Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to steel plates.
• Patent Document 1 describes a steel sheet having a predetermined chemical composition, in a cross section parallel to the rolling direction and thickness direction of the steel sheet, a metal structure at a depth position of 1/4 of the sheet thickness from the surface is, in area %, retained austenite: 10% or more, tempered martensite: 60 to 80%, and martensite: less than 20%, and in the tempered martensite and martensite, the density of low-angle grain boundaries having a crystal orientation difference of 2 degrees or more and less than 20 degrees is 0.20 to 1.0 ⁇ m -1 and the density of high-angle grain boundaries having a crystal orientation difference of 20 to 50 degrees is 0.30 to 0.60 ⁇ m -1 , and the ratio A L /A N of the particle density A L of the retained austenite in the rolling direction to the particle density A N in the thickness direction is 0.80 to 1.0.
• Patent Document 1 also teaches that by setting the low-angle grain boundary density to 0.20 to 1.0 ⁇ m ⁇ 1 , transformation-induced plasticity of austenite occurs at high stress, resulting in an excellent balance between strength and ductility, while by setting the high-angle grain boundary density to 0.30 to 0.60 ⁇ m ⁇ 1 , the progression of ductile fracture is suppressed, resulting in excellent impact properties, and further by setting the particle density ratio A L /A N to 0.80 to 1.0, the retained austenite becomes isotropically dispersed, resulting in excellent bendability and impact properties.
• Patent Document 2 describes a high-strength steel plate having a specified chemical composition, in which the steel structure is, in terms of area percentage, 35% to 80% polygonal ferrite and 5% to 25% martensite, and in terms of volume percentage, 8% or more retained austenite, the average crystal grain size of the polygonal ferrite is 6 ⁇ m or less, the average crystal grain size of the martensite is 3 ⁇ m or less, the average crystal grain size of the retained austenite is 3 ⁇ m or less, the average aspect ratios of the crystal grains of the polygonal ferrite, the martensite and the retained austenite are each 2.0 or less, and further the value obtained by dividing the Mn amount (mass %) in the retained austenite by the Mn amount (mass %) in the polygonal ferrite is 2.0 or more.
• Patent Document 2 teaches that in order to ensure good ductility, it is necessary to increase the amount of stable retained austenite in which Mn is concentrated, and therefore it is extremely important that the value obtained by dividing the amount of Mn (mass%) in the retained austenite by the amount of Mn (mass%) in the polygonal ferrite is 2.0 or more.
• Patent Document 2 teaches that when the amount of C in the retained austenite satisfies the relationship between the amount of Mn in the retained austenite and the amount of C, 0.09 x [amount of Mn] - 0.130 - 0.140 ⁇ [amount of C] ⁇ 0.09 x [amount of Mn] - 0.130 + 0.140 (where [amount of C]: amount of C in the retained austenite, [amount of Mn]: amount of Mn in the retained austenite), a large amount of so-called stable retained austenite is obtained, which causes the deformation-induced transformation (TRIP) phenomenon, which is the main factor in improving ductility, to occur intermittently until the end of the processing of the steel sheet, and this makes it possible to achieve high strength and even better elongation.
• TRIP deformation-induced transformation
• Patent Document 2 is mainly intended for steel sheets with a tensile strength of 590 MPa or more.
• the automotive industry and other sectors are demanding further weight reduction in steel sheets, and in order to achieve such weight reduction, it is necessary to make the steel sheets stronger than ever before. Therefore, there remains a high demand for steel sheets that can improve elongation even when the strength is increased to the same level or even more than before.
• the present invention aims to provide a steel sheet that can achieve high strength and improved elongation through a new structure.
• the inventors conducted research with a particular focus on the microstructure of steel plate.
• the inventors discovered that by forming the microstructure of a steel plate having a specified chemical composition mainly from tempered martensite, ferrite, and retained austenite, it is possible to achieve high strength while improving elongation, and further, by limiting the Mn concentration in the ferrite within a specified range, it is possible to suppress solid solution strengthening of the ferrite, and by concentrating Mn in the retained austenite to stabilize the retained austenite, it is possible to significantly improve the elongation of the steel plate, thus completing the present invention.
• the present invention which has achieved the above object is as follows.
• the steel plate has a microstructure in which the average Mn concentration in the retained austenite divided by the Mn concentration in the base metal is 1.150 to 2.000.
• the chemical composition is, in mass%, Cr: 0.001-2.000%, Mo: 0.001 to 1.000%, Ti: 0.001 to 0.500%, Nb: 0.001-0.500%, B: 0.0001 to 0.0100%, Cu: 0.001 to 1.000%, Ni: 0.001 to 1.000%, W: 0.001-0.100%, V: 0.001-1.000%, Ta: 0.001-0.100%, Co: 0.001 to 3.000%, Sn: 0.001 to 1.000%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, Mg: 0.0001-0.050%, Zr: 0.0001 to 0.050%, Ca: 0.0001-0.0500%, Y: 0.0001-0.0500%, La: 0.0001 to 0.0500%, Ce: 0.0001 to 0.0500%, and Bi: 0.0001 to 0.0500%
• the steel sheet according to the above (1) characterized in that it contains at least one of the following: (3) The steel plate according to (1) or (2)
• the present invention provides a steel sheet that can achieve high strength and improved elongation.
• the steel plate according to the embodiment of the present invention has, in mass%, C: 0.20-0.35%, Si: 0.01-2.00%, Mn: 1.40-4.00%, P: 0.1000% or less, S: 0.0200% or less, Al: 2.00% or less, N: 0.0200% or less, O: 0.0200% or less, Cr: 0-2.000%, Mo: 0-1.000%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0100%, Cu: 0 to 1.000%, Ni: 0-1.000%, W: 0-0.100%, V: 0-1.000%, Ta: 0-0.100%, Co: 0-3.000%, Sn: 0-1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0-0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%
• a value obtained by dividing the average Mn concentration in the ferrite by the Mn concentration in the base material is 0.980 or less;
• the alloy is characterized by having a microstructure in which the average Mn concentration in the retained austenite divided by the Mn concentration in the base metal is 1.150 to 2.000.
• TRIP steel sheets (TRANSFORMATION INDUCED PLASTICITY), which utilizes the transformation-induced plasticity of retained austenite, are known as a means of achieving both high strength and improved elongation.
• retained austenite is included in the microstructure, the elongation of the steel sheet is generally improved due to the TRIP effect, in which the steel sheet is transformed into martensite by processing-induced transformation during deformation. Therefore, the inventors conducted research, focusing in particular on microstructures containing retained austenite, in order to achieve both high strength and improved elongation of steel sheets.
• the inventors have found that by optimizing the chemical composition of a steel sheet and configuring the microstructure of the steel sheet to mainly contain tempered martensite, which is a hard and tough structure, ferrite, which is a soft structure, and retained austenite having a TRIP effect, more specifically, by configuring the microstructure of the steel sheet to contain, in terms of area ratio, 40-65% tempered martensite, 15-40% ferrite, and 10-20% retained austenite, it is possible to improve elongation while ensuring sufficient strength.
• tempered martensite which is a hard and tough structure
• ferrite which is a soft structure
• retained austenite having a TRIP effect more specifically, by configuring the microstructure of the steel sheet to contain, in terms of area ratio, 40-65% tempered martensite, 15-40% ferrite, and 10-20% retained austenite, it is possible to improve elongation while ensuring sufficient strength.
• the inventors have found that in addition to containing a predetermined proportion of C and Mn, which are effective in increasing the strength of the steel sheet, controlling the total amount of Si and Al to within a range of 1.00-2.20% by mass is effective in both increasing strength and improving elongation. It is believed that by controlling the total amount of Si and Al within such a range, C can be concentrated in the retained austenite. More specifically, the microstructure of the steel sheet according to the embodiment of the present invention includes bainite: 10 to 30% by area ratio in addition to tempered martensite, ferrite, and retained austenite.
• the concentrated C not only stabilizes the retained austenite and contributes to improving the elongation of the steel sheet, but also increases the hardness when the retained austenite transforms into martensite by processing-induced transformation, and is therefore considered to contribute to increasing the strength of the steel sheet.
• the inventors conducted an investigation focusing on ferrite as a soft structure that can particularly contribute to improving elongation among microstructures, and the specific form of retained austenite that exhibits the TRIP effect.
• the inventors discovered that by limiting the Mn concentration in ferrite to a predetermined range, more specifically by controlling the value obtained by dividing the average Mn concentration in ferrite by the Mn concentration of the base material (i.e., the Mn content of the steel sheet) to 0.980 or less and making the Mn concentration in ferrite smaller than the average Mn concentration in the entire steel sheet, it is possible to significantly suppress solid solution strengthening of ferrite by Mn, thereby improving the elongation of the steel sheet.
• the inventors have discovered that in relation to lowering the Mn concentration in ferrite, it is possible to stabilize the retained austenite by concentrating Mn in the retained austenite, more specifically by controlling the average Mn concentration in the retained austenite divided by the Mn concentration of the base material (i.e., the Mn content of the steel sheet) to 1.150 to 2.000, and that in combination with the effect of lowering the Mn concentration in ferrite, it is possible to significantly improve the elongation of the steel sheet.
• the steel sheet according to the embodiment of the present invention it is extremely important to moderately concentrate Mn in the retained austenite, that is, to control the value obtained by dividing the average value of the Mn concentration in the retained austenite by the Mn concentration in the base material to 1.150 to 2.000.
• the steel sheet according to the embodiment of the present invention it is possible to reliably achieve both the contradictory properties of high strength and excellent elongation, and therefore the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field where both properties are required to be achieved.
• C is an element that secures a predetermined amount of martensite and improves the strength of the steel sheet.
• the C content is set to 0.200% or more.
• the C content is set to 0.350% or less.
• the content may be up to 0.320%, up to 0.300% or up to 0.280%.
• Silicon is an element that improves the strength of steel sheet by solid solution strengthening.
• the silicon content is set to 0.01% or more.
• the silicon content is set to 0.10% or more.
• the Si content may be 0.30% or more, 0.50% or more, or 0.80% or more.
• the Si content is set to 2.00% or less.
• the Si content is set to 1.80% or less, 1.50% or less, 1.20% or less, less than 1.20%, 1.10% or less. % or less, or 1.00% or less.
• Mn is an element that improves hardenability and contributes to improving the strength of steel sheets. Mn is also an element that concentrates in the retained austenite to stabilize the retained austenite, thereby improving the elongation. In order to fully obtain the above effects, the Mn content is set to 1.40% or more.
• the Mn content is set to 1.60% or more, 1.80% or more, 2.00% or more, 2.20% or more, 2.50% or more, 2.70% or more, 2.80% or more, 2.90% or more, 2.10% or more, 2.20% or more, 2.30% or more, 2.40% or more, 2.50% or more, 2.60% or more, 2.70% or more, 2.80% or more, 2.9 ...10% or more, 2.20% or more, 2.30% or more, 2.40% or more, 2.50% or more, 2.60% or
• the residual austenite may be excessively stabilized, which may result in a decrease in elongation and/or Alternatively, the solid solution strengthening of ferrite may progress, similarly resulting in a decrease in elongation, or the load on equipment such as a rolling mill during production may increase, resulting in a decrease in manufacturability.
• the Mn content may be 3.80% or less, 3.50% or less, 3.20% or less, or 3
• P is an impurity element that embrittles welds and deteriorates plating properties. Therefore, the P content is set to 0.1000% or less.
• the P content is set to 0.0600% or less, 0. The lower the P content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. If the P content is reduced to less than 0.0001%, the production cost increases significantly, which is economically disadvantageous. Therefore, the P content is set to 0.0001% or more, 0.0002% or more, or 0.0005% or more. It's fine if there is.
• S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. For this reason, the S content is set to 0.0200% or less. The lower the S content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the S content of practical steel sheets is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which will be economically disadvantageous. Therefore, the S content is set to 0.0001% or more, 0.0002% or more. It may be 0.0005% or more.
• Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel.
• the Al content may be 0%, but in order to fully obtain these effects,
• the Al content is preferably 0.001% or more.
• the Al content is preferably 0.005% or more, 0.01% or more, 0.10% or more, more than 0.20%, 0.25% or more, or 0. 30% or more.
• excessive Al content may form coarse oxides and reduce toughness. Therefore, the Al content is set to 2.00% or less.
• the content may be up to 1.80%, up to 1.50%, up to 1.30% or up to 1.00%.
• N is an element that can cause blowholes during welding. Therefore, the N content is set to 0.0200% or less.
• the N content is set to 0.0180% or less, 0.0150% or less, 0.
• O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0200% or less.
• the O content is set to 0.0180% or less, 0.0150% or less, 0.
• O is set to less than 0.0001%. If the O content is reduced, the production cost increases significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary, for the purpose of improving characteristics.
• the steel sheet may contain Cr: 0-2.000%, Mo: 0-1.000%, Ti: 0-0.500%, Nb: 0-0.500%, B: 0-0.0100%, Cu: 0-1.000%, Ni: 0-1.000%, W: 0-0.100%, V: 0-1.000%, Ta: 0-0.100%, Co: 0-3.000%, Sn : 0-1.000%, Sb: 0-0.500%, As: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca: 0-0.0500%, Y: 0-0.0500%, La: 0-0.0500%, Ce: 0-0.0500%, and Bi: 0-0.0500% may be included.
• These optional elements are described in detail below.
• Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet, similar to Mn.
• the Cr content may be 0%, but in order to obtain the above effect, the Cr content should be 0.001% or more.
• the Cr content may be 0.010% or more, 0.100% or more, or 0.200% or more.
• the Cr content is preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, or 0.500% or less.
• Mo is an element that contributes to increasing the strength of steel sheets. This effect can be obtained even with a small amount of Mo.
• the Mo content may be 0%, but in order to obtain the above effect, The Mo content is preferably 0.001% or more.
• the Mo content may be 0.010% or more, 0.020% or more, 0.050% or more, or 0.100% or more. If Mo is contained excessively, hot workability may deteriorate, and productivity may decrease. Therefore, the Mo content is preferably 1.000% or less.
• the Mo content is 0.800%. It may be 0.400% or less or 0.200% or less.
• Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite.
• the Ti content may be 0%, but in order to obtain these effects, the Ti content must be less than 0.
• the Ti content is preferably 0.001% or more.
• the Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.050% or more.
• an excessive Ti content However, the effect of adding Ti becomes saturated, and there is a risk of increasing the manufacturing cost. Therefore, the Ti content is preferably 0.500% or less, and more preferably 0.400% or less, 0.200% or less, or 0.100% or less. It may be the following.
• Nb is an element that is effective in controlling the morphology of carbides, and is also an element that is effective in refining the structure and improving the toughness of steel plates. These effects can be obtained even in small amounts.
• the Nb content may be 0%, but in order to obtain the above effects, the Nb content is preferably 0.001% or more.
• the Nb content is preferably 0.005% or more or 0.010% or more.
• the Nb content is set to 0.500% or less.
• the Nb content may be 0.200% or less, 0.100% or less, or 0.060% or less.
• B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. B is also an element that is beneficial for increasing the strength of steel. These effects are only seen in small amounts.
• the B content may be 0%, but in order to obtain the above effects, the B content is preferably 0.0001% or more. The B content is preferably 0.0005% or more.
• the B content is preferably 0.0100% or less.
• the B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• Cu is an element that contributes to improving the strength of the steel sheet. This effect can be obtained even with a small amount of Cu.
• the Cu content may be 0%, but in order to obtain the above effect, the Cu content must be 0%.
• the Cu content is preferably 0.001% or more.
• the Cu content may be 0.005% or more, 0.010% or more, or 0.050% or more.
• excessive Cu content may cause red shortness. This may lead to a decrease in productivity in hot rolling. Therefore, the Cu content is preferably 1.000% or less. It may be 0.300% or less or 0.100% or less.
• Ni is an element effective in improving the strength of a steel sheet.
• the Ni content may be 0%, but in order to obtain the above effect, the Ni content is preferably 0.001% or more.
• the Ni content may be 0.005% or more, or 0.010% or more.
• the Ni content is set to 1.000% or more.
• the Ni content may be 0.800% or less, 0.400% or less, or 0.200% or less.
• W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets.
• the W content may be 0%, but in order to obtain these effects, the W content must be 0.001% or more.
• the W content may be 0.005% or more, or 0.010% or more.
• the W content is preferably 0.
• the W content is preferably 0.080% or less, 0.040% or less, or 0.020% or less.
• V is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel plate.
• the V content may be 0%, but in the above In order to obtain the effect, the V content is preferably 0.001% or more.
• the V content may be 0.005% or more, 0.010% or more, or 0.050% or more. However, if V is contained excessively, a large amount of precipitates may be formed, which may reduce toughness. Therefore, the V content is preferably 1.000% or less.
• the V content is preferably 0.400% or less. It may be 0.200% or less or 0.100% or less.
• Ta is an element that is effective in controlling the morphology of carbides and improving the strength of steel sheets.
• the Ta content may be 0%, but in order to obtain these effects, the Ta content should be 0.001%.
• the Ta content may be 0.005% or more, or 0.010% or more.
• the Ta content is preferably 0.100% or less.
• the Ta content is preferably 0.080% or less, 0.040% or less, or 0.020% or less. It's fine if there is.
• Co is an element that is effective in improving the strength of steel sheets.
• the Co content may be 0%, but in order to obtain the above effect, the Co content must be 0.001% or more.
• the Co content may be 0.005% or more, 0.010% or more, or 0.100% or more.
• the Co content is preferably 3.000% or less. % or less.
• Sn is an element that may be contained in a steel sheet when scrap is used as the raw material for the steel sheet. In addition, Sn may cause embrittlement of ferrite. Therefore, the smaller the Sn content, the better.
• the Sn content may be 0.100% or less, 0.040% or less, or 0.020% or less.
• the Sn content may be 0%, but Sn Reducing the Sn content to less than 0.001% leads to an excessive increase in refining costs. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.010% or more. good.
• Sb is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet.
• Sb may strongly segregate at grain boundaries and cause embrittlement of the grain boundaries. Therefore, the smaller the Sb content, the better, and it is preferably 0.500% or less.
• the Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less.
• the Sb content may be 0%, but reducing the Sb content to less than 0.001% will lead to an excessive increase in refining costs. % or more, or 0.010% or more.
• As is an element that can be contained in a steel sheet when scrap is used as the raw material for the steel sheet.
• As is an element that strongly segregates at grain boundaries, and the lower the As content, the better.
• the As content is preferably 0.050% or less, and may be 0.040% or less or 0.020% or less.
• the As content may be 0%, but the As content Reducing As to less than 0.001% leads to an excessive increase in refining costs, so the As content may be 0.001% or more, 0.005% or more, or 0.010% or more.
• Mg controls the morphology of sulfides and oxides, and contributes to improving the bending formability of steel sheets. This effect can be obtained even with a small amount.
• the Mg content may be 0%, but if the above effect is not achieved, the Mg content may be 0%.
• the Mg content is preferably 0.0001% or more.
• the Mg content may be 0.0005% or more, 0.001% or more, or 0.005% or more. Even if Mg is contained in an excessive amount, the effect is saturated, and the inclusion of more Mg in the steel sheet than necessary leads to an increase in manufacturing costs. For this reason, the Mg content is preferably 0.050% or less. may be 0.040% or less, 0.020% or less, or 0.010% or less.
• Zr is an element that can control the morphology of sulfides with a small amount.
• the Zr content may be 0%, but in order to obtain the above effects, the Zr content is preferably 0.0001% or more.
• the content may be 0.0005% or more, 0.001% or more, or 0.005% or more.
• the Zr content is preferably 0.050% or less.
• the Zr content is preferably 0.040% or less, 0.020% or less, or 0.010% or less. Good too.
• Ca, Y, La and Ce are elements that can control the morphology of sulfides even in small amounts.
• the contents of Ca, Y, La and Ce may be 0%, but in order to obtain the above effects, Ca, Y,
• the La and Ce contents are each preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, 0.0020% or more, or 0.0030% or more. Even if these elements are contained in excess, the effect is saturated, and the inclusion of more than necessary in the steel sheet leads to an increase in manufacturing costs. Therefore, the Ca, Y, La and Ce contents are each set to 0.0500% or less. It is preferable that the content be 0.0200% or less, and it may be 0.0100% or less, or 0.0060% or less.
• Bi is an element that has the effect of improving formability by refining the solidification structure.
• the Bi content may be 0%, but in order to obtain such an effect, the Bi content should be 0.0001%.
• the Bi content is preferably 0.0500% or less, and even if it is 0.0400% or less, 0.0200% or less, or 0.0100% or less, good.
• the remainder excluding the above elements consists of Fe and impurities.
• Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel plate according to the embodiment of the present invention.
• the chemical composition of the steel sheet according to the embodiment of the present invention is set so that the total content of Si and Al is 1.00% or more, that is, [Si] + [Al] ⁇ 1.00. is controlled by. From the viewpoint of further enhancing these effects, the total content of Si and Al is preferably 1.20% or more, and may be 1.40% or more or 1.60% or more.
• the total content of Si and Al is set to 2.20% or less, that is, [Si] + [Al] ⁇ 2.20.
• Si and Al The total content may be 2.10% or less, 2.20% or less, 1.90% or less, 1.80% or less, or 1.70% or less.
• the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
• the chemical composition of the steel plate may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
• C and S may be measured using the combustion-infrared absorption method
• N may be measured using the inert gas fusion-thermal conductivity method
• O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
• the microstructure of the steel plate according to the embodiment of the present invention will be described.
• the structure fraction is expressed as an area fraction, so the unit "%" of the structure fraction means area %.
• the microstructure is controlled at 1/4 of the plate thickness of the steel plate.
• the 1/4 of the plate thickness of the steel plate means a region between a surface at 1/8 depth and a surface at 3/8 depth of the plate thickness from the rolling surface of the steel plate.
• all the structure fractions mean values at 1/4 of the plate thickness.
• the area ratio of ferrite is set to 15% or more. From the viewpoint of improving elongation, the higher the area ratio of ferrite, the more preferable it is, and it may be, for example, 20% or more, 25% or more, or 30% or more. On the other hand, if ferrite is contained excessively, the desired strength may not be achieved in the steel sheet. Therefore, the area ratio of ferrite is set to 40% or less. The area ratio of ferrite may be 38% or less, 36% or less, or 34% or less.
• tempered martensite 40-65%
• tempered martensite is a hard structure, it contributes to improving strength.
• tempered martensite is a tough structure compared to as-quenched martensite, which is hard but relatively brittle, it also contributes to improving elongation.
• the area ratio of tempered martensite is set to 40% or more. From the viewpoint of improving strength, the higher the area ratio of tempered martensite, the more preferable it is, for example, 42% or more, 45% or more, or 48% or more. On the other hand, if tempered martensite is excessively contained, the strength may become too high and the elongation may decrease.
• the area ratio of tempered martensite is set to 65% or less. From the viewpoint of improving elongation, the lower the area ratio of tempered martensite, the more preferable it is, for example, 60% or less, 55% or less, or 50% or less.
• the retained austenite is a structure that improves the strength and elongation of the steel sheet by the TRIP effect, which transforms into martensite by processing-induced transformation during deformation of the steel sheet.
• the area ratio of the retained austenite content is set to 10% or more. From the viewpoint of improving elongation, the higher the area ratio of the retained austenite, the more preferable it is, and it may be, for example, 12% or more, 14% or more, or 16% or more.
• the area ratio of the retained austenite is set to 20% or less.
• the area ratio of the retained austenite may be 19% or less or 18% or less.
• the microstructure of the steel plate according to the embodiment of the present invention is mainly composed of the above-mentioned ferrite, tempered martensite, and retained austenite, and these structures mainly achieve improvement in strength and improvement in elongation, but in addition to these structures, bainite is also included as an essential structure. If the area ratio of bainite is too small, the amount of C discharged into austenite during bainite transformation is insufficient, and C concentration in the retained austenite becomes insufficient, which may adversely affect improvement in strength and/or improvement in elongation. Therefore, the area ratio of bainite is 10% or more. The area ratio of bainite may be 12% or more, 15% or more, 18% or more, or 20% or more.
• bainite is also a relatively hard structure, it may contribute to improvement in strength, but if it is excessively contained, the ratio of other structures such as ferrite, tempered martensite, and retained austenite decreases, and as a result, the desired strength and/or elongation may not be achieved. Therefore, the area ratio of bainite is 30% or less. The area ratio of bainite may be 28% or less, 25% or less, or 22% or less.
• the remaining structure other than ferrite, tempered martensite, retained austenite, and bainite may be 0% in area ratio, but if a remaining structure exists, the remaining structure is pearlite and as-quenched martensite.
• the area ratios of the remaining structure, i.e., pearlite and as-quenched martensite may be 10% or less, for example, 8% or less, 6% or less, 4% or less, or 2% or less.
• the area ratios of pearlite and as-quenched martensite may be 0.5% or more, or 1% or more, respectively.
• Identification of each metal structure and calculation of the area ratio are performed by EBSD (Electron Back Scattering Diffraction), X-ray measurement, corrosion using Nital reagent or Lepera solution, and by observing a 100 ⁇ m ⁇ 100 ⁇ m region of the steel sheet cross section perpendicular to the sheet surface at a magnification of 1000 to 50000 times using a scanning electron microscope.
• the measurement points are three places, and the average value is calculated.
• the area ratio of ferrite is measured by the following method. That is, the range of 1/8 to 3/8 thickness centered at the position of 1/4 of the sheet thickness from the surface of the steel sheet is measured at intervals (pitch) of 0.2 ⁇ m by EBSD attached to a scanning electron microscope.
• the value of the local misorientation average (Grain Average Misorientation: GAM) is calculated from the measurement data. Then, the area and area ratio of ferrite are measured for the area where the local misorientation average value is less than 0.5°.
• the local misorientation average is a value obtained by calculating the misorientation between adjacent measurement points in an area surrounded by grain boundaries with a crystal misorientation of 5° or more, and averaging the calculated misorientation between all measurement points in the crystal grain.
• the area ratio of bainite is calculated by taking a sample from a cross section of the steel plate perpendicular to the plate surface, polishing the observed surface, etching the surface with nital solution, observing the area of 1/8 to 3/8 of the plate thickness centered at 1/4 with a field emission scanning electron microscope (FE-SEM), and calculating the area ratio with known image analysis software.
• the area ratio can be calculated using, for example, the "Analyze” function of "ImageJ" as the image analysis software.
• “ImageJ” is an open source, public domain image processing software that is widely used among those skilled in the art.
• Bainite is a collection of lath-shaped crystal grains that does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, and the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction.
• a group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by 5° or less.
• Bainite surrounded by grain boundaries with an orientation difference of 15° or more is counted as one bainite grain.
• the area ratio of tempered martensite is calculated using the observation surface and measurement method used for calculating the area ratio of bainite.
• tempered martensite cementite exists inside the martensite lath, but there are two or more types of crystal orientations of martensite lath and cementite, and cementite has multiple variants, so tempered martensite can be identified.
• the area ratio of tempered martensite identified in this way is calculated by the point counting method.
• the area ratio of as-quenched martensite is determined by first etching the same observation surface as that used for the identification of ferrite with a RePella liquid, and the same region as that used for the identification of ferrite is determined as the observation region.
• the as-quenched martensite and the retained austenite are not corroded by corrosion with the RePella liquid. Therefore, the observation region corroded by the RePella liquid is observed with an FE-SEM, and the uncorroded region is determined as the as-quenched martensite and the retained austenite.
• the total area ratio of the as-quenched martensite and the retained austenite identified in this manner is calculated by the point counting method.
• the volume ratio of the retained austenite calculated as follows is regarded as the area ratio of the retained austenite, and the area ratio of the as-quenched martensite is calculated by subtracting the volume ratio of the retained austenite calculated as follows from the total area ratio.
• the area ratio of the retained austenite is calculated by measuring the diffraction intensity using X-rays in a sample in which a region of 100 ⁇ m from the surface layer in the sheet thickness direction is removed by electrolytic polishing or chemical polishing. Specifically, the measurement is performed using MoK ⁇ rays as characteristic X-rays, and the volume ratio of the retained austenite is calculated from the integrated intensity ratio of the obtained diffraction peaks of (200) and (211) of the bcc phase and (200), (220) and (311) of the fcc phase.
• the region where plate-like carbides are arranged in a row at intervals of 0.5 ⁇ m or less is defined as pearlite, and the area ratio of pearlite is calculated using the "Analyze" function of the image analysis software "ImageJ" mentioned above.
• the lower the value the more preferable it is, and it may be, for example, 0.970 or less, 0.960 or less, or 0.950 or less.
• the lower limit is not particularly limited, since it is difficult to make the Mn concentration in ferrite 0%, the value obtained by dividing the average value of the Mn concentration in ferrite by the Mn concentration of the base material may be, for example, 0.800 or more, 0.830 or more, 0.850 or more, 0.870 or more, 0.890 or more, 0.900 or more, or 0.910 or more.
• the average Mn concentration in ferrite is determined using an electron probe microanalyzer (EPMA) as follows. Specifically, a sample is first taken with the plate thickness cross section perpendicular to the plate surface of the steel plate as the observation surface, and a 100 ⁇ m x 100 ⁇ m area is observed within the range of 1/8 to 3/8 of the plate thickness centered at 1/4 of the plate thickness in an electron channeling contrast image taken with a FE-SEM (field emission scanning electron microscope) to determine the position of the ferrite in this observation area. Next, the observation surface is wet polished with emery paper and polished with diamond abrasive grains with an average particle size of 1 ⁇ m, and then chemically polished.
• EPMA electron probe microanalyzer
• an indentation is made in a 100 x 100 ⁇ m square in the observation area using a Vickers hardness test, and the indentation is used as a marker.
• the Mn concentration in the ferrite is measured using an EPMA.
• the equipment used for the measurements is a JXA-8500F manufactured by JEOL. Crystal orientation information is obtained under conditions of an acceleration voltage of 7 kV and a measurement point interval of 80 nm, and the parts determined to be BCC within the observation area are measured. From the data obtained in this way, the Mn concentration in the ferrite is calculated using the calibration curve method, and the calculated value is determined as the average Mn concentration in the ferrite.
• the value obtained by dividing the average value of the Mn concentration in the retained austenite by the Mn concentration of the base material is controlled to 1.150 to 2.000.
• the retained austenite can be appropriately stabilized.
• the higher the value the more preferable it is, and it may be, for example, 1.200 or more, 1.250 or more, or 1.300 or more.
• the value obtained by dividing the average value of the Mn concentration in the retained austenite by the Mn concentration in the base material is set to 2.000 or less, and may be, for example, 1.800 or less or 1.600 or less.
• the average value of the Mn concentration in the retained austenite is also determined using EPMA in a similar manner.
• the FCC phase is separated using crystal orientation information obtained in the same manner as described above to determine the average value of the Mn concentration in ferrite.
• the separated FCC phase is the retained austenite.
• the Mn concentration in the retained austenite is measured using EPMA.
• the measurement device used is the JXA-8500F manufactured by JEOL. Crystal orientation information is obtained under conditions of an acceleration voltage of 7 kV and a measurement point interval of 80 nm, and the parts of the region that are determined to be FCC are measured. From the data obtained in this manner, the calibration curve method is used to determine the Mn concentration in the retained austenite, and the obtained value is determined as the average value of the Mn concentration in the retained austenite.
• the average value of the C concentration in the retained austenite is controlled to 0.80 mass% or more.
• the effect of the C concentration in the retained austenite described above that is, the improvement in elongation due to the stabilization of the retained austenite and the high strength due to the hardness improvement of the processing-induced martensite, can be particularly remarkable.
• the average value of the C concentration in the retained austenite may be, for example, 1.00 mass% or less, 0.98 mass% or less, 0.96 mass% or less, or 0.95 mass% or less.
• the average value of the C concentration in the retained austenite is also determined using EPMA in the same manner as above. Specifically, in the same manner as described above for determining the average value of the Mn concentration in the retained austenite, crystal orientation information is obtained and the parts of the region that are determined to be FCC are measured. From the data obtained in this manner, the C concentration in the retained austenite is found using the calibration curve method, and the found value is determined as the average value of the C concentration in the retained austenite.
• the steel plate includes a plate thickness center portion and a surface layer softened portion disposed on one or both sides of the plate thickness center portion, and the surface layer softened portion has an average thickness of 10 ⁇ m or more and an average Vickers hardness of 0.90 times or less of the average Vickers hardness at the plate thickness 1/2 position.
• the effect of providing a surface layer softened portion on one or both sides of the steel plate can be fully exhibited by having an average thickness of 10 ⁇ m or more.
• the average thickness of the surface layer softened portion may be any value of 10 ⁇ m or more, for example, 15 ⁇ m or more, 30 ⁇ m or more, 40 ⁇ m or more, 50 ⁇ m or more, 70 ⁇ m or more, or 100 ⁇ m or more.
• the upper limit is not particularly limited, the average thickness of the surface-softened portion is generally 30% or less of the plate thickness.
• the average thickness of the surface-softened portion may be 25% or less, 20% or less, 15% or less, or 10% or less of the plate thickness, and more specifically, may be 450 ⁇ m or less, 400 ⁇ m or less, 300 ⁇ m or less, 250 ⁇ m or less, 200 ⁇ m or less, or 150 ⁇ m or less.
• the average Vickers hardness of the surface softened portion may be any average Vickers hardness of 0.90 times or less of the average Vickers hardness at the 1/2 thickness position.
• the average Vickers hardness of the surface softened portion may be 0.85 times or less, 0.80 times or less, 0.70 times or less, or 0.60 times or less of the average Vickers hardness at the 1/2 thickness position.
• the average Vickers hardness of the surface softened portion is generally 0.10 times or more of the average Vickers hardness at the 1/2 thickness position, and may be, for example, 0.15 times or more or 0.20 times or more.
• the "average thickness of the surface-softened portion” and the “average Vickers hardness of the surface-softened portion” are determined as follows. First, the Vickers hardness is measured at regular intervals (for example, every 5% of the thickness, or every 3%, 2.5%, 1% or 0.5% as necessary) in the thickness direction from the 1/2 position of the steel plate toward the surface of the surface-softened portion with an indentation load of 100 g. Then, the Vickers hardness is measured at a total of three or more points, for example, five or ten points, on a line perpendicular to the thickness from that position with an indentation load of 100 g, and the average value of these is taken as the average Vickers hardness at that thickness direction position.
• the distance between each measurement point arranged in the thickness direction and the direction perpendicular thereto is four or more times the distance of the indentation if possible.
• a distance four or more times the distance of the indentation means a distance four or more times the length of the diagonal line of the rectangular opening of the indentation made by the diamond indenter when measuring the Vickers hardness. If it is difficult to stamp linearly from the surface to the plate thickness direction while keeping the distance between each measurement point at least four times the indentation, stamping may be performed in a zigzag pattern from the surface to the plate thickness direction while keeping the distance between each measurement point at least four times the indentation.
• the surface side of that position is defined as a surface softened portion, and the average thickness ( ⁇ m) of the surface softened portion and its proportion (%) in the plate thickness are determined.
• the Vickers hardness of 10 randomly determined points in the surface softened portion is measured with a load of 100 g, and the average value of the measured values is calculated to determine the average Vickers hardness of the surface softened portion.
• the surface softened portions are located on both sides of the plate thickness center, the average thickness and average Vickers hardness of the surface softened portion on the other side are determined by measuring in the same manner as described above.
• the steel plate according to the embodiment of the present invention has a thickness of, for example, 0.6 to 6.0 mm, but is not particularly limited thereto.
• the thickness may be 0.8 mm or more, 1.0 mm or more, or 1.2 mm or more, but is not particularly limited thereto.
• the thickness may be 4.0 mm or less, 3.0 mm or less, 2.5 mm or less, or 2.0 mm or less.
• the thickness of the steel plate is measured by a micrometer.
• the steel sheet according to the embodiment of the present invention may further have a plating layer on the surface for the purpose of improving corrosion resistance.
• the plating layer may be any appropriate plating layer, for example, a hot-dip plating layer or an electroplating layer.
• the hot-dip plating layer may be, for example, a hot-dip galvanizing layer, a hot-dip zinc alloy plating layer (a hot-dip plating layer composed of an alloy of zinc and additional elements such as Si and Al), or an alloyed hot-dip galvanizing layer (alloyed plating layer) obtained by alloying these platings.
• the hot-dip galvanizing layer and the hot-dip zinc alloy plating layer are preferably plating layers containing less than 7% by mass of Fe, and the alloyed plating layer is preferably a plating layer containing 7% by mass or more and 15% by mass or less of Fe.
• the components other than zinc and Fe are not particularly limited, and various configurations can be adopted within the usual range.
• the plating layer may be, for example, an aluminum plating layer.
• the coating amount of the plating layer is not particularly limited and may be a general coating amount.
• the above-mentioned steel sheet characteristics do not apply to the plating on the surface.
• the evaluation of the chemical composition, microstructure fraction, and softened surface area of plated steel sheets is performed by immersing the steel sheets in a strong alkaline solution and removing the plating layer on the surface.
• a high tensile strength for example, a tensile strength of 980 MPa or more
• the tensile strength is preferably 1080 MPa or more or 1180 MPa or more, more preferably 1250 MPa or more or 1350 MPa or more.
• the upper limit is not particularly limited, but for example, the tensile strength may be 1780 MPa or less, 1700 MPa or less, 1600 MPa or less, or 1500 MPa or less.
• the steel plate of the embodiment of the present invention despite having such a very high tensile strength, it is possible to reliably and sufficiently improve the elongation compared to a steel plate not including such a specific combination by the specific combination of chemical composition and microstructure described above.
• the steel plate of the embodiment of the present invention can achieve a total elongation of 14% or more, preferably 15% or more, more preferably 16% or more.
• the upper limit is not particularly limited, but for example, the total elongation may be 30% or less or 25% or less.
• the tensile strength and total elongation are measured by conducting a tensile test in accordance with JIS Z 2241:2011 on a JIS No. 5 test piece taken in a direction in which the longitudinal direction of the test piece is parallel to the rolling direction perpendicular to the steel plate.
• the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet.
• the steel sheet according to the embodiment of the present invention can reliably achieve both the contradictory properties of high strength and excellent elongation.
• it is also possible to significantly improve bendability. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields that require both high strength and workability, and is particularly useful for use in parts in the automotive field.
• an automobile part including the steel sheet according to the embodiment of the present invention is provided. Examples of automobile parts include frame parts, bumpers, and other structural parts and reinforcing parts that require strength.
• the method for producing a steel sheet according to an embodiment of the present invention includes: a hot rolling step comprising: heating a slab having the chemical composition described above in relation to the steel sheet to a temperature of 1200-1400°C, finish rolling it, then coiling it at a temperature of 500-700°C, and dwelling the coiled coil in a temperature range of 600-750°C for 1.0-5.0 hours, the end temperature of said finish rolling being 900°C or higher; a pickling step of pickling the obtained hot-rolled steel sheet;
• the present invention is characterized in that it includes a cold rolling step of cold rolling the pickled hot-rolled steel sheet at a rolling reduction of 20 to 90%, and a step of annealing the obtained cold-rolled steel sheet, the annealing step including heating the cold-rolled steel sheet and holding it at a maximum heating temperature of 780 to 900°C for 30 to 500 seconds, followed by cooling and retention, the cooling step including primary cooling from the maximum heating temperature to a primary cooling stop temperature of 650°C or higher at an
• the slab to be subjected to hot rolling may be any cast slab, and is not limited to a specific cast slab, such as a continuous cast slab or a slab produced by a thin slab caster.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• the heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling.
