EP4575008A1 - Tôle d'acier épaisse, et procédé de fabrication de celle-ci - Google Patents

Tôle d'acier épaisse, et procédé de fabrication de celle-ci Download PDF

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Publication number
EP4575008A1
EP4575008A1 EP23854722.8A EP23854722A EP4575008A1 EP 4575008 A1 EP4575008 A1 EP 4575008A1 EP 23854722 A EP23854722 A EP 23854722A EP 4575008 A1 EP4575008 A1 EP 4575008A1
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steel plate
cooling
content
mass
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German (de)
English (en)
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EP4575008A4 (fr
Inventor
Masao YUGA
Yuya Sato
Shunichi Murakami
Yoshiaki Murakami
Hitoshi Sueyoshi
Yoshihiro HYODO
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JFE Steel Corp
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JFE Steel Corp
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • This disclosure relates to a steel plate, in particular, to a steel plate that has high strength, low yield ratio, and high toughness with excellent toughness of the bond portion at a welded joint even in large-heat input welding with an amount of welding heat input exceeding 40 kJ/mm.
  • the steel plate of this disclosure is also suitable for industrial production and can be used extremely well as a construction steel material.
  • This disclosure also relates to a method of producing the steel plate.
  • steel plates used are required to have low yield ratio (ratio of yield stress to tensile strength).
  • a lower yield ratio improves the plastic deformation capacity, which contributes to improvement in seismic safety of the structures.
  • a process using multi-stage heat treatment has been put to practical use as a process of producing steel plates with reduced yield ratio.
  • steel plates after hot rolling are reheated to the two-phase region of ferrite and austenite and subjected to quenching and then tempering.
  • it was difficult to achieve both high strength and low yield ratio because the yield stress increases due to the recovery of the microstructure by tempering.
  • the steel plates used are required to have excellent toughness of the heat-affected zone as well as excellent toughness of the steel plates themselves (toughness of base metal).
  • a welded joint in particular, the bond portion at the welded joint has excellent toughness.
  • the most serious problem when large-heat input welding is applied to the steel material is the deterioration of the toughness of the bond portion at the welded joint.
  • coarsening of austenite crystal grains is most pronounced in the bond portion because of exposure to high temperatures just below the melting point during large-heat input welding.
  • the coarsened austenite crystal grains then transform into a brittle upper bainitic microstructure due to the temperature drop after welding.
  • the toughness is reduced by the formation of coarse martensite austenite constituent (MA), which is an embrittlement microstructure. Therefore, if the toughness of the bond portion at the welded joint in large-heat input welding can be improved, the safety of steel structures can be greatly enhanced.
  • MA coarse martensite austenite constituent
  • the steel plates are required to have excellent toughness of the heat-affected zone.
  • Various techniques have been proposed to meet such a requirement.
  • JPH06-248337A proposes a technique to produce high-tension steel by quenching a steel sheet after hot rolling, heating and quenching the steel sheet again to the two-phase region of ferrite and austenite, and then subjecting steel sheet to tempering treatment.
  • JP2001-226740A (PTL 2) proposes a high-tension steel sheet with low yield ratio, having a specific chemical composition and an amount of retained austenite of 1.0 % or more.
  • JP2018-090872A proposes a high-strength steel plate with low yield ratio, having a specific chemical composition, a microstructure containing bainite and martensite austenite constituent, and a controlled equivalent circular diameter and average aspect ratio of prior austenite grains.
  • the toughness of the welded portion was considered.
  • evaluations were conducted only at relatively low heat input of 5 kJ/mm or 15 kJ/mm, and the toughness of the heat-affected zone in large-heat input welding with an amount of welding heat input exceeding 40 kJ/mm was not considered.
  • the volume fraction of ferrite and martensite tends to change depending on the production conditions and the position in the steel sheet. Therefore, the production conditions need to be strictly adjusted to obtain the desired product, and the high operational load makes the technique proposed in PTL 2 unsuitable for industrial production.
  • the technique proposed in PTL 3 achieves high toughness of bond portion in large-heat input welding with an amount of welding heat input exceeding 40 kJ/mm, in addition to low yield ratio and high strength.
  • the area fraction of martensite austenite constituent must be 5 % or more to achieve the above properties.
  • it is necessary to control the formation of martensite austenite constituent by performing a first water cooling step, an air cooling step, and a second water cooling step under controlled temperature conditions. During cooling, temperature variations are likely to occur in the longitudinal direction and width direction of the steel sheet. Thus, extremely strict adjustment of production conditions is required to control the microstructure during such a cooling process, which has a high operational burden.
  • the present disclosure was made in view of the above-mentioned circumstances. It could be helpful to provide a steel plate that has high strength, low yield ratio, and high toughness with excellent toughness of the bond portion at a welded joint even in large-heat input welding with an amount of welding heat input exceeding 40 kJ/mm, and is suitable for industrial production.
