EP4775705A1 - Steel sheet and outside sheet member - Google Patents

Abstract

The present invention provides a high strength steel sheet excellent in appearance after molding and workability during molding through a novel configuration.

A steel sheet according to the present invention has a specific chemical composition, and is characterized in that: when an area ratio of a hard phase in a region having a depth from the position corresponding to 3/8 the sheet thickness to the position corresponding to 5/8 the sheet thickness is Vm, an area ratio of the hard phase in a region having a depth from the surface of the steel sheet to the position corresponding to 1/8 the sheet thickness is 0.20 Vm to 0.80 Vm; the metal structure in the region having a depth from the position corresponding to 3/8 the sheet thickness to the position corresponding to 5/8 the sheet thickness contains 75-97% of ferrite and 3-25% of hard phase in area %; and the standard deviation of the area ratio of the hard phase in a direction orthogonal to the rolling direction and the sheet thickness direction in the region having a depth from the position corresponding to 3/8 the sheet thickness to the position corresponding to 5/8 the sheet thickness is 0.85% or less.

Description

FIELD
• The present invention relates to a steel sheet.
BACKGROUND
• In the automobile industry, lighter weight of car bodies has been sought from the viewpoint of improving fuel efficiency. To both lighten the weight of car bodies and secure safety in collisions, making the steel sheet used higher in strength is one effective method. Due to such a backdrop, a high strength steel sheet is being developed.
• In relation to this, PTL 1 discloses a high strength hot dip galvanized steel sheet having a hot dip galvanized layer on the surface of the steel sheet used as the base sheet, in which hot dip galvanized steel sheet, the base sheet contains, by mass%, C: 0.02 to 0.20%, Si: 0.7% or less, Mn: 1.5 to 3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1 to 1.0%, N: 0.010% or less, and Cr: 0.03 to 0.5%, has an annealing surface oxidation index A, defined by the numerical formula A=400Al/(4Cr+3Si+6Mn) having the contents of Al, Cr, Si, and Mn as the corresponding terms, of 2.3 or more, having a balance of Fe and unavoidable impurities, furthermore, the structures of the base sheet are comprised of ferrite and second phases, and the second phases are mainly martensite. Furthermore, PTL 1 describes that the above high strength hot dip galvanized steel sheet has an excellent surface quality and a 590 MPa or more tensile strength suitable for mainly applications as structural members, rockers, and other structural parts of automobiles.
[CITATION LIST] [PATENT LITERATURE]
• [PTL 1] Japanese Unexamined Patent Publication No. 2005-220430
SUMMARY [TECHNICAL PROBLEM]
• In recent years, in the face of demands for further improvement of fuel efficiency, the need has been rising for lighter weight not only in structural members and other structural parts described in PTL 1, but also roofs, hoods, fenders, doors, and other external sheet parts. These external sheet parts, unlike the above such structural parts, are visible to the eye, therefore not only the strength and other properties, but also the beauty and surface quality are important. Therefore, excellent appearance after shaping is sought. On the other hand, in relation to such demands for lighter weight, further higher strength and greater thinness are being sought for the steel sheet used for these external sheet parts. In addition, along with the increasing complexity of shapes of these external sheet parts, the surface of a steel sheet after shaping has tended to more easily be formed with irregularities. If such irregularities are formed, the quality of appearance falls.
• Specifically, for example, in the case of DP steel (dual phase steel) comprised of soft ferrite phases (first phases) and hard second phases mainly comprised of martensite such as described in PTL 1, the yield strength (below, sometimes referred to as "YS") is low and the workability at the time of press forming and other shaping is excellent, but uneven deformation, where the soft phases comprised of ferrite and their surroundings deform preferentially at the time of working, easily occurs. For such a reason, if using such DP steel for external sheet parts, fine irregularities are formed at the surface of the steel sheet after shaping, whereby defects in appearance called "ghost lines" and defects in appearance called "stretcher strain" sometimes occurred.
• Further, even if optimizing the various conditions at the time of production of a steel sheet so as to suppress these defects in appearance, depending on the conditions at the time of production of steel sheet, the characteristic low YS of DP steel could not be obtained and the workability at the time of shaping was liable to fall.
• Therefore, the present invention has as an object thereof the use of a novel constitution to provide a high strength steel sheet excellent in appearance after shaping and workability at the time of shaping.
[SOLUTION TO PROBLEM]
• The present invention includes the following aspects:
(Aspect 1)
• A steel sheet,
  in which
• a chemical composition of the steel sheet contains, by mass%,
  • C: 0.030 to 0.100%,
  • Mn: 1.00 to 2.50%,
  • Si: 0.005 to 1.500%,
  • Al: 0.005 to 0.700%,
  • P: 0.100% or less,
  • S: 0.0200% or less,
  • N: 0.0150% or less,
  • O: 0.0100% or less,
  • Cr: 0 to 0.80%,
  • Mo: 0 to 0.50%,
  • B: 0 to 0.0100%,
  • Ti: 0 to 0.100%,
  • Nb: 0 to 0.100%,
  • V: 0 to 0.50%,
  • Ni: 0 to 1.00%,
  • Cu: 0 to 1.00%,
  • W: 0 to 1.00%,
  • Sn: 0 to 1.00%,
  • Sb: 0 to 0.200%,
  • As: 0 to 0.200%,
  • Ca: 0 to 0.0100%,
  • Mg: 0 to 0.0100%,
  • Zr: 0 to 0.0100%,
  • REM: 0 to 0.0100%, and
  • balance: Fe and impurities,
• an area ratio of a hard phase at a region having a depth from a surface of the steel sheet to a 1/8 depth position of sheet thickness is 0.20 Vm to 0.80 Vm when an area ratio of a hard phase at a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is Vm, and
• a microstructure of the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness contains, by area%, ferrite: 75 to 97% and the hard phase: 3 to 25% and a standard deviation of the area ratio of the hard phase in a direction perpendicular to a rolling direction and sheet thickness direction at the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is 0.85% or less.
(Aspect 2)
• The steel sheet according to the above aspect 1, wherein the chemical composition contains one or more elements selected from the group comprising, by mass%:
• Cr: 0.01 to 0.80%,
• Mo: 0.01 to 0.50%,
• B: 0.0001 to 0.0100%,
• Ti: 0.001 to 0.100%,
• Nb: 0.001 to 0.100%,
• V: 0.01 to 0.50%,
• Ni: 0.01 to 1.00%,
• Cu: 0.01 to 1.00%,
• W: 0.01 to 1.00%,
• Sn: 0.001 to 1.00%,
• Sb: 0.001 to 0.200%,
• As: 0.001 to 0.200%,
• Ca: 0.0001 to 0.0100%,
• Mg: 0.0001 to 0.0100%,
• Zr: 0.0001 to 0.0100%, and
• REM: 0.0001 to 0.0100%.
(Aspect 3)
• The steel sheet according to the above aspect 1 or the above aspect 2, wherein an index A expressed by the following formula (1) is 0.45% or more: $$\mathrm{A}=\left[\mathrm{Si}\right]+10\left[\mathrm{P}\right]+0.6\left[\mathrm{Al}\right]+8\left[\mathrm{Ti}\right]+9\left[\mathrm{Nb}\right]$$ where, [Si], [P], [Al], [Ti], and [Nb] are contents of elements in units of mass%, 0% when the corresponding elements are not contained.
(Aspect 4)
• The steel sheet according to any one of the above aspect 1 to the above aspect 3, wherein the following formula (2) is satisfied: $$\left(\mathrm{TS}-180000/\mathrm{TS}\right)/\mathrm{Vm}\ge 35$$ where, TS indicates a tensile strength in units of MPa and Vm indicates an area ratio of a hard phase at the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness.
(Aspect 5)
• The steel sheet according to any one of the above aspect 1 to the above aspect 4, wherein
• a mean grain size of the ferrite in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is 5.0 to 30.0 µm and
• a mean grain size of the hard phase in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is 1.0 to 5.0 µm.
(Aspect 6)
• The steel sheet according to any one of the above aspect 1 to the above aspect 5, wherein the hard phase in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is comprised of a least one of martensite, bainite, tempered martensite, and pearlite.
(Aspect 7)
• An outer panel member including the steel sheet according to any one of the above aspect 1 to the above aspect 6.
[ADVANTAGEOUS EFFECTS OF INVENTION]
• According to the present invention, it is possible to provide a high strength steel sheet excellent in appearance after shaping and workability at the time of shaping.
DESCRIPTION OF EMBODIMENTS
• Below, preferred embodiments of the steel sheet of the present invention will be explained in detail. Note that in this Description, the various numerical ranges will mean the ranges including the upper and lower limit values unless otherwise indicated.
• In roofs, doors, and other outer panel members, i.e., outer panel parts, DP steel with a relatively low yield strength (YS) is often used from the viewpoint of avoiding surface defects called surface strain occurring at the time of press forming etc. However, DP steel, as explained above, easily suffers from uneven deformation where the soft phases and their surroundings preferentially deform at the time of press forming and other working. For this reason, when using such DP steel for external sheet parts, fine irregularities occur at the surface of the steel sheet after shaping whereby sometimes defects in appearance called "ghost lines" occur. Explaining the occurrence of such ghost lines in more detail, first, at the time of press forming and other working, the steel sheet deforms so that soft phase parts comprised of ferrite become recessed while hard phase parts mainly comprised of martensite etc. do not become recessed or rather rise up in projecting shapes. Due to this, fine irregularities are formed at the surface of the steel sheet after shaping. The fine irregularities are formed by projecting parts extending generally along the rolling direction and recessed parts extending generally along the rolling direction so as to be arranged in the sheet width direction perpendicular to the rolling direction. Further, when polishing the surface of the steel sheet after shaping, the projecting parts of the fine irregularities of the surface of the steel sheet are shaved down, whereby stripe pattern ghost lines extending in the rolling direction of the steel sheet appear. Note that, the rolling direction can be easily identified based on the stretched direction of crystal grains of the steel sheet. Further, in this Description, the "direction perpendicular to the rolling direction and sheet thickness direction" means a direction vertical to each of the rolling direction and sheet thickness direction.
• The inventors first studied in detail how to suppress such ghost lines by focusing on the form of the hard phase at the microstructure of the steel sheet. As a result, the inventors found out that in a steel sheet like DP steel where a soft phase and a hard phase are mixed, the hard phase connected in a stripe (stripe-shaped hard phase) is present in the microstructure causing the ghost lines to remarkably form. Furthermore, the inventors discovered that by suppressing the formation of such a stripe-shaped hard phase to make the hard phase in the microstructure disperse more evenly, it is possible to sufficiently maintain the high strength due to the hard phase while suppressing formation of fine irregularities at the surface of the steel sheet after shaping and as a result suppressing the formation of ghost lines.
• Explained in more detail, the inventors discovered that to suppress the formation of the stripe-shaped hard phase, it is effective to decrease the Mn segregation at the time of solidification in the slab casting step of solidifying the molten steel to cast a slab. Therefore, the inventors further studied in detail a technique for reducing Mn segregation from the two viewpoints of center segregation and micro segregation.
• The inventors first thought that to reduce the center segregation of Mn, it would be effective to suppress the flow of molten steel at the time of slab casting and engaged in various studies on this. Explained in more detail, at the time of solidification, molten steel starts solidifying from the surface. Finally, the center part solidifies. At this time, the solid phases of the molten steel are discharged from the liquid phase, therefore at this stage, the Mn in the liquid phase becomes more concentrated. If the molten steel flows at the time of solidification, such concentrated Mn parts finally easily collect at the center part in the process of solidification. As a result, the center segregation of Mn becomes remarkable. Therefore, the inventors discovered that when producing a steel sheet, it is possible to suitably control the conditions at the time of solidification and suppress such flow of the molten steel to thereby remarkably suppress the center segregation of Mn.
• Next, the inventors thought that to reduce micro segregation of Mn, it would be effective to promote the diffusion of Mn at the time of solidification and engaged in various studies on this. To promote the diffusion of Mn, it is effective to build in structures in which Mn can easily diffuse. Therefore, the inventors focused on the δ phases with fast speeds of diffusion of Mn and engaged in experiments to investigate the degree of effect on micro segregation of Mn at the different elements in the steel so as to make the mode of solidification the δ solidification. As a result, the inventors discovered that if the contents of C and Mn become higher, at the time of solidification, δ solidification does not occur, therefore the speed of diffusion of Mn falls and micro segregation increases, while if the contents of Si, Al, Cr, and Mo become higher, diffusion of Mn at the time of solidification is promoted and micro segregation can be reduced.
• Using the above such technique of reducing Mn segregation, ghost lines can be suppressed to a certain extent, but to more sufficiently obtain the effect of suppression of ghost lines, the inventors further studied the technique for suppression of ghost lines in addition to the above such technique of reducing Mn segregation. As a result, the inventors obtained the finding that by reducing the hard phase fraction of a steel sheet, it is possible to reduce ghost lines even if center segregation of Mn remains to a certain extent.
• Further, the inventors obtained the finding that formation of ghost lines is also greatly affected by the solidified structures formed, even if the center segregation of Mn is small, if coarse equiaxed crystals are formed in the solidified structures, negative segregation of Mn occurs, the variation in hard phase fraction in a direction perpendicular to the rolling direction and sheet thickness direction becomes greater, and ghost lines become easily formed. Furthermore, the inventors obtained the finding that a technique different from the conventional measure against center segregation, i.e., the technique of reducing the equiaxed crystal fraction and controlling the solidified structures to columnar crystal structure, can be used to suppress negative segregation of Mn and suppress ghost lines.
• Based on these findings, the inventors discovered that the technique of controlling the solidified structures at the time of casting to columnar crystal structures and reducing the center segregation of Mn at the time of solidification, one of the causes for formation of the stripe-shaped hard phase, reducing the hard phase fraction, and reducing the variation in the same can be used to sufficiently maintain the high strength while remarkably suppressing ghost lines.
• While the above such technique can be used to sufficiently improve ghost lines, it was learned that defects in appearance called "stretcher strain" still can occur. Furthermore, it was learned that even if controlling the various conditions at the time of production of steel sheet to within specific ranges as explained above so as to suppress these defects in appearance, depending on the conditions, the low YS characteristic of DP steel cannot be obtained and the workability at the time of shaping can fall.
• Therefore, the inventors engaged in intensive studies on a technique for achieving both suppression of defects in appearance after shaping, including not only ghost lines, but also stretcher strain, and workability at the time of shaping. As a result, the inventors discovered that by controlling the area ratio of the hard phase at a region having a depth from the surface of the steel sheet to the 1/8 depth position of sheet thickness , i.e., the hard phase fraction at the surface layer of the steel sheet, and the area ratio of the hard phase and the variation thereof at a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness, i.e., the hard phase fraction and the variation thereof at the sheet thickness center part of the steel sheet, to within specific ranges, a high strength steel sheet excellent in both the appearance after shaping and the workability at the time of shaping can be obtained.
