JP7722607B2 - Hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

The present invention relates to a hot-rolled steel sheet with excellent scale adhesion, which is used in automobiles, home appliances, building materials, etc., and a method for manufacturing the same. The present invention is particularly suitable as a material for parts of construction and industrial machinery that undergo processing such as temper rolling, bending, press forming, and laser cutting. The present invention relates to a hot-rolled steel sheet with excellent scale adhesion, which has excellent adhesion between the scale and base steel interface and resistance to powdery peeling, and a method for manufacturing the same.

Hot-rolled steel sheets are typically hot-rolled at high temperatures in an oxidizing atmosphere, which inevitably leads to the formation of scale (iron oxides) on the surface. When black hot-rolled steel sheets with this scale still attached undergo processes such as temper rolling, bending, press forming, and laser cutting, some of the scale peels off. This can result in poor processing, contamination of the processing line, and surface defects in the processed product. To avoid these problems, there is a growing demand for hot-rolled steel sheets with excellent surface scale adhesion, and this demand is growing stronger. In particular, powder-like scale peeling is a major cause of contamination of the processing line and deterioration of surface quality. Therefore, in order to improve scale adhesion, it is necessary to suppress powder-like scale peeling.

For example, Patent Document 1 discloses a steel material having a composition containing, by mass%, C: 0.01 to 0.3%, Si: 0.20% or less, Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.10% or less, Al: 0.10% or less, Cr: 0.01 to 2.0%, with the balance being Fe and unavoidable impurities, which is rough-rolled, descaled, and then finish-rolled at a finish-rolling outlet temperature of 800 to 950°C so as to satisfy the following formula (1): an average cooling rate from the end of finish rolling to the start of coiling: 3°C/s or more, and a cooling rate of 80°C/s or more. /s or less, and then coiled at a coiling temperature of 430 to 580°C, a hot-rolled steel sheet with excellent scale adhesion has been proposed, characterized in that it has, from the base steel side, a magnetite layer and, in an upper layer of the magnetite layer, magnetite grains and/or a eutectoid transformed structure of iron and magnetite, the average grain size of the magnetite grains and/or the average block size of the eutectoid transformed structure is 3 μm or more and 8 μm or less, and the mass fraction of wüstite contained in the scale layer is 10% or less.
|T ₂ −T ₁ |≦50° C. and |T ₃ −T ₂ |≦50° C. (1)
However, in the above formula (1),
T ₁ : Temperature (°C) of the steel plate after finish rolling, 30 m from the longitudinal end and at the center in the width direction
_T2 : Temperature (°C) of the longitudinal center and width center of the steel sheet after finish rolling
T ₃ : The temperature (° C.) of the steel plate after finish rolling at a point 30 m from the tail end in the longitudinal direction and at the center in the width direction.

Patent Document 2 also proposes a hot-rolled steel sheet with excellent scale-tightness, which is obtained by heating a slab containing, by mass, C: 0.02-0.20%, Mn: 0.1-2.0%, Si: 0.3% or less, P: 0.03% or less, S: 0.03% or less, Ni: 0.03-0.3%, Cu: 0.04-0.5%, Cr: 0.03-0.3%, with the remainder being Fe and unavoidable impurities, at 1100°C or higher, completing hot rolling in a temperature range of 800°C to 950°C, and then coiling at 400°C to 650°C. This results in a surface roughness at the interface between the steel sheet surface scale and the steel sheet base metal of 300 or more irregularities of 0.5 μm or more per inch of length.

Patent Document 3 proposes a black hot-rolled steel sheet with excellent blackness, which contains 0.001 to 0.20 mass% C, 0.001 to 0.50 mass% Si, 0.05 to 2.0 mass% Mn, 0.05 mass% or less P, 0.05 mass% or less S, and 0.01 to 0.10 mass% sol. Al, with the balance being Fe and unavoidable impurities, and which has a scale with a thickness of more than 4 μm on the surface of the hot-rolled steel sheet, the scale having a composition containing 50% or more Fe ₃ O ₄ by volume, and which does not contain precipitated Fe in a region from the scale surface to a depth of at least 2 μm in the thickness direction.

Patent No. 6760425 Patent No. 4153734 Patent No. 4061996

The technology described in Patent Document 1 uses a steel material with a predetermined chemical composition, and adjusts the finish rolling exit temperature during hot rolling, the cooling rate after rolling, and the coiling temperature. This optimizes the average particle size of magnetite grains in the upper layer of the magnetite layer on the base steel side of the scale layer and/or the average block size of the eutectoid transformation structure of iron and magnetite. Furthermore, by controlling the temperature in the longitudinal direction of the steel sheet immediately after finish rolling, uniform scale adhesion in the longitudinal direction is improved. However, there is no mention of a method for suppressing powdery scale peeling.

The technology described in Patent Document 2 proposes a hot-rolled steel sheet with excellent scale tightness by hot-rolling steel to which predetermined amounts of Ni, Cu, and Cr have been added, and controlling the surface roughness of the interface between the surface scale of the steel sheet and the base steel of the steel sheet within a predetermined range. However, while this improves the adhesion at the interface between the scale layer and the base steel, there is concern that scale spalling may occur on the surface or inside the scale, reducing adhesion. Furthermore, no method is mentioned for suppressing powder-like scale spalling.

