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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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  • C21D1/84—Controlled slow cooling
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties 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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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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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
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high-strength steel sheet and its manufacturing method.
• tensile strength 980 MPa or more and uniform elongation of 6% or more
• it has excellent bending workability, and is suitable as a material for trucks and passenger car frames, suspension parts, etc. It relates to a high strength steel sheet and a method for producing the same. .
• Patent Documents 1 to 6 have the following problems.
• Patent Documents 1 and 2 a tensile strength of 980 MPa or more cannot be obtained.
• hot-rolled steel sheets are considered to have excellent workability, and "elongation" is used as an index of workability.
• This "elongation” is also called total elongation (El), and represents the elongation at the time when the test piece breaks in the tensile test.
• El total elongation
• necking occurs before breakage occurs. If necking occurs, the plate thickness becomes thin locally, resulting in product defects during press molding. Therefore, high total elongation alone is not sufficient to achieve excellent press formability.
• Patent Documents 1 and 2 do not refer to bending workability.
• Patent Documents 3 to 5 are said to yield high-strength steel sheets with excellent bending workability, but all of them focus only on cracks that occur on the outside of bending. If cracks occur during bending, regardless of whether they are on the outside or inside of the bend, the cracks will become fatigue crack initiation points, which may reduce the durability of the part. It cannot be said that ensuring sexuality is sufficient.
• Patent Document 6 With the technology described in Patent Document 6, it is said that a high-strength steel sheet with excellent bending workability can be obtained, but attention is focused only on cracks that occur on the inner side of bending. If cracks occur during bending, regardless of whether they are on the outside or inside of the bend, those cracks can become fatigue crack initiation points, reducing the durability of the part. Otherwise, the performance of parts cannot be ensured.
• the present invention has been made in view of the above-mentioned actual situation, and an object thereof is to provide a high-strength steel sheet having tensile strength, press formability, and bending workability, and a method for manufacturing the same.
• the present inventors have found that a tensile strength of 980 MPa or more and a virtual stress-strain curve of steel sheets having various yield stresses and uniform elongations are created, and the stress-strain curve is used.
• the inventors studied the optimal steel sheet structure in order to obtain a tensile strength of 980 MPa or more and a uniform elongation of 6% or more.
• the main phase is upper bainite, and a microstructure containing an appropriate amount of a hard secondary phase containing fresh martensite and/or retained austenite results in a high strength of 980 MPa or more and a uniform elongation of 6% or more. It has been shown that it is possible to combine
• the upper bainite referred to here is an aggregate of lath-shaped ferrite with an orientation difference of less than 15°, and a structure having Fe-based carbides and/or retained austenite between lath-shaped ferrites (however, between lath-shaped ferrites (including the case of not having Fe-based carbides and/or retained austenite).
• lath-like ferrite has a lath-like shape and a relatively high dislocation density inside. electron microscopy).
• Fresh martensite is martensite that does not contain Fe-based carbides.
• Fresh martensite and retained austenite have similar contrast in SEM, but are distinguishable using electron backscatter diffraction (EBSD) methods.
• the inventors investigated the bending workability of high-strength steel sheets having a tensile strength of 980 MPa or more and a uniform elongation of 6% or more. Specifically, steel sheets with a tensile strength of 980 MPa or more and a uniform elongation of 6% or more, manufactured by different manufacturing methods, were subjected to a 90° V bending test to observe the fracture surface of bending cracks and the microstructure in the vicinity of the cracks. On the outside of the bending, the fracture surface of the crack was ductile fracture, and many voids were observed in the microstructure near the crack.
• the crack fracture surface is brittle fracture surface, and voids are not observed in the microstructure near the crack. Therefore, an improvement in ductility can suppress external bending cracks, and an improvement in compression embrittlement resistance can suppress internal bending cracks. Therefore, it was found that it is necessary to control the microstructure of the surface layer region and its neighboring region where bending cracks can occur.
• the present invention has been made based on further studies based on the above findings, and the gist thereof is as follows. [1] % by mass, C: 0.05 to 0.20%, Si: 0.5 to 1.2%, Mn: 1.5-4.0%, P: 0.10% or less, S: 0.03% or less, Al: 0.001 to 2.0%, N: 0.01% or less, O: 0.01% or less and B: 0.0005 to 0.010% or less, with the balance being Fe and unavoidable impurities,
• the microstructure includes upper bainite with an area ratio of 80% or more and fresh martensite and/or retained austenite with a total area ratio of 2% or more in the surface layer region from the steel plate surface to the position of 1/10 of the plate thickness,
• the internal region from the plate thickness 1/10 position to the plate thickness 3/10 position contains upper bainite with an area ratio of 70% or more and fresh martensite and / or retained austenite with a total area ratio of 3% or more,
• the component composition further contains, in % by mass, Cr: 1.0% or less, and Mo: 1.0% or less, The high-strength steel sheet according to [1], containing at least one of [3]
• the component composition further contains, in % by mass, Cu: 2.0% or less, Ni: 2.0% or less, Ti: 0.3% or less, The high-strength steel sheet according to [1] or [2], containing at least one of Nb: 0.3% or less and V: 0.3% or less.