• The present invention was completed based on the above such findings and includes the following aspects of the embodiments:
• Below, preferred embodiments of the steel sheet of the present invention will be explained in detail.
<Steel Sheet>
• The steel sheet according to one embodiment of the present invention has a chemical composition containing, by mass%,
• C: 0.030 to 0.100%,
• Mn: 1.00 to 2.50%,
• Si: 0.005 to 1.500%,
• Al: 0.005 to 0.700%,
• P: 0.100% or less,
• S: 0.0200% or less,
• N: 0.0150% or less,
• O: 0.0100% or less,
• Cr: 0 to 0.80%,
• Mo: 0 to 0.50%,
• B: 0 to 0.0100%,
• Ti: 0 to 0.100%,
• Nb: 0 to 0.100%,
• V: 0 to 0.50%,
• Ni: 0 to 1.00%,
• Cu: 0 to 1.00%,
• W: 0 to 1.00%,
• Sn: 0 to 1.00%,
• Sb: 0 to 0.200%,
• As: 0 to 0.200%,
• Ca: 0 to 0.0100%,
• Mg: 0 to 0.0100%,
• Zr: 0 to 0.0100%,
• REM: 0 to 0.0100%, and
• balance: Fe and impurities.
• Further, the steel sheet of the present embodiment has an area ratio of a hard phase at a region having a depth from a surface of the steel sheet to a 1/8 depth position of sheet thickness of 0.20 Vm to 0.80 Vm when an area ratio of a hard phase at a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is Vm.
• Furthermore, the steel sheet of the present embodiment has a microstructure of the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness containing, by area%, ferrite: 75 to 97% and the hard phase: 3 to 25% and a standard deviation of the area ratio of the hard phase in a direction perpendicular to a rolling direction and sheet thickness direction at the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness of 0.85% or less.
• The steel sheet of the present embodiment, as stated above, has a specific chemical composition and has a specific microstructure where the microstructure at a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness (i.e., sheet thickness center part) has a lower hard phase fraction than conventional DP steel and where the standard deviation of the hard phase fraction (i.e., the variation in hard phase fraction) at the sheet thickness center part in a direction perpendicular to the rolling direction and sheet thickness direction is small. Such a specific microstructure, as explained later, can be obtained by employing a specific chemical composition and casting conditions.
• As explained above, the steel sheet of the present embodiment has such a unique microstructure, i.e., a microstructure with a small center segregation of Mn at the time of solidification and on top of this a small hard phase fraction and variation of the same, therefore it is possible to sufficiently maintain a high strength while suppressing formation of fine irregularities at the steel sheet surface after shaping. Due to this, the steel sheet of the present embodiment enables a high strength to be sufficiently maintained while remarkably suppressing formation of ghost lines and stretcher strain.
• Furthermore, the steel sheet of the present embodiment has an area ratio of a hard phase at a region having a depth from the surface of the steel sheet to the 1/8 depth position of sheet thickness, i.e., the hard phase fraction of the surface layer, of 0.20 Vm to 0.80 Vm with respect to the area ratio of the hard phase Vm of the sheet thickness center part. If the hard phase fraction of the surface layer is 0.20 Vm or more, it is possible to secure a low yield strength (YS) and high tensile strength (TS), therefore it is possible to obtain a high strength steel sheet excellent in workability at the time of shaping. Furthermore, due to this, the stretcher strain formed at the time of shaping of a steel sheet also becomes much easier to suppress. Further, if the hard phase fraction of the surface layer of the steel sheet is 0.80 Vm or less, an excellent bendability can be secured, therefore it is possible to obtain a steel sheet excellent in workability at the time of shaping.
• Note that, in order to control the hard phase fraction of the surface layer of the steel sheet to 0.20 Vm to 0.80 Vm in range, it is necessary to employ the respective conditions explained below in the hot rolling step, pickling step, and annealing step at the time of production of the steel sheet.
• Here, the hard phase fraction of the surface layer of the steel sheet being "0.20 Vm" means 0.20 time (0.20×Vm) the hard phase fraction Vm of the sheet thickness center part. Further, the same is also true for 0.80 Vm etc. Note that, in this Description, the area ratio of the hard phase (%) and the hard phase fraction (%) are synonymous.
• In the above way, the steel sheet of the present embodiment becomes a high strength steel sheet achieving both suppression of defects in appearance after shaping including not only ghost lines, but also stretcher strain and workability at the time of shaping, i.e., a high strength steel sheet excellent in both the appearance after shaping and workability at the time of shaping.
• Below, the steel sheet of the present invention will be explained in more detail. Note that, in the following explanation, the "%" of the units of contents of the elements and index A mean "mass%" unless otherwise indicated. Further, in this Description, the "to" showing a numerical range, unless otherwise indicated, is used in a sense including the numerical values shown before and after it as a lower limit value and upper limit value.
• Further, in this Description, the "x/y depth position of sheet thickness (where, "x" and "y" are natural numbers satisfying x<y)" means a position moved from the surface of the steel sheet in the sheet thickness direction to the sheet thickness direction by exactly a distance (depth) of x/y of the sheet thickness toward the center part of the steel sheet. For example, if the sheet thickness of the steel sheet is 2 mm, the "1/8 depth position of sheet thickness" means a position from the surface of the steel sheet in the sheet thickness direction to a depth of 0.25 mm. Note that, when the steel sheet has a plating layer at the surface, the "surface of the steel sheet" in the definition of a "x/8 depth position from the surface of the steel sheet" means the interface of the steel sheet and plating layer, while the "sheet thickness" means the sheet thickness of the steel sheet (base material) minus the plating layer.
(Chemical Composition)
• The steel sheet of the present embodiment, as stated above, contains
• C: 0.030 to 0.100%,
• Mn: 1.00 to 2.50%,
• Si: 0.005 to 1.500%,
• Al: 0.005 to 0.700%,
• P: 0.100% or less,
• S: 0.0200% or less,
• N: 0.0150% or less,
• O: 0.0100% or less,
• Cr: 0 to 0.80%,
• Mo: 0 to 0.50%,
• B: 0 to 0.0100%,
• Ti: 0 to 0.100%,
• Nb: 0 to 0.100%,
• V: 0 to 0.50%,
• Ni: 0 to 1.00%,
• Cu: 0 to 1.00%,
• W: 0 to 1.00%,
• Sn: 0 to 1.00%,
• Sb: 0 to 0.200%,
• As: 0 to 0.200%,
• Ca: 0 to 0.0100%,
• Mg: 0 to 0.0100%,
• Zr: 0 to 0.0100%,
• REM: 0 to 0.0100%, and
• balance: Fe and impurities.
• Below, the elements in the chemical composition will be explained in detail:
[C: 0.030 to 0.100%]
• C is an element raising the strength of the steel sheet. To sufficiently obtain such an effect, the C content is 0.030% or more. The C content may also be 0.032% or more, 0.034% or more, or 0.035% or more. On the other hand, if excessively containing C, diffusion of Mn at the time of solidification is obstructed and micro segregation of Mn sometimes cannot be sufficiently suppressed. Therefore, the C content is 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, or 0.080% or less.
[Mn: 1.00 to 2.50%]
• Mn is an element raising the quenchability of the steel and contributing to improvement of the strength. To sufficiently obtain such an effect, the Mn content is 1.00% or more. The Mn content may also be 1.02% or more, 1.04% or more, or 1.05% or more. On the other hand, if excessively containing Mn, diffusion of Mn at the time of solidification is obstructed and micro segregation of Mn sometimes cannot be sufficiently suppressed. Therefore, the Mn content is 2.50% or less. The Mn content may also be 2.40% or less, 2.30% or less, or 2.20% or less.
[Si: 0.005 to 1.500%]
• Si is a deoxidizing element of steel and a solution strengthening element effective for raising the strength without detracting from the ductility of the steel sheet. Further, Si is an element effective for promoting diffusion of Mn at the time of solidification to reduce the micro segregation of Mn. To sufficiently obtain these effects, the Si content is 0.005% or more. The Si content may also be 0.008% or more, 0.010% or more, or 0.012% or more. On the other hand, if excessively containing Si, the peelability of the scale falls and sometimes surface defects are formed. Therefore, the Si content is 1.500% or less. The Si content may also be 1.200% or less, 1.000% or less, or 0.800% or less.
[Al: 0.005 to 0.700%]
• Al is an element functioning as a deoxidizer and a solution strengthening element effective for raising the strength of steel. Further, Al is also an element effective for promoting the diffusion of Mn at the time of solidification and reducing micro segregation of Mn. To sufficiently obtain these effects, the Al content is 0.005% or more. The Al content may also be 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if excessively containing Al, the castability deteriorates and sometimes the productivity falls. Therefore, the Al content is 0.700% or less. The Al content may also be 0.600% or less, 0.500% or less, 0.400% or less, or 0.300% or less.
[P: 0.100% or Less]
• P is an element entering in the production process. Further, P is also a solution strengthening element. The P content may also be 0%. However, reducing the P content to less than 0.0001% requires time for refining and invites a drop in productivity. Therefore, the P content may also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if excessively containing P, the toughness of the steel sheet sometimes falls. Therefore, the P content is 0.100% or less. The P content may also be 0.080% or less, 0.075% or less, or 0.070% or less.
[S: 0.0200% or Less]
• S is an element entering in the production process. The S content may also be 0%. However, reducing the S content to less than 0.0001% requires time for refining and invites a drop in productivity. Therefore, the S content may also be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, if excessively containing S, Mn sulfides are formed and sometimes the steel sheet is made to fall in ductility, hole expandability, stretch flangeability, and/or bendability and other shapeability. Therefore, the S content is 0.0200% or less. The S content may also be 0.0150% or less, 0.0100% or less, or 0.0080% or less.
[N: 0.0150% or Less]
• N is an element entering in the production process. The N content may also be 0%. However, reducing the N content to less than 0.0001% requires time for refining and invites a drop in productivity. Therefore, the N content may also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if excessively containing N, nitrides are formed and sometimes the steel sheet is made to fall in ductility, hole expandability, stretch flangeability, and/or bendability and other shapeability. Therefore, the N content is 0.0150% or less. The N content may also be 0.0100% or less, 0.0080% or less, or 0.0050% or less.
[O: 0.0100% or Less]
• O is an element entering in the production process. The O content may also be 0%. However, reducing the O content to less than 0.0001% requires time for refining and invites a drop in productivity. Therefore, the O content may also be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, if excessively containing O, coarse oxides are formed and sometimes the steel sheet is made to fall in ductility, hole expandability, stretch flangeability, and/or bendability and other shapeability. Therefore, the O content is 0.0100% or less. The O content may also be 0.0070% or less, 0.0050% or less, or 0.0030% or less.
• The basic chemical composition of the steel sheet of the present embodiment is as explained above. Furthermore, in the present embodiment, the steel sheet may also contain one or more of the following optional elements in place of part of the balance of Fe in accordance with need. Below, these optional elements will be explained in detail.
[Cr: 0 to 0.80%]
• Cr is an element raising the quenchability of steel and contributing to improvement of the strength of the steel sheet. Further, Cr is an element effective for promoting diffusion of Mn at the time of solidification and reducing micro segregation of Mn. The Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.01% or more. The Cr content may also be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if excessively containing Cr, sometimes coarse Cr oxides becoming starting points of fracture are formed. Therefore, the Cr content is preferably 0.80% or less. The Cr content may also be 0.75% or less, 0.70% or less, or 0.65% or less.
[Mo: 0 to 0.50%]
• Mo is an element suppressing phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. Further, Mo is an element effective for promoting diffusion of Mn at the time of solidification and reducing micro segregation of Mn. The Mo content may also be 0%, but to obtain these effects, the Mo content is preferably 0.01% or more. The Mo content may also be 0.05% or more or 0.07% or more. On the other hand, if excessively containing Mo, the hot workability falls and sometimes the productivity is lowered. Therefore, the Mo content is preferably 0.50% or less. The Mo content may also be 0.45% or less, 0.40% or less, or 0.35% or less.
[B: 0 to 0.0100%]
• B is an element suppressing phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. The B content may also be 0%, but to obtain such an effect, the B content is preferably 0.0001% or more. The B content may also be 0.0005% or more, 0.0008% or more, or 0.0010% or more. On the other hand, if excessively containing B, B precipitates are formed and sometimes the strength of the steel sheet falls. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
[Ti: 0 to 0.100%]
• Ti has an element having an effect of reducing the amounts of S, N, and O forming coarse inclusions acting as starting points of fracture. Further, Ti is also a precipitation strengthening element having the effect of refining structures and raising the balance of the steel sheet strength and shapeability. The Ti content may also be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may also be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, if excessively containing Ti, coarse Ti sulfides, Ti nitrides, and/or Ti oxides are formed and sometimes the shapeability of the steel sheet falls. Therefore, the Ti content is preferably 0.100% or less. The Ti content may also be 0.080% or less, 0.075% or less, or 0.070% or less.
[Nb: 0 to 0.100%]
• Nb is a precipitation strengthening element contributing to improvement of the strength of the steel sheet due to strengthening by precipitates, refinement strengthening by suppressing growth of ferrite grains, and/or dislocation strengthening by suppression of recrystallization. The Nb content may also be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may also be 0.003% or more, 0.004% or more, or 0.005% or more. On the other hand, if excessively containing Nb, nonrecrystallized ferrite increases and sometimes the shapeability of the steel sheet falls. Therefore, the Nb content is preferably 0.100% or less. The Nb content may also be 0.080% or less, 0.070% or less, or 0.060% or less.
[V: 0 to 0.50%]
• V is an element contributing to improvement of the strength of the steel sheet due to strengthening by precipitates, refinement strengthening by suppressing growth of ferrite grains, and/or dislocation strengthening by suppression of recrystallization. The V content may also be 0%, but to obtain these effects, the V content is preferably 0.01% or more. The V content may also be 0.02% or more. On the other hand, if excessively containing V, nitrides precipitate in large amounts and sometimes the shapeability of the steel sheet falls. Therefore, the V content is preferably 0.50% or less. The V content may also be 0.40% or less, 0.30% or less, or 0.20% or less.
[Ni: 0 to 1.00%]
• Ni is an element suppressing phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. The Ni content may also be 0%, but to obtain such an effect, the Ni content is preferably 0.01% or more. The Ni content may also be 0.03% or more or 0.05% or more. On the other hand, if excessively containing Ni, sometimes the weldability of the steel sheet falls. Therefore, the Ni content is preferably 1.00% or less. The Ni content may also be 0.60% or less, 0.50% or less, or 0.40% or less.
[Cu: 0 to 1.00%]
