Steel sheet and method for manufacturing the same

Abstract

As a steel sheet excellent in energy absorption at the time of crushing deformation, a steel sheet having a prescribed chemical composition and a steel structure, in which a plurality of step differences having a height difference exceeding 5.0 μm exist at intervals of 2.0 mm or less on the surface of the sheet.

Description

TECHNICAL FIELD

[0001] The present application discloses a steel sheet and a method for manufacturing the same. BACKGROUND

[0002] In recent years, in order to achieve improvement in fuel efficiency of automobiles, weight reduction of automobile bodies by application of high-strength steel sheets is being promoted. In addition, in order to ensure safety of passengers, high-strength steel sheets are also becoming frequently used in automobile bodies instead of mild steel sheets. In the future, in order to further promote weight reduction of automobile bodies, it is necessary to improve the strength level of high-strength steel sheets more than ever.

[0003] In addition, it is required that automobile parts deform to exhibit high energy absorption at the time of collision of an automobile. In order to improve the energy absorbed by deformation of an automobile part at the time of collision of an automobile, it is preferable to prevent cracking of steel generated in crushing deformation of an automobile part. Therefore, it is required that a steel sheet for use in an automobile part is high in strength and exhibits excellent energy absorption at the time of crushing deformation. However, in the conventional art, although workability and the like of high-strength steel sheets have been studied (for example, Patent Documents 1 to 3 below), sufficient study has not been made on energy absorption at the time of crushing deformation.

[0004] In Patent Document 1, a method is disclosed in which a hot-rolled steel strip containing C: 0.3 to 1.3%, Si: 0.03 to 0.35%, Mn: 0.20 to 1.50%, and the remainder consisting essentially of Fe and inevitable impurities is cold-rolled at a reduction of 20% or more and 85% or less, and then annealed by repeatedly heating at a rate of 20 to 100°C / Hr to Ac1 point to Ac1 point + 50°C and holding for 8Hr or less, and then cooling at a rate of 50°C / Hr or less to Ar1 point or less, using a bell-type batch annealing furnace in a gas atmosphere consisting essentially of 75% by volume or more of hydrogen and the remainder of nitrogen and inevitable impurities, thereby preventing the generation of baking defects and softening, and inexpensively producing a high-carbon cold-rolled steel strip excellent in workability.

[0005] In Patent Document 2, a steel sheet for working excellent in painting freshness is disclosed, characterized in that a surface of a steel sheet is formed into a concave-convex rough surface, a wavelength λ of a concave-convex pattern in the rough surface is set to 500 μm or less, and a center line average roughness Ra is set to a range of 1 to 5 μm.

[0006] A steel sheet disclosed in Patent Literature 3 has a prescribed chemical composition, a metal structure containing polygonal ferrite at 40.0% or more and less than 60.0% by area ratio, bainite ferrite at 30.0% or more, residual austenite at 10.0% or more and 25.0% or less, and martensite at 15.0% or less, in the above residual austenite, the ratio of residual austenite having an aspect ratio of 2.0 or less, a length of long axis of 1.0 μm or less, and a length of short axis of 1.0 μm or less is 80.0% or more, in the above bainite ferrite, the ratio of bainite ferrite having an aspect ratio of 1.7 or less and an average value of crystal orientation difference of 0.5° or more and less than 3.0° in a region surrounded by a crystal boundary having a crystal orientation difference of 15° or more is 80.0% or more, and the connectivity D value of the above martensite, the above bainite ferrite, and the above residual austenite is 0.70 or less.

[0007] Prior Art Documents

[0008] Patent Literature

[0009] Patent Literature 1: Japanese Patent Laid-Open No. 10-204540

[0010] Patent Literature 2: Japanese Patent Laid-Open No. 4-253503

[0011] Patent Literature 3: Japanese Patent No. 6791838 SUMMARY

[0012] Problems to be Solved by the Invention

[0013] The present application discloses a steel sheet having excellent energy absorption property at crushing deformation and a method for manufacturing the same in view of the above-described actual situation.

[0014] Means for Solving the Problems

[0015] The present inventors have intensively studied a method for solving the above-described problems and have found that a steel sheet exhibiting excellent energy absorption property at crushing deformation can be obtained by increasing the surface unevenness of the steel sheet and introducing a starting point of deformation to the surface of the steel sheet. Furthermore, it has been confirmed that in a steel sheet having a smooth surface, deformation locally occurs at crushing, and the energy absorption occasionally decreases.

[0016] Furthermore, the present inventors have found that the above-described steel sheet can be manufactured by a continuous manufacturing method characterized by increasing the surface unevenness of a hot-rolled sheet by working under hot-rolling conditions and passing through an annealing process without making the unevenness completely smooth.

[0017] Furthermore, through repeated research, the inventors have also realized that steel plates with improved energy absorption during crushing deformation due to their surface irregularities are difficult to manufacture even with simple adjustments to hot rolling and annealing conditions. Instead, they can only be manufactured by optimizing a series of processes, such as hot rolling and annealing.

[0018] The main points of this invention are as follows.

[0019] (1) A type of steel plate,

[0020] It has the following chemical composition: containing, by mass %

[0021] C: 0.05% to below 0.15%

[0022] Si: 0.01~2.00%

[0023] Mn: 0.10~4.00%

[0024] P: below 0.0200%

[0025] S: Below 0.0200%

[0026] Al: 0.001~1.000%

[0027] N: below 0.0200%

[0028] Ti: 0~0.500%

[0029] Co: 0~0.500%

[0030] Ni: 0~0.500%

[0031] Mo: 0–0.500%

[0032] Cr: 0–2.000%

[0033] O: 0~0.0100%

[0034] B: 0~0.0100%

[0035] Nb: 0~0.500%

[0036] V: 0~0.500%

[0037] Cu: 0~0.500%

[0038] W: 0~0.1000%

[0039] Ta: 0~0.1000%

[0040] Sn: 0~0.0500%

[0041] Sb: 0 to 0.0500%,

[0042] As: 0 to 0.0500%,

[0043] Mg: 0 to 0.0500%,

[0044] Ca: 0 to 0.0500%,

[0045] Y: 0 to 0.0500%,

[0046] Zr: 0 to 0.0500%,

[0047] La: 0 to 0.0500% and

[0048] Ce: 0 to 0.0500%,

[0049] the remainder consisting of Fe and impurities,

[0050] the steel sheet having a steel structure containing, in terms of area fraction,

[0051] ferrite, pearlite and bainite: 0% or more and 60.0% or less, and

[0052] retained austenite: 0% or more and 1.0% or less,

[0053] the remainder consisting of martensite and tempered martensite,

[0054] a plurality of step differences having a height difference exceeding 5.0 pm exist at intervals of 2.0 mm or less on the surface of the sheet.

[0055] (2) The steel sheet according to the above (1),

[0056] having the above chemical composition containing, in terms of mass%,

[0057] Ti: 0.001 to 0.500%,

[0058] Co: 0.001 to 0.500%,

[0059] Ni: 0.001 to 0.500%,

[0060] Mo: 0.001 to 0.500%,

[0061] Cr: 0.001 to 2.000%

[0062] O: 0.0001 to 0.0100%

[0063] B: 0.0001 to 0.0100%,

[0064] Nb: 0.001 to 0.500%,

[0065] V: 0.001 to 0.500%,

[0066] Cu: 0.001 to 0.500%,

[0067] W: 0.0001 to 0.1000%,

[0068] Ta: 0.0001 to 0.1000%,

[0069] Sn: 0.0001 to 0.0500%,

[0070] Sb: 0.0001 to 0.0500%,

[0071] As: 0.0001 to 0.0500%,

[0072] Mg: 0.0001 to 0.0500%,

[0073] Ca: 0.0001 to 0.0500%,

[0074] Y: 0.0001 to 0.0500%,

[0075] Zr: 0.0001 to 0.0500%,

[0076] La: 0.0001 to 0.0500%, and

[0077] Ce: 0.0001 to 0.0500% of one or two or more.

[0078] (3) A method for manufacturing a steel sheet, the method for manufacturing a steel sheet comprising:

[0079] hot-rolling a steel slab having the chemical composition described in (1) or (2) above to obtain a hot-rolled sheet;

[0080] coiling the hot-rolled sheet;

[0081] pickling the hot-rolled sheet; and

[0082] annealing the hot-rolled sheet without cold-rolling, or annealing after cold-rolling,

[0083] the hot-rolling includes rolling the sheet at a reduction rate exceeding 30% and 70% or less while supplying a lubricant between the roll and the sheet in the preceding stand from the final stand of the finishing mill,

[0084] the temperature at the time of coiling the hot-rolled sheet is 700°C or less,

[0085] In the case where the above-described cold rolling is performed, the reduction in the above-described cold rolling is 0.1 to 20%.

[0086] (4) The production method according to the above-described (3), wherein

[0087] In the above-described annealing, a film layer formed of zinc, aluminum, magnesium, or an alloy thereof is formed on the front and back surfaces of the sheet.

[0088] Effects of the Invention

[0089] According to the present application, it is possible to provide a steel sheet excellent in energy absorption at crushing deformation and a production method thereof. BRIEF DESCRIPTION OF DRAWINGS

[0090] Figure 1 Schematically represents the form of a step difference on the surface of a steel sheet.

[0091] Figure 2 Is a schematic diagram for explaining the difference between "maximum height roughness Rz" and the so-called "step difference" in the present application. DETAILED DESCRIPTION

[0092] Hereinafter, embodiments of the present application will be described. Note that the intention of these descriptions is to simply exemplify the embodiments of the present application, and the present application is not limited to the following embodiments.

