CN115003842A

CN115003842A - High-tension steel plate with excellent base metal toughness and joint toughness and manufacturing method thereof

Info

Publication number: CN115003842A
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: 2022-09-02
Anticipated expiration: 2041-03-25
Also published as: CN115003842B; WO2021200572A1; JP2021161524A; JP7519202B2; KR20220141863A

Abstract

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

Description

High-tension steel plate with excellent base metal toughness and joint toughness and manufacturing method thereof

Technical Field

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

Background

Thick steel plates for LPG tanks and the like used in low-temperature environments are required to have high strength and excellent toughness at low temperatures (hereinafter referred to as "low-temperature toughness"). Further, it is also required that a weld joint (hereinafter, sometimes simply referred to as "joint" or "joint") between the weld metal and the Heat Affected Zone (HAZ) is excellent in low-temperature toughness (hereinafter, sometimes referred to as "joint toughness"). In particular, in recent years, high toughness at extremely low temperatures has been required from the viewpoint of safety.

Here, the addition of the alloy is effective for improving the strength, but on the other hand, causes a decrease in toughness. Therefore, it is extremely difficult to achieve both strength and toughness. One means for improving both strength and toughness is to add Ni. However, as represented by 3.5% Ni steel and 9% Ni steel, this effect cannot be exhibited to the maximum extent unless a large amount of Ni is added. Therefore, a thick steel plate excellent in base metal strength, low-temperature toughness, and low-temperature toughness of a joint portion, in which the amount of Ni is further suppressed, has been studied.

For example, patent document 1 discloses a high-tension steel sheet having a yield stress of 420MPa or more and excellent low-temperature toughness in a welding heat affected zone of a multilayer welded part at small to middle line energies, which is suitable for steel structures such as ships, marine structures, pressure vessels, and penstocks, and a method for manufacturing the same. In patent document 1, a high-tension steel plate having a predetermined composition and having excellent low-temperature toughness in a welding heat-affected zone is obtained by controlling the hardness of a center segregation portion of the steel plate.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 2012-184500

Disclosure of Invention

Problems to be solved by the invention

However, the technique disclosed in patent document 1 may not be sufficient in toughness at a lower temperature, where the toughness evaluation temperature of the base material and the joint is-40 ℃.

An embodiment of the present invention has been made in view of the above circumstances, and an object thereof is to provide a high-tension steel sheet having high strength, excellent base metal toughness at a lower temperature, and excellent low-temperature toughness of a weld 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-tension steel plate excellent in base metal toughness and joint toughness, including:

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% inclusive,

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% by mass or more and 0.070% by mass or less,

Cu: 0.10 to 0.40 mass% inclusive,

Nb: 0.010 to 0.060 mass%,

Ni: 0.40 to 0.80 mass%,

Ti: 0.005% by mass or more and 0.025% by mass or less, and

n: 0.0020 to 0.0080 mass%,

the balance consisting of iron and unavoidable impurities,

a parameter PY represented by the following formula (1) is 1.300 or more and 2.500 or less,

with respect to the entire metal structure,

the surface 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.

Parameter PY ═ 10 × ([ Nb ] +3 × [ C ]) × (2 × [ Si ] + [ Cu ] + [ Ni ] + [ Mo ]) … (1)

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

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

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%.

Embodiment 3 of the present invention is a method for producing a high-tensile-strength steel sheet excellent in base metal toughness and joint toughness according to

embodiment

1 or 2, wherein,

comprising a step of heating a steel having the composition described in embodiment 1 or embodiment 2 at 1000 ℃ to 1250 ℃, and a hot rolling step after the heating,

the hot rolling process comprises:

a step of reducing the pressure at a cumulative reduction ratio of 30% or more in a temperature range of 900 ℃ or higher;

a step wherein the pressure is reduced at a cumulative reduction ratio of 20% to 80% in a temperature range of Ar3 to less than 900 ℃;

and cooling the steel sheet to a cooling stop temperature of 500 ℃ or higher (the cooling start temperature-20 ℃) or lower at an average cooling rate of 1 ℃/sec or higher and 10 ℃/sec or lower from the cooling start temperature of (Ar 3-30 ℃) or higher.

In this case, the amount of the solvent to be used,

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, excellent base metal toughness at a lower temperature, and excellent low-temperature toughness of a welded joint portion at the time of welding can be obtained.

Drawings

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

FIG. 2 is a graph showing the relationship between the parameter PY and the MA-plane integral rate of the linker portion.