In the technology described in Patent Document 3, a steel slab having a predetermined chemical composition is heated, then hot-rolled to a finish rolling outlet temperature of 800°C or higher, and then cooled to 650°C or lower at a cooling rate of 50°C/s or higher, and coiled at 600°C or higher. This advantageously achieves blackening of the scale while maintaining its adhesion. However, to blacken the scale, the region from the scale surface to a depth of at least 2 μm in the thickness direction is controlled to contain no precipitated Fe, raising concerns that powdery scale spalling may not be sufficiently suppressed.

The present invention aims to solve the above problems and provide a hot-rolled steel sheet with excellent scale adhesion, including adhesion between the scale and base steel interface and resistance to powdery peeling, and a manufacturing method thereof. The thickness of the hot-rolled steel sheet in this invention is more than 2.0 mm and not more than 20 mm, and preferably more than 5.0 mm and not more than 20 mm.

The inventors first investigated the cause of powder-like flaking of scale on conventional hot-rolled steel sheets. As a result, they found that the powder-like flaking scale is primarily composed of magnetite and is caused by destruction within the scale. In other words, by appropriately controlling the magnetite that forms on the surface of the scale, it is possible to suppress powder-like flaking.

The scale formed during hot rolling is formed in the following order from the surface layer side at high temperatures _: hematite, magnetite _{( Fe3O4} ), and wustite (FeO). Of these, wustite undergoes eutectoid transformation during cooling after coiling, resulting in the formation of an eutectoid-transformed structure consisting of magnetite and precipitated Fe (4FeO → _Fe3O4 + Fe). In conventional technology, cracks occur at the interface between the magnetite formed at high temperatures and the eutectoid-transformed structure, causing powdery scale peeling.

Therefore, the present inventors have conducted extensive research into means for solving the above problems and achieving excellent adhesion between the scale and the base steel and resistance to powdery peeling, and have arrived at the following findings.
(i) A steel material having a predetermined chemical composition is subjected to rough hot rolling, followed by descaling, and then finish rolling at a finish rolling outlet temperature of 800 to 950°C, thereby appropriately controlling the scale thickness and suppressing the occurrence of cracks in the scale.
(ii) After cooling at an average cooling rate of 5°C/s or more in the temperature range from the finish rolling delivery temperature to 750°C, the steel sheet is cooled at an average cooling rate of 1°C/s or more and 30°C/s or less in the temperature range from 750°C to the start of coiling, thereby suppressing excessive growth of scale at high temperatures.
(iii) Coiling temperature: Coiling is performed at 500 to 650°C, and the temperature is held within a coiling temperature range of −50°C to the coiling temperature for 100 minutes or more. During coiling, the steel sheet surface is isolated from the atmosphere, and the temperature is held within a temperature range in which wüstite is stably formed, thereby reducing some or all of the hematite and magnetite formed on the surface of the scale to wüstite. After coiling, the steel sheet is held within a temperature range of −50°C to the coiling temperature for 100 minutes or more, thereby allowing the eutectoid transformation of wüstite to proceed sufficiently, and an eutectoid transformed structure is also formed on the surface of the scale. As a result, adhesion between the scale and the base steel interface is improved, and the amount of magnetite formed on the surface of the scale is reduced, thereby suppressing powder-like peeling.

The present invention has been made based on the above findings, and specifically provides the following.
[1] A hot-rolled steel sheet having a composition, by mass%, of C: 0.01 to 0.30%, Si: 0.50% or less, Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.10% or less, sol.Al: 0.10% or less, and N: 0.015% or less, with the balance being Fe and unavoidable impurities, with scale on the steel sheet surface, the average thickness of the scale being 20 μm or less, and the scale having an area ratio of magnetite: 20% or more, a eutectoid transformation structure of iron and magnetite: 40% or more, and wustite: 15% or less, and a structure containing 90% or more (including 100%) of the total of magnetite, wustite, and a eutectoid transformation structure of iron and magnetite, and an area ratio of precipitated Fe in the region from the outermost layer of the scale to 1/2 of the scale thickness in the thickness direction is 2% or more.
[2] The component composition further contains, in mass%, Cu: 1.0% or less, Ni: 0.50% or less,
Cr: 2.0% or less.
[3] The hot-rolled steel sheet according to [1] or [2], wherein the chemical composition further contains, in mass%, one or more of Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, and Sb: 0.03% or less.
[4] A method for producing a hot-rolled steel sheet, comprising rough hot rolling a steel material having the chemical composition according to [1] or [2], descaling the steel material, and then finish rolling the steel material at a finish rolling outlet temperature of 800 to 950°C. The steel material is cooled in a temperature range from the finish rolling outlet temperature to 750°C at an average cooling rate of 5°C/s or more, and then cooled in a temperature range from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less. The steel material is coiled at a coiling temperature of 500 to 650°C, and held for 100 minutes or more in a temperature range of not less than -50°C and not more than the coiling temperature.
[5] A method for producing a hot-rolled steel sheet, comprising the steps of rough hot rolling a steel material having the chemical composition described in [3], descaling the steel material, and then finish rolling the steel material at a finish rolling outlet temperature of 800 to 950°C, cooling the steel material from the finish rolling outlet temperature to 750°C at an average cooling rate of 5°C/s or more, cooling the steel material from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less, coiling the steel material at a coiling temperature of 500 to 650°C, and holding the steel material in a temperature range of -50°C or more and the coiling temperature or less for 100 minutes or more.