[0093] <Steel Sheet>

[0094] The steel sheet of the present embodiment is characterized by having the following chemical composition: containing, in mass%,

[0095] C: 0.05 to less than 0.15%,

[0096] Si: 0.01 to 2.00%,

[0097] Mn: 0.10 to 4.00%,

[0098] P: 0.0200% or less,

[0099] S: 0.0200% or less,

[0100] Al: 0.001 to 1.000%,

[0101] N: 0.0200% or less,

[0102] Ti: 0 to 0.500%,

[0103] Co: 0 to 0.500%,

[0104] Ni: 0 to 0.500%,

[0105] Mo: 0 to 0.500%,

[0106] Cr: 0 to 2.000%,

[0107] O: 0 to 0.0100%,

[0108] B: 0 to 0.0100%,

[0109] Nb: 0 to 0.500%,

[0110] V: 0 to 0.500%,

[0111] Cu: 0 to 0.500%,

[0112] W: 0 to 0.1000%,

[0113] Ta: 0 to 0.1000%,

[0114] Sn: 0 to 0.0500%,

[0115] Sb: 0 to 0.0500%,

[0116] As: 0 to 0.0500%,

[0117] Mg: 0 to 0.0500%,

[0118] Ca: 0 to 0.0500%,

[0119] Y: 0 to 0.0500%,

[0120] Zr: 0 to 0.0500%,

[0121] La: 0 to 0.0500% and

[0122] Ce: 0 to 0.0500%,

[0123] the remainder consisting of Fe and impurities, the steel sheet having a steel structure containing, in terms of area fraction,

[0124] ferrite, pearlite and bainite: 0% or more and 60.0% or less, and

[0125] retained austenite: 0% or more and 1.0% or less,

[0126] the remainder consisting of martensite and tempered martensite,

[0127] a plurality of step differences having a height difference exceeding 5.0 μm exist at intervals of 2.0 mm or less on the sheet surface.

[0128] First, the reason for limiting the chemical composition of the steel sheet of the present embodiment will be described. Here, "%" of the components means mass %. Further, in the present specification, "~" indicating a numerical range is used with the meaning that the numerical values recited before and after it are included as lower limit values and upper limit values, unless otherwise specified.

[0129] (C: 0.05 to less than 0.15%)

[0130] C is an element that cheaply increases the tensile strength, and is an extremely important element for suppressing phase transformation from austenite to ferrite, bainite, pearlite in the continuous annealing process, and controlling the strength of the steel. In the case where the C content is 0.05% or more, such an effect is easily obtained. The C content can also be 0.07% or more. On the other hand, if C is excessively contained, by the increase in the area ratio of residual austenite, a processing-induced phase transformation occurs with a small amount of deformation at the time of crush deformation, and thus sometimes leads to a decrease in absorbed energy. In the case where the C content is less than 0.15%, such a problem is easily avoided. The C content can also be 0.13% or less.

[0131] (Si: 0.01 to 2.00%)

[0132] Si is an element that functions as a deoxidizer, and suppresses the precipitation of carbides during the cooling process in the cold-rolling annealing. In the case where the Si content is 0.01% or more, such an effect is easily obtained. The Si content can also be 0.10% or more. On the other hand, if Si is excessively contained, with the increase in the strength of the steel, the machinability decreases, and further, coarse oxides are dispersed in the surface layer of the hot-rolled sheet, and it is difficult to obtain the desired unevenness on the surface of the steel sheet after the cold-rolling annealing, and thus sometimes the absorbed energy at the time of crush deformation decreases. In the case where the Si content is 2.00% or less, such a problem is easily avoided. The Si content can also be 1.60% or less.

[0133] (Mn: 0.10 to 4.00%)

[0134] Mn is a factor that affects the ferrite phase transformation of the steel, and is an element effective for the increase in strength. In the case where the Mn content is 0.10% or more, such an effect is easily obtained. The Mn content can also be 0.60% or more. On the other hand, if Mn is excessively contained, with the increase in the strength of the steel, the machinability decreases, and further, coarse oxides are dispersed in the surface layer of the hot-rolled sheet, and it is difficult to obtain the desired unevenness on the surface of the steel sheet after the cold-rolling annealing, and thus sometimes the absorbed energy at the time of crush deformation decreases. In the case where the Mn content is 4.00% or less, such a problem is easily avoided. The Mn content can also be 3.00% or less.

[0135] (P: 0.0200% or less)

[0136] P is an element that promotes the concentration of Mn into the un-solidified portion during the solidification of the molten steel, and is an element that reduces the Mn concentration of the negative segregation portion and promotes the increase in the area ratio of ferrite, and is more preferably less. In addition, if P is excessively contained, it leads to brittle fracture of the steel as the strength of the steel increases, and sometimes promotes the reduction in the absorbed energy at the time of press-forming. The P content can be 0%, or can be 0.0001% or more, or can be 0.0010% or more, and in addition, can be 0.0200% or less, or can be 0.0180% or less.

[0137] (S: 0.0200% or less)

[0138] S is an element that generates non-metallic inclusions such as MnS in the steel, and leads to a reduction in the ductility of the steel member, and is more preferably less. In addition, if S is excessively contained, it leads to the generation of cracking with non-metallic inclusions as the starting point at the time of press-forming, and it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold-rolling annealing, and sometimes reduces the absorbed energy at the time of press-forming. The S content can be 0%, or can be 0.0001% or more, or can be 0.0005% or more, and in addition, can be 0.0200% or less, or can be 0.0180% or less.

[0139] (Al: 0.001 to 1.000%)

[0140] Al is an element that stabilizes ferrite as a deoxidizer of the steel, and is added as needed. In the case where the Al content is 0.001% or more, such an effect is easily obtained. The Al content can be 0.010% or more. On the other hand, if Al is excessively contained, the ferrite transformation and bainite transformation during cooling in annealing are sometimes excessively promoted, and the strength of the steel sheet is reduced. In addition, if Al is excessively contained, coarse and large amounts of Al oxides are generated on the surface of the steel sheet during the middle of hot-rolling, it is difficult to obtain the desired unevenness on the surface of the steel sheet, and sometimes leads to a reduction in the absorbed energy at the time of press-forming. In the case where the Al content is 1.000% or less, such a problem is easily avoided. The Al content can be 0.800% or less.

[0141] (N: 0.0200% or less)

[0142] N is an element that forms coarse nitrides in the steel sheet, which reduces the workability of the steel sheet. In addition, N is an element that is a cause of generation of blowholes at the time of welding. Furthermore, if N is excessively contained, a large amount of AlN or TiN is generated by combining with Al and Ti, and these nitrides inhibit the contact of the steel sheet surface with the roll in hot rolling, so it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling annealing, and sometimes leads to a decrease in absorbed energy at the time of press-forming deformation. The N content can be 0%, or can be 0.0001% or more, or can be 0.0010% or more, and in addition, can be 0.0200% or less, or can be 0.0160% or less.

[0143] The basic chemical composition of the steel sheet in the present embodiment is as described above. Furthermore, the steel sheet in the present embodiment can also contain at least one of the following optional elements as needed. These elements can also not be contained, so the lower limit thereof is 0%.

[0144] (Ti: 0 to 0.500% or less)

[0145] Ti is a strengthening element. By precipitate strengthening, fine grain strengthening due to growth inhibition of the crystal grains, and dislocation strengthening by inhibition of recrystallization, the strength of the steel sheet is increased. On the other hand, if Ti is excessively contained, the precipitation of coarse carbides increases, and these carbides inhibit the contact of the steel sheet surface with the roll in hot rolling, so it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling annealing, and sometimes leads to a decrease in absorbed energy at the time of press-forming deformation. The Ti content can be 0%, or can be 0.001% or more, or can be 0.005% or more, and in addition, can be 0.500% or less, or can be 0.400% or less.

[0146] (Co: 0 to 0.500% or less)

[0147] Co is an element effective for the control of the morphology of carbides and the increase in strength, and is added as needed in order to control the strength. On the other hand, if Co is excessively contained, a large amount of fine Co carbides are precipitated, and these carbides inhibit the contact of the steel sheet surface with the roll in hot rolling, so it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling annealing, and sometimes leads to a decrease in absorbed energy at the time of press-forming deformation. The Co content can be 0%, or can be 0.001% or more, and in addition, can be 0.500% or less, or can be 0.400% or less.

[0148] (Ni: 0 to 0.500% or less)

[0149] Ni is a strengthening element, and is effective for increasing quenchability. In addition, since it brings about an increase in wettability of the steel sheet and the plated layer, and promotes alloying reaction, it can also be added. On the other hand, if Ni is excessively contained, it has an influence on the peelability of the scale at the time of hot rolling, promotes the generation of damage on the surface of the steel sheet, and it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes results in a decrease in absorbed energy at the time of crush deformation. The Ni content can also be 0%, and can also be 0.001% or more, and in addition, can also be 0.500% or less, and can also be 0.400% or less.

[0150] (Mo: 0 to 0.500% or less)

[0151] Mo is an element effective for increasing the strength of the steel sheet. In addition, Mo is an element having an effect of suppressing the ferrite phase transformation generated at the time of heat treatment using a continuous annealing apparatus or a continuous hot dip galvanizing apparatus. On the other hand, if Mo is excessively contained, many fine Mo carbides are precipitated, and these carbides inhibit the contact of the surface of the steel sheet with the roll in hot rolling, and it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes results in a decrease in absorbed energy at the time of crush deformation. The Mo content can also be 0%, and can also be 0.001% or more, and in addition, can also be 0.500% or less, and can also be 0.400% or less.

[0152] (Cr: 0 to 2.000% or less)

[0153] Cr is an element effective for suppressing the pearlite phase transformation and increasing the strength of the steel, like Mn, and is added as necessary. On the other hand, if Cr is excessively contained, the generation of residual austenite is promoted, and the starting point of fracture at the time of crush deformation increases due to the presence of excessive residual austenite, and sometimes results in a decrease in absorbed energy at the time of crush deformation. The Cr content can also be 0%, and can also be 0.001% or more, and in addition, can also be 2.000% or less, and can also be 1.500% or less.

[0154] (O: 0 to 0.0100% or less)

[0155] O Since oxides are formed, processability is deteriorated, and thus the content needs to be suppressed. In particular, oxides exist mostly as inclusions, and if granular and coarse oxides exist on the surface of the steel sheet, cracking of the surface of the steel sheet and generation of fine iron powder are caused in hot rolling, and desired unevenness is difficult to obtain on the surface of the steel sheet after cold rolling and annealing, and sometimes reduction of absorbed energy at the time of press-forming is caused. The content of O can also be 0.0100% or less, and can also be 0.0080% or less. Note that it is desirable that the content of O be 0%, but controlling the content of O to be less than 0.0001% can possibly cause an increase in manufacturing cost as the refining time increases. From the viewpoint of preventing an increase in manufacturing cost, the content of O can also be 0.0001% or more, and can also be 0.0010% or more.

[0156] (B: 0 to 0.0100% or less)

[0157] B is an element that suppresses generation of ferrite and pearlite and promotes generation of low-temperature phase transformation structures such as bainite or martensite in the cooling process from austenite. In addition, B is an element that is beneficial to high-strength of steel, and is added as necessary. On the other hand, if B is excessively contained, coarse B oxides are generated in the steel, and the B oxides suppress contact of the surface of the steel sheet with the roll in hot rolling, and thus desired unevenness is difficult to obtain on the surface of the steel sheet after cold rolling and annealing, and sometimes reduction of absorbed energy at the time of press-forming is caused. The content of B can also be 0%, and can also be 0.0001% or more, and can also be 0.0010% or more, and in addition, can also be 0.0100% or less, and can also be 0.0080% or less.