FIG. 3 is a graph showing the relationship 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 the relationship between the parameter PY and the strength-toughness balance (TV).

Detailed Description

As a result of intensive studies, the present inventors have found that a high-tensile steel sheet having high strength, excellent base material toughness at a lower temperature than conventional, and excellent low-temperature toughness at a welded joint portion 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, sometimes referred to as "FR") to 20% or more.

1. Chemical composition of components

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 as 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 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, if C is excessively contained, MA is formed, which causes a decrease in base material toughness and a decrease in HAZ toughness (i.e., toughness of the HAZ portion), and also deteriorates weldability. Therefore, the C content is 0.06 mass% or less. The C content is preferably 0.05 mass% or less. The term "MA" as used herein is an abbreviation for martensite-austenit constancy, and refers to a composite structure of martensite and austenite. "MA" is also referred to as "island martensite".

[ Si: more than 0 mass% and not more than 0.50 mass% ]

Si is an element effective as a deoxidizing material, and is also an element 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, and more preferably 0.10 mass% or more. On the other hand, if Si is excessively contained, MA is formed to lower the toughness of the base material and the HAZ toughness, and therefore the Si content is 0.50 mass% or less. The Si content is preferably 0.35 mass% or less, and more preferably 0.30 mass% or less.

[ Mn: 0.90 to 1.60 mass% ]

Mn is an element effective in stabilizing austenite, lowering the transformation temperature, and refining the structure by rolling. Further, Mn is also an effective element for increasing the strength. Therefore, Mn is contained by 0.90 mass% or more. The Mn content is preferably 1.10 mass% or more, and more preferably 1.20 mass% or more. On the other hand, if 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 mass% and not more than 0.03 mass% ]

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

[ S: more than 0 mass% and not more than 0.01 mass% ]

S is an element that forms MnS and deteriorates the toughness of the base material. Therefore, S needs to be suppressed to 0.01 mass% or less. The S content is preferably 0.005 mass% or less. S is industrially difficult to reach 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. In order to exhibit this effect, Al is contained by 0.020 mass% or more. The Al content is preferably 0.025 mass% or more. On the other hand, if Al is excessively contained, coarse alumina inclusions are formed, and the toughness is lowered. Therefore, the Al content is 0.070 mass% or less. The Al content is preferably 0.065 mass% or less, and more preferably 0.060 mass% or less.

[ Cu: 0.10 to 0.40 mass% ]

Cu is an element effective for strength improvement. 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 contained excessively, cracks are 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 by 0.010 mass% or more. The Nb content is preferably 0.015 mass% or more, and more preferably 0.020 mass% or more. On the other hand, if Nb is excessively contained, MA is formed, and the 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 the steel sheet, and is also effective for improving the HAZ toughness. If 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 by 0.40 mass% or more. The Ni content is preferably 0.45 mass% or more, and more preferably 0.50 mass% or more. On the other hand, if the Ni content is excessive, the strength increasing effect becomes excessive compared to the effect of suppressing ductile fracture at low temperature, and the low-temperature toughness deteriorates. 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% by mass or more and 0.025% by mass or less ]

Ti is a strong nitride-forming element, and TiN can be finely precipitated in a small amount, thereby exerting the effect of refining crystal grains. In order to exert this effect, 0.005 mass% or more of Ti is contained. 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 that forms AlN and TiN, suppresses coarsening of austenite grains during heating before hot rolling and during welding, and is effective for improving base material toughness and HAZ toughness. In order to exhibit this effect, N is contained by 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.

[ balance ]

The balance being iron and unavoidable impurities. As the inevitable impurities, it is allowable to mix in trace elements (for example, As, Sb, Sn, etc.) introduced depending on the conditions of raw materials, manufacturing facilities, etc. For example, as P and S, the smaller the content is, the more preferable the content is, and therefore, the impurities are inevitable. Therefore, in the present specification, the term "unavoidable impurities" constituting the balance means a concept excluding elements that are specified for the composition range.

Any other element may be further included 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 manner are exemplified.

[ from B: more than 0 mass% and less than 0.0015 mass%, Ca: more than 0 mass% and less than 0.003 mass%, and Mo: more than 0 mass% and 0.50 mass% or less of the group selected)

If necessary, the compound may contain a compound selected from the group consisting of B: more than 0 mass% and less than 0.0015 mass%, Ca: more than 0 mass% and less than 0.003 mass%, and Mo: more than 0 mass% and not more than 0.50 mass%.