  • the steel plate of this disclosure can be used extremely well as a construction steel material and contributes to the increase in size and improvement in seismic resistance of steel structures.
  • the steel plate of this disclosure can be produced in a process with low operational load and is suitable for industrial production.
  • the toughness of an unwelded steel plate itself is sometimes referred to as "toughness of base metal" to distinguish it from the toughness of bond portion after welding.
  • a steel plate of this disclosure and a steel material used for producing the steel plate need to have the chemical composition described above.
  • the following describes each of the components contained in the above chemical composition. Note that “%” indicating the content of each component is “mass%”, unless otherwise stated.
  • C is an element that has an effect of increasing the strength of the steel plate.
  • the C content is therefore 0.010 % or more, preferably 0.020 % or more, and more preferably 0.030 % or more.
  • the C content exceeds 0.14 %, the formation of coarse martensite austenite constituent and cementite is promoted, resulting in a decrease in toughness of base metal and a significant degradation in toughness of the bond portion.
  • the C content is therefore 0.14 % or less, preferably 0.10 % or less, and more preferably 0.08 % or less.
  • Si is an element that functions as a deoxidizer and has an effect of increasing the strength of the steel plate. To achieve the effect, the Si content is 0.01 % or more. On the other hand, when the Si content exceeds 0.50 %, the formation of coarse martensite austenite constituent is promoted, and a decrease in toughness of base metal and toughness of the bond portion becomes apparent.
  • the Si content is therefore 0.50 % or less, and preferably 0.35 % or less.
  • Mn is an element that has an effect of increasing the strength of the steel plate.
  • high strength, low yield ratio, and high toughness can be achieved by controlling a Mn concentration distribution as described below.
  • the Mn content is therefore 0.9 % or more, and preferably 1.2 % or more.
  • the Mn content exceeds 3.0 %, the area fraction of a Mn-enriched portion increases to form coarse MA, resulting in a decrease in toughness of base metal.
  • the heat-affected zone hardens to significantly decrease the toughness of bond portion.
  • the Mn content is therefore 3.0 % or less, and preferably 2.6 % or less.
  • the chemical composition described here is an average composition of the steel plate. Therefore, the above Mn content is used as the "average Mn content" in the specification of the Mn concentration distribution described below.
  • the P content is an element that degrades the toughness of base metal and the toughness of bond portion.
  • the P content is desirably reduced as much as possible.
  • the P content exceeds 0.015 %, the toughness of base metal and the toughness of bond portion significantly decrease. This is thought to be due to the segregation of P in the Mn-enriched portion at high P content, resulting in hardening of the microstructure.
  • the P content is therefore 0.015 % or less.
  • no particular lower limit is placed on the P content, and the P content may be 0 %. Excessive reduction, however, leads to higher costs. Therefore, from the viewpoint of production cost, the P content is preferably 0.001 % or more.
  • the S content is an element that degrades the toughness of base metal.
  • the S content is desirably reduced as much as possible.
  • the S content is higher than 0.0050 %, the desired toughness of base metal and toughness of bond portion cannot be achieved.
  • the S content is therefore 0.0050 % or less.
  • no particular lower limit is placed on the S content, and the S content may be 0 %. Excessive reduction, however, leads to higher costs. Therefore, from the viewpoint of production cost, the S content is preferably 0.0003 % or more.
  • Al is an element that acts as a deoxidizer. Al also fixes N in steel as AlN to contribute to the improvement in toughness of base metal. To achieve the effect, the Al content is 0.002 % or more, and preferably 0.010 % or more. On the other hand, when the Al content exceeds 0.080 %, the toughness of base metal decreases. The Al content is therefore 0.080 % or less, and preferably 0.060 % or less.
  • Ti is an element that functions as a deoxidizer and contributes to the improvement in strength of the steel plate. Ti also combines with N to precipitate as TiN, a nitride that is stable even at high temperatures. Therefore, the pinning effect of TiN prevents austenite grain coarsening when heated, resulting in an improvement in toughness of base metal and toughness of bond portion. To achieve the effect, the Ti content is 0.003 % or more, and preferably 0.005 % or more. On the other hand, when the Ti content exceeds 0.030 %, the toughness of base metal and the toughness of bond portion deteriorate. The Ti content is therefore 0.030 % or less, preferably 0.025 % or less, and more preferably 0.020 % or less.
  • N combines with Al or Ti to precipitate a nitride.
  • the nitride inhibits the coarsening of austenite grains to improve the toughness of base metal and bond portion.
  • the N content is 0.0015 % or more, and preferably 0.0025 % or more.
  • the toughness of base metal and bond portion are rather reduced due to the increase in solute N content.
  • the N content is therefore 0.0080 % or less, preferably 0.0065 % or less, and more preferably 0.0060 % or less.
  • the chemical composition according to one of the embodiments can contain the above elements, with the balance being Fe and inevitable impurities.
