Technical field
[0001]
The present invention relates to a steel plate with high strength and excellent weldability and a method for manufacturing the same.
Background technology
[0002]
When welding galvanized steel sheets with a spot welder, molten zinc may cause cracks in the steel sheets. These cracks are called LME cracks (liquid metal embrittlement cracks), and are caused by molten zinc penetrating into the steel plate along the grain boundaries of the steel.
[0003]
Although many inventions related to high-strength steel sheets have been disclosed so far, there are few disclosed examples of techniques for suppressing spot weld LME cracking. (For example, see Patent Documents 1 and 2)
[0004]
Patent Document 1 discloses a steel sheet with improved strength and galling resistance by dispersing fine oxides containing Si and/or Mn in the steel sheet surface layer to increase hardness, and oxides are generated on the steel sheet surface layer. A technique is disclosed in which the hot rolling conditions are controlled so as to allow the oxides to be removed completely, and the pickling conditions are controlled so as not to completely remove the oxides. However, Patent Document 1 does not disclose a technique for suppressing LME.
[0005]
In Patent Document 2, an internal oxide layer having a certain depth is provided on the surface layer of the steel sheet to function as a hydrogen trap site, and the surface layer is softened to improve the balance between strength and ductility, bendability, and delayed fracture resistance. A steel sheet is disclosed, and a technique of performing annealing in an oxidizing and reducing atmosphere while leaving an internal oxide layer formed by hot rolling with a constant thickness even after pickling and cold rolling is disclosed. However, Patent Document 2 does not disclose any technology for suppressing LME.
prior art documents
patent literature
[0006]
Patent Document 1: JP 2013-60630 A
Patent document 2: JP 2016-130355 A
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0007]
In view of the above circumstances, an object of the present invention is to provide a steel sheet with high strength and excellent weldability, and a method for manufacturing the same.
Means to solve problems
[0008]
The present inventors have conducted intensive research on methods for solving the above problems, and have clarified that "strain" has a large effect on the occurrence of LME cracking. For example, even with the same energization cycle (thermal history), if spot welding is performed so as to increase the amount of plastic deformation of the steel sheet, LME cracking occurs significantly. The reason why LME cracking is likely to occur with an increase in "strain" is considered to be that "invasion of molten zinc into the inside of the steel sheet" described above is likely to occur. Therefore, by preventing an increase in strain in the surface layer of the steel sheet, it is possible to suppress the occurrence of spot weld LME cracking. The present inventors have found a means of providing a strength difference in the plate thickness direction in order to prevent an increase in strain in the surface layer of the steel plate. Specifically, when a steel plate is subjected to rapid heating during spot welding, it was found that the austenite grain size is affected by the block diameter of the raw material before welding. , A soft layer (second layer) with a large block diameter is provided inside the plate thickness of the hard outermost layer, and a hard layer (third layer) with a finer block diameter than this soft layer is provided on the inner plate thickness side. set up. In this way, by adopting a three-layer structure in which the block diameter is tilted from the thickness surface layer to the thickness center layer, even during spot welding, when subjected to deformation, the block diameter is large and the soft layer ( The second layer) bears the strain, making it possible to suppress an excessive increase in strain in the outermost layer (first layer). In addition, by providing a difference in block diameter in the plate thickness direction, penetration of cracks into the outermost layer is suppressed in the hole expanding process, so it is possible to obtain high hole expanding properties.
[0009]
In addition, the present inventors have found that it is difficult to manufacture a layered steel sheet in which an appropriate difference in block diameter is provided in the sheet thickness direction, even if the hot rolling conditions and annealing conditions are devised in a single manner. Through various researches, the inventors also found that manufacturing can only be achieved by achieving optimization in a so-called integrated process such as rolling and annealing, and completed the present invention.
[0010]
The gist of the present invention is as follows.
[0011]
(1) mass%,
C: 0.20-0.40%,
Si: 0.01 to 1.00%,
Mn: 0.10 to 4.00%,
P: 0.0200% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0200% or less,
Co: 0 to 0.5000%,
Ni: 0 to 1.0000%,
Mo: 0 to 1.0000%,
Cr: 0 to 2.0000%,
O: 0 to 0.0200%,
Ti: 0 to 0.500%,
B: 0 to 0.0100%,
Nb: 0 to 0.5000%,
　V: 0 to 0.5000%,
Cu: 0 to 0.5000%,
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%
and has a chemical composition with the balance consisting of Fe and impurities,
In terms of area ratio,
　Total of ferrite, pearlite and bainite: 0 to 10.0%, and
　Total of martensite and tempered martensite: 80.0-100.0%
having a microstructure containing
In the cross-sectional structure cut in the width direction orthogonal to the rolling direction,
the block diameter in the first depth region of 1 to 10 μm from the surface is 5.0 μm or less,
The block diameter in the second depth region of 10 to 60 μm from the surface is 6.0 to 20.0 μm,
A steel plate having a block diameter of less than 6.0 μm in the third depth region from 60 μm to 1/4 of the plate thickness from the surface.
(2) The chemical composition is mass%,
Co: 0.0001 to 0.5000%,
Ni: 0.0001 to 1.0000%,
Mo: 0.0001 to 1.0000%,
Cr: 0.0001 to 2.0000%,
O: 0.0001 to 0.0200%,
Ti: 0.0001 to 0.500%,
B: 0.0001 to 0.0100%,
Nb: 0.0001 to 0.5000%,
　V: 0.0001 to 0.5000%,
Cu: 0.0001 to 0.5000%,
W: 0.0001 to 0.1000%,
Ta: 0.0001 to 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%,
Y: 0.0001 to 0.0500%,
Zr: 0.0001 to 0.0500%,
La: 0.0001 to 0.0500%, and
Ce: 0.0001 to 0.0500%
The steel sheet according to (1) above, containing one or more selected from the group consisting of:
(3) The steel sheet according to (1) or (2) above, wherein the area ratio of retained austenite in the microstructure is 10.0% or less.
(4) A plating layer containing zinc, aluminum, magnesium, an alloy of any combination thereof, or an alloy of at least one of these elements and iron is formed on at least one surface of the steel sheet. The steel sheet according to any one of (1) to (3) above.
(5) A step of hot-rolling a steel slab having the chemical composition described in (1) or (2) above, and then coiling at 500°C or higher;
a step of pickling the obtained hot-rolled steel sheet to remove oxide scale existing on the surface of the hot-rolled steel sheet, wherein the amount of removal of the surface layer of the hot-rolled steel sheet is less than 5.00 μm;
a step of cold-rolling the hot-rolled steel sheet at a rolling reduction of 30 to 90%;
An annealing process in which the obtained cold-rolled steel sheet is held in an atmosphere with a dew point of -20 to 20°C in a temperature range of 740 to 900°C for 40 to 300 seconds.
A method of manufacturing a steel plate, comprising:
(6) In the annealing step, plating containing zinc, aluminum, magnesium, an alloy of any combination thereof, or an alloy of at least one of these elements and iron on at least one surface of the cold-rolled steel sheet. The method for producing a steel sheet according to (5) above, wherein a layer is formed.
The invention's effect
[0012]
According to the present invention, it is possible to provide a steel plate with high strength and excellent weldability and a method for manufacturing the same.
MODE FOR CARRYING OUT THE INVENTION
[0013]
Embodiments of the present invention will be described below. It should be noted that these descriptions are intended to be merely examples of embodiments of the present invention, and the present invention is not limited to the following embodiments.
