermined component ranges and the parameter PY are satisfied, MA may remain in the base material structure while ensuring the base material strength and the toughness of the joint. MA becomes the starting point of fracture, and may deteriorate the toughness of the base material, and deteriorate the strength-toughness balance. The above-described influence of MA can be reduced by ensuring that the total area Fraction (FR) of ferrite having an equivalent circle diameter of 7.5 μm or less is 20% or more with respect to the entire metal structure. The total area fraction is preferably 25% or more, more preferably 30% or more. The upper limit of the total area fraction is not particularly limited, but is about 80% when the chemical composition and the production conditions are taken into consideration. The method for measuring the total area fraction is described below.

[ residual tissue ]

The rest structure is at least one selected from the group consisting of pearlite, bainite, cementite, retained austenite, martensite, and MA. The area fraction of MA of the base material is preferably 10% or less, more preferably 8% or less, and even more preferably 6% or less, from the viewpoint of securing toughness. The area fraction of MA of the base material is preferably 0% from the viewpoint of securing toughness, but is necessarily generated within the present composition range. Therefore, MA in the base material may be 0.5% or more in terms of area fraction, and may be 0.6% or more. Ferrite having an equivalent circle diameter of 7.5 μm or less may be present as long as the total area fraction of ferrite is 20% or more.

As described above, in the embodiment of the present invention, since the parameter PY is set to 2.500 or less, MA fraction in the joint structure is reduced and low-temperature toughness of the joint portion is improved when welding is performed using the high-tensile steel sheet of the embodiment of the present invention. Therefore, if the parameter PY is controlled in the above manner, the MA fraction in the joint structure is not particularly limited, but is preferably 3.5% or less, more preferably 3.2% or less.

3. Characteristics of

The characteristics of the high-tensile steel sheet (base material) according to the embodiment of the present invention and the characteristics of the joint portion when welding using the high-tensile steel sheet according to the embodiment of the present invention are described in detail below.

3-1. Characteristics of steel sheet

(1) Balance of strength and Toughness (TV)

The high-tensile steel sheet according to the embodiment of the present invention has excellent balance between strength and toughness. That is, the high tensile strength steel sheet according to the embodiment of the present invention has high strength and is superior to conventional low temperature toughness. In the evaluation of the strength-toughness balance, a parameter TV represented by the following formula (2) was used. When the parameter TV is-4000 or less, the balance of strength and toughness is excellent.

TV＝3×vTrs－7×TS…(2)

Wherein,

vTrs: fracture morphology transition temperature (DEG C) of base material

TS: tensile Strength (MPa) of parent Material

(2) Tensile Strength (TS), yield Strength (YP), fracture morphology transformation temperature (vTrs)

The characteristics of the steel sheet may satisfy the above-described parameter TV. The Tensile Strength (TS) is preferably 515MPa or more, more preferably 520MPa or more. The yield strength (YP) is preferably 360MPa or more, more preferably 380MPa or more. The fracture morphology transition temperature (vTrs) is preferably-80℃or lower, more preferably-90℃or lower.

3-2. Characteristics of the joint

The high-tensile steel sheet according to the embodiment of the present invention has excellent low-temperature toughness at a joint portion formed when welding is performed at a line energy of 10kJ/mm or more and 11kJ/mm or less. Specifically, the impact absorption work vE at-62 ℃ at the joint _－62℃ 27J or more. vE _－62℃ Preferably 30J or more, more preferably 40J or more.

4. Method of manufacture

Next, a method for manufacturing a high tensile steel sheet according to an embodiment of the present invention will be described.

The present inventors have found that a steel having a predetermined chemical composition can be subjected to hot rolling described later to obtain a high-tensile steel sheet having the desired microstructure and, as a result, the desired properties. Details thereof are described below.

After the steel sheet having the above chemical composition was heated, hot rolling was performed under the following conditions. In the heating step before rolling, it is preferable to heat the steel sheet such as a slab at 1000 to 1250 ℃.