• the slab used in the manufacture of the steel plate according to the embodiment of the present invention contains a relatively large amount of alloying elements, so it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature.
• the end temperature of the finish rolling is important in terms of controlling the microstructure of the steel plate. If the end temperature of the finish rolling is in the two-phase temperature range of (austenite + ferrite), the microstructure becomes more non-uniform, and the formability after heat treatment may decrease.
• the end temperature of the finish rolling is set to 900 ° C. or higher. Although there is no particular upper limit, it is preferable that the end temperature of the finish rolling is set to, for example, 1100 ° C. or lower in order to suppress the coarsening of austenite.
• the finish-rolled hot-rolled steel sheet is coiled at a temperature of 500 to 700° C. If the coiling temperature of the hot-rolled steel sheet exceeds 700° C., the microstructure becomes more nonuniform, and the steel sheet becomes hard to be obtained after heat treatment. Therefore, the coiling temperature is set to 700°C or less. If the coiling temperature is less than 500°C, the strength of the hot-rolled sheet becomes excessively high and the cold rollability is impaired. The temperature is set to 500°C.
• the coil retention control after coiling is very important in controlling the value obtained by dividing the average Mn concentration in ferrite by the Mn concentration in the base material (Mn F /Mn A ) and the value obtained by dividing the average Mn concentration in retained austenite by the Mn concentration in the base material (Mn ⁇ /Mn A ) within the desired range in the microstructure of the finally obtained steel sheet.
• ferrite, pearlite and bainite are obtained by winding a finish-rolled hot-rolled steel sheet into a coil at a temperature of 500 to 700 ° C.
• Mn can be concentrated from bainitic ferrite in ferrite and bainite into carbides contained in pearlite and bainite, and a distribution of Mn concentration can be generated. Carbide with concentrated Mn is likely to become retained austenite after subsequent annealing, and Mn increases chemical stability and contributes to improving elongation. If the retention temperature is less than 600°C, Mn cannot be sufficiently diffused, and therefore the concentration of Mn in the carbides contained in pearlite and bainite cannot be promoted.
• the average value of the Mn concentration in the retained austenite cannot be sufficiently increased, and in connection with this, the average value of the Mn concentration in the ferrite cannot be sufficiently reduced.
• the desired Mn F /Mn A value and/or Mn ⁇ /Mn A value cannot be obtained in the microstructure of the finally obtained steel sheet. Therefore, the lower limit of the retention temperature is set to 600°C.
• the retention temperature exceeds 750°C, Mn is excessively concentrated, making it difficult to exhibit the TRIP effect, so the upper limit of the retention temperature is set to 750°C.
• the method of retaining the coil after winding at 600 to 750°C is not limited to a specific method, and may include reheating the coil or covering the coil with a highly insulating box.
• the lower limit of the holding time is set to 1.0 hour.
• the upper limit of the holding time is set to 5.0 hours.
• the solution used for pickling may be any solution used in ordinary pickling, such as 5 vol. % or more hydrochloric acid or sulfuric acid. Pickling may be performed once or may be performed multiple times as necessary.
• the pickled hot-rolled steel sheet is subjected to cold rolling at a reduction rate of 20 to 90% to obtain a cold-rolled steel sheet.
• the reduction rate of the cold rolling is preferably 30% or more.
• the reduction rate of the cold rolling is preferably 80% or less.
• the number of rolling passes and the reduction rate for each pass are not particularly limited, and may be appropriately set so that the reduction rate of the cold rolling is within the above range.
• the dew point of the furnace atmosphere during the holding at the maximum heating temperature may be increased to -30 ° C. or higher.
• the annealing process in such an atmosphere, it is possible to promote the decarburization reaction from the steel sheet surface.
• a surface layer softened portion having an average thickness of 10 ⁇ m or more and an average Vickers hardness of 0.90 times or less of the average Vickers hardness at the sheet thickness 1/2 position.
• Primary cooling [Average cooling rate from maximum heating temperature to first cooling stop temperature of 650°C or higher: 1.0 to 20.0°C/sec]
• the primary cooling is performed at an average cooling rate of 1.0 to 20.0 ° C./sec from the maximum heating temperature to a primary cooling stop temperature of 650 ° C. or more. If the primary cooling stop temperature is less than 650 ° C., excessive ferrite is generated, the area ratio of tempered martensite cannot be obtained sufficiently, and the strength is reduced. For this reason, the lower limit of the cooling stop temperature is set to 650 ° C.
• the lower limit of the average cooling rate is set to 1.0 ° C./sec. If the average cooling rate exceeds 20.0 ° C./sec, ferrite cannot be generated, and the effect of primary cooling cannot be exhibited.
• the lower limit of the secondary cooling stop temperature is set to 100°C.
• the secondary cooling stop temperature is 300°C or higher, sufficient martensite cannot be obtained in the secondary cooling stage. For this reason, the desired amount of tempered martensite cannot be obtained even by the subsequent stationary operation. In this case, a relatively large amount of untransformed austenite may remain after the dwell operation, and such untransformed austenite may be subsequently cooled and ultimately produce a large amount of as-quenched martensite. Therefore, the upper limit of the secondary cooling stop temperature is set to less than 300°C.
• the lower limit of the stay temperature is set to 300°C.
• the tempering proceeds excessively and sufficient strength cannot be obtained.
• the upper limit of the stay temperature is set to 450°C.
• the stay time is necessary to promote the bainite transformation. If the stay time is less than 100 seconds, the bainite transformation is not completed, sufficient retained austenite cannot be obtained, and/or the desired C concentration in the retained austenite cannot be obtained.
• the steel sheet may be subjected to a plating treatment such as electroplating or vapor deposition plating, and further may be subjected to an alloying treatment after the plating treatment.
• the steel sheet may also be subjected to a surface treatment such as formation of an organic film, film lamination, organic salt or inorganic salt treatment, or non-chromium treatment.
• hot-dip galvanizing When hot-dip galvanizing is performed on a steel sheet as a plating process, the steel sheet is heated or cooled to a temperature that is at least 40°C lower than the temperature of the galvanizing bath and at most 50°C higher than the temperature of the galvanizing bath, and then the steel sheet is passed through the galvanizing bath.