  • the inevitable impurities include oxygen (O).
  • the content of oxygen contained as inevitable impurities is preferably 0.0030 % or less.
  • 4.83C + Mn is 1.4 mass% or more, and preferably 1.7 mass% or more.
  • 4.83C + Mn exceeds 3.3 mass%, the effect is saturated. Therefore, 4.83C + Mn is 3.3 mass% or less.
  • TiN has a pinning effect that suppresses austenite grain growth in the heat-affected zone and improves the toughness of bond portion.
  • Ti/N is 2.0 or more, and preferably 2.4 or more.
  • Ti/N exceeds 4.3, the toughness of base metal and the toughness of bond portion deteriorate due to the formation of TiC particles and the coarsening of TiN. Therefore, Ti/N is 4.3 or less, and preferably 4.0 or less.
  • P CM is 0.30 mass% or less, preferably 0.28 mass% or less, and more preferably 0.26 mass% or less:
  • P cm [C] + [Si]/30 + [Mn]/20 + [Cu]/20 + [Ni]/60 + [Cr]/20 + [Mo]/15 + [V]/10 + 5[B] where the brackets in the above formula indicate a content (mass%) of an element enclosed in the brackets and have a value of 0 if such an element is not contained.
  • P CM is preferably 0.15 mass% or more, more preferably 0.17 mass% or more, and further preferably 0.19 mass% or more.
  • the above chemical composition can optionally further contain at least one selected from the group consisting of Cu, Ni, Cr, Mo, W, Nb, V, B, Ca, REM, Mg, and Zr.
  • Cu is an element that further improves the strength while maintaining the high toughness of the steel plate and can be optionally contained depending on the strength required.
  • the Cu content exceeds 3.0 %, hot brittleness occurs to deteriorate the surface characteristics of the steel plate. Therefore, when Cu is contained, the Cu content is 3.0 % or less.
  • the Cu content is preferably 2.0 % or less.
  • no particular lower limit is placed on the Cu content.
  • the Cu content is preferably 0.01 % or more, and more preferably 0.05 % or more.
  • Ni is an element that further improves the strength while maintaining the high toughness of the steel plate and can be optionally contained depending on the strength required.
  • the Ni content exceeds 3.0 %, the effect of addition is saturated, which is economically disadvantageous. Therefore, when Ni is contained, the Ni content is 3.0 % or less.
  • the N content is preferably 2.0 % or less.
  • no particular lower limit is placed on the Ni content.
  • the Ni content is preferably 0.01 % or more, and more preferably 0.10 % or more.
  • Cr is an element that further improves the strength of the steel plate and can be optionally contained depending on the strength required.
  • the Cr content exceeds 3.0 %, the toughness of the base metal and the bond portion deteriorates. Therefore, when Cr is contained, the Cr content is 3.0 % or less.
  • the Cr content is preferably 2.0 % or less.
  • no particular lower limit is placed on the Cr content.
  • the Cr content is preferably 0.01 % or more, and more preferably 0.10 % or more.
  • Mo is an element that further improves the strength of the steel plate and can be optionally contained depending on the strength required.
  • Mo content exceeds 1.5 %, the toughness of the base metal and the bond portion deteriorates.
  • quench cracks are more likely to occur during the process of producing the steel plate, resulting in reduced manufacturability. Therefore, when Mo is contained, the Mo content is 1.5 % or less, and preferably 1.0 % or less.
  • no particular lower limit is placed on the Mo content.
  • the Mo content is preferably 0.01 % or more, and more preferably 0.10 % or more.
  • W is an element that further improves the strength of the steel plate and can be optionally contained depending on the strength required.
  • the W content exceeds 3.0 %, the toughness of the base metal and the bond portion deteriorates. Therefore, when W is contained, the W content is 3.0 % or less, and preferably 2.0 % or less.
  • no particular lower limit is placed on the W content.
  • the W content is preferably 0.01 % or more, and more preferably 0.10 % or more.
  • Nb is an element that further improves the strength of the steel plate and can be optionally contained depending on the strength required.
  • the Nb content exceeds 0.10 %, the toughness of the base metal and the bond portion decreases. Therefore, when Nb is contained, the Nb content is 0.10 % or less, and preferably 0.05 % or less.
  • the Nb content is preferably 0.005 % or more.
  • V 0.10 % or less
  • V is an element that further improves the strength of the steel plate and can be optionally contained depending on the strength required.
  • the V content exceeds 0.10 %, the toughness of the base metal and the bond portion decreases. Therefore, when V is contained, the V content is 0.10 % or less, and preferably 0.05 % or less.
  • no particular lower limit is placed on the V content. However, in terms of sufficiently achieving the strength improving effect by V, the V content is preferably 0.005 % or more.
  • B is an element that has an action of further increasing the strength of the steel plate by improving the quench hardenability. B also has an effect of further improving the toughness of bond portion by sticking solute nitrogen as a nitride in the heat-affected zone during large-heat input welding.