[ step of pressing at a cumulative reduction of 30% or more in a temperature range of 900 ℃ or more ]

In order to refine the austenite grains, it is necessary to heat the austenite grains to a recrystallization temperature range and then to sufficiently press the austenite grains. The cumulative reduction can be applied over a recrystallization temperature range: a reduction of 30% or more (hereinafter, this cumulative reduction is referred to as "1 st cumulative reduction") can accumulate dislocations in austenite grains, and new grains can be generated by using the dislocations as a driving force. In a steel sheet having such a chemical composition, recrystallization occurs by applying pressure in a high temperature region (recrystallization temperature range) of 900 ℃ or higher. In order to effectively exhibit the above-described effects, the 1 st cumulative reduction is set to 30% or more, preferably 35% or more. The 1 st cumulative reduction is usually 80% or less.

[ step of pressing at a cumulative reduction of 20% to 80% in a temperature range of Ar3 to 900 ℃ inclusive ]

Next, in order to increase the deformation zone in which ferrite can nucleate, it is necessary to perform sufficient pressing also in the unrecrystallized temperature range. If the pressure is applied at a lower temperature than the recrystallization temperature range, austenite grains cannot form new grains, and a flat structure is formed, and the deformed bands are introduced into the grains. In order to effectively exhibit the above-described effects, the rolling reduction in the unrecrystallized temperature range is such that the cumulative rolling reduction (hereinafter, this cumulative rolling reduction is referred to as "the 2 nd cumulative rolling reduction") is 20% or more, preferably 25% or more in the temperature range of Ar3 or more and less than 900 ℃. The 2 nd cumulative reduction is usually 80% or less.

Here, ar3 (°c) is calculated from the following formula.

Further, when the rolling is performed in a two-phase temperature range lower than the unrecrystallized temperature range, the strength of the steel sheet increases, but the stress concentration accompanying the work strengthening becomes remarkable, and the toughness of the steel sheet deteriorates. Therefore, it is preferable not to perform the pressing in the two-phase temperature range.

The above-described 1 st cumulative reduction and 2 nd cumulative reduction are calculated by the following formulas.

The 1 st cumulative reduction (%) = (H1-H2)/h1×100

The 2 nd cumulative reduction (%) = (H2-t)/h2×100

In the above, the first step of,

h1 is the plate thickness (for example, slab thickness) at the beginning of rolling in the temperature range of 900 ℃ or higher,

h2 is a plate thickness at the end of rolling in a temperature range of 900 ℃ or more=ar3 or more and a plate thickness at the beginning of rolling in a temperature range of less than 900 ℃,

t is the finished thickness in mm.

[ step of cooling to a cooling stop temperature of 500 ℃ or higher and (cooling start temperature-20 ℃) or lower at an average cooling rate of 1 ℃ or higher and 10 ℃ or lower from a cooling start temperature of (Ar 3-30 ℃) or higher ]

Then, the steel is cooled from a cooling start temperature of (Ar 3-30 ℃) to a cooling stop temperature of (cooling start temperature-20 ℃) or higher than 500 ℃ at an average cooling rate of 1 ℃/sec or higher and 10 ℃/sec or lower. If the cooling is performed from a cooling start temperature lower than (Ar 3-30 ℃ C.), the grain boundary ferrite precipitates and coarsens, and the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less decreases. Therefore, the cooling is performed from a cooling start temperature of (Ar 3-30 ℃) or higher. The cooling start temperature is preferably (Ar 3-20deg.C) or higher, more preferably (Ar 3-10deg.C) or higher. From the viewpoint of securing the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less, the cooling start temperature is preferably (Ar 3+60℃ C.) or less, more preferably (Ar 3+40 ℃ C.) or less. In order to achieve ferrite refinement, the lower the cooling stop temperature is, the better. Therefore, the cooling stop temperature is not higher than (cooling start temperature-20 ℃ C.), preferably not higher than (cooling start temperature-30 ℃ C.), and more preferably not higher than (cooling start temperature-40 ℃ C.). On the other hand, if the cooling stop temperature is low, the MA amount increases. Therefore, the cooling stop temperature is 500℃or higher, preferably 510℃or higher, and more preferably 520℃or higher. In order to suppress ferrite growth by accelerated cooling, it is necessary to set the average cooling rate to 1.0 ℃/sec or more, preferably 1.2 ℃/sec or more, and more preferably 1.5 ℃/sec or more. On the other hand, if the average cooling rate is too high, the desired ferrite fraction cannot be ensured, and the toughness is lowered. Therefore, the average cooling rate is 10 ℃ per second or less, preferably 9.0 ℃ per second or less, and more preferably 8.0 ℃ per second or less. After the above-mentioned accelerated cooling, the material can be cooled to room temperature, for example.

