Title of the invention: Hot-rolled steel sheet and its manufacturing method
Technical field
[0001]
The present invention relates to a hot-rolled steel sheet and its manufacturing method.
This application claims priority based on Japanese Patent Application No. 2020-018844 filed in Japan on February 06, 2020, the content of which is incorporated herein.
Background technology
[0002]
In recent years, in order to reduce carbon dioxide (CO2) emissions from automobiles, efforts have been made to reduce the weight of automobile bodies by using high-strength steel plates. Also, in order to ensure the safety of passengers, high-strength steel sheets are being used more often than mild steel sheets for automobile bodies. Furthermore, recently, due to further tightening of fuel consumption regulations and environmental regulations such as NO X, the number of plug-in hybrid vehicles and electric vehicles is expected to increase. These next-generation automobiles need to be equipped with large-capacity batteries, and further weight reduction of the vehicle body is required.
[0003]
In order to further reduce the weight of the car body, it is possible to replace steel sheets with lightweight materials such as aluminum alloys, resins, and CFRP, or to increase the strength of steel sheets. However, the use of ultra-high-strength steel sheets is realistic for mass-produced mass-produced cars, excluding luxury cars.
[0004]
For underbody parts (for example, lower arms) where hot-rolled steel sheets are mainly used, the application of high-strength steel sheets of 540 MPa class or higher (tensile strength of 540 MPa or higher) is progressing in order to reduce weight. On the other hand, even if the strength is high, if the plate thickness is thin, the rigidity may be insufficient. As a countermeasure against insufficient rigidity, changing the shape and structure of the parts has been considered, but in this case, the shape and structure of the parts become complicated. Therefore, high-strength steel sheets that are used to reduce the weight of automobile bodies are required to have high strength, as well as improved workability and fatigue properties.
[0005]
For example, Patent Document 1 discloses a method for producing a hot-rolled steel sheet having high strength and excellent surface properties, formability (ductility, burring properties), and notch fatigue properties. In Patent Document 1, in order to suppress the tiger stripe-shaped scale pattern that deteriorates the surface properties, the Si content is reduced, and polygonal ferrite that is precipitation-strengthened by Ti carbide and 1 to 10% of low-temperature transformation generation are used. High ductility and burring properties are achieved by using a composite structure consisting of materials.
[0006]
In addition, Patent Document 2 discloses a high-strength hot-rolled steel sheet with excellent ductility, fatigue properties, and corrosion resistance, and a method for manufacturing the same. In Patent Document 2, the Si content is reduced in order to suppress the tiger-stripe scale pattern that deteriorates the surface properties. Further, the fatigue characteristics are improved by controlling the size of the Ti carbides so that the mass of Ti carbides having a circle-equivalent grain size of 7 nm or more and 20 nm or less is 50% or more of the mass of all Ti carbides. Moreover, Patent Document 2 describes that this hot-rolled steel sheet has good chemical conversion treatability and corrosion resistance after painting.
prior art documents
patent literature
[0007]
Patent Document 1: International Publication No. 2014/051005
Patent Document 2: Japanese Patent Application Laid-Open No. 2016-204690
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0008]
Including the above-mentioned Patent Documents 1 and 2, when trying to obtain high strength and good workability and fatigue properties, the content of alloying elements is usually increased.
Even with such high-strength steel sheets with increased alloying elements, if chemical conversion treatment such as zinc phosphate treatment is performed under ideal operating conditions, problems with chemical conversion treatability do not occur in many cases. Industrially, however, in the chemical conversion treatment of automobile parts and the like, a plurality of parts are continuously chemically treated using the same chemical conversion treatment liquid. In this case, the chemical conversion treatment solution gradually deteriorates, and the chemical conversion treatment may not be performed under ideal operating conditions.
As a result of studies by the present inventors, it was found that in a high-strength steel sheet containing a relatively large amount of alloying elements (for example, a tensile strength of 490 MPa or more), when chemical conversion treatment is performed using a deteriorated chemical conversion treatment solution, chemical conversion treatability is reduced. It is not necessarily sufficient, and the surface of the steel plate after chemical conversion treatment is exposed to the base iron, which is a skeletal part, and when the paint is applied to the surface of the steel plate, the adhesion between the paint and the steel plate is said to be poor. Found a problem.
When the chemical conversion treatability is lowered by using a deteriorated chemical conversion treatment solution, among the operating conditions for chemical conversion treatment, for example, the control value of free acidity is strictly controlled, and a large amount of accelerators that improve the chemical conversion treatability are used. etc. must be taken, which causes an increase in manufacturing cost and a decrease in productivity. Therefore, even with high-strength steel sheets, even if the chemical conversion treatment solution deteriorates and the chemical conversion treatment conditions vary, if good chemical conversion treatability can be obtained, that is, good corrosion resistance after painting can be obtained under a wide range of chemical conversion treatment operating conditions. If so, it becomes unnecessary to strictly control the operating conditions of the chemical conversion treatment, and it is possible to prevent an increase in manufacturing cost and a decrease in productivity.
The present invention has been made in view of the above problems. An object of the present invention is to provide a hot-rolled steel sheet excellent in chemical conversion treatability and a method for producing the same.
Means to solve problems
[0009]
The present inventors investigated the reason why the chemical conversion treatability of high-strength steel sheets deteriorates depending on the conditions. As a result, oxides such as Si and Al, or concentrated layers such as Mn and Cu are formed on the surface or near the surface of the high-strength steel sheet even after pickling. It was thought that inhibiting the elution of Fe would reduce the chemical conversion treatability. As a result of further studies by the present inventors, it was found that by partially concentrating (locally concentrating) Ni in the surface layer of the steel sheet, the elution of Fe is promoted and the chemical conversion treatability is improved.
The present invention has been completed based on the above findings, and the gist thereof lies in the following hot-rolled steel sheets.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition in mass% of C: 0.01 to 0.30%, Si: 0.01 to 3.00%, and Mn: 0.20. ~3.00%, P: 0.030% or less, S: 0.030% or less, Al: 0.001-2.000%, N: 0.0100% or less, Ni: 0.02-0.50 %, Nb: 0-0.060%, V: 0-0.20%, Ti: 0-0.20%, Cu: 0-0.20%, Cr: 0-0.20%, Mo: 0 ~1.00%, B: 0-0.0020%, W: 0-0.50%, Mg: 0-0.010%, Ca: 0-0.0100%, REM: 0-0.0100% , O: 0 to 0.0100%, Zr: 0 to 0.500%, Co: 0 to 0.500%, Zn: 0 to 0.500%, and Sn: 0 to 0.500%, The balance is Fe and impurities, and the Ni content is 0.5% by mass or more among the measurement points when elemental analysis is performed on the surface area of ​​250 μm × 250 μm using EPMA at a measurement pitch of 1 μm. The percentage of certain measurement points is 10-70%.