  • the B content exceeds 0.0050 %, the quench hardenability is excessively high, and the toughness of the base metal and the bond portion is rather reduced. Therefore, when B is contained, the B content is 0.0050 % or less, and preferably 0.0020 % or less.
  • no particular lower limit is placed on the B content. However, in terms of sufficiently achieving the effect of addition of B, the B content is preferably 0.0003 % or more.
  • Ca is an element that has an effect of further improving the toughness of base metal by refinement of crystal grains and can be optionally contained depending on the toughness of base metal required.
  • the Ca content exceeds 0.005 %, the effect of addition is saturated. Therefore, when Ca is contained, the Ca content is 0.005 % or less.
  • no particular lower limit is placed on the Ca content.
  • the Ca content is preferably 0.001 % or more.
  • REM rare earth metal
  • the REM content is preferably 0.002 % or more.
  • Mg is an element that has an effect of further improving the toughness of base metal by refinement of crystal grains and can be optionally contained depending on the toughness of base metal required.
  • the Mg content exceeds 0.005 %, the effect of addition is saturated. Therefore, when Mg is contained, the Mg content is 0.005 % or less.
  • no particular lower limit is placed on the Mg content.
  • the Mg content is preferably 0.001 % or more.
  • Zr is an element that has an effect of further improving the toughness of base metal by refinement of crystal grains and can be optionally contained depending on the toughness of base metal required.
  • the Zr content exceeds 0.020 %, the effect of addition is saturated. Therefore, when Zr is contained, the Zr content is 0.020 % or less.
  • no particular lower limit is placed on the Zr content.
  • the Zr content is preferably 0.002 % or more.
  • the steel plate of this disclosure has a microstructure that contains bainite and martensite austenite constituent, with an area fraction of bainite of 80.0 % or more.
  • the reason for limiting the microstructure to the above range is described below.
  • area fraction in the following description refers to an area fraction relative to the whole microstructure, unless otherwise specified.
  • the above microstructure refers to a microstructure at a 1/4 thickness position of the steel plate.
  • Bainite is a microstructure necessary to improve the strength and toughness of the steel plate, as described below.
  • martensite austenite constituent MA
  • MA martensite austenite constituent
  • a large amount of mobile dislocation is introduced around MA, which suppresses the increase in yield stress. Therefore, to achieve both high strength and low yield ratio, the microstructure needs to contain bainite and martensite austenite constituent.
  • the area fraction of bainite is 80.0 % or more, preferably 85.0 % or more, and more preferably 90.0 % or more.
  • the area fraction of bainite is preferably 99.0 % or less. The area fraction of bainite can be measured by the method described in Examples.
  • the toughness of bond portion decreases when the area fraction is 5.0 % or more. This is thought to be because of the following reasons. That is, since the bond portion is heated to a high temperature close to the melting point during welding, MA contained in the steel plate is once decomposed by the heating. However, during the cooling process after welding, MA is regenerated in the bond portion. The amount of MA regenerated in this process increases with the amount of MA contained in the steel plate prior to welding. When the amount of MA regenerated in the bond portion is high, the toughness of bond portion decreases.
  • the area fraction of martensite austenite constituent in the microstructure of the steel plate is less than 5.0 %, preferably 4.9 % or less, more preferably 4.7 % or less, and further preferably 4.5 % or less, in order to improve the toughness of bond portion.
  • the area fraction of MA is preferably 1.0 % or more, and more preferably 2.0 % or more.
  • MA may have any size. However, when MA is excessively coarse, the toughness of the steel plate decreases. Therefore, from the viewpoint of further improving the toughness, the average equivalent circular diameter of MA is preferably 5.0 ⁇ m or less, and preferably 4.0 ⁇ m or less. On the other hand, no particular lower limit is placed on the average equivalent circular diameter of MA. However, the average equivalent circular diameter of MA usually may be 0.8 ⁇ m or more, or 1.0 ⁇ m or more.
  • the area fraction and average equivalent circular diameter of MA can be determined by applying LePera corrosion ( Journal of Metals, March, 1980, p. 38-39 ) to a steel sheet as a sample, observing the steel sheet using a scanning electron microscope (SEM) at a magnification of 1000 ⁇ , and then analyzing captured images using image interpretation equipment.
  • LePera corrosion Journal of Metals, March, 1980, p. 38-39
  • SEM scanning electron microscope
  • the microstructure according to one of the embodiments may consist of bainite and martensite austenite constituent.
  • the microstructures according to other disclosed embodiments may also contain another structure, in addition to bainite and martensite austenite constituent.
  • the other structure may be any structure without limitation.
  • the other structure may be at least one selected from the group consisting of ferrite, pearlite, martensite, or retained austenite.
  • the total area fraction of the other structure is preferably 19 % or less, more preferably 15 % or less, and further preferably 10 % or less.