• Cu is an element present in the steel in the form of fine particles and contributing to improvement of the strength of the steel sheet. The Cu content may also be 0%, but to obtain such an effect, the Cu content is preferably 0.01% or more. The Cu content may also be 0.03% or more or 0.05% or more. On the other hand, if excessively containing Cu, sometimes the weldability of the steel sheet falls. Therefore, the Cu content is preferably 1.00% or less. The Cu content may also be 0.80% or less, 0.60% or less, or 0.40% or less.
[W: 0 to 1.00%]
• W is an element suppressing phase transformation at a high temperature and contributing to improvement of the strength of the steel sheet. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.01% or more. The W content may also be 0.02% or more. On the other hand, if excessively containing W, sometimes the hot workability falls and the productivity falls. Therefore, the W content is preferably 1.00% or less. The W content may also be 0.80% or less, 0.60% or less, or 0.40% or less.
[Sn: 0 to 1.00%]
• Sn is an element suppressing coarsening of the grains and contributing to improvement of the strength of the steel sheet. The Sn content may also be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more. The Sn content may also be 0.004% or more. On the other hand, if excessively containing Sn, sometimes embrittlement of the steel sheet is caused. Therefore, the Sn content is preferably 1.00% or less. The Sn content may also be 0.80% or less, 0.60% or less, or 0.40% or less.
[Sb: 0 to 0.200%]
• Sb is an element suppressing coarsening of the grains and contributing to improvement of the strength of the steel sheet. The Sb content may also be 0%, but to obtain such an effect, the Sb content is preferably 0.001% or more. The Sb content may also be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, if excessively containing Sb, sometimes embrittlement of the steel sheet is caused. Therefore, the Sb content is preferably 0.200% or less. The Sb content may also be 0.180% or less, 0.170% or less, or 0.160% or less.
[As: 0 to 0.200%]
• As is an element effective for improvement of the corrosion resistance in an acidic environment although not having as remarkable an effect as Sb and Sn. The As content may also be 0%, but to obtain such an effect, the As content is preferably 0.001% or more. The As content may also be 0.002% or more, 0.004% or more, or 0.006% or more. On the other hand, if excessively containing As, sometimes the hot workability falls. Therefore, the As content is preferably 0.200% or less. The As content may also be 0.180% or less, 0.170% or less, or 0.160% or less.
[Ca: 0 to 0.0100%], [Mg: 0 to 0.0100%], [Zr: 0 to 0.0100%], and [REM: 0 to 0.0100%]
• Ca, Mg, Zr, and a REM are elements contributing to improvement of the shapeability of a steel sheet. The Ca, Mg, Zr, and REM contents may also be 0%, but to obtain such an effect, the Ca, Mg, Zr, and REM contents are preferably respectively 0.0001% or more. The Ca, Mg, Zr, and REM contents may also be respectively 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if excessively containing these elements, sometimes the ductility of the steel sheet falls. Therefore, the Ca, Mg, Zr, and REM contents are preferably respectively 0.0100% or less. The Ca, Mg, Zr, and REM contents may also respectively be 0.0080% or less, 0.0070% or less, or 0.0060% or less.
• Note that, in this Description, "REM" is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.
• Regarding the above optional elements, in the present embodiment, the chemical composition of the steel sheet preferably contains one or more elements selected from the group comprising
• Cr: 0.01 to 0.80%,
• Mo: 0.01 to 0.50%,
• B: 0.0001 to 0.0100%,
• Ti: 0.001 to 0.100%,
• Nb: 0.001 to 0.100%,
• V: 0.01 to 0.50%,
• Ni: 0.01 to 1.00%,
• Cu: 0.01 to 1.00%,
• W: 0.01 to 1.00%,
• Sn: 0.001 to 1.00%,
• Sb: 0.001 to 0.200%,
• As: 0.001 to 0.200%,
• Ca: 0.0001 to 0.0100%,
• Mg: 0.0001 to 0.0100%,
• Zr: 0.0001 to 0.0100%, and
• REM: 0.0001 to 0.0100%.
• If a steel sheet contains such optional elements, it is possible to more reliably maintain a high strength while remarkably suppressing the formation of ghost lines and other defects in appearance after shaping and further to provide a steel sheet excellent in workability at the time of shaping.
• In the steel sheet in the present embodiment, the balance besides the constituents explained above is comprised of Fe and impurities. Here, the "impurities" are constituents etc. entering due to various factors in the production process such as ore, scrap, and other starting materials when industrially producing the steel sheet. As impurities, for example, H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po may be mentioned. The impurities may be contained in a total of 0.100% or less.
[Index A: 0.45% or More]
• In the present embodiment, the chemical composition of the steel sheet is preferably one where the index A expressed by the following formula (1) is 0.45% or more: $$\mathrm{A}=\left[\mathrm{Si}\right]+10\left[\mathrm{P}\right]+0.6\left[\mathrm{Al}\right]+8\left[\mathrm{Ti}\right]+9\left[\mathrm{Nb}\right]$$ where, [Si], [P], [Al], [Ti], and [Nb] are contents of elements in units of mass%, 0% when the corresponding elements are not contained.
• The index A is an index determined by the contents of solution strengthening elements of Si, P, and Al and the precipitation strengthening elements of Ti and Nb. The larger the value, the higher the strength that can be obtained by a smaller hard phase fraction. By making the index A 0.45% or more, it is possible to control the hard phase fraction of the steel sheet to a certain level or less while obtaining a high strength.
• The index A may also be 0.48% or more, 0.50% or more, or 0.52% or more. Note that, the upper limit of the index A is not particularly prescribed, but, for example, the index A may also be 1.50% or less, 1.20% or less, or 1.00% or less.
• The chemical composition of the steel sheet according to an embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, a 35 mm square test piece can be taken from near the 1/2 depth position of the thickness of the steel sheet and an ICPS-8100 made by Shimadzu or other measurement apparatus can be used to measure the piece under conditions based on a calibration curve prepared in advance to thereby identify the chemical composition of the steel sheet. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.
(Microstructure) [Area Ratio of Hard Phase at Region Having Depth From Surface of Steel Sheet to 1/8 Depth Position of Sheet Thickness: 0.20 Vm to 0.80 Vm]
• In the present embodiment, in the microstructure of the steel sheet, the area ratio of the hard phase at the region having a depth from the surface of the steel sheet to the 1/8 depth position of sheet thickness (i.e., the hard phase fraction of the surface layer) is 0.20 Vm to 0.80 Vm when defining the area ratio of hard phase at the region having a depth from the 3/8 depth position of sheet thickness to the 5/8 depth position (i.e., the hard phase fraction of the sheet thickness center part) as Vm. If the hard phase fraction of the surface layer of the steel sheet is 0.20 Vm or more, it is possible to secure a low yield strength (YS) and a high tensile strength (TS), therefore, it is possible to obtain a high strength steel sheet excellent in workability at the time of shaping. Furthermore, due to this, it is possible to easily suppress stretcher strain generated at the time of shaping the steel sheet. Further, if the hard phase fraction of the surface layer of the steel sheet is 0.8 Vm or less, it is possible to secure a good bendability, therefore it is possible to obtain a steel sheet excellent in workability at the time of shaping. From the viewpoint of further raising these effects, the area ratio of hard phase of the surface layer may be 0.30 Vm or more, 0.35 Vm or more, 0.40 Vm or more, or 0.45 Vm or more. On the other hand, from the viewpoint of further raising the workability at the time of shaping, the area ratio of hard phase of the surface layer may also be 0.79 Vm or less, 0.78 Vm or less, 0.77 Vm or less, or 0.76 Vm or less.
[Microstructure at Region Having Depth From 3/8 Depth Position to 5/8 Depth Position of Sheet Thickness: Ferrite: 75 To 97% and Hard Phase: 3 to 25%]
• In the present embodiment, the microstructure at the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness of the steel sheet (i.e., sheet thickness center part) contains, by area%, 75 to 97% of ferrite and hard phase: 3 to 25%. By making the microstructure of the sheet thickness center part of the steel sheet such a specific composite structure, it is possible to maintain the strength of the steel sheet in a suitable range, more specifically achieve a 500 MPa or more tensile strength, while suppressing defects in appearance after shaping. From the viewpoint of better raising the strength of the steel sheet, the area fraction of the hard phase at the sheet thickness center part may also be 4% or more, 5% or more, 6% or more, 7% or more, or 8% or more. Similarly, the area fraction of the ferrite at the sheet thickness center part may also be 96% or less, 95% or less, or 94% or less. On the other hand, from the viewpoint of better improving the appearance after shaping, the area fraction of the hard phase at the sheet thickness center part may also be 22% or less, 20% or less, or 18% or less. Similarly, the area fraction of the ferrite at the sheet thickness center part may also be 78% or more, 80% or more, 82% or more, 85% or more, 90% or more, or 92% or more.
• In the steel sheet of the present embodiment, "hard phase" mean a structure harder than ferrite. For example, it is comprised of at least one of martensite, bainite, tempered martensite, and pearlite. From the viewpoint of raising the strength of the steel sheet, the hard phase in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is preferably comprised of at least one of martensite, bainite, and tempered martensite, more preferably is comprised of martensite. The microstructure of the steel sheet preferably has little retained austenite. Specifically, the retained austenite is preferably, by area%, less than 1% or less than 0.5%, more preferably 0%.
(Identification of Microstructure and Calculation of Area Fraction)
• The microstructure is identified and the area fraction is calculated in the following way. First, the rolling direction of the obtained steel sheet is identified. The rolling direction of the steel sheet can be determined in the following way.
• A test piece is taken so as to enable examination of a cross section parallel to the surface of the steel sheet. The cross section of the test piece taken in the sheet thickness direction is polished to a mirror finish, then is examined using an optical microscope. The examined plane is made a plane parallel with the surface of the steel sheet at any depth position in a range in the sheet thickness direction from a 1/4 depth position to 1/2 depth position of sheet thickness. In the examined plane, the direction parallel to the stretched direction of the grains is judged to be the rolling direction.
• The direction perpendicular to the rolling direction and sheet thickness direction of the steel sheet is made the sheet width direction while the length of the steel sheet in the sheet width direction is made the sheet width W. A sample for examination of the microstructure (micro structures) is taken from the W/4 position or 3W/4 position of the sheet width W (i.e., W/4 position in sheet width direction from either end part of the steel sheet in the sheet width direction). The size of the sample is, for example, 20 mm in the rolling direction × 20 mm in the sheet width direction × thickness of steel sheet.
• Next, an FE-SEM (field emission type scan electron microscope, for example, "JSM-7200F" made by JEOL, acceleration voltage: 15 kV, light source: FE, apparatus resolution: 1.2 nm) is used to examine the microstructure. At this time, the examined field is made a 100 µm×100 µm field corresponding to the examined position in the sheet thickness direction of the steel sheet. In other words, when examining the microstructure of a region having depth from the surface of the steel sheet to a 1/8 depth position of sheet thickness, a 50 µm × 100 µm region in the range from the surface of the steel sheet to the 1/8 depth position of sheet thickness (region of 50 µm in sheet thickness direction and 100 µm in direction perpendicular to sheet thickness direction in examined field) in the sheet thickness direction of the steel sheet is examined. On the other hand, when examining the microstructure of a region having a depth from a 3/8 depth position to a 5/8 depth position of sheet thickness (i.e., sheet thickness center part), a 100 µm×100 µm region in the range from the 3/8 depth position to 5/8 depth position of sheet thickness of the steel sheet is examined. Note that, in the examination of any region, the resolution is made 1280×960 pixels. For preparation of the sample, the sheet thickness cross section in the sheet width direction is polished as the examined plane and etched by Nital corrosion. Next, the "micro structures" are classified from power 500 or 1000X SEM photographs. Ferrite and the hard phase can be differentiated by the difference in brightness.
• The steel sheet etched by Nital corrosion is examined at the above 100 µm×100 µm region by a power of 500X or 1000X. The images are analyzed using the image analysis software Image J (Ver.1.54f) to find the area fraction of the hard phase. Specifically, the secondary electron images of the microstructure are binarized by the differences in brightness. The black parts of the image data are deemed ferrite and the not corroded white parts are deemed hard phase to calculate the area fraction of the hard phase. In each microstructure, three measurement locations are selected. In these measurement locations, image analysis is performed in the same way as the above to measure the area fractions of the hard phase. The area fractions of these are arithmetically averaged to calculate the mean value. This mean value is made the area fraction of the hard phase, while the balance is made the area fraction of the ferrite. Note that, when examining the microstructure of the region having a depth from the surface of the steel sheet to the 1/8 depth position of sheet thickness and when examining the microstructure of the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness of the steel sheet, only the examined fields in the sheet thickness direction of the steel sheet differ. It is similarly possible to measure the area fraction of the hard phase.
• Further, if measurement of the area fraction of the retained austenite is necessary, it is possible to measure the area fraction of the retained austenite by X-ray diffraction of the above examined plane. Specifically, Co-Kα rays are used to find the integrated intensities of a total of six peaks of α(110), α(200), α(211), γ(111), γ(200), γ(220) at the 1/2 depth position of sheet thickness, the intensity averaging method is used to calculate the volume fraction of the retained austenite, and the obtained volume fraction of the retained austenite is made the area fraction of the retained austenite.
[In Region Having Region Having Depth From 3/8 Depth Position to 5/8 Depth Position of Sheet Thickness, Standard Deviation of Area Ratio of Hard Phase in Direction Perpendicular to Rolling Direction and Sheet Thickness Direction: 0.85% or Less]
• In the present embodiment, in the microstructure of the steel sheet, in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness (i.e., sheet thickness center part), the standard deviation of the hard phase fraction in a direction perpendicular to the rolling direction and sheet thickness direction is 0.85% or less. Note that, the "standard deviation of the hard phase fraction" means the standard deviation of the area fraction of the hard phase themselves. As stated above, ghost lines and other defects in appearance after shaping are greatly related to not only Mn segregation, but also the solidified structures. For example, even if the center segregation of Mn is small, if coarse equiaxed crystals are formed in the solidified structures, negative segregation of Mn occurs, the variation in the hard phase fraction in the direction perpendicular to the rolling direction and sheet thickness direction becomes greater, and sometimes the defects in appearance after shaping become worse. However, in the steel sheet of the present embodiment, in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness , the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and sheet thickness direction is 0.85% or less, i.e., the variation in the hard phase fraction in the direction perpendicular to the rolling direction and sheet thickness directions is a certain level or less, therefore it is possible to remarkably suppress defects in appearance after shaping.