• the component composition further contains, in % by mass, Sb: 0.005-0.020% The high-strength steel sheet according to any one of [1] to [3], containing [5]
• the component composition further contains, in % by mass, Ca: 0.01% or less, The high-strength steel sheet according to any one of [1] to [4], containing at least one of Mg: 0.01% or less and REM: 0.01% or less.
• a hot-rolled steel sheet is obtained by hot rolling under the conditions that the total rolling reduction in the temperature range of RC1 or less is 25% or more and 80% or less, and the finish rolling end temperature is (RC2-50°C) or more (RC2 + 120°C) or less, Time from the end of hot rolling to the start of cooling of the hot-rolled steel sheet: within 2.0 s, average cooling rate at 3/10 thickness position: 15 ° C./s or more, cooling stop temperature: Trs or more, (Trs + 250 ° C.
• the hot-rolled steel sheet after cooling is coiled at a coiling temperature of Trs or more and (Trs + 250 ° C.) or less, A method for producing a high-strength steel sheet by cooling to 100°C or less at an average cooling rate of 20°C/s or less.
• Trs a coiling temperature of Trs or more and (Trs + 250 ° C.) or less
• Trs a coiling temperature of Trs or more and (Trs + 250 ° C.) or less
• Trs a high-strength steel sheet by cooling to 100°C or less at an average cooling rate of 20°C/s or less.
• RC1, RC2, and Trs are defined by the following formulas (1), (2), and (3), respectively.
• RC1 (°C) 900 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V
• RC2 (°C) 750 + 100 x C + 100 x N + 10 x Mn + 350 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1000 x Nb + 150 x V
• Trs (° C.) 500-450 ⁇ C-35 ⁇ Mn-15 ⁇ Cr-10 ⁇ Ni-20 ⁇ Mo (3)
• each element symbol in the above formulas (1), (2), and (3) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.
• a high-strength steel sheet having a tensile strength of 980 MPa or more, press formability, and bending workability can be obtained.
• the high-strength steel sheet of the present invention has high tensile strength, it is excellent in press formability and can be press-formed without forming defects such as necking and cracking.
• the high-strength steel sheet of the present invention is applied to members of trucks and passenger cars, it is possible to reduce the weight of automobile bodies by reducing the amount of steel used while ensuring safety, thereby contributing to the reduction of environmental load.
• excellent press formability means having a uniform elongation of 6% or more.
• excellent bending workability means that R/t, which is the ratio of the limit bending radius R and the plate thickness t at which cracks of 50 ⁇ m or more in depth do not occur on both the outer side and the inner side of the bend in the 90° V bending test, is 1.5. 5 or less.
• C 0.05-0.20%
• C is an element that has the effect of improving the strength of steel.
• C promotes the formation of bainite by improving hardenability and contributes to high strength.
• C also contributes to high strength by increasing the strength of martensite.
• the C content In order to obtain a tensile strength of 980 MPa or more, the C content must be 0.05% or more. Therefore, the C content should be 0.05% or more, preferably 0.06% or more.
• the C content should be 0.20% or less, preferably 0.18% or less.
• Si 0.5-1.2% Si has the effect of suppressing the formation of Fe-based carbides and suppresses precipitation of cementite during upper bainite transformation.
• C is distributed in untransformed austenite, and by cooling after coiling in the hot rolling process, untransformed austenite becomes fresh martensite and/or retained austenite, and the desired fresh martensite and/or retained austenite are obtained. be able to.
• the Si content should be 0.5% or more.
• the Si content is 0.6% or more.
• the Si content exceeds 1.2%, fresh martensite and/or retained austenite are formed more than the desired area ratio, and as a result, the desired upper bainite area ratio cannot be obtained. may worsen sexuality. Therefore, the Si content should be 1.2% or less, preferably 1.1% or less.
• Mn 1.5-4.0% Mn stabilizes austenite and contributes to the generation of fresh martensite and/or retained austenite. In order to obtain such effects, the Mn content must be 1.5% or more. Therefore, the Mn content is set to 1.5% or more, preferably 1.7% or more. On the other hand, when the Mn content exceeds 4.0%, fresh martensite and/or retained austenite are excessively generated, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in deterioration of bendability. Therefore, the Mn content should be 4.0% or less, preferably 3.8% or less.
• P 0.10% or less
• P is an element that forms a solid solution and contributes to an increase in the strength of steel.
