The present invention relates to a hot-rolled steel sheet and a method for producing the same.
　The present application claims priority based on Japanese Patent Application No. 2018-197936 filed in Japan on October 19, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　In recent years, in order to reduce the amount of carbon dioxide (CO 2 ) emitted from automobiles, the weight of automobile bodies has been reduced by using high-strength steel plates. Further, in order to ensure the safety of passengers, high-strength steel sheets are often used in addition to mild steel sheets for automobile bodies.
[0003]
　More recently, fuel efficiency regulations and NO X by further tightening of environmental regulations, such as the increase in plug-in hybrid and electric vehicles is expected. In these next-generation automobiles, it is necessary to install a large-capacity battery, and it is necessary to further reduce the weight of the vehicle body. Automakers are also actively developing technologies for reducing the weight of vehicle bodies with the aim of reducing fuel consumption. However, it is not easy to reduce the weight of the vehicle body because the emphasis is on improving the collision resistance characteristics in order to ensure the safety of the occupants.
[0004]
　In order to further reduce the weight of the car body, it may be possible to replace the steel plate with a lightweight material such as aluminum alloy, resin, CFRP, or to further increase the strength of the steel plate, but from the viewpoint of material cost and processing cost, , It is realistic to use ultra-high-strength steel sheets for mass-produced cars, excluding luxury cars.
　Therefore, in order to achieve both weight reduction of the vehicle body and collision resistance, it is being studied to thin the member by using a high-strength steel plate. Therefore, a steel sheet having both high strength and excellent formability is strongly desired, and some techniques have been conventionally proposed in order to meet these demands. Among them, steel sheets containing retained austenite have been studied many times because they exhibit excellent ductility due to transformation-induced plasticity (TRIP).
[0005]
　For example, Patent Document 1 describes high strength for automobiles having excellent collision resistance and moldability, in which retained austenite having an average crystal particle size of 5 μm or less is dispersed in ferrite having an average crystal particle size of 10 μm or less. Steel plates are disclosed. In a steel sheet containing retained austenite in its metal structure, austenite undergoes martensitic transformation during processing and exhibits a large elongation due to transformation-induced plasticity, but the formation of hard martensite impairs hole expansion. Patent Document 1 discloses that not only ductility but also hole expansion property is improved by miniaturizing ferrite and retained austenite.
[0006]
　Patent Document 2 discloses a high-strength steel plate having excellent elongation and stretch flangeability and a tensile strength of 980 MPa or more, in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains. There is.
[0007]
　Patent Documents 3 and 4 disclose a high-strength hot-rolled steel sheet having excellent ductility and stretch flangeability, and a method for producing the same. According to Patent Document 3, after cooling to a temperature range of 720 ° C. or lower within 1 second after the completion of hot rolling, and staying in a temperature range of more than 500 ° C. and 720 ° C. or lower for a residence time of 1 to 20 seconds, 350 to A method for producing a high-strength hot-rolled steel sheet having good ductility and stretch flangeability, which is wound in a temperature range of 500 ° C., is disclosed. Further, Patent Document 4 describes grains mainly composed of bainite, having an appropriate amount of polygonal ferrite and retained austenite, and surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in a steel structure excluding retained ausnite. A high-strength hot-rolled steel sheet having an average particle size of 15 μm or less and having good ductility and stretch flangeability is disclosed.
[0008]
　On the other hand, recently, LCA (Life Cycle Assessment) has attracted attention, and attention has been paid not only to the environmental load during driving of automobiles but also during manufacturing.
[0009]
　For example, in the painting of automobile parts, zinc phosphate treatment, which is a kind of chemical conversion treatment, has been applied as a base treatment. The zinc phosphate treatment is low cost and has excellent coating film adhesion and corrosion resistance. However, the zinc phosphate treatment liquid contains phosphoric acid as a main component and metal components such as zinc salt, nickel salt, and manganese salt. Therefore, there is concern about the environmental load caused by phosphorus and metals in the waste liquid that is discarded after use. In addition, a large amount of sludge containing iron phosphate as a main component that precipitates in the chemical conversion treatment tank has become a large environmental load as industrial waste.
[0010]
　Therefore, recently, a zirconium-based chemical conversion treatment liquid has been used as a chemical conversion treatment liquid that can reduce the environmental load. The zirconium-based chemical conversion treatment solution does not contain phosphate and does not require the addition of metal salts. Therefore, the amount of sludge generated is extremely small. For example, Patent Documents 5 and 6 describe techniques for forming a chemical conversion treatment film on a metal surface using a zirconium chemical conversion treatment liquid.
Prior art literature
Patent documents
[0011]
Patent Document 1: Japanese Patent Application Laid-Open No. 11-61326
Patent Document 2: Japanese Patent
Application Laid-Open No. 2005-179703 Patent Document 3: Japanese Patent Application Laid-Open No. 2012-251200
Patent Document 4: Japanese Patent Application Laid-Open No. 2015-124410 JP
Patent Document 5: Japanese Patent 2004-218074 JP
Patent Document 6: Japanese Patent 2008-202149 JP
Outline of the invention
Problems to be solved by the invention
[0012]
　Even if a zirconium-based chemical conversion treatment liquid is used, conventional high-strength steel sheets up to the strength class of 780 MPa can obtain corrosion resistance and coating film adhesion comparable to zinc phosphate treatment. However, in an ultra-high-strength steel sheet having a tensile strength of 980 MPa or more, the amount of alloying elements contained is large, so that zirconium-based chemical crystals are insufficiently adhered to the steel sheet surface, resulting in good corrosion resistance and coating film adhesion. I can't get it.
　Further, in ultra-high-strength steel sheets having excellent collision resistance, such as the steel sheets disclosed in Patent Documents 1 to 4 described above, there is still no method for sufficiently improving the coating film adhesion when a zirconium-based chemical conversion treatment liquid is used. Not proposed.
[0013]
　The present invention has been devised in view of the above-mentioned problems, and an object of the present invention is to have a tensile strength of 980 MPa or more, high press formability (ductility and stretch flangeability), and good toughness. A hot-rolled steel sheet and its heat, which is a high-strength steel sheet and has chemical conversion treatment properties and coating adhesion equal to or higher than those when a zirconium-based chemical conversion treatment solution is used. The purpose of the present invention is to provide a manufacturing method capable of stably manufacturing rolled steel sheets.
Means to solve problems
[0014]
　The present inventors have conducted diligent studies to solve the above problems and obtained the following findings.
[0015]
　The present invention has been made based on these findings, and the gist thereof is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition represented by an average value of the entire plate thickness direction in mass%, C: 0.100 to 0.250%, Si: 0.05. ~ 3.00%, Mn: 1.00 to 4.00%, Al: 0.001 to 2.000%, Ni: 0.02 to 2.00%, Nb: 0 to 0.300%, Ti: 0 to 0.300%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, V: 0 to 0.500%, Cr: 0 to 2.00%, Mg: 0 to 0.0200 %, Ca: 0 to 0.0200%, REM: 0 to 0.1000%, B: 0 to 0.0100%, Bi: 0 to 0.020%, Zr, Co, Zn, and W. Or two or more types: 0 to 1.000% in total, Sn: 0 to 0.050%, P: 0.100% or less, S: 0.0300% or less, O: 0.0100% or less, N: 0 When the content is 1000% or less, the balance is Fe and impurities, the following formula (i) is satisfied, and the thickness is t, the metallographic structure at the position t / 4 from the surface is the area fraction. 77.0 to 97.0% of baynite or tempered martensite, 0 to 5.0% of ferrite, 0 to 5.0% of pearlite, 3.0% or more of retained austenite, 0 to 0 to martensite. In the metal structure containing 10.0%, the average crystal grain size excluding the retained austenite is 7.0 μm or less, and the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0 × 10 6 / mm. It is 2 or more, the tensile strength is 980 MPa or more, and the average Ni concentration on the surface is 7.0% or more.
　0.05% ≤ Si + Al ≤ 3.00% ... Equation (i)
　The element represented by the above formula (i) is the mass% of the element contained in the hot-rolled steel sheet.
(2) The hot-rolled steel sheet according to (1) above may have a chemical composition of% by mass and Ni: 0.02 to 0.05%.
(3) In the hot-rolled steel sheet according to (1) or (2) above, the hot-rolled steel sheet has an internal oxide layer, and the average depth of the internal oxide layer is 5. It may be 0 μm or more and 20.0 μm or less.
(4) In the hot-rolled steel sheet according to any one of (1) to (3) above, the standard deviation of the arithmetic mean roughness Ra of the surface of the hot-rolled steel sheet is 10.0 μm or more and 50.0 μm or less. You may.
(5) The hot-rolled steel sheet according to any one of (1) to (4) above has a chemical composition of V: ​​0.005 to 0.500% and Ti: 0.005 to 0. It may contain 300% of one or two.
(6) The hot-rolled steel sheet according to any one of (1) to (5) above has a chemical composition of Nb: 0.005 to 0.300% and Cu: 0.01% to 2 in mass%. One or two or more of 0.00%, Mo: 0.01% to 1.000%, B: 0.0001 to 0.0100%, Cr: 0.01% or more, 2.00% or less. It may be contained.
(7) The hot-rolled steel sheet according to any one of (1) to (6) above has a chemical composition of% by mass, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0. It may contain one or more of 0200% and REM: 0.0005 to 0.1000%.
(8) In the method for producing a hot-rolled steel sheet according to another aspect of the present invention, a steel piece having the chemical composition described in (1) above has at least a preheating zone, a heating zone, and a heat equalizing zone. In a heating furnace equipped with a formula burner, a heating step of heating to 1150 ° C. or higher, and the heated steel pieces so that the finishing temperature is T2 ° C. or higher obtained by the following formula (ii) and 850 to A hot-rolling step of hot-rolling to obtain a hot-rolled steel plate so that the cumulative reduction rate in the temperature range of 1100 ° C. is 90% or more, and cooling is started within 1.5 seconds after the hot-rolling step. The primary cooling step of cooling the hot-rolled steel sheet to a temperature T3 ° C. or lower represented by the following formula (iii) at an average cooling rate of 50 ° C./sec or more, and the temperature represented by the following formula (iv) are T4. When the temperature is set to ° C., the secondary cooling step of cooling from the cooling stop temperature of the primary cooling step to the winding temperature of (T4-100) ° C. to (T4 + 50) ° C. at an average cooling rate of 10 ° C./sec or more, and the winding It has a winding step of winding at a taking temperature, and in the heating step, the air ratio in the preheating zone is set to 1.1 to 1.9.