• From the viewpoint of more reliably suppressing defects in appearance after shaping, in a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness, the standard deviation of the hard phase fraction in a direction perpendicular to the rolling direction and sheet thickness direction may be 0.80% or less, 0.75% or less, 0.70% or less, or 0.65% or less. Note that, the lower limit of the standard deviation of the hard phase fraction is not particularly prescribed, but, for example, the standard deviation of the hard phase fraction may also be 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.
• Note that, regarding the hard phase fraction, preferably the ratio of the mean value of the area ratio of hard phase and the standard deviation thereof becomes 0.10 or less (i.e., the standard deviation of the hard phase fraction/mean value of area ratio of hard phase≤0.10). The above ratio is preferably 0.09 or less, 0.08 or less, or 0.07 or less. The lower limit of the above ratio is 0, however, if necessary, the lower limit may also be 0.01.
• The standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and sheet thickness direction of the microstructure of the sheet thickness center part can be found as follows: First, a 100 µm×100 µm region parallel to the direction perpendicular to the rolling direction and sheet thickness direction of the steel sheet and between the 3/8 depth position to 5/8 depth position of sheet thickness of the cross section vertical to the surface of the steel sheet is examined by an FE-SEM (field emission type scan electron microscope, for example, "JSM-7200F" made by JEOL, acceleration voltage: 15 kV, light source: FE, apparatus resolution: 1.2 nm) by a power of 500X or 1000X to obtain a secondary electron image. Note that, the resolution is made 1280×960 pixels. This secondary electron image is analyzed using the image analysis software Image J (Ver.1.54f) in the same way as the area fraction of the hard phase in the above-mentioned "Identification of Microstructure and Calculation of Area Fraction" to measure the area fraction of the hard phase at the W/4 position to 3W/4 position of the sheet width W for each 100 µm in the range of 8 mm in the direction perpendicular to the rolling direction and sheet thickness direction (sheet width direction) of the steel sheet and calculate the standard deviation. Note that, the range of examination in the direction perpendicular to the rolling direction and sheet thickness direction (sheet width direction) may be less than 8 mm or may be more than 8 mm. However, the lower limit of the range of examination of the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and sheet thickness direction (sheet width direction) is made 4 mm and the upper limit is made 12 mm.
[Relationship of Tensile Strength and Hard Phase Fraction of Steel Sheet: (TS-180000/TS)/ Vm≥35]
• In the present embodiment, the steel sheet preferably satisfies the following formula (2). $$\left(\mathrm{TS}-180000/\mathrm{TS}\right)/\mathrm{Vm}\ge 35$$
• Here, TS indicates a tensile strength (MPa) of the steel sheet, while Vm indicates an area ratio of a hard phase at a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness (area%).
• If the steel sheet satisfies such a formula (2), even if center segregation of Mn remains to a certain extent, variation in the hard phase fraction in the direction perpendicular to the rolling direction and sheet thickness direction is easily suppressed, therefore as a result it is possible to more easily make defects in appearance after shaping harder to occur.
• The tensile strength (TS) of the steel sheet can be measured by taking from the steel sheet a No. 5 tensile test piece of JIS Z2241: 2011 having a direction perpendicular to the rolling direction and sheet thickness direction as a longitudinal direction and performing a tensile test based on JIS Z2241: 2011. Note that, if it is not possible to obtain the above No. 5 tensile test piece from the steel sheet of the measured sample (for example, if the size of the measured sample is small), instead of a No. 5 tensile test piece, it is possible to employ a tensile test piece of any size having a direction perpendicular to the rolling direction and sheet thickness direction as a longitudinal direction.
[Mean Grain Size of Ferrite at Region Having Depth From 3/8 Depth Position to 5/8 Depth Position of Sheet Thickness: 5.0 to 30.0 µm]
• In the steel sheet of the present embodiment, the mean grain size of the ferrite in the microstructure at a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness (i.e., sheet thickness center part) of the steel sheet is preferably 5.0 to 30.0 µm. If controlling the mean grain size of the ferrite to within such a fine range, it is possible to further improve the appearance after shaping of the steel sheet. The mean grain size of the ferrite may also be 6.0 µm or more or 7.0 µm or more. Similarly, the mean grain size of the ferrite may also be 27.0 µm or less, 21.0 µm or less, 15.0 µm or less, or 11.0 µm or less.
• The mean grain size of the ferrite at the sheet thickness center part is determined in the following way.
• First, the KAM (kernel average misorientation) value of the ferrite is calculated by KAM analysis and GAIQ (grain average image quality) analysis in measurement by an electron probe micro analyzer (EPMA) - a technique for crystal analysis using an SEM.
• As the SEM examination apparatus, a field emission type scan electron microscope (for example, "JSM-7001F" made by JEOL) is used. For example, "OIM Analysis 7" made by TSL is used for EBSD analysis. In EBSD analysis, a 50 µm×50 µm region between the 3/8 depth position to 5/8 depth position of sheet thickness of the steel sheet with a cross section of thickness polished to a mirror finish is analyzed an intervals (pitch) of 0.05 µm. KAM analysis is analysis defining the average value of the misorientation (°) of the measurement point of "a certain one pixel" with all adjoining pixels as the KAM value of that "certain one pixel". It is possible to prepare a KAM map based on the local crystal misorientation. Such KAM analysis is used to analyze the KAM value in the ferrite. A region with ferrite present in the results of EBSD measurement is judged by the following GAIQ analysis.
• The results of EBSD measurement obtained under the same measurement conditions as the KAM value are analyzed for GAIQ (grain average image quality) to obtain a GAIQ value. Based on this GAIQ value, the image is divided into regions with a GAIQ value of 50000 or more and regions with a GAIQ value of less than 50000. A region (grain) with a GAIQ value of 50000 or more is judged as a region with ferrite present. On the other hand, a region (grain) with a GAIQ value of less than 50000 is judged as a region with a hard phase present. Note that, GAIQ analysis is analysis defining the average value in one pixel of the IQ value showing the clarity of the Kikuchi pattern of a measurement point of "a certain one pixel" with all adjoining pixels as the GAIQ value of that grain.
• The circle equivalent diameters are calculated for all of the grains (ferrite grains) positioned in regions corresponding to ferrite judged by GAIQ analysis in the above results of EBSD measurement. Here, the grains are regions surrounded by grain boundaries of interfaces of regions with crystal orientations different by 15° or more. Next, these are arithmetically averaged to obtain values deemed the mean grain size of the ferrite.
• The above measurement of the mean grain size of the ferrite is performed for five locations. The arithmetic average value of these is determined as the mean grain size of the ferrite.
[Mean Grain Size of Hard Phase in Region Having Depth From 3/8 Depth Position to 5/8 Depth Position of Sheet Thickness: 1.0 to 5.0 µm]
• In the steel sheet of the present embodiment, the mean grain size of the hard phase in the microstructure at a region having a depth from a 3/8 depth position to 5/8 depth position of the sheet thickness (i.e., sheet thickness center part) of the steel sheet is preferably 1.0 to 5.0 µm. By controlling the mean grain size of the hard phase to within such a fine range, it is possible to further improve the appearance after shaping of the steel sheet. The mean grain size of the hard phase may also be 1.2 µm or more, 1.5 µm or more, 1.7 µm or more, or 2.0 µm or more. Similarly, the mean grain size of the hard phase may also be 4.7 µm or less, 4.5 µm or less, 4.2 µm or less, 4.0 µm or less, 3.5 µm or less, or 3.0 µm or less.
• The mean grain size of the hard phase of the sheet thickness center part is determined in the following way.
• First, a sample having a steel sheet cross section in a direction vertical to the sheet surface is taken. The steel sheet cross section is etched by a Nital reagent. That cross section is made the examined plane. In the examined plane, a 100 µm×100 µm region in the range of the sheet thickness 3/8 position to 5/8 position is made the examined region. The hard phase is identified using an FE-SEM (for example, JSM-7200F made by JEOL, measured at acceleration voltage 15kV). Specifically, the image analysis software Image J (Ver.1.54f) is used to binarize the microstructure by differences in brightness and identify the hard phase. Specifically, when using a Nital reagent, the black parts of the image data are ferrite and the not corroded white parts are the hard phase. Next, the circle equivalent diameters of all of the identified the hard phase is calculated. This operation is performed at three examined regions. The circle equivalent diameters of all of the obtained the hard phase is arithmetically averaged and the obtained value is determined as the mean grain size of the hard phase.
(Sheet Thickness)
• In the present embodiment, the thickness of the steel sheet is not particularly limited and can be suitably determined in accordance with the type of the final product etc. For example, the steel sheet may also have a 0.1 to 2.0 mm sheet thickness. A steel sheet having such a thickness is suitable when used as a material of a door, hood, or other cover member. Note that, the thickness of the steel sheet may also be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the thickness of the steel sheet may also be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by making the thickness of the steel sheet 0.2 mm or more, the additional effect can be obtained that maintaining the shape of the part flat becomes easy and the dimensional precision and shape precision can be improved. On the other hand, by making the sheet thickness 1.0 mm or less, the effect of lightening the member becomes remarkable. The thickness of the steel sheet is measured by a micrometer.
(Plating Layer)
• The steel sheet of the present embodiment may further have a plating layer at the surface thereof for the purpose of improvement of the corrosion resistance etc. If the steel sheet has a plating layer, the plating layer may be provided on one surface or both surfaces of the steel sheet. Further, the plating layer may be either of a hot dip coating layer or an electroplating layer. As the hot dip coating layer, for example, a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), hot dip aluminum plating layer, hot dip Zn-Al alloy plating layer, hot dip Zn-Al-Mg alloy plating layer, hot dip Zn-Al-Mg-Si alloy plating layer, etc. may be mentioned. As the electroplating layer, for example, an electrogalvanized layer (EG), electro Zn-Ni alloy plating layer, etc. may be mentioned. Among these, the plating layer is preferably a hot dip galvanized layer, hot dip galvannealed layer, or electrogalvanized layer. The amount of deposition of the plating layer is not particularly limited and may also be a general amount of deposition.
(Mechanical Properties)
• According to the steel sheet of the present embodiment having the above specific chemical composition and microstructure, it is possible to achieve a high tensile strength, specifically a 500 MPa or more tensile strength. The tensile strength of the steel sheet is preferably 540 MPa or more, more preferably 600 MPa or more. The upper limit of the tensile strength is not particularly prescribed, but, for example, the tensile strength may also be 980 MPa or less or 850 MPa or less. By making the tensile strength 850 MPa or less, the workability when press forming the steel sheet can be more easily secured.
• The steel sheet of the present embodiment is excellent in press forming and other workability and can maintain an excellent appearance even after shaping despite having a high strength, specifically a 500 MPa or more tensile strength. In a preferable embodiment, an outer panel member including the steel sheet according to an embodiment of the present invention, in particular an outer panel member of an automobile, is provided. As examples of outer panel members of automobiles, members where high beauty is sought such as roofs, hoods, fenders, and doors may be mentioned. These outer panel members, in particular outer panel members of automobiles, need only include the steel sheet according to an embodiment of the present invention at least at parts of these outer panel members. The features of the chemical composition and microstructure explained previously are satisfied at least at parts of these outer panel members. At portions of the steel sheet relatively low in extent of working at press forming and other shaping, the features of the microstructure do not particularly change before and after shaping.
• <Method of Production of Steel Sheet>
• Next, a preferable method of production of the steel sheet according to one embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for producing the steel sheet according to an embodiment of the present invention and is not intended to limit the steel sheet to one produced by a method of production such as explained below.
• The method of production of the steel sheet of the present embodiment includes at least a casting step of casting a slab having the above specific chemical composition, a hot rolling step of hot rolling the cast slab, a pickling step of removing an oxide film (scale) formed at the time of hot rolling, a cold rolling step of cold rolling the hot rolled steel sheet after pickling, and an annealing step of holding the cold rolled steel sheet in a predetermined atmosphere at a predetermined temperature region.
• Furthermore, the method of production of the steel sheet of the present embodiment may optionally include, in addition to the above steps, a cooling step of cooling the cold rolled steel sheet after annealing, a plating step of forming a plating layer on the surface of the cold rolled steel sheet after cooling, a skin pass rolling step of skin pass rolling the steel sheet after the plating step, and other steps.
• Below, the preferred conditions etc. of these steps will be explained.
(Casting Step)
• In the method of production of the steel sheet of the present embodiment, the casting step is a step of casting a slab having the above specific chemical composition. The casting step uses a continuous casting machine provided with a plurality of adjoining reduction rolls in the conveyance direction of the slab and having a roll pitch of adjoining reduction rolls of 290 mm or less to perform light rolling reduction. Note that, in this Description, "light rolling reduction" indicates rolling reduction having a rolling reduction gradient of 0.6 mm or more per meter in the direction of progression of casting.
• The steel sheet of the present embodiment, as explained above, has as an essential requirement a unique microstructure comprised of a hard phase fraction lower than conventional DP steel and having a small variation in hard phase fraction in the direction perpendicular to the rolling direction and sheet thickness direction of the sheet thickness center part. To obtain such a unique microstructure, it is important to control the solidified structures at the time of casting to become columnar crystals. Specifically, in the casting step, by making the superheat ΔT (difference of molten steel temperature and solidification temperature of molten steel) of molten steel having the above specific chemical composition 25°C or more and, furthermore, making the segment pressing force 450 tons or more, it is possible to suppress even center segregation while using a method different from the conventional measure against center segregation of control of the solidified structures to columnar crystal structures with an equiaxed crystal rate of 15% or less. The superheat ΔT is preferably 30°C or more. Further, the superheat ΔT is preferably 40°C or less. Note that, the molten steel temperature is the molten steel temperature in the tundish and can be found by actual measurement. The solidification temperature can be found from the chemical composition of the molten steel utilizing a known solidification temperature estimation formula.
• Note that, the conventional measure for center segregation comprises making the superheat ΔT as small as possible (making it at least less than 25°C) and making the equiaxed crystal rate increase (making it at least increase to more than 15%), but a sufficient effect of improvement cannot be obtained by such a conventional measure. In the present embodiment, by employing casting conditions completely different from the conventional measure, i.e., the unique casting conditions of a superheat ΔT of 25°C or more and a segment pressing force of 450 tons or more to control the solidified structures to columnar crystal structures, it is possible to suppress negative segregation of Mn. As a result, micro segregation of Mn can be improved and ghost lines and other defects in appearance after shaping can be sufficiently suppressed.