The present invention makes it possible to easily and inexpensively produce hot-rolled steel sheets with excellent scale adhesion, providing significant industrial benefits. Furthermore, the present invention also prevents peeling at the interface between the scale and the base steel, significantly contributing to improved product surface quality, prevention of product processing defects, and an improved working environment. Furthermore, it also solves issues such as reduced resistance to powdery peeling.

The hot-rolled steel sheet and its manufacturing method of the present invention are described in detail below. Note that the present invention is not limited to the following embodiments. The following embodiments include those that can be easily substituted by a person skilled in the art, or those that are substantially identical.

The hot-rolled steel sheet of the present invention contains the following chemical composition. Note that "%," the unit of content in the chemical composition, means "mass %" unless otherwise specified.

C: 0.01-0.30%
C is an element useful for ensuring strength. If the content is less than 0.01%, the effect of ensuring strength is small, so the C content is set to 0.01% or more. If the C content exceeds 0.30%, CO gas is generated at the interface between the scale and the base steel, causing peeling at the interface between the scale and the base steel during rolling and resulting in scale defects, so the C content is set to 0.30% or less. From the viewpoint of adhesion at the interface between the scale and the base steel, the C content is preferably 0.20% or less.

Si: 0.50% or less Si is an element that acts as a deoxidizer, and to obtain this effect, a content of 0.01% or more is preferable. However, if the Si content exceeds 0.50%, Si concentrates at the interface between the scale and the base steel, forming a Si oxide layer. Scale spalling is likely to occur at the interface between this Si oxide layer and the scale layer formed thereon. For this reason, the Si content is set to 0.50% or less. Preferably, it is 0.20% or less.

Mn: 0.01-2.0%
Mn is an element that not only neutralizes solute S, which causes embrittlement during hot working, by converting it into MnS, but also has the effect of improving strength. If the amount is less than 0.01%, the effect is small, while if it is contained in excess of 2.0%, it not only reduces toughness but also forms Mn-based oxides at the interface between the scale and the base steel, causing a decrease in scale adhesion. For this reason, the Mn amount is set to 0.01 to 2.0%. The preferred lower limit is 0.05% or more. The preferred upper limit is 1.5% or less.

P: 0.10% or less P is an element that is desirably kept as low as possible because it has a detrimental effect on grain boundary embrittlement. P also forms a very brittle oxide layer at the interface between the scale and the base steel, reducing the adhesion at the interface between the scale and the base steel. If the P content exceeds 0.10%, these detrimental effects become greater, so the P content is set to 0.10% or less. Preferably, it is set to 0.05% or less. Note that P does not necessarily have to be contained, but from the viewpoint of production costs, the P content is preferably set to 0.001% or more.

S: 0.10% or less S is an element that significantly deteriorates hot workability and toughness. Furthermore, S concentrates at the interface between the scale and the base steel, reducing the adhesion between the scale and the base steel interface. If the S content exceeds 0.10%, these adverse effects become significant, so the S content is set to 0.10% or less. Preferably, it is set to 0.05% or less. Note that S does not necessarily need to be contained, but from the viewpoint of production costs, the S content is preferably set to 0.0001% or more.

Sol. Al: 0.10% or less Sol. Al is an element that acts as a deoxidizer. The sol. Al content may be 0.00%, but to obtain this effect, it is preferable to contain 0.01% or more. On the other hand, if it is contained in excess of 0.10%, oxide-based inclusions increase and cleanliness decreases. For this reason, the sol. Al content is set to 0.10% or less, preferably 0.06% or less.

N: 0.015% or less N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces the hot ductility of steel and the surface quality. If the N content exceeds 0.015%, the surface quality deteriorates significantly. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.010% or less. Note that N does not necessarily need to be contained, but from the viewpoint of manufacturing costs, the N content is preferably 0.0001% or more. More preferably, the N content is 0.001% or more.

The above chemical components are essential components of the hot-rolled steel sheet of the present invention. In addition to the above chemical components, the hot-rolled steel sheet of the present invention may contain one or more of Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less, as necessary, to improve various properties.

Cu: 1.0% or less Cu is an element that concentrates at the interface between the scale and the base steel to promote grain boundary oxidation, promotes the formation of irregularities at the interface between the scale and the base steel, and improves adhesion at the interface between the scale and the base steel. To achieve these effects, it is preferable to include 0.01% or more of Cu. However, if the Cu content exceeds 1.0%, molten Cu may penetrate into the austenite grain boundaries of the base steel during heating, which may cause deterioration of surface properties due to hot embrittlement. For this reason, if Cu is included, it should be 1.0% or less. Preferably, it is 0.8% or less.