T2 (° C.) = 868-396 x [C] -68.1 x [Mn] + 24.6 x [Si] -36.1 x [Ni] -24.8 x [Cr] -20.7 x [Cu] ] + 250 x [Al] ... (ii)
T3 (° C.) = 770-270 x [C] -90 x [Mn] -37 x [Ni] -70 x [Cr] -83 x [Mo] ... (Iii)
T4 (° C.) = 591-474 x [C] -33 x [Mn] -17 x [Ni] -17 x [Cr] -21 x [Mo] ... (iv)
　However, each formula [Element symbol] in the inside indicates the content (mass%) of each element in the steel piece.
(9) In the method for producing a hot-rolled steel sheet according to (8) above, the air ratio in the heating zone may be 0.9 to 1.3 in the heating step.
(10) In the method for producing a hot-rolled steel sheet according to (8) or (9) above, the air ratio in the heat equalizing zone may be 0.9 to 1.9 in the heating step.
(11) In the method for producing a hot-rolled steel sheet according to (9) or (10) above, the air ratio in the preheating zone may be larger than the air ratio in the heating zone.
(12) In the method for producing a hot-rolled steel sheet according to any one of (8) to (10) above, the hot-rolled steel sheet after the winding step is subjected to 1 to 10% by mass of a temperature of 20 to 95 ° C. A pickling step of pickling with a hydrochloric acid solution under the condition of a pickling time of less than 30 to 60 seconds may be provided.
Effect of the invention
[0016]
　According to the above aspect of the present invention, an ultra-high strength steel sheet having a tensile strength of 980 MPa or more, high press formability (ductility and stretch flangeability), and good toughness, when a zirconium-based chemical conversion treatment liquid is used. However, it is possible to obtain a hot-rolled steel sheet having a chemical conversion treatment property and a coating film adhesion equal to or higher than those in the case of using a zinc phosphate chemical conversion treatment solution. Since the steel sheet according to the present invention is excellent in chemical conversion treatment property and coating film adhesion, it is excellent in corrosion resistance after coating. It also has excellent ductility and stretch flangeability. Therefore, the steel sheet according to the present invention is suitable for automobile parts that require high strength, moldability, and corrosion resistance after painting.
A brief description of the drawing
[0017]
FIG. 1 is an example of surface EPMA measurement results of a hot-rolled steel sheet and a comparative hot-rolled steel sheet according to the present embodiment. (Measurement conditions: Acceleration voltage: 15 kV, Irradiation current: 6 × 10 -8 A, Irradiation time: 30 ms, Beam diameter: 1 μm)
[Fig. 2] Mechanism in which Ni concentrated on the surface becomes precipitated nuclei of zirconium-based chemical crystals. It is a figure which shows.
[Fig. 3] Fig. 3 is a diagram showing the mechanism by which the surface roughness of a hot-rolled steel sheet changes.
Mode for carrying out the invention
[0018]
　The present inventors have good chemical conversion treatment and coating film adhesion by chemical conversion treatment using a zirconium-based chemical conversion treatment liquid on an ultra-high-strength steel plate having a tensile strength of 980 MPa or more and sufficient ductility and stretch flangeability. We have conducted extensive research on the conditions under which and can be stably obtained. As a result of the examination, it was found that the oxide on the surface layer of the steel sheet has a great influence on the chemical conversion treatment property and the coating film adhesion. Specifically, it is as follows.
　The steel sheet is usually pickled before the chemical conversion treatment. However, it was found that oxides such as Si and Al were formed on the surface of the ultra-high-strength steel sheet even after normal pickling, which deteriorated the zirconium-based chemical conversion treatment property and the coating film adhesion. .. As a result of further studies by the present inventors, in order to improve the chemical conversion treatment property and the coating film adhesion, the formation of oxides such as Si and Al is suppressed, and the surface layer of the steel sheet is used as a precipitation core of zirconium-based chemical crystals. It was found that forming a concentrated layer of Ni is effective.
　Further, the present inventors consider the content of a small amount of Ni and the heating conditions in the heating step prior to hot rolling, assuming that the process for manufacturing a general hot-rolled steel sheet is inexpensive and mass-produced. It has been found that it is possible to form a Ni-concentrated layer on the surface layer of the steel sheet after pickling (before the chemical conversion treatment) by limiting the amount.
[0019]
　Hereinafter, the hot-rolled steel sheet according to the present embodiment will be described in detail.
[0020]
[Components of Steel Sheet]
　First, the reason for limiting the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described. Unless otherwise specified,% with respect to the content of the component indicates mass%.
　In addition, the display of the element name used in each formula in the present specification shall indicate the content (mass%) of the element in the steel sheet, and if it is not contained, 0 shall be substituted.
[0021]
C: 0.100 to 0.250%
　C has an action of promoting the formation of bainite and an action of stabilizing retained austenite. If the C content is less than 0.100%, it becomes difficult to obtain the desired bainite surface integral and retained austenite surface integral. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.120% or more, or 0.150% or more.
　On the other hand, when the C content exceeds 0.250%, pearlite is preferentially produced to insufficiently produce bainite and retained austenite, and a desired area fraction of bainite and an area fraction of retained austenite can be obtained. It becomes difficult. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.220% or less, or 0.200% or less.
[0022]
Si: 0.05 to 3.00%
　Si has the effect of delaying the precipitation of cementite. By this action, the amount of austenite remaining untransformed, that is, the surface integral of the retained austenite can be increased, and the strength of the steel sheet can be increased by solid solution strengthening. Further, Si has an action of making the steel sound by deoxidation (suppressing the occurrence of defects such as blow holes in the steel). If the Si content is less than 0.05%, the effect of the above action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, or 1.00% or more.
　On the other hand, when the Si content exceeds 3.00%, the surface texture and chemical conversion treatment property, ductility and weldability of the steel sheet are remarkably deteriorated, and the A3 transformation point is remarkably increased. This makes it difficult to perform hot rolling in a stable manner. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, or 2.50% or less.
[0023]
Mn: 1.00 to 4.00%
　Mn has an action of suppressing the ferrite transformation and promoting the formation of bainite. If the Mn content is less than 1.00%, the desired surface integral of bainite cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more, more preferably 1.80% or more.
　On the other hand, when the Mn content exceeds 4.00%, the completion of the bainite transformation is delayed, so that carbon concentration to austenite is not promoted, the formation of retained austenite becomes insufficient, and the desired surface integral of retained austenite is used. It becomes difficult to obtain the rate. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, or 3.50% or less.
[0024]
Ni: 0.02% to 2.00%
　Ni is one of the important elements in the hot-rolled steel sheet according to the present embodiment. Ni is concentrated in the vicinity of the steel sheet surface near the interface between the steel sheet surface and the scale under specific conditions, mainly in the heating process of the hot rolling process. When the zirconium-based chemical conversion treatment is performed on the surface of the steel sheet, this Ni becomes the precipitation nuclei of the zirconium-based chemical conversion treatment film, and promotes the formation of a film having no scale and good adhesion. If the Ni content is less than 0.02%, there is no effect, so the Ni content is set to 0.02% or more. The effect of improving adhesion can be obtained not only for a zirconium-based chemical conversion treatment film but also for a conventional zinc phosphate chemical conversion treatment film. In addition, the adhesion to the hot-dip galvanized layer by the hot-dip galvanized treatment and the base material of the alloyed galvanized layer that has been alloyed after plating is also improved.
　On the other hand, even if the Ni content exceeds 2.00%, not only the effect is saturated but also the alloy cost increases. Therefore, the Ni content is set to 2.00% or less. It is preferably 0.50% or less, 0.20% or less, or 0.05% or less.
[0025]
Al: 0.001 to 2.000%
　Al has an action of deoxidizing the steel to make the steel sheet sound, like Si. In addition, Al has an action of promoting the formation of retained austenite by suppressing the precipitation of cementite from austenite. If the Al content is less than 0.001%, the effect of the above action cannot be obtained. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.010% or more.
　On the other hand, if the Al content exceeds 2.000%, the above effects are saturated and economically unfavorable. Therefore, the Al content is set to 2.000% or less. The Al content is preferably 1.500% or less, or 1.300% or less.
[0026]
P: 0.100% or less
　P is an element generally contained as an impurity, but it is also an element having an action of increasing the strength by strengthening the solid solution. P may be positively contained, but P is an element that is easily segregated, and when the P content exceeds 0.100%, the moldability and toughness are significantly reduced due to the grain boundary segregation. Therefore, the P content is limited to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content does not need to be specified, but is preferably 0.001% from the viewpoint of refining cost.
[0027]
S: 0.0300% or less
　S is an element contained as an impurity and forms sulfide-based inclusions in the steel to reduce the formability of the hot-rolled steel sheet. When the S content exceeds 0.0300%, the moldability is significantly lowered. Therefore, the S content is limited to 0.0300% or less. The S content is preferably 0.0050% or less. The lower limit of the S content does not need to be specified, but is preferably 0.0001% from the viewpoint of refining cost.
[0028]
N: 0.1000% or less
　N is an element contained in steel as an impurity and is an element that lowers the moldability of the steel sheet. When the N content exceeds 0.1000%, the moldability of the steel sheet is significantly lowered. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, and more preferably 0.0700% or less. The lower limit of the N content does not need to be specified in particular, but as will be described later, when one or more of Ti and V are contained to refine the metal structure, precipitation of carbonitride is promoted. The N content is preferably 0.0010% or more, and more preferably 0.0020% or more.
[0029]
O: 0.0100% or less
　O forms a coarse oxide that becomes a starting point of fracture when it is contained in a large amount in steel, and causes brittle fracture and hydrogen-induced cracking. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0080% or less and 0.0050% or less. The O content may be 0.0005% or more, or 0.0010% or more, in order to disperse a large number of fine oxides when the molten steel is deoxidized.
[0030]
　The balance of the chemical composition of the hot-rolled steel sheet according to the present embodiment is basically composed of Fe and impurities, but the hot-rolled steel sheet according to the present embodiment is composed of Nb, Ti, V, Cu, in addition to the above elements. Cr, Mo, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W and Sn may be contained as optional elements. When the above optional element is not contained, the content is 0%. Hereinafter, the above optional elements will be described in detail.
[0031]
　In the present embodiment, the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., and are allowed as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment. do.
[0032]
Nb: 0 to 0.300%
　Nb forms carbonitride, or solid solution Nb delays grain growth during hot rolling, resulting in low temperature toughness through finer grain size of hot-rolled steel sheet. It is an element that contributes to the improvement of. When this effect is obtained, the Nb content is preferably 0.005% or more.