• The equiaxed crystal rate (%) can be calculated by taking an etch print of the sheet thickness cross section in the width direction of the slab (direction perpendicular to conveyance direction and sheet thickness direction), determining the interfaces of the columnar crystal structures and equiaxed crystal structures by visual observation, measuring the thickness (mm) of the equiaxed crystal structures of the sheet thickness center part of the slab and the thickness (mm) of the slab, dividing the thickness of the equiaxed structures by the thickness of the slab, and multiplying the result by 100.
• Further, in the casting step, it is possible to use a continuous casting machine with a roll pitch of the adjoining plurality of reduction rolls of 290 mm or less for light rolling reduction to thereby suppress flow of molten steel at the time of solidification and reduce the concentration of Mn at the center part. Due to this, it is possible to suppress the center segregation of Mn. Note that, the roll pitch of the adjoining plurality of reduction rolls is preferably 280 mm or less.
(Hot Rolling Step)
• In the method of production of the steel sheet of the present embodiment, the hot rolling step is a step of hot rolling the cast slab. In this step, the slab fallen in temperature to less than 1100°C after casting is preferably heated to 1100°C or more before hot rolling. By making the heating temperature 1100°C or more, the rolling reaction force in the hot rolling does not become excessively large and the targeted product thickness can be easily obtained. The upper limit of the heating temperature is not particularly prescribed, but from the viewpoint of economy, the heating temperature is preferably made less than 1300°C. In the hot rolling step, the heated slab is rough rolled and finish rolled.
• Note that, the finish rolling end temperature is preferably 950°C or less. By making the finish rolling end temperature 950°C or less, it is possible to reduce the mean grain size of the hot rolled steel sheet and final product and becomes possible to secure a sufficient yield strength and secure a high surface quality after shaping.
• Further, to control the hard phase fraction of the surface layer of the steel sheet to 0.20 Vm to 0.80 Vm in range, the air cooling time "t" (s) after the finish rolling is made a time satisfying the following formula (3). $$0.067\mathrm{t}\le \ln \left(1300/\left(\mathrm{FT}+273\right)\right)$$ where, in formula (3), "t" indicates the air cooling time (s) after finish rolling and FT indicates the surface temperature (°C) of the steel sheet after finish rolling.
• If the air cooling time after the finish rolling satisfies the above formula (3), the internal oxide layer becomes easier to remove in the later pickling step, therefore as a result it is possible to keep the internal oxide layer at the smallest extent.
• The thus obtained hot rolled steel sheet is coiled at a predetermined coiling temperature (CT). To control the hard phase fraction of the surface layer of the steel sheet to 0.20 Vm to 0.80 Vm in range, the coiling temperature is set to 580°C or less in temperature. By making the coiling temperature a 580°C or less temperature, it is possible to make formation of an internal oxide layer more difficult. Note that, the coiling temperature may also be, for example, a 450°C or more temperature.
(Pickling Step)
• In the method of production of the steel sheet of the present embodiment, the pickling step is a step of removing scale formed at the time of hot rolling. In the pickling step, the continuously conveyed hot rolled steel sheet is dipped in a pickling tank in which an acidic washing solution is filled to thereby remove the scale formed at the surface of the hot rolled steel sheet. As the acidic washing solution, for example, hydrochloric acid, sulfuric acid, etc. can be used.
• In the pickling step, the pickling time and pickling temperature are set in accordance with the chemical composition (Si and Mn) of the steel sheet and coiling temperature (CT). However, to control the hard phase fraction of the surface layer of the steel sheet to 0.20 Vm to 0.80 Vm in range, the pickling time and pickling temperature are set to satisfy the following formula (4). $$0.4\times \left(\mathrm{T}-15\right)\times \left({\mathrm{t}}_{\mathrm{A}}-20\right)\ge \left(5\left[\mathrm{Si}\right]+0.3\left[\mathrm{Mn}\right]-0.6\right)\times \left(\mathrm{CT}+273\right)$$ where, in formula (4), T indicates the pickling temperature (°C), t_A indicates the pickling time (s), [Si] indicates the content of Si (mass%), [Mn] indicates the content of Mn (mass%), and CT indicates the coiling temperature after hot rolling (°C).
• If the pickling time t_A and pickling temperature T satisfy the above formula (4), the internal oxide layer becomes further easily removed, therefore it is possible to greatly reduce the internal oxide layer as a result.
• Note that, the pickling time t_A can be freely changed by adjusting the conveyance speed of the hot rolled steel sheet.
(Cold Rolling Step)
• In the method of production of the steel sheet of the present embodiment, the cold rolling step is a step of cold rolling the hot rolled steel sheet after pickling. In the cold rolling step, for example, the hot rolled steel sheet is preferably cold rolled so that the cumulative rolling reduction becomes 50 to 90%. By controlling the cumulative rolling reduction to such a range, it is possible to secure the desired sheet thickness and sufficiently secure uniformity of quality in the sheet width direction.
(Annealing Step)
• In the method of production of the steel sheet of the present embodiment, the annealing step is a step of holding the cold rolled steel sheet in a predetermined atmosphere at a predetermined temperature region. In the annealing step, it is preferable to perform annealing treatment heating and holding the cold rolled steel sheet to a 750 to 900°C soaking temperature. By making the soaking temperature 750°C or more, it becomes possible to make recrystallization of ferrite and reverse transformation from ferrite to austenite sufficiently proceed and obtain the desired microstructure in the final product. On the other hand, by making the soaking temperature 900°C or less, it is possible to make the crystal grains denser and obtain sufficient strength.
• Furthermore, in this annealing step, to control the hard phase fraction of the surface layer of the steel sheet to 0.20 Vm to 0.80 Vm in range, the oxygen potential and other conditions are controlled so that the three conditions of the following (i) to (iii) are satisfied in humidifying annealing, i.e., high dew point annealing.
1. (i) heating zone oxygen potential: -0.9≤log(pH₂ O/pH₂ )≤-0.3
2. (ii) soaking zone oxygen potential: -2.4≤log(pH₂ O/pH₂ )≤-0.8
3. (iii) heating zone exit side and soaking zone temperature: Ac1+20≤T≤Ac3-30
where, pH₂ O is the steam partial pressure (Pa) of the heating zone atmosphere and soaking zone atmosphere, pH₂ is hydrogen partial pressure (Pa) of the heating zone atmosphere and soaking zone atmosphere, and T is the temperature of the heating zone exit side and soaking zone (°C). Further, the Ac1 point (°C) and Ac3 point (°C) are found by the following formulas in accordance with the chemical composition of the steel sheet. $$\mathrm{Ac}1=751-27\mathrm{C}+18\mathrm{Si}-12\mathrm{Mn}-23\mathrm{Cu}-23\mathrm{Ni}+24\mathrm{Cr}+23\mathrm{Mo}-40\mathrm{V}-6\mathrm{Ti}+230\mathrm{Nb}-169\mathrm{Al}$$ $$\mathrm{Ac}3=911-436\mathrm{C}+30\mathrm{Si}-25\mathrm{Mn}-5\mathrm{Cr}+15\mathrm{Mo}+136\mathrm{Ti}-19\mathrm{Nb}+101\mathrm{Al}$$
• In the annealing step, by controlling the oxygen potential and other conditions so as to satisfy the above three conditions in high dew point annealing, it is possible to easily control the hard phase fractions of the surface layer of the steel sheet and the sheet thickness center part to within the above specific ranges.
(Cooling Step)
• In the method of production of the steel sheet of the present embodiment, the cooling step is the step of cooling the cold rolled steel sheet after annealing. In the cooling step, it is preferable to cool the cold rolled steel sheet so that the mean cooling speed from the soaking temperature becomes 5 to 50°C/s. By making the mean cooling speed 5°C/s or more, it becomes possible to suppress excessive transformation to ferrite and to increase the amount of formation of martensite and other hard phase to obtain the desired strength. Further, by making the mean cooling speed 50°C/s or less, it becomes possible to more uniformly cool the steel sheet in the sheet width direction (direction perpendicular to the rolling direction and sheet thickness direction).
(Plating Step)
• In the method of production of the steel sheet of the present embodiment, the plating step is a step of plating the surface of the cold rolled steel sheet after cooling so as to form a plating layer on the surface of the steel sheet. As the plating treatment, for example, hot dip coating, alloyed hot dip coating, electroplating, and other treatment may be mentioned. For example, as the plating treatment, the steel sheet surface may be hot dip galvanized or may be hot dip galvanized, then alloyed. The specific conditions of the plating and alloying are not particularly prescribed. Any suitable conditions known to persons skilled in the art can be employed. For example, the alloying temperature may be 450 to 600°C.
• Note that, in the method of production of the steel sheet of the present embodiment, the plating step is a step performed for the purpose of improvement of the corrosion resistance. It is not an essential step, therefore if the steel sheet does not contain a plating layer, such a plating step need not be performed.
• Note that, the present invention is not limited to the above embodiment or the following examples etc. They can be suitably combined, replaced, changed, etc. to an extent not deviating from the object and gist of the present invention.
• Below, examples will be illustrated to more specifically explain the present invention, but the present invention is not limited to only these examples.
EXAMPLES
• In the following examples, steel sheets according to an embodiment of the present invention were produced under various conditions. The properties of the tensile strength workability, and appearance after shaping of each of the obtained steel sheets etc. were investigated.
• First, a continuous casting machine provided with a plurality of reduction rolls arranged a 290 mm or less roll pitches was used for continuous casting performing light reduction having a reduction gradient of 0.6 mm or more per meter in the direction of casting progression to thereby cast each slab having a chemical composition shown in Table 1. The balance besides the constituents shown in Table 1 are Fe and impurities. The various conditions in this casting step are shown in Table 2. In this Table 2, the "ΔT" in the casting conditions is the condition of the superheat ΔT≥25°C, while the "pressing force" in the casting conditions is the condition of the segment pressing force≥450 tons. In the examples, cases satisfying these conditions are labeled as "OK (okay)" while cases not satisfying these conditions are labeled as "NG (no good)". These are respectively shown in Table 2.
• Next, each obtained slab was subjected to a hot rolling step. The various conditions in the hot rolling step are shown in Table 2.
• In Table 2, the "air cooling time" in the hot rolling conditions of each steel sheet is a condition where the air cooling time after finish rolling satisfies the following formula (3), while the "coiling temperature" of the hot rolling conditions is the condition of coiling temperature (CT)≤580°C. $$0.067\mathrm{t}\le \ln \left(1300/\left(\mathrm{FT}+273\right)\right)$$ in formula (3), "t" indicating the air cooling time (s) after finish rolling and "FT" indicating the surface temperature of the steel sheet after finish rolling (°C).
• In the examples, cases satisfying these conditions are labeled as "OK" while cases not satisfying these conditions are labeled as "NG". These are respectively shown in Table 2.
• Note that, regarding Steel Sheet No. 1, a slab having the chemical composition shown in Table 1 was heated and held at 1230°C for 30 minutes or more, then was finish rolled at a temperature of 900°C, was air cooled after finish rolling for 1 second, and was coiled at a temperature of 550°C to prepare a thickness 2.3 mm hot rolled steel sheet.
• Furthermore, after the hot rolling step, the hot rolled steel sheet was subjected to a pickling step. The various conditions in the pickling step are shown in Table 2. In this Table 2, the "time and temperature" of the pickling conditions are conditions where the pickling time and pickling temperature satisfy the following formula (4): $$0.4\times \left(\mathrm{T}-15\right)\times \left({\mathrm{t}}_{\mathrm{A}}-20\right)\ge \left(5\left[\mathrm{Si}\right]+0.3\left[\mathrm{Mn}\right]-0.6\right)\times \left(\mathrm{CT}+273\right)$$ where, in formula (4), T indicates the pickling temperature (°C), t_A indicates the pickling time (s), [Si] indicates the content of Si (mass%), [Mn] indicates the content of Mn (mass%), and CT indicates the coiling temperature after hot rolling (°C).
• In the examples, cases satisfying this condition are labeled as "OK" while cases not satisfying it are labeled as "NG". These are shown in Table 2.
• Note that, in the Steel Sheet No. 1, the hot rolled steel sheet prepared in the above way was pickled by a hydrochloric acid concentration of 10%, a temperature of 70°C, and a time of 100 seconds.
• Further, after pickling, the hot rolled steel sheet was subjected to a cold rolling step (cumulative rolling reduction 80%), an annealing step (soaking temperature 800°C), and cooling step (mean cooling speed 10°C/s) to prepare each steel sheet. The surface of each obtained steel sheet was suitably plated to form a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), or electrogalvanized layer (EG) shown in Table 2.
• Note that, in Steel Sheet No. 1, after the pickling, the steel sheet was cold rolled down to a sheet thickness of 0.45 mm. Furthermore, the obtained cold rolled steel sheet was hot dip galvannealed by CGL (continuous galvannealing line). At the CGL, the steel sheet was heated under conditions of a temperature of the heating zone of 780°C, a dew point of -2°C, a hydrogen concentration of 2.6 vol%, a temperature of the soaking zone of 780°C, a dew point of -40°C, and a hydrogen concentration of 2.7 vol%, then was hot dip galvanized and then was alloyed at 560°C.
• Note that, in Table 2, Steel Sheet Nos. 8 and 33 are examples of steel sheets not plated on their surfaces.
• Here, the various conditions in the annealing step are shown in Table 2. In this Table 2, the "heating zone oxygen potential", "soaking zone oxygen potential", and "heating zone exit side and soaking zone temperature" of the annealing conditions are conditions satisfying the three conditions of the following (i) to (iii):
1. (i) heating zone oxygen potential: -0.9≤log(pH₂ O/pH₂ )≤-0.3
2. (ii) soaking zone oxygen potential: -2.4≤log(pH₂ O/pH₂ )≤-0.8
3. (iii) heating zone exit side soaking zone temperature: Ac1+20≤T≤Ac3-30
• In each example, cases satisfying these conditions are indicated as "OK" and cases not satisfying then are indicated as "NG" and shown in Table 2.
• A sample was taken from each steel sheet produced as explained above and analyzed for chemical composition, whereupon it was confirmed it was not changed from the chemical composition of the slab shown in Table 1. Note that, the underlines attached to the chemical compositions in Table 1 show outside the scope of the present invention. Further, the underlines attached to the various numerical values etc. in Table 2 show outside the scope of the present invention, production conditions under which the steel sheet of the present invention cannot be obtained, or various properties of the steel sheet not being preferable. Note that, in Table 2, "ratio of hard phase fractions at 3/8 depth position to 5/8 depth position of steel sheet: Vs/ Vm" means the ratio Vs/Vm between the hard phase fraction Vs of the region having depth from the surface of the steel sheet to the 1/8 depth position of sheet thickness and the hard phase fraction Vm at the region having a depth from the 3/8 depth position to 5/8 depth position of sheet thickness . Therefore, this ratio Vs/Vm being 0.20 to 0.80 and the "area ratio of the hard phase at the region having a depth from the surface of the steel sheet to the 1/8 depth position of sheet thickness of 0.20 Vm to 0.80 Vm" are synonymous.
[Table 1]
• Table 1