B has the effect of reducing the amount of dissolved N that adversely affects toughness by generating BN. Therefore, B can 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 content of B is too large, precipitates of B increase, and toughness deteriorates conversely. Therefore, when B is contained, the B content is 0.0015 mass% or less. The content of B is preferably 0.0012% by mass or less.

Ca is an element effective for improving the toughness of the steel sheet by controlling inclusions. 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 the 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 may be 0.50 mass% or less. The Mo content is preferably 0.40 mass% or less.

[ Mg, REM, Zr: about 0.0010% by mass or less in total ]

Elements forming oxides of Mg, REM (Rare Earth Metal), Zr, etc. may be contained at an unavoidable impurity level of about 0.0010 mass% or less in total, because of little influence on the characteristics.

[ 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, whereby recrystallization of austenite grains is suppressed and a non-recrystallized region is expanded. Therefore, Nb and C are elements contributing to promotion of ferrite refinement by rolling. Si, Cu, Ni, and Mo constituting the parameter PY are elements that contribute to the refinement of the structure by rolling by stabilizing austenite and lowering the ferrite nucleation temperature. The present inventors have conducted experiments in consideration of these elements contributing to the refinement of ferrite, and have found the parameter PY accordingly. When the parameter PY is less than 1.300, the strength-toughness balance deteriorates (i.e., 1 or more of the strength and the low-temperature toughness of the base material deteriorate). 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 investigation for reducing MA in the joint tissue, it was found that the parameter PY has a relationship with the MA fraction. That is, if 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.

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

Wherein [ ] represents the content (mass%) of each element, and the content of an element not included is set to zero.

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

The inventors investigated the relationship between the low-temperature toughness of the joint and the structure of the joint in order to ensure 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 energy vE at-62 ℃ was measured _－62℃ . FIG. 1 shows this vE _－62℃ Graph showing the relationship with MA (island martensite) fraction of the joint. In embodiments of the present invention, it was found that to achieve a targeted vE _－62℃ In order to achieve such excellent low-temperature toughness as 27J or more, as shown in fig. 1, it is necessary to suppress the fraction of MA in the structure of the joint to 4 area% or less.

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

On the other hand, the embodiment of the present invention aims to balance the strength and toughness of the base material. Therefore, the present inventors have further studied intensively that MA may remain in the base material structure even if the parameter PY is satisfied as described above to secure the joint toughness. The remaining MA deteriorates the low-temperature toughness of the base material, and as a result, the balance of the strength and toughness of the base material also deteriorates. 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 further studied it 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 above-mentioned 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 present inventors have confirmed the influence of the increase in the area ratio of the fine ferrite on the balance of strength and toughness of the base material. In the embodiment of the present invention, a parameter TV calculated from the tensile strength of the base material and the fracture morphology transition temperature is used as an index of the strength-toughness balance. If the parameter TV is-4000 or less, the balance between strength and toughness is good. The details of the parameter TV will be described later. Fig. 4 is a diagram showing a relationship between the parameter PY and the parameter TV. As shown in FIG. 4, it was confirmed that the strength and toughness were well balanced when the parameter PY was 1.300 or more and the parameter TV was-4000 or less. As described above, the present inventors have found that the balance of the strength and toughness of the base material can be improved together with the low-temperature toughness of the joint portion by setting the parameter PY to 1.300 or more and 2.500 or less.

2. Metal structure of base material

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

[ area fraction of ferrite: more than 60%)

In order to improve the balance of strength and toughness by fine ferrite 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, and more preferably 78% or more. The area fraction of ferrite is preferably 99% or less, more preferably 98% or less, in consideration of the chemical composition of the steel sheet according to the embodiment of the present invention and the production method. The method of measuring the area fraction of ferrite will be described later.

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

As described above, in order to obtain the toughness of the linker, it is necessary to satisfy the predetermined composition range and the parameter PY. However, even if the predetermined composition range and parameter PY are satisfied to secure the base material strength and the joint toughness, MA may remain in the base material structure. MA becomes a starting point of fracture, and deteriorates the toughness of the base material, and deteriorates the balance of strength and toughness. The above-mentioned influence of MA can be reduced by securing a total area Fraction (FR) of ferrite having an equivalent circle diameter of 7.5 μm or less of 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% in consideration of the chemical composition and the production conditions. The method of measuring the total area fraction will be described later.

[ residual texture ]

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

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

3. Characteristics of

Hereinafter, 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 is performed using the high-tensile steel sheet according to the embodiment of the present invention will be described in detail.