　On the other hand, even if the Nb content exceeds 0.300%, the above effect is saturated and the economic efficiency is lowered. Therefore, if necessary, the Nb content is set to 0.300% or less even when Nb is contained.
[0033]
One or two
　Ti and V selected from the group consisting of Ti: 0 to 0.300% and V: 0 to 0.500% are both precipitated as carbides or nitrides in the steel and pinned. It has the effect of refining the metal structure by the effect. Therefore, one or two of these elements may be contained. In order to obtain the effect of the above action more reliably, it is preferable that the Ti content is 0.005% or more, or the V content is 0.005% or more. However, even if these elements are excessively contained, the effect of the above action is saturated and it is economically unfavorable. Therefore, even when it is contained, the Ti content is 0.300% or less, and the V content is 0.500% or less.
[0034]
One or more
　Cu selected from the group consisting of Cu: 0 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 1.000%, and B: 0 to 0.0100%. , Cr, Mo, and B all have an action of enhancing hardenability. Further, Cr has an action of stabilizing retained austenite, and Cu and Mo have an action of precipitating carbides in steel to increase the strength.
[0035]
　Cu has an action of enhancing hardenability and an action of precipitating as carbide in steel at a low temperature to increase the strength of the steel sheet. In order to obtain the effect of the above action more reliably, the Cu content is preferably 0.01% or more, and more preferably 0.03% or more or 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracks in the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less and 1.00% or less.
[0036]
　Cr has an action of enhancing hardenability and an action of stabilizing retained austenite. In order to obtain the effect of the above action more reliably, the Cr content is preferably 0.01% or more, or 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical conversion treatment property of the steel sheet is significantly lowered. Therefore, the Cr content is set to 2.00% or less.
[0037]
　Mo has an action of enhancing hardenability and an action of precipitating carbides in steel to increase strength. In order to obtain the effect of the above action more reliably, the Mo content is preferably 0.010% or more, or 0.020% or more. However, even if the Mo content exceeds 1.000%, the effect of the above action is saturated and economically unfavorable. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.500% or less and 0.200% or less.
[0038]
　B has an effect of enhancing hardenability. In order to obtain the effect of this action more reliably, the B content is preferably 0.0001% or more, or 0.0002% or more. However, if the B content exceeds 0.0100%, the moldability of the steel sheet is significantly lowered, so the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.
[0039]
One or more
　Ca, Mg and REM selected from the group consisting of Ca: 0 to 0.0200%, Mg: 0 to 0.0200% and REM: 0 to 0.1000% are all intervening. By adjusting the shape of the object to a preferable shape, it has an effect of improving the formability of the steel sheet. Therefore, one or more of these elements may be contained. In order to obtain the effect of the above action more reliably, it is preferable that the content of any one or more of Ca, Mg and REM is 0.0005% or more. However, when the Ca content or Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which in turn lowers the formability of the steel sheet. May cause you to. Therefore, the Ca content and Mg content are set to 0.0200% or less, and the REM content is set to 0.1000% or less.
　Here, REM refers to a total of 17 elements composed of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.
[0040]
Bi: 0 to 0.020%
　Bi may be contained in steel because it has an effect of improving moldability by refining the solidified structure. In order to obtain the effect of this action more reliably, the Bi content is preferably 0.0005% or more. However, even if the Bi content exceeds 0.020%, the effect of the above action is saturated, which is economically unfavorable. Therefore, the Bi content is 0.020% or less. The Bi content is preferably 0.010% or less.
[0041]
One or more of Zr, Co, Zn and W: 0 to 1.000% in total
Sn: 0 to 0.050% For
　Zr, Co, Zn and W, the present inventors have these elements. It has been confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if the total content of 1.000% or less is not impaired. Therefore, one or more of Zr, Co, Zn and W may be contained in a total of 1.000% or less.
　Further, the present inventors have confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if a small amount of Sn is contained, but if Sn is contained, defects occur during hot rolling. Since it becomes easy, the Sn content is set to 0.050% or less.
[0042]
　0.05% ≤ Si + Al ≤ 3.00% In the
　hot-rolled steel sheet according to the present embodiment, the content of each element is controlled within the above range, and then Si + Al is controlled so as to satisfy the following formula (1). There is a need.
　0.05% ≤ Si + Al ≤ 3.00% ... Equation (1) When
　Si + Al is less than 0.05%, scale defects such as scales and spindle scales occur,
　while Si + Al exceeds 3.00%. If there is, the effect of improving the chemical conversion treatment property and the coating film adhesion will not be exhibited even if Ni is contained.
[0043]
　The content of each element in the hot-rolled steel sheet described above is the average content in the total plate thickness determined by ICP emission spectroscopic analysis using chips according to JISG1201: 2014.
[0044]
[Metallic Structure of Steel Sheet]
　Next, the metal structure (microstructure) of the hot-rolled steel sheet according to the present embodiment will be described.
　In the hot-rolled steel sheet according to the present embodiment, the metal at a position at a depth of 1/4 of the sheet thickness (t / 4 when the plate thickness is t (mm)) from the surface of the steel sheet in a cross section parallel to the rolling direction of the steel sheet. In terms of area fraction (area%), bainite and tempered martensite total 77.0 to 97.0%, ferrite 0 to 5.0%, pearlite 0 to 5.0%, and retained austenite. By containing 3.0% or more of martensite and 0 to 10.0% of martensite, a tensile strength of 980 MPa or more and high press formability (ductility and stretch flangeability) can be obtained. In the present embodiment, the reason for defining the metal structure at a depth of 1/4 of the plate thickness from the surface of the steel sheet in the cross section parallel to the rolling direction of the steel sheet is that the metal structure at this position is a typical metal structure of the steel sheet. Because it shows.
[0045]
Total area fraction of
　bainite and tempered martensite : 77.0-97.0% Bainite and tempered martensite are the most important metallographic structures in this embodiment.
　Bainite is a collection of lath-shaped crystal grains. The bainite includes an upper bainite which is an aggregate of laths containing carbides between laths and a lower bainite containing iron-based carbides having a major axis of 5 nm or more inside. The iron-based carbides precipitated in the lower bainite belong to a single variant, i.e., a group of iron-based carbides extending in the same direction. Tempering martensite is a collection of lath-shaped crystal grains, and contains iron-based carbides having a major axis of 5 nm or more inside. The iron carbides in tempered martensite belong to a plurality of variants, i.e., a group of iron carbides extending in different directions. Since it is difficult to distinguish between lower bainite and tempered martensite by the measurement method described later, it is not necessary to distinguish between the two in this embodiment.
[0046]
　As described above, bainite and tempered martensite are hard and homogeneous metal structures, which are the most suitable metal structures for steel sheets to have both high strength and excellent stretch flangeability. If the total surface integral of bainite and tempered martensite is less than 77.0%, the steel sheet cannot have both high strength and excellent stretch flangeability. Therefore, the total surface integral of bainite and tempered martensite shall be 77.0% or more. The total surface integral of bainite and tempered martensite is preferably 85.0% or more, more preferably 90.0% or more. Since the hot-rolled steel sheet according to the present embodiment contains 3.0% or more of retained austenite, the total area fraction of bainite and tempered martensite is 97.0% or less.
[0047]
Surface integral of ferrite: 0 to 5.0%
　Ferrite is a lumpy crystal grain and is a metal structure that does not contain a substructure such as a lath inside. When the area fraction of soft ferrite exceeds 5.0%, the interface between ferrite and bainite or tempered martensite, which tends to be the starting point of void generation, and the interface between ferrite and retained austenite increase, especially in steel sheets. The extensibility of the ferrite is reduced. Therefore, the surface integral of ferrite shall be 5.0% or less. The surface integral of ferrite is preferably 4.0% or less, 3.0% or less, or 2.0% or less. In order to improve the stretch flangeability of the steel sheet, the surface integral of ferrite is preferably reduced as much as possible, and the lower limit thereof is 0%.
[0048]
Area fraction of pearlite: 0 to 5.0%
　Pearlite is a lamellar metal structure in which cementite is deposited in layers between ferrites, and is a soft metal structure as compared with bainite. When the area fraction of pearlite exceeds 5.0%, the interface between pearlite, which tends to be the starting point of voids, and bainite or tempered martensite, and the interface between pearlite and retained austenite increase, especially for steel sheets. Stretchable flangeability is reduced. Therefore, the surface integral of pearlite is set to 5.0% or less. The surface integral of pearlite is preferably 4.0% or less, 3.0% or less, or 2.0% or less. In order to improve the stretch flangeability of the steel sheet, the surface integral of the pearlite is preferably reduced as much as possible, and the lower limit thereof is 0%.
[0049]
Area fraction of martensite: 0 to 10.0% In
　this embodiment, martensite is defined as a metallographic structure in which carbides having a diameter of 5 nm or more are not deposited between laths and in laths. Martensite (so-called fresh martensite) is a very hard structure and greatly contributes to increasing the strength of steel sheets. On the other hand, when martensite is contained, the interface between martensite and the parent phases bainite and tempered martensite becomes the starting point of void generation, and the stretch flangeability of the steel sheet is particularly lowered. Further, since martensite has a hard structure, it deteriorates the low temperature toughness of the steel sheet. Therefore, the martensite surface integral ratio shall be 10.0% or less. Since the hot-rolled steel sheet according to the present embodiment contains a predetermined amount of bainite and tempered martensite, a desired strength can be ensured even when martensite is not contained. In order to obtain the desired stretch flangeability of the steel sheet, the surface integral of martensite is preferably reduced as much as possible, and the lower limit thereof is 0%.
[0050]
　The bainite, tempered martensite, ferrite, pearlite and martensite constituting the metal structure of the hot-rolled steel sheet according to the above embodiment can be identified by the following methods, the position of the bainite, and the area of ​​the hot-rolled steel sheet. Measure the fraction.
　First, a Nital reagent and a reagent disclosed in JP-A-59-219473 are used to corrode a cross section of a steel sheet parallel to the rolling direction. Regarding the corrosion of the cross section, specifically, a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol was used as liquid A, and 1 to 25 g of sodium thiosulfate and 1 to 5 g of nitric acid were dissolved in 100 ml of water. The solution is solution B, and solution A and solution B are mixed at a ratio of 1: 1 to form a mixed solution, and nitric acid at a ratio of 1.5 to 4% is further added and mixed with respect to the total amount of this mixed solution. The prepared liquid is used as a pretreatment liquid. Further, a liquid obtained by adding and mixing the above-mentioned pretreatment liquid in a ratio of 10% with respect to the total amount of the 2% nital liquid to the 2% nital liquid is used as a post-treatment liquid. A cross section parallel to the rolling direction of the steel sheet is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, then immersed in the posttreatment liquid for 3 to 20 seconds, washed with water and dried. , Corrodes the above cross section.