  Steel	Chemical composition (mass%), balance: Fe and im purities														Remarks
  	C	Mn	Si	Al	P	S	N	O	Cr	Mo	B	Ti	Nb	Others
  A	0.048	1.52	0.158	0.092	0.014	0.0018	0.0024	0.0009	0.32	0.11		0.017			Inv. steel
  B	0.055	1.18	0.382	0.108	0.018	0.0016	0.0035	0.0012	0.29	0.28		0.019			Inv. steel
  C	0.044	1.75	0.261	0.026	0.028	0.0023	0.0025	0.0010		0.14					Inv. steel
  D	0.069	1.31	0.013	0.302	0.055	0.0011	0.0054	0.0019	0.45		0.0020	0.021	0.011	Sn: 0.02	Inv. steel
  E	0.041	1.86	0.081	0.084	0.024	0.0008	0.0025	0.0008			0.0018	0.018			Inv. steel
  F	0.035	1.09	0.292	0.034	0.042	0.0020	0.0031	0.0012		0.25	0.0022	0.012		V: 0.18	Inv. steel
  G	0.060	1.62	0.154	0.035	0.029	0.0021	0.0044	0.0009	0.58		0.0019	0.011	0.006	W: 0.02, Ni: 0.09	Inv. steel
  H	0.047	2.03	0.186	0.189	0.067	0.0013	0.0056	0.0019							Inv. steel
  I	0.051	1.21	0.725	0.254	0.019	0.0031	0.0035	0.0015	0.42	0.07			0.015	Ni: 0.05, Sn: 0.007, Sb: 0.08	Inv. steel
  J	0.060	1.72	0.151	0.054	0.014	0.0028	0.0021	0.0011		0.09	0.0014	0.035		REM: 0.0027	Inv. steel
  K	0.058	1.74	0.285	0.051	0.036	0.0021	0.0038	0.0008	0.53			0.014		Mg: 0.0015	Inv. steel
  L	0.062	1.73	0.293	0.057	0.024	0.0024	0.0035	0.0015	0.44	0.07	0.0013	0.023		Zr: 0.0015, Ca: 0.0018	Inv. steel
  M	0.078	1.55	0.112	0.243	0.020	0.0064	0.0034	0.0012	0.25	0.34		0.024			Inv. steel
  N	0.048	1.31	0.260	0.026	0.031	0.0009	0.0022	0.0024			0.0026	0.015		W: 0.12	Inv. steel
  O	0.055	1.26	0.153	0.154	0.019	0.0019	0.0065	0.0008		0.21		0.062			Inv. steel
  P	0.049	1.34	0.078	0.054	0.041	0.0015	0.0047	0.0015	0.18	0.12			0.051		Inv. steel
  Q	0.057	1.05	0.294	0.078	0.035	0.0041	0.0035	0.0011	0.61			0.012	0.008	Cu: 0.39, Ni: 0.22	Inv. steel
  R	0.055	1.38	0.102	0.213	0.009	0.0014	0.0029	0.0012	0.25			0.024		As: 0.07	Inv. steel
  S	0.118	1.31	0.291	0.034	0.025	0.0028	0.0035	0.0015	0.40	0.10					Comp. steel
  T	0.079	2.71	0.125	0.033	0.038	0.0026	0.0030	0.0014		0.07		0.013			Comp. steel
  U	0.052	1.35	1.620	0.291	0.008	0.0013	0.0041	0.0011	0.25		0.0013	0.012			Comp. steel
  V	0.023	1.85	0.156	0.030	0.018	0.0024	0.0033	0.0015		0.08			0.012		Comp. steel
  W	0.055	0.85	0.127	0.135	0.015	0.0013	0.0029	0.0010	0.16	0.10		0.015			Comp. steel