The high-tensile steel sheet according to the embodiment of the present invention can be applied to a so-called thick steel sheet. The thickness is about 6mm or more, preferably 10mm or more. The upper limit of the plate thickness is not particularly limited, but is usually about 40mm or less.

Examples

Steel sheets satisfying the chemical compositions shown in table 1 were heated at the heating temperatures shown in table 2, and then hot-rolled under the conditions shown in table 2 to produce thick steel sheets. In Table 2, the "average cooling rate" refers to an average cooling rate from a cooling start temperature of (Ar 3-30 ℃) to a cooling stop temperature of not less than 500 ℃ and not more than (cooling start temperature-20 ℃). The "cooling stop temperature" refers to a stop temperature of cooling at the "average cooling rate" described above. The thickness of the produced thick steel plate is also shown in table 2. In table 1, the line (-) indicates that the chemical composition was not detected. In table 1 and table 3 described below, underlined values indicate that the range of the embodiment of the present invention is deviated.

[ Table 1 ]

[ Table 2 ]

[ observation of Metal Structure ]

Samples were extracted from the thick steel plate so that a plate thickness cross section including the front and rear surfaces of the steel plate was observed parallel to the rolling direction and perpendicular to the surface of the steel plate. In the observation of the metal structure, the corrosion was performed by using a 3% nital solution or a lepera solution according to the observation object, and the position 6mm to 7mm from the surface was observed. Using an optical microscope, 1 region with 600 μm X800 μm field of view was observed at 100 times magnification. The area fraction of ferrite, the area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less, and the area fraction of MA were measured by image analysis. The test piece is qualified by a test piece in which the area fraction of ferrite is 60% or more and the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less is 20% or more.

[ elongation test of base Material ]

From the t (plate thickness)/4 portion, a test piece No.4 of JIS Z2201 was extracted in the direction of right angle to roll, and a tensile test was performed in accordance with the procedure of JIS Z2241 to measure the Tensile Strength (TS) and yield strength (YP).

[ evaluation of Low temperature toughness of base Material ]

From the surface of each steel sheet, the position of the steel sheet facing the thickness direction of 6mm to 7mm was set to be the same as the center of the pendulum impact test piece, and the test piece was extracted with the longitudinal direction of the test piece at right angles to the rolling direction. Then, a pendulum impact test was performed in accordance with the neck of JIS Z2242, and the fracture morphology transition temperature vTrs was measured. The measurement results are shown in table 3. The strength-toughness balance (TV) was calculated from the above formula (2), and is shown in table 3. The samples having a strength-toughness balance (TV) of-4000 or less were evaluated as excellent (acceptable) in the strength-toughness balance.

[ evaluation of Low temperature toughness of Joint ]

Welding was performed at a line energy of 10kJ/mm or more and 11kJ/mm or less, and a test piece was extracted from the obtained weld. The test piece is extracted by making the position of the weld joint from the same surface as the base material toward the thickness direction of 6 mm-7 mm the same as the center of the pendulum impact test piece, making the longitudinal direction of the test piece be perpendicular to the welding line direction, and making the longitudinal direction of the test piece be perpendicular to the rolling direction. Then, a pendulum impact test was performed in accordance with JIS Z2242 to obtain an impact absorption energy (vE) at-62 ℃ _－62℃ ) The joint was evaluated for low-temperature toughness. vE _－62℃ The test pieces of 27J or more were evaluated as excellent (acceptable) in low-temperature toughness.

In addition, the tissue of the joint portion was also observed. Specifically, the sample at the joint was etched using a 3% nital solution or lepera solution according to the observation object, and the grain boundary and MA were developed. Then, at a position 6mm to 7mm in the plate thickness direction from the surface, the visualized tissue was observed at 400 times magnification using an optical microscope, and 1 field of view corresponds to a region of 200 μm×160 μm. The area fraction of MA was calculated by image analysis software.

These evaluation results are shown in table 3.

[ Table 3 ]

The results of Table 3 were examined. No.1 to 3, 5, 6, 9 to 15 are examples of the invention which fully satisfy the requirements of the embodiments of the present invention. Since the alloy has a predetermined chemical composition and a predetermined metal structure, the alloy has high strength and excellent low-temperature toughness, that is, the balance of strength and toughness is excellent, and the low-temperature toughness of the joint portion is also excellent.