  • the steel plate according to one of the embodiments consists of, with area fraction:
  • the steel plate of this disclosure has a Mn concentration distribution that satisfies the following conditions (1) to (3).
  • the Mn concentration distribution in this disclosure refers to a Mn concentration distribution at a 1/4 thickness position of the steel plate:
  • the area fraction of the average concentration region of Mn is 90 % or more, the hard microstructure containing MA is insufficient to achieve the desired strength.
  • the enrichment of Mn for the Mn-enriched region is insufficient, making the effect of yield stress reduction due to MA formation insufficient.
  • the yield ratio is also likely to be high. Therefore, the area fraction of the average concentration region of Mn is less than 90 %, preferably 85 % or less, and more preferably 80 % or less.
  • no particular lower limit is placed on the area fraction of the average concentration region of Mn.
  • the area fraction of the average concentration region of Mn is too low, the size of the Mn-enriched region increases, resulting in the coarsening of MA formed in the enriched region.
  • the area fraction of the average concentration region of Mn is preferably 50 % or more, and more preferably 60 % or more.
  • the "average concentration region of Mn" is defined here as a region with a Mn concentration of 0.9 times to 1.1 times the average Mn content (mass%).
  • the area fraction of the Mn-enriched region is 1.0 % or more, preferably 1.5 % or more, more preferably 2.0 % or more, and further preferably 6.2 % or more.
  • no particular upper limit is placed on the area fraction of the Mn-enriched region.
  • the area fraction of the Mn-enriched region is preferably less than 50 %, more preferably less than 40 %, and further preferably 20 % or less.
  • the Mn-enriched region is eliminated by heating during welding.
  • the existence of the enriched region does not adversely affect the toughness of bond portion after welding. Therefore, this disclosure can achieve both excellent mechanical properties of the steel plate and high toughness of bond portion.
  • Average equivalent circular diameter of Mn-enriched region 7.0 ⁇ m or less
  • the average equivalent circular diameter of the Mn-enriched region is larger than 7.0 ⁇ m, MA formed within the Mn-enriched region also becomes coarse, resulting in reduced toughness of the steel plate. Therefore, the average equivalent circular diameter of the Mn-enriched region is 7.0 ⁇ m or less, and preferably 4.0 ⁇ m or less. On the other hand, no particular lower limit is placed on the average equivalent circular diameter of the Mn-enriched region. However, the average equivalent circular diameter of the Mn-enriched region generally may be 1.0 ⁇ m or more, or 1.5 ⁇ m or more.
  • the Mn concentration distribution can be measured using an electron probe microanalyzer (EPMA). Specifically, a test piece is taken from the steel plate so that the observation position is at a 1/4 thickness position. The Mn concentration distribution in the test piece is measured by the EPMA to calculate the area fraction of the average concentration region of Mn, the area fraction of the Mn-enriched region, and the average equivalent circular diameter of the Mn-enriched region. The measurement of the Mn concentration distribution by the EPMA is performed in two or more randomly selected fields of view, with the size of one field of view being 50 ⁇ m ⁇ 50 ⁇ m and 250 ⁇ 250 measurement points per field of view.
  • EPMA electron probe microanalyzer
  • the steel plate of this disclosure has a Charpy absorbed energy at 0 °C: vE 0 of 70 J or more.
  • the Charpy absorbed energy is one of the indices of toughness.
  • the steel plate of this disclosure with vE 0 of 70 J or more produces excellent seismic safety even when used in high-rise buildings.
  • vE 0 is preferably 80 J or more, and more preferably 100 J or more.
  • a higher vE 0 is better from the viewpoint of seismic resistance.
  • no particular upper limit is placed on vE 0 .
  • vE 0 generally may be 250 J or less, 220 J or less, or 210 J or less.
  • the Charpy absorbed energy at 0 °C of the above steel sheet can be measured in accordance with the provisions of JIS Z 2242 using a V-notch test piece taken from a 1/4 thickness position of the steel plate in accordance with the provisions of JIS Z 2202.
  • the yield stress is preferably 600 MPa or more, and preferably 620 MPa or more. No particular upper limit is also placed on the yield stress.
  • the yield stress may be, for example, 900 MPa or less, 880 MPa or less, or 850 MPa or less.
  • the tensile strength (TS) of the steel plate of this disclosure is preferably 780 MPa or more, and more preferably 800 MPa or more. No particular upper limit is also placed on the tensile strength. However, the tensile strength may be, for example, 1100 MPa or less, or 1000 MPa or less.
  • the yield ratio is preferably 85 % or less.
  • the yield ratio may be, for example, 70 % or more, or 75 % or more.
  • the yield ratio is here a value expressed as a percentage of the ratio of yield stress (YS) to tensile strength (TS), i.e., YS/TS ⁇ 100 (%).
