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
• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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

Definitions

• Patent Document 1 discloses a hot-dip galvanized steel sheet having a steel structure containing, in terms of area%, more than 40% and 95% or less of ferrite, and one or more selected from a group of ultra-hard phases consisting of martensite, retained austenite, and cementite in a total amount of 3% to 20%.
• the steel structure has an average grain size of 2.5 ⁇ m or less, an average grain size of the ultra-hard phase group of 2.0 ⁇ m or less, an average interval of ultra-hard phases, which is the average value of the closest distance of the ultra-hard phase group, of 2.0 ⁇ m or less, and a number density of Ti-B-based precipitates having a circle equivalent diameter of 1 ⁇ m or more of 500 pieces/ mm2 or less.
• the steel structure has mechanical properties such as tensile strength: 780 MPa or more, total elongation: 10% or more, hole expansion ratio: 35% or more, and a minimum inner radius at which cracks do not occur in a bending test at a bending angle of 180°: 3.5 times or less of the sheet thickness.
• Patent Document 2 discloses a hot-rolled steel sheet in which the microstructure contains, by volume, 70% or more of martensite, tempered martensite, and bainite in total, and 5-20% of retained austenite, and in the surface layer region extending from the surface to a position 1/10 of the sheet thickness, the sum of the average pole density of the orientation group consisting of ⁇ 211 ⁇ 111> to ⁇ 111 ⁇ 112> and the pole density of the crystal orientation ⁇ 110 ⁇ 001> is 6.0 or less, the solute carbon concentration in the retained austenite is 0.5 mass% or more, and the tensile strength is 980 MPa or more.
• the inventor found that the cracked area had been subjected to bending and unbending deformation by pressing, and that after this bending and unbending deformation, deformation in a direction perpendicular to the bending strain caused localized shrinkage, resulting in fracture.
• the inventor found that in order to prevent this fracture, it is necessary for the material to have excellent formability, and in particular, a high critical fracture thickness reduction rate, after being pre-strained by bending and unbending deformation.
• the critical thickness reduction rate at break is the value calculated from the thickness of the tensile test piece before break and the minimum thickness of the tensile test piece after break. If the critical thickness reduction rate at break after pre-straining is low, it is not preferable because it may break early in the subsequent process or the durability and crash resistance properties may deteriorate when used as a part.
• Patent Documents 1 and 2 do not take into consideration the critical sheet thickness reduction rate at break after pre-straining.
• the present disclosure has been made in consideration of the above problems, and aims to provide a steel sheet that has high strength and yield ratio, excellent ductility and hole expandability, and a high critical fracture thickness reduction rate after pre-straining, as well as a part that uses the same.
• the gist of the present invention is as follows. [1] Chemical composition, in mass%, C: 0.090-0.210%, Si: 0.20-1.00%, Mn: 1.95-2.55%, P: 0.060% or less, S: 0.005% or less, Al: 0.01-0.26%, N: 0.0070% or less, O: 0 to 0.010%, Ti: 0.10-0.18%, Nb: 0.01-0.04%, B: 0.0001 to 0.0030%, Cr: 0 to 0.47%, Mo: 0 to 0.12%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, V: 0 to 0.30%, Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0-0.010%, Mg: 0 to 0.010%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, REM: 0-0.01
• a steel plate characterized in that, when a difference between a maximum number and a minimum number of MAs present on a line segment per 100 ⁇ m for each of the structural photographs is calculated, and an average value of the differences between the maximum number and the minimum number of MAs in the structural photographs of the five fields of view is calculated, the average value is 8 to 16.
• the chemical composition is, in mass%, Cr: 0.01-0.47%, Mo: 0.01-0.12%, Cu: 0.01-0.40%, Ni: 0.01 to 0.30%, V: 0.01 to 0.30%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, Zr: 0.001 to 0.050%, Ca: 0.001-0.010%, Mg: 0.001-0.010%, Bi: 0.001 to 0.010%, Co: 0.001 to 0.010%, W: 0.001-0.100%, Zn: 0.001-0.010%, REM: 0.001-0.010%,
• the steel sheet according to [1] characterized in that it contains one or more of Sb: 0.001 to 0.010% and Ta: 0.001 to 0.010%.