Ni: 0.50% or less Like Cu, Ni is an element that concentrates at the interface between the scale and the base steel to promote grain boundary oxidation, promotes the formation of irregularities at the interface between the scale and the base steel, and improves adhesion at the interface between the scale and the base steel. To achieve these effects, it is preferable to contain 0.01% or more of Ni. However, if the Ni content exceeds 0.50%, the above effects saturate, and there is a concern that costs will increase. For this reason, if Ni is contained, it should be 0.50% or less. Preferably, it is 0.40% or less.

Cr: 2.0% or less Cr has the effect of increasing strength, hardenability, and corrosion resistance. Furthermore, Cr concentrates at the interface between the scale and the base steel, roughening the interface so that the scale penetrates the base steel, thereby improving adhesion at the interface between the scale and the base steel. To achieve this effect, a Cr content of 0.01% or more is preferable. On the other hand, if the Cr content exceeds 2.0%, the above effect saturates, so if Cr is contained, it should be 2.0% or less. A more preferable lower limit is 0.07% or more. A more preferable upper limit is 1.0% or less. The most preferable lower limit is 0.12% or more. The most preferable upper limit is 0.8% or less.

In the present invention, if necessary, one or more of the following may be further contained: Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, and Sb: 0.03% or less.

Mo: 1.0% or less Mo has the effect of improving strength and hardenability and suppressing softening due to tempering. To obtain these effects, it is preferable to contain Mo in an amount of 0.1% or more. On the other hand, if the content exceeds 1.0%, the strength increases excessively and the toughness and formability deteriorate. Therefore, if Mo is contained, the amount is set to 1.0% or less.

Nb: 0.1% or less Nb is an element that improves the strength and toughness of the base material, and in order to obtain this effect, it is preferable to contain 0.003% or more. On the other hand, if it is contained in excess of 0.1%, it may actually cause a decrease in toughness. Therefore, if Nb is contained, the amount is set to 0.1% or less.

V: 0.1% or less V is an element that improves the strength and toughness of the base material, and in order to obtain this effect, it is preferable to contain 0.003% or more. On the other hand, if it is contained in excess of 0.1%, it may actually cause a decrease in toughness. Therefore, if V is contained, the amount is set to 0.1% or less.

Ti: 0.03% or less Ti is an element that improves the strength and toughness of the base material and is also effective in ensuring toughness in the weld heat affected zone. To obtain these effects, it is preferable to contain 0.001% or more of Ti. On the other hand, if the content exceeds 0.03%, there is a risk of causing a decrease in toughness. Therefore, if Ti is contained, the amount is set to 0.03% or less.

B: 0.01% or less B is an element that improves the hardenability of steel, and this effect can increase strength. To obtain this effect, it is preferable to contain 0.0005% or more of B. On the other hand, if the content exceeds 0.01%, this effect saturates, so if B is contained, the amount should be 0.01% or less.

Sb: 0.03% or less Sb concentrates in the surface layer when the material is heated, and has the effect of suppressing a decrease in the C content in the surface layer during heating. To achieve this effect, it is preferable to include 0.001% or more of Sb. On the other hand, if the content exceeds 0.03%, Sb becomes a liquid metal when the material is heated, corroding the prior austenite grain boundaries and reducing the adhesion between the scale and the base steel interface. For this reason, if Sb is included, the content should be 0.03% or less.

The balance other than the above chemical components consists of Fe and unavoidable impurities. The allowable unavoidable impurities are O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less.

When the above optional components are contained in amounts below the preferred lower limit, the optional elements contained below the lower limit do not impair the effects of the present invention. Therefore, when the above optional elements are contained in amounts below the preferred lower limit, they are considered to be included as unavoidable impurities.

Next, we will explain the scale structure of the hot-rolled steel sheet of the present invention.

Average scale thickness of 20 μm or less If the average scale thickness exceeds 20 μm, strain is applied to the outermost layer of the scale and to the interface between the scale and the base steel when the steel sheet is processed, resulting in a decrease in adhesion between the scale and the base steel and a decrease in resistance to powdery peeling. For this reason, the average scale thickness is set to 20 μm or less. It is preferably 18 μm or less, and more preferably 15 μm or less. There is no particular lower limit for the average scale thickness, but from the viewpoint of ensuring stable cut surface quality during processing such as laser cutting, it is preferably 3 μm or more. It is more preferably 5 μm or more.

Magnetite: 20% or more Magnetite has high compatibility with the base steel and contributes to improving adhesion at the interface between the scale and the base steel. If it is less than 20%, this effect cannot be sufficiently obtained, so the area ratio of magnetite is set to 20% or more. Preferably, it is 30% or more. There is no particular upper limit, but if the area ratio of magnetite exceeds 70%, the amount of magnetite in the surface layer of the scale, which is the cause of powder-like spalling, increases, and resistance to powder-like spalling may be impaired. For this reason, the area ratio of magnetite is preferably 70% or less. More preferably, it is 60% or less. Note that the magnetite here can be distinguished from the magnetite contained in the eutectoid transformation structure of iron and magnetite.