　Next, the above-mentioned features are included by observing at least three 40 μm × 30 μm regions at a magnification of 1000 to 100,000 times using a scanning electron microscope at a depth of 1/4 of the plate thickness from the steel plate surface. Each phase in the metallographic structure is identified based on whether or not, the existence position of each phase is confirmed, and the area fraction is measured.
[0051]

　Surface integral of retained austenite : 3.0% or more Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Residual austenite has the effect of increasing the ductility of the steel sheet due to transformation-induced plasticity (TRIP). If the surface integral of the retained austenite is less than 3.0%, the effect of the above action cannot be obtained and the ductility of the steel sheet deteriorates. Therefore, the surface integral of retained austenite is set to 3.0% or more. The surface integral of the retained austenite is preferably 5.0% or more, more preferably 7.0% or more, still more preferably 8.0% or more. The upper limit of the area fraction of retained austenite does not need to be specified in particular, but since the area fraction of retained austenite that can be secured in the chemical composition of the hot-rolled steel sheet according to the present embodiment is approximately 20.0% or less, it remains. The upper limit of the area fraction of austenite may be 20.0%.
[0052]
　Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Backscatter Diffraction) analysis, and magnetic measurement methods, and the measured values ​​may differ depending on the measurement method. .. In this embodiment, the surface integral of retained austenite is measured by X-ray diffraction.
　In the measurement of the residual austenite area fraction by X-ray diffraction in the present embodiment, first, Co-Kα rays are used in a cross section parallel to the rolling direction of the steel sheet at a depth of 1/4 of the thickness of the steel sheet. The integrated intensity of a total of 6 peaks of α (110), α (200), α (211), γ (111), γ (200), and γ (220) was obtained and calculated using the intensity averaging method to remain. Obtain the volume fraction of austenite. Assuming that the volume fraction and the surface integral are equal, this is taken as the surface integral of retained austenite.
　In the present embodiment, the area fractions of bainite, tempered martensite, ferrite, pearlite, and martensite (area fractions other than retained austenite) and the area fractions of retained austenite are measured by different measurement methods. The sum of the two area fractions may not be 100.0%. If the total of the surface integrals other than the retained austenite and the surface integrals of the retained austenite does not reach 100.0%, the above two surface integrals are adjusted so that the total becomes 100.0%. For example, when the total of the area fraction other than retained austenite and the area fraction of retained austenite is 101.0%, in order to make the total of both 100.0%, other than the retained austenite obtained by measurement. The value obtained by multiplying the area fraction of the above by 100.0 / 101.0 is defined as the area fraction other than the retained austenite, and the area fraction of the retained austenite obtained by the measurement is multiplied by 100.0 / 101.0. The value is defined as the area fraction of retained austenite.
　If the sum of the surface integrals other than the retained austenite and the area fractions of the retained austenite is less than 95.0% or more than 105.0%, the area fractions are measured again.
[0053]
Average grain size of
　metal structure excluding retained austenite : 7.0 μm or less Average crystal grain size (main phase bainite and tempered martensite, ferrite, pearlite, and martensite) excluding retained austenite Hereinafter, it may be simply referred to as an average crystal grain size), and thus the low temperature toughness is improved. If the average crystal grain size exceeds 7.0 μm, vTrs ≦ −50 ° C., which is an index of low temperature toughness required for steel sheets for undercarriage parts of automobiles, cannot be satisfied. Therefore, the average crystal grain size is set to 7.0 μm or less. The lower limit of the average crystal grain size is not particularly limited, but the smaller the average crystal grain size is, the more preferable it is, and it may be more than 0 μm. However, since it may be practically difficult to set the average crystal grain size to less than 1.0 μm from the viewpoint of manufacturing equipment, the average crystal grain size may be 1.0 μm or more.
[0054]
　In the present embodiment, the crystal grains are defined by using the EBSP-OIM TM (Electron Backscatter Diffraction Pattern Microscopic) method. In the EBSP-OIM method, a highly inclined sample is irradiated with an electron beam in a scanning electron microscope (SEM), the Kikuchi pattern formed by backscattering is photographed with a high-sensitivity camera, and the photographed photograph is image-processed by a computer. By doing so, the crystal orientation of the irradiation point can be measured in a short waiting time. The EBSP-OIM method is performed using a device that combines a scanning electron microscope and an EBSP analyzer and an OIM Analysis (registered trademark) manufactured by AMETEK. In the EBSP-OIM method, the fine structure and crystal orientation of the sample surface can be quantitatively analyzed. The analyzable area of ​​the EBSP-OIM method is an area that can be observed by SEM. Although it depends on the resolution of the SEM, according to the EBSP-OIM method, analysis can be performed with a minimum resolution of 20 nm. Since the threshold value of the large-angle grain boundary, which is generally recognized as a crystal grain boundary, is 15 °, in the present embodiment, a crystal grain having an orientation difference of 15 ° or more between adjacent crystal grains is defined as one crystal grain. The crystal grains are visualized from the mapped image, and the average crystal grain size of the area average calculated by OIM Analysis is obtained.
[0055]
　When measuring the average crystal grain size of the metal structure at a depth of 1/4 of the plate thickness from the surface of the steel plate in the cross section parallel to the rolling direction of the steel plate, at least 10 fields of 40 μm × 30 μm are viewed at a magnification of 1200 times. The average crystal grain size is defined as the average of the grain sizes (effective crystal grain sizes) of crystals having an orientation difference of 15 ° or more between adjacent crystal grains. In this measurement method, since the area fraction is small for structures other than the main phase, it is judged that the effect is small, and the average grain boundaries of bainite and tempered martensite, which are the main phases, and ferrite, pearlite, and martensite No distinction from average grain size. That is, the average crystal grain size measured by the above-mentioned measuring method is the average crystal grain size of bainite, tempered martensite, ferrite, pearlite and martensite. In the measurement of the effective crystal grain size of pearlite, the effective crystal grain size of ferrite in pearlite is measured instead of the effective crystal grain size of the pearlite block.
　Since the crystal structure of the retained austenite is FCC and the other microstructures are BCC, which are different from each other, the average crystal grain size of the metal structure excluding the retained austenite can be easily measured by EBSP.
[0056]
Average number of iron-based carbides with a diameter of 20 nm or more Density: 1.0 × 10 6 pieces / mm 2 or
　more The reason why iron-based carbides with a diameter of 20 nm or more are contained in steel is 1.0 × 10 6 pieces / mm 2 or more. This is to increase the low temperature toughness of the matrix and obtain a balance between excellent strength and low temperature toughness. The iron-based carbide in the present embodiment means one containing Fe and C and having a major axis length of less than 1 μm. That is, coarse carbides precipitated between cementite and bainite lath in pearlite having a major axis length of 1 μm or more are not included in this embodiment. When the matrix is ​​as-quenched martensite, the strength is excellent but the low temperature toughness is poor, so it is necessary to improve the low temperature toughness. Therefore, by precipitating a predetermined number or more of iron-based carbides in the steel by tempering or the like, the low-temperature toughness of the main phase is improved, and the low-temperature toughness (vTrs ≤ -50 ° C) required for steel sheets for undercarriage parts of automobiles is reduced. Achieve.
[0057]
　When the present inventors investigated the relationship between the low temperature toughness of a steel plate and the number density of iron-based carbides, the number density of iron-based carbides in the metal structure was set to 1.0 × 10 6 pieces / mm 2 or more. In particular, it was clarified that excellent low temperature toughness can be obtained by setting the number density of iron-based carbides in tempered martensite and lower bainite to 1.0 × 10 6 / mm 2 or more. Therefore, in the present embodiment, the number density of iron-based carbides is 1.0 × 10 6 / mm in the metal structure at a depth of 1/4 of the sheet thickness from the surface of the steel sheet in the cross section parallel to the rolling direction of the steel sheet. 2 or more. The number density of iron-based carbides is preferably 5.0 × 10 6 pieces / mm 2 or more, and more preferably 1.0 × 10 7 pieces / mm 2 or more.
　Further, the size of the iron-based carbides precipitated on the hot-rolled steel sheet according to the present embodiment is as small as 300 nm or less, and most of them are precipitated in the lath of martensite and bainite, so it is estimated that the low temperature toughness is not deteriorated. ..
[0058]
　To measure the number density of iron-based carbides, a sample is taken with the cross section parallel to the rolling direction of the steel plate as the observation surface, the observation surface is polished, night tar etching is performed, and the depth position is 1/4 of the plate thickness from the steel plate surface. The range of the plate thickness of 1/8 to 3/8 centered on the above is observed with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope). Observation is performed in 10 fields or more at a magnification of 200,000 times, and the number density of iron-based carbides having a diameter of 20 nm or more is measured.
[0059]
Average Ni concentration on the surface: 7.0% or more In
　order to obtain excellent chemical conversion treatment properties and coating adhesion of the zirconium-based chemical conversion treatment film even on the surface of ultra-high strength steel sheets after pickling (before chemical conversion treatment), It is preferable that oxides such as Si and Al on the surface of the pickling plate are reduced to a harmless level. In order to obtain the above effect only by controlling oxides such as Si and Al, Ar, He, N 2 in the preheating zone of the heating furnace in order to suppress the oxidation of the slab surface as much as possible in the heating process of hot rolling. It is necessary to create a substantially non-oxidizing atmosphere using an inert gas such as, or to set the air ratio to incomplete combustion of less than 0.9. However, when it is inexpensive and mass-produced in the process of manufacturing a general hot-rolled steel sheet, it is not possible to create a substantially non-oxidizing atmosphere using an inert gas in the heating process of hot rolling. It is possible. Further, if the air ratio is set to less than 0.9 to control oxides such as Si and Al, the heat loss due to incomplete combustion increases remarkably, the thermal efficiency of the heating furnace itself decreases, and the production cost increases. Problems arise.
　The present inventors assume that the above-mentioned chemical composition, structure, and tensile strength of 980 MPa or more, excellent ductility and elongation flangeability are obtained on the premise of application of a manufacturing process capable of inexpensive mass production. In, the adhesion of the coating film after the chemical conversion treatment using the zirconium-based chemical conversion treatment liquid was examined. Since the hot-rolled steel sheet is usually subjected to chemical conversion treatment after pickling, the steel sheet after pickling was evaluated in this embodiment as well. In this embodiment, pickling is carried out using a 1 to 10% by mass hydrochloric acid solution at a temperature of 20 to 95 ° C. and a pickling time of less than 30 to 60 seconds. If no scale is formed on the surface, evaluation may be performed without pickling.