• P is also an element that causes slab cracks during hot rolling by segregating at austenite grain boundaries during hot rolling. In addition, it segregates at grain boundaries to reduce uniform elongation. For this reason, it is preferable to keep the P content as low as possible, but the P content up to 0.10% is permissible. Therefore, the P content should be 0.10% or less.
• the lower limit is not particularly limited, but if the P content is less than 0.0002%, production efficiency is lowered, so 0.0002% or more is preferable.
• S 0.03% or less S combines with Ti and Mn to form coarse sulfides, which hasten the generation of voids, thereby lowering the uniform elongation. Therefore, it is preferable to keep the S content as low as possible, but an S content of up to 0.03% is permissible. Therefore, the S content is made 0.03% or less.
• the lower limit is not particularly limited, but if the S content is less than 0.0002%, production efficiency is lowered, so 0.0002% or more is preferable.
• Al 0.001-2.0%
• Al is an element that acts as a deoxidizing agent and is effective in improving the cleanliness of steel. If the Al content is less than 0.001%, the effect is not sufficient, so the Al content should be 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more.
• Al, like Si, has the effect of suppressing the formation of Fe-based carbides and suppresses precipitation of cementite during upper bainite transformation. This contributes to the generation of fresh martensite and/or retained austenite during cooling after winding.
• an excessive content of Al causes an increase in oxide-based inclusions and lowers the uniform elongation. Therefore, the Al content should be 2.0% or less, preferably 1.0% or less, and more preferably 0.1% or less.
• N 0.01% or less N precipitates as a nitride by combining with a nitride-forming element, and generally contributes to grain refinement.
• N combines with Ti at high temperatures to form coarse nitrides, a content exceeding 0.01% causes a decrease in uniform elongation. Therefore, the N content is set to 0.01% or less.
• the lower limit is not particularly limited, but if the N content is less than 0.0002%, production efficiency is lowered, so 0.0002% or more is preferable.
• O 0.01% or less O forms oxides and deteriorates moldability, so the content must be suppressed. In particular, when O exceeds 0.01%, this tendency becomes remarkable. Therefore, the O content should be 0.01% or less, preferably 0.005%, more preferably 0.003%.
• the lower limit is not specified, but if it is less than 0.00005%, production efficiency may be remarkably lowered, so 0.00005% or more is preferable.
• B 0.0005 to 0.010%
• B is an element that segregates at prior austenite grain boundaries, suppresses the formation of ferrite, promotes the formation of upper bainite, and contributes to the improvement of the strength of the steel sheet.
• the B content In order to develop these effects, the B content must be 0.0005% or more. Therefore, the B content is set to 0.0005% or more, preferably 0.0006%, and more preferably 0.0007%.
• the B content exceeds 0.010%, the above effects are saturated. Therefore, the B content is 0.010% or less, preferably 0.009% or less, more preferably 0.008% or less.
• the balance consists of Fe and unavoidable impurities.
• unavoidable impurities include Zr, Co, Sn, Zn, and W.
• the component composition contains at least one of Zr, Co, Sn, Zn, and W as unavoidable impurities, the total content of these elements is preferably 0.5% or less.
• the chemical composition of the high-strength steel sheet of the present invention can optionally contain at least one of the elements listed below.
• Cr 1.0% or less
• Cr is a carbide-forming element that segregates at the interface between the upper bainite and the untransformed austenite during the upper bainite transformation after winding, thereby reducing the driving force of the bainite transformation and causing the upper bainite to segregate. It has the effect of stopping metamorphosis. Untransformed austenite remaining after the transformation to upper bainite stops becomes fresh martensite and/or retained austenite by cooling after winding. Therefore, when Cr is added, Cr also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is obtained when Cr is preferably 0.1% or more.
• the Cr content exceeds 1.0%, fresh martensite and/or retained austenite are excessively generated, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in deterioration of bendability. Therefore, when Cr is added, the Cr content is 1.0% or less, preferably 0.9% or less, and more preferably 0.8% or less.
• Mo 1.0% or less Mo promotes formation of bainite through improvement of hardenability and contributes to strength improvement of the steel sheet.
• Mo like Cr, is a carbide-forming element, and segregates at the interface between the upper bainite and the untransformed austenite during the upper bainite transformation after winding, thereby reducing the transformation driving force of the bainite and cooling the winding. It contributes to the later generation of fresh martensite and/or retained austenite.
• Mo exceeds 1.0%, fresh martensite and/or retained austenite are excessively generated, and as a result, the desired area ratio of upper bainite cannot be obtained, which deteriorates uniform elongation. .
• This effect is obtained when Mo is preferably 0.1% or more. Therefore, when Mo is added, the Mo content is 1.0% or less, preferably 0.9% or less, and more preferably 0.8% or less.