• The microstructure and various properties in the obtained surface layer and sheet thickness center part of each steel sheet were measured and evaluated by the methods explained above or below.
(Tensile Strength and Yield Ratio)
• A No. 5 tensile test piece of JIS Z2241: 2011 having a direction perpendicular to the rolling direction and sheet thickness direction as the longitudinal direction thereof was taken from each steel sheet and subjected to a tensile test based on JIS Z2241: 2011 to measure the tensile strength (MPa) and yield stress (0.2% yield strength) of the steel sheet. Furthermore, from these measurement results, the yield ratio YR (%) defined by the (yield stress) / (tensile strength) was calculated. Further, from these results, a steel sheet with a tensile strength of 500 MPa or more was evaluated as a high strength steel sheet and a steel sheet with a yield ratio of 0.70 or less was evaluated as a steel sheet excellent in workability.
(Workability)
• An adhesion and bending test was conducted by the pressure bend method of the metal material bending test method of JIS Z2248: 2022 and the workability was evaluated. A width 30 mm and length 100 mm strip-shaped test piece was taken from the steel sheet so that the direction parallel to the rolling direction of the steel sheet became the axial direction of the bending test. After the adhesion and bending test, the test piece was visually evaluated for any occurrence of cracks at the ridge parts of the bent peaks. Cases with no cracks were deemed as passing and indicated as "OK" in Table 2. Cases with cracks recognized by the above visual evaluation were deemed as failing and indicated as "NG" in Table 2.
(Appearance After Shaping)
• The appearance after shaping a steel sheet was evaluated by the degree of ghost lines formed at the surface of an outer door after press forming and the presence of any stretcher strain. The surface after press forming was ground by a grindstone. Stripe patterns comprised of the streak patterns formed on the surface at intervals of the order of several mm and extending in the rolling direction are judged as ghost lines and were evaluated by a score of 1 to 5 based on the following criteria corresponding to the degree of formation of the streak patterns. On the surface of the steel sheet ground down after press forming, any 100 mm×100 mm region was visually checked. Cases where no streak patterns extending in the rolling direction could be confirmed at all were evaluated as "1", cases where the maximum length of the streak patterns was 20 mm or less were evaluated as "2", cases where the maximum length of the streak patterns was more than 20 mm and 50 mm or less were evaluated as "3", cases where the maximum length of the streak patterns was more than 50 mm and 70 mm or less were evaluated as "4", and cases where the maximum length of the streak patterns was more than 70 mm were evaluated as "5". Cases evaluated as "3" or less were judged as excellent in appearance after shaping and passing. On the other hand, cases evaluated as "4" or more were judged as inferior in appearance after shaping and failing.
• Further, when visually examining the surface after press forming, cases where surface defects comprised of uneven local line shapes or tree shapes called stretcher strain can be confirmed were evaluated as "SS" and judged as inferior in appearance after shaping and failing.
• Referring to Table 2, in the comparative example of Steel Sheet No. 3 performing the casting step under conditions of ΔT=15°C, the standard deviation of the hard phase fraction of the microstructure of the sheet thickness center part became 0.89%, ghost lines formed, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 4 performing the annealing step under conditions of a value of the heating zone oxygen potential becoming -1.5, the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.83 (hard phase fraction Vs of microstructure of surface layer became 0.83 Vm) and the workability became inferior.
• In the comparative example of Steel Sheet No. 5 performing the annealing step under conditions of a value of the heating zone oxygen potential becoming -0.6, the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.00 (hard phase fraction Vs of microstructure of surface layer became 0.00 Vm), the yield ratio was high, stretcher strain occurred, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 6 performing the annealing step under conditions of heating and soaking zone temperature of 860°C, the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.17 (hard phase fraction Vs of microstructure of surface layer became 0.17 Vm), the yield ratio was high, stretcher strain occurred, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 14 performing the casting step under conditions of a pressing force of 350 tons, the result was the standard deviation of the hard phase fraction of the microstructure of the sheet thickness center part became 0.88%, ghost lines formed, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 15 performing the hot rolling step under conditions of a finish rolling temperature of 930°C and an air cooling time of 1.7 seconds, i.e., conditions not satisfying the above formula (3), the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.17 (hard phase fraction Vs of microstructure of surface layer became 0.17 Vm), the yield ratio was high, stretcher strain occurred, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 16 performing the hot rolling step under conditions of a coiling temperature of 670°C, the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.14 (hard phase fraction Vs of microstructure of surface layer became 0.14 Vm), the yield ratio was high, stretcher strain occurred, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 17 performing the pickling step under conditions of a coiling temperature of 570°C, a pickling temperature of 60°C, and a pickling time of 100 seconds, i.e., conditions not satisfying the above formula (4), the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.14 (hard phase fraction Vs of microstructure of surface layer became 0.14 Vm), the yield ratio was high, stretcher strain occurred, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 21 performing the hot rolling step under conditions of a coiling temperature of 690°C, performing the pickling step under conditions of a pickling temperature of 65°C and a pickling time of 80 seconds, i.e., conditions not satisfying the above formula (4), and performing the annealing step under conditions of a value of the soaking zone oxygen potential of -0.5, the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.00 (hard phase fraction Vs of microstructure of surface layer became 0.00 Vm), the yield ratio was high, stretcher strain occurred, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 22 performing the hot rolling step under conditions of a finish rolling temperature of 920°C and an air cooling time of 1.5 seconds, i.e., conditions not satisfying the above formula (3), and performing the pickling step under conditions of a coiling temperature of 570°C, a pickling temperature of 55°C, and a pickling time of 90 seconds, i.e., conditions not satisfying the above formula (4), the result was the ratio Vs/Vm of the hard phase fraction Vs of the microstructure of the surface layer and the hard phase fraction Vm of the microstructure of the sheet thickness center part became 0.13 (hard phase fraction Vs of microstructure of surface layer became 0.13 Vm), the yield ratio was high, stretcher strain occurred, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 29 using the comparative steel S with a content of C of 0.118%, the result was the standard deviation of the hard phase fraction of the microstructure of the sheet thickness center part became 0.87%, ghost lines formed, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 30 using the comparative steel T with a content of Mn of 2.71%, the result was the ferrite fraction and hard phase fraction of the microstructure of the sheet thickness center part became 73% and 27%, the standard deviation of the hard phase fraction of the microstructure of the sheet thickness center part became 0.91%, the workability became inferior, ghost lines formed, and the appearance after shaping became inferior.
• In the comparative example of Steel Sheet No. 31 using the comparative steel U with a content of Si of 1.620%, the result was stretcher strain occurred and the appearance after shaping was inferior.
• In the comparative example of Steel Sheet No. 32 using the comparative steel V with a content of C of 0.023%, the result was the ferrite fraction and hard phase fraction of the microstructure of the sheet thickness center part became 98% and 2%, the tensile strength fell, and the strength became inferior.
• In the comparative example of Steel Sheet No. 33 using the comparative steel W with a content of Mn of 0.85%, the result was the tensile strength fell and the strength became inferior and the yield ratio became higher.
• On the other hand, in the invention examples of Steel Sheet Nos. 1, 2, 7 to 13, 18 to 20, and 23 to 28, it was possible to maintain a high strength of a tensile strength of 500 MPa or more while obtaining excellent workability at press forming etc. and remarkably suppress the formation of ghost lines and stretcher strain at the surface of a steel sheet even in the case where strain was imparted by press forming.