　As a result of the examination, in the measurement using FE-EPMA, if the average Ni concentration on the surface is 7.0% or more in mass%, even if oxides such as Si and Al remain on the surface of the pickling plate, It was found that the coating peeling width evaluated by the method described later for all the samples was within 4.0 mm, which is the standard, and the coating film adhesion was excellent. Further, in such a case, no scale was observed in the chemical conversion-treated film. On the other hand, the coating peeling width was more than 4.0 mm in all the samples having an average Ni concentration of less than 7.0% on the surface.
　This is because, as shown in FIG. 2, the Ni-concentrated portion 3 is formed on the surface of the steel sheet, so that a potential difference is generated between the locally concentrated Ni on the surface and the base iron 1. It is considered that this is because Ni serves as a precipitation nucleus of the zirconium-based chemical crystal, and thus the formation of the zirconium-based chemical crystal 4 is promoted. The base iron 1 refers to the steel plate portion excluding the scale 2.
[0060]
　Therefore, in the hot-rolled steel sheet according to the present embodiment, the average Ni concentration on the surface (the surface after pickling and before the chemical conversion treatment) is 7.0% or more. When the average Ni concentration on the surface is 7.0% or more, even if oxides such as Si and Al remain on the surface, it is sufficient to form a precipitation nucleus of zirconium-based chemical crystals. In order to make the average Ni concentration on the surface 7.0% or more, in the heating process of hot rolling, Fe is selectively oxidized to some extent on the surface of the steel sheet, so that the ground iron side of the interface between the scale and the base iron In addition, it is necessary to concentrate Ni, which is less likely to be oxidized than Fe.
[0061]
　The average Ni concentration on the surface of the steel sheet is measured using a JXA-8530F field emission electron probe microanalyzer (FE-EPMA). The measurement conditions are acceleration voltage: 15 kV, irradiation current: 6 × 10-8 A, irradiation time: 30 ms, beam diameter: 1 μm. The measurement is performed on a measurement area of ​​900 μm 2 or more from a direction perpendicular to the surface of the steel sheet, and the Ni concentration in the measurement range is averaged (the Ni concentration at all measurement points is averaged).
　FIG. 1 shows an example of the surface EPMA measurement result.
　Ni is mainly concentrated on the ground iron side of the interface between the scale and the ground iron. In addition, pickling is usually performed before the chemical conversion treatment. Therefore, when the scale is formed on the surface of the target steel sheet, the measurement is performed after pickling in the same manner as in the case of being subjected to chemical conversion treatment.
[0062]
　The coating film adhesion of the above-mentioned pickling plate is evaluated according to the following procedure. First, the produced steel sheet is pickled and then subjected to a chemical conversion treatment to attach a zirconium-based chemical conversion treatment film. Further, an electrodeposition coating having a thickness of 25 μm is applied to the upper surface thereof, and a coating baking process is performed at 170 ° C. for 20 minutes. Then, under the salt spray conditions shown in JIS Z 2371: 2015, 5% salt spray at a temperature of 35 ° C. was continuously performed for 700 hours, and then a tape having a width of 24 mm (Nichiban 405A-24 JIS Z 1522) was placed on the notch. : 2009) is pasted in parallel with the cut portion with a length of 130 mm, and the maximum peeling width of the coating film when this is peeled off is measured.
[0063]
Internal oxidation layer in the hot-rolled steel sheet is present (the base steel area oxide is generated internally), the average depth from the surface of the hot-rolled steel sheet of the inner oxide layer is more than 5.0 .mu.m, less 20.0 .mu.m
　Ni in the surface layer Even if there is a thickened portion, if the coating ratio of oxides such as Si and Al is too large on the surface of the hot-rolled steel sheet, “scale” on which the zirconium-based chemical conversion treatment film does not adhere tends to occur. In order to suppress this, it is desirable that the oxidation of Si, Al, etc. is not an external oxidation that forms an oxide outside the ground iron, but an internal oxidation that forms an oxide inside.
　The present inventors observed the cross section of only a sample having an average Ni concentration of 7.0% or more on the surface with an optical microscope, and observed the coating peeling width and the average depth of the internal oxide layer from the steel sheet surface (internal oxide layer). The relationship between the positions of the lower ends of the above) was investigated. As a result, all the samples having an average depth of the internal oxide layer of 5.0 μm or more had a coating peeling width of 3.5 mm or less, whereas the average depth of the internal oxide layer was less than 5.0 μm. In all the samples, the paint peeling width was more than 3.5 mm and 4.0 mm or less.
　Therefore, in order to obtain better coating film adhesion, the average depth of the internal oxide layer from the surface of the hot-rolled steel sheet is preferably 5.0 μm or more and 20.0 μm or less.
　If the average depth of the internal oxide layers such as Si and Al is less than 5.0 μm, the effect of suppressing “scale” to which the zirconium-based chemical conversion treatment film does not adhere is small. On the other hand, when the average depth exceeds 20.0 μm, not only the effect of suppressing “skeleton” on which the zirconium-based chemical conversion coating does not adhere is saturated, but also the hardness of the surface layer decreases due to the formation of a decarburized layer that occurs at the same time as internal oxidation. There is a concern that fatigue durability will deteriorate.
[0064]
　The average depth of the internal oxide layer is a mirror surface after embedding in a resin sample by cutting out a surface parallel to the rolling direction and the plate thickness direction as an embedding sample at the position of 1/4 or 3/4 in the plate width direction of the pickling plate. After polishing, 12 or more visual fields are observed with an optical microscope in a visual field of 195 μm × 240 μm (corresponding to a magnification of 400 times) without etching. When a straight line is drawn in the plate thickness direction, the position where it intersects the surface of the steel plate is taken as the surface, and the depth of the internal oxide layer (position of the lower end) of each field of view with respect to that surface is measured and averaged at 5 points per field of view. , The average value is calculated by excluding the maximum value and the minimum value from the average value of each visual field, and this is taken as the average depth of the internal oxide layer.
[0065]
　Standard deviation of arithmetic mean roughness Ra of the surface of hot-rolled steel sheet after pickling under predetermined conditions: 10.0 μm or more and 50.0 μm or less In the
　zirconium-based chemical conversion treatment film, the film thickness is several μm. The film thickness is very thin compared to the zinc acid acid film, and it is about several tens of nm. This difference in film thickness is due to the fact that the zirconium-based chemical conversion-treated crystals are extremely fine. When the chemical conversion-treated crystal is fine, the surface of the chemical conversion-treated crystal is very smooth, and it is difficult to obtain a strong adhesion to the coating film due to the anchor effect as seen in the zinc phosphate-treated film.
　However, as a result of the studies by the present inventors, it has been found that the adhesion between the chemical conversion coating film and the coating film can be improved by forming irregularities on the surface of the steel sheet.
　Based on these findings, the present inventors pickled a sample having an average Ni concentration of 7.0% or more and an internal oxide layer having an average depth of 5.0 μm or more before performing a zirconium-based chemical conversion treatment. The relationship between the standard deviation of the arithmetic mean roughness Ra of the surface of the plate and the adhesion of the coating film was investigated. As a result, all the samples in which the standard deviation of the arithmetic mean roughness Ra of the surface of the pickling plate was 10.0 μm or more and 50.0 μm or less had a coating peeling width of 3.0 mm or less. The standard deviation of the arithmetic mean roughness Ra of the surface of the pickling plate was less than 10.0 μm or more than 50.0 μm, and the paint peeling width was more than 3.0 mm and less than 3.5 mm.
　Therefore, it is preferable that the standard deviation of the arithmetic mean roughness Ra of the surface of the steel sheet after pickling is 10.0 μm or more and 50.0 μm or less.
　If the standard deviation of the arithmetic mean roughness Ra of the steel sheet surface is less than 10.0 μm, a sufficient anchor effect cannot be obtained. On the other hand, if the standard deviation of the arithmetic mean roughness Ra of the steel sheet surface after pickling exceeds 50.0 μm, not only the anchor effect is saturated, but also zirconium on the uneven valleys and mountain sides of the steel sheet surface after pickling. System chemical conversion treatment Crystals are less likely to adhere and "scale" is more likely to occur.
　The surface roughness of the steel sheet varies greatly depending on the pickling conditions, but in the hot-rolled steel sheet according to the present embodiment, it takes less than 30 to 60 seconds using a 1 to 10% by mass hydrochloric acid solution at a temperature of 20 to 95 ° C. It is preferable that the standard deviation of the arithmetic average roughness Ra of the surface of the hot-rolled steel sheet after pickling under the condition of pickling time is 10.0 μm or more and 50.0 μm or less.
[0066]
　For the standard deviation of the arithmetic mean roughness Ra, the value obtained by measuring the surface roughness of the pickling plate by the measuring method described in JIS B 0601: 2013 is adopted. After measuring the arithmetic mean roughness Ra on the front and back of 12 or more samples, the standard deviation of the arithmetic average roughness Ra of each sample is calculated, and the average value is the standard deviation excluding the maximum and minimum values. Is calculated.
[0067]
　The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 0.8 to 8.0 mm. If the thickness of the steel sheet is less than 0.8 mm, it may be difficult to secure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the steel plate according to the present invention may be 0.8 mm or more. It is more preferably 1.2 mm or more, still more preferably 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to miniaturize the metal structure, and it may be difficult to secure the steel structure described above. Therefore, the plate thickness may be 8.0 mm or less. More preferably, it is 6.0 mm or less.
[0068]
　The hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanization and electroZn—Ni alloy plating. Examples of the hot-dip plating layer include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, and the like. NS. The amount of plating adhered is not particularly limited and may be the same as the conventional one. Further, it is also possible to further enhance the corrosion resistance by subjecting an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
[0069]
[Manufacturing Method] The
　hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure can be manufactured by the following manufacturing method.
[0070]
　In order to obtain the hot-rolled steel sheet according to the present embodiment, after heating and hot rolling under predetermined conditions, acceleration cooling is performed to a predetermined temperature range, and after winding, the outermost periphery of the coil and the inside of the coil are cooled. It is important to control the history. It is also important to control the air ratio in the heating furnace during slab heating before hot rolling.
[0071]
　In the method for manufacturing a hot-rolled steel sheet according to the present embodiment, the following steps (I) to (VI) are sequentially performed. The temperature of the slab and the temperature of the steel plate in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.
(I) Heat the slab above 1150 ° C.