• the chemical composition of the high-strength steel sheet of the present invention can optionally contain at least one of the elements listed below.
• Cu 2.0% or less
• Cu is an element that forms a solid solution and contributes to increasing the strength of steel. Further, Cu promotes the formation of bainite through improvement of hardenability and contributes to strength improvement. This effect is obtained when Cu is preferably 0.01% or more.
• the Cu content exceeds 2.0%, the surface properties of the high-strength steel sheet are deteriorated, and the bendability of the high-strength steel sheet is deteriorated. Therefore, when Cu is added, the Cu content is 2.0% or less, preferably 1.9% or less, and more preferably 1.8% or less.
• Ni 2.0% or less
• Ni is an element that forms a solid solution and contributes to increasing the strength of steel.
• Ni promotes the formation of bainite through improvement of hardenability and contributes to strength improvement. This effect is obtained when Ni is preferably 0.01% or more.
• the Ni content exceeds 2.0%, fresh martensite and/or retained austenite excessively increase, and as a result, the desired area ratio of upper bainite cannot be obtained. deteriorate. Therefore, when Ni is added, the Ni content should be 2.0% or less, preferably 1.9% or less, and more preferably 1.8% or less.
• Ti 0.3% or less
• Ti is an element that acts to improve the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Ti forms nitrides in the high temperature range of austenite. As a result, precipitation of BN is suppressed, and B becomes a solid solution. Therefore, when Ti is added, Ti also contributes to ensuring the hardenability necessary for forming upper bainite, and the strength is improved. This effect is obtained when Ti is preferably 0.01% or more. However, when the Ti content exceeds 0.3%, a large amount of Ti nitrides are formed, which reduces the uniform elongation. Therefore, when Ti is added, the Ti content should be 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.
• Nb 0.3% or less
• Nb is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening.
• Nb like Ti, raises the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite unrecrystallized region, refining the grain size of upper bainite, fresh martensite and / Or contribute to an increase in the area ratio of retained austenite.
• Nb like Cr, is a carbide-forming element, and segregates at the interface between upper bainite and untransformed austenite during the upper bainite transformation after winding, thereby reducing the transformation driving force of bainite and untransformed austenite.
• Nb it is an element that has the effect of stopping the upper bainite transformation while leaving the The untransformed austenite is then cooled to become fresh martensite and/or retained austenite. Therefore, when Nb is added, Nb also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is obtained when Nb is preferably 0.01% or more. However, when the Nb content exceeds 0.3%, fresh martensite and/or retained austenite excessively increase, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in a decrease in uniform elongation. Therefore, when Nb is added, the Nb content should be 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.
• V 0.3% or less
• V is an element that acts to improve the strength of the steel sheet by precipitation strengthening and solid solution strengthening. Further, similarly to Ti, V raises the recrystallization temperature of austenite during hot rolling, thereby enabling rolling in the austenite non-recrystallization region and contributing to refinement of the grain size of upper bainite.
• V is a carbide-forming element, and segregates at the interface between upper bainite and untransformed austenite during upper bainite transformation after winding, thereby reducing the transformation driving force of bainite and untransformed austenite.
• V is an element that has the effect of stopping the upper bainite transformation while leaving the The untransformed austenite is then cooled to become fresh martensite and/or retained austenite. Therefore, when V is added, V also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is obtained when V is preferably 0.01% or more. However, when the V content exceeds 0.3%, fresh martensite and/or retained austenite excessively increase, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in a decrease in uniform elongation. Therefore, when V is added, the V content is 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.
• the chemical composition of the high-strength steel sheet of the present invention can optionally contain the following elements.
• Sb is an element that has the effect of suppressing nitridation of the surface of the steel material (slab) when the steel material (slab) is heated.
• Sb precipitation of BN in the surface layer of the steel material can be suppressed.
• the remaining solid solution B contributes to ensuring the hardenability necessary for the formation of bainite and thereby improving the strength of the steel sheet.
• the Sb content is 0.005% or more, preferably 0.006% or more, more preferably 0.007% or more, in order to obtain the above effect.
• the Sb content exceeds 0.020%, the toughness of the steel is lowered, and slab cracks and hot rolling cracks may occur. Therefore, when Sb is added, the Sb content is 0.020% or less, preferably 0.019% or less, and more preferably 0.018% or less.
• the chemical composition of the high-strength steel sheet in the present invention can optionally contain at least one of the elements listed below.
• the elements listed below contribute to further improvement of properties such as press formability.
• Ca 0.01% or less Ca controls the shape of oxide- and sulfide-based inclusions, and contributes to the suppression of cracking at sheared edge surfaces of steel sheets and the further improvement of bending workability. This effect is obtained when Ca is preferably 0.001% or more. However, if the Ca content exceeds 0.01%, the amount of Ca-based inclusions increases and the cleanliness of the steel deteriorates, which may rather cause shear edge cracks and bending cracks. Therefore, when Ca is added, the Ca content is set to 0.01% or less.
• Mg 0.01% or less Like Ca, Mg controls the shape of oxide- and sulfide-based inclusions, and contributes to the suppression of cracking at sheared edge surfaces of steel sheets and the further improvement of bending workability. This effect is obtained when Mg is preferably 0.001% or more. However, if the Mg content exceeds 0.01%, the cleanliness of the steel deteriorates, which may rather cause shear edge cracks and bending cracks. Therefore, when Mg is added, the Mg content is made 0.01% or less.
• REM 0.01% or less Like Ca, REM (rare earth metal) controls the shape of oxide- and sulfide-based inclusions, and contributes to the suppression of cracking at sheared edge surfaces of steel sheets and the further improvement of bending workability. do. This effect is obtained when REM is preferably 0.001% or more. However, if the REM content exceeds 0.01%, the cleanliness of the steel deteriorates, which may rather cause shear edge cracks and bending cracks. Therefore, when REM is added, the REM content is made 0.01% or less.
• the high-strength steel sheet of the present invention includes upper bainite with an area ratio of 80% or more and fresh martensite with a total area ratio of 2% or more and / or residual Top bainite with an area ratio of 70% or more and fresh martensite with a total area ratio of 3% or more and/or residual It contains austenite and has an average crystal grain size of 6 ⁇ m or less in the surface layer region from the steel plate surface to the plate thickness 1/10 position, and the hardness (HV1) of the surface layer region from the steel plate surface to the plate thickness 1/10 position.
• the difference (HV2-HV1) in the hardness (HV2) of the inner region from the 1/10th thickness position to the 3/10th thickness position is 5% or more and 15% or less with respect to [0.3 ⁇ tensile strength (MPa)] It has a certain microstructure.
• the soft upper part By finely dispersing hard fresh martensite and/or retained austenite in bainite, ductility can be improved and external bending cracks can be suppressed.
• the surface layer should have an area fraction of upper bainite of 80% or more and an area fraction of fresh martensite and/or retained austenite of 2% or more.
• the area ratio of upper bainite is 85% or more, and the area ratio of fresh martensite and/or retained austenite is 3% or more.
• the bendability may decrease, so the total area ratio of fresh martensite and/or retained austenite is It is preferable to make it 20% or less. It is more preferably 18% or less, still more preferably 15% or less.
• the bainite transformation progresses quickly, and the concentration of C for forming fresh martensite and/or retained austenite is less than in the interior. If the concentration of C is small, martensite transformation is suppressed. As a result, the area ratio of fresh martensite and/or retained austenite in the surface layer region of the steel sheet is smaller than that in the interior.
• upper bainite is included as a main phase in the inner region from the 1/10 thickness position to the 3/10 thickness position. If the area ratio of upper bainite is less than 70%, a tensile strength of 980 MPa or more and a uniform elongation of 6% or more cannot be achieved. Therefore, the area ratio of upper bainite is set to 70% or more, preferably 80% or more.
• fresh martensite and/or retained austenite are included in the internal region from the 1/10 thickness position to the 3/10 thickness position.
• Fresh martensite has the effect of improving uniform elongation by promoting work hardening and delaying the onset of plastic instability.
• Retained austenite can increase uniform elongation by TRIP (Transformation Induced Plasticity) effect.
• the total area ratio of fresh martensite and/or retained austenite is set to 3% or more, preferably 4% or more.
• the microstructure near the center of the plate thickness after the 3/10th position of the plate thickness has little effect on bendability, but from the viewpoint of ductility, the area ratio of upper bainite is preferably 60% or more.
• Fresh martensite/tempered martensite/retained austenite and the like may be contained up to 40% due to Mn segregation at the thickness center.
• Average crystal grain size in the surface layer region from the steel plate surface to the position of 1/10 of the plate thickness 6 ⁇ m or less
• Bending inner cracks are brittle fractures due to strong compression. That is, if the resistance to compression embrittlement is improved, internal bending cracks can be suppressed. Compression embrittlement is less likely to occur due to the refinement of crystal grains.
• the average crystal grain size in the surface layer region should be 6 ⁇ m or less, preferably 5 ⁇ m or less. As the average grain size becomes smaller, the effect of improving resistance to compression embrittlement can be obtained. Therefore, the average crystal grain size in the surface layer region is preferably 2 ⁇ m or more.
• the effect of improving the uniform elongation of fresh martensite and/or retained austenite and the control of the surface layer microstructure can only be achieved by combining inhibitory effects.
• the difference (HV2-HV1) between the hardness (HV1) of the surface layer region from the steel plate surface to the 1/10 thickness position and the hardness (HV2) of the inner region from the 1/10 thickness position to the 3/10 thickness position 5% or more and 15% or less with respect to [0.3 ⁇ tensile strength (MPa)]
• MPa tensile strength
• the difference between the hardness of the surface layer region (HV1) and the hardness of the inner region (HV2) (HV2-HV1) is 0.3 ⁇ tensile strength (MPa ) to 5% or more. It is preferably 6% or more, more preferably 7% or more.
• MPa tensile strength
• the difference between the hardness of the surface layer region and the hardness of the inner region is set to 15% or less with respect to 0.3 ⁇ tensile strength (MPa). It is preferably 14% or less, more preferably 13% or less.
• MPa tensile strength
• the microstructure can further contain any structure (hereinafter referred to as "other structures") other than upper bainite, fresh martensite, and retained austenite.
• other structures such structure
• the total area ratio of other structures is preferably 3% or less.
• the total area ratio of upper bainite, fresh martensite, and retained austenite in the microstructure is preferably 97% or more.
• Other structures include, for example, cementite, polygonal ferrite, pearlite, tempered martensite, and lower bainite.
• the high-strength steel sheet of the present invention has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and R / t (limit bending radius R and plate thickness t at which cracks with a depth of 50 ⁇ m or more do not occur on both the outside and inside of the bend. ratio) is 1.5 or less. Therefore, the high-strength steel sheet of the present invention has excellent press formability despite its high tensile strength, and can be press-formed without causing forming defects such as necking and cracking. The durability of the part can be ensured without large cracks occurring on both the outside and inside of the bend. Therefore, safety can be ensured when applied to members of trucks and passenger cars.
• microstructure, hardness, and mechanical properties of the present invention can be determined by the measurement methods described in Examples below.
• the high-strength steel sheet of the present invention can be produced by sequentially subjecting a steel material to the following treatments (1) to (5). Each step will be described below. (1) heating (2) hot rolling (3) cooling (first cooling) (4) Winding (5) Cooling (second cooling)
• the steel material any material can be used as long as it has the chemical composition described above.
• the chemical composition of the finally obtained high-strength steel sheet is the same as the chemical composition of the steel material used.
• a steel slab can be used as the steel material.
• the manufacturing method of the steel material is not particularly limited. For example, molten steel having the above chemical composition can be melted by a known method such as a converter, and a steel material can be obtained by a casting method such as continuous casting.
• a method other than the continuous casting method such as an ingot casting-blooming rolling method, can also be used.
• scrap may be used as a raw material.
• the steel material may be directly subjected to the next heating step after being manufactured by a method such as a continuous casting method, or may be subjected to the heating step after being cooled into a hot piece or a cold piece. good.
• the steel material is heated to a heating temperature of 1150°C or higher.
• a heating temperature 1150°C or higher.
• carbonitride-forming elements such as Ti exist as coarse carbonitrides in steel materials.
• the presence of this coarse and non-uniform precipitates is generally required for high-strength steel sheets for truck and passenger car parts (e.g. shear edge crack resistance, bending workability, burring workability, etc.). aggravate. Therefore, it is necessary to heat the steel material prior to hot rolling to dissolve coarse precipitates.
• the heating temperature of the steel material must be 1150° C. or higher in order to sufficiently dissolve the coarse precipitates.
• the heating temperature of the steel material becomes too high, slab flaws will occur and the yield will decrease due to scale off.
• the heating temperature of the steel material it is preferable to set the heating temperature of the steel material to 1350° C. or lower.
• the lower limit of the heating temperature of the steel material is more preferably 1180°C or higher, and still more preferably 1200°C or higher.
• the upper limit of the heating temperature of the steel material is more preferably 1300° C. or lower, and still more preferably 1280° C. or lower.
• the heating from the viewpoint of uniforming the temperature of the steel material, it is preferable to raise the temperature of the steel material to the heating temperature and then maintain it at the heating temperature.
• the time for which the heating temperature is maintained (holding time) is not particularly limited, but from the viewpoint of improving the temperature uniformity of the steel material, it is preferably 1800 seconds or longer.
• the holding time exceeds 10000 seconds, the amount of scale generation increases. As a result, entrapment of scales and the like is likely to occur in subsequent hot rolling, leading to a decrease in yield due to defective surface defects. Therefore, the retention time is preferably 10000 seconds or less, more preferably 8000 seconds or less.
• Hot rolling Next, the heated steel material is hot rolled to form a hot rolled steel sheet.
• Hot rolling may consist of rough rolling and finish rolling.
• the conditions are not particularly limited.
• descaling is preferably performed prior to finish rolling in order to remove surface scales. Descaling may be performed between stands in the finish rolling.
• the total rolling reduction in the temperature range of RC1 or less is 25% or more and 80% or less
• the finishing temperature of finish rolling should be (RC2-50°C) or more and (RC2+120°C) or less.
• RC1 is the austenite 50% recrystallization temperature estimated from the component composition
• RC2 is the austenite lower limit recrystallization temperature estimated from the component composition. If the total rolling reduction of RC1 or less is less than 25%, the average crystal grain size becomes large and good bending workability cannot be obtained. On the other hand, when the total rolling reduction in the temperature range of RC1 or less exceeds 80%, the dislocation density of austenite is high, the ductility of the bainite structure transformed from austenite in a state of high dislocation density is poor, and the uniform elongation is 6% or more. is not obtained. Therefore, the total rolling reduction in the temperature range of RC1 or less is set to 25% or more and 80% or less.
• finish rolling finish temperature (RC2-50°C) or more and (RC2+120°C) or less. If the finish rolling finish temperature is lower than (RC2-50° C.), bainite transformation occurs from austenite in a state of high dislocation density. Since upper bainite transformed from austenite with a high dislocation density has a high dislocation density and poor ductility, the uniform elongation decreases. Also, when the rolling end temperature is low and the rolling is performed at the two-phase region temperature of ferrite + austenite, the uniform elongation decreases. Therefore, the finishing temperature of finish rolling should be (RC2-50° C.) or higher.
• the finish rolling finish temperature is set to (RC2+120° C.) or less.
• RC1 and RC2 are defined by the following formulas (1) and (2).
• RC1 (°C) 900 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V
• RC2 (°C) 750 + 100 x C + 100 x N + 10 x Mn + 350 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1000 x Nb + 150 x V (2)
• each element symbol in the above formulas (1) and (2) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.
• Cooling (first cooling) Next, the obtained hot-rolled steel sheet is cooled (first cooling). At that time, the time from the end of hot rolling (end of finish rolling) to the start of cooling (cooling start time) is set within 2.0 seconds. If the cooling start time exceeds 2.0 seconds, grain growth of austenite grains occurs and a tensile strength of 980 MPa or more cannot be secured. The cooling start time is preferably within 1.5 seconds.
• the average cooling rate at the plate thickness 3/10 position shall be 15°C/s or more.
• different microstructures are created between the surface layer and the inside. Due to the rapid cooling of the surface layer, the bainite transformation of the surface layer starts early, and the formation of martensite and retained austenite due to the enrichment of C is less than in the inside. If the average cooling rate in cooling is less than 15 ° C. / s, the surface layer is not cooled sufficiently rapidly, and the upper bainite with an area ratio of 80% or more and the total area ratio of 2% or more fresh martensite and / or residual An austenite surface layer structure cannot be obtained.
• the average cooling rate is set to 15° C./s or higher, preferably 20° C./s or higher, more preferably 50° C./s or higher.
• the upper limit of the average cooling rate is not particularly limited, but if the average cooling rate is too high, it becomes difficult to manage the cooling stop temperature. Therefore, the average cooling rate is preferably 200° C./s or less.
• the average cooling rate is defined based on the average cooling rate on the surface of the steel sheet.
• the average cooling rate of the surface layer - the average cooling rate at the plate thickness 3/10 position of 10 ° C./s or more by satisfying the average cooling rate of the surface layer - the average cooling rate at the plate thickness 3/10 position of 10 ° C./s or more, the formation of martensite and retained austenite due to the enrichment of C in the surface layer is reduced to the plate thickness. Less than the 3/10 position. As a result, a soft surface layer structure can be created.
• the cooling rate is slower than the surface layer, and the progress of bainite transformation is slower than that in the surface layer. be able to. That is, a difference in hardness between the surface layer and the inside can be realized.
• the average cooling rate of the plate thickness 3/10 position surface layer is less than 10 ° C / s, the above effect is not observed, so the average cooling rate of the surface layer - plate thickness
• the average cooling rate at the 3/10 position is 10° C./s or more.
• the average cooling rate is obtained by (temperature at the start of cooling - temperature at the end of cooling)/cooling time.
• the temperature of the surface layer is actually measured with a thermometer.
• the temperature at the 3/10 thickness position is obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis and correcting the result with the actual surface temperature of the steel sheet.
• cooling forced cooling may be performed so as to achieve the above average cooling rate.
• the cooling method is not particularly limited, but for example, water cooling is preferable.
• the cooling stop temperature shall be Trs or higher and (Trs + 250°C) or lower.
• Trs When the cooling stop temperature is less than Trs, the microstructure becomes tempered martensite or lower bainite. Tempered martensite and lower bainite are both high-strength structures, but their uniform elongation is remarkably low. Therefore, the cooling stop temperature is set to Trs or higher.
• the cooling stop temperature is set to (Trs+250° C.) or less.
• each element symbol in the above formula (3) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.
• the cooled hot-rolled steel sheet is coiled under the conditions of a coiling temperature of Trs or more and (Trs+250° C.) or less. If the coiling temperature is lower than Trs, martensite transformation or lower bainite transformation proceeds after coiling, and desired fresh martensite and/or retained austenite cannot be obtained. Therefore, the winding temperature should be Trs or higher. On the other hand, if the coiling temperature is higher than (Trs+250° C.), ferrite is generated, and a tensile strength of 980 MPa cannot be obtained. Therefore, the winding temperature is set to (Trs+250° C.) or less.
• Cooling After winding, it is further cooled to 100° C. or lower at an average cooling rate of 20° C./s or lower (second cooling).
• the average cooling rate affects the formation of fresh martensite and/or retained austenite.
• the average cooling rate is set to 20° C./s or less, preferably 2° C./s or less, more preferably 0.02° C./s or less.
• the lower limit of the average cooling rate is not particularly limited, it is preferably 0.0001° C./s or more.
• Cooling can be performed to any temperature below 100°C, but cooling to about 10 to 30°C (for example, room temperature) is preferable. It should be noted that the cooling can be performed in any form, for example, it may be performed in the state of a wound coil.
• the high-strength steel sheet of the present invention can be manufactured by the above procedure.
• the winding and subsequent cooling may be carried out in accordance with a conventional method. For example, temper rolling may be applied, or pickling may be applied to remove scales formed on the surface.
• Molten steel having the composition shown in Table 1 was melted in a converter, and a steel slab was produced as a steel material by continuous casting.
• the obtained steel material was heated to the heating temperature shown in Table 2, and then the heated steel material was subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot-rolled steel sheet.
• Table 2 shows the finishing temperature of hot rolling.
• the obtained hot-rolled steel sheet was cooled under the conditions of the average cooling rate and the cooling stop temperature shown in Table 2 (first cooling).
• the cooled hot-rolled steel sheet was coiled at the coiling temperature shown in Table 2, and the coiled steel sheet was cooled at the average cooling rate shown in Table 2 (second cooling) to obtain a high-strength steel sheet.
• skin-pass rolling and pickling were performed as post-treatments. The pickling was carried out at a temperature of 85° C. using an aqueous solution of hydrochloric acid having a concentration of 10% by mass.
• a test piece was taken from the obtained high-strength steel sheet, and the microstructure, surface roughness, and mechanical properties were evaluated according to the procedure described below.
• a test piece for microstructure observation was taken from the obtained high-strength steel sheet so that the thickness cross-section parallel to the rolling direction was the observation surface.
• the surface of the obtained test piece was polished, and the surface was corroded using an etchant (3 vol.% nital solution) to expose the microstructure.
• the obtained SEM images were analyzed by image processing to quantify the area ratios of upper bainite (UB), polygonal ferrite (F), and tempered martensite (TM).
• UB upper bainite
• F polygonal ferrite
• TM tempered martensite
• fresh martensite (M) and retained austenite ( ⁇ ) are difficult to distinguish by SEM, so they were identified using an electron backscatter diffraction (EBSD) method, and each area ratio and average crystal Particle size was determined.
• Table 3 shows the measured area ratio of each microstructure and the average crystal grain size of the surface layer structure. Table 3 also shows the total area ratio (M+ ⁇ ) of fresh martensite and retained austenite.
• Hardness measurement From the obtained high-strength steel sheet, a sample for hardness measurement is taken so that the thickness cross section parallel to the rolling direction becomes the hardness measurement cross section, and the surface layer region from the steel plate surface to the thickness 1/10 position and the thickness 1/ The hardness of the internal region from the 10th position to the plate thickness 3/10th position was measured. The hardness of the surface layer region from the surface of the steel plate to the position of 1/10 of the plate thickness was measured at a position 50 ⁇ m away from the surface with an indentation interval of 250 ⁇ m. The hardness of the inner region from the 1/10 thickness position to the 3/10 thickness position was measured at the 1/5 thickness position with an indentation interval of 250 ⁇ m. All hardness measurement conditions were a load of 100 g, a holding time of 10 s, and an average of 5 measurement points.
• Test test A JIS No. 5 test piece (gauge length, GL: 50 mm) was taken from the obtained high-strength steel sheet so that the tensile direction was perpendicular to the rolling direction. Using the obtained test piece, a tensile test was performed in accordance with the provisions of JIS Z 2241, yield strength (yield point, YP), tensile strength (TS), yield ratio (YR), total elongation (El), Similar elongation (u-El) was determined. The tensile test was performed twice for each high-strength steel sheet, and the average of the obtained measured values is shown in Table 3 as the mechanical properties of the high-strength steel sheet. In the present invention, when TS was 980 MPa or more, it was evaluated as high strength. Moreover, when the uniform elongation was 6% or more, the press formability was evaluated as good.

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