3-1. Characteristics of the Steel sheet

(1) Balance of Strength and Toughness (TV)

The high-tension steel sheet according to the embodiment of the present invention has an excellent balance between strength and toughness. That is, the high-tension 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 (. degree. C.) of parent material

TS: tensile Strength (MPa) of mother Material

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

The properties of the steel sheet may satisfy the above 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 joint part

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

4. Manufacturing method

Next, a method for manufacturing a high-tensile-strength 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, which will be described later, to obtain a high-tensile steel sheet having the desired microstructure described above, and as a result, the desired properties described above can be obtained. The details thereof will be described below.

After heating, the steel sheet having the above chemical composition was hot-rolled under the following conditions. In the heating step before rolling, a steel sheet such as a slab is preferably heated at, for example, 1000 to 1250 ℃.

[ Process of reducing the pressure at a cumulative reduction ratio of 30% or more in a temperature range of 900 ℃ or higher ]

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

[ Process of reducing at a cumulative reduction ratio 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 be nucleated, it is necessary to perform sufficient rolling reduction also in the unrecrystallized temperature range. When the pressure is applied at a temperature lower than the recrystallization temperature range, austenite grains cannot form new grains, and a flat structure is formed, and a deformation zone is introduced into the grains. In order to effectively exhibit the above-mentioned effects, the non-recrystallization temperature range is a temperature range of not less than Ar3 and less than 900 ℃ and the cumulative reduction (hereinafter, the cumulative reduction is referred to as "2 nd cumulative reduction") is not less than 20%, preferably not less than 25%. The 2 nd cumulative rolling reduction is usually 80% or less.

Here, Ar3 (c) was calculated by the following formula.

When the rolling is performed in a two-phase temperature range lower than the unrecrystallized temperature range, although the strength of the steel sheet is improved, stress concentration due to work strengthening becomes remarkable, and the toughness of the steel sheet deteriorates. Therefore, it is preferable not to conduct the pressure in the two-phase temperature range.

The 1 st and 2 nd cumulative reduction ratios are calculated by the following equations.

Cumulative reduction ratio of No.1 (%) - (H1-H2)/H1X 100

Cumulative reduction ratio of No. 2 (%) - (H2-t)/H2X 100

In the above-mentioned description,

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

h2 is the plate thickness at the end of rolling in the temperature range of 900 ℃ or higher, Ar3 or higher and lower than the plate thickness at the start of rolling in the temperature range of 900 ℃,

t is the finished thickness in mm.

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

Then, the steel sheet is cooled from a cooling start temperature of (Ar 3-30 ℃) or higher to a cooling stop temperature of (cooling start temperature-20 ℃) or higher at an average cooling rate of 1 ℃/sec or higher and 10 ℃/sec or lower. When the cooling is performed at a cooling start temperature lower than (Ar 3-30 ℃), 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 at a cooling start temperature of (Ar 3-30 ℃ C.) or higher. The cooling initiation temperature is preferably (Ar 3-20 ℃ C.) or higher, more preferably (Ar 3-10 ℃ C.) or higher. The cooling initiation temperature is preferably (Ar3+60 ℃ C.) or lower, and more preferably (Ar3+40 ℃ C.) or lower, from the viewpoint of ensuring the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or lower. In order to refine the ferrite, the cooling stop temperature is preferably as low as possible. Therefore, the cooling stop temperature is (cooling start temperature-20 ℃ C.) or lower, preferably (cooling start temperature-30 ℃ C.) or lower, and more preferably (cooling start temperature-40 ℃ C.) or lower. 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 the growth of ferrite by accelerated cooling, the average cooling rate needs to be 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, a desired ferrite fraction cannot be secured, and the toughness is lowered. Therefore, the average cooling rate is 10 ℃/sec or less, preferably 9.0 ℃/sec or less, and more preferably 8.0 ℃/sec or less. After the accelerated cooling, the steel sheet 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 composition 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" means an average cooling rate from a cooling start temperature of not less than (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 at which cooling is stopped at the "average cooling rate". Table 2 also shows the thickness of the produced thick steel plate. In addition, the expression of line (-) in table 1 means that the chemical composition was not detected. In table 1 and table 3 described later, underlined numerical values indicate ranges that deviate from the embodiments of the present invention.