[0158] (Nb: 0 to 0.500% or less)

[0159] Nb is an element that is effective for control of the morphology of carbides, and is also an element that is effective for improvement of toughness because the structure is refined by addition thereof. On the other hand, if Nb is excessively contained, many fine and hard Nb carbides are precipitated, and these carbides suppress contact of the surface of the steel sheet with the roll in hot rolling, and thus desired unevenness is difficult to obtain on the surface of the steel sheet after cold rolling and annealing, and sometimes reduction of absorbed energy at the time of press-forming is caused. The content of Nb can also be 0%, and can also be 0.001% or more, and in addition, can also be 0.500% or less, and can also be 0.400% or less.

[0160] (V: 0 to 0.500% or less)

[0161] V is a strengthening element. By precipitate strengthening, fine particle strengthening using growth inhibition of ferrite grains, and dislocation strengthening by inhibition of recrystallization, the strength of the steel sheet is increased. On the other hand, if V is excessively contained, the precipitation of carbonitride increases, and these carbonitrides inhibit the contact of the steel sheet surface with the roll during hot rolling, so it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes the absorbed energy at the time of press-forming deformation is reduced. The V content can also be 0%, or 0.001% or more, and in addition, it can be 0.500% or less, or 0.400% or less.

[0162] (Cu: 0 to 0.500% or less)

[0163] Cu is an element effective for the increase in the strength of the steel sheet. On the other hand, if Cu is excessively contained, the steel becomes brittle during hot rolling, and becomes unable to be hot-rolled. Furthermore, the contact of the steel sheet surface with the roll during hot rolling is inhibited by the Cu layer concentrated on the surface of the steel sheet, so it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes the absorbed energy at the time of press-forming deformation is reduced. The Cu content can also be 0%, or 0.001% or more, and in addition, it can be 0.500% or less, or 0.400% or less.

[0164] (W: 0 to 0.1000% or less)

[0165] W is effective for the increase in the strength of the steel sheet, and the precipitates and crystals containing W become hydrogen trapping sites. On the other hand, if W is excessively contained, coarse carbides are generated, and these carbides inhibit the contact of the steel sheet surface with the roll during hot rolling, so it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes the absorbed energy at the time of press-forming deformation is reduced. The W content can also be 0%, or 0.0001% or more, or 0.0010% or more, and in addition, it can be 0.1000% or less, or 0.0800% or less.

[0166] (Ta: 0 to 0.1000% or less)

[0167] Ta is an element effective for the control of the morphology of carbides and the increase in the strength, like Nb, V, and W, and is added as necessary. On the other hand, if Ta is excessively contained, many fine Ta carbides are precipitated, and these carbides inhibit the contact of the steel sheet surface with the roll during hot rolling, so it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes the absorbed energy at the time of press-forming deformation is reduced. The Ta content can also be 0%, or 0.0001% or more, or 0.0010% or more, and in addition, it can be 0.1000% or less, or 0.0800% or less.

[0168] (Sn: 0 to 0.0500% or less)

[0169] Sn is an element contained in steel when scrap iron is used as a raw material, and is more preferably less. If Sn is excessively contained, cracking of the surface of the steel sheet and generation of fine iron powder occur in hot rolling, and it is difficult to obtain desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes, reduction in absorbed energy at the time of press-forming deformation occurs. The Sn content can also be 0.0500% or less, or 0.0400% or less. Note that it is desirable that the Sn content be 0%, but controlling the Sn content to be less than 0.0001% can possibly lead to an increase in manufacturing cost as the refining time increases. From the viewpoint of preventing an increase in manufacturing cost, the Sn content can also be 0.0001% or more, or 0.0010% or more.

[0170] (Sb: 0 to 0.0500% or less)

[0171] Sb is an element contained when scrap iron is used as a steel raw material, like Sn. Sb causes embrittlement of grain boundaries and reduction in ductility due to strong segregation in the grain boundaries, and is more preferably less. Furthermore, if Sb is excessively contained, cracking of the surface of the steel sheet and generation of fine iron powder occur in hot rolling, and it is difficult to obtain desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes, reduction in absorbed energy at the time of press-forming deformation occurs. The Sb content can also be 0.0500% or less, or 0.0400% or less. Note that it is desirable that the Sb content be 0%, but controlling the Sn content to be less than 0.0001% can possibly lead to an increase in manufacturing cost as the refining time increases. From the viewpoint of preventing an increase in manufacturing cost, the Sb content can also be 0.0001% or more, or 0.0010% or more.

[0172] (As: 0 to 0.0500% or less)

[0173] As is an element that is contained when scrap iron is used as a steel raw material, and strongly segregates in grain boundaries, like Sn and Sb, and is more preferably less. Furthermore, if As is excessively contained, cracking of the surface of the steel sheet and generation of fine iron powder occur in hot rolling, and it is difficult to obtain desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes, reduction in absorbed energy at the time of press-forming deformation occurs. The As content can also be 0.0500% or less, or 0.0400% or less. Note that it is desirable that the As content be 0%, but controlling the As content to be less than 0.0001% can possibly lead to an increase in manufacturing cost as the refining time increases. From the viewpoint of preventing an increase in manufacturing cost, the As content can also be 0.0001% or more, or 0.0010% or more.

[0174] (Mg: 0 to 0.0500% or less)

[0175] Mg is an element that can control the morphology of sulfides by trace addition, and is added as needed. On the other hand, if Mg is excessively contained, coarse inclusions are formed, which inhibit the contact of the steel sheet surface with the roll in hot rolling, and thus it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes leads to a decrease in absorbed energy at the time of press-forming deformation. The Mg content can also be 0%, or can be 0.0001% or more, or can be 0.0010% or more, and in addition, can be 0.0500% or less, or can be 0.0400% or less.

[0176] (Ca: 0 to 0.0500% or less)

[0177] Ca is effective not only as a deoxidizing element but also for the control of the morphology of sulfides. On the other hand, if Ca is excessively contained, cracking of the steel sheet surface and the generation of fine iron powder are caused in hot rolling, and it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes leads to a decrease in absorbed energy at the time of press-forming deformation. The Ca content can also be 0%, or can be 0.0001% or more, or can be 0.0010% or more, and in addition, can be 0.0500% or less, or can be 0.0400% or less.

[0178] (Y: 0 to 0.0500% or less)

[0179] Y, like Mg and Ca, is an element that can control the morphology of sulfides by trace addition, and is added as needed. On the other hand, if Y is excessively contained, coarse Y oxides are generated, which inhibit the contact of the steel sheet surface with the roll in hot rolling, and thus it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes leads to a decrease in absorbed energy at the time of press-forming deformation. The Y content can also be 0%, or can be 0.0001% or more, or can be 0.0010% or more, and in addition, can be 0.0500% or less, or can be 0.0400% or less.

[0180] (Zr: 0 to 0.0500% or less)

[0181] Zr, like Mg, Ca, and Y, is an element that can control the morphology of sulfides by trace addition, and is added as needed. On the other hand, if Zr is excessively contained, coarse Zr oxides are generated, which inhibit the contact of the steel sheet surface with the roll in hot rolling, and thus it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes leads to a decrease in absorbed energy at the time of press-forming deformation. The Zr content can also be 0%, or can be 0.0001% or more, or can be 0.0010% or more, and in addition, can be 0.0500% or less, or can be 0.0400% or less.

[0182] (La: 0 to 0.0500% or less)

[0183] La is an element effective for the morphology control of sulfides by trace addition, and is added as necessary. On the other hand, if La is excessively contained, La oxide is generated, which inhibits the contact of the steel sheet surface with the roll in hot rolling, and thus it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes leads to a decrease in the absorbed energy at the time of press-forming deformation. The La content can also be 0%, or 0.0001% or more, or 0.0010% or more, and in addition, it can be 0.0500% or less, or 0.0400% or less.

[0184] (Ce: 0 to 0.0500% or less)

[0185] Ce is an element capable of controlling the morphology of sulfides by trace addition, like La, and is added as necessary. On the other hand, if Ce is excessively contained, Ce oxide is generated, which inhibits the contact of the steel sheet surface with the roll in hot rolling, and thus it is difficult to obtain the desired unevenness on the surface of the steel sheet after cold rolling and annealing, and sometimes leads to a decrease in the absorbed energy at the time of press-forming deformation. The Ce content can also be 0%, or 0.0001% or more, or 0.0010% or more, and in addition, it can be 0.0500% or less, or 0.0400% or less.

[0186] In the steel sheet in the present embodiment, the remaining portion of the above-described components is Fe and impurities. The so-called impurities are components and the like mixed due to various factors of the manufacturing process, represented by raw materials such as ores, scrap iron, and the like, when the steel sheet of the present embodiment is industrially manufactured.

[0187] Next, the characteristics of the steel structure and the characteristics of the steel sheet of the present embodiment are described.

[0188] (total of area ratio of ferrite, pearlite, and bainite: 0 to 60.0%)

[0189] The total of the area ratios of ferrite, pearlite, and bainite is a structure effective for improving the strength and ductility balance of the steel sheet, but when contained in a large amount, it can cause a decrease in local ductility and a decrease in absorbed energy at the time of press-forming. In addition, from the viewpoint of effectively improving the strength of the steel, it is more preferable that the area ratios of ferrite, pearlite, and bainite be smaller. The total of the area ratios of ferrite, pearlite, and bainite can be 0%, or can be 1.0% or more, and in addition, can be 60.0% or less, can be 55.0% or less, or can be 50.0% or less. Note that although the productivity is slightly decreased, by controlling the continuous manufacturing conditions with high precision, the total of the area ratios of ferrite, pearlite, and bainite can be set to 0%.

[0190] (Area ratio of retained austenite: 0 to 1.0%)

[0191] The area ratio of retained austenite is a structure effective for improving the strength and ductility balance of the steel sheet. On the other hand, if the area ratio of retained austenite is too large, the proportion of chemically unstable austenite becomes large, and processing-induced phase transformation occurs with a small amount of deformation at the time of press-forming, and thus the absorbed energy can sometimes decrease. The area ratio of retained austenite can be 0%, or can be 0.1% or more, and in addition, can be 1.0% or less, or can be 0.8% or less.