[0014]

　The steel sheet according to the embodiment of the present invention, in mass%,
C: 0.20-0.40%,
Si: 0.01 to 1.00%,
Mn: 0.10 to 4.00%,
P: 0.0200% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0200% or less,
Co: 0 to 0.5000%,
Ni: 0 to 1.0000%,
Mo: 0 to 1.0000%,
Cr: 0 to 2.0000%,
O: 0 to 0.0200%,
Ti: 0 to 0.500%,
B: 0 to 0.0100%,
Nb: 0 to 0.5000%,
　V: 0 to 0.5000%,
Cu: 0 to 0.5000%,
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%
and has a chemical composition with the balance consisting of Fe and impurities,
In terms of area ratio,
　Total of ferrite, pearlite and bainite: 0 to 10.0%, and
　Total of martensite and tempered martensite: 80.0-100.0%
having a microstructure containing
In the cross-sectional structure cut in the width direction orthogonal to the rolling direction,
the block diameter in the first depth region of 1 to 10 μm from the surface is 5.0 μm or less,
The block diameter in the second depth region of 10 to 60 μm from the surface is 6.0 to 20.0 μm,
It is characterized by a block diameter of less than 6.0 μm in the third depth region from 60 μm to 1/4 of the plate thickness from the surface.
[0015]
First, the reason for limiting the chemical composition of the steel sheet according to the embodiment of the present invention will be explained. Here, "%" for components means % by weight. Furthermore, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as lower and upper limits, unless otherwise specified.
[0016]
(C: 0.20-0.40%)
C is an element that inexpensively increases tensile strength and is an extremely important element for controlling the strength of steel. In order to sufficiently obtain such effects, the C content is made 0.20% or more. The C content may be 0.22% or more, 0.24% or more, or 0.28% or more. On the other hand, an excessive C content may promote the occurrence of LME. Therefore, the C content should be 0.40% or less. The C content may be 0.38% or less, 0.36% or less, or 0.34% or less.
[0017]
(Si: 0.01 to 1.00%)
Si is an element that acts as a deoxidizing agent and suppresses the precipitation of carbides during the cooling process during cold-rolled sheet annealing. In order to sufficiently obtain such effects, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, or 0.20% or more. On the other hand, an excessive Si content increases the strength of the steel and decreases the hole expandability, and furthermore, coarse oxides are dispersed in the surface layer of the hot-rolled steel sheet. Since the desired particle size distribution cannot be obtained in, the LME resistance may be reduced. Therefore, the Si content is set to 1.00% or less. The Si content may be 0.90% or less, 0.80% or less, or 0.70% or less.
[0018]
(Mn: 0.10 to 4.00%)
　Mn is a factor that affects the ferrite transformation of steel and is an element effective in increasing strength. get enough of these effectsTherefore, the Mn content is set to 0.10% or more. The Mn content may be 0.50% or more, 0.90% or more, or 1.50% or more. On the other hand, if Mn is excessively contained, the steel strength increases and the hole expansibility decreases, and further, coarse oxides are dispersed in the surface layer of the hot-rolled steel sheet, and the surface layer of the steel sheet after cold-rolled steel annealing. Since the desired particle size distribution cannot be obtained in, the LME resistance may be reduced. Therefore, the Mn content should be 4.00% or less. The Mn content may be 3.30% or less, 3.00% or less, or 2.70% or less.
[0019]
(P: 0.0200% or less)
P is an element that strongly segregates at ferrite grain boundaries and promotes grain boundary embrittlement. Since the P content is preferably as small as possible, it is ideally 0%. However, excessive reduction of the P content causes a significant increase in cost, so the P content may be 0.0001% or more, 0.0010% or more, or 0.0040% or more. On the other hand, an excessive P content may increase the strength of the steel and cause embrittlement of the steel, further reducing the LME resistance. Therefore, the P content should be 0.0200% or less. The P content may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.
[0020]
(S: 0.0200% or less)
S is an element that forms non-metallic inclusions such as MnS in steel and causes a decrease in ductility of steel parts. Ideally, the S content is 0% because the smaller the S content, the better. However, since an excessive reduction in the S content causes a significant increase in cost, the S content may be 0.0001% or more, 0.0002% or more, 0.0010% or more, or 0.0050% or more. There may be. On the other hand, if the S content is excessive, cracks originating from nonmetallic inclusions may occur during cold forming, and the LME resistance may be lowered. Therefore, the S content should be 0.0200% or less. The S content may be 0.0180% or less, 0.0150% or less, or 0.0100% or less.
[0021]
(Al: 1.000% or less)
Al is an element that acts as a deoxidizing agent for steel and stabilizes ferrite, and may be contained as necessary. Since Al may not be contained, the lower limit of the Al content is 0%. In order to sufficiently obtain the effect, the Al content is preferably 0.001% or more, and may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, when Al is contained excessively, ferrite transformation and bainite transformation are excessively accelerated in the cooling process of cold-rolled steel sheet annealing, which may reduce the strength of the steel sheet. Therefore, the Al content is set to 1.000% or less. The Al content may be 0.900% or less, 0.800% or less, or 0.700% or less.
[0022]
(N: 0.0200% or less)
N is an element that forms coarse nitrides in the steel sheet and reduces the workability of the steel sheet. Also, N is an element that causes blowholes during welding. Ideally, the N content is 0% because the smaller the N content, the better. However, since an excessive reduction in the N content causes a significant increase in manufacturing costs, the N content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0050% or more. may be On the other hand, when N is excessively contained, it combines with Al and Ti to form a large amount of AlN or TiN, and these nitrides refine the austenite grain size and the block diameter during cold-rolled steel annealing. It may become impossible to control the inclination of the block diameter in the surface layer in the plate thickness direction. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0160% or less, 0.0100% or less, or 0.0080% or less.
[0023]
The basic chemical composition of the steel sheet in this embodiment is as described above. Furthermore, the steel sheet in the present embodiment may contain at least one of the following optional elements in place of part of the remaining Fe, if necessary. Since these elements do not have to be contained, the lower limit is 0%.
[0024]
(Co: 0 to 0.5000%)
Co is an element effective for controlling the morphology of carbides and increasing the strength, and may be contained as necessary for controlling solute carbon. In order to sufficiently obtain these effects, the Co content is preferably 0.0001% or more. The Co content may be 0.0010% or more, 0.0100% or more, or 0.0400% or more. On the other hand, when Co is contained excessively, a large number of fine Co carbides precipitate, and these carbides refine the block diameter along with the austenite grain size during cold-rolled steel annealing. tilt control may become impossible. Therefore, the Co content is preferably 0.5000% or less. The Co content may be 0.4000% or less, 0.3000% or less, or 0.2000% or less.
[0025]
(Ni: 0 to 1.0000%)
Ni is a strengthening element and effective in improving hardenability. In addition, it may be contained as necessary because it brings about improvement in wettability and promotion of alloying reaction. In order to sufficiently obtain these effects, the Ni content is preferably 0.0001% or more. The Ni content may be 0.0010% or more, 0.0100% or more, or 0.0500% or more. On the other hand, an excessive Ni content may adversely affect manufacturability during manufacturing and hot rolling, and may deteriorate hole expansibility. Therefore, the Ni content is preferably 1.0000% or less. The Ni content may be 0.8000% or less, 0.5000% or less, or 0.200% or less.
[0026]
(Mo: 0 to 1.0000%)
Mo is an element effective in improving the strength of steel sheets. In addition, Mo is an element that has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. In order to sufficiently obtain these effects, the Mo content is preferably 0.0001% or more. The Mo content may be 0.0010% or more, 0.0100% or more, or 0.0500% or more. On the other hand, if Mo is contained excessively, a large number of fine Mo carbides precipitate, and these carbides refine the block diameter along with the austenite grain size during cold-rolled steel annealing. tilt control may become impossible. Therefore, the Mo content is preferably 1.0000% or less. The Mo content may be 0.9000% or less, 0.8000% or less, or 0.700% or less.
[0027]
(Cr: 0 to 2.0000%)
Cr, like Mn, is an element that suppresses pearlite transformation and is effective in increasing the strength of steel, and may be contained as necessary. In order to sufficiently obtain such effects, the Cr content is preferably 0.0001% or more. The Cr content may be 0.0010% or more, 0.0100% or more, or 0.0500% or more. On the other hand, an excessive Cr content promotes the formation of retained austenite, which may deteriorate the hole expansibility. Therefore, the Cr content is preferably 2.0000% or less. The Cr content may be 1.8000% or less, 1.6000% or less, or 1.000% or less.