(II) Hot rolling so that the cumulative reduction rate is 90% or more in total in the temperature range of 850 to 1100 ° C. and the finishing temperature is T2 (° C.) or higher represented by the following formula (2). To do.
(III) Cooling is started within 1.5 seconds after the completion of hot rolling, and accelerated cooling is performed at an average cooling rate of 50 ° C./sec or more to a temperature T3 (° C.) or less represented by the following formula (3).
(IV) Cool from the cooling stop temperature of accelerated cooling to the take-up temperature at an average cooling rate of 10 ° C./sec or more.
(V) Winding is performed at (T4-100) ° C. to (T4 + 50) ° C. with respect to the temperature T4 (° C.) represented by the following formula (4).
[0072]
T2 (° C.) = 868-396 x [C] -68.1 x [Mn] + 24.6 x [Si] -36.1 x [Ni] -24.8 x [Cr] -20.7 x [Cu] ] + 250 x [Al] ... (2)
T3 (° C.) = 770-270 x [C] -90 x [Mn] -37 x [Ni] -70 x [Cr] -83 x [Mo] ... (3)
T4 (° C.) = 591-474 x [C] -33 x [Mn] -17 x [Ni] -17 x [Cr] -21 x [Mo] ... (4)
　However, each formula [Element symbol] in the inside indicates the content (mass%) of each element in the steel piece.
　The content of each element in the steel piece is obtained by using spark discharge emission spectroscopy (cantback, QV) on a sample taken from molten steel.
[0073]
[Heating step] As the
　slab (steel piece) to be subjected to hot rolling, a slab obtained by continuous casting, a slab obtained by casting / slab, etc. can be used, and if necessary, they are hot-worked or cooled. An interprocessed product can be used.
　The temperature of the slab used for hot rolling (slab heating temperature) is 1150 from the viewpoint of Ni concentration on the slab surface, increase in rolling load during hot rolling, and material deterioration due to insufficient cumulative rolling reduction inside the slab. The temperature should be above ℃. From the viewpoint of suppressing scale loss, the slab heating temperature is preferably 1350 ° C. or lower. If the slab to be subjected to hot rolling is a slab obtained by continuous casting or a slab obtained by bulk rolling and is in a high temperature state (1150 ° C. or higher), it is directly subjected to hot rolling without heating. May be good.
[0074]
　However, in order to obtain excellent coating film adhesion, it is important to control the air ratio of each zone of the heating furnace in slab heating as follows. In order to control the air ratio in each zone, it is preferable that the burner equipment of the heating furnace is a heat storage type burner. This will be described later because the heat storage type burner has a higher soaking property of the furnace temperature and a higher controllability of each zone than the conventional burner, and in particular, the air ratio in each zone can be strictly controlled. This is because the heating furnace can be controlled.
[0075]
　The preferred air ratio of each zone will be described.
By
　setting the air ratio in the preheating zone to 1.1 or more, Ni is concentrated on the surface of the hot-rolled steel sheet after pickling, and the average Ni The concentration can be 7.0% or more.
　The scale growth behavior of the slab surface in the heating furnace is evaluated by the formation scale thickness, and the linear law, which is the rate-determining rate of oxygen supply from the atmosphere on the slab surface, and the diffusion rate-determining rate of iron ions in the scale, depending on the air ratio (oxygen partial pressure). It is classified as a parabolic law. In order to promote the growth of the scale of the slab to some extent and form a sufficient Ni-concentrated layer on the surface layer in the limited material furnace time in the heating furnace, the growth of the scale thickness needs to follow the parabolic law.
　If the air ratio in the preheating zone is less than 1.1, the scale growth does not become a parabolic rule, and a sufficient Ni-concentrated layer is formed on the surface layer of the slab in the limited material furnace time in the heating furnace. Can't. In this case, the average Ni concentration on the surface of the hot-rolled steel sheet after pickling does not become 7.0% or more, and good coating film adhesion cannot be obtained.
[0076]
　On the other hand, if the air ratio in the preheating zone exceeds 1.9, not only the scale-off amount increases and the yield deteriorates, but also the heat loss due to the increase in exhaust gas increases, the thermal efficiency deteriorates, and the production cost increases. ..
　The amount of scale produced in the heating furnace is dominated by the atmosphere of the preheating zone immediately after insertion of the heating furnace, and even if the atmosphere of the subsequent zone changes, the scale thickness is hardly affected. Therefore, it is very important to control the scale growth behavior in the preheating zone.
[0077]
In order to
　form the internal oxide layer, it is necessary to control the air ratio in the heating zone in the heating process, and the air ratio in the heating zone should be 0.9 or more. By setting the value to 1.3 or less, the average depth of the internal oxide layer can be set to 5.0 to 20.0 μm.
　If the air ratio in the heating zone is less than 0.9, the average depth of the internal oxide layer cannot be obtained more than 5.0 μm. On the other hand, when the air ratio in the heating zone exceeds 1.3, not only the average depth of the internal oxide layer exceeds 20.0 μm, but also the hardness of the surface layer decreases due to the formation of the decarburized layer, resulting in fatigue durability. Is concerned about deterioration.
[0078]
In
　order to control the unevenness of the steel sheet surface after pickling , the air ratio in the heat equalizing zone, which is the zone immediately before extraction in the heating process, is controlled. Is effective. In the preheating zone, Ni, which is less likely to be oxidized than Fe, is concentrated on the ground iron side at the interface between the scale and the base iron. The Ni-concentrated layer having the Ni-concentrated portion suppresses oxidation in the surface layer, but suppresses external oxidation in the subsequent heating zone and promotes internal oxidation. After that, by controlling the air ratio in the soaking zone, for example, as shown in FIG. 3, the scale 2 erodes into the grain boundaries 5 and the like where diffusion is easy, and the ground caused by the difference in the concentration of Ni. Due to the difference in Ni concentration on the surface of iron 1, the way of oxidation of the interface between scale 2 and base iron 1 becomes non-uniform, so that the unevenness of the interface between scale 2 and base iron 1 becomes large. Further, although not shown in FIG. 3, unevenness is also generated by suppressing the erosion of the grain boundaries by the scale 2 by the Ni-concentrated portion 3 around the internal oxide 6. When this steel sheet is pickled, the scale 2 is removed, and the surface of the hot-rolled steel sheet has a predetermined roughness.
　By setting the air ratio in the soaking zone to 0.9 or more and 1.9 or less, after hot rolling, for example, using a hydrochloric acid solution of 1 to 10% by mass at a temperature of 20 to 95 ° C. for 30 to 60 seconds. The standard deviation of the arithmetic average roughness Ra of the surface of the hot-rolled steel sheet after pickling under the condition of less than the pickling time can be 10.0 μm or more and 50.0 μm or less.
　If the air ratio in the soaking zone is less than 0.9, the oxygen potential is not reached to selectively generate oxide nuclei at the grain boundaries where diffusion is easy. Therefore, the standard deviation of the arithmetic mean roughness Ra of the surface of the steel sheet after pickling does not exceed 10.0 μm. On the other hand, when the air ratio in the soaking zone exceeds 1.9, the depth of the selectively oxidized grain boundaries in the plate thickness direction becomes too deep, which is the standard of the arithmetic mean roughness Ra of the steel sheet surface after pickling. The deviation exceeds 50.0 μm.
[0079]
Air ratio in
　preheating zone> Air ratio in heating zone Control of the air ratio in the preheating zone is important for controlling the Ni concentration on the surface of the hot-rolled steel sheet after pickling. On the other hand, control of the air ratio in the heating zone is important for controlling the degree of formation of the internal oxide layer. Therefore, it is necessary to promote the growth of the scale of the slab to some extent in the preheating zone in the limited furnace time to form a sufficient Ni-concentrated layer on the surface layer. For that purpose, a relatively high air ratio is required in which the growth of scale thickness follows a parabolic law. On the other hand, in order to control the average depth of the internal oxide layer within a preferable range, it is necessary to suppress the air ratio to be relatively low in the heating zone and suppress the rapid growth of the internal oxide layer. Further, if the air ratio is high in the heating zone, there is a concern that a decarburized layer is formed and grown, the hardness of the surface layer is lowered, and the fatigue durability is deteriorated. Therefore, it is preferable that the air ratio in the preheating zone is higher than the air ratio in the heating zone.
[0080]
[Hot rolling process] For
　hot rolling, it is preferable to use a reverse mill or a tandem mill for multi-pass rolling. In particular, from the viewpoint of industrial productivity, it is more preferable that at least the final several steps are hot-rolled using a tandem mill.
[0081]
Tempering rate for hot rolling: Cumulative rolling rate of 90% or more (reduction of plate thickness) in
　the temperature range of 850 to 1100 ° C. By rolling, the recrystallized austenite grains are mainly refined, and the accumulation of strain energy in the unrecrystallized austenite grains is promoted, and the average crystal grains of bainite and tempered martensite, which are the main phases, are promoted. The diameter becomes finer. Therefore, hot rolling is performed in a temperature range of 850 to 1100 ° C. so as to have a cumulative reduction rate of 90% or more (a plate thickness reduction due to rolling is 90% or more). The cumulative rolling reduction in the temperature range of 850 to 1100 ° C. is a percentage of the difference between the inlet plate thickness before the first pass in rolling in this temperature range and the outlet plate thickness after the final pass in rolling in this temperature range.
[0082]
Hot rolling completion temperature (finishing temperature): T2 (° C.) or higher The
　hot rolling completion temperature is T2 (° C.) or higher. By setting the completion temperature of hot rolling to T2 (° C.) or higher, excessive increase of ferrite nucleation sites in austenite can be suppressed, and ferrite in the final structure (metal structure of hot-rolled steel sheet after production) can be suppressed. The area fraction of the above can be suppressed to less than 5.0%.
[0083]
[Primary cooling step]
Accelerated cooling after completion of hot rolling: Cooling was started within 1.5 seconds
　, and the particles were refined by cooling hot rolling to T3 (° C.) at an average cooling rate of 50 ° C./sec or higher . Accelerated cooling is started within 1.5 seconds after the completion of hot rolling in order to suppress the growth of austenite crystal grains.