(2) The hot-rolled steel sheet described in (1) above has the chemical composition of Nb: 0.003-0.060%, V: 0.01-0.20%, Ti: 0.01-0. 20%, Cu: 0.01-0.20%, Cr: 0.01-0.20%, Mo: 0.01-1.00%, B: 0.0005-0.0020%, W: 0 .01-0.50%, Mg: 0.001-0.010%, Ca: 0.0010-0.0100%, REM: 0.0010-0.0100%, and O: 0.0005-0. It may contain one or more selected from the group consisting of 0100%.
(3) In the hot-rolled steel sheet described in (2) above, the chemical composition may contain Si: 0.50 to 3.00%.
(4) The chemical composition of the hot-rolled steel sheet described in (2) above may contain Si: 0.01 to less than 0.50% and Al: 0.050 to 2.000%.
(5) The hot-rolled steel sheet according to (2) above, wherein the chemical composition contains Si: 0.01 to less than 0.50%, Al: 0.001 to less than 0.050%, and Si and Total with Al: May be less than 0.50 to 0.55%.
(6) The hot-rolled steel sheet according to any one of the above (3) to (5) has an O content of 0.5% by mass or more among the measurement points when the elemental analysis of the surface is performed. The percentage of measurement points may be 30% or less.
(7) The hot-rolled steel sheet according to any one of (1) to (6) above, wherein the chemical composition contains Cu: 0.01 to 0.20%, and Ni/Cu: 0.50 or more. may be
(8) The hot-rolled steel sheet according to any one of the above (1) to (7) has a Ni content of 0.5% by mass or more among measurement points when the elemental analysis of the surface is performed. An average interval between the measurement points may be 3 to 10 μm.
(9) The hot-rolled steel sheet according to any one of (1) to (8) above may have a rust preventive oil film on the surface.
(10) The hot-rolled steel sheet according to any one of (1) to (8) above may have a chemical conversion coating on the surface.
(11) A method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention includes a heating step of heating a steel slab having the chemical composition according to (1) or (2) in a heating furnace; a descaling step of descaling the steel slab; and a hot rolling step of hot rolling the steel slab after the descaling step to obtain a hot-rolled steel sheet. After the surface temperature reaches 1100 ° C. or higher, it is held in an atmosphere with an air ratio of 0.9 or higher for 60 minutes or longer, the extraction temperature is set to 1180 ° C. or higher, and in the descaling step, the surface temperature is 1170 ° C. or higher. The steel billet is descaled at least once with an injection pressure of 5 to 50 MPa, and the surface temperature of the steel billet is maintained at 1100° C. or higher for 20 to 240 seconds after the descaling is completed.
Effect of the invention
[0010]
According to the above aspect of the present invention, a hot-rolled steel sheet with excellent chemical conversion treatability and a method for producing the same can be obtained. With the hot-rolled steel sheet of the present invention, a good chemical conversion coating can be obtained even when the chemical conversion treatment conditions vary.
Brief description of the drawing
[0011]
FIG. 1 is a diagram for explaining the mechanism by which Ni locally concentrated in a surface layer promotes the formation of chemical crystals;
MODE FOR CARRYING OUT THE INVENTION
[0012]
A hot-rolled steel sheet according to one embodiment of the present invention (hot-rolled steel sheet according to this embodiment) will be described below.
The hot-rolled steel sheet according to the present embodiment has a predetermined chemical composition, and a surface area of ​​250 μm × 250 μm is subjected to elemental analysis using EPMA at a measurement pitch of 1 μm. The proportion of measurement points with a content of 0.5% by mass or more is 10 to 70%.
The hot-rolled steel sheet according to this embodiment may have a chemical conversion treatment film and/or an electrodeposition coating film on the surface. Moreover, the hot-rolled steel sheet according to the present embodiment may have a rust preventive oil film on the surface.
[0013]

The reasons for limiting the chemical composition are explained below. "%" regarding chemical composition is mass % unless otherwise specified. In addition, in the following numerical limitation range sandwiching "-", in principle, the values ​​at both ends are included in the range as the lower limit and the upper limit. On the other hand, numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range.
[0014]
C: 0.01-0.30%
C is added to the steel sheet by structural strengthening by forming low temperature transformation products, or by precipitation strengthening by forming precipitates with Ti, Nb and/or V when Ti, Nb and/or V are included. It is an element that contributes to increasing the strength of steel. If the C content is less than 0.01%, it is not possible to obtain a strength of preferably 300 MPa or more, more preferably 490 MPa or more, and even more preferably 540 MPa or more as the strength required for the steel sheet. Therefore, the C content is made 0.01% or more. The C content is preferably 0.03% or more, more preferably 0.05% or more.
On the other hand, when the C content exceeds 0.30%, the area ratio of the hard layer of low-temperature transformation products and cementite increases, and workability decreases. Therefore, the C content is made 0.30% or less. The C content is preferably 0.25% or less, more preferably 0.20% or less.
[0015]
Si: 0.01-3.00%
　Si is used as an element that improves strength and is an important element involved in the formation of ferrite. In addition, Si is an effective source for deoxidation. It is raw. Therefore, the Si content is set to 0.01% or more. When using structure control to generate ferrite, the Si content is preferably 0.50% or more, more preferably 0.80% or more.
On the other hand, when the Si content increases, the ferrite temperature range expands to the high temperature side. In addition, regarding high-temperature oxidation of steel, Si affects the growth rate and properties of scale. Si in the steel sheet forms Fe2SiO4 on the surface of the steel sheet during hot rolling. If the content is excessive, it concentrates on the surface of the steel sheet, and the concentrated layer cannot be completely removed even after pickling, which affects chemical conversion treatability. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, more preferably 2.00% or less. The Si content may be less than 0.50% if no texture control is used to produce ferrite.
[0016]
　Mn: 0.20 to 3.00%
　Mn is an element that contributes to increasing the strength of the steel sheet by strengthening ferrite. Moreover, when the Mn content increases, the austenite temperature range expands to the low temperature side, and the ferrite + austenite two-phase temperature range expands. Moreover, Mn is an element that has the effect of suppressing hot tearing due to S by bonding with S and fixing S as MnS. In order to obtain these effects, the Mn content is set to 0.20% or more. In order to obtain a strength of 300 MPa or more, which is preferable as the strength required for steel sheets, the Mn content is preferably 0.30% or more. In order to obtain a strength of 490 MPa or more, which is more preferable as the strength required for steel sheets, the Mn content is more preferably 0.90% or more. The Mn content is more preferably 1.20% or more in order to obtain a strength of 540 MPa or more, which is more preferable as the strength required for steel sheets.