[0028]
(O: 0 to 0.0200%)
　O forms an oxide and deteriorates workability, so it is necessary to suppress the content. In particular, oxides often exist as inclusions, and if they exist on the punched end face or cut face, they form notch-like scratches or coarse dimples on the end face. , induces stress concentration and becomes a starting point for crack formation, resulting in significant deterioration of workability. For this reason, the O content may be 0%, but excessive reduction causes a significant increase in cost, which is economically undesirable. Therefore, the O content is preferably 0.0001% or more. The O content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, when O is contained excessively, fracture progresses easily starting from coarse oxides, which may deteriorate the hole expandability. Therefore, the O content is preferably 0.0200% or less. The O content may be 0.0160% or less, 0.0100% or less, or 0.0050% or less.
[0029]
(Ti: 0 to 0.500%)
Ti is a strengthening element, and contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of crystal grains, and strengthening dislocations by suppressing recrystallization. In order to sufficiently obtain such effects, the Ti content is preferably 0.0001% or more. The Ti content may be 0.001% or more, 0.005% or more, 0.010% or more, or 0.030% or more. On the other hand, if Ti is contained excessively, the precipitation of coarse carbides increases, which may deteriorate the hole expansibility. Therefore, the Ti content is preferably 0.500% or less. The Ti content may be 0.400% or less, 0.200% or less, or 0.100% or less.
[0030]
(B: 0 to 0.0100%)
B is an element that suppresses the formation of ferrite and pearlite in the cooling process from austenite and promotes the formation of low temperature transformation structures such as bainite or martensite. Moreover, B is an element useful for increasing the strength of steel, and may be contained as necessary. However, if the B content is too low, the improvement effect such as high strength may not be obtained sufficiently. Furthermore, the identification of less than 0.0001% requires meticulous attention in analysis, and reaches the lower limit of detection depending on the analyzer. Therefore, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, an excessive B content may lead to the formation of coarse B oxides in the steel, which may become starting points for the generation of voids during cold forming, degrading the hole expansibility. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
[0031]
(Nb: 0 to 0.5000%)
Nb is an element that is effective in controlling the morphology of carbides, and is also an element that is effective in improving toughness because its addition refines the structure. In order to sufficiently obtain these effects, the Nb content is preferably 0.0001% or more. The Nb content may be 0.0010% or more, 0.0100% or more, or 0.0200% or more. On the other hand, when Nb is contained excessively, a large number of fine and hard Nb carbides precipitate, and these carbides refine the block diameter along with the austenite grain size during cold-rolled steel annealing. It may become impossible to control the tilt in the thickness direction. Therefore, the Nb content is preferably 0.5000% or less. The Nb content may be 0.4000% or less, 0.2000% or less, or 0.1000% or less.
[0032]
(V: 0 to 0.5000%)
V is a strengthening element, and contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite grains, and strengthening dislocations by suppressing recrystallization. In order to sufficiently obtain such effects, the V content is preferably 0.0001% or more. The V content may be 0.0010% or more, 0.0100% or more, or 0.0200% or more. On the other hand, if V is contained excessively, precipitation of carbonitrides increases, which may deteriorate the hole expansibility. Therefore, the V content is preferably 0.5000% or less. The V content may be 0.4000% or less, 0.2000% or less, or 0.1000% or less.
[0033]
(Cu: 0 to 0.5000%)
Cu is an element effective in improving the strength of steel sheets. In order to sufficiently obtain such effects, the Cu content is preferably 0.0001% or more. The Cu content may be 0.0010% or more, 0.0100% or more, or 0.0200% or more. On the other hand, if Cu is contained excessively, the steel material becomes embrittled during hot rolling, and hot rolling may become impossible. In addition, steelThe strength of is remarkably increased, and the hole expansibility may be deteriorated. Therefore, the Cu content is preferably 0.5000% or less. The Cu content may be 0.4000% or less, 0.2000% or less, or 0.1000% or less.
[0034]
(W: 0 to 0.1000%)
W is a very important element because it is effective in increasing the strength of steel sheets, and precipitates and crystallized substances containing W act as hydrogen trap sites. In order to sufficiently obtain these effects, the W content is preferably 0.0001% or more. The W content may be 0.0010% or more, 0.0050% or more, or 0.0100% or more. On the other hand, an excessive W content promotes the formation of voids during cold working starting from coarse carbides, which may reduce the hole expansibility. Therefore, the W content is preferably 0.1000% or less. The W content may be 0.0800% or less, 0.0600% or less, or 0.0400% or less.
[0035]
(Ta: 0 to 0.1000%)
Ta, like Co, is an element effective in controlling the morphology of carbides and increasing the strength, and may be contained as necessary. In order to sufficiently obtain these effects, the Ta content is preferably 0.0001% or more. The Ta content may be 0.0010% or more, 0.0050% or more, or 0.0100% or more. On the other hand, when Ta is contained excessively, a large number of fine Ta carbides precipitate, which may reduce the hole expansibility. Therefore, the Ta content is preferably 0.1000% or less. The Ta content may be 0.0800% or less, 0.0600% or less, or 0.0400% or less.
[0036]
(Sn: 0 to 0.0500%)
Sn is an element contained in steel when scrap is used as a raw material, and the smaller the amount, the better. Therefore, the Sn content may be 0%, but excessive reduction leads to an increase in refining costs. Therefore, the Sn content is preferably 0.0001% or more. The Sn content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, when Sn is excessively contained, the steel sheet may become embrittled, resulting in a decrease in hole expansibility. Therefore, the Sn content is preferably 0.0500% or less. The Sn content may be 0.0400% or less, 0.0200% or less, or 0.0100% or less.
[0037]
(Sb: 0 to 0.0500%)
Sb, like Sn, is an element contained when scrap is used as a raw material for steel. Sb strongly segregates at grain boundaries and causes grain boundary embrittlement and ductility deterioration. However, excessive reduction leads to an increase in refining costs. Therefore, the Sb content is preferably 0.0001% or more. The Sb content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, an excessive Sb content may cause a decrease in hole expandability. Therefore, the Sb content is preferably 0.0500% or less. The Sb content may be 0.0400% or less, 0.0200% or less, or 0.0100% or less.
[0038]
(As: 0 to 0.0500%)
As, like Sn and Sb, is contained when scrap is used as a raw material for steel, and is an element that strongly segregates at grain boundaries. Therefore, the As content may be 0%, but excessive reduction leads to an increase in refining cost. Therefore, the As content is preferably 0.0001% or more. The As content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, an excessive As content may lead to a decrease in hole expansibility. Therefore, the As content is preferably 0.0500% or less. The As content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0039]
(Mg: 0 to 0.0500%)
Mg is an element that can control the morphology of sulfides by adding a small amount, and may be contained as necessary. In order to sufficiently obtain such effects, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, when Mg is contained excessively, formation of coarse inclusions may lead to deterioration of hole expansibility. Therefore, the Mg content is preferably 0.0500% or less. The Mg content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0040]
(Ca: 0 to 0.0500%)
In addition to being useful as a deoxidizing element, Ca is also effective in controlling the morphology of sulfides. In order to sufficiently obtain these effects, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, if Ca is contained excessively, the hole expansibility may deteriorate. Therefore, the Ca content is preferably 0.0500% or less. The Ca content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0041]
(Y: 0 to 0.0500%)
Y is an element that can control the morphology of sulfides by adding a small amount like Mg and Ca, and may be contained as necessary. In order to sufficiently obtain such effects, the Y content is preferably 0.0001% or more. The Y content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, when Y is contained excessively, coarse Y oxides are formed, and the hole expansibility may be lowered. Therefore, the Y content is preferably 0.0500% or less. The Y content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0042]
(Zr: 0 to 0.0500%)
Zr, like Mg, Ca and Y, is an element capable of controlling the morphology of sulfides by adding a very small amount, and may be contained as necessary. In order to sufficiently obtain such effects, the Zr content is preferably 0.0001% or more. The Zr content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, when Zr is contained excessively, coarse Zr oxides are formed, which may deteriorate the hole expansibility. Therefore, the Zr content is preferably 0.0500% or less. The Zr content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0043]
(La: 0 to 0.0500%)
La is an element effective in controlling the morphology of sulfides when added in a very small amount, and may be contained as necessary. In order to sufficiently obtain such effects, the La content is preferably 0.0001% or more. The La content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, when La is contained excessively, La oxide is generated, which may lead to a decrease in hole expansibility. Therefore, the La content is preferably 0.0500% or less. The La content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0044]
(Ce: 0 to 0.0500%)
Ce, like La, is an element that can control the morphology of sulfides by adding a small amount, and may be contained as necessary. In order to sufficiently obtain such effects, the Ce content is preferably 0.0001% or more. The Ce content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, when Ce is contained excessively, Ce oxide is generated, which may lead to a decrease in hole expansibility. Therefore, the Ce content is preferably 0.0500% or less. The Ce content may be 0.0400% or less, 0.0300% or less, or 0.0200% or less.