[0192] (Remaining portion: martensite and tempered martensite)

[0193] Martensite and tempered martensite are structures extremely effective for increasing the strength of the steel sheet, and it is more preferable that the area ratio thereof be higher. For example, the remaining portion other than the above-described structures can also be composed of martensite and tempered martensite. The total of the area ratios of martensite and tempered martensite can be 30.0% or more, can be 35.0% or more, can be 40.0% or more, can be 45.0% or more, or can be 50.0% or more, and in addition, can be 100%, or can be 99.0% or less. Note that although the productivity is decreased, by controlling the continuous manufacturing conditions with high precision, the total of the area ratios of martensite and tempered martensite can be set to 100%.

[0194] (Surface unevenness)

[0195] The spacing of step differences with a height difference exceeding 5.0 μm on the surface of the steel plate is an important factor that functions as the starting point for bending deformation of the steel plate when subjected to crushing deformation. A shorter spacing is preferred. Specifically, in the steel plate of this embodiment, it is important that multiple step differences with a height difference exceeding 5.0 μm exist on the surface of the steel plate at intervals of 2.0 mm or less. This interval can also be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, 1.0 mm or less, 0.7 mm or less, or 0.4 mm or less. It should be noted that if the interval is less than 0.01 mm, the surface of the steel plate may sometimes become serrated. In this regard, the interval can also be 0.01 mm or more, or 0.05 mm or more. Furthermore, in the steel plate of this embodiment, multiple step differences with a height difference exceeding 5.0 μm must exist dispersedly on the surface of the steel plate at the above-mentioned intervals. Especially when multiple step differences with a height difference of 7.0 μm or more or 10.0 μm or more are dispersed on the surface of the steel plate at the aforementioned intervals, the energy absorption capacity of the steel plate during crushing deformation is even more excellent. There is no particular limitation on the upper limit of the height difference of the step difference; for example, it can be 20.0 μm or less, 15.0 μm or less, or 10.0 μm or less. In the steel plate of this embodiment, multiple step differences with a height difference of more than 5.0 μm can also exist at intervals of 2.0 mm or less in 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more of the surface area of ​​the steel plate.

[0196] Figure 1 An example of "a step difference with a height difference exceeding 5.0 μm" is shown. Figure 1 This describes the shape of the step difference when observing a cross-section in the thickness direction of a steel plate. For example... Figure 1 As shown, unevenness can be repeatedly formed on the surface of the steel plate along the rolling direction, with the height difference of specific steps exceeding 5.0 μm and including multiple such steps within a range of 2.0 mm, i.e., the interval between the steps is less than 2.0 mm. In this invention, at least a portion of the multiple steps may contain a so-called negative angle portion (lower recessed portion). Furthermore, in this invention, the heights of the multiple steps may differ from each other; for example, their heights may be irregular (random). Furthermore, the shapes of the multiple steps may also differ from each other. Furthermore, the intervals of the multiple steps are not necessarily fixed and may also be irregular (random). Such a step shape can be formed by the method described later.

[0197] It should be noted that the term "step difference with a height difference exceeding 5.0 μm" used in this application is a different concept from general surface roughness such as maximum height roughness Rz or arithmetic mean roughness Ra. For example, "maximum height roughness Rz" is... Figure 2 As shown in (A), this refers to the distance between the most convex and the most concave parts of the surface roughness (maximum height difference). Furthermore, the distribution (interval) of surface roughness cannot be specifically defined by the "maximum height roughness Rz". Additionally, the "arithmetic mean roughness Ra" is ultimately the average surface roughness, and its maximum value is unclear. Furthermore, the distribution (interval) of surface roughness cannot be specifically defined by the "arithmetic mean roughness Ra". In contrast, the "step difference with a height difference exceeding 5.0 μm" mentioned in this application... Figure 2 As shown in (B), it means that the height difference of “one step difference” exceeds 5.0 μm, and that the step difference must exist in multiples at intervals of less than 2.0 mm.

[0198] (Yield Strength)

[0199] To improve the lightweight of structures using steel as a raw material and the yield point at which plastic deformation begins, the yield strength of the steel raw material is preferably high. On the other hand, if the yield strength is too high, the shape change caused by elastic deformation after plastic processing, the so-called springback effect, may become greater, resulting in reduced formability. The yield strength of the steel plate in this embodiment is not particularly limited, but it can be 500 MPa or more, 550 MPa or more, or 1100 MPa or less, or 1050 MPa or less.

[0200] (tensile strength)

[0201] To improve the lightweight nature of structures using steel as a raw material and the resistance of the structure during plastic deformation, the steel raw material is preferably one with high work hardening capacity, exhibiting maximum strength. On the other hand, if the tensile strength is too high, it may sometimes become prone to fracture due to low energy during plastic deformation, reducing formability. The tensile strength of the steel plate is not particularly limited, but it can be above 900 MPa, above 980 MPa, or below 1470 MPa, below 1410 MPa, below 1350 MPa, or below 1310 MPa.

[0202] (Total elongation rate)

[0203] When a steel sheet as a raw material is cold-formed to manufacture a structure, ductility becomes necessary in order to finish into a complicated shape. If the total elongation is too low, the raw material sometimes cracks in cold-forming. On the other hand, the higher the total elongation is, the more preferable it is, but if the total elongation is excessively increased, a large amount of residual austenite becomes necessary in the steel structure, whereby the absorbed energy at the time of crushing deformation sometimes decreases. The total elongation of the steel sheet is not particularly limited, but can be 5% or more, can be 8% or more, and in addition, can be 20% or less, can be 18% or less.

[0204] (Expanding property)

[0205] When a steel sheet as a raw material is cold-formed to manufacture a structure, expanding property becomes necessary in order to finish into a complicated shape, in addition to the ductility. If the expanding property is too small, the raw material sometimes cracks in cold-forming. On the other hand, the higher the expanding property is, the more preferable it is, but if the expanding property is excessively increased, a large amount of residual austenite becomes necessary in the steel structure, whereby the absorbed energy at the time of crushing deformation sometimes decreases. The expanding rate of the steel sheet is not particularly limited, but can be 20% or more, can be 25% or more, and in addition, can be 90% or less, can be 80% or less.

[0206] (Bending property)

[0207] When a steel sheet as a raw material is cold-formed to manufacture a structure, bending property becomes necessary in order to finish into a complicated shape. As an index of the bending property, for example, a VDA bending angle a obtained from a test according to the provisions of Standard 238-100 of the Verband der Automobilindustrie (VDA) becomes an index. If the VDA bending angle is too small, the raw material sometimes cracks in cold-forming. The higher the bending property is, the more preferable it is. The VDA bending angle of the steel sheet is not particularly limited, but can be 45° or more, can be 50° or more. Note that the VDA bending angle presented here is a characteristic value at a sheet thickness of 1.4 mm, and even with the same steel sheet, a higher bending angle value can be obtained at a sheet thickness lower than 1.4 mm. In addition, in the case where the sheet thickness exceeds 1.4 mm, it is preferable to remove the face of one side of the sheet by plane grinding, to finish the sheet thickness to 1.4 mm, and to obtain the bending angle with the ground face as the inside of the bend and the unground face as the outside of the bend.

[0208] (Sheet thickness)

[0209] The plate thickness is a factor that affects the rigidity of the formed steel member, and the greater the plate thickness, the higher the rigidity of the member. If the plate thickness is too small, it sometimes leads to a decrease in rigidity, and the press formability is decreased due to the inevitable non-ferrous inclusions present inside the steel sheet. On the other hand, if the plate thickness is too large, the press forming load increases, leading to a decrease in productivity and damage to the mold. The plate thickness of the steel sheet is not particularly limited, but can be 0.2 mm or more, and can be 6.0 mm or less. Note that the "steel sheet" in the present application can also be a single-layer steel sheet. The "single-layer steel sheet" herein refers to a steel sheet that is not a so-called multi-layer steel sheet, and refers to a steel sheet in which the joining interfaces of the base steel sheets to each other are not observed in the plate thickness direction when the cross section of the steel sheet is observed. For example, it is a steel sheet composed of one slab. The "plate thickness" of the above-described steel sheet is preferably the plate thickness of the single-layer steel sheet. In addition, the single-layer steel sheet can have a surface treatment layer such as a plated layer on its surface. That is, the "steel sheet" in the present application can also be a steel sheet having a single-layer steel sheet and a surface treatment layer.

[0210] Next, the observation and measurement method of the above-described prescribed structure, and the measurement and evaluation method of the above-described prescribed characteristics will be described.

[0211] (Measurement method of the total of the area ratios of ferrite, pearlite, and bainite)

[0212] The microstructure observation was performed by scanning electron microscopy (SEM). Before the observation, the sample for the microstructure observation was polished by wet polishing using sandpaper and polishing using diamond grains having an average particle size of 1 μm, and the observation surface was finished to a mirror surface, and then the microstructure was etched using a 3% nitric acid alcohol solution. The magnification for the observation was set to 3000 times, and 10 images of a field of view of 30 μm x 40 μm at a position of 1 / 4 of the thickness from the surface side of the steel sheet were randomly taken. The ratio of the microstructure was determined by a point counting method. For the obtained microstructure images, lattice points arranged at intervals of 3 μm in the vertical direction and 4 μm in the horizontal direction were defined in total of 100 points, and the microstructure present under the lattice points was discriminated, and the ratio of the microstructure contained in the steel sheet was determined from the average of 10. The ferrite was a blocky grain, and did not contain iron-based carbides having a length of 100 nm or more in the interior. The bainite was a collection of lath-shaped grains, and did not contain iron-based carbides having a length of 20 nm or more in the interior, or contained iron-based carbides having a length of 20 nm or more in the interior, and the carbides belonged to a single variant, i.e., a group of iron-based carbides extending in the same direction. Here, the group of iron-based carbides extending in the same direction means iron-based carbides having a difference of 5° or less in the extending direction of the group of iron-based carbides. The bainite counted one bainite grain from the bainite surrounded by grain boundaries having an orientation difference of 15° or more. Here, regarding the "grain boundaries having an orientation difference of 15° or more", the SEM-EBSD was used to determine by the following steps. The observation surface of the sample for the measurement was finished to a mirror surface by polishing before the measurement using SEM-EBSD, and further, the strain generated by the polishing was removed, and similarly to the observation using SEM described above, a field of view of 30 μm x 40 μm at a position of 1 / 4 of the thickness from the surface side of the steel sheet was set as the measurement range, and the crystal orientation data of B.C.C. iron was obtained by SEM-EBSD. The measurement using EBSD was performed using an EBSD detector attached to the SEM, and the interval (STEP) of the measurement was set to 0.05 μm. At this time, as the data acquisition software for the crystal orientation in the present application, software "OIM Data Collection TM (ver. 7)" manufactured by TSL SOLUTIONS Co., Ltd. or the like was used. In the crystal orientation MAP data of B.C.C. iron obtained under this measurement condition, regions having a confidence value (CI value) of less than 0.1 were excluded, and boundaries having a crystal orientation difference of 15° or more were determined as crystal grain boundaries. Note that the bainite can also be said to be a mixed microstructure of bainite ferrite composed of a body-centered cubic structure of iron and iron-based carbides (Fe3C). The bainite ferrite is distinguished from the ferrite described above. The pearlite was a microstructure containing cementite precipitated in a columnar shape, and a region taken in bright contrast in a two-time electron image was set as the pearlite, and the area ratio was calculated.