• a part comprising the steel plate according to [1] or [2].
• the above aspects of the present disclosure make it possible to provide a steel sheet having high strength and yield ratio, excellent ductility and hole expandability, and a high critical fracture thickness reduction rate after pre-straining, as well as a part using the same.
• FIG. 1 is a diagram showing a cracked portion of a lower arm made of a high-strength steel plate.
• FIG. 13 is a diagram for explaining a method for measuring the difference between the maximum number and the minimum number of MAs.
• FIG. 13 is a diagram showing the relationship between the average value of the difference between the maximum and minimum numbers of MA in structural photographs of five fields of view in the examples and the critical sheet thickness reduction rate at fracture after pre-straining.
• 11A to 11C are diagrams for explaining a manufacturing method of the hat part.
• the inventors discovered that in order to achieve a high critical fracture thickness reduction rate after pre-straining while ensuring high strength and yield ratio, as well as excellent ductility and hole expandability by having a desired metal structure, it is important to favorably control the average grain size of the prior austenite grains and to favorably control the distribution of the MA, which is a hard phase.
• a steel plate according to one embodiment of the present disclosure (hereinafter, sometimes referred to as the steel plate according to this embodiment) will be described.
• this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of this disclosure.
• the chemical composition of the steel plate according to this embodiment is, in mass%, C: 0.090-0.210%, Si: 0.20-1.00%, Mn: 1.95-2.55%, P: 0.060% or less, S: 0.005% or less, Al: 0.01-0.26%, N: 0.0070% or less, O: 0-0.010%, Ti: 0.10-0.18%, Nb: 0.01-0.04%, B: 0.0001-0.0030%, and the balance: Fe and impurities.
• C 0.090-0.210%
• Si 0.20-1.00%
• Mn 1.95-2.55%
• P 0.060% or less
• S 0.005% or less
• Al 0.01-0.26%
• N 0.0070% or less
• O 0-0.010%
• Ti 0.10-0.18%
• Nb 0.01-0.04%
• B 0.0001-0.0030%
• Fe and impurities are described in detail below.
• C 0.090-0.210%
• C is an element necessary for obtaining a desired strength of the steel sheet. If the C content is less than 0.090%, the desired strength cannot be obtained. Therefore, the C content is set to 0.090% or more.
• the C content is preferably 0.100% or more, 0.120% or more, or 0.150% or more.
• the C content is set to 0.210% or less.
• the C content is preferably 0.200% or less or 0.180% or less.
• Si 0.20-1.00%
• Si is an element that improves the strength of steel sheet by solid solution strengthening. If the Si content is less than 0.20%, the desired strength or hole expandability cannot be obtained. Therefore, the Si content is set to 0.20% or more.
• the Si content is preferably 0.40% or more or 0.50% or more.
• the Si content is set to 1.00% or less.
• the Si content is preferably 0.90% or less, 0.70% or less, or 0.60% or less.
• Mn 1.95-2.55%
• Mn is an element necessary for improving the strength of the steel sheet. If the Mn content is less than 1.95%, the area ratio of ferrite becomes too high, and the desired strength and hole expandability cannot be obtained. Therefore, the Mn content is set to 1.95% or more.
• the Mn content is preferably 2.00% or more or 2.10% or more. On the other hand, if the Mn content exceeds 2.55%, the hole expandability of the steel sheet deteriorates. Therefore, the Mn content is set to 2.55% or less.
• the Mn content is preferably 2.50% or less or 2.40% or less.
• P 0.060% or less
• P is an element that segregates in the center of the thickness of the steel plate.
• P is also an element that embrittles the welded portion. If the P content exceeds 0.060%, the hole expandability of the steel plate deteriorates. Therefore, the P content is set to 0.060% or less.
• the P content is preferably 0.030% or less or 0.020% or less. The lower the P content, the better, and 0% is preferable. However, if the P content is excessively reduced, the dephosphorization cost increases significantly. Therefore, the P content may be 0.001% or more or 0.005% or more.
• S 0.005% or less
• S is an element that embrittles the slab when present as a sulfide.
• S is also an element that deteriorates the formability of the steel sheet. If the S content exceeds 0.005%, the hole expandability of the steel sheet deteriorates. Therefore, the S content is set to 0.005% or less.
• the S content is preferably 0.004% or less or 0.003% or less. The lower the S content, the better, and 0% is preferable. However, if the S content is excessively reduced, the desulfurization cost increases significantly. Therefore, the S content may be 0.0005% or more, or 0.001% or more.
• Al acts as a deoxidizer and is an element that improves the cleanliness of steel. If the Al content is less than 0.01%, a sufficient deoxidizing effect is not obtained, and a large amount of inclusions (oxides) is formed in the steel sheet. Such inclusions deteriorate the formability of the steel sheet. Therefore, the Al content is set to 0.01% or more.
• the Al content is preferably 0.02% or more, 0.03% or more, or 0.05% or more.
• the Al content is set to 0.26% or less.
• the Al content is preferably 0.25% or less, 0.20% or less, or 0.15% or less.
• N 0.0070% or less
• N is an element that forms coarse nitrides in steel and deteriorates the hole expandability of the steel sheet. If the N content exceeds 0.0070%, the hole expandability of the steel sheet deteriorates. Therefore, the N content is set to 0.0070% or less.
• the N content is preferably 0.0060% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower the N content, the better, and 0% is preferable. However, if the N content is excessively reduced, the cost of denitrification increases significantly. Therefore, the N content may be 0.0005% or more or 0.0010% or more.
• O is an element that forms coarse oxides that become the starting point of fracture when contained in a large amount in steel. If the O content exceeds 0.010%, cracks are likely to occur in the slab. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.005% or less or 0.001% or less. Since O may not be contained, the O content may be 0%.
• Ti 0.10-0.18%
• Ti is an element that forms fine nitrides in steel to increase the strength of the steel sheet. If the Ti content is less than 0.10%, the desired strength cannot be obtained. Therefore, the Ti content is set to 0.10% or more. The Ti content is preferably 0.12% or more or 0.13% or more. On the other hand, if the Ti content exceeds 0.18%, the hole expandability of the steel sheet deteriorates. Therefore, the Ti content is set to 0.18% or less. The Ti content is preferably 0.16% or less or 0.15% or less.
• Nb 0.01-0.04%
• Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling.
• Nb is also an element that increases the yield ratio of a steel sheet by forming fine carbides. If the Nb content is less than 0.01%, the desired yield ratio cannot be obtained. Therefore, the Nb content is set to 0.01% or more.
• the Nb content is preferably 0.02% or more. On the other hand, if the Nb content exceeds 0.04%, the hole expandability of the steel sheet deteriorates. Therefore, the Nb content is set to 0.04% or less.
• the Nb content is preferably 0.03% or less.
• B 0.0001-0.0030%
• B is an element that suppresses the formation of ferrite during the cooling process and increases the strength of the steel sheet. If the B content is less than 0.0001%, the desired strength cannot be obtained. Therefore, the B content is set to 0.0001% or more.
• the B content is preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more.
• the B content is set to 0.0030% or less.
• the B content is preferably 0.0025% or less or 0.0020% or less.
• the remainder of the chemical composition of the steel plate according to this embodiment is made up of Fe and impurities.
• impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment.
• the steel plate according to this embodiment may contain the following optional elements in place of a portion of Fe.
• the lower limit of the content is 0%.
• Each optional element is described below.
• Cr:0.01 ⁇ 0.47% Cr is an element that exerts an effect similar to that of Mn.
• the Cr content is preferably 0.01% or more.
• the Cr content exceeds 0.47%, the above effect saturates, so the Cr content is set to 0.47% or less.
• Mo 0.01 ⁇ 0.12%
• Mo is an element that forms fine carbides in steel to increase the strength of the steel sheet.
• the Mo content is preferably 0.01% or more.
• the Mo content exceeds 0.12%, the above effect saturates, so the Mo content is set to 0.12% or less.
• Cu 0.01 ⁇ 0.40%
• Cu has the effect of increasing the hardenability of the steel sheet and the effect of increasing the strength of the steel sheet by precipitating in the steel as carbides at low temperatures.
• the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 0.40%, grain boundary cracking of the slab may occur, and therefore the Cu content is set to 0.40% or less.
• Ni 0.01 ⁇ 0.30%
• Ni has the effect of increasing the hardenability of the steel sheet and increasing the strength of the steel sheet.
• Ni has the effect of effectively suppressing grain boundary cracking of the slab caused by Cu.
• the Ni content is 0.01% or more. Since Ni is an expensive element, it is economically undesirable to include a large amount of Ni. Furthermore, even if the Ni content exceeds 0.30%, the above effects are saturated. Therefore, the Ni content is set to 0.30% or less.
• V is an element that forms fine carbides in steel to increase the strength of the steel sheet.
• the V content is preferably 0.01% or more.
• the V content is set to 0.30% or less.
• Sn 0.001-0.040%
• Sn has the effect of suppressing the generation of oxides that are the starting points of fracture, thereby improving the hole expandability of the steel sheet.
• the Sn content is preferably 0.001% or more.
• the Sn content exceeds 0.040%, the above effect saturates, so the Sn content is set to 0.040% or less.
• the As content is preferably 0.001% or more.
• excessive As content may cause slab cracking, and this effect becomes significant when the As content exceeds 0.100%, so the As content is set to 0.100% or less.
• Zr 0.001-0.050%
• Zr has the effect of increasing hole expandability.
• the Zr content is preferably 0.001% or more.
• the Zr content exceeds 0.050%, the above effect saturates, so the Zr content is set to 0.050% or less.
• Ca 0.001-0.010%
• Ca has the effect of dispersing a large number of fine oxides during deoxidation of molten steel and refining the structure of the steel sheet.
• Ca also has the effect of fixing S in the steel as spherical CaS, suppressing the generation of elongated inclusions such as MnS, and improving the hole expandability of the steel sheet.
• the Ca content is preferably 0.001% or more.
• the Ca content exceeds 0.010%, the above effect saturates, so the Ca content is set to 0.010% or less.
• Mg 0.001-0.010%
• Mg has the effect of improving the hole expandability of the steel sheet by adjusting the shape of inclusions in the steel to a preferable shape.
• the Mg content is preferably 0.001% or more.
• the Mg content exceeds 0.010%, the above effect saturates, so the Mg content is set to 0.010% or less.
• Bi 0.001-0.010%
• Bi has the effect of improving the hole expandability of a steel sheet by refining the solidification structure.
• the Bi content is preferably 0.001% or more.
• the Bi content exceeds 0.010%, the above effect saturates, so the Bi content is set to 0.010% or less.
• W 0.001 ⁇ 0.100%
• W has the effect of increasing the strength of the steel sheet by solid solution strengthening.
• the W content is preferably 0.001% or more.
• the W content exceeds 0.100%, the above effect saturates, so the W content is set to 0.100% or less.
• Zn 0.001-0.010%
• Zn has the effect of increasing the strength of the steel sheet by solid solution strengthening, and in order to reliably obtain this effect, the Zn content is preferably 0.001% or more.
• the Zn content is set to 0.010% or less.
• REM 0.001-0.010% REM has the effect of increasing the yield ratio of a steel sheet by adjusting the shape of inclusions in the steel to a preferred shape.
• the REM content is preferably 0.001% or more.
• the REM content exceeds 0.010%, slab cracking may occur, so the REM content is set to 0.010% or less.
• REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of the REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.
• Sb 0.001-0.010%
• Sb has the effect of suppressing the generation of oxides that are the starting points of fracture, thereby improving the ductility and hole expandability of the steel sheet.
• the Sb content is preferably 0.001% or more.
• the Sb content is set to 0.010% or less.
• Ta 0.001-0.010%
• Ta has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel, similar to V.
• the Ta content is preferably 0.001% or more.
• the Ta content is set to 0.010% or less.
• the chemical composition of the above-mentioned steel sheet is analyzed using a spark discharge optical emission spectrometer etc.
• C and S are analyzed using the combustion-infrared absorption method
• N is analyzed using the inert gas fusion-thermal conductivity method
• O is analyzed using the inert gas fusion-non-dispersive infrared absorption method.
• the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.
• the metal structure of the steel plate according to this embodiment in a region from the surface to a depth of 1/8 of the sheet thickness to a depth of 3/8 of the sheet thickness from the surface, the metal structure is, in area %, tempered martensite: 80.0 to 99.0%, MA: 1.0 to 20.0%, ferrite and bainite: 10.0% or less in total, and pearlite: 2.0% or less, the average grain size of prior austenite grains is 25 ⁇ m or less, and in the region, a structural photograph is obtained by continuously photographing five fields of view of 100 ⁇ m in the rolling direction ⁇ 100 ⁇ m in the sheet thickness direction in the rolling direction, and when 10 or more lines are drawn in the rolling direction at equal intervals of 10 ⁇ m or more in the 45° direction with respect to the sheet thickness direction in the structural photograph, the difference between the maximum and minimum numbers of MA present on the line segments per 100 ⁇ m of the line segment for each of the structural photographs is obtained, and the average
• the region from 1/8 of the plate thickness depth from the surface to 3/8 of the plate thickness depth from the surface is, in other words, the region that starts at 1/8 of the plate thickness depth from the surface and ends at 3/8 of the plate thickness depth from the surface.
• the reason for specifying the metal structure in this region is that the metal structure in this region represents a typical metal structure of a steel plate.
• the cross section is observed using a BSE image (or COMPO image)
• a clear difference in contrast is confirmed between the plating layer, coating film, etc. and the base steel (steel sheet). Therefore, the position where the contrast changes from the outermost surface can be identified as the interface between the steel sheet and the plating layer, coating film, etc.
• the interface is identified by the same method.
• a plane parallel to a plane rotated in 5° increments in the range of 0° to 180° around the plate thickness direction is observed by the same method as above.
• the average value of the length of the major axis of the multiple inclusions in each cross section is calculated for each cross section.
• the cross section in which the average value of the length of the major axis of the obtained inclusions is maximum is identified.
• the direction parallel to the major axis direction of the inclusions in that cross section is determined to be the rolling direction.
• the rolling direction of the parts is also determined in a similar manner.
• the rough rolling is preferably performed so that the total reduction is 60 to 85%.
• the total rolling reduction in rough rolling can be expressed as (1- t3 /t2) x 100 (%), where t2 is the inlet thickness in the first pass of rough rolling and t3 is the outlet thickness in the last pass of rough rolling.
• the rough rolling completion temperature (the delivery temperature of the final stage of rough rolling) is preferably set to more than 1220° C. By setting the rough rolling completion temperature to more than 1220° C., recrystallization can be promoted, and as a result, the average grain size of prior austenite grains can be preferably controlled.
• the steel plate was deemed to have high strength and passed the test. On the other hand, if the tensile strength was less than 1180 MPa, the steel plate was deemed to not have high strength and failed the test.
• the steel plate was deemed to have excellent hole expansion properties and was judged to have passed. On the other hand, if the hole expansion ratio was less than 45%, the steel plate was deemed to have poor hole expansion properties and was judged to have failed.
• Tables 1A to 4B show that the steel sheets according to the present invention have high strength and yield ratio, as well as excellent ductility and hole expansion properties, and have a high critical thickness reduction rate at break after pre-straining.
• the lower arm (part) was manufactured by press processing.
• the flat portion of the lower arm was evaluated in the same manner as described above.
• the measurement results and evaluation results were the same as those shown in Tables 4A and 4B.
• the above aspects of the present disclosure make it possible to provide a steel sheet having high strength and yield ratio, excellent ductility and hole expandability, and a high critical fracture thickness reduction rate after pre-straining, as well as a part using the same.

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