[0045]
In addition, in the steel sheet of the present embodiment, the rest of the components described above are Fe and impurities. Impurities are components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel sheet according to the present embodiment is industrially manufactured.
[0046]
Next, the characteristics of the structure and properties of the steel sheet according to the embodiment of the present invention will be described.
[0047]
(Total of ferrite, pearlite and bainite: 0 to 10.0%)
Ferrite, pearlite, and bainite are factors that cause a decrease in the strength of the steel sheet as well as a decrease in the hole expansibility, and the smaller the area ratio thereof, the better. Therefore, the total area ratio of ferrite, pearlite and bainite is 10.0% or less, and may be 8.0% or less, 6.0% or less, 5.0% or less, or 0%. However, controlling to 0% requires highly accurate control of the consistent manufacturing conditions, which may lead to a decrease in productivity. Therefore, the total area ratio of ferrite, pearlite and bainite may be 0.3% or more or 0.5% or more.
[0048]
(Total of martensite and tempered martensite: 80.0 to 100.0%)
Martensite and tempered martensite are structures that are extremely effective in increasing the strength of steel sheets, and in order to ensure strength and hole expansibility, the higher the area ratio, the better. Therefore, the total area ratio of martensite and tempered martensite is 80.0% or more, and may be 85.0% or more, 90.0% or more, 95.0% or more, or 100.0%. However, controlling to 100.0% requires highly accurate control of consistent manufacturing conditions, which may lead to a decrease in productivity. Therefore, the total area ratio of martensite and tempered martensite may be 99.5% or less or 99.0% or less.
[0049]
(Retained austenite: 0 to 10.0%)
As described above, the microstructure of the steel sheet according to the embodiment of the present invention is, in area ratio, the total of ferrite, pearlite and bainite: 0 to 10.0%, and the total of martensite and tempered martensite: 80.0. It may contain up to 100.0%, and may be composed only of them, or the remaining tissue may be present. If a residual structure exists, it preferably consists of retained austenite: 0 to 10.0% in terms of area ratio. Retained austenite is a structure effective for improving the balance of strength and ductility of a steel sheet, but if contained in a large amount, it may lead to a decrease in local ductility and deteriorate hole expansibility. Therefore, in order to reliably improve properties such as hole expansibility, the area ratio of retained austenite in the microstructure is preferably 10.0% or less, 9.0% or less, 8.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, 2.0% or less, 1.0% or less, 0.9% or less, 0.8% or less, 0.6% or less, 0.6% or less It may be 4% or less or 0%. However, controlling to 0% requires highly accurate control of the consistent manufacturing conditions, which may lead to a decrease in productivity. Therefore, the area ratio of retained austenite in the microstructure may be 0.1% or more or 0.3% or more.
[0050]
(Block diameter in the first depth region of 1 to 10 μm from the steel plate surface: 5.0 μm or less)
The block diameter in the first depth region of 1 to 10 μm in the plate thickness direction from the steel plate surface is an important factor for increasing the hot deformation resistance of the steel plate during spot welding. Here, the first depth region and the second and third depth regions described later are regions in a cross-sectional structure obtained by cutting the steel plate in the width direction perpendicular to the rolling direction of the steel plate and in the direction perpendicular to the steel plate surface. It means. When a steel plate is subjected to rapid heating during spot welding, the austenite grain size in the area heated by hot deformation, ie spot welding, is affected by the block diameter of the material before welding. That is, the finer the block diameter of the material, the finer the austenite grain size in the region heated during spot welding. Due to the effect of refining the austenite grain size, it is possible to suppress an excessive increase in strain in the outermost layer of the welding material during spot welding. blocks in the first depth regionIf the hook diameter is large, this effect cannot be obtained, and LME occurs during spot welding. Therefore, the block diameter in the first depth region is set to 5.0 μm or less, preferably 4.0 μm or less, more preferably 3.0 μm or less. Although the lower limit of the block diameter in the first depth region is not particularly limited, it is generally 0.1 μm or more or 0.3 μm or more.
[0051]
(Block diameter in the second depth region of 10 to 60 μm from the steel plate surface: 6.0 to 20.0 μm)
The block diameter in the second depth region of 10 to 60 μm from the steel plate surface is an important factor for suppressing strain concentration on the steel plate surface layer during spot welding. When the block diameter in the second depth region is sufficiently coarse with respect to the block diameter in the first depth region, hot deformation during spot welding occurs in the first depth region and the second depth region. Sometimes there is a difference in the amount of distortion that they are responsible for. Specifically, the second depth region bears more strain than the first depth region, so strain occurring in the first depth region can be suppressed. This effect cannot be obtained unless the block diameter in the second depth region is sufficiently larger than the block diameter in the first depth region. As a result, the steel sheet invites the occurrence of LME during spot welding. Therefore, the block diameter in the second depth region is 6.0 μm or more, and may be 8.0 μm or more or 10.0 μm or more. On the other hand, if the block diameter in the second depth region is too large, the deformation resistance during spot welding is excessively lowered. Therefore, if the block diameter in the second depth region is too large, the amount of deformation in the second depth region significantly increases during spot welding, the amount of strain generated in the first depth region increases, and the LME causes the occurrence of Therefore, the block diameter in the second depth region is set to 20.0 μm or less, preferably 18.0 μm or less, and more preferably 15.0 μm or less.
[0052]
(Block diameter in the third depth region from 60 μm to ¼ of the plate thickness from the steel plate surface: less than 6.0 μm)
The block diameter in the third depth region from 60 μm to ¼ of the plate thickness from the steel plate surface is an important factor for suppressing strain concentration on the steel plate surface layer during spot welding. In order to disperse the strain generated in the second depth region during spot welding not in the plate thickness direction but in the plane parallel to the rolling direction and the width direction of the steel plate, the third depth region is formed more than the second depth region. It is also necessary to form a hard layer with a fine block diameter. With such a configuration, the hot deformation resistance in the third depth region during spot welding is higher than that in the second depth region. Thus, by making the block diameters of the first depth region and the third depth region smaller than the block diameter of the second depth region, the first depth region and the third depth region The hot deformation resistance is greater than the hot deformation resistance of the second depth region. For this reason, the strain that occurs during spot welding is concentrated in the second depth region, and the occurrence of strain in the first depth region and the third depth region can be suppressed. If the block diameter in the third depth region is larger than that in the second depth region, strain generated in the second depth region during spot welding will also be dispersed in the third depth region. As a result, the strain is dispersed in the plate thickness direction, and this effect cannot be obtained, resulting in the occurrence of LME during spot welding. Therefore, the block diameter in the third depth region is less than 6.0 μm, preferably 5.0 μm or less, more preferably 3.0 μm or less. Although the lower limit of the block diameter in the third depth region is not particularly limited, it is generally 0.1 μm or more or 0.3 μm or more.
[0053]
(Plating layer)
The steel sheet according to the embodiment of the present invention may include a plating layer on at least one surface, preferably both surfaces, for the purpose of improving corrosion resistance. The plating layer may be a plating layer having any composition known to those skilled in the art, and is not particularly limited, but may contain, for example, zinc, aluminum, magnesium, or an alloy made of any combination thereof. . Also, the plating layer may or may not be alloyed. When alloying treatment is performed, the plating layer may contain an alloy of at least one of the above elements and iron diffused from the steel sheet. Also, the amount of the plating layer deposited is not particularly limited, and may be a general amount of deposition.