Eutectoid-transformed structure of iron and magnetite: 40% or more The eutectoid-transformed structure of iron and magnetite has high compatibility between magnetite and precipitated Fe and the base steel, and therefore contributes to improving adhesion at the interface between the scale and the base steel. Furthermore, when the surface layer of the scale contains a eutectoid-transformed structure of iron and magnetite, the magnetite in the surface layer of the scale, which is the cause of powder-like spalling, is reduced, thereby contributing to improving resistance to powder-like spalling. This effect cannot be fully achieved at an area ratio of less than 40%, so the area ratio of the eutectoid-transformed structure of iron and magnetite is set to 40% or more. It is preferably 45% or more, and more preferably 50% or more.

Wüstite: 15% or less In addition to magnetite and the eutectoid-transformed structure of iron and magnetite, wüstite may remain untransformed at room temperature. Wüstite is more brittle than magnetite at room temperature, and cracks may form in the scale, impairing the scale's resistance to powder peeling. Furthermore, wüstite has lower compatibility with the base steel compared to magnetite or the eutectoid-transformed structure of iron and magnetite, which can impair the adhesion at the interface between the scale and the base steel. For this reason, the area ratio of wüstite is set to 15% or less. Preferably, it is 10% or less, and more preferably 7% or less. The area ratio of wüstite may be 0%.

In addition to wüstite, a hematite layer may form in the outermost layer of the hot-rolled steel sheet. However, the formation of the hematite layer does not impair the effects of the present invention, so it is acceptable for the layer to be present. Because the hematite layer can cause surface defects such as red scale, the mass fraction of hematite is preferably 10% or less. The mass fraction of hematite can be considered an area fraction.

Total of magnetite, wustite, and eutectoid transformation structure of iron and magnetite: 90% or more (including 100%)
If a large amount of phases other than magnetite, wustite, and the eutectoid-transformed structure of iron and magnetite are contained, such as hematite, the total fraction of magnetite, wustite, and the eutectoid-transformed structure of iron and magnetite may be less than 90%. In such cases, this may cause surface defects and reduce the adhesion and powder-like spalling resistance of the interface between the scale and the base steel. From the viewpoint of ensuring surface quality, the desired adhesion and powder-like spalling resistance of the interface between the scale and the base steel, the total fraction of magnetite, wustite, and the eutectoid-transformed structure of iron and magnetite is set to 90% or more.

The remaining structure other than magnetite, wüstite, and the eutectoid transformation structure of iron and magnetite includes hematite as well as non-ferrous oxides, such as Si-based oxides and Cr-based oxides, but as long as the total area ratio of these is 10% or less, the effects of the present invention are not impaired. Furthermore, these remaining structures can be measured by X-ray diffraction analysis in the same manner as the method for measuring the mass fraction of hematite, described below.

Precipitated Fe in the region from the outermost layer of the scale to 1/2 of the scale thickness in the thickness direction: 2% or more. By temperature control, as described below, part or all of the hematite and magnetite formed on the scale surface at high temperatures is reduced to wüstite. After coiling, the coil is held at a temperature range of −50°C or higher and lower than the coiling temperature for 100 minutes or more to sufficiently promote the eutectoid transformation of wüstite, thereby forming an eutectoid-transformed structure on the surface layer of the scale. As a result, adhesion between the scale and the base steel is improved, and the amount of magnetite formed on the scale surface is reduced, thereby suppressing powdery spalling. The amount of eutectoid-transformed structure on the surface layer side can be expressed as the area ratio of precipitated Fe in the region from the outermost layer of the scale to 1/2 of the scale thickness in the thickness direction. From the viewpoint of suppressing powdery spalling, precipitated Fe in the region from the outermost layer of the scale to 1/2 of the scale thickness in the thickness direction is set to 2% or more, preferably 3% or more. There is no particular upper limit for the amount of precipitated Fe in the region from the outermost layer of the scale to half the thickness of the scale in the thickness direction, but it is preferably 50% or less.

Next, we will explain the method for measuring the scale structure of the hot-rolled steel sheet of the present invention.

The area percentages of magnetite, eutectoid transformation structure of iron and magnetite, wüstite, and precipitated Fe are measured by cutting a cross section of the steel sheet perpendicular to the surface and parallel to the rolling direction and mirror-polishing it. Then, using a scanning electron microscope (SEM) to observe the backscattered electron image of the cross section of the scale at 3000x magnification, magnetite appears as the darkest region, the base steel and precipitated Fe appear as the brightest, and wüstite appears as an intermediate region. The average thickness of the scale is calculated by measuring the scale thickness at three random locations using the SEM and averaging the results. The area percentages of magnetite, wüstite, and eutectoid transformation structure of iron and magnetite are calculated based on the total area of the oxide scale being 100%. If there are voids in the oxide scale, they are excluded from the calculation of the area percentage.

The mass fraction of hematite is determined by measuring the integrated intensity of the diffraction peaks of each phase in the scale using an X-ray diffractometer with a CoK _α source. The mass fraction can be calculated from the ratio of _the integrated intensity of each phase in the test sample to that in a standard sample (a mixture of equal weights _of Fe, FeO (wustite), _Fe2O3 (hematite), and _Fe3O4 (magnetite)) using the following formula (2):
Mass fraction of phase A=(I _A /R _A )×100/((I _Fe /R _Fe )+(I Fe O /R _{Fe O} )+(I _{Fe 2 O 3} /R Fe _{2 O 3} )+(I _{Fe 3 O 4} /R _{Fe 3 O 4} )) (2)
However, in the above formula (2),
I _A : Integrated intensity of phase A in the test sample R _A : Integrated intensity of phase A in the standard sample A: Fe, FeO, Fe ₂ O ₃ , or Fe ₃ O ₄ .
The mass fraction of hematite detected using the X-ray diffractometer is regarded as an area fraction and is subtracted from the area of the entire oxide scale, which is 100%, and the area fractions of magnetite, the eutectoid transformed structure, and wüstite are calculated from the remaining areas.

Next, we will explain the manufacturing method of the hot-rolled steel sheet of the present invention.

The temperatures specified in each process in this invention refer to the surface temperature of the slab (steel slab) or steel plate, and can be measured using a radiation thermometer, etc. Furthermore, unless otherwise specified, the average cooling rate is ((cooling start temperature - cooling stop temperature) / cooling time).

In the present invention, the method for producing the steel material having the above-mentioned composition does not need to be particularly limited, and any commonly used method can be used. For example, it is desirable to melt molten steel having the above-mentioned composition in a converter or electric furnace, and then produce a steel material such as a slab by a casting method such as continuous casting. However, there is no problem in using an ingot-blooming rolling method. Typically, the steel material is heated and then hot-rolled. This heating is sufficient as long as sufficient solid solution is achieved, and the material is preferably heated to the Ac ₃ point or higher. Specifically, a typical slab heating temperature range of 1060°C to 1300°C is appropriate. In the case of a slab produced by a continuous casting method, direct rolling may be used, in which the slab is rolled as is or after being held in order to prevent temperature reduction.

The hot rolling process consists of rough rolling and finish rolling. Rough rolling only requires that a sheet bar of the specified dimensions be produced, and the conditions for rough rolling do not need to be particularly limited. Furthermore, to perform finish rolling at a specified temperature, the material being rolled may be heated midway using a heating means such as a sheet bar heater. Before finish rolling, scale formed on the surface of the sheet bar is removed by conventional descaling using high water pressure, etc., at the entry side of the finish rolling mill.

Next, finish rolling is performed. If the finish rolling entry temperature exceeds 1100°C, the scale thickness increases, which may reduce the adhesion at the interface between the scale and the base steel and the scale's resistance to powdery peeling. On the other hand, if the finish rolling entry temperature is 950°C or lower, the rolling load increases significantly, which may reduce productivity. Furthermore, as the product thickness increases, the finish rolling entry thickness also increases. For example, if the product thickness exceeds 5.0 mm, a long time is required before finish rolling can begin, which may reduce productivity. Therefore, the finish rolling entry temperature is preferably 1100°C or lower, and more preferably 1050°C or lower. Furthermore, it is preferable that the lower limit of the finish rolling entry temperature be 950°C or higher.

Finishing rolling outlet temperature: 800 to 950°C
If the finish rolling exit temperature is less than 800°C, cracks occur due to a decrease in scale ductility. This reduces the adhesion between the scale and the base steel, and these cracks promote the reoxidation of the scale, generating hematite, which reduces the scale's resistance to powder spalling. Furthermore, the scale structure becomes finer, and the hardness of the scale itself increases, reducing the adhesion between the scale and the base steel and the resistance to powder spalling. On the other hand, if the finish rolling exit temperature exceeds 950°C, excessive scale growth increases the scale thickness, reducing the adhesion between the scale and the base steel and the resistance to powder spalling. Furthermore, the grain size of each phase in the scale structure increases, reducing the scale's resistance to powder spalling. Therefore, the finish rolling exit temperature is set to 800 to 950°C. The preferred lower limit is 820°C or higher. The preferred upper limit is 930°C or lower.

Cooling at an average cooling rate of 5°C/s or more in the temperature range from the finish rolling exit temperature to 750°C. Scale grows faster in high-temperature regions, so to prevent excessive scale growth from reducing the adhesion between the scale and the base steel and the scale's resistance to powdery spalling, it is necessary to rapidly cool the high-temperature region immediately after finish rolling. If the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is less than 5°C/s, the scale will grow excessively, causing a reduction in the adhesion between the scale and the base steel and the scale's resistance to powdery spalling. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is set to 5°C/s or more, preferably 7°C/s or more. On the other hand, if the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C exceeds 80°C/s, the scale structure will become finer, which may reduce the scale's adhesion. Furthermore, a decrease in the ductility of the scale can cause cracks, which can promote the reoxidation of the scale, resulting in the formation of hematite, which can cause a decrease in the scale's resistance to powdery spalling. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is preferably 80°C/s or less, and more preferably 50°C/s or less.