[0213] (Method for distinguishing between martensite and tempered martensite)

[0214] 444For martensite and tempered martensite, observation is performed using a scanning electron microscope and a transmission electron microscope, and an organization containing Fe-based carbide inside is identified as tempered martensite, and an organization containing almost no carbide is identified as martensite. As for the Fe-based carbide, it is reported that Fe-based carbides having various crystal structures can be contained, and any Fe-based carbide can be contained. Depending on the heat treatment conditions, various Fe-based carbides can sometimes exist. In the present application, the area ratio Al of ferrite, pearlite, and bainite is measured by the above method, the area ratio A2 of residual austenite is measured by the method described later, and the remaining portion obtained by subtracting the sum of the area ratios Al and A2 from 100% is regarded as the area ratio of the sum of martensite and tempered martensite. 445

[0215] 446(Method for measuring area ratio of residual austenite) 447

[0216] 448The area ratio of residual austenite is determined by X-ray measurement as follows. First, a portion from the surface of the steel sheet to 1 / 4 of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and measurement is performed by using Mo Kα rays as characteristic X-rays to the chemically polished surface. Then, the area ratio of residual austenite in the center portion of the sheet thickness is calculated using the following formula from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the body-centered cubic lattice (bcc) phase, and (200), (220), and (311) of the face-centered cubic lattice (fcc) phase. 449

[0217] 450Sγ = (I200f + I220f + I311f) / (I200b + I211b) x 100 451

[0218] 452(Sγ is the area ratio of residual austenite in the center portion of the sheet thickness, I200f, I220f, and I311f represent the intensities of the diffraction peaks of (200), (220), and (311) of the fcc phase, respectively, and I200b and I211b represent the intensities of the diffraction peaks of (200) and (211) of the bcc phase, respectively.) 453

[0219] 454The sample for X-ray diffraction can be measured by thinning the steel sheet from the surface to a predetermined sheet thickness by mechanical polishing or the like, and then removing strain by chemical polishing or electrolytic polishing or the like, while adjusting the sample in accordance with the above method so that the appropriate surface becomes the measurement surface in the range of 1 / 8 to 3 / 8 of the sheet thickness. Of course, the above limitation of X-ray intensity is satisfied not only for the vicinity of 1 / 4 of the sheet thickness, but also for as much thickness as possible, so that the material anisotropy is further reduced. However, by performing measurement at a distance of 1 / 8 to 3 / 8 from the surface of the steel sheet, the material properties of the entire steel sheet can be roughly represented. Therefore, 1 / 8 to 3 / 8 of the sheet thickness is set as the measurement range. 455

[0220] 456(Measurement method of interval of surface unevenness (step difference of height difference exceeding 5.0 μm))

[0221] The height difference of the unevenness on the surface of the steel sheet and the interval of the distribution thereof were measured by a scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope). Before observation using the SEM, a sample for microstructure observation having a length of 20 mm in the rolling direction was embedded in a resin, and a face (TD face: Transversal Direction face) parallel to the rolling direction and perpendicular to the sheet thickness direction was finished to a mirror surface by polishing. The observation magnification of the SEM was set to 1000 times, and a field of view within an observation range in which the steel sheet and the resin were simultaneously taken in over 110 μm in the rolling direction and over 70 μm in the sheet thickness direction within 20 mm in the rolling length direction was taken, and a continuous photograph in which the surface unevenness of the steel sheet was taken in was obtained. In the continuous photograph, a site in which the height difference of the surface unevenness of the steel sheet exceeded 5 μm within a range of 20 μm in the length of the rolling direction was defined as "step difference having a height difference exceeding 5.0 μm on the surface of the steel sheet", and the average of the interval between the top of the step difference within the photographing range of the continuous photograph, that is, the interval between the top and the top within 20 mm in the length of the rolling direction was set as "interval of step difference having a height difference exceeding 5.0 μm on the surface of the steel sheet". Note that, in the present application, a slight unevenness having a height difference of 1.0 μm or less was not defined as "step difference".

[0222] Note that, even after the steel sheet is formed / processed into a certain member, by taking a part (e.g., a flat portion) of the formed / processed member and analyzing the surface state thereof, it is possible to determine whether the member has a step difference having a height difference exceeding 5.0 μm at an interval of 2.0 mm or less in the state of the steel sheet before the forming / processing.

[0223] (Measurement method of yield strength, tensile strength, and total elongation)

[0224] The tensile test for measuring the yield strength, the tensile strength, and the total elongation was performed according to JIS Z 2241, using JIS No. 5 test pieces taken from a direction in which the length direction of the test piece became parallel to the rolling direction of the steel sheet.

[0225] (Measurement method of hole expandability)

[0226] As to the hole expandability, a circular hole having a diameter of 10 mm was punched under the condition that the clearance became 12.5%, the burr became the punch side, and the forming was performed using a 60° conical punch, and the hole expansion ratio λ (%) was evaluated. The hole expansion test was performed 5 times under each condition, and the average value thereof was set as the hole expansion ratio.

[0227] <Methods for manufacturing steel plates>

[0228] The steel plate manufacturing method of this embodiment uses materials with the composition range described above, and is characterized by continuous management of hot rolling, cold rolling, and annealing. Specifically, the steel plate manufacturing method of this embodiment is characterized by including the following steps: hot rolling a steel billet (slab billet) having the same chemical composition as described above using a pre-finishing mill with a lubricant at a predetermined reduction rate using a mill preceding the final finishing mill; coiling; pickling the obtained hot-rolled plate; cold rolling; and then annealing. More specifically, the steel plate manufacturing method of this embodiment is characterized by including:

[0229] A hot-rolled plate is obtained by hot rolling a steel slab with the above-mentioned chemical composition;

[0230] The hot-rolled sheet is coiled up;

[0231] The hot-rolled plates were pickled; and

[0232] The hot-rolled plates described above are annealed without cold rolling, or annealed after cold rolling.

[0233] The aforementioned hot rolling includes rolling the plate in the stand preceding the final stand of the finishing mill at a reduction rate of more than 30% and less than 70%, while supplying lubricant between the rolls and the plate.

[0234] The temperature at which the hot-rolled sheet is coiled is below 700℃.

[0235] In the case of the above-described cold rolling, the reduction rate during cold rolling is 0.1% to 20%. Hereinafter, each process will be described in detail, focusing on the key points of this embodiment.

[0236] (Reduction rate in the previous stand starting from the final stand of the finishing mill)

[0237] The reduction in the preceding stand from the final stand of the finishing mill is a factor that affects the surface state of the steel sheet. Here, by supplying a lubricant (for example, a water solvent mixed with a lubricant) to the surface of the rolled material (sheet) before rolling in the preceding stand from the final stand, high surface pressure is applied in a state in which the lubricant remains on the sheet surface, and rolling is performed, thereby intermittently imparting partial sliding and contact between the sheet and the roll surface during rolling, and the surface unevenness of the sheet can be improved. If the reduction is too small, the surface pressure between the sheet and the roll during rolling is insufficient, and thus it becomes impossible to form the desired surface unevenness on the finally obtained steel sheet. Further, if the reduction is too large, the surface pressure generated between the sheet and the roll during rolling becomes excessively high, and the frequency of contact between the sheet and the roll increases compared to sliding, and thus it is difficult to impart the desired surface unevenness to the finally obtained steel sheet. From the above viewpoint, in the present embodiment, the reduction in the preceding stand from the final stand of the hot rolling is more than 30% and 70% or less, and is preferably 35% or more and 60% or less. Note that in the final stand of the finishing mill, it is difficult to perform large reduction in order to correct the shape of the sheet. The reduction in the final stand of the finishing mill may, for example, be 20% or less.

[0238] Note that in the stand before the final stand, by supplying a lubricant and performing reduction at a reduction of 30% or more to form a step difference on the surface of the sheet, and thereafter, by controlling so that the cumulative reduction up to the final stand becomes light reduction (for example, cumulative reduction of 20% or less), it is also possible to form the desired step difference on the surface of the hot-rolled steel sheet after finishing. In this regard, the large reduction for improving the surface unevenness of the sheet can also be performed by a stand further upstream than the preceding stand of the final stand. However, on the upstream side in the finishing, the temperature of the sheet is high, and the shape of the surface of the sheet is easily changed by reduction. That is, after the large reduction, it is necessary to control the cumulative reduction while taking into account the effect of the temperature. In this regard, after supplying a lubricant and performing large reduction of 30% or more in the preceding stand of the final stand on the downstream side in the finishing, and performing light reduction in the final stand, the method of adjusting the shape of the sheet is likely to form the desired step difference on the surface of the steel sheet.

[0239] As the above-described lubricant, various lubricants can be used. For example, as a component of the lubricant, ester, mineral oil, polymer, fatty acid, S-based additive, Ca-based additive can be contained. The viscosity of the lubricant is preferably 250 mm 2 or less. The lubricant can also be used by being mixed with water as described above. The supply amount of the lubricant is not particularly limited, and for example, the surface of the steel sheet can be adhered with 0.1 g / m 2 or more or 1.0 g / m 2 or more and 100.0 g / m 2 or less or 50.0 g / m2 The lubricant. The means for supplying the lubricant is not particularly limited, and for example, the lubricant can be supplied by spraying onto the surface of the sheet.