[0054]
(Tensile strength: TS)
Regarding the tensile strength, it is preferable that the steel material has a large work hardening ability and exhibits the maximum strength in order to reduce the weight of the structure using steel as a material and to improve the resistance of the structure against plastic deformation. preferably has a tensile strength of 1200 MPa or more. If the tensile strength is low, the effect of reducing the weight and improving the deformation resistance of the steel-based structure is reduced. In this regard, a steel sheet having the above chemical composition and structure can reliably achieve a tensile strength of 1200 MPa or more. The tensile strength of the steel sheet is preferably 1280 MPa or higher, more preferably 1350 MPa or higher or 1400 MPa or higher, most preferably 1500 MPa or higher. On the other hand, if the tensile strength is too high, the material is prone to brittle fracture during plastic deformation, resulting in poor formability. Therefore, the tensile strength of the steel sheet is generally 2300 MPa or less, and may be 2100 MPa or less, 2000 MPa or less, or 1900 MPa or less. Tensile strength is measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate, and performing a tensile test in accordance with JIS Z 2241 (2011).
[0055]
(Total elongation: t-El)
In addition to high strength and excellent weldability, according to certain embodiments of the present invention, it is also possible to improve total elongation, e.g. It is possible to achieve the above total elongation. Although the upper limit is not particularly limited, for example, the total elongation may be 25.0% or less or 20.0% or less. Elongation is necessary in order to finish a complicated shape when a structure is manufactured by cold forming a steel plate as a raw material. Therefore, a steel sheet capable of achieving such a high total elongation is very useful for manufacturing structures. Total elongation is measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate, and performing a tensile test in accordance with JIS Z 2241 (2011).
[0056]
(Hole expansion value: λ)
According to certain embodiments of the present invention, in addition to high strength and excellent weldability, it is also possible to improve hole expansibility, e.g. It is possible to achieve hole expansion values of % or better. Such a high hole expansion value can be reliably achieved by setting the area ratio of retained austenite in the microstructure to 10.0% or less. Although the upper limit is not particularly limited, for example, the hole expansion value may be 90.0% or less or 80.0% or less. When a steel plate is cold-formed to manufacture a structure, elongation and hole expansibility are required in order to finish it into a complicated shape. Therefore, a steel sheet capable of achieving such a high hole expansion value is very useful for manufacturing structures. The hole expansion value is determined as follows. First, a circular hole with a diameter of 10 mm (initial hole: hole diameter d0 = 10 mm) was punched out in the test piece under the condition that the clearance was 12.5%, so that the burr was on the die side, and the vertical angle was 60 °. Expand the initial hole until a crack penetrating the plate thickness occurs with a conical punch, measure the hole diameter d1mm at the time of cracking, and obtain the hole expansion value λ (%) of each test piece using the following formula . This hole expansion test is performed 5 times, and the average value thereof is determined as the hole expansion value λ.
　　　λ=100×(d1-d0)/d0
[0057]
(Thickness)
The thickness of the steel plate is a factor that affects the rigidity of the steel member after forming, and the greater the plate thickness, the higher the rigidity of the member. Therefore, from the viewpoint of increasing rigidity, a plate thickness of 0.2 mm or more is preferable. The plate thickness may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. On the other hand, if the plate thickness is too thick, the forming load during hole-expanding forming increases, which may lead to wear of the mold and a decrease in productivity. Therefore, a plate thickness of 6.0 mm or less is preferable. The plate thickness may be 5.0 mm or less or 4.0 mm or less.
[0058]
Next, the observation and measurement methods for the tissue specified above will be described.
[0059]
(Evaluation method for area ratio of ferrite, pearlite and bainite)
The tissue observation is performed with a scanning electron microscope. Prior to observation, the sample for structure observation was wet-polished with emery paper and polished with diamond abrasive grains having an average particle size of 1 μm, and after finishing the observation surface to a mirror surface, the structure was etched with a 3% nitric acid alcohol solution. Keep The observation magnification is set to 3000 times, and 10 images of a field of view of 30 μm×40 μm at a position of 1/4 of the plate thickness from the surface are randomly photographed. Tissue ratios are determined by the point counting method. A total of 100 lattice points arranged at intervals of 3 μm in length and 4 μm in width are determined on the obtained structure image, and the structure existing under the lattice points is determined. Ask for Ferrite is a massive crystal grain that does not contain iron-based carbide having a major axis of 100 nm or more. Bainite is an aggregate of lath-shaped crystal grains that does not contain iron-based carbides with a major axis of 20 nm or more inside, or contains iron-based carbides with a major axis of 20 nm or more inside, and the carbide is a single variant, That is, they belong to a group of iron-based carbides elongated in the same direction. Here, the iron-based carbide group extending in the same direction means that the difference in the extending direction of the iron-based carbide group is within 5°. A bainite surrounded by grain boundaries with an orientation difference of 15° or more is counted as one bainite grain. Pearlite is a structure containing cementite that is precipitated in rows, and the area ratio is calculated using pearlite as a region photographed with bright contrast in a secondary electron image.
[0060]
(Evaluation method for area ratio of martensite and tempered martensite)
For tempered martensite, the 1/4 position of the plate thickness from the surface is observed with a scanning and transmission electron microscope, and those containing carbides containing a large amount of Fe (Fe-based carbides) are classified as tempered martensite and carbides. is identified as martensite. Regarding Fe-based carbides, those having various crystal structures have been reported, but any Fe-based carbide may be contained. A plurality of types of Fe-based carbides may exist depending on the heat treatment conditions.
[0061]
(Evaluation method for area ratio of retained austenite)
The area ratio of retained austenite is determined as follows by X-ray measurement. First, a portion from the surface of the steel sheet to the position of 1/4 of the plate thickness is removed by mechanical polishing and chemical polishing, and the chemically polished surface is measured using MoKα rays as characteristic X-rays. Then, from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the body-centered cubic (bcc) phase and (200), (220) and (311) of the face-centered cubic (fcc) phase, the following Calculate the area ratio of retained austenite using the formula.
　Sγ = (I 200f + I 220f + I 311f) / (I 200b + I 211b) x 100
where Sγ is the area fraction of retained austenite, I 200f, I 220f and I 311f are the intensities of the (200), (220) and (311) diffraction peaks of the fcc phase, respectively, and I 200b and I 211b indicates the intensity of the (200) and (211) diffraction peaks of the bcc phase, respectively.
[0062]
(Evaluation method of block diameter in first to third depth regions)
The block diameter (μm) is obtained from the crystal orientation map obtained by the FESEM-EBSP method without distinguishing between martensite blocks and bainite blocks. Specifically, a plane parallel to the width direction perpendicular to the rolling direction in the steel plate surface layer is cut out by FIB (focused ion beam), and a field of view of 30 μm in the rolling direction and 90 μm in the plate thickness direction is measured at a pitch of 0.1 μm. conduct. For EBSP measurementThe orientation of αFe is identified from the Kikuchi line pattern collected from the specimen. A crystal orientation diagram is obtained from the orientation of αFe. This crystal orientation map is divided into three regions of 1 to 10 μm (first depth region), 10 to 60 μm (second depth region), and 60 to 90 μm (third depth region) in the plate thickness direction. , in the crystal orientation diagram after division, identify a region surrounded by an orientation difference of 15° or more from adjacent crystals. A region surrounded by an orientation difference of 15° or more is defined as one grain of a block. Calculate the equivalent circle diameter from the area of each block. The average value of the equivalent circle diameters in the field of view is calculated and used as the block diameter.