Cooling in the temperature range from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less In the temperature range from 750°C to the start of coiling, scale growth is relatively slower than in the high-temperature region immediately after finish rolling, but it is necessary to suppress deterioration in adhesion at the interface between the scale and the base steel and in resistance to scale powder spalling due to excessive scale growth. If the average cooling rate in the temperature range from 750°C to the start of coiling is less than 1°C/s, scale will grow excessively, causing deterioration in adhesion at the interface between the scale and the base steel and in resistance to scale powder spalling. For this reason, the average cooling rate in the temperature range from 750°C to the start of coiling is set to 1°C/s or more, preferably 3°C/s or more. On the other hand, if the average cooling rate in the temperature range from 750°C to the start of coiling exceeds 30°C/s, the scale structure will become finer and the stress difference with the base steel will increase. This causes cracks in the scale, reducing the adhesion between the scale and the base steel interface, and these cracks promote reoxidation of the scale, producing hematite and reducing the scale's resistance to powdery spalling. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is set to 30°C/s or less, preferably 20°C/s or less.

Winding temperature: 500-650℃
After the cooling process, the steel sheet is coiled at a coiling temperature of 500 to 650°C. By insulating the steel sheet surface from the atmosphere during coiling and maintaining it within a temperature range where wüstite is stably formed, some or all of the hematite and magnetite formed on the surface of the scale are reduced to wüstite. By maintaining a temperature range of −50°C or higher and lower than the coiling temperature after coiling for 100 minutes or longer, the eutectoid transformation of wüstite is sufficiently promoted, and an eutectoid-transformed structure is also formed on the surface of the scale. As a result, adhesion between the scale and the base steel is improved, and the amount of magnetite formed on the surface of the scale is reduced, thereby suppressing powdery spalling. If the coiling temperature is lower than 500°C, the eutectoid transformation of wüstite does not occur sufficiently, and excessive wüstite remains at room temperature. As a result, adhesion between the scale and the base steel and the scale's resistance to powdery spalling are reduced. Furthermore, the reduction of hematite and magnetite in the scale surface layer to wüstite does not proceed sufficiently, resulting in insufficient eutectoid transformation of the scale surface layer and reduced powder spalling resistance. If the coiling temperature exceeds 650°C, the scale grows excessively, reducing the adhesion at the interface between the scale and the base steel and the scale's powder spalling resistance. Therefore, the coiling temperature is set to 500°C or higher and 650°C or lower, with a preferred lower limit of 530°C or higher. A preferred upper limit is 630°C or lower.

Holding for 100 Minutes or More in a Temperature Range of Not Less than Coiling Temperature -50°C and Not More than Coiling Temperature After coiling, holding for 100 minutes or more in a temperature range of not less than Coiling Temperature -50°C and not more than Coiling Temperature allows the eutectoid transformation of wüstite to proceed sufficiently. If the holding time in the temperature range of not less than Coiling Temperature -50°C and not more than Coiling Temperature is less than 100 minutes, the eutectoid transformed structure formed on the surface layer of the scale becomes insufficient, and resistance to powdery spalling decreases. Furthermore, if the coiling temperature is held at less than -50°C, the eutectoid transformation of wüstite does not occur sufficiently, and excessive wüstite remains at room temperature. As a result, the adhesion at the interface between the scale and the base steel and the resistance to powdery spalling of the scale decrease. Therefore, the holding time in the temperature range of not less than Coiling Temperature -50°C and not more than Coiling Temperature is set to 100 minutes or more. Preferably, it is 120 minutes or more. Furthermore, if the holding time is long, internal oxidation at the interface between the scale and the base steel may proceed excessively, which may result in a decrease in scale adhesion. Therefore, the holding time in the temperature range of not less than −50° C. the coiling temperature and not more than the coiling temperature is preferably 300 minutes or less.

After winding, it is preferable to place the coil in a coil box or cover it to prevent oxidation of the outermost circumference and edges.

Furthermore, hot-rolled steel sheets wound into a coil may be deformed using a roller leveler, tension leveler, etc., to undergo shape correction. For example, for a hot-rolled steel sheet with a thickness of 12 mm, two upper rolls with a diameter of 250 mm and three lower rolls are arranged, and shape correction is performed with a pressing depth of 2 mm.

The following describes an example of the present invention.

Steels with the composition shown in Table 1 were melted and cast to produce steel materials. These steel materials were hot-rolled under the conditions shown in Table 2 to produce hot-rolled coils with thicknesses of 6 to 18 mm. The resulting hot-rolled coils were subjected to shape correction using a leveler, and then cut to a specified length to produce hot-rolled sheets. Test specimens were taken from each portion of the resulting hot-rolled sheets, and the scale structure and scale adhesion were evaluated using the methods described below.

The area ratios of magnetite, eutectoid transformation structure of iron and magnetite, wüstite, and precipitated Fe were measured by cutting a cross section of the steel sheet through the thickness perpendicular to the surface and parallel to the rolling direction and mirror-polishing it. Then, using a scanning electron microscope (SEM) to observe the backscattered electron image of the cross section of the scale at a magnification of 3000x, the area ratios were measured. In the backscattered electron image of the SEM, magnetite appears as the darkest region, the base steel and precipitated Fe appear as the brightest region, and wüstite appears as an intermediate region. The average thickness of the scale was determined by measuring the scale thickness at three random locations using the SEM and averaging the results.