[ TABLE 1 ]

[ TABLE 2 ]

[ Observation of Metal Structure ]

The samples were taken from the thick steel plate so that the thickness section including the front and back surfaces of the steel plate could be observed in parallel to the rolling direction and perpendicular to the surface of the steel plate. In the observation of the metal structure, the position 6mm to 7mm from the surface was observed by etching using a 3% nital solution or a lepera solution depending on the object to be observed. Using an optical microscope, 1 region with a visual field of 600. mu. m.times.800. mu.m was observed at a magnification of 100 times. 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 sample was qualified as a sample in which the area fraction of ferrite was 60% or more and the total area fraction of ferrite having an equivalent circle diameter of 7.5 μm or less was 20% or more.

[ tensile test of base Material ]

A test piece No.4 of JIS Z2201 was taken from a portion of t (plate thickness)/4 in the direction perpendicular to the rolling direction, and a tensile test was carried out in the same manner as in JIS Z2241 to measure the Tensile Strength (TS) and yield strength (YP).

[ evaluation of Low-temperature toughness of base Material ]

The test piece was taken from the surface of each steel plate at a position of 6mm to 7mm in the plate thickness direction, which was the same as the center of the charpy impact test piece, and at a right angle to the rolling direction in the longitudinal direction of the test piece. Then, a pendulum impact test was carried out in accordance with JIS Z2242, and the fracture morphology transition temperature vTrs was measured. The measurement results are shown in table 3. Further, the strength-toughness balance (TV) was calculated from the above formula (2) and is shown in table 3. The test piece having a strength-toughness balance (TV) of-4000 or less was evaluated as excellent (acceptable) in strength-toughness balance.

[ evaluation of Low-temperature toughness of Joint ]

A test piece was taken from the resultant welded article by welding at a linear energy of 10kJ/mm to 11 kJ/mm. The test piece is extracted from the joint of the welded material in the same manner as the base materialThe position of the surface facing 6mm to 7mm in the thickness direction of the sheet was the same as the center of the pendulum impact test piece, and the test piece was taken with the longitudinal direction of the test piece at right angles to the weld line direction and at right angles to the rolling direction. Then, a pendulum impact test was carried out in accordance with JIS Z2242 to determine the energy of impact absorption (vE) at-62 ℃ _－62℃ ) The low-temperature toughness of the joint portion was evaluated. vE _－62℃ The sample having a hardness of 27J or more was evaluated to be excellent in low-temperature toughness (acceptable).

In addition, the tissue of the joint portion was also observed. Specifically, depending on the object to be observed, the sample at the joint portion was etched using a 3% nital solution or a lepera solution to expose the crystal grain boundaries and MA. Then, 1 region corresponding to 200. mu. m.times.160. mu.m in visual field was observed at a magnification of 400 times for the visualized tissue at a position of 6mm to 7mm from the surface in the thickness direction of the sheet. The area fraction of MA was calculated by image analysis software.

The evaluation results are shown in table 3.

[ TABLE 3 ]

The results of Table 3 were examined. Nos. 1 to 3, 5, 6, and 9 to 15 are invention examples which completely satisfy the requirements of the embodiment of the present invention. The steel sheet has a predetermined chemical composition and a predetermined metal structure, and therefore has high strength and excellent low-temperature toughness, that is, has an excellent balance of strength and toughness, and also has excellent low-temperature toughness at the joint portion.

On the other hand, nos. 4, 7 and 8 do not satisfy certain specifications of the embodiment of the present invention, so the characteristics deteriorate. Specifically, in No.4, 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, so that the balance of strength and toughness is deteriorated. In nos. 7 and 8, since the parameter PY is high, the MA fraction of the tab portion becomes high, and the low-temperature toughness of the tab portion deteriorates.

This application is accompanied with Japanese patent application having filing date of 2020, 4/2, and Japanese patent application No. 2020-. Japanese patent application No. 2020 and 066825 are incorporated herein by reference.

Claims

1. A high tensile strength steel plate excellent in base metal toughness and joint toughness, which comprises

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%, and,

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% by mass or more and 0.070% by mass or less,

Cu: 0.10 to 0.40 mass% inclusive,

Nb: 0.010 to 0.060 mass%,

Ni: 0.40 to 0.80 mass%,

Ti: 0.005% by mass or more and 0.025% by mass or less, and

n: 0.0020 to 0.0080 mass%,

the balance consisting of iron and unavoidable impurities,

with respect to the entire metal structure,

the surface 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,

Wherein [ ] represents the content of each element in mass%, and the content of an element not included is set to zero.

2. The high-tension steel plate excellent in base material toughness and joint toughness according to claim 1, further comprising

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%.

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

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

the hot rolling process comprises:

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

in this case, the number of the first and second,

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

where Ar3 has a unit of ℃, [ ] indicates the content of each element in mass%, and the content of an element not included is set to zero.