  • the above yield stress and tensile strength can be measured by a tensile test in accordance with JIS Z 2241, using a JIS No. 4 tensile test piece taken from a 1/4 thickness position of the steel plate.
  • the yield ratio can be calculated from the yield stress and tensile strength measured by the above method.
  • the Charpy absorbed energy at 0 °C (vE 0 ) of the bond portion is preferably 47 J or more.
  • No particular upper limit is also placed on vE 0 in the bond portion.
  • vE 0 generally may be 150 J or less.
  • vE 0 in the bond portion is a value measured by producing a welded joint by electroslag welding with an amount of welding heat input of 40 kJ/mm or more and then using a JIS No. 4 Charpy impact test piece taken from the welded joint so that the notch position is at the bond portion. More specifically, vE 0 in the bond portion can be measured by the method described in Examples.
  • the plate thickness of the above steel plate can be any thickness.
  • the plate thickness of the steel plate is preferably 6 mm or more, more preferably 9 mm or more, and further preferably 12 mm or more. From the viewpoint of the response to increased height in building structures, the plate thickness is preferably 40 mm or more, and more preferably 60 mm or more. On the other hand, no particular upper limit is also placed on the plate thickness of the steel plate. However, the plate thickness is preferably 100 mm or less.
  • the temperature means a temperature at the center of the plate thickness (position of plate thickness ⁇ 1/2).
  • the temperature at the center of the plate thickness can be determined by heat transfer calculation from the surface temperature of the steel plate measured by a radiation thermometer.
  • the temperatures under the heating conditions and cooling conditions after the hot rolling step are temperatures at a 1/4 thickness position.
  • the heating rate and the cooling rate also mean an average heating rate and an average cooling rate calculated based on the temperatures at the 1/4 thickness position, respectively.
  • the steel material may be, for example, steel slab. No particular limitation is placed on the method of producing the steel material. However, the steel material can be produced, for example, by melting and casting steel having the above-described chemical composition. The melting can be performed by any method using a converter, an electric furnace, an induction furnace, or the like. The casting is preferably performed by continuous casting in terms of productivity, but also can be performed by ingot casting and blooming.
  • the above steel material is hot rolled to form a steel plate.
  • the hot rolling can be performed under any conditions.
  • the steel material is heated to a specific heating temperature and then rolled.
  • the heating may be performed after the steel material obtained by a method such as casting is once cooled. Alternatively, the obtained steel material may be directly subjected to the heating without cooling it.
  • the microstructure and properties of the steel plate are controlled in the reheating step and the second cooling step after the hot rolling. Therefore, no particular limitation is placed on the heating temperature in the hot rolling step.
  • the heating temperature can be any temperature. However, when the heating temperature is less than 1000 °C, the load on the rolling mill during hot rolling may increase due to the high deformation resistance of the steel material, making it difficult to perform hot rolling. Therefore, the heating temperature is preferably 1000 °C or more. On the other hand, when the heating temperature is higher than 1250 °C, oxidation of the steel is more pronounced, resulting in increased loss due to oxidation and lower yield rate. Therefore, the heating temperature is preferably 1250 °C or less.
  • the rolling finish temperature is preferably 1000 °C or less.
  • the rolling finish temperature is preferably 750 °C or more.
  • first cooling step the steel plate obtained in the above hot rolling step is cooled.
  • cooling in this first cooling step can be performed under any conditions without any particular limitations in order to control the microstructure and properties of the steel plate in the subsequent reheating step and second cooling step.
  • a cooling stop temperature in the first cooling step may be Ac1 point or less.
  • the cooling stop temperature is preferably 500 °C or less. When the cooling stop temperature is 500 °C or less, coarsening of precipitates can be suppressed, and the Mn-enriched region can be generated more uniformly.
  • the cooling stop temperature is more preferably 250 °C or less. On the other hand, no lower limit is also placed on the cooling stop temperature.
  • the steel plate can be cooled to any temperature. However, since excessive cooling reduces the productivity, the cooling stop temperature is preferably 0 °C or more, more preferably 10 °C or more, and further preferably 20 °C or more. Typically, the cooling stop temperature is preferably the room temperature or more.
  • the cooling in the first cooling step can be performed by any method with no particular limitation.
  • the cooling can be performed by one or both of air cooling and water cooling.
  • the cooling is preferably performed by water cooling.
  • the water cooling is more preferably performed by at least one selected from the group consisting of spray cooling, mist cooling, or laminar cooling.
  • the steel plate after the first cooling step is heated to a reheating temperature in a specific heating pattern and held at the reheating temperature.
  • the heating process Mn is distributed into austenite formed by reverse transformation from bainite and martensite through the temperature range from Ac1 point or more to Ac3 point or less (two-phase region), resulting in microscopic variations in Mn concentration.