The mass fraction of hematite was determined by measuring the integrated intensity of the diffraction peaks of each phase in the scale using an X-ray diffractometer with a CoK _α radiation source, and was calculated from the ratio of the integrated intensity of each phase in the test sample to that in a standard sample (a mixture of equal weights of Fe _{, FeO (wustite), Fe2O3 ( hematite} ), and _Fe3O4 (magnetite)) using the following formula (2):
Mass fraction of phase A=(I _A /R _A )×100/((I _Fe /R _Fe )+(I Fe O /R _{Fe O} )+(I _{Fe 2 O 3} /R Fe _{2 O 3} )+(I _{Fe 3 O 4} /R _{Fe 3 O 4} )) (2)
However, in the above formula (2),
I _A : Integrated intensity of phase A in the test sample R _A : Integrated intensity of phase A in the standard sample A: Fe, FeO, Fe ₂ O ₃ , or Fe ₃ O ₄ .

In this invention, scale adhesion was evaluated by measuring the adhesion at the interface between the scale and the base steel and the resistance to powder peeling. Specifically, test specimens were taken from the hot-rolled sheet after leveling, and tape was applied to the steel sheet surface to peel the scale. Evaluation was based on the amount of exposed base steel on the steel sheet surface and the amount of scale adhered to the tape. The peeled tape was then attached to a transparent sheet, scanned, and the amount of peeled scale was measured using image processing. For exposed base steel, images of the steel sheet surface were taken with a camera, and the area ratio of exposed base steel, i.e., the percentage of the area of exposed base steel relative to the area of the peeled tape, was measured. When the area ratio of exposed base steel was less than 10% and the area ratio of scale adhered to the peeled tape was less than 10%, the interfacial adhesion and resistance to powder peeling were deemed excellent, and this was marked with a circle in Table 2. On the other hand, when the area ratio of exposed base steel on the steel sheet surface was 10% or more, the interfacial adhesion between the scale and base steel was deemed poor, and this was marked with an X in Table 2. Furthermore, when the area ratio of scale adhering to the peeled tape was 10% or more, it was determined that the resistance to powder peeling was poor, and this was recorded as x in Table 2.

The examples of the present invention shown in Table 2 had excellent interfacial adhesion between the scale and the base steel and resistance to powdery scale peeling, while the comparative examples had poor interfacial adhesion between the scale and the base steel and/or resistance to powdery scale peeling.

Claims (5)

In mass%,
C: 0.01-0.30%,
Si: 0.50% or less,
Mn: 0.01-2.0%,
P: 0.10% or less,
S: 0.10% or less,
sol. Al: 0.10% or less,
N: 0.015% or less, with the balance being Fe and unavoidable impurities;
The steel sheet has scale on the surface,
The average thickness of the scale is 20 μm or less,
The scale is an area ratio,
Magnetite: 20% or more,
Eutectoid transformation structure of iron and magnetite: 40% or more,
Wustite: 15% or less,
The structure has a total of 90% or more (including 100%) of magnetite, wustite, and eutectoid transformation structures of iron and magnetite,
A hot-rolled steel sheet, wherein the area ratio of precipitated Fe in a region extending from the outermost surface layer of the scale to half the thickness of the scale in the thickness direction is 2% or more. The component composition further comprises, in mass%,
Cu: 1.0% or less,
Ni: 0.50% or less,
The hot-rolled steel sheet according to claim 1, further comprising at least one of the following: Cr: 2.0% or less. The component composition further comprises, in mass%,
Mo: 1.0% or less,
Nb: 0.1% or less,
V: 0.1% or less,
Ti: 0.03% or less,
B: 0.01% or less,
Sb: 0.03% or less,
The hot-rolled steel sheet according to claim 1 or 2, comprising one or more of the following: The method for producing a hot-rolled steel sheet according to claim 1 or 2, wherein a steel material having the above-mentioned composition is
After hot rough rolling, descaling is performed.
Finish rolling is performed at a temperature on the delivery side of the finish rolling mill at 800 to 950°C.
After cooling in the temperature range from the finish rolling delivery temperature to 750 ° C. at an average cooling rate of 5 ° C./s or more,
Cooling the temperature range from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less,
Winding temperature: Winding at 500 to 650°C,
Maintaining the temperature range of -50°C or higher and lower than the winding temperature for 100 minutes or more;
A manufacturing method for hot-rolled steel sheets. The method for producing a hot-rolled steel sheet according to claim 3, further comprising the steps of:
After hot rough rolling, descaling is performed.
Finish rolling is performed at a temperature on the delivery side of the finish rolling mill at 800 to 950°C.
After cooling in the temperature range from the finish rolling delivery temperature to 750 ° C. at an average cooling rate of 5 ° C./s or more,
Cooling the temperature range from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less,
Winding temperature: Winding at 500 to 650°C,
Maintaining the temperature range of -50°C or higher and lower than the winding temperature for 100 minutes or more;
A manufacturing method for hot-rolled steel sheets.