[0063]

The steel sheet manufacturing method according to the embodiment of the present invention is characterized by consistent management of hot rolling, cold rolling, and annealing conditions using materials within the above-described composition ranges. Specifically, the method of manufacturing a steel sheet according to an embodiment of the present invention includes the steps of hot rolling a billet having the same chemical composition as the chemical composition described above for the steel sheet, and then coiling at 500° C. or higher;
a step of pickling the obtained hot-rolled steel sheet to remove oxide scale existing on the surface of the hot-rolled steel sheet, wherein the amount of removal of the surface layer of the hot-rolled steel sheet is less than 5.00 μm;
a step of cold-rolling the hot-rolled steel sheet at a rolling reduction of 30 to 90%;
An annealing process in which the obtained cold-rolled steel sheet is held in an atmosphere with a dew point of -20 to 20°C in a temperature range of 740 to 900°C for 40 to 300 seconds.
is characterized by including Each step will be described in detail below.
[0064]
(Hot rolling and winding process)
In this process, a steel billet having the same chemical composition as the chemical composition described above for the steel plate is subjected to hot rolling. The steel slabs to be used are preferably cast by continuous casting from the viewpoint of productivity, but may be produced by ingot casting or thin slab casting. Further, the cast steel slab may optionally be subjected to rough rolling before finish rolling for plate thickness adjustment and the like. Conditions for such rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured. Hot rolling is not particularly limited, but is generally carried out under such conditions that the completion temperature of finish rolling is 650° C. or higher. This is because if the finish rolling completion temperature is too low, the rolling reaction force increases, making it difficult to stably obtain a desired plate thickness. The upper limit is not particularly limited, but generally the finish rolling completion temperature is 950° C. or less.
[0065]
(winding temperature)
After hot rolling, the obtained hot-rolled steel sheet is coiled at a coiling temperature of 500°C or higher. The coiling temperature is a factor that controls the formation of oxide scale and oxides on the surface layer of the hot-rolled steel sheet and affects the strength of the hot-rolled steel sheet. By coiling at a coiling temperature of 500°C or higher, oxides (internal oxides) are generated in the surface layer of the hot-rolled steel sheet, and the oxides can be crushed and finely dispersed by subsequent cold rolling. . The finely dispersed oxide can suppress grain growth in the first depth region of the surface layer of the steel sheet. Therefore, it is possible to produce a structure in which the block diameter is controlled to be inclined from the thickness surface layer toward the thickness center layer after the cold-rolled steel sheet is annealed. However, if the hot-rolled steel sheet is coiled at a relatively low temperature, it is not possible to generate sufficient oxides in the thickness direction on the surface layer of the hot-rolled steel sheet. For this reason, it becomes impossible to promote the crushing and fine dispersion of oxides in the steel plate surface layer in the subsequent pickling and cold rolling processes, and it is possible to control the block diameter along with the prior austenite grain size in the steel plate surface layer after cold-rolled steel annealing. become unable. Therefore, the winding temperature is 500° C. or higher, preferably 530° C. or higher, and more preferably 550° C. or higher or 560° C. or higher. By coiling at a relatively high temperature of over 550°C, particularly 560°C or higher, the formation of internal oxides in the surface layer of the hot-rolled steel sheet can be further promoted, and the subsequent cold rolling can reduce the fineness of the internal oxides. It is possible to remarkably enhance the effect of dispersing the grains and thus suppressing grain growth in the first depth region. The upper limit of the coiling temperature is not particularly limited. The coarse oxides are not crushed and remain coarse even after cold-rolled sheet annealing, which may cause a decrease in hole expansibility. Therefore, the winding temperature is preferably 700° C. or lower, more preferably 670° C. or lower.
[0066]
(Pickling process)
The wound hot-rolled steel sheet is rewound and subjected to pickling. By pickling, the oxide scale existing on the surface of the hot-rolled steel sheet can be removed, and the chemical convertability and the platability of the cold-rolled steel sheet can be improved. Oxide scale refers to an oxide layer (external oxide layer) formed on the surface of a steel sheet. Fayalite (Fe 2 SiO 4 ) etc. In addition, the pickling promotes dissolution of the surface layer of the steel sheet, and does not dissolve or completely dissolve oxides (internal oxides) generated under the oxide scale in the surface layer of the hot-rolled steel sheet, that is, inside the steel sheet. By crushing and finely dispersing those undissolved oxides by cold rolling, the structure of the surface layer of the steel sheet can have a gradient function after annealing. In order to control the amount of dissolution of steel in order to leave the oxides in the steel formed under the oxide scale of the hot-rolled steel sheet, the pickling may be performed once or in multiple steps. Mechanical polishing with a grinding brush or the like may be performed before or after pickling. Alternatively, instead of measuring the change in plate thickness before and after pickling, the removal amount of the steel sheet surface layer may be obtained from the change in coil weight before and after pickling. If the amount of removal of the steel sheet surface layer is too large, the amount of crushed oxides present in the steel sheet surface layer after cold rolling is reduced, so that the desired grain size distribution cannot be obtained in the steel sheet surface layer after cold-rolled steel sheet annealing. Decrease LME resistance. Therefore, the removal amount of the steel sheet surface layer by pickling is set to less than 5.00 μm, preferably 4.00 μm or less or 3.50 μm or less. As described above, the coiling temperature is set to 500 ° C. or higher to promote the formation of internal oxides, and the amount of removal of the steel plate surface layer by subsequent pickling is suppressed to less than 5.00 μm. Certain combinations of temperature and removal of less than 5.00 μm by pickling can ensure an internal oxide layer thickness of 1.00 μm or more after pickling and before cold rolling, resulting in It is possible to ensure that the internal oxide is finely dispersed and that grain growth is inhibited in the first depth region. The thickness of the internal oxide layer after pickling and before cold rolling should be 1.00 μm or more, and the upper limit is not particularly limited, but may be, for example, 15.00 μm or less. When the internal oxide layer is thick and coarse oxides are abundant, these coarse oxides are not sufficiently crushed by cold rolling and remain coarse even after cold-rolled steel annealing. may cause decline. Therefore, from the viewpoint of improving the hole expansibility, the thickness of the internal oxide layer after pickling and before cold rolling is preferably 10.00 μm or less. Here, the thickness of the internal oxide layer is the distance from the surface of the steel sheet to the farthest position where the internal oxide exists when proceeding from the surface of the steel sheet in the thickness direction of the steel sheet (direction perpendicular to the surface of the steel sheet). It means. The lower limit of the removal amount of the surface layer of the steel sheet is not particularly limited, and may be 0 μm. However, if the removal amount is less than 0.01 μm, the oxide scale may partially remain on the surface of the steel sheet. There is a risk of causing a decline. Therefore, from the viewpoint of improving hole expansibility, etc., the removal amount of the surface layer of the steel sheet is preferably 0.01 μm or more, 0.10 μm or more, 0.20 μm or more, 0.30 μm or more, 0.40 μm or more, It may be 0.50 μm or more, 0.60 μm or more, 0.80 μm or more, or 1.00 μm or more.
[0067]
(Cold rolling process)
Next, the obtained hot-rolled steel sheet is subjected to cold rolling. The reduction rate in cold rolling is such that in steel sheets with oxides remaining on the surface layer, the oxides are finely dispersed by crushing and oxidized in a first depth region of 1 to 10 μm from the steel sheet surface after cold-rolled steel annealing. It is an extremely important control factor in order to obtain the effect of miniaturizing the block diameter by finely dispersing substances. If the rolling reduction is less than 30%, the oxide crushing effect cannot be obtained, and the block diameter in the first depth region cannot be controlled to 5.0 μm or less. Therefore, the rolling reduction is 30% or more, preferably 35% or more or 40% or more. On the other hand, when the rolling reduction is more than 90%, the thickness of the oxide layer formed in the surface layer of the hot-rolled steel sheet becomes extremely thin after cold rolling, so that the desired grain size is obtained in the surface layer of the steel sheet after cold-rolled steel sheet annealing. Distribution cannot be obtained, and LME resistance is lowered. Therefore, the rolling reduction is set to 90% or less, preferably 85% or less or 80% or less. In the method for manufacturing a steel sheet according to the embodiment of the present invention, the formation of internal oxides is promoted, and the internal oxides are finely refined while mainly removing the external oxide layer by relatively weak pickling to leave the internal oxides. Decentralization is important. In the present method, such fine dispersion of internal oxides is specified for cold rolling at a coiling temperature of 500° C. or higher, a pickling removal amount of less than 5.00 μm, and a rolling reduction of 30 to 90%. This is achieved by a combination of The fine dispersion of internal oxides based on such a specific combination of manufacturing conditions and the effect of suppressing grain growth in the first depth region have not been known in the past. that has been made clear.