　Accelerated cooling (primary cooling) is started within 1.5 seconds after the completion of hot rolling, and cooling is performed at an average cooling rate of 50 ° C./sec or higher to T3 (° C.) or lower, thereby suppressing the formation of ferrite and pearlite. However, the area fraction of bainite and tempered martensite can be increased. As a result, the uniformity in the metal structure is improved, and the strength and stretch flangeability of the steel sheet are improved. The average cooling rate here is the temperature drop width of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel plate is taken out from the cooling equipment) at the start of accelerated cooling. It is the value obtained by dividing from the time required until the end of accelerated cooling. In accelerated cooling after the completion of hot rolling, the time to start cooling is more than 1.5 seconds, the average cooling rate is less than 50 ° C / sec, and the cooling stop temperature is more than T3 (° C). Then, the ferrite transformation and / or the pearlite transformation inside the steel plate becomes remarkable, and it becomes difficult to obtain a metal structure mainly composed of bainite and tempered martensite. Therefore, the accelerated cooling after the completion of hot rolling starts cooling within 1.5 seconds after the completion of hot rolling, and cools to T3 (° C.) or less at an average cooling rate of 50 ° C./sec or more. The upper limit of the cooling rate is not particularly specified, but if the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, the average cooling rate is preferably 300 ° C./sec or less. Further, the cooling shutdown temperature of accelerated cooling is preferably (T4-100) ° C. or higher.
[0084]
[Secondary cooling process]
Average cooling rate from the cooling stop temperature of the primary cooling to the take-up temperature: 10 ° C / sec or more From
　the cooling stop temperature of accelerated cooling in order to keep the area fraction of pearlite to less than 5.0%. The average cooling rate to the winding temperature is 10 ° C./sec or more (secondary cooling). As a result, the surface integral of bainite and tempered martensite is increased, and the balance between the strength and stretch flangeability of the steel sheet can be improved. The average cooling rate here means a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of accelerated cooling to the winding temperature by the time required from the stop of accelerated cooling to winding. If the average cooling rate is less than 10 ° C./sec, the surface integral of pearlite increases, the strength decreases, and the ductility decreases. Therefore, the average cooling rate from the cooling stop temperature of the accelerated cooling to the winding temperature is set to 10 ° C./sec or more. Although the upper limit is not particularly specified, the average cooling rate is preferably 300 ° C./sec or less in consideration of the plate warpage due to thermal strain.
[0085]
[
Winling step] Winding temperature: (T4-100) ° C. to (T4 + 50) ° C. The
　winding temperature is (T4-100) ° C. to (T4 + 50) ° C. If the take-up temperature is less than (T4-100) ° C, carbon emission from bainite and tempered martensite into austenite does not proceed and austenite is not stabilized, so residual austenite with an area fraction of 3.0% or more Is difficult to obtain, and the ductility of the steel sheet is reduced. In addition, the low temperature toughness of the steel sheet also deteriorates due to the decrease in the number density of iron-based carbides. Further, when the winding temperature exceeds (T4 + 50) ° C., carbon discharged from bainite and tempered martensite is excessively precipitated in steel as iron-based carbides, so that carbon is sufficiently concentrated in austenite. It is also disadvantageous to increase the C concentration in retained austenite to 0.50% by mass or more. Therefore, the winding temperature is (T4-100) ° C. to (T4 + 50) ° C.
[0086]
　After winding, it may be cooled to room temperature by a usual method.
[0087]
[Pickling step]
[Skin pass step]
　Skin pass rolling with a reduction ratio of 0.1% or more and 2.0% or less may be performed for the purpose of improving ductility by straightening the shape of the steel sheet and introducing movable dislocations. Further, the obtained hot-rolled steel sheet may be pickled, if necessary, for the purpose of removing scale adhering to the surface of the obtained hot-rolled steel sheet. In the case of pickling, it is preferable to pickle with a 1 to 10 wt% hydrochloric acid solution at a temperature of 20 to 95 ° C. under a pickling time of less than 30 to 60 seconds.
　Further, after pickling, the obtained hot-rolled steel sheet may be subjected to skin pass or cold rolling with a reduction ratio of 10% or less in-line or offline.
[0088]
　According to the above manufacturing method, the hot-rolled steel sheet according to the present embodiment can be manufactured.
Example
[0089]
　Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
[0090]
　Steel Nos. Of Table 1A and Table 1B. Steels having the component compositions shown in A to W were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. The obtained slab was heated to the temperatures shown in Tables 2A and 2B using a heat storage type burner. At that time, the air ratios in the preheating zone (preheating zone), the heating zone (heating zone), and the soaking zone (equal tropical) were controlled as shown in Tables 2A and 2B.
[0091]
　The heated slab was hot-rolled at the cumulative rolling ratio and finishing temperature as shown in Tables 2A and 2B. After hot rolling, cooling was performed at the timing and cooling conditions as shown in Tables 2A and 2B, and after cooling, winding was performed.
　No. 2 and No. No. 8 was hot-dip galvanized.
[0092]
　The obtained production No. The metallographic structure of the hot-rolled steel sheets 1 to 38 was observed, and the area fraction and average crystal grain size of each phase were determined.
[0093]
　The surface integral of each phase was determined by the following method.
　A cross section of the steel sheet parallel to the rolling direction was corroded using a Nital reagent and a reagent disclosed in JP-A-59-219473. Regarding the corrosion of the cross section, specifically, a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol was used as liquid A, and 1 to 25 g of sodium thiosulfate and 1 to 5 g of nitric acid were dissolved in 100 ml of water. The solution is solution B, and solution A and solution B are mixed at a ratio of 1: 1 to form a mixed solution, and nitric acid at a ratio of 1.5 to 4% is further added and mixed with respect to the total amount of this mixed solution. The prepared liquid was used as a pretreatment liquid. Further, a liquid obtained by adding and mixing the above-mentioned pretreatment liquid in a ratio of 10% with respect to the total amount of the 2% nital liquid to the 2% nital liquid is used as a post-treatment liquid. A cross section parallel to the rolling direction of the steel sheet is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, then immersed in the posttreatment liquid for 3 to 20 seconds, washed with water and dried. , The above cross section was corroded.
　Next, by observing at least three 40 μm × 30 μm regions at a magnification of 1000 to 100,000 times using a scanning electron microscope or a transmission electron microscope at a depth of 1/4 of the plate thickness from the surface of the steel plate. Bainite, tempered martensite, ferrite, pearlite and martensite were identified in the metal structure from the shape and carbide state, the existence position of each phase was confirmed, and the area fraction was measured.
　In addition, the surface integral of retained austenite was measured using X-ray diffraction. Specifically, first, in a cross section parallel to the rolling direction of the steel sheet at a depth of 1/4 of the thickness of the steel sheet, α (110), α (200), α ( The integrated intensity of a total of 6 peaks of 211), γ (111), γ (200), and γ (220) was obtained, and the area fraction of retained austenite was obtained by calculating using the intensity averaging method.
[0094]
　The average crystal grain size was determined by the following method.
　Defined using the EBSP-OIM (Electron Backscatter Diffraction Pattern) method The crystal grains with an orientation difference of 15 ° or more between adjacent crystal grains are defined as one crystal grain, and the crystal grains are visualized by the mapped image. Then, the average crystal grain size was determined. When measuring the average crystal grain size of the metal structure at a depth of 1/4 of the plate thickness from the surface of the steel plate in the cross section parallel to the rolling direction of the steel plate, 10 fields of 40 μm × 30 μm were measured at a magnification of 1200 times. The average grain size of crystals having an orientation difference of 15 ° or more between adjacent crystal grains (effective grain size) was defined as the average crystal grain size.
[0095]
　Further, the obtained hot-rolled steel sheet was pickled with a 1 to 10 mass% hydrochloric acid solution at a temperature of 20 to 95 ° C. under a pickling time of less than 30 to 60 seconds, and then pickled on the surface. The Ni concentration, the number density of iron-based carbides, the average depth of the internal oxide layer, and the arithmetic mean roughness of the surface were determined.
[0096]
　The Ni concentration on the surface was determined by the following method.
　Using a JXA-8530F field emission electron probe microanalyzer (FE-EPMA), the target hot-rolled steel sheet was analyzed for Ni concentration over a measurement area of ​​900 μm 2 or more from the direction perpendicular to the surface of the steel sheet . The Ni concentration in the measurement range was averaged. At this time, the measurement conditions were an acceleration voltage: 15 kV, an irradiation current: 6 × 10-8 A, an irradiation time: 30 ms, and a beam diameter of 1 μm.
[0097]
　The number density of iron-based carbides was determined by the following method.
　A sample is taken with the cross section parallel to the rolling direction of the steel plate as the observation surface, the observation surface is polished, and night tar etching is performed. A 10-field observation was performed in a range of 3/8 using a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope) at a magnification of 200,000 times, and the number density of iron-based carbides was measured.
[0098]
　The average depth of the internal oxide layer was determined by the following method.
　A surface parallel to the rolling direction and the plate thickness direction is cut out as an embedding sample at a position 1/4 or 3/4 of the plate width direction of the pickling plate, and after embedding in the resin sample, mirror polishing is performed and the pickling plate is optical without etching. Twelve fields of view were observed with a microscope in a field of view of 195 μm × 240 μm (corresponding to a magnification of 400 times). When a straight line is drawn in the plate thickness direction, the position where it intersects the surface of the steel plate is taken as the surface, and the depth of the internal oxide layer (position of the lower end) of each field of view with respect to that surface is measured and averaged at 5 points per field of view. , The average value was calculated by excluding the maximum value and the minimum value from the average value of each visual field, and this was taken as the average depth of the internal oxide layer.
[0099]
　The standard deviation of the arithmetic mean roughness of the surface was calculated by the following method.
　After measuring the surface roughness of the pickling plate by the measurement method described in JIS B 0601: 2013, the arithmetic average roughness Ra of the front and back sides of 12 or more samples is measured, and then the standard deviation of the arithmetic average roughness Ra of each sample is calculated. Then, the average value was calculated from the standard deviation excluding the maximum value and the minimum value.
[0100]
　In addition, the obtained production No. Tensile strength, toughness (vTrs), ductility, and stretch flangeability were determined as mechanical properties of the steel sheets 1 to 38.
[0101]
　Tensile strength and ductility (total elongation) were determined by collecting a test piece of JIS No. 5 from a hot-rolled steel sheet and conducting a tensile test in accordance with JIS Z 2241: 2011. Tensile strength (TS) indicates the tensile strength of JIS Z 2241: 2011. The total elongation (t-EL) indicates the total elongation at break of JIS Z 2241: 2011.
　It was judged that preferable characteristics were obtained when the tensile strength was 980 MPa or more and the ductility was 12.0% or more.
[0102]
　The toughness was determined by the following method. The transition temperature was determined according to the Charpy impact test method for metallic materials described in JIS Z 2242: 2005.
　When vTrs was −50 ° C. or lower, it was judged that preferable characteristics were obtained.