• This hot-dip galvanizing process produces a steel sheet with a hot-dip galvanized layer on its surface, i.e., a hot-dip galvanized steel sheet.
• the hot-dip galvanized layer has a chemical composition that is, for example, Fe: 7% by mass to 15% by mass, with the balance being Zn, Al, and impurities.
• the hot-dip galvanized layer may also be a zinc alloy.
• the hot-dip galvanized steel sheet is heated to a temperature of 460°C or higher and 600°C or lower. If the temperature is less than 460°C, alloying may be insufficient. On the other hand, if the temperature is more than 600°C, alloying may be excessive, resulting in deterioration of corrosion resistance.
• This type of alloying treatment results in a steel sheet having an alloyed hot-dip galvanized layer on the surface, i.e., an alloyed hot-dip galvanized steel sheet.
• the steel plate according to the embodiment of the present invention can be manufactured by the method exemplified above. Note that the above embodiments are merely examples of concrete ways of implementing the present invention, and the technical scope of the present invention should not be interpreted in a limiting manner based on them. In other words, the present invention can be implemented in various forms without departing from its technical concept or main features.
• steel plates according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength and elongation properties of the resulting steel plates were investigated.
• molten steel was cast by continuous casting to form slabs having various chemical compositions shown in Table 1, and these slabs were heated to the heating temperatures shown in Table 2 and hot-rolled.
• Hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finishing temperature and coiling temperature of the finish rolling were as shown in Table 2.
• the coiled coil was held at the maximum temperature shown in Table 2 for the time shown in Table 2.
• the obtained hot-rolled steel sheet was pickled, and then cold-rolled at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet having a thickness of 1.4 mm.
• the obtained cold-rolled steel sheet was annealed under the conditions shown in Table 2, consisting of heating, primary cooling, secondary cooling, and holding operations.
• the dew point in the furnace during holding at the maximum heating temperature was as shown in Table 2.
• hot-dip galvanizing was appropriately performed as a plating treatment, and some of them were further subjected to an alloying treatment.
• the properties of the resulting steel plates were measured and evaluated using the following methods.
• TS tensile strength
• El total elongation
• Bendability is evaluated by the ratio R/t of the limit bending radius R to the sheet thickness t.
• the limit bending radius R was determined by preparing a No. 1 test piece described in JIS Z 2204:1996 so that the direction perpendicular to the rolling direction was the longitudinal direction (the bending ridge line coincided with the rolling direction), and performing a V-bend test in accordance with JIS Z 2248:2022.
• the angle between the die and the punch was set to 60°, and the bending test was performed by changing the tip radius of the punch in 0.5 mm increments, and the punch tip radius at which bending was possible without generating cracks was determined as the limit bending radius R.
• crashworthiness was judged based on TS, El, and R/t. That is, the crashworthiness was evaluated as follows: TS of 980 MPa or more, El of 14% or more, and R/t of 1.5 or less were all satisfied, with a ⁇ ; two were satisfied with a ⁇ ; and one or less was satisfied with an ⁇ .
• Comparative Examples 22 and 26 the C and Mn contents were low, respectively, and therefore TS was reduced.
• Comparative Examples 23 and 27 the C and Mn contents were high, respectively, and therefore the strength of the hot-rolled sheet was too high, and therefore cold rolling could not be performed appropriately.
• Comparative Example 24 the total content of Si and Al was low, which is thought to have caused insufficient C concentration in the untransformed austenite during bainite transformation. As a result, the desired amount of retained austenite could not be obtained, and El was reduced.
• Comparative Example 25 the total content of Si and Al was high, which caused excessive strength and/or excessive stabilization of the retained austenite, which resulted in excessive rolling load during cold rolling, and thus caused cracks in the steel sheet.
• Comparative Example 31 the maximum heating temperature in the annealing process was low, so austenitization was insufficient and the desired amount of tempered martensite could not be obtained. As a result, TS was reduced.
• Comparative Example 32 the holding time at the maximum heating temperature in the annealing process was short, so austenitization was similarly insufficient and the desired amount of tempered martensite could not be obtained. As a result, TS was reduced.
• Comparative Example 33 the first cooling stop temperature in the annealing process was low, so excessive ferrite was generated, and the desired amount of tempered martensite was not obtained in relation to this, and TS was reduced.
• Comparative Example 34 the average cooling rate of the first cooling in the annealing process was slow, so excessive ferrite was generated in the same way, and the desired amount of tempered martensite was not obtained in relation to this, and TS was reduced.
• Comparative Example 35 the second cooling stop temperature in the annealing process was low, so a lot of tempered martensite was generated, and the residual austenite was not obtained in relation to this, and El was reduced.
• Comparative Example 36 it is considered that a large amount of as-quenched martensite was generated from the untransformed austenite remaining after the retention operation because the secondary cooling stop temperature in the annealing process was high, and as a result, El was reduced.
• Comparative Example 37 it is considered that the retention time in the temperature range of 300 to 450°C in the annealing process was short, and therefore C enrichment in the untransformed austenite during bainite transformation was insufficient. As a result, the desired amount of retained austenite could not be obtained, and El was reduced. In Comparative Example 38, the average cooling rate of the secondary cooling in the annealing process was slow, and therefore a large amount of bainite was generated, and therefore the desired amount of tempered martensite could not be obtained. As a result, TS was reduced.
• Comparative Example 39 it is considered that the maximum temperature (retention temperature) of the coil after coiling was low, while the retention time of the coil after coiling was long, and therefore appropriate enrichment of Mn could not be achieved.
• the average value of the Mn concentration in the retained austenite could be increased, the average value of the Mn concentration in the ferrite could not be sufficiently reduced, and El was reduced in relation to this.
• the coil was held for a short time after winding, which is thought to be why Mn could not be sufficiently diffused and the concentration of Mn in the carbides contained in pearlite and bainite could not be promoted. As a result, the average Mn concentration in the retained austenite could not be sufficiently increased, and El decreased accordingly.
• the steel plates have a specified chemical composition and are configured to contain, by area ratio, 40-65% tempered martensite, 15-40% ferrite, and 10-20% retained austenite, and the value obtained by dividing the average Mn concentration in ferrite by the Mn concentration in the base material is controlled to 0.980 or less, and the value obtained by dividing the average Mn concentration in retained austenite by the Mn concentration in the base material is controlled to 1.150-2.000.
• Examples 1 to 15, 19 to 21, 41, and 42 which have a surface softened portion with an average thickness of 10 ⁇ m or more and an average Vickers hardness of 0.90 times or less the average Vickers hardness at the 1/2 plate thickness position, R/t is 1.5 or less, and therefore the bendability is high. Furthermore, TS is 980 MPa or more and El is 14% or more, so very high crash resistance properties were achieved.

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