Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/16—Controlling or regulating processes or operations
• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to steel plates and exterior panel members.
• panel parts for the exterior of the vehicle body require a high r-value, including processing elements such as draw forming at corners (corner ends) or embossed parts of door handles.
• Patent Document 1 discloses a steel sheet with excellent deep drawability, in which the composition is adjusted and the coefficient of friction between the steel sheet and the rolls at least in the final rolling stand during hot rolling is controlled to increase the area ratio of crystal grains whose ⁇ 111 ⁇ plane orientation is parallel to the steel sheet surface from the surface layer of the steel sheet to 1/4 of the sheet thickness.
• Patent Document 2 discloses a steel sheet with excellent deep drawability that develops a ⁇ 111 ⁇ recrystallized texture by adjusting the amount of C and Nb added and controlling the cooling conditions after hot rolling and the heating rate during annealing.
• Patent Document 3 discloses a steel sheet with excellent stretch formability, in which the rolling conditions after solidification and the cooling conditions after hot rolling are controlled to achieve an accumulation degree of 3.0 or more in the (111) ⁇ 112> orientation of ferrite, and an accumulation degree of 5.0 or less in the (252) ⁇ 2-11> orientation of martensite and tempered martensite.
• Patent Document 4 discloses a steel sheet with an excellent r45 value and ultimate deformability, which develops gamma fiber by adjusting the rolling conditions and coiling conditions in the final three stages of finish hot rolling and performing an appropriate cold rolling process.
• the present invention aims to provide a steel sheet that has improved tensile strength, ductility, and deep drawability, and also has excellent appearance after forming.
• the inventors conducted a study focusing on both the chemical composition and metal structure of the steel sheet.
• the inventors discovered that by forming the metal structure of the steel sheet with a first phase of ferrite and a second phase of hard phase in a predetermined ratio, it is possible to improve ductility while achieving the desired high strength.
• the inventors discovered that by developing a specific texture in the metal structure mainly composed of such ferrite, it is possible to significantly improve deep drawability.
• the inventors discovered that by optimizing the chemical composition of the steel sheet to suppress or reduce microsegregation of Mn and reduce the variation of the second phase in a predetermined direction of the metal structure, and further by controlling the arithmetic mean height Sa of the steel sheet surface within a predetermined range, it is possible to significantly suppress the occurrence of poor appearance due to minute irregularities on the steel sheet surface even when strain is applied by press forming or the like, and thus completed the present invention.
• the present invention which has achieved the above object, is as follows.
• the chemical composition is, in mass%, V: 0.001 to 0.15%, Ni: 0.001 to 0.50%, Cu: 0.001 to 0.50%, W: 0.001 to 0.15%, Sn: 0.001 to 0.10%, Sb: 0.001 to 0.10%, Ca: 0.0001-0.005%, Mg: 0.0001 to 0.0050%, Zr: 0.0001 to 0.015%, Te: 0.0001 to 0.0100%, and REM: 0.0001 to 0.010%
• the steel sheet according to the above (1) characterized in that it contains at least one of the following: (3)
• the present invention makes it possible to provide a steel sheet that has improved tensile strength, ductility, and deep drawability, and also has excellent appearance after forming.
• the steel sheet according to the embodiment of the present invention has a chemical composition, in mass%, C: 0.03-0.12%, Si: 0.005-1.500%, Mn: 1.0-2.8%, P: 0.100% or less, S: 0.020% or less, N: 0.010% or less, Al: 0.005-0.700%, O: 0.010% or less, Cr: 0.10-0.50%, Mo: 0.05-0.30%, B: 0.0005-0.005%, Ti: 0.01 to 0.25%, Nb: 0.01-0.40%, V: 0 to 0.15%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, W: 0 to 0.15%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Ca: 0-0.005%, Mg: 0 to 0.0050%, Zr: 0 to 0.015%, Te: 0 to 0.0100%, REM: 0 to 0.010%, and the balance: Fe and impurities; In the following: 0.03-0.
• DP steel which is a mixture of a soft phase made of ferrite and a hard phase made of martensite, etc.
• non-uniform deformation is likely to occur during processing such as press forming, in which the soft phase and its surroundings are preferentially deformed, and fine irregularities may occur on the surface of the steel sheet after forming, resulting in poor appearance.
• elements such as Mn may be added to improve the hardenability of the steel sheet.
• Mn is an element that easily segregates in the form of stripes in steel sheets.
• Mn-enriched regions such as central segregation and microsegregation are formed during casting, and the enriched regions are stretched in the rolling direction by hot rolling or cold rolling, causing Mn to segregate in the form of stripes.
• Mn segregation due to such Mn segregation, there are regions with high and low hardenability in the steel sheet.
• a relatively large amount of striped hard phases are generated in the metal structure of the steel sheet after quenching. In this case, the occurrence of poor appearance is particularly noticeable.
• properties such as deep drawability are also required for outer plate members.
• the inventors therefore conducted research focusing on both the chemical composition and metal structure of the steel plate in order to satisfy these characteristics.
• the inventors discovered that by configuring the metal structure of the steel plate with a specified ratio of a first phase ferrite and a second phase hard phase, more specifically by configuring the metal structure with area percentages of ferrite: 70-97% and second phase: 3-30%, it is possible to achieve the desired high strength while improving ductility.
• the outer plate members generally have a shape similar to a large rectangle. For this reason, from the viewpoint of ensuring material yield, it is common to take the members from the material steel plate so that the longitudinal direction of the member coincides with the rolling direction or width direction of the material steel plate. In relation to this, since many of the outer plate members include processing elements of deep drawing at the corners, in order to improve deep drawability, it is particularly important to increase the r value in the 45° direction to the rolling direction (hereinafter referred to as the "r45 value").
• the r45 value the change in plate thickness is small and the dimensional change in the direction perpendicular to the plate thickness direction is large, so a high r value generally means excellent deep drawability.
• control of the r value is important not only for deep drawability but also for improving the appearance of the member after forming.
• the plate thickness variation is small for each part of the member, regardless of the direction of the strain introduced during forming.
• the r-value which is an index of the resistance to reduction in thickness, does not depend on the strain direction, that is, that the anisotropy of the r-value calculated from the r-values in the rolling direction, the 45° direction relative to the rolling direction, and the direction perpendicular to the rolling direction (hereinafter referred to as the " ⁇ r value" is small.
• the inventors have studied the improvement of deep drawability and appearance after forming from this perspective.