[0240] (temperature at which the hot-rolled sheet is coiled)

[0241] The temperature at which the hot-rolled sheet is coiled is a factor that controls the generation state of the scale on the hot-rolled sheet and affects the strength of the hot-rolled sheet. In order to maintain the surface unevenness generated in the hot-rolling, the thickness of the scale generated on the surface of the hot-rolled sheet is preferably thin, and thus the coiling temperature is preferably low. Note that in the case of extremely lowering the coiling temperature, special equipment becomes necessary. Further, if the coiling temperature is excessively high, as described above, the scale generated on the surface of the hot-rolled sheet becomes significantly thick, and thus the convex portions of the unevenness formed on the surface of the hot-rolled sheet by the hot-rolling enter the scale, and the scale is removed by the subsequent pickling, and as a result, it is difficult to form the desired unevenness on the surface of the hot-rolled sheet. From the above viewpoint, the temperature at which the hot-rolled sheet is coiled is 700°C or lower, and can be 680°C or lower, and further, can be 0°C or higher, and can be 20°C or higher.

[0242] (reduction rate in cold rolling)

[0243] The reduction rate in cold rolling is an important factor in order to control the shape of the hot-rolled sheet and the unevenness on the surface of the steel sheet. In the case where cold rolling is performed, if the reduction rate is excessively small, the shape defect of the hot-rolled sheet cannot be corrected, and the bending of the steel sheet remains, and thus sometimes leads to a decrease in manufacturability in the subsequent annealing process, and a decrease in absorbed energy at the time of crushing deformation of a member formed into a square tube shape. On the other hand, if the reduction rate in cold rolling is excessively large, the convex portions of the unevenness formed on the surface of the hot-rolled sheet by rolling are crushed by cold rolling, and it is difficult to obtain the desired surface unevenness after the subsequent annealing. From the above viewpoint, in the case where cold rolling is performed, the reduction rate in this cold rolling is 0.1 to 20%. It is preferably 0.3% or more and 18.0% or less.

[0244] On the other hand, the hot-rolled sheet can be directly annealed without performing cold rolling. In this case, a steel sheet having a desired surface unevenness is also easily obtained in the end.

[0245] Hereinafter, a preferred embodiment of a method for manufacturing a steel sheet excellent in energy absorption at the time of crushing deformation will be described in detail. The following description is an example of a preferred embodiment of the finishing temperature of hot-rolling, heat treatment in annealing, and plating treatment, and does not limit the method for manufacturing the steel sheet of the present embodiment in any way.

[0246] (finishing temperature of hot-rolling)

[0247] The finish rolling temperature of hot rolling is a factor that gives an effect to the control of the texture of the original austenite grain size. From the viewpoint of the development of the rolling texture of austenite, the generation of anisotropy of the properties of steel material, the finish rolling temperature is preferably 650°C or higher, and in addition, from the viewpoint of the suppression of the bias of the texture generated by abnormal grain growth of austenite, the finish rolling temperature is preferably set to, for example, 940°C or lower.

[0248] (Annealing atmosphere)

[0249] In order to prevent the diffusion of an easily oxidizable element to the surface of the steel sheet and promote internal oxidation, the control of the oxygen potential in the heating zone at the time of annealing is important. Specifically, the annealing is preferably performed in an atmosphere containing 0.1 to 30% by volume of hydrogen and H2O having a dew point of -40 to 20°C, with the remainder being nitrogen and impurities. More preferably, it is an atmosphere containing 0.5 to 20% by volume of hydrogen and H2O having a dew point of -30 to 15°C, and further preferably, it is an atmosphere containing 1 to 10% by volume of hydrogen and H2O having a dew point of -20 to 10°C.

[0250] (Annealing temperature)

[0251] In the case where the maximum heating temperature at the time of annealing is too low, sometimes the carbonitride formed at the time of hot rolling is excessively time-consuming to re-dissolve, the carbonitride, or a part thereof, remains, or martensite is not sufficiently obtained after cooling, and thus it is difficult to ensure the strength of the steel sheet. On the other hand, excessive high temperature heating not only leads to an increase in cost, but also the shape of the sheet becomes poor at the time of high temperature passage, or the life of the roll is reduced to induce a failure. From the above viewpoints, the maximum heating temperature at the time of annealing (annealing holding temperature) is preferably 750°C or higher, and in addition, it is preferably 900°C or lower.

[0252] (Annealing holding time)

[0253] At the time of annealing, it is preferable to maintain for 5 seconds or more at the above-mentioned heating temperature. This is because if the holding time is too short, sometimes the progress of the austenite phase transformation of the base steel sheet becomes insufficient, and the decrease in strength becomes significant. In addition, the recrystallization of the ferrite structure becomes insufficient, and the unevenness of hardness also becomes large. From these viewpoints, the holding time is more preferably 10 seconds or more. Further preferably, it is 20 seconds or more.

[0254] (Cooling rate after annealing)

[0255] In the cooling after the above annealing, it is preferable to cool from 750°C to 550°C or lower at an average cooling rate of 100°C / s or less. The lower limit of the average cooling rate is not particularly limited, but for example, it is preferably 2.5°C / s. The reason for setting the lower limit of the average cooling rate to 2.5°C / s is to suppress the generation of ferrite phase transformation in the base material steel sheet and to soften the base material steel sheet. In the case where the average cooling rate is too slow, the strength easily decreases. It is more preferable to be 5°C / s or more, further preferable to be 10°C / s or more, and further preferable to be 20°C / s or more. Note that, when it is 750°C or more, the ferrite phase transformation is not easily generated significantly, and thus the cooling rate is not limited. Also, when it is 550°C or less, a low-temperature phase transformation structure can be obtained, and thus the cooling rate is not limited. In the case where the cooling rate is too fast, a low-temperature phase transformation structure is also generated in the surface layer of the steel sheet, and becomes a cause of hardness unevenness. In this regard, the average cooling rate is preferably 100°C / s or less, more preferably 50°C / s or less, and further preferably 20°C / s or less.

[0256] (Cooling stop temperature and reheating after annealing)

[0257] Further, after the above cooling, further cooling to a temperature of 25°C to 550°C is performed, and then, in the case where the cooling stop temperature is lower than the plating bath temperature, reheating to a temperature region of 350°C to 550°C and retention can be performed. If the cooling is performed in the above temperature range, martensite is generated from untransformed austenite in the cooling. Thereafter, by performing reheating, the martensite is tempered, and precipitation of carbides in the hard phase, recovery / rearrangement of dislocations, and improvement of hydrogen embrittlement resistance are caused. The lower limit of the cooling stop temperature is set to 25°C because excessive cooling not only requires a large equipment investment, but also the effect is saturated.

[0258] (Retention temperature)

[0259] Further, after the reheating and before the plating bath immersion, the steel sheet in a temperature region of 350°C to 550°C can also be retained. The retention in this temperature region not only contributes to the tempering of the martensite, but also eliminates the temperature unevenness in the width direction of the sheet and improves the appearance after plating. Note that, in the case where the cooling stop temperature is 350°C to 550°C, the retention can be performed without reheating.

[0260] (Retention time)

[0261] The time for the retention is preferably set to 30 seconds or more and 300 seconds or less in order to obtain the effect thereof.

[0262] (Tempering)

[0263] In a series of annealing processes, reheating can also be started after the cold-rolled sheet or the steel sheet on which plating treatment has been performed on the cold-rolled sheet is cooled to room temperature or halfway through the cooling (Ms or below), and held in a temperature range of 150°C or higher and 400°C or lower for 2 seconds or more. If this process is utilized, tempered martensite is produced by tempering the martensite generated in the cooling after the reheating, and the resistance to hydrogen embrittlement can be improved. In the case where the tempering process is performed, in the case where the holding temperature is too low or the holding time is too short, the martensite is not sufficiently tempered, and there is little change in the microstructure and mechanical properties. On the other hand, if the holding temperature is too high, the dislocation density in the tempered martensite decreases, resulting in a decrease in the tensile strength. Therefore, in the case where tempering is performed, it is preferable to hold in a temperature range of 150°C or higher and 400°C or lower for 2 seconds or more. The tempering can also be performed in a continuous annealing apparatus or by an additional apparatus offline after the continuous annealing. At this time, the tempering time varies depending on the tempering temperature. That is, the lower the temperature, the longer the time, and the higher the temperature, the shorter the time.

[0264] (Plating)

[0265] For the steel sheet, hot-dip galvanizing can also be performed by heating or cooling as necessary to (zinc plating bath temperature - 40)°C to (zinc plating bath temperature + 50)°C. By the hot-dip galvanizing process, a hot-dip galvanizing layer is formed on the surface of the steel sheet. In this case, the corrosion resistance of the cold-rolled steel sheet is improved, and thus is preferable. For example, in the manufacturing method of the present embodiment, a coating layer formed of zinc, aluminum, magnesium, or an alloy thereof can also be formed on the front and back surfaces of the sheet in the annealing. Alternatively, the coating layer can also be formed on the front and back surfaces of the sheet after the annealing.

[0266] (Temperature of steel sheet after immersion in plating bath)

[0267] In the case where alloying treatment is performed on the hot-dip galvanizing layer, the steel sheet on which the hot-dip galvanizing layer is formed is heated to a temperature range of 450 to 550°C. If the alloying temperature is too low, it is possible that the alloying does not sufficiently proceed. On the other hand, if the alloying temperature is too high, the alloying excessively proceeds, the Fe concentration in the plated layer exceeds 15% through the generation of Γ phase, and thus it is possible that the corrosion resistance deteriorates. The alloying temperature is more preferably 470°C or higher and more preferably 540°C or lower. The alloying temperature needs to be changed depending on the composition of the steel sheet and the degree of formation of the internal oxidation layer, and thus it is only necessary to confirm the Fe concentration in the plated layer to perform the setting.

[0268] (Composition of plating bath)

[0269] The composition of the plating bath is preferably: Zn as the main component, and an effective Al amount (a value obtained by subtracting the total Fe amount from the total Al amount in the plating bath) of 0.050 to 0.250 mass%. If the effective Al amount in the plating bath is too small, it is likely that the progress of the immersion of Fe into the plated layer will proceed excessively, and the plating adhesion will decrease. On the other hand, if the effective Al amount in the plating bath is too large, it is likely that Al-based oxides that hinder the movement of Fe atoms and Zn atoms will be generated at the boundary between the steel sheet and the plated layer, and the plating adhesion will decrease. The effective Al amount in the plating bath is more preferably 0.065 mass% or more, and more preferably 0.180 mass% or less.