  • the average heating rate in the temperature range from Ac1 point to Ac3 point is higher than 2.0 °C/s, Mn distribution does not sufficiently progress. As a result, the desired Mn concentration distribution cannot be achieved. Therefore, in the reheating step, the steel plate after the first cooling step is heated at the average heating rate in the temperature range from Ac1 point to Ac3 point at the 1/4 thickness position: 2.0 °C/s or less.
  • no particular lower limit is placed on the average heating rate.
  • the heating rate is excessively slow, the effect of controlling the heating rate is saturated, and the time required for heating increases, resulting in reduced productivity. Therefore, the average heating rate is preferably 0.01 °C/s or more.
  • the stay time in the temperature range from Ac3 point - 100 °C to Ac3 point is 60 seconds or more.
  • the stay time is preferably 60 minutes or less.
  • the heating pattern in the above heating process just has to satisfy the above conditions, and no particular limitation is placed on the other conditions.
  • the temperature may be continuously raised to the reheating temperature or intentionally kept in the two-phase region.
  • Reheating temperature Ac3 point or more, Ac3 point + 60 °C or less
  • the steel plate is heated to a reheating temperature of Ac3 point or more and Ac3 point + 60 °C or less.
  • a reheating temperature of Ac3 point or more and Ac3 point + 60 °C or less.
  • the reheating temperature is Ac3 point or more.
  • the reheating temperature is Ac3 point + 60 °C or less, more preferably Ac3 point + 55 °C or less, and further preferably Ac3 point + 50 °C or less.
  • the steel plate is heated up to the above reheating temperature and then held at the re-heating temperature for a specific holding time.
  • the holding time is less than 10 minutes, the average equivalent circular diameter of the Mn-enriched region in the steel plate to be finally obtained cannot be 7.0 ⁇ m or less. This may be due to the short holding time, which increased the variation in grain size of the reverse transformation austenite, resulting in a non-uniformity in size of the Mn-enriched region. Therefore, the holding time is 10 minutes or more.
  • no particular upper limit is placed on the holding time. However, the holding time is preferably 100 minutes or less, as excessive long holding time reduces productivity.
  • any heating method can be used.
  • An example of the heating method is furnace heating.
  • furnace heating a general heat treatment furnace can be used without any particular limitations.
  • the steel plate after the reheating step is cooled. Specifically, the steel plate after the reheating step is subjected to accelerated cooling to an accelerated cooling stop temperature of 100 °C to 600 °C at an average cooling rate at the 1/4 thickness position: 1.0 °C/s to 200.0 °C/s, and then air cooled to a temperature of 100 °C or less.
  • Average cooling rate 1.0 °C/s to 200.0 °C/s
  • Accelerated cooling under the above conditions can transform reverse transformation austenite to bainite to obtain a bainite-dominated microstructure.
  • the average cooling rate is less than 1.0 °C/s, ferrite is formed, and the area fraction of bainite thus cannot be 80.0 % or more. Therefore, the average cooling rate is 1.0 °C/s or more, and preferably 5.0 °C/s or more.
  • the average cooling rate is higher than 200.0 °C/s, it is difficult to control the temperature at each position in the steel plate, which leads to material property variations in the plate transverse direction and rolling direction, resulting in material variations such as strength properties. Therefore, the average cooling rate is 200 °C/s or less, preferably 150.0 °C/s or less, and more preferably 100.0 °C/s or less.
  • any method can be performed for cooling.
  • the accelerated cooling is preferably performed by one or both of air cooling and water cooling, and water cooling is more preferred.
  • Any water-based method e.g., spray cooling, mist cooling, laminar cooling, etc.
  • Mist cooling is preferably used as the method of the water cooling.
  • Accelerated cooling stop temperature 100 °C to 600 °C
  • the accelerated cooling stop temperature is 100 °C or more, and preferably 200 °C or more.
  • the accelerated cooling stop temperature is 600 °C or less, and preferably 500 °C or less.
  • the steel plate is further air cooled to a temperature of 100 °C or less.
  • the air cooling may be allowed to naturally cool, i.e., natural cooling, rather than forced cooling.
  • the steel plate may be cooled to the temperature of 100 °C or less, but it is not necessary to stop air cooling at a specific temperature.
  • the steel plate usually may be air cooled to the room temperature (ambient temperature).
  • cooling rate in the air cooling may be performed at any rate.
  • the cooling rate in the air cooling may be less than 1.0 °C/s, or 0.5 °C/s or less.
  • No particular lower limit is also placed on the cooling rate in the air cooling.
  • the cooling rate may be 0.001 °C/s or more, 0.01 °C/s or more, or 0.07 °C/s or more.
  • the air cooling time may be more than 300 s, 310 s or more, or 320 s or more.
  • no particular upper limit is also placed on the air cooling time.
  • the air cooling time may be, for example, 24 hours or less, or 12 hours or less. The time taken from the start of air cooling to reach 100 °C is defined here as the air cooling time, in the second cooling step.