Claims (7)

1. A steel sheet, in which

   a chemical composition of the steel sheet contains, by mass%,

   C: 0.030 to 0.100%,

   Mn: 1.00 to 2.50%,

   Si: 0.005 to 1.500%,

   Al: 0.005 to 0.700%,

   P: 0.100% or less,

   S: 0.0200% or less,

   N: 0.0150% or less,

   O: 0.0100% or less,

   Cr: 0 to 0.80%,

   Mo: 0 to 0.50%,

   B: 0 to 0.0100%,

   Ti: 0 to 0.100%,

   Nb: 0 to 0.100%,

   V: 0 to 0.50%,

   Ni: 0 to 1.00%,

   Cu: 0 to 1.00%,

   W: 0 to 1.00%,

   Sn: 0 to 1.00%,

   Sb: 0 to 0.200%,

   As: 0 to 0.200%,

   Ca: 0 to 0.0100%,

   Mg: 0 to 0.0100%,

   Zr: 0 to 0.0100%,

   REM: 0 to 0.0100%, and

   balance: Fe and impurities,

   an area ratio of a hard phase at a region having a depth from a surface of the steel sheet to a 1/8 depth position of sheet thickness is 0.20 Vm to 0.80 Vm when an area ratio of a hard phase at a region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is Vm, and

   a microstructure of the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness contains, by area%, ferrite: 75 to 97% and the hard phase: 3 to 25% and a standard deviation of the area ratio of the hard phase in a direction perpendicular to a rolling direction and sheet thickness direction at the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is 0.85% or less.
2. The steel sheet according to claim 1, wherein the chemical composition contains one or more elements selected from a group comprising, by mass%:

   Cr: 0.01 to 0.80%,

   Mo: 0.01 to 0.50%,

   B: 0.0001 to 0.0100%,

   Ti: 0.001 to 0.100%,

   Nb: 0.001 to 0.100%,

   V: 0.01 to 0.50%,

   Ni: 0.01 to 1.00%,

   Cu: 0.01 to 1.00%,

   W: 0.01 to 1.00%,

   Sn: 0.001 to 1.00%,

   Sb: 0.001 to 0.200%,

   As: 0.001 to 0.200%,

   Ca: 0.0001 to 0.0100%,

   Mg: 0.0001 to 0.0100%,

   Zr: 0.0001 to 0.0100%, and

   REM: 0.0001 to 0.0100%.
3. The steel sheet according to claim 1 or 2, wherein an index A expressed by the following formula (1) is 0.45% or more: $$\mathrm{A}=\left[\mathrm{Si}\right]+10\left[\mathrm{P}\right]+0.6\left[\mathrm{Al}\right]+8\left[\mathrm{Ti}\right]+9\left[\mathrm{Nb}\right]$$ where, [Si], [P], [Al], [Ti], and [Nb] are contents of elements in units of mass%, 0% when corresponding elements are not contained.
4. The steel sheet according to any one of claims 1 to 3, wherein the following formula (2) is satisfied: $$\left(\mathrm{TS}-180000/\mathrm{TS}\right)/\mathrm{Vm}\ge 35$$ where, TS indicates a tensile strength of the steel sheet in units of MPa and Vm indicates an area ratio of a hard phase at the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness.
5. The steel sheet according to any one of claims 1 to 4, wherein

   a mean grain size of the ferrite in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is 5.0 to 30.0 µm and

   a mean grain size of the hard phase in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is 1.0 to 5.0 µm.
6. The steel sheet according to any one of claims 1 to 5, wherein the hard phase in the region having a depth from a 3/8 depth position to 5/8 depth position of sheet thickness is comprised of a least one of martensite, bainite, tempered martensite, and pearlite.
7. An outer panel member including the steel sheet according to any one of the claims 1 to 6.