[0103]
　For the stretch flangeability, the hole widening value was obtained by the hole widening test method described in JSS Z 2256: 2010, and this was used as an index of the stretch flangeability.
　When the hole expanding property was 45% or more, it was judged that preferable characteristics were obtained.
[0104]
　Further, the above-mentioned hot-rolled steel sheet after pickling was degreased, thoroughly washed with water, and immersed in a zirconium chemical conversion treatment bath. The chemical conversion treatment bath contained (NH 4 ) 2 ZrF 6 : 10 mM (mmol / l) and a metal salt of 0 to 3 mM, had a pH of 4 (NH 3 , HNO 3 ), and had a bath temperature of 45 ° C. The processing time was 120.
[0105]
　The chemical conversion treatability and coating film adhesion of the hot-rolled steel sheet after the chemical conversion treatment were evaluated.
[0106]
　The chemical conversion processability was evaluated by the following method. The surface of the steel plate after the chemical conversion treatment was observed with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope). Specifically, 10 fields of view were observed at a magnification of 10000 times, and the presence or absence of "scale" to which the chemical conversion-treated crystals were not attached was observed. The observation was performed at an acceleration voltage of 5 kV, a probe diameter of 30 mm, and tilt angles of 45 ° and 60 °. Tungsten coating (ESC-101, Elionix) was applied for 150 seconds to impart conductivity to the sample.
　When no scale was observed in all the visual fields, it was judged that the chemical conversion processability was excellent (OK in the table).
[0107]
　The coating film adhesion was evaluated by the following method.
　25 μm thick electrodeposition coating is applied to the upper surface of the hot-rolled steel sheet after chemical conversion treatment, and after coating baking treatment at 170 ° C for 20 minutes, the electrodeposition coating film is lengthened to reach the base metal with a sharp-pointed knife. A 130 mm cut is made, and under the salt spray conditions shown in JIS Z 2371, 5% salt spray at a temperature of 35 ° C. is continuously performed for 700 hours, and then a tape having a width of 24 mm (Nichiban 405A-24) is placed on the cut portion. JIS Z 1522) was pasted in parallel with the cut portion in a length of 130 mm, and the maximum peeling width of the coating film when this was peeled off was measured.
　When the maximum coating film peeling width was 4.0 mm or less, it was judged that the coating film adhesion was excellent.
[0108]
　The results are shown in Tables 3A, 3B and 3C.
　As can be seen from Table 3A, Table 3B, and Table 3C, Production No. In Nos. 1 to 4, 8 to 11, 20 to 32, even if the tensile strength is 980 MPa, chemical conversion treatment using a zirconium-based chemical conversion treatment liquid is performed while ensuring the mechanical properties required for steel sheets for automobiles. However, a chemical conversion treatment film having good chemical conversion treatment property and excellent coating film adhesion was obtained.
　On the other hand, the production No. in which the Ni concentration in the component, the metallographic structure, or the surface is not within the range of the present invention. In 5 to 7, 12 to 19, and 33 to 38, the mechanical properties were not sufficient, or the chemical conversion processability and / or the coating film adhesion was inferior. (For reference, in Table 3C, the values ​​outside the scope of the present invention and the characteristics that did not reach the target are also underlined.)
[0109]
[Table 1A]

[0110]
[Table 1B]

[0111]
[Table 2A]

[0112]
[Table 2B]

[0113]
[Table 3A]

[0114]
[Table 3B]

[0115]
[Table 3C]

Industrial applicability
[0116]
　According to the present invention, it is an ultra-high strength steel sheet having a tensile strength of 980 MPa or more and high press formability (ductility and elongation flangeability), and even when a zirconium-based chemical conversion treatment liquid is used, a zinc phosphate chemical conversion treatment liquid is used. It is possible to obtain a hot-rolled steel sheet having chemical conversion treatment property and coating adhesion which are equal to or higher than those in the case of using. Since the steel sheet according to the present invention is excellent in chemical conversion treatment property and coating film adhesion, it is excellent in corrosion resistance after coating. It also has excellent ductility and stretch flangeability. Therefore, the present invention is suitable for automobile parts that require high strength, moldability, and corrosion resistance after painting.
Code description
[0117]
　1 Ground iron (steel plate)
　2 Scale
　3 Ni Concentrated part
　4 Zirconium-based chemical crystals
　5 Grain boundaries
　6 Internal oxides

The scope of the claims
[Claim 1]
　The chemical composition represented by the average value of the entire plate thickness direction is
　　C: 0.100 to 0.250%,
　　Si: 0.05 to 3.00%,
　　Mn: 1.00 to 4.00 in mass%. %,
　　Al: 0.001 to 2.000%,
　　Ni: 0.02 to 2.00%,
　　Nb: 0 to 0.300%,
　　Ti: 0 to 0.300%,
　　Cu: 0 to 2.00% ,
　　Mo: 0 to 1.000%,
　　V: 0 to 0.500%,
　　Cr: 0 to 2.00%,
　　Mg: 0 to 0.0200%,
　　Ca: 0 to 0.0200%,
　　REM: 0 to 0.1000%,
　　B: 0 to 0.0100%,
　　Bi: 0 to 0.020%,
　　one or more of Zr, Co, Zn, and W: 0 to 1.000% in total,
　　Sn : 0 to 0.050%,
　　P: 0.100% or less,
　　S: 0.0300% or less,
　　O: 0.0100% or less,
　　N: 0.1000% or less,
When the
　　balance is composed of Fe and impurities, the following formula (1) is satisfied, and the
　thickness is t, the metallographic structure at the position of t / 4 from the surface is bainite or baked in an area fraction. Contains 77.0-97.0% of reconstituted martensite, 0-5.0% of ferrite, 0-5.0% of pearlite, 3.0% or more of retained austenite, and 0-10.0% of martensite. In the
　metallographic structure,
　　the average crystal grain size excluding the retained austenite is 7.0 μm or less, and
　　the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0 × 10 6 pieces / mm 2 or more. A hot-rolled steel sheet having a
　tensile strength of 980 MPa or more and an
　average Ni concentration of 7.0% or more on the surface
.
　0.05% ≤ Si + Al ≤ 3.00% ..... Formula (1)
　The element shown in the above formula (1) is the mass% of the element contained in the hot-rolled steel sheet.
[Claim 2]
　The hot-rolled steel sheet according to claim 1, 　wherein the chemical composition is, in mass%,
　Ni: 0.02 to 0.05%
.
[Claim 3]

Claim 1 or 2 is characterized in that an 　internal oxide layer is present in the hot-rolled steel sheet, and the average depth of the internal oxide layer is 5.0 μm or more and 20.0 μm or less from the surface of the hot-rolled steel sheet. The hot-rolled steel sheet described in.
[Claim 4]
　The hot-rolled steel sheet according
to any one of claims 1 to 3 , wherein the standard deviation of the arithmetic mean roughness Ra of the surface of the hot-rolled steel sheet is 10.0 μm or more and 50.0 μm or less .
[Claim 5]
Claims 1 to 4 　, wherein the chemical composition contains one or two kinds of
V: 0.005 to 0.500% and
Ti: 0.005 to 0.300% in mass%.
The hot-rolled steel sheet according to any one of the above items.
[Claim 6]
　The chemical composition is, in mass%,
Nb: 0.005 to 0.300%,
Cu: 0.01% to 2.00%,
Mo: 0.01% to 1.000%,
B: 0.0001 to The hot rolling according to any one of claims 1 to 5, which contains one or
more of 0.0100%, Cr: 0.01% or more, and 2.00% or less.
Steel plate.
[Claim 7]
　The chemical composition, by
mass%,
Mg: 0.0005 ~
0.0200%, Ca: 0.0005 ~ 0.0200%, REM: 0.0005 ~ 0.1000%,
1 or two or more of The hot-rolled steel sheet according to any one of claims 1 to 6, wherein the hot-rolled steel sheet contains.
[Claim 8]
　A heating step of heating a steel piece having the chemical composition according to claim 1 to 1150 ° C. or higher in a heating furnace equipped with a heat storage type burner having at least a preheating zone, a heating zone, and a soaking zone, and
　heating. The steel pieces are hot-rolled so that the finishing temperature is T2 ° C. or higher obtained by the following formula (2) and the cumulative reduction rate in the temperature range of 850 to 1100 ° C. is 90% or higher. After the hot-rolling step of obtaining a hot-rolled steel sheet and the hot-rolling step
　, cooling is started within 1.5 seconds, and the average cooling rate is 50 ° C./sec or more, which is represented by the following formula (3). When the primary cooling step of cooling the hot-rolled steel sheet to a temperature of T3 ° C. or lower and the
　temperature represented by the following formula (4) are T4 ° C., the cooling stop temperature of the primary cooling step is (T4-100) ° C. ~ and (T4 + 50) ℃ secondary cooling step to the coiling temperature is cooled at an average cooling rate of more than 10 ° C. / sec,
　and the winding step of winding in the winding temperature,
have,
　in the heating step, the A
method for producing a hot-rolled steel sheet, characterized in that the air ratio in the preheating zone is 1.1 to 1.9 .
T2 (° C.) = 868-396 x [C] -68.1 x [Mn] + 24.6 x [Si] -36.1 x [Ni] -24.8 x [Cr] -20.7 x [Cu] ] + 250 x [Al] ... (2)
T3 (° C.) = 770-270 x [C] -90 x [Mn] -37 x [Ni] -70 x [Cr] -83 x [Mo] ...・ (3)
T4 (° C.) = 591-474 x [C] -33 x [Mn] -17 x [Ni] -17 x [Cr] -21 x [Mo] ... (4)
　However, the [element ] in each formula Symbol] indicates the content of each element in the steel piece in mass%.
[Claim 9]
　The
method for producing a hot-rolled steel sheet according to claim 8, wherein in the heating step, the air ratio in the heating zone is 0.9 to 1.3 .
[Claim 10]
　The
method for producing a hot-rolled steel sheet according to claim 8 or 9, wherein in the heating step, the air ratio in the heat equalizing zone is 0.9 to 1.9 .
[Claim 11]
　The
method for producing a hot-rolled steel sheet according to claim 9 or 10 , wherein the air ratio in the preheating zone is larger than the air ratio in the heating zone .
[Claim 12]
　A pickling step of pickling the hot-rolled steel sheet after the winding step with a 1 to 10 mass% hydrochloric acid solution at a temperature of 20 to 95 ° C. under a pickling time of less than 30 to 60 seconds. The
method for producing a hot-rolled steel sheet according to any one of claims 8 to 11, wherein the hot-rolled steel sheet is provided.