[0068]
In order to further promote the fine dispersion of oxides on the surface layer of the steel sheet in the cold rolling process, it is preferable to apply greater shear deformation to the surface layer of the steel sheet during cold rolling. In order to give greater shear deformation to the surface layer of the steel sheet, for example, the cold rolling process is performed under a rolling load of 800 ton/m while supplying lubricating oil having a coefficient of friction of less than 0.10 between the steel sheet and the rolling rolls. It is desirable to include performing the rolling which is the above. In a continuous cold rolling mill composed of multistage rolling stands, at least one rolling mill should perform rolling with a friction coefficient of less than 0.10 and a rolling load of 800 ton/m or more. In addition, when rolling is performed in multiple steps, at least one of the rolling operations has a coefficient of friction of less than 0.10 and a rolling load of 800 ton/m or more. good. When the coefficient of friction is 0.10 or more or the rolling load is less than 800 tons/m, the amount of shear deformation is relatively small, and it may not be possible to sufficiently promote the fine dispersion of oxides on the surface layer of the steel sheet. In addition, the smaller the coefficient of friction and/or the higher the rolling load, the greater the amount of shear deformation applied to the surface layer of the steel sheet. Therefore, the coefficient of friction is preferably 0.08 or less, and may be 0.06 or less, 0.04 or less, or 0.02 or less. Although the lower limit of the coefficient of friction is not particularly limited, the coefficient of friction may be 0.01 or more, for example. Additionally, the rolling load may be 1000 ton/m or greater, 1200 ton/m or greater, 1300 ton/m or greater, 1400 ton/m or greater, or 1600 ton/m or greater. Although the upper limit of the rolling load is not particularly limited, the rolling load may be, for example, 2000 tons/m or less.
[0069]
(annealing process)
Finally, the obtained cold-rolled steel sheet is subjected to a predetermined annealing (also referred to as "cold-rolled sheet annealing") to obtain the steel sheet according to the embodiment of the present invention. The cold-rolled sheet annealing will be described in detail below.
[0070]
(Dew point in the temperature range of 740 to 900°C)
In cold-rolled steel annealing, by controlling the dew point at 740 to 900 ° C., decarburization is promoted in the second depth region of 10 to 60 μm from the steel plate surface, thereby increasing the mobility of austenite grain boundaries. , it is possible to coarsen the block diameter in the second depth region. If the dew point is too low, the decarburization amount in the second depth region is insufficient, the austenite grain boundary mobility does not increase, and the austenite grain size and block diameter in the second depth region become coarse. is hindered. Therefore, the lower limit of the dew point should be -20°C or higher, preferably -15°C or higher.Above. On the other hand, when the dew point is high, the amount of decarburization in the second depth region becomes excessive, and the austenite grain boundary mobility increases significantly. The block diameter is remarkably coarsened. Therefore, the upper limit of the dew point is 20°C or lower, preferably 15°C or lower.
[0071]
(Holding time in the temperature range of 740 to 900°C)
In the cold-rolled sheet annealing, by controlling the holding time in the temperature range of 740 to 900 ° C., decarburization is promoted in the second depth region, thereby increasing the mobility of the austenite grain boundaries, It is possible to coarsen the austenite grain size and block diameter in the depth region of 2. Here, the holding time means the time during which the temperature stays in the temperature range of 740 to 900°C, and thus includes the time when the temperature is gradually increased between 740 to 900°C. be. If the holding time is short, the decarburization amount in the second depth region is insufficient, the austenite grain boundary mobility does not increase, and the austenite grain size and block diameter in the second depth region become coarse. is hindered. Therefore, the lower limit of the retention time is set to 40 seconds or longer, preferably 60 seconds or longer. On the other hand, if the holding time is long, the amount of decarburization in the second depth region becomes excessive, and the mobility of the austenite grain boundary increases significantly, so the austenite grain size in the second depth region And the block diameter is remarkably coarsened. Therefore, the upper limit of the holding time is 300 seconds or less, preferably 250 seconds or less.
[0072]
(average cooling rate)
Preferred embodiments of cooling after annealing, tempering and plating are described in detail below. The following descriptions are mere examples of preferred embodiments of cooling, tempering and plating after annealing, and do not limit the steel sheet manufacturing method in any way. Cooling after the annealing is preferably carried out from 750° C. to 550° C. at an average cooling rate of 100° C./sec or less. Cooling at an average cooling rate of 100° C./sec or less makes it possible to suppress variation in hardness. The average cooling rate may be 80°C/sec or less, or 50°C/sec or less. The lower limit of the average cooling rate is not particularly limited, but from the viewpoint of ensuring sufficient strength, it may be, for example, 2.5 ° C./sec, preferably 5 ° C./sec or more, more preferably 10 ° C./sec. above, and most preferably above 20° C./sec.
[0073]
(cooling stop temperature)
The above cooling is stopped at a temperature of 25 to 550 ° C. (cooling stop temperature), followed by reheating to a temperature range of 350 to 550 ° C. when this cooling stop temperature is lower than the plating bath temperature. You may let When cooling is performed in the above temperature range, martensite is formed from untransformed austenite during cooling. After that, by performing reheating, martensite is tempered, and carbide precipitation and dislocation recovery/rearrangement occur in the hard phase, and hydrogen embrittlement resistance is improved.
[0074]
(Residence temperature and residence time)
After reheating and before immersion in the plating bath, the steel sheet may be retained in the temperature range of 350-550°C. Retention in this temperature range not only contributes to martensite tempering, but also eliminates temperature unevenness in the width direction of the sheet and improves the appearance after plating. When the cooling stop temperature is 350 to 550° C., the residence may be performed without reheating. When residence is performed, the residence time is preferably 10 to 600 seconds.
[0075]
(Tempering)
Tempering is a series of annealing steps in which the cold-rolled sheet or the steel sheet obtained by plating the cold-rolled sheet is cooled to room temperature, or during cooling to room temperature (however, the martensitic transformation start temperature (Ms) or less). may be carried out by starting reheating at , and holding the temperature in the temperature range of 150 to 400° C. for 2 seconds or more. According to such treatment, hydrogen embrittlement resistance can be improved by tempering the martensite generated during cooling after reheating into tempered martensite. The tempering may be performed in the continuous annealing equipment, or may be performed off-line in another equipment 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.
[0076]
(Plating)
The cold-rolled steel sheet during or after the annealing process is heated or cooled to (galvanizing bath temperature -40) ° C. to (galvanizing bath temperature +50) ° C. and hot-dip galvanized as necessary. may The hot dip galvanizing process forms a hot dip galvanized layer on at least one surface, preferably both surfaces, of the cold rolled steel sheet. In this case, the corrosion resistance of the cold-rolled steel sheet is improved, which is preferable. Even with hot-dip galvanization, the LME resistance of the cold-rolled steel sheet can be sufficiently maintained.
[0077]
(Plating bath immersion plate temperature)
The temperature of the steel plate immersed in the galvanizing bath (the temperature of the steel plate when immersed in the hot-dip galvanizing bath) ranges from 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to 50°C lower than the hot-dip galvanizing bath temperature. A temperature range up to high temperatures (galvanizing bath temperature + 50°C) is preferred. If the plating bath immersion plate temperature is lower than the hot-dip galvanizing bath temperature of −40° C., the heat removal during immersion in the plating bath is large, and part of the molten zinc solidifies, which may deteriorate the appearance of the plating, which is not desirable. If the plate temperature before immersion is lower than the hot-dip galvanizing bath temperature of -40°C, heat the plate further before immersion in the galvanizing bath by any method to control the plate temperature to the hot-dip galvanizing bath temperature of -40°C or higher. may be immersed in the plating bath. Moreover, if the plating bath immersion plate temperature exceeds the hot-dip galvanizing bath temperature +50°C, an operational problem is induced due to the increase in the plating bath temperature.