• the inventors have conducted studies from the viewpoints of reducing Mn segregation, reducing the variation of the second phase (hard phase) in the metal structure, and surface properties. As a result, the inventors have found that the microsegregation of Mn can be significantly suppressed or reduced by optimizing the chemical composition of the steel sheet, more specifically, by controlling the chemical composition of the steel sheet to contain, in mass%, C: 0.12% or less, Si: 0.005% or more, Mn: 2.8% or less, Al: 0.005% or more, Cr: 0.10% or more, and Mo: 0.10% or more.
• the inventors have found that the generation of minute irregularities on the surface of the steel sheet after forming can be significantly suppressed by reducing the variation of the second phase in a specified direction of the metal structure, more specifically, by controlling the standard deviation of the area ratio of the second phase in a direction inclined at 45 degrees to the rolling direction to 0.75% or less.
• the inventors have found that by controlling the arithmetic mean height Sa of the steel sheet surface within the range of 0.10 to 0.50 ⁇ m, it is possible to suppress or reduce the occurrence of poor appearance due to minute irregularities on the steel sheet surface, even when strain is applied by press forming or the like.
• C (C: 0.03-0.12%) C is necessary to generate a second phase other than ferrite, and is an effective element for increasing the strength of the steel sheet.
• the C content is set to 0.03% or more.
• the C content may be 0.04% or more, 0.05% or more, or 0.06% or more.
• the C content is set to 0.12% or less.
• the C content may be 0.10% or less, 0.09% or less, or 0.08% or less.
• Si is a deoxidizing element for steel and is a solid solution strengthening element effective for increasing the strength of steel plate without impairing the ductility.
• Si is also an element effective for promoting the diffusion of Mn during solidification and reducing the microsegregation of Mn.
• the Si content is set to 0.005% or more.
• the Si content may be 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, or 0.400% or more.
• the Si content is set to 1.500% or less.
• the Si content may be 1.200% or less, 1.000% or less, 0.800% or less, or 0.500% or less.
• Mn is an element necessary for improving the hardenability of steel to obtain a second phase, which is a hard phase including martensite, and contributes to improving strength.
• the Mn content is set to 1.0% or more.
• the Mn content may be 1.1% or more, 1.3% or more, 1.5% or more, or 1.8% or more.
• the Mn content is set to 2.8% or less.
• the Mn content may be 2.6% or less, 2.4% or less, 2.2% or less, or 2.0% or less.
• P is a solid solution strengthening element and is an element mixed in during the manufacturing process.
• the lower limit is not particularly limited and may be 0%.
• the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the P content is set to 0.100% or less.
• the P content may be 0.060% or less, 0.040% or less, or 0.020% or less.
• S is an element that is mixed in during the manufacturing process.
• the lower limit is not particularly limited and may be 0%.
• the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• excessive S content may form Mn sulfides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the S content is set to 0.020% or less.
• the S content may be 0.015% or less, 0.010% or less, 0.006% or less, or 0.003% or less.
• N is an element that is mixed in during the manufacturing process.
• the lower limit is not particularly limited and may be 0%.
• the N content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the N content is set to 0.010% or less.
• the N content may be 0.008% or less, 0.005% or less, or 0.003% or less.
• Al is an element that functions as a deoxidizer and is a solid solution strengthening element that is effective in increasing the strength of steel.
• Al is also an element that is effective in promoting the diffusion of Mn during solidification and reducing the microsegregation of Mn.
• the Al content is set to 0.005% or more.
• the Al content may be 0.010% or more, 0.050% or more, 0.100% or more, or 0.200% or more.
• the Al content is set to 0.700% or less.
• the Al content may be 0.500% or less or 0.300% or less.
• O is an element that is mixed in during the manufacturing process.
• the lower limit is not particularly limited and may be 0%.
• the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the O content is set to 0.010% or less.
• the O content may be 0.008% or less, 0.005% or less, or 0.003% or less.
• Cr 0.10-0.50% Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel plate, and is also an element that is effective in promoting the diffusion of Mn during solidification and reducing the microsegregation of Mn.
• the Cr content is set to 0.10% or more.
• the Cr content may be 0.12% or more, 0.15% or more, 0.18% or more, 0.20% or more, or 0.25% or more.
• the Cr content is set to 0.50% or less.
• the Cr content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.
• Mo 0.05-0.30%
• Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet, and is also an element that is effective in promoting the diffusion of Mn during solidification and reducing microsegregation of Mn.
• the Mo content is set to 0.05% or more.
• the Mo content may be 0.08% or more or 0.10% or more.
• the Mo content is set to 0.30% or less.
• the Mo content may be 0.25% or less, 0.20% or less, or 0.15% or less.
• B has the effect of suppressing recrystallization and coarsening of austenite, promoting flattening, and facilitating the acquisition of the ⁇ 223 ⁇ 252> orientation of the hot-rolled sheet.
• B is an element that has the effect of increasing the recrystallization temperature during annealing and suppressing randomization of the texture.
• the B content is set to 0.0005% or more.
• the B content may be 0.001% or more or 0.002% or more.
• B content is set to 0.005% or less.
• the B content may be 0.004% or less or 0.003% or less.
• Ti is an element that precipitates as carbides in the hot-rolled sheet structure, reduces solute carbon, makes it easier to obtain the ⁇ 211 ⁇ 011> orientation in the cold-rolled steel sheet, and contributes to improving the r45 value and ⁇ r. Ti also has the effect of suppressing the recrystallization and coarsening of austenite, promoting the flattening of austenite in the hot rolling process, and making it easier to obtain the ⁇ 223 ⁇ 252> orientation of the hot-rolled sheet. Ti is also a precipitation strengthening element that has the effect of refining the structure and improving the strength-formability balance of the steel sheet. In order to fully obtain such effects, the Ti content is set to 0.01% or more.
• the Ti content may be 0.05% or more, 0.08% or more, 0.10% or more, or 0.12% or more.
• the Ti content is set to 0.25% or less.
• the Ti content may be 0.22% or less, 0.20% or less, 0.18% or less, or 0.15% or less.
• Nb 0.01-0.40% Nb precipitates as carbides or nitrides and has the effect of suppressing recrystallization and coarsening of austenite.
• Nb has the effect of promoting flattening of austenite in the hot rolling process, making it easier to obtain the ⁇ 223 ⁇ 252> orientation of the hot-rolled sheet, suppressing recrystallization during annealing, and suppressing randomization of the texture.
• the Nb content is set to 0.01% or more.
• the Nb content may be 0.02% or more, 0.03% or more, or 0.05% or more.
• the Nb content is set to 0.40% or less.
• the Nb content may be 0.35% or less, 0.30% or less, 0.20% or less, or 0.10% or less.
• the basic chemical composition of the steel plate according to the embodiment of the present invention is as described above. Furthermore, the steel plate may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.
• V is an element that contributes to improving the strength of the steel sheet due to strengthening by precipitation, grain refinement strengthening by suppressing the growth of ferrite crystal grains, and/or dislocation strengthening by suppressing recrystallization.
• the V content may be 0%, but in order to obtain the above effect, the V content is preferably 0.001% or more.
• the V content may be 0.005% or more, 0.01% or more, or 0.05% or more.
• the V content is preferably 0.15% or less.
• the V content may be 0.12% or less, 0.10% or less, or 0.08% or less.
• Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the 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, 0.01% or more, or 0.05% or more.
• excessive Ni content may reduce the weldability of the steel sheet. For this reason, the Ni content is preferably 0.50% or less.
• the Ni content may be 0.40% or less, 0.20% or less, or 0.10% or less.
• Cu is an element that exists in the steel in the form of fine particles and contributes to improving the strength of the steel sheet.
• the Cu content may be 0%, but in order to obtain the above effect, the Cu content is preferably 0.001% or more.
• the Cu content may be 0.005% or more, 0.01% or more, or 0.05% or more.
• excessive Cu content may reduce the weldability of the steel sheet. For this reason, the Cu content is preferably 0.50% or less.
• the Cu content may be 0.40% or less, 0.20% or less, or 0.10% or less.
• W is an element that has the effect of suppressing recrystallization and coarsening of austenite, promoting flattening, and facilitating the acquisition of the ⁇ 223 ⁇ 252> orientation of the hot-rolled sheet.
• the W content may be 0%, but in order to obtain the above effect, the W content is preferably 0.001% or more.
• the W content may be 0.005% or more, 0.01% or more, or 0.05% or more.
• excessive W content may deteriorate hot workability and reduce productivity. For this reason, the W content is preferably 0.15% or less.
• the W content may be 0.12% or less, 0.10% or less, or 0.08% or less.
• Sn is an element that suppresses the coarsening of crystal grains and contributes to improving the strength of the steel sheet.
• the Sn content may be 0%, but in order to obtain the above effect, the Sn content is preferably 0.001% or more.
• the Sn content may be 0.005% or more, 0.01% or more, or 0.03% or more.
• excessive Sn content may generate coarse oxides and cause embrittlement of the steel sheet. For this reason, the Sn content is preferably 0.10% or less.
• the Sn content may be 0.08% or less, 0.06% or less, or 0.04% or less.
• Sb is an element that suppresses the coarsening of crystal grains and contributes to improving the strength of the steel sheet.
• the Sb content may be 0%, but in order to obtain the above effect, the Sb content is preferably 0.001% or more.
• the Sn content may be 0.005% or more, 0.01% or more, or 0.03% or more.
• the Sb content is preferably 0.10% or less.
• the Sb content may be 0.08% or less, 0.06% or less, or 0.04% or less.
• Ca is an element mixed in as a deoxidizer.
• the Ca content may be 0%, but excessive reduction leads to increased manufacturing costs. Therefore, the Ca content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive Ca content may generate coarse oxides and cause embrittlement of the steel sheet. Therefore, the Ca content is preferably 0.005% or less.
• the Ca content may be 0.004% or less, 0.003% or less, or 0.002% or less.
• Mg is an element that can control the form of sulfides by adding a small amount, and is added as necessary.
• the Mg content may be 0%, but excessive reduction leads to increased manufacturing costs. Therefore, the Mg content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
• the Mg content is desirably 0.0050% or less.
• the Mg content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less.
• Zr 0-0.015%
• Zr is an element mixed in as a deoxidizer.
• the Zr content may be 0%, but excessive reduction leads to increased manufacturing costs. Therefore, the Zr content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive inclusion of Zr may generate coarse oxides, which may cause embrittlement of the steel sheet. Therefore, the Zr content is preferably 0.015% or less.
• the Zr content may be 0.010% or less, 0.005% or less, or 0.003% or less.
• Te (Te: 0 ⁇ 0.0100%) Te is an element that can control the form of sulfides by adding a small amount, and is added as necessary.
• the Te content may be 0%, but excessive reduction leads to increased manufacturing costs. Therefore, the Te content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive Te content may generate coarse inclusions and cause embrittlement of the steel sheet. Therefore, the Te content is desirably 0.0100% or less.
• the Te content may be 0.0090% or less, 0.0080% or less, 0.0070% or less, or 0.0050% or less.
• REM 0-0.010%
• the REM content may be 0%, but excessive reduction leads to increased manufacturing costs. For this reason, the REM content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive REM content may generate coarse oxides and cause embrittlement of the steel sheet. For this reason, the REM content is preferably 0.010% or less. The REM content may be 0.008% or less, 0.005% or less, or 0.003% or less.
• REM is a collective term for 17 elements, namely, scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.
• the remainder of the steel plate of this embodiment is Fe and impurities.
• Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate of this embodiment is industrially manufactured.
• 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 metal structure of the steel sheet contains, in terms of area %, 70 to 97% ferrite and 3 to 30% second phase, and is composed of, for example, only 70 to 97% ferrite and 3 to 30% hard phase. If the ferrite area ratio is less than 70%, ductility decreases. For this reason, the ferrite area ratio is set to 70% or more. From the viewpoint of improving ductility, the higher the ferrite area ratio, the more preferable it is, and it may be, for example, 75% or more, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if it exceeds 97%, the desired strength may not be obtained.
• the ferrite area ratio is set to 97% or less.
• the ferrite area ratio may be 96% or less, 93% or less, 90% or less, or 87% or less. From the viewpoint of strength-ductility balance, the ferrite area ratio is preferably 70 to 90%.
• the area ratio of the second phase which is a hard phase
• the desired strength cannot be obtained. Therefore, the area ratio of the second phase is set to 3% or more.
• the higher the area ratio of the second phase the more preferable it is, and it may be, for example, 5% or more, 7% or more, 10% or more, or 15% or more.
• the area ratio of the second phase is set to 30% or less.
• the area ratio of the second phase may be 27% or less, 25% or less, 20% or less, or 18% or less.
• the second phase refers to a structure consisting of at least one of martensite, bainite, tempered martensite, pearlite, and retained austenite.
• the lower the standard deviation of the area ratio of the second phase the more preferable it is, and for example, it may be 0.70% or less, 0.65% or less, 0.60% or less, or 0.55% or less.
• the lower limit is not particularly limited and may be 0%.
• the standard deviation of the area ratio of the second phase may be 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more.
• the following method is used to identify the rolling direction of the steel plate.
• the S concentration is measured using an electron probe microanalyzer (EPMA) (for example, JXA-8230 manufactured by JEOL Ltd.).
• the measurement conditions are an acceleration voltage of 15 kV and a measurement pitch of 1 ⁇ m, and a distribution image is measured in a 500 ⁇ m square range in the center of the plate thickness.
• extended areas with high S concentration are determined to be inclusions such as MnS.
• observations are made in three or more fields of view spaced 100 ⁇ m or more apart.
• a surface parallel to the surface rotated in 5° increments in the range of 0° to 180° around the axis of the plate thickness direction is observed by the above method.
• the average value of the length of the major axis of the multiple inclusions in each cross section obtained is calculated for each cross section, and the cross section in which the average value of the length of the major axis of the inclusions is the largest is identified.
• the direction parallel to the major axis of the inclusions in that cross section is determined to be the rolling direction.
• the cluster strengths of ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> may be 5.0 or more, 6.0 or more to 7.0 or more, or 8.0 or more, respectively.
• the accumulation intensities of ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> may be 18.0 or less, 16.0 or less, or 15.0 or less, respectively.
• Measurement of the accumulation strength by ODF is performed as follows. First, a cross section parallel to the rolling direction and thickness direction of the steel sheet is cut out, and the surface is prepared by mechanical grinding and electrolytic polishing. If the rolling direction of the steel sheet is unclear, the rolling direction may be identified by the method using the inclusions described above. The same applies to other measurements. Next, the crystal orientation is measured at 1.00 ⁇ m intervals in a 100 ⁇ m ⁇ 100 ⁇ m region at a depth position of 1/2 the sheet thickness by the SEM-EBSD method.
• ODF crystal orientation distribution function
• the average grain size of ferrite in the steel sheet is the average grain size of ferrite generated during annealing and subsequent cooling.
• the density and grain size of the hard phase change depending on the average grain size of ferrite.
• the average grain size of ferrite is preferably 5.0 to 30.0 ⁇ m.
• the average grain size of ferrite when the average grain size of ferrite is 5.0 ⁇ m or more, aggregation of the hard phase after ferrite generation is less likely to occur, and strain during forming can be prevented from becoming non-uniform, and the appearance after forming can be further improved.
• the average grain size of ferrite when the average grain size of ferrite is 30.0 ⁇ m or less, the variation in the grain size of ferrite is reduced, and strain during forming can be prevented from becoming non-uniform, and the appearance after forming can be further improved.
• the average grain size of ferrite may be 8.0 ⁇ m or more, 9.0 ⁇ m or more, or 10.0 ⁇ m or more.
• the average grain size of ferrite may be 28.0 ⁇ m or less, 25.0 ⁇ m or less, 20.0 ⁇ m or less, 16.0 ⁇ m or less, 14.0 ⁇ m or less, or 12.0 ⁇ m or less.
• the average grain size of the hard phase in the steel sheet is the average grain size of the hard phase, which is at least one of martensite, bainite, tempered martensite, pearlite, and retained austenite generated during annealing and subsequent cooling.
• the average grain size of the hard phase is preferably 1.0 to 5.0 ⁇ m.
• the average grain size of the hard phase when the average grain size of the hard phase is 1.0 ⁇ m or more, aggregation of the hard phase is less likely to occur, and strain during forming can be prevented from becoming non-uniform, and the appearance after forming can be further improved.
• the average grain size of the hard phase when the average grain size of the hard phase is 5.0 ⁇ m or less, the variation in grain size of the hard phase is reduced, and strain during forming can be prevented from becoming non-uniform, and the appearance after forming can be further improved.
• the average grain size of the hard phase may be 1.2 ⁇ m or more, 1.5 ⁇ m or more, 1.7 ⁇ m or more, or 2.0 ⁇ m or more.
• the average crystal grain size of the hard phase may be 4.8 ⁇ m or less, or 4.5 ⁇ m or less, 4.2 ⁇ m or less, 4.0 ⁇ m or less, 3.8 ⁇ m or less, 3.6 ⁇ m or less, or 3.4 ⁇ m or less.
• the arithmetic mean height Sa of the steel sheet surface (the surface of the plating layer when a plating layer is present on the surface of the steel sheet) is 0.10 to 0.50 ⁇ m.
• the arithmetic mean height Sa is preferably 0.45 ⁇ m or less, more preferably 0.43 ⁇ m or less, and most preferably 0.40 ⁇ m or less or 0.35 ⁇ m or less.
• the arithmetic mean height Sa may be 0.15 ⁇ m or more, 0.20 ⁇ m or more, or 0.25 ⁇ m or more.
• a preferred range of Sa for further improving the appearance after forming is, for example, 0.20 to 0.45 ⁇ m.
• the measurement of Sa is carried out as follows. First, a test piece is cut out from a position 100 mm or more away from the end face of the steel sheet, and then the unevenness of the surface of the steel sheet (if a plating layer is present on the surface of the steel sheet, the surface of the plating layer) is measured in an 8 mm square area using a laser microscope (for example, Keyence's "VK-X3000"). The test piece may be cut out from a position 10 mm or more away from the end face of the steel sheet depending on the size of the measurement object.
• the measurement magnification is 20 times, the resolution in the XY plane is 5 ⁇ m, and the resolution in the Z space plane is 0.1 nm, and the measurements are performed in a linked manner.
• the entire measurement area is subjected to low-pass filtering with a cutoff value of 0.25 mm using a Gaussian filter specified in JIS B0681-2:2018, and the arithmetic mean height Sa is obtained.
• Sa is the absolute average of z(x, y) in the reference region (A) defined in JIS B0681-2:2018, 4.1.7 "Arithmetic mean height of the scale limited surface".
• the arithmetic mean height Sa is measured in three or more regions (8 mm square regions) spaced apart by 100 ⁇ m or more, and the arithmetic mean value of each measurement is obtained.
• the thickness of the steel plate is not particularly limited, but for example, the steel plate may have a thickness of 0.1 to 2.0 mm.
• a steel plate having such a thickness is suitable for use as a material for a cover member such as a door or a hood.
• the thickness of the steel plate may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more.
• the thickness of the steel plate may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less.
• the thickness of the steel plate 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and an additional effect of improving the dimensional accuracy and shape accuracy can be obtained.
• the thickness 1.0 mm or less the weight reduction effect of the member becomes remarkable.
• the thickness of the steel plate is measured by a micrometer.
• the steel sheet according to the embodiment of the present invention may be a cold-rolled sheet (cold-rolled steel sheet), but may further include a plating layer on the surface for the purpose of improving corrosion resistance or the like.
• the plating layer may be either a hot-dip plating layer or an electroplating layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplating layer on its surface.
• the hot-dip plating layer includes, for example, a hot-dip galvanized layer (GI), a hot-dip galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• the electroplating layer includes, for example, an electrogalvanized layer (EG), an electrogalvanized Zn-Ni alloy plating layer, and the like.
• the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
• the coating weight of the plating layer is not particularly limited and may be a general coating weight.
• the white parts (relatively bright contrast areas) of the obtained image data are the structure of the second phase (martensite, bainite, tempered martensite, pearlite, and retained austenite) and the black parts (relatively dark contrast areas) are ferrite
• the image is analyzed in a field of view of the total plate thickness x 5 mm to determine the area ratio of ferrite and the second phase.
• the method of determining the white parts and black parts can be performed, for example, using image analysis software ImageJ.
• the binarization threshold value that indicates black when the structure is ferrite and white when the structure is not ferrite is determined using a method that adopts the average brightness value as the threshold value, as described in "Glasbey, CA (1993), "Analysis of histogram-based thresholding algorithms", CVGIP: Graphical Models and Image Processing 55:532-537".
• Image analysis is performed in three or more visual fields spaced at intervals of 100 ⁇ m or more, and the arithmetic mean value of the measured values in each visual field is taken as the area ratio of ferrite and the second phase in the observation surface.
• the standard deviation of the area ratio of the second phase is calculated from the combined SEM images measured above.
• the steel sheet is divided into 100 ⁇ m (0.1 mm) in the 45° direction relative to the rolling direction of the steel sheet, and the area ratio of the second phase in the entire sheet thickness is calculated for each divided range.
• the standard deviation of the area ratio of the second phase is calculated based on the area ratio of the second phase calculated from each of the total of 500 divided images.
• the image analysis is performed in three or more fields of view spaced 100 ⁇ m or more apart, and the arithmetic mean value of the measured values in each field of view is taken as the standard deviation of the area ratio of the second phase in the observation surface.
• the number of ferrite particles is calculated by image analysis in a field of view of the total plate thickness ⁇ 5 mm.
• the average area ratio per ferrite particle is calculated by dividing the ferrite area ratio by the number of ferrite particles.
• the circle equivalent diameter is calculated from this average area ratio, and the obtained circle equivalent diameter is regarded as the average grain size of ferrite.
• the image analysis is performed in three or more fields of view spaced 100 ⁇ m apart from each other, and the arithmetic mean value of the measured values in each field of view is regarded as the average grain size of ferrite in the observation surface.
• the steel plate of this embodiment which is composed of a steel plate having the above-mentioned specific chemical composition and metal structure and the above-mentioned specific arithmetic mean height Sa formed on the surface thereof, can achieve high strength and ductility (elongation), specifically, a tensile strength of 540 MPa or more and a total elongation of 15.0% or more.
• the total elongation of the steel sheet is preferably 16.0% or more, and more preferably 18.0% or more.
• the upper limit of the total elongation of the steel sheet is not particularly limited, but from the viewpoint of productivity, the total elongation may be, for example, 30.0% or less or 25.0% or less.
• the tensile strength (TS) and total elongation (EL) can be measured as follows. First, a tensile test piece according to JIS Z 2241:2022 No. 5 is cut from the steel plate to be measured, with the longitudinal direction perpendicular to the rolling direction. This test piece is then used to perform a tensile test in accordance with JIS Z 2241:2022, which allows the tensile strength TS (MPa) and butt elongation (total elongation) EL (%) to be measured.
• TS tensile strength
• EL total elongation
• Tensile test pieces are taken from three or more locations spaced at least 100 ⁇ m apart, and the arithmetic mean values of the measured values for each tensile test piece are taken as the measurement results for the tensile strength (TS) and total elongation (EL) of the steel plate to be measured.
• TS tensile strength
• EL total elongation
• the steel sheet of this embodiment has an excellent surface property even after forming because the Sa of the steel sheet of this embodiment is controlled to 0.10 to 0.55 ⁇ m after forming, and therefore has an excellent appearance after forming.
• the steel sheet of this embodiment can have surface characteristics in which the post-forming Sa is 0.10 to 0.55 ⁇ m.
• the post-forming Sa is the average value of the height difference (absolute value) of each point with respect to the average surface of the surface after strain during forming is imparted, and more specifically, it is the absolute value average of z (x, y) in the reference region (A) defined in 4.1.7 "arithmetic mean height of the scale limited surface" of JIS B0681-2:2018. If the post-forming Sa is 0.55 ⁇ m or less, the post-forming appearance is excellent. Note that the post-forming Sa may be 0.10 ⁇ m or more from the viewpoint of productivity.
• the post-forming Sa can be measured as follows. First, cut out a tensile test piece of JIS Z2241:2022 No. 5 with the direction perpendicular to the rolling direction as the longitudinal direction from the steel plate to be measured. At this time, the test piece is cut out from a position 100 mm or more away from the end face of the steel plate. Depending on the size of the measurement object, the test piece may be cut out from a position 10 mm or more away from the end face of the steel plate. Next, apply a tensile strain of 5% in the longitudinal direction to the above test piece by a tensile test conforming to JIS Z 2241:2022.
• the measurement conditions at this time are a measurement magnification of 20 times, a resolution of 5 ⁇ m in the XY plane, and a resolution of 0.1 nm in the Z space plane, and measure in a linked manner.
• the entire measurement area is subjected to low-pass filtering with a cutoff value of 0.25 mm using a Gaussian filter standardized in JIS B0681-2:2018 to obtain the arithmetic mean height Sa.
• the arithmetic mean height Sa is measured in three or more areas spaced at intervals of 100 ⁇ m or more, and the arithmetic mean value of each measurement is obtained.
• the arithmetic mean height Sa obtained in this way is the post-molding Sa.
• the r45 value of the steel sheet is preferably 1.40 or more, more preferably 1.50 or more.
• the upper limit of the r45 value of the steel sheet is not particularly limited, but may be 3.00 or less, 2.50 or less, or 2.00 or less.
• the ⁇ r of the steel sheet is preferably 0.40 or less, more preferably 0.35 or less, or 0.30 or less.
• the r45 value and ⁇ r which are the plastic strain ratios, were measured in accordance with the provisions of JIS Z 2254: 2008. Test pieces were taken from three or more locations spaced at intervals of 100 ⁇ m or more, and the arithmetic mean values of the measured values for each test piece were taken as the measurement results of the r45 value and ⁇ r, respectively, of the steel plate to be measured.
• the steel plate according to the embodiment of the present invention can achieve high strength, for example, tensile strength of 540 MPa or more, improved ductility and deep drawability, and even excellent appearance after forming. For this reason, the steel plate according to the embodiment of the present invention is particularly useful for use in parts in technical fields where these properties are required.
• an exterior plate member particularly an automobile exterior plate member, is provided that includes the steel plate according to the embodiment of the present invention. Examples of the exterior plate members of an automobile include roofs, hoods, fenders, doors, and the like, which require high designability.
• exterior plate members particularly the exterior plate members of an automobile, only need to include the steel plate according to the embodiment of the present invention in at least a portion of these exterior plate members, and therefore at least a portion of these exterior plate members satisfy the above-mentioned chemical composition and metal structure characteristics.
• the characteristics of the metal structure do not change particularly before and after forming.
• the method for manufacturing steel sheet of this embodiment includes a casting process for casting a steel piece (slab) having the above-mentioned specific chemical composition, a hot rolling process for hot rolling the cast steel piece, a cold rolling process for cold rolling the hot rolled sheet, a primary annealing process for holding the cold rolled sheet at a predetermined temperature range, a secondary annealing process for holding the cold rolled sheet at a predetermined temperature range under a predetermined atmosphere, a cooling process for cooling the annealed cold rolled sheet, an optional plating process for forming a plating layer on the surface of the cooled cold rolled sheet, and a skin pass rolling process.
• These processes are not particularly limited, and may be carried out under any appropriate conditions selected as appropriate so as to obtain the above-mentioned specific metal structure. Preferred conditions for these processes are described below.
• the casting process it is preferable to cast the slab by a continuous casting method from the viewpoint of productivity. It is also important to control the solidification structure during casting to be columnar. Specifically, in the casting process, the superheat ⁇ T (the difference between the molten steel temperature and the solidification temperature of the molten steel) of the molten steel having the above-mentioned specific chemical composition is set to 25° C.
• the segment pressing force is set to 450 tons or more, so that the solidification structure can be controlled to a columnar crystal structure with an equiaxed crystal ratio of 15% or less and the central segregation of Mn can also be suppressed.
• a columnar crystal structure with an equiaxed crystal ratio of 15% or less is effective in suppressing local Mn concentration, and from the viewpoint of further promoting the formation of such a structure, it is more preferable that the superheat ⁇ T is 30° C. or more.
• the upper limit of the superheat is not particularly limited, but it may be, for example, 50° C. or less.
• Heating temperature 1050-1300°C
• the heating temperature is set to 1050°C or higher because Ti needs to be dissolved in the steel. If the heating temperature exceeds 1300°C, the life of the heating furnace may be shortened. For this reason, the heating temperature is set to 1300°C or lower.
• 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.
• Total reduction in the final three stages of finish rolling 40-100%
• the heated slab or the slab that has been rough-rolled as required is then subjected to finish rolling.
• the finish rolling is performed using a tandem rolling mill consisting of a plurality of rolling stands, for example, three or more, preferably four to six.
• the total reduction in the final three stages of finish rolling is set to 40% or more in order to develop the ⁇ 223 ⁇ 252> orientation of the hot-rolled sheet and thus to increase the accumulation strength of ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> in the final product.
• the ⁇ 223 ⁇ 252> orientation of the hot-rolled sheet is the crystal orientation that becomes the nucleus of ⁇ 211 ⁇ 011> in the subsequent introduction of shear bands by cold rolling and recrystallization by annealing. Therefore, by developing the ⁇ 223 ⁇ 252> orientation in the hot-rolled sheet, it is possible to increase the accumulation of ⁇ 211 ⁇ 011> and the like in the final product after cold rolling and annealing.
• the total reduction in the final three stages (the sum of the reductions in each stage) is 100% or less. At least the final three stages of rolling including the final rolling stand are performed in a temperature range from the finish rolling end temperature to the finish rolling end temperature + 100°C.
• the effective rolling index is set to 1.2 or more. There is no particular upper limit for the effective rolling index, but the larger the effective rolling index, the more the austenite texture can be developed.
• the austenite becomes equiaxed and the texture weakens, which results in a weakened texture of the ferrite after cooling.
• the following method can be used to determine a combination that flattens the grains while enhancing the texture in the final three stages of finishing rolling.
• dT eff-p which is a value related to the time it takes for 50% of austenite to recrystallize after F1 rolling
• dT eff-p which is a value related to the time it takes for 50% of austenite to recrystallize after F1 rolling
• t ini is the plate thickness (mm) at the F1 entry side
• t F1 is the plate thickness (mm) after F1 rolling
• W Ti is the Ti content (mass%) contained in the steel.
• dT eff-p is greater than 2.0, it is determined that no recrystallization occurred in F1, and dT eff-sa, which is a value related to the time it takes for 50% of austenite to recrystallize after rolling F2, is calculated from the accumulated rolling strain up to F2 using the following formula (iv).
• t F2 is the plate thickness (mm) after F2 rolling.
• t F3 is the plate thickness (mm) after F3 rolling.
• the effective rolling index C eff-tc is calculated from the accumulated rolling strains of F2 and F3 by the following formula (viii).
• the K value is determined by the following formula (ix) instead of the above formula (ii).
• WMo , WV , WW , WZr and WNb are the contents (mass%) of Mo, V, W, Zr and Nb contained in the steel, respectively, and 0 is substituted if no elements are contained.
• the effective rolling index obtained by the above procedure is 1.2 or more, austenite grains that are flat in the rolling direction and have a well-developed texture can be effectively obtained.
• the effective rolling index is preferably 1.5 or more from the viewpoint of further developing the texture.
• Average cooling rate after finishing rolling 15 to 200°C/sec
• the average cooling rate after the finish rolling is set to 15°C/s or more. If the average cooling rate exceeds 200°C/s, the strength of the hot-rolled sheet becomes excessively high, and the load on the cold rolling machine in the subsequent cold rolling process becomes excessive. Therefore, the average cooling rate is set to 200°C/s or less.
• the end temperature of the finish rolling is preferably 800° C. or higher.
• the end temperature of the finish rolling is 800° C. or higher, a desired standard deviation of the area ratio of the second phase can be achieved in the final product due to the formation of a more uniform metal structure, and in this regard, a higher quality post-forming appearance can be obtained.
• the end temperature of the finish rolling is preferably 950° C. or lower.
• the coiling temperature of the hot-rolled coil is set to 300°C or higher. If the coiling temperature exceeds 600°C, the ⁇ 223 ⁇ 252> texture of the hot-rolled sheet cannot be sufficiently developed, and in this connection, the accumulation strength of ⁇ 211 ⁇ 011> and/or ⁇ 332 ⁇ 113> of the final product cannot be sufficiently increased. In addition, coarse ferrite and pearlite are generated in the hot-rolled sheet structure, which increases the structural non-uniformity of the final product and deteriorates the appearance after forming. For this reason, the coiling temperature is set to 600°C or lower.
• Cold rolling process Cold rolling ratio
• the obtained hot-rolled sheet is appropriately pickled to remove scale, and then subjected to a cold rolling process.
• the cold rolling reduction By setting the cold rolling reduction to 65% or more, the cold rolling strain is accumulated and the grains are refined in the next annealing process, thereby improving the uniformity of the structure, and the r45 value can be increased to a desired value while ensuring the appearance after forming.
• the cold rolling reduction is set to 90% or less.
• the average heating rate of the first annealing is set to 0.01° C./s or more.
• the average heating rate exceeds 5.0° C./s, carbides are generated first, causing the recrystallized structure to become random, the integration strength of ⁇ 211 ⁇ 011> and the like to decrease, and the r45 value and/or ⁇ r to deteriorate. For this reason, the average heating rate is set to 5.0° C./s or less.
• Maximum heating temperature for primary annealing 600 to Ac1°C If the maximum heating temperature of the primary annealing is less than 600°C, recrystallization does not proceed sufficiently, and the formation of the ⁇ 211 ⁇ 011> texture of ferrite, which is effective in reducing ⁇ r, is inhibited. For this reason, the maximum heating temperature is set to 600°C or higher. On the other hand, if the maximum heating temperature exceeds the Ac1 transformation point and becomes high, carbides dissolve, austenite transformation occurs, and the texture becomes random, resulting in deterioration of the r45 value and/or ⁇ r. For this reason, the maximum heating temperature is set to Ac1°C or lower.
• the Ac1 point can be calculated from the chemical composition of the steel sheet using the following formula (1) described in Leslie, "Leslie Steel Materials Science,” supervised by Shigeyasu Koda, translated by Maruzen Co., Ltd., 1985, p. 273.
• Ac1(°C) 723-10.7 ⁇ Mn-16.9 ⁇ Ni+29.1 ⁇ Si+16.9 ⁇ Cr...(1)
• the element symbols in the above formula represent the content (mass %) of each element.
• First annealing holding time 5 to 60 minutes
• the holding time of the first annealing is less than 5 minutes, fine carbides with low Mn concentration cannot be precipitated after recrystallization to develop the ⁇ 211 ⁇ 011> texture, and as a result, ⁇ r and the like may not be sufficiently reduced. For this reason, the holding time is set to 5 minutes or more.
• the holding time exceeds 60 minutes, Mn is concentrated in the carbides, and the carbides cannot be dissolved during annealing, making it impossible to uniformly generate the second phase in the subsequent second annealing step. As a result, it becomes impossible to sufficiently reduce the standard deviation of the area ratio of the second phase in the final product. For this reason, the holding time is set to 60 minutes or less.
• the cold-rolled sheet that has been subjected to the primary annealing is then heated in the secondary annealing step to a maximum heating temperature of Ac1 to 900°C at an average heating rate of 2 to 30°C/sec in an atmosphere with a dew point of -30 to 20°C and a hydrogen concentration of 2 to 20% (nitrogen balance), and is held at the maximum heating temperature for 30 to 500 seconds.
• a maximum heating temperature of Ac1 to 900°C at an average heating rate of 2 to 30°C/sec in an atmosphere with a dew point of -30 to 20°C and a hydrogen concentration of 2 to 20% (nitrogen balance)
• the recrystallization of ferrite and the reverse transformation from ferrite to austenite can be appropriately advanced, the crystal grains can be densified, and the steel sheet surface can be sufficiently reduced, so that the final product can have the desired metal structure fraction and integration strength of the texture, and can achieve an excellent appearance.
• Average heating rate of secondary annealing 2 to 30°C/sec
• the average heating rate of the secondary annealing is set to 2°C/s or more.
• the average heating rate exceeds 30°C/s, condensation occurs in the equipment, hindering the operation of the equipment. Therefore, the average heating rate is set to 30°C/s or less.
• the hydrogen concentration in the atmosphere for the secondary annealing is set to 2% or more.
• the hydrogen concentration exceeds 20%, it becomes impossible to maintain the dew point at 20°C or less. Therefore, the hydrogen concentration is set to 20% or less.
• Dew point for secondary annealing -30 to 20°C If the dew point of the secondary annealing is less than -30°C, the surface is not sufficiently reduced, the plating wettability of the base steel sheet deteriorates, and the appearance after forming deteriorates. Therefore, the dew point of the secondary annealing is set to -30°C or higher. On the other hand, if the dew point is more than 20°C, condensation occurs in the equipment, hindering the operation of the equipment. Therefore, the dew point is set to 20°C or lower.
• Maximum heating temperature for secondary annealing Ac1 to 900°C If the maximum heating temperature of the secondary annealing is less than Ac1°C, the recrystallization of ferrite and the reverse transformation from ferrite to austenite will be insufficient. Therefore, the maximum heating temperature is set to Ac1°C or higher. On the other hand, if the maximum heating temperature exceeds 900°C, the crystal grains cannot be densified, and as a result, the desired metal structure fraction and therefore the desired mechanical properties cannot be obtained in the final product. Therefore, the maximum heating temperature is set to 900°C or lower.
• the holding time of the secondary annealing is set to 30 seconds or more.
• the holding time exceeds 500 seconds, the crystal grains will not be densified, and as a result, the desired metal structure fraction and sufficient strength will not be obtained in the final product. Therefore, the holding time is set to 500 seconds or less.
• the cold-rolled sheet that has been subjected to the second annealing is then cooled in the next cooling step at an average cooling rate of 3 to 30° C./sec to a cooling stop temperature of 450 to 650° C.
• Average cooling rate 3-30°C/sec
• the average cooling rate is set to 3°C/sec or more.
• the average cooling rate exceeds 30°C/sec, transformation accompanied by new nucleation occurs, and the accumulation of ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> becomes insufficient. Therefore, the average cooling rate is set to 30°C/sec or less.
• the cold-rolled sheet after the cooling step is optionally subjected to an alloying treatment in the next plating step.
• the alloying treatment temperature is set to 450°C or higher.
• the alloying treatment temperature is set to 600°C or lower.
• the alloying treatment temperature is preferably 460 to 550°C.
• alloying treatment time 10 to 1000 seconds
• the alloying treatment time in the plating step is less than 10 seconds, the alloying treatment cannot be stably achieved. Therefore, the alloying treatment time is set to 10 seconds or more. On the other hand, if the alloying treatment time exceeds 1000 seconds, the productivity decreases. Therefore, the alloying treatment time is set to 1000 seconds or less.
• the above manufacturing method makes it possible to obtain the steel plate according to an embodiment of the present invention.
• a steel plate according to one embodiment of the present invention i.e., a steel plate according to an example of the present invention
• a steel plate for comparison therewith i.e., a steel plate according to a comparative example
• a slab having the chemical composition shown in Table 1 below was cast by a continuous casting method under conditions of a segment pressing force of 450 tons or more so that the superheat ⁇ T of the molten steel was the value shown in Table 1.
• the obtained slab was subjected to a hot rolling process under the conditions shown in Table 2 below.
• the hot rolling was performed by performing rough rolling and finish rolling. More specifically, the rough rolling was performed under the same conditions in all examples and comparative examples, and the finish rolling was performed using a tandem rolling mill consisting of six rolling stands. In all examples and comparative examples, the final three stages of rolling in the finish rolling were performed in a temperature range from the finish rolling end temperature to the finish rolling end temperature + 100°C.
• Step Plate Evaluation For the obtained steel sheets, the metal structure, the surface characteristics before forming (i.e., arithmetic mean height Sa before forming), the mechanical properties (i.e., tensile strength and total elongation), the deep drawability (i.e., r45 value and ⁇ r) and the surface characteristics after forming (i.e., Sa after forming) were measured, and each property and the appearance after forming were evaluated.
• the surface characteristics before forming i.e., arithmetic mean height Sa before forming
• the mechanical properties i.e., tensile strength and total elongation
• the deep drawability i.e., r45 value and ⁇ r
• the surface characteristics after forming i.e., Sa after forming
• Comparative Example 2 the finishing rolling end temperature was low, so a uniform metal structure could not be formed, and the standard deviation of the area ratio of the second phase in the final product exceeded 0.75%. As a result, the appearance after forming was deteriorated.
• the coiling temperature was high, so the ⁇ 223 ⁇ 252> texture of the hot-rolled sheet could not be sufficiently developed, and coarse ferrite and the like were generated in the hot-rolled sheet structure, which is thought to have increased the structural non-uniformity of the final product.
• Comparative Example 6 since the superheat ⁇ T in the casting process was low, the solidification structure during casting did not become columnar crystals, and it is considered that local Mn enrichment could not be sufficiently suppressed. As a result, the standard deviation of the area ratio of the second phase exceeded 0.75%, and the appearance after forming was deteriorated. In Comparative Example 8, since the cold rolling rate was high, it is considered that the texture was randomized due to discontinuous recrystallization. As a result, the integrated strength of ⁇ 211 ⁇ 011> could not be sufficiently increased in the final product, and the standard deviation of the area ratio of the second phase exceeded 0.75%, and the deep drawability and the appearance after forming were deteriorated.
• Comparative Example 12 the total reduction in the final three stages of finish rolling was low, so it is believed that the ⁇ 223 ⁇ 252> texture of the hot-rolled sheet could not be sufficiently developed. In relation to this, the accumulated strength of ⁇ 211 ⁇ 011> could not be sufficiently increased in the final product, and the deep drawability was reduced. In Comparative Example 14, the effective rolling index of the hot-rolling process was low, so it is believed that the ⁇ 223 ⁇ 252> texture of the hot-rolled sheet could not be sufficiently developed. In relation to this, the accumulated strength of ⁇ 211 ⁇ 011> could not be sufficiently increased in the final product, and the deep drawability was reduced.
• Comparative Example 15 the average cooling rate of the hot-rolling process was slow, so it is believed that the ⁇ 223 ⁇ 252> texture of the hot-rolled sheet could not be sufficiently developed. In relation to this, the accumulated strength of ⁇ 211 ⁇ 011> could not be sufficiently increased in the final product, and the deep drawability was reduced. In Comparative Example 17, it is considered that the average heating rate in the first annealing step was high, so that carbides were generated first, and the recrystallized structure was randomized. As a result, the integrated strength of ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> was reduced in the final product, and the deep drawability was reduced.
• Comparative Example 18 it is considered that the maximum heating temperature in the first annealing step was low, so that recrystallization did not proceed sufficiently. As a result, the integrated strength of ⁇ 211 ⁇ 011> was reduced in the final product, and the deep drawability was reduced. In Comparative Example 19, it is considered that the maximum heating temperature in the first annealing step was high, so that carbides were dissolved and austenite transformation occurred, and the texture was randomized. As a result, the integrated strength of ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> was reduced in the final product, and the deep drawability was reduced.
• Comparative Example 23 the average cooling rate in the cooling process was fast, so the accumulation of ⁇ 211 ⁇ 011> and ⁇ 332 ⁇ 113> was insufficient in the final product, and the deep drawability was reduced.
• the skin pass rolling rate was low, so the desired arithmetic mean height Sa was not obtained in the final product, and the appearance after forming was reduced.
• Comparative Example 25 it is believed that the maximum heating temperature in the secondary annealing process was high, so the crystal grains could not be densified. As a result, the area ratio of the second phase in the final product was high, and the ductility was reduced.
• Comparative Examples 26 and 28 it is believed that the C and Mn contents were high, respectively, so the diffusion of Mn during solidification was inhibited, and the microsegregation of Mn could not be sufficiently suppressed. As a result, the appearance after forming was reduced. In Comparative Examples 27 and 29, it is believed that the Si and Al contents were low, so the diffusion of Mn during solidification was inhibited, and the microsegregation of Mn could not be sufficiently suppressed. As a result, the appearance after forming was similarly reduced. In Comparative Examples 30 to 32, the B, Ti, and Nb contents were low, which is believed to be why the ⁇ 223 ⁇ 252> texture of the hot-rolled sheet could not be sufficiently developed. In relation to this, the integrated strength of ⁇ 211 ⁇ 011> and/or ⁇ 332 ⁇ 113> could not be sufficiently increased in the final product, resulting in poor deep drawability.

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