  • the steel plate of this disclosure can be produced by controlling the chemical composition, in particular, the C and Mn contents, within specific ranges and by appropriately controlling the heating conditions in the reheating step after hot rolling. Therefore, the steel plate of this disclosure is easier to produce and more suitable for industrial production, compared with the steel plate in PTL 3, which requires controlling the conditions during the cooling process after reheating to control the formation of MA.
  • a steel plate can be produced by sequentially applying the steps (a) to (e) to a steel material having the above-described chemical composition:
  • the steel plate after the first cooling step is heated to a heat treatment temperature of Ac3 point or more and 1050 °C or less, held at the heat treatment temperature for a holding time of 5 minutes or more, and then cooled to a cooling stop temperature of 500 °C or less.
  • the heat treatment can achieve both high strength and low yield ratio at an even higher level.
  • the reason for this may be as follows.
  • the reheating step after the heat treatment step homogenizes the present position of the Mn-enriched portion to increase the frequency of formation of the reverse transformation nucleation site. As a result, the formation of microscopic variations in Mn concentration is promoted to increase the final area fraction of the Mn-enriched region. Specifically, the area fraction of the enriched region can be 6.2 % or more.
  • MA is formed in the Mn-enriched region, leading to both high strength and low yield ratio.
  • Heat treatment temperature Ac3 point or more, 1050 °C or less
  • desired bainite and martensitic microstructure can be obtained by ensuring quench hardenability.
  • the desired toughness of base metal cannot be achieved. This is thought to be due to the formation of coarse ferrite during heat treatment, resulting in the formation of an upper bainitic microstructure with coarse carbides in the finally obtained microstructure.
  • the desired toughness of base metal cannot be achieved also when the heat treatment temperature is higher than 1050 °C. This is thought to be due to the formation of coarse bainite and coarse martensite during heat treatment, resulting in the formation of a coarse bainitic microstructure in the finally obtained microstructure.
  • the holding time at the heat treatment temperature is 5 minutes or more, and preferably 10 minutes or more, to reduce the variation in austenite grain size.
  • no particular upper limit is placed on the holding time.
  • the holding time is preferably 100 minutes or less, and more preferably 60 minutes or less, as the effect will be saturated even if the holding time is excessively long.
  • Any heating method can be used for heating in the heat treatment step, as long as the heat treatment temperature and the holding time can be controlled as described above.
  • An example of the heating method that can be used is furnace heating.
  • furnace heating a general heat treatment furnace can be used without any particular limitations.
  • Cooling stop temperature 500 °C or less
  • the steel plate After holding the steel plate at the above heat treatment temperature, the steel plate is cooled down to the cooling stop temperature of 500 °C or less.
  • the austenite formed in the heat treatment step is transformed to a low temperature transformation phase of bainite and martensite by the cooling, and the subsequent reheating step can achieve further high strength and low yield ratio.
  • the cooling stop temperature is 500 °C or less, preferably 400 °C or less, and more preferably 200 °C or less.
  • no particular lower limit is placed on the cooling stop temperature.
  • the cooling stop temperature is preferably 0 °C or more, more preferably 10 °C or more, and further preferably 20 °C or more.
  • the cooling stop temperature is preferably the room temperature or more.
  • the cooling can be performed by any method such as air cooling or water cooling. Any water-based method can be used for the water cooling, such as spray cooling, mist cooling, or laminar cooling.
  • the steel plate after cooling may be subjected to the next reheating step.
  • the steel plate after the first cooling step may be subjected to the next reheating step without heat treatment.
  • a further tempering step can be optionally applied after the reheating step, such as for the purpose of correcting the shape of the steel plate.
  • a lower cooling stop temperature in the cooling step after reheating can be expected to further improve the toughness of the base metal due to the effect of tempering.
  • the heating temperature is preferably 200 °C to 500 °C. Cooling of the steel sheet after the above tempering step can be performed by any method, since the cooling rate does not change the material properties.
  • Each steel slab was heated to 1150 °C and then hot rolled to be a steel plate with the thickness presented in Tables 2 and 3 (hot rolling step).
  • the rolling finish temperature in the hot rolling is presented in Tables 2 and 3.
  • the obtained steel plate was cooled to the cooling stop temperature presented in Tables 2 and 3 by the cooling method presented in Tables 2 and 3 (first cooling step).

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JP6988836B2 (ja) * 2019-01-28 2022-01-05 Ｊｆｅスチール株式会社 超低降伏比高張力厚鋼板およびその製造方法
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JP7533408B2 (ja) * 2021-09-21 2024-08-14 Ｊｆｅスチール株式会社 鋼板およびその製造方法

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TWI857697B (zh) 2024-10-01
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KR20250011693A (ko) 2025-01-21
JPWO2024038684A1 (fr) 2024-02-22
JP7444343B1 (ja) 2024-03-06
WO2024038684A1 (fr) 2024-02-22
TW202411438A (zh) 2024-03-16

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