[0270] (Temperature of steel sheet when immersed in plating bath)

[0271] The temperature of the steel sheet when immersed in the hot-dip galvanizing bath is preferably in the temperature range from a temperature 40°C lower than the temperature of the hot-dip galvanizing bath (hot-dip galvanizing bath temperature - 40°C) to a temperature 50°C higher than the temperature of the hot-dip galvanizing bath (hot-dip galvanizing bath temperature + 50°C). If this temperature is lower than the hot-dip galvanizing bath temperature - 40°C, the heat removal when the plating bath is immersed is sometimes large, and a part of the molten zinc solidifies, deteriorating the appearance of the plating. In the case where the temperature of the sheet before immersion is lower than the hot-dip galvanizing bath temperature - 40°C, the temperature of the sheet can be controlled to be 40°C or higher than the hot-dip galvanizing bath temperature by further heating the sheet by an arbitrary method before immersion in the plating bath, and then the sheet can be immersed in the plating bath. Further, if the temperature of the steel sheet when immersed in the plating bath exceeds the hot-dip galvanizing bath temperature + 50°C, it is likely that problems in the operation accompanying the rise in the temperature of the plating bath will be induced.

[0272] (Pretreatment)

[0273] In order to further improve the plating adhesion, plating of Ni, Cu, Co, Fe alone or a plurality of them can be performed on the base steel sheet before annealing in the continuous hot-dip galvanizing production line.

[0274] (Post-treatment)

[0275] On the surface of the hot-dip galvanizing steel sheet and the alloyed hot-dip galvanizing steel sheet, upper plating or various treatments such as chromate treatment, phosphate treatment, lubricity improving treatment, and weldability improving treatment can be performed for the purpose of improving the paintability and the weldability.

[0276] (Skin reduction rate)

[0277] Further, in order to improve the ductility by correction of the shape of the steel sheet or introduction of movable dislocations, skin pass rolling can be performed. The reduction rate of the skin pass rolling after the heat treatment is preferably in the range of 0.1 to 2.0%. If it is less than 0.1%, the effect is small and control is difficult, so it is the lower limit. If it exceeds 2.0%, the productivity is significantly reduced, so it is set as the upper limit. The skin pass rolling can be performed on-line or off-line. Further, the skin pass rolling at the target reduction rate can be performed at one time or can be performed in multiple times. Further, since the strength of the steel sheet after annealing is higher than that of the hot-rolled sheet, the change in the surface unevenness when rolling is given at the same reduction rate is not the same, but from the viewpoint of maintaining the unevenness formed in the hot-rolled sheet, the total of the cold reduction rate and the skin pass rolling rate is preferably 20% or less.

[0278] According to the manufacturing method described above, the steel sheet of the embodiment described above can be obtained.

[0279] Example

[0280] Examples of the present application are shown below. The present application is not limited to this one example. The present application can employ various conditions as long as the purpose thereof is achieved without departing from the gist thereof.

[0281] Example 1

[0282] Steels having various chemical compositions were melted to manufacture billets. These billets were inserted into a furnace heated to 1220°C, given a homogenization treatment for 60 minutes, and taken out to the atmosphere, and hot-rolled to obtain steel sheets having a sheet thickness of 1.8 mm. In the hot-rolling, the reduction rate in the preceding stand from the final stand of the finishing mill was 35%, and the lubricant was supplied between the roll and the sheet in the preceding stand from the final stand, and the finishing was completed at a temperature of 910°C, and cooled to 550°C and coiled. Next, the scale of the hot-rolled steel sheet was removed by pickling, cold-rolled at a reduction rate of 12.0%, and the sheet thickness was finished to 1.4 mm. Further, the cold-rolled steel sheet was annealed, specifically, warmed up to 860°C, and the holding time in the temperature range was set to 130 seconds. Next, the cold-rolled steel sheet after the annealing was cooled and held at 280°C, and then skin pass rolling was performed. The chemical compositions obtained by analyzing samples collected from each of the obtained steel sheets are shown in Tables 1-1 to 1-4. Note that the remainder other than the components shown in Tables 1-1 to 1-4 is Fe and impurities. Further, Tables 2-1 and 2-2 are evaluation results of the properties of the steel sheets after the above-described processing heat treatment.

[0283] Note that in Tables 2-1 and 2-2, the measurement methods for the "area fraction of the structure of the cold-rolled and annealed sheet", the "tensile properties (tensile strength, total elongation, hole expansibility)", and the "interval of the step difference having a height difference of more than 5.0 μm on the surface of the sheet" are as described above.

[0284] As for the "absorbed energy at the time of axial crushing", it is evaluated by the axial crushing test of a hat-shaped member (50 mm square, 300 mm in length, joined to a back sheet of the same raw material at an interval of 30 mm by spot welding). First, the steel sheet obtained by the operation as described above is subjected to bending processing, and a molded product having the prescribed open cross-sectional shape as described above is produced. The end portion of the molded product is fixed, and a 900 kg weight is accelerated to fall from a height of 2 m so as to collide with the end side of the molded product at a speed of 22 km / h in the axial direction. From the load-displacement curve at the time of the axial crushing test, the impact absorbed energy until 100 mm crushing is calculated. The evaluation criteria for the absorbed energy are as described below. If the energy absorption is more than that indicated by OK (Δ), it can be said to be suitable for automobile use.

[0285] OK (pass): absorbed energy more than 5.5 kJ

[0286] OK (Δ): absorbed energy more than 4.5 kJ and 5.5 kJ or less

[0287] NG (fail): absorbed energy 4.5 kJ or less

[0288]

[0289]

[0290]

[0291]

[0292]

[0293]

[0294] From the results shown in Tables 2-1 and 2-2, the following is understood.

[0295] It is considered that AN-1, because of the too small C content in the steel, promotes the phase transition from austenite to ferrite, bainite, and pearlite at the time of annealing, and the tempered martensite and the martensite are insufficient, so that the strength of the steel is reduced. As a result, the absorbed energy at the time of axial crushing deformation of the finally obtained steel sheet is reduced.

[0296] It is considered that AO-1 has an increased area ratio of residual austenite due to an excessive C content in the steel, and thus a processing-induced phase transformation occurs at a small amount of deformation at the time of the axial crush deformation. As a result, the absorbed energy at the time of the axial crush deformation of the finally obtained steel sheet decreases.

[0297] It is considered that AP-1 has an increased strength of the steel due to an excessive Si content in the steel, and on the other hand, a decreased workability, and further, a dispersion of coarse oxides in the surface layer of the hot-rolled sheet, and thus it is difficult to obtain the desired unevenness at the time of the hot-rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crush deformation decreases.

[0298] It is considered that AQ-1 has an increased strength of the steel due to an excessive Mn content in the steel, and on the other hand, a decreased workability, and further, a dispersion of coarse oxides in the surface layer of the hot-rolled sheet, and thus it is difficult to obtain the desired unevenness at the time of the hot-rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crush deformation decreases.

[0299] It is considered that AR-1 has an increased strength of the steel due to an excessive P content in the steel, and on the other hand, a brittle fracture of the steel. As a result, the absorbed energy at the time of the axial crush deformation of the finally obtained steel sheet decreases.

[0300] It is considered that AS-1 becomes easy to generate a cracking with a non-metallic inclusion as a starting point at the time of the hot-rolling, and the cracking peels off from the steel sheet at the middle of the hot-rolling, and the surface of the steel sheet is abraded by fine powdered iron at the time of the hot-rolling, and thus it is difficult to obtain the desired unevenness at the time of the hot-rolling. Further, it is considered that it becomes easy to generate a cracking with a non-metallic inclusion as a starting point at the time of the crush deformation. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crush deformation decreases.

[0301] It is considered that AT-1 promotes a ferrite transformation and a bainite transformation during the cooling process of the annealing due to an excessive Al content in the steel, and thus the strength of the steel decreases, and the surface of the steel sheet is abraded by coarse and large amounts of Al oxides formed on the surface of the steel at the middle of the hot-rolling at the time of the hot-rolling, and thus it is difficult to generate a moderate deformation at the time of the hot-rolling, and it is difficult to obtain the desired unevenness. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crush deformation decreases.

[0302] It is considered that AU-1 has an excessive generation of nitrides in the steel due to an excessive N content in the steel, and the contact of the surface of the sheet with the roll is inhibited by the nitrides at the time of the hot-rolling, and thus it is difficult to obtain the desired unevenness at the time of the hot-rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crush deformation decreases.

[0303] It is considered that AV-1 generates coarse carbides in excess in the steel because of the excessive Ti content in the steel, and the contact of the plate surface with the roll is inhibited by the carbides in the hot rolling, and thus it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0304] It is considered that AW-1 generates Co carbides in excess in the steel because of the excessive Co content in the steel, and the contact of the plate surface with the roll is inhibited by the Co carbides in the hot rolling, and thus it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0305] It is considered that AX-1 affects the peeling property of the scale at the time of the hot rolling because of the excessive Ni content in the steel, and the generation of the damage in the plate surface is promoted. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0306] It is considered that AY-1 generates Mo carbides in excess in the steel because of the excessive Mo content in the steel, and the contact of the plate surface with the roll is inhibited by the Mo carbides in the hot rolling, and thus it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0307] It is considered that AZ-1 promotes the generation of residual austenite because of the excessive Cr content in the steel, and the starting point of the fracture at the time of the axial crushing deformation increases due to the presence of the excessive residual austenite. As a result, the absorbed energy at the time of the axial crushing deformation is reduced.