[0078]
(Composition of plating bath)
The composition of the plating bath is preferably composed mainly of Zn, and the effective Al content (the value obtained by subtracting the total Fe content from the total Al content in the plating bath) is preferably 0.050 to 0.250% by mass. If the effective Al content in the plating bath is less than 0.050% by mass, Fe may excessively penetrate into the plating layer, resulting in deterioration of plating adhesion. On the other hand, when the effective Al amount in the plating bath exceeds 0.250% by mass, an Al-based oxide that inhibits the movement of Fe atoms and Zn atoms is generated at the boundary between the steel sheet and the coating layer, resulting in poor coating adhesion. may decrease. The effective Al content in the plating bath is more preferably 0.065% by mass or more, and more preferably 0.180% by mass or less. The plating bath may contain elements such as Mg in addition to Zn and Al.
[0079]
(Holding temperature after immersion in plating bath)
When the hot-dip galvanized layer is alloyed, it is preferable to heat the steel sheet on which the hot-dip galvanized layer is formed to a temperature range of 470 to 550°C. If the alloying temperature is lower than 470°C, the alloying may not proceed sufficiently. On the other hand, if the alloying temperature exceeds 550° C., the alloying progresses excessively, and the Fe concentration in the coating layer exceeds 15% due to the generation of the Γ phase, which may deteriorate the corrosion resistance. More preferably, the alloying temperature is 480° C. or higher, and even more preferably 540° C. or lower. The alloying temperature needs to be changed according to the chemical composition of the steel sheet and the degree of formation of the internal oxide layer, so it can be set while checking the Fe concentration in the coating layer. On the other hand, when the hot-dip galvanized layer is not alloyed, the holding temperature after immersion in the plating bath may be less than 470.degree.
[0080]
(plating pretreatment)
In order to further improve the coating adhesion, the base steel sheet may be coated with Ni, Cu, Co, or Fe, either singly or in combination, prior to annealing in the continuous hot-dip galvanizing line.
[0081]
(Post-plating treatment)
The surface of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet is subjected to upper layer plating and various treatments such as chromate treatment, phosphate treatment, and lubricity improvement for the purpose of improving paintability and weldability. Treatment, weldability improvement treatment, etc. can also be applied.
[0082]
(Skin pass reduction rate)
In addition, skin pass rolling may be performed for the purpose of improving ductility by straightening the steel sheet shape or introducing mobile dislocations. The rolling reduction of skin pass rolling after heat treatment is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so 0.1% is the lower limit. If it exceeds 1.5%, the productivity drops significantly, so 1.5% is made the upper limit. A skin pass may be performed in-line or off-line. Moreover, the skin pass with the target rolling reduction may be performed at once, or may be performed in several steps.
[0083]
According to the manufacturing method described above, the steel sheet according to the embodiment of the present invention can be obtained.
[0084]
Examples according to the present invention are shown below. The present invention is not limited to this one conditional example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
Example
[0085]
(Example 1)
Steel slabs were manufactured by melting steel with various chemical compositions. These steel slabs were placed in a furnace heated to 1220° C., held for 60 minutes for homogenization, taken out into the air, and hot rolled to obtain a steel plate with a thickness of 2.6 mm. The completion temperature of finish rolling in hot rolling was 890°C, and the steel was cooled to 540°C and coiled. Subsequently, the oxide scale of this hot-rolled steel sheet was removed from the surface layers of both sides of the steel sheet to a thickness of 3.0 μm on one side by pickling (the thickness of the internal oxide layer after pickling and before cold rolling is shown in Table 2). ), cold-rolled at a rolling reduction of 50%, and finished to a sheet thickness of 1.4 mm. Table 2 shows the rolling load of the rolling mill to which the highest rolling load was applied in cold rolling and the friction coefficient of the lubricating oil used in the rolling mill. Furthermore, when the cold-rolled steel sheet was annealed and specifically heated to 880° C., the temperature range of 740 to 900° C. was controlled to an atmosphere with a dew point of 8° C., and the holding time in that temperature range was 130 seconds. . Next, the cold-rolled steel sheets were cooled and held under the conditions shown in Table 2, and then subjected to skin-pass rolling. Table 1 shows the chemical compositions obtained by analyzing the samples taken from each of the obtained steel sheets. The balance other than the components shown in Table 1 is Fe and impurities. Table 2 shows the evaluation results of the properties of the steel sheets subjected to the above heat treatment.

The scope of the claims
[Claim 1]
in % by mass,
C: 0.20-0.40%,
Si: 0.01 to 1.00%,
Mn: 0.10 to 4.00%,
P: 0.0200% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0200% or less,
Co: 0 to 0.5000%,
Ni: 0 to 1.0000%,
Mo: 0 to 1.0000%,
Cr: 0 to 2.0000%,
O: 0 to 0.0200%,
Ti: 0 to 0.500%,
B: 0 to 0.0100%,
Nb: 0 to 0.5000%,
　V: 0 to 0.5000%,
Cu: 0 to 0.5000%,
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%
and has a chemical composition with the balance consisting of Fe and impurities,
In terms of area ratio,
　Total of ferrite, pearlite and bainite: 0 to 10.0%, and
　Total of martensite and tempered martensite: 80.0-100.0%
having a microstructure containing
In the cross-sectional structure cut in the width direction orthogonal to the rolling direction,
the block diameter in the first depth region of 1 to 10 μm from the surface is 5.0 μm or less,
The block diameter in the second depth region of 10 to 60 μm from the surface is 6.0 to 20.0 μm,
A steel plate having a block diameter of less than 6.0 μm in the third depth region from 60 μm to 1/4 of the plate thickness from the surface.
[Claim 2]
The chemical composition, in % by mass,
Co: 0.0001 to 0.5000%,
Ni: 0.0001 to 1.0000%,
Mo: 0.0001 to 1.0000%,
Cr: 0.0001 to 2.0000%,
O: 0.0001 to 0.0200%,
Ti: 0.0001 to 0.500%,
B: 0.0001 to 0.0100%,
Nb: 0.0001 to 0.5000%,
　V: 0.0001 to 0.5000%,
Cu: 0.0001 to 0.5000%,
W: 0.0001 to 0.1000%,
Ta: 0.0001 to 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%,
Y: 0.0001 to 0.0500%,
Zr: 0.0001 to 0.0500%,
La: 0.0001 to 0.0500%, and
Ce: 0.0001 to 0.0500%
The steel sheet according to claim 1, containing one or more selected from the group consisting of
[Claim 3]
The steel sheet according to claim 1 or 2, wherein the area ratio of retained austenite in the microstructure is 10.0% or less.
[Claim 4]
2. A plating layer containing zinc, aluminum, magnesium, an alloy of any combination thereof, or an alloy of at least one of these elements and iron is formed on at least one surface of the steel sheet. 4. The steel plate according to any one of 1 to 3.
[Claim 5]
A step of hot-rolling a steel billet having the chemical composition according to claim 1 or 2 and then winding it at 500°C or higher,
a step of pickling the obtained hot-rolled steel sheet to remove oxide scale existing on the surface of the hot-rolled steel sheet, wherein the amount of removal of the surface layer of the hot-rolled steel sheet is less than 5.00 μm;
a step of cold-rolling the hot-rolled steel sheet at a rolling reduction of 30 to 90%;
An annealing process in which the obtained cold-rolled steel sheet is held in an atmosphere with a dew point of -20 to 20°C in a temperature range of 740 to 900°C for 40 to 300 seconds.
A method of manufacturing a steel plate, comprising:
[Claim 6]
In the annealing step, a coating layer containing zinc, aluminum, magnesium, an alloy composed of any combination thereof, or an alloy of at least one of these elements and iron is formed on at least one surface of the cold-rolled steel sheet. The method for manufacturing a steel sheet according to claim 5, wherein