On the other hand, nos. 4, 7 and 8 do not satisfy a certain rule of the embodiment of the present invention, so that the characteristics deteriorate. Specifically, in the case of No.4, since the parameter PY is small and the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less is small, the strength-toughness balance is deteriorated. No.7 and 8, because the parameter PY is high, the MA fraction of the joint portion becomes high, and the low-temperature toughness of the joint portion deteriorates.

The present application is accompanied by Japanese patent application No. 2020-066825, which claims priority from Japanese patent application No. 2020, 4/2. Japanese patent application No. 2020-066825 is incorporated herein by reference.

Claims

1. A high-tensile steel sheet excellent in base material toughness and joint toughness, which comprises the following components:

c:0.02 mass% or more and 0.06 mass% or less;

si: more than 0 mass% and not more than 0.50 mass%;

mn:0.90 mass% or more and 1.60 mass% or less;

p: more than 0 mass% and not more than 0.03 mass%;

s: more than 0 mass% and not more than 0.01 mass%;

al:0.020 mass% or more and 0.070 mass% or less;

cu:0.10 mass% or more and 0.40 mass% or less;

nb:0.010 mass% or more and 0.060 mass% or less;

ni:0.40 mass% or more and 0.80 mass% or less;

ti:0.005 mass% or more and 0.025 mass% or less;

n:0.0020 mass% or more and 0.0080 mass% or less; and

the balance: iron and unavoidable impurities are present in the steel,

the parameter PY expressed by the following formula 1 is 1.300 to 2.500,

with respect to the overall metallic structure,

the area fraction of ferrite is 60% or more,

ferrite having an equivalent circle diameter of 7.5 μm or less has a total area fraction of 20% or more,

the strength-toughness balance parameter TV is-4000 or less,

the hot rolling step in the production of the steel sheet comprises cooling from a cooling start temperature of Ar3-30 ℃ to a cooling stop temperature of 500 ℃ or more and the cooling start temperature of-20 ℃ or less at an average cooling rate of 2.94 ℃ per second or more and 10 ℃ per second or less,

the parameter PY=10× ([ Nb ] +3× [ C ])× (2× [ Si ] + [ Cu ] + [ Ni ] + [ Mo ]) … 1

Ar3＝868－369×[C]+24.6×[Si]－68.1×[Mn]－36.1×[Ni]－20.7×[Cu]－24.8×[Cr]+29.6×[Mo]

Wherein Ar3 is represented by the unit of °c, [ ] represents the content of each element in mass%, and the content of the element not contained is set to zero.

2. A high-tensile steel sheet excellent in base material toughness and joint toughness, which comprises the following components:

c:0.02 mass% or more and 0.06 mass% or less;

si: more than 0 mass% and not more than 0.50 mass%;

mn:0.90 mass% or more and 1.60 mass% or less;

p: more than 0 mass% and not more than 0.03 mass%;

s: more than 0 mass% and not more than 0.01 mass%;

al:0.020 mass% or more and 0.070 mass% or less;

cu:0.10 mass% or more and 0.40 mass% or less;

nb:0.010 mass% or more and 0.060 mass% or less;

ni:0.40 mass% or more and 0.80 mass% or less;

ti:0.005 mass% or more and 0.025 mass% or less;

n:0.0020 mass% or more and 0.0080 mass% or less;

from B: above 0 mass% and below 0.0015 mass%, ca: above 0 mass% and below 0.003 mass%, and Mo: more than 0 mass% and not more than 0.50 mass% of one selected from the group consisting of; and

the balance: iron and unavoidable impurities are present in the steel,

the parameter PY expressed by the following formula 1 is 1.300 to 2.500,

with respect to the overall metallic structure,

the area fraction of ferrite is 60% or more,

the strength-toughness balance parameter TV is-4000 or less,

3. A method for producing a high-tensile steel sheet excellent in base material toughness and joint toughness according to claim 1 or claim 2, wherein,

comprising a step of heating a steel having the composition described in claim 1 or claim 2 at a temperature of 1000 ℃ or more and 1250 ℃ or less, and a hot rolling step after the heating,

the hot rolling process includes:

a step of cooling the steel sheet from a cooling start temperature of Ar3-30 ℃ to a cooling stop temperature of-20 ℃ or lower at an average cooling rate of 2.94 ℃ to 10 ℃ inclusive to 500 ℃ inclusive,

here the number of the elements to be processed is,