[0308] It is considered that BA-1 generates granular coarse oxides on the surface of the steel plate because of the excessive O content in the steel, and the cracking of the surface of the steel plate and the generation of fine iron powder are caused in the hot rolling, and thus it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0309] It is considered that BB-1 generates B oxides in the steel because of the excessive B content in the steel, and the contact of the plate surface with the roll is inhibited by the B oxides in the hot rolling, and thus it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0310] It is considered that BC-1 generates many Nb carbides in the steel because of the excessive amount of Nb in the steel, and the contact of the plate surface with the roll in the hot rolling is inhibited by the Nb carbides, so it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0311] It is considered that BD-1 generates many carbonitrides in the steel because of the excessive amount of V in the steel, and the contact of the plate surface with the roll in the hot rolling is inhibited by the carbonitrides, so it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0312] It is considered that BE-1 concentrates Cu on the plate surface because of the excessive amount of Cu in the steel, and the contact of the plate surface with the roll in the hot rolling is inhibited by the concentrated Cu, so it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0313] It is considered that BF-1 generates carbides in the steel because of the excessive amount of W in the steel, and the contact of the plate surface with the roll in the hot rolling is inhibited by the carbides, so it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0314] It is considered that BG-1 generates carbides in the steel because of the excessive amount of Ta in the steel, and the contact of the plate surface with the roll in the hot rolling is inhibited by the carbides, so it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0315] It is considered that BH-1 causes cracking of the plate surface and generation of fine iron powder in the hot rolling because of the excessive amount of Sn in the steel, so it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0316] It is considered that BI-1 causes cracking of the plate surface and generation of fine iron powder in the hot rolling because of the excessive amount of Sb in the steel, so it is difficult to obtain the desired unevenness in the hot rolling. As a result, the desired unevenness cannot be formed on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0317] It is considered that BJ-1 causes cracking of the surface of the steel sheet and generation of fine iron powder in the hot rolling due to the excessive As content in the steel, and it is difficult to obtain the desired unevenness in the hot rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0318] It is considered that BK-1 forms coarse inclusions in the steel due to the excessive Mg content in the steel, and the contact of the surface of the sheet with the roll in the hot rolling is inhibited by the inclusions, and it is difficult to obtain the desired unevenness in the hot rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0319] It is considered that BL-1 causes cracking of the surface of the steel sheet and generation of fine iron powder in the hot rolling due to the excessive Ca content in the steel, and it is difficult to obtain the desired unevenness in the hot rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0320] It is considered that BM-1 generates Y oxide in the steel due to the excessive Y content in the steel, and the contact of the surface of the sheet with the roll in the hot rolling is inhibited by the Y oxide, and it is difficult to obtain the desired unevenness in the hot rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0321] It is considered that BN-1 generates Zr oxide in the steel due to the excessive Zr content in the steel, and the contact of the surface of the sheet with the roll in the hot rolling is inhibited by the Zr oxide, and it is difficult to obtain the desired unevenness in the hot rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0322] It is considered that BO-1 generates La oxide in the steel due to the excessive La content in the steel, and the contact of the surface of the sheet with the roll in the hot rolling is inhibited by the La oxide, and it is difficult to obtain the desired unevenness in the hot rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0323] It is considered that BP-1 generates Ce oxide in the steel due to the excessive Ce content in the steel, and the contact of the surface of the sheet with the roll in the hot rolling is inhibited by the Ce oxide, and it is difficult to obtain the desired unevenness in the hot rolling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel sheet, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0324] As to A-1 to AM-1 in which the contents of the respective elements are within the prescribed range, a desired structure is obtained in the finally obtained steel sheet, and a desired unevenness is formed on the surface of the steel sheet, and as a result, the energy absorption property at the time of axial crush deformation is excellent.

[0325] (Example 2)

[0326] Further, in order to investigate the influence of the manufacturing conditions, the steel grades A to AM in which excellent properties were confirmed in Example 1 were taken as targets, the manufacturing conditions described in Table 3 were given to the processing heat treatment, cold-rolled steel sheets having a sheet thickness of 1.4 mm were produced, and the properties of the steel sheets after cold-rolling and annealing were evaluated. Here, the steel sheets on which plating was performed were immersed in a hot-dip galvanizing bath and then held at the temperatures shown in Tables 3-1 to 3-4, and alloyed hot-dip galvanized steel sheets on which an alloy plating layer of iron and zinc was given to the surface of the steel sheet were produced. Furthermore, during the period until the steel sheet held at each holding temperature was cooled to room temperature in the annealing of the cold-rolled sheet, tempering treatment in which the steel sheet temporarily cooled to 150°C was reheated and held for 2 seconds or more was given. The results obtained are shown in Tables 3-1 to 3-4. Note that the evaluation method of the properties was the same as in the case of Example 1.

[0327]

[0328] Table 3-2

[0329]

[0330]

[0331] Table 3-4

[0332]

[0333] From the results shown in Tables 3-1 to 3-4, the following was found.

[0334] It is considered that, in A-2 and AI-2, the convex portions of the unevenness formed on the surface of the sheet by hot-rolling were crushed by cold-rolling because the reduction rate in cold-rolling was too large. As a result, a desired unevenness could not be formed on the surface of the finally obtained steel sheet, and the absorbed energy at the time of axial crush deformation decreased.

[0335] It is considered that, in G-2, the lubricant was not supplied in the preceding stand from the final stand of the finishing mill in hot-rolling, and as a result, it became difficult for the sheet to slide with the roll. As a result, a desired unevenness could not be formed on the surface of the finally obtained steel sheet, and the absorbed energy at the time of axial crush deformation decreased.

[0336] It is considered that S-2 and AB-3 are such that the reduction ratio in the preceding stand from the final stand of the finishing mill is excessively large in hot rolling, and thus the surface pressure generated between the plate and the roll in the rolling is excessively high, and the frequency of contact between the plate and the roll is increased compared with the sliding. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0337] It is considered that AH-2 and O-3 are such that the temperature at the time of coiling the hot-rolled plate is excessively high, and thus the scale generated on the surface of the hot-rolled plate is significantly thickened, the convex portions of the unevenness formed on the surface of the hot-rolled plate by the hot rolling are entered into the scale, and the convex portions are disappeared by the removal of the scale by the subsequent pickling. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0338] It is considered that N-3 and T-3 are such that the reduction ratio in the preceding stand from the final stand of the finishing mill is excessively small in hot rolling, and thus the surface pressure between the plate and the roll at the time of the hot rolling is insufficient and it is difficult to form the unevenness. As a result, it is not possible to form the desired unevenness on the surface of the finally obtained steel plate, and the absorbed energy at the time of the axial crushing deformation is reduced.

[0339] It is understood from the results of Example 1 and Example 2 above that the energy absorption at the time of the axial crushing deformation of the steel plate satisfying the following necessary conditions (I) to (III) is excellent.

[0340] (I) has the following chemical composition: contains C: 0.05 to 0.15%, Si: 0.01 to 2.00%, Mn: 0.10 to 4.00%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, Ti: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0 to 0.500%, Mo: 0 to 0.500%, Cr: 0 to 2.000%, O: 0 to 0.0100%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 0.500%, W: 0 to 0.1000%, Ta: 0 to 0.1000%, Sn: 0 to 0.0500%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Y: 0 to 0.0500%, Zr: 0 to 0.0500%, La: 0 to 0.0500%, and Ce: 0 to 0.0500% by mass, and the remainder is composed of Fe and impurities.

[0341] (II) has a steel structure containing, in terms of area ratio, ferrite, pearlite, and bainite in total: 0% or more and 60.0% or less, and residual austenite: 0% or more and 1.0% or less, the remaining portion being composed of martensite and tempered martensite.

[0342] (III) a plurality of step differences having a height difference exceeding 5.0 μm exist at intervals of 2.0 mm or less on the surface of the sheet.

[0343] Further, it is known that the steel sheet satisfying the above necessary conditions (I) to (III) can be manufactured by a continuous manufacturing method characterized by enhancing the unevenness of the surface of the hot-rolled sheet with respect to the work under the hot-rolling conditions, and passing through an annealing process without completely smoothing the unevenness. Specifically, it can be said that the steel sheet can be manufactured by the following manufacturing method.

[0344] A manufacturing method of a steel sheet, comprising:

[0345] hot-rolling a steel slab having the chemical composition of (I) above to obtain a hot-rolled sheet;

[0346] coiling the hot-rolled sheet;

[0347] pickling the hot-rolled sheet; and

[0348] annealing the hot-rolled sheet without cold-rolling, or annealing after cold-rolling,

[0349] the hot-rolling includes rolling the sheet at a reduction rate exceeding 30% and 70% or less from the preceding stand of the final stand of the finishing mill while supplying a lubricant between the roll and the sheet,

[0350] the temperature at the time of coiling the hot-rolled sheet is 700°C or less,

[0351] in the case where the cold-rolling is performed, the reduction rate in the cold-rolling is 0.1 to 20%.

Claims

1. A steel sheet having a chemical composition containing, in mass%, C: 0.05 to less than 0.15%, Si: 0.01 to 2.00%, Mn: 0.10 to 4.00%, P: 0.0200% or less, S: 0.0200% or less, Al：0.001～1.000％、 N: 0.0200% or less, Ti: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0 to 0.500%, Mo: 0 to 0.500%, Cr：0～2.000％、 O：0～0.0100％、 B：0～0.0100％、 Nb: 0 to 0.500%, V：0～0.500％、 Cu: 0 to 0.500%, W：0～0.1000％、 Ta: 0 to 0.1000%, Sn: 0 to 0.0500%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Y：0～0.0500％、 Zr：0～0.0500％、 La: 0 to 0.0500%, and Ce: 0 to 0.0500%, the remainder consisting of Fe and impurities, the steel sheet having a steel structure containing, in area ratio, ferrite, pearlite, and bainite: 0% or more and 60.0% or less, and retained austenite: 0% or more and 1.0% or less, the remainder consisting of martensite and tempered martensite, a plurality of step differences having a height difference of 5.0 μm or more at intervals of 2.0 mm or less being present on the surface of the sheet.

2. The steel sheet according to claim 1, having the chemical composition containing, in mass%, one or two or more of Ti: 0.001 to 0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 0.500%, Mo: 0.001 to 0.500%, Nb: 0.001 to 0.500%, Cr：0.001～2.000％ O：0.0001～0.0100％ B：0.0001～0.0100％、 Cu: 0.001 to 0.500%, V：0.001～0.500％、 Ta: 0.0001 to 0.1000%, W：0.0001～0.1000％、 Sn: 0.0001 to 0.0500%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0500%, Ca: 0.0001 to 0.0500%, La: 0.0001 to 0.0500%, and Y：0.0001～0.0500％、 Zr：0.0001～0.0500％、 Ce: 0.0001 to 0.0500%.

3. A method of manufacturing a steel sheet, the method of manufacturing a steel sheet comprising: hot-rolling a steel slab having the chemical composition according to claim 1 or 2 to obtain a hot-rolled sheet; coiling the hot-rolled sheet; pickling the hot-rolled sheet; and annealing the hot-rolled sheet without cold-rolling or after cold-rolling, the hot-rolling including rolling the sheet at a reduction rate of more than 30% and 70% or less in a preceding stand from a final stand of a finishing mill while supplying a lubricant between the roll and the sheet, the temperature at the time of coiling the hot-rolled sheet being 700°C or less, in the case of cold-rolling, the reduction rate in the cold-rolling being 0.1 to 20%. in the annealing, a coating layer formed of zinc, aluminum, magnesium, or an alloy thereof is formed on the front and back surfaces of the sheet. ​ 4. The manufacturing method according to claim 3, wherein, ​

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