On the other hand, when the Mn content exceeds 3.00%, manufacturing problems such as cracking in the slab during casting occur. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, more preferably 2.00% or less.
[0017]
　P: 0.030% or less
Although the P content is preferably as small as possible, if the P content exceeds 0.030%, the segregation of P into grains becomes significant, and local ductility deteriorates due to grain boundary embrittlement. Therefore, the P content is made 0.030% or less. The P content is preferably 0.020% or less, more preferably 0.015% or less.
The P content may be 0%, but if the P content is less than 0.005%, the cost will increase significantly. Therefore, the lower limit of the P content may be 0.005%.
[0018]
　S: 0.030% or less
Although the S content is preferably as small as possible, if the S content exceeds 0.030%, the weldability, the manufacturability during casting and hot rolling, and the hole expansibility are adversely affected. Therefore, the S content is made 0.030% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less.
The S content may be 0%, but if the S content is less than 0.002%, the cost increases significantly. Therefore, the lower limit of the S content may be 0.002%.
[0019]
Al: 0.001-2.000%
Al, like Si, is an element involved in deoxidation and formation of ferrite. Moreover, when the Al content increases, the ferrite temperature range expands to the high temperature side. In addition, Al is an element that suppresses the formation of coarse cementite and contributes to the improvement of hole expansibility. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.020% or more, more preferably 0.030% or more. Also, when using structure control to generate ferrite, it is preferable to set the Al content to 0.050% or more.
On the other hand, if the Al content exceeds 2.000%, the number of Al-based coarse inclusions increases, resulting in deterioration of workability and surface defects. Also, the nozzle of the tundish during casting tends to be clogged. Therefore, the Al content is set to 2.000% or less. The Al content is preferably 1.200% or less, more preferably 1.000% or less, still more preferably 0.400% or less. The Al content may be less than 0.050% if no microstructure control is used to produce ferrite.
[0020]
　N: 0.0100% or less
N is an element that reduces ductility if it remains in the steel as solute nitrogen. Also, N combines with Ti to form TiN, but if the N content is large, coarse TiN precipitates and the hole expansibility deteriorates. Therefore, the smaller the N content, the better. If the N content exceeds 0.0100%, the above adverse effects become pronounced. Therefore, the N content is made 0.0100% or less. The N content is preferably 0.0060% or less, more preferably 0.0040% or less. The N content may be 0%, but if the N content is less than 0.0010%, the cost increases significantly. Therefore, the lower limit of the N content may be 0.0010%.
[0021]
Ni: 0.02-0.50%
Ni is the most important element in the hot-rolled steel sheet according to this embodiment. When manufacturing hot-rolled steel sheets, a specific operation is performed mainly in the heating process of heating the billet that is the basis of the hot-rolled steel sheet in a heating furnace and the descaling process of descaling the heated billet. By setting the conditions, Ni is locally concentrated on the surface layer side of the steel sheet in the vicinity of the interface between the steel sheet surface and the scale. When the surface of the steel sheet is subjected to a chemical conversion treatment such as zinc phosphate treatment, a difference in ionization tendency occurs between the Ni-enriched area and the surrounding Ni-unenriched area. As a result, the Fe surrounding the locally concentrated Ni dissolves out onto the surface of the steel sheet and becomes the precipitation nuclei of the chemical conversion treatment film (chemical conversion film). Formed, it is possible to improve the adhesion between the paint and the steel plate.
If the Ni content is less than 0.02%, the above effects cannot be obtained (skeleton occurs and the chemical crystal size increases), so the Ni content is made 0.02% or more. For example, if the Ni content is less than 0.02%, local concentration of Ni does not occur, so the elution of iron into the chemical bath is not promoted, the chemical crystal size increases, and the paint adhesion deteriorates. . The Ni content is preferably 0.05% or more.
On the other hand, when the Ni content exceeds 0.50%, Ni covers the entire surface of the steel sheet (no longer local enrichment), and the above effect cannot be obtained. Also, the cost increases. Therefore, the Ni content is set to 0.50% or less. The Ni content is preferably 0.45% or less, more preferably 0.40% or less.
[0022]
The hot-rolled steel sheet according to the present embodiment basically contains the above elements with the balance being Fe and impurities, but may contain the following elements within the content range described later. The following elements are optional elements that do not necessarily need to be contained, and may not be contained.
[0023]
Cu: 0-0.20%
Cu is an element that contributes to increasing the strength of the steel sheet. Therefore, it may be contained. The Cu content is preferably 0.01% or more in order to contribute to an increase in strength. The Cu content is preferably 0.02% or more, more preferably 0.04% or more.
On the other hand, Cu has a low melting point and concentrates at the interface between the scale and the base iron through the austenite grain boundaries. If the Cu content is too high, a Cu-enriched layer is formed and the zinc phosphating property is deteriorated. If the Cu content exceeds 0.20%, the Cu-enriched layer covers the entire surface of the steel sheet, which significantly deteriorates the chemical conversion treatability. Therefore, the Cu content is set to 0.20% or less. The Cu content is preferably 0.15% or less, more preferably 0.10% or less. In addition, when Ni/Cu<0.50, a Cu-enriched layer tends to be uniformly formed on the entire surface of the steel sheet, and the chemical conversion treatability deteriorates, so Ni/Cu≧0.50. is preferred.
[0024]
Nb: 0-0.060%
　V: 0 to 0.20%
Ti: 0-0.20%
Cr: 0-0.20%
Mo: 0-1.00%
W: 0-0.50%
Nb, V, Ti, Cr, Mo, Nb, and W are elements that increase the strength of the steel sheet by precipitation strengthening and/or solid solution strengthening. Therefore, it may be contained. To obtain these effects, the Nb content is preferably 0.003% or more, more preferably 0.005% or more, still more preferably 0.010% or more, and still more preferably 0.015% or more. Also, the V content is preferably 0.01% or more. The Ti content is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.10% or more, and still more preferably 0.15% or more. The Cr content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. Mo content is preferably 0.01% or more, more preferably 0.02% or more. The W content is preferably 0.01% or more, more preferably 0.02% or more.
On the other hand, the Nb content is over 0.060%, the V content is over 0.20%, the Ti content is over 0.20%, the Cr content is over 0.20%, and the Mo content is 1.00% Even if the W content exceeds 0.50%, the above effect is saturated and the economy is lowered. Therefore, even if they are contained, the Nb content is 0.060% or less, the V content is 0.20% or less, the Ti content is 0.20% or less, the Cr content is 0.20% or less, and the Mo content is is 1.00% or less, and the W content is 0.50% or less. The Nb content is preferably 0.055% or less, more preferably 0.050% or less. The V content is preferably 0.15% or less, more preferably 0.08% or less. The Ti content is preferably 0.18% or less, more preferably 0.17% or less. The Cr content is preferably 0.18% or less, more preferably 0.15% or less. The Mo content is preferably 0.70% or less, more preferably 0.05% or less. The W content is preferably 0.40% or less, more preferably 0.03% or less.
[0025]
B: 0-0.0020%
B is an element that has the effect of improving the hardenability and increasing the fraction of the low-temperature transformation product phase. Therefore, when it is desired to exhibit the effect of improving the hardenability, the B content may be 0.0005% or more. The B content is preferably 0.0010% or more, preferably 0.0015% or more.
On the other hand, even if the B content exceeds 0.0020%, not only does the effect saturate, but there is an increased concern that cracks will occur in the slab during the cooling process after continuous casting. Therefore, even when it is contained, the B content is made 0.0020% or less.
[0026]
　Mg: 0 to 0.010%
Ca: 0-0.0100%
REM: 0-0.0100%
　Mg, Ca, and REM are elements that improve workability by controlling the form of non-metallic inclusions that act as starting points for fracture and cause deterioration in workability. Therefore, it may be contained. When obtaining the above effects, the Mg content is preferably 0.001% or more, the Ca content is preferably 0.0010% or more, and the REM content is preferably 0.0010% or more.
On the other hand, when the Mg content exceeds 0.010%, the Ca content exceeds 0.0100%, and the REM content exceeds 0.0100%, the above effects become saturated and economic efficiency decreases. Therefore, even when they are contained, the Mg content is 0.010% or less, the Ca content is 0.0100% or less, and the REM content is 0.0100% or less. The Mg content is preferably 0.005% or less, the Ca content is preferably 0.0070% or less, and the REM content is preferably 0.0070% or less.
[0027]
O: 0 to 0.0100%
　O is an element that disperses a large number of fine oxides when deoxidizing molten steel. Therefore, it may be contained. When obtaining the above effects, the O content is preferably 0.0005% or more. The O content is preferably 0.0010% or more, more preferably 0.0020% or more.
On the other hand, O is an element that, if the content is too high, forms coarse oxides that act as fracture starting points in steel, causing brittle fracture and hydrogen-induced cracking. Therefore, O content shall be 0.0100% or less. From the viewpoint of weldability, the O content is preferably 0.0030% or less.
[0028]
Zr: 0-0.500%
Co: 0-0.500%
Zn: 0 to 0.500%
Sn: 0-0.500%
Even if Zr, Co, Zn, and Sn are contained in an amount of 0.500% or less, the effect of the hot-rolled steel sheet according to this embodiment is not impaired. Therefore, one or more of Zr, Co, Zn and Sn may each be contained in an amount of 0.500% or less.
[0029]
The content of each element in the hot-rolled steel sheet according to the present embodiment (including the case where the surface has a chemical conversion treatment film or an antirust oil film) was obtained by ICP emission spectroscopic analysis using chips according to JIS G1201: 2014. , is the average content in the entire plate thickness. The C content and S content are obtained by a well-known high-frequency combustion method (combustion-infrared absorption method). The O content is determined using the well-known inert gas fusion-nondispersive infrared absorption method.
[0030]

The present inventors investigated the reason why the chemical conversion treatability of high-strength steel sheets is lowered. As a result, oxides such as Si and Al, or concentrated layers such as Mn and Cu are formed on the surface or surface layer of the high-strength steel sheet even after pickling, and these are the elution of Fe during chemical conversion treatment. It was thought that by inhibiting , the chemical conversion treatability would decrease especially in the state where the chemical conversion treatment conditions deteriorated due to variations during operation.
On the other hand, by partially concentrating Ni on the surface layer of the steel sheet (not all over), a potential difference occurs between Ni--Fe, and the elution of Fe around the Ni-concentrated layer is promoted. In other words, the remaining Ni and the elution of its surroundings serve as precipitation nuclei for the chemical conversion coating, forming a coating with a small chemical crystal size without generating scales, and improving the chemical conversion treatment properties. For example, as shown in FIG. 1, by forming a Ni-enriched layer 4 on the surface of the steel sheet, (in FIG. 1, an oxide such as Si or Al, or an enriched layer 3 such as Mn or Cu remains. A potential difference is generated between the locally concentrated Ni on the surface and the base iron 1 (regardless of the presence or absence of these), and chemical crystals 5 are precipitated from the portion where this potential difference is generated. This is probably because nuclei are crystallized and formation of chemically formed crystals 5 is promoted. The base steel 1 refers to the steel plate portion excluding the scale 2 .
Specifically, among the measurement points when elemental analysis is performed on a 250 μm × 250 μm region of the surface at a measurement pitch of 1 μm using EPMA, the measurement points where the Ni content is 0.5% by mass or more. When the ratio of is 10 to 70%, the chemical conversion treatability is improved.
If the percentage of measurement points where the Ni content is 0.5% by mass or more is less than 10%, the Fe elution promotion effect is not sufficient, and the chemical conversion treatability is not sufficiently improved.
In addition, if the ratio of measurement points with a Ni content of 0.5% by mass or more exceeds 70%, Ni will be present on the surface of the steel sheet in a nearly uniform state, and the above effects cannot be obtained sufficiently.
When the hot-rolled steel sheet according to the present embodiment has a chemical conversion treatment film (including the case where it is electrodeposition-coated and has an electrodeposition coating film), it is difficult to perform elemental analysis on the surface of the hot-rolled steel sheet. There is In this case, a rectangular area of ​​10 μm in the thickness direction and 500 μm in the width direction from the surface of the steel sheet in the cross section in the thickness direction is subjected to elemental analysis using EPMA at a measurement pitch of 1 μm. , If the percentage of measurement points with a Ni content of 0.5% by mass or more is 10 to 70%, an elemental analysis was performed on a 250 μm × 250 μm area of ​​the surface using EPMA at a measurement pitch of 1 μm. Of the measurement points in the case, it can be considered that the percentage of measurement points at which the Ni content is 0.5% by mass or more is 10 to 70%. The reason for this is that Ni has a three-dimensionally substantially isotropic distribution in a range of 10 μm in the thickness direction from the surface (surface layer portion).
[0031]
The measurement points where the Ni content is 0.5% by mass or more are preferably distributed in a mottled manner on the surface of the steel sheet.
Specifically, it is preferable that the average interval between regions having a Ni content of 0.5% by mass or more is 3 to 10 μm. If the average interval is less than 3 μm or more than 10 μm, the elution of Fe around the Ni-enriched portion is less likely to be promoted.
[0032]
The average distance between regions with a Ni content of 0.5% by mass or more is measured as follows. Ni content is 0.5% by mass or more among measurement points when elemental analysis is performed on a 250 μm × 250 μm region on the surface of the hot-rolled steel sheet according to the present embodiment using EPMA at a measurement pitch of 1 μm. The average value of the intervals between the adjacent measurement points is defined as the average interval between the Ni 0.5% by mass or more regions.
[0033]
Hot-rolled steel sheets are often pickled before chemical conversion treatment. ) using a hydrochloric acid solution for 30 to 60 seconds), Ni is locally concentrated as described above. Therefore, the chemical convertibility is excellent even after pickling.
In addition, the hot-rolled steel sheet according to the present embodiment may have a rust-preventive oil film formed on its surface in order to prevent oxidation and the like until chemical conversion treatment is performed after pickling.
[0034]
The measurement conditions for elemental analysis of a 250 μm × 250 μm region on the surface at a measurement pitch of 1 μm using EPMA and for obtaining the average distance between regions having a Ni content of 0.5% by mass or more are as follows. For example:
Using a device of tungsten electron gun type (model number: JXA-8800RL) manufactured by JEOL Ltd., acceleration voltage: 15 kV, irradiation current: 6 × 10 -8 A, irradiation time: 15 ms, beam diameter: 0.5 μm. .
In addition, the same conditions may be applied when elemental analysis is performed using EPMA at a measurement pitch of 1 μm on a cross section in the plate thickness direction.
[0035]
The effect of improving the chemical conversion treatability of the hot-rolled steel sheet according to the present embodiment by locally concentrating Ni is effective for any steel sheet.
However, in the case of a steel sheet containing a large amount of Si or Al in order to increase strength or improve formability, a large amount of Si or Al oxide is formed on the surface of the steel sheet. Chemical convertability is lowered. So, for example,
1) When the Si content is 0.50% or more,
2) Even if the Si content is less than 0.50%, if the Al content is 0.050% or more,
3) Even if the Si content is less than 0.50% and the Al content is less than 0.050%, if the total content of Si and Al is 0.50% or more,
In particular, the effect of improving the chemical conversion treatability is large.
[0036]
Even if the hot-rolled steel sheet according to the present embodiment is subjected to chemical conversion treatment and electrodeposition coating, the above-mentioned local concentration of Ni hardly changes. That is, the distribution of the area where the Ni content is 0.5% by mass or more near the boundary between the chemical conversion coating and the hot-rolled steel sheet in the chemically treated hot-rolled steel sheet (corresponding to the vicinity of the surface of the hot-rolled steel sheet to be the base sheet) is the same as the surface of the hot-rolled steel sheet as the base sheet. For this reason, the measurement results by the following method are the ratio of measurement points with a Ni content of 0.5% by mass or more on the surface of the hot-rolled steel sheet that is the original sheet before chemical conversion treatment (relative to the surface of the hot-rolled steel sheet (synonymous with the results of the measurements performed).
[0037]

In the hot-rolled steel sheet according to the present embodiment, it is preferable that the percentage of measurement points at which the oxygen content is 0.5% by mass or more is 30% or less among the measurement points when elemental analysis is performed.
When elemental analysis was performed, oxides of Si and Al were formed at the measurement points where the oxygen content was 0.5% by mass or more, and the ratio of these measurement points was 30% or less. , indicates that oxides such as Si and Al are less generated. Oxides impede the elution of Fe during chemical conversion treatment, thereby lowering the chemical conversion treatment properties. more improved.
[0038]
When performing elemental analysis, EPMA analysis is performed on an area of ​​250 μm × 250 μm at a measurement pitch of 1 μm, targeting elements with the number of atoms equal to or greater than the number of atoms of B (boron). Then, when the total mass of elements having an atomic number equal to or greater than that of B is taken as 100%, the percentage of measurement points where the Ni content is 0.5% by mass or more is calculated.
When performing EPMA analysis on a steel sheet, if a rust-preventing oil film is formed on the surface of the steel sheet, a solvent such as acetone or alcohol is used to remove the rust-preventing oil film so that the surface of the steel sheet can be measured. do. When scale is formed, pickling is performed under normal pickling conditions (for example, using a hydrochloric acid solution of 1 to 10 wt% (% by weight) at a temperature of 20 to 95°C for 30 to 60 seconds). then measure.
EPMA analysis, for example, using a tungsten electron gun type (model number: JXA-8800RL) equipment manufactured by JEOL Ltd., acceleration voltage: 15 kV, irradiation current: 6 × 10 -8 A, irradiation time: 15 ms, beam diameter: 0.5 kV. It is carried out under the condition of 5 μm.
[0039]
The structure (microstructure) of the hot-rolled steel sheet according to the present embodiment is not limited. Regardless of the phase of the structure, the local concentration of Ni improves the phosphatability.
In addition, the effect of improving chemical conversion treatability by local concentration of Ni is large in high-strength steel sheets containing many alloying elements. For example, in a hot-rolled steel sheet having a tensile strength of 300 MPa or more, the effect becomes clear, and in a hot-rolled steel sheet having a tensile strength of 490 MPa or more, the effect is large, and in a hot-rolled steel sheet having a tensile strength of 540 MPa or more , more effective.
The thickness of the hot-rolled steel sheet according to this embodiment is not limited, but is, for example, 1.2 to 10.0 mm.
[0040]
A method for manufacturing a hot-rolled steel sheet according to this embodiment will be described below.
The hot-rolled steel sheet according to this embodiment can be manufactured by a manufacturing method having the following steps.
(i) Heating step of heating steel billets in a heating furnace
(ii) a descaling step of descaling the heated billet;
(iii) a hot rolling step of hot-rolling the billet after the descaling step to obtain a hot-rolled steel sheet;
I will explain each process.
[0041]
The casting process (steel billet manufacturing process) that precedes hot rolling is not particularly limited. That is, after smelting by a blast furnace, an electric furnace, etc., various secondary smelting is performed to adjust the above-described components, and then casting is performed by ordinary continuous casting or ingot casting.
"You can use scrap as a raw material."
[0042]
[Heating process]
[Descaling process]
In the heating process, steel pieces such as slabs are heated in a heating furnace. Thereafter, descaling is performed before the hot rolling process. Local enrichment of Ni is achieved primarily during this heating and descaling step.
Specifically, by promoting the oxidation of the surface of the steel slab in the heating process and selectively oxidizing Fe, Ni, which is less oxidized than Fe, is concentrated on the base iron side of the interface between the scale and the base iron. Let After that, descaling is performed to remove preferentially generated oxides to some extent, and the Ni is further locally concentrated by holding in a predetermined temperature range for a predetermined time or longer.
[0043]
In the heating process, after the surface temperature of the steel slab reaches 1100°C or higher, it is held in an atmosphere with an air ratio of 0.9 or higher for 60 minutes or longer to raise the extraction temperature to 1180°C or higher.
In order to form a sufficient Ni-enriched layer on the surface in the heating furnace, it is necessary to promote the growth of scale on the billet. When the air ratio of the heating furnace is less than 0.9, the growth of scale follows the parabolic law, but the growth of scale slows down during the limited time in the heating furnace. Therefore, a sufficient Ni-enriched layer cannot be formed at the interface between the scale and the base iron. The air ratio may vary depending on the position in the heating furnace and the time-dependent change in the period during which the billet is heated. If it is 0.9 or more, the air ratio when the billet is heated is 0.9 or more Therefore, it is preferable.
On the other hand, if the air ratio is more than 1.5, the amount of scale off increases, the yield increases, and the heat loss due to the increase in exhaust gas increases, the thermal efficiency deteriorates, and the production cost rises. Therefore, it is preferable that the air ratio is 1.5 or less. The air ratio may vary depending on the position in the heating furnace and the period during which the billet is heated. If it is 1.5 or less, the air ratio when the billet is heated becomes 1.5 or less, which is preferable.
Also, if the steel slab surface temperature is kept at 1100°C or higher for less than 60 minutes, the scale does not grow and a sufficient Ni-enriched layer cannot be formed at the interface between the scale and the base iron.
On the other hand, if the holding time exceeds 240 minutes, the amount of scale off increases, the yield decreases, and the surface layer of the steel sheet decarburizes, deteriorating the properties of the steel sheet, which is not preferable.
The reason why the extraction temperature is 1180°C or higher is that it is necessary to ensure the surface temperature of the steel slab in the descaling process that is performed after the heating process. If the interval time from the heating process to the descaling process is long, the extraction temperature should be set at 1200° C. or higher in order to secure the surface temperature of the billet.
In this embodiment, the extraction temperature is the calculated temperature at a position 5 mm in the thickness direction of the billet from the upper surface of the billet when heat transfer is calculated by dividing the billet in the thickness direction from the atmosphere temperature of the heating furnace. , or the calculated temperature at a position 5 mm in the thickness direction of the billet from the lower surface of the billet, whichever is lower.
[0044]
In the descaling step, the steel billet having a surface temperature of 1170°C or higher is subjected to descaling at least once with an injection pressure of 5 to 50 MPa. Also, the surface temperature of the steel slab is maintained at 1100° C. or higher for 20 to 240 seconds after the descaling is completed.
[0045]
In the descaling process, the scale layer formed by the heating process is removed. This scale layer exists in a mixed state of oxides of Fe and oxides of other elements, and is generally in a molten state in a temperature range of 1170°C or higher, but is solidified and solid in a temperature range of less than 1170°C. state, making it difficult to remove by descaling. Especially when the scale contains Si, the composite oxide of Fe 2 SiO 4 exists at the same time as the oxide of Fe, and the composite oxide of Fe 2 SiO 4 enters between the oxides of Fe. Strong scale. Therefore, in the hot-rolled steel sheet manufacturing method according to the present embodiment, descaling is performed at least once while the temperature of the billet is 1170° C. or higher. However, if the descaling injection pressure is less than 5 MPa, scale cannot be sufficiently removed. In addition, if the descaling injection pressure exceeds 50 MPa, Ni concentrated near the interface during heating is also removed. Therefore, the injection pressure is set to 5 to 50 MPa.
It is preferable that the descaling be performed at an ejection force per unit time/unit width of 50 to 700 MN/(m·s). The injection force per unit time/unit width is obtained by multiplying the descaling pressure (MPa), the descaling time (seconds), and the length (m) of the steel sheet to be descaled.
[0046]
After performing descaling, the surface temperature of the steel piece is maintained at 1100°C or higher for 20 to 240 seconds after the completion of this descaling (primary descaling). By holding at 1100° C. or higher for 20 seconds or longer, the surface of the steel sheet is oxidized again and Ni is concentrated at the interface.
If the holding time at 1100°C or higher is less than 20 seconds, the concentration of Ni becomes insufficient. Therefore, the retention time is set to 20 seconds or longer. The holding time is preferably 30 seconds or more.
On the other hand, if the holding time after descaling exceeds 240 seconds, the thickness of the scale increases and the chemical convertibility deteriorates, as does productivity. Therefore, the retention time is set to 240 seconds or less. The retention time is preferably 180 seconds or less.
After maintaining the surface temperature of the steel billet at 1100°C or higher, the steel billet is rolled.
After the surface temperature of the billet is held at 1100° C. or higher for 20 to 240 seconds from the completion of descaling, secondary descaling is performed one or more times in addition to the previous descaling (primary descaling). You can go to This secondary descaling can remove scale layers that are formed during holding. However, even if the secondary descaling is performed so as not to remove the concentrated Ni, the injection pressure is set to 5 to 50 MPa, which is the same as the primary descaling. The surface temperature of the billet before secondary descaling may be 1170°C or higher, or may be less than 1170°C.
After the secondary descaling is completed, the time for which the surface temperature of the steel slab is maintained at 1100°C or higher may be between 20 and 240 seconds, or may be less than 20 seconds.
If the surface temperature of the steel slab is held at 1100°C or higher for more than 240 seconds after secondary descaling is completed, the thickness of the scale increases and the chemical conversion treatability and productivity decrease.
As described above, with respect to secondary descaling, the temperature before descaling and the time for holding at 1100° C. or higher after that are not limited, but the secondary descaling is performed at a steel slab surface temperature of 1170° C. or higher once or more. If the surface temperature of the billet is maintained at 1100°C or higher for 20 to 240 seconds after the completion of the secondary descaling, the time to maintain the surface temperature of the billet at 1100°C or higher from the completion of the primary descaling. may be less than 20 seconds.
Thus, whether only the primary descaling was performed or both the primary descaling and the secondary descaling were performed, the surface temperature of the billet was maintained at 1100°C or higher for a total of 20 seconds or longer from the completion of descaling. you should keep it. However, in terms of characteristics, when descaling and subsequent holding at 1100° C. or higher are repeated, it is preferable that the holding time for one or more times is 20 seconds or longer.
[0047]
[Hot rolling process]
The hot rolling conditions for the hot rolling process performed after the descaling process are not particularly limited. The hot rolling conditions may be appropriately adjusted according to the required plate thickness and mechanical properties. There are no restrictions on the cooling conditions after rolling. It may be cooled to normal temperature (to 100° C. or less), or may be wound up without cooling and left to cool in a coil state.
[0048]
According to the manufacturing method described above, the hot-rolled steel sheet according to the present embodiment can be manufactured.
Example
[0049]
The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.
[0050]
Slabs having chemical compositions shown in Tables 1A to 1C were heated under the heating conditions shown in Tables 2A to 2C, and descaling was performed under the descaling conditions shown in Tables 2A to 2C.
Under the heating conditions, as described above, the minimum air ratio at each position in the heating furnace and the maximum air ratio at each position in the heating furnace are the values ​​shown in Tables 2A to 2C. Combustion control was performed as follows.
As the descaling conditions, only primary descaling was performed for Steel 2, Steel 34, Steels 42 to 72, and Steels 75 to 82 and 86. Tables 2A to 2C show the surface temperature of the billet before the primary descaling, the primary descaling pressure, and the injection force per unit time/unit width. In addition, the time for which the surface temperature of the steel slab after completing the primary descaling was maintained at 1100° C. or higher was set as the conditions shown in Tables 2A to 2C. Tables 2A to 2C show the minimum slab surface temperatures during the period from the completion of the primary descaling to the rolling of the slab.
For Steel 1, Steel 3 to Steel 33, Steel 35 to Steel 41, Steel 73, Steel 74, Steel 83 to 85, Steel 87, and Steel 88, secondary descaling was performed after primary descaling. rice field. The surface temperature of the billet before primary descaling, the primary descaling pressure and injection force per unit time/unit width, and the secondary descaling pressure and injection force per unit time/unit width are shown in Table 2A- Shown in Table 2C (where secondary descaling was performed, the pressure is listed in the pressure column for secondary descaling). When secondary descaling is performed, the longer of the times during which the surface temperature of the steel slab after completion of descaling is held at 1100°C or higher, for each of the primary descaling and the secondary descaling. The conditions shown in Tables 2A to 2C were the total value of the holding time at 1100° C. or higher after descaling. Of the primary descaling and the secondary descaling, the descaling in which the surface temperature of the billet after descaling is held at 1100° C. or higher is the longer one. Tables 2A to 2C show the minimum surface temperature of the billet during the period from to rolling the billet.
After descaling, finish rolling was performed at a rolling end temperature of 800°C or higher. After hot finish rolling, a part of the steel was cooled to 100° C. or less, and a part of the steel was coiled without being cooled and left to cool in a coil state.
[0051]
[Table 1A]

[0052]
[Table 1B]

[0053]
[Table 1C]

[0054]
[Table 2A]

[0055]
[Table 2B]

[0056]
[Table 2C]

[0057]
The obtained hot-rolled steel sheet is pickled using a hydrochloric acid solution of 1 to 10 wt% (% by weight) at a temperature of 20 to 95 ° C. for 30 to 60 seconds. , Targeting elements having the number of atoms greater than or equal to that of B, EPMA analysis is performed on a 250 μm × 250 μm area under the above conditions at a measurement pitch of 1 μm, and the elements having the number of atoms greater than or equal to B When the total mass is 100%, the ratio of measurement points with a Ni content of 0.5% by mass or more, and the ratio of measurement points with an oxygen content of 0.5% by mass or more and the Ni content An average interval in the region of 0.5% by mass or more was obtained.
The results are shown in the surface texture columns of Tables 3A to 3C.
In Tables 3A to 3C, the reason why the average interval of the area having a Ni content of 0.5% by mass or more on the pickling surface is ≤ 1 (μm) is that the average interval was smaller than the measurement pitch and could not be measured. indicates
[0058]
In addition, the tensile strength was evaluated for the obtained hot-rolled steel sheets.
Tensile strength (TS) is the direction perpendicular to the rolling direction (sheet width direction) at either W/4 or 3W/4 position in the sheet width direction from one end of the steel sheet, where W is the sheet width. Using a JIS Z 2241:2011 No. 5 test piece sampled in the longitudinal direction, it was measured in accordance with JIS Z 2241:2011.
Tables 3A to 3C show the results of tensile strength (TS) together with the thickness of the hot-rolled steel sheets.
[0059]
In addition, the obtained hot-rolled steel sheet was pickled under the pickling conditions described above. After that, the pickled hot-rolled steel sheet described above was subjected to chemical conversion treatment under the following conditions assuming a chemical conversion treatment solution that had deteriorated due to continuous use, etc., and the chemical conversion treatability was evaluated. Although the effect of the present steel sheet can be exhibited regardless of the zinc phosphate-based chemical conversion treatment solution, the evaluation was performed under the following conditions as an example.
(1) Degreasing treatment:
Nippon Paint Pharmaceutical Solution: SD400
Chemical solution temperature: 42°C
Time to spray the chemical solution on the surface of the test piece: 120 seconds
(2) Surface adjustment treatment:
Nippon Paint Pharmaceutical Materials: 5N-10
Immersion time of chemicals: 20 seconds
(3) Chemical conversion treatment:
Nippon Paint Pharmaceutical Liquid: Surfdyne DP4000
Chemical solution temperature (chemical bath temperature): 35°C
Bath time: 60 seconds
Free acidity: 0.5pt
Total acidity (TA): 25pt
Accelerator: 2.0pt
(4) Water washing treatment:
City water (spray injection)
City water temperature: 25℃
Water washing time: 30 seconds
(5) Pure water washing treatment:
Deionized water (spray injection)
Deionized water temperature: 25°C
Pure water washing time: 30 seconds
Here, the free acidity is obtained by adding 3 drops of bromophenol blue to 10 ml of the chemical conversion treatment solution, performing neutralization titration with 0.1 N sodium hydroxide until the color changes from yellow green to blue green, and 1 ml of 1 N sodium hydroxide is defined as 1 pt. In addition, the total acidity means that 10 ml of the chemical conversion treatment solution contains phenol Three drops of turrain are added, and neutralization titration is carried out with 0.1 N sodium hydroxide until the color changes from colorless to pink.
The effect of improving the chemical conversion treatability of this steel sheet can be exhibited with other model numbers and chemical conversion solutions of other companies, regardless of whether the chemical conversion treatment solution was used under the chemical conversion treatment conditions shown above.
As a result of the chemical conversion treatment, when no scales were observed and the size of the chemical crystals was 10 μm or less, it was judged that the chemical conversion treatability was excellent. This is because even after chemical conversion treatment, if the base iron is exposed, that is, if there is a scale, the adhesion between the steel plate and the paint is reduced, and after chemical conversion treatment This is because if the size of the chemical crystals exceeds 10 μm, the cohesive failure of the zinc phosphate coating itself reduces the coating adhesion.
When the size of the chemical crystals is 10 μm or less, the adhesion between the paint and the steel plate and the corrosion resistance after peeling of the coating film are improved. Corrosion resistance after film peeling is further improved. In this example, there is no slack and the chemical crystal size is 5 μm or less is evaluated as A (invention example), and there is no skeletal structure and the chemical crystal size is more than 5 μm and 10 μm or less is evaluated as B (invention example). Alternatively, the case where the size of the chemically formed crystal exceeds 10 μm even if there is no slack, was evaluated as C (comparative example).
[0060]
In addition, although not shown in the table, before the evaluation of the chemical conversion treatability, the hot-rolled steel sheet subjected to the chemical conversion treatment was subjected to a thickness of 10 μm in the thickness direction and 500 μm in the width direction from the surface of the steel sheet in the cross section in the thickness direction. EPMA analysis is performed at a measurement pitch of 1 μm for elements having the number of atoms equal to or greater than that of B in the rectangular area, and the total mass of elements having the number of atoms equal to or greater than the number of atoms of B is taken as 100%. When the percentage of measurement points where the Ni content was 0.5% by mass or more was obtained, the Ni content measured for a 250 μm × 250 μm area of ​​the surface before chemical conversion treatment after pickling was It was equivalent to the percentage of measurement points that were 0.5% by mass or more.
[0061]
SEM was used to observe scales. Specifically, 3 fields of 250 μm × 250 μm on each of the front and back sides of the steel plate after chemical conversion treatment are checked with an SEM to see if there are any surfaces where the base iron is exposed, thereby preventing the generation of skeletal scales. Existence was investigated.
Similarly, the size of the chemically formed crystal is obtained by determining the grain size (diameter) of the chemically formed crystal for the region of 250 μm × 250 μm when performing the SEM observation described above, and the average value of the grain size (diameter) of the chemically formed crystal. was taken as the size of the chemical crystal.
Among the 6 fields of view observed, the worst results are shown in the property column of chemical products in Tables 3A to 3C.
[0062]
[Table 3A]

[0063]
[Table 3B]

[0064]
[Table 3C]

[0065]
As shown in Tables 1A to 1C and Tables 3A to 3C, the ratio of measurement points having a chemical composition within the range of the present invention and having a surface Ni content of 0.5% by mass or more is 10 to 70%. All the examples of the present invention had no scales, the size of the chemical crystals was 10 μm or less, and were excellent in chemical conversion treatability.
On the other hand, in the comparative examples in which one or more of the chemical composition and the percentage of measurement points where the Ni content on the surface is 0.5% by mass or more are outside the scope of the present invention, skeletal The crystal size was more than 10 μm, and the chemical convertibility was not sufficient.
Industrial applicability
[0066]
According to the present invention, a hot-rolled steel sheet with excellent chemical conversion treatability and a method for producing the same can be obtained. The hot-rolled steel sheet of the present invention has high industrial applicability because a good chemical conversion coating can be obtained even when the chemical conversion treatment conditions vary.
Code explanation
[0067]
1 Subrailway
2 Scale
3. Oxides such as Si and Al, or concentrated layers such as Mn and Cu
4 Ni-enriched layer
5 Chemical crystal
The scope of the claims
[Claim 1]
The chemical composition, in mass%,
C: 0.01-0.30%,
Si: 0.01 to 3.00%,
Mn: 0.20 to 3.00%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.001 to 2.000%,
N: 0.0100% or less,
Ni: 0.02-0.50%,
Nb: 0 to 0.060%,
　V: 0 to 0.20%,
Ti: 0 to 0.20%,
Cu: 0 to 0.20%,
Cr: 0 to 0.20%,
Mo: 0 to 1.00%,
B: 0 to 0.0020%,
W: 0-0.50%,
　Mg: 0 to 0.010%,
Ca: 0 to 0.0100%,
REM: 0-0.0100%,
O: 0 to 0.0100%,
Zr: 0 to 0.500%,
Co: 0 to 0.500%,
Zn: 0 to 0.500%, and
Sn: 0-0.500%
and the balance consists of Fe and impurities,
The percentage of measurement points with a Ni content of 0.5% by mass or more among the measurement points when elemental analysis is performed on a 250 μm × 250 μm region of the surface using EPMA at a measurement pitch of 1 μm is 10 to 10. is 70%;
A hot-rolled steel sheet characterized by:
[Claim 2]
The chemical composition is
Nb: 0.003 to 0.060%,
　V: 0.01 to 0.20%,
Ti: 0.01 to 0.20%,
Cu: 0.01 to 0.20%,
Cr: 0.01 to 0.20%,
Mo: 0.01 to 1.00%,
B: 0.0005-0.0020%,
W: 0.01-0.50%,
　Mg: 0.001 to 0.010%,
Ca: 0.0010 to 0.0100%,
REM: 0.0010 to 0.0100%, and
O: 0.0005-0.0100%
containing one or more selected from the group consisting of
The hot-rolled steel sheet according to claim 1.
[Claim 3]
The chemical composition is
　　Si: 0.50 to 3.00%,
The hot-rolled steel sheet according to claim 2, containing
[Claim 4]
The chemical composition is
　　Si: less than 0.01 to 0.50%,
Al: 0.050-2.000%,
The hot-rolled steel sheet according to claim 2, containing
[Claim 5]
The chemical composition is
　　Si: less than 0.01 to 0.50%,
Al: less than 0.001 to 0.050%,
contains
　　Total of Si and Al: 0.50 to less than 0.55%,
The hot-rolled steel sheet according to claim 2, wherein
[Claim 6]
Among the measurement points when the elemental analysis of the surface is performed, the percentage of measurement points where the O content is 0.5% by mass or more is 30% or less,
The hot rolled steel sheet according to any one of claims 3 to 5.
[Claim 7]
The chemical composition is
　　Cu: 0.01 to 0.20%,
contains
Ni/Cu: 0.50 or more,
The hot-rolled steel sheet according to any one of claims 1 to 6.
[Claim 8]
Among the measurement points when the elemental analysis of the surface is performed, the average interval of the measurement points where the Ni content is 0.5% by mass or more is 3 to 10 μm,
The hot-rolled steel sheet according to any one of claims 1 to 7.
[Claim 9]
The hot-rolled steel sheet according to any one of claims 1 to 8, characterized by having a rust preventive oil film on the surface.
[Claim 10]
The hot-rolled steel sheet according to any one of claims 1 to 8, characterized by having a chemical conversion coating on the surface.
[Claim 11]
a heating step of heating the steel slab having the chemical composition according to claim 1 or 2 in a heating furnace;
a descaling step of descaling the heated billet,
a hot rolling step of hot-rolling the billet after the descaling step to obtain a hot-rolled steel sheet;
with
　In the heating process,
After the surface temperature of the billet reaches 1100°C or higher, it is held in an atmosphere with an air ratio of 0.9 or higher for 60 minutes or longer,
　　The extraction temperature is 1180℃ or higher,
　In the descaling process,
　　Perform descaling with an injection pressure of 5 to 50 MPa at least once on the billet with a surface temperature of 1170°C or higher,
The surface temperature of the billet is maintained at 1100°C or higher for 20 to 240 seconds after the descaling is completed,
A method for manufacturing a hot-rolled